Prior studies have concluded that CHLa maxima in some estuaries arise through retention of exogenous phytoplankton because abundance cannot be explained by local growth where turbidity is high (Cole et al. 1992; Lapierre and Frenette 2008). Our metabolism data suggest that is not the case in the tidal freshwater segment of the James River Estuary. Net plankton metabolism was positive at all sites (GPPp > Rp) with highest values observed at the CHLa maximum. NPP e was correlated with the mean irradiance of the water column indicating that light availability determined primary production. Chlorophyll-specific production was
similar among sites suggesting that changes in light utilization efficiency (e.g., due to species shifts; Lehman 2007) did not account for inter-site variation in NPP. The CHLa maximum occurred at the transition from a narrow, deep channel to a broad channel with shallow lateral areas. In the narrow upper segment, light availability and NPP were limited by large water depths despite greater water clarity (low turbidity and attenuation). At the site of the CHLa maximum, average values of underwater irradiance were
two–threefold higher and exceeded previously reported light limitation thresholds for river and estuarine phytoplankton
(5–10 E m−2 day−1; Koch et al. 2004; Oliver and Merrick 2006; Whalen and Benson 2007). The region of elevated