Sources of organic matter in seagrass-colonized sediments: A stable isotope study of the silt and clay fraction from Posidonia oceanica meadows in the western Mediterranean

The origin of sedimentary organic matter in 22 sandy beds of Posidonia oceanica (L.) Delile on the coast of the Iberian Peninsula and the Balearic Islands in the northwestern Mediterranean Sea was investigated using natural abundance stable isotope measurements of carbon and nitrogen (δ¹³C and δ¹⁵N) in fine (<63 μm grain size) surface sediments and in the local primary sources of detrital organic matter, seston, above- and below-ground seagrass tissues and bulk epiphytes. The δ¹³C measurements provided the greatest power for resolving primary sources of the fine sedimentary organic matter in these shallow coastal ecosystems. Their use in an end member, isotopic mass balance approach showed that sestonic particles were as important as seagrass-derived detrital material as a source of sedimentary organic carbon in the P. oceanica meadows. The δ¹⁵N measurements for the sediments exhibited patterns of isotopic depletion on a regional scale relative to the local primary sources of organic matter, which could be associated with occurrence of microbial fixation of molecular nitrogen (N₂), indicating potential contribution of this process to the pool of sedimentary nitrogen in the P. oceanica meadows.