Philippa Kaur

Bacterial density and growth potential in the rearing water were related to the microbial carrying capacityQuantifying the bacterial density in each tank verified that we obtained a higher bacterial load in the systems with added organic material. The bacterial density was, on average, 7.8× higher in the systems with high compared to low bacterial carrying capacity. This difference was particularly evident at 2 (34.8×, Kruskal–Wallis p = 0.0008) and 9 DPH (9.1×, Kruskal–Wallis p = 0.0007) (Fig. 1). The bacterial density increased throughout the experiment for the tanks with low microbial carrying capacity (treatment group MMS−, FTS−), reflecting increased larval feeding and defecation. Contrastingly, the bacterial density was relatively stable over time in the MMS+ treatment and even decreased over time in the FTS+ treatment. When averaging the densities at 11 and 15 DPH within each rearing treatment, we observed that the ‘MMS+ to FTS+’ had a considerable difference in the bacterial density between incoming and rearing water (24.2×). In contrast, this difference was below 8.2× in all other treatment tanks. Such differences in density indicated that some communities were below the microbial carrying capacity of the systems. We thus investigated the growth potential to determine if carrying capacity was reached in the rearing water.Figure 1Bacterial density (million bacterial cells mL−1) at various days post-hatching (DPH) in incoming and rearing tank water. Note that the y-axis is log scaled. Colours indicate the rearing treatment, and shape signifies rearing (filled circle) and incoming water (filled triangle).Full size imageThe bacterial net growth potential in the intake and rearing water was quantified as the number of cell doublings after incubation for 3 days11. Generally, the FTS− and MMS− rearing water had net growth potential with an average of 0.2 and 0.1, respectively (Supplementary Fig. 2). In contrast, the rearing water of the FTS+ and MMS+ had a negative net growth potential with averages of −0.2 and −0.06, respectively. In the case of negative net growth potential, the bacterial density decreased during the incubation. A negative net growth potential suggested that the rearing water bacterial communities were at the tank’s microbial carrying capacity at the time of sampling. Thus, the bacterial communities were at the carrying capacity of the high (+) carrying capacity systems and below in the low (−) systems. To gain a deeper understanding of the bacterial community characteristics the 16S rRNA gene of the bacterial community was sequenced at 1 and 9 DPH.Initial rearing condition did not leave a legacy effect on bacterial α-diversityThe bacterial α-diversity of the rearing water was investigated at 1 and 12 DPH (Fig. 2). At 1 DPH, the richness was comparable between the FTS−, FTS+ and MMS+ treatments, but on average, 1.5× higher for the MMS− treatment (307 vs 205 ASVs, Tukey’s test p  More

G.M., L.O.A., L.V.G. and L.E.O.C.A. thank the São Paulo Research Foundation (FAPESP) for funding (grants 2019/25701-8, 2020/08916-8, 2016/02018-2 and 2020/15230-5, respectively). L.O.A. and L.E.O.C.A. thank the National Council for Scientific and Technological Development (CNPq) for funding (grants 314473/2020-3 and 314416/2020-0, respectively). G.d.O. thanks the University of South Alabama Faculty Development Council Grant for funding (grant 279600-2022). More

We investigated whether ricefield mosquito larval control and/or rice cultivation practices are associated with malaria vector densities through a systematic review and meta-analysis. Forty-seven experimental studies were eligible for inclusion in the qualitative analysis and thirty-three studies were eligible for the meta-analysis. It was demonstrated that the use of fish, chemical and biological larvicides in rice fields were effective in controlling larval malaria vector densities at all developmental stages. Intermittent irrigation, however, could only significantly reduce late-stage larvae. Based on a limited number of studies, meta-analyses on other forms of larval control such as monomolecular surface films (MSFs), neem, copepods and Azolla failed to demonstrate any consistent reduction in anopheline numbers. Similarly, rice cultivation practices such as plant variety and density, type of levelling and pesticide application were not generally associated with reduced malaria vectors. Nonetheless, in one study, minimal tillage was observed to reduce average numbers of larvae throughout a cropping season. In another study, herbicide application increased larval abundance over a 4-week period, as did one-time drainage in a third study.
Despite their different modes of action, the use of chemical and bacterial larvicides and MSFs were all relatively effective measures of larval control in rice fields, varying between a 57% to 76% reduction in vector abundance compared to no larviciding. Their effects were highest (often reaching 100% reduction) only shortly following application but did not persist for longer than two weeks. These larvicides mostly had short residual half-lives because they were applied to paddy water which was naturally not completely stagnant: there was a small but constant process of water loss (through drainage, evapotranspiration and percolation) and replacement through irrigation. Hence, even with a residual formulation, weekly re-application would be needed for sustained control47,40,41,50. This would be very labour- and cost-intensive to scale-up, to ensure that larvicides are evenly distributed across vast areas (even at plot/sub-plot level) throughout at least one 5-month long rice-growing season per year42,51. Aerial application (including unmanned aerial vehicles), although widely used in the US and Europe, is unlikely to be a feasible delivery system for smallholders in SSA, even in large irrigation schemes26,27,48,49. Furthermore, if synthetic organic chemicals were to be considered for riceland malaria vector control, their management in the current landscape of insecticide resistance across Africa must be considered.Biological control using fish was found to be, in general, slightly more effective than (chemical, bacterial and MSF) larviciding. The degree of effectiveness was dependent on the fish species and their feeding preferences: surface-feeding, larvivorous species provided better anopheline control than bottom-feeding selective feeders4,43. Selecting the most suitable fish for local rice fields is not straightforward; many criteria need to be considered4,52,53. Generally, fish were well-received by rice farmers, perceived to contribute to increased yield by reducing weeds and pests and providing fertiliser through excrement43,44. This was reportedly also observed in Guangxi, China, where a certain proportion of the field had to be deepened into a side-trench where the fish could take shelter when the fields were drained. Even with this reduction in rice production area, carp rearing still increased yields by 10% and farmer’s income per hectare by 70%53. Unfortunately, none of the eligible studies in this review had included yield or water use as an outcome. Future entomological studies need to measure these critical agronomic variables so that studies of vector control in rice can be understood by, and transferred to, agronomists. In SSA, irrigated rice-fish farming can be scaled up provided that an inventory of fish species suitable for specific locations is available and that water is consistently available in fields (an important limiting factor in African irrigation schemes)54. Lessons can be learnt from successful large-scale rice-fish systems in Asia, where they have served as win–win solutions for sustainable food production and malaria control16,55.Overall, there was only limited evidence that intermittent irrigation is effective at reducing late-instar anopheline larvae in rice fields. This finding contrasts with prior reviews, which found mixed results (regardless of larval stage) but emphasised that success was site-specific4,17,56. This contrast is presumably due to the inclusion criteria of our systematic review. These reviews excluded studies in various geographical settings and some older studies that reported successful anopheline control with intermittent irrigation but lacked either a contemporaneous control arm, adequate replication or adequate differentiation between culicines and anophelines16,57,50,51,52,61. It seems, from our review, that intermittent irrigation does not prevent the recruitment of early instars (and in one case, may have encouraged oviposition31) but tends to prevent their development into late-stage immatures. This important conclusion is, however, based only on four studies; more evidence is urgently needed where future trials should consider the basic principles of modern trials with adequate replication, controls and differentiation between larval instars and species.Generally, it is observed that drainage, passive or active, did not reliably reduce overall numbers of mosquito immatures. In India and Kenya, closer inspection revealed that soils were not drying sufficiently, so any stranded larvae were not killed31,46. Highlighted by van der Hoek et al.29 and Keiser et al.17, water management in rice fields is very dependent on the physical characteristics of the soil and the climate and is most suited to places that not only favour rapid drying, but also have a good control of water supply17,56. Moreover, repeated drainage, although directed against mosquitoes, can also kill their aquatic predators62. Since mosquitoes can re-establish themselves in a newly flooded rice field more quickly than their predators, intermittent irrigation with more than a week between successive drying periods can permit repeated cycles of mosquito breeding without any predation pressure. Its efficacy against malaria vectors is therefore highly reliant on the timing of the wetting and drying periods. Further site-specific research on timing, especially with regards to predator–prey interactions within the rice agroecosystem, is required to find the perfect balance.Another limitation in intermittent irrigation is that it cannot be applied during the first two to three weeks following transplanting, because rice plants must remain flooded to recover from transplanting shock. Unfortunately, this time coincides with peak vector breeding. Thus, other methods of larval control would be required to fill this gap. To agronomists, intermittent irrigation provides benefits to farmers, as it does not penalise yield but significantly reduces water consumption. Nonetheless, farmer compliance seems to be variable, especially in areas where water availability is inconsistent and intermittent irrigation would potentially require more labour31,32,39. Importantly, rice farmers doubted their ability to coordinate water distribution evenly amongst themselves, suggesting that there may be sharing issues, as in the “tragedy of the commons”63. Instead, they said that they preferred to have an agreed authority to regulate water46.No general conclusions could be made on the effect on malaria vectors of other rice cultivation practices (apart from water management) because only one study was eligible for each practice. Nevertheless, these experiments on pesticide application, tillage and weed control, as well as another study on plant spacing (not eligible since glass rods were used to simulate rice plants), do illustrate that small changes in agronomic inputs and conditions can have considerable effects on mosquito densities, not just rice yield36,38,64. Moreover, in partially- or shallowly-flooded plots, the larvae are often concentrated in depressions (usually footprints), suggesting that rice operations which leave or remove footprints (e.g. hand-weeding, drum seeders, levelling) will influence vector breeding4.Our study has some important limitations. First, in most trials, the units of intervention were replicate plots of rice, and success was measured as a reduction in larval densities within treated plots. This design focuses on the identification of effective and easy-to-implement ways of growing rice without growing mosquitoes, on the assumption that higher vector densities are harmful. However, from a public health perspective, the need for epidemiological outcomes is often, and reasonably, stressed22,65. Nonetheless, from a farmers’ perspective, it is also important to consider whether the vectors emerging from their rice fields significantly contribute to the local burden of malaria and to determine how this contribution can be minimised. There is evidence that riceland vectors do increase malaria transmission, since human biting rates are much higher in communities living next to rice schemes than their non-rice counterparts66 and that additional riceland vectors may intensify transmission and malaria prevalence in rice communities15. Hence, when investigating how rice-attributed malaria risk can be minimised, mosquito abundance as measured in the experimental rice trials is a useful indicator of potential impact on epidemiological outcomes.Second, larval density was not always separated into larval developmental stages. This can be misleading because some interventions work by reducing larval survival (but not by preventing oviposition) and development to late instars and pupae. Therefore, an intervention could completely eliminate late-stage larvae and pupae but have little effect on the total number of immatures. This was illustrated in our meta-analyses of intermittent irrigation in Table 3 and Supplementary Table 5, and could have been the case for some studies that failed to demonstrate consistent reductions in overall anopheline numbers but did not differentiate between larval instars34,45,67,60,69. We infer that when monitoring mosquito immatures in rice trials, it is important to distinguish between larval instars and pupae. Pupae should always be counted separately since its abundance is the most direct indicator of adult productivity70.Third, experimental trials rarely reported the timing of intervention application or accounted for different rice-growing phases, or “days after transplantation”, in the outcome. Both aspects are important to consider since an intervention may be suited to control larvae during certain growth phases but not others. This is illustrated by Djegbe et al.38, where, compared to deep tillage, minimal tillage could significantly reduce larvae during the early stages of rice cultivation but not during tillering and maturation38. In contrast, other interventions, such as Azolla and predatory copepods, took time to grow and accumulate, and were more effective during the later stages of a rice season45,67,71. This differentiation is important because it can identify components that could potentially form a complementary set of interventions against riceland malaria vectors, each component being effective at different parts of the season. Since rice fields, and hence the dynamics of riceland mosquito populations, vary from place to place, this set of interventions must also be robust. Special attention must be paid to the early stages of rice cultivation, particularly the first few weeks after transplanting (or sowing), since, with many vector species, a large proportion of adult mosquitoes are produced during this time.Fourth, the analysis of entomological counts is often inadequate. Many studies failed to provide the standard deviation (or any other measure of error) for larval counts and could not be included in the quantitative analysis. Often, due to the extreme (and not unexpected) variability of larval numbers, sample sizes were insufficient to calculate statistically significant differences between treatments. Fifth, a high risk of bias was found across both CTS and CITS studies, including high heterogeneity and some publication bias. Study quality was, in general, a shortcoming and limited the number of eligible studies for certain interventions, including intermittent irrigation. Moreover, there are conspicuous a priori reasons for bias in such experimental trials: trial locations are frequently chosen to maximise the probability of success.Finally, few studies were conducted in African countries, where the relationship between rice and malaria is most important because of the efficiency, and the “rice-philic” nature, of the vector An. gambiae s.l.15. In particular, there was a lack of studies on the effectiveness and scalability of biological control and rice cultivation practices. There is also very little information (particularly social science studies) on the views and perspectives of African rice farmers on mosquitoes in rice and interventions to control them72,73.In the future, as malaria declines (particularly across SSA), the contribution of rice production to increased malaria transmission is likely to become more conspicuous15. Unless this problem is addressed, rice growing will probably become an obstacle to malaria elimination. Current default methods of rice production provide near-perfect conditions for the larvae of African malaria vectors. Therefore, we need to develop modified rice-growing methods that are unfavourable to mosquitoes but still favourable for the rice. Although larviciding and biological control may be appropriate, their unsustainable costs remain the biggest barrier to uptake amongst smallholder farmers. Future investigations into riceland vector control should pay more attention to interventions that may be useful to farmers.Supported by medical entomologists, agronomists should lead the research task of identifying cultivation methods that achieve high rice productivity whilst suppressing vector productivity. Rice fields are a major global source of greenhouse gases, and agronomists have responded by successfully developing novel cultivation methods that minimise these emissions while maintaining yield. We need the same kind of response from agronomists, to achieve malaria control co-benefits within rice cultivation. At present, only a few aspects of rice cultivation have been investigated for their effects on mosquitoes, and the potential of many other practices for reducing anopheline numbers are awaiting study. Due to the spatial and temporal heterogeneity of rice agroecosystems, it is likely that no single control method can reduce mosquito numbers throughout an entire cropping season and in all soil types and irrigation methods. Thus, effective overall control is likely to come from a combination of local, site-specific set of complementary methods, each of which is active and effective during a different phase of the rice-growing season. More

Notwithstanding constraints on the amount of hard data, according to our integrated analysis, the developmental environment and ecology of reef communities have an important impact on the appearance of reefs.Analysis of environmental conditions for reef developmentSettings of reef developmentThe F/F extinction event in Late Devonian caused the complete recession of the reef-building communities based on stromatoporoid-coral assemblages7,17. The Carboniferous is generally considered to be a sub-optimal period for the development of framed reefs. After the biological mass extinction, microorganisms and algae rebuilt new reef-building ecosystems18,19. Some short-term biological frame reefs developed with low diversity, limited reef-building organisms, small sizes, and restricted distribution20. Harsh climate and marine conditions occurred in the Mississippian, including extensive marine hypoxia, repeated glacial and interglacial climate changes, and frequent changes of sea level and seawater surface temperature, potentially hindering the recovery of Early Carboniferous metazoan reefs7,21.Metazoans gradually began to participate in reef building in Early Viséan. A large number of biogenic structures formed by corals and bryozoans began to appear, including a small number of sponge reefs/mounds in the middle and late stage of Viséan. The richness and biodiversity of the Mississippian post-zoobenthic reefs flourished in the late Viséan during which corals, bryozoans, sponges, calcareous microorganisms, and some calcareous algae became the main builders3 and large-scale reefs could also be seen in some areas although most of the Viséan metazoan reefs were tabular or laminar. Thus, the metazoan skeletal reefs in the middle to late Viséan were considered to have been resurrected due to relatively warm climatic conditions and higher sea levels after a period of complete disintegration at the end of the Devonian and recession at the beginning of the Carboniferous7.Consequently, the coral reefs in the study area were the products of shallow benthic communities thriving in relatively favourable conditions of Late Viséan-Serpukhovian, which was common for reef development at that time7. Thus, it is expected that more synchronous reefs would to be identified in southern China, or even in the study area in the future.Paleogeography of reefsLangping is located in Dian-Qian-Gui Basin22 regionally (Fig. 2), in the eastern end of Tethys tectonic domain and at the interjunction of Tethys and Pacific structure globally. The Carboniferous Dian-Qian-Gui Basin is adjacent to the Tethys Basin. During the Early Carboniferous, the continent of Gondwana was close to the equator but was separated from the northern continent by the Tethys, where the tropical currents flowed freely from east to west. The benthic warm-water organisms were distributed widely with high abundance and diversity on both sides of the shallow shelf of Tethys.Figure 2Paleogeographic map of southern China in Viséan-Serpukhovian (modified from Feng23, Yao8, and Maillet24). This figure was obtained from articles by Feng23, Yao8 and Maillet24 respectively. The author modified the picture with CorelDRAW (version 2022, and the URL link: https://www.coreldraw.com/cn/). QG Qian-Gui Basin, DQGX Dian-Qian-Gui-Xiang platform.Full size imageViséan-Serpukhovian ecosystems experienced dramatic climate changes and widespread glaciation25. However, the Viséan was also a key layer for a variety of biological structures, with abundant coral reefs and a high diversity of shallow benthic communities, peaking in the late Viséan. Newly discovered post-faunal reefs in Tianlin were mainly formed in the late Viséan-Serpukhovian period, which coincided with frequent sea level fluctuations and possible glacial changes. It seems counterintuitive that tropical coral communities developed during glacial period. However, recent studies suggest that the persistent warm ocean currents on the platform helped some coral species survive from Carboniferous glacial events24. While other areas of symbiotic reefs were poorly developed, Tianlin may provide an ecological sanctuary for corals associated with ocean currents26.Sedimentation of reef developmentAccording to the regional geological structure, the slope model for Langping paleocarbonate platform was obviously different from that of steep slope platform margin, which could be directly affected by waves. Langping palaeo-platform could be regarded as one of the small blocks (block fault barrier) separated from a large platform (continental margin sea basin)13. The relative positions of these blocks were crucial for the emergence and growth of reefs.In situ development of mud-crystalline tuffs and muddy tuffs with weak hydrodynamic conditions is common in the Langping area, and evaporites are poorly developed. There were patch reefs and reef layers in different sizes in the wide intraclast beach, where obviously developed reef beach complexes were rare. The fragments of carbonate base broken by storm in the clastic beach haven’t been observed. The study area is considered to be gentle-slope open platform27,28 based on sedimentary characteristics. It suggests that the study area was far away from the margin of steep-slope platform that directly affected by waves, and more consistent with less energetic internal environments of gentle-slope platform.On the vast platform of Langping gentle slop, deep water lead to low water energy. While in the coastal area, the water energy was relatively strong, thus coarse-grained bioclastic beach and a small amount of point reef could be developed. The beaches were irregular-shaped due to long term transportation and reformation effects of waves and water flow, showing low and gentle slope angles. Dispersed reef-beach complexes at the platform margin slightly impacted inner-platform seawater and the water flows smoothly29.Therefore, it can be assumed that Langping reefs developed along the intertidal shallows of the terrace. The seawater around Langping carbonate platform in Late Viséan-Serpukhovian was relatively shallow while the water flow was strong. Remains of crinoids, brachiopods, a few foraminifera, and solitary corals were likely broken by strong currents, and deposited in situ with a small amount of gravels and lime-mud (Fig. 3). The clastic beach was unstable, suggesting large-scale wave-resistant structures could not be formed quickly30 due to insufficient cohesive and consolidating organisms. In addition, the circumferential impact of water in extensive terraces leads to mud-lime deposition, which is detrimental to most benthic organisms. However, bondstone was more likely to be formed by some binding algaes in the platform (Fig. 4). Therefore, neither the surrounding or the inner region of the platform could provide favourable living conditions for coral reefs to develop over for a long term. The gently sloping terrace environment of Lanping resulted in significant differences in growth size, wave resistance and reef-building capacity between corals in the study area and those on the edge of the steeply sloping terrace.Figure 3Clastic beaches in the Langping. Various clastic beaches developed in the study area. Diverse composition, fragmentation degree and sorting of the clast indicate different water conditions of formation. This figure is modified by the author from field photos with CorelDRAW (version 2022, and the URL link: https://www.coreldraw.com/cn/).Full size imageFigure 4Algal bondstone in the Langping. Bondstone formed by various algaes living in still water. Morphology of bondstone correlates water environment and deposition of mud. Vast algal bondstones indicate deep water and high deposition rate. This figure is modified by the author from field photos with CorelDRAW (version 2022, and the URL link: https://www.coreldraw.com/cn/).Full size imageAt the same time, the warm climate of the late Viséan-Serpukhovian, the good circulation of seawater around the Langping platform, and the abundant supply of oxygen and nutrients were a series of favourable conditions that facilitated the growth of reef-building corals, which led to uplifts being formed on clastic beach, including patch reefs and reef layers with certain sizes. These uplifts impeded waves and provided a protected nearshore environment, though they were much smaller than those developed at the steep-slope platform margin. The inhabitants on the beach could not resist strong waves. These rises were therefore known as reef-beach complexes and could only persist where waves and currents were mild28. They were essentially different from the framework coral reefs which developed on steep-slope platform margin that reflected changed hydrodynamic conditions, nutrient sources, reef sizes, and growth rates.Another potentially favourable factor in the study area could be the deeper water area in the gentle-slope sedimentary environment, which could provide more stable conditions and reduce the damaging effects of global glacial events and large scale sea level fluctuations on reef-building communities25. The frequent fault activities in Dian-Qian-Gui Basin caused the rise and fall of equivalent sea level. More influence of sea-level fluctuations and hydrodynamic conditions would be exerted on Langping platform due to its small size. Furthermore, reef growth promoted by reef-building communities would be frequently disturbed. The sediments displaying evidence of multicycle sedimentation, different components, and diversely fragile clasts in the study area provided direct evidence of frequently changing environment.Alternatively, the sedimentary environment of Langping platform provided conditions favorable for reef-building communities to develop and reefs to grow rapidly. These factors directly or indirectly determined the ecology of reef-building communities and the general appearance of reef development in the study area.Overall, the environmental factor is the primary factor affecting the overall development trend of reefs.Inferred ecological characteristics of reef communitiesResponse of reef-building corals to hydrodynamic conditionsHydrodynamic conditions are very important factors for reef development, which directly determine the abundance and distribution of each reef-building population and are key factors influencing sedimentation and reef growth, and was particularly evident at Langping. Evidence from the fossils suggested the reef-building corals were also changed in response (Table 1). The hydrodynamic condition changes during the development of reefs are inferred based on analysis of the vertical sediments and microfacies changes of coral reefs in the study area31. How these ancient reef-building corals adapted to hydrodynamic conditions was reconstructed combining the evolution of reef-building communities with the study.Table.1 General situation of reef-building coral population in Langping.Full size tableThe Xiadong coral reef started with colonization and expansion of Diphyphyllum on the bioclastic hard substrates32,33. They grew vertically into upright clusters (Fig. 5A) and were insensitive to more sediment in a relatively calm, turbid water environment34. The relatively dense clumped Siphonodendron and massive Lithostrotion (Fig. 5B) were better suited to the turbulent water environment, becoming dominant over time, with Diphyphyllum subordinate with the continuous increase of the water energy, as indicated by the characteristics of sediment particles from fine to coarse. After flourishing for a period, the Siphonodendron–Lithostrotion assemblage eventually waned, likely due to the failure to adapt to the increasing hydrodynamic conditions. Diphyphyllum had persisted combined with Syringopora, to maintain the growth of the reef. However, this assemblage subsequently declined as a result of strong hydrodynamic conditions and finally died out in response to continuous falling of sea level. Consequently, the reefs stopped developing.Figure 5Sketch of coral cluster with upright growing morphology. Most reef-building corals in Langping grow vertically into cluster colonies. This type of morphology is very favourable for corals to get more living space and is important to reef-building. (A) Cluster coral individuals grow uprightly with certain distance between each other. (B) Polygonal columnar coral individuals grow closely to resist strong water flow. This figure is made by the author with CorelDRAW (version 2022, and the URL link: https://www.coreldraw.com/cn/).Full size imageThe Longjiangdong multi-layer reef was composed of three relatively independent, flat reef layers, suggesting three distinct periods of reef development. Diverse species were identified in the reef, with colonial coral Diphyphyllum contributing greatly to reef growth. Diphyphyllum clusters colonized in patchy form on substrates composed of bioclasts or lithic gravels (Fig. 6A). The first reef-building process was brief, ending under high-energy water conditions after a period of growing (Fig. 6B). Subsequently the hydrodynamic conditions became weaker and favorable. Then Diphyphyllum once again flourished. Diphyphyllum clumps in the unit grew closely together in strong currents, with larger and more sparse individuals than in the lower units. A relatively low energy environment was formed between the Diphyphyllum clusters (Fig. 6C). Subsequently, Diphyphyllum could only grow in a limited area of suitability due to the disturbance of high-energy water brought about by short-term sea-level rise and fall. Afterwards, the environment became more favourable and Diphyphyllum expanded rapidly. As a result, the upper unit of Longjiangdong coral reef was formed, in which Diphyphyllum individuals were slightly larger than those in the first two units. Finally, because the kinetic energy of the water continued to weaken, the plaster deposition forced the whole coral reef to stop growing (Fig. 6D).Figure 6Micrographs of sediments in different positions of reef. (A) Calcareous bioclastic limestone, with biological particles accounting for about 70% of the debris. Abundant and diverse organisms indicate a medium-energy environment of the subtidal zone. Samples were taken from the bioclastic beach at the reef base. (B) Slightly larger bioclastics but lower biologic content than that in (A) suggest an increasing water energy. (C) Various bioclastic particles account for about 80% of the clastic particles contained in the calcareous bio-granular rock. The obviously small benthos indicate a low-energy environment in the subtidal zone barriered by the Diphyphyllum clusters. (D) Bioclastic grainstone is mainly composed of marl, with fine clastic particles (about 30%) and bedding. Low biomass indicates a low-energy environment of the subtidal zone. This figure is modified by the author with CorelDRAW (version 2022, and the URL link: https://www.coreldraw.com/cn/). Meaning of the letters in the figure: C crinoids, BF brachiopods, F foraminifera, B bryozoan, P pelletoid, MF mollusk shell fragment.Full size imageLongjiangdong patch reef started to develop in a relatively deep water environment. Diphyphyllum initially colonized and expanded in favorable conditions with the increase of water energy. Then the reef-builders transitioned from a single coral species to an assemblage of Diphyphyllum–Caninia–Lithostrotionella. These three coral species grew independently and contribute almost equally to the structure of the reef. However, the structure and function of the coral community were not yet stable enough. It was easily influenced by the weakening hydrodynamics and the increasing sedimentation, resulting in only small patch reefs.The Xinzhai layer reef was initialized by colonization and expansion of Lonsdaleia on bioclastic beach. Large coral clusters were formed in the presence of turbulent water. With the weakening of hydrodynamic conditions, an unknown branchlike organism and Antheria communities continued to develop separately in this area. Slender branchlike organisms expanded rapidly in these low-energy water environments until they were replaced by some individual corals as hydrodynamic energy increased. Each builder was short-lived in this layer reef, departing from the reef just at the beginning of colonization and expansion, due to rapidly changed hydrodynamic conditions.The evolution of reef-building corals in these four reefs indicated that both the coral assemblages and coral individuals would constantly adapt to the changing hydrodynamic conditions in Langping as sea level rose and fell. Although this was a reactive adjustment of coral populations in response to long-term environmental impacts, it was clearly positive for the building and development of coral reefs.Impact of disturbance on reef communitiesDisturbance is a relatively discontinuous event, which is ubiquitous in nature. It may indirectly affect the composition and population structure of reef communities by changing the environmental conditions, thus affect the structure and function of reef communities, even the evolution of the reef35. The major disturbances evident in these Mississippian framework reefs were associated with frequent changes of water flow, and drastic changes of climate and weather. These seem to be most obvious in the Langping platform due to its small size, with more frequent environmental influence evident on the reef communities in the study area.The most direct effect of disturbance events on coral reefs is the disruption of continuously evolving reef communities, which is common in coral reef studies. After the interruption caused by disturbances, some communities gradually recover due to the absence of continuous disturbance, or the dominant biota may be substituted by invading communities. The winner after interruption is decided by random factors to a large extent, in a ‘Competitive lottery’36. The conditions for the emergence of ‘Competitive lottery’ also include the need for species in a community to have similar abilities to invade discontinuities and to tolerate environmental conditions.Certainly, low-intensity disturbance does not necessarily produce discontinuity, but medium-intensity disturbance without discontinuity could directly impact on community species diversity. According to the ‘Moderate disturbance hypothesis’, moderate disturbance is conducive to a higher level of community diversity37. In environmental conditions with moderate intensity of disturbance, most species will not disappear entirely. The dominant pioneer species will also be restrained by disturbance to a certain extent, so large number of species can coexist, attaining the highest diversity35.The reef-builders in Langping are diverse compared with the Late Carboniferous reefs in Ziyun County10, which also developed in Dian-Qian-Gui Basin. More than 4 reef-building corals are identified in Xiadong reef, while 4 and 3 are in Xinzhai layer reef, Longjiangdong patch reef respectively. These reef-building corals, mostly Diphyphyllum, Lithostrotion, Siphonodendron and Lonsdaleia, were distributed irregularly in the reefs. Their ecological niche and function were likely similar and none of them was obviously dominant in the community (Fig. 7). This is in line with ‘Competitive lottery’ theory and the ‘Moderate disturbance hypothesis’.Figure 7Different species occupied the discontinuity surface irregularly. (A) Different reef-builders colonized and grew on the same hard substrate. (B) and (C) show detailed morphology of colony corals of (A). (D) Colony corals and a large number of individual corals grew together in a limited area, indicating equal colonization on the newly formed discontinuity surface. This figure is modified by the author from field photos with CorelDRAW (version 2022, and the URL link: https://www.coreldraw.com/cn/).Full size imageThe stability of a classical reef ecosystem includes the ability to withstand external disturbances and the ability to return to its original state once the disturbance is removed37,38. It is generally accepted that communities with high diversity are always more stable although ecosystem stability is not absolutely correlated to biodiversity35.There have been no reef-building corals with strong resistance and rapid recovery ability in the communities in Langping. None of these corals succeeded in developing into dominant species that can build reef shelves, which made the reefs in Langping mostly appear in the form of small patch reefs or reef layers. However, formation of the large reef in Xiadong Village, patch reef in Longjiangdong Village, and layer reef in Xinzhai Village were all related to their relatively high diversity of reef-building corals. Compared with the situation where only one reef-building organism dominated the Bianping large coral reef, Wengdao large phylloid algal reef and Ivanovia cf. manchurica patch reefs in Ziyun County10, Guizhou province, the different coral assemblages in Langping area could effectively adapt to changing hydrodynamic conditions and maintain reef growth.Species diversity increased by disturbance stabilized the ecosystemas shown during the construction of coral reefs in Langping.Effects of non-reef-builders on reef-building coralsBesides reef-building corals, there were a large number of reef-dwellers and off-reef organisms in the study area. Reef-dwellers referred to the species that didn’t directly contribute to reef growth in the community, mainly including various benthos and algaes39. Off-reef organisms are not part of the reef-building community, but also play an important role in participating in energy flow and providing organic matter to the reef ecosystem40.Common reef-dwelling organisms include crinoids, brachiopods, gastropods, various algae, foraminifera, bryozoans and individual corals. Crinoids were overwhelmingly dominant in numbers in the reef samples studied here.Carboniferous echinodermata in Guangxi Province reached its peak in Middle-to-Late Mississippian. In terms of amount and distribution, thick limestone with echinodermata debris in the carbonate platform were often dominated by crinoids41,42,43,44,45,46. The large number of crinoids in Langping excluded other metazoans and restricted the development of benthic reef-builders in Late Viséan-Serpukhovian in Langping, leading to poorly developed reef-building communities.Microorganisms and algaes had limited success in stablishing on the moving clastic beach in frequently disturbed water. There has not been obvious evidence of extensive “algal turf” in the coastal area of Langping platform. Only a few corals bonded by algal mats were observed47 (Fig. 8). In addition to their significant contribution to primary productivity, macroalgae were considered to play an important role in two aspects of coral reef ecosystems. One was to promote reef construction by its own binding and consolidation48,49. The other was to create a good condition for zoobenthos larvae to dwell and develop, thereby improving species diversity50. The limited productivity of algae in Langping constrained coral reef trophic inputs, which may then have limited populations of dependent metazoans. As a result, algaes and other metazoans were unable to achieve a variety of reef-building patterns, such as bonding, bounding, entanglement51,52. The reef framework in the study area was not stable in the presence of strong water flow, and the biological communities could not deal with frequent environmental changes, which were directly related to poor development of calcareous algae.Figure 8Micrographs of microbes and algaes. (A) Encrustations (indicated by black arrows) with distinct thickness around coral clusters formed by microbe and algal mats through bonding mud. The encrustations were formed before the clastic deposition (indicated by white arrows), showing the corals were living then. Microbes and algaes inside of the dense coral clusters had little impact on corals. (B) Single polarized micrograph showed clear and smooth boundaries of coral individuals without encrustation or drilling hole made by microbes or algaes. Few corals surrounded by bonding algaes could be observed in Langping, indicating that algaes were poorly developed between coral clusters. This figure is modified by the author from field photos with CorelDRAW (version 2022, and the URL link: https://www.coreldraw.com/cn/).Full size imageInfluence of coral morphology on reef developmentThe accumulation of reef structure had obvious impact on communities. Large reef structures could support abundance and diverse biota by modifying local environments and creating diverse conditions. Consequently the reef-building communities thrived between disturbances, stabilizing reef construction. In terms of large reef, the framework-building corals would play a key role in reef construction regardless of which kind of patterns was adopted. Therefore, reef-building corals with large size, rapid growth vertically, and strong resistance would become the biggest contributors to reef frame construction.The main reef-building corals in Langping were composed of Diphyphyllum, Lithostrotion, Siphonodendron, and Lonsdaleia, etc., being the dominant builders. These corals were similar in morphology such as cluster colony, thick and strong skeleton, and densely packed individuals (Fig. 9), which enabled them to resist water flow. At the same time, the upright colonies were adaptable to relatively calm water, being insensitive to mud deposition. The ecological characteristics of the Langping corals matched the gently sloping environment, the deep water environment and the rapidly changing energy of the currents. These cluster corals were able to colonize hard substrates and expand rapidly, thus altering the surrounding environment. The visible carbonate uplifts were formed with a large amount of benthos grouped into reef-building communities. These distinct uplifts constructed by coral clusters in different water conditions are composed of coral reefs of different sizes and appearance in the study area.Figure 9Main reef-building corals in the study area. (A) Diphyphyllum, (B) Lithostrotion, (C) Siphonodendron, (D) Lonsdaleia. (A) Rapidly grew clusters of main reef-building corals. The strong individuals are packed tightly when growing to support each other. This figure is modified by the author from field photos with CorelDRAW (version 2022, and the URL link: https://www.coreldraw.com/cn/).Full size imageThe complex and diverse local environments formed by large coral reefs can significantly increase benthic populations and improve reef species diversity. As a result, the nutrient flow in the community becomes complicated, and nutrients could be recycled effectively by reducing loss caused by water flow. Therefore, the overall productivity of large coral reef communities was always high. Complex trophic structure satisfied most of the benthos in the community with sufficient nutrients and inorganic salts.The morphology of reef-building corals in Langping enabled them to become predominant species in various water environments, which promoted the continued domed growth of coral reefs and facilitates the development of reef-building communities that form a variety of reefs. It suggests that the morphology of reef-building corals was a key prerequisite for reef development.In conclusion, coral reef communities are always constrained and influenced by environmental conditions. However, the ecology of the inhabitants is also an important factor in the formation of coral reefs. More

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Model validationBased on the time-series validations of water temperature and DO concentration, model accuracy improved gradually, despite several discrepancies at the beginning of the simulation (Supplementary Fig. S1). The model is primarily driven by a set of boundary data, including wind speed, solar radiation, and precipitation data24,25. From this perspective, more high-quality boundary data promotes better numerical reproducibility. However, meteorological data collection was challenging due to the early observation equipment limitations and low observational accuracy compared to current data. The temporal inconsistency of accuracy in observational data has been eliminated to a large extent by fitting a regression curve24. Spatial resolution is the other issue. Possessing spatially constant values for all boundary conditions complicates the numerical reproducibility of variations on finer scales.The relationship between turnovers and the curve shape of water temperature versus DO concentration is theoretically sound27,28. In the last stage of stratification in the lake, water temperature and DO concentration near the bottom are more likely to slightly increase due to thermal diffusion and DO supplies from the upper water. If a turnover occurs, the whole column of water is mixed strongly (Supplementary Fig. S3). Bottom water temperature decreases due to surface water cooling, and DO concentration increases, due to surface water replenishment and increased oxygen solubility. If the turnover fails, only the partial column of water is mixed, causing a delay in the timing of deep-water renewal (Supplementary Fig. S3). However, the upper water in later months, like that in March, has been rapidly warmed, resulting in an increase in the bottom water temperature. For example, in 2007 and 2016, the simulated water temperature and DO concentration fluctuated within a limited range in February and then skyrocketed in March, after mixing with the warmed surface water (blue points in Supplementary Fig. S4). On the other hand, explicit definitions of turnover timing are challenging. The threshold used to judge turnover timing is reliable because the results matched the observation. The turnover timing varied by 36 days in Lake Biwa during the simulation period, which is comparable to that observed in other lakes, such as approximately 21 days in Heiligensee, Germany over a 17-year timespan29, 16 days in Lake Washington over a 40-year timespan30, and 28 days in Blelham Tarn over a 41-year timespan31.Variables affecting the mixing regimeDetermining variables that affect the mixing regime is essential to improve understanding and enable future projections16,17,18. Air temperature, wind speed, cloud cover, precipitation, water density, and lake transparency are all potential variables. We, here, compared the above variables to the turnover timing in Lake Biwa. The meteorological inputs in this study provided data for air temperature, wind speed, cloud cover, and precipitation. Water density and particulate organic carbon (POC) concentration representing lake transparency were the model’s outputs. The annual averages and cold season (November–April) values of the above variables were calculated over the simulation period (Supplementary Fig. S6). Annual averages illustrate general long-term warming trends18, while cold season values particularly determine the timing of turnover17. However, in Lake Biwa, air temperature during the cold season fluctuated greatly compared to the annual averages. A random forest analysis17 has been conducted between the turnover timing and the above two variable sets (cold season values versus annual averages) in Lake Biwa, and the cold season values better explained the turnover timing (35.39% versus 18.48%). The results agree with the conclusion drawn from the previous sensitivity tests, which indicated the relative importance of air temperature and solar radiation during winter based on 40 scenarios32.The importance of variables was estimated based on the random forest analysis using the cold season data (Fig. 4a). Wind speed dominates the timing of turnover, which is consistent with the previous studies17,25. The POC concentration, the difference in water density between the surface and bottom, and cloud cover have moderate effects on the timing of turnover. However, air temperature is less important, which is contrary to the turnover mechanism17,24,32. A re-confirmation was conducted of the relationship between turnover timing and air temperature (Fig. 4b and Supplementary Fig. S7). The cool air generally encourages an early turnover, albeit with several anomalies. The turnover timing between 1976 and 1990 remained constant independent of climate change, and the period coincidently had a substantial nutrient fluctuation (Fig. 3). As a result, it is essential to investigate the nutrient status further.Figure 4Analysis results of the relationship between potential variables and turnover timing: (a) the importance of variables importance using a random forest analysis, and (b) the relationship between the cold season air temperature and the timing of turnover. Variable importance is calculated using the percentage increase in mean square error (MSE) and the increase in node purity. Higher values illustrate the greater importance of the variable. Variables include air temperature (AT), precipitation (pptn.), cloud cover (CC), the difference in density (DD), POC, and wind speed (WS).Full size imageLake nutrient concentrationsBecause phosphorus is the limiting nutrient in Lake Biwa and DIP concentrations can be effectively limited by regulating external loadings as practiced (Fig. 3), DIP concentrations become the focus of this discussion for nutrient status. However, the DIP concentrations disproportionately responded to the external loadings of total phosphorus (TP) in Lake Biwa. Although external TP loading itself fails to determine lake phosphorus concentrations due to the hydrodynamics of lakes33, Lake Biwa exhibited insignificant changes in the inflow rate or the retention time (and see an example of the surface flow in Supplementary Fig. S8). Therefore, it can be assumed that the hydraulic loading remained constant, and the input nutrient concentrations were proportionate to the external nutrient loadings in Lake Biwa. This finding contradicts a recent meta-analysis that highlighted a deterministic relationship between input nutrient concentrations and lake nutrient concentrations, based on steady-state mass balance models6. The possible reason is the dynamics of the lake’s ecosystem22, which have been considered in this study. For example, the surface DIP concentrations were almost nonexistent regardless of the external TP loadings in Lake Biwa, supporting that phosphorus is the limiting nutrient in Lake Biwa34,35. The low DIP concentrations at the surface may be caused by the rapid recycling of phosphorus because the amount of phosphorus available for phytoplankton is easily affected by the feedback mechanism between phytoplankton photosynthesis and the phosphorus released from the water35,36.Hypoxia and strategiesThe variations in DO concentration are the public’s top concern as it relates to hypoxia, a key indicator of water quality. Lake bottom, among all water depths, is more sensitive to small changes in oxygen conditions12. In Lake Biwa, the annual minimum DO concentrations ranged from 2 to 5.5 mg/L over the last 60 years (Supplementary Fig. S9). The decrease in DO concentrations in the early period, typically till the 1980s, was mainly caused by nutrient enrichments (Fig. 3). The nutrient enrichment-induced heavy eutrophication eventually accelerates the rate of DO depletion2. After eutrophication was controlled in the 1980s, climate change became the dominant stressor23. There remains much uncertainty surrounding the relationship between climatic variables-related turnover timing and hypoxia in Lake Biwa12. We, therefore, first investigate the relationship between hypoxia and turnover timing, and then concentrate on nutrients to alleviate hypoxia.Although the relationship between turnover timing and DO concentrations is quite weak (R2 = 0.10), there is a general decrease in DO concentrations with increasing turnover timing (Fig. 5a). On the other hand, a linear relationship has been found between DIP concentrations and DO concentrations, with an R2 of 0.67 (Fig. 5b). The slope of –0.841 μgP/mgDO means an increase in DIP concentrations by approximately 0.841 μgP/L causes a decrease in DO concentrations by 1 mg/L. Note that the simulation results were compared over the whole period, and eutrophication-induced hypoxia differs theoretically from climate-induced hypoxia. Additional testing has been conducted to distinguish the effects of two stressors (eutrophication- and climate-induced hypoxia; Supplementary Fig. S10). Before 1980 when eutrophication progressed, the annual minimum DO concentrations and the DIP concentrations had a stronger linear relationship (R2 = 0.89). Although waste-water treatment has improved conditions in the lake, climate change induced alteration of turnover timing may adversely influence water quality. However, the relationship weakened dramatically with an R2 of 0.10 after 1980, when climate change dominated hypoxia. The lower R2 value indicates that climate-related hypoxia is more complex as concluded previously37,38. The two possibilities are as follows. First, there can be a legacy of hypoxia related to eutrophication. The DO recovery at the bottom of Lake Biwa was complicated by the low DO concentration in 1980 and the delayed timing of turnover; similar phenomena have been observed in the Lake of Zurich22. Second, ecosystem dynamics could help explain the difficulty in predicting hypoxia at the bottom. Phytoplankton fully exploits phosphorus at the surface, as explained above, then the death and sinking of the surface phytoplankton are accompanied by the sedimentation of phosphorus to the bottom as modeled. Bacteria break down the sinking phytoplankton, releasing phosphorus and consuming DO in the process. Additional DO consumption lowers the bottom DO concentration, which in turn encourages phosphorus release from the sediment in a low DO environment22. Such unfavorable feedback between DIP and DO concentrations are strengthened by prolonged stratification and eventually accelerates the development of hypoxia. However, future research is necessary because this numerical model simplified the relationship between water and sediment. The sinking of organic carbon into sediment is integrated in the model, and due to the decomposition of organic carbon in the sediment, nutrients are released into and oxygen is depleted in the water. Despite that, the trends between DO and DIP concentrations stay the same under climate change (Fig. 5b), and thus controlling lake phosphorus is beneficial to the Lake Biwa hypoxia.Figure 5The linear regression results of the relationship: (a) between turnover timing and annual minimum concentration of DO, (b) between the annual minimum concentration of DO and annual average concentration of DIP. The simulation results at the monitoring station were used for analysis.Full size image More

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