Shifts in habitat templates for lotic microalgae linked to interannual variation in snowmelt intensity

We investigated the importance of snowmelt as an organizing factor for epil ithic microalgae in a high-altitude montane stream during 3 yr (1995-1997) of differing melt characteristics. Changes in algal biomass and taxonomic s tructure in two stream reaches that differed in hydrologic characteristics were assessed relative to variation in stream-water nutrient content, tempe rature, surface-water/groundwater exchange, and algivorous macroinvertebrat e assemblages. Melt-induced increases in stream discharge were large and ab rupt in 1995, nearly undetectable in 1996, and protracted and of intermedia te magnitude in 1997. In 1995, algal and grazer densities were significantl y reduced by melt discharge. Postmelt grazer recovery was slow, and the per centage of live cells in the diatom assemblage increased abruptly; algal bi omass initially increased and subsequently varied with stream-water N:P rat io. In 1996, snowmelt produced no evident proximate effects. Initial high g razer densities declined throughout the summer. In midsummer, an N:P ratio of 140 in the upstream reach corresponded to a more than fourfold increase in algal biovolume and a shift to dominance by large diatoms. Downstream, a lower N:P peak induced no algal response. In 1997, grazer densities declin ed during protracted melt runoff and increased sharply during melt recessio n, concurrent with decreases in live diatom percentage and algal biovolume. N:P declined in 1997 from similar to 16:1 to values indicative of N limita tion and correlated with decreases in algal biovolume. Our results show tha t effects of snowmelt in montane streams reflect both initial melt-induced mortality that sets initial conditions for succession and melt-induced aqui fer recharge that controls nutrient supply in the months following peak mel t discharge. The influence of these two components extends beyond snowmelt recession into summer base flow, suggesting that variation in melt characte ristics generates interannual differences in the functioning of these syste ms.