Watershed Rules of Life
Rivers are the circulatory systems of the continents, delivering land-derived water, pollutants and elements to the ocean. They are also sites with unique organisms, food webs, and microbial communities that can transform these materials during transport to the ocean. Microbes are the engines for these transformations, with considerable capacity to remove or alter important elements (e.g., carbon, nitrogen, phosphorus, mercury, arsenic), produce greenhouse gasses, and support food webs. However, due to large changes in water flow from day to day, and the big differences in the types of streams and rivers across the landscape, understanding how riverine microbes impact the chemistry of rivers is difficult. This project will research a set of "Watershed Rules of Life" that govern the establishment of riverine water column microbial communities that are critical to understanding rivers as circulatory systems.The project targeted the emergent property of microbial functional trait distributions across the physical templates of watersheds and in response to hydrologic variability. The project sampled the Connecticut, Deschutes, Gunnison and Willamette watersheds, which are similar in size but differ in climate and geomorphology. Within each watershed, sampling focused on streams of different sizes, and the impact of reservoirs and hydrologic variability. The proposed research developed mathematical expressions that relate multiple indices for the development and activity of planktonic microbial communities to stream order, stream hydraulics (e.g., velocity and depth), flow variability, and temperature. These indices are based on metagenomics for genomic diversity, metatranscriptomics for gene expression, and microbial 16S rRNA gene amplicon sequencing for phylogenetic diversity in riverine samples. Microbial indices were compared to modeled estimates of the time scale for water column microbial community development (i.e., residence time) and microbial transformation rates (e.g., denitrification). It hypothesized that geomorphology (e.g., depth, velocity), discharge variability, and temperature are the main determinants of when and where distinct planktonic microbial riverine communities emerge in the water column of river networks.