Formation, degradation, and mass : volume ratios of detritus derived from decaying phytoplankton

The mass of non-living particulate organic matter, or detritus, often excee ds that of living plankton in pelagic environments; in coastal waters the d ifference can be 10-fold. Relatively little is known about the dynamics of this pool of organic matter because it has not been previously possible to accurately determine its magnitude. A recent approach utilizing fluorescenc e microscopy (Verity et al. 1996) provided estimates of the volume of detri tus. Here, laboratory experiments were conducted to estimate carbon:volume (C:V) and nitrogen:volume (N:V) conversion ratios of detritus formed by 2 p hytoplankton species when incubated in darkness in the presence of bacteria . The volume of detritus was measured directly, along with total particulat e organic carbon (POC) and nitrogen (PON), and that contained in the associ ated phytoplankton and bacterial communities. Detrital carbon and nitrogen were estimated by difference and compared to detrital volume, from which co nversion factors were calculated. The physiological state of the bacteria w as assessed using a recently developed fluorescent stain and molecular prob e protocol (Williams et al. 19983. The 2 phytoplankton cultures were degrad ed at different rates. At the times of peak bacterial abundances, most of t he remaining bulk POC and PON was in the form of bacteria cells. Conversion efficiencies, however were only 8 to 9 % (carbon) and 10 to 11% (nitrogen) . The fraction of the bacterial community composed of active cells was inve rsely related to the C:N ratio of the bulk particulate matter in both cultu res, although with different absolute values. C:N ratios of detritus, disti nct from surrounding bacteria and phytoplankton, were typically 35 to 50 bu t varied during the 56 d incubations because bacteria selectively degraded PON compared to POC. C:V and N:V ratios were typically 0.09 to 0.11 and 0.0 02 to 0.004 pg mum(-3) in fresh detritus, respectively, and ratios declined as the detritus degraded in the presence of bacteria. Mean C:V and N:V rat ios were 0.05 to 0.11 and 0.0014 to 0.0031, respectively. These ratios indi cate that detritus derived from phytoplankton cultures contains reduced den sities of organic carbon and nitrogen compared to living plankton. They pro vide the means to directly estimate the carbon and nitrogen content of natu ral detritus, although the C:V and N:V ratios of cultures need to be compar ed to those estimated from natural plankton communities.