Northern Adriatic sea mass-balance model: trophic structure of a highly-fished and eutrophic ecosystem

A trophic network static model of the Northern Adriatic sea (Italy, Slovenia, Croatia) was developed in order to assess trophic structure, anthropogenic impact and ecosystem stress level. The Northern Adriatic sea is a shallow and eutrophic basin which has been heavily fished for many years. The river Po is the largest river in the region and discharges a remarkable load of nitrogen and phosphorous into the system. The model, constructed within the EU-sponsored INCOFISH project and using the well known mass balance software Ecopath, provides a snapshot of biomasses and energy fluxes in the ecosystem during the 1990’s. Ecosystem structure was analysed by aggregating fluxes in discrete trophic levels sensu Lindemann, using mixed trophic impact matrix analysis and calculating different indices specifically to evaluate ecosystem maturity sensu Odum. The analysed system is highly productive, showing strong energy losses through respiration. The network structure is linear, with little recycling. The trophic structure is characterized by high primary production and the phytoplankton-based cycle has the highest efficiency of energy transfer within the system. However, trophic fluxes are mainly based on the detritus cycle. The importance of detritus does not point out the maturity of the system, as a strong detritus uptake may be due to both the input of the Po river and the low depth of the basin, the latter allowing resuspension and recirculation of dead organic matter. Mixed trophic impact analysis shows that top predators have a negligible effect on the system. Rather the system appears to be wasp-waist controlled: small pelagic fish and zooplankton are the main groups affecting the trophic network. Zooplankton has a strong limiting effect on pelagic fish (which in turn impact the higher trophic levels) by limiting the energy transfer from bacteria and detritus that they feed upon. The microbial loop is as important as grazing with respect to the magnitude of the fluxes. Phytoplankton has some impact on the network and, despite its abundance, appears to be a key resource. The frequent eutrophication-caused blooms may be too intense for phytoplankton, causing population crashes before grazers can exploit this resource. In general, the system appears to be strongly impacted by human activities and at an immature stage. The strong pelagic primary production due to eutrophication could provide the energy to sustain the heavily fished higher levels of the trophic web.