Dc. Biggs et Ll. Sanchez, NUTRIENT ENHANCED PRIMARY PRODUCTIVITY OF THE TEXAS-LOUISIANA CONTINENTAL-SHELF, Journal of marine systems, 11(3-4), 1997, pp. 237-247
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 Jul
y, 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 t
he nitrate-rich Mississippi-Atchafalaya river system and from the Trin
ity River had reached highest recorded outflows in many years; near-su
rface nitrate concentrations over the Texas-Louisiana shelf remained w
ell above the 0.05 mu M l(-1) limit of detection into July. In contras
t, near-surface nitrate concentrations were close to or at the limit o
f detection in October. Reflecting this variation in nitrate inventory
, in July the production index (P/B ratio) at stations near riverine a
nd 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 g
enerally was 2- to 3-fold tower. Thus, primary productivity of the Tex
as-Louisiana continental margin appears to be moderate when not enhanc
ed by the high ''new'' nitrogen nutrient loads that enter via riverine
/estuarine outflows. Since the volume of freshwater discharge varies m
arkedly on seasonal and interannual time scales, we suggest that ''dis
charge driven'' is a more appropriate description of the primary produ
ctivity of this subtropical continental margin than is its spatial par
tition into regions of high (250-500), medium (150-250) and low (100-1
50 mg C m(-2) d(-1)) mean production.