Zooplankton induced decrease in inorganic phosphorus uptake by plankton in an oligotrophic lake

Abstract

Experiments conducted on samples collected from a large oligotrophic lake revealed the following: (1) excretion rates of PO inf4 sup3− by single Daphnia thorata were below detection (5 pmol animal−1 min−1) in 20 ml of oligotrophic lake water over a period of 10 min, (2) experimental addition of D. thorata to 20 ml aliquots of lake water decreased community-wide microbial uptake of PO inf4 sup3− on two occasions (as measured by 32PO inf4 sup3− incorporation), and (3) the presence of D. thorata increased uptake by organisms smaller than 1µm, and decreased uptake by large phytoplankton. The specific mechanism for these responses remains unclear, but the results imply that when phytoplankton larger than 1µm encounter cm scale patches of water recently occupied by Daphnia they may experience decreased PO inf4 sup3− availability rather than elevated concentrations of PO inf4 sup3− caused by excretion. We show that 32P uptake experiments using natural plankton assemblages can be influenced by the presence or absence of large zooplankton, and that neither grazing, turbulence, nor PO inf4 sup3− excretion can account for this influence.