Trophic basis of invertebrate production in 2 streams at the Hubbard Brook experimental forest

Many forest stream food webs have leaf litter as the primary food resource, but instream primary production can also be quantitatively important, in part because it is more easily assimilated. We estimated the trophic basis of invertebrate production in 2 streams at Hubbard Brook Experimental Forest: Bear Brook (BB), a 2nd-order closed canopy stream, and Main Hubbard Brook (HB), a 5th-order open-canopy stream. We combined secondary production measurements for 1 y with gut content analyses to estimate the fraction of total secondary production derived from various food sources. Secondary production was low in both streams: 4.2 g ash-free dry mass (AFDM) m^{-2 y-1} in BB and 3.0 g AFDM m^-2y^-1 in HB. The amount of primary consumer secondary production derived from algae was 5% in BB and 28% in HB, with the remainder derived from organic detritus. Higher algal availability and lower benthic organic matter storage resulted in a higher fraction of algal consumption in HB relative to BB. Predators consumed ∼72 to 92% of total secondary production, producing high predatory losses of insect production. Algal production was not a large food source in either stream because of low availability, possibly caused by shading in BB and possibly nutrient limitation in both streams.