Structure of the aerobic food chain in a meromictic lake dominated by purple sulfur bacteria
Overmann, J.; Hall, K.J.; Northcote, T.G.; Ebenhoeh, W.; Chapman, M.A.; Beatty, T.
Archiv fur Hydrobiologie 144(2): 127-156
ISSN/ISBN: 0003-9136 Accession: 011410186
The biomass and seasonal variability of all groups of planktonic organisms was investigated in saline meromictic Mahoney Lake (British Columbia, Canada). After ice-break, meltwater formed a stable surface layer which resulted in steep vertical gradients of temperature, salinity, and oxygen concentration. This secondary chemocline persisted until August when it was detected at 4 m depth. Pronounced vertical differences were also observed for heterotrophic flagellates, ciliates, rotifers, calanoid copepods, and growth rates of chemotrophic bacterioplankton. No strong correlation between the different components of the microbial loop was found, but phytoplankton primary production and ciliates numbers were tightly correlated (r2 = 0.649, p<0.001). Compared to phytoplankton biomass (<200 mgC cntdot m-2), the biomass of bacterioplankton in the epilimnion measured in autumn was more than one order of magnitude higher (up to 5,650 mgC cntdot m-2) and chemoheterotrophic bacterial production exceeded photosynthesis by a factor of 7.1. By comparison, the biomasses of flagellates and ciliates remained very low throughout the year (maxima, 5.52 and 3.4 mgC cntdot m-2) whereas that of the calanoid copepod Diaptomus connexus rose to unexpected high levels (681 mgC cntdot m-2), parallel to the increase in biomass of aerobic chemoheterotrophic bacteria. Calanoid copepods represent the highest trophic level in the planktonic food web of this fishless lake. Taken together our results indicated an unusual pattern of carbon flow in Mahoney Lake. Maximum rates of carbon transfer through the microbial loop and the grazing food chain were determined by radiolabeling techniques at four times. The efficiency of organic carbon transfer from chemotrophic bacteria to zooplankton was 3 to 12 %, while the transfer efficiency from phytoplankton reached 117 %. As a result, carbon fluxes to higher trophic levels through the microbial loop and the conventional grazing food chain were similar. A comparison of the carbon supply and demand revealed that additional food sources of zooplankters must exist in Mahoney Lake. Concomitantly to the autumnal increase in chemotrophic bacterial and zooplankton biomass, a massive upwelling of the purple sulfur bacterium Amoebobacter purpureus from the chemocline into the mixolimnion was observed. Several lines of evidence indicate that the carbon fixed by anoxygenic photosynthesis is a direct, significant food supply for higher trophic levels in the oxic part of Mahoney Lake.