MEPS 212:53-67 (2001)  -  doi:10.3354/meps212053

ABSTRACT: The short-term variability of photosynthesis-irradiance (P-E) relationships and integrated primary production at 3 stations (Stns A, B, C in a coastal-offshore gradient) located across the Western Alboran Sea Gyre (SW Mediterranean) were investigated during a High Frequency Flux Experiment (1 to 16 May 1998). Photosynthetic production of total, particulate and dissolved organic carbon (TOC, POC and DOC respectively) was considered separatedly in 34 experiments. Photosynthetic parameters were calculated for TOC and POC data. For POC measurements, maximum photosynthetic rate (PB_{m POC}) and the initial slope of the P-E curve (α) varied between 0.59 and 6.25 mg C (mg chl a)^-1 h^-1 and 0.0023 and 0.0182 mg C (mg chl a)^-1 h^-1 (µmol photons m^-2 s^-1)^-1, respectively, and were significantly correlated, showing a similar decrease with depth. No photoinhibition was detected at any depth and the light saturation parameter (E_k) was on average 251 µmol photons m^-2 s^-1, suggesting that phytoplankton assemblages were acclimated to high irradiance levels in the whole area. Percent extracellular release (PER) obtained for each experimental irradiance (PER_E = DOC/[POC+DOC]) showed an inverse relationship with irradiance when data from all the P-E experiments were pooled. A light-saturated chl a-normalized DOC production rate (PB_{m DOC}) was calculated as PB_{m TOC} - PB_{m POC}, and PER_m values calculated as PB_{m DOC}/PB_{m TOC} tended to increase with depth. Integrated particulate and total primary production varied considerably (23- and 35-fold, respectively), but tended to decrease along a coastal-offshore gradient. During the cruise, a subsurface phytoplankton bloom was advected into Stn A, increasing primary production rates up to 1 g C m^-2 d^-1. Average particulate primary production was 632 mg C m^-2 d^-1 at Stn A, 388 mg C m^-2 d^-1 at Stn B and 330 mg C m^-2 d^-1 at Stn C, with dissolved primary production ranging from 4 to 44% of total values. Integrated primary production was significantly correlated with surface chl a but not with integrated chl a. This finding has implications for the estimation of regional primary production based on remote sensing of near-surface pigments. Altogether, the results confirmed that the high hydrodynamism of the Alboran Sea was associated with marked biological variability and episodic high primary production events.

KEY WORDS: Phytoplankton · P-E relationships · Short-term variability · Primary production · DOC · Upwelling