Submerged macrophyte-bed effects on water-column phosphorus, chlorophyll a, and bacterial production

Submerged macrophytes are a major component of freshwater ecosystems, yet their net effect on water column phosphorus (P), algae, and bacterioplankton is not well understood. A 4-month mass-balance study during the summer quantified the net effect of a large (similar to5.5 ha) undisturbed macrophyte bed on these water-column properties. The bed is located in a slow-flowing (0.05-0.1 cm s(-1)) channel between two lakes, allowing for the quantification of inputs and outputs. The P budget for the study period showed that, despite considerable short-term variation, the macrophyte bed was a negligible net sink for P (0.06 mg m(-2) day(-1), range from -0.76 to +0.79 mg m(-2) day(-1)), demonstrating that loading and uptake processes in the weedbed roughly balance over the summer. Chlorophyll a was disproportionately retained relative to particulate organic carbon (POC), indicating that the algal component of the POC was preferentially trapped. However, the principal contribution of the weedbed to the open water was a consistent positive influence on bacterioplankton production over the summer. Conservative extrapolations based on measured August specific exports (m(-2) day(-1)) of P and bacterial production exiting the weedbed applied to five regional lakes varying in lake morphometry and macrophyte cover suggest that even in the most macrophyte dominated of lakes (66% cover), P loading from submerged weedbeds never exceeds 1% day(-1) of standing epilimnetic P levels, whereas subsidization of bacterioplankton production can reach upward of 20% day(-1). The presence of submerged macrophytes therefore differentially modifies algae and bacteria in the water column, while modestly altering P dynamics over the summer.