Seasonal Variation in Feeding and Defensive Traits of Rotifers in Two Tropical Reservoirs

Rotifers are a key group in freshwater food webs, with diverse feeding and defensive traits that enable them to dominate tropical zooplankton. However, whether these traits trade off at the community level remains unclear. We investigated rotifers monthly in two tropical reservoirs in South China, with different hydraulic retention times (HRTs) from 2023 to 2024, comparing patterns between the dry and wet seasons. Functional diversity (Rao’s Q) and community-weighted sum (CWS) of traits were measured for both feeding and defensive traits. Rao’s Q for both traits showed no significant differences between the dry and wet seasons in either reservoir. Feeding trait composition, based on CWS, differed significantly between the dry and wet seasons in both reservoirs, whereas defensive trait composition changed significantly only in the shorter-HRT reservoir (SZ). The relative abundance of suctor feeders was significantly negatively correlated with that of loricate and loricate-spined rotifers, particularly in the wet season. RDA and hierarchical partitioning revealed that the change in feeding traits was mainly driven by water temperature and cladocerans, whereas that in defensive traits was mainly governed by predation pressure from cyclopoid copepods. Rotifers achieve functional trait trade-offs through species turnover. Under high predation pressure, communities may shift from species with more efficient feeding traits to those with stronger defensive traits, thereby enhancing survival.