Department of Civil and Environmental Engineering, University of California, Davis, CA, 95616, USA.
Department of Civil and Environmental Engineering, Portland State University, Portland, OR, 97207, USA.
J Environ Manage. 2020 May 1;261:110233. doi: 10.1016/j.jenvman.2020.110233. Epub 2020 Mar 2.
Transport and fate of phytoplankton blooms and excessive nutrients along salinity and turbidity gradients of a river-estuary continuum could determine when and where impaired water quality occurs. However, the general spatiotemporal patterns, underlying mechanisms and their implication for water quality management are not well understood. This study reveals typical seasonal variations and longitudinal patterns of phytoplankton, dissolved oxygen (DO) and nutrients (C, N, and P) in the lower St. Johns River estuary in Florida based on 23 years of data and a model which spans 3 years. Evident declines in freshwater phytoplankton and DO concentrations were observed in the freshwater-saltwater transition zone and the estuarine turbidity maxima along the river-estuary continuum. Observations show that most cyanobacteria blooms originating from upstream lakes collapsed in the freshwater-saltwater transition zone where salinity was greater than 1 ppt, but data analysis and model simulation both indicate salinity stress was not the sole reason, other factors such as changes in hydrodynamics and river morphology also contributed to the bloom crashes. Inorganic nutrients (ammonium, nitrate, and phosphate) exhibited inverse longitudinal patterns with phytoplankton. Due to algal uptake, summer concentrations of inorganic nutrients were low in the freshwater, but substantially elevated in the marine reach as a result of large point source inputs and nutrient regeneration from organic detritus. However, because of strong river-ocean mixing, the dramatic increase in nutrients did not promote a phytoplankton bloom in the local marine reach. The nutrients were eventually transported into coastal waters or oceans where they fueled phytoplankton blooms. Our findings highlight that strategies for nutrient reduction and phytoplankton bloom management should be developed beyond local reaches and across a river-estuary-ocean continuum, exploring the possibility that freshwater phytoplankton blooms and excessive nutrients may be transported to downstream estuaries, coastal waters and even oceans that are vulnerable to poor water quality.
沿河流-河口连续体的盐度和浊度梯度，浮游植物水华和过量营养物质的输运和归宿，可能决定水质恶化发生的时间和地点。然而，一般的时空格局、潜在机制及其对水质管理的意义尚不完全清楚。本研究基于 23 年的数据和一个跨越 3 年的模型，揭示了佛罗里达州圣约翰斯河下游河口浮游植物、溶解氧（DO）和营养物质（C、N 和 P）的典型季节性变化和纵向格局。在淡水-海水过渡带和河口浊度最大值处，观察到淡水浮游植物和 DO 浓度明显下降，这种情况沿河流-河口连续体延伸。观测结果表明，大部分源自上游湖泊的蓝藻水华，在盐度大于 1ppt 的淡水-海水过渡带崩溃，但数据分析和模型模拟均表明盐度胁迫不是唯一原因，其他因素如水动力和河流形态的变化也促成了水华的崩溃。无机营养物质（氨、硝酸盐和磷酸盐）与浮游植物呈相反的纵向分布模式。由于藻类吸收，夏季在淡水段的无机营养物质浓度较低，但由于大量点源输入和有机碎屑的营养再生，在海洋段的浓度显著升高。然而，由于强烈的河海混合，营养物质的急剧增加并没有在当地海洋段促进浮游植物水华的发生。这些营养物质最终被输送到沿海水域或海洋，在那里它们为浮游植物水华提供了燃料。我们的研究结果强调，应超越局部范围，在河流-河口-海洋连续体上制定减少营养物质和浮游植物水华管理的策略，探索淡水浮游植物水华和过量营养物质可能被输送到下游河口、沿海水域甚至容易受到水质恶化影响的海洋的可能性。
