Phytoplankton functional response to spatial and temporal differences before and after the regime shift of Caohai Lake, China

This study aimed to explore the functional responses of phytoplankton to the ecosystem regime shift of Caohai Lake, a typical plateau karst lake located in Weining County, Guizhou Province, China. Since 2020, the lake has undergone a significant transition from a clear-water steady state dominated by submerged macrophytes (e.g., Potamogeton lucens L., Myriophyllum verticillatum L.) to a turbid-water steady state dominated by phytoplankton, accompanied by ecological degradation such as extensive macrophyte die-off and severe eutrophication. To address this, water and phytoplankton samples were collected in July 2018 (pre-regime shift) and July 2023 (post-regime shift). A suite of environmental variables was measured, including electrical conductivity (EC), total dissolved solids (TDS), total nitrogen (TN), total phosphorus (TP), ammonia nitrogen (NH₃-N), turbidity (Tur), chlorophyll a (Chl a), pH, and dissolved oxygen (DO). Statistical analyses, including one-way analysis of variance (ANOVA), redundancy analysis (RDA), analysis of similarities (ANOSIM), and similarity percentages (SIMPER) analysis, were applied to examine differences in environmental conditions and phytoplankton communities between the two periods. Results showed significant post- regime shift changes in environmental conditions: EC, TDS, TN, TP, NH₃-N, Tur, and Chl a increased significantly (P < 0.05), while pH and DO decreased. Phytoplankton cell density rose markedly from 15.48 × 10⁶ cells/L (2018) to 58.86 × 10⁶ cells/L (2023), Cyanobacteria remained the dominant class in both periods, the cell density were 13.58 × 10⁶ cells/L and 48.42 × 10⁶ cells/L respectively, indicating a dramatic expansion post-regime shift. The composition of phytoplankton functional groups (FGs) shifted from pre-regime shift dominant groups Lo/M/N/S1/X1 to post-regime shift C/F/J/Lo/M, with Lo (e.g., Merismopedia sp.) and M (e.g., Microcystis sp.) being the most abundant across both periods. RDA indicated that the main environmental parameters affect Cyanobacteria, Bacillariophyceae and Chlorophyceae were NH₃-N, Sal, Tur, Cond, Chl a, TDS and TN. SIMPER analysis revealed an average dissimilarity of 53.30% in FGs between 2018 and 2023, with Lo (29.42%) and M (17.78%) contributing most to this difference. In conclusion, the aggravation of eutrophication following the regime shift was the key driver of changes in the phytoplankton community of Caohai Lake. Nutrient enrichment created a turbid, eutrophic habitat that favored the proliferation of FGs adapted to such conditions, highlighting the critical role of nutrient regulation in restoring the lake’s degraded ecosystem.