Colloidal organic carbon and trace metal (Cd, Fe, and Zn) releases by diatom exudation and copepod grazing

Abstract

Colloidal macromolecular organic compounds are important intermediaries between solution and particle phases and play a critical role in the biogeochemistry of trace metals and organic carbon. The releases of colloidal organic carbon and trace metals (Cd, Fe, and Zn) mediated by copepod grazing and decomposition, and direct diatom exudation, were examined using a radiotracer approach. The colloidal phase was operationally defined in this study as the size fraction between 5 kDa and 0.2 μm and the dissolved phase as the ≤0.2 μm filter passing phase. About 13–60% of dissolved carbon exuded by the diatom Thalassiosira pseudonana was partitioned into the colloidal phase, and this fraction increased considerably as the diatom cells grew older. A lower fraction of dissolved 14C (12–23%) excreted by the copepods Acartia erythraea was detected in the colloidal phase compared to carcass (13–35%) and feces decomposition (21–34%). In contrast to carbon, a lower fraction of regenerated dissolved Cd (1–11%) and Zn (0–20%) from copepods and diatoms was consistently detected in the colloidal phases. Copepod excretion and carcass decomposition resulted in more colloidal Fe (51–91%) than diatom exudation (46–62% for Thalassiosira weissflogii, and 3–33% for T. pseudonana) and copepod feces decomposition (16–30%). Copepod ( Calanus sinicus) grazing reduced the colloidal fraction of dissolved 14C, although a higher concentration of the diatom's ( T. weissflogii) carbon was regenerated into the dissolved phase. The grazing of these copepods did not have any influence on the colloidal metal partitioning. The release of trace metals and carbon was enhanced by a higher density of copepod's grazing. Thus, different biological processes (grazing, excretion, exudation, and decomposition) may contribute differently to the production and dynamics of colloidal carbon and metals in planktonic systems.