Decomposition of Planktonic Algae in an Oligotrophic Lake

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Abstract
A new procedure (dip incubation) was used to measure both the dissolution and mineralization (to CO2) of killed, 14C-labeled phytoplankton in an oligotrophic lake. This procedure, based on a series of 24-h incubations in bottles, avoids the problem of extremely long incubations and can be used to measure, independently, the dissolution and mineralization rates of algal detritus of any recent age. In Mirror Lake, New Hampshire [USA] the conversion of particulate C to CO2 occurred only in the presence of living microorganisms and was strongly influenced by the age of the algal detritus and the water temperature. There was a pronounced annual cycle of the conversion of particulates to CO2 with highest rates (average rate for 6 d [days] of 6% d-1) during summer and lowest rates (average rate for 6 d of 0.5% d-1) in winter. In comparison to these average rates, the initial rates of both dissolution and mineralization were higher and much less dependent on temperature. These initial rates, taken alone, would give an erroneously high estimate of the rate of decomposition and underestimate the importance of temperature as a factor controlling decomposition. An annual cycle of these measurements was used in conjunction with models of the age structure of particulate detritus to estimate the metabolism of algal C by planktonic microbes. The estimate (3.6-4.3 .mu.g C l-1 d-1) is comparable to previous measurements of total microbial respiration (of all C-sources) in both amount (3.1 .mu.g C l d-1) and annual pattern. This comparison suggests that algal C is a major C-source for planktonic bacteria in the epilimnion of Mirror Lake.