Seasonal and spatial changes in the sediment organic matter of a semi-enclosed marine system (W-Mediterranean Sea)

The composition of sediment organic matter and the related role of microphytobenthic biomass have been investigated during one-year in a semi-enclosed marine system (Marsala lagoon, Mediterranean Sea). Monthly samples from June 1993 to May 1994 were analysed for carbohydrate, protein, lipid, photosynthetic pigment and total organic matter. The three main biochemical classes of organic compounds (i.e. carbohydrates, proteins and lipids) showed higher concentrations than reported in the literature. However, photosynthetic pigment was quite low, compared to other enclosed marine basins or Mediterranean coastal lagoons. As a result, the contribution of primary organic carbon to the sedimentary biopolymeric fraction of organic matter was low (on average 2.2%), indicating that most of the sedimentary organic matter in the study area originated from sources other than micro-algae. The sedimentary organic matter, dominated by carbohydrates (on average 51.2%) followed by proteins (39.0%) and lipids (9.8%), as well as the low protein to carbohydrate ratio, indicate the presence of large amounts of non-living and/or aged organic matter. Comparing data on spatial distribution of sedimentary and suspended organic matter, the dynamic balance of resuspension vs. sedimentation along a north-south axis is invoked as one major factor affecting the distribution and composition of the main classes of organic compounds. The macroalgal and vascular plant coverage is suggested to be another major factor affecting both amounts and composition of sedimentary organic matter. The northern area, characterised by partially unvegetated sediments, showed higher amounts of proteins, whilst moving southward and approaching a luxuriant Posidonia oceanica reef, carbohydrates became more important relative to proteins. As only the biopolymeric fraction of sediment organic matter showed significant seasonal changes, the quantity of sediment OM behaves as an emerging property. By contrast, OM quality is strictly connected to algal coverage as well as to episodic inputs of primary organic matter from deposited phytoplankton and/or microphytobenthos. The uncoupling between large amounts and relatively low nutritional value of sedimentary OM suggests that this particular environment behaves as a detrital 'trap'.