Net photosynthesis of Cladophora glomerata was measured at three sites in lower Green Bay, Lake Michigan, from late spring through summer, 1971. Lower levels of productivity occurred early in the season at two of the sites, when water temperatures were lowest. At the third site water temperature and productivity varied the least. Contrary to other reports, we found that Cladophora made relatively efficient use of low illumination. Under statistically similar temperature and irradiance levels, productivity was higher with increasing proximity to the mouth of the Fox River. Nitrogen, calcium, strontium, sodium, and zinc also were highest in concentration in plants receiving the greatest amount of effluents from the Fox River in comparison with the site receiving the least. Site differences in productivity appear to be related to nutrient levels, whereas seasonal differences in productivity are probably most closely related to seasonal temperature differences.
The photosynthetic responses of Myriophyllum spicatum and Vallisneria americana to total dissolved inorganic carbon (DIC) concentration (pH 8.0; 25°C; saturating light) indicate that both species are sensitive to changes in DIC concentrations at natural lake levels, and that M. spicatum has a somewhat lower half-saturation constant than V. americana for that relationship.This difference is probably not ecologically significant. Carbon uptake rates of V. americana decline by 61% from pH 7 to 8, but change little from pH 8 to 9, whereas those of M. spicatum decline by 35% from pH 7 to 8 and by a further 58% from pH 8 to 9. These decreases do not parallel the corresponding decline in free CO, availability over this pH range. Carbon compensation points expressed as total DIC, free COz, or the atmospheric CO2 concentration in equilibrium with the corresponding free CO2 level are strongly pH-dependent and do not differ substantially between M. spicatum and V. americana.Submersed macrophytes, unlike emergent or floating-leaved macrophytes, are entirely dependent on the availability of dissolved inorganic carbon for photosynthesis. Literature summarized by Sculthorpe (1967) and Raven (1970) has suggested that the absorption of dissolved inorganic carbon (DIC) is a major problem facing a submersed photosynthetic organ, partly because rates of gas diffusion in solution may be five orders of magnitude slower than in air. Furthermore, the DIC system of alkaline lakes consists of several forms which are not equally available as photosynthetic carbon sources. Carbonic acid dissociates to form significant quantities of HC03-and C03+, and the proportion of free CO, declines rapidly as pH rises from 4.5. Photosynthesis is generally more rapid with free CO, than with the equivalent concentration of HC03-, and COs2-does not
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