Preliminary insights on the fine-scale responses in larval Gilchristella aestuaria (Family Clupeidae) and dominant zooplankton to estuarine harmful algal blooms

Abstract Estuarine ecosystems are increasingly threatened by eutrophication. In a warm-temperate South African estuary, modified hydrodynamics and agricultural return flows drive eutrophication which has resulted in recurrent harmful algal blooms (HABs). These bloom conditions provided an opportunity to investigate the fine-scale temporal responses of the most dominant larval fish (Clupeidae: Gilchristella aestuaria) and zooplankton (Copepoda: Pseudodiaptomus hessei, Paracartia longipatella; Mysida: Mesopodopsis wooldridgei, Rhopalophthalmus terranatalis) species to HABs (Raphidophyceae: Heterosigma akashiwo). Data were collected within the mesohaline zone during spring and analysed according to HAB phases (‘bloom’, ‘decay’, ‘intermediate’). During blooms, low densities of predominantly postflexion G. aestuaria and high prey (P. hessei) and predator (R. terranatalis) densities were recorded. The density of G. aestuaria was higher during the ‘decay’ phase and consisted mainly of preflexion and newly hatched individuals. Gilchristella aestuaria dynamics were best explained by temperature, R. terranatalis density, supersaturated dissolved oxygen and bloom biomass. The HAB also resulted in a mismatch between larval G. aestuaria and its main prey, P. hessei. Recurrent HABs threaten estuarine ecosystem function by driving the loss of forage fish. This study provided insight into the fine-scale consequences that HABs have on plankton dynamics and is relevant in a future of global change where eutrophication will intensify.