FROM BROWN TO GREEN: LONG-TERM TRENDS IN PARTICULATE ORGANIC AND INORGANIC MATTER IN MISSOURI RESERVOIRS

Contrasting with the trend observed in Northern lakes, visible changes in the color of Missouri reservoirs have resulted in a transition from brown to green waters; however, the root cause of this change is unknown. We analyzed two long-term water quality monitoring programs to investigate decadal trends (10-35 yrs) in the ratio between particulate organic matter (POM) and inorganic matter (PIM) to total suspended solids (TSS) in 158 reservoirs that span a broad anthropogenic gradient in land cover in the Midwest USA. We observed a statistically significant increase in the POM/TSS (i.e. greening) in 42 reservoirs with an average rate of 0.014 yr^-1. Boosted Regression Tree analysis reveals that the shift toward greener waters was observed in reservoirs likely experiencing increasing transparency and total nitrogen concentrations, mediated by warming surface water temperatures, watershed to reservoir area ratio (W_area/R_area) and precipitation anomaly (PDSI; Palmer Drought Severity Index). However, the relative importance of the drivers differed depending on the land cover type. In the northern agricultural reservoirs, surface water temperature emerged as a key factor, while in the southern forest reservoirs, the effect of total nitrogen (TN) was more pronouced. Additionally, in urban and mixed reservoirs, W_area/R_area, TN and PDSI together had the largest relative influence. Our findings reveal that the phenomenon of shifting lake water color is not yet widespread in Midwest reservoirs but may likely represent the future of these regional aquatic ecosystems.

Primary Presenter: Lorena Pinheiro-Silva, University of Maryland Center for Environmental Science / University of Missouri-Columbia (lsilva@umces.edu)

Authors:

Lorena Pinheiro-Silva, University of Maryland Center for Environmental Science and University of Missouri-Columbia  (lsilva@umces.edu)

Greg Silsbe, University of Maryland Center for Environmental Science  (gsilsbe@umces.edu)

David Richardson, State University of New York at New Paltz  (richardsond@newpaltz.edu)

Rebecca North, University of Missouri-Columbia  (northr@missouri.edu)