Potential eutrophication assessment in rivers; relationship between produced and exported loads

In contrast with biological processes in lacustrine environments, the physical characteristics of river waters greatly complicate the relationship between nutrient input and primary productivity. In this paper a new approach to quantify this relationship is developed, linking land use within the watershed to the bioavailability of nutrients in the receiving waters. It estimates by two different ways an enrichment index derived from N/P ratios. A load model links land-use within the watershed area to nutrient export, and allows the estimation of the potential enrichment index from the ratio of calculated annual exports of nitrogen and phosphorus. This index value correlates well with the real enrichment index representing the annual mean value of the corresponding measured ratios in running waters. The latter index can then be related to nutrient availability expressed by chl-a and the algal growth potential. This model, established from 7 sampling stations on the Yamaska-North river, allowed assessment of nutrient bioavailability in the river water as a function of hydrological (low or high water flow) as well as of seasonal (summer or winter) events.