Written by Ken Wagner, Ph.D., CLM, for the Belgrade Lakes Association Winter 2014/2015 Newsletter
Algae are everywhere! No, it’s not a second rate horror movie, but an ecological fact. On land, algae form a coating on many surfaces, but do not exhibit the bulk biomass we see in trees. In streams algae form coatings on just about any surface and may produce tufts or trailing growths on rocks. Algae can also be found growing on many surfaces in lakes, including docks, buoys, ropes and the lake bottom. However, the lake algae we often notice most are those that discolor the water and form surface scums and mats; nuisance algae are often members of the phytoplankton, or free-floating algae.
Phytoplankton form an essential part of lake ecology. A lake without phytoplankton is not likely to be very productive. This may suit swimmers just fine, but fisherman and birdwatchers would be very disappointed. It is only when the phytoplankton produced by the lake are not consumed by the next level in the food web that they build up and impair both recreation and habitat value. Some types of algae are more likely to do this than others, but a wide variety of algae can form blooms or mats. It is important to know something about both the amount and type of phytoplankton when developing a lake management plan.
Phytoplankton can be quantified in a number of ways, the simplest being by Secchi transparency, but this may not actually measure phytoplankton when substantial amounts of suspended sediment are present. Most often professionals and volunteers use chlorophyll a as a surrogate for algal quantity. Chlorophyll a is a green pigment found in all plants capable of photosynthesis, the process by which plants convert the sun’s energy into biomass. However, not all types of algae have similar ratios of chlorophyll a to biomass, and each major algal group has its own set of pigments that include other forms of green chlorophyll as well as other pigments that produce red, yellow and blue colors. Consequently, while chlorophyll a is a very useful measurement, it tells only part of the story and does not provide a lot of information that could be very important in choosing a management strategy. While other quantitative measures are also potentially useful, it makes a lot of sense to simply look at the algae – under great magnification – to see what types are present. This is what phycologists (people who study algae) and their microscopes do.
At the very least, lake managers need to know which algae can dominant as bloom formers to take appropriate action. The most common bloom-forming algal groups are the blue-greens (Cyanophyta, actually cyanobacteria), green algae (Chlorophyta), and diatoms (Bacillariophyta), although the four other common algal groups include bloom-forming algae too. Knowing the composition of a bloom can make a real difference in which algaecide to apply at what dose for short-term control, and matters in determining what caused the bloom and how to stop it from happening again (long-term control).
Within the blue-green group, there are several major types that are commonly associated with blooms and each has its own properties and requirements that may lead to different impacts on recreation and habitat. It may actually be necessary to identify the alga to the species level to determine what the best course of action will be. Knowing the types of algae present will also shed light on the effective level of nutrients in the lake. Nutrient levels can be measured directly, but how they affect lake uses can often be best discerned through phytoplankton analysis. The actual quantity of algae will be determined by available phosphorus levels in most lakes, but the ratio of available forms of multiple nutrients often determines which algae will dominate. Diatoms require high silica, and many green algae require high nitrogen as ammonium or nitrate. Yet some blue-greens can fix abundant gaseous nitrogen (Figure 2), freeing them from any nitrogen limits and allowing severe bloom formation if enough phosphorus is available. Consequently, phosphorus control is the preferred method of algae control; there can’t be blooms if phosphorus availability is low.
There is a catch, however. Just because phosphorus in the water column is low does not mean that phosphorus is not available. Phosphorus in surficial sediments may be taken up by algae growing right on the sediment, and those algae may store up extra phosphorus before rising in the water column. When they get up into the lighted upper water layer, they can continue to grow and bloom using the phosphorus they brought with them, sort of an algal picnic! Eventually the bloom will subside if there is not enough phosphorus in the water column to support it, but some portion of the phosphorus brought up with the algae will remain and may support other algae, possibly at bloom levels. Rising algae are therefore vectors of fertilization, not just a symptom.
Control of phosphorus can be accomplished by multiple methods, and the first stop is always the watershed. If there is too much phosphorus in runoff, there may be too much phosphorus in the lake. Eventually, however, the input phosphorus either leaves the lake or winds up in the surficial sediments. When oxygen is low, reactions allow some of that phosphorus to come back out of the water column and be recycled. Consequently, even the best watershed management program will not prevent blooms if enough phosphorus has accumulated. This internal load must be removed or neutralized. Dredging is the ideal technical solution, but is often infeasible due to engineering difficulties, disposal limitations, and overall cost. Oxygenation can keep most of the phosphorus in the sediment, and makes better fish habitat in deep water, but must be repeated each summer at considerable cost. If affordable, however, this is a great option. Phosphorus inactivation, most often with aluminum compounds, will bind up the phosphorus in forms that do not become available under fluctuating oxygen levels, and has enjoyed considerable success throughout the USA in lakes where the internal load is the dominant source of phosphorus, including multiple lakes in Maine. Oxygenation and phosphorus inactivation are being evaluated for the Belgrade Lakes to supplement watershed management.