GEOGRAPHIC SETTING

The SBIC is a network of long and deep steep-sided fjords on the north-central coast of mainland British Columbia (Figure 1), about 40 km northeast of Port Hardy, Vancouver Island. The complex lies between latitudes 50°50.2' N and 51°10.6' N, and longitudes 126°30.2' W and 127°40.5' W, and opens to Queen Charlotte Sound via Slingby and Schooner Channels.

The main arms of the complex are the east-west trending Seymour and Belize inlets, which reach inland ~ 70 and 50 km, respectively. Alison Sound is a smaller inlet that extends 20 km off the northern margin of Belize Inlet, in a northeasterly direction before turning towards the east. Small rivers and creeks provide freshwater to the heads of all inlets in the SBIC, with Belize Creek in Belize Inlet, Waump Creek in Alison Sound and the Seymour River in Seymour Inlet being amongst the most important streams. The freshwater input of these rivers, together with numerous waterfalls along the inlets, peaks during the snow-melt period that starts in May and plays an important role in circulation in the SBIC (Thomson 1981).

The climate of coastal British Columbia is mild, with winter temperatures typically remaining above the freezing point accompanied by frequent rains, and warm days and cool nights in summer (Hare and Thomas 1979). Mean annual precipitation in the SBIC region is 3120 mm (ranging between 2009 mm and 3943 mm), with an average annual temperature of 9.1°C (ranging between 5.4° to 9.4°C) (Green and Klinka 1994).

Weather systems are seasonally dominated by the counter-clockwise circulating winds that accompany the Aleutian Low (AL) in winter and the clockwise circulating winds that occur with the North Pacific High (NPH) in summer. The mouth of the SBIC is situated at the northern border of the Coastal Upwelling Domain (CUD) (Ware and McFarlane 1989), where from May through September the northwesterly winds of the NPH displace warm water from the surface, promoting the upwelling of cold, nutrient-rich deep water that greatly enhances productivity in the surface layers. However, because the SBIC is at the northern boundary of the domain and opens to Queen Charlotte Sound instead of the open ocean, the influence of upwelling within the SBIC is virtually nil as compared to more exposed inlets on the west coast of Vancouver Island (Thomson 1981).

The AL is responsible for the frequency and intensity of storms that promote cold weather and precipitation in winter (Cayan and Peterson 1989; Miller et al. 1994). Conversely, during the spring and summer, when the NPH moves northward and the AL dissipates, less precipitation and clear skies are the dominant features in the area (Hare and Thomas 1979; Thomson 1981).

Maximum depths within the SBIC are greater than 600 m in Seymour Inlet, ~ 300 m in Belize Sound, and ~ 150 m in Alison Sound. Oxygen concentrations in the bottom waters of the SBIC range from high oxic in the main arms (more than 6 mL/L) to anoxic conditions in Alison Sound (less than 0.1 mL/L; Kaiho 1994).

An important characteristic of these bathymetrically U-shaped basins is the presence of sills in most of them, formed by crushed rock and silt that were deposited as moraines by advancing glaciers (Schafer et al. 1989). The sills reduce the input of oxygen-rich ocean water into the inlets, which together with a low-salinity wedge caused by riverine input at the surface, results in reduced mixing between surface and bottom waters. The outcome is a stratified water column and the development of estuarine circulation. It is typical in the SBIC and in similar partially mixed estuaries to have the salinity of the surface layer increase down-inlet away from freshwater sources, and the salinity of bottom waters to increase slightly towards the head of the inlet (Pickard and Stanton 1980; Thomson 1981). The restricted circulation in these systems enhances the trapping of organic and inorganic material borne by runoff, making these environments effective nutrient traps. Disturbance at the sediment water interface is minimized due to the low oxygen conditions, the high residence time and slow circulation of the bottom water, resulting in undisturbed sediments that create an ideal setting for paleoenvironmental research.

The main sill at the mouth of the SBIC, the Nakwakto Rapids, is only 34 m deep and 300 m wide, forming a major bottleneck during tidal cycles (Department of Energy 1979) (Figure 1). During the ebb tide current flow through Nakwakto Rapids can reach velocities of 8 m/s, making it one of the strongest tidal currents in the world (Thomson 1981). This tidal constriction at the Nakwakto Rapids is so restrictive that it is impossible for sea level with the SBIC to equalize with that of Queen Charlotte Sound during ebb tidal flow, resulting in a tidal range of more than 2 m in Queen Charlotte Sound and a maximum of only 1.3 m in the interior of the SBIC (Fisheries and Oceans 2003).

There are three sills in Alison Sound (Figure 1), which further restrict circulation into this inlet. The outermost one, located at the juncture of Alison Sound with Belize Inlet, is 31 m deep; the second, just inside the mouth of Alison Sound, is 30 m deep, while the last sill, located near the head of the inlet, is only 17 m deep. This restricted circulation reduces the bottom water oxygen concentration to < 1mL/L, the lowest values recorded in the SBIC. Within Belize Inlet oxygen values do not go below 3 mL/L.