Food web structure in a harsh glacier-fed river

Abstract

Glacier retreat is occurring across the world, and associated river ecosystems are expected to respond more rapidly than those in flowing waters in other regions. The river environment directly downstream of a glacier snout is characterised by extreme low water temperature and unstable channel sediments but these habitats may become rarer with widespread glacier retreat. In these extreme environments food web dynamics have been little studied, yet they could offer opportunities to test food web theories using highly resolved food webs owing to their low taxonomic richness. This study examined the interactions of macroinvertebrate and diatom taxa in the Ödenwinkelkees river, Austrian central Alps between 2006 and 2011. The webs were characterised by low taxon richness (13-22), highly connected individuals (directed connectance up to 0.19) and short mean food chain length (2.00-2.36). The dominant macroinvertebrates were members of the Chironomidae genus Diamesa and had an omnivorous diet rich in detritus and diatoms as well as other Chironomidae. Simuliidae (typically detritivorous filterers) had a diet rich in diatoms but also showed evidence of predation on Chironomidae larvae. Food webs showed strong species-averaged and individual size structuring but mass-abundance scaling coefficients were larger than those predicted by metabolic theory, perhaps due to a combination of spatial averaging effects of patchily distributed consumers and resources, and/or consumers deriving unquantified resources from microorganisms attached to the large amounts of ingested rock fragments. Comparison of food web structural metrics with those from 62 published river webs suggest these glacier-fed river food web properties were extreme but in line with general food web scaling predictions, a finding which could prove useful to forecast the effects of anticipated future glacier retreat on the structure of aquatic food webs.

Figure 1. Ödenwinkelkees river sampling site.

Stream width in the foreground is approximately 5 m.

Figure 2. Food webs collected in Ödenwinkelkees river.

(a) 2006, (b) 2008, (c) 2011 and (d) composite of the three webs. Green nodes represent primary producers, red nodes represent consumers. Green lines represent feeding links and blue lines represent cannibalistic links. Food web diagrams created using FoodWeb3D .

Figure 3. Path length (d) distributions for the four food webs.

Path lengths were calculated as the fewest links connecting each pair of taxa, measured for each pair of nodes.

Figure 4. Relationship between taxon richness (S) and number of links (L).

Ödenwinkelkees food webs are shown in black and reported literature values as open circles. Grey symbols highlight the alpine stream food webs collected by Lavandier & Décamps .

Figure 5. Cumulative distribution of trophic links per species for the four food webs.

Y-axis indicates number of trophic links. All linear relationships P<0.05.

Figure 6. Food web body size relationships derived from the 2011 food web.

(a) relationship between body mass of individual consumers and resources; (b) relationship between body mass of taxon and trophic height; (c) relationship between species averaged body mass (M) and abundance (N); (d) relationship between individual M and N* (normalized abundance, see methods). (Error terms in parentheses for b and c denote 95% confidence intervals; NB. Data shown for log₂ integer approach only); (e) upper angle (A_upper) versus lower angle (A_lower) of all three- node chains within food webs; (f) two-span as a function of between angle (A_between) in three node chains. See Table 1 caption for definition of these measures.