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Figure 9.4. Relationship between phytoplankton biomass (as chlorophyll-fl) and freshwater discharge, both averaged for the period from May 15 through October 1; error bar is SD. The solid circles are for the period from 1986-1991, prior to the introduction of the zebra mussel ("Pre"). The open circles are for the period 1993-2000 ("Post") and the open triangle is for 1992, the transitional year (see text).

variation at a single site (Fig. 9.5). The spatial structure is complex and has changed dramatically in response to the zebra mussel invasion, as we saw for the seasonal cycle. Even following the invasion, however, there is intriguing spatial structure that we do not fully understand. Clearly a primary determinant is water column depth. Within the tidal-freshwater portion, which is well mixed, the shallower reaches tend to have the highest volumetric biomass. Deeper water column depth causes cells to spend more time in the dark. Depth at a given reach interacts with other factors such as input from upstream and advective losses. A model that includes these factors along with grazing rates and phytoplankton growth rates is able to reproduce the major features of the chlorophyll pattern along the length of the river both before and the first few years after the zebra mussel invasion (Caraco et al., 1997).

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