Autochthonous resources are the main driver of consumer production in dystrophic boreal lakes

Abstract

Dystrophic lakes are widespread in temperate regions and intimately interact with surrounding terrestrial ecosystems in energy and nutrient dynamics, yet the relative importance of autochthonous and allochthonous resources to consumer production in dystrophic lakes remains controversial. We argue that allochthonous organic matter quantitatively dominates over photosynthetic autotrophs in dystrophic lakes, but that autotrophs are higher in diet quality and more important for consumers as they contain essential polyunsaturated fatty acids (PUFA). In a field study, we tested the hypotheses that (1) autochthonous primary production is the main driver for consumer production, despite being limited by light availability and low nutrient supplies, and greater supply of allochthonous carbon, (2) the relative contribution of autotrophs to consumers is directly related to their tissue PUFA concentrations, and (3) methane-oxidizing bacteria (MOB) provide an energy alternative for consumers. Pelagic and benthic consumer taxa representing different trophic levels were sampled from five dystrophic lakes: isopod Asellus aquaticus, megalopteran Sialis lutaria, dipteran Chaoborus flavicans, and perch Perca fluviatilis. Based on carbon and nitrogen stable isotopes, the relative contributions of autochthonous (biofilms and seston) and allochthonous (coarse particulate and dissolved organic matter) resources and MOB to these taxa were 47-79%, 9-44% and 7-12% respectively. Results from fatty acid (FA) analyses show that the relative omega3-FA and PUFA concentrations increased with trophic level (Asellus < Sialis and Chaoborus < Perca). Also, eicosapentaenoic-acid (EPA), omega3-FA and PUFA concentrations increased with the autochthonous contribution in consumers, i.e., a 47-79% biofilm and/or seston diet resulted in tissue EPA of 4.2-18.4, omega3 FAs of 11.6-37.0 and PUFA of 21.6-61.0 mg/g dry mass. The results indicate that consumers in dystrophic lakes predominantly rely on energy from autotrophs and that their PUFA concentrations are dependent on the relative contribution of these autochthonous resources. The limited energy support from MOB suggests they are not negligible and are potentially an integral part of the food webs. Our findings show that autochthonous resources are the main driver of secondary production even in dystrophic lakes and offer new insights into the functioning of these ecosystems.

Reconciling the role of organic matter pathways in aquatic food webs by measuring multiple tracers in individuals

“…By measuring dietary source pathways leading to individuals, our results show that the full spectrum of organic matter sources available to these river food webs is used to some degree, consistent with findings from small temperate lakes , Lau et al 2014. Animals exploited these different pathways during different life stages, with smaller individuals coupled to the water column where they fed on plankton rich in PUFA that supported somatic growth and development (Arts and Kohler 2009).…”

Periphyton support for littoral secondary production in a highly humic boreal lake

“…Similar results have recently been reported from dystrophic lakes in Sweden (Lau et al 2014) and humic bog pools in Estonia (van Duinen et al 2013). The restricted light penetration into humic lakes commonly constrains pelagic and benthic primary production and ultimately secondary production (Karlsson et al 2009, Jones et al 2012), but littoral vegetation can provide substrata for substantial periphyton growth that can sustain rich invertebrate communities.…”

Terrestrial support of lake food webs: Synthesis reveals controls over cross-ecosystem resource use

“…Concurrently, we have discovered the conditions that make high allochthony possible, helping to explain the tremendous discrepancy observed across stable isotope studies of lake food webs over the last two decades ( 6 – 8 , 16 , 21 , 23 , 25 , 26 , 28 ). The lowest mean estimate (11%) of allochthony reported here exceeds that observed by others, possibly because our nonrandom sample of study sites largely lacked clear deepwater and eutrophic lakes where primary production is relatively high ( 5 , 25 ).…”

“…Second, we tested whether the model was robust to missing end members. Other resources, such as MOB, certainly contributed to secondary production ( 16 , 26 , 28 , 62 , 64 ). Therefore, we simulated data with 10, 20, and 40% use of MOB, which had distinct isotopic signatures from terrestrial or pelagic resources (methods S3).…”