Cycling of Dissolved Organic Matter in the Ocean

Abstract

The ocean is a major global reservoir of organic matter. Photosynthetic microalgae and bacteria are the major sources of organic matter in the ocean, and it is estimated that 45% (0.5 × 10¹⁷ g C year⁻¹) of global primary production occurs in the euphotic zone of the ocean (Hedges 1992). The contribution of organic matter from the continents to the ocean is ~100-fold lower than the input from marine primary production. Rivers discharge 0.4 × 10¹⁵ g C annually to the ocean, and aeolian transport contributes another 0.1 × 10¹⁵ g C annually (Hedges 1992). About 8 × 10¹⁷ g C resides in ocean waters and surface sediments (Hedges 1992), and, given the above mentioned annual fluxes, a mean residence time of 16 years is calculated for organic carbon in the ocean. This relatively short mean residence time indicates that most organic matter cycles rapidly in the ocean. In contrast, however, the average ¹⁴C “age” of dissolved organic carbon (DOC) in the deep ocean is 4000–6000 years B.P. (Williams and Druffel 1987; Bauer et al. 1992), indicating that some organic matter has a long residence time in the ocean. These apparently contradictory observations indicate that residence times of the diverse mixture of organic molecules in seawater are highly variable. This chapter will review various aspects of the marine carbon cycle and provide new insights into the composition and transformations of dissolved organic matter (DOM) in the ocean.