Classical alpine stream types on the equator: are they different?

Abstract

Alpine headwater streams can be classified according to origin: those fed by groundwater (spring streams or krenals), by glacial melt-water (glacial streams or kryals), or by surficial runoff from rain-water and snow melt ( occasionally denoted rhithrals, but in the following called "runoff streams"; WARD 1994). FüREDERetal. (2002)and BROWN et al. (2003)provide comprehensive treatments on alpine stream types. In temperate mountains, glacial streams have daily cycles and large seasonal variations in discharge (reaching a maximum during late summer glacial ablation). Runoff streams have a predictable peak in discharge in spring and early sumrner dueto melting of accumulated snow or during the seasons ofmaximum rainfall, while spring streams are relatively stable with respect to physicochemical parameters. Differences in temperature and hydrological regime between these alpine stream types are reflected in the composition and richness oftheir benthic macroinvertebrate faunas, forming characteristic biocoenoses (WARD 1994 ). These characteristic coenoses seem most distinct during high-flow conditions in summer (MILNER et al. 2001). Equatorial alpine streams differ from those at temperate/ arctic latitudes in a number of ways. Due to a tree line situated ;?: 3000 m a.s.l., they lie at considerably higher elevations than temperate ones. This does not necessarily imply lower water temperatures than in temperate alpine streams, but it does produce a lower saturation of dissolved oxygen due to the reduced atmospheric pressure at higher altitudes, and this is suspected to limit taxon richness (JACOBSEN et al. 2003 ). In addition, in equatorial mountains glacial ablation does not vary much with season and snow does not accumulate. The main variation in discharge of equatorial glacier streams is the characteristic die! cycle in flow-regime due to day-time melting and night-time freezing of the glacier. Equatorial alpine runoff streams have more stochastic, shortterm variation in discharge, driven by rain showers occurring year-round. The absence of pronounced seasonality in flow suggests a less harsh, but also a less predictable, physical environment in equatorial runoff and glacial streams coropared to temperate ones. Spring streams probably show the least differences in flow dynamics between equatorial and temperate regions. Information on macroinvertebrate comrnunities in equatorial alpine runoff streams is still relatively scarce, while practically nothing is known about alpine springs and glacier-fed streams in the tropics (WARD 1994, JACOBSEN 2008). Here 1 provide preliminary data on the physical environment and macroinvertebrate fauna in runoff, spring, and glacial streams in an equatorial alpine region. My aim was to compare comrnunity structure and richness of these stream types in the light of results from alpine streams at temperate latitudes. If differences in environmental conditions among stream types are less drastic at the equator, then 1 expected stream-type related differences in macroinvertebrate assemblages to be less pronounced than reported elsewhere (KowNACKA & KOWNACKI 1972, HIEBER et al. 2005).