Liebig formulated the so-called "law of the minimum", which states that only one factor limits the growth of a population at any particular time, although the identity of that factor can shift over time and space. Nitrogen availability sometimes limits primary production. In other places, however, phosphorus is the limiting nutrient, particularly in freshwater ecosystems, in which case controlling phosphorus inputs is a good management goal.

Recent experiments on stormwater detention ponds have shown a mixed pattern of nutrient limitation. It appears that shifts in the loading of nitrogen and, perhaps, phosphorus have led to a situation where nitrogen and phosphorus are co-limiting in pond ecosystems with substantial anthropogenic impacts.

We will test the hypotheses that 1) one element limits phytoplankton growth in local stormwater detention ponds and 2) phosphorus is the limiting element in ponds with low total phosphorus contents by doing a replicated nutrient manipulation experiment.

Procedure

We will conduct a nutrient "deletion" experiment using water from one or more locations. Basically we will add all but one macronutrient (N, P, or Si) to each of a set of experimental vessels, and will measure the response of phytoplankton biomass as chlorophyll a.

Fill sample bottles with raw, freshly collected pond water. Supplement the nutrient contents of these vessels (labeled properly) as follows, using solutions and volumes as you are instructed:

• To one set of five bottles add nitrate and phosphate.
• To another set of five bottles add silicate and phosphate.
• To another set of five bottles add nitrate and silicate.
• To another set of five bottles add nitrate and phosphate and silicate (+ control).
• To a set of five bottles add distilled water (- control).

Filter samples from five additional bottles of raw pond water immediately onto glass fiber filters. These are initial samples. Label and store the filters as you are instructed. These filters will be extracted for chlorophyll a analyses later. We will use chlorophyll a concentrations as an estimator of phytoplankton biomass.

Place the capped and labeled vessels in water-filled pans on the rotating table in the greenhouse. Sample chlorophyll a as above from each vessel after seven days have elapsed. Be sure to shake the bottles thoroughly before sampling!

Measure and calculate chlorophyll a values. Be prepared to put your data up on the board for class discussion, after which reports will be due at a date to be determined.

View results from previous years' experiments: 1996 or 1997. View additional results.

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