Aggregation of organic matter in coastal waters: A dilemma of using a Couette flocculator

Aggregation of organic matter (OM) plays an important role in regulating the efficiency of biological pump. We investigated the effect of OM aggregation on bulk and compound-specific composition of particulate and dissolved samples from the coast of Avery Point, Connecticut, USA. Samples were incubated in a Couette device for 6 h in the dark at 8 rpm, which generated a mean shear of 3.4 s−1. The shear induced a net aggregation (1.69 µM-C, equivalent to 7.2% of initial particulate organic carbon; POC) and an even larger gross aggregation (7.43 µM-C, equivalent to 31.5% of initial POC) in 6 h. However, in a blank test, we found that Couette devices released a significant amount of uncharacterized dissolved organic matter (DOM) within 6 h of the experiment. Blank tests have not been part of previous studies, but released DOM has to be taken into account when evaluating experimental data employing Couette devices. The relative abundance (as % of organic C and N) of labile POC components (i.e. amino acids and neutral aldoses in this study) decreased significantly in the treatment. There was no compound-specific selective degradation of amino acids or neutral aldoses in the particulate phase. The decrease of labile organic components and the increase of uncharacterized OM in the particulate phase, indicating that aggregation processes accumulate uncharacterized OM and lead to a more refractory POC. Further, our data indicate that aggregation processes may accumulate the element not essential for microbial growth under the specific environmental conditions. In this study, we found particulate C accumulates when microbial growth is N-limited.