Patterns of Dissolved Oxygen, Productivity and Respiration in Old Woman Creek Estuary, Erie County, Ohio during Low and High Water Conditions

Old Woman Creek wetland is composed of a stream and freshwater estuary and may act as a sink or transformer of nutrients entering Lake Erie. Primary productivity and respiration are indicators of ecosystem level trophic conditions and may be linked to the estuary’s effectiveness as a nutrient sink or transformer. Old Woman Creek National Estuarine Research Reserve has been collecting water chemistry data at 15 minute intervals using data loggers since 1995. Both diel and seasonal trends in water temperature, dissolved oxygen, and water depth at selected sites in the creek and estuary were related to physical and biological processes. Daily primary productivity and respiration were estimated from diurnal changes in dissolved oxygen under both high and low water conditions. Mean water depth was higher in 1997 and 1998 (0.94-1.2 m) than in 2003 and 2004 (0.5-0.66 m). Water temperature was generally 1-2o C higher in the open lower estuary and mouth than in the creek and upper estuary which is more shaded. Typical diurnal fluctuations in dissolved oxygen ranged from less than 20% to greater than 150% saturation. Primary productivity rates in the creek and upper estuary were lower (0.5-3.5 gO2/m 2/day) than in the lower estuary sites (2.010.1 g O2/ m2/day). Through both periods of high water and low water, the GPP/R ratio was less than one, thus negating our hypothesis that the ratio would be greater than 1 during high water years when the estuary was primarily an open water system. OHIO J SCI 108(3): 31-43, 2008 1Address correspondence to linda P. Cornell, Department of Natural and Social Sciences, BGSU firelands, 1 University Dr., Huron, OH 44839. email: [email protected]. INTRODUCTION Primary productivity and respiration vary widely across different ecosystems and are indicators of ecosystem trophic conditions. If primary productivity exceeds respiration, the system is autotrophic and internal production of organic matter dominates. When respiration exceeds primary productivity, the system is heterotrophic and relies on an outside source of organic matter. Diurnal changes in dissolved oxygen reflect system metabolism and have been used to determine primary productivity and respiration for many ecosystems (Odum 1956, Odum and Hoskins 1958, Mitsch and Kaltenborn 1980, Caffrey 2003). In most freshwater wetlands, phytoplankton is considered a minor component (Mitsch and Gosselink, 1993). In earlier studies of the coastal marshes along lake erie, phytoplankton was considered a significant, but not the major source of fixed carbon (Herdendorf, 1987). In an earlier study, Reeder (1990) determined that phytoplankton was the major source of carbon in Old Woman Creek estuary, resulting in a grazer based food web. Shallow, plankton-dominated wetlands, like Old Woman Creek (OWC) near lake erie, are ideal systems to use diurnal patterns in dissolved oxygen because, being shallow, they have a narrow euphotic zone with high chlorophyll and dramatic oxygen fluctuations (Mitsch and Reeder 1989). Coastal wetlands like OWC estuary protect lake erie by intercepting nutrients and other pollutants before they enter the lake. ecosystem metabolism may be linked to the estuary’s effectiveness as a nutrient sink or transformer (Klarer and Millie 1989 , Heath 1987, Mitsch and Reeder 1991). Changing water levels in Great lakes have had a dramatic impact on the flora in the adjacent coastal wetlands (Keddy and Reznicek 1986). During the years of high lake erie water levels (1970s-1999), the OWC estuary was primarily an open water system with aquatic macrophytes, particularly Nelumbo lutea covering about one-third or less of the surface of the estuary (Whyte 1996, Herdendorf et al. 2006). With declining lake water levels beginning in 1999, the estuary shifted from an open water system to one dominated by emergent vegetation (Trexel-Kroll 2002). Open water areas of the estuary declined from about 70% of the surface area of the estuary during the mid 1990s to less than 30% of the surface area in 2000 (Herdendorf et al. 2006). The drop in water levels and the resulting shift in vegetation should be reflected in the production/respiration ratio in the estuary. Before the drop in water levels, the estuary was dominated by open water and plankton communities (Reeder 1990; Herdendorf et al. 2006). After this drop, emergent macrophytes became the dominant community. This should result in a shift from a grazer based food web to a detrital based food web. In such a food web, the production/respiration ratio would be lower than in a grazer based system, as the primary source of organic carbon is outside the water column and therefore does not contribute to the measured production. It is hypothesized that with the drop in water levels in 1999/2000, and the subsequent dominance of aquatic macrophytes, the production/respiration ratio would decrease as the estuary shifted from a grazer based food web to one that was detrital based.