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RESEARCH PRODUCT

The Use of Carbon Stable Isotopes to Investigate the Origin and Distribution of Suspended and Sedimentary Organic Matter in a Semi-enclosed Mediterranean Marine System

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The natural stable isotope values of different primary sources have been used to trace the fate of organic carbon that enters in the marine food webs of Stagnone di Marsala (Italy). Water and sediment samples were collected monthly (March 1996 – February 1997) at 3 stations, characterized by different amounts of vegetal cover and analysed to determine total organic matter (OM), phytopigments, biopolymeric organic carbon (BPC) and stable carbon isotopic composition (δ13C), the latter measured also in main primary producers. Sedimentary OM accumulated in summer, while total suspended organic matter reached highest concentrations in May and December. The concentration of chlorophyll-a carbon in the sediments and suspended material (on average 227 ± 165 μg C g−1 and 14 ± 8.7 μg Cl−1 respectively) were highest in spring and autumn. Sedimentary BPC concentrations showed several peaks throughout the year, suspended BPC concentrations peaked in May and in summer. The δ13C from suspended particulate matter and sediments (on average −19.5 ± 2.7‰ and −13.0 ± 3.1‰ respectively) followed a gradient of enrichment moving towards the inner part of the Stagnone. The δ13C of primary sources ranged from −21.4‰ (phytoplankton) to −4.5‰ (seagrass detritus). The Dauby’s model (1989) was used in order to estimate the contribution of each primary carbon source to sedimentary and particulate reservoirs. In the northern basin, OM in sediments was mainly influenced by macroalgae carbon (46%), while that in the water column was influenced by phytoplankton (60%) and microphytobenthic (30%) carbon. In the southern basin, main sedimentary carbon sources were seagrasses and their detritus (61%, mainly Posidonia), while in the water column were phytoplankton and microphytobenthos (50% and 24% respectively). The role of some physical constraints (wind and tidal energy) in influencing isotopic composition of organic matter is discussed.