Does quality trump quantity? Ecosystem-level consequences of subsidies in a forested stream

Background/Question/Methods

High-quality subsidies like terrestrial invertebrate prey for fish may have low magnitude fluxes when compared to other inputs to streams (e.g., terrestrial leaf litter subsidies), but have been shown to have disproportionate effects on community dynamics and food webs.  Although small fluxes appear unimportant when considered in the context of an ecosystem-level budget, the consequences of low quantity/high quality subsidies for ecosystem processes like rates of primary and secondary production have rarely been considered.  We asked, what is the relative contribution of two terrestrial subsidies that vary in quality and quantity (leaf litter vs. terrestrial invertebrates) to animal production and organic matter flow through the food web of a forested stream? To address this question, we compiled existing data from previous studies in Horonai Stream (Hokkaido, Japan) that had never been analyzed in an ecosystem context, including fluxes leaf litter and terrestrial invertebrate prey, monthly estimates of benthic macroinvertebrate and periphyton standing crops, and seasonal estimates of fish populations and diets. We also collected new data including spring and fall estimates of whole-stream metabolism.  We synthesized this data to calculate secondary production and trophic basis of production of fishes and to construct a partial stream organic matter budget. 

Results/Conclusions

The organic matter budget of Horonai stream is dominated by DOC with equal input and output of 2910 g ash-free dry mass (AFDM)/m²/yr, followed by inputs of leaf litter at 274 g AFDM/m²/yr.  The annual flux of terrestrial invertebrate biomass is 13.4 g AFDM/m²/yr, which equals only 5% of the leaf litter flux, but they are temporally offset with peaks occurring in August/September and October /November, respectively.  Annual fish production was 2.6 g AFDM/m²/yr and peaked during the spring and summer (0.002 g AFDM/m²/d). Terrestrial invertebrates contribute 44% of the organic matter budget of the fish assemblage, yet this is a small portion of the overall stream organic matter budget. As expected, whole-stream metabolism suggested that rates of primary production in this stream are low.  However, standing crop of primary producers can reach 22 g AFDM/m² in November and December when invertebrate abundance is low, and results of previous experiments demonstrated that primary producer standing crops are closely controlled by cascading trophic interactions from fish to invertebrates, which are in turn mediated by the supply of terrestrial prey to fish. Future analyses will focus on these potential indirect effects of terrestrial subsidies on primary and invertebrate production.