Virginia Institute of Marine Science, William & Mary, Gloucester Point, VA 23062, USA.
Virginia Institute of Marine Science, William & Mary, Gloucester Point, VA 23062, USA.
Sci Total Environ. 2021 May 15;769:145157. doi: 10.1016/j.scitotenv.2021.145157. Epub 2021 Jan 14.
Shoreline erosion supplies sediments to estuaries and coastal waters, influencing water clarity and primary production. Globally, shoreline erosion sediment inputs are changing with anthropogenic alteration of coastlines in populated regions. Chesapeake Bay, a prime example of such a system where shoreline erosion accounts for a large proportion of sediments entering the estuary, serves here as a case study for investigating the effects of changing sediment inputs on water clarity. Long-term increases in shoreline armoring have contributed to decreased erosional sediment inputs to the estuary, changing the composition of suspended particles in surface waters. This study examined the impact of shoreline erosion on water clarity using a coupled hydrodynamic-biogeochemical model. Experiments were conducted to simulate realistic shoreline conditions representative of the early 2000s, increased shoreline erosion, and highly armored shorelines. Together, reduced shoreline erosion and the corresponding reduced rates of resuspension result in decreased concentrations of inorganic particles, improving water clarity particularly in the lower Bay and in dry years where and when riverine sediment influence is low. This clarity improvement relaxed light limitation, which increased organic matter production. Differences between the two extreme experiments revealed that in the mid-estuary in February to April, surface inorganic suspended sediment concentrations decreased 3-7 mg L, while organic suspended solids increased 1-3 mg L. The resulting increase in the organic-to-inorganic ratio often had opposite effects on clarity according to different metrics, improving clarity in mid-Bay central channel waters in terms of light attenuation depth, but simultaneously degrading clarity in terms of Secchi depth because the resulting increase in organic suspended solids decreased the water's transparency. This incongruous water clarity effect, the spatial extent of which is defined here as an Organic Fog Zone, was present in February to April in all years studied, but occurred farther south in wet years.
海岸侵蚀为河口和沿海水域提供沉积物，影响水的清澈度和初级生产力。在全球范围内，由于人类对人口密集地区海岸线的改变，海岸侵蚀沉积物的输入正在发生变化。切萨皮克湾就是这样一个系统的主要范例，该系统中，海岸侵蚀导致大量沉积物进入河口，这里作为一个案例研究，调查了沉积物输入变化对水清澈度的影响。长期以来，海岸线的防护设施增加导致进入河口的侵蚀性沉积物减少，改变了地表水悬浮颗粒的组成。本研究使用耦合水动力-生物地球化学模型来研究海岸侵蚀对水清澈度的影响。进行了实验来模拟具有代表性的现实海岸线条件，包括 21 世纪初、增加的海岸侵蚀和高度防护的海岸线。减少的海岸侵蚀和相应减少的再悬浮速率共同导致无机颗粒浓度降低，从而改善水的清澈度，特别是在下游湾和在河流泥沙影响较低的干旱年份。这种清澈度的改善缓解了光限制，从而增加了有机物的产生。两个极端实验之间的差异表明，在中河口的 2 月至 4 月，表面无机悬浮泥沙浓度降低了 3-7mg/L，而有机悬浮固体增加了 1-3mg/L。根据不同的指标，有机与无机的比例的增加通常对清澈度有相反的影响，在光衰减深度方面，改善了中湾中心航道的清澈度，但同时由于有机悬浮固体的增加降低了水的透明度，从而降低了塞奇深度的清澈度。这种不协调的水清澈度效应，其空间范围在此定义为有机雾区，在所有研究年份的 2 月至 4 月都存在，但在湿润年份发生在更靠南的地方。
