By Bruce Paakh
Water Quality Specialist, Minnesota Pollution Control Agency

Algal populations in Minnesota lakes are in a state fo continuous change. At any one time there may be fifteen or more species of algae present in the water column. The changes that occur in the algal community composition throughout the growing season are similar to the changes that occur in the plant community in terrestrial ecosystems. If you were to take a walk through a forest every week from early spring to late fall, you would notice that on each trip different plants would be in one of the various stages (i.e. emerging, growing rapidly, flowering, going to seed, or dying). The succession of plants that occurs throughout the growing season is often referred to as the seasonal succession. Algae act the same. Each species has a growth period during the spring, summer or fall that they prefer. Although the plant may be present in the water column throughout the year there is a specific time when each species will tend to "bloom" or grow rapidly.

There are three common groups or Phyla of algae that typically mke up the majority of algae present in the lake. They are the diatoms, green algae, and bluegreen algae. The typical seasonal scenario that occurs in Minnesota lakes involves the diatoms dominating the water column during the early spring when cold water conditions exist. As the spring progresses many varieties of green algae tend to increase in density during may and through June. The bluegreen algae prefer warmer water temperatures and will usually dominate during the months of July and August. As water temperatures begin to cool during September and into October the various species of diatoms resume their dominance in the water column.

Each specie of algae has a set of optimum conditions for growth. When all of the conditions are present, the algae may respond and grow in numbers at a rapid rate. The period of optimum conditions for a specie can last for part of a day or for several weeks. Weather conditions (light intensity, temperature, rainfall), nutrient levels, water temperature and predation by zooplankton all have an affect on the abundance of the various species present.

Some varieties of bluegreen algae are very large in contrast to the relatively small diatoms and greens. When bluegreens bloom in significant proportion, the high density of algae present can cause severe nuisance conditions that include floating scums, very poor clarity and odors. When the algae die off they can create obnoxious odors and have resulted in fish kills due to decreased oxygen levels during the decomposition of dead algae.

The summer algal dynamics can take place with few people being aware of it unless a significant bloom occurs that noticeably affects water quality. Volunteers of the Citizen Lake Monitoring Program (those persons collecting Secchi disk data) are familiar with the seasonal variability of the Secchi readings. These variations in clarity are most often due to changes in the type and density of algae. An obvious effect that algal dynamics has on the Secchi clarity is during August when you will often find your lowest readings when the bluegreens dominate. Another thing to look for is a period or two during the year when the clarity greatly improves for a week or so. This often occurs during June or July when a dominant specie of green algae dies off. If another specie isn't ready to take over and bloom at that time, you may find a period of very good water quality.

Monitoring lake level clarity with the Secchi disk for several summers will enable you to look for seasonal trends in water clarity. The monitoring of rainfall, lake level and water temperature over the same period may enable you to detect trends in the way that your lake responds to various summer conditions.