44Human activities have greatly increased the transport of biologically available N through 45 watersheds to potentially sensitive coastal ecosystems. Lentic water bodies (lakes and 46 reservoirs) have the potential to act as important sinks for this reactive N as it is 47 transported across the landscape because they offer ideal conditions for N burial in 48 sediments or permanent loss via denitrification. However, the patterns and controls on 49 lentic N removal have not been explored in great detail at large regional to global scales. 50In this paper we describe, evaluate, and apply a new, spatially explicit, annual-scale, climate and the density of lentic systems. Although reservoirs occupy just 6% of the 60 global lentic surface area, we estimate they retain approximately 33% of the total N 61 removed by lentic systems, due to a combination of higher drainage ratios (catchment 62 surface area : lake or reservoir surface area), higher apparent settling velocities for N, and 63 greater N loading rates in reservoirs than in lakes. Finally, a sensitivity analysis of 64 NiRReLa suggests that, on-average, N removal within lentic systems will respond more 65 strongly to changes in land use and N loading than to changes in climate at the global 66 scale. 67

In order to investigate the factors controlling viral abundance, 22 lakes In Quebec were surveyed. We measured viral and bacterial abundance, bacterial production, chlorophyll a, total phosphorus and DOC (dissolved organic carbon) concentrations. Regression models built with these data were compared to models based on literature data, which to date have been collected largely from marine sites Positive empirical relationships were found between viral abundance and (1) chlorophyll a concentrations, (2) bacterial abundances, (3) bacterial production, and (4) total phosphorus concentration. There was little to no trend in the virus-to-bacteria ratio with increasing trophy. Analysis of covariance revealed significant differences between relations in marine and freshwater systems. The virusto-bacteria ratio was significantly higher in freshwater (mode = 22.5) than marine environments (mode = 2.51, and there were significantly more bacteria per unit chlorophyll in our freshwater samples. We suggest that this difference is related to the increased dependence of freshwater bacteria on allochthonous material relative to marine systems, as well as the increased relative importance of photosynthetic cyanobacteria in lakes.

Large-scale commercialization of the Haber-Bosch (HB) process is resulting in intensification of nitrogen (N) fertilizer use worldwide. Globally N fertilizer use is far outpacing that of phosphorus (P) fertilizer. Much of the increase in N fertilizers is also now in the form of urea, a reduced form of N. Incorporation of these fertilizers into agricultural products is inefficient leading to significant environmental pollution and aquatic eutrophication. Of particular concern is the increased occurrence of harmful algal blooms (HABs) in waters receiving nutrient enriched runoff. Many phytoplankton causing HABs have physiological adaptive strategies that make them favored under conditions of elevated N : P conditions and supply of chemically reduced N (ammonium, urea). We propose that the HB-HAB link is a function of (1) the inefficiency of incorporation of N fertilizers in the food supply chain, the leakiness of the N cycle from crop to table, and the fate of lost N relative to P to the environment; and (2) adaptive physiology of many HABs to thrive in environments in which there is excess N relative to classic nutrient stoichiometric proportions and where chemically reduced forms of N dominate. The rate of HAB expansion is particularly pronounced in China where N fertilizer use has escalated very rapidly, where soil retention is declining, and where blooms have had large economic and ecological impacts. There, in addition to increased use of urea and high N : P based fertilizers overall, escalating aquaculture production adds to the availability of reduced forms of N, as does atmospheric deposition of ammonia. HABs in both freshwaters and marginal seas in China are highly related to these overall changing N loads and ratios. Without more aggressive N control the future outlook in terms of HABs is likely to include more events, more often, and they may also be more toxic.

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Terrestrial ecosystems contribute significant amounts of dissolved organic carbon (DOC) to aquatic ecosystems. Temperate lakes vary in DOC concentration as a result of variation in the spatial configuration and composition of vegetation within the watershed, hydrology, and within-lake processes. We have developed and parameterized a spatially explicit model of lake DOC concentrations, using data from 428 watersheds in the Adirondack Park of New York. Our analysis estimates watershed loading to each lake as a function of the cover type of each 10 ϫ 10 m grid cell within the watershed, and its flow-path distance to the lake. The estimated export rates for the three main forest cover types were 37.7-47.0 kg C·ha Ϫ1 ·yr Ϫ1 . The four main wetland cover types had much higher rates of export per unit area (188.4-227.0 kg C·ha Ϫ1 ·yr Ϫ1 ), but wetlands occupied only 11%, on average, of watershed area. As a result, upland forests were the source of ϳ70% of DOC loading. There was evidence of significant interannual variation in DOC loading, correlated with interannual variation in precipitation. Estimated net in situ DOC production within the lakes was extremely low (Ͻ1 kg C·ha Ϫ1 ·yr Ϫ1 ). Many of the lakes have large watersheds relative to lake volume and have correspondingly high flushing rates. As a result, losses due to lake discharge generally had a larger effect on lake DOC concentrations than in-lake decay. Our approach can be readily incorporated within a GIS framework and allows examination of scenarios such as loss of wetlands, alterations in forest management, or increases in conserved areas, as a function of the unique configuration of individual watersheds.

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[1] Nitrous oxide (N 2 O) is a potent greenhouse gas produced during nitrogen cycling. Global nitrogen enrichment has resulted in increased atmospheric N 2 O concentrations due in large part to increased soil emissions. There is also a potentially important flux from streams, rivers and estuaries; although measurements of these emissions are sparse, and role of aquatic ecosystems in global N 2 O budgets remains highly uncertain. Using the longest-term measurements of N 2 O fluxes from streams to date, we found annual fluxes from 14 sites in five streams of south-central Ontario, Canada varied widely-from net uptake of 3.2 ± 0.2 (standard deviation) mmol N 2 O m −2 d −1 to net release of 776 ± 61 mmol N 2 O m −2 d −1 . N 2 O consumption was associated with very low nitrate concentrations (<2.7 mM). Mean annual (log-transformed) N 2 O emissions from our study streams (across sites and years) were positively related to nitrate concentrations (r 2 = 0.59).This nitrate-N 2 O relationship can be generalized across all 20 streams (in Canada, Japan, Mexico, and the midwestern United States) for which published data now exist and could provide a new basis for the IPCC to calculate agricultural emissions from streams. In addition to predicting annual emissions, we present the first measurements of N 2 O concentrations under ice in streams. Nitrate was a strong predictor of N 2 O % saturation during periods of ice cover (r 2 = 0.89), when gas exchange is negligible. Given the small surface area of streams within a catchment and the fact our measured areal fluxes are comparable to reported fluxes from agricultural soils, this suggests streams are a small regional N 2 O source.Citation: Baulch, H. M., S. L. Schiff, R. Maranger, and P. J. Dillon (2011), Nitrogen enrichment and the emission of nitrous oxide from streams, Global Biogeochem. Cycles, 25, GB4013,