Net heterotrophy in productive prairie wetlands with high DOC concentrations

Abstract

Millions of productive wetlands dot the North American Great Plains. Although these wetlands are key ecological features of the Canadian prairies, their microbial food webs remain relatively unstudied. Over 3 yr, pelagic primary (PP) and bacterial production (BP) and biomass were monitored in 2 wetlands in Central Saskatchewan, Canada. PP ranged from 8.8 to 3911.0 mgC m-3 d-1 (x̄ = 612.0 mg C m-3 d -1), while rates of BP ranged from 8.2 to 678.0 mg C m-3 d-1 (x̄ = 140.0 mg C m-3 d-1). Nutrients, light and temperature did not appear as major factors influencing these rates. Seasonal mean ratios of PP:BP revealed that both wetlands were net autotrophic (x̄PP:BP = 7.4; range 0.08 to 42.6), which was not surprising given their eutrophic status. On a smaller temporal scale, these wetlands were, on average, net heterotrophic (PP:BP < 1) on 33 % of the sampling dates. High pelagic bacterial carbon demand (greater than twice the phytoplankton carbon production) indicated that bacterial metabolism was not dependent on autochthonous carbon sources. Biological availability of dissolved organic carbon (DOC) was low in both wetlands (x̄ = 3.8%). But this percentage of high ambient concentrations was enough to satisfy bacterial carbon demand. Based on seasonal averages, BP and bacterial numbers decreased across increasing trophic gradients (as chl a or total phosphorus). This phenomenon could also be observed across a seasonal gradient of changing chl a concentrations. Prairie wetlands not only experience periods of net heterotrophy, but bacterial carbon demand and production per unit biomass are high, suggesting an important role for bacteria in the metabolism of these eutrophic ecosystems.