Acidification effects on the plankton size spectrum: an in situ mesocosm experiment

Triplicate in situ enclosures containing plankton from a small glacial kettle lake were either untreated (pH>8) or were acidified to pH 6.5, 5.5 or 4.5 over 7 days using H2SO4. Plankton size spectra were constructed, in order to quantify acidification impacts on mean phytoplankton size (MESDp), mean zooplankton size (MESDz), and phytoplankton-zooplankton size difference (the P-Z distance). Acidification to pH 6.5 did not significantly affect the size spectrum parameters. However, at pH 5.5 and 4.5, MESDp increased, MESDz declined, and the P-Z distance was greatly reduced. These changes reflected a simultaneous shift to large phytoplankton (Peridinium) and small zooplankton (rotifers and nauplii) dominance at low pH.