Ciclo biogeochimico dei nutrienti

The interactions of mussel aquaculture with physico-chemical properties and the biogenic elemental composition of the surface sediment were studied at a long-line farm, located in a 12 km2 area, which included the farm (4 km2) characterized by relevant freshwater inputs along the North-Western Adriatic coastline. Negative redox potentials were detected at two stations directly affected by the farm. Total phosphorus content reached the highest concentration at stations located beneath the farm. OC/TP and TN/TP showed similar patterns, which was characterized by average values underneath the farm (respectively 45.5 and 4.5) that were significantly lower than those measured at the control stations. Our hypothesis, that these patterns could be related to organic matter enrichment of the surface sediment underneath the farm, due to intense mussel biodeposition, was confirmed by the mass accumulation rates, estimated to be respectively 23, 2.3, 0.94 and 0.26 g m
-2 y-1 for organic carbon, total nitrogen, total and organic phosphorus. Data concerning the sediment chemistry were normalized with respect to the pelite fraction thus increasing the capability of detecting the environmental impact of the farm under variable background sedimentary fluxes.

Benthic metabolism and carbon and nutrient cycling at the sedimentewater interface were studied seasonally in the Marano (sitesMB andMC) and Grado Lagoon (sites ART and BAR), northern Adriatic Italy, using porewater vertical profiles and daily fluxes of O2, DIC, DOC, NO3-
, NH4+, PO4-3
and SiO4-4
measured in situ deployed transparent and dark benthic chambers. Diffusive and benthic fluxes of solutes were evaluated on a seasonal basis. Sites MC and ART were characterized by higher Corg. contents due to input of riverine organic matter and mariculture, respectively. The Corg., Ntot., Ptot., Porg. and Sibiog. contents decreased along the sediment cores while porewater concentrations of DIC, DOC,NO3-
, NH4+, PO4-3
and SiO4-4
increased along the sediment cores at all study sites due to the degradation of labile sedimentary matter. Higher concentrations of all porewater solutes and higher diffusive fluxes were observed in warmer periods. Benthic fluxes of O2, DIC, NO3-
, NH4+, PO4-3
and SiO4-4
showed intensive seasonal variations. Based on O2 and DIC metabolism, the lagoon sediments were highly heterotrophic except at BAR being in trophic balance or weakly heterotrophic. NO3-
and SiO4-4
exhibited influxes due to intense microphytobenthic assimilation, mostly by diatoms, and denitrification while extremely low PO4-3
fluxes suggest P as a limiting factor. The great difference observed between the diffusive and the in situ benthic fluxes suggests the importance of bioturbation and that the pertinent processes occur at the sedimentewater interface. Tentative annual budgets of carbon and nutrients in surface sediments of studied sites indicate that their cycling, compared to burial flux, is more intensive at the sedimentewater interface. These basic benthic biogeochemical processes can be important to better understand the trace metal cycling, especially Hg mobilization and sequestration, in the lagoon environment.

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.