Monique Nunes

Benthic diatoms are reliable indicators of estuarine ecosystem health. These communities can be used to track water quality changes in estuarine lakes that are unique, yet highly susceptible to anthropogenic perturbations due to their high accumulation potential. This study investigated the abiotic drivers of the benthic diatom community structure in the near-natural Kosi Bay Estuarine Lake, a Ramsar Wetland of International Importance in summer/spring of 2016 and 2022. A total of 159 taxa, belonging to 57 genera were recorded. The composition of benthic diatom assemblages was related to salinity variability, with marine species dominating the tidal lower estuarine reaches and freshwater taxa in the distal and oligohaline lake compartments. Taxa capable of attachment to different substrates that enable resilience to tidal disturbance, such as Amphora, Halamphora, Mastogloia, and Seminavis, were dominant in the tidally influenced lower lake compartments. Higher species diversity (H’ > 2) was observed in proximity to the estuarine mouth, with regular sediment disturbance favouring increased diversity. An increased availability of DIP (≥ 0.01 mg L⁻¹) and sediment organic content (1.1–3.7%), coupled with low flow velocities, favoured the proliferation (RA > 10%) of Cocconeis spp. in the Phragmites-dominated upper lake compartments. Nutrient tolerant taxa, such as Navicula gregaria, Nitzschia frustulum, and Tabularia tabulata, were present in the estuarine lake. However, further investigations are required to determine whether their presence is due to inherent estuarine variability or a precursor to future anthropogenic change. Studies such as these that provide baseline information are essential to help inform effective management and recovery plans of rare estuarine ecosystem types.

Context: Artificial breaching of intermittently closed estuaries has become more frequent in the face of global-change pressures. Aims: This study aimed to determine whether the ecological health of the Great Brak Estuary has been affected by the prolonged loss of marine connectivity arising from below-average inflow and failed breaching attempts. Methods: We characterised primary eutrophic symptoms (inorganic nutrients, dissolved oxygen, microalgae) typical of the various mouth states, i.e. open, closed and semi-closed. Key results: Initially, low inflow and closed mouth conditions facilitated the widespread occurrence of macroalgal blooms (Cladophora glomerata). Phytoplankton bloom conditions (>20 µg Chl-a L-1) ensued only in response to favourable hydrodynamic conditions (e.g. increased water residency, halocline formation) and increased nutrient availability from fluvial sources and macroalgal dieback. These blooms occurred in brackish conditions and comprised numerous taxa, including Cyclotella atomus var. marina, Cryptomonas sp. and Prorocentrum cordatum. Widespread hypoxia (<2 mg L-1) occurred during the semi-closed mouth phase because of the reduced flushing potential associated with the preceding high flow conditions. Conclusions: Global-change pressures and ineffective breaching practices will promote eutrophic conditions in intermittently closed estuaries in the future. Implications: Allocating sufficient environmental flows is key to preventing ecosystem degradation.

Primary producer responses to cumulative regulating factors were investigated in a temporarily closed microtidal estuary. We hypothesised that the phytoplankton chlorophyll-a concentration (Chl a) ...

The occurrence of harmful algal blooms (HABs) is increasing in frequency and intensity in South African estuaries because of eutrophication. This study used DNA metabarcoding to identify the benthi...