Categorizing zonal productivity on the continental shelf with nutrient-salinity ratios

Coastal ocean productivity is often dependent on riverine sources of nutrients, yet it can be difficult to determine how far the influence of the river extends. The northern Gulf of Mexico (GOM) receives freshwater and nutrients discharged mainly from the Mississippi and Atchafalaya Rivers. We used nutrient/salinity relationships to (i) differentiate the nutrient inputs of the two rivers and (ii) determine the potential extent of the zones where productivity is affected by each. We identified three different zones: one close to the coast having a linear nutrient/salinity relationship where physical forcing (river flow) dominates, one offshore with nutrient (N or Si) concentrations <1 μM, and one between them with variable nutrient concentrations largely controlled by consumption by autotrophs. While in the GOM salinity/nutrient relationships varied systematically with distance from the two rivers in winter, this was not seen in summer. Thus, the methodology is not always applicable directly, because the boundaries of the different regions vary with river flow, overall nutrient flux, and grids of stations at the regional spatial scale (15–20 km in the GOM), rather than single sections are needed to determine boundaries.