Stoichiometric imbalance affects competitive superiority in marine herbivorous zooplankton communities

Abstract

Traditionally, abiotic drivers and predator–prey interactions have been considered critical to shaping the taxonomic composition of a marine zooplankton community, whereas the role of indirect biotic processes, such as exploitative competition, remains unexamined. The competitive performance of a zooplankton taxon can be limited by biochemical mismatches between its requirements and available prey, known as stoichiometric imbalance. Such imbalance impairs zooplankton growth and reproduction, thereby reducing their competitiveness. Stoichiometric imbalance is known to shape zooplankton communities in freshwater systems; however, its role in marine systems has largely been examined only at the species or individual levels under experimental conditions. To fill this gap, we measured the family‐level taxonomic composition and carbon : nitrogen : phosphorus ratios (C : N : P) of herbivorous zooplankton and quantified their stoichiometric imbalance relative to their prey in the southern East China Sea. We examined three hypotheses: Hypothesis 1—Intra‐family stoichiometry of zooplankton is independent of and less variable than phytoplankton; Hypothesis 2—Zooplankton show interfamily variation in stoichiometry; and Hypothesis 3—A higher degree of stoichiometric imbalance reduces the relative abundance of a zooplankton family in the community. Our results showed zooplankton stoichiometry was stable within but variable among families. Family‐level relative abundance was significantly associated with phosphorus imbalance. Despite seasonal shifts in dominant families, the negative relationship between stoichiometric imbalance and relative abundance remained consistent, suggesting that stoichiometric imbalance is an important factor modulating zooplankton community composition across seasons. Notably, phosphorus excess did not negatively affect relative abundance, a pattern that contrasts with the traditional stoichiometric knife‐edge hypothesis.