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Cryptobiotic crusts (biological soil crusts) are a dominant component of a unique system of dunes and intermittently inundated pans and playas situated on Pleistocene Lake Thompson bed at Edwards Air Force Base. California. Previous work reported on basic ecological parameters including distribution, abundance, and species composition of these algal (cyanobacterial)dominated crusts. As a step toward building an ecological model in support of the military installation natural resources management plan, we determined the photosynthetic rate of the algal crusts under controlled laboratory conditions using a portable photosynthesis system. We examined the relation between the environmental conditions of light intensity, crust moisture content. antecedent crust moisture content, and atmospheric CO2 concentration, on photosynthetic rates.

Maximal photosynthetic rates were 4.97 and 7.75 mumol m(-2) s(-1) for cryptobiotic crusts collected from dunes (upland crusts) and from pans (aquatic remnant crusts), respectively, which corresponds well with the upper range of previous reports for such algal crusts. Light saturation levels were about 1500 and 1500-2000 mumol m(-2) s(-1) for cryptobiotic crusts from both the dunes and the pans respectively. Optimal moisture content of the crusts for photosynthesis was 15-30% and 90-100% for crusts from dunes and pans respectively: both lower and higher moisture contents resulted in decreased rates of photosynthesis. With increasing atmospheric CO2 concentrations up to at least 1000 ppm, photosynthesis of both sorts of crusts increased linearly to rates several times that of rates at natural CO2 concentrations.