Phosphorus enrichment enhances planktonic carbon transfer efficiency in warm eutrophic systems dominated by cyanobacteria or chlorophytes by alleviating stoichiometric constraint

In eutrophic systems dominated by chlorophytes or cyanobacteria, low carbon transfer efficiency from phytoplankton to zooplankton is typically attributed to biochemical deficiencies. However, under high light and nutrients, these groups can exhibit strong carbon fixation relative to phosphorus acquisition. How this nutrient use efficiency affects herbivore efficiency (HE; zooplankton-to-phytoplankton productivity ratio) in tropical eutrophic systems remains unclear. We conducted two nutrient-manipulation mesocosm experiments using plankton communities from a tropical eutrophic reservoir during the flood (chlorophyte-dominated) and dry (cyanobacteria-dominated) seasons. In both experiments, unamended mesocosms exhibited low HE (< 0.05) and low zooplankton-to-phytoplankton biomass ratio (Z : P; < 0.15), along with substantial C : P, C : N, and n-3 highly unsaturated fatty acids (n-3 HUFA) mismatches between phytoplankton and zooplankton. Phosphorus enrichment improved HE and Z : P by reducing C : P mismatch, despite concurrently increasing C : N and n-3 HUFA mismatches. Notably, P addition triggered transient HE spikes exceeding 0.1, coinciding with Z : P values of ~ 0.5 in the cyanobacteria-dominated system and > 1 in the chlorophyte-dominated system. In contrast, N enrichment reduced C : N mismatch but increased C : P mismatch, with negligible effects on HE and Z : P. These findings suggest that phytoplankton phosphorus use efficiency is an important driver of HE in tropical eutrophic systems. Under low planktivorous fish predation, transient high carbon transfer from chlorophytes or cyanobacteria to herbivores may occur when zooplankton C : P requirements are met and ample complementary foods compensate for inherent biochemical deficiencies. With ongoing eutrophication and climate change expected to intensify cyanobacterial dominance in tropical regions, these stoichiometric and biochemical constraints on HE are likely to shape food web dynamics and ecosystem functioning.