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Summertime in the BelgradesContentsfor Printing Article Summaries |
What's The Deal With Phosphorus?by Peter Kallin At this time of year, I go to a lot of lake association and road association annual meetings to talk about water quality. I frequently get asked some variant of the question, "What's the deal with phosphorus?" We have previously discussed the concept of "eutrophication," or nutrient enrichment. Even with fully intact upland forests and vegetated wetland buffers to control sediment erosion, some sediment and nutrients are going to enter the lake. These are natural processes and a certain level of nutrients is necessary to support aquatic life. The problem comes when nutrient loading exceeds what the lake really needs to maintain a healthy aquatic ecosystem. This situation is analogous to maintaining a healthy diet and active lifestyle. If you consistently overeat, you will gain weight and suffer other health problems including premature aging. To remain healthy, you need to limit your caloric input to what your metabolism requires. It is the same with lakes. A lake will normally exist in a relatively clean state for tens of thousands of years with the healthy nutrient loading provided by Mother Nature. But if the nutrient load is excessive, that clean stage of life, which limnologists refer to as "oligotrophic" or "mesotrophic," can be reduced to less than 100 years. To keep our lakes from premature aging, we need to put the lakes on a diet. In Maine, the key to protecting water quality is limiting phosphorus. This week I will explain the concept of a "limiting nutrient" and why controlling phosphorus is important. Aquatic ecosystems vary in complexity but are all based on a food chain that has phytoplankton and other plants at the bottom. Phytoplankton are minute floating plant cells that create biomass through the process of photosynthesis. Using energy from sunlight, algae form plant cells by combining carbon dioxide (CO2) and water (H2O) with some nitrogen (N) and phosphorus (P). In the 1950's, an oceanographer named Redfield studied marine algae and determined that on average they were comprised of carbon, nitrogen, and phosphorus in a ratio of 106:16:1. For freshwater algae, the ratio is 113:15:1. This means that for every 113 molecules of carbon dioxide the plant uses, it needs 15 molecules of nitrogen, and one molecule of phosphorus to create plant biomass. Since light, carbon dioxide and nitrogen are in ample supply in most freshwater systems, the amount of plant growth that can take place is limited by the amount of phosphorus available. Phosphorus is the "limiting nutrient" in the system. Think of a car manufacturer who needs a ton of steel and four tires to manufacture a car. Even with twenty tons of steel, if he only has four tires, he can only make one car until he gets more tires. He is limited by his tire supply. In the same way, the productivity of freshwater aquatic systems is generally limited by the amount of phosphorus available. Phosphorus is critical to life and helps organisms store energy. It exists in multiple oxidation states and readily combines with oxygen. Under aerobic conditions, phosphorus is not especially soluble in water so precipitation percolating or filtering through the soil will not dissolve much phosphorus even if it is relatively abundant in the soil, as it is around here. Most of the phosphorus in the soils is strongly bound to iron or aluminum minerals under normal oxygenated conditions and these minerals don't give up their phosphorus easily. It is a different story however if the soil is eroded and washes into the lake. Once in the lake, some phosphorus rich sediments settle to the bottom but soluble, bioavailable phosphorus in the water column is quickly taken up by some type of algae, either green or blue-green, which reduces water clarity, or perhaps by macrophyte (large) plants. These algae will be grazed by zooplankton, which in turn will be eaten by fish, transferring the phosphorus up the food chain. Some will be returned to the water through excretion, starting the cycle again, but some will be washed downstream, be removed by human or other fishermen, or settle to the bottom where it is at least temporarily not "bioavailable." Limnologists refer to these various places phosphorus can be as "compartments" and phosphorus is always cycling between compartments. The bottom line is that some phosphorus is always coming into the lake, some is being stored as biomass, and some is always leaving. If more is coming in than going out, phosphorus will accumulate and plants and algae will thrive. It's a bit like tracking the amount of money in your check book. Some cash is always coming in and some is leaving and the actual amount in your account will vary over time. If you have more coming in than going out, your account will grow and you can afford to spend more. The most complicated situation for our lakes occurs in the summertime when the surface waters warm and the lake stratifies into a warm surface layer called the "epilimnion" and a colder, deeper layer called the "hypolimnion." I'll talk a little bit more about why this happens in future columns, but anyone who swims in the lakes in the summer and dives down until the water suddenly changes temperature has experienced it firsthand. Under these thermally stratified conditions, there is little mixing between the layers and the oxygen that is entering the lake from the atmosphere never makes it to the bottom of the lake. At the same time, organic matter continues to fall to the bottom and uses up oxygen as it decomposes. As a result, the bottom waters go anoxic (no oxygen). Under these conditions, the iron in the sediments is reduced from the ferric (+3) form, which is nearly insoluble, to the ferrous (+2) form, which is highly soluble. When the iron dissolves, the phosphorus that had been bound up in the non bioavailable particulate state is suddenly released into the water column as bioavailable SRP. If a mixing event occurs such as a sudden cooling of the surface water or a strong wind that can cause upwelling, this bioavailable phosphorus is suddenly stirred into the upper epilimnion where it can trigger an algae bloom because the limiting nutrient is no longer limited. This is when East Pond suddenly turns green or the swimmers in Great Pond start complaining about Gloeotrichia. This phosphorus loading from the sediments is called "internal loading" and is difficult to control. In future columns we will discuss ways of taking advantage of Mother Nature's natural systems to reduce phosphorus loading. In the mean time, do everything you can to reduce erosion and prevent sediment from entering the lakes. In simple terms, erosion control equals phosphorus control. If you have shorefront property and want to reduce the amount of phosphorus washing into the lake from your property, please contact the BRCA Youth Conservation Corps Director, Clark Freeman, at (Updated: June 24, 2011) Peter Kallin is Executive Director of the Belgrade Regional Conservation Alliance (BRCA). He can be contacted at | ||