Trophic interactions across lake–stream boundaries in mountain lakes

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Fish can affect the mass flux of invertebrates across aquatic ecosystem boundaries, as shown by the effects of fish on export of zooplankton from lakes to outlet streams in 12 oligotrophic lakes in Rocky Mountain National Park (RMNP), Colorado (6 lakes and streams with fish; 6 without fish). Lakes without fish exported to streams 5 times as much zooplankton biomass as lakes with fish because of strong suppression of large zooplankton in lakes with fish. Of the zooplankton biomass exported to streams from both types of lakes, 95% was lost within 500 m of the lakes; the greatest rate of loss occurred within 10 m. Higher rates of loss for large zooplankton in streams below lakes without fish suggest that fish predation is not the primary cause for loss of zooplankton below lakes. In streams with and without fish, small zooplankton were lost at much lower rates downstream than large zooplankton. Daylight and nocturnal sampling showed much higher exports of zooplankton and, especially for large taxa, from lakes to streams at night, presumably because of diel vertical migration. Stream locations near a lake outlet had high total macroinvertebrate biomass, especially for filtering and gathering collectors as well as detritivores, indicating that macroinvertebrate predation is likely related to zooplankton mass flux. Large zooplankton also could have high vulnerability to interception by the channel substrate, which could stimulate detritivory. Biomass of all macroinvertebrates, and specifically of the filtering and gathering collectors, was lower in streams with fish because fish suppressed the export of zooplankton food for macroinvertebrates from the lakes and directly consumed macroinvertebrates. Therefore, the RMNP lakes and their outlet streams demonstrate trophic compression of stream macroinvertebrates by fish, during which macroinvertebrates are simultaneously pressured by increasing top-down forces from direct predation and reduced bottom-up forces because zooplankton are suppressed by fish in lakes.