Microphytobenthic biomass, species composition and nutrient availability in sublittoral sediments of the Gulf of Trieste (northern Adriatic Sea)

Monthly sampling was carried out during a 2-year study (2003-2004) in order to analyse benthic microalgal composition, abundance (ABU) and biomass (BIOM). Temperature and photosynthetically available radiation (PAR) at the bottom were recorded during sampling. In the overlying water nitrite (NO2−), nitrate (NO3−), ammonium (NH4+), phosphate (PO43−) and silicate (SiO3−) were analysed. The sediment consisted of 8.2% sand, 18.3% clay and 73.5% silt. BIOM showed its maximum in August 2004, while the minimum was recorded in October 2003. The microphytobenthic community was mainly composed of diatoms. Among all the 103 Bacillariophyceae taxa identified in the sediment, we distinguished 67 benthic, 8 epiphytic and 8 planktonic species. Among diatoms Nitzschia and Navicula were the most abundant genera (30.8 and 26.2%, respectively). The linear regression between total diatom ABU and PAR was statistically significant (r = 0.66, p < 0.001). A seasonal pattern of monthly samplings was highlighted both from cluster analysis and principal component analysis (PCA). The latter revealed two diatom assemblages: a winter assemblage and a spring-summer one. The genera Diploneis and Pinnularia showed an inverse relation against temperature. Considering the nutrient ratios the development of microphytobenthos (MPB) appeared to be potentially co-limited by Si and P almost throughout the study period. A clear inverse relation between NO2−, NO3− and BIOM was emphasised by the moving average and the PCA. The inverse relation among nutrients and light availability suggested that the photosynthetic activity of benthic diatoms in spring and summer may be one of the processes controlling sediment-water nutrient fluxes.