Alaskan Stream Research Articles

Observations from moorings in the Aleutian Passes: temperature, salinity and transport

AbstractBetween May 2001 and September 2003, a series of moorings were deployed in four of the Aleutian Passes – Tanaga Pass (12 months of data), Akutan Pass and Seguam Pass (18 months), and Amukta Pass (36 months). Instruments on each mooring measured temperature, salinity and current velocity. Tidal currents dominated the flow in each pass, including a strong fortnightly component in the three deeper passes (Tanaga, Seguam, and Amukta). Net transport in each of the passes was northward, varying from 0.1 × 106 m3 s−1 in Akutan Pass and 0.4 × 106 m3 s−1 in Seguam to >4.0 × 106 m3 s−1 in Amukta Pass. The transport in Amukta Pass, calculated from current meters, was approximately five times as large as previously estimated from hydrographic surveys. At monthly and longer periods, the variability in transport in Amukta Pass was related to the position and strength of the Alaskan Stream southeast of the pass. Vertical mixing was examined in Akutan and Seguam Passes. Strong tidal currents mix the water column top‐to‐bottom over the shallow sills in the passes, a depth of 80 m in Akutan and 140 m in Seguam Pass, providing a critical source of nutrients to the Bering Sea ecosystem.

Zooplankton distribution, abundance and biomass relative to water masses in eastern and central Aleutian Island passes

AbstractConductivity–temperature–depth (CTD), acoustic and net samples were taken through a series of passes in the eastern and central Aleutian Island archipelago to document regional differences in the physical and biological conditions. The water column in central passes had elevated salinity and depressed temperatures, indicating upwelling of deep water through the passes. The zooplankton community included the oceanic genera Neocalanus and Eucalanus, and the euphausiid Euphausia pacifica, all oceanic taxa. Eastern passes had significantly lower salinity and elevated temperature relative to the central passes, suggesting a more neritic environment. Calanus marshallae, Pseudocalanus, Acartia and Thysanoessa inermis, common neritic species, were observed in the eastern passes. Canonical correlation indicated that up to 50% of the observed variance in mean zooplankton abundance in the passes could be explained by salinity and temperature. Elevated sound scattering was observed in fronts and eddies in the passes, particularly at the northern ends of the passes, in association with elevated zooplankton abundance and biomass. The central passes were characterized as an oceanic environment, influenced primarily by Alaskan Stream water with its associated interzonal copepod species, while the eastern passes were influenced by the Alaska Coastal Current, which contains a mixture of oceanic and neritic zooplankton species.

Marine environment of the eastern and central Aleutian Islands

AbstractTo examine the marine habitat of the endangered western stock of the Steller's sea lion (Eumetopias jubatus), two interdisciplinary research cruises (June 2001 and May to June 2002) measured water properties in the eastern and central Aleutian Passes. Unimak, Akutan, Amukta, and Seguam Passes were sampled in both years, and three additional passes (Umnak, Samalga, and Tanaga) were sampled in 2002. In the North Pacific (and to a lesser extent in the Bering Sea), a strong front in water properties was observed near Samalga Pass in June of both years, with significantly warmer, fresher, and more nitrate‐poor water east of Samalga Pass than west of the pass. These water properties reflect differences in source waters (Alaska Coastal Current versus Alaskan Stream), mixing depth, and Bering Sea influence. Strong cross‐Aleutian gradients were also observed with warmer, fresher water on the North Pacific side of the archipelago. The nutrient content of the waters flowing through the passes, combined with the effects of mixing within the passes, influences the transport of nutrients into the Bering Sea. As water moves away from the strong mixing of the passes and becomes more stratified, phytoplankton can take advantage of the enhanced nutrient concentrations. Thus, the northern side of the Aleutian Islands (especially in the lee of the islands) appears to be more productive. Combined with evidence of coincident changes in many ecosystem parameters near Samalga Pass, it is hypothesized that Samalga Pass forms a physical and biogeographic boundary between the eastern and central Aleutian marine ecosystems.

Interdecadal changes in mesoscale eddy variance in the Gulf of Alaska circulation: Possible implications for the Steller sea lion decline

A distinct change in the ocean circulation of the Gulf of Alaska after the 1976–77 climate shift is studied in an eddy‐permitting primitive equation model forced by observed wind stresses from 1951–99. When the Aleutian Low strengthens after 1976–77, strong changes occur in the mean velocity of the Alaskan Stream and in its associated mesoscale eddy field. In contrast, the Alaska Current and the eddy flows in the eastern Gulf remain relatively unchanged after the shift. Since mesoscale eddies provide a possible mechanism for transporting nutrient‐ rich open‐ocean waters to the productive shelf region, the flow of energy through the food web may have been altered by this physical oceanographic change. This climate‐driven mechanism, which has a characteristic eastwest spatial asymmetry, may potentially help to explain changes in forage fish quality in diet diversity of Steller sea lions whose populations have declined precipitously since the mid‐1970s in the western Gulf while remaining stable in the eastern Gulf.

Productivity changes in the Bering Sea during the late Quaternary

Changes in biological productivity of the Bering Sea have been evaluated for the late Quaternary based on biogenic opal, calcium carbonate, and microfossils in two piston cores. Biological productivity increased during Marine Isotope Stage (MIS) 1 after the last glaciation. During the last deglaciation, two pronounced peaks of calcium carbonate content were observed at ca. 2400 m water depth, which can be explained by the ‘‘CaCO3 compensation hypothesis’’ and coccolithophore blooms. Simultaneous with the CaCO3 peaks, oxygen-poor deep water was apparently distributed in the Aleutian Basin based on benthic foraminiferal assemblages. The low-O2 events seem to be related to deep-water circulation and/or elevated productivity in the northwest Pacific. After the CaCO3 peak events, biogenic opal contents and diatoms increased gradually, associated with enhanced vertical mixing with an inflow of the Alaskan Stream through the eastern Aleutian Arc passes. r 2005 Elsevier Ltd. All rights reserved.

Effects of Salmon-Borne Nutrients on Riparian Soils and Vegetation in Southwest Alaska

Spawning Pacific salmon (Oncorhynchus spp.) contribute marine-derived nutrients to riparian ecosystems, potentially affecting characteristics of the associated soils and vegetation. We quantified these effects by comparing soil and vegetative characteristics upstream and downstream of natural migratory barriers on ten spawning streams in southwest Alaska. Mean δ15N values—indicative of salmon-borne nutrients—were significantly higher in the O horizon and surface mineral soils downstream of barriers (near spawning reaches) than in soils upstream of barriers (near non-spawning reaches). However, the mean total N concentration in surface mineral soil was lower downstream than upstream. Mean foliar δ15N values were higher downstream for three plant species (Picea glauca, Salix alaxensis and Arctagrostis latifolia) with contrasting physiognomies. Mean overstory stem density was 100% higher downstream, primarily due to a fivefold difference in the density of large-diameter willows (Salix spp.). Mean understory stem density was 47% lower downstream, also driven by a difference in willow density. Mean ground layer non-vascular and dwarf shrub species covers were 28% and 73% lower downstream, respectively. Of the ten soil and vegetative characteristics that differed upstream to downstream, two (O horizon and Picea glauca δ15N) were correlated with the density of spawning salmon. Collectively, the data suggest that salmon-borne nutrients alter riparian soils and vegetation, while factors unrelated to salmon are responsible for the ultimate expression of many community characteristics.

Analysis of 11 Bioengineered Stream Bank Erosion Control Structures in Alaska

This paper reports the results of a study of the use of bioengineered erosion control structures on Alaskan streams and rivers. Field investigations of hydraulic and vegetation conditions at 11 study sites around the state of Alaska were conducted to determine the performance of these structures. Root wads, live staking, brush layering, and coir logs were the primary bioengineering methods used for erosion control at the study sites. A one-dimensional numerical computer model was applied at each site to estimate the magnitude of average bed and bank shear stresses (tractive force) apparent to the erosion control structures at the 50- and 100-year design flood levels. Discharge records and field flood indicators were checked to correlate structure condition to flow history. Damage at existing structures was attributed to flowing ice, undermining of toe protection, buoyancy effects, and failure of construction fabrics. Root wad structures in good condition were located in areas with high boat wake occurrence but low channel tractive forces. The findings of the study suggest that the types of bioengineered erosion control structures studied have not been proven to offer reliable bank erosion protection during flooding conditions on channels with high tractive forces.

Seasonal cross-shelf distribution of major zooplankton taxa on the northern Gulf of Alaska shelf relative to water mass properties, species depth preferences and vertical migration behavior

The cross-shelf distribution of major zooplankton species was examined on the northern Gulf of Alaska (GOA) shelf during the production season for four years, between October 1997 and October 2001. The zooplankton community on the northern GOA shelf consisted of oceanic and neritic species of the North Pacific subarctic species complex. Cross-shelf distribution of the major zooplankton species was influenced by their depth preferences, vertical migration behavior, salinity-temperature preferences, and by cross-shelf water-mass distribution and movement. The neritic community, dominated by Pseudocalanus spp., Metridia pacifica and Calanus marshallae, had highest abundances on the inner shelf, in the Alaska Coastal Current, and in the adjacent fjords in late spring and early summer. The oceanic community, which contained primarily Neocalanus cristatus and Eucalanus bungii, was observed in the Alaskan Stream and adjacent waters near the shelf break. A mid-shelf transition zone contained a mixture of oceanic and neritic species. Prince William Sound (PWS) contained a unique species complex of large mesopelagic copepods, amphipods and shrimp. Neocalanus flemingeri and Oithona similis were abundant in all four regions during spring and early summer. The transition zone commonly crossed much of the shelf between the shelf break and the ACC, but satellite images and CTD data indicate that occasionally a narrow shelf-break front can form, in which case distinct zooplankton species groups are observed on either side of the front. Satellite data also revealed numerous large and small eddies, which probably contribute to cross-shelf mixing in the transition zone.

Analysis of 11 Bioengineered Stream Bank Erosion Control Structures in Alaska

This paper reports the results of a study of the use of bioengineered erosion control structures on Alaskan streams and rivers. Field investigations of hydraulic and vegetation conditions at 11 study sites around the state of Alaska were conducted to determine the performance of these structures. Root wads, live staking, brush layering, and coir logs were the primary bioengineering methods used for erosion control at the study sites. A one-dimensional numerical computer model was applied at each site to estimate the magnitude of average bed and bank shear stresses (tractive force) apparent to the erosion control structures at the 50- and 100-year design flood levels. Discharge records and field flood indicators were checked to correlate structure condition to flow history. Damage at existing structures was attributed to flowing ice, undermining of toe protection, buoyancy effects, and failure of construction fabrics. Root wad structures in good condition were located in areas with high boat wake occurrence but low channel tractive forces. The findings of the study suggest that the types of bioengineered erosion control structures studied have not been proven to offer reliable bank erosion protection during flooding conditions on channels with high tractive forces.

Responses in dissolved nutrients and epilithon abundance to spawning salmon in southeast Alaska streams

Spawning Pacific salmon (Oncorhynchus spp.) historically transported massive quantities of marine-derived nutrients (MDN) into nutrient-poor streams of the Pacific Northwest. In southeast Alaska, we measured the effects of MDN on streamwater chemistry and epilithon standing stock (1) through time during two consecutive years in one stream, Fish Creek; (2) over space in six salmon streams; and (3) in a controlled mesocosm experiment. In Fish Creek during strong salmon runs in 2000 and 2001, streamwater concentrations of ammonium (NH ) increased 101 4 fold and soluble reactive phosphorus (SRP) increased by 4- to 7-fold in the presence of salmon. NH and SRP also 1 4 increased with distance downstream in the salmon reach, likely related to a ‘carcass-loading’ effect. In contrast, nitrate (NO ) and dissolved organic carbon concentrations varied only with discharge. In 2000, epilithon chlorophyll 2 3 a increased by 20-fold during the salmon run, whereas no significant change was observed in 2001. Over space, the multistream survey revealed consistent increases in NH and SRP, but no pattern in epilithon response to the 1 4 salmon run. In the mesocosm experiment, NH , SRP, and epilithon standing stock all increased in the presence of 1 4 salmon carcasses in artificial streams. Overall, salmon clearly increased the concentrations of important dissolved nutrients in southeast Alaska streams. Responses in epilithon were more variable, however, suggesting that multiple environmental factors including light and disturbance likely regulate epilithon growth in salmon streams. Nutrient mass transport estimates revealed that a substantial amount of MDN (46%‐60% depending on element) is exported directly back to the estuarine environment, suggesting that salmon represent a key marine‐freshwater coupling in nutrient cycling.

Bear Predation on Pacific Salmon Facilitates Colonization of Carcasses by Fly Maggots

The carcasses of Pacific salmon can be an important food resource for aquatic and terrestrial organisms, depending on where the carcasses are located. We hypothesized that the availability of sockeye salmon (Oncorhynchus nerka) carcasses to ovipositing flies (Family: Calliphoridae) would be facilitated by brown bears (Ursus arctos) moving salmon from the water to terrestrial environments. We further hypothesized that the proportion of carcasses in terrestrial environments (hence accessible to flies) would vary among streams as a function of stream size. In three small southwestern Alaska streams during 2 y, flies oviposited on about half of the carcasses in terrestrial habitats (44% of those on gravel, 54% in grass and 57% in the forest). Only 28% of the partially submerged carcasses and 0.5% of the fully submerged carcasses were colonized. The proportion of carcasses deposited outside the stream was strongly and positively influenced by the level of bear predation and negatively affected by stream complexity (in deeper more complex streams more carcasses remained in the water). Based on data from the past 12 y on sockeye salmon abundance and predation by bears (averaging about 40% of the salmon), we estimated that on average 204, 551 and 839 carcasses were colonized per km annually in the three streams (12 to 24% of the total number of carcasses). In these sites, where salmon are abundant and highly available to bears, a large fraction of the tissue is not consumed and so is available to flies. Fly maggots, numbering up to 50,000 per carcass, can consume virtually an entire salmon within 5 d. Thus bears as consumers and transporters of carcasses facilitate the scavenging of carcasses by maggots, increasing the transfer marine-derived nutrients from aquatic to riparian habitats.

Salmonids on the Fringe: Abundance, Species Composition, and Habitat Use of Salmonids in High-Gradient Headwater Streams, Southeast Alaska

Abstract We evaluated the species distribution, abundance, and habitat relationships of salmonids in small first- to second-order headwater streams in southeast Alaska. Streams were separated into three zones based on gradient and sampled during the spring, summer, and fall. Dolly Varden Salvelinus malma were found in all streams where fish were present. They were the dominant species in moderate- (mean gradient = 5.5%) and high-gradient (mean gradient = 12.9%) zones. Coho salmon Oncorhynchus kisutch fry and parr were the dominant species in the low-gradient zone (mean gradient = 3.1%) but were present in higher-gradient zones. Small numbers of steelhead O. mykiss parr were present in all three zones in the spring and fall. Few were captured during the summer. Coastal cutthroat trout O. clarkii were found primarily in one stream and in all three zones. The density of all species decreased as gradient increased. Anadromous Dolly Varden in spawning condition were observed in the fall up to the highest acces...

Macroinvertebrate assemblages in streams of interior Alaska following alluvial gold mining

AbstractAlluvial gold mining, or placer mining, represents a major disturbance to stream ecosystems in interior Alaska. This process typically involves re‐routing the stream to one side of the valley while the gravels are removed across the floodplain for processing. The stream is then re‐directed back into a reclaimed new channel after mining has ceased in that section of stream. Macroinvertebrate colonization of reclaimed stream reaches is necessary for benthic community recovery following mining. Macroinvertebrate community assemblages at 36 sites on six streams in interior Alaska were examined. Sites with different mining histories were compared to control sites. TWINSPAN classification and ordination of sites using macroinvertebrate abundance indicated a greater similarity of sites within watersheds than between watersheds even though each watershed had a history of mining including recently mined sites. Significant differences were found for total invertebrate abundance, percentage dominant taxon and mean total biomass of macroinvertebrates between control sites and sites that were recently mined (≤10 years) and sites mined 12–50 years ago. Total macroinvertebrate abundance and biomass was lower and taxon dominance higher in both groups of sites with a history of mining. No significant difference in these or other measures was found for sites where mining occurred upstream. There was also no significant relationship between time since mining and the biotic measures used to summarize macroinvertebrate community structure. Although taxon richness recovered rapidly, no individual taxon was found to be a specific indicator of macroinvertebrate recovery since mining. Copyright © 2004 John Wiley & Sons, Ltd.

Testing the Ability of a Temperature‐Based Model to Predict the Growth of Age‐0 Arctic Grayling

AbstractWe tested the ability of a temperature‐based growth model for European grayling (also known as grayling) Thymallus thymallus to predict the growth rate of age‐0 Arctic grayling T. arcticus in two reaches of a small Alaskan stream. We compared observed and predicted lengths at weekly intervals in both reaches during the summers of 2000 and 2001, calculating predicted lengths from information on daily mean water temperature and the length of larval fish at the start of the growing season. In one reach, the model accurately predicted growth over the course of both summers and the differences in growth between summers. In the other reach, observed length fell below predicted length at 4– 5 weeks of age in both years. We hypothesize that the model works well when prey abundance does not limit growth but fails when low prey density limits the fish's daily ration.

Variation in responses to spawning Pacific salmon among three south‐eastern Alaska streams

Summary1. Pacific salmon are thought to stimulate the productivity of the fresh waters in which they spawn by fertilising them with marine‐derived nutrients (MDN). We compared the influence of salmon spawners on surface streamwater chemistry and benthic biota among three south‐eastern Alaska streams. Within each stream, reaches up‐ and downstream of barriers to salmon migration were sampled during or soon after spawners entered the streams.2. Within streams, concentrations of dissolved ammonium and soluble reactive phosphorus (SRP), abundance of epilithon (chlorophyll a and ash‐free dry mass) and biomass of chironomids were significantly higher in reaches with salmon spawners. In contrast, biomass of the mayflies Epeorus spp. and Rhithrogena spp. was significantly higher in reaches lacking spawners.3. Among streams, significant differences were found in concentrations of dissolved ammonium, dissolved organic carbon, nitrate and SRP, abundance of epilithon, and the biomass of chironomids and Rhithrogena. These differences did not appear to reflect differences among streams in spawner density, nor the changes in water chemistry resulting from salmon spawners.4. Our results suggest that the ‘enrichment’ effect of salmon spawners (e.g. increased streamwater nutrient concentrations) was balanced by other concurrent effects of spawners on streams (e.g. sediment disturbance). Furthermore, the collective effect of spawners on lotic ecosystems is likely to be constrained by conditions unique to individual streams, such as temperature, background water chemistry and light attenuation.

The relative importance of prey density and social dominance in determining energy intake by bears feeding on Pacific salmon

We quantified foraging behavior of brown bears (Ursus arctos) feeding on adult chum (Oncorhynchus keta) and pink salmon (Oncorhynchus gorbuscha) at three small coastal streams in southeastern Alaska from streamside tree stands. These observations revealed that social dominance was much more important in determining intake rates among bears than salmon densities. Each small stream supported one large, socially dominant bear that directly displaced other bears in aggressive encounters or was avoided in "passive deferrals". Although the number of fish killed per foraging bout was positively correlated with salmon density, energy intake was determined primarily by foraging effort, as dominant bears visited the stream more often and foraged for longer periods than subdominant bears. Capture efficiency (fish captured per minute searching) was highly variable and increased only marginally with salmon density and among social ranks. Subdominant bears were more vigilant, used a smaller fraction of each stream, and carried salmon much farther into the forest prior to consumption, presumably to minimize interactions with other bears. Social dominance may play an important role in regulating reproductive success when salmon densities are low and may have important implications for managers in bear-viewing areas.

Bed load transport in managed steep‐gradient headwater streams of southeastern Alaska

Bed load transport was investigated in four headwater streams in southeastern Alaska subjected to different management and disturbance regimes. Bed load yield was positively correlated to peak discharge during the fall 1999 monitoring period. Fine bed load materials (1–11 mm) that were supplied from hillslope sources were equally mobilized during most storm events. Medium‐sized bed materials (11–200 m) were only partially mobilized even during large storms, whereas large particles (>200 mm) were immobile and often formed interlocking channel structures. The transport distances of medium‐size materials depended on amount of channel obstructions (e.g., woody debris) and sediment supply conditions; both of these factors were influenced by the occurrence of mass movement, timber harvesting, and the related recovery processes. The highest total bed load yield was observed in a channel affected by a debris flow in 1993. Total sediment yields are similar among channels with old‐growth, clear cut (logged 4 years before monitoring), and young alder (affected by landslides and debris flows in 1961) riparian stands. By comparing the old‐growth and young alder channels, it appears that bed load yield recovers from debris flow disturbances in about 40 years; however, recovery of channel conditions (e.g., reach types and woody debris) may take much longer. Effects of timber harvesting on bed load transport are controlled by sediment linkages between hillslopes and channels related to the occurrence of mass movement.

The hydraulic characteristics and geochemistry of hyporheic and parafluvial zones in Arctic tundra streams, north slope, Alaska

Sodium bromide and Rhodamine WT were used as conservative tracers to examine the hydrologic characteristics of seven tundra streams in Arctic Alaska, during the summers of 1994–1996. Continuous tracer additions were conducted in seven rivers ranging from 1st to 5th order with samples collected from instream, hyporheic, and parafluvial locations. Tracer data was used as input for a computer model to estimate hydrologic characteristics of each study reach. While solute concentrations during the tracer additions indicated that steady-state or “plateau” conditions had been reached, interstitial samples indicated that there were additional hyporheic and parafluvial zones that had not been fully labeled at the time of apparent steady state in the stream channel (plateau). Exchange between channel and hyporheic water was a function of location within a pool–riffle sequence, with rapid downwelling at the head of riffles and delayed upwelling in riffle tails. The extent of exchange between channel and hyporheic water was positively correlated with apparent streambed hydraulic conductivity. Tracer additions indicated interstitial velocities ranging from 0.030 to 0.075 cm s −1 and hydraulic conductivities from 2.4 to 12.2 cm s −1. Hyporheic and in-channel samples were collected for N, P, DO, and CO 2 analyses in conjunction with conservative tracer additions in four of the stream reaches for which the interstitial velocities were also determined. Transformation rates based on these data indicated that there was rapid nitrification of mineralized organic N and production of ammonium, phosphate, and carbon dioxide in the interstitial zones of all four reaches. Dissolved oxygen did not appear to be limiting in the reaches studied. The hyporheic zone of all four reaches was a source of nitrate, carbon dioxide, and ammonium to the channel water based on the average concentration of upwelling waters. Increased contact time with hyporheic and parafluvial zones was related to decreased temperature and increased conductivity. Net nitrogen flux from the hyporheic zone was equivalent to 14–162% of benthic N uptake requirements for the Kuparuk River. These observations are important because we expected that the presence of continuous permafrost in this Arctic environment would limit the importance of hyporheic processes, either physically (i.e., through the presence of a restricting thaw bulb in the permafrost) or biogeochemically (i.e., through low temperatures). Instead, we found that biogeochemical processes in the hyporheic zone of these Arctic streams are at least as important as it is in similar temperate stream ecosystems.

Spring growth of caddisflies (Limnephilidae : Trichoptera) in response to marine-derived nutrients and food type in a Southeast Alaskan stream

The short-term stimulation of production, due to marine-derived nutrients (MDN) from spawning salmon, is well documented for certain trophic levels in stream communities (e.g., algae and insect biomass). The effect of these nutrients on the stream ecosystem as a whole, however, remains unclear especially later in the year. Trichopterans have been shown to feed on salmon and other fish carcasses and there is evidence for greater growth rates in the presence of salmon tissue. To address the question of long-term MDN subsidy on trichopterans, we investigated the growth of three limnephilid caddisflies in the spring in the Harris River on Prince of Wales Island, Southeast Alaska. The Harris River has a natural waterfall barrier to salmon and receives large runs of pink (O. gorbuscha) and chum (O. keta) salmon each fall. We selected two shredding caddisflies (Onocosmoecus unicolor) and (Psychoglypha spp.) and one facultative scraper, (Dicosmoecus atripes) for our study. We had two objectives : 1) compare the spring growth of larval caddisflies in a stream section that receives a large autumn run of salmon with their growth in a stream section that is blocked from receiving salmon (due to an impassable waterfall), and 2) compare the growth of shredders with that of a facultative scraper when provided either leaves or biofilm on rocks as food. Insects were placed in growth boxes in May 2001 with either conditioned alder leaves or stream rocks as food sources. The boxes were placed along with temperature loggers in both the salmon (below the waterfall) and non-salmon (above the waterfall) reaches. The boxes were removed 40 days later. In-stream samples were taken of each caddisfly initially and at the end of the experiment to establish in-stream growth versus growth in the boxes. All larvae were coaxed from their cases, measured for total wet length, dried and weighed. Only D. atripes and Psychoglypha spp. were growing during our experiment and both showed very high relative growth rates in the Harris River. Psychoglypha spp. and O. unicolor were both significantly larger in the leaf boxes and D. atripes was significantly larger in the rock boxes. Both D. atripes and Psychoglypha spp. had significantly greater relative growth rates between food types (on biofilm on rocks and leaves respectively). These results support the notion that D. atripes are most likely facultative scrapers at least in their first year of growth. None of these caddisflies showed differences in their final mean weights or relative growth rates between stream sections, suggesting no effect of MDN on their spring growth in the Harris River. Further research on caddisfly communities in the fall and winter will help clarify if MDN has an influence on the abundance and life history of these species closer to the salmon run. This study questions the long-term influence of MDN on stream communities, particularly those populations that do most of their production in the spring, months after salmon carcasses are no longer visible.

Influence of decomposing Pacific salmon carcasses on macroinvertebrate growth and standing stock in southeastern Alaska streams

We compared macroinvertebrate growth rates and standing stock in the absence and presence of meat from Pacific salmon (Oncorhynchus spp.) carcasses in microcosm and natural stream rearing experiments in southeastern Alaska. In microcosm experiments, the presence of salmon meat increased growth rates and standing stock for the shredder Zapada cinctipes and the collector Psychoglypha subborealis, but not the predator Rhyacophila sp., or the scraper Cinygmula sp. In natural stream experiments, the presence of salmon meat increased the growth rate and standing stock of P. subborealis, but increased only the growth rate of Z. cinctipes. Macroinvertebrate responses to inputs of salmon-derived organic material can vary by species, which may reflect their feeding ecology. Macroinvertebrate taxa belonging to the collector functional-feeding group are likely to be important in transferring the effects of spawning salmon to the rest of the food web in southeastern Alaska streams.