Botany Department, Nelson Mandela University, Port Elizabeth 6031, South Africa; DST/NRF Research Chair in Shallow Water Ecosystems, Institute for Coastal and Marine Research, Nelson Mandela University, Port Elizabeth 6031, South Africa.
Botany Department, Nelson Mandela University, Port Elizabeth 6031, South Africa; DST/NRF Research Chair in Shallow Water Ecosystems, Institute for Coastal and Marine Research, Nelson Mandela University, Port Elizabeth 6031, South Africa.
Sci Total Environ. 2019 Nov 25;693:133601. doi: 10.1016/j.scitotenv.2019.133601. Epub 2019 Jul 26.
Upstream anthropogenic perturbations can dramatically affect estuarine water quality, especially in small systems where water retention times are high. Management of these modified ecosystems often requires direct interventions to prevent detrimental long-term effects. The heavily urbanized and temporarily closed Zandvlei Estuary in South Africa has a long history of anthropogenic manipulation, and therefore acts as an ideal case-study to gauge reactive management practices. Continued deterioration of estuarine water quality and ecological functioning prompted the adoption of an adaptive management approach. Subsequent management interventions to increase salinity variability and nutrient removal included the (1) adoption of a mouth management plan, (2) regular harvesting of submerged vegetation, and (3) removal of flow obstructions. Physico-chemical and nutrient monitoring data (2010-2018) were assessed to document the response of phytoplankton to these interventions. Time-series analysis indicated an upward trend in salinity throughout the estuary. This corresponded with a temporal decline in phytoplankton biomass levels and was supported by model results that highlighted a strongly inverse relationship with salinity. The frequency of high-biomass events (>80 μg Chl-a l) also declined in each of the designated estuarine zones. In April 2012, an extensive harmful algal bloom (HAB) of Prymnesium parvum (Prymnesiophyceae) was recorded (~530 μg Chl-a l) in the estuary, culminating in hypoxic conditions (O < 2 mg l) and fish kills. However, the subsequent mechanical breaching of the mouth has prevented reoccurrences of P. parvum. Similarly, more saline conditions arising from increased marine connectivity reduced the abundance of the previously dominant Bacillariophyceae and Chlorophyceae classes. The overall improvement in water quality validates the efficacy of management interventions aimed at restoring and conserving ecosystem functionality. Yet, these efforts are a deviation from natural estuarine functioning and thus a 'catchment to coast' approach that incorporates upstream nutrient mitigation measures is needed to achieve sustainable long-term management objectives.
上游人为干扰会显著影响河口水质，尤其是在水滞留时间较长的小型系统中。这些经人为改造的生态系统通常需要直接干预，以防止产生有害的长期影响。南非高度城市化且暂时封闭的赞德河河口具有悠久的人为干预历史，因此是评估反应性管理实践的理想案例研究地点。由于河口水质和生态功能持续恶化，促使人们采用适应性管理方法。为了增加盐度变异性和养分去除，随后采取了一些管理干预措施，包括：（1）采用河口管理计划；（2）定期收割沉水植被；（3）清除水流障碍物。本文评估了 2010-2018 年期间的理化和营养监测数据，以记录浮游植物对这些干预措施的响应。时间序列分析表明，整个河口的盐度呈上升趋势。这与浮游植物生物量水平的时间下降相对应，并得到模型结果的支持，模型结果突出表明浮游植物生物量与盐度呈强烈的反比关系。每个指定的河口区域中，高生物量事件（>80μgChl-a l）的频率也都有所下降。2012 年 4 月，在河口记录到了广泛的中肋骨条藻（Prymnesiophyceae）有害藻华（HAB）（~530μgChl-a l），最终导致缺氧条件（O<2mg l）和鱼类死亡。然而，随后的河口人工决堤防止了中肋骨条藻的再次发生。同样，由于海洋连通性增加导致的更咸的条件减少了以前占主导地位的硅藻纲和绿藻纲的丰度。水质的整体改善证明了旨在恢复和保护生态系统功能的管理干预措施的有效性。然而，这些努力偏离了自然河口的功能，因此需要采取“集水区到海岸”的方法，纳入上游养分缓解措施，以实现可持续的长期管理目标。
