Nicola K. Carrasco

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

Background The St. Lucia Estuary, Africas largest estuarine lake, is currently experiencing unprecedented freshwater deprivation which has resulted in a northward gradient of drought effects, with hypersaline conditions in its northern lakes. Methodology/Principal Findings This study documents the changes that occurred in the biotic communities at False Bay from May 2010 to June 2011, in order to better understand ecosystem functioning in hypersaline habitats. Few zooplankton taxa were able to withstand the harsh environmental conditions during 2010. These were the flatworm Macrostomum sp., the harpacticoid copepod Cletocamptus confluens, the cyclopoid copepod Apocyclops cf. dengizicus and the ciliate Fabrea cf. salina. In addition to their exceptional salinity tolerance, they were involved in a remarkably simple food web. In June 2009, a bloom of an orange-pigmented cyanobacterium (Cyanothece sp.) was recorded in False Bay and persisted uninterruptedly for 18 months. Stable isotope analysis suggests that this cyanobacterium was the main prey item of F. cf. salina. This ciliate was then consumed by A. cf. dengizicus, which in turn was presumably consumed by flamingos as they flocked in the area when the copepods attained swarming densities. On the shore, cyanobacteria mats contributed to a population explosion of the staphylinid beetle Bledius pilicollis. Although zooplankton disappeared once salinities exceeded 130, many taxa are capable of producing spores or resting cysts to bridge harsh periods. The hypersaline community was disrupted by heavy summer rains in 2011, which alleviated drought conditions and resulted in a sharp increase in zooplankton stock and diversity. Conclusions/Significance Despite the current freshwater deprivation crisis, the False Bay region has shown to be resilient, harboring a unique biodiversity with species that are capable of enduring harsh environmental conditions. However, further freshwater deprivation may extend beyond the physiological thresholds of this community, as well as other unique biodiversity components which this system sustains.

Diet of selected fish species in the freshwater-deprived St Lucia Estuary, South Africa, assessed using stable isotopes

Abstract The St Lucia Estuary, Africas largest estuarine lake, is the most important juvenile fish nursery on the south-east African coastline and is currently experiencing an unprecedented crisis. Prolonged freshwater deprivation has resulted in hypersaline conditions in the northern regions of the lake, as well as low water levels. The mixed model SIAR (Stable Isotope Analysis in R) was used to determine the likely contribution of each of the available carbon sources to the diets of selected fish species. This was complemented with ad-hoc gut content analysis of representative specimens. Two diverse regions of the lake system were investigated: Charters Creek, which has been severely affected by the desiccation process, and the Mouth, which is somewhat protected due to the freshwater input from the Mfolozi and Mpate rivers. The mysid Mesopodopsis africana was found to be important in the diet of a number of fish species, namely, Leiognathus equula, Ambassis ambassis and Gerres acinaces. The copepod Pseudodiaptomus stuhlmanni and the mysid Rhopalophthalmus tropicalis were most important in the diet of Gilchristella aestuaria. Mozambique tilapia (Oreochromis mossambicus) exhibited the most variable diet, the flexibility of which may aid survival during harsh conditions and could explain its current dominance throughout the system.

Salinity tolerance of the bivalve Solen cylindraceus (Hanley, 1843) (Mollusca: Euheterodonta : Solenidae) in the St Lucia Estuary

ABSTRACT Solen cylindraceus (Hanley, 1843) is an infaunal bivalve that in the St Lucia Estuary is currently restricted to the southern part of its South Lake, having disappeared from the northern reaches due to persisting hypersaline conditions (>70 ‰) and air exposure at low water levels. The system experiences marked fluctuations in salinity due to quasi-decadal changes from wet to dry periods. In this study, the salinity tolerance of S. cylindraceus is determined using both shock and gradual change tests. Animals were collected at Catalina Bay (eastern shores of South Lake) and acclimated under laboratory conditions to naturally occurring salinities of 50 ‰ and 45 ‰ for the shock and gradual test, respectively. Mortalities were recorded for animals exposed to a sudden change in salinity, using eight different treatments ranging from 0 to 80 ‰. The second test involved exposing bivalves to a gradual change in salinity, using eight different treatments from 0 to 85 ‰. In the shock test, the lower salinity tolerance limit for S. cylindraceus was 30 ‰ and the upper 60 ‰, while in the gradual test, these limits were 15 and 65 ‰, respectively. The time it took for 50 % of animals to die increased from the shock to the gradual test for 10, 20 and 70 ‰, and decreased for 0 and 80 ‰. This knowledge may be useful towards predicting major crises in the S. cylindraceus populations, as drought and flood events alternate in the region. Major losses will be expected when salinities exceed 65 ‰ during dry phases or drop below 15 ‰ during flood events.

Impact of a flood event on the zooplankton of an estuarine lake

ABSTRACT Shallow coastal lakes are prone to large fluctuations in physico-chemical variables such as salinity and turbidity. This is now escalating in response to global change. A flood event in March 2014 resulted in a silt plume spreading through part of Lake St Lucia (South Africa). To determine the impact of this event on zooplankton, the Narrows region of St Lucia was sampled on a monthly basis from March to September 2014. For comparative purposes, data from samples collected prior to the flood event were included in the analyses. Analysis of similarity (ANOSIM) revealed dissimilarities in zooplankton community structure among the sampling occasions. The March 2014–May 2014 period was characterized by the highest abundance of freshwater species. Conversely, the abundance of the resident St Lucia copepods Acartiella natalensis and Oithona brevicornis was lowest during this time, and highest in September 2014. The other dominant copepod Pseudodiaptomus stuhlmanni prevailed in March 2014, but declined markedly in April. As of September 2014, P. stuhlmanni had yet to regain its pre-flood densities. The BIOENV procedure, which relates biological and environmental data, revealed that turbidity, salinity and dissolved oxygen were responsible for the observed changes in zooplankton community structure during the study period. Careful management of turbidity and salinity is stressed, as both factors are major drivers of the biota of St Lucia and similar systems worldwide.

Meso-Zooplankton Movement through the Newly Constructed Mfolozi Channel into and out of the St Lucia Estuary, South Africa

The historical canalization of the Mfolozi River resulted in the St Lucia Estuary losing its main fresh-water source. This, combined with the recent drought, led to low water levels, hypersaline conditions and dramatic losses in biodiversity. The excavation of a beach spillway between the estuary and the Mfolozi River mouth in 2012, combined with the commencement of a wet phase in the preceding year, has alleviated the freshwater deprivation crisis. This study compares the meso-zooplankton composition within the channel on both flood and ebb tides, providing a preliminary understanding of the role of the Mfolozi channel in the migration and recruitment of organisms. Abundance was higher on ebb tides, while species richness was higher on flood tides. A total of 53 zooplankton taxa were recorded in the channel. Of these, one had never been recorded in St Lucia before, namely the gastropod Pterotrachea cf. hippocampus. Additionally, the ctenophore Pleurobrachia cf. pileus and the mysid Gastrosaccus gordonae were again recorded for the first time since the late 1970s. Results show that the beach spillway has facilitated recruitment into St Lucia from the ocean and the Mfolozi River. Further research is needed to ascertain what effect this recruitment has on long-term zooplankton community structure.

Ingestion rates and grazing impact of the brackwater mussel Brachidontes virgiliae in Lake St Lucia, iSimangaliso Wetland Park, South Africa

Bivalves feed on a combination of phytoplankton and zooplankton and have the potential to impact considerably the planktonic biomass, especially when they occur in high densities, such as in oyster and mussel beds. The brackwater mussel Brachidontes virgiliae is numerically dominant during wet phases within Africa’s largest estuarine lake, St Lucia, in the iSimangaliso Wetland Park on the east coast of South Africa. The ingestion rates and potential grazing impact of this small mussel (maximum shell length = 2.5 cm) were estimated for both the wet and dry seasons using an in situ gut fluorescence technique. Ingestion rates were higher during the wet season (5.78 µg pigment ind.−1 d−1) than during the dry season (4.44 µg pigment ind.−1 d−1). This might be explained by the increased water temperature and food availability during the wet season. Because of the patchy distribution of mussel populations, there could be higher localised grazing impact near mussel aggregations. Results showed a potential grazing impact of up to 20 times the available phytoplankton biomass at specific sites. These high grazing impacts have the potential to deplete phytoplankton stocks in the lake, especially during wet phases in the northern reaches, where mussel densities are highest. This needs to be factored into ecological models of Lake St Lucia, because the system might function differently during increased flood events.

Effects of zooplankton grazing on the bloom-forming Cyanothece sp. in a subtropical estuarine lake

Modern anthropogenic modifications to aquatic environments, specifically through changes in trophic state and hydrodynamic conditions, have facilitated increasing frequency and duration of cyanobacterial blooms world-wide. This study investigated the potential for top-down grazing control by mesozooplankton on a persistent bloom-forming (18 months) cyanobacterium, Cyanothece sp., isolated from Lake St Lucia (South Africa). Seasonal surveys and in situ experiments were conducted to assess the zooplankton community structure and its grazing impact on Cyanothece sp. cells, respectively. Grazing experiments showed that zooplankton can have high grazing impacts on Cyanothece sp., but that increased salinity levels are responsible for the observed decrease in total abundances and the shift to a halotolerant community. Most typical estuarine zooplankton species at St Lucia disappear when salinity increases above 100. Additionally, the halotolerant zooplankton species, specifically Fabrea salina, occurred in low densities at the time of the study. This would result in a major drop of grazing control by zooplankton on Cyanothece sp. The results, which indicate that zooplankton can potentially play a key role in top-down grazing control of cyanobacterial bloom, are relevant in the global context where the occurrence of persistent cyanobacterial blooms is on the rise.