Abstract
Stream ecological theory predicts that the use of allochthonous resources declines with increasing channel width, while at the same time primary production and autochthonous carbon use by consumers increase. Although these expectations have found support in several studies, it is not well known how terrestrial runoff and/or inputs of primary production from lakes alter these longitudinal patterns. To investigate this, we analyzed the diet of filter-feeding black fly and caddisfly larvae from 23 boreal streams, encompassing gradients in drainage area, land cover and land use, and distance to nearest upstream lake outlet. In five of these streams, we also sampled repeatedly during autumn to test if allochthony of filter feeders increases over time as new litter inputs are processed. Across sites, filter-feeder autochthony was 21.1–75.1%, did not differ between black fly and caddisfly larvae, was not positively related to drainage area, and did not decrease with distance from lakes. Instead, lake and wetland cover promoted filter-feeder autochthony independently of stream size, whereas catchment-scale forest cover and forestry reduced autochthony. Further, we found no seasonal increase in allochthony, indicating low assimilation of particles derived from autumn litter fall. Hence, catchment properties, rather than local conditions, can influence levels of autochthony in boreal streams.
Highlights
In aquatic systems, in situ primary production provides basal resources that are high in essential nutrients, and important for growth and physiology of consumers (Brett et al, 2009; Gladyshev et al, 2009)
In contrast to our hypothesis, we found a significant (t = - 2.230, P = 0.026) negative trend in black fly larvae dD values over time, but this was mainly driven by sites DJA and VEB (Fig. 2)
In contrast to our first hypothesis, i.e., that the level of autochthony would increase with increasing stream size, we found no systematic change in autochthony/allochthony with increasing drainage area
Summary
In situ (i.e., autochthonous) primary production provides basal resources that are high in essential nutrients, and important for growth and physiology of consumers (Brett et al, 2009; Gladyshev et al, 2009). Even in lakes and large rivers, which are often autochthonous based (Carpenter et al, 1985; Thorp et al, 1998), inputs of terrestrial dissolved organic matter (DOM) can serve as an important resource (Berggren et al, 2010; Fey et al, 2015). Because terrestrially derived resources are of relatively low quality to stream consumers, these resources differ in their ability to support production at higher trophic levels (Brett et al, 2017). It is, important to understand what drives the availability and use of these different types of resources
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