Vegetation dynamics and primary production in saline, lacustrine wetlands of a Rocky Mountain basin

In lacustrine wetlands of the semiarid Laramie Basin (Wyoming, USA), we inv estigated whether dominance by Chara spp. versus dominance by Potamogeton p ectinatus alters the amount and form of organic carbon available to consume rs. In these two wetland states, we compared relative biomass and productio n of different producer types (macrophytes, epiphyton, epipelon, and phytop lankton). Based on chlorophyll a measurements, light profiles, and a model of primary production, differences in relative biomass and production of di fferent algal types depended mainly on combined effects of macrophyte growt h form and water depth. Canopy density (kg m(-3)) of Chara spp. was at leas t twice that of P. pectinatus (1.3-3 versus 0.16-0.52 kg m(-3)), and Chara habitats also supported a much higher biomass of epiphyton than did P. pect inatus (80-224 versus 9-24 mg Chi a m(-2)). However, the lower canopy densi ty of P. pectinatus, by providing better light conditions for phytoplankton and epipelon, led to similar total algal production. Models estimated 66-1 33 g C m(-2) assimilated by algae in Chara habitats and 66-105 g C m(-2) as similated by algae in P. pectinatus habitats. Differences in the form of al gal production might have trophic correlates. In Chara habitats where most algal production is epiphytic, earlier studies showed that macroinvertebrat e consumers were mostly scrapers and epiphytic deposit-feeders (snails and amphipods). In I. pectinatus habitats where phytoplankton and epipelon are more important, consumers were mostly filter-feeders and often benthic depo sit-feeders (cladocerans, copepods, and chironomid larvae). Despite these t axonomic differences, total invertebrate biomass between states was similar and may reflect similarities in total algal production. Our results indica te that the state shift in vegetation among saline, irrigation-driven wetla nds is associated with major changes in relative production of different al gal types, despite continued coverage of submersed macrophytes. (C)2000 Els evier Science B.V. All rights reserved.