Ratios of community respiration to photosynthesis and rates of primary production in Lake Erie via oxygen isotope techniques

To evaluate levels of primary production and community metabolism in Lake Erie, we conducted incubations with ¹⁸O-labelled water and determined the ratio of respiration to primary production (R:P) during the summer and early fall of 2002. The epilimnion of Lake Erie was characterized by δ¹⁸O values less than 0.7‰ at all times that reflects a strong contribution of O₂ from primary production. High δ¹⁸O values (maximum of 6.6‰) were common in the O ₂ depleted waters of the hypolimnion as a consequence of isotope fractionation during respiration. Hypoxic conditions were evident in the central basin in August when the fractional abundance of O₂ reached a minimum value of 0.04. Rates of primary production varied from 0.23 to 1.76 mmol-O₂·m^-3·h^-1, decreased from west to east in August, and were at a minimum in the central basin in September. Ratios of respiration to photosynthesis (R:P) (determined from the abundance and isotopic composition of dissolved O₂) in the epilimnion as low as 0.45 in July were strong evidence of net autotrophy and preceded the development of hypoxia in August. Net heterotrophy prevailed in August and September. The absolute values of R:P ratios were not indicative of trophic state, however, the wide range of R:P ratios (0.45 to 20.40), fraction of O ₂ saturation (0.04 to 1.36), and δ¹⁸O-O₂ (-6.0 to 7.5‰) values were all indicative of a eutrophic system. An average isotope fractionation factor for respiration of 7.9‰ was determined from samples in the hypolimnion in July and August. Based on unique fractionation factors for respiration in the water column (23.5‰) and in sediments (3‰) we calculate that 61% of hypolimnion O₂ respiration occurs within sediments and 39% occurs within the water column.