Primary production of microalgae in sediments measured by oxygen microprofile, H14CO3 [carbon isotope labeled bicarbonate ion] fixation, and oxygen exchange methods [Denmark]

A platinum microelectrode was used to measure changes in dissolved oxygen profiles in surface sediment exposed to light and darkness. Estimates of production by microalgae thus derived were compared with those derived from conventional methods of measuring changes in dissolved oxygen and H14CO3- uptake. For each particular depth in the sediment, the primary production can be determined within a few seconds by the new method. Undisturbed sediment cores were used for the H14CO3- fixation assay of benthic photosynthesis. Measurements of the specific activity of HCO3- within the photic zone showed a steep gradient of H14CO3- at the sediment surface. Calculations of benthic photosynthesis taking the actual specific activity into account yielded 2-5 times higher estimates than calculations using the specific activity in the overlying water. In a highly oxidized sediment, the three methods yielded almost identical results at low light intensities ($200 \muEinst \cdot m^-2 \cdot s^-1$). The oxygen methods grossly underestimated the primary productivity at higher light intensities, due to bubble formation. The conventional oxygen method grossly underestimated the primary productivity in sulfuretum-type sediments as compared to the two other methods. In addition to photosynthesis, the oxygen consumption by the sediment was determined by oxygen microprofiles.