Frederick J. Wrona, Terry D. Prowse, James D. Reist, John E. Hobbie, Lucie M.J. Le ´ vesque and Warwick F. Vincent Climate Change Effects on Aquatic Biota, Ecosystem Structure and Function Climate change is projected to cause significant alter- ations to aquatic biogeochemical processes, (including carbon dynamics), aquatic food web structure, dynamics and biodiversity, primary and secondary production; and, affect the range, distribution and habitat quality/quantity of aquatic mammals and waterfowl. Projected enhanced permafrost thawing is very likely to increase nutrient, sediment, and carbon loadings to aquatic systems, resulting in both positive and negative effects on fresh- water chemistry. Nutrient and carbon enrichment will enhance nutrient cycling and productivity, and alter the generation and consumption of carbon-based trace gases. Consequently, the status of aquatic ecosystems as carbon sinks or sources is very likely to change. Climate change will also very likely affect the biodiversity of freshwater ecosystems across most of the Arctic. The magnitude, extent, and duration of the impacts and responses will be system- and location-dependent. Pro- jected effects on aquatic mammals and waterfowl include altered migration routes and timing; a possible increase in the incidence of mortality and decreased growth and productivity from disease and/or parasites; and, probable changes in habitat suitability and timing of availability. INTRODUCTION Climate change is very likely to have both direct and indirect consequences on the biota and the structure and function of arctic freshwater ecosystems. Changes in key physical and chemical parameters at the landscape scale as described by (1), (2), and (3) are very likely to affect aquatic community and ecosystem attributes such as species richness, biodiversity, range, and distribution, and consequently alter corresponding food web structures and primary and secondary production levels. The magnitude and extent of the ecological conse- quences of climate change in arctic freshwater ecosystems will depend largely on the rate and magnitude of change in three primary environmental drivers: the timing, magnitude, and duration of the runoff regime; temperature; and alterations in water chemistry such as nutrient levels, DOC, and particulate organic matter loadings (4, 5, 6). Below we discuss the projected effects climate change is expected to have on: biological communities, biodiversity and adaptive responses; aquatic food web structure and dynamics; primary and secondary production; carbon dynamics; and, aquatic birds and mammals. Effects on Biological Communities, Biodiversity, and Adaptive Responses Climate change will probably produce significant effects on the biodiversity of freshwater ecosystems throughout the Arctic and possibly initiate varying adaptive responses. The magnitude, extent, and duration of the impacts and responses will be system- and location-dependent, and difficult to separate from other environmental stressors. Biodiversity is related to, or affected by, factors including: the variability of regional and local climate; the availability of local resources (e.g., water, nutrients, trace elements, energy, substrate) affecting the productivity potential; the nature, timing, and duration of disturbance regimes in the area (e.g., floods, catastrophic water loss, fire); the original local and regional ‘‘stock’’ of species and their dispersal opportunities or barriers; the physiological capacity of individuals and populations to cope with new environmental conditions (e.g., physiological thresholds and tolerances); the levels of spatial heterogeneity (habitat fragmen- tation) and connections among aquatic systems; the intensity of biotic interactions such as competition, predation, disease, and parasitism; phenotypic and genotypic flexibility in reproductive and life-history strategies (e.g., facultative versus obligatory anadromy for certain fish; plasticity in sexual versus asexual reproductive strategies in aquatic invertebrate and plant species); and the overall genetic variability and adaptive capacity of the species (7, 8, 9). Many arctic freshwater systems are exposed to multiple environmental stressors or perturbations including point- and/ or non point-source pollution (e.g., long-range aerial transport of contaminants; see 10); altered hydrologic regimes related to impoundments and diversions; water quality changes from landscape alterations (e.g., mining, oil and gas exploration); and biological resource exploitation (e.g., subsistence and commer- cial fisheries and harvesting of waterfowl and mammals; (see 11), to name a few. These stressors, along with climate variability, can synergistically contribute to the degradation of biological diversity at the species, genetic, and/or habitat– ecosystem levels (7, 9, 12, 13).There is little evidence to suggest that climate change will slow species loss. There is growing evidence, however, that climate change will contribute to accelerated species losses at regional and global levels (9) and that the effects of alterations in the biodiversity of ecosystem structure and function are likely to be more dependent on given levels of functional diversity than on the total number of species (14). Moreover, both the number and type of functional units present in a community largely affect ecosystem resilience and vulnerability to change (9). Ambio Vol. 35, No. 7, November 2006 359 Ó Royal Swedish Academy of Sciences 2006 http://www.ambio.kva.se