Assessing the trophic state and eutrophication of coastal marine systems: a new approach based on the biochemical composition of sediment organic matter.

We used a biochemical approach based on the analysis of the quality and quantity of sedimentary organic matter for identifying new descriptors of the trophic state and environmental quality of coastal marine systems. A large-scale study, including 99 stations, belonging to 33 transects, was carried out along 250 km of the Apulian coasts (Mediterranean Sea) in March and September 2000. The investigated area covered a wide range of anthropogenic impacts (industrial ports, tourist harbours, areas affected by power plants and industrial wastes, mariculture areas). Other sites, including marine protected areas (i.e., without any apparent impact), were used as "control". Water column and benthic parameters provided different indications and classifications of the trophic state of coastal marine systems. We found that phytopigment content of the sediments changed in response to all different sources of anthropogenic impact and resulted in a useful descriptor of the trophic state and environmental quality. Highest sediment chlorophyll-a concentrations, indicating conditions of increasing eutrophication, were found in areas impacted by the discharge of heated waters from a power plant. In particular, the contribution of the autotrophic biomass to the biopolymeric carbon pool appeared to be a good descriptor of the decreasing environmental quality. Independently from the sampling period or the pollution source such contribution was significantly lower in transects subjected to anthropogenic impact than in control areas. Differences in trophic conditions were evident both in terms of quantity (i.e., total organic matter content) and quality (i.e., biochemical composition) of sediment organic matter. In particular, sediment protein concentration appeared to be a good descriptor of the trophic state of the benthic systems at different spatial scales. Multivariate (MDS) analysis allowed identifying areas characterised by hypertrophic, eutrophic and meso-oligotrophic conditions and to define relative threshold levels. A classification of the trophic state of coastal systems based on protein and carbohydrate concentrations is proposed.