Nutrient Pollution in Headwaters

Canopy photos from a plant-based ("autotrophic") headwater stream (left) and a light-limited detritus-based ("heterotrophic") headwater stream (right).

My students and I are just beginning to explore the effects of anthropogenic nutrient pollution on headwater spring and stream food-webs. In a world where increasing urbanization and its associated nutrient pollution pose a global threat to aquatic ecosystems, most of what we know about the impact of these nutrients comes from studies in what ecologists call “autotrophic” habitats, where plants make up the base of the food web. We know strikingly little about the effects of such nutrients on light-limited detritus-based (or “heterotrophic”) habitats where microbes are the base food webs. Detritus-based systems include shaded headwater streams and springs that are essential to river-system function and have their own unique and specialized fauna, including threatened amphibians. Headwaters are often hotspots of endemism but are afforded little legal protection under the Clean Water Act and, therefore, are of particular conservation concern.

The Georgetown Salamander (Eurycea naufragia) inhabits light-limited detritus-based springs in Williamson County, Texas

Increased nutrient concentrations in ecosystems due to human activities – anthropogenic nutrient loading (ANL) – is a phenomenon affecting aquatic systems worldwide at an alarming rate. In autotrophic (plant-based) systems, ANL can alter freshwater and marine habitat by converting clear (or, “oligotrophic”) systems to turbid (or, “eutrophic”, murky) systems. Eutrophication can change the aquatic environment by increasing sedimentation, temperature and turbidity, and by lowering oxygen concentration. It can also have profound effects on food webs biological communities. Dramatic changes in food webs of headwater streams is especially concerning if springs are home to endemic or endangered species, such as the Eurycea salamanders than inhabit headwater springs of central Texas.

My student Travis Bartholomew at a shaded, detritus-based spring at the Brackenridge Field Laboratory in Austin, Texas.

My students and I have begun preliminary ecological comparisons of a pair of headwater stream sites in Travis County, Texas that will help us to refine methods and provide a baseline for studying a larger suite of pristine vs. polluted headwater springs in the near future. We are comparing water quality, invertebrate diversity and abundance and rates of leaf-litter breakdown at one light-limited detritus-based site at the Brackenridge Field Laboratory in Austin, Texas and one unshaded autotrophic site at the Bee Caves Ecology Laboratory. This work has been generously funded by the Bee Caves Ecology Endowment Award.

Here is Travis collecting leaf litter bags from our unshaded autotrophic headwater site at the Bee Caves Ecolab

I am currently collaborating with students Travis Bartholomew, Elena Konovalova, Kofi Amoako and Marie Gibson on a manuscript for peer review on the results of this research project.

Gillespie H, Bartholomew T, Konovalova E, Amoako K, Gibson M. In Prep. The role of autotrophic and heterotrophic respiration in nutrient-enriched light-abundant vs. light-limited headwater springs.