Forests fuel fish growth in freshwater deltas

Abstract

Aquatic ecosystems are fuelled by biogeochemical inputs from surrounding lands and within-lake primary production. Disturbances that change these inputs may affect how aquatic ecosystems function and deliver services vital to humans. Here we test, using a forest cover gradient across eight separate catchments, whether disturbances that remove terrestrial biomass lower organic matter inputs into freshwater lakes, thereby reducing food web productivity. We focus on deltas formed at the stream-lake interface where terrestrial-derived particulate material is deposited. We find that organic matter export increases from more forested catchments, enhancing bacterial biomass. This transfers energy upwards through communities of heavier zooplankton, leading to a fourfold increase in weights of planktivorous young-of-the-year fish. At least 34% of fish biomass is supported by terrestrial primary production, increasing to 66% with greater forest cover. Habitat tracers confirm fish were closely associated with individual catchments, demonstrating that watershed protection and restoration increase biomass in critical life-stages of fish.

Allochthonous dissolved organic carbon in river, lake and coastal systems: transport, function and ecological role

“…The changes in microbial food structure as a result of allochthonous carbon inputs are suggested to influence higher trophic levels with studies showing substantial subsidies from terrestrial sources in lakes (Tanentzap et al 2014;Karlsson et al 2015) whereas others argue that autochthonous sources of carbon are the dominant form (Brett et al 2009). Roach (2013) in a review and meta-analysis of food sources for large rivers using stable isotope data found that algal sources dominated but also found that many consumers did assimilate material from C 3 plants in large rivers with high sediment load and low transparency during high-flow pulses.…”

“…More recently, paradigms about autochthonous carbon dominating lakes are being questioned (Cole et al 2011). The significance of allochthonous DOC to aquatic ecosystems is being further recognised with the understanding that terrestrial subsidies are playing a much greater role in some systems than previously thought (Tanentzap et al 2014).…”

Bridging between litterbags and whole-ecosystem experiments: a new approach for studying lake sediments

“…Our interest was in manipulating the quantity and quality of terrestrial organic matter (tOM) that accumulated in littoral sites under different water qualities. Previously, we discovered that the productivity of bacteria, zooplankton, and young-of-the-year fish in a single lake environment was greater beneath catchments that received larger inputs of tOM (Tanentzap et al, 2014). We were subsequently interested in testing how tOM was processed and mobilized into aquatic food webs at much greater resolution.…”

“…Vertically-migrating animals were collected on two nights toward the end of our pore water sampling using a 500-mL funnel trap deployed at a height of 5 cm above the centre of each sediment sample as in Tanentzap et al, (2014). Traps were collected the morning after deployment, immediately filtered through an 80-μm mesh, and live-sorted into pure zooplankton.…”

Carbon transfer from dissolved organic carbon to the cladoceranBosmina: a mesocosm study

“…Despite evidence of terrestrial support for secondary production in lakes (Berggren et al, 2010a,b;Tanentzap et al, 2015), the transfer of t-DOC to higher trophic levels remains contentious (Ducklow et al, 1986;Brett et al, 2012;Kelly et al, 2016).…”