Strong links between phytoplankton community physiology and dimethylsulphoniopropionate (DMSP) concentrations in the sub-tropical/tropical Atlantic

We present an extensive dataset of dimethylsulphide (DMS, n = 651) and dimethylsulphoniopropionate (DMSP, n = 590) from the Atlantic Meridional Transect programme. These data are used to derive representative depth profiles that illustrate observed natural variations and can be used for DMS and DMSP model-validation in oligotrophic waters. To further understand our dataset, we interpret the data with a wide range of accompanying parameters that characterise the prevailing biogeochemical conditions and phytoplankton community physiology, activity, taxonomic composition, and capacity to cope with light stress. No correlations were observed with typical biomarker pigments for DMSP-producing species. However, strong correlations were found between DMSP and primary production by cells >2 µm in diameter, and between DMSP and some photo-protective pigments. These parameters are measures of mixed phytoplankton communities, so we infer that such associations are likely to be stronger in DMSP-producing organisms. Further work is warranted to develop links between community parameters, DMS and DMSP at the global scale.

Seasonality and interannual variability of the European Slope Current from 20 years of altimeter data compared with in situ measurements

The European Slope Current (SC) is a major section of the warm poleward flow from the Atlantic to the Arctic, which also moderates the exchange of heat, salt, nutrients and carbon between the deep ocean and the European shelf seas. The mean structure of the geostrophic flow, seasonality, interannual variability and long-term trend of SC are appraised with an unprecedented continuous 20-year satellite altimeter dataset. Comparisons with long term in situ data showed a maximum correlation of r2=0.51 between altimeter and Acoustic Doppler Current Profilers (ADCP), with similar results for drogued buoy data. Mean geostrophic currents were appraised more comprehensively than previous attempts, and the paths of 4 branches of the North Atlantic Current (NAC) and positions of 5 eddies in the region were derived quantitatively. A consistent seasonal cycle in the flow of the SC was found at all 8 sections along the European shelf slope, with maximum poleward flow in the winter and minimum in the summer. The seasonal difference in the altimetry current speed amounted to ~8-10 cm s-1 at the northern sections, but only ~5 cm s-1 on the Bay of Biscay slopes. This extended altimeter dataset indicates significant regional and seasonal variations, and has revealed new insights into the interannual variability of the SC. It is shown that there is a peak poleward flow at most positions along a ~2000 km stretch of the continental slope from Portugal to Scotland during 1995-1997, but this did not clearly relate to the extreme negative North Atlantic Oscillation (NAO) in the winter of 1995-1996. The speed of the SC exhibited a long term decreasing trend of ~1% per year. By contrast the NAC showed no significant trend over the 20-year period. Major changes in the NAC occurred three times, and these changes followed decreases in the NAO index.

Variations in the distributions of the Centropages chierchiae and Temora stylifera (Copepoda: Calanoida) in the north-eastern Atlantic Ocean and western european shelf waters.

Centropages chierchiae and Temora stylifera occurred rarely in the Continuous Plankton Recorder (CPR) survey in the Bay of Biscay, Celtic Sea, and English Channel before 1988. By 2000 they were found frequently and in abundance. The seasonal cycles of abundance of these species differ, C. chierchiae occurring mainly in the summer while T. stylifera was found most frequently in late autumn or winter towards the northern limits of its distribution. The increase in abundance of both species is related to temperature. However, in the years when it was found in the samples, the frequency of occurrence of C. chierchiae was correlated positively with the strength of the shelf edge current and negatively with the North Atlantic Oscillation (NAO) while the reverse was true for T. stylifera.

Continuous Plankton Records - Seasonal-Variations In The Distribution And Abundance Of Plankton In The North-Atlantic Ocean And The North-Sea

Charts are presented of the seasonal variations in the distribution of four phytoplankton and five zooplankton taxa in the North Atlantic and the North Sea. The main factors determining the seasonal variations appear to be the distribution of the main overwintering stocks, the current system and, in some instances, temperature control of the rate of population increase. Information is presented about the variation with latitude (over the range from 34° N to 65 ° N) of the seasonal regime of the plankton. On the assumption that there is a relationship between nutrient supply and vertical temperature stratification the main features of this variability can be interpreted. In the south (to about 43° N) nutrient limitation plus grazing appear to be dominant, resulting in a bimodal seasonal cycle of phytoplankton. North of about 60° N the system appears to be limited by the size of the phytoplankton stocks being grazed primarily by Calanus Finmarchicus and Euphausiacea. In an extensive zone, from about 44° N to 60° N, it would appear that the spring bloom of phytoplankton is under-exploited by grazing while in summer the zooplankton graze the daily production of the phytoplankton, the stocks of which are probably maintained by in situ nutrient regeneration. The implications, for at least this mid-latitude zone, that rates and fluxes of processes, as opposed to density dependent interactions between stocks, play a major role in the dynamics of the seasonal cycle is consistent with previously reported observations suggesting that physical environmental factors play a major role in determining year-to-year fluctuations in the abundance of the plankton.