The three dominant colors of freshwaters - blue, green and brown - are important ecosystem indicators. IES scientists are discovering how these conditions arise and are maintained, and their research findings are providing new ways of thinking about lakes and their watersheds, as well as new tools for managers charged with protection of vital water resources.
Brown Lake: Tuesday Lake, Michigan  Not all water is the same color, as even the most casual observers of lakes, ponds, streams will agree. Clear water that often appears blue from a distance is the most pleasing to the human eye - fabled clear waters like Lake Tahoe and Crater Lake are prized for their beauty and purity. The Hudson River presents a different impression because its waters hold high concentrations of silt, and consequently are muddy brown. And then there is green: in lakes and ponds choked with algae, the photosynthetic pigments of these microscopic plants selectively absorb red and blue light so that the human eye sees just the remaining green light. Finally, it is not uncommon to find lakes and streams that appear brown or even black - something between tea- and coffee-colored - because they contain high concentrations of dissolved organic compounds that come from the decomposition of forest and wetland plants. Brown water lakes are quite common especially in areas like the Adirondack Mountains of New York.
These different colored waters indicate differences in basic functioning of aquatic ecosystems. IES scientists are investigating what factors control lake-water color, specifically asking what causes lakes to have blue, green, or brown water, and how do lakes change between these conditions? The scientists are also exploring ways to manage undesirable lake conditions; for example, is it possible to make a green lake, teeming with algae, turn brown? Or, even better, blue?
The addition of nutrients in the form of nitrogen and phosphorus has long been known to determine whether a lake is blue or green. Just as fertilizing a lawn makes grass green, increasing the input of nitrogen and phosphorus to lakes increases the concentration of algae suspended in the water and the rooted plants that live in shallow water. This greening of the waters is a common problem that is often related to human development in the watershed. Even Lake Tahoe has become greener over the last few decades because of development in the basin that has led to increased nutrient inputs.
Green Lake: Lake Magog, Quebec  The obvious solution to this problem is to turn off the tap of nutrient inputs. Managing such a change, however, can often be difficult or even in some cases practically impossible. Are there alternatives to reducing nutrients? IES scientists Michael Pace and Jonathan Cole have participated in a series of whole lake experimental manipulations designed to test if differences in fish populations in lakes can encourage population growth of specific herbivores that are particularly good algal grazers. By grazing excess algal growth, certain lake herbivores could turn green lakes a more desirable color. The presence or absence of these herbivores is influenced by predation from small but not large fish.
The manipulation experiments tested the idea that fish at the top of the food web can limit, by predation, the small fish and invertebrates that prey on the most effective herbivores. If top predator fish can limit smaller fish large populations of herbivores develop and effectively graze algae. The lake experiments supported this idea. When largemouth bass were present and the investigators added nutrients, the lakes did not turn green. Smaller fish were reduced, and the herbivores were able to control algae. In the absence of largemouth bass, however, lakes became enriched with algae.
The conclusion? Lakes turn green because of nutrients, but it is possible to manage food webs in lakes using fish to help control excess plant growth. This research provides a tool for managers who wish to limit phytoplankton in green lakes, especially when nutrient reductions or removals are difficult.
While green water lakes are often choked with plant growth, brown water lakes tell us a different story. They are typically mildly acidic with elevated mercury levels and high amounts of dissolved organic matter. Rain that is polluted with sulfuric and nitric acids can make these lakes so acidic that they become clear! Under these conditions, the compounds that give the lakes their brown color coagulate and precipitate. The clarity of the water exposes the aquatic organisms to harmful ultraviolet light. Inhabitants of low-light brown water lakes may be poorly adapted to this stress, and local population extinctions may result.
Brown water lakes tend to have fish with greater mercury contamination, a condition that is true even for remote wilderness areas of the Adirondack Mountains. The causes of this problem are complex, but are related to deposition of mercury from air pollution and mercury's tendency to "biomagnify," or increase in concentration as it moves through the food web.
IES scientists Michael Pace and Charles Canham are currently doing research on brown and blue lakes in the Adirondack Park region of New York. The goal of this project is to determine how landscape features and lake conditions interact to determine water color. Their study involves over 500 watersheds that have been carefully mapped by the Adirondack Park Agency using aerial photography and satellite imagery. A computer model has been developed that uses the data on land cover to predict the amount of dissolved organic matter in lakes and the fate of this material within lakes.
This work has led to new knowledge about the controls of brown and blue lake color. For example, wetlands produce more dissolved organic matter per unit area than upland forests. Upland forest areas are still important sources of organic matter to lakes because they are the dominant land cover type in most watersheds. Small lakes in big watersheds can be brown even though there may be few wetlands because upland forests produce enough organic matter to color the water. The modeling approach developed in this study provides a tool for large-scale land managers, such as the Adirondack Park Agency, to assess how development or climate change might impact water resources. |
