Increased nutrient loading associated with rapid population growth is the leading cause of deteriorating water quality in urbanized estuaries globally. Small estuaries are particularly sensitive to changes when connection with the marine environment is restricted, or lost, because of high water retention. The temporarily closed Hartenbos Estuary (South Africa) is an example of how such pressures can culminate in a severely degraded ecosystem. Wastewater treatment work (WWTW) discharges introduce substantial volumes of freshwater (8,000 m3 d–1) and nutrient loads (38 kg DIN d–1 and 22 kg DIP d–1) into this estuary. This constant inflow has necessitated frequent artificial breaching (inducing alternating states) of the estuary mouth to prevent flooding of low-lying developments and, occasionally, to mitigate against extreme events such as fish kills and sewage spills. This study investigated the efficacy of artificial mouth breaching practices in eliciting responses in selected abiotic and biotic parameters. Microalgal (phytoplankton and benthic diatoms), benthic macrofauna and fish community dynamics were assessed in response to mouth state and water quality conditions using a seasonal monitoring programme. The hypereutrophic nature of the Hartenbos Estuary was highlighted by persistent high-biomass phytoplankton accumulations (>100 μg Chl-a l–1), extreme dissolved oxygen conditions (0.4–20.5 mg O2 l–1) and the predominance of harmful algal bloom (HAB) events comprising Nannochloropsis sp. and Heterosigma akashiwo. Artificial breaching of the mouth facilitated limited tidal exchange and occurred approximately bimonthly once water levels exceeded 1.9 m above mean sea level (MSL). Current pressures and management interventions have culminated in an ecosystem void of natural fluctuations and instead characterised by low diversity and shifts between undesirable states. This is highlighted by the near year-round dominance of only a few opportunistic species/groups tolerant of adverse conditions (e.g., Nannochloropsis sp., Halamphora coffeiformis, oligochaetes, estuarine round herring Gilchristella aestuaria, and southern mullet Chelon richardsonii). Therefore, catchment-scale interventions such as the diversion of WWTW discharges and restoration of hydrodynamic variability are management priorities for improving the health and biodiversity of small, closed microtidal systems such as the Hartenbos Estuary.

Low river inflow conditions during 2009/2010 resulted in the mouth of the Great Brak Estuary remaining closed for almost two years. The low water level in Wolwedans Dam resulted in no annual environmental flow releases being made, causing mouth closure. The response of primary producers to this prolonged period of mouth closure was investigated in 2010/2011. Urban and agricultural development in the river catchment and along the estuary banks had increased the nutrient inputs into the estuary. Mouth closure, combined with elevated nutrient concentrations, increased the growth of both macroalgae and microalgae, but little change was observed in the submerged macrophytes. Macroalgal mats covered large areas of open-water surface, smothering the salt marsh and causing a decrease in its cover. These results have important implications for the management of temporarily open/closed estuaries, as increased development, freshwater abstraction and reduced river flow will result in prolonged periods of closure and reduced tidal exchange, which is likely to lead to eutrophication.

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In shallow estuaries, artificial substrates provide a means of assessing the response of the diatom community to water chemistry. The shallow St Lucia Estuary was historically connected to the Mfolozi River at the mouth. This connection was severed during the early 1950s due to sediment input from the agriculturally impacted Mfolozi River. A connection has recently been re-established and the potential impact of dissolved inorganic nutrients from the Mfolozi River needs to be determined, as it may alter the ecological integrity of the St Lucia Estuary which forms part of a UNESCO World Heritage Site. This study found that the epilithic algal biomass and diatom species composition grown on glass slides were good indicators of nutrient enrichment in the estuary. Over a 28-day study period an above-average rainfall event resulted in an increase of nutrient-enriched freshwater flow into the estuary. No significant biomass differences were recorded in either the natural phytoplankton or microphytobenthos communities because of high variability. By contrast, the epilithic algal biomass accumulation on the glass slides was highest following the freshwater input. Statistical analyses indicated that the accumulation of the epilithic community on the glass slides was mostly affected by DIN and salinity. Diversity index scores for both the natural phytoplankton and microphytobenthos were variable, while the epilithic diatom index scores consistently declined from Day 7 (H’ = 1.2) to Day 28 (H’ = 0.7). These data indicate that epilithic algae grown on glass slides can be used as an effective monitoring tool to detect nutrient-induced changes as a supplementary method in this highly variable estuary.

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