Small-scale study of primary productivity during spring in a Mediterranean coastal area (Gulf of Lions)

Euphotic zone concentrations of chlorophyll, NO3−, NH4+ and fluxes of nitrogen (15NO−3 and 15NH+4) and carbon (H14CO−3) were measured in the northwestern part of the Mediterranean Sea during the High-Frequency Flux (HFF) experiment. The study site (700 km2) was investigated with a set of five daily cruises during March–May, 1997 to study the importance of short-term changes in water masses on productivity. Hydrographic data showed a transition between typical winter conditions (cold, mixed water column) and early summer (thermal stratification of surface layer) MAW (Modified Atlantic Water) predominance dominated with low NO3− (<0.50 μM) and chl (<1 mg m−3), low integrated primary production (IPP <400 mg C m−1 d−1) and a regeneration regime ( f-ratios∼0.30, NH4+ uptake prevailing). This regime changed within a few weeks as a result of the intrusion of NO3− rich Coastal Water (CW) and offshore Levantine Intermediate Waters (LIW) onto the continental shelf. Transient CW, moved into the area and caused an increase in chl (1.5 mg m−3) and IPP (400–600 mg C m−1 d−1) in the western part of the study site. Intrusion of LIW into the euphotic layer resulted in a large increase in production (1154 mg C m−1 d−1) and f-ratios (>0.45) suggesting that production was supported by NO3−. Throughout the study, vertical profiles of carbon assimilation were typical for early spring with highest rates in the surface layer (45 mg C m−3 d−1 at 20 m) decreasing to <2 mg C m−3 d−1 at 60 m. Nitrogen uptake profiles exhibited similar profiles with uptake rates ranging from 10 to 75 and 45 to 220 nM d−1 for NO3− and NH4+, respectively. No increase in NO3− uptake was observed near the nitracline indicating NO3− uptake was light limited during spring. Over the course of the study, new production was estimated of 4.9 g C m−2, an exceptional low rate for a spring period.