Growth Patterns of a Chironomid Shredder on Fresh and Senescent Tag Alder Leaves in Two Michigan Streams

Abstract

ABSTRACT A chironomid, Brillia flavifrons Johannsen (Orthocladinii) previously thought to specialize on decaying wood, consumed Tag Alder leaves (Alnus rugosa (Du Roy)) in two Michigan streams, Schwartz Creek and the Ford River. Schwartz Creek's substrate is primarily sand and the principle allochthonous inputs are White Cedar (Thuja occidentalis L.) needles and wood. The Ford River's substrate contains cobble and gravel. The primary inputs are deciduous leaves, with the majority being Tag Alder. Fresh Tag Alder leaves are commonly found in streams in the western Upper Peninsula, owing primarily to high beaver activity and secondarily to wind activity. Substantial green leaf inputs occur at a time when the previous year's leaves have been processed. In Schwartz Creek, B. flavifrons may be the primary shredder species, as the biomass of B. flavifrons exceeded the biomass of all other shredder insect species combined on fresh green Tag Alder leaf packs and autumn senescent Tag Alder leaf packs. This chironomid appears to prefer to feed on fresh leaves: Larval mean biomass and field growth rates were significantly higher on fresh than on autumn leaves. In the Ford River, B. flavifrons is not the primary shredder, as the biomass of other shredder species exceeded the biomass of B. flavifrons on both leaf types. In the Ford River, B. flavifrons appears to prefer to feed on fresh leaves more than autumn leaves: Larval mean sizes were significantly higher on fresh than on autumn leaves. We hypothesize that (1) fresh green leaves are chemically richer and have surfaces that are richer in microflora and microfauna to account for the increased growth rates and/or higher mean biomass values for B. flavifrons on fresh green leaves, and that 2) increased resource competition by other shredder species in the Ford River may account for the lower growth rates and/or lower mean biomass values of B. flavifrons observed on both leaf types in that river.