Effects of Natural Disturbance on Arctic Stream Communities

Geomorpholgy, water chemistry, water source, and disturbance regime are physical factors that determine community structure of streams in arctic Alaska. Adjacent headwater catchments of the Lvishak River on the North Slope are drained by a groundwater spring and a mountain stream and were studied during the summer of 2002. The spring stream has relatively constant temperature and discharge, whereas discharge in the mountain stream is strongly affected by seasonal changes in precipitation and air temperature. Estimates of bed movement, flood frequency and substrate particle size suggest that the mountain stream has a more intense disturbance regime than the spring stream. Although disturbance regimes contrast strongly, invertebrate taxonomic richness (22-24 taxa) and food-web connectance (0.107 and 0.127) are similar. Food-web connectance is comparable to literature values for food webs of similar size. Trophic structure was also difference between the two streams, with four trophic levels in the mountain stream and 5 in the spring stream. Estimates of biomass, however, show that predators account for 40% of macroinvertebrate biomass in the spring stream but only 7% in the mountain stream. Nutrient availability, invertebrate grazing, and disturbance are the most important factors affecting algal community structure in streams. Artificial substrata made from untwisted, natural fiber ropes were used to measure algal colonization in the Ivishak spring and mountain streams. Twenty-three of a total 28 algal species were found in both streams. However, chlorphyll levels were higher on average in the mountain stream (30.7μg cm-1 rope) than in the spring stream (18.7 μg cm-1 rope). Reductions in chlorophyll levels remained relatively constant over the summer and may have been regulated by invertebrate grazing. Contrast in disturbance regime between these two arctic streams appear to have a minor effect on community structure, but a substantial effect on function. As nutrient concentrations are similar in both streams and neither stream freezes during winter, bed movement in the mountain streams is the most important abiotic factor affecting the stream community. Grazing by invertebrates, however, may be an important biotic control on algal communities in the low-intensity disturbance spring stream. These actors appear to have strong effects on function but not structure in the food webs and algal communities of these two streams