Comparison of fatty acids and elemental nutrients in periphyton, invertebrates, and cutthroat trout (Oncorhyncus clarki) in conifer and alder streams of western Washington state

This is the portpriint version of the article. The original publication is available at www.springerlink.comOrganism growth and reproduction are often limited by nutrient availability in freshwater ecosystems where, in some cases, food webs are primarily supported by allochthonous organic matter. Therefore, we hypothesized that the composition of riparian vegetation would influence the variability of N, P and fatty acid content of in-stream consumers. Specifically, we predicted that organisms living in alder streams would have higher levels of N, P, and polyunsaturated fatty acids than organisms in coniferous streams. To determine this, we sampled fresh and aged leaf litter, periphyton, invertebrates, and cutthroat trout (Oncorhynchus clarki) from 6 streams in western Washington state: 3 streams had high densities of nitrogen-fixing red alder (Alnus rubra) in the riparian zone, whereas 3 had high densities of conifers. We found fresh alder litter had twice the total polyunsaturated fatty acid concentrations of hemlock vegetation while there were few statistical differences among aged alder and aged hemlock vegetation. Multidimensional plots showed fatty acid profiles were unique to vegetation and fish while periphyton and invertebrates shared the same multidimensional space. We used a mixed model to determine the relative importance of vegetation type (fixed factor: conifer or alder), trophic levels (fixed factor: periphyton, primary consumer, or fish) and streams (random factor) on individual fatty acid concentrations. Total polyunsaturated fatty acids, 16:0, 20:1, 20:3n6 and total n3 were the only fatty acids influenced by stream vegetation (vegetation + stream model or full model. 67% of the fatty acids were best supported by the trophic +stream model. Nitrogen, P, Ca, Fe, C:N, N:P and C:N:P were all best supported by the trophic level + stream model and Zn was the only nutrient supported best by the full model. Correlations of n3 and n6 fatty acid concentrations between periphyton and primary consumers, and primary consumers with trout indicated several fatty acid metrics, such as n3:n6, showed food resources may affect relative fatty acid abundances of consumers. Although vegetation type did not influence relative fatty acids of stream organisms, the importance of trophic level likely indicates organisms have different physical requirements for fatty acids. The significance of a random factor, ‘stream,’ suggests that the relative abundances of fatty acids in periphyton, invertebrates and trout are more related than similar organisms from another stream

Do fall additions of salmon carcasses benefit food webs in experimental streams?

This is the postprint version of the article. The printed version of the article can be found here: http://www.springerlink.com/content/jl0122219v283124/Research showing that salmon carcasses support the productivity and biodiversity of aquatic and riparian ecosystems has been conducted over a variety of spatial and temporal scales. In some studies, carcasses were manipulated in a single pulse or loading rate or manipulations occurred during summer and early fall, rather than simulating the natural dynamic of an extended spawning period, a gradient of loading rates, or testing carcass effects in late fall-early winter when some salmon stocks in the US Pacific Northwest spawn. To address these discrepancies, we manipulated salmon carcass biomass in 16 experimental channels located in the sunlit floodplain of the Cedar River, WA, USA between mid-September and mid-December, 2006. Total carcass loads ranged from 0–4.0 kg/m2 (0, 0.001, 0.01, 0.1, 0.5, 1.0, 2.0 and 4.0 kg/m2, n = 2 per treatment) and were added to mimic the temporal dynamic of an extended spawning period. We found little evidence that carcasses influenced primary producer biomass or fish growth; however, nutrients and some primary consumer populations increased with loading rate. These effects varied through time, however. We hypothesize that the variable effects of carcasses were a result of ambient abiotic condition, such as light, temperature and disturbance that constrained trophic response. There was some evidence to suggest peak responses for primary producers and consumers occurred at a loading rate of *1.0–2.0 kg/m2, which was similar to other experimental studies conducted during summer

Changing forests-changing streams: Riparian forest stand development and ecosystem function in temperate headwaters

Light availability influences temperature, primary production, nutrient dynamics, and secondary production in aquatic ecosystems. In forested freshwater ecosystems, shading by streamside (riparian) vegetation is a dominant control on light flux and represents an important interaction at the aquatic-terrestrial interface. Changes in forest structure over time, particularly tree mortality processes that gradually increase light penetration through maturing forest canopies, are likely to influence stream light fluxes and associated ecosystem functions. We provide a set of conceptual models describing how stream light dynamics change with the development of complex canopy structure and how changes in light availability are likely to affect stream ecosystem processes. Shortly after a stand-replacing event, light flux to the stream is high, but light fluxes decline as canopies reestablish and close. Tree density, the degree of understory growth, patterns of tree mortality, and small-scale disturbances interact as drivers of multiple pathways of forest structural development. Changes in canopy structure will, in turn, influence stream light, which is expected to impact primary production and stream nutrient dynamics as well as the amount of autochthonous carbon supporting aquatic food webs. Ultimately, these conceptual models stress the importance of recovery from historic forest disturbances as well as future forest change as important factors influencing the long-term trajectories of ecosystem processes in headwaters

Linking resource availability and habitat structure to stream organisms: an experimental and observational assessment

An experiment and a mark-recapture field study of juvenile coho salmon ( Oncorhynchus kisutch ) were conducted to identify controls of key energy flow chains in river food webs. In the small-scale experiment, we investigated the individual and interactive effects of physical habitat structure (PHS) as small wood and resource availability (tissue of adult Chinook salmon, O. tshawytscha ) on nutrients, algae, invertebrates, and fish predators including juvenile coho. In the field, we quantified the effects of natural variation in prey availability (invertebrate drift biomass), PHS (wood), and local fish density on summer growth of juvenile coho across multiple stream reaches. Adding salmon tissue to experimental channels resulted in strong bottom-up effects on select invertebrates including increased population biomass of chironomids and baetids, the numerically dominant invertebrates, and faster growth of juvenile coho. We link the enhanced growth of coho to chironomid productivity: for instance, adult chironomid flux was 4.3 3 higher and coho consumption of these animals 3 3 higher in salmon-subsidized channels. PHS in experimental channels was associated with reduced algal biomass, potentially in response to increased invertebrate consumption, and invertebrate flux or export. The field study revealed coho growth was negatively related to PHS and total fish density and positively related to Diptera drift biomass; however, the effects of fish density and drift biomass on coho growth were relatively weak. The field study also indicated that prey resource availability and coho growth were associated with differences in canopy cover, with prey biomass and coho growth 2–4 3 higher in reaches receiving more sunlight. As in the experiment, coho in natural stream reaches predominantly fed on adult chironomids and other Diptera, indicating that these taxa and life-stages are a key link between the benthic food web and mobile vertebrate predators. Our study showed that bottom-up processes initiated by salmon subsidies and possibly light flux determined key trophic interactions in the Cedar River food web. Moreover, we speculate that PHS may modify some of these interactions indirectly through its effects on the movement of organisms through the environment

Comparison of fatty acids and elemental nutrients in periphyton, invertebrates, and cutthroat trout (Oncorhyncus clarki) in conifer and alder streams of western Washington state

Organism growth and reproduction are often limited by nutrient availability in freshwater ecosystems where, in some cases, food webs are primarily supported by allochthonous organic matter. Therefore, we hypothesized that the composition of riparian vegetation would influence the variability of N, P and fatty acid content of in-stream consumers. Specifically, we predicted that organisms living in alder streams would have higher levels of N, P, and polyunsaturated fatty acids than organisms in coniferous streams. To determine this, we sampled fresh and aged leaf litter, periphyton, invertebrates, and cutthroat trout (Oncorhynchus clarki) from 6 streams in western Washington state: 3 streams had high densities of nitrogen-fixing red alder (Alnus rubra) in the riparian zone, whereas 3 had high densities of conifers. We found fresh alder litter had twice the total polyunsaturated fatty acid concentrations of hemlock vegetation while there were few statistical differences among aged alder and aged hemlock vegetation. Multidimensional plots showed fatty acid profiles were unique to vegetation and fish while periphyton and invertebrates shared the same multidimensional space. We used a mixed model to determine the relative importance of vegetation type (fixed factor: conifer or alder), trophic levels (fixed factor: periphyton, primary consumer, or fish) and streams (random factor) on individual fatty acid concentrations. Total polyunsaturated fatty acids, 16:0, 20:1, 20:3n6 and total n3 were the only fatty acids influenced by stream vegetation (vegetation + stream model or full model. 67% of the fatty acids were best supported by the trophic +stream model. Nitrogen, P, Ca, Fe, C:N, N:P and C:N:P were all best supported by the trophic level + stream model and Zn was the only nutrient supported best by the full model. Correlations of n3 and n6 fatty acid concentrations between periphyton and primary consumers, and primary consumers with trout indicated several fatty acid metrics, such as n3:n6, showed food resources may affect relative fatty acid abundances of consumers. Although vegetation type did not influence relative fatty acids of stream organisms, the importance of trophic level likely indicates organisms have different physical requirements for fatty acids. The significance of a random factor, ‘stream,’ suggests that the relative abundances of fatty acids in periphyton, invertebrates and trout are more related than similar organisms from another stream

Do fall additions of salmon carcasses benefit food webs in experimental streams?

Research showing that salmon carcasses support the productivity and biodiversity of aquatic and riparian ecosystems has been conducted over a variety of spatial and temporal scales. In some studies, carcasses were manipulated in a single pulse or loading rate or manipulations occurred during summer and early fall, rather than simulating the natural dynamic of an extended spawning period, a gradient of loading rates, or testing carcass effects in late fall-early winter when some salmon stocks in the US Pacific Northwest spawn. To address these discrepancies, we manipulated salmon carcass biomass in 16 experimental channels located in the sunlit floodplain of the Cedar River, WA, USA between mid-September and mid-December, 2006. Total carcass loads ranged from 0–4.0 kg/m2 (0, 0.001, 0.01, 0.1, 0.5, 1.0, 2.0 and 4.0 kg/m2, n = 2 per treatment) and were added to mimic the temporal dynamic of an extended spawning period. We found little evidence that carcasses influenced primary producer biomass or fish growth; however, nutrients and some primary consumer populations increased with loading rate. These effects varied through time, however. We hypothesize that the variable effects of carcasses were a result of ambient abiotic condition, such as light, temperature and disturbance that constrained trophic response. There was some evidence to suggest peak responses for primary producers and consumers occurred at a loading rate of *1.0–2.0 kg/m2, which was similar to other experimental studies conducted during summer

Linking resource availability and habitat structure to stream organisms: an experimental and observational assessment

An experiment and a mark-recapture field study of juvenile coho salmon ( Oncorhynchus kisutch ) were conducted to identify controls of key energy flow chains in river food webs. In the small-scale experiment, we investigated the individual and interactive effects of physical habitat structure (PHS) as small wood and resource availability (tissue of adult Chinook salmon, O. tshawytscha ) on nutrients, algae, invertebrates, and fish predators including juvenile coho. In the field, we quantified the effects of natural variation in prey availability (invertebrate drift biomass), PHS (wood), and local fish density on summer growth of juvenile coho across multiple stream reaches. Adding salmon tissue to experimental channels resulted in strong bottom-up effects on select invertebrates including increased population biomass of chironomids and baetids, the numerically dominant invertebrates, and faster growth of juvenile coho. We link the enhanced growth of coho to chironomid productivity: for instance, adult chironomid flux was 4.3 3 higher and coho consumption of these animals 3 3 higher in salmon-subsidized channels. PHS in experimental channels was associated with reduced algal biomass, potentially in response to increased invertebrate consumption, and invertebrate flux or export. The field study revealed coho growth was negatively related to PHS and total fish density and positively related to Diptera drift biomass; however, the effects of fish density and drift biomass on coho growth were relatively weak. The field study also indicated that prey resource availability and coho growth were associated with differences in canopy cover, with prey biomass and coho growth 2–4 3 higher in reaches receiving more sunlight. As in the experiment, coho in natural stream reaches predominantly fed on adult chironomids and other Diptera, indicating that these taxa and life-stages are a key link between the benthic food web and mobile vertebrate predators. Our study showed that bottom-up processes initiated by salmon subsidies and possibly light flux determined key trophic interactions in the Cedar River food web. Moreover, we speculate that PHS may modify some of these interactions indirectly through its effects on the movement of organisms through the environment

The influence of metal exposure history and ultraviolet-B radiation on benthic communities in Colorado Rocky Mountain streams

Interest in understanding the influence of ultraviolet-B (UVB; 280–320 nm) radiation in aquatic ecosystems has increased since the early 1990s. Pollution from historic mining operations coupled with physicochemical characteristics of Rocky Mountain streams that increase exposure of benthic communities to UVB provided an opportunity to examine how UVB interacted with heavy metal contamination to structure stream communities. We integrated a series of UVB addition experiments done in stream microcosms with a large-scale UVB shading experiment to test the hypothesis that effects of UVB were greater on benthic communities from metal-polluted streams than from reference streams. Microcosm experiments involved short-term exposure (7–10 d) of natural benthic macroinvertebrate communities collected from reference and metal-contaminated sites to lamp-generated UVB. In all cases, abundance decreased in UVB-treated streams compared to controls. Moreover, effects of UVB addition were significantly greater on communities from metal-polluted sites than from reference sites. The field experiment involved shading portions of the streambed from UVB for 60 d at 12 streams along a Zn gradient. Median Zn concentration at these sites ranged between 5 and 530 μg/L, and mean UVB reaching the streambed varied from 6.5 to 29.0 J/cm2. Results of the field experiment indicated that removal of UVB significantly increased total macroinvertebrate abundance and abundance of grazers, mayflies, caddisflies, Orthocladiinae midges, and the mayfly Baetis bicaudatus compared to controls. Grazer abundance was significantly greater in UVB removal treatments compared to controls, but UVB removal had no effect on algal biomass. As with the microcosm experiments, the effects of UVB removal on benthic communities were generally greater at metal-polluted sites than at reference sites. We speculate that the energetic cost of regulating metals might inhibit the ability of some organisms to repair efficiently DNA damaged by UVB exposure. Our results demonstrate that benthic communities in Colorado Rocky Mountain streams are negatively influenced by UVB radiation and that communities subjected to long-term metal exposure are more sensitive to UVB than are reference communities. As a consequence, the effects of increased UVB radiation reaching the earth’s surface might be more severe than previously considered in systems receiving multiple stressors

Do fall additions of salmon carcasses benefit food webs in experimental streams?

Research showing that salmon carcasses support the productivity and biodiversity of aquatic and riparian ecosystems has been conducted over a variety of spatial and temporal scales. In some studies, carcasses were manipulated in a single pulse or loading rate or manipulations occurred during summer and early fall, rather than simulating the natural dynamic of an extended spawning period, a gradient of loading rates, or testing carcass effects in late fall-early winter when some salmon stocks in the US Pacific Northwest spawn. To address these discrepancies, we manipulated salmon carcass biomass in 16 experimental channels located in the sunlit floodplain of the Cedar River, WA, USA between mid-September and mid-December, 2006. Total carcass loads ranged from 0–4.0 kg/m2 (0, 0.001, 0.01, 0.1, 0.5, 1.0, 2.0 and 4.0 kg/m2, n = 2 per treatment) and were added to mimic the temporal dynamic of an extended spawning period. We found little evidence that carcasses influenced primary producer biomass or fish growth; however, nutrients and some primary consumer populations increased with loading rate. These effects varied through time, however. We hypothesize that the variable effects of carcasses were a result of ambient abiotic condition, such as light, temperature and disturbance that constrained trophic response. There was some evidence to suggest peak responses for primary producers and consumers occurred at a loading rate of *1.0–2.0 kg/m2, which was similar to other experimental studies conducted during summer