Many freshwater systems receive substantial inputs of terrestrial organic matter. Terrestrially derived dissolved organic carbon (t‐DOC) inputs can modify light availability, the spatial distribution of primary production, heat, and oxygen in aquatic systems, as well as inorganic nutrient bioavailability. It is also well‐established that some terrestrial inputs (such as invertebrates and fruits) provide high‐quality food resources for consumers in some systems. In small to moderate‐sized streams, leaf litter inputs average approximately three times greater than the autochthonous production. Conversely, in oligo/mesotrophic lakes algal production is typically five times greater than the available flux of allochthonous basal resources. Terrestrial particulate organic carbon (t‐POC) inputs to lakes and rivers are comprised of 80%–90% biochemically recalcitrant lignocellulose, which is highly resistant to enzymatic breakdown by animal consumers. Further, t‐POC and heterotrophic bacteria lack essential biochemical compounds that are critical for rapid growth and reproduction in aquatic invertebrates and fishes. Several studies have directly shown that these resources have very low food quality for herbivorous zooplankton and benthic invertebrates. 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 contribution. Amino acid stable isotope analyses for large river food webs indicate that most upper trophic level essential amino acids are derived from algae. Similarly, profiles of essential fatty acids in consumers show a strong dependence on the algal food resources. Primary production to respiration ratios are not a meaningful index to assess consumer allochthony because respiration represents an oxidised carbon flux that cannot be utilised by animal consumers. Rather, the relative importance of allochthonous subsidies for upper trophic level production should be addressed by considering the rates at which terrestrial and autochthonous resources are consumed and the growth efficiency supported by this food. Ultimately, the biochemical composition of a particular basal resource, and not just its quantity or origin, determines how readily this material is incorporated into upper trophic level consumers. Because of its highly favourable biochemical composition and greater availability, we conclude that microalgal production supports most animal production in freshwater ecosystems.

Ohio River Zooplankton were collected monthly or quarterly for 1 yr from littoral and pelagic (main channel) areas in three navigation pools (constricted and floodplain) with four intrapool locations (lower pool and above, below, and within major tributaries). Total densities were minimal (1.64/L) when discharge and turbidities peaked (December–April) but were relatively high otherwise (21.63/L). Seasonal rises in rotifer density preceded increases in cladocerans and copepods; rotifers were nearly twice as abundant as cladocerans and copepods. Diversity (species richness) was not correlated with temperature, velocity, or turbidity. Density was positively linked with temperature and negatively correlated with river velocity and turbidity. Diversities in littoral and pelagic areas were not different, but densities were higher nearshore (mostly copepods and cladocerans); rotifers were usually more common in the main channel. Neither proximity to low-head navigation dams nor channel morphology had detectable effects on zooplankton. Although total diversity and density were similar in tributaries and the main channel, rotifers were more abundant in tributaries, and the Ohio contained more cladocerans and copepods. Densities in the Ohio were greater below than above tributaries for rotifers and cladocerans but lower for copepods, primarily because of nauplii.