What Makes a Lake Turn Green?
November 26, 2018 -
LEA Lake News-by Amanda Pratt and Ben Peierls, Summer 2018: NAPLES- The dreaded algae bloom-a thick green scum carpeting the water like astroturf. Itís what lake stewards the world over are trying to prevent. But in order to prevent blooms from happening, we need to have an appreciation and understanding of how and why they happen. It starts with the fuel that sparks the rampant growth of algae: nutrients such as phosphorus and nitrogen.
Most lawn fertilizers are made with nitrogen, phosphorus, and potassium because these elements are needed by plants to grow. Bloom-forming algae are basically tiny aquatic plants, so they react to fertilizers in a similar way to land plants. And because they are so small, they are able to grow rapidly as long as they have nutrients (particularly phosphorus).
Nutrients don't just come from fertilizer, although lawn fertilizer can be a major source if it is not carefully applied. In terms of quantity, most of the phosphorus that enters a lake comes from stormwater. In Maine, erosion from roads, driveways, and ditches provide the biggest source of nutrients to our waters. Poorly maintained septic systems can also leach nutrients along with bacteria and pathogens. These are examples of human-induced nutrient loading, but there are plenty of natural sources as well, from natural soil erosion to animal waste. Absent human influence, lakes generally remain in equilibrium with the natural sources of nitrogen and phosphorus they receive. However, human activities can greatly increase nutrient loads, throwing off the delicate balance within the lake.
Imagine a lake watershed that is largely forested, with a few cabins dotting its shoreline. One day, a large stand of trees in the watershed is cut and cleared for development, leaving a swath of bare soil. Soon after, a large rainstorm moves through. The area where the trees were harvested was on a slope, and as the rains wash over the bare ground, soil particles are dislodged and travel down the hill, forming a muddy rivulet that joins a small stream and its deposited directly into the lake.
The tiny soil particles carry phosphorus, some of which is immediately taken up by algae within the lake, while some sinks to the bottom of the lake. This concentrated pulse of phosphorus drives rapid algae growth, unlike natural sources of phosphorus which tend to be more diffuse and are released slowly into the water, which means less phosphorus is available for algae growth at any one time. This can be likened to a blood sugar spike: simple sugars (lawn fertilizers) are going to increase your blood sugar (phosphorus levels) more than complex sugars (decomposing leaves) will.
In freshwater systems the active form of phosphorus-the one algae need to grow-is recycled rapidly and therefore very scarce. This means that any new phosphorus added to the system is taken up very quickly. In fact, we measure phosphorus in parts per billion-that's how little there is in the water. We worry less about nitrogen because, although it is more readily available, it cannot be utilized for growth without sufficient phosphorus present. By limiting phosphorus inputs, we stop algae from getting all the necessary materials needed for growth.
At LEA, we directly and indirectly measure both phosphorus and algae in our lakes using several methods. We measure total phosphorus and chlorophyll-a in the upper layer of each lake's water column. Chlorophyll-a is the green pigment in algae. Higher chlorophyll-a concentrations indicate higher levels of algae. We also use a fluorometer to measure chlorophyll fluorescence at discrete depths throughout the water column, which gives us an indication of where algae are concentrated. While chlorophyll-a and fluorometry are proxies for algae concentration, that is only part of the story. Several local lake associations provide support for LEA to count and identify algae in select lakes. Knowing what kinds of algae are present in lakes over time allows us to assess water quality with a greater level of detail.
Some lakes, despite high nutrient levels, do not experience algae blooms. There are a huge array of factors that can affect how phosphorus impacts a lake ecosystem. The Maine DEP and researchers at the University of Maine in Orono are currently working on a Lake Vulnerability Index to try ti rank lakes that are most at risk of algae blooms. factors that affect lake vulnerability include lake area and volume, watershed area, watershed land use, lake sediment chemistry, dissolved organic carbon levels, stratification patterns, and flushing rate.
LEA staff are staying informed about the Vulnerability Index, especially as it can be applied to lakes in our service area. Along with our continuing testing and advanced data collection, the Index will provide insights into which or lakes are most at risk of "turning green" and what we can do about it.
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