Abstract Invasive alien aquatic plants (IAAPs) are rapidly spreading in South African estuaries because of anthropogenic nutrient loading and persistent freshwater conditions. This study investigated changes in IAAP cover in relation to development pressures in the temporarily closed, subtropical uThongathi Estuary. Changes over time (1937-2018) were assessed using historical aerial photographs and rectified satellite imagery. The IAAP cover was assessed quarterly in relation to river flow, physico-chemical and nutrient monitoring data (2015-2018). The estuary floodplain has been transformed by sugarcane farming, urban development, uncultivated fields and sand mining operations, occupying 27% of the estuarine functional zone. In the past, agricultural return flow and urban runoff facilitated the sporadic growth of IAAPs in the estuary. Since 1983, discharges from the Tongaat Central Wastewater Treatment Works (WWTW) increased daily nutrient loads (current contribution: 1.8 kg d−1 N and 9.63 kg d−1 P), resulting in persistent IAAP growth in the estuary. Discharge from the WWTW represents 15% of the estuary volume and the increased inflow has changed the estuary functionality to a predominantly open, low salinity state. Aerial photograph analysis showed that IAAPs have established along the entire length of the estuary (∼7 km) and are abundant during all mouth states. They are flushed out of the estuary during high flow events (≥ 5 m3s−1) but steadily re-establish (within 3 months) after floods. The likely source of IAAPs are heavily infested upstream dams. A nutrient management reduction strategy integrated with manual/biological control methods and increased salinity (> 5) are needed to restore estuary function and prevent infestations.

中文翻译：

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小型封闭河口外来入侵水生植物的变化

摘要 由于人为营养负荷和持续的淡水条件，外来入侵水生植物 (IAAP) 正在南非河口迅速蔓延。本研究调查了与暂时封闭的亚热带 uThongathi 河口的开发压力相关的 IAAP 覆盖变化。使用历史航拍照片和修正后的卫星图像评估了随时间（1937-2018 年）的变化。IAAP 覆盖范围每季度评估一次，涉及河流流量、物理化学和营养监测数据（2015-2018 年）。河口泛滥平原经过甘蔗种植、城市发展、荒地和采砂作业改造，占河口功能区的27%。过去，农业回流和城市径流促进了河口 IAAP 的零星增长。自 1983 年以来，汤加中央污水处理厂 (WWTW) 的排放增加了每日养分负荷（当前贡献：1.8 kg d-1 N 和 9.63 kg d-1 P），导致河口 IAAP 持续增长。WWTW 的排放量占河口体积的 15%，增加的流入量已将河口功能改变为主要开放、低盐度的状态。航拍照片分析表明，IAAP 沿河口全长（约 7 公里）建立，并且在所有河口状态下都很丰富。它们在高流量事件（≥ 5 m3s-1）期间被冲出河口，但在洪水后稳定重建（3 个月内）。IAAP 的可能来源是受到严重侵染的上游大坝。结合人工/生物控制方法和增加盐度的营养管理减少策略 (>