Intraspecific Physiological Alterations in Phytoplankton Declined the Food Nutritional Quality in Tropical Nutrient-Manipulated Aquatic Mesocosms

The polyunsaturated fatty acid (PUFA) content of phytoplankton determines the nutritional quality of aquatic basal food resources and influences food-web energy transfer. Field studies suggest that phytoplankton PUFA content is associated with taxonomic characteristics, with nutrient enrichment promoting cyanobacteria dominance to reduce phytoplankton PUFA content. Nevertheless, laboratory evidence indicates that nutrient enrichment can also alter the fatty acid composition of single algae species. The role of intraspecific physiological alterations has long been overlooked. To partition the relative importance of taxonomic and intraspecific physiological alterations on phytoplankton nutritional quality, nutrient-manipulated mesocosm experiments were conducted in spring and winter using natural plankton communities from a tropical eutrophic reservoir. Fatty acid analysis showed that the nutritional quality of the phytoplankton community declined in mesocosms after nutrient additions. Piecewise structural equation modeling (pSEM) was employed to disentangle direct effects (representing intraspecific physiological alterations) and indirect effects (via the Shannon–Wiener index, representing taxonomic alterations) of phosphorus enrichment on ω-3 PUFA. Within each season, phosphorus enrichment exerted a significant direct negative effect on ω-3 PUFA, regardless of whether cyanobacteria dominated in winter or chlorophytes dominated in spring. In the pooled cross-seasonal model, community diversity showed a significant positive effect on ω-3 PUFA, with a stronger standardized effect than that of phosphorus enrichment, whereas phosphorus enrichment still exerted a significant direct negative effect. Our results indicate that intraspecific physiological alterations in phytoplankton may play a crucial role in the changes in phytoplankton nutritional quality following additional nutrient loading, particularly in tropical, eutrophic waters. However, the underlying biochemical mechanisms remain uncertain because direct molecular or enzymatic evidence was not obtained in this study.