In a natural environment, aquatic plants get the nutrients they need from water running off the land. As the water erodes the surrounding watershed, it carries soil particles containing the nutrients the plants need to thrive. The particles form the sediments that slowly fill the lake and provide a fertile foundation for rooted plants. In a natural process, over the course of thousands of years, the transport of sediments and nutrients from the land to the lake transform a glacial lake with a barren lakebed into a weed-filled wetland in a process called eutrophication. Human development in the watershed and along the lakeshore greatly accelerates this process.
Risks from excess nutrients
Cyanobacteria bloom on Lake Champlain (from 2020 Vermont Clean Water Initiative Report)
Aquatic plants and algae are part of a healthy lake ecosystem, providing food and shelter for fish and other aquatic fauna. However, as nutrient concentrations in a lake increase to high levels, plant growth can become a nuisance and unsightly. Thick stands of aquatic plants can make boat navigation difficult. Invasive aquatic plants become harder to control. Algae reduce water clarity. Algae blooms are ugly and can turn toxic, forcing the closure of swimming beaches. Heavy plant growth or algae blooms can result in oxygen depletion, resulting in fish kills. Loss of natural beauty and recreational opportunity has been shown to have a significant impact on shoreline property values and local economies. Lake Champlain and many other Vermont lakes are experiencing some of these problems.
Transport of phosphorus into a lake
An essential nutrient for aquatic plants and algae is phosphorus. Phosphorus is in the soil, and it moves with soil particles that are transported by water erosion. This process is enhanced when stormwater is allowed to flow quickly downslope on the soil surface directly into the lake or its tributaries. Natural shoreline vegetation and duff (undisturbed accumulations of leaves, needles, and branches) can slow the downward flow of stormwater and allow it to infiltrate deep into the soil. When this happens, the water may still enter the stream or lake as groundwater, but the phosphorus and sediments are filtered out.
An important factor in the transport of phosphorus is the land use within the lake’s watershed. Heavily fertilized farmland can be a major source of phosphorus pollution in lakes if there is an insufficient buffer between the fields and the shoreline of the lake or tributaries. Natural vegetation buffers slow down the stormwater flow and allow more of the water to be absorbed into the soil before reaching the shoreline. Stores of manure from livestock need careful handling to avoid polluting nearby streams with phosphorus.
Near the lakeshore, buildings, roads, and other impervious surfaces can result in significant erosion and phosphorus transport, unless stormwater runoff is carefully controlled in a way that discourages surface runoff and encourages infiltration. In the absence of a shoreline buffer, mowed lawns near the lake edge are insufficient to effectively prevent direct surface stormwater runoff and associated phosphorus pollution.
The pie charts below show the estimates for relative contributions to phosphorus pollution from various sources for Lake Champlain and inland Lake Eden. The story for each lake is unique.
Contributions from various souces of phosphorus loading in Lake Champlain (2020 Clean Water Initiative Report)
Contributions of phosphorus and sediment loading to Lake Eden from various land uses (from the Lake Eden Watershed Plan).
Troubling phosphorus trends
Using data from lake water samples gathered by both volunteers and staff, the Vermont Department of Environmental Conservation routinely monitors phosphorus concentrations in many Vermont lakes. With data for some lakes spanning decades, trends in phosphorus concentration are now apparent (see https://dec.vermont.gov/watershed/lakes-ponds/monitor/lay-monitoring#Reports and “Is Vermont Losing Its Oligotrophic Lakes?” NALMS Lakeline – Summer 2018). Polluted lakes with the highest levels of phosphorus at the start of the program (1980s) have generally improved. This is likely the result of more lake-friendly farming practices, the banning of phosphorus in lawn fertilizers and detergents, and the improved treatment of wastewater by municipalities. However, many lakes, with low to moderate phosphorus concentrations at the start, have seen a steady increase in this nutrient. Similar trends are seen in many other states.
Potential causes for these troubling trends can be listed, however, quantifying the relative contributions is difficult. Experts rely on modeling with assumptions about the phosphorus contributions depending on the land use and the relative area of various land uses near the lake and in the watershed. Each lake is unique in these characteristics.
A further complication is the existence of “legacy phosphorus” bound to deep lake sediments. Under conditions of oxygen depletion called anoxia, this phosphorus can be released into the water. Anoxic conditions occur in many deep lakes in Vermont during periods of thermal stratification. The stratification prevents the mixing of deep water with oxygen-rich surface water during the warm summer months. Bacterial decay of organic matter in the deep sediments depletes the oxygen resulting in the release of legacy phosphorus. In the fall, as the surface water cools and sinks, mixing resumes, and the released phosphorus that has accumulated in the deep water can reach the surface. This may result in algae blooms in the late fall, even on some lakes with normally low concentrations of phosphorus.
Impacts of climate change – Climate change certainly contributes to the phosphorus problem. Precipitation in Vermont is increasing. More importantly, the amount of precipitation in major storms is increasing. These major events cause most of the erosion and hence account for most of the phosphorus inputs into our waterways. Water temperatures are on the rise, increasing the risk of algae blooms in the late summer and fall. Summer stratification periods are getting longer, increasing the risk of anoxic conditions and the release of legacy phosphorus in deep lakes.
State and Federal Governments are aware of the risks that phosphorus poses to our valuable lake resources. In response to these risks and to satisfy mandates from the Environmental Protection Agency, the State of Vermont has committed to reducing phosphorus loading to specific target values over the next 20 years and is making good progress towards these goals. The investment in the Vermont Clean Water Initiative, including federal contributions, reached over $80M for the year 2020, and since 2016 the investment in the program has been over $240M (see https://anrweb.vt.gov/DEC/CleanWaterDashboard/). This funding went to implement lake-friendly agricultural projects, improve wastewater treatment, reduce erosion along roads and shorelines, and restore wetlands and floodplains. Some of this funding is available to lake associations to improve conditions in their watersheds.
What can your lake association do to help?
Funding is available from the Clean Water Initiative and through the Lake Champlain Basin Program (see https://www.lcbp.org/about-us/grants-rfps/request-for-proposals-rfps/) to create a “Watershed Action Plan”. The Plan, usually created by an expert consulting firm, 1) identifies a large number of specific phosphorus reduction projects within the lake’s watershed, 2) prioritizes them using accepted criteria that consider reduction potential, cost, and practicality, and 3) provides preliminary engineering designs and cost estimates for a few of the projects with the highest priority. Such plans may also use established techniques to estimate the relative phosphorus contributions of different land use types within the watershed.
In addition, lake associations can encourage individual members to take action to reduce stormwater runoff and erosion from their own properties. An excellent way to do this is to follow the guidelines available from the Vermont Lake Wise Program (see https://dec.vermont.gov/watershed/lakes-ponds/lakeshores-lake-wise). Individuals should also be encouraged to regularly maintain septic systems, avoid yard and garden products containing phosphorus, and obey the Vermont Shoreland Protection Act regulations (see https://dec.vermont.gov/sites/dec/files/wsm/lakes/docs/Shoreland/lp_ShorelandHandbook.pdf).
(This article by Dave Johnson, FOVLAP board member, first appeared on page 5 of the Spring 2021 newsletter)