Introduction

Biogeochemical cycles of carbon (C) and other elements depend on biological processes, which operate at different rates depending on environmental conditions. In the Arctic, the cycles are generally slower than in most other ecosystems because of climatic constraints and strong seasonality. However, within the Arctic there is also a pronounced variation in ecosystem structure and function on different scales. Some of the biological variability is associated with easily interpreted latitudinal and climatic gradients of progressively shorter growing season and lower temperatures toward the north. For instance, differences of 1 or 2 weeks in the time of snow-melt or a growing-season mean temperature difference of less than 1°C are much more significant in the high Arctic than at lower latitudes with longer and milder growing seasons. This creates latitudinal gradients of, e.g., decreasing stocks of plant biomass and decreasing ecosystem nutrient content and productivity from the southern toward the northern Arctic. The latitudinal constraints, at the same time, make life and ecosystem processes increasingly dependent on stochastic, between-year variation in temperature from lower to higher latitudes.

However, local variations in environment can constrain ecosystem function just as much as large-scale regional differences can, due to the narrow range within which the biota must operate. Such local heterogeneity can translate into gradients over short distances that are just as pronounced as the large-scale climatic constraints (Shaver et al., 1996; Shaver and Jonasson, in press).

In the following, we give a brief biogeochemical characterization of arctic ecosystems, which we contrast with neighboring ecosystems further south. We also discuss the biogeochemical variability among arctic ecosystems, the controls that have led to their formation, and shorter-term controls on their stability. We emphasize those processes and interactions among ecosystem components that are least well understood and that would most greatly improve our understanding of the controls over ecosystem structure and function. Finally, we give a brief summary of responses in organisms to environmental manipulations across a range of contrasting arctic ecosystems. On the basis of these responses, we seek to identify the sources of major ecosystem controls at various levels of complexity and discuss common and distinctive controls of the biogeochemical functioning of the ecosystem types.

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