At our study site on the Nyack floodplain of the Flathead River, Montana, large numbers of aquatic invertebrates, from microscopic copepods to large stoneflies and crustaceans, are found in groundwater pumped from wells distributed across the floodplain. Compared to the nearby river channel, which receives solar energy and food from algal production and terrestrial sources such as leaves, the groundwater must import food and nutrients from the river or through the floodplain soil. Our research has shown that food resources in the groundwater are very low. So, how can so many invertebrates live in an area with so little food? To address this question, we are developing a carbon budget for the groundwater system investigating the various organic matter sources; from river water, from overlying soils, or as buried plant matter as the river shifts its channel over time. This organic matter is consumed by microbes, which in turn feed the invertebrates. With funding from this proposal, we shall refine our understanding of the organic budget in two ways: through (1) respiration experiments, (consumption of oxygen to produce carbon dioxide by invertebrates and bacteria & fungi), and; (2) use of stable isotopes of carbon and nitrogen (that are unique to river vs. soil sources). The respiration experiments will let us calculate the energy used by various organisms and compare these to measurements from water samples taken from across the whole aquifer. In another experiment, using a constructed groundwater chamber, we will use respiration to tell us how strongly the organisms respond to carbon additions from soil or river water sources.
Intellectual Merit: this proposal will significantly increase our understanding of the basic biology of organisms that live in river-connected river floodplain aquifers.
Broader Impacts: In the west, mountain valleys and floodplains are where human population, agriculture, and drinking water overlap. By looking at a pristine aquifer, the research will tell us how groundwater systems work in ways important to understanding human impacts
