Trophic interactions across lake–stream boundaries in mountain lakes

Abstract

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.

Influences of fish on food web structure and function in mountain lakes

“…Zooplankton were not included in the present study because for small taxa, enough organic material could not be collected at a fine enough taxonomic resolution for stable isotope analyses. Fine taxonomic resolution is necessary for understanding trophic position and flow of material of zooplankton because small taxa, which are abundant in high elevation lakes (Detmer et al., ), can be herbivores ( Daphnia rosea ), omnivores ( Diacyclops thomasi ), or predators ( Asplanchna ). Invertebrate and fish samples were rinsed with deionised water, frozen at −20°C, and lyophilised.…”

“…The effects of introduced fish on invertebrate body size, biomass, and community composition in historically fishless lakes also are well documented. In fishless lakes, introduced fish strongly reduce abundance of large invertebrates and total biomass of invertebrates (Carlisle & Hawkins, 1998;Detmer, McCutchan, & Lewis, 2017a;Knapp, Corn, & Schindler, 2001;Schilling, Loftin, & Huryn, 2009). Several studies have suggested that changes in food web structure are probably through the addition of fish (Reissig, Trochine, Queimalinos, Balseiro, & Modenutti, 2006;Schindler, Geib, & Williams, 2000), but none are supported by data of assimilated material, such as can be obtained through naturally abundant stable isotopes or fatty acids, and production.…”