Vertical migration, entrainment and photosynthesis of the freshwater dinoflagellate Peridinium cinctum in a shallow urban lake

Vertical migration, entrainment and photosynthesis of Peridinium cinctum were investigated in the Torrens Lake, South Australia. Cell distribution was a function of swimming speed, shear velocity and the surface mixed layer depth. During the morning, when the wind speed was low, cells migrated upwards, at a velocity of 2.35 ×10−4 m s−1 and accumulated at 0.7 m (827 μmol m−2 s−1 at 13:00 h). After 13:00 h, cells migrated downwards to 3.3–3.6 m at 1.85 × 10−4 m s−1. As wind speed increased in the afternoon, the shear velocity of the surface mixed layer exceeded the swimming speed, leading to a homogeneous cell distribution within the surface mixed layer. Below the surface mixed layer, a deep chlorophyll maximum persisted. The observations were found to fit in an entrainment function (ψ), where if ψ > 1, cells are disentrained, and if ψ < 1, cells are entrained. The maximum effective quantum yield of photosystem II (F′v/F′m) was used as an index to monitor the photosynthetic performance of P. cinctum and the ratio of bottle (fixed depth) to lake F′v/F′m indicated whether or not migration or mixing was enhancing photosynthesis or preventing photoinhibition. A small (0.1–0.2 units) but significant depression in F′v/F′m was observed in P. cinctum in bottle and lake surface samples during stratified conditions. However, this recovered to initial values later in the day. The ratio of bottle to lake F′v/F′m was consistent with P. cinctum migration patterns. A comparison between modelled daily photosynthetic rates of the observed migrating and a theoretical homogeneous population revealed that migration would not increase photosynthetic rates within the Torrens Lake. The net rates of O2 production in the water column from dawn to dusk for the migrating and evenly distributed cell populations were 2574 and 3120 mg m−2, respectively.