A tracer approach to investigation of the nitrogen (N) cycle of streams, first developed at the Arctic LTER, has transformed scientific understanding of the nitrogen cycle and food web structure in flowing waters.
The experimental addition of low levels of phosphorus to an arctic stream created a gradual transformation of the tundra stream ecosystem from a cobble-bottom stream covered with diatom-dominated biofilm to a moss-dominated bottom that hosted a different community composition of invertebrates. This transformation was not predicted and was a surprise because for the first seven or eight years there was little evidence that such a major change was occurring. The long-term transformation due to a very low level of fertilization may have revealed a general principle of how streams respond to disturbance.
Starting in summer 1983 whole-stream fertilization of the Kuparuk River with a constant drip addition of enough phosphorus to raise the stream concentration to 10 ug/l stimulated algal production, increased bacterial activity and the abundance of insects such as Brachycentrus and Baetis (Peterson et al. 1985). The increased insect abundance stimulated the growth of fish in the fertilized area relative to the reference area. However, in spite of consistent responses to fertilization, the populations of algae and insects varied widely from year to year in both the fertilized and reference areas (Slavik et al. 2004) and much of this variability correlated with year to year variation in discharge. Thus, river discharge interacted strongly with nutrient levels to control stream ecosystem productivity.
Just when scientists thought they understood the major controls on stream ecosystem function, an amazing transformation occurred. After seven or eight years of fertilization the river bottom for several kilometers downstream of the phosphorus addition point was overgrown by a "carpet" of the moss Hygrohypnum (Bowden et al. 1994). Hygrohypnum provides a large amount of surface area for algae and moss biomass, which dwarf by orders of magnitude the biomass of algae in the reference area. The moss fronds create a matrix that hosts an insect community quite different in abundance and composition from that found in the rocky-bottom reference area (Slavik et al. 2004). Chironomids, Brachycentrus and a large mayfly (Ephemerella) are more abundant by an order of magnitude in the mossy fertilized area. In contrast, other common insects including Baetis, black flies, and Orthocladius are less abundant in the mossy area than in the reference area. An unexplained surprise is that in spite of higher insect biomass in the fertilized area, the growth of fish is no longer significantly greater than in the reference area.
This experiment is the only stream enrichment study that has continued long enough to document such profound changes in ecosystem function. However, similar long-term fertilization of tundra vegetation at the Toolik Lake LTER site has also found major shifts in plant species dominance over several decades. Most studies have not been conducted long enough to reveal how ecosystems ultimately respond to low level chronic stress or disturbance.
Long-term effects of PO4 fertilization on the distribution of bryophytes in an arctic river. Freshwater Biology. 32:445-454. 1994.