A comparative study of decomposition, oxygen consumption and nutrient release for selected aquatic plants occurring in an estuarine environment

The rates of decomposition and nutrient regeneration were compared among six aquatic plants representing examples from phytoplankton (Chlorella sp.), macroalgae (Ulva lactuca), submersed vascular macrophytes (Myriophyllum spicatum, Potamogeton perfoliatus, and Ruppia maritima) and marsh grasses (Spartina alterniflora). These plants, which were obtained from the Choptank River estuary, Maryland, (except for Chlorella which was a laboratory culture) were placed in 1 mm mesh bags and incubated in aquaria with ambient water under dark, aerated, temperature controlled (20 + 3°C) conditions for 93 d. The rank in decomposition rates based on both decrease in original mass (decrease in chlorophyll a for Chlorella) and associated oxygen consumption was phytoplankton > macroalga > submersed macrophytes > emergent macrophyte, and rates were directly proportional to the initial nitrogen content of the plant tissues. Nitrogen content of all the plant tissues increased during decomposition, yet reductions of C:N ratios were only observed for those plants with initial C:N > 20. N:P ratios generally increased due to a much higher leaching for P (10-40% of initial P) compared with N (1 to 10% of original N). The leached P was equally distributed between dissolved inorganic and organic forms. Generally, the magnitude of P and N leaching rate was not related to respective initial nutrient concentrations of the plant, nor to the plant's structural integrity (C:N ratio). Total N and P dissolved in the water column plus that in plant material remaining in the mesh bags at the experiment's termination accounted for 7 to 48% of their original respective quantities for submersed macrophytes compared with 8294% for Spartina.