ZOOPLANKTON INDUCED DECREASE IN INORGANIC PHOSPHORUS UPTAKE BY PLANKTON IN AN OLIGOTROPHIC LAKE

Experiments conducted on samples collected from a large oligotrophic lake revealed the following: (1) excretion rates of PO4(3-) 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 PO4(3-) on two occasions (as measured by (PO4(3-))-P-32 incorporation), and (3) the presence of D. thorata increased uptake by organisms smaller than 1-mu-m, and decreased uptake by large phytoplankton. The specific mechanism for these responses remains unclear, but the results imply that when phytoplankton larger than 1-mu-m encounter cm scale patches of water recently occupied by Daphnia they may experience decreased PO4(3-) availability rather than elevated concentrations of PO4(3-) caused by excretion. We show that P-32 uptake experiments using natural plankton assemblages can be influenced by the presence or absence of large zooplankton, and that neither grazing, turbulence, nor PO4(3-) excretion can account for this influence.