Undergraduate Honors Theses

Thesis Defended

Spring 2018

Document Type


Type of Thesis

Departmental Honors


Ecology and Evolutionary Biology

First Advisor

Boswell Wing

Second Advisor

Erin Tripp

Third Advisor

Barbara Demmig-Adams


Gnammas are potholes in solid bedrock that periodically fill with rainwater to form ephemeral pools. In order to understand the ecological and evolutionary processes at play in these dynamic ecosystems, the spatial and temporal controls on gnamma hydrology must be constrained. Previous studies on the relationships between hydro-regime and biota in gnamma environments posit evaporation as the sole process of water loss. I evaluated this assumption about gnamma water balance via stable isotope analysis of gnamma waters in the Colorado Rocky Mountains. My study aimed to constrain the processes of water loss in gnammas; that is, whether gnammas lose water over time through evaporation, seepage through the rock substrate, or a combination of both processes. I conducted two week-long experiments with water samples taken from a suite of gnammas of a variety of sizes. The stable isotope compositions of the water samples were interpreted with a derivation of Craig and Gordon’s (1965) evaporative fractionation model. Water loss in gnammas was found to be attributable to a combination of both evaporation and seepage, with ~10 - 30% of water lost by means of evaporation. Although previously overlooked, water loss via seepage through the rock substrate plays a substantial role in the water balance of gnammas. From this new understanding of gnamma water balance, I developed a conceptual model of gnamma development that highlights the importance of aridity and the residence time of water in the gnamma. This model also predicts that the environmental controls of gnamma development should vary as a function of gnamma size. The constraints provided here on gnamma hydrology should allow for the application of gnammas as model systems to explore species richness-area relationships, insular biogeography concepts, and other geobiological hypotheses.