Structure of the aerobic food chain in a meromictic lake dominated by purple sulfur bacteria

The biomass and seasonal variability of all groups of planktonic organisms was investigated in saline meromictic Mahoney Lake (British Columbia, Canad a). After ice-break, meltwater formed a stable surface layer which resulted in steep vertical gradients of temperature, salinity, and oxygen concentra tion. This secondary chemocline persisted until August when it was detected at 4 m depth. Pronounced vertical differences were also observed for heter otrophic flagellates, ciliates, rotifers, calanoid copepods, and growth rat es of chemotrophic bacterioplankton. No strong correlation between the diff erent components of the microbial loop was found but phytoplankton primary production and ciliates numbers were tightly correlated (r(2) = 0.649, p<0. 001). Compared to phytoplankton biomass (<200 mgC.m(-2)), the biomass of ba cterioplankton in the epilimnion measured in autumn was more than one order of magnitude higher (up to 5.650 mgC.m(-2)) and chemoheterotrophic bacteri al production exceeded photosynthesis by a factor of 7.1. By comparison, th e biomasses of flagellates and ciliates remained very low throughout the ye ar (maxima, 5.52 and 3.4 mgC.m(-2)) whereas that of the calanoid copepod Di aptomus connexus rose to unexpected high levels (681 mgC.m(-2)), parallel t o 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 t he microbial loop and the grazing food chain were determined by radiolabeli ng techniques at four times. The efficiency of organic carbon transfer from chemotrophic bacteria to zooplankton was 3 to 12 %, while the transfer eff iciency from phytoplankton, reached 117 %. As a result, carbon fluxes to hi gher trophic levels through the microbial loop and the conventional grazing food chain were similar. A comparison of the carbon supply and demand reve aled that additional food sources of zooplankters must exist in Mahoney Lak e. Concomitantly to the autumnal increase in chemotrophic bacterial and zoo plankton biomass, a massive upwelling of the purple sulfur bacterium Amoebo bacter purpureus from the chemocline into the mixolimnion was observed. Sev eral lines of evidence indicate that the carbon fixed by anoxygenic photosy nthesis is a direct, significant food supply for higher trophic levels in t he oxic part of Mahoney Lake.