IDENTIFYING HYDROLOGIC VARIABILITY WITHIN WETLANDS USING STABLE ISOTOPES AND OSTRACODES

Isotopic analysis of water and biota in wetlands can reveal a hydrologic complexity not apparent in the major ion hydrochemistry. In a study of Mantua Bog, a fen wetland in northeast Ohio, major ion hydrochemistry, oxygen and carbon isotopes, and ostracode shell chemistry were measured biweekly from July to November of 1999, along a groundwater flow path. Sampling stations included a perennial spring in a wooded upland, a wet meadow with no standing water, a cattail marsh, and a small unnamed stream. Although the major ion hydrochemistry showed no significant differences from station to station or throughout the sampling period, the isotopes and ostracode fauna showed significant differences. d13C values were higher by up to 2 per mil in the spring, wet meadow, and stream, where groundwater discharge predominates, than in the cattail marsh. Considerable decay of cattail plants (Typha latifolia) occurs in the cattail marsh, and this constant return of carbon-12 from decaying plants to the water may lead to lower d13C values. Ostracode species assemblages varied with the sampling stations, including Cavernocypris wardi, Cypridopsis okeechobei, and Candona crogmaniana in the spring, Microdarwinula species and Scottia browniana in the wet meadow, Candona compressa and Eucandona euplectella in the cattail marsh, and Physocypria globula in the stream. The oxygen isotopic composition of the ostracode shells shows how the spring and stream have consistently lower d18O values (-7.13 to -7.38 per mil) with less variability than the wet meadow and cattail marsh (-4.68 to -7.38 per mil), where variable moisture content and evapotranspiration play a greater role. The range of variability for d18O of shells from a single species is only 0.4 per mil in the stream and spring, whereas in the wet meadow and cattail marsh it is 2 per mil. The ostracodes record the expected variability in the oxygen and carbon isotope composition of water in microenvironments in a wetland, along a hydrologic flow path.