Photosynthesis of cryptobiotic soil crusts in a seasonally inundated system of pans and dunes in the western Mojave Desert, CA: Field studies

Abstract

Cryptobiotic soil crusts form the dominant cover of a unique system of dunes and intermittently inundated pans and playas situated on the Pleistocene Lake Thompson bed, in the western Mojave Desert, CA. Recent work has reported on basic ecological parameters including distribution, abundance, and species composition of these algal (cyanobacterial)-dominated crusts as well as on their photosynthetic rates in laboratory studies. As a step toward building an ecological model for a natural resource management plan, we determined the photosynthetic rate of the crusts under field environmental conditions. We examined the relationship between the environmental conditions of crust moisture content (MC), temperature, and ambient CO 2 concentration on photosynthetic rates during the mid-spring and the late autumn. Maximal photosynthetic rates ( P max), which did not vary greatly with season, were 3.57 and 3.99 μmol CO 2 m −2 s −1 for crusts on dunes and in pans, respectively. Mean photosynthetic rates for three types of crust on dunes, smooth and flat, pedicillated, and unconsolidated, were 1.31, 1.47, and 1.29 μmol CO 2 m −2 s −1, respectively. P max of crusts on dunes and pans occurred at soil MCs of 110% and 20%, respectively, both lower and higher MCs resulted in decreased rates of photosynthesis. The relation between atmospheric CO 2 concentration and net photosynthesis was linear ( r 2 = 0.88 and 0.97 for crusts on dunes and pans, respectively) through at least 1000 ppm CO 2. We measured positive net photosynthesis at temperatures from 0 to 46 °C and found no clear optimum within this range when crusts were provided with optimal moisture and light conditions. Photosynthetic rates in this field study were somewhat lower than those measured in the laboratory, but both optimum MC and the relation between ambient CO 2 and net photosynthetic rate were similar. Respiration rates were generally less than 10% those of photosynthetic rates. Based on data herein and climatic information, we estimated the potential net primary productivity of crusts on dunes to be 11.7 g C m −2 yr −1. The photosynthetic rate of the unconsolidated crust type, for which no data have been published, was similar to the other crust types. This finding, together with previous information that the chlorophyll values of this and other crust forms were similar, suggests that ecological studies, as well as conservation and restoration efforts, give these the same attention as the other crust types.