Short term primary production in western Mediterranean Sea phytoplankton communities subjected to the combined stress of high irradiance and low nutrients during summer stratification

Abstract

Thermal stratification of the water column promotes contrasting conditions with respect to irradiance level and nutrient concentration for phytoplankton growth, demanding more research on which environmental factor has more importance in determining the photosynthetic performance of the communities. For this purpose, two research surveys were performed in several coastal stations located in the Alboran Sea (Western Mediterranean Sea) at the end of 2008 and 2009 summers. Primary productivity was estimated by determining photosynthetic electron transport rates (ETRB) and inorganic carbon assimilation rates (PPCB) at two different depths within the water column, surface and chlorophyll a vertical maximum depth (CM). At the surface, communities presented similar photo-acclimation features during the two surveys, which were different from those obtained for the CM samples that grew at more favorable nutrient and light conditions. The acclimation response to surface conditions consisted in the reduction in the chlorophyll a (Chl a) cell content, and the increase in the Chl a specific light absorption coefficient [a*(λ)], initial slope of ETRB vs. Irradiance curves (αB) and light saturation parameter (Ek), indicating that the communities were acclimated to stress conditions (i.e. high PAR and UVR dose and low nutrient availability). However, the in situ primary productivity was unaffected; on the contrary, both PPCB and ETRBin situ increased with respect to the values obtained for communities growing at CM depth. Furthermore, ΦETR,C (i.e. the molar ratio of transported e− to assimilated carbon) in surface samples was similar to CM samples (8.0 ± 3.5 compared with 7.5 ± 5.3 mol e− [mol−1 C], respectively). The analysed phytoplankton communities were mainly dominated by diatom. This corroborates that diatoms possess a greater capacity to cope with high irradiance levels, even at low nutrient concentrations registered during this study at surface waters.