Alaskan Stream Research Articles

Marine carbon and nitrogen in southeastern Alaska stream food webs: evidence from artificial and natural streams

Incorporation of marine-derived nutrients (MDN) into freshwater food webs of southeastern Alaska was studied by measuring the natural abundance of nitrogen and carbon stable isotopes in biota from artificial and natural streams. Biofilm, aquatic macroinvertebrates (detritivores, shredders, and predators), and fish (coho salmon, Oncorhynchus kisutch, and cutthroat trout, Oncorhynchus clarki) were sampled from streams in which Pacific salmon (Oncorhynchus spp.) carcasses had been artificially placed or were present naturally. In the presence of carcasses, all trophic levels incorporated marine-derived nitrogen (range, 22–73% of total N) and carbon (range, 7–52% of total C). In general, chironomid midges assimilated more marine-derived nitrogen and carbon than did other consumers. The assimilation of MDN by aquatic organisms and subsequent isotopic enrichment (5–6‰ for 15N, 3–4‰ for 13C) were similar in experimentally and naturally carcass-enriched streams. For specific taxa, however, percent assimilation for marine nitrogen and carbon were often dissimilar, possibly because of fractionation or transfer inefficiencies. These results suggest that pathways of MDN incorporation into stream food webs include both consumption of salmon material by macroinvertebrates and fish and uptake of mineralized MDN by biofilm. Incorporation of MDN into multiple trophic levels demonstrates the ecological significance of annual returns of anadromous fishes for sustaining the productivity of freshwater food webs.

Decadal variability of the North Pacific Polar Front: Subsurface warming versus surface cooling

Over 200 hydrographic sections are used to trace the Polar Front defined as the southern boundary of the “pure” subarctic stratification with a pronounced, extremely cold, subsurface temperature minimum underlain by a temperature maximum. The front extends from 40°N off Japan to 57°N in the Gulf of Alaska where it retroflects and continues WSW with the Alaskan Stream. The front's decadal variability from 1977–1999 is examined along 150°E, 170°E, 175.5°E, and 180°E. At these longitudes the front is relatively stable, except for 170°E, where it shifts north‐south by 400 km every 6 years. Most time series reveal a subsurface warming of ∼1°C per decade, and a surface cooling, of the front. Since the subsurface temperature minimum is a remnant of winter convection, the subsurface warming signals an amelioration of the winter climate, whereas the summer climate becomes colder.

Export of invertebrates and detritus from fishless headwater streams in southeastern Alaska: implications for downstream salmonid production

1. We examined the export of invertebrates (aquatic and terrestrial) and coarse organic detritus from forested headwaters to aquatic habitats downstream in the coastal mountains of southeast Alaska, U.S.A. Fifty‐two small streams (mean discharge range: 1.2–3.6 L s−1), representing a geographic range throughout southeast Alaska, were sampled with 250‐μm nets either seasonally (April, July, September) or every 2 weeks throughout the year. Samples were used to assess the potential subsidy of energy from fishless headwaters to downstream systems containing fish.2. Invertebrates of aquatic and terrestrial origin were both captured, with aquatic taxa making up 65–92% of the total. Baetidae, Chironomidae and Ostracoda were most numerous of the aquatic taxa (34, 16 and 8%, respectively), although Coleoptera (mostly Amphizoidae) contributed the greatest biomass (30%). Mites (Acarina) were the most numerous terrestrial taxon, while terrestrial Coleoptera accounted for most of the terrestrial invertebrate biomass.3. Invertebrates and detritus were exported from headwaters throughout the year, averaging 163 mg invertebrate dry mass stream−1 day−1 and 10.4 g detritus stream−1 day−1, respectively. The amount of export was highly variable among streams and seasons (5–6000 individuals stream−1 day−1 and <1–22 individuals m−3 water; <1–286 g detritus stream−1 day−1 and <0.1–1.7 g detritus m−3 water). Delivery of invertebrates from headwaters to habitats with fish was estimated at 0.44 g dry mass m−2 year−1. We estimate that every kilometre of salmonid‐bearing stream could receive enough energy (prey and detritus) from fishless headwaters to support 100–2000 young‐of‐the‐year (YOY) salmonids. These results illustrate that headwaters are source areas of aquatic and terrestrial invertebrates and detritus, linking upland ecosystems with habitats lower in the catchment.

Mass loss and macroinvertebrate colonisation of Pacific salmon carcasses in south‐eastern Alaskan streams

1. We examined the spatial and temporal dynamics of pink salmon (Oncorhynchus gorbuscha) carcass decomposition (mass loss and macroinvertebrate colonisation) in south‐eastern Alaskan streams. Dry mass and macroinvertebrate fauna of carcasses placed in streams were measured every two weeks over two months in six artificial streams and once after six weeks in four natural streams. We also surveyed the macroinvertebrate fauna and wet mass of naturally occurring salmon carcasses.2. Carcass mass loss in artificial streams was initially rapid and then declined over time (k=–0.033 day–1), and no significant differences were found among natural streams.3. Several macroinvertebrate taxa colonised carcasses, but chironomid midge (Diptera: Chironomidae) and Zapada (Plecoptera: Nemouridae) larvae were found consistently and were the most abundant (on average 95 and 2%, respectively, of the invertebrates found). Chironomid abundance and biomass increased over time, whereas Zapada abundance and biomass did not. Significant differences in abundance were found among natural streams for Baetis (Ephemeroptera: Baetidae) and Sweltsa (Plecoptera: Chloroperlidae) larvae, while no significant differences were found for chironomid and Zapada abundance or biomass.4. Our results suggest that salmon carcasses initially undergo a high rate of mass loss that tapers off with time. Chironomid and Zapada larvae are likely to be important in mediating nutrient and energy transfer between salmon carcasses and other components of the freshwater‐riparian food web in south‐eastern Alaskan streams.

Larval distribution of offshore spawning flatfish in the Gulf of Alaska: potential transport pathways and enhanced onshore transport during ENSO events

Offshore and deepwater spawning flatfish species in the Gulf of Alaska, such as arrow- tooth flounder Atheresthes stomias and Pacific halibut Hippoglossus stenolepis, have juvenile nurs- eries that are inshore, in bays or at the mouths of bays. Larvae must emigrate from their spawning areas along the continental slope and outer shelf towards shore, in a direction across the prevailing Alaskan Stream. Using a 20 yr time series of data from ichthyoplankton surveys in the Gulf of Alaska, we examine patterns of variability in larval halibut and flounder distributions that may reflect pro- cesses resulting in successful recruitment to nursery areas. Several patterns can be observed in these data. Eggs and the smallest-sized larvae are distributed deep in the water column along the outer shelf and slope. Larger larvae tend to be located in the upper water column and farther inshore over the continental shelf. Larger larvae are also associated with deep-sea valleys and troughs that pene- trate the shelf. Thus, these topographic features may serve as transport pathways to juvenile nursery grounds. ENSO (El Nino-Southern Oscillation) conditions and warm-year anomalies are linked to recruitment strength of Pacific halibut. Variability in larval transport as related to ENSO and other conditions that enhance onshore advection may play an important role in the recruitment of flatfishes to their nursery grounds. The results of this analysis indicate that larvae of both species are more abundant in coastal areas during El Nino events, and that a higher proportion of larvae are trans- ported inshore during El Nino years.

Winter Mixed Layer and Formation of Dichothermal Water in the Bering Sea

The temperature minimum layer, called “dichothermal water”, is a characteristic feature of the North Pacific subarctic gyre. In particular, dichothermal water having a density of approximately 26.6 sigma-theta (σθ), which corresponds to the densest water outcropping in winter in the North Pacific, is seen in the Bering Sea. In order to clarify the water properties, and the area in which and the process by which the dichothermal water is formed, a new seasonal mean gridded climatological dataset with a fine resolution for the Bering Sea and adjacent seas has been prepared using historically accumulated hydrographic data. Although the waters of the Alaskan Stream have temperature minimum layers, their temperature inversions are very weak in climatologies and the core densities of the temperature minimum layers are much lighter than 26.6σθ. On the other hand, in the Bering Sea one can see the robust structure of temperature minimum layers, the core density of the dichothermal water being around 26.6σθ. In addition, it has been found that the properties of the dichothermal water observed in the warming season are almost the same as those in the winter mixed layer. That is, the dichothermal waters are formed in the winter mixed layer in the Bering Sea. Since these waters are found in the Kamchatka Strait, i.e., the main exit of the Bering Sea waters, it can be supposed that the dichothermal waters are exported from the Bering Sea to the Pacific Ocean by the Kamchatka Current.

Size- and Sex-Selective Mortality of Adult Sockeye Salmon: Bears, Gulls, and Fish Out of Water

Abstract Pacific salmon Oncorhynchus spp. exist as reproductively isolated breeding populations and display marked variation in size and age at maturity within and among populations. We conducted a detailed study of mortality on adult sockeye salmon O. nerka in Hansen Creek, a small stream in southwestern Alaska, to test the prediction that predation by brown bears Ursus arctos and death by stranding during upstream migration would selectively remove males and large (old) individuals. During the 3 years of this study, 13.7% of the mature salmon died in shallow water at the creek's mouth before reaching the spawning grounds. Males had proportionally higher mortality at the mouth than females (17.6% versus 10.8%), and the fish of both sexes that died at the mouth were larger and older than those that survived to ascend the creek. Of the fish that died at the mouth, females were more often attacked (chiefly scavenged) by glaucous-winged gulls Larus glaucescens than males, and the gulls tended to attack fish ...

Mesoscale variability in the boundary currents of the Alaska Gyre

Measurements of sea-surface height anomalies acquired during the GEOSAT, ERS-1, and TOPEX altimeter missions show that the boundary currents of the Alaska gyre exhibit interannual variability with respect to the occurrence, size, and propagation of mesoscale, eddy-like features. Observations and model results suggest that eddies are generated in the Alaska Current during years in which the wind forcing in the eastern Gulf of Alaska promotes strong downwelling along the British Columbia–Alaska coast. Wind forcing conditions that support eddy formation and intensification often occur in years that coincide with El Niño–Southern Oscillation events. Eddy variability is significantly more deterministic in the Alaska Current than in the Alaskan Stream.

Interannual variability of upper ocean vorticity balances in the Gulf of Alaska

A high‐resolution numerical ocean model is used to examine the interannual variability of the upper ocean vorticity budget in the Gulf of Alaska. A circulation equation is derived for a layer representative of the upper ocean. The equation is a balance of the area integral of vorticity over the domain with the time‐ and area‐integrated vorticity flux through the layer top, bottom, and lateral boundaries and dissipation. Time series of each component of the equation are constructed and examined for interannual variability. Other native and derived model variables are analyzed to facilitate explanation of observed interannual variability. The model data show that interannual fluctuations in the time rate of change of the circulation in the Gulf of Alaska are small compared to the rate at which the dominant sources and sinks feed and drain vorticity from the domain. Variability on interannual timescales is predominantly due to atmospheric and oceanic teleconnections originating with the El Niño‐Southern Oscillation. Direct forcing of the wind on the ocean is the strongest driving mechanism for interannual variability of the gyre circulation. However, large fluctuations in the wind forcing are partially balanced by other processes. El Niño events excite downwelling coastal Kelvin waves that propagate northward along the eastern continental margin of the northeast Pacific Ocean. Arrival of the Kelvin waves at the peak of the annual cycle of the Alaskan Current triggers baroclinic instabilities that result in the formation of a coast‐wide train of large anticyclonic eddies. The more southerly of these eddies tend to propagate to the southwest, advecting negative vorticity out of the Gulf of Alaska. This net gain of vorticity partially balances the contemporaneous reduction of vorticity input to the region by the wind stress curl. Excess vorticity is advected out of the region in the Alaskan Stream to maintain the circulation over longer timescales.

Spatial and Temporal Variability in a Vertical Section across the Alaskan Stream and Subarctic Current

In the central North Pacific Subarctic Gyre, CTD hydrographic measurements were carried out yearly in late June from 1990 to 1998 at 9 stations along 180° meridian from 48°N to 51.2°N. Vertical sections of 9-year means, anomalies for each year and others of potential temperature, salinity, potential density and geostrophic velocity (referred to 3000 m) were calculated based on this data set. Empirical Orthogonal Function (EOF) analysis was adopted in the investigation of spatial characteristics and its temporal variation in vertical sections. The spatial distribution of the 1st mode EOF of velocity shows the westward Alaskan Stream and the eastward Subarctic Current. This mode explains 37.6% of the total variance. Two positive maxims appear in its amplitude in 1991 and 1997, which is similar to the variation in volume transport of the eastward Subarctic Current. These variations are closely related to the vertical movement of Ridge Domain deep water.

Estimating Fish Populations by Removal Methods with Minnow Traps in Southeast Alaska Streams

Abstract Passive capture methods, such as minnow traps, are commonly used to capture fish for mark–recapture population estimates; however, they have not been used for removal methods. Minnow traps set for 90-min periods during three or four sequential capture occasions during the summer of 1996 were used to capture coho salmon Oncorhynchus kisutch fry and parr, Dolly Varden Salvelinus malma, cutthroat trout O. clarki, and juvenile steelhead O. mykiss to estimate population size with the Zippin or generalized removal method. More than 45% of the total catch was obtained during the first capture occasion, and in most cases, the catch during the fourth occasion was less than 15% of the total catch. In most pools, the probability of capture was greater than 0.4 but was lower for coho salmon fry than for coho salmon parr and other species. Mean population estimates for coho salmon parr made with concurrent mark–recapture and removal methods differed significantly in small streams. Estimates from mark–recaptur...

Disturbance of Small-Stream Habitat by Spawning Sockeye Salmon in Alaska

Abstract We examined how the spatially and temporally predictable disturbance caused by a spawning run of native sockeye salmon Oncorhynchus nerka temporarily affected benthic and invertebrate drift communities of two small streams in southwestern Alaska. Surface gravel size temporarily increased as a consequence of this mechanical disturbance, and periphyton biomass decreased in one stream. The postspawning rise in periphyton biomass in Bear Creek (larger sockeye salmon run) likely reflects nutrient release by decomposing sockeye salmon. Total benthic invertebrate and larval chironomid density decreased during spawning in one stream, but most invertebrate taxa in both streams did not exhibit clear spawning-related impacts. In contrast, invertebrate drift almost always increased during daylight hours when sockeye salmon were present, but it was difficult to separate mechanical effects of sockeye salmon acting via redd construction versus swimming. Total daytime drift density during spawning was four times...

Multi‐year meanders and eddies in the Alaskan Stream as observed by TOPEX/Poseidon altimeter

The birth, life and decay of long‐lived anticyclonic eddies, or meanders, in the Alaskan Stream are observed from TOPEX/Poseidon (T/P) satellite. Their surface height anomalies approach 72 cm, average diameter is about 160km and they live for 1 to 3 years, propagating along the Stream with mean speed of 2.5 km d−1. Of the six observed, the last three were formed after the first T/P observations in September 1992. Of these, two evolved from eddies near Alaskan Panhandle in winter, drifting across the northern Gulf of Alaska before entering the Alaskan Stream, while another formed in the Stream west of Shelikof Strait.

Population processes responsible for larger-fish-upstream distribution patterns of Arctic grayling (Thymallus arcticus) in interior Alaskan runoff rivers

During the summer months, Arctic grayling (Thymallus arcticus) in Alaskan streams adopt a larger-older-fish-upstream distribution pattern. In this paper, I analyse data from two large interior Alaskan rivers to determine how population processes maintain this size and age gradient. These analyses support the hypothesis that age-phased recruitment and growth-dependent movement are primarily responsible for this distribution pattern. Age-phased recruitment describes the way that the mean age of fish recruiting to a reach increases upstream, from ages 0-1 in the lower river to ages 3-7 in the headwaters. This process begins with the concentration of spawning fish, and the resultant fry, in the lower reaches of the river. Downstream movement during the first year of life further concentrates young fish in the lower river. Over time, the distribution of this cohort broadens steadily as individuals move further upstream, so that fish recruiting to headwater reaches are 3-7 years old. This process contributes to both size and age gradients. Growth-dependent movement magnifies the size gradient by sorting fast-growing fish into the upper river and slow-growing fish into the lower river. This sorting results from the fact that individuals making long-distance upstream movements tend to have grown particularly rapidly that year, while individuals making long-distance downstream movements tend to have grown especially slowly that year. I rejected the hypothesis that age and size gradients are the result of whole-stream gradients in growth or mortality acting on a sedentary population. However, there was some evidence that fish did grow more slowly in the lowest 40 km of one river, although this made only a minor contribution to the size gradient and growth rates were remarkably constant for the next 120 km. There was no suggestion that spatial variation in mortality rate contributes towards the size or age gradient, but natural and sampling variability could have obscured small but significant differences between reaches.

Near-surface circulation of the northeast Pacific Ocean derived from WOCE-SVP satellite-tracked drifters

Statistics of the near-surface circulation in the northeast Pacific Ocean were derived from the trajectories of nearly 100 surface drifters tracked between August 1990 and December 1995 as part of the World Ocean Circulation Experiment's (WOCE) Surface Velocity Program (SVP). Drifters were drogued within the mixed layer (15 m drogue depth) or near the top of the permanent halocline (120 m). All branches of the Alaskan Gyre were well-sampled at both depths, revealing a weak Subarctic Current, a bifurcation of the Subarctic Current near 48°N, 130°W at 15 m depth, and strong, variable flow in the Alaska Current and Alaskan Stream. At 120 m depth, northward flow in the Alaska Current occurred much farther offshore than within the mixed layer. The drifter trajectories revealed interannual variability, with evidence of an intensified Alaskan Gyre during the winters of 1991–92 and 1992–93 and more southerly transport during winter 1994–95. A minimum in eddy kinetic energy was found at both depths within the northern branch of the Subtropical Gyre. Eddy kinetic energies were nearly twice as high in the mixed layer compared to below, and were 2–3 times larger in winter than in summer throughout most of the near-surface Alaskan Gyre. High eddy energies observed near the eastern perimeter of the Alaskan Gyre may be due to the offshore intrusion of eddies formed by coastal current instabilities. Taylor's theory of single-particle dispersion was applied to the drifter ensembles to estimate Lagrangian decorrelation scales and eddy diffusivities. Both the initial dispersion and random walk regimes were identified in the dispersion time series computed for several regions of both ensembles. The integral time scales and eddy diffusivities computed from the dispersion scale linearly with r.m.s. velocity, which is consistent with drifter studies from the Atlantic. An exception is the meridional integral time scales, which were nearly constant throughout the study area and at both drogue depths. The magnitudes of the derived eddy statistics are comparable to those derived from surface drifters in other parts of the world ocean. These are the first Lagrangian estimates of particle dispersion over a broad region of the near-surface North Pacific, and the consistency of the results with previous studies from the Atlantic lends credence to the idea that the simplifying assumptions of Taylor (1921) (Proceedings of the London Mathematical Society Series A 20, 196–221) are reasonably valid throughout the upper ocean. This bodes well for the effective parameterization of near-surface diffusivities in general circulation models. Finally, the drifter-derived velocity statistics were used to speculate on the source regions of waters of possible coastal origin observed at offshore stations during the field studies of the Canadian Joint Global Ocean Flux Study.

Egg burial depth by sockeye salmon (Oncorhynchus nerka): implications for survival of embryos and natural selection on female body size

We studied the relationship between female sockeye salmon (Oncorhynchus nerka) body size and egg burial depth in a small Alaskan stream to better understand the ways in which stream-bed scour or digging by other females might destroy embryos. Two different measurements of egg burial depth were taken: depth from the original stream-bed surface to the top of the egg pocket and depth from the disturbed substrate directly above the egg pocket to the top of the egg pocket. The former may represent the depth to which stream-bed scouring would have to go to reach the eggs, but the latter represents the depth to which a second female would have to dig to disturb the egg pocket. Larger females buried their eggs deeper, relative to the original substrate level, than smaller females. This suggests that streams with frequent scour events would select for larger females. However, mean depth from the disturbed substrate level was significantly shallower than mean depth from the original stream-bed level, suggesting that even the smallest females could dig deep enough to disturb the egg pockets of the largest females. Finally, the egg burial depth - fish size relationship that we observed was compared with published data on other salmonid species, revealing considerable variation but a clear positive relationship between female size and burial depth. Because embryonic survival is affected by scour and nest disturbance, and because changes in fish body size, density, and flow regime can affect the vulnerability of embryos to such mortality, we recommend further, standardized measurements of the relationship between egg burial depth and female body size.

Influence of Riparian Vegetation on Invertebrate Assemblages in a Recently Formed Stream in Glacier Bay National Park, Alaska

Influence of the development of riparian vegetation on benthic invertebrate assemblages was analyzed in a recently formed stream in southeast Alaska. Several features of riparian interaction were documented: 1) invertebrate use of willow catkins entering streams in summer, 2) invertebrate use of submerged alder roots as a substrate for attachment and as a source of building material for caddisfly cases, and 3) retention of leaf litter by salmon carcasses. The development of riparian vegetation markedly influenced colonization of the stream by certain invertebrate taxa and thereby played an important role in the successional sequence of macroinvertebrates and overall assemblage development in this new stream.

On Seasonal and Year to Year Variation in Flow of the Alaskan Stream in the Central North Pacific

The Alaskan Stream is the westward boundary current of the North Pacific subarctic gyre. In the central region of the North Pacific, the Alaskan Stream serves as a connection between the Alaskan gyre, Western subarctic gyre and Bering Sea gyre. Its volume transport is very important in estimating the magnitude of the subarctic circulation in the North Pacific. In order to clarify its seasonal and interannual variation, we conducted observations along a north-south section at 180° during June from 1990 to 1997. Moorings were deployed from 1995 to 1997. Hydrographic casts were made at intervals of ∼37 km to a depth of 3000 m. Moorings were set between CTD stations, with Moor1 (Moor2) at the center (southern edge) of the Alaskan Stream. Geostrophic volume transport (referred to 3000 m) revealed large interannual variability in the Alaskan Stream. Average volume transport over the 8 years was 27.5 × 106 m3s-1 with a standard deviation of 6.5 × 106 m3s-1. Maximum transport was 41.0 × 106 m3s-1 (1997) and minimum was 21.7 × 106 m3s-1 (1995). Stable westward flows were observed at Moor1 1500 m (259°, 11.7 cm s-1) and 3000 m (240°, 3.7 cm s-1, 1996–1997 year average). The ratio of eddy to mean kinetic energy (KE′/\(\overline {KE} \)) was very small (<0.6) throughout the year. A relatively weak and unstable westward flow was observed at Moor2 at 3000 m depth. Conversely, the average flow direction at Moor2 5000 m was eastward.

A Recent Full-Depth Survey of the Alaskan Stream

We examine results from a cruise in May 1997. CTD casts to near the bottom were made south of the Aleutian Islands, across Amchitka Pass, and north of the islands. We computed a westward geostrophic speed of 123 cm s−1 at 173.5°W in the Alaskan Stream. The computed volume transport there, referred to the bottom, was 25×106m3s−1. On other similar sections, transports were 8–15 × 106 m3s−1. Various complex variations in geopotential height along the Stream apparently altered the cross-stream gradients, and hence the transports. Rotational tendencies were also present. Northward inflow through Amchitka Pass was quite strong (6 × 106 m3s−1). Data north of the islands supported the existence of a zero-velocity reference level of variable depth.

Modification of the intermediate waters in the northeastern subpolar Pacific

The modification of the intermediate layers in and around the Alaskan Gyre are investigated in relation to general circulation features of the region, using tracer data from the World Ocean Circulation Experiment P17N line. The Alaskan Gyre (AG) core lies between the North Pacific Current and the Alaskan Stream (AS) in the 52–54.5°N range and to the west of 144°W. The North Pacific Current feeds the Transition Zone (TZ) and the Western Subpolar Waters (WSW), which consequently constitute the inflow to the region from the northwestern Pacific. The Alaska Current (AC) is observed to the west of 144°W, between the North American coast and the AG. The AC halocline (σθ &lt; 26.7) has distinct property distributions (low chlorofluorocarbon and oxygen saturations and high nutrient concentrations), which are correlated with eddies that are characterized by higher salt content and warmer temperatures (most significantly at 26.0 σθ). The AG is found to act as a reservoir where water mass modification takes place and the AS transports a mixture of waters from the AG and the AC west‐ward out of the region. The residence time estimate for the intermediate and upper layer water in the AG (σθ ≤ 27.2) is about 2 years, which corresponds to a volume exchange between the gyre and the surrounding circulation of 4.7 Sv in the 0–1000 m range. The layers above 26.9 σθ (∼0–200 m) are modified by vertical mixing mostly of WSW in the AG; the flux of modified waters out of the AG is approximately 0.9 Sv. Comparison of the present data with the 1984–1989 period suggests that mixing of water from different regimes has not changed dramatically.