Origins of Dissolved Organic Matter in Southern California Coastal Waters: Experiments on the Role of Zooplankton

Abstract

The contribution of zooplankton to flows of dissolved organic C was studied for the food web of coastal plankton assemblages off southern California, USA. Seawater samples including active macrozooplankton were labeled with 14CO2. The time course of 14CO2 incorporation into size fractions of the plankton and dissolved organic materials was determined. Changes in extracellular concentrations of free primary amines and dissolved saccharides were also measured. The time course of 3H-thymidine incorporation into bacteria was determined along with bacterial cell counts. By comparing C flows in samples with macrozooplankton and without macrozooplankton, the role of macrozooplankton in the flux of C to bacteria and dissolved organic matter was evaluated. Of the 14C-carbon, .gtoreq. 1/2 in the bacterial size fraction could be due to small photosynthetic forms or the remains of microplankton disrupted by the filtration used in the size fractionation. Moreover, most of the C flowing to bacteria from the macrozooplankton was not labeled by 14C and was proportional to the number of macrozooplankton. Conservative estimates of this unlabeled C flux to bacterial biomass due to macrozooplankton were 3 and 10% of the 14C assimilated photosynthetically. Potential artefacts resulting from experimental manipulations are considered.