Coowe M. Walker

Allochthonous inputs from grass-dominated wetlands support juvenile salmonids in headwater streams: evidence from stable isotopes of carbon, hydrogen, and nitrogen

Abstract.  We used dual-isotope mixing models (&dgr;13C/&dgr;15N and &dgr;2H/&dgr;15N) in a Bayesian framework to partition allochthonous and autochthonous energy sources for salmonids in 2 headwater streams in the Kenai Lowlands, Alaska (USA). Our 1st objective was to estimate the production base for juvenile coho salmon (Oncorhynchus kisutch) and Dolly Varden (Salvelinus malma). We hypothesized that consumers would be reliant on both autochthonous (filamentous algae and periphyton) and allochthonous sources, but that autochthonous sources would dominate because of the open canopy and lower-quality litter inputs provided by the riparian wetland vegetation, primarily bluejoint grass (Calamagrostis canadensis). Our 2nd objective was to evaluate the utility of stable H isotopes for tracing energy pathways in a northern-latitude ecosystem. We hypothesized that &dgr;2H-based models would provide more precise estimates of source partitioning than &dgr;13C-based models because of greater source separation. Allochthonous source contributions consistently exceeded autochthonous sources for all fish species and size classes at both study sites. However, diet shifted during ontogeny, and larger Dolly Varden relied more on autochthonous sources than did smaller individuals of both species. Last, we found good correspondence and similar levels of precision between the &dgr;13C- and &dgr;2H-based models despite greater source separation by &dgr;2H. Our results highlight the importance of allochthonous sources in headwater streams, and we suggest that litter inputs from grasses may be an under-appreciated subsidy to salmon production. Stable H isotopes can be an effective foodweb tracer in northern-latitude streams, but source partitioning results were not sufficiently different from stable C isotope models for us to recommend unequivocally using them to replace or enhance &dgr;13C in similar studies.

Landscape and Wetland Influences on Headwater Stream Chemistry in the Kenai Lowlands, Alaska

Headwater streams are typically closely connected with the surrounding watershed landscape, making them sensitive to local watershed conditions. Headwater streams of the Kenai Lowlands in Alaska provide important rearing habitat for juvenile salmon and other biota, and understanding the connections between surrounding landscapes and stream conditions will improve management capabilities. We conducted field evaluations of 30 headwater stream sites on the Kenai Lowlands of Alaska, stratified across watersheds and wetland types, and combined these results with GIS analysis of 12 landscape metrics. Flow-weighted slope (FWS), which is an indicator of the combined influence of percentage cover and topographic position of wetlands, was the best predictor of stream chemistry. Our results revealed distinct differences in water chemistry among headwaters that are largely driven by topography and the amount of wetland in the upstream drainage area. Streams with a high FWS (higher gradient, low wetness) had higher dissolved oxygen and dissolved inorganic nitrogen (mostly nitrate-N), and lower temperatures. Lower FWS streams (low gradient, high wetness) had higher dissolved organic carbon, temperatures, and ammonium and lower dissolved oxygen, all of which were consistent with strong connections between wetlands in watershed and headwater streams. The flow-weighted slope metric is a landscape feature that can be easily derived from GIS, and can be used as a spatially explicit approach for predicting landscape connections to headwater streams on the Kenai Lowlands.

Catchment topography and wetland geomorphology drive macroinvertebrate community structure and juvenile salmonid distributions in south-central Alaska headwater streams

Abstract.  Conservation and management of headwater streams amid rapid global change require an understanding of the spatial and environmental factors that drive species distributions and associated ecosystem processes. We used a hierarchical analytical framework to model effects of catchment-scale topography and wetland geomorphic classes on stream physical habitat, chemistry, and macroinvertebrate and fish communities in 30 headwater streams across the Kenai Lowlands, southcentral Alaska, USA. We identified 135 macroinvertebrate taxa, 122 of which were aquatic insects, of which 79 were dipterans. We collected only 6 species of fish, but juvenile coho salmon and Dolly Varden were collected in 17 and 25 of the 30 streams and reached densities >500 and 1300/km, respectively. Flow-weighted slope, an indicator of water residence time and gradient, was the best catchment-scale correlate of macroinvertebrate and fish community structure, and its effect was mediated by wetland geomorphic classes and numerous water chemistry, substrate composition, and channel geomorphology variables measured at the reach scale. Many macroinvertebrate taxa showed high fidelity to different levels of the topographic gradient, resulting in high &bgr; diversity but relatively similar levels of &agr; diversity across the gradient. Juvenile salmonids were segregated among streams by both species and age classes. Coho salmon fry and parr (<10 cm total length [TL]) had significant unimodal distributions that peaked in streams with intermediate slopes and gravel substrate, whereas presmolts (≥10 cm) were found only in lowest-sloping streams with mostly peat substrate and deep, slow channels. Large Dolly Varden (≥8 cm) were found across the entire gradient but were most abundant in high-sloping catchments, whereas small Dolly Varden (<8 cm) followed a similar distribution but were absent from the lowest-gradient sites with low flow velocity, dissolved O2, and gravel substrate. Predictive modeling indicated that all of the 547 km of headwater streams in the study area might serve as potential habitat for ≥1 species and age class of salmonids. Our study should assist in development of catchment management tools for identifying and prioritizing conservation efforts in the region and may serve as a framework for other studies concerning biodiversity and focal species conservation in headwater streams.

Estuarine Environments as Rearing Habitats for Juvenile Coho Salmon in Contrasting South-Central Alaska Watersheds

Abstract For Pacific salmon, estuaries are typically considered transitional staging areas between freshwater and marine environments, but their potential as rearing habitat has only recently been recognized. The objectives of this study were two-fold: (1) to determine if Coho Salmon Oncorhynchus kisutch were rearing in estuarine habitats, and (2) to characterize and compare the body length, age, condition, and duration and timing of estuarine occupancy of juvenile Coho Salmon between the two contrasting estuaries. We examined use of estuary habitats with analysis of microchemistry and microstructure of sagittal otoliths in two watersheds of south-central Alaska. Juvenile Coho Salmon were classified as estuary residents or nonresidents (recent estuary immigrants) based on otolith Sr : Ca ratios and counts of daily growth increments on otoliths. The estuaries differed in water source (glacial versus snowmelt hydrographs) and in relative estuarine and watershed area. Juvenile Coho Salmon with evidence of es...

Multiple Scales of Influence on Wetland Vegetation Associated with Headwater Streams in Alaska, USA

Vegetation of wetlands adjacent to headwater streams on the Kenai Lowlands was dominated by Calamagrostis canadensis, indicating that it is a keystone species that influences stream-wetland interactions across a wide range of geomorphic settings from which headwater streams have their origin. We sampled 30 sites as part of a project to determine the relationships between landscape features and the biological and chemical characteristics of headwater streams and their associated wetlands. In this paper we consider vegetation in wetlands adjacent to headwater streams. Calamagrostis canadensis was the only species that occurred at all sites and only a few species were widespread and abundant across the range of sites sampled. Nonmetric multidimensional scaling of species importance values indicated that the distribution of sites and species was primarily related to stream-reach scale environmental and biological factors. Sixteen stream-reach factors were significantly correlated with the distribution of sites and species on one axis of the ordination. Headwater streams that were located in relatively flat areas with extensive wetlands had species characteristic of nutrient poor wetlands and sites located in steep valleys with narrow wetlands had species characteristic of uplands and wetlands on mineral soils. The distribution of sites and species on the second ordination axis was interpreted to be a response to biological interactions; primarily the negative relationship between C. canadensis and the diversity of other species. We concluded that large-scale watershed features of the landscape are less important than local scale factors in determining the characteristics of vegetation in headwater stream-wetland complexes in the Kenai Lowlands. There was no evidence, however, that differences in the stream-reach scale conditions across the study sites resulted in distinct plant communities associated with the headwater wetlands even though the headwater streams had their origin in different landscape settings.

Watershed influences on the structure and function of riparian wetlands associated with headwater streams - Kenai Peninsula, Alaska

Riparian wetlands are dynamic components of landscapes. Located between uplands and aquatic environments, riparian habitats intercept sediments and nutrients before they enter aquatic environments. They are a source of organic matter and nutrients to aquatic systems, and they provide important habitat for animals, often serving as corridors for the movement of animals between habitats in fragmented landscapes. In this project, we focused on the structure and function of riparian wetlands associated with headwater streams in Alaska that serve as nursery habitats for juvenile salmonids. We asked whether or not the structure and function of headwater wetlands differed between watersheds with and without nitrogen-fixing Alder (Alnus spp.). We found that the aboveground biomass of riparian vegetation was higher in the watershed with Alder, but the largest differences were in the litter layer and belowground where vegetation in the watershed with no Alder had significantly higher root biomass. Interstitial water chemistry also differed between the study sites with significantly higher inorganic N and significantly different characteristics of colored dissolved organic matter at the site with Alder on the watershed. The biomass of litter that hung over the creek bank was less at the site with Alder on the watershed and an in situ decomposition experiment showed significant differences between the two systems. Results of the research demonstrates that watershed characteristics can impact the ecology of headwater streams in ways that had not been previously recognized.

Catchment-scale alder cover controls nitrogen fixation in boreal headwater streams

Alder (Alnus spp.) is a woody plant with bacterial symbionts that fix atmospheric N2 into bioavailable N. We studied 12 North American boreal headwater streams spanning a steep gradient of catchment alder cover (0–27%) to test the hypothesis that increasing inputs of inorganic N associated with increasing alder cover would reduce or eliminate in-stream benthic N2 fixation. We measured N2 fixation rates, chlorophyll a, and ash-free dry mass (AFDM) of periphyton in early (May) and late (August) summer 2011. Dissolved inorganic N (DIN) concentrations, composed almost entirely of NO3/NO2-N, ranged from below detection limits to nearly 2 mg/L and were strongly predicted by catchment alder cover in both months. Higher N2 fixation rates were observed in August than in May. N2 fixation rates declined sharply when alder cover exceeded ~2% of catchment cover, corresponding to 20 to 40 µg/L DIN. This pattern also was evident among 3 streams with contrasting catchment alder cover sampled approximately every 2 wk during the study. The stream with no catchment alder cover exhibited a steady increase in N2 fixation rates over the summer, whereas the streams with low and high alder in their catchments had very low N2 fixation rates that did not vary over time. The influence of alder cover on periphyton biomass was not clear. The strong regulation of alder cover on in-stream N availability leads us to suggest that alder presence may be an important terrestrial regulator of stream N cycling.

Nitrogen Subsidies from Hillslope Alder Stands to Streamside Wetlands and Headwater Streams, Kenai Peninsula, Alaska

We examined nitrogen transport and wetland primary production along hydrologic flow paths that link nitrogen-fixing alder (Alnus spp.) stands to downslope wetlands and streams in the Kenai Lowlands, Alaska. We expected that nitrate concentrations in surface water and groundwater would be higher on flow paths below alder. We further expected that nitrate concentrations would be higher in surface water and groundwater at the base of short flow paths with alder and that streamside wetlands at the base of alder-near flow paths would be less nitrogen limited than wetlands at the base of long flow paths with alder. Our results showed that groundwater nitrate-N concentrations were significantly higher at alder-near sites than at no-alder sites, but did not differ significantly between alder-far sites and no-alder sites or between alder-far sites and alder-near sites. A survey of N stable isotope signatures in soils and foliage in alder-near and no-alder flow paths indicated the alder-derived nitrogen evident in soils below alder is quickly integrated downslope. Additionally, there was a significant difference in the relative increase in plant biomass after nitrogen fertilization, with the greatest increase occurring in the no-alder sites. This study demonstrates that streamside wetlands and streams are connected to the surrounding landscapes through hydrologic flow paths, and flow paths with alder stands are potential “hot spots” for nitrogen subsidies at the hillslope scale. (KEY TERMS: surface water/groundwater interactions; surface water hydrology; groundwater hydrology; watershed management; Alnus; Calamagrostis; connectivity; nitrogen; N.) Callahan, Michael K., Dennis F. Whigham, Mark C. Rains, Kai C. Rains, Ryan S. King, Coowe M. Walker, Jasmine R. Maurer, and Steven J. Baird, 2017. Nitrogen Subsidies from Hillslope Alder Stands to Streamside Wetlands and Headwater Streams, Kenai Peninsula, Alaska. Journal of the American Water Resources Association (JAWRA) 1-15. DOI: 10.1111/1752-1688.12508

Use of glacier river-fed estuary channels by juvenile Coho Salmon: transitional or rearing habitats?

Estuaries are among the most productive ecosystems in the world and provide important rearing environments for a variety of fish species. Though generally considered important transitional habitats for smolting salmon, little is known about the role that estuaries serve for rearing and the environmental conditions important for salmon. We illustrate how juvenile coho salmon Oncorhynchus kisutch use a glacial river-fed estuary based on examination of spatial and seasonal variability in patterns of abundance, fish size, age structure, condition, and local habitat use. Fish abundance was greater in deeper channels with cooler and less variable temperatures, and these habitats were consistently occupied throughout the season. Variability in channel depth and water temperature was negatively associated with fish abundance. Fish size was negatively related to site distance from the upper extent of the tidal influence, while fish condition did not relate to channel location within the estuary ecotone. Our work demonstrates the potential this glacially-fed estuary serves as both transitional and rearing habitat for juvenile coho salmon during smolt emigration to the ocean, and patterns of fish distribution within the estuary correspond to environmental conditions.

Erratum to: Multiple Scales of Influence on Wetland Vegetation Associated with Headwater Streams in Alaska, USA

Vegetation of wetlands adjacent to headwater streams on the Kenai Lowlands was dominated by Calamagrostis canadensis, indicating that it is a keystone species that influences stream-wetland interactions across a wide range of geomorphic settings from which headwater streams have their origin. We sampled 30 sites as part of a project to determine the relationships between landscape features and the biological and chemical characteristics of headwater streams and their associated wetlands. In this paper we consider vegetation in wetlands adjacent to headwater streams. Calamagrostis canadensis was the only species that occurred at all sites and only a few species were widespread and abundant across the range of sites sampled. Nonmetric multidimensional scaling of species importance values indicated that the distribution of sites and species was primarily related to stream-reach scale environmental and biological factors. Sixteen stream-reach factors were significantly correlated with the distribution of sites and species on one axis of the ordination. Headwater streams that were located in relatively flat areas with extensive wetlands had species characteristic of nutrient poor wetlands and sites located in steep valleys with narrow wetlands had species characteristic of uplands and wetlands on mineral soils. The distribution of sites and species on the second ordination axis was interpreted to be a response to biological interactions; primarily the negative relationship between C. canadensis and the diversity of other species. We concluded that large-scale watershed features of the landscape are less important than local scale factors in determining the characteristics of vegetation in headwater stream-wetland complexes in the Kenai Lowlands. There was no evidence, however, that differences in the stream-reach scale conditions across the study sites resulted in distinct plant communities associated with the headwater wetlands even though the headwater streams had their origin in different landscape settings.