HYDROLOGIC HETEROGENEITY WITHIN WETLANDS LINKED TO OSTRACODE SPECIES DISTRIBUTIONS: BIOLOGICAL SIGNALS OF GROUNDWATER-SURFACE WATER INTERACTIONS

Wetlands are characterized by a mosaic of ground water - surface water interactions that can often be highly variable in time and space. Even small wetlands can show a variety of recharge and discharge areas, ephemeral or perennial, with a range of flow rates. Ostracodes (microscopic crustaceans, 500 to 700 um average length) occupy microhabitats within wetlands that can be defined and distinguished by their physical hydrology, although not by their major ion hydrogeochemistry. In studies of fen wetlands located in one of the most extensive glacial sand & gravel aquifers in Ohio, we have found that the hydrologic heterogeneity of a wetland can be identified by its ostracode assemblages, and vice versa. In perennial rheocrene settings within the fens where flow rates are relatively constant and water temperature approaches mean annual air temperature, Cavernocypris wardi, Cypridopsis okeechobei, Ilyocypris bradyi, Candona crogmaniana and Microdarwinula species are found. Within the same fens, in locations where groundwater seepage from shallower sources is combined with recharge, a different faunal assemblage is identifiable, including Cypridopsis vidua, Scottia browniana, Candona compressa, Darwinula stevensoni, and Physocypria globula. Streams within the wetlands are characterized by Ilyocypris bradyi and Physocypria globula, and recharge dominated sites are marked by Cypria opthalmica, Cypridopsis vidua, and Physocypria globula.

Although sites within each wetland are not distinguishable on the basis of major ion hydrochemistry, they are distinguished from one another on the basis of the proportion of groundwater discharge and flow rate, the oxygen isotope signature, and the ostracode faunal assemblage. In summary, hydrologic dynamics within wetlands can be identified and monitored by their ostracode faunal assemblages and associated isotopic signatures.