THE EFFECT OF DOC SOURCES ON BACTERIAL CARBON CYCLING IN TIDAL CREEKS.


Katherine MacMillin and Linda Blum.


Bacteria from tidal creeks on the Chesapeake Bay and ocean sides of the lower Delmarva peninsula were examined for their response to the addition of DOC derived from Spartina alterniflora or phytoplankton. Water samples collected from Hungars Creek, a bayside creek, and from Phillips Creek, a seaside creek, were incubated with both types of DOC. The flasks were monitored for 48 hours and samples were collected to measure bacterial abundance, biovolume and productivity as well as DOC concentration. The response of the bacteria to the two sources of DOC was similar; however, DOC enrichment affected the seaside bacteria differently than the bayside bacteria (Figure 1). After 12 hours, the biomass (abundance x biovolume x a carbon conversion factor) of seaside bacteria increased over 4-fold. After 48 hours, the biomass of seaside bacteria returned to near the initial value. In contrast, the biomass of bayside bacteria increased only 35% after 12 hours and remained elevated through 48 hours. The distribution of seaside bacterial biovolumes also changed following enrichment. After 12 hours, the smaller size classes (<0.05del-m3) contained relatively fewer cells and large cells were more abundant. After 48 hours the biovolume distributions resembled the initial frequency distributions: smaller cells were present in large numbers and the larger cells were generally absent. The average biovolume and biovolume distributions of bayside bacteria did not change during the enrichment experiment. The changes in bacterial biomass were accompanied by increased bacterial productivity (as measured using tritiated thymidine incorporation) and a decrease in DOC in both seaside and bayside samples. These results are consistent with the hypothesis that seaside bacterial productivity may be substrate limited while grazing on bacterial cells may control the cell-size frequency distributions.

This work was supported in part by NOAA Office of Sea Grant, U.S. Department of Commerce, under Grant No. NA90AA-D-SG045 to the Virginia Graduate Marine Science Consortium and Virginia Sea Grant College Program, and a Bannon Foundation Graduate Student Research Grant. Logistical support was provided by the Virginia Coast Reserve Long-Term Ecological Research Project.