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.