472 F. S. Chapin III et al. 



through regular annual fluxes but through the sea spray from rare sum- 

 mer or autumn storms and through geologic and successional processes 

 such as the cyclic occurrence of frost action and draining and coloniza- 

 tion of phosphorus-rich lake basins. In the absence of such renewing in- 

 puts, the rates of decomposition, nutrient cycling, and primary produc- 

 tion decline, and the standing crops of all trophic groups diminish. This 

 situation is most clearly seen in the basins of low-centered polygons on 

 the coastal tundra (Chapter 3). 



Nitrogen accrues at Barrow primarily through nitrogen fixation, as 

 in most ecosystems (Figure 12-2b). Blue-green algae associated with 

 mosses account for the bulk of nitrogen fixation (Alexander and Schell 

 1973). Even in the Arctic, nitrogen fixation is strongly temperature- 

 dependent, so that the total annual nitrogen input to coastal tundra is 

 10-fold smaller than precipitation inputs alone in temperate latitudes 

 (Barsdate and Alexander 1975). In fact, the total annual nitrogen input is 

 only 5% of that which annually cycles through the vegetation in coastal 

 tundra as compared with an estimated 21% in the shortgrass prairie 

 (Woodmansee et al. 1978). 



The annual input of carbon is l.O^^o of the total amount in the sys- 

 tem. The annual input of nitrogen is only about 0.01 '^o of the total eco- 

 system nitrogen content, and the input of phosphorus is about 0.0014% 

 of the total phosphorus content. Thus, at the current input rates, it 

 would take 10,000 years to regenerate the present standing crop of nitro- 

 gen and over 70,000 years to regenerate the present standing crop of 

 phosphorus. By contrast, at Hubbard Brook (Likens et al. 1977) the an- 

 nual inputs of nitrogen and phosphorus are about 1% and 0.01% of the 

 total standing crop respectively, about 100- and 7-fold higher than in 

 coastal tundra. This comparison emphasizes the importance of nutrient 

 conservation within the tundra system. 



Nutrient Loss 



In the Arctic, where chmate dictates that nutrient input must be 

 small, the system can remain in steady state only if it has characteristics 

 that lead to low rates of nutrient loss. Some of these characteristics are 

 associated with climate and landform, others with development of the 

 system during succession. Low precipitation and flat terrain reduce the 

 amount of runoff, so that nutrient loss from the coastal tundra is small. 

 Ninety-five percent of summer precipitation normally evaporates (Brown 

 et al. 1970). Only during the ten days of snowmelt is runoff from the Bar- 

 row tundra appreciable, and at this time the organic mat readily absorbs 

 available nutrients such as ammonium and phosphate (Chapin et al. 

 1978). Permafrost prevents downward leaching of nutrients. 



