Fate of organic matter in faecal pellets egested by epifaunal mesograzers in a Sargassum forest and implications for biogeochemical cycling

Fate of organic matter of the faecal pellets produced by epifaunal mesograzers in a Sargassum forest was elucidated by determination of the organic carbon and nitrogen flux and fatty acid composition of the 3 types of faecal pellets, i.e. fresh, sinking and precipitated. The organic carbon contents of those 3 types averaged 21.4, 6.0 and 5.5% dry weight, respectively, showing a marked difference between the fresh and sinking types. The same trend was found for their organic nitrogen content. A dissolution-decomposition experiment ascertained the rapid loss of organic matter after the pellets were egested. The ratio of the unsaturated fatty acids to total fatty acids (UFA:TFA) decreased in the order of the fresh, sinking and precipitated pellets, whereas the bacterial biomarker fatty acids increased in that order. The daily sinking fluxes of the faecal pellets were 197 to 639 mg C and 15 to 53 mgN m(-2) d(-1) for organic carbon and nitrogen, respectively. This flux accounted for 7.4 to 13.4% (C) and 9.4 to 20.3% (N) of the primary production in the corresponding season of the Sargassum forest. The estimated residence time of the organic matter in the pellets together with the result of the dissolution-decomposition experiment indicated that about half of the organic carbon and nitrogen egested as faecal pellets was laterally exported to the pelagic zone by the exchange of water. Those possible lateral export fluxes accounted for 4.1 to 7.4% (C) and 4.4 to 9.5% (N) of Sargassum production.