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[1] A coupled physical-biological model is used to simulate the ecosystem characteristics in Lake Michigan. The physical model is the unstructured grid, Finite-Volume Coastal Ocean Model (FVCOM). The biological model is a NPZD model, including phosphorus as the nutrient, which is the limiting element in Lake Michigan, phytoplankton, zooplankton and detritus. The models are driven by observed hourly meteorological forcing in 1998 and the model results are calibrated by satellite and in situ data. The main physical and ecological phenomena in the spring of 1998 are captured. During March to May, a circle-like phytoplankton bloom appears in southern Lake Michigan, which looks like a 'doughnut'. The formation mechanisms of the prolonged spring bloom are investigated. It is confirmed that the phytoplankton bloom is forced by rapidly increasing temperature and light intensity in spring. The thermal front that develops in spring inhibits the transport of nutrients and phytoplankton from the nearshore to the deeper water. The wind-driven gyre circulation in southern Lake Michigan induces significant offshore transport, which contributes to the establishment of the circular bloom.