The combined influences of intensive mussel aquaculture and watershed nutrient inputs on nitrogen dynamics in Tracadie Bay, Prince Edward Island, Canada, were examined using a nitrogen budget and an ecosystem model. Budget calculations, and inputs and parameters for the model were based on extensive field data. Both approaches showed that mussel aquaculture has a dominant influence on all aspects of the nitrogen cycle and dramatically alters pathways by which nitrogen reaches the phytoplankton and benthos. A large proportion of phytoplankton production is supported by land-derived nitrogen and this anthropogenic input is important for sustaining existing levels of mussel production. The amount of nitrogen removed in the mussel harvest is small compared with agricultural nitrogen inputs and the amounts excreted and biodeposited on the seabed. Mussel biodeposition greatly increases the flux of nitrogen to the benthos, with potentially serious eutrophication impacts. Results from the observation-based nitrogen budget and dynamic model were compared and both support the above conclusions. However, the ability of the model to test different scenarios and to provide additional information (e.g. fluxes) over a finer spatial scale led to insights unattainable with a nitrogen budget. For example, food appears to be less available to mussels at the head of the Bay than at the mouth, despite the lower density of grow-out sites in the former location. The number of fundamental ecosystem processes influenced by the mussels and the complexity of their interactions make it difficult to predict the effects of mussels on many ecosystem properties without resorting to a model.

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The distribution of sea ice meltwater and meteoric water in the eastern Canadian Arctic has been studied by oxygen isotope techniques. The distribution pattern of sea ice meltwater is presented. A comparison of the relative amounts of sea ice meltwater and meteoric water in the surface layer shows that more than 25% of all the samples with sea ice input contained more sea ice input than meteoric input. Sea ice meltwater/meteoric water ratios as high as 4.7 have been observed. The depth of sea ice meltwater penetration varies from 50 m in Baffm Bay to 140 m in Lancaster Sound. Calculated sea ice thicknesses range from 0.5 to 4.5 m with a mean of 1.5 m, in good agreement with ice core data. The significance of sea ice meltwater for chemical, physical, and biological oceanography is briefly discussed. The principles and limitations of using oxygen isotopes to detect brines are discussed in the Baffm Bay setting. The isotopic compositions of possible source waters for Baffm Bay bottom water are examined. INTRODUCTION In arctic regions the composition of surface seawater is in-. fluenced by freshwater derived from both the melting of sea ice and the input of meteoric waters•precipitation, land runoff, and glacial melt. A full understanding of the chemistry and physics of such regions would require the knowledge of the spatial and temporal distributions of these low-salinity inputs. At the present time it is not possible to determine the individual contributions of each of these four sources in reducing the surface salinity and causing other surface layer changes. Stable isotope measurements can, however, distinguish between sea ice meltwater and the combined input of meteoric water from the other three sources. Redfield and Friedman [1969] showed that the D/H ratios of freshwaters derived from the melting of sea ice are substantially heavier (i.e., higher) than those derived from meteoric waters. Since 180/160 ratios behave in the same way [e.g., Craig, 1961], this distinction is equally well made by 180/•60 measurements. The isotopic difference between meteoric waters and seawater has long been recognized [Epstein and Mayeda, 1953]. However, freshwater derived from the melting of sea ice has an isotopic composition very similar to that of the seawater from which it was formed IRedfield and Friedman, 1969; O'Neil, 1968; Tan and Fraser, 1976]. It is therefore possible to use the oxygen isotope-salinity relationship to determine in situ concentrations of sea ice meltwater and meteoric water. Redfield and Friedman [1969] used D/H ratios to demonstrate the presence of sea ice meltwater in arctic surface waters. The present paper describes an 180/•60 study of the distribution of sea ice meltwater and meteoric water in the eastern Canadian Arctic based on samples collected in 1976 and 1977. This work extends that of Redfield and Friedman [1969] to a wider, more intensively sampled geographic area. Furthermore, we provide the first quantitative estimates of sea ice melt concentrations and their variations with depth and loca...

The relationship between the isotopic composition of seawater and salinity and the variation of the isotopic composition of precipitation with latitude have frequently been used to identify freshwater sources and to tag water masses. Underlying these applications is the assumption that the isotopesalinity relationship is affected only by the mixing of salt and fresh waters. Simple box models are used to show how /j180-salinity (/J-S) relationships in the upper water column vary seasonally in areas where sea ice forms or melts. During melting, the/J-S relationship takes a number of different shapes, but it returns to a straight line late in the melting season. During freezing, the/J-S shape stays linear, but its slope varies. The models show that ice-related processes can produce very large changes in the 18 18 /J O value of apparent freshwater components (/J Os=o), the parameter usually used to label water masses. At the end of the melting season the /j18Os= 0 value may be 10%o more positive than at the beginning' at the end of the freezing season, /j18Os= 0 may be more than 20%o more negative than at the beginning. Thus the models provide explanations for both anomalously heavy/j18Os= 0 values in the Labrador Current (-7.6%o) and anomalously light values in the Canadian arctic archipelago and the East Greenland Current (•-50%o). They can also explain the negative slope occasionally observed in /J-S plots. Caution must be exercised in using •j18Os= 0 to identify source waters derived from the Labrador Sea, which receives significant amounts of freshwater from Baffin Bay and Hudson Strait which, in many cases, have been altered by more than a single season of melting/freezing activity.

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