Time series of wind-stress data, AVHRR and SeaWiFS satellite images, and in situ data from seven cruises are used to assemble a coherent picture of the hydrographic variability of the seas off the Northwest Iberian Peninsula from the onset (September-October) to the cessation (February-May) of the Portugal coastal counter current (PCCC). During this period the chemistry and the biology of the shelf, slope and ocean waters between 40°and 43°N have previously been undersampled. Novel information extracted from these observations relate to:1. The most frequent modes of variability of the alongshore coastal winds, covering event, seasonal and long-term scales; 2. The conspicuous cycling between stratification and homogenisation observed in PCCC waters, which has key implications for the chemistry and biology of these waters; 3. The seasonal evolution of nitrite profiles in PCCC waters in relation to the stratification cycle; 4. The Redfield stoichiometry of the remineralisation of organic matter in Eastern North Atlantic Central Water (ENACW)-the water mass being transported by the PCCC; 5. The separation of coastal (mesotrophic) from PCCC (oligotrophic) planktonic populations by a downwelling front along the shelf, which oscillates to and fro across the shelf as a function of coastal wind intensity and continental runoff; and 6. The photosynthetic responses of the PCCC and coastal plankton populations to the changing stratification and light conditions from the onset to the cessation of the PCCC.

Appropriate conditions have been achieve d for the accurate, rapid, and highly precise shipboard simultaneous determination of dissolved organic carbon and total dissolved nitrogen in seawater by high tem perature catalytic oxidation. A nitrogen-specific Antek 705D chemiluminescence detector and a CO 2 -specific LiCor Li6252 IRGA have been coupled inseries with a Shimadzu TOC-5000 organic carbon analy ser. Precision of both sim ultaneous measurements is 1.5%, i.e., ±1 µmol C l -1 and ± 0.3 µmol N l -1 , respectively. Quality of analysis is not compromised by vibrations associated with ocean going research vessels.

The bioavailability and bacterial degradation rates of dissolved organic matter (DOM) were determined over a seasonal cycle in Loch Creran (Scotland) by measuring the decrease in dissolved organic carbon (DOC), nitrogen (DON) and phosphorous (DOP) concentrations during long-term laboratory incubations (150 days) at a constant temperature of 14ºC. The experiments showed that bioavailable DOC (BDOC) accounted for 29 ± 11 % of DOC (average ± SD), bioavailable DON (BDON) for 52 ± 11% of DON and bioavailable DOP (BDOP) for 88 ± 8 % of DOP. The seasonal variations in DOM concentrations were mainly due to the bioavailable fraction. BDOP was degraded at a rate of 12 ± 4 % d-1 (average ± SD) while the degradation rates of BDOC and BDON were 7 ± 2 % d-1 and 9 ± 2 % d-1 respectively, indicating a preferential mineralization of DOP relative to DON and of DON relative to DOC. Positive correlations between concentration and degradation rate of DOM suggested that the higher the concentration the faster DOM would be degraded. On average, 77 ± 9 % of BDOP, 62 ± 14 % of BDON and 49 ± 19 % of BDOC were mineralized during the residence time of water in Loch Creran, showing that this coastal area exported C-rich DOM to the adjacent Firth of Lorne. Four additional degradation experiments testing the effect of varying temperature on bioavailability and degradation rates of DOM were also conducted throughout the seasonal cycle (summer, autumn, winter and spring). Apart from the standard incubations at 14ºC, additional studies at 8ºC and 18°C were also conducted. Bioavailability did not change with temperature, but degradation rates were stimulated by increased temperature, with a Q 10 of 2.6 ± 1.1 for DOC and 2.5 ± 0.7 for DON (average ± SD).

No abstract

No abstract