The Fundamental Role of Periphyton in Supplying Dietary Energy to Alpine Stream Food Webs: Current Knowledge, Limitations and Future Perspectives

Abstract

Periphyton is a key basal resource in stream food webs. It provides consumers with more easily digestible carbon than terrestrial resources, and is a major source of long-chain polyunsaturated fatty acids (LC-PUFA), which are essential for consumers. In particular, omega-3 and omega-6 LC-PUFA are important for somatic growth and reproduction of aquatic consumers and for the neural development of animals. In this review we synthetize the current knowledge about biomass and nutritional quality of periphyton in alpine and subalpine streams. Our discussion is centered on streams of the European Alps, where spatial and temporal patterns of periphyton biomass and nutritional quality have been more extensively studied, but emphasises that similar patterns may also occur in other mountain region worldwide. Furthermore, we highlight the limitations of the current knowledge, and outline future research requirements for a comprehensive assessment of periphyton's role as a resource for consumers. Existing studies show that regional climate and environmental conditions drive seasonal patterns of periphyton biomass in high-elevation streams. In the European Alps, periphyton biomass peaks typically occur in spring and/or autumn when conditions are more favourable (Windows of Opportunity). Studies also suggests that streams with different nutrient and disturbance regimes can exhibit different levels of periphyton biomass. For example, surveyed glacial streams were on average less productive (mean = 23.2 mg Chl-a m−2) than springs and groundwater-fed streams (mean = 66.3 mg Chl-a m−2). The nutritional quality of periphyton also varies seasonally and spatially. In subalpine streams, the content of the omega-3 LC-PUFA eicosapentaenoic acid (EPA) in periphyton is higher in autumn, when temperatures are lower, than in summer and increases longitudinally with a moderate increase in nitrogen and phosphorus concentrations of stream water. Seasonal and spatial variations of periphyton LC-PUFA content are also affected by light intensity (canopy cover). Specifically, the content of the omega-6 LC-PUFA arachidonic acid (ARA) decreases at higher light intensities. Overall, the assessment of periphyton biomass and nutritional quality in alpine and subalpine streams has received little attention so far. Especially in high-elevation catchments, periphyton quality has not been studied yet. This review presents the ecological importance of dietary energy of periphyton in alpine and subalpine streams for herbivores and for consumers at higher trophic levels, including fish, and terrestrial consumers, such as insectivorous birds and other vertebrates. The assessment of the current biomass and nutritional status of periphyton is necessary to predict how mountain stream ecosystems will respond to current and future climatic scenarios.