Effects of community composition and size structure on light absorption and nutrient uptake of phytoplankton in contrasting areas of the Alboran Sea

Biomass size distribution, light absorption properties and carbon and nitrogen uptake rates were analysed in phytoplankton assemblages along coast-offshore gradients. During sampling, the coastal stations were affected by upwelling. In contrast, the offshore stations were located at the western Alboran anticyclonic gyre core. Surface nitrate concentration was >1 mu M at the coastal stations and lower than the detection limit at the offshore stations. Further more, the lower limit of the upper mixed layer was deeper at the coast (ca. 20-30 m depth) than off shore (80-100 m depth). In terms of biomass, the coastal communities were dominated by diatoms; dinoflagellates and picoplankton contributed to phytoplankton biomass by <30 and 7%, respectively. At the offshore stations, diatom abundance decreased while dinoflagellate abundance did not change significantly in comparison to the coastal stations, with dinoflagellates be coming the dominant group in terms of biomass. The mean length of the microplankton cells increased from the coast to offshore due to a decrease in abundance of diatoms <50 mu m in length. The coastal communities featured lower chl a specific absorption coefficients in the blue spectral band than found in the offshore communities. In contrast, the biomass-specific uptake rates of inorganic carbon and nitrate estimated for the coastal communities (3.5 nmol C mu g(-1) POC h(-1) and 0.2 nmol N mu g(-1) POC h(-1), respectively) were higher than for the offshore communities (1.0 nmol C mu g(-1) POC h(-1) and 0.03 nmol N mu g(-1) POC h(-1), respectively). These differences in light absorption efficiency and nutrient uptake rates were significantly correlated with the change in the mean cell size of the communities moving from the coast to offshore, which were mainly due to the decrease in biomass of diatoms <50 mu m which effectively increased the dinoflagellate contribution to phytoplankton biomass.