Evidence for the seasonal accumulation of carbon-rich dissolved organic material, its scale in comparison with changes in particulate material and the consequential effect on net CN assimilation ratios

Abstract

Abstract An analysis is made of the seasonal cycles of particulate and dissolved organic carbon and nitrogen. Data are taken from four published works (Duursma, E.K., 1961. Neth. J. Sea Res., 1: 1–147; Parsons T.R. LeBrasseur, R.J. and Barraclough, W.E., 1970. J. Fish. Board Can., 27: 1251–1264; Banoub, M.W. and Williams P.J. leB., 1973. J. Mar. Biol. Assoc. UK, 53: 695–703; and Butler, E.I., 1979. Estuarine Coastal Mar. Sci., 8: 195–197) and one internal report (Scripps Inst. Oceanogr. Ref., 68-20). They illustrate that the seasonal increase of the dissolved organic fraction (DOM) at all the sites studied is comparable to or exceeds that of the particulate fraction. The analysis revealed a mid-to late-summer accumulation of C-rich DOM, with C N ratios twice or more the Redfield value of 6.6. It is illustrated that the formation of this C-rich material would give rise to net inorganic carbon and nitrogen assimilation ratios as high as 14:1, as such the could be the basis of high reported C N assimilation ratios. Nitrogen limitation is speculated as a cause for the accumulation of the C-rich material. It is argued that systematic errors in the analytical methods would not give the patterns observed. The significance of accumulation of C-rich DOM in late summer is discussed in relation to reports which infer DOC to be a major form of carbon export from the euphotic zone. It is argued that the weak biochemical coupling of C and N in DOM as compared with the particulate fraction would loosen the link between the export of organic carbon and the import of inorganic nitrogen with important consequences on the biogeochemical control of carbon export from the upper ocean.