Effects of an extraordinarily harsh winter on macroinvertebrates and fish in boreal streams

Trout density and macroinvertebrate species richness, community structure and abundance were studied before and after an extraordinarily harsh winter with low temperatures and thin snow cover, in nine medium-sized streams in central Sweden. Reductions in trout density (77 %), macroinvertebrate abundance (44 %) and taxonomic richness (43 %) were documented. Large reductions were recorded in most taxa, but with blackflies, Orthocladiinae and Lepidostoma hirtum (Trichoptera) being notable exceptions. Reductions in macroinvertebrate abundance were positively related to the number of days with supercooled water. A partial least squares regression analysis showed a positive relationship between number of days of supercooling and distance to an upstream lake, and negative relationships with stream width and catchment area. Most taxa recovered to pre-disturbance levels after three years. The proportion of less vagile taxa (crustaceans, molluscs, water mites, oligochaetes, flatworms, leeches and beetles combined) was still significantly reduced after three years. Extraordinarily harsh winters can be described as pulse disturbances, inducing temporary effects on total abundance and taxonomic richness, but also with long-term effect on certain taxa, including those that are less vagile.