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 microphy tobenthic biomass have been investigated during one-year in a semi-enclosed marine system (Marsala lagoon, Mediterranean Sea). Monthly samples from Ju ne 1993 to May 1994 were analysed for carbohydrate, protein, lipid, photosy nthetic 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 li terature. However, photosynthetic pigment was quite low, compared to other enclosed marine basins or Mediterranean coastal lagoons. As a result, the c ontribution of primary organic carbon to the sedimentary biopolymeric fract ion of organic matter was low (on average 2.2%), indicating that most of th e sedimentary organic matter in the study area originated from sources othe r than micro-algae. The sedimentary organic matter, dominated by carbohydra tes (on average 51.2%) followed by proteins (39.0%) and lipids (9.8%), as w ell as the low protein to carbohydrate ratio, indicate the presence of larg e amounts of non-living and/or aged organic matter. Comparing data on spati al distribution of sedimentary and suspended organic matter, the dynamic ba lance 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 i s suggested to be another major factor affecting both amounts and compositi on of sedimentary organic matter. The northern area, characterised by parti ally unvegetated sediments, showed higher amounts of proteins, whilst movin g southward and approaching a luxuriant Posidonia oceanica reef, carbohydra tes became more important relative to proteins. As only the biopolymeric fr action of sediment organic matter showed significant seasonal changes, the quantity of sediment OM behaves as an emerging property. By contrast, OM qu ality is strictly connected to algal coverage as well as to episodic inputs of primary organic matter from deposited phytoplankton and/or microphytobe nthos. The uncoupling between large amounts and relatively low nutritional value of sedimentary OM suggests that this particular environment behaves a s a detrital 'trap'.