overwinter Temperatures stoneflies 15QdgF hibernate hypolimnion 15QdgC hibernaculum thermalling frost sporidia germinate 2QdgC sabkha bottom anestrus chinook freshet Qdg colder 12QdgC buds solifluction microcrustaceans Precipitation Diapause Acephala 68QdgF solstice austral midwater 30QdgC dormancy germination