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Secchi Disk Transparency
First developed by Professor P.A. Secchi in 1865 for a Vatican-financed Mediterranean oceanographic expedition, the Secchi disk has since become a standard piece of equipment for lake scientists. It is simply a weighted circular disk 20 centimeters (about eight inches) in diameter with four alternating black and white sections painted on the surface.
The disk is attached to a measured line that is marked off either in meters (subdivided by tenths of meter), if using metric units, or feet (subdivided by tenths of feet or inches), if using English units.
The Secchi disk is used to measure how deep a person can see into the water. It is lowered into the lake by the measured line until the observer loses sight of it. The disk is then raised until it reappears. The depth of the water where the disk vanishes and reappears is the Secchi disk reading.
In extremely clear lakes, disk readings greater than 10 meters can be measured. On the other hand, lakes affected by large amounts of algal growth, suspended sediments, or other conditions often have readings of less than one-half meter.
In some shallow lakes, it is impossible to get a Secchi disk reading because the disk hits the bottom before vanishing from sight. This means the true Secchi disk reading is greater than the depth of the lake in that particular location.
Unfortunately, Secchi disk data are among the most abused and misinterpreted measurements in monitoring programs because people often directly equate Secchi disk readings with algal density. There are, how ever, many other factors found both inside and outside the lake that affect how deep a person can see into the water.
Inside the lake, water transparency can be reduced by:
Chlorophyll a is the green photosynthetic pigment found in the cells of all algae. By taking a measured sample of lake water and extracting the chlorophyll a from the algae cells contained in that sample, monitors can get a good indication of the density of the algal population.
The chlorophyll a concentration cannot be considered a precise mea surement of algal density, however, because the amount of chlorophyll a found in living cells varies among algal species. Thus, two lakes can have identical densities of algae yet have significantly different concentra tions of chlorophyll a because they are dominated by different species.
Direct comparability, even within a single lake, is further complicated by the fact that the amount of chlorophyll a in an algal cell varies with light conditions. Healthy algal cells constantly try to maintain chlorophyll concentrations at a level for maximum photosynthetic efficiency. Chlorophyll in a cell usually decreases during high light conditions and increases during the night or low light conditions.
Similarly, a cell that is sinking down into the water column (away from the sun) may also produce more chlorophyll to compensate for the lower light levels found at greater depths. Changing seasons also create higher or lower light conditions according to the position of the sun which, in turn, affects chlorophyll production.
Despite these drawbacks, the ease of sampling and relatively low cost of analysis makes chlorophyll a an attractive parameter for characterizing the algal density in lakes, especially for lake monitoring programs.
Chlorophyll a is analyzed in a laboratory from a sample. The simplest protocol is to ship the water sample to the labora tory for analysis.
Alternatively, some monitoring programs have personnel pass a measured volume of lake water through a filtering apparatus containing a prepared filter paper disk. The filter paper lets the water pass through but retains the algae cells on its surface. The personnal then removes the disk and places it in a special tube to be forwarded to the laboratory for chlorophyll a analysis.
In some instances, this procedure may produce lower than actual results for chlorophyll a concentrations if the filtering procedure is not followed exactly. QA/QC considerations will determine if this method is a feasible alternative for a monitoring program.
Phosphorus is one of several essential nutrients that algae need to grow and reproduce. In many lakes, phosphorus is in short supply. Therefore, it often serves as a limiting factor for algal growth.
Phosphorus migrates to lake water from only a few natural sources. As a result, lakes located in pristine wilderness settings rarely have problems with algal blooms. Humans, on the other hand, use and dispose of phosphorus on a daily basis. Phosphorus is found in such common items as fertilizers, foods, and laundry detergents.
Lakes with developed watersheds often receive a portion of this human-generated phosphorus through runoff, septic leachate, and other sources. This phosphorus fertilizes the water and can stimulate increased algal growth.
Algae most readily consume a form of phosphorus known as orthophosphate, the simplest form of phosphorus found in natural waters. In fact, orthophosphate is so quickly taken up by a growing algal population that it often is found only in low concentrations in lakes.
Phosphorus is found in lakes in several forms other than orthophosphate. For example, when phosphorus is absorbed by algae, it becomes organically bound to a living cell. When the cell dies, the phosphorus is still bound to particles even as it settles to the lake bottom. Once the decomposer organisms break down the cell, the phosphorus can become attached to calcium, iron, aluminum, and other ions.
Under anoxic conditions, chemical reactions can release phosphorus from the sediments to the overlying waters. Spring or fall overturn may then redistribute it back to the surface where it can be taken up by another algal cell.
Phosphorus, therefore, is in a constant state of flux as environmental conditions change and plants and animals live, die, and decompose in the lake. Because the forms of phosphorus are constantly changing and recycling, it is generally most appropriate for citizen monitoring programs to measure all forms of phosphorus together. This one "umbrella" measurement is known as total phosphorus.
This manual describes a method that instructs the personnel to collect a water sample, transfer it into a sample bottle that contains an acid preservative, and then ship it to a laboratory for total phosphorus analysis.
Alternatively, there are test kits on the market for total phosphorus analysis. To conduct the test, however, personnel must be well-trained and possess special laboratory equipment. For these reasons, phosphorus test kits are not generally appropriate for monitoring programs.
In some instances, orthophosphate may be a parameter of interest since it is the form of phosphorus available for uptake by algae. Like total phosphorus, orthophosphate is best measured in a laboratory.
Where to Sample
Analyzed together, the three parameters Secchi disc, chlorophyll a, and total phosphorus can provide information on the quantity of free-floating algae, the critical nutrient that feeds the population, and how the algae affect water transparency. Where the parameters are sampled on the water surface and in the water column is an important consideration when planning a program to monitor algal conditions.
A lake and its water quality are not uniform from shore to shore or from surface to bottom. Lake morphometry, exposure to winds, incoming streams, watershed development, and human activity can greatly influence the algal conditions found at any one location in the lake.
Thus, the planning committee or supervising staff is challenged to select sample locations that will best characterize the algal condition in accordance with the goals and objectives of the monitoring program. Increasing the number of sampling sites will reduce uncertainty, but it will come at increased cost.
Where to Sample on the Water Surface
The majority of citizen monitoring programs are designed to measure average algal conditions in the lake's pelagic (deep, open water) zone. For these programs, the number and location of sampling sites are most influenced by the size and shape of the lake basin.
In most cases, a site over the deepest section of the lake best represents average conditions. In natural lakes that are circular in shape, the deepest section is usually near the middle. In reservoirs, the deepest section is usually near the dam.
Many lakes, however, possess significant arms or bays. In this instance, it is often useful to sample the deepest section in each individual arm or bay. In many cases, monitors will find a significant difference between sites, especially if one arm of the lake is more populated.
Some monitoring programs, on the other hand, are designed to characterize the algal condition at its worst location. For these types of programs, certain known problem areas may be targeted for sampling. For example, a particular bay may be monitored because it "collects" algae and other materials because of prevailing winds.
More often, however, "worst area" sampling is designed to monitor how point or nonpoint sources of nutrient pollution affect water quality and algal growth. Examples of potential sources of nutrient pollution include farms, residential developments, and sewage outfalls. This monitoring can provide evidence that specific watershed management efforts are needed to manage the algal population.
The number and location of sampling sites can also be influenced by the basic goal of the program. A program managed primarily for public education, for example, may wish to include stations for various non -scientific reasons such as their proximity to residential neighborhoods or convenience of access.
Sample site selection should be consistent within a program in order to get results worthy of lake-to-lake comparison. For example, if the deepest part of the lake is chosen as the location for sampling, all the lakes in the program should then be sampled at the deepest site.
To select the location of a sampling site, the manager must possess some preliminary information about the lake including:
For shoreline or near-shore stations, finding the site will probably not be a problem. Many programs, however, will require to sample over the deepest portion of the lake. This usually means the monitoring site will be somewhere in the middle of the waterbody.
Two simple ways to find the site are by:
On land, personnel know where they are located by finding familiar landmarks. The same process can be used on water, except that the landmarks are located on the shoreline. On an initial training trip, the personnel must designate an "official" site location.
Once securely anchored at the site, the personnel should pick out two permanent landmarks on shore (a dwelling, tall tree, large rocks) that align one behind the other. This alignment forms an imaginary bearing line through the objects to the site.
Then, at about a 90 degree angle, two more aligning landmarks should be identified. These landmarks then form a second bearing line to the sampling site. The personnel should mark these landmarks and bearing lines on their lake map for future reference. They should also practice finding the site location with the program manager.
To further verify that personnel have found the proper sampling site, the program manager may also require that they perform a depth check using the anchor rope, a weighted calibrated sounding line, or an electric "fish-finder" apparatus that indicates bottom depth.
Marker Buoy Method
If the lake is small and protected from strong winds and waves, a marker buoy may be the simplest way to designate a sample site location. In many public lakes, however, it is illegal to set out buoys without proper permits. The rules and regulations regarding buoys should be checked before any placement.
There is a risk that a marker buoy will be moved by winds, waves, and
/or lake users.
Where to Sample in the Water Column
Free-floating algae grow and reproduce in the photic zone. This zone constitutes the upper portion of the water column where sunlight penetrates and stimulates photosynthesis in the algal cells. In programs designed to measure the algal condition of a lake, water samples are taken from the photic zone and analyzed in a laboratory for their chlorophyll a and total phosphorus content.
Where these samples are taken in the photic zone is another important decision that must be made by the planning committee. There are two basic choices for water sampling in the photic zone. you can collect:
Point sampling refers to the collection of a water sample from a specific depth in the water column. Also known as grab sampling, it is the method most often used in monitoring programs.
When measuring the algal condition parameters, a sample is usually taken at a selected depth between one-half and two meters. (Water samples are generally not collected directly at the surface because floating substances such as pollen and gasoline residue will contaminate them.)
If a depth of one-half meter is selected, personnel can collect the sample by simply submerging the sample bottle to about elbow depth. For deeper point sampling, some type of water sampler instrument must be used.
When the messenger reaches the sampler, it hits a trigger mechanism and the two stoppers snap shut, trapping the sample of water from that depth. The sampler is then hauled back into the boat and the sample water poured into a container.
The goals of the monitoring program and how the water quality data will be used will help the planning committee determine where a point sample should be collected. A depth of one meter is selected many times as a representative depth of photic zone conditions for chlorophyll a and total phosphorus analyses.
If a water sampler is used, other depths in the water column also can be easily sampled by the personnel. Total phosphorus is an especially interesting parameter to monitor at different points in the water column, in addition to the upper layer photic zone.
As discussed in Chapter 2, phosphorus is released from bottom sediments under anaerobic conditions. If the lake is strongly stratified in the summer, and wind energy does not mix the water column, the bottom-released phosphorus cannot reach the photic zone and stimulate increased algal growth. In some lakes, however, summer stratification occasionally breaks down and the bottom phosphorus does reach the surface waters, causing sudden algal blooms.
This internal loading of phosphorus is often important when analyzing the algal condition of productive lakes. For this reason, the planning commit tee should consider having personnel collect point samples from the bottom and middle of the water column for total phosphorus analysis, as well as in the photic zone.
An integrated sample combines water from a range of depths in the water column. It is essentially a mixture of point samples designed to represent more of the photic zone than a single sample. The simplest way for personnel to collect an integrated sample is to use a hose and bucket.
Basically, a measured length of hose is weighted on one end and then lowered into the lake. While the hose descends, it collects a vertical column of water. By plugging the surface end and then bringing the lowered end to the surface with a line, an intact column of water can then be emptied into a bucket and a sample drawn for laboratory analysis.
A basic drawback is that this method can not be easily standardized.
Frequency of Sampling
There is usually a change in the quantity and species of algae occur ring in a lake throughout the year. Often algal density increases in the spring and early summer as water temperatures increase and nutrients become available in the well-lit upper layer as a result of spring overturn.
When summer arrives and the lake stratifies, the algae population may change as the supply of orthophosphate in the upper layer becomes depleted and/or microscopic animals (zooplankton) graze on the population. After the summer, fall overturn can once again bring fresh nutrients to the well-lit upper zone and stimulate increased algal growth.
A variety of other factors can also affect algal habitat and growth response, especially during the summer growing season. Storms can churn the lake and cause a temporary upwelling of nutrients from the lake bottom. Phosphorus-rich runoff can escape from residential or agricultural areas after rainstorms, drain into the lake, and stimulate growth. On the other hand, chemicals or herbicides that are toxic to algae may be released to the water and cause a (planned or unplanned) population crash.
The planning committee should base its decision on how often to sample on data quality criteria, costs, and other practical considerations.
Many citizen monitoring programs have found it appropriate to sample algal conditions on a two-week or bi-monthly cycle. In most cases, this time period has proven adequate to monitor changes in the algal parameters.
However, if conditions are known to change at more frequent intervals (if lake water flushes quickly through an inlet and outlet), the committee may determine that weekly sampling is more appropriate.
More frequent sampling also improves the odds of measuring a short-term event such as an algal bloom or a sudden pulse of phospho rus input because of storm runoff or a sewage plant bypass. Expense becomes a key factor when determining sampling frequency because each sampling round will increase program costs.
Length of the Sampling Season
An ideal monitoring program runs year-round to collect the full amount of seasonal data on the lake. A more practical sampling period for citizen monitoring is from spring overturn to the end of the summer growing season.
Spring overturn is important because it is when wind action circulates the entire volume of water. Importantly, citizens can sample the spring algal blooms that are sometimes observed as a result of increased nutrient availability and warming water temperatures.
The summer growing season corresponds with the main recreational season. It is during this time that increased algal growth is most objectionable because it can interfere with swimming, water-skiing, fishing, and other activities.
Fall overturn is another time when the water circulates and algal blooms typically occur. This season is not as important to the general public, however, because it comes at the end of the recreational season. Thus, fall algal blooms are not usually perceived as a problem. Personnel interest also wanes as the weather turns cooler and more unpredictable. For these rea sons, it is often prudent to stop monitoring at the end of summer.
How to Sample
Presented in this section are suggested procedures for sampling Secchi disk depth, chlorophyll a, and total phosphorus concentration for a citizen monitoring program. Basically, these sampling activities are divided into four main segments.
Under no circumstances should personnal be on the water during rain or electrical storms, high winds (white caps), or other unsafe conditions.