Besides light, a supply of carbon dioxide is essential to normal photosynthetic production. However, while the necessity of an instantaneous light source to the fixation of carbon is self-evidently axiomatic, the potential limitation of photosynthetic production by inorganic carbon has been a surprisingly contentious issue. Kuentzel (1969) considered the carbon supply to be one of several factors crucial to the development of algal blooms in response to lake enrichment. Shapiro's (1973) experimental demonstrations of the ability of bloom-forming Cyanobac-teria to grow at high pH levels (indicative of deficiencies in the reserves of CO2 in solution) provided very strong support for this view. On the other hand, Schindler's (1971) whole-lake manipulations in the Experimental Lakes Area of Canada pointed to the direct linkage of production responses to added phosphorus and nitrogen. His data were persuasive. Moreover, the intuitive supposition of an adjacent, effectively infinite reserve of atmospheric carbon dioxide, readily soluble in water, was enough to allay most doubts that carbon availability is a significant constraint upon the yields of aquatic biomass.
However, matters did not rest there and much important research has ensued. Crucially, the first point that must be recognised is that these arguments are not, in fact, directly opposed, nor are they mutually exclusive. The aquatic sources of inorganic carbon are, indeed, ultimately plentiful and renewable and do not constitute a biomass-limiting constraint. At the same time, it has to be accepted that the ambient concentration of dissolved carbon dioxide is highly variable, that it is inextricably linked to the pH-dependent bicarbonate system and that algal production is, anyway, a principal driver in the transformations. The potential role of the carbon supply as a rate-limiting constraint on photosyn-thetic behaviour is plainly indicated.
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