Arctic Tundra Stream Research Articles

Do beaver ponds increase methane emissions along Arctic tundra streams?

Beaver engineering in the Arctic tundra induces hydrologic and geomorphic changes that are favorable to methane (CH4) production. Beaver-mediated methane emissions are driven by inundation of existing vegetation, conversion from lotic to lentic systems, accumulation of organic rich sediments, elevated water tables, anaerobic conditions, and thawing permafrost. Ground-based measurements of CH4 emissions from beaver ponds in permafrost landscapes are scarce, but hyperspectral remote sensing data (AVIRIS-NG) permit mapping of ‘hotspots’ thought to represent locations of high CH4 emission. We surveyed a 429.5 km2 area in Northwestern Alaska using hyperspectral airborne imaging spectroscopy at ∼5 m pixel resolution (14.7 million observations) to examine spatial relationships between CH4 hotspots and 118 beaver ponds. AVIRIS-NG CH4 hotspots covered 0.539% (2.3 km2) of the study area, and were concentrated within 30 m of waterbodies. Comparing beaver ponds to all non-beaver waterbodies (including waterbodies >450 m from beaver-affected water), we found significantly greater CH4 hotspot occurrences around beaver ponds, extending to a distance of 60 m. We found a 51% greater CH4 hotspot occurrence ratio around beaver ponds relative to nearby non-beaver waterbodies. Dammed lake outlets showed no significant differences in CH4 hotspot ratios compared to non-beaver lakes, likely due to little change in inundation extent. The enhancement in AVIRIS-NG CH4 hotspots adjacent to beaver ponds is an example of a new disturbance regime, wrought by an ecosystem engineer, accelerating the effects of climate change in the Arctic. As beavers continue to expand into the Arctic and reshape lowland ecosystems, we expect continued wetland creation, permafrost thaw and alteration of the Arctic carbon cycle, as well as myriad physical and biological changes.

Nutrients influence seasonal metabolic patterns and total productivity of Arctic streams

AbstractThe seasonality of gross primary production (GPP) in streams is driven by multiple physical and chemical factors, yet incident light is often thought to be most important. In Arctic tundra streams, however, light is available in saturating amounts throughout the summer, but sharp declines in nutrient supply during the terrestrial growing season may constrain aquatic productivity. Given the opposing seasonality of these drivers, we hypothesized that “shoulder seasons”—spring and autumn—represent critical time windows when light and nutrients align to optimize rates of stream productivity in the Arctic. To test this, we measured annual patterns of GPP and biofilm accumulation in eight streams in Arctic Sweden. We found that the aquatic growing season length differed by 4 months across streams and was determined largely by the timing of ice‐off in spring. During the growing season, temporal variability in GPP for nitrogen (N) poor streams was correlated with inorganic N concentration, while in more N‐rich streams GPP was instead linked to changes in phosphorus and light. Annual GPP varied ninefold among streams and was enhanced by N availability, the length of ice‐free period, and low flood frequency. Finally, network scale estimates of GPP highlight the overall significance of the shoulder seasons, which accounted for 48% of annual productivity. We suggest that the timing of ice off and nutrient supply from land interact to regulate the annual metabolic regimes of nutrient poor, Arctic streams, leading to unexpected peaks in productivity that are offset from the terrestrial growing season.

Benthic Biomonitoring in Arctic Tundra Streams: A Community-Based Approach in Iqaluit, Nunavut, Canada

<p><span style="font-family: 'Times New Roman';">Recent residential, commercial, and industrial development in the catchments of several Arctic streams has heightened the need to assess these freshwater systems accurately. It was imperative to develop methods that would be both effective at judging ecological condition of tundra streams and suitable for use by local groups. An investigation of two streams influenced by urbanization in Iqaluit, Nunavut, was carried out between July and August each year in 2007 – 09. Simple summary metrics (e.g., Shannon Index) and multivariate analysis (DCA, RD A) both demonstrated biological impairment in the benthic community at site locations downstream of urbanized portions of a local stream. This impairment was characterized by a loss of diversity and a dramatic shift of the benthic community to one dominated by chironomids from the subfamily Orthocladiinae. Elevated levels of total nitrogen (TN) and total phosphorus (TP) and several metals (Zn, Sr, Rb, Al, Co, Fe) were also found to be significantly related to benthic assemblages within these disturbed areas. This investigation also addressed taxonomic sufficiency, indicating that while family-level taxonomic identifications were sensitive enough to differentiate between pristine and impacted stream sites, a more precise taxonomic identification of the dominant benthos taxa (Insecta: Diptera: Chironomidae) to sub-family/tribe level identified a significant shift towards pollution-tolerant taxa. This higher taxonomic resolution will allow for the adaptation of protocols and the use of simple summary metrics to be effective for a community-based biomonitoring program in Arctic tundra streams. </span></p>

Growth, condition and energy stores of Arctic grayling fry inhabiting natural and artificial constructed Arctic tundra streams

Juvenile north-temperate and Arctic fishes are faced with trade-offs between energy allocation to growth and energy storage (primarily lipids) prior to over-wintering. We determined classical morphometric (fork length, body weight and condition factor) and biochemical (whole body triglycerides, muscle RNA/DNA ratio, muscle proteins) measures of growth and condition in individual young-of-the-year (YOY) Arctic grayling ( Thymallus arcticus ). Grayling were collected just prior to over-wintering in late August (approximately 50 days after swim-up) from two natural streams and five locations within a 3.4 km long artificial stream constructed as a fish habitat compensation project and diversion channel for the diamond mining industry in Northwest Territories, Canada (64°45′N). Fork lengths, body weights and whole body triglyceride levels in grayling collected from all sites along the artificial stream were significantly lower than fish collected from one of the natural streams. Condition factor (weight-at-length) was not different among grayling collected from natural and artificial streams. Muscle proteins were lower in grayling collected from four sites along the artificial stream compared to the natural streams. In contrast, muscle RNA/DNA ratios were greater in grayling collected from two sites in the artificial stream compared to natural streams. There were no consistent differences in any variable among grayling collected at the five artificial stream sites or among grayling collected from the two natural streams. The higher RNA/DNA ratios and lower fork lengths, whole body triglycerides and muscle proteins in grayling inhabiting the artificial stream are consistent with energy still being primarily allocated to growth in these fish at this late stage of summer. Individuals that are both larger and possess greater energy storage in the form of triglycerides are more likely to survive the long over-wintering period at this latitude. Our results suggest that YOY grayling using the artificial stream as nursery habitat will likely face increased over-winter mortality, thus raising concerns over the use of fish presence, spawning and rearing as criteria for the initial success of artificial streams as habitat compensation measures in Arctic tundra regions. Further research is needed to determine the potential consequences of reduced size and energy storage in juvenile fishes in order to assess the viability of stream fish habitat compensation and restoration projects associated with industrial development in Arctic tundra regions.

Hyporheic exchange and water chemistry of two arctic tundra streams of contrasting geomorphology

The North Slope of Alaska's Brooks Range is underlain by continuous permafrost, but an active layer of thawed sediments develops at the tundra surface and beneath streambeds during the summer, facilitating hyporheic exchange. Our goal was to understand how active layer extent and stream geomorphology influence hyporheic exchange and nutrient chemistry. We studied two arctic tundra streams of contrasting geomorphology: a high‐gradient, alluvial stream with riffle‐pool sequences and a low‐gradient, peat‐bottomed stream with large deep pools connected by deep runs. Hyporheic exchange occurred to ∼50 cm beneath the alluvial streambed and to only ∼15 cm beneath the peat streambed. The thaw bulb was deeper than the hyporheic exchange zone in both stream types. The hyporheic zone was a net source of ammonium and soluble reactive phosphorus in both stream types. The hyporheic zone was a net source of nitrate in the alluvial stream, but a net nitrate sink in the peat stream. The mass flux of nutrients regenerated from the hyporheic zones in these two streams was a small portion of the surface water mass flux. Although small, hyporheic sources of regenerated nutrients help maintain the in‐stream nutrient balance. If future warming in the arctic increases the depth of the thaw bulb, it may not increase the vertical extent of hyporheic exchange. The greater impacts on annual contributions of hyporheic regeneration are likely to be due to longer thawed seasons, increased sediment temperatures or changes in geomorphology.

Transient storage as a function of geomorphology, discharge, and permafrost active layer conditions in Arctic tundra streams

Transient storage of solutes in hyporheic zones or other slow‐moving stream waters plays an important role in the biogeochemical processes of streams. While numerous studies have reported a wide range of parameter values from simulations of transient storage, little field work has been done to investigate the correlations between these parameters and shifts in surface and subsurface flow conditions. In this investigation we use the stream properties of the Arctic (namely, highly varied discharges, channel morphologies, and subchannel permafrost conditions) to isolate the effects of discharge, channel morphology, and potential size of the hyporheic zone on transient storage. We repeated stream tracer experiments in five morphologically diverse tundra streams in Arctic Alaska during the thaw season (May–August) of 2004 to assess transient storage and hydrologic characteristics. We compared transient storage model parameters to discharge (Q), the Darcy‐Weisbach friction factor (f), and unit stream power (ω). Across all studied streams, permafrost active layer depths (i.e., the potential extent of the hyporheic zone) increased throughout the thaw season, and discharges and velocities varied dramatically with minimum ranges of eight‐fold and four‐fold, respectively. In all reaches the mean storage residence time (tstor) decreased exponentially with increasing Q, but did not clearly relate to permafrost active layer depths. Furthermore, we found that modeled transient storage metrics (i.e., tstor, storage zone exchange rate (αOTIS), and hydraulic retention (Rh)) correlated better with channel hydraulic descriptors such as f and ω than they did with Q or channel slope. Our results indicate that Q is the first‐order control on transient storage dynamics of these streams, and that f and ω are two relatively simple measures of channel hydraulics that may be important metrics for predicting the response of transient storage to perturbations in discharge and morphology in a given stream.

Recovery of three arctic stream reaches from experimental nutrient enrichment

Summary1. Nutrient enrichment and resulting eutrophication is a widespread anthropogenic influence on freshwater ecosystems, but recovery from nutrient enrichment is poorly understood, especially in stream environments. We examined multi‐year patterns in community recovery from experimental low‐concentration nutrient enrichment (N + P or P only) in three reaches of two Arctic tundra streams (Kuparuk River and Oksrukuyik Creek) on the North Slope of Alaska (U.S.A.).2. Rates of recovery varied among community components and depended on duration of enrichment (2–13 consecutive growing seasons). Biomass of epilithic algae returned to reference levels rapidly (within 2 years), regardless of nutrients added or enrichment duration. Aquatic bryophyte cover, which increased greatly in the Kuparuk River only after long‐term enrichment (8 years), took 8 years of recovery to approach reference levels, after storms had scoured most remnant moss in the recovering reach.3. Multi‐year persistence of bryophytes in the Kuparuk River appeared to prevent recovery of insect populations that had either been positively (e.g. the mayfly Ephemerella, most chironomid midge taxa) or negatively (e.g. the tube‐building chironomid Orthocladius rivulorum) affected by this shift in dominant primary producer. These lags in recovery (of >3 years) were probably driven by the persistent effect of bryophytes on physical benthic habitat.4. Summer growth rates of Arctic grayling (both adults and young‐of‐year) in Oksrukuyik Creek (fertilised for 6 years with no bryophyte colonisation), which were consistently increased by nutrient addition, returned to reference rates within 1–2 years.5. Rates of recovery of these virtually pristine Arctic stream ecosystems from low‐level nutrient enrichment appeared to be controlled largely by duration of enrichment, mediated through physical habitat shifts caused by eventual bryophyte colonisation, and subsequent physical disturbance that removed bryophytes. Nutrient enrichment of oligotrophic Arctic stream ecosystems caused by climate change or local anthropogenic activity may have dramatic and persistent consequences if it results in the colonisation of long‐lived primary producers that alter physical habitat.

LONG-TERM RESPONSES OF THE KUPARUK RIVER ECOSYSTEM TO PHOSPHORUS FERTILIZATION

A long-term stream fertilization experiment was performed to evaluate the potential eutrophication of an arctic stream ecosystem. During 16 years of summer phosphorus (H3PO4) fertilization, we observed a dramatic change in the community structure of the Kuparuk River on the North Slope of Alaska. A positive response to fertilization was observed at all trophic levels with increases in epilithic algal stocks, some insect densities, and fish growth rates. After approximately eight years of P fertilization, bryophytes (mosses) replaced epilithic diatoms as the dominant primary producers in the Kuparuk River. The moss impacted NH4+ uptake rates, benthic gross primary production, habitat structure, and insect abundance and species composition. This study documents the long-term changes in an arctic tundra stream in response to nutrient enrichment. Predicting stream ecosystem responses to chronic perturbation requires long-term observation and experiments.

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.

Effects of desiccation and temperature/irradiance on the metabolism of 2 arctic stream bryophyte taxa

Long-term (13 y) experimental P enrichment of a 4th-order reach of the Kuparuk River (North Slope, Alaska) had little effect on the distribution, abundance, or metabolism of a common, clump-forming bryophyte (Schistidium [Grimmia] agassizii), but promoted rapid and extensive growth by mat-forming, streamer bryophytes (Hygrohypnum alpestre and H. ochraceum) that formerly were rare in the river. In previous research we showed that there were important physiological differences (i.e., photosynthetic efficiency) that helped explain why S. agassizii was able to persist in the river under cold, oligotrophic conditions, whereas H. alpestre and H. ochraceum were not. Here we hypothesize that S. agassizii should be more tolerant than Hygrohypnum spp. to desiccation stress, and that Hygrohypnum spp. would take greater advantage (e.g., greater rates of net primary production) than S. agassizii of elevated temperatures. We did laboratory experiments using small-scale metabolism chambers to test this hypothesis. Desiccation affected net photosynthesis more in Hygrohypnum spp. than in S. agassizii. Hygrohypnum spp. had a greater tolerance to temperatures >20°C than S. agassizii. Further, net photosynthetic rates (at light saturation) were significantly higher for Hygrohypnum spp. (1676–6342 μg O2 g−1 dry mass [dm] h−1) than for S. agassizii (428–1163 μg O2 g−1dm h−1) at all temperatures measured. In fact, S. agassizii showed minimal response to increases in light availability and recovered rapidly from desiccation, but was inhibited by high temperature. In contrast, Hygrohypnum spp. were susceptible to desiccation, and responded strongly to increased light and temperature. The physiological attributes we measured helped explain the distribution of our test species in arctic tundra streams.

Influence of Environmental Variability on the Growth of Age-0 and Adult Arctic Grayling

We examined the influence of river discharge and temperature on growth of adult and age-0 Arctic grayling Thymallus arcticus in the Kuparuk River (1985–1996) and Oksrukuyik Creek (1990–1996) during the Arctic growing season (late June to mid-August). Grayling growth was compared against six physical variables (river, nutrient level, mean summer discharge, mean summer temperature, consecutive days with extreme low flow, and total days with extreme high flow) using stepwise regression analysis. Fifty-six percent of the variance in adult grayling growth rate in both rivers was associated with nutrient level and mean summer discharge. Sixty percent of the variance in age-0 grayling weight at 40 d was associated with nutrient level, mean summer discharge, mean summer temperature, and river. The correlation with discharge was positive for adult grayling growth but negative for age-0 grayling weight. This study suggests that river discharge and water temperature may influence long-term survival and population dynamics of grayling in Arctic tundra streams.

Biological Responses to Fertilization of Oksrukuyik Creek, a Tundra Stream

Experimental additions of phosphorus or phosphorus plus nitrogen to Oksrukuyik Creek, an arctic tundra stream, stimulated production at all trophic levels relative to an unfertilized reach. Epilithic chlorophyll concentration, primary production, and respiration in the epilithic community increased in fertilized riffles, and biomass of diatoms and filamentous macroalgae increased considerably. Production and density of Baetis spp. and Orthocladius rivulorum, 2 key grazing larval insects, increased in the fertilized zone. Production and density of larval black flies, the major filter feeders in the Creek, were not affected significantly by enrichment. Fertilization increased summer growth of adult and young-of-the-year Arctic grayling Thymallus arcticus. Mean summer discharge was an important regulator of epilithic chlorophyll concentration, macroalgal biomass, Orthocladius production, and possibly fish growth. We compared our results to a similar fertilization experiment conducted in the Kuparuk River, a larger arctic tundra stream. In both systems, production increased at every trophic level, although responses of specific primary producers and insects differed, possibly because of differences in flow regimes.

Comparison of Epilithic Algal and Bryophyte Metabolism in an Arctic Tundra Stream, Alaska

Phosphorus has been added to a reach of the Kuparuk River, Alaska, from late June to mid August every year since 1983. The P-fertilized reach of the river is now extensively colonized by 2 bryophytes (Schistidium agassizii and Hygrohypnum spp.), whereas only S. agassizii is common in unfertilized reaches of the river. We compared photosynthesis rates of epilithic algae, S. agassizii, and Hygrohypnum spp. under reference and P-enriched conditions to extend our long-term records of bryophyte dynamics within the fertilized reach and to describe the physiological differences between major primary producers. Rates of primary production were determined from changes in dissolved oxygen concentration during light and dark incubations of algal and bryophyte samples in closed chambers. Net primary productivity per unit total chlorophyll a (CHL_t a) was greater for epilithic algae (2.0-6.0 mg O₂ mg^-1 CHL_t a h^-1) than for bryophytes (0.2-1.7 mg O₂ mg^-1 CHL_t a h^-1). However, the greater biomass of the bryophyte community in the fertilized reach yielded area-specific productivity rates for Hygrohypnum spp. that were 2 to 4 times greater than areal rates of epilithic algal productivity. Bryophytes accounted for 80% of primary production in P-fertilized reaches but only 9% in reference reaches. Thus, increased bryophyte abundance in response to P increased total net primary production from 2.3 g C/h to 6.3 g C/h. Photosynthesis-irradiance (PI) parameters obtained by fitting data to a hyperbolic tangent model differed between bryophyte species, between reference and fertilized stream reaches, and over the season. Differences in PI relationships between S. agassizii and Hygrohypnum spp. suggest that they use different life strategies (subsistence vs opportunism, respectively) to exist in the arctic environment.

The trophic interactions of young Arctic grayling (Thymallus arcticus) in an Arctic tundra stream

1. Young (0+) Arctic grayling (Thymallus arcticus) have the potential to control the trophic structure of Arctic tundra streams through consumption, nutrient excretion and the modification of prey behaviour. The effect of young grayling on three trophic levels (algae, invertebrates and fish) was investigated by manipulating fish density and by fertilizing the river with phosphorus (P).2. Nutrients, epilithic chlorophyll a, benthic invertebrates and fish biomass were measured within each fish density treatment (0, 4, and 40 fish m–2) within the P‐limited reference zone and the P‐enriched fertilized zone of the Kuparuk River, Alaska.3. Epilithic chlorophyll a increased with increased fish density in both reference and fertilized zones, while mayfly density decreased with increased fish density in the fertilized zone only. Final mean mass of young grayling in the 40 fish m–2 cages was lower than mean mass in the 4 fish m–2 cages.4. Young grayling may produce a top‐down cascading trophic effect in areas where nutrients are not limited.5. River nutrient status and river discharge may modify the strength of top‐down control by young grayling.

Microfaunal response to fertilization of an arctic tundra stream

SUMMARY1. As part of a whole‐system study, the response of the heterotrophic microfaunal community colonizing artificial substrata (polyfoam units) to fertilization of an arctic tundra stream was followed for 6 weeks during the summer.2. Dominant heterotrophic microfauna observed included amoebae (approximately 40% of colonizing biomass), rotifers (36% of biomass) and ciliates (25% of biomass).3. Biomass of heterotrophic microfauna on artificial substrata was not significantly different in a control reach and an experimental reach fertilized with phosphorus (loading rate ten times ambient), but in a reach fertilized with both phosphorus and nitrogen (loading rates ten times ambient) biomass was double that of the control and phosphorus‐fertilized reaches. The lack of response in the phosphorus reach was probably due to greater insect grazing as a result of previous phosphorus fertilization of this reach.4. Abundance of microfauna on epilithic surfaces in the river was higher on rocks from pools than on rocks from riffle areas, but abundance on the artificial substrata was higher than on the natural rocks.5. The results suggest that microfauna of arctic tundra streams are regulated by grazers and that their importance in transfers among trophic levels is greater in pools than in riffles.

Controls on production of bryophytes in an arctic tundra stream

SUMMARY1. Two bryophyte taxa (Hygrohypnum spp. and, to a lesser extent, Fontinalis neomexicana) were abundant in riffles within phosphorus‐fertilized reaches of the Kuparuk River (North Slope, Alaska), but were much less common in fertilized pools and virtually absent in unfertilized reaches of the river. We conducted field experiments using stems and clumps of both species and artificial bryophytes to test the hypotheses that bryophyte growth was strongly limited by low phosphorus concentrations in unfertilized reaches, and limited by epiphytes in fertilized pools.2. Stem tips of Hygrohypnum spp. did not elongate when grown in unfertilized pool and riffle environments. In fertilized reaches, Hygrohypnum elongated significantly, although there was no significant difference in elongation of stem tips placed in pools [2.5 ± 0.9 cm (SD)] as opposed to riffles (2.8 ± 0.9 cm) for 32 days.3. Stem tips of F. neomexicana elongated significantly in all sites. There was a significant difference in elongation of stem tips in control and fertilized riffles (2.1 ± 1.1 and 4.7 ± 0.1 cm, respectively) but not in tips grown in control and fertilized pools (2.8 ± 0.8 and 2.7 ± 0.9 cm, respectively).4. Biomass increments in clumps of these same species followed similar patterns except in fertilized pools. Hygrohypnum spp. lost weight in control riffle environments and did not grow in pools, but accumulated 181 ± 44 and 335 ± 200% of initial biomass in fertilized riffles in 1992 (over 32 days) and 1993 (over 44 days), respectively. F. neomexicana accumulated 38 ± 39 and 98 ± 47% of initial biomass in 1992 in unfertilized and fertilized riffles, respectively. Total phosphorus concentrations of both bryophytes in 1992 were significantly greater when grown in fertilized riffles than control riffles.5. Artificial mosses (untwisted, natural fibre rope) and clumps of Hygrohypnum spp. were used to assess effects of flow regime on derrital and epiphyte accumulation in the fertilized zone. Epiphyte and detrital mass was 4–4.5 times greater on average on artificial mosses in slow‐flowing pool environments than in fast‐flowing riffle environments. Epiphyte chlorophyll a was 4 times greater on Hygrohypnum clumps in pools than in riffles. This difference was probably brought about by increased detrital deposition and reduced grazing by invertebrates in pools. It is likely that both Hygrohypnum spp. and F. neomexicana could grow throughout the river, but are limited strongly by low phosphorus concentrations in unfertilized reaches and secondarily by detritus accumulation and interference competition with epiphytic algae in fertilized pools.

Epilithic chlorophyll a, photosynthesis, and respiration in control and fertilized reaches of a tundra stream

Photosynthesis and respiration by the epilithic community on cobble in an arctic tundra stream, were estimated from oxygen production and consumption in short-term (4–12 h), light and dark, chamber incubations. Chlorophyll a was estimated at the end of each incubation by quantitatively removing the epilithon from the cobble.

Black fly (Diptera: Simuliidae) response to phosphorus enrichment of an arctic tundra stream

Black fly (Diptera: Simuliidae) response to phosphorus enrichment of an arctic tundra stream