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Excess Nutrients Foster the Growth of Phytoplankton

Low summer dissolved oxygen levels threaten life at the bottom of the Severn. Excessive growth of phytoplankton leads to low dissolved oxygen levels in water near the bottom of the Severn in the summer. This stresses all animal life, and while mobile organisms like fish can swim away from regions of oxygen-depleted water, bottom dwelling organisms generally do not have this ability and the more sensitive species die off. The process by which oxygen is used up after phytoplankton blooms is known as eutrophication, which has been documented in many bodies of water world-wide suffering from excess nutrients (nitrogen and phosphorus).
Estuaries are susceptible to eutrophication in the summer because the lighter, warmer fresh water layers over the denser salt water and forms a stable gradient, resisting vertical mixing. Wind and waves disrupt this gradient and promote mixing, but in sheltered areas like the Severn such mixing of the salt layers is minimal. Data from the Severn shows salinity gradients are present, even if they are not as pronounced as those in the deeper Chesapeake. Measurements of dissolved oxygen in the Severn show that levels are low enough to threaten life on the bottom in the deeper parts of the River every summer. While fish can generally swim away from regions of low oxygen, many other organisms lack this ability. Most estuarine life forms can tolerate low oxygen levels for hours or even a day, and a spectrum of sensitivities exists among benthic (bottom-dwelling) organisms. However, prolonged hypoxia (low oxygen) becomes fatal even to resistant organisms like oysters. If nutrient levels increase, the abundance and richness of life in the Severn will clearly suffer from these oxygen-depleting effects of eutrophication.

Phytoplankton and epiphytic algae block sunlight from reaching the leaves of SAV.

As described, during the 1990s submerged aquatic vegetation (SAV) in the Severn River mainstem recovered much of the acreage it had formerly occupied. SAV is a critical element in the ecology of the Chesapeake, providing shelter for fish and crabs, food for wintering waterfowl, and protecting shorelines from wave-induced erosion. Nevertheless, SAV has not come back in many parts of the watershed, and 2000 saw a major set-back in SAV acreage, apparently because of a particularly strong spring dinoflagellate (a type of phytoplankton) bloom. These microscopic organisms grow thick enough to discolor the water, absorbing light otherwise available to SAV. In addition to increasing phytoblankton growth, excess nutrients foster the growth of epiphytic algae on SAV. Just as epiphytes grow on rainforest trees, these algae grow directly on the leaves of SAV and thus absorb sunlight which would otherwise benefit the SAV. The Severn's SAV recovery of the 1990s is clearly fragile, and enhanced water clarity is critical to continued progress. Excess nutrients drive all of the above problems, and are probably the major threat to the quantity and quality of life in the Severn.

Response to the nutrient threat.The sources of nitrogen and phosphorus in the Severn are numerous, and to a considerable extent are brought into the tidal river by Bay water. Local municipal sewage treatment conforms to high nutrient reduction standards, but individual septic systems serve much of the watershed and these probably contribute substantially to local nitrogen loading. Studies are needed on which nutrients are most critical in fostering phytoplankton growth in the Severn and what methods can provide the most cost-efficient means of reducing them. The Severn's nutrient problems are linked to those of the northern Chesapeake generally, and concerted action throughout the watershed is required for improvement.

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