Estuarine, Coastal and Shelf Science 199 (2017) 96e104 Contents lists available at ScienceDirect Estuarine, Coastal and Shelf Science journal homepage: www.elsevier.com/locate/ecss Using oxygen isotopes to establish freshwater sources in Bedford Basin, Nova Scotia, a Northwestern Atlantic fjord * Elizabeth A. Kerrigan, Markus Kienast, Helmuth Thomas, Douglas W.R. Wallace Dalhousie University, Department of Oceanography, 1355 Oxford Street, PO Box 15000, B3H 4R2, Halifax, NS, Canada article info abstract Article history: A weekly time-series of oxygen isotope (d18O) measurements was collected over a 16-month period from Received 10 March 2017 near-surface (1 m) and near-bottom (60 m) waters of Bedford Basin, a coastal fjord adjacent to the Available online 8 September 2017 Scotian Shelf, off eastern Canada. The time-series was complemented with d18O measurements of local precipitation (rain and snow), river, and wastewater runoff. The isotopic composition of precipitation Keywords: displayed strong seasonality with an average (volume-weighted) d18O value of À5.39‰ (±0.96) for Bedford Basin summer and a depleted value of À10.37‰ (±2.96) over winter. Winter precipitation exhibited more Fjord depleted and variable d18O of solid precipitation relative to rainfall. The annual, amount-weighted Oxygen isotopes d18 À ‰ ± Salinity average O of Sackville River discharge ( 6.49 0.82) was not statistically different from precipi- À ‰ ± Seasonality tation ( 7.24 0.92), but exhibited less seasonal variation. Freshwater end-members (zero-salinity 18 Freshwater inputs intercepts) estimated from annual and seasonal regressions of d O versus salinity (S) for Bedford Basin near-surface samples were consistent with the d18O of summer precipitation and the annual, amount- weighted average for the Sackville River. However, the isotopically depleted signature of winter pre- cipitation was not observed clearly in near-surface waters of Bedford Basin, which might reflect isotope enrichment during sublimation from accumulated snowfall prior to melting and discharge, or retention and mixing within the drainage basin. In near bottom waters, most of the d18O-S variation (average freshwater end-member: 7.47‰ ± 2.17) could be explained by vertical mixing with near-surface waters (average freshwater end-member: À6.23‰ ± 0.34) and hence with locally-derived freshwater. However the near-bottom d18O-S variation suggested an additional contribution of a freshwater end-member with a d18OofÀ15.55‰ ± 2.3, consistent with a remotely-derived freshwater end-member identified previ- ously for the Scotian Shelf. Residuals from a long-term regression of d18O-S were generally within the range expected due to analytical uncertainty (±0.05); however near-surface waters exhibited seasonal variability of small amplitude, which was consistent with the timing and d18O variability of local freshwater inputs. © 2017 Published by Elsevier Ltd. 1. Introduction alongshore nutrient transport and stratification-dependent bio- logical production over continental shelves. The alongshore conti- The freshwater balance of the northern North Atlantic Ocean is nuity of this boundary current was first identified on the basis of an changing as a result of changing Arctic sea-ice cover and increased analysis of oxygen isotopes of water (Fairbanks, 1982). melting of glacial ice (e.g., on Greenland; Bamber et al., 2012). Much The oxygen isotope composition of H2O molecules has been of this Arctic-derived freshwater is transported southwards along used widely as a tracer of the hydrological cycle (Craig, 1961), ocean the east coast of North America, as far as Cape Hatteras, by a freshwater sources (e.g., Craig and Gordon, 1965; Fairbanks, 1982; 5000 km long boundary current (Chapman and Beardsley, 1989). Khatiwala et al., 1999), and sources of deep water masses (Bauch This long-range influence of Arctic freshwater potentially impacts et al., 1995). Craig and Gordon (1965) first used d18O as an ocean- ographic tracer and established that, when paired with salinity (S), it can be used to characterize mixing of water masses and to * Corresponding author. distinguish different sources of freshwater (e.g., precipitation, river E-mail addresses: [email protected] (E.A. Kerrigan), [email protected] discharge, meltwater, etc.). (M. Kienast), [email protected] (H. Thomas), [email protected] For the North Atlantic as a whole, freshwater originating from (D.W.R. Wallace). https://doi.org/10.1016/j.ecss.2017.09.003 0272-7714/© 2017 Published by Elsevier Ltd. E.A. Kerrigan et al. / Estuarine, Coastal and Shelf Science 199 (2017) 96e104 97 Arctic outflow dominates, with a d18O end-member of À21‰ (Khatiwala et al., 1999). Oxygen isotope analysis of the waters over the Scotian Shelf and Scotian Slope, off the east coast of Canada, revealed that the main freshwater input has its origin at high- latitudes (Fairbanks, 1982; Khatiwala et al., 1999). This Arctic freshwater source is complemented in spring and summer by the outflow of the Gulf of St. Lawrence (Shadwick and Thomas, 2011). Khatiwala et al. (1999) noted that the 18O-salinity signature and resulting freshwater end-member of waters over the Scotian Shelf (À15.55‰; Fairbanks, 1982)areinfluenced strongly by sea-ice for- mation and brine rejection over the Labrador Shelf and by the heavier freshwater end-member derived from the St. Lawrence River (À10.3‰). The two main sources of freshwater on the Scotian Shelf, high-latitude (i.e. Arctic) runoff and St. Lawrence River water (SLRW), contribute freshwater in an approximately 2:1 ratio (Khatiwala et al., 1999). A number of previous studies have used the d18O-S relationship within estuaries and fjords (Martin and Letolle, 1979; Austin and Inall, 2002), typically focussing on distinguishing various sources and sinks (precipitation, evaporation, river discharge, groundwater input, ice-melt) (e.g., Azetsu-Scott and Tan, 1997; Corlis et al., 2003; Fig. 1. General movement of water masses along the Scotian Shelf (Khatiwala et al., MacLachlan et al., 2007; Stalker et al., 2009; Turk et al., 2016; 1999; Shadwick and Thomas, 2011). SLEW: St. Lawrence Estuary Water, LShW: Lab- Whitney et al., 2017). The d18O-S relationship allows characteriza- rador Shelf Water, WSW: Warm Slope Water, LSW: Labrador Slope Water, GS: Gulf e tion of freshwater inputs with distinct d18O signatures in an estuary. Stream. Halifax Line stations are indicated along with station numbers (1 7) and the fi d18 grey star identi es Bedford Basin. The bedrock geology surrounding Bedford Basin However, few studies have examined the seasonality of the O-S includes granite but is composed largely of very fine-to medium-grained metasand- relationship. stone and slate, with relatively low groundwater permeability and yield (Kennedy and Here we present results from a 16-month, weekly time-series of Drage, 2009). d18O collected from Bedford Basin, a coastal fjord adjacent to the Scotian Shelf, together with measurements of local river water, wastewater, and precipitation. The data provide insight into the 1993). Near-surface salinity within Bedford Basin is typically in e distribution and fate of freshwater sources as well as a reference the range of 29 30.5 and hence lower than surface salinities of e point against which to evaluate longer-term changes in freshwater 30 31.5 measured at HL-2 (see Fig. 1). The annual average 3 inputs, both local and remote, that might arise due to climate discharge of fresh water into Halifax Harbour is 15.7 m /s (Buckley change. and Winters, 1992) and is supplied from a watershed with area of 281 km2 (Li and Harrison, 2008). The largest single source is the 2. Study site Sackville River, which enters Bedford Basin at its northwestward end and has an average discharge of 5.41 m3/s (Buckley and 3 3 2.1. Scotian Shelf Winters, 1992), ranging from 2 m /s to 9 m /s in the summer (JulyeSeptember) and spring (March and April) respectively The Scotian Shelf is a 700 km long region of the continental shelf (Fournier, 1990). The remaining freshwater discharge is via a off Nova Scotia, varying in width from 120 to 240 km, and covering number of small streams and sewers (Buckley and Winters, 1992) 120,000 km2 with an average depth of 90 m. This region is bound by with c. 16% of the total delivered by the Halifax wastewater treat- the Laurentian Channel to the northeast and by the Northeast ment system, whose water originates from two lakes, Pockwock Channel and the Gulf of Maine to the southwest (Shadwick and Lake and Lake Major, as well as a number of smaller reservoirs Thomas, 2011)(Fig. 1). Three major water masses contribute to (Fader and Buckley, 1995). Direct precipitation onto Bedford Basin 3 Scotian Shelf waters: (1) Warm Slope Water (WSW), a mixture of contributes an additional 0.8 m /s (Kerrigan, 2015). The contribu- Labrador Slope Water (LSW) and Gulf Stream (GS) water, (2) Lab- tion of submarine groundwater discharge (SGD) to overall fresh- rador Shelf Water (LShW), derived from the inner branch of the water input is not known. However given the low permeability Labrador Current, and (3) St. Lawrence Estuary Water (SLEW), bedrock (Fig. 1), a groundwater recharge ratio for the catchment which is influenced by the St. Lawrence River (Khatiwala et al., area of 0.12 (Gavin Kennedy, Nova Scotia Department of Natural 1999; Shadwick and Thomas, 2011). Resources, personal communication, July 2017), and the likelihood that much of the recharge is delivered to Bedford Basin via rivers 2.2. Bedford Basin and streams, we consider additional input by SGD to be small for this water body. fi Bedford Basin is a fjord (volume: 5.6 Â 108 m3, area: 17 km2, Bedford Basin is strati ed by temperature seasonally, and by max. depth: 71 m) situated on the Atlantic coast of Nova Scotia, salinity almost year-round due to local freshwater inputs (Li and Canada, and separated from the adjacent Scotian Shelf by a 10 km Harrison, 2008).