Most simply, a combination of calm, sunny weather, warm water temperatures and excess nutrients cause algal blooms. Warmer temperatures increase the rate of algae growth and increase thermal stratification (layers of water that are defined by temperature). The warming of the water surface accentuates this stratification and can limit the physical ability of winds to mix the water. Mild winters combined with warmer springs and autumns lengthen the growing season and lead to a longer season for potential harmful algae blooms.

Phosphorus, an essential nutrient for plant growth, is the primary factor controlling the nutrient enrichment of freshwater bodies. Phosphorus levels above 0.02 mg/L can stimulate algal growth and lead to freshwater systems becoming eutrophic. Eutrophic waters have an excess of nutrients, frequently due to runoff that carries phosphorus or nitrogen. The nutrients that flow into the lake can cause primary producers like algae to grow. The dense growth of plant material blocks light which decreases oxygen levels. Not all algal blooms are harmful, but some cyanobacteria or blue-green algae species can produce toxins. These toxins can have adverse health impacts on animals, aquatic ecosystems and humans, such as elevated incidence rates of liver cancer in people who regularly draw their drinking water from eutrophic sources.

Oxygen levels are also important. Hypoxic (less than 2 to 3 mg/L oxygen) conditions occur with the decomposition of the algae, leading to potentially disastrous effects on fish and aquatic health. In eutrophic systems (waters that have excess nutrients), bottom waters become hypoxic, and phosphorus in those bottom sediments can be released, diffusing back into the water column. This negative feedback loop can worsen the situation and lead to more algae growth.

In the case of Mirror Lake last fall, the algal bloom was caused by a perfect combination of these conditions: calm and sunny weather, warm temperatures unusual for late autumn, and excess bioavailable nutrients.

The algal bloom that occurred on Mirror Lake coincided with algal blooms that occurred in Lake George and across the state.

Although road salt was not the immediate cause of this algal bloom, it plays a supporting role, making the lake more susceptible to algal blooms by storing phosphorus enriched sediments at the bottom of the lake that can become plant available in low oxygen conditions.

At the Ausable River Association, we are committed to the protection of Mirror Lake. Working with our partners and with support from the village of Lake Placid, the town of North Elba, private donors, and the Lake Placid LEAF grant, our Salt Use Reduction Initiative will help ensure the viability of this precious freshwater ecosystem.

(Leanna Thalman is a water quality associate at the Ausable River Association.)