Summertime Nutrient Supply to Near-Surface Waters of the Northeastern Gulf of Mexico: 1998, 1999, and 2000

In the summers of 1998, 1999, and 2000, deep water eddies induced strong anticyclonic currents along the upper slope and outer shelf from the Mississippi River delta to the west Florida shelf. Those currents transported Mississippi River discharge eastward along the outer shelf and slope, reversing the normal offshore increase in salinity, with the exception of a few regions very near the coast that were influenced by the discharges from other rivers or bays. The entrainment of low-salinity river water resulted in anomalously high chlorophyll a concentrations in the upper 15 m over the outer shelf and upper slope, in contrast to the concentrations that typically occur over deep water in the subtropics in summer. Nitrate concentrations in this surface water were quite low except near the mouths of rivers, which act as point sources for nutrients; presumably, this was because of the rapid utilization of nitrate by phytoplankton. A significant supply of nutrients to the euphotic zone at regions quite removed from these point sources resulted from eddies intruding onto or formed over the slope. These caused mid-depth water rich in nutrients to be uplifted to within the euphotic zone, the uplift depending on the location and intensity of the eddies. Based on measurements at approximately 100 stations on each cruise, estimates were made of the quantity of nitrate and silicate in the upper 15 m of the water column and in the depth interval from 15m to 60 m, the nominal depth of the euphotic zone. Study results suggest that the nitrate and silicate in the near-surface interval of 0-15 m largely resulted from riverine discharge and subsequent advection, while the nutrients between 15 and 60 m resulted from uplift of waters by circulation features. The euphotic zone occupied at least the upper 60 m of the water column, but standing stocks of nitrate and silicate in the 15- to 60-m layer were between two and six times those in the upper 15 m on all three cruises and appeared to depend on the strength and relative proximity to the shelf break of local anticyclonic features. The effects of these circulation features were potentially significant in supplying nutrients to the euphotic zone during these summers