Thaliacean Community Dominance Shift in the Northern South China Sea

Abstract

Thaliaceans are pivotal gelatinous zooplankton that accelerate carbon export and structure pelagic food webs through rapid filtration and production of dense fecal pellets and carcasses. While taxon‐specific contributions to the biological pump are recognized, their ecological niches and drivers of distribution remain poorly resolved. Here, we analyzed a 5‐year data set (2007–2011) from annual surveys in the northern South China Sea, spanning contrasting hydrographic regimes shaped by the Pearl River plume, Yuedong upwelling, and Taiwan Bank upwelling. The results showed that thaliacean aggregations were associated with distinct hydrographic processes that enhance chlorophyll a via either upwelling or terrestrial nutrient discharge. The results revealed clear basin‐scale niche partitioning between doliolids and salps. Doliolids dominated high‐chlorophyll a environments linked to river discharge and upwelling. In contrast, salps dominated in high‐salinity, moderate‐chlorophyll a offshore waters. Interannual comparisons revealed that community structure shifted between wet and dry years in the Pearl River Estuary, coinciding with monsoon‐driven changes in precipitation regimes. During wet years, doliolid habitats expanded in association with enhanced freshwater discharge, whereas salp dominance increased during droughts, concurrent with offshore seawater intrusion. Hierarchical partitioning analysis of the redundancy analysis results indicated that, among the measured environmental factors, sea surface salinity and sea surface chlorophyll a concentration contributed most to this distribution pattern. By linking hydroclimatic forcing to thaliacean biogeography, this study provides a basis for understanding potential ecosystem responses to climate variability and offers new insights into the connections between hydroclimatic variability and plankton‐mediated biogeochemical cycles.