What do we look for?
Lake users are commonly concerned about the amount of aquatic plant growth in the lake shallows, and the increase in density and abundance of microscopic plant “algae” growth (detected as greener water), and water transparency decreases. This is knows as “eutrophication,” a natural process by which all lakes age and progress from clear, pristine lakes to green, nutrient enriched lakes on a geological time frame of thousands of years. Much like the fertilizers applied to lawns, nutrients that enter our lakes stimulate plant growth and culminate in greener (and in turn less clear) waters.
Lakes are categorized into what are known as “trophic states”, or levels of lake plant and algae productivity.
Oligotrophic lakes are considered “unproductive” pristine systems and are characterized by:
- high water clarities,
- low nutrient concentrations,
- low algae concentrations,
- minimal levels of aquatic plant “weed” growth,
- and high dissolved oxygen concentrations near the lake bottom.
Eutrophic lakes are considered “highly productive” enriched systems characterized by:
- low water transparencies,
- high nutrient concentrations,
- high algae concentrations,
- large stands of aquatic plants,
- very low dissolved oxygen concentrations near the lake bottom.
Mesotrophic lakes have qualities between those of oligotrophic and eutrophic lakes and are characterized by:
- moderate water transparencies,
- moderate nutrient concentrations
- moderate algae growth, moderate aquatic plant “weed” growth, and
- decreasing oxygen levels.
All lakes will naturally exhibit varying degrees of productivity, which can be a good thing for a robust fishery, where bass will thrive in a more mesotrophic environment, but trout need cold, clear, clean waters. Newfound Lake is still considered an oligotrophic and pristine lake.
However, human related activities (cultural eutrophication) can augment the aging process and result in a transition from a pristine system to an enriched system in a matter of years rather than the natural transition over thousands of years. Cultural eutrophication is particularly a concern for northern New England lakes where large tracts of once forested or agricultural lands are being developed, with the potential for increased sediment and nutrient loadings into our lakes.
We use a Secchi Disk to measure the water transparency. Light penetrates deeper if there is little dissolved and/or particulate matter. So, the deeper the depth of Secchi Disk disappearance, the more transparent the lake water. Transparencies to greater than 4 meters (approximately 12-14’) are indicative of an oligotrophic lake. As a deep, spring-fed, geological lake, Newfound is known for its clarity to depths over 5 (?) meters. However, this measurement will be affected by large storm events that wash a lot of soil into the lake, such as in the spring of 2006 after the “Mother’s Day floods.”
The chlorophyll a concentration is a measure of the standing crop of phytoplankton, and is often used to classify lakes into categories of productivity, called trophic states. Greater levels of the point sample, in conjunction with microscopic examination of the samples confirm the presence of such a population of algae. These populations should be monitored as they may be an indication of increased nutrient loading into the lake.
The dissolved color of lakes is generally due to dissolved organic matter from humic substances, which are naturally occurring compounds leached from decayed vegetation. Such substances do not generally threaten water quality except as they diminish sunlight penetration into deep waters. Increases in dissolved watercolor can be an indication in increased development within the watershed as many land clearing activities (construction, deforestation, and the resulting increased run-off) add additional organic material to lakes. Natural fluctuations also occur when storm events increase drainage from wetland areas.
Of the two “nutrients” most important to the growth of aquatic plants, nitrogen and phosphorus, it is generally observed that phosphorus is the more limiting to plant growth, and therefore the more important to monitor and control. The total phosphorus includes dissolved phosphorus as well as phosphorus which adheres to suspended soils and plankton. As little as 10 parts per billion (ppb) can cause an algal bloom. Generally, in more pristine lakes like Newfound, phosphorus values are higher after spring runoff or as a result of human activities (agriculture, logging, sediment erosion from construction, etc.) as the summer progresses or after major storm events.
pH and Alkalinity
pH expresses the acidic level of lake water, ranging from 1 (very acidic) to 14 (very “basic” or alkaline). Most aquatic organisms tolerate a limited range of pH, and most fish require a pH of 5.5 or higher for successful growth and reproduction.
Alkalinity is a measure of the “buffering” capacity of the lake water. The higher the value the more acid that can be neutralized. Typically lakes in NH have low alkalinities due to the absence of carbonates and other natural buffering minerals in the bedrock and soils of lake watersheds. Decreasing alkalinity can have serious effects on lake ecosystem. A decline in alkalinity below 5.0 can cause a decline of the condition and reproduction rate of fish while producing an increase in nuisance species of algae.
Alkalinity levels are most critical in the spring when acid loadings from snowmelt and fun-off are high, and many aquatic species are in their early, most susceptible states of their life cycle.
Specific Conductivity (See “Tributary Monitoring”)
The specific conductance of a water sample indicates concentrations of dissolved salts. Leaking septic systems, deicing salt runoff from highways, fertilizers and other pollutants can increase the conductivity of the water.
Dissolved Oxygen and Free Carbon Dioxide
Oxygen is an essential component for the survival of aquatic life. Submergent plants and algae take in carbon dioxide and create oxygen through photosynthesis by day. Respiration by both animals and plants uses up oxygen continually and creates carbon dioxide. Oxygen in the lower waters is important for maintaining a fit, reproducing, cold water fishery such as for trout and salmon. Carp and catfish can survive very low oxygen conditions. The typical pattern of clear, unproductive lakes is a slight decline in oxygen as summer progresses.
Light available for photosynthetic organisms is measured with an underwater photometer, which is like the light meter of a camera. The “photic zone” of a lake is the volume of water capable of supporting photosyntheses.
These microbial organisms represent diverse life forms, containing photosynthetic and non-photosynthetic types, and include bacteria, algae, crustaceans and insect larvae. The composition and concentration of phytoplankton can be indicative of the trophic status of a lake, and will vary over the seasons and life cycle of the organism. These are the food sources lowest on the food chain in a lake, and Newfound Lake contains a diverse, healthy population. However, in nutrient rich lakes, nuisance green algae and/or bluegreen bacteria might dominate.
There are three groups of zooplankton that are generally prevalent in lakes: the protozoa, rotifers and crustaceans. Analysis is generally restricted to the larger crustaceans. Crustacean zooplankton are very sensitive to pollutants, and are commonly used to indicate the presence of toxic substances in water. Zooplankton can be an important food source for some fish, and play a part in the recycling of nutrients within the lake.
Macroinvertebrates generally refer to the aquatic insect community living near the bottom sediments while other invertebrate groups such as the crayfish, leeches and the aquatic worms are also included. They undergo seasonal cycles, and the mayflies are probably the most well known example of a seasonal aquatic macroinvertebrate.
Macrointertebrates are also sensitive to environmental conditions such as streamflow, temperature, and food availability and are most representative of particular habitats along streams.
Macrointertebrates are an essential component to a healthy aquatic habitat. They help decompose organic matter such as leaves and twigs, and also serve as a food source for many fish species. Macroinvertebrate data are useful in discerning the more impacted areas within a watershed where corrective efforts should be directed.
The assessment of fish species “health” is another biological indicator of water quality. Because fish are at the top of the food chain, their condition should reflect not only water quality changes that affect them directly, but also those changes that affect their food supply (See “Fisheries”). The NH Fish and Game Department does annual assessments of fish health and populations on Newfo