EFFECTS OF DIFFERENT SUBMERSED MACROPHYTES ON SEDIMENT BIOGEOCHEMISTRY

Porewater phosphate levels in submersed macrophyte grassbeds varied am ong years in the upper Chesapeake Bay (Maryland, USA) coincident with macrophyte species variation during these same years (1990, 1993, 1995 ). When native, deep-rooted Vallisneria americana Michx. was a codomin ant in the grassbed, the porewater phosphate concentrations were signi ficantly lower (P < 0.001) than concentrations when the exotic, shallo w-rooted species Hydrilla verticillata (L.f.) Royle and Myriophyllum s picatum L. were codominants. There were significant relationships (P < 0.001) between solid-phase inorganic phosphorus and reactive metals ( Fe, Mn) in both native and exotic grassbeds. However, the slopes of th e regression relationships between years were significantly different (P < 0.001), suggesting greater retention of inorganic phosphorus in s ediments when V. americana was a codominant at the site. In addition, significant relationships between reactive manganese and iron in the s ediments were observed, but the coefficient of determination was stati stically greater (P < 0.001) when V. americana was a codominant at the site. Furthermore, plant cores of V. americana and H. verticillata ha d noticeably different sediment redox profiles, with the oxidation-red uction status of V. americana sediments being more oxidized in the roo t zone (i.e., +125 mV vs -5 mV at 4 cm depth). These data suggest that macrophyte species composition can alter sediment biogeochemistry res ulting in varying porewater phosphate and solid-phase phosphorus and m etal levels. Possible explanations for these biogeochemical difference s may be attributed to morphological differences among macrophyte spec ies (i.e., root/shoot ratio, canopy type, growth form) and differences in root oxygenation capabilities.