Dissolved and particulate organic matter source-age characterization in the upper and lower Chesapeake Bay: A combined isotope and biochemical approach

KW Tang
KML Hutalle
HP Grossart

Abstract

Literature suggests that zooplankton carcasses are prevalent at times in both freshwater and marine environments, and could be important substrate sources for water column microbes (Dubovskaya et al. 2003, Hydrobiologia 504:223-227; Tang et al. 2006b, Estuar Coast Shelf Sci 68:499-508). We conducted laboratory experiments to investigate the decomposition of copepod carcasses by ambient microbes from Lake Dagow, Germany. Bacteria rapidly colonized and decomposed the carcasses, mainly from the inside. The ambient bacterial abundance increased 2-fold or more at the peak of decomposition, but decreased afterward, presumably due to protozoan grazing. Initial increase in ambient bacteria was faster at 20 degrees C than at 6 degrees C; however, this did not differ between aerobic and anaerobic conditions at 20 degrees C, suggesting that pelagic bacteria in Lake Dagow were equally adapted to both aerobic and anaerobic conditions. When bacteria were suppressed by antibiotics, the carcasses were colonized and decomposed by a massive amount of fungi; much of the fungal mass remained attached to the outside of the carcasses. DGGE analyses showed that bacterial and fungal communities of the decomposing carcasses were very different from those of natural copepod samples, indicating a shift in the microbial community at the onset of decomposition. The bacterial composition remained relatively stable, whereas the fungal composition varied greatly over time and between treatments. The ambient protease activity increased with bacterial abundance, and was at most 4 to 18 times higher than in the control (lake water). Except for the antibiotics treatment, re-suspension of carcasses in the water increased the measured protease activity by as much as 4- to 7-fold, indicating that protease activity was highly localized within the decomposing carcasses. Our study shows that copepod carcasses support high bacterial growth and enzymatic activities. Colonization and decomposition of the carcasses by fungi point to a previously unknown ecological role for aquatic fungi that deserves further investigation.