Spatial and temporal variations in the diet of the mysid Mesopodopsis africana in the St. Lucia Estuary (South Africa)

Abstract

This study presents one of the few known examples where a mysid species has been observed modifying its diet rapidly and under natural conditions in response to environmental changes. Mesopodopsis africana is a dominant mysid in many estuaries along the east coast of South Africa, and a key species in the St. Lucia Estuary, Africa's largest estuarine lake. St. Lucia is currently undergoing severe desiccation owing to freshwater deprivation. Lack of freshwater input has dampened the effect of temporal variations, while different regions have become more spatially heterogeneous. The mixed model SIAR v 4.0 (stable isotope analysis in R) was used to determine the likely contribution of each of the available carbon sources to the diet of M. africana. The copepod Pseudodiaptomus stuhlmanni made a significant contribution to M. africana's diet in the Mouth region. At Catalina Bay, mysids mostly utilized particulate organic matter (POM), while at Charters Creek they were most closely associated with the macroalga Cladophora sp. The sensitivity of Charters Creek to drought effects is emphasized here, as well as the important role M. africana plays in this habitat as an omnivore, increasing the connectance and, hence, sustaining its food web. While the Mouth and Narrows are partly protected from drought effects, the northern lakes have experienced further increases in salinity during the past decade, forcing the periodical exclusion of this mysid from much of the system. This has lead to severe effects on the food webs that the mysid supports under normal conditions. KEY WORDS: Stable isotopes · South Africa · iSimangaliso Wetland Park · Trophic relations · Key speciesResale or republication not permitted without written consent of the publisher Mar Ecol Prog Ser 417: 127-138, 2010 ences cyclical wet and dry phases, each lasting between 4 and 10 yr (Begg 1978). Past agricultural practices, combined with the below-average rainfall the area has received since 2002, have resulted in the estuary being cut off from the ocean. Low freshwater input and high evaporation rates have led to the persistence of a reversed salinity gradient, with hypersaline conditions in the upper reaches. These harsh conditions combined with the closed-mouth state, have resulted in only the most adaptive species remaining (Carrasco et al. 2010). Dominant zooplankton within the St. Lucia Estuary include the mysid Mesopodopsis africana and the calanoid copepods Pseudodiaptomus stuhlmanni and Acartia natalensis. While the copepods are numerically dominant, mysids tend to be more important in terms of biomass (Wooldridge 1999). Due to the ongoing drought, M. africana has disappeared from large sections of the estuarine lake, currently occurring only in its mouth region and in parts of the Narrows and South Lake (Fig. 1).The present study examines the implications of harsh environmental conditions on the key zooplankton species Mesopodopsis africana. Knowledge of food web properties, such as trophic processes, enables inferences to be made ab...

Importance of suspended particulate organic matter in the diet of Nephrops norvegicus (Linnaeus, 1758)

“…Also, the δ 13 C/δ 15 N bi-plot in our study showed no missing dietary items, which might have been expected had SOM been important in the diet. Other studies have also shown distinct POM and SOM signatures 28,43 . Future studies combining fatty acid analysis and SIA may further disentangle the various sources of organic particulates and their relative importance, including sources found inside lobster burrows (e.g.…”

“…A final aim was to determine Nephrops' trophic position based on new SIA results from the present study. The SIA data were analysed within a Bayesian framework, an approach which is increasingly used to address ecological problems [27][28][29][30][31][32][33][34] .…”

Development of a Halotolerant Community in the St. Lucia Estuary (South Africa) during a Hypersaline Phase

“…[23], [24]). Microalgae were then filtered onto previously combusted (450°C, 6 hours) Whatman GF/F filters.…”

Biodiversidade, distribuição, alimentação e papel trófico de misídeos marinhos (Crustacea, Peracarida, Mysida)

“…Mysids usually are omnivorous and may feed on cyanobacteria ), benthic algae (Webb et al 1988;Kibirige et al 2003;Carrasco & Perissinotto 2010), phytoplankton (Jerling & Wooldridge 1995;David et al 2006a;Metillo et al 2007;, detritus (Zagursky & Feller 1985;David et al 2006a;Carleton & McKinnon 2007;, microand mesozooplankton, mainly copepods, but also cladocerans, rotifers and meroplankton (Aaser et al 1995;Jerling & Wooldridge 1995;David et al 2006b;Gorokhova & Lehtiniemi 2007;Winkler et al 2007;Barz & Hirche 2009;Carrasco & Perissinotto 2010, benthic invertebrates (harpacticoid copepods; , zooplankton benthic eggs (Viitasalo & Viitasalo 2004;Viitasalo 2007;Karlson & Viitasalo-Frösen 2009;, and mysids from the same or different species (Wittmann 1978;Fenton 1992;Johnston & Ritz 2001).…”

“…Their wide diet varies seasonally (Viherluoto et al 2000; according to food availability Winkler & Greve 2004;Carrasco & Perissinotto 2010), species (David et al 2006a;Winkler et al 2007;Vilas et al 2008), and ontogenetic stage (Froneman 2001;Gorokhova & Lehtiniemi 2007;Lesutienė et al 2007), with herbivory and carnivory generally being predominant in juveniles and adults, respectively (Branstrator et al 2000;Viherluoto et al 2000). Mysids are suspension-feeders when feeding on detritus or algae (Webb et al 1987), but may change to raptorial feeding when preying on other animals, depending on prey size and swimming behavior (Wooldridge & Webb 1988;).…”