Lake Sagamore is in good health. Yet because of its small size and “softness,” it is relatively vulnerable. Two main factors protect our fragile lake: first, the care taken by residents to minimize human impact; and second, the fact that the lake watershed — an area of some 12 square miles extending beyond the Taconic Parkway — is almost entirely covered with healthy forest. (Also, see Handbook, Part II, Section C.)

The ecology of the Lake in recent years has been fairly good. Water quality has remained within the healthy limits for a lake such as ours, allowing the food chain to function normally. The primary recyclers and producers – bacteria and photosynthesizing algae – support abundant and diverse consumers, from tiny freshwater crustaceans to otters.

Submerged water weeds are a natural growth, but in large numbers they limit the ability of residents to enjoy the lake. To the extent that the Lake Association acts to reduce weed growth, we are choosing to upset a natural process. Whether this is desirable is an ethical question, and it cannot be denied that some organisms benefit even as others are harmed. The rapid increase in weeds over the past six years, however, is a striking ecological shift, and the plant ecology itself has also shifted, from mixed stands with milfoil, Vallisneria and Elodea, to almost 100 per cent Potamogeton illinoensis , a native pondweed. At present, the Association is attempting to control weed growth, as already noted (see Handbook, Part II Section C).

Acidity and hardness

Acidity is expressed on a Ph scale from 0 to 12, acid to alkaline, with “neutral” at 6.0. The lake water regularly measures slightly alkaline, between 6.7 to 7.0, due to the relatively high level of free oxygen and bacterial activity in the water. The mild alkalinity is normal, and indicates that “acid rain” from airborne pollution is not currently a factor. On the other hand, the level of dissolved lime in the water is quite low, around 40 mg/L (where 60 mg/L defines “soft” water), indicating that there is very little capacity to neutralize any acid input. For this reason the lake remains highly susceptible to acidification, either by eutropification (increased decay of vegetable matter in the lake due to overproduction — see below) by significant disturbance to the forest leading to increased decay in the watershed, or by regional air pollution.

Nutrients and oxygen

The basic problem faced by living lakes is to keep the cycle of growth and decay in equilibrium. The major destabilizing factor is the supply of nutrients that support the growth of one-celled photosynthesizers – not plants, but blue-green bacteria, diatoms, cladophora, and dinoflagellates, collectively known as algae – at the base of the food chain. The nutrients of greatest concern are the two so-called “limiting” nutrients — nitrate (NO3) and phosphate (PO4). Other essential nutrients (iron, sulfur, aluminum, calcium, magnesium, organic matter) are always in abundant supply in normal lakes, so it is the abundance of the first two that sets the pace for the entire biological activity of the lake. Studies in 2001 and 2003 show that most of the algae in the lake are types found in nutrient-rich water. This is a danger sign and reflects the fact that while Lake Sagamore presently has a low nitrate level, it has a phosphate level that is not far below maximum for a healthy lake. The fact that the water is oxygenated all the way to the bottom, however, is an encouraging indication that the lake is not in immediate danger of eutropification.

Eutropification

Eutropification is the head-on train wreck of lake ecology caused by overabundant nutrients. Lakes can vary widely in water quality, however, and still stay “normal” as long as production, consumption and recycling stay in balance. But if there are high levels of the limiting nutrients, nitrates and phosphates, oxygen levels will be lowered by decay of overabundant algae. When that occurs, seasonal overturn can lead to a massive algal bloom. If the mass of dead algae is great enough, the normal recycling bacteria use up all available oxygen — in a lake already low in oxygen they will use it up faster — and then die in turn. The job of recycling all that dead matter then goes to bacteria that do not require oxygen. These are called anaerobic, or airless, bacteria. These are the same bacteria that inhabit sewers, septic systems and, in fact, our intestines. These anaerobic bacteria generate methane gas as an essential part of their metabolic cycle. This entire process is eutropification. The critical consequence of eutropification — besides the smell — is the death of all the oxygen-dependent life in the water.

Limiting those nutrients, nitrates and phosphates, is critical if eutrophication is to be avoided. Lawn fertilizer is of course loaded with these same limiting nutrients — nitrates and phosphates. Residents should carefully consider the tradeoff between lush lawns and an open sewer for a lake when deciding how much, where, or even whether to fertilize. Faulty septic systems that are loaded with phosphate-rich detergent and nitrogenous household manure can also contribute to potentially disastrous nutrient overloads.

Sedimentation

In 2004, the Association commissioned its first full-scale hydrological survey of the lake bottom to determine which parts of Lake Sagamore might need dredging. Much to our surprise, the engineers reported that accumulations of sand and clay sediment in the 60 years since the lake was created were negligible in all parts of the lake, except in very local areas where road material had washed in. “Muck” (organic sludge and fine clay) was little more than a foot deep even in the most protected coves. The conclusion we draw from these findings is that disturbances due to building, gardening, and waterside landscaping around the lake have not been significant, to date, in adding sediment to the lake floor. We can also conclude that erosion in our forested watershed is almost nonexistent and that as long as this protective cover remains relatively intact, Lake Sagamore will not have a serious sedimentation problem requiring disruptive and expensive remediation.