The BC Coastline Is a Labyrinth, a Palimpsest of Ancient Episodes of Subduction and Glaciations

Home , British Columbia Coast, Hecate Strait, Queen Charlotte Strait

Bangarang February 2014 Backgrounder1

British Columbia’s Coast (Geography, meteorology & oceanography)

Eric Keen

Abstract

The BC coastline is a labyrinth, a palimpsest of ancient episodes of subduction and glaciations. Its position relative to adjacent ocean gyres and their corresponding atmospheric air masses, which oscillate in latitudinal position throughout the year, brings about a strong seasonal signal that dictates ecosystem processes. These systems induce seasonal freshwater input from land, an estuarine-type two way flow (seaward at surface, landward at depth), persistent alongshore surface currents close to the coast, persistent summertime upwelling regimes, and stark patchiness in water properties, nutrients, and productivity in space and time. In the summer the BC shelf experiences a weak southward current, weak upwelling, a highly stratified water column, estuarine circulation, and low sea level. In the winter, the southward drift of gyres and air masses brings about strong northward currents, strong downwelling, a well-mixed water column, westward bottom currents, and high sea level. These characteristics summarize processes both within the fjords that perforate the shoreline and on the mesoscale of the continental shelf as a whole. But at each scale there are unique interruptions; for example, katabatic outlfows within the fjords and mesoscale eddies and jets on the shelf. On all scales, these dynamic physical processes drive outrageously productive ecosystems.

Contents

Geography South to North Inshore to Off Meteorology Regional Processes Climate Gradients in the Study Area Interannual Variability State of the BC Ocean Oceanography Ocean gyres Seasonal Signals Mesoscale Eddies Coastal Surface Currents Tides Water Properties

1 Bangarang Backgrounders are imperfect but rigorous reviews – written in haste, not peer-reviewed – in an effort to organize and memorize the key information for every aspect of the project. They will be updated regularly as new learnin’ is incorporated.

Geography

The coastal corridor of British Columbia (BC) is host to a highly productive ecosystem characterized by a broad intracoastal zone of protected waters and labyrinthine inland passages. Contiguous with the fjord-gouged shoreline of the Gulf of Alaska (GOA), it comprises an ecosystem of more than 27,300 km in total length (Sebert & Munro 1972, Thomson 1981), contains the largest coastal rainforest in the temperate world, supports multi- million-dollar fishing and ecotourism industries, and provides subsistence for many First Nations societies.

South to North BC geography can be readily considered in thirds: the south, central and north coast. The south coast stretches from the Strait of Juan de Fuca (which separates the Olympic Peninsula from Vancouver Island) to the tip of Vancouver Island. The predominant feature of the south coast is Vancouver Island, whose rugged west coast has a markedly different climate and oceanography than the protected waters between it and the mainland (an inland sea called the Strait of Georgia). North of the Strait of Georgia, between central Vancouver Island and the coastal ranges of the BC mainland, one finds a labyrinth of passages punctuated with tidal rapids. These tight corridors open up to the north into the relatively open Queen Charlotte Strait, between northern Vancouver Island and the mainland, then into Queen Charlotte Sound and the north Pacific.

The central coastline is a tightly packed archipelago, with a continuous waterway (the Inside Passage) separating outer islands from the mainland. This otherwise closed coastline is divided into discrete regions by several broad sounds (Fitz Hugh, Milbanke, and Laredo), each of which has branching inlets that cut deep into the mainland. Offshore of this fjordland, a broad, shallow continental shelf extends out to the north-south trending Haida Gwaii archipelago (Queen Charlotte Islands), thus creating a bounded but volatile region known as Hecate Strait.

The divide between the central and north coast is somewhat arbitrary, but this author places the divide at Princess Royal Island, an enormous island encircled by deep fjord channels. From there, two parallel bands of nearshore islands create two protected passageways north to the Prince Rupert area (and onward to the Alaska border, with some large entrances uniting the passages on occasion): an inland and outside route. Just north of Princess Royal Island are the Douglas and Gardner fjord systems, known collectively as the Kitimat Fjord System. These fjords open to Hecate Strait in the broad amphitheater of Caamano Sound. This area’s intracoastal zone, from Caamano to the mouths of Douglas and Gardner fjords, comprises the marine territory of the Gitga’at First Nation and the study area for the Bangarang Project.

Inshore to Off The BC central and north coast is a fjordland, and as such we can think of its continental shelf in terms of three subregions (sensu Mackas 1992): the inner shelf back (including the fjords), the shelf break and slope, and the blue waters offshore. As fjords, the coastal inlets are semi-enclosed systems, restricted from interactions with open waters by shallow sills and narrow entrances. Water properties within these inlets are quite distinct from more open shelf waters because of their isolation (Burrell 1986, Crawford et al. 2007). Fjord oceanography is covered in a Backgrounder.

Compared to its Atlantic counterpart, Canada’s Pacific continental shelf is narrow. This is due to the fact that the Pacific coast has been a subduction zone in recent geologic history. With the exception of the broad, shallow basins of Hecate Strait and Queen Charlotte Sound, the width of the shelf off British Columbia rarely reaches 95km and it typically less than 45km (Thomson 1981). The shelf reaches its maximum off southern Vancouver Island, where it is also more bathymetrically complex than neighboring coastal regions to the north and south (Mackas 1992). To the north, a continuous seaway is formed over the continental shelf of the central BC coast by Queen Charlotte Sound, Hecate Strait, and Dixon Entrance (Thomson 1981). The seaward funneling of land- derived sediments from the coastal mountains has resulted in a gently falling continental slope within Queen Charlotte Sound. The central BC shelf region is dominated by a series of banks and troughs, remnants of glacial scouring and turbidity currents (Crawford et al. 2007). Some of these features, such as Morsby Trough, have been identified as highly productive foraging areas for baleen whales (Nichol and Ford 2011).

The BC shelf’s shallow basins represent a hybrid of oceanic and neritic environments (Thomson 1981). Freshwater drainage integrated along the fjordland coast creates an inshore-offshore salinity gradient analogous to those found within individual fjords. Related to this hydraulic gradient, a thin seaward surface current at the surface and a slower landward current at depth, analogous to estuarine circulation found in fjords. These two examples hint at the fractal nature of geography and oceanography found on the BC coast as one scales up from fjord to fjordland to coastline (Thomson 1981).

Meteorology

Regional Processes

On the BC coast, the annual seasonal cycle is the strongest environmental signal (Mackas 1992), governed by the interaction of stacked oceanographic and atmospheric systems. Over the offshore waters of BC, there are two juxtaposed atmospheric systems: the subtropical North Pacific High and, to the north, the subpolar Aleutian Low. The North Pacific high is dry, stable and fair-weather. Winds rush clockwise out of the North Pacific High, deflected to the right by Coriolis forces. The Aleutian low is wet and stormy. Winds rush counter-clockwise into the Aleutian Low.

Weather occurs at the transition zone between these two systems, which is generally an east-west front located somewhere along the BC coast. The transition zone drifts north and south throughout the year, orchestrating patterns in coastal winds, storms and temperature. Additionally, these winds contribute to driving the gyres and the North Pacific Current (see Oceanography section below).

Winter, October - April In the winter, the Aleutian Low propagates south, and storms dominate the winter climate. Air will circle counterclockwise into this depression, causing southwesterly or southeasterly coastal winds (Thomson 1981, Royer 1982). The storms of the Aleutian Low, forced to linger over the coast by the trappings of coastal mountain ranges, produce large amounts of precipitation, much of which is stored as snow and ice on the coastal range during winter and is subsequently discharged into coastal inlets in the spring (Coyle et al. 2005).

The onset of autumn storms generally occurs in late September or October (Crawford et al. 2007). Precipitation doubles in the autumn relative to summer (Fissel et al. 2010). Storm-force winds (gusting to 90-100 knots), accompanied by 8-10m high waves, occur several times each winter (Environment Canada 1992). Waves over 20m in height occur in the open coastal waters (Crawford et al. 2007). The Aleutian Low reaches its lowest pressure, with the strongest counterclockwise winds, in January. The winter is a dangerous time to be out at sea in BC (Environment Canada 1992, Lange 2003).

Near the coast, the two systems abut high pressure systems held aloft over the continent. These high pressure air masses are composed of cold Arctic air that is “eager”, so to speak, to descend the coastal ranges and rush out to sea. It is held in place, however, as long as the North Pacific High has drifted north (in the summer). But when the North Pacific High drifts back south (like in winter), the BC coast becomes exposed to the Aleutian Low, into which continental high pressure systems rush readily. These outflows generate extremely high (60-100 knots, Fissel et al. 2010), freezing winds in coastal inlets (60-100 knots, Fissel et al. 2010). Such northeasterly “katabatic” outflows can generate rough seas even in protected inlets (up to 3m in Knight and Rivers Inlets, Province of British Columbia 2001).

In the Gitga’at marine territory, rainfall peaks in October and November and the highest winds occur later in the winter (Fissel et al. 2010). From October to April, prevailing wind direction is from the north, northeast, or southeast (Fissel et al. 2010). Winter waves in Caamano Sound can reach more than 6m in height, with waves >4m not uncommon (Fissel et al. 2010). Enormous quantities of snow fall in the coast mountains (Kildala Pass, at 1600m, can get 20m of snow in a winter! MacDonald et al. 1983).

Summer, May – September

The North Pacific High shifts northward and displaces the Aleutian Low, resulting in a clockwise flow of air from the high’s center that causes weak northwesterly coastal winds. Southwest winds are also common (Fissel et al. 2010). Storms are also deflected to the north, bringing about relatively fair weather on the BC coast (Fissel et al. 2010). The lowest annual precipitation levels occur in June through August (Fissel et al. 2010, Pickard and Stanton 1980), when the North Pacific High reaches its greatest intensity (Thomson 1981).

Off the British Columbia coast, the speed of the wind tends to vary cyclically in strength every 3 days or so due to the eastward passage of smaller highs, lows, and associated frontal systems. Maximum wind speeds vary with season and are greatest in fall and winter, somewhat less in spring, and least in summer (Thomson 1981).

In the Gitga’at marine territory, mild marine conditions in the summer bring about intermittent scattered showers (Fissel et al. 2010). The sheltered inland waterways experience small wind waves thanks to weaker winds and reduced fetch. But when strong winds do occur they can generate steep, choppy wave conditions up to 2m in height, especially if meeting an opposing tidal current (Crawford et al. 2007).

Climate Gradients in the Study Area

The Gitga’at marine territory encompasses an intracoastal corridor that spans from protected inlets that reach deep into the mainland to outer islands that are exposed to Hecate Strait and the north Pacific. Being nested as it is within a coastal margin, moist marine air is trapped by the coastal ranges and islands and brings copious amounts of precipitation (Fissel et al. 2010). At the seaward limit the climate is notably marine: moderate air temperature fluctuations with intense storms in the fall and winter (Fissel et al. 2010). At the landward limit temperatures exhibit a greater range (warmer summers and colder winters), marine storm winds are abated, and strong Arctic outflows can occur in winter (Fissel et al. 2010).

From Fissel et al. 2010: Seasonal temperature variability is more dramatic inshore (red line, Kitimat), than it is in Hartley Bay (yellow line), and even less variable on the exposed outer coast (green line, Bonila Island).

Interannual Variability

The BC coast is affected by interannual climatic oscillations, including the Pacific Decadal Oscillation (Mantua et al. 1997) and the three-to-seven year El Nino/La Nina oscillation. These interannual cycles influence biotic regime shifts and ecosystem dynamics in general (Kang 2005, Schwing et al. 2005). In addition to these decadal oscillations, longer term variability, such as the current trend of global warming, influence the BC coast.

State of the BC Ocean

Despite a steady and indisputable warming trend in global temperatures averages, the ocean temperature near the BC coast has declined fairly steadily since the late 1980’s, especially after 1998 (Irvine and Crawford 2009).

Stronger summertime North Pacific Highs and stronger winter Aleutian Lows have brought about stronger upwelling and downwelling regimes, respectively, for the previous decade. The northeast Pacific Ocean has been experiencing La Nina conditions since 2007 (Irvine and Crawford 2009). In 2012, however, conditions weakened and may signal the end of a relatively consistent decade for BC climate (Irvine and Crawford 2013).

Oceanography

“From an oceanographic point of view it is a hybrid region, similar in many respects to the offshore waters but considerably modified by estuarine processes characteristic of the protected inland coastal waters.” (Thomson 1981)

The BC Coast is a dynamic, unique setting for ongoing oceanographic research (Crawford and Greenan 2007).

Ocean Gyres

The coast of BC lies at a transition zone between two large oceanographic gyres, the southern East Pacific Gyre and the northern Alaska Gyre, whose positions shift north and south according to seasonal changes in climate and basic-scale circulatory processes (Crawford et al. 2007). At the transition zone between these two gyres is the slow (5-10 cm/s), east-flowing North Pacific Current (also called the Subarctic Current). When this current reaches the BC coast, it splits; one part turns south to become the equatorward California Current upwelling system, and the remainder turns north to become the poleward Alaska Current downwelling system, which itself is an extension of the California Countercurrent (Mackas et al. 2000). At the split itself, waters meander and eddy.

When these two gyres shift north or south, the location of their split will change, and in turn the direction of current flow at a point on the coast may change. This is hugely significant to regional patterns in productivity, since current direction determines upwelling rates, which largely determines nutrient availability for phytoplankton.

Broadly speaking, the BC coastal currents are largely wind- and freshwater-runoff-driven. However, scale matters greatly if trying to determine their relative influence. Over large scales in space and time, currents are dictated by regional oscillations in oceanic gyres and atmospheric systems. On the mesoscale in space and time, these larger processes are interrupted by both episodic eddies induced by strong currents flowing around topography offshore,and also freshwater inputs nearshore. On smaller spatial scales, currents are governed by coastal morphology and the bathymetry of the continental shelf (Crawford et al. 2007). On small temporal scales, tides dictate the direction and flow of currents.

From Crawford et al. 2007:

Seasonal Signals

Summer The North Pacific and Alaska Gyres meet in a transition zone halfway up the BC coast, causing weak and variable currents persist on the north coast all summer long. In such circumstances, summer winds, though lighter than in winter, have more influence over the flow direction of offshore surface waters. Prevailing northwest winds therefore induce a slight southeast flow on the coast, but light variable winds can bring about current reversals too (Crawford et al. 2007). In confined waterways, however, wind-forcing is negligible and estuarine circulation goes on largely without interruption (Thomson 1981). This reduced wind mixing, along with surface heating and high freshwater runoff, bring about a structured water column with a strong thermocline that can deepen to 20m or more (Crawford et al. 2007) in the coast’s fjords.

Wind-forced southward currents that are sustained become deflected offshore by Coriolis forces (i.e., Ekman transport), bringing about a very weak upwelling regime on the shelf that draws cold, nutrient–rich water from the deep up into the coastal photic zone (Crawford et al. 2007, Freeland 1983; Mackas 1992; Thomas and Emery 1986, Benmank et al. 1981). Upwelling thus draws deeper water onto the shelf. Offshore winds and increased spring- and summer-time freshwater outflow from inlets combine with the upwelling process (Thomson 1981) to drive the estuarine circulation pump. As surface waters are pushed offshore by winds and upwelling, deep, cold and saline waters are drawn onto the shelf in a landward bottom current (Thomson 1981, Mackas et al. 2000). As a result, even as surface (<50m) waters warm in the summer sun, deeper waters in the

shelf seaway are colder and more saline than in winter (Mackas 1992). The seaward deflection of the coastal current – combined with high pressure weather systems (the “reverse barometer effect”, Crawford et al. 2007) – depresses the summer sea level.

Winter In step with the atmospheric highs and lows, the ocean gyres drift south in winter. The strengthening (Crawford et al. 2007) Alaskan Gyre butts against the majority of the coast. Nearshore currents are therefore predominantly north-flowing and stronger. Surface waters are pushed into Queen Charlotte Sound and Hecate Strait. Coriolis forces deflect this current to the right, “heaping” waters up against the BC shore. This, combined with prevailing low pressure systems, elevate sea level as much as 30cm in the winter (Crawford 1980, Crawford et al. 2007). This heaped sea level is more pronounced in the shallow, northern terminus of Hecate Strait, and causes lowered sea levels on the west side of Hecate Strait on the shores of Haida Gwaii (Crawford et al. 2007).

The wintertime landward current opposes outflowing, buoyant freshwater runoff from the coast and downwells strongly, resulting in low surface productivity. However, downwelling institgates thorough vertical mixing (Crawford et al. 2007, Crawford and Dewey 1989), brings oxygen-rich waters to the seafloor, and displaces bottom water into a westward bottom current. As a result the coastal waters are “primed” for inordinate spring plankton blooms.

From Crawford et al. 2007:

Mesoscale Eddies

Seasonal instabilities and pronounced bathymetric features generate frequent meanders and mesoscale eddies in the BC coast’s alongshore currents (Thomson 1984; Ikeda et al. 1984). These eddies are anti-cyclonic mesoscale vortices, 200km or more in diameter, that typically form in the winter along the eastern continental margin of the Gulf of Alaska, west of Haida Gwaii (Crawford 2002). Strong outflow currents ripping around Cape St. James are reliable sources of one to two such eddies every winter (Crawford et al. 2007b). These eddies have warm waters in their cores and a baroclinic depth structure with fresher waters down to 1000m or more. These Haida Eddies generally propagate southwest (Crawford 2002) and have a “lifetime” of 1-3 years (Mackas & Galbraith 2002). Eddies are larger in strong El Nino winters (Crawford 2002).

Similar but smaller oceanographic features reliably occur in certain areas of the BC coast. When northwesterly winds are strong, persistent cooler waters can occur on the western side of land masses like Haida Gwaii (Crawford et al. 2007). Waters flushing out of Caamano Sound north of Aristazabal Island (the ‘Aristazabal Island Plume’), have a strong nutrient and productivity signature that persists most of the year and is funneled into troughs that traverse Hecate Strait (Crawford et al. 1995; Crawford 2001).

These mesoscale features contribute to cross-shelf mixing and the redistribution of zooplankton taxa (Coyle et al. 2005). The eddies entrain plankton- and larvae-rich coastal waters and whisk them offshore. They then slowly “leak” the nutrients and plankton from within their cores as they drift and degrade (Crawford et al. 2007b). Upon the birth of a Haida Eddy, shelf species within it decline in abundance quickly as they are brought into unfamiliar oceanic waters. Slope species tend to persist for longer (Mackas & Galbraith 2002). Offshore biomass anomalies have been observed to lag behind changes in the continental shelf by 1 or 2 years (Mackas et al. 2000), which could be associated with these eddies. This could be thought of as an multi-annual analog to the hypothesis (Peterson & Miller 1976, Peterson et al. 1979, Mackas 1992, Peterson 1998) that each summer upwelling season, a large amount of zooplankton originating on the continental shelf are subsequently advected offshore.

Coastal surface currents

Over the inner continental shelf, coastal inputs drive a year-round, poleward, low-salinity, buoyancy-driven nearshore current (Mackas et al. 2000, Coyle and Pinchuk 2005): the Alaska Coastal Current (ACC). Much of the discharge fueling this fixture comes from the Columbia (47 degrees N) and Fraser Rivers (49 N)(Thomson 1981). Their discharge is flushed out of the Strait of Juan de Fuca in its estuarine circulation (Freeland 1983; Thomson et al. 1989) and is added to by runoff as it travels up the coast (especially in winter)(Mackas 1992). Because of estuarine circulation at the eastern end of Juan de Fuca Strait (which can be modulated by the tides; Masson and Cummins 2000), these alongshore flows also carry very high nutrient loads (Mackas et al. 1980; Crawford and Dewey 1989).

The ACC continues up the inner continental shelf of the Gulf of Alaska (GOA). Reduced discharge (due to freezing temperatures) and onshore Ekman transport confines the ACC to a narrow region near the coast in the winter (Royer 1982). Much of the remainder of the shelf is dominated by the prevailing onshore transport of water masses, which contain a random mixture of overwintering zooplankton species (Coyle et al. 2005). A transitional zone also exists, from the ACC margin out to the shelf break, of increasing salinity, outside of which oceanic water masses flow westward along the slope (Coyle et al. 2005). ACC geography also has a seasonal signature: As the storm activity subsides during the spring and early summer, the wind-driven onshore Ekman transport relaxes, solar radiation melts the snow pack. The snowmelt increases discharge and elevates surface temperatures (Coyle et al. 2005), which results in a broadening and shoaling of the ACC. In the spring, therefore, the ACC covers a larger portion of the shelf environment (Weingartner et al. 2005, Williams 2003).

Tides

On the BC coast, large tidal currents tend to dominate the water movement over time scales of hours; however the oscillatory motion of the ebb/flood cycle can result in little net movement (Crawford et al. 2007). The flooding tide flows northeast into Queen Charlotte Sound, then turns northwards and progresses up Hecate Strait. Due to the deflection caused by the Coriolis force, the flooding tidal wave “leans” up against the mainland coast. There is a corresponding downward gradient in both sea level and tidal range out to sea (Thomson 1981). The ebb outflow is generally in the opposite direction, south (Crawford et al. 2007). However, there are some channels, such as Grenville Channel to the northwest of the Kitimat Fjord System, where tidal currents flood from both ends, meet in the middle, then ebb out to both mouths. Offshore, components of the tidal wave wrap around Haida Gwaii to meet in the area of Dixon Entrance, resulting in exacerbated tidal ranges. The mean tide range increases from about 3 m in Queen Charlotte Sound to about 5 m midway up Hecate Strait (Crawford et al. 2007). Tide ranges over 7m are encountered near Prince Rupert during spring tides (Thomson 1981). Tidal ranges increase in the narrow coastal fjords of the mainland (Thomson 1981). A 6.2m tide has been observed in the Gitga’at Territory (Fissel et al. 2010). An 8m tide was observed nearby in Principe Channel (Fissel et al. 2010).

Tidal currents can result in strong mixing zones, particularly over the shallow banks in Queen Charlotte Sound, Hecate Strait, and the northern and southern tips of Haida Gwaii (Cape St. James and Rose Spit; Crawford et al. 2007). Vigorous tidally-induced mixing occurs in the bottlenecks within coastal inlets, where waterways are

constricted either vertically by sills or laterally with narrowing channel walls – or both. In such scenarios, hydraulic flows can become established, resulting in stationary or slow propagating surface manifestations of internal tidal waves (Cummins et al. 2003; see the Fjords Backgrounder). These features, and others such as tidal fronts in both confined channels and open waters, all serve to mix waters, accumulate nutrients, aggregate plankton, and attract an ecosystem of predators (Crawford et al. 2007).

From Crawford et al. 2007:

Water Properties

Salinity Although the largest meltwater will not come until the summer and spring rains grow rare, the lowest surface salinities occur in spring before the flow regime has switched to upwelling and its associated seaward transport (Freeland et al. 1980, Mackas 1992). This is because the onshore-flowing currents of winter confine freshwater discharge to the coastal inlets and shelf, preventing their diffusion into the ocean.

Upwelling signals begin on the continental shelf, where they remain strongest throughout the summer season. After the transition to upwelling (April-September), freshwater discharge is pulled out to sea and high-salinity water is entrained with it from below, reducing overall coastal salinity (Mackas 1992). A cross-shelf salinity gradient is therefore present most of the year (Royer 1998), strongest just before the onset of the upwelling season (April-May). This might prevent lateral mixing of zooplankton communities with particular salinity requirements, and may contribute to horizontal heterogeneity in shelf zooplankton communities (Coyle and Pinchuk 2005).

As upwelling weakens and discharge abates in the late fall, surface salinity slowly increases.

Temperature On the shelf and slope, upwelling induces seasonal signatures in sea-surface temperature (SST). Surface temperature decreases as upwelling becomes established.

Within the protected areas of the coast the variability of water temperatures depends on the intensity of solar heating as well as the degree of vertical mixing by wind and tide. In turbulent tidal channels like Juan de Fuca Strait and Johnstone Strait the water is always cold. Only in isolated nooks and crannies, such as Sooke Basin, does any appreciable summer warming take place (Thomson 1981).

Productivity The lowest surface nutrient concentrations on the shelf occur at the end of the winter current regime, in March or April in Juan de Fuca Strait (a rainwater-driven system) and May-June in all snow-melt-driven regions to the north and on the mainland (Mackas 1992, Thomson 1981).

The onset of the upwelling season causes nitrate and phytoplankton concentrations to increase (Mackas et al. 2000). This establishes a persistent cross-shelf gradient in nitrate and chlorophyll. Throughout the summer upwelling regime, nitrate concentrations increase until they reach an annual maximum in August-September (island inlets, e.g. Juan de Fuca area) or October-November (mainland fjords) (Mackas 1992). Over the course of the season, these signals slowly transgress seaward (Mackas 1992, Mackas et al. 2000).

In shelf waters, iron is a limiting nutrient to phytoplankton productivity, resulting in expansive High Nutrient Low Chlorophyll (HNLC) areas in the Gulf of Alaska and BC waters (Crawford et al. 2007). Mesoscale eddies, which advect iron-rich coastal waters offshore, can constitute a minority of Gulf of Alaska waters but contain a majority of the region’s phytoplankton (Crawford et al. 2007). As these eddies decay, offshore phytoplankton can then access the nutrients that were within in steady and regular doses (Crawford et al. 2007).

Despite large amounts of suspended sediments and tannins in the freshwater discharge that restricts light penetration in fjord waters, high phytoplankton levels can be maintained throughout an inlet (Gilmartin 1964, Hooge & Hooge 2002). High average chlorophyll concentrations occur through the spring, summer, and early autumn on the inner shelf (Mackas 1992). Offshore, early spring phytoplankton blooms that are not associated with upwelling or offshore transport deplete nutrients locally in March and April. Concentrations then increase throughout the summer as nutrients are advected in with the seaward transport of coastal and upwelled waters (Mackas 1992). In the late summer or early fall a smaller secondary bloom may occur in Alaskan coastal waters, but heavily silted systems such as glacial fjords are thought to have suppressed fall blooms (Burrell 1986).

At the shelf break, upper layer alongshore currents are strong (Mackas 1992). These flows include, in southern BC, the Shelf Break Current and the California Undercurrent (below 150m), which extends northward to the central and north coast (Thomson 1984, Ikeda et al. 1984), and the Alaska Coastal Current (Coyle and Pinchuk 2005). These flows are high in nutrients and phytoplankton biomass during the summer (Denman et al. 1981, Sirnard and Mackas 1989). Seaward of these currents, waters are relatively oligotrophic.

Literature Cited

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