Effects of Environmental Stressors on the Spring Zooplankton Community in Qinzhou Bay, China

Multiple environmental stressors can directly or indirectly affect zooplankton community structure in marine coastal waters. However, quantifying the relative importance of eutrophication-mediated pathways remains challenging. In this study, we surveyed zooplankton, phytoplankton, and water conditions at 24 stations in Qinzhou Bay during spring and used correlation analysis and structural equation modelling (SEM) to examine the association between eutrophication and zooplankton biomass. The results showed that zooplankton biomass increased from the Inner Bay to the Outer Bay; whereas, zooplankton evenness decreased. Biomass was significantly negatively correlated with both the eutrophication index and zooplankton evenness; a single small copepod numerically dominated the assemblage, so high biomass and low evenness co-occurred as expressions of dominance. The SEM showed an acceptable fit and linked eutrophication to lower zooplankton biomass mainly through heavy-metal enrichment, community dominance (low evenness), and reduced phytoplankton density. The total association between eutrophication and biomass was −0.40, primarily indirect rather than through a significant direct path (−0.13, p = 0.51), with heavy-metal enrichment and community dominance (low evenness) as the main statistical mediators. Because the data come from a single spring survey of 24 stations, these path coefficients should be interpreted as correlational associations requiring multi-season confirmation.