Microphytobenthos in the Gulf of Trieste (Northern Adriatic Sea): relationship with labile sedimentary organic matter and nutrients

Seasonal and inter-annual variations of microphytobenthos and inorganic nutrients were examined in sediments sited at 20 m depth in the Gulf of Trieste (Northern Adriatic Sea). Seasonal changes in water-soluble carbohydrates and proteins were studied in order to obtain information on quality and quantity of labile organic matter and the role of microphytobenthos in producing these compounds. Annual variations in microphytobenthos abundance in the surface sediment layer (0–1 cm) in the months from November 1992 to October 1993 and from July 1995 to June 1996 were similar. The vertical distribution of microphytobenthos abundance revealed a decrease with sediment depth. The depth profiles of water-soluble carbohydrates concentrations revealed a marked decrease with sediment depth. The profiles of protein concentration showed a more irregular pattern than carbohydrates without a progressive decline with sediment depth. In the upper 7 cm of surface sediment, a significant correlation between microphytobenthos abundance and both water-soluble carbohydrates and protein concentration was noticed during the months from July 1995 to March 1996. This positive relationship between the biopolymeric fraction and the microphytobenthos abundance suggest that water-soluble charbohydrates, as exocellular polymeric substances, mostly originate from the metabolic activity of microphytobenthos, whereas the proteins might be a good indicator of the microalgal biomass. Proteins in lower sediment layers could be associated with benthic heterotrophs too. The inorganic nutrient concentration, at the sediment overlying water, evidenced temporal variations with similar annual patterns. Positive correlation between microphytobenthos abundance and ammonium and silicate concentrations, at the water-sediment interface, were found. The lack of correlation between microphytobenthos and nitrite/nitrate concentration can be explained considering that, for their growth, microalgae use ammonium rather than nitrite/nitrate ions as inorganic N sources. The silicate concentration was clearly linked to microphytobenthos abundance since benthic microalgae were mainly composed of diatoms. The significant correlation between microphytobenthos and phosphate, observed when the N:P ratio was lower than 34:1, may be related to phosphorous limitation.