FACTORS CONTROLLING PRIMARY PRODUCTION IN LARGE TEMPERATE AND TROPICAL GREAT LAKES

Primary production sets the upper boundary on how much carbon is available for sedimentation. Average annual primary production was 55, 160, 210 and 4,000 g C/m²/y for Lakes Superior, Erie, Malawi and Victoria respectively. Primary production in the Laurentian great lakes is light limited for at least 7 months of the year. During the stratified season these lakes are phosphorus limited as indicated by particulate C:P molar ratios. Lake Erie (C:P 175), is on average only 20 m deep and internal regeneration prevents the phytoplankton from being as strongly P limited as Lake Superior (C:P 320). The tropical great lakes experience light limitation during windy season. During the stratified season primary production in Lake Malawi is nitrogen limited as indicated by the particulate C:N molar ratio (9.9) due to the high rates of nitrogen loss from the permanently anoxic hypolimnion. Lake Victoria although much shallower than Lake Malawi (average depth 40 m), also has an anaerobic hypolimnion and is nitrogen deficient. However high anthropogenic P loading to Lake Victoria permits development of extremely high concentrations of nitrogen fixing cyanobacteria (average chlorophyll 35 mg/m³) and particulate C:N ratios in Lake Victoria (7.9) are not indicative of N limitation. The range of annual primary production in these great lakes spans the range of marine primary productivity from oligotrophic open oceans (50 g C/m²/y) to coastal estuaries (1200 g C/m²/y). Great lakes and ocean basins appear to have equal potential for producing carbon for burial.