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

Abstract

The mass of non-living particulate organic matter, or detritus, often exceeds that of liv- ing plankton in pelagic environments; in coastal waters the difference can be 10-fold. Relatively little is known about the dynamics of this pool of organic matter because it has not been previously possi- ble to accurately determine its magnitude. A recent approach utilizing fluorescence microscopy (Ver- ity et al. 1996) provided estimates of the volume of detritus. Here, laboratory experiments were con- ducted to estimate carbon:volume (C:V) and nitrogen:volume (N:V) conversion ratios of detritus formed by 2 phytoplankton species when incubated in darkness in the presence of bacteria. The vol- ume of detritus was measured directly, along with total particulate organic carbon (POC) and nitro- gen (PON), and that contained in the associated phytoplankton and bacterial communities. Detrital carbon and nitrogen were estimated by difference and compared to detrital volume, from which con- version factors were calculated. The physiological state of the bacteria was assessed using a recently developed fluorescent stain and molecular probe protocol (Williams et al. 1998). The 2 phytoplankton cultures were degraded at different rates. At the times of peak bacterial abundances, most of the 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 com- posed of active cells was inversely related to the C:N ratio of the bulk particulate matter in both cul- tures, although with different absolute values. C:N ratios of detritus, distinct from surrounding bac- teria and phytoplankton, were typically 35 to 50 but 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 0.002 to 0.004 pg µm -3 in fresh detritus, respectively, and ratios declined as the detritus degraded in the presence of bacteria. Mean C:V and N:V ratios were 0.05 to 0.11 and 0.0014 to 0.0031, respectively. These ratios indicate that detritus derived from phytoplankton cultures contains reduced densities of organic carbon and nitrogen compared to living plankton. They provide the means to directly estimate the carbon and nitrogen content of natural detritus, although the C:V and N:V ratios of cultures need to be compared to those estimated from natural plankton communities.