Land sources of marine organic matter

Abstract

Organic matter in seawater is ultimately derived from various species of marine plants and river run-off and airborne materials of terrestrial origin. Direct addition of organic matter, especially due to oil spills, is also an important source for organic matter in seawater. Recent advances in the studies of the chemistry and biochemistry of particulate organic matter in seawater clearly indicate that most of the organic materials produced by the photosynthetic reaction of phytoplankton is decayed before the particulate matter has reached the ocean floor. This fact suggests that the processes of primary production may not contribute much to the input of organic matter to the deep ocean. The average concentration of organic carbon in river water is estimated to be 10 mg C/l. Thus, annual input of organic matter to the ocean can be calculated as 3.0 · 10 14g C. However, two opposite arguments on this point have been reported. Several workers have stated that the contribution of river-dissolved organic matter (i.e., humic substances) to the ocean must be ignored because of rapid precipitation of the organic matter during the mixing processes of river water with seawater, while Sholkovitz (1976) stated that only 3–10% of organic matter from river water is precipitated in contact with seawater. Thus, more studies on the mechanism of the flocculation in estuaries are needed. The organic matter carried by winds may be somewhat more important to the organic matter in the ocean than is that of the rivers. The isolation, identification and elucidation of the organic compounds from airborne particles of urban areas and from exhaust gases derived from internal combustion engines have been developed extensively during the last decade. Very limited information as to the contribution of these organic compounds to the ocean is available at the present time. Organic materials from land sources must be transported to the ocean mainly by trade winds, since pesticides and PCBs have been determined in marine aerosols collected at Barbados. Quantitative estimation of the contribution of airborne particles to the ocean is an important problem to solve in the future in view of the mass-balance of the organic matter in the ocean.