Nitrogen and phosphorus cycling by nymphaea tuberosa and ceratophyllum demersum in Lake Onalaska, Navigation Pool 7 of the Upper Mississippi River

Abstract

Nitrogen and phosphorus cycling by Nymphaea tuberosa and Ceratophyllum demersum was studied in Lake Onalaska, Navigation Pool 7of the Upper Mississippi River, for a one-year period. The influence these macrophytes exerted on the sediments, water, and on the exchange between sediments and water was determined. Three sampling stations were studied; a monotypic stand of Nymphaea tuberosa, a monotypic stand of Ceratophyllum demersum, and a control area representative of the majority of Lake Onalaska. Sampling occurred once every two weeks. At each sampling time representative samples of sediments, water, and plant material were collected and analyzed for nutrient content. Biomass at each sampling site was also determined. Multiple correlations among sediment, water, and plant nutrient concentrations were calculated, as were the absorption and loss of nutrients from the sediments, water, and plant material. In both macrophytes, cycling was dependent upon the extent of plant growth above the sediment-water interface, on the ability of the plant to assimilate sediment nutrients and function as a nutrient sink, and on the release of the nutrients upon senescence of the plant. Nymphaea tuberosa obtained the majority of its nutrients from the sediments in which it was rooted. It retained average cellular concentrations of 679.78 mg phosphorus/m2 and 4021 mg nitrogen/m2 and mineralized 28.44 kg phosphorus/year and 127.4 kg nitrogen/year. Ceratophyllum demersum absorbed all of its nutrients from the water; however, the source of those nutrients were from both the sediments and water. The macrophyte upset the natural sediment-water equilibrium by retaining an average of 910.84 mg phosphorus/m2 and 3595.88 mg nitrogen/m2 and caused the sediments to transfer nutrients to the water to re-establish equilibrium. Ceratophyllum demersum mineralized a total of 2.72 kg phosphorus/year and 88.07 kg nitrogen/year. Both macrophytes increased the rate of mobilization of nitrogen and phosphorus from the sediments to the water.