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

Abstract

In lacustrine wetlands of the semiarid Laramie Basin (Wyoming, USA), we investigated whether dominance by Chara spp. versus dominance by Potamogeton pectinatus alters the amount and form of organic carbon available to consumers. In these two wetland states, we compared relative biomass and production of different producer types (macrophytes, epiphyton, epipelon, and phytoplankton). Based on chlorophyll a measurements, light profiles, and a model of primary production, differences in relative biomass and production of different algal types depended mainly on combined effects of macrophyte growth form and water depth. Canopy density (kg m −3) of Chara spp. was at least 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. pectinatus (80–224 versus 9–24 mg Chl a m −2). However, the lower canopy density of P. pectinatus, by providing better light conditions for phytoplankton and epipelon, led to similar total algal production. Models estimated 66–133 g C m −2 assimilated by algae in Chara habitats and 66–105 g C m −2 assimilated by algae in P. pectinatus habitats. Differences in the form of algal production might have trophic correlates. In Chara habitats where most algal production is epiphytic, earlier studies showed that macroinvertebrate consumers were mostly scrapers and epiphytic deposit-feeders (snails and amphipods). In P. pectinatus habitats where phytoplankton and epipelon are more important, consumers were mostly filter-feeders and often benthic deposit-feeders (cladocerans, copepods, and chironomid larvae). Despite these taxonomic differences, total invertebrate biomass between states was similar and may reflect similarities in total algal production. Our results indicate that the state shift in vegetation among saline, irrigation-driven wetlands is associated with major changes in relative production of different algal types, despite continued coverage of submersed macrophytes.