Effects of aquatic bryophytes and long-term fertilization on arctic stream insects

Abstract

The effect of bryophytes on stream macroinvertebrates has gained attention in many temperate systems, but is not well understood relative to the role of mineral substrates. Long-term fertilization of the Kuparuk River in arctic Alaska has resulted in dense growths of bryophytes (the moss Hygrohypnum), and a recovery zone created in 1996 left an unfertilized reach of moss-dominated riffles. We examined aquatic insect density, drift, and growth during 1996 and fine and coarse particulate organic matter (FPOM and CPOM) in transport in the reference, recovery, and fertilized zones of the Kuparuk River, to quantify the impact of moss on invertebrate assemblages, epiphytic diatoms, and detrital export. We also used enclosures to examine the effects of fertilization, position in the streambed, and substrate type on insect densities. Moss increased densities of Ephemerella aurivilla and Chironomidae, but had no effect on Baetis spp. or Simuliidae densities. Drift densities of Baetis spp. and Ephemerella were similar in the 3 fertilization regimes, yet ratios of Ephemerella drift densities to benthic densities were highest in the unfertilized zone. Both Baetis and Ephemerella grew larger in the fertilized zone, likely the result of increased ephiphytic diatom densities rather than the presence of moss. Enclosure experiments showed that substrate type (i.e., bare rock, natural and artificial moss) had no effect on any taxa except for Ephemerella, which had higher densities in complex (moss) microhabitats compared to bare-rock microhabitats. The presence of moss beds did not significantly alter FPOM concentrations, but CPOM in transport was greater downstream of moss beds in the recovery zone compared to the fertilized zone. Nutrient-induced moss beds affect arctic stream invertebrate communities by enhancing selected taxa through increased habitat complexity (e.g., Ephemerella, Chironomidae), and by enhancing growth of selected taxa through increased periphyton abundance (e.g., Ephemerella, Baetis). However, taxa that showed positive density responses to mosses did so because of increased habitat complexity, rather than enhanced food resources.