Longitudinal variation in the nutritional quality of basal food sources and its effect on invertebrates and fish in subalpine rivers

1. There is growing recognition of the importance of food quality over
quantity for aquatic consumers. In streams and rivers, most previous
studies considered this primarily in terms of the quality of terrestrial
leaf litter and importance of microbial conditioning. However, many recent
studies suggest that algae are a more nutritional food source for riverine
consumers than leaf litter. To date, few studies have quantified
longitudinal shifts in the nutritional quality of basal food resources in
river ecosystems and how these may affect consumers. 2. We conducted a
field investigation in a subalpine river ecosystem in Austria to
investigate longitudinal variations in diet quality of basal food sources
(submerged leaves and periphyton) and diet source dependence of stream
consumers (invertebrate grazers, shredders, filterers and predators, and
fish). Fatty acid (FA) profiles of basal food sources and their consumers
were measured. 3. Our results indicate systematic differences between the
FA profiles of terrestrial leaves and aquatic biota, i.e., periphyton,
invertebrates and fish. Submerged leaves contained very low proportions of
long-chain polyunsaturated fatty acids (LC-PUFA), which were conversely
rich in aquatic biota. While the FA composition of submerged leaves
remained similar among sites, the LC-PUFA of periphyton increased
longitudinally, which was associated with increasing nutrients from
upstream to downstream. 4. Longitudinal variations in periphyton LC-PUFA
were reflected in the LC-PUFA of invertebrate grazers and shredders, and
further tracked by invertebrate predators and fish. However, brown trout
(Salmo trutta) contained a large proportion of docosahexaenoic acid (DHA,
22:6ω3), a LC-PUFA almost entirely missing in basal sources and
invertebrates. The fish accumulated eicosapentaenoic acid (EPA, 20:5ω3)
from invertebrate prey and may use this FA to synthesize DHA. 5. Our
results provide a nutritional perspective for river food web studies,
emphasizing the importance of algal resources to consumer somatic growth
and the need to account for the longitudinal shifts in the quality of
these basal resources.