Effects of seasonal stratification on turbulent mixing in a hypereutrophic coastal lagoon
Effects of seasonal stratification on turbulent mixing in a hypereutrophic coastal lagoon
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DOI：
10.4319/lo.2010.55.1.0172
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发表时间：
2010-01-01
影响因子：
4.5
通讯作者：
Drake, Jeana L.
中科院分区：
文献类型：
--
作者：
Cousins, Mary;Stacey, Mark T.;Drake, Jeana L.
研究了一个浅海、潮汐阻塞的沿海泻湖中垂直湍流混合率、密度结构、无机营养物浓度和浮游植物生物量的季节变化。项目地点，罗迪欧泻湖，位于加利福尼亚州金门国家娱乐区，近年来经历了蓝藻结核菌和铜绿微囊藻的强烈繁殖。从2006年3月到2008年4月，沿着泻湖的一个样带收集的每月测量数据显示，在冬季，从分水岭输入的淡水和从风暴潮输入的咸水都达到最高时，咸水强烈分层。在此条件下，浮力频率的平方大于0.5 s（-2）。在夏季，较弱的日温度分层是该富营养化泻湖强光吸收特性的结果。风是湍流混合的主要驱动力。虽然水深小于2.5 m会导致垂直混合的快速发生，但有限的取水量和强密度梯度减少了风应力和底应力的耦合。垂直湍流扩散系数在斜斜上降低了3个数量级，而斜斜内部和下方的水柱只是间歇性地表现出活跃的湍流。盐基分层的年循环以及伴随而来的沉积物和上覆水柱之间的营养物质湍流交换的减少抑制了营养物质从泻湖中冲洗出来，并导致浮游植物生物量过剩。
Seasonal variations in vertical turbulent mixing rates, density structure, inorganic nutrient concentrations, and phytoplankton biomass were investigated in a shallow, tidally choked coastal lagoon. The project site, Rodeo Lagoon, is located in the Golden Gate National Recreation Area, California, and has experienced intense blooms of the cyanobacteria Nodularia spumigena and Microcystis aeruginosa in recent years. Monthly measurements collected along a transect of the lagoon from March 2006 to April 2008 show it is strongly stratified by brackish water in winter, when freshwater inputs from the watershed and saltwater inputs from storm surge are both at their highest. The squared buoyancy frequency exceeds 0.5 s(-2) under these conditions. In summer, weaker diurnal temperature stratification is the result of strong light absorption characteristics in this hypereutrophic lagoon. Wind is the dominant driver of turbulent mixing. Although water depths of less than 2.5 m lead to the expectation of rapid vertical mixing, limited fetch and strong density gradients reduce the coupling of wind stress and bottom stress. The vertical turbulent diffusivity is reduced by as much as three orders of magnitude across the pycnocline, and the water column in and below the pycnocline shows active turbulence only intermittently. The annual cycle of salt-based stratification and accompanying reduction in turbulent exchange of nutrients between the sediments and overlying water column inhibit the flushing of nutrients out of the lagoon and contribute to excessive phytoplankton biomass.