Characterization and ecology of selected micro-estuaries and micro-outlets in the Eastern Cape province, South Africa

South Africa has more than 200 Temporarily Open/Closed Estuaries (TOCEs) along its approximately 3000 km coastline. In addition, there are approximately 200 micro-estuaries and micro-outlets on the subcontinent, many of which share some of the characteristics with TOCEs but differ in that they are supplied by localised coastal stream catchments with a more limited supply of freshwater and less connectivity to the marine environment. Between 2015 and 2016, this study endeavoured to investigate the physico-chemical properties, microalgal dynamics as well as the seasonal species abundance and composition of aquatic invertebrates and fishes in four micro-estuaries and four micro-outlets along the warm-temperate section of the Eastern Cape coast. The micro-outlets were dominated by oligohaline or freshwater conditions, while the micro-estuaries were mostly mesohaline. Microalgal biomass in these systems was influenced by temperature, nutrient conditions and light availability. Chironomidae and Ephemeroptera dominated benthic macro-invertebrate communities in the micro-outlets, while Amphipoda and Mysida were most dominant in micro-estuaries. Copepoda and Amphipoda dominated the zooplankton assemblages in both system types. Nekton communities in micro-outlets were dominated by juvenile Myxus capensis and tadpoles (mostly Xenopus laevis), while Oreochromis mossambicus, Gilchristella aestuaria, M. capensis and Mugil cephalus were the four most common species in the micro-estuaries. Seasonal variations in physico-chemical parameters and community composition of both the nekton and invertebrates were recorded during this study. Micro-estuaries and micro-outlets exhibited relatively lower diversity and abundances of biota than TOCEs. Furthermore, the micro-estuaries have a low estuarine function when compared to TOCEs, while the micro-outlets have very limited estuarine characteristics and were dominated by freshwater biota. The micro-systems are smaller in size and contain lower proportions of estuary-associated taxa when compared to larger estuarine systems such as TOCEs and permanently open estuaries. The results of this study confirm that there are distinctions between the biota of iv the two micro-system types, thus making biotic assemblages, together with physico-chemical parameters, a viable tool for differentiating between micro-estuaries and micro-outlets. This study represents the first investigation into the ecological functioning of micro-estuaries and micro-outlets. As a whole, micro-systems serve as nursery areas for freshwater-associated invertebrate taxa. Due to their limited periods of stability, micro-estuaries serve as intermediate nursery areas for 0+ juveniles of estuary-associated marine fish which, after less than a year, then migrate to larger estuarine systems located nearby. The small catchment size and short period of stability in these systems heightens their level of sensitivity to water flow alterations through dams, direct water abstraction, afforestation and watercourse diversion, while their lower buffering capacity makes them more susceptible to pollution and nutrient inputs from intensive agricultural activities and urban expansion

Predation by alien largemouth bass, Micropterus salmoides Lacepéde 1802 (Centrarchidae: Perciformes), on indigenous marine fish species in the Kowie System, South Africa

Estuaries serve as nursery areas for a large number of estuary-associated fish species. Some of these taxa also use river catchments as nursery areas. During the upstream migration of this latter group, the juveniles are prone to predation by native and alien predatory fish inhabiting the system. The rate of invasion of ecosystems by alien organisms can be directly linked to anthropogenic influences, including both intentional and unintentional introductions by alien organisms into new regions. The largemouth bass, Micropterus salmoides, is a facultative piscivorous fish that has been successfully introduced worldwide for the main purpose of sport fishing. Where introduced, it has been found to negatively impact native fish and invertebrate species through predation, competitive exclusion and displacement of indigenous fish species. The aim of this thesis was to investigate the predatory impact of largemouth bass on the estuary-associated Cape moony Monodactylus falciformis, Cape stumpnose Rhabdosargus holubi and freshwater mullet Myxus capensis in the lower Kowie River of the Eastern Cape Province, South Africa. A combination of approaches was employed during this study. Stomach contents, stable isotopes and fatty acid analyses were employed for the reconstruction of the diet of largemouth bass. Acoustic telemetry was used to elucidate largemouth bass movements, particularly their ability to enter the upper reaches of the Kowie Estuary. Stomach contents and stable isotopes analyses showed that M. salmoides exhibit an ontogenic shift in diet, with small and medium sized individuals consuming the higher proportion of fish prey, while large sized individuals mostly consumed invertebrates such as crabs (Potamonautes sidneyi) and Odonata larvae, while consuming only a small proportion of estuary-associated fish. Fatty acid analysis only showed a direct connection between the fatty acid profiles of largemouth bass and those of M. capensis and M. falciformis. The acoustic telemetry results indicated that some M. salmoides individuals periodically move into the upper reaches of the estuary following river flood events. The results obtained from all these approaches highlight the risk posed by largemouth bass introductions on indigenous fish species, particularly those that enter the areas occupied by these top predators

Connectivity through allochthony: Reciprocal links between adjacent aquatic and terrestrial ecosystems in South Africa

An important aspect of the dynamics of nutrients and pollutants in natural systems is captured in the concept of allochthony, founded on the observation that nutrients and energy in a variety of forms are transferred between adjacent habitats, com-munities and ecosystems that are not routinely considered as connected. Different forms of nutrients and energy move across the conceptual boundaries of habitats via organisms’ activities or physical processes such as wind or water currents, and these transfers can represent important food subsidies. Such cross-partition ecolog-ical subsidies can augment the nutritional condition, biomass and biodiversity of communities, particularly where local production (or autochthony) alone may be inadequate to support local food webs. Furthermore, organic subsidies can influ-ence population dynamics, community interactions and ecosystem processes, and can represent dominant flux inputs in ecosystem budgets. Our intention was to ex-plore organic nutrient fluxes in relation to a primarily lotic (i.e. flowing) aquatic sys-tem at the scale of a hydrological catchment

Movement behaviour of alien largemouth bass Micropterus salmoides in the estuarine headwater region of the Kowie River, South Africa

The movement behaviour of largemouth bass Micropterus salmoides in the estuarine headwater region of the Kowie River, South Africa, was investigated using passive acoustic telemetry. Ten adult fish were tagged and released in four discrete pools below a weir that precluded possible upriver migration. Their residency and movement patterns were monitored using 27 moored acoustic receivers spanning both riverine and estuarine habitats between January and November 2013. Small-scale movements ranging from 50 to 400 m occurred between pools in the headwater region, where tagged fish remained resident (did not leave their tagging pool) during the warmer (summer) months. The onset of autumn and winter correlated with declining water temperatures and half of the tagged largemouth bass made downstream excursions into the slightly warmer upper reaches of the estuary. The penetration of these alien piscivores into estuarine headwaters potentially provide additional predator pressure on the juveniles of migratory estuary-associated native marine fish species, which utilise these areas as nurseries. The presence of the alien fish in South African estuarine headwaters may impair the migration of juvenile fishes between the estuarine and freshwater environments

Movement behaviour of alien largemouth bass Micropterus salmoides in the estuarine headwater region of the Kowie River, South Africa

The movement behaviour of largemouth bass Micropterus salmoides in the estuarine headwater region of the Kowie River, South Africa, was investigated using passive acoustic telemetry. Ten adult fish were tagged and released in four discrete pools below a weir that precluded possible upriver migration. Their residency and movement patterns were monitored using 27 moored acoustic receivers spanning both riverine and estuarine habitats between January and November 2013. Small-scale movements ranging from 50 to 400 m occurred between pools in the headwater region, where tagged fish remained resident (did not leave their tagging pool) during the warmer (summer) months. The onset of autumn and winter correlated with declining water temperatures and half of the tagged largemouth bass made downstream excursions into the slightly warmer upper reaches of the estuary. The penetration of these alien piscivores into estuarine headwaters potentially provide additional predator pressure on the juveniles of migratory estuary-associated native marine fish species, which utilise these areas as nurseries. The presence of the alien fish in South African estuarine headwaters may impair the migration of juvenile fishes between the estuarine and freshwater environments.Keywords: alien invasive, estuary-dependent, estuarine fish, fishway, residency, telemetr

Benthic diatom-based indices and isotopic biomonitoring of nitrogen pollution in a warm temperate Austral river system

Highlights: • BDI, BIWQ, CEC, IDAP and SLA indices were strongly explained by sediment variables. • DES and SHE indices were explained by water and physical variables. • Isotope values at all sites indicated inorganic to organic impacts. • Microphytobenthos (MPB) δ15N values were significantly correlated to TDI, SPI and IDAP indices. • MPB δ15N values are strong indicators of nitrogen pollution across spatiotemporal gradients. Abstract: Rivers are impacted by pollutants from anthropogenic activities such as urbanisation and agricultural practices. Whilst point source pollution has been widely studied and in some cases remediated, non-point pollutant sources remain pervasive, particularly in developing countries that lack economic and human capacity. Monitoring of pollution levels in many regions is additionally challenged by a lack of robust indicators for nitrogen inputs, however, diatom community indices and analysis of variation in microphytobenthos (MBP) stable isotope analysis variations have potential. The present study investigates variations and utilities in benthic diatom indices and MPB δ15N along different river sections (n = 31) of an austral river between two seasons (wet and dry), testing for relationships with key environmental variables (physical, water and sediment), in the context of N monitoring. One hundred and eighteen diatom taxa belonging to 36 genera were identified, with physical (water flow), water (nitrate, P and total dissolved solids) and sediment (B, Ca, Cr, Na, N, P, SOM, Pb and Zn) variables correlating to one or more of the 12 diatom indices presented. In particular, Biological Diatom Index, Biological Index of Water Quality, Central Economic Community, Index of Artois-Picardie Diatom (IDAP) and Sládeček’s Index were strongly explained by sediment variables, whilst Descy’s Pollution Index and Schiefele and Schreiner’s Index were explained by water and physical variables. While MPB δ15N were within the “no impact” level in the wet and dry seasons at reference (i.e. unpolluted) sites, all sites located in agricultural or urban areas, and downstream of sewage discharges, had a wider range that encompassed increasing organic impacts (“inorganic impacts” to high “organic impacts”). Temperature and turbidity (negative), as well as dissolved oxygen, waterway width and depth (positive), significantly affected MPB δ15N, while effects of chemistry variables were less apparent. Overall, we found that MPB δ15N signatures were significantly correlated with Trophic Diatom Index, the Specific Pollution sensitivity Index and the Artois-Picardie Diatom Index, suggesting the utility of diatoms and MPB δ15N in assessments of aquatic pollution. In turn, MPB δ15N values are strong indicators of N pollution across spatial and seasonal gradients. Thus, the results showed the effects of sediment variables on diatoms to be strong, indicating that sediment rather than water characteristics more strongly structure diatom communities. Thus, sediments variables should be sampled when conducting bioassessment studies