We measured net ecosystem CO 2 exchange (NEE), plant biomass and growth, species composition, peat microclimate, and litter decomposition in a fertilization experiment at Mer Bleue Bog, Ottawa, Ontario. The bog is located in the zone with the highest atmospheric nitrogen deposition for Canada, estimated at 0.8-1.2 g N m À2 yr À1 (wet deposition as NH 4 and NO 3 ). To establish the effect of nutrient addition on this ecosystem, we fertilized the bog with six treatments involving the application of 1.6-6 g N m À2 yr À1 (as NH 4 NO 3 ), with and without P and K, in triplicate 3 m  3 m plots. The initial 5-6 years have shown a loss of first Sphagnum, then Polytrichum mosses, and an increase in vascular plant biomass and leaf area index. Analyses of NEE, measured in situ with climate-controlled chambers, indicate that contrary to expectations, the treatments with the highest levels of nutrient addition showed lower rates of maximum NEE and gross photosynthesis, but little change in ecosystem respiration after 5 years. Although shrub biomass and leaf area increased in the high nutrient plots, loss of moss photosynthesis owing to nutrient toxicity, increased vascular plant shading and greater litter accumulation contributed to the lower levels of CO 2 uptake. Our study highlights the importance of long-term experiments as we did not observe lower NEE until the fifth year of the experiment. However, this may be a transient response as the treatment plots continue to change. Higher levels of nutrients may cause changes in plant composition and productivity and decrease the ability of peatlands to sequester CO 2 from the atmosphere.

The large accumulation of organic matter in peatlands is primarily caused by slow rates of litter decomposition. We determined rates of decomposition of major peat-forming litters of vascular plants and mosses at five sites: a poor fen in New Hampshire and a bog hummock, a poor fen, a beaver pond margin and a beaver pond in Ontario. We used the litterbag technique, retrieving triplicate litterbags six or seven times over 3-5 years, and found that simple exponential decay and continuous-quality non-linear regression models could adequately characterize the decomposition in most cases. Within each site, the rate of decomposition at the surface was generally Typha latifolia leaves = Chamaedaphne calyculata leaves = Carex leaves > Chamaedaphne calyculata stems > hummock Sphagnum = lawn/hollow Sphagnum, with exponential decay constant (k) values generally ranging from 0.05 to 0.37 and continuous-quality model initial quality (q 0 ) values ranging from 1.0 (arbitrarily set for Typha leaves) to 0.7 (Sphagnum). In general, surface decay rates were slowest at the bog hummock site, which had the lowest water table, and in the beaver pond, which was inundated, and fastest at the fens. The continuous-quality model site decomposition parameter (u 0 ) ranged from 0.80 to 0.17. Analysis of original litter samples for carbon, nitrogen and proximate fractions revealed a relatively poor explanation of decomposition rates, as defined by k and q 0 , compared to most well-drained ecosystems. Three litters, roots of sedge and a shrub and Typha leaves, were placed at depths of 10, 30 and 60 cm at the sites. Decomposition rates decreased with depth at each site, with k means of 0.15, 0.08 and 0.05 y )1 at 10, 30and 60 cm, respectively, and u 0 of 0.25, 0.13 and 0.07. These differences are primarily related to the position of the water table at each site and to a lesser extent the cooler temperatures in the lower layers of the peat. The distinction between bog and fen was less important than the position of the water table.These results show that we can characterize decomposition rates of surface litter in northern peatlands, but given the large primary productivity below-ground in these ecosystems, and the differential rates of decomposition with depth, subsurface input and decomposition of organic matter is an important and relatively uncertain attribute.

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Beaver, as allogenic ecosystem engineers, are capable of extensively altering surrounding habitats resulting in significant effects on stream water quality. This paper presents insight into downstream effects of beaver impoundments (both actively maintained and abandoned by beaver) and accumulated sediments on water quality, especially in protected watersheds serving as a drinking water supply. In a tributary un-inhabited by beaver compared to those with active beaver ponds, we found significantly (p < 0Ð001) lower concentrations of NO 3 -N (respectively: mean (standard error)) without beaver 0Ð11 (0Ð005) mg/l and with beaver 0Ð14 (0Ð002) mg/l), NH 4 -N (0Ð007 (0Ð002) and 0Ð24 (0Ð002) mg/l), dissolved organic carbon (DOC) (3Ð47 (0Ð081) and 8Ð37 (0Ð203) mg/l), chlorophyll (2Ð38 (0Ð066) and 6Ð96 (0Ð203) µg/l), and temperature (16Ð43 (0Ð101) and 19Ð85 (0Ð123)°C), but higher pH (7Ð17 (0Ð02) and 6Ð72 (0Ð022)), PO 4 (0Ð20 (0Ð01) and 0Ð17 (0Ð014) mg/l) concentration, and dissolved oxygen (DO) saturation (100Ð8 (0Ð708) and 67Ð6 (1Ð74)%). Within the beaver ponds, we observed a high abundance of predatory zooplankton, which help to control algae and feed on detritus within the pond ecosystem. We intentionally breached an abandoned beaver dam in the field and conducted a sediment resuspension experiment in the laboratory to assess the potential for increases in turbidity and nutrients from beaver pond sediments as would be expected in the event of a catastrophic dam breach. We found that abandoned beaver ponds have a stronger influence on downstream water quality than actively maintained ponds owing to higher turbidity with finer sediments and sharply increased stream water temperature during a dam failure.