River flow, zooplankton and dominant zooplanktivorous fish dynamics in a warm‐temperate South African estuary

Abstract

The possible links between river flow, zooplankton abundance and the responses of zooplanktivorous fishes to physico-chemical and food resource changes are assessed. To this end, the seasonal abundance, distribution and diet of the estuarine round-herring Gilchristella aestuaria and Cape silverside Atherina breviceps were studied in the Kariega Estuary. Spatio-temporal differences were determined for selected physico-chemical variables, zooplankton abundance and zooplanktivorous fish abundance and distribution. Results indicated that, following a river flood event in winter (>30 m s ), altered physico-chemical conditions occurred throughout the estuary and depressed zooplankton stocks. Abundance of G. aestuaria was highest in spring, with this species dominant in the upper and middle zones of the estuary, while A. breviceps was dominant in summer and preferred the middle and lower zones. The catch per unit of effort of both zooplanktivores also declined significantly following the flooding, thus suggesting that these fishes are reliant on zooplankton as a primary food source for healthy populations. Copepods dominated the stomach contents of both fish species, indicating a potential for strong interspecific competition for food, particularly in the middle reaches. Temporal differences were evident in dietary overlap between the two zooplanktivorous fish species and were correlated with river flow, zooplankton availability and fish distribution. The findings of this study emphasize the close trophic linkages between zooplankton and zooplanktivorous fishes under changing estuarine environmental conditions, particularly river flow and provide important baseline information for similar studies elsewhere in South Africa and the rest of the world.

Environmental Flow Requirements of Estuaries: Providing Resilience to Current and Future Climate and Direct Anthropogenic Changes

“…The widespread degradation of estuaries is notably caused due to eutrophication (Pinckney et al, 2001;Davis and Koop, 2006;Maier et al, 2009;Howarth et al, 2011) and has been the focus of research for many decades (e.g., Barlow et al, 1963;Caperon et al, 1971;Livingston, 1996). However, we also highlight the degradation estuaries have experienced as a consequence of modifications to freshwater flow regimes (Arthington, 2012;Kiwango et al, 2015;Stein et al, 2021), resulting in a decline in estuarine habitat quality due to altered ecosystem processes (Pinckney et al, 2001;Mbandzi et al, 2018). This can induce problems, such as eutrophication (see Section 4), due to biological responses (e.g., algal blooms, seagrass dieback) associated with modified ecosystem structure and function (Cottingham et al, 2018;Scharler et al, 2020).…”

Effect of a glyphosate formulation on freshwater plankton: A community combined metric approach

“…The observed cooccurrence of different tolerances and competitiveness within the zooplankton community determines the community structure. This attribute is a good indicator of environmental disturbance, since zooplankton community plays a key role in freshwater environments as intermediate links in trophic networks (Mbandzi et al, 2018). Consequently, the changes in this community structure can affect other trophic levels through top-down and bottom-up effects (Carpenter and Kitchells, 1995).…”

eDNA metabarcoding vs metagenomics: an assessment of dietary competition in two estuarine pipefishes

“…These include water abstraction for agricultural activities, pollution, and urban development (Morant & Quinn, 1999). Nutrients provided by freshwater input are a primary determinant in the trophic structure of estuarine ecosystems by contributing to planktonic productivity (Allanson et al, 2000) and to the functioning of pelagic food webs in particular (Grange et al, 2000; Mbandzi et al, 2018). However, some catchments in South Africa now retain more than 50% of the freshwater that the estuaries would receive under natural conditions (Wooldridge & Callahan, 2000).…”