Overwintering populations of Anabaena, Aphanizomenon and Microcystis as potential inocula for summer blooms

Overwintering cyanobacterial populations of Nostocales and Microcystis were investigated in six freshwater reservoirs in Northwestern Spain during two consecutive winters.Surface sediments hosted 10 3 -10 5 akinetes mL 21 and 10 2 -10 4 Microcystis colonies mL 21 .Sediments from deeper areas close to dam accumulated 2-fold (Microcystis) and 11-fold (akinetes) greater concentrations than those at the shallower upstream areas.Anabaena spp.and Microcystis aeruginosa dominated the sediment pool, with minor amounts of akinetes of Aphanizomenon (Aph.flos-aquae, Aph.gracile) and benthic Nostocales (Cylindrospermum, Nostoc and Trichormus).Our study confirms the dual benthic-pelagic overwintering of Anabaena, Aphanizomenon and Microcystis, found in the pelagial at 7.5-9.88C.This study also provides an insight into the little known annual cycle of potential cyanotoxin-producers Aph.gracile and Anabaena circinalis.Our estimates show that: (i) only a small fraction (,1%) of the sediment pool of akinetes and Microcystis was resupended in the bottom water during winter which, however, may be sufficient inocula to build up the summer maxima under realistic in situ growth rates; and (ii) the time required for the development of summer populations is mainly driven by growth rates, and therefore by the environmental conditions faced by the inoculum, with a lower influence (although greater for Microcystis than for Nostocales) of the inoculum size.