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Mouth state is a key determinant of estuarine processes in arid and semi-arid regions. While this factor is well researched for periodically open systems, it is much less researched for estuarine lakes. Here we used a 4-year data set to determine spatio-temporal patterns in responses of benthic macrofauna to changes in mouth state in a subtropical estuarine lake system during a drought cycle, with a focus on understanding the ecological mechanisms driving community change. We also assessed the effects of changes in mean levels of environmental factors relative to their statistical variability (measured as standard deviation, SD) in driving changes in species richness and abundance of dominant macrofauna. Results showed that greatest variability in physico-chemical factors and macrofaunal assemblages occurred in the upper lake sections, especially under closed mouth conditions. At the community level, changes in salinity under different mouth states played an important structuring role, but were less important at the level of individual species, being important for only 1 of the 6 dominant species. Low oxygen levels under closed mouth conditions, particularly in the lake complex, were also an important determinant of macrofaunal community structure and species richness. A major finding was that variability (SD) in environmental factors was more important in determining species richness and abundance of dominant macrofauna than changes in mean levels. This suggests that, while dominant species in the systems can tolerate significant environmental changes, the rate and magnitude of change exert an important control over species abundance.

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 damp- ened the effect of temporal variations, while different regions have become more spatially hetero- geneous. 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 Pseudo- diaptomus 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.

Phytoplankton and microphytobenthic biomass were investigated over a 4 yr period in the drought-stricken St Lucia Estuary, KwaZulu-Natal, South Africa. This is the largest estuarine lake in Africa and part of the iSimangaliso Wetlands Park, South Africa's first World Heritage Site and a Ramsar Wetland of International Importance. The present study aimed at identifying the factors con- trolling the extreme variability in its ecological functioning. Water-column chlorophyll a (chl a) con- centrations averaged (±SD) 32.3 ± 53.4 mg m -3 , with the highest value reaching 413 mg m -3 . Within the sediment, average (±SD) chl a concentration was 201 ± 377 mg m -2 , with a maximum of 2576 mg m -2 . Both are among the highest values so far reported in the literature for any estuarine ecosystem and by far the highest values reported for any South African estuary. Areal phytoplankton biomass was highest in the Mouth and Narrows region, declining in the South and North lakes, while micro- phytobenthos showed the reverse trend. Ordinations and cluster analysis revealed that phytoplankton biomass differed between the southern and northern halves of the system, while differences in micro- phytobenthic biomass were found between the Narrows and the rest of the system. Dissolved inor- ganic nitrogen to phosphorus ratios were greater and lower than the Redfield optimum (16) 60% and 27% of the time, respectively, indicating that microalgal production may be limited by availability of either P or N for part of the year, with a peak in growth during summer. Water level and mouth state are key factors controlling microalgal biomass within the estuary. During the closed phase, the shal- low waters and vast surface area of the lakes region led to the development of hypersaline conditions and temperatures occasionally approaching 50°C. The negative effects of such extreme conditions on grazers, as well as negative correlations between grazer densities and microalgal biomass, have been documented in previous work on the system. This would suggest that the interaction between the physical environment and grazers may be important in regulating microalgal biomass in the estuary.

The St. Lucia Estuary is currently experiencing a drought-induced crisis, resulting in the system having been closed off from the sea for approximately 8 yr. This closure was interrupted by a brief open-mouth phase, induced by a unique combination of extreme climatic events. The primary aim of the present study was to compare zooplankton dynamics during open- and closed-mouth con- ditions. Sampling was undertaken during quarterly surveys from February 2006 to November 2008. During the closed-mouth phase, up to 70% of the lake bed was dry and salinities in the northern lakes often exceeded 90, making these areas largely uninhabitable for zooplankton. However, in the lower regions where drought effects were less harsh, zooplankton were characterized by high densi- ties and biomass of typical estuarine taxa such as the copepods Pseudodiaptomus stuhlmanni, Acar- tia natalensis and the mysid Mesopodopsis africana. Of the 69 taxa recorded during the study period, only 27 were present during the closed-mouth phase. Under open-mouth conditions, previously excluded marine taxa (e.g. the prawn Penaeus indicus and fish larvae), once again re-entered the system, increasing its diversity significantly. A unique occurrence after mouth re-closure was the colonisation of the mouth area by swarms of the tunicate Oikopleura dioica (>10 3 ind. m -3 ), while pre- viously dominant zooplankton grazers virtually disappeared. These findings emphasize the complex- ity of the system and stress the need for further research into the potential impacts of environmental and climate changes on this key African estuarine lake.

Gastrosaccus brevifissura is a key crustacean species in most South African estuaries. The biomass, distribution, grazing rates and feeding preferences of this species were investigated in the Mpenjati Estuary, on the east coast of South Africa. The species exhibits a very marked diurnal migration behaviour, with biomass varying within the range 0.005 to 0.26 g m -3 dry wt (DW) during the daytime and 0.002 to 1.53 g m -3 DW during the night. The estimated total population grazing impact of the mysid ranged between 22 and 32% of the available phytoplankton in summer and win- ter, respectively. This suggests that G. brevifissura may not meet all its metabolic demands by con- suming phytoplankton alone. Other possible food sources available in the estuary include plant detri- tus (DTR), microheterotrophs and benthic microalgae (BMA). In particular, the vertical migration behaviour of G. brevifissura allows close spatial association with the benthic microalgae during most of the day. Indeed, feeding experiments suggest that G. brevifissura is able to feed efficiently on settled as well as on resuspended benthic microalgae. This is supported by results from stable isotope analysis (δ 13 C and δ 15 N), which show that benthic microalgae contribute 68 and 24% to the total diet of G. brevifissura in winter and summer, respectively. This also suggests that other food sources are needed by this species to meet all its energetic demands. Carbon rations obtained from in situ graz- ing methods are 35 to 44% (winter) and 1.6 to 3.9% (summer). When these values are compared to the previously estimated basal metabolic requirements for the mysid, it follows that G. brevifissura is able to meet all its energetic requirements from a pure autotrophic diet only during the winter. On the other hand, during summer, these values are barely enough to meet the basal metabolic rate of this species.