Trends
These graphs are useful in examining the yearly and long-term fluctuations of Secchi transparency within an individual lake. The values are calculated by subtracting the long-term average from each year's summer average Secchi value. Secchi depth is one of the best parameters for detecting trends in lake trophic status as long as you can assume that changes in clarity are primarily due to changes in the amount of algae in the water. For some lakes, increased turbidity (loss of clarity) may be due to sediment particulates from river inflows, shoreline erosion, or even resuspension of bottom sediments in shallow lakes.

Algae can be affected by changes in the:
· amount of nutrients
· fisheries composition
· aquatic macrophyte community
· climate (precipitation, runoff, lake levels, temperature, sunlight).

Long-term patterns can sometimes be seen in lakes (consistently increasing or decreasing values over time). Sometimes cyclical patterns are seen which may be related to the above factors. Because so many factors can affect water clarity it can be difficult to determine what's happening to a lake but looking at Secchi data is usually the best we've got.

Nutrients

More nutrients usually mean more algae, unless the lake has already been "over fertilized". The biggest sources of nutrients entering our lakes are from direct nutrient wash-in from the watershed during rainstorms and from the mixing of high nutrient, low oxygen bottom water up into surface waters during windstorms and during spring and fall turnover.

Fisheries Composition

Zooplankton eat algae and fish eat zooplankton. Walleye, bass, northerns and muskies eat "zoops" in their larval and juvenile stages. Minnows and panfish eat them throughout their life cycles. Fish especially like a species of zooplankton, called Daphnia that are the most efficient grazers of algae. When Daphnia reach their peak in summer they can effectively "clear" the water of algae. Too many zooplankton-eating fish can really diminish the Daphnia population.

One lake restoration method actually involves trying to increase the number of zooplankton in the lake to maximize this clearing effect. This involves netting or even poisoning (rotenone treatment) the small, zooplankton eating fish. Or, large, fish-eating predators can be added to the lake to accomplish the same thing. The idea is to increase the number of zooplankton so more algae are eaten and therefore the lake clears up. These techniques are called "biomanipulation". They have been successful in many urban lakes in Europe and North America although they require very careful long-term management and can be expensive. There is no cheap quick fix for excessive nutrient loadings from the watershed!

Macrophytes

Lily pads and pondweeds also need nutrients to grow. Although bottom-rooted lake plants get most of their nutrients from the sediments they can compete with algae for water column nutrients. Perhaps more importantly, they also shade the water column, which decreases the amount of light available to algae. These plants also provide a refuge or a place to hide for zooplankton so they can avoid being eaten by fish. Healthy shallow lakes or shallow zones in deeper lakes often have flourishing macrophyte communities. In these zones you can often see that the water is actually quite clear among the plants.

Climate

Ultimately, day-to-day variations and seasonal changes in weather have big effects on algae. Changes in physical factors such as temperature and light as well as indirect effects on nutrients and on other biological communities in the lake are all influenced by climate.

Why Is it Important?

The secchi disk depth provides a low "tech" method for assessing the clarity of a lake. A Secchi disk is a circular plate divided into quarters painted alternately black and white. The disk is attached to a rope and lowered into the water until it is no longer visible. Secchi disk depth, then, is a measure of water clarity. Higher Secchi readings mean more rope was let out before the disk disappeared from sight and indicates clearer water. Lower readings indicate turbid or colored water. Clear water lets light penetrate more deeply into the lake than does murky water. This light allows photosynthesis to occur and oxygen to be produced. The rule of thumb is that light can penetrate to a depth of about 2 - 3 times the Secchi disk depth.

Clarity is affected by algae, soil particles, and other materials suspended in the water. However, Secchi disk depth is primarily used as an indicator of algal abundance and general lake productivity. Although it is only an indicator, Secchi disk depth is the simplest and one of the most effective tools for estimating a lake's productivity.

Reasons for Natural Variation

Secchi disk readings vary seasonally with changes in photosynthesis and therefore, algal growth. In most lakes, Secchi disk readings begin to decrease in the spring, with warmer temperature and increased growth, and continue decreasing until algal growth peaks in the summer. As cooler weather sets in and growth decreases, Secchi disk readings increase again. (However, cooler weather often means more wind. In a shallow lake, the improved clarity from decreased algal growth may be partly offset by an increase in concentration of sediments mixed into the water column by wind.) In lakes that thermally stratify, Secchi disk readings may decrease again with fall turnover. As the surface water cools, the thermal stratification created in summer weakens and the lake mixes. The nutrients thus released from the bottom layer of water may cause a fall algae bloom and the resultant decrease in Secchi disk reading.

Rainstorms also may affect readings. Erosion from rainfall, runoff, and high stream velocities may result in higher concentrations of suspended particles in inflowing streams and therefore decreases in Secchi disk readings. On the other hand, temperature and volume of the incoming water may be sufficient to dilute the lake with cooler, clearer water and reduce algal growth rates. Both clearer water and lower growth rates would result in increased Secchi disk readings.

The natural color of the water also affects the readings. In most lakes, the impact of color may be insignificant. But some lakes are highly colored. Lakes strongly influenced by bogs, for example, are often a very dark brown and have low Secchi readings even though they may have few algae.

Expected Impact of Pollution

Pollution tends to reduce water clarity. Watershed development and poor land use practices cause increases in erosion, organic matter, and nutrients, all of which cause increases in suspended particulates and algae growth. Secchi disk depth is usually reported in feet to the nearest tenth of a foot, or meters to the nearest tenth of a meter. Secchi disk readings can be used to determine a lake's trophic status. Though trophic status is not related to any water quality standard, it is a mechanism for "rating" a lake's productive state since unproductive lakes are usually much clearer than productive lakes.