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

• DOI: 10.1016/j.ecss.2023.108514
• Publication Date: 2023-09-30T07:38:54Z
• Authors: P. Nodo; A.-R. Childs; P. Pattrick; D.A. Lemley; N.C. James
• Keywords: Hypoxia; Demersal fish; Fish kill; Demersal zone

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 functioning 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 bottom-water oxygen depletion (<4 mg/l). Bi-seasonal sampling physico-chemical parameters demersal fishes between February 2018 September 2019 allowed for quantification low dissolved mg/l) hypoxic (<2 events impact (DO) on overall fish species abundance, richness distribution. Low DO conditions were typically observed bottom waters vertically-stratified meso-to polyhaline middle upper reaches estuaries, respectively. Hypoxic (min. 0.5 recorded Estuary during study, whereas lowest was 2.4 mg/l. resulted a decrease abundance Estuary, while declined events, little effect abundance. Fish only completely avoided areas where <1 mg/l or >10 As such, concentrations did not have noticeable distribution, although Rhabdosargus holubi Caffrogobius gilchristi when <3 results from this study provide much-needed insights into spatial distribution species.