7.3 Chlorophyll A

Chlorophyll A is a green pigment extracted from algae and is used to measure the amount of algae in the lake. It is found in plants, algae, and cyanobacteria. Its name is derived from ancient Greek: chloros = green and phyllon = leaf. Chlorophyll absorbs mostly in the blue and to lesser extent red portions of the electromagnetic spectrum, hence its intense green color.

Fig 7.8 Plant cell containing ChlA

7.3.1 Algae

The algae (singular alga) consist of several different groups of living organisms that capture light energy through photosynthesis, converting inorganic substances into simple sugars with the captured energy. Algae have been traditionally regarded as simple plants, and some are closely related to the higher plants. Others appear to represent different protist groups, alongside other organisms that are traditionally considered more animal-like (protozoa).

Algae range from single-celled organisms to multi-cellular organisms, some with fairly complex differentiated forms called seaweeds in marine environments. All lack leaves, roots, flowers, and other organ structures that characterize higher plants. They are distinguished from other protozoa in that they are photoautotrophic, although this is not a hard and fast distinction as some groups contain members that are mixotrophic, deriving energy both from photosynthesis and uptake of organic carbon either by osmotrophy, myzotrophy, or phagotrophy. Some unicellular species rely entirely on external energy sources and have reduced or lost their photosynthetic apparatus.

All algae have photosynthetic machinery ultimately derived from the cyanobacteria, and so produce oxygen as a by-product of photosynthesis, unlike the non-cyanobacterial photosynthetic bacteria.

Algae are usually found in damp places or bodies of water and thus are common in terrestrial as well as aquatic environments. However, terrestrial algae are usually rather inconspicuous and far more common in moist, tropical regions than dry ones, because algae lack vascular tissues and other adaptions to live on land. Algae can endure dryness and other conditions in symbiosis with a fungus as lichen. The various sorts of algae play significant roles in aquatic ecology. Microscopic forms that live suspended in the water column, called phytoplankton, provide the food base for most marine food chains. In very high densities (so-called algal blooms) they may discolor the water and out compete or poison other life forms. The seaweeds grow mostly in shallow marine waters. Some are used as human food or are harvested for useful substances such as agar or fertilizer. The study of algae is called phycology or algology.

7.4 Secchi Disk

Secchi is one of the best parameters for determining the overall health of the lake. Transparency varies in response to algae, watershed runoff, precipitation, water cleanliness and other factors.

Created in 1865 by Pietro Angelo Secchi, the Secchi disk is a device used to measure water transparency in open waters of lakes, bays, and the ocean. The pattern shown in the image is drawn or painted onto a card or acrylic, mounted on a pole or line, and lowered slowly down in the water. The depth at which the pattern on the disk is no longer visible is taken as a measure of the transparency of the water. This measure is known as the Secchi depth and is related to water turbidity.

Turbidity is a cloudiness or haziness of water (or other liquid) caused by individual particles that are too small to be seen without magnification, thus being much like smoke in air. Liquids can contain suspended solid matter consisting of particles of many different sizes (total suspended solids). While some suspended material will be large enough and heavy enough to settle rapidly to the bottom of a container if a liquid sample is left to stand, very small particles will settle only very slowly or not at all if the sample is regularly agitated or the particles are colloidal. These small solid particles cause the liquid to appear turbid. Measurement of turbidity is a key test of water quality.

Secchi disk readings do not provide an exact measure of transparency, as there can be errors due to the sun's glare on the water, or one person may see the disk at one depth, but another, with better eyesight, may see it at a greater depth. However a Secchi disk is an inexpensive and straight-forward method of measuring water clarity. Because of the potential for variation between practitioners, methods should be standardized as much as possible.

A Secchi disk measurement should always be taken off the shady side of a boat or dock between 9 AM and 3 PM (Lind, 1979). According to Cole (1994), the period for best results is between 10 AM and 2 PM. The same observer should take secchi depth measurements in the same manner every time.

One can approach the measurement by lowering the disk beyond a point of disappearance, then raising it and lowering it slightly to set the Secchi depth. Another method is to record the depth at which the disk disappears, lower another few feet, then record the depth at which the disk reappears as it is slowly brought up. The Secchi depth is taken as the average of the two values.

Secchi disk measurements have been an integral component of Minnesota 's lake water quality assessment programs for some time; lake residents make periodic measurements and submit their readings to state and local agencies. The aggregated longitudinal data are used to reveal general trends in water quality.

Next page: Chapter 7.5 Temperature, Dissolved Oxygen, Conductivity