Factors In The Decline of Submerged Aquatic Vegetation In The Patuxent River With An Emphasis on the Role of Nutrients

Abstract

Literature concerning Patuxent River SAV was reviewed to determine the factors most likely responsible for plant decline. Ruppia maritima, Potamogeton perfoliatus, Na'as flexilis, Elodea canadensis, and Myriophyllum spicatum were dominant in the river in the early 1960's. In the 1970's, only 4 species were observed including P. perfoliatus, R. maritima, and Zannichellia palustris. The depth to which light penetrates the water column is considered a major factor in SAV decline. Suspended sediment, resulting from runoff from surrounding land, blocks light from penetrating the water column. Light penetration is also blocked by phytoplankton growth stimulated by high nutrient levels in the water column. Hurley reported light must penetrate at least 0.5m for SAV survival. These levels correspond to a chlorophyll a level of less than 0.015 mg/l. To achieve this level of chlorophyll a, phosphorus concentrations within the river must be less than 0.02 mg/1 and nitrogen less than 0.15 mg/l. A 1985 ban on phosphorus in detergents and the upgrading of sewage treatment plants (point sources) for phosphorus and nitrogen removal have resulted in decreased loading rates of these nutrients into the Patuxent River. Unfortunately, controlling point sources alone does not result in the required chlorophyll a limits. Nonpoint sources, primarily from agricultural land, also need to be controlled. Conservation tillage covers 75 percent of the Patuxent River Basin cropland and has resulted in a decrease of loading rates to 260 kg/day phosphorus and 4438 kg/day nitrogen. By the author's calculations, increasing conservation tillage to 100 percent of the cropland will not result in river nutrient concentration reductions sufficient to lower chlorophyll a to the necessary level. However, a mathematical model needs to be developed to more accurately measure how nutrient loadings correlate to nutrient and chlorophyll a concentrations specifically in the Patuxent River.