Modelling of pore water nutrient distribution and benthic fluxes in shallow coastal waters (Gulf of Trieste, Northern Adriatic)

Pore waters, extracted monthly from short cores at two sedimentologically and biologically different locations (AA1 and F) in the Gulf of Trieste (Northern Adriatic), were analyzed for NH4+, NO3-, PO43- Si(OH)4, DIC and periodically for DOC, DON and DOP. Nutrient concentrations were used to model vertical profiles using a diffusion-reaction model which included the macrofaunal influence on sediment-water exchange rates. Winter nutrient profiles showed nearly an exponential increase, or decrease in the case of NO3-, in nutrient concentrations with depth while the profiles from other seasons exhibited concentration maximum at 3-6 cm, a minimum around 8-10 cm, and then, except for NO3-, a gradual increase or constant values. This vertical distribution is attributed to seasonal variations in the benthic infauna activity, mostly composed of polychaetes and bivalves and concentrated in the top 4-5 cm, being less active during the winter. The vertical profiles of DOC and DON showed the vertical distribution described above in all periods, while DOP was similar to that of PO43-. The comparison of modelled fluxes of nutrients across the sediment-water interface at the location AA1 and those measured using in situ benthic chamber showed quite good agreement for NH4+ and PO43- fluxes but not for NO3- and Si(OH)(4). Discrepancies could be caused by a topography effect and for Si(OH)4 by an additional dissolution of the solid phase balancing the diffusive loss into burrows and lessening the effect of bioturbation.