Metabolism of the dominant autotrophs of the North American Great Lakes

Summary

This investigation of photosynthetic and respiratory rates of natural communities encountered in the North American Great Lakes reveals an interesting contrast among the lakes and between enriched near-shore areas and offshore areas. Lake Superior with its soft water (0.7 meq titratable base) and its oligotrophic nutrient supply (phosphorus is often less than 1 μg/1) shows the lowest yields (∼ 70 moles of CO₂ fixed/m²/day). Lake Michigan's yields are about 3-fold higher. Lake Huron, which is a mixture of Superior and Michigan waters, exhibits a photosynthetic yield intermediate between that of Michigan and Superior. As the waters enter western Lake Erie they receive spectacular nutrient injections from the city of Detroit, and from the agricultural and urban areas of southeastern Michigan and northwestern Ohio. Phosphorus supplies rise to about 35 μg/liter and photosynthetic yields to 500 mmol/m²/day.

The western basin yields are similar to those encountered in enriched nearshore areas of the upper lakes such as Green Bay, Wisconsin (in Lake Michigan) and Saginaw Bay, Michigan (in Lake Huron). The final lake in the system, Lake Ontario, exhibits yields closely similar to those of eastern Lake Erie.

The filamentous algal communities found on the lake shores exhibit photosynthetic yields of about 50 micromoles of CO₂ absorbed per ml of algal biomass per hour, regardless of species composition (Ulothrix or Cladophora) or of locality, with one exception; namely western Lake Erie, in which yields were 3-fold higher. Respiration rates of the filamentous community were about 6 micromoles of CO₂ evolved per ml of algae hour except in western Lake Erie, where the rate was about twice as high, suggesting that the nutrient enrichment of western Lake Erie supports enhanced metabolic rates in the filamentous littoral zone community.

Growth rates of the diatom communities of Lake Michigan and western Lake Erie indicate that the time for a population to double in offshore waters is about twice as long (24 days) as in enriched near-shore areas (13 days). There is also evidence of species differences in this parameter with Asterionella and Tabellaria species exhibiting 3-fold higher growth rates than Stephanodiscus species (∼ 42 days).