Abstract
Secchi depth (SD), a primary metric to assess trophic state, is controlled in many lakes by algal densities, measured as chlorophyll-a (chl-a) concentration. Two other optically related water quality variables also directly affect SD: non-algal suspended solids (SSNA) and colored dissolved organic matter (CDOM, expressed as the absorption coefficient at 440 nm, a440). Using a database of ~1,460 samples from ~625 inland lake basins in Minnesota and two other Upper Midwest states, Wisconsin and Michigan, we analyzed relationships among these variables, with special focus on CDOM levels that influence SD values and the Minnesota SD standards used to assess eutrophication impairment of lakes. Log-transformed chl-a, total suspended solids (TSS), and SD were strongly correlated with each other; log(a440) had major effects on log(SD) but was only weakly correlated with log(chl-a) and log(TSS). Multiple regression models for log(SD) and 1/SD based on the three driving variables (chl-a, SSNA, and CDOM) explained ~80% of the variance in SD in the whole data set, but substantial differences in the form of the best-fit relationships were found between major ecoregions. High chl-a concentrations (> 50 μg/L) and TSS (> 20 mg/L) rarely occurred in lakes with high CDOM (a440 > ~4 m−1), and all lakes with a440 > 8 m−1 had SD ≤ 2.0 m despite low chl-a values (<10 μg/L) in most lakes. Further statistical analyses revealed that CDOM has significant effects on SD at a440 values > ~ 4 m−1. Thus, SD is not an accurate trophic state metric in moderately to highly colored lakes, and Minnesota's 2-m SD criterion should not be the sole metric to assess eutrophication impairment in warm/cool-water lakes of the Northern Lakes and Forest ecoregion. More generally, trophic state assessments using SD in regions with large landscape sources of CDOM need to account for effects of CDOM on SD.
| Original language | English (US) |
|---|---|
| Article number | e01871 |
| Journal | Ecological Applications |
| Volume | 29 |
| Issue number | 3 |
| DOIs | |
| State | Published - Apr 2019 |
Bibliographical note
Funding Information:This work was supported in part by National Science Foundation grant (CBET 1510332), the Minnesota Environmental and Natural Resources Trust Fund, as recommended by the Legislative-Citizen Commission on Minnesota Resources, the Minnesota Pollution Control Agency's lake water quality assessment program, and the University of Minnesota's Office of the VP for Research and Retirees Association, U-Spatial Program, Sea Grant Program, and Agricultural Experiment Station. We thank numerous collaborators, research staff, and students for assistance in sample collection and analysis and colleague Marvin Bauer for advice and encouragement throughout the larger project, of which this paper is a component.
Funding Information:
This work was supported in part by National Science Foundation grant (CBET 1510332), the Minnesota Environmental and Natural Resources Trust Fund, as recommended by the Legislative-Citizen Commission on Minnesota Resources, the Minnesota Pollution Control Agency’s lake water quality assessment program, and the University of Minnesota’s Office of the VP for Research and Retirees Association, U-Spatial Program, Sea Grant Program, and Agricultural Experiment Station. We thank numerous collaborators, research staff, and students for assistance in sample collection and analysis and colleague Marvin Bauer for advice and encouragement throughout the larger project, of which this paper is a component.
Publisher Copyright:
© 2019 by the Ecological Society of America
Keywords
- Secchi depth
- Upper Midwest
- chlorophyll a
- colored dissolved organic matter
- dissolved colored organic matter
- ecoregion
- lakes
- total suspended solids
- trophic state
