Evaluating the influence of macrophytes on algal and bacterial production in multiple habitats of a freshwater wetland

Algal 14C uptake and bacterial 3H leucine incorporation were measured over 20 months to assess the influence of macrophytes on the spatial distribution and magnitude of microbial production and the relative importance of algae versus macrophytes to whole-system energy flow in a southeastern U.S. wetland. Algal and bacterial production were determined for water column and plant-, sediment-, and wood-surface microhabitats in four zones defined by aquatic vascular plant composition: no macrophyte, floating leaved (Nymphaea odorata), heterophyllous (Proserpinaca palustris), or emergent macrophyte (Juncus effusus) zones. We combined production data with detailed habitat measurements to estimate production at meter-squared and whole-wetland scales and compared microbial C fixation to concurrently determined rates of macrophyte production. Production on plant surfaces was significantly lower than on wood and benthic sediments in all zones. At a meter-squared scale, 79% of algal production and 74% of bacterial production occurred on sediments, with epiphytes contributing ,6% to both algal and bacterial rates. With the exception of phytoplankton in the Nymphaea zone and bacteria on Juncus zone sediments, production in the water column or on plant or sediment surfaces did not significantly differ among macrophyte zones. Thus, plant type did not affect the spatial distribution of microbial activity except in the Juncus marsh, where the limited area and volume of water per square meter reduced production at larger spatial scales. However, the magnitude of bacterial production was influenced by macrophytes, as bacterial carbon demand greatly exceeded the amount supplied by algal production. At the whole-ecosystem scale, macrophytes overwhelmed algal production, which accounted for only 4–10% of total wetland C fixation. Although significant progress has been made in understanding rates and determinants of algal and bacterial production and the contribution of these assemblages to energy flow in pelagic systems over the past two decades, microbial dynamics in other settings have received far less attention (Vadeboncoeur et al. 2002). This lack of information is particularly acute for wetlands (Goldsborough and Robinson 1996; Kirschner and Velimirov 1999), where studies of energy flow have focused on aquatic macrophytes rather than algae and where, conversely, bacterial populations are often viewed in terms of biogeochemical processes rather than productivity (but see Findlay et al. 1998). Consequently, 1 Corresponding author ([email protected]). Acknowledgments We are indebted to S. M. Evces for her tireless contributions to all aspects of this study. R. G. Wetzel generously provided supplies, equipment, and a wide range of other support and advice throughout this project, and G. M. Ward provided meteorological and hydrologic data. Thanks to L. S. W. Lindblom for access to her data. Pond surface area and volume estimates and wetland maps were made by Joe Partlow at the Department of Biological Sciences GIS Laboratory. This article has been greatly improved by the comments of S. R. Carpenter, P. J. Mulholland, and two anonymous reviewers. This research was supported by NSF-EPSCoR grant OSR-9108761. substantial gaps exist in our understanding of how rates of algal and bacterial production vary among different habitats, over time, or in comparison to vascular plants in wetland ecosystems. Aquatic vascular plants are conspicuous, defining features of freshwater wetlands, and the presence and type of macrophytes have profound effects on these systems. With respect to microbial production, Goldsborough and Robinson (1996) and Wetzel and Søndergaard (1998) argued that establishment of macrophyte beds should shift the location of greatest algal and bacterial growth from the water column and benthic sediments to plant surfaces because of the multiple advantages associated with an epiphytic lifestyle. These benefits include direct access to nutrients and labile organic compounds from the plant, avoidance of particle settling and anoxia in the sediments, high insolation, and availability of extensive surface area (Wetzel 2001). Spatial patterns of microbial production in wetlands may be further modified by the type or growth form of macrophytes present. Architectural differences among species determine the amount of surface area available for microbial colonization and create pronounced spatial variation in light, temperature, water current, and nutrient conditions within and between macrophyte beds (e.g., Wilcock et al. 1999). Moreover, if microbes rely on plant-derived organic matter, then patterns of production are likely to reflect differences