Comparative assessment of large-scale marine eutrophication: North Sea area and Adriatic Sea as case studies

Abstract

In the last 50 yr, marginal seas under freshwater influence have become affected by eutrophication. The large areas involved render its assessment difficult. In this paper, a spatial and comparative approach is presented assuming that hydromorphology together with organic matter production control the various consequences of nutrient overenrichment. We have developed 2 eutrophication indices of shallow marine ecosystems using hydrodynamic modelling and optical satellite remote-sensing. The indices are tested in systems characterised by contrasting physical regimes, i.e. the Adriatic and North Sea. The physically sensitive area index (PSA) integrates the supporting factors of eutrophication, i.e. the physical conditions driving primary production in the upper layer and oxygen availability near the bottom. The eutrophication risk index (EUTRISK) characterises the spatial distribution of potential hypoxia at a monthly timescale. It is based on an oxygen budget between the physical capacity to store and renew the oxygen near the seabed and the flux of organic matter reaching the sediment, estimated primarily from satellite-derived chlorophyll a data. The indices identify 3 main types of eutrophicated waters: (1) eutrophic and sensitive, (2) mesotrophic and sensitive and (3) eutrophic and resistant. In Category 1 oxygen depletion occurs recurrently because both the main pressure and supporting factors are unfavourable (e.g. coastal waters south of the Po river mouth). In Category 2, severe hypoxia or anoxia occur due to particularly adverse physical conditions, even when primary production is relatively low (e.g. the Kattegat and NE part of the northern Adriatic Sea). Category 3 comprises areas where severe hypoxia near the seabed is prevented by permanent tidally induced vertical mixing; however nutrient overenrichment is recognised as leading to alterations in the food-web structure and functioning (e.g. Bay of Seine). The EUTRISK index forms the basis of a comparative tool for the assessment and monitoring of marine eutrophication risk. The PSA index, which provides an understanding of physical vulnerability on a large scale, indicates the relative threshold of nutrient discharge, i.e. the assimilative capacity of marine ecosystems. This first assessment of the underlying processes controlling marine eutrophication is a prerequisite for efficient restoration strategies on a large scale.