Nutrient and Lower Food Web Dynamics:
A Coupled Biological and Physical Modeling Study

Changsheng Chen, Lixia Wang, Rubao Ji, David J. Schwab, Dmitry Beletsky, J. W. Budd, Gary L. Fahnenstiel, T. H. Johengen, H. Vanderploeg, Brian Eadie, Wayne Gardner, Peter Lavrenyev, and Marie Bundy

The plume ecosystem of Lake Michigan was examined using a 3-D coupled physical and biological model. The physical model is the Princeton Ocean Model (POM) and the biological model is a phosphorus-controlling lower trophic level food web model including 7 state variables (phosphorus, small and large phytoplankton, small and large zooplankton, bacteria, and detritus). The coupled model was forced by the 1998 and 1999 real-time wind and net heat flux under wintertime climatological initial conditions and ran prognostically with input of the real-time suspended sediment concentration derived directly from temporally and spatially interpolated satellite imagery (with correction using data samples).

The model results show that the spatial distribution of the biological field was closely related to the physical environment associated with 3D circulation and mixing. The different spatial patterns of the chlorophyll-a concentration found in southern Lake Michigan in 1998 and 1999 were well coherent with the spatial distribution of the model-predicted 3D circulation for these two years.

The suspended sediments in the reflective, recurrent coastal plume had a significant impact on the spatial distribution and temporal variation of the nutrients and plankton in southern Lake Michigan in 1998 and 1999 spring seasons. The nutrients released from suspended sediments were critical to maintain the nutrient level in southern Lake Michigan. The growth of phytoplankton in the plume depended on the availability of nutrients and light, but the cross-shelf decrease tendency of phytoplankton was controlled by a ratio of the euphotic depth to the mixed layer depth. The photosynthesis inhibition due to both light intensity and suspended sediments led to a subsurface maximum of primary production, but had little influences on the spatial distribution of phytoplankton biomass as a result of strong vertical mixing.

The cross-plume fluxes of nutrients and phytoplankton were mainly driven by episodic wind events with a period of about 5 to 7 days: offshore during northerly winds and onshore during southerly winds. The flux estimates among biological variables in the food web system suggested that the microbial food web played an important role in the secondary production in southern Lake Michigan. The lower trophic level food web system could be divided into two decoupled loops: (1) detritus-bacteria-microzooplankton-large zooplankton and (2) nutrient-phytoplankton-detritus.

The model-predicted spatial distributions of nutrients and phytoplankton were in reasonable agreement with observations taken during the 1988 and 1999 EEGLE interdisciplinary cruises, suggesting that the model was robust to capture the basic characteristics of the plume ecosystem in southern Lake Michigan.