Carbon and Nutrient Budgets 479 



Virtually the entire coastal landscape has at some time been part of this 

 thaw lake cycle. Nutrient cycles and nutrient budgets of terrestrial sys- 

 tems and their yearly variations must be viewed in this cyclic successional 

 context. Furthermore, the extensive interdigitation of aquatic and terres- 

 trial systems has important implications for the function of coastal tun- 

 dra as a landscape unit. Our concept of the exchanges between terrestrial 

 and aquatic nutrient pools and changes in nutrient availability during the 

 thaw lake cycle is shown in Figure 12-4. 



The annual carbon and nitrogen budgets of wet meadow tundra sug- 

 gest a gradual accumulation of these elements through time, and this view 

 is supported by the accumulation of peat. Some of this accumulation 

 may have occurred in the past. In contrast, tundra ponds exhibit negative 

 carbon balances and virtually no peat accumulation in the sediments 

 (Stanley and Daley 1976). The more effective recycling of organic carbon 

 in ponds than in wet meadow may result from higher temperatures, bet- 

 ter aeration, more intensive aquatic grazing, a more elaborate food web, 

 or some combination of these factors (Barsdate et al. 1974, Hobbie 1980). 

 Considerable organic matter of terrestrial origin decomposes in tundra 

 ponds, including dissolved and particulate organic matter that flows into 

 the ponds during snowmeh and some of the accumulated soil organic 

 matter at the pond margin. Small tundra ponds may enlarge, although 

 others may be invaded by emergent macrophytes. Similarly, large tundra 

 thaw lakes enlarge by erosion and eventually drain. 



Nitrogen accumulates slowly in both terrestrial and aquatic systems. 

 On land, nitrogen accumulates primarily in peat. When organic nitrogen 

 enters the aquatic system, either through runoff or erosion, much of the 

 carbon portion of the organic matter is respired, but the nitrogen cycles 

 through the aquatic system and is eventually deposited in the sediments 

 as ammonium. 



In contrast to carbon and nitrogen, phosphorus may exhibit a net 

 loss from wet meadow tundra but accumulates in sediments of small 

 ponds as iron hydroxy-phosphate compounds (Prentki 1976). The phos- 

 phorus in the sediments of the lakes and ponds is insoluble and is not 

 recycled within the aquatic system to any significant extent. 



Lakes and ponds eventually drain, either through the process of en- 

 largement or when captured by a headward eroding stream (Britton 1957; 

 Chapter 6). When lake sediments are exposed, they are characterized by 

 low organic accumulation and presumably by relatively high phosphorus 

 and nitrogen availability. In spite of low rates of seed production in 

 coastal tundra, complete cover of drained lake basins can be attained 

 within 20 to 25 years by seedling establishment and subsequent vegetative 

 spread (Dennis 1968, Peterson 1978, Webber 1978). Such early succes- 

 sional communities are relatively productive, presumably because of 

 high nutrient availability in the lake sediments. 



