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Estimating ammonium uptake rates of several species of duckweed with the AquaFluor Handheld Fluorome

Parameters: Ammonium

The Department of Ecology & Evolution at Stony Brook University is located on Long Island, NY. It was founded in 1969 and is one of the first departments of its kind in the world. The department is home to approximately 18 faculty, 50 PhD students, and over a dozen masters students.

“Duckweed” is the common name for a number of species (e.g., Lemna sp., Spirodela sp., Wolffia sp.) of tiny plants that float directly on the surface of freshwater lakes and ponds. The rapid growth of these plants under high nutrient (eutrophic) conditions can make for a love-hate relationship with duckweed. On one hand, duckweed has the potential for applied uses with numerous societal benefits. Engineered applications of duckweed such as wastewater treatment, biofuel production, ecotoxicity assays, and agriculture feed production are currently being developed or are in use. On the other hand, duckweed can become a nuisance in natural environments. Many small, eutrophic ponds can be completely covered by these floating plants. When duckweeds dominate, light and dissolved oxygen levels in waterbody are low, causing negative impacts on submerged aquatic vegetation and other groups of organisms. For both applied uses and management of nuisance duckweed it is important to know the performance of different duckweed species over a range of environmental conditions. Michael conducted laboratory experiments to examine growth rates, nutrient uptake rates, and competitive interactions of multiple species of these tiny, floating plants.

Uptake rates of ammonium were estimated for three species of duckweeds in monoculture as well as all two- and three-species combinations (polycultures) in a laboratory experiment in August 2013. The three species studied were Lemna minor, Spirodela polyrhiza, and Wolffia brasiliensis. All co-occur in the northeast US. The experiment was conducted in multiwell plates. Each well (35 mm diameter, 12 mL volume) acted as an individual replicate for a treatment combination. Plants were grown on a liquid nutrient media (Barko-Smart) supplemented with the major nutrients nitrate, ammonium, and phosphate at a level of 5.0 mg N / L (50:50 ammonium to nitrate) and 0.83 mg P / L. The nutrient media was replenished every other day and the experiment lasted 10 days. Growth rates (and relative growth rates) were determined by digital image analysis of pictures taken every other day. Water samples were collected from each replicate on day 2 and day 8 for nutrient analysis. Ammonium concentration was quantified via the method of fluorometric determination with the AquaFluor Handheld Fluorometer (Kérouel & Aminot 1997, Holmes et al. 1999).

In total, over 200 samples were measured for ammonium during the experiment. Significant ammonium uptake rates were observed by all species of duckweeds in this experiment relative to wells that did not contain any plants (i.e., “Blank”) (ANOVA, df = 7, p 0.05). Michael has previously measured ammonium concentration via the colorimetric indophenol blue method (Parsons et al. 1984). Unfortunately, this method is not accurate at low ammonium levels. The method of fluorometric determination of ammonium used with the AquaFluor Handheld Fluorometer provides much lower detection limits. Also, the reagents are inexpensive and the laboratory protocol is simple.

Author: Michael McCann

Institution: Department of Ecology & Evolution, Stony Brook University, Stony Brook, NY USA

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