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
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.