About How Our lake Works: More About the Lake Water Quality

ABOUT HOW OUR LAKE WORKS: MORE ABOUT THE LAKE WATER QUALITY

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.)



A quick look underwater – a two-layered lake

Lake Sagamore, with maximum depths of around 18 feet, is a persistently stratified lake. Every summer, a layer of warm, well-aerated, sunlit water forms on top of the cooler and thus heavier water at depth. The two layers mix somewhat during storms, when wind and rain move things around, but the disturbance never reaches the depths and the layers soon reform. During the winter, the lake is also stratified, but this time with the coldest water at the top, not the bottom. In the spring and again in the fall, as you might therefore expect, the cold and the warm trade places. This occurs because water is at its densest at 39 degrees F, seven degrees above freezing. It expands like a normal substance as it warms from this point – but it also expands as it cools. When it freezes it expands even more (which is why ice floats). Thus, as the sun warms frigid Lake Sagamore in the spring, the surface water gets heavier, and sinks to the bottom! This brings bottom water to the top, where it warms up in its turn, and sinks. This goes on for several weeks, and is called the “spring overturn.” Eventually the whole lake gets to 39 degrees, and overturn stops. Since the surface layer can’t sink any more, it just keeps heating up and getting even less sinkable — and the two layers are formed for the rest of the season. Actually, the bottom water also warms up, but more slowly, so that in late summer the bottom water may be up to 65 degrees while the surface layer comes close to 80. In the fall, the water on top cools down, and a “fall turnover” proceeds until the whole lake is once more at the same magic temperature of 39 degrees. From this point on, the top layer rapidly gets colder and less dense, while the bottom layer cools more slowly, in a mirror image of the summer stratification.

 

The depth to which sunlight warms the lake depends on the clarity of the water, which in turn depends on nutrients, because turbidity is not due to suspended dirt and mud but to the green clouds of one-celled algae (mainly diatoms) that float in the upper few feet of water. Lake Sagamore is not overly nutritious, so that algae are usually not a nuisance. During overturns, however, nutrients that have been out of reach all season are brought into the surface zone and the result is an “algal bloom,” which only dissipates after the water stratifies. In early summer the thermocline between warm and cold lies at a depth of around four feet, but the warm layer thickens over the summer and the thermocline sinks to around nine feet by end of August. (Swimmers who find their feet swishing in cold water as they tread may also be hovering over one of the springs in the lake floor.)



Ecological balance

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.