Response of demersal fishes to low dissolved oxygen events in two eutrophic estuaries

Abstract

Coastal ecosystems serve as important nursery areas, yet they are facing many challenges. For example, eutrophication-induced hypoxia is one of the major threats to the functioning of coastal ecosystems, particularly estuaries. The Sundays and Swartkops estuaries both experience persistent eutrophic conditions, with frequent phytoplankton blooms (>20 μg Chl-a l−1) that result in instances of bottom-water oxygen depletion (<4 mg/l). Bi-seasonal sampling of both physico-chemical parameters and demersal fishes between February 2018 and September 2019 in the Swartkops and Sundays estuaries allowed for the quantification of low dissolved oxygen (<4 mg/l) and hypoxic (<2 mg/l) events in both estuaries and the impact of low dissolved oxygen (DO) and hypoxic events on overall fish species abundance, richness and distribution. Low DO conditions were typically observed in the bottom waters of the vertically-stratified meso-to polyhaline middle and upper reaches of the Sundays and Swartkops estuaries, respectively. Hypoxic conditions ( DO = 0.5 mg/l) were recorded in the Sundays Estuary during the study, whereas in the Swartkops Estuary the lowest DO recorded was 2.4 mg/l. Low DO events resulted in a decrease in fish species richness and overall abundance in the Sundays Estuary, while in the Swartkops Estuary fish species richness declined during low DO events, with little effect on fish abundance. Fish only completely avoided areas where DO was <1 mg/l or >10 mg/l. As such, the low DO concentrations recorded in the Swartkops Estuary did not have a noticeable impact on fish distribution, although the abundance of Rhabdosargus holubi and Caffrogobius gilchristi declined in the upper reaches of the Swartkops Estuary when DO was <3 mg/l. The results from this study provide much-needed insights into the impact of low DO and hypoxic conditions on the spatial distribution of demersal fish species.