Before settlement, the ponds on Cape Cod had few nutrients. That's because the sandy soil was rather sterile--nutrients blew or washed out to sea before they could get into the ponds.
Few nutrients in the ponds means very clear water--and good swimming, because you don't get tangled up in aquatic plants. With low levels of nutrients, here aren't as many plants fueling the food chain, so there are fewer fish.
Nitrogen and phosphorus are the nutrients that threaten Hawksnest Pond. They come from the air, the groundwater, and any organic matter that gets into the lakes--such as human or pet waste, or muddy runoff from bare land or paved areas. Before settlement, there was very little overland runoff. Vegetation around the shore filtered any rainwater that did flow in.
Since settlement, household septic systems have leaked nutrients--about 70% of the nitrogen in Cape Cod groundwater comes from septic systems. Nitrogen flows as fast as the groundwater--about a foot a day--but phosphorus creeps more slowly. Nutrients from septic systems eventually reach the ponds, depending on the direction of local groundwater flow.
Nitrogen and phosphorus (N and P) behave rather differently. N is readily soluble in water, but P is not. So the P tends to sit in bottom sediments, getting recycled back into the water from time to time. Runoff can't carry P to a pond unless the P attaches to tiny soil particles. In other words, P has to hitchkike on muddy water. And, the soil stores phosphorus--that's why soil erosion is so dangerous to water quality. Once P gets into a pond, it's very difficult to get it out of the sediments.
P is usually the problem in freshwater ponds, while N is the problem in estuaries. When levels of P are high relative to N, that's a condition that favors toxic algae blooms.
Nutrients have two harmful effects on ponds.
- They stimulate the growth of plants, making water sports less enjoyable.
- They unbalance the lake, in favor of microscopic algae (and dangerous blue-green algae). You get "boom and bust" cycles in the pond. The algae grow explosively, then die off. The rotting remains of the algae stink, and use up all the oxygen, often causing die-offs of fish.