Effects of River Inputs on Near-Shore Silicate Dynamics in Lake Michigan

Abstract

Silicate plays a significant role as a nutrient in aquatic ecosystems especially by limiting the growth of certain phytoplankton, including diatoms which are important to food webs. Despite this importance, the cycling of silica in freshwater lakes is poorly understood, especially in near-shore regions. It is not known to what extent near-shore silicate concentrations are affected by benthic sources, or runoff from significant riverine sources. We explored this in near-shore Lake Michigan using flow data for the Milwaukee River during 2008-2011 and comparing with dissolved silicate (dSi) data. There were significant fluctuations in both near-shore (2-30 μM) and riverine (100-250 μM) dSi on a weekly time scale. To examine the contribution of these major sources of silicate to silicate dynamics, we compared the silicate concentration in the near-shore region to silicate supplied by the river (using river flow data sets) and silicate supplied by deep water upwelling (using wind speed, and direction). Weak relationships were found between riverine outflow and near-shore dSi but some correlations were found between upwelling indicators and near-shore dSi. We are currently applying a hydrodynamic model (based on the Princeton Ocean Model) to integrate silicate dynamics with near-shore mixing.