No abstract

Worldwide, cyanobacterial blooms are becoming more frequent, exacerbated by eutrophication and other anthropogenic actions and also associated with global climate change. In June 2009, a widespread bloom of the unicellular cyanobacterium Cyanothece sp. appeared in North Lake and False Bay of Lake St Lucia, a large (360 km 2 ) estuarine lake system in KwaZulu-Natal, South Africa, and persisted for 18 mo. It remains unclear how the bloom status was maintained for so long. This study investigated aspects of the nutrient (N and P) assimilation of Cyanothece sp. and how these may relate to maintaining a persistent bloom state during hypersaline conditions. The effects of salinity and nutrient limitation on the nutrient uptake dynamics of Cyanothece sp. were evaluated with 15 NO 3 − uptake, PO 4 3− uptake and 15 N 2 fixation experiments. Nitrogen fixation was observed in this Cyanothece sp. isolate from St Lucia. Highest nutrient assimilation rates in all experiments were recorded at the lowest salinities, decreasing progressively up to a salinity of 120, with very little activity observed above this level. No 15 N 2 fixation was measured above this salinity. Results indicate that Cyanothece sp. was well suited to take advantage of the conditions present during the onset of the bloom at salinities <100. However, once salinity increased above 120, nutrient uptake abilities would have been drastically reduced. Regardless, cells still survived under these extreme saline conditions, as most of their potential grazers and autotrophic competitors disappeared from the St Lucia Estuary.

No abstract

The St Lucia Estuary, a UNESCO World Heritage Site, has experienced prolonged mouth closure since 2002. Until recently it was hypersaline, but heavy rainfall between October 2010 and January 2011 led to a large overall decrease in salinity. A stable isotope study was conducted in March 2011 on selected fish species from the nearshore environments at Charter's Creek and the estuary mouth region to determine the influence of flooding events on their diet. Oreochromis mossambicus, the dominant fish species at both sites, had a generalised diet and occupied the lowest trophic level both before and after flooding. Monodactylus argenteus, Leiognathus equula and Gerres acinaces from the mouth area displayed a largely zooplanktivorous diet and showed some changes in prey species between drought and post-flood conditions.