Application of an in situ gradostat for a natural phytoplankton community in a eutrophic environment

Abstract

The in situ gradostat ( Higashi et al., 1998) was applied to study the effect of oligotrophication on a eutrophic phytoplankton community. The in situ gradostat was launched in a eutrophic experimental pond (2360 ± 166 μg NO 3–N litre −1). Distilled water was pumped into the central (first) chamber and diffused radially outwards into the second and third chambers through the perforated walls, so as to achieve a nutrient gradient of every limnological trophic level from the first to the third chambers within 150 h. With this apparatus, the impact of a nitrate gradient on a natural phytoplankton community in eutrophic Lake Kasumigaura was determined. The mean growth rate of a eutrophic phytoplankton species Oscillatoria tenuis, which often dominates eutrophic environment such as this, was the lowest (−0.3 ± 0.5 day −1 at 209 ± 80 μg NO 3–N litre −1) in the oligotrophic chamber, and intermediate in the mesotrophic chamber (−0.3 ± 0.4 day −1 at 679 ± 130 μg NO 3–N litre −1) and higher in the eutrophic chamber (0.7 ± 0.9 day −1 at 2018 ± 212 μg NO 3–N litre −1). In contrast, the mean growth rate of a non-eutrophic phytoplankton species, Synedra ulna, was higher in the experimentally oligotrophic environment (0.4 ± 0.4 day −1) than that in the mesotrophic environment (−0.7 ± 0.5 day −1).