Mangrove dispersal disrupted by projected changes in global seawater density

Mangrove forests thrive along tropical and subtropical shorelines and their distribution extends to warm temperate regions¹. They are globally recognized for the valuable ecosystem services they provide² but are expected to be substantially influenced by climate change-related physical processes in the future^3,4. Under warming winter temperatures, poleward expansion is predicted for mangroves^5,6, with potential implications for ecosystem structure and functioning, as well as human livelihoods and well-being^7,8. The global distribution, abundance and species richness of mangroves is governed by a broad range of biotic and environmental factors, including temperature and precipitation⁹ and diverse geomorphological and hydrological gradients¹⁰. Climate and aspects related to coastal geography (for example, floodplain area) determine the availability of suitable habitat for establishment^11,12. However, the potential for mangroves to track changing environmental conditions and expand their distributions ultimately depends on dispersal^11,13. The importance of dispersal in controlling mangrove distributions has been demonstrated by mangrove distributional responses to historical climate variability¹⁴, past mangrove (re)colonization of oceanic islands¹⁵ and from the long-term survival of mangrove seedlings planted beyond natural range limits¹⁶. As such, quantifying changes in the factors that influence dispersal is important for understanding climate-driven distributional responses of mangroves under future climate conditions.

In mangroves, dispersal is accomplished by buoyant seeds and fruits (hereafter referred to as ‘propagules’). In combination with prevailing currents, the spatial scale of this process, ranging from local retention to transoceanic dispersal over thousands of kilometres¹³, is determined by propagule buoyancy¹⁷, that is, the density difference between that of propagules and the surrounding water. Hence, the course of dispersal trajectories for propagules from these species depends on the interaction between spatiotemporal changes in both propagule density and that of the surrounding water, rendering this process sensitive to climate-driven changes in coastal and open-ocean water properties. The biogeographic implications of such density differences were recognized more than a century ago by Henry Brougham Guppy, who discussed¹⁸ ‘the far-reaching influence on plant-distribution and on plant-development that the relation between the specific weight of seeds and fruits and the density of sea-water must possess’.

Since the time of Guppy’s early observations, climate change from human activities has driven pronounced changes in ocean temperature and salinity, with further changes predicted throughout the twenty-first century¹⁹. Ocean density is a nonlinear function of temperature, salinity and pressure²⁰; therefore, these changes may influence dispersal patterns of mangrove propagules by altering their buoyancy and floating orientation. As Guppy noted¹⁸, ‘[for] plants whose seeds or fruits are not much lighter than seawater […] the effect of increased density of the water is to extend the flotation period’ or ‘to increase the number that floated for a given period’. Guppy also reported that the seedlings of the widespread mangrove genera Rhizophora and Bruguiera present exceptional examples of propagules with densities somewhere between seawater and freshwater¹⁸. Previous studies of the impacts of climate change on mangroves have focused on factors such as sea level rise, altered precipitation regimes and increasing temperature and storm frequency^4,21,22,23 but the potential impact of climate-driven changes in seawater properties on mangroves has not yet been examined. This is somewhat surprising, as the ocean is the primary dispersal medium of this ‘sea-faring’ coastal vegetation and dispersal is a key process that governs a species’ response to climate change by changing its geographical range. This knowledge gap contrasts with recent efforts to expose links between climate change and dispersal in other ecologically important marine taxa such as zooplankton and fish species^24,25,26,27.

In this study, we investigate predicted changes in sea surface temperature (SST), sea surface salinity (SSS) and sea surface density (SSD) for coastal waters bordering mangrove forests (hereafter referred to as ‘coastal mangrove waters’), over the next century. Using a biogeographic classification system for coastal and shelf areas²⁸, we examine spatiotemporal changes in these surface ocean properties, with a particular focus on the world’s two major mangrove diversity hotspots: (1) the Atlantic East Pacific (AEP) region, including all of the Americas, West and Central Africa and (2) the Indo West Pacific (IWP) region, extending from East Africa eastwards to the islands of the central Pacific¹. Finally, we synthesize available data on the density of mangrove propagules for different mangrove species and explore the potential impact of climate-driven changes in SSD on propagule dispersal.

To assess changes in SST and SSS throughout the global range of mangrove forests, we used present (2000–2014) and future (2090–2100) surface ocean properties from the Bio-ORACLE database^29,30. SSD estimates were derived from these variables using the UNESCO EOS-80 equation of state polynomial for seawater³¹. Changes in SST, SSS and SSD (Fig. 1) were calculated for four representative concentration pathways (RCPs) and derived for coastal waters closest to the 583,578 polygon centroids from the 2015 Global Mangrove Watch (GMW) database³². After removing duplicates, our dataset contained 10,108 unique mangrove occurrence locations, with corresponding present conditions and predicted future changes in mean SST, SSS and SSD. Under the low-warming scenario RCP 2.6, mean SST of coastal mangrove waters is predicted to change by +0.64 (±0.11) °C and mean SSS by −0.06 (±0.25) practical salinity units (PSU). Combined, this results in an average change in mean SSD of −0.25 (±0.20) kg m⁻³ in coastal mangrove waters by the late twenty-first century (Supplementary Table 1). These values roughly double under RCP 4.5 (Supplementary Table 2), while under RCP 6.0, a change of +1.69 (±0.14) °C in mean SST, −0.21 (±0.42) PSU in mean SSS and −0.71 (±0.32) kg m⁻³ in mean SSD is predicted (Supplementary Table 3). Under RCP 8.5, our study predicts a change in SST of +2.84 (±0.21) °C (range 2.11–4.01 °C), a change in SSS of −0.30 (±0.74) PSU (−2.01–1.26 PSU) and a corresponding change in SSD of −1.17 (±0.56) kg m⁻³ (−2.53–0.03 kg m⁻³) (Supplementary Table 4).

Spatial variability in predicted surface ocean property changes was examined by considering the two major mangrove bioregions (AEP and IWP) (Fig. 2) and using the Marine Ecoregions of the World (MEOW) biogeographic classification²⁸ (Fig. 3). Both the range and changes in mean SST were comparable for the AEP and IWP mangrove bioregions, for all respective RCP scenarios (Fig. 2a and Supplementary Tables 1–4). Under RCP 8.5, mean SST in both mangrove bioregions is predicted to warm ~2.8 °C by 2100, which is roughly 4.5 times the predicted increase in mean SST under RCP 2.6 (Supplementary Tables 1 and 4). Predictions for the RCP 8.5 scenario are generally consistent with reported global ocean temperature trends³³ and show that the greatest warming occurs in coastal waters near the Galapagos Islands (change in mean SST of 3.92 ± 0.06 °C). Pronounced SST increases are also predicted for Hawaii (change in mean SST of 3.36 ± 0.05 °C), the Southeast Australian Shelf (3.30 ± 0.25 °C), Northern and Southern New Zealand (3.25 ± 0.07 °C and 3.34 ± 0.02 °C, respectively), Warm Temperate Northwest Pacific (3.27 ± 0.16 °C), the Red Sea and Gulf of Aden (3.24 ± 0.08 °C), Somali/Arabian Coast (3.23 ± 0.15 °C), South China Sea (3.07 ± 0.10 °C), the Tropical East Pacific (3.09 ± 0.15 °C) and the Warm Temperate Northwest Atlantic (3.14 ± 0.13 °C) (Fig. 3b and Supplementary Tables 4).

Predicted SSS changes exhibit an opposite trend in the AEP and IWP bioregions, with increased salinity in the AEP and reduced salinity in the IWP under global warming (RCP 2.6–RCP 8.5; Fig. 2b); this is reflected in contrasting SSD changes in both mangrove bioregions (Fig. 2c) and associated with predicted global changes in precipitation, with extensions of the rainy season over most of the monsoon domains, except for the American monsoon³⁴. Under RCP 8.5, the spatially averaged change in mean SSS is +0.51 (±0.57) PSU in the AEP and −0.68 (±0.44) PSU in the IWP region. The maximum decrease in mean SSS (−2.01 PSU) is predicted for the Gulf of Guinea in the AEP bioregion (Fig. 3c and Supplementary Table 4). Within the IWP, the Western Indian Ocean region shows little or no changes in SSS, which contrasts with the pronounced freshening trends predicted in the eastern part of this ocean basin and the Tropical West Pacific (Figs. 1b and 3c). Increased freshening is predicted in the Bay of Bengal (SSS change: −1.17 ± 0.43 PSU), the Sunda Shelf (SSS change: −1.21 ± 0.29 PSU) and the Western Coral Triangle province (mean SSS change: −0.80 ± 0.17 PSU) (Fig. 3c and Supplementary Table 4). Within the AEP, salinity increases exceed +0.96 PSU in the Tropical Northwestern Atlantic, +0.80 in the Warm Temperate Northwest Atlantic and +0.68 in the West African Transition (Fig. 3c and Supplementary Table 4). The spatial heterogeneity in SSS across the global range of mangrove forests corresponds with observed changes in SSS³⁵. Trends in SSD (Fig. 3d) strongly track changes in SSS (Fig. 3c) rather than SST. All RCP scenarios predict an overall decrease in SSD for both mangrove bioregions; however, the predicted decrease in SSD in the IWP region was a factor of 2 (RCP 6.0) and 2.5 (RCP 2.6, RCP 4.5 and RCP 8.5) stronger than in the AEP (Figs. 2 and 3d and Supplementary Tables 1–4).

Propagule density values from our literature survey range from <600 kg m⁻³ to >1,080 kg m⁻³ for different mangrove species (Fig. 4 and Supplementary Table 5). The low densities reported for Heritiera littoralis propagules provide a strong contrast with the near-seawater propagule densities reported for Avicennia and members of the Rhizophoraceae (Bruguiera, Rhizophora and Ceriops). Floating characteristics of the latter may be particularly sensitive to changes in SSD. To illustrate the potential influence of changing ocean conditions on mangrove propagule dispersal, we considered threshold water density values (1,020 and 1,022 kg m⁻³) that are within the range where elongated propagules of important mangrove genera tend to change floating orientation (Fig. 4a). More specifically, we determined the ocean surface area with an SSD below or equal to these thresholds under different climate change scenarios (Fig. 5). Under RCP 8.5, the ocean surface covered by mangrove coastal waters (coastal waters bordering present mangrove forests) with a density ≤1,020 kg m⁻³ increases ~27% by 2100, notably more so in the IWP (~37%) than in the AEP (~6%) (Supplementary Table 6). A threshold of 1,022 kg m⁻³ results in increases of roughly +11% (global), +12% (IWP) and +8% (AEP) (Supplementary Table 7). Similar spatial patterns are observed for open-ocean waters within the global latitudinal range of mangroves (Fig. 5 and Supplementary Figs. 1 and 2).

Our study shows changes in the physical and chemical properties of coastal mangrove waters by the end of the twenty-first century that could affect the distribution of propagules from widespread mangrove genera (Avicennia, Bruguiera, Ceriops and Rhizophora) and probably more so within the IWP region, the primary hotspot of mangrove diversity, compared to the AEP. Propagules from these genera have densities that are close to that of seawater and experimental evidence shows that propagules in species of these genera typically become denser as they age^36,37,38,39. For mangroves in large parts of the IWP, as well as the Gulf of Guinea in the AEP, declines in SSD could therefore promote local sinking rates and reduce the probability of successful long-distance dispersal due to earlier propagule sinking before reaching suitable establishment zones (Fig. 4). Indeed, future changes in surface ocean properties may impact dispersal differently at different scales. More specifically, short-distance dispersal may be promoted and successful long-distance dispersal reduced since (1) freshening of local waters near the propagule release site and associated sinking can reduce the fraction of propagules reaching open-ocean waters; and (2) older propagules that have travelled longer distances are at risk of offshore sinking due to the combined effect of increased propagule density during transport^36,37,38,39 and fresher coastal waters; as such, increased ocean freshening and warming and resulting changes in SSD are likely to reduce the tail of the dispersal kernel—which captures long-distance dispersal events. Upon sinking, a propagule is at least temporarily eliminated from the dispersing cohort and prone to mortality associated with stranding in unsuitable conditions. Empirical evidence of mangrove distributions limited by sinking of propagules under increasingly fresher conditions in estuaries was shown for mangroves along the Nakara River in Japan³⁶. Such effects are of considerable importance, since some of the most pronounced changes in SSS and SSD are near major river outlets where many of the most extensive mangrove areas in the world are found¹, such as the Ganges–Brahmaputra delta. In contrast to the wide-ranging mangrove genera Avicennia, Bruguiera, Ceriops and Rhizophora, mangrove species such as H. littoralis and Xylocarpus granatum are unlikely to be affected by these ocean changes as their propagules possess very low densities¹³.

An additional level of complexity is that the floating orientation of propagules from several widespread mangrove genera (Rhizophora and Ceriops) could move between horizontal and vertical due to small changes in density^38,40,41. Changes in floating orientation have been associated with the replacement of air in intercellular tissue by water via lenticels³⁸; however, while the exact anatomical and physiological mechanisms underlying changes in mangrove propagule buoyancy are not yet fully understood, experimental and modelling studies showed that these changes in floating orientation can strongly alter dispersal trajectories at the landscape (10²–10³ m), regional (10³–10⁵ m), as well as the biogeographic scale (10⁵–10⁷ m), via the relative effects of ocean and wind forces on propagule transport^42,43,44.

Besides effects of changes in SSD, propagule dispersal may also be directly impacted by SSS or SST. In Avicennia marina, propagule sinking has been associated with the shedding of the pericarp (Steinke, 1975, as cited in ref. ⁴⁵) and the time required for pericarp shedding and the separation of the cotyledons increases with increasing salinity (Downton, 1982, as cited in ref. ⁴⁵). As such, future ocean freshening might decrease floating periods and potentially dispersal distances in this species. Increases in SSS might also result in higher propagule mortality rates and lower germination success⁴⁰, whereas lower SSS could reduce propagule viability by increasing the incidence of fungal infestation and rotting⁴⁶. Finally, projected increases in SST may facilitate mangrove expansion to higher latitudes in some regions by reducing the negative effect of colder oceanic waters on propagule viability⁴⁷. For example, an earlier study on the western South Atlantic latitudinal mangrove range limit reported that temperatures <20 °C may limit the viability of mangrove propagules during their dispersal along this coast and during subsequent establishment⁴⁸. However, since empirical data on potential direct effects of SSS and SST on mangrove propagule dispersal are deficient, effects of these variables require further investigation.

It is important to note that changes indicated by our study are based on changes in time-mean surface ocean properties and that the actual variability in SSD around these mean values could be higher. Since mangroves thrive in a broad range of coastal settings, including estuaries, deltas, lagoons and open coast, their propagules already encounter a wide range of water densities today but our findings clearly illustrate that for important mangrove regions worldwide exposure to lower-density waters will occur more frequently in the future. While our results suggest changes in dispersal patterns following climate-driven changes in coastal and open-ocean surface-water properties, the effects also depend on adaptive capacity. Under current ocean conditions, propagule densities that are just slightly lower than that of seawater are probably adaptive, since this ensures that they sink in favourable coastal environments, that is, along coastal zones where SSS and SSD are lower due to freshwater influx from rivers. Yet, in future oceans the spatial extent of these lower-density waters may expand further offshore, altering buoyancy characteristics and the spatial distribution of propagules. The demographic costs associated with ending up in unsuitable habitat (for example, due to offshore sinking) can get balanced when populations evolve to have lower propagule densities. As such, there is a need for quantitative data for inter- and intra-population variation in the critical SSD at which mangrove propagules sink, which could reflect underlying genetic variation in propagule density that can fuel evolutionary change. Overall, our results suggest that we may be entering a density-induced transition phase as part of the Anthropocene, highlighting the importance of considering future ocean property changes in evaluating the impacts of climate change on mangrove ecosystems. Such information will complement knowledge on the effects of other impacting factors and is important for predicting how altered environmental conditions will affect these sensitive forests. Finally, while we considered mangroves as a model system in this study, our findings may be relevant also for other coastal taxa producing sea-drifted propagules, such as seagrasses⁴⁹ and coastal strand communities (Terminalia catappa, Barringtonia asiatica, Thespesia populnea, Hibiscus tiliaceus, Pisonia grandis, Pandanus spp. and so on).

Source: Ecology - nature.com