The occurrence of CHLa maxima in estuaries is variously attributed to localized conditions that favor enhanced growth rates or reduced loss rates. The latter include retention effects whereby entrainment of exogenous phytoplankton promotes their accumulation despite low localized growth rates. Circulation patterns in some estuaries are characterized by a surface layer of lowsalinity water flowing seaward over a bottom layer of higher salinity water moving landward. Hydrodynamic entrapment occurs as dense particles sink into the landward bottom current (Moon and Dunstan 1990; Simons et al. 2006; Lapierre and Frenette 2008). Where CHLa maxima co-occur with the estuarine turbidity maximum, retention has been invoked as a mechanism to explain high
phytoplankton abundance despite low light availability (Cole et al. 1992; Kocum et al. 2002; Muylaert et al. 2005). Alternatively, localized areas of high CHLa may reflect enhanced growth rates in response to favorable light and nutrient conditions. In estuaries, the transition from turbid, nutrient-rich riverine waters to clear, nutrient-poor marine waters favors peak production at a transition point where release from light limitation allows phytoplankton to maximize nutrient utilization. For example, rates of primary
production in the tidal freshwater portion of the Schelde Estuary were nearly an order of magnitude higher than in the saline zone (Muylaert et al. 2005). Thus, both growthand loss-related processes have been invoked as mechanisms accounting for the occurrence and magnitude ofCHLa maxima in estuaries