Importance of physical processes on near-surface nutrient distributions in summer in the northeastern Gulf of Mexico

Abstract

As part of the northeastern Gulf of Mexico chemical oceanography and hydrography study, data on salinity, nutrients, and surface chlorophyll were collected three times per year over the northeastern Gulf of Mexico along 11 cross-shelf (normal to bathymetry) transects from the Mississippi River Delta to Tampa Bay. The transects covered the shelf from the 10-m to the 1000-m isobath. In all three summers of 1998, 1999, and 2000, eddies over the continental margin induced strong, eastward anticyclonic flows over the outer shelf and slope. This flow strongly affected the distribution of coastal water on the shelf in all three years, drawing Mississippi River water into the circulation along the 1000-m isobath and reversing the normal offshore salinity gradient. The entrainment of low salinity river water resulted in anomalously high nutrient and chlorophyll a concentrations in the upper 10-20 m over the outer shelf and upper slope. This is in contrast to the typically low standing stocks of phytoplankton that occur over deep water in subtropical summertime conditions. Vertical nutrient distributions indicated that there was uplift of mid-depth water to shallower depth caused by the presence of either cyclones, regions of diverging flow at eddy peripheries, or bottom Ekman layer transport. Comparison of near-surface nutrient distributions in the low salinity entrainment features with those in the region of uplifted mid-depth water showed that the intrusion of an anticyclonic secondary eddy onto this subtropical continental margin caused deep water rich in nutrients to rise to depths as shallow as 35 or 10 m depending on the location and the intensity of the anticyclonic and cyclonic eddies. Thus, the impact of both Mississippi River water and anticyclonic and cyclonic eddies on nutrient distribution may be important in enhancing productivity over the study region during summer. On average, during each summer, the amount of nitrate reaching the outer shelf and slope in the nutrient-depleted, low-salinity river water transported by eddy circulation, provided 15 to 32 % as much nitrate as came from uplifted mid-depth water from either cyclones, diverging flow at eddy peripheries, or bottom Ekman layer transport.