The ‘Bryophyte’ story in the Kuparuk River: Surprise in long-term Arctic stream studies

The ‘flagship’ experiment in the Streams component of the LTER and OPP projects has been the long-term, low-level fertilization of the Kuparuk River. This experiment began in 1983 and is the longest controlled stream fertilization experiment in the world. Early experiments we did confirmed that phosphorus is generally the limiting nutrient for primary production in tundra streams. Other nutrients – e.g., ammonium and nitrate – are also low, but phosphorus is regularly so low that it is below the detection limits of the most sensitive methods available to us. More importantly, very small additions of phosphorus stimulate short-term primary production. The amounts of phosphorus required to do this are quite small, and so our ‘treatment’ levels (~0.3 uM) are well below levels that are considered the normal background in developed temperate areas.

Nevertheless, this subtle manipulation of the ecosystem has caused a number of profound changes to the community structure and functions of this river. Many of these changes would have remained hidden had we not persisted with this long-term experiment – now 21 years old. For example, in the first few years of this experiment we observed luxuriant growth of filamentous algae in the river, which confirmed the earlier, ‘bench-top’ experiments we had done. However, in the third year of this experiment, the filamentous algal growth disappeared despite the continued addition of phosphorus. In fact, unless we add more phosphorus or some nitrogen, we never again saw filamentous algal growth in the river. It turns out that one of the key insects in these rivers is a Baetiid mayfly that has a 3-year life cycle. Thus, it took this insect 3 years to fully exploit the new resource presented by the increased primary production in the fertilized reach of the river. This critical interaction was not one we anticipated.

A greater surprise, however, awaited us. Beginning in 1990 – seven years after the phosphorus enrichment experiment began – we noticed a dramatic and unexpected increase in the abundance of bryophytes (mosses) in riffles within phosphorus-enriched reach the Kuparuk River. Although one species of bryophyte was common in the river prior to fertilization, the increased abundance was largely due to several species that are not common in the main river, but are common in nutrient-rich, headwater springs and seeps. These bryophytes are now the dominant sink for P and the most important primary producers in the fertilized reach of the river. The story of how they came to dominate in this reach is fascinating.

Through groundbreaking ecophysiological experiments we did in the late 1990’s we found that the sparse and inconspicuous moss species that had always been present in the Kuparuk River (Schistidium agassizii) was essentially unresponsive to increases in light, temperature or nutrient conditions. It’s strategy was to hold on, grow slowly, and never expect good conditions - just survive. In stark contrast, the ‘invading’ mosses (Hygrohynum ochraceum and H. alpestre) have the capacity to respond vigorously to more light, higher temperature, and – especially – more nutrients. These species are present in nutrient-rich springs and lake outlets in the headwaters of the Kuparuk River. Interestingly, we have never seen fruiting bodies on these species. However, both species are well known to grow vigorously from vegetative fragments. It is now apparent that fragments of these two species have always been ‘raining’ downstream. However, they could not establish because the conditions were not suitable (primarily low phosphorus). Our fertilization experiment, however, changed all of this. And yet, it took over 7 years for just the right fragment, to travel just the right distance, and lodge in just the right way to establish. Once it did, the moss community ‘exploded’ and in a period of 3 years the new mosses became the dominant primary produces in the system. Primary producer biomass increased from mg per meter squared in the form of a diatom biofilm to nearly a kilogram of dry matter per meter squared as a luxuriant moss mat!

This long term effect is far from an academic curiosity. We have found that the presence of the new bryophytes has completely altered the nature of organic matter production in the fertilized reach. Insect biomass has increased dramatically, though community structure has changed less dramatically. And we think that our data are showing that fish in this altered reach are able to grow better due to the greater food resources there.

This unexpected long-term effect has opened our eyes to several important matters. First, we have come to a realization of just how poorly we understand the role of bryophytes in stream ecosystems. Second, our results illustrate clearly how critically important it is to maintain long-term manipulative experiments. Aside from the expected increase in filamentous algae in the first 3 years of this experiment, none of the subsequent changes were expected or would have been detected in a typical 2-3 year project period. Finally, we have begun to wonder if the changes we have observed are indicative of what we might expect to see in a future, changed environment in the Arctic, where nutrient supply rates to streams might be higher. To fully assess this possibility we need to work closely with our other colleagues in the LTER/OPP projects who are studying how land, riparian, and lake systems process nutrients and are likely to respond to changes in the environment.

Key published papers about this story:

Bowden, W.B., B.J. Peterson, J. Finlay, and J. Tucker. 1992. Epilithic oxygen production and consumption in a fertilized arctic stream. Hydrobiologia 240:121-131. Also reprinted in W.J. O'Brien (ed.) Toolik Lake: Ecology of an Aquatic Ecosystem in Arctic Alaska. Development in Hydrobiology. Volume 78. Kluwer Academic Publishers. Boston.

Hershey, A., W.B. Bowden, L. Deegan, J.E. Hobbie, B.J. Peterson, G. Kipput, G. Kling, M. Lock, M. Miller, R. Vestal. 1997. The Kuparuk River: a long-term study of biological and chemical processes in an arctic river. In: A. Milner and M. Oswood (eds.). Freshwaters of Alaska. Ecological Synthesis. Ecological Studies Series, Volume 119. Springer-Verlag. New York..

Bowden, W.B., J.C. Finlay and P.E. Maloney. 1994. Longterm effects of PO4 fertilization on the distribution of bryophytes in an arctic stream. Freshwater Biology 32:445-454.

Finlay, J.C. and W.B. Bowden. 1994. Controls on production of bryophytes in an arctic tundra stream. Freshwater Biology 32:455-466.

Harvey, C.J., B.J. Peterson, W.B. Bowden, L.A. Deegan, J.C. Finlay, A.E. Hershey, and M.C. Miller. 1997. Organic matter dynamics in the Kuparuk River, a tundra river in Alaska, USA. Journal of the North American Benthological Society 16:18-23.

Arscott, D.B, W.B. Bowden, J.C. Finlay. 1998. Comparison of epilithic algal and bryophyte metabolism in an arctic tundra stream, Alaska. Journal of the North American Benthological Society 17(2): 210-227.

The Stream Bryophyte Group (Bowden, organizer and lead author). 1999. Roles of bryophytes in stream ecosystems. Journal of the North American Benthological Society. 18(2):151-184.

Arscott, D.B., W.B. Bowden, and J.C. Finlay. 2000. Effects of desiccation and temperature/irradiance on the metabolism of 2 Arctic stream bryophyte taxa. Journal of the North American Benthological Society 19(2):263-273.

Slavik, K., B.J. Peterson, L.A. Deegan, W.B. Bowden, A.E. Hershey, John Hobbie. 2004. Long-term responses of the Kuparuk River to phosphorus fertilization. Ecology 85(4):939-954.