Nitrogen inputs restructure ecosystems and can interact with other agents of ecological change and potentially intensify them. To examine the effects of nitrogen combined with those of elevation and competition, in 2005 we mapped vegetation and elevation within experimental plots that have been fertilized since 1970 in Great Sippewissett salt marsh, Cape Cod, MA, USA and compared the resulting effects on marsh vegetation. Decadal-scale chronic nutrient enrichment forced changes in cover and spatial distribution of different species. With increasing enrichment, there was a shift in species cover primarily involving loss of Spartina alterniflora and an increase in Distichlis spicata. Percent cover of near monocultures increased with nitrogen fertilization, owing mainly to the proliferation of D. spicata. The experimental fertilization prompted a shift from the short form of S. alterniflora to taller forms, hence increasing above-ground biomass, where this species managed to remain. Chronic enrichment increased upper and lower limits of the elevation range within which certain species occurred. The shift to increased cover of D. spicata was also associated with faster accretion of the marsh surface where this species was dominant, but not where S. alterniflora was dominant. Interactions among nutrient supply, elevation, and competition altered the direction of competitive success among different species of marsh plants, and forced changes in the spatial distribution and composition of the salt marsh plant communities. The results imply that there will be parallel changes in New England salt marshes owing to the widespread eutrophication of coastal waters and the increasing sea level rise. Knowing the mechanisms structuring marsh vegetative cover, and their role in modification of salt marsh accretion, may provide background with which to manage maintenance of affected coastal wetlands.

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Assessments of the potential for metal and radionuclide contamination of arctic organisms have been based in part on experiments on organisms collected from temperate zones, even though adaptation to Arctic environments may enhance bioaccumulation of contaminants. Here, we compare the potential for blue mussels Mytilus edulis originating from the Arctic and from temperate zones to accumulate trace metals from filtered algal food at 2 temperatures. Geographic origin had little effect on either the percentage of metal assimilated from food (the assimilation efficiency, AE) or the rate at which ingested metal was excreted (measured by the efflux constant, k e ). By comparison, experimental temperature had much larger effects, causing AE for Ag, Am and Zn to be 122 to 945% higher, and k e for Cd and Co to be 50 to 80% lower at 2°C than at 12°C. The effect of temperature on the trophic accumulation factor (TAF = AE/k e ) was even more pronounced and systematic, with all metals characterized by larger TAFs at 2°C than at 12°C. The effects on the TAF were largest for the non-essential metals Ag and Am (6 to 7-fold), smallest for the required elements Co, Se and Zn (2 to 3-fold) and intermediate for Cd (4-fold). Geographic origin affected the TAF for Cd only, with temperate mussels displaying slightly higher potential for biomagnification. Our study suggests that, with the exception of Cd, mussels from temperate and Arctic zones accumulate metals from food similarly, and that temperature has a much larger impact on accumulation than population origin.

Nitrogen retention by salt marshes has been suggested as a means of mitigating the delivery of land-derived nitrogen loads to coastal waters. As land-derived nitrogen loads increase, it is unclear whether there is an upper limit to the amount of nitrogen retained by salt marshes. A long-term fertilization study in the Great Sippewissett Marsh on Cape Cod, USA, has been examining the changes to salt marsh vegetation and sediment processes as a result of increased nitrogen loading. To determine whether decadal-scale changes in nitrogen loading and sources are recorded in salt marsh sediments, we examined sediment δ 15 N and %N profiles from below low and high marsh vegetation in control and fertilized plots in Great Sippewissett Marsh. As expected, we found little change in δ 15 N values in control plots. Nitrogen burial, calculated using %N values in bulk sediments, was higher in fertilized plots, but did not increase over time. However, δ 15 N values in fertilized plots were higher than in control plots and increased over time, becoming heavier than the source fertilizer and continuing to increase linearly. The continuous increase in sediment δ 15 N values in fertilized plots over the fertilizer δ 15 N value suggests that denitrifying bacteria are responding to the increased nitrogen load and fractionating the available nitrogen. The nitrogen that remained unaccounted for by burial led us to conclude that 47 to 80% of the fertilizer nitrogen was denitrified.

Arctic waters are contaminated with metals and radionuclides from diverse sources. Here we present experimental results evaluating the bioaccumulation of dissolved metals by the mussel Mytilus edulis, widely used as a food source and also as a bioindicator of coastal contamination. We used radioisotopes to determine the uptake and excretion of Ag, Am, Cd, Co and Zn by M. edulis collected from both Arctic and temperate waters at 2 and 12°C. For both mussel populations, uptake rates (k u ) of Cd, Co and Zn were 50 to 60% slower at 2°C than at 12°C, mainly attributable to differences in mussel filtration rates at these temperatures, whereas uptake of Ag and Am was not affected by temperature. For both populations, the first-order loss constant for all metals from the slowest exchanging pool (k e ) was 20 to 50% smaller at 2°C than at 12°C. As well, the loss of Cd and Zn was significantly slower in the temperate mussels than in Arctic mussels. Because temperature had similar effects on uptake and efflux, the concentration factor for all metals from water (CF w = k u /k e ) did not vary significantly between temperatures, but CF w values for Cd and Zn in temperate mussels were 15-fold and 2.6-fold higher, respectively, than in Arctic mussels. Thus, while temperature does not exert a major influence on net bioaccumulation of aqueous metal by M. edulis, Arctic mussels may accumulate dissolved Cd and Zn to a lesser degree than temperate mussels. KEY WORDS: Contaminants · Kinetic modeling · Radionuclides · Bioaccumulation · Concentration factorsResale or republication not permitted without written consent of the publisher