Midsummer Photosynthetic Carbon Budget for Old Woman Creek Wetland, Ohio: Relative Contribution of Aquatic Macrophytes Versus Phytoplankton

Few data are available on net productivity rates in Laurentian Great Lakes wetland communities. We used several methods (Licor photosynthesis system and various radiotracer methods) to estimate midsummer carbon photoassimilation rates among important phytoplanktonic and aquatic macrophyte assemblages in Old Woman Creek National Estuarine Research Reserve (OWC) on Lake Erie near Huron, OH, during 1993-1995Our data suggested that the majority of carbon flow into the OWC estuary (approximately 66-99% of the total) occurred through aquatic macrophytes, especially the dominant floatingleaved species Nelumbo lutea and the emergent species Phragmites australis. OHIO J SCI 99 (2): 6-9, 1999 INTRODUCTION Coastal wetlands associated with the Laurentian Great Lakes play an important role as "metabolic gates" in processing organic and inorganic materials before they reach recipient waters of the Great Lakes (Wetzel 1992). This is especially true in the western basin of Lake Erie, a body of water essentially ringed by coastal wetlands (Stuckey 1989, Herdendorf 1992). However, information on community-level energetics in Great Lakes coastal wetlands, including photosynthetic budgets, are conspicuously lacking. A consensus research priority for Great Lakes researchers (summarized by Krieger and others 1990, 1992) has thus been the development of an estuarine carbon budget model that incorporates contributions from all photoautotrophs, including aquatic macrophytes as well as algae. In inland wetlands and marine coastal wetlands, annual primary productivity rates are among the highest of any community in the biosphere and the majority of carbon fixation into the ecosystem occurs via macrophytic fixation (reviewed by Wetzel 1983, Mitsch and Gosselink 1993). In the only published primary productivity studies we are aware on a Lake Erie wetland to date, Reeder and Mitsch (1989) and Reeder (1990, 1994) estimated annual carbon fixation rates in the Old Woman Creek National Estuarine Research Reserve (OWC) near Huron, OH. Using peak biomass transect sampling of dominant aquatic macrophyte Nelumbo lutea (American lotus) and a light bottle:dark oxygen evolution assay method to estimate phytoplanktonic productivity, they concluded that, in contrast to "typical" wetlands, annual net primary production by open-water plankton communities (366 g m yr) greatly exceeded that of lotus beds (approximately 75 g nr yr). However, both aboveground biomass sampling and oxygen evolution techniques represent indirect methods for measuring photosynthetic carbon flow rates into photoautotrophs, and may underestimate total fixation in systems dominated by rhizomatous macrophytes with large under'Manuscript received 24 June 1998 and in revised form 8 December 1998 (#98-10). Present Address: Associated Colleges of the South, 1975 Century Blvd. NE, Suite 10, Atlanta, GA 30345. ground biomass. The goal of this study was to determine midsummer photosynthetic C flux through various macrophytic and algal components of OWC using a variety of direct measurement techniques. MATERIALS AND METHODS Description of Study Site The Old Woman Creek National Estuarine Research Reserve and State Nature Preserve (OWC) represents one of the few remaining undeveloped coastal wetland systems along the south shore of Lake Erie. It is located in Erie County, OH, at the edge of the Western Basin and the southernmost point on the Great Lakes. Classified as a freshwater estuary, it is characterized by its drowned river mouth as it enters Lake Erie (Herdendorf 1990). The estuary proper is approximately 56 hectares and extends about 2 km south of Lake Erie. Its watershed encompasses an area of approximately 69 km. Water level fluctuations are controlled by a barrier sand beach and lake levels. The physical environment of OWC has been the focus of several studies (Klarer and Millie 1989, and reviewed by Whyte 1996). Extant plant communities may be the result of a prolonged period of high water that existed through the 1980s and has continued into the late 1990s. Additional information on the flora and limnology of OWC may be found in Klarer and Millie (1992), Whyte (1996), and Whyte and Francko (1997, 1999). Primary Productivity Field measurements of carbon photoassimilation rates in OWC phytoplankton and aquatic macrophytes were conducted in July 1993 and July and August 1994 and 1995, to coincide with the annual wetland peak biomass period (Whyte 1996). Measurements were conducted at mid-day via a "circuit sampling" protocol we developed to permit short-term measurements of all photoautotroph compartments within a single 60-90 min time frame, so that instantaneous C-assimilation rates could be directly compared to determine the relative proportion of C-fixation accounted for by major groups of primary producers at peak biomass. All field experimentation was conducted in the NW embayment of OWC. OHIO JOURNAL OF SCIFNCK I). A. FRANCKO AND R. S. WMYTF A Licor 6200 portable photosynthesis system (1000ml chamber) was used to measure CO;-fixation rates in aerial leaves of Nelumbo lulea and Pbragmites australis (giant reed), the dominant floating-leaved and emergent macrophytes in OWC, respectively. Replicate 30-second incubations of leaves that remained attached to the plant (N = 5 leaves of each species on each sampling date) were used to provide adequate replication for statistical analysis. The section of plant leaf enclosed by the chamber was excised for later measurement of leaf surface area and conversion of rate functions into Limol C taken up nr leaf surface s ' . Radiotracer techniques utilizing C-bicarbonate were employed to estimate photosynthetic rates in plant/algal samples where the Licor chamber could not be used. We utilized a sealed air chamber technique for measuring photosynthetic carbon flow into the upper surfaces of Nelumbo floating leaves. Chamber systems (350-ml) were constructed from magnetic-closure filter holder systems. The reservoir chamber was sealed at the top with parafilm and placed on the upper surface of the leaf a few cm from the petiole. The seal at the surface of the leaf was maintained by placing the magnetic ring filter holder on the underside of the floating leaf, restricting air exchange into the chamber without damaging the leaf. Ampules of C-bicarbonate solution (5 (iCi ml") were opened immediately before use and a 100-|il aliquot was added to replicate chambers placed on different leaves at time zero. In preliminary experiments we found that 30-min incubations produced adequate C-incorporation with minimal reduction in chamber CO2 partial pressure. A chamber covered with foil was run concurrently to serve as a dark control. After incubation chambers were collected and placed in a darkened box for transport to the shoreline where samples were processed for transport to the Miami campus. Aqueous C bottle assays were used for analyses of the dominant submerged aquatic macrophyte species (Potamogeton pectinatus [sago pondweed] and Ceratophyllum demersum [coontail]) and their attached epiflora, and for open water/littoral zone phytoplankton samples, using standard methodology (Francko and Wetzel 1984; Wetzel and Likens 1991). Assays were conducted in replicate 250-ml polycarbonate screw-cap flasks spiked with 100 |il of non-acidified, amputated C-bicarbonate solutions. Light/dark flasks containing macrophyte material (approximately 0.1 g fresh wt, collected immediately before assays) or whole littoral zone or pelagic waters, were incubated at the 10-cm and 50-cm depths for 30 to 60 min (attached to stakes driven into the littoral/pelagic sediment), removed, placed in dark boxes, and returned to the shoreline where samples were fixed for transport back to Miami University for liquid scintillation spectrographic analysis. Due to methodological difficulties, epiflora attached to lotus petioles and benthic algae associated with littoral sediments were not sampled in our investigation. On the shoreline, planktonic samples were filtered though 0.2-(im pore-sized Nuclepore filters (0.3 atm vacuum), and triplicate filters from each bottle were placed in glass scintillation vials containing 0.5 ml of 0.5 N perchloric acid to solubilize cells and drive off unincorporated inorganic carbon. Portions of macrophytes samples were withdrawn from incubation bottles, blotted dry, weighed, and immediately added to perchloric acid-containing vials. In Nelumbo leaves, 1-cm leaf disk samples (N = 5 from each leaf) were excised with a cork borer from leaf sections exposed to C and disks were placed in perchloric acid as above (Francko 1986). Ancillary measurements of PAR and water temperature were made during the time of incubation and water samples were collected in 1-liter brown polyethylene bottles for analyses of pH and alkalinity. Radiolabel incorporation in fixed samples was measured using a Beckman model 1800 liquid scintillation counter. Data for each water or plant sample on each sampling date were pooled and mean radiolabel uptake rates (+/SD) were converted to total C-assimilation rates using a C:C uptake ratio method (Francko and Wetzel 1984, Wetzel and Likens 1991). From the above data, it was possible to estimate photosynthetic rates in each compartment per unit time for the whole estuary, using aerial and ground-truth data on macrophyte bed sizes and volumes of water present in open water/littoral zones to calculate compartment sizes (Whyte 1996, Whyte and Francko 1997). The mean depth of OWC is 0.5 m and in this extremely turbid system (approximately 30-35 NTU; Whyte 1996) limited phytoplanktonic production occurs below the 0.5-m depth. To compute total phytoplanktonic Cassimilation rates in the entire water column, we plotted rate functions for the 0.1-m and 0.5-m depths within both the open water/pelagic zone (defined as all waters outside lotus beds, or 64% of the estuarine surface area) and the littoral zone (within lotus beds), and used the resultant slope to compute mean fixation in the top 50 cm and bottom 50 cm volumes of water. By summing these values and dividing by two, an estimate of total areal fixation by phytoplankton per unit time could be derived. During the course of our study, Nelumbo beds covered 36% of the estuary surface area, and within a bed, aerial and floating leaves accounted for an average of 1.6 nr and 0.7 mr of leaf surface area, respectively, per m of bed surface area. In 1993, Phragmites beds containing approximately 10.2 nr of leaf surface area per m of bed area were found in about 2% of the estuarine surface area, and by 1994 and 1995 that percentage had increased to 5%. We estimated that submerged aquatic macrophytes occurred in about 5% of the estuarine surface area (approximately 10 kg fresh wt biomass m water surface) in 1993 and 1995, and were absent from the flora in 1994.