Low parasite biomass in oligotrophic streams differs from previous estimates in aquatic ecosystems

Parasites may mediate ecosystem functioning through a number of direct and indirect mechanisms, but the importance of parasitism at the ecosystem scale is poorly understood. Measuring the density of free-living and parasitic consumers in units that are directly comparable provides a first step toward understanding the importance of parasitism to ecosystem processes. I sampled 2 streams in the New Jersey Pine Barrens seasonally for 1 y to measure the biomass density of all major consumer groups, including macroparasites infecting fish and macroinvertebrates. Parasites made up a small percentage of consumer biomass in Pine Barrens streams, representing just 0.00643 to 0.00733% of total consumer biomass annually. These low values contrast with higher estimates from other aquatic ecosystems, where parasite biomass exceeds that of some free-living consumers. The mean biomass densities of all consumer groups differed significantly between the 2 streams, perhaps because of stream characteristics, such as productivity or pH. Comparison of parasite biomass density in these 2 streams with that in 3 other types of aquatic ecosystems reveals substantial variation both within and among ecosystem types. Methodological differences among published studies complicate comparisons of parasite biomass across ecosystems. I reviewed the methods used in previous studies on parasite biomass and argue for a consistent and transparent method for future research. Comparing the biomass of free-living and parasitic consumers is a first step toward understanding the ecosystem-level importance of parasitism, but more work is needed to understand the specific mechanisms by which parasites influence ecosystem processes and the magnitude of parasite effects.