Julie Granger, faculty member at University of Connecticut in Marine Sciences, will be presenting "Inshore nutrient trapping increases the fertility of the Southern Benguela Upwelling System".
Seminar abstract: The southern Benguela upwelling system (SBUS) supports high rates of primary productivity that sustain important commercial fisheries. The exceptional fertility of this system is reportedly fueled not only by upwelled nutrients, but also by nutrients regenerated on the broad and shallow continental shelf. We measured nutrient concentrations and the nitrogen (N) and oxygen (O) isotope ratios (δ15N and δ18O) of nitrate along four zonal lines in the SBUS in late summer and early winter to evaluate the extent to which regenerated nutrients augment the shelf nutrient reservoir. During summer upwelling, a decrease in on-shelf nitrate δ18O revealed that, on average, 30% of the subsurface nutrients derived from in situ remineralization. While a similar proportion of the on-shelf nitrate was regenerated in the more quiescent winter (35%), this signal extended further along the mid-shelf, such that the total regenerated nitrate burden was greater during winter. A shoreward increase in subsurface nitrate δ15N and a greater N deficit in on-shelf bottom waters further indicated N loss to benthic (and in some instances, water column) denitrification coincident with the on-shelf remineralization. Our data show that remineralized nutrients get trapped on the SBUS shelf in summer through early winter, enhancing the nutrient pool that can be upwelled to support surface production. We hypothesize that this process is aided by a number of hydrographic fronts that impede the lateral exchange of surface waters. The extent to which nutrients remain trapped on the shelf likely has implications for the occurrence of hypoxic events in the SBUS.
