WHAT DIFFERENCE DOES A TINY CHANGE IN HYDROLOGIC BALANCE MAKE?

Cottonwood Lakes Area (CLA) in the prairie pothole region of North Dakota has been studied by USGS and other workers since 1967. We monitored two semi-permanent wetlands, P1 and P8, for 3 years (2001-2004) to see how different are climate records of two wetlands with a tiny difference in their hydrologic balance. P1 and P8 are separated by less than 50m residing in the same surroundings and under the same climate. The hydrochemistry of P1 and P8 are characterized as SO4>Mg>HCO3>Ca~Na. Water replenishment by snowmelt and loss by evaporation are the primary hydrologic controls, but groundwater is the main source of solutes supplemented by dissolution of previously precipitated salt minerals when water level is increasing. Depending on the climate and the season, P1 is partially surrounded by a groundwater trough or mound, becoming a groundwater discharge or recharge wetland although groundwater remains a minor component of the hydrologic balance (Rosenberry and Winter, 1997, J. Hydrol. 191:266-269). P1 responds directly to short-term changes in effective moisture and at different times prior to our study was fringed by cattails or became dry. P8 has minor but persistent groundwater recharge & discharge and surface water inflow & outflow. It hasn’t gone dry and is fringed by cattails. The groundwater buffers P8 from short-term changes in effective moisture, so it is a more stable and predictable environment relative to P1.

From July 2001 to July 2003, water and ostracode samples were collected biweekly to monthly from several fixed locations in each wetland for the total of 7 to 20 collections per location. Short sediment cores were also taken from P1 and P8. P1 water is consistently higher in δ¹⁸O by 2 to 5‰ and in TIC δ¹³C by 4 to as much as 12‰ compared to P8 due to smaller groundwater contribution. These differences are reflected in authigenic carbonate sediments, which can lead to a very different perceived climate. The P1 ostracode assemblages were composed of 1 abundant (C. vidua), 1 common (P. unicaudata), 1 present (C. renoensis) and 6 rare species. In contrast, P8 assemblages had 4 common (C. vidua, P. unicaudata, C. decora and C. serena), 3 present (C. compressa, C. renoensis and C. stagnalis) and 4 rare species. There appears to be a relation between ostracode population structure and physicochemical variability of water bodies.