The fate of terrestrial organic matter in two Scottish sea lochs

Sea lochs are zones of rapid organic matter (OM) turnover. Most of this OM is of allochthonous origin, being introduced into the lochs via freshwater input. In this study the behaviour of terrestrially derived OM was elucidated using a combination of parameters which indicate OM diagenesis in the near surface sediments from two Scottish sea lochs, Loch Creran and Loch Etive. Alkaline CuO oxidation was used to determine lignin phenols which serve as biomarkers for terrestrial OM in sediments. Stable carbon isotope, total carbon and nitrogen and total OM (including the labile and refractory fractions) compositions were also determined. Lignin materials in the lochs were generally highly degraded and undergo little degradation further seaward. The vanillic acid to vanillin ratio, (Ad/Al)v in the lochs ranged from 0.52 to 2.69. However, there was a fraction of relatively fresh, land-derived OM, still undergoing degradation adding to the carbon cycling in the lochs, as indicated by the Rp values (ratio of refractory to total OM) and OC/N ratios in the surface sediments. The hydrological and hydrodynamic regimes in Loch Creran result in several phenomena such as the transportation of terrestrial debris via hydrodynamic sorting processes and the promotion of surface sediment diagenesis by bioturbation. Frequent water renewal results in better water circulation and oxygenation which facilitate OM decomposition. In Loch Etive the less frequent renewal gives rise to a more constant OM diagenesis along the loch.