ABSTRACT: Coastal eutrophication has been identified as an important ecological problem in many regions. Yet simple, generalizable models, such as those available for the management of lake eutrophication, do not exist for estuaries. As a first step inthe development of estuarine eutrophication models, we generated chlorophyll a:nutrient regression models for 15 estuaries in Prince Edward Island, Canada. Total phosphorus and total nitrogen account for 65 and 72% of the variance in chlorophyll(chl), respectively. However, when these models are compared to similar models for lakes, the yield of chl per unit nutrient is between 1 and 2 orders of magnitude lower in estuaries than in lakes. As herbivory and turbidity are likely contributors tothis low yield, we used a mass-balance approach to model phytoplankton biomass as a function of primary production and losses due to flushing, sedimentation and herbivory. In the 6 estuaries with mussel aquaculture, 45 to 88% of the chl deficit could beaccounted for by herbivory. In the remaining 9 estuaries, turbidity accounted for 35 to 75% of the chl deficit. Considering both herbivory and turbidity, the mass-balance accounted for on average 68% of the chl deficit for the 15 estuaries. We alsogenerated an empirical model predicting the deficit as a function of herbivory and turbidity; this model accounted for 50% of the variation in the deficit. The analysis suggests that chl:nutrient relations can be generalized across fresh and estuarineaquatic systems if turbidity and herbivory are considered.