WATER COLUMN IMPACTS OF MACROPHYTE DECOMPOSITION BENEATH FIBERGLASS SCREENS

The use of fiberglass screens to control nuisance growth of aquatic plants is a relatively new method which is rapidly increasing in usage. Laboratory experiments and field enclosure studies were used to assess potential nutrient regeneration and exertion of O2 demand resulting from the decomposition of macrophyte [Myriophyllum spicatum] material beneath these screens. Nutrient regeneration was a function of plant nutrient content and sediment retention capacity. Due to the high C:N ratio of most macrophytes relative to that of decomposing organisms, the regeneration of N did not occur. Decomposition of macrophytes with a moderate P content resulted in the retention of the plant P within the sediment. In the laboratory the use of plants with an abnormally high P content and sediment with a low mineral fraction, resulted in the regeneration of much of the plants'' initial P content to the water column. The potential for water column O2 depletion was a function of the rate of plant death, the initial plant biomass, the magnitude of the natural diurnal O2 fluctuation and the rate of regeneration or dilution via water movement. Overall the potential for adverse water column impacts is low under typical field conditions. Postulated impacts would be dependent upon site-specific characteristics such as area of treatment, water mixing, plant biomass and nature of the substrate. Placement of the screens in the early spring prior to the development of a large biomass should be sufficient totally to offset impacts associated with plant decomposition.