Nutrient enhanced primary productivity of the Texas-Louisiana continental shelf

Light bottle and dark bottle C-14 uptake was measured on deck in 4-6 h shipboard incubations at 12 locations on the NW continental margin of the Gulf of Mexico in July and at 9 locations in October 1990. In July, rates of P-max m(-3) h(-1) were higher than previously reported for the Texas-Louisiana shelf, and daily production calculated from these 4-6 h incubations was 1-1.4 g C m(-2) d(-1) at most inner and middle shelf locations. However, in May-June 1990 freshwater discharge from the nitrate-rich Mississippi-Atchafalaya river system and from the Trinity River had reached highest recorded outflows in many years; near-surface nitrate concentrations over the Texas-Louisiana shelf remained well above the 0.05 mu M l(-1) limit of detection into July. In contrast, near-surface nitrate concentrations were close to or at the limit of detection in October. Reflecting this variation in nitrate inventory, in July the production index (P/B ratio) at stations near riverine and estuarine nutrient sources and over the inner shelf reached 40 mg C fixed m(-3) h(-1).(mg chl m(-3))(-1), whereas in October this index generally was 2- to 3-fold tower. Thus, primary productivity of the Texas-Louisiana continental margin appears to be moderate when not enhanced by the high ''new'' nitrogen nutrient loads that enter via riverine/estuarine outflows. Since the volume of freshwater discharge varies markedly on seasonal and interannual time scales, we suggest that ''discharge driven'' is a more appropriate description of the primary productivity of this subtropical continental margin than is its spatial partition into regions of high (250-500), medium (150-250) and low (100-150 mg C m(-2) d(-1)) mean production.