Nutrient enhanced primary productivity of the Texas-Louisiana continental shelf

Light bottle and dark bottle 14C 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 Pmax m−3 h−1 were higher than previously reported for the Texas-Louisiana shelf, and daily production calculated frm 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 μM 1−1 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 lower. 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.