Subsamples from the bioassays are filtered to measure changes in chlorophyll and nutrient concentrations.The bioassay procedure uses unfiltered water samples collected during Maryland DNR sampling periods.  Samples were stored overnight at air temperature and reduced light conditions.  The next morning, the roughly 5 gallons of water (20 liters), is divided into 5 transparent containers called cubetainers.  The cubetainers are nearly 1 gallon in size (3 liters).

Two of the cubetainers are maintained as "controls".  Control samples do not have nutrients added to them in the experiments.  The remaining three cubetainers receive experimental treatments.

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The treatments are:

* the addition of nitrogen (1 container),

* the addition of phosphorus (1 container),

* a combination nitrogen and phosphorus additions (1 container).

Phosphorus and nitrogen are key nutrients that are managed for to improve water quality.  If a nutrient is limiting the growth of the algae, there is not enough of the nutrient to maximize algal growth.  By experimentally adding nutrients to such a sample, we expect to see algal growth far more than samples that have no nutrients added.  This is the basis for the bioassay measurement, comparing growth of algae between untreated (control) samples and treated (nutrient addition) samples.

Once the samples have been treated, all 5 cubetainers were placed outside under natural temperature conditions but 60% of light conditions and allowed to grow for 48-72 hours.  The growth of algae in the 5 containers is measured at the end of the experiment and comparisons of the 2 controls and the 3 treatments are made to assess which nutrient or combination of nutrients is limiting the growth of the algae.

Suppose the algae in the controls and samples grow the same amount?  This suggests there was already an abundance of nutrients in the water; the addition of nutrients had no affect over the controls.  So another key factor controlling algal growth that must be considered is light.  Photosynthesis depends on the ability of chlorophyll to acquire energy from the sunlight to allow the chemical reactions necessary to convert carbon dioxide and water to produce glucose and oxygen.  Glucose is then used by the plant for energy in growth and reproduction.

6 CO2 + 6 H2O --------------------------------> C6H12O6 + 6 O2
In the presence of light and chlorophyll

If the water is clouded or colored by sediments, this lowers the amount of light can reach the algae and can also limit the growth of algae.  Ideally, we want to keep sediments low in the water for a healthier ecosystem, but we also need to keep nutrients low when the water is clear, otherwise the algae will grow uncontrollably.  The result of the uncontrolled algal growth and problems associated with such algal overproduction is termed eutrophication.

For more detailed information on the methods, please see Fisher & Gustafson, 2000 (filename: 2002_level1_report.pdf, 11,982 kb).

For more information, please contact Renee Karrh at (410) 260-8628.

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