Effects of bottom trawling on the quantity and biochemical composition of organic matter in coastal marine sediments (Thermaikos Gulf, northwestern Aegean Sea)

The effects of bottom trawling on quantity and the biochemical composition of sediment organic matter were assessed, along a gradient of trophic state in a coastal area of the Gulf of Thermaikos (Aegean Sea). Total organic carbon concentrations and other organic variables (such as biopolymeric C concentrations) displayed a significant increase immediately after the initiation of trawling activities, reaching unexpectedly high values when compared to a control (i.e. samples collected before trawling events). The lack of significant changes in photopigment content of the sediments, observed after trawling events, indicates that primary production processes did not change markedly. Since also sediment properties did not vary significantly during the period of investigation, the observed changes in sediment organic content can be related with organic matter uplift from deeper sediment layers, caused by reworking (bottom up transfer) and/or reduced C and N consumption by benthic biota. Trawling determined also significant changes in the biochemical composition of sediment organic matter. Indeed, sediment resuspension increased the hydrolysable fractions of protein and carbohydrate pools, likely to be the result of increased degradation rates under oxic conditions. Such an effect indicates that trawling may increase the quality and bioavailability of organic C to consumers; this, in turn, could modify the energetic and trophic state of the benthic systems. We conclude that bottom trawling might have important trophodynamic consequences, for benthic microbial and meiofaunal assemblages.