Planktonic grazers are a potentially important source of marine dissolved organic carbon

Abstract

A series of laboratory experiments was performed to measure dissolved organic carbon (DOC) production during herbivorous grazing by heterotrophic protists (ciliate Strombidinopsis acuminatum, dinoflagellate Oxyrrhis marina) and copepods (Calanus pacificus). DOC production by phytoplankton was also measured. Experiments were performed in artificial seawater to provide a low DOC background against which changes in DOC concentration could be measured directly. We found that DOC production during grazing was high, i.e. 16-37% of algal C content was released as DOC during an ingestion event. Bacterial growth rates were stimulated by grazer activity, most likely due to increased availability of labile DOC; breakage of fecal pellets by copepods may also have yielded DOC. In contrast, DOC production by phytoplankton was low, ranging from 3 to 7% of algal C content per day. Generalizing from these rates, a simple budget shows that grazer DOC production should be 4-6 times greater than phytoplankton DOC production in any region of the ocean where grazing is the dominant phytoplankton loss process. Both phytoplankton and grazer species influenced the carbohydrate composition of the DOC produced. Dissolved carbohydrates averaged 30 and 22% of total DOC in phytoplankton-only and grazer-containing treatments, respectively, and most variability in carbohydrate content was due to variations in polysaccharide levels. We conclude that planktonic grazers are potentially a major source of DOC in the marine environment.