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

Euphotic zone concentrations of chlorophyll, NO3-, NH4+ and fluxes of nitro gen ((NO3-)-N-15 and (NH4+))-N-15 and carbon ((HCO3-)-C-14) were measured i n the northwestern part of the Mediterranean Sea during the High Frequency Flux (HFF) experiment. The study site (700 km(2)) was investigated with a s et of five daily cruises during March-May, 1997 to study the importance of short-term changes in water masses on productivity. Hydrographic data showe d a transition between typical winter conditions (cold, mixed water column) and early summer (thermal stratification of surface layer) MAW (Modified A tlantic Water) predominance dominated with low NO3- (<0.50 mu M) and chl (< 1 mg m(-3)), low integrated primary production (IPP <400 mg C m(-1) d(-1)) and a regeneration regime (f-ratios similar to 0.30, NH4+ uptake prevailing ). This regime changed within a few weeks as a result of the intrusion of N O3- 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 r esulted in a large increase in production (1154mg C m(-1) d(-1)) and f-rati os (>0.45) suggesting that production was supported by NO3-. Throughout the study, vertical profiles of carbon assimilation were typical for early spr ing with highest rates in the surface layer (45 mg C m(-3) d(-1) at 20 m) d ecreasing to <2 mg C m(-3) d(-1) at 60 m. Nitrogen uptake profiles exhibite d 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 obse rved near the nitracline indicating NO3- uptake was light limited during sp ring. 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. (C) 2000 Elsevier Scie nce Ltd. All rights reserved.