How important are terrestrial organic carbon inputs for secondary production in freshwater ecosystems?

Abstract

1. Many freshwater systems receive substantial inputs of terrestrial organic matter.Terrestrially derived dissolved organic carbon (t-DOC) inputs can modify lightavailability, the spatial distribution of primary production, heat, and oxygen inaquatic systems, as well as inorganic nutrient bioavailability. It is also well-estab-lished that some terrestrial inputs (such as invertebrates and fruits) provide high-quality food resources for consumers in some systems.2. In small to moderate-sized streams, leaf litter inputs average approximately threetimes greater than the autochthonous production. Conversely, in oligo/me-sotrophic lakes algal production is typically five times greater than the availableflux of allochthonous basal resources.3. Terrestrial particulate organic carbon (t-POC) inputs to lakes and rivers are com-prised of 80%–90% biochemically recalcitrant lignocellulose, which is highly resis-tant to enzymatic breakdown by animal consumers. Further, t-POC andheterotrophic bacteria lack essential biochemical compounds that are critical forrapid growth and reproduction in aquatic invertebrates and fishes. Several stud-ies have directly shown that these resources have very low food quality for her-bivorous zooplankton and benthic invertebrates.4. Much of the nitrogen assimilated by stream consumers is probably of algal origin,even in systems where there appears to be a significant terrestrial carbon contri-bution. Amino acid stable isotope analyses for large river food webs indicate thatmost upper trophic level essential amino acids are derived from algae. Similarly, profiles of essential fatty acids in consumers show a strong dependence on thealgal food resources.5. Primary production to respiration ratios are not a meaningful index to assessconsumer allochthony because respiration represents an oxidised carbon fluxthat cannot be utilised by animal consumers. Rather, the relative importance ofallochthonous subsidies for upper trophic level production should be addressedby considering the rates at which terrestrial and autochthonous resources areconsumed and the growth efficiency supported by this food.6. Ultimately, the biochemical composition of a particular basal resource, and notjust its quantity or origin, determines how readily this material is incorporatedinto upper trophic level consumers. Because of its highly favourable biochemicalcomposition and greater availability, we conclude that microalgal production sup-ports most animal production in freshwater ecosystems.