An assessment of the contribution of terrestrial organic matter to total organic matter in sediments in Scottish sea lochs

Abstract

The biogeochemical cycling of carbon in Lochs Creran and Etive was investigated by using lignin as the biomarker for terrestrial organic matter. The oxygen uptake rate, percentage organic matter due to loss on ignition, Rp index, total carbon, total organic carbon (TOC), total nitrogen, C/N ratio, and phosphate content were used as the proxies to determine the biodegradability of the sediment organic matter. Total lignin in Lochs Creran and Etive ranged from 0.03 to 0.55mg/g, and A (mg/lOOmg OC) ranged from 0.37 to 0.98; the syringyl/vanillyl and cinnamyl/vanillyl ratios for both lochs ranged from 0.16-1.61 and 0-1.47, respectively. It was found that non-woody angiosperm tissues predominated in the lochs. The concentrations of lignin and the proxies were constant at individual locations, indicating continual but intermittent input of terrestrial materials into the lochs. However, total lignin and these proxies decreased significantly from the head to the mouth and outside the lochs, indicating the importance of the Rivers Creran and Etive in contributing terrestrial materials into the lochs and the importance of terrestrial organic matter in fuelling the biogeochemical cycling of organic matter in the lochs. Overall the effect of the hydrodynamic and hydrological regimes of the lochs, fish and shellfish farms, and bioturbation on the sediment organic matter was investigated. From the head to mouth of the lochs, in Creran, lignin contributed to 0.69% and 0.47% TOC at LC0 and LC6 ; in Etive, lignin contributed to 0.91% and 0.33% TOC at RE2 and Camas Nathais, respectively. The input of organic matter into Creran was 1.9 x 109 g/year. Of this, 1.2 x 109 g/year (63.16%) was labile fraction and 7.0 x 108 g/year (36.84%) consisted of refractory organic matter. There were overall 5.0 x 108 gC/year input of total carbon and 2.5 x 106 g/year input of lignin materials into Creran.