Department of Biology, George Mason University, 4400 University Drive, Fairfax, Virginia 22030, and Center for Environmental and Estuarine Studies, University of Maryland System, Chesapeake Biological Laboratory, Solomons, Maryland 20688-0038.
Appl Environ Microbiol. 1990 Mar;56(3):747-57. doi: 10.1128/aem.56.3.747-757.1990.
The mesohaline portion of the Chesapeake Bay is subject to annual summertime hypoxia and anoxia in waters beneath the pycnocline. This dissolved oxygen deficit is directly related to salinity-based stratification of the water column in combination with high levels of autochthonously produced organic matter and a very high abundance of metabolically active bacteria. Throughout the water column in the lower, mesohaline part of the bay, between the Potomac and Rappahannock rivers, near the southern limit of the mainstem anoxia, bacterial abundance often exceeded 10 x 10 cells per ml and bacterial production exceeded 7 x 10 cells per liter per day during summer. Bacterial biomass averaged 34% (range, 16 to 126%) of the phytoplankton biomass in summer. These values are equal to or greater than those found farther north in the bay, where the oxygen deficit is more severe. Seasonal variations in bacterial abundance and production were correlated with phytoplankton biomass (lag time, 7 to 14 days), particulate organic carbon and nitrogen, and particulate biochemical oxygen demand in spring; but during summer, they were significantly correlated only with dissolved biochemical oxygen demand. During summer, dissolved biochemical oxygen demand can account for 50 to 60% of the total biochemical oxygen demand throughout the water column and 80% in the bottom waters. There is a clear spring-summer seasonal shift in the production of organic matter and in the coupling of bacteria and autochthonous organic matter. The measurement of dissolved, microbially labile organic matter concentrations is crucial in understanding the trophic dynamics of the lower mesohaline part of the bay. The absolute levels of organic matter in the water column and the bacterial-organic carbon relationships suggest that a lower bay source of organic matter fuels the upper mesohaline bay oxygen deficits.
切萨皮克湾的半咸水部分在夏季会出现底层缺氧和无氧现象，这种现象发生在密度跃层以下的水域。这种溶解氧亏缺与水柱基于盐度的分层直接相关，同时也与高水平的自生有机物质和大量代谢活跃的细菌有关。在湾流下部的半咸水区域的整个水柱中，在波托马克河和拉帕汉诺克河之间，靠近主缺氧带的南部边界，细菌丰度通常超过每毫升 10 x 10 个细胞，夏季细菌的比生产率超过每升每天 7 x 10 个细胞。在夏季，细菌生物量平均占浮游植物生物量的 34%（范围为 16%至 126%）。这些值与湾流北部更严重缺氧地区的数值相等或更大。夏季细菌丰度和比生产率的季节性变化与浮游植物生物量（滞后时间为 7 至 14 天）、颗粒有机碳和氮以及春季的颗粒生化需氧量相关；但在夏季，它们仅与溶解生化需氧量显著相关。在夏季，溶解生化需氧量可以占整个水柱总生化需氧量的 50%至 60%，在底层水中占 80%。有机物的产生和细菌与自生有机物的耦合在春季到夏季有明显的季节性转变。测量溶解的、微生物可利用的有机物浓度对于理解湾流下部半咸水区域的营养动态至关重要。水柱中有机物的绝对水平和细菌-有机碳关系表明，湾流下部的有机物是导致上部半咸水湾缺氧的原因。
