We sampled two streams in southwestern Nova Scotia from 1983 to 2004 and one stream from 1992 to 2004 for total organic carbon (TOC) and nitrogen (TN) in order to investigate if changes in catchment exports could be determined over the sampling periods, and if so what were the controlling factors. We Wrst show that early TOC measurements underestimated concentrations due to analytical shortcomings and then produce a correction to adjust values to more accurate levels. Our trend results showed that TOC concentrations decreased in the two streams with the longest record, from 1980 to 1992 when acid deposition to the area decreased most rapidly, and have remained constant since then. TOC exports only decreased at one site over the total sampling period. As expected, we also measured seasonal changes in exports, with the autumn period showing TOC and TN exports as high as during spring snowmelt. We found that only 24% of deposition N is exported from the larger catchments, most of it in organic form, while the smallest catchment exported 16%. We also show a constant increase in TN from 1994 to the present at all three sites sampled. Our results do not support the hypothesis that reductions in sulfur acidiWcation lead to increases in catchment organic carbon mobilization to streams.

We assembled 831 data points for total mercury (Hg(t)) and 277 overlapping points for methyl mercury (CH3Hg+) in surface waters from Massachussetts, USA to the Island of Newfoundland, Canada from State, Provincial, and Federal government databases. These geographically indexed values were used to determine: (a) if large-scale spatial distribution patterns existed and (b) whether there were significant relationships between the two main forms of aquatic Hg as well as with total organic carbon (TOC), a well know complexer of metals. We analyzed the catchments where samples were collected using a Geographical Information System (GIS) approach, calculating catchment sizes, mean slope, and mean wetness index. Our results show two main spatial distribution patterns. We detected loci of high Hg(t) values near urbanized regions of Boston MA and Portland ME. However, except for one unexplained exception, the highest Hg(t) and CH3Hg+ concentrations were located in regions far from obvious point sources. These correlated to topographically flat (and thus wet) areas that we relate to wetland abundances. We show that aquatic Hg(t) and CH3Hg+ concentrations are generally well correlated with TOC and with each other. Over the region, CH3Hg+ concentrations are typically approximately 15% of Hg(t). There is an exception in the Boston region where CH3Hg+ is low compared to the high Hg(t) values. This is probably due to the proximity of point sources of inorganic Hg and a lack of wetlands. We also attempted to predict Hg concentrations in water with statistical models using catchment features as variables. We were only able to produce statistically significant predictive models in some parts of regions due to the lack of suitable digital information, and because data ranges in some regions were too narrow for meaningful regression analyses.

Atlantic Canada receives the lowest acid deposition amounts in eastern North America, but has some of the most acidic surface waters on the continent, due to the low buffering provided by regional bedrock and wetlands that produce natural organic acids. Southwestern and eastern parts of Nova Scotia combine poor buffering, high organic acidity, and higher acid deposition, to produce extremely low surface water pH and acid neutralization capacity (ANC) values. Although sulfate deposition is decreasing, concurrent reductions in dissolved base cations, as well as the acid-base characteristics of natural organic acids, are not allowing the recovery of ANC or surface water pH. Spring-time acid pulses occur in Atlantic Canada, though these have been reduced in severity with decreases in winter acid deposition, while autumnal low pH pulses caused by organic acids are a regular occurence in Nova Scotia and must be separated from mineral acidity pulses. Geochemical modeling using both critical load and dynamic approaches, nevertheless predict improvements in the water chemistry of Nova Scotia lakes within the next 20 years. However, re-establishment of pre-acidification water chemistry in most of its lakes will require greater reductions in S emissions than are currently planned in Canada and the United States.Résumé : La côte Atlantique du Canada reçoit les plus faibles dépositions de pluies acides du nord-est Américain, mais possède les eaux de surface les plus acides du continent, dues au faible pouvoir tampon de la roche mère et aux terres humides qui génèrent naturellement des substances organiques acides. Le sud-ouest et l'est de la Nouvelle-É cosse combinent une haute acidité organique et la plus importante déposition acide, ce qui engendre des eaux de surface qui engendre des pH de l'eau de surface et des capacités de neutralisation de l'acide (ANC) extrêmement faibles. Bien que la déposition des sulfates soit à la baisse, une réduction simultanée des cations basiques, ainsi que les caractéristiques acide-base des acides organiques naturels, ne permettent pas la récupération de l'ANC ou du pH des eaux de surface. Des poussées printanières d'acidité surviennent sur la côte Atlantique du Canada, bien que la sévérité de celles-ci ait été réduite avec une diminution des précipitations acides hivernales, alors que les poussées de forte acidité automnales causées par les acides organiques reviennent régulièrement en Nouvelle-É cosse, et doivent être distinguées des poussées d'acidité minérale. Le modelage géochimique, à l'aide des approches dynamique et de charge critique, prédisent néanmoins des améliorations de la chimie de l'eau des lacs en Nouvelle-É cosse, au cours des 20 prochaines années. Cependant, le retour à la chimie de l'eau d'avant l'acidification nécessitera des réductions plus élevées des émissions de S que celles couramment envisagées, au Canada et aux Etats-Unis.Mots-clés : côte Atlantique du Canada, acidification, eau douce, carbone organique total, acidité organique.[Traduit par la Rédaction]