The response of microalgal biomass and community composition to environmental factors in the Sundays Estuary

Abstract

The Sundays Estuary is permanently open to the sea and experiences regular freshwater inflow in the form of agricultural return flows with large supplies of nutrients. The objectives of this study were to measure microalgal biomass and community composition and relate these to freshwater inflow, water quality and other environmental variables. These data can then be used in setting the ecological water requirements of the estuary. Surveys in August 2006, March 2007, February, June and August 2008 showed that salinity less than 10, expressed in practical salinity units, mostly occurred from 12.5 km from the mouth in the middle reaches of the estuary, which was also where the highest water column chlorophyll a (>20 µg·ℓ Phytoplankton blooms were found during Weeks 1, 4 and 5 from the middle to the upper reaches of the estuary. It was shown that diatoms occurred in blooms during warm, calm conditions whereas wind-mixing and reduced temperature, as a result of a cold front during 17 to 19 March 2009, promoted the dominance of flagellates throughout the estuary although they were present at all times. Dominant diatom species (Cylindrotheca closterium, Cyclotella atomus and Cyclostephanus dubius) indicated brackish, nutrient-rich water. Nanoplankton (2.7 -20 µm) was dominant during each week sampled and contributed 55 -79% to the phytoplankton biomass. Maximum benthic chlorophyll a was found 12.5 km from the mouth. This study is the first to show successive chlorophyll a blooms consisting of different phytoplankton groups in an estuary, an indication of the eutrophic state of the system.

Resilience of estuarine phytoplankton and their temporal variability along salinity gradients during drought and hypersalinity

“…Marshall and Alden (1990) also concluded that salinity gradients in estuaries were more important than variation in nutrient concentrations in determining the composition of phytoplankton communities. This is however not always the case especially in freshwater dominated estuaries and lakes such as the Nyara Estuary (Walker et al, 2001) and Sundays Estuary (Kotsedi et al, 2012) in South Africa, or the Santragachi lake in, India (Ghosh et al, 2012), where phytoplankton community structure and blooms were mainly due to high nutrient concentrations.…”

Nutrient enrichment as a threat to the ecological resilience and health of South African microtidal estuaries

“…Consequently, persistent phytoplankton blooms and nuisance macroalgal growth have been recorded in several estuaries (e.g. Kotsedi et al 2012;Human et al 2016;Lemley et al 2017;. Nationally, deteriorating water quality has been linked to an increase in fish kills and the spread of invasive species (e.g.…”

Deterioration in the water quality of an urbanised estuary with recommendations for improvement

“…The eutrophication of estuaries caused by human-induced nutrient enrichment is a global environmental concern because of the ecological and economic value of these ecosystems (Cloern et al, 2016). Eutrophication in South Africa appears to be increasing with persistent phytoplankton blooms being reported in a number of other estuaries (Snow et al, 2000;Kotsedi et al, 2012;Kaselowski and Adams, 2013;Lemley et al, 2018). By applying the pelagic component of a eutrophic condition index, proposed by Lemley et al (2015), the eutrophic condition of the Swartkops Estuary could be assessed (Table 1).…”