Submersed macrophyte effects on nutrient exchanges in riverine sediments

Submersed macrophytes are important in nutrient cycling in marine and lacus trine systems, although their role in nutrient exchange in tidally-influenc ed riverine systems is not well studied. In the laboratory, plants signific antly lowered porewater nutrient pools of riverine sediments compared with bare controls. Deep-rooted Vallisneria americana lowered the porewater nutr ients to a greater extent than the shallow-rooted Potamogeton pectinatus. V . americana showed significantly higher tissue nutrient content (N in roots , P in leaves) than P. pectinatus. Porewater nutrients in the river increas ed from spring to summer (1995) when vegetation was at its peak (for porewa ter PO4-P, p < 0.05). In 1996, porewater nutrients were higher during peak plant biomass in the summer than in the fall (for porewater PO4-P, p < 0.05 ). In the summer (1995) vegetated patches had significantly greater porewat er PO4-P than bare patches. We hypothesize that the concentrating of partic ulates in riverine grassbeds and subsequent microbial processing may provid e an indirect source of nutrients for submersed macrophytes. In tidally-inf luenced riverine systems, biological mechanisms such as root uptake of nutr ients and lateral oxygen release may be masked by the interaction of physic al forces (i.e., tides, currents) with the structure of the grassbeds.