Interactions Between Physical and Biological Processes in Riverine Landscapes IV: Ecosystem Response to Physical Processes and Disturbance
Presiding: J M Buffington, USDA Forest Service; C V Baxter, Colorado State University; A E Rosenberger, University of Idaho and USDA Forest Service
B43B-01 INVITED 13:30h
Floods, Food Webs, and Fluxes in a Northern California channel network
Following flood scour, re-assembling river food webs respond to solute and particle fluxes. During periods of relatively low flow and high biomass, food webs will also intercept and alter these fluxes. We are studying these interactions in the South Fork Eel River and its tributaries in northern California, a steep drainage system subject to a Mediterranean climate regime. During the biologically productive summer low flow season, large algal blooms in sunlit channels tend to follow bed scouring winter floods. These blooms have strong impacts on consumers and predators in local food webs, and on local fluxes, including cross-habitat food web exchange mediated by insect emergence. We are just beginning to study the effects of blooms on fluxes through channels over larger spatial and temporal scales. Year-to-year variation in the magnitude of algal blooms will produce corresponding variation in pulsed releases of organic matter, stored temporarily in the beds of deep pools, and ultimately, in this incised system, in off shore ocean sediments. A small but ecologically significant component, however, may be stored as biomass in long lived vertebrate predators, including bats and salmonids. The ultimate goal of quantifying relationships among flow regimes, ecosystem productivity, food web responses, and fluxes through watersheds will require prolonged cross-disciplinary collaborations, but may become increasingly feasible due to access to new sensor, tracing and mapping technologies.
http://www.nced.umn.edu/
B43B-02 13:45h
Spatial subsidies in riverine food webs: consequences of disturbance and environmental change for stream fishes in Alaska and the Pacific Northwest
Freshwater food webs in Alaska and other parts of the Pacific Northwest rely heavily on nutrient, detritus and prey subsidies from marine and terrestrial ecosystems. Adult salmon provide tons of marine biomass to riverine ecosystems each year when they spawn and decompose; riparian forests provide terrestrial invertebrates to streams, which at times comprise over half of the food ingested by stream-resident salmonids; and up-slope, fish-less headwater streams are a year-round source of prey and detritus to fish-bearing food webs in valley bottoms. Disturbance and environmental change influence these resource subsidies. Fishing, dams, and ocean currents affect salmon returns to fresh water, and riparian forest management (fish-bearing and upslope fishless streams) ultimately affects the flow of prey to fishes. Following disturbances such as timber harvesting, regenerating red alder riparian forests elevate terrestrial invertebrate inputs to streams by up to four times compared to forests lacking alder. Because fish-bearing food webs receive resource subsidies from multiple sources, fish communities may be able to absorb short-term loss of subsidies from any given source. Understanding the effects of disturbance and environmental change on nutrient and food subsidies to freshwater food webs will broaden our knowledge of ecosystem function and improve resource management options.
B43B-03 14:00h
Effects of fire and subsequent channel-reorganizing events on invertebrate drift and rainbow trout diet in small headwater streams 10 years post-disturbance
Studies examining the effects of fire on the biota of streams are often confined to immediate post-disturbance impacts; however it is also important to consider longer-term effects of fire and fire-related channel disturbances, including both negative and positive influences on stream communities. Fire and subsequent debris flows and hyperconcentrated flows destroy streamside vegetation and alter the channel morphology such that streams are wider and shallower with larger, less mobile substrate. Increased light, high temperatures, and altered stream morphology have the potential to greatly impact invertebrate communities, invertebrate drift, and drift-feeding fish diet. The goal of our study was to determine the effects of wildfire and wildfire-related disturbance on the amount and composition of stream invertebrate drift and how that translates to the diet of resident fishes 10 years post-disturbance. In the summer and fall of 2003, we set drift nets and examined the diet of fishes in 9 streams: 3 unburned; 3 burned (1992-4); and 3 burned with a subsequent channel disturbance (1992-4). Key questions include: does the taxonomic composition (richness, functional feeding groups), origin (terrestrial or aquatic), or total production (biomass) of invertebrate drift and fish diet vary with burn history? Does the composition and biomass of invertebrate drift indicate main sources of energy (allochthonous vs. autochthonous) for headwater streams affected by fire? Differences among streams in channel morphology, streamside vegetation, light input, and temperature did not correspond to consistent or marked differences in invertebrate drift productivity and only slight differences in functional feeding group composition. However, preliminary data suggest that taxon richness, though similar among burned and unburned streams, is lowest in burned and disturbed streams. Although there is a terrestrial component to fish diet in all three treatment groups, in the summer, there is a greater terrestrial contribution in burned streams; while fish in unburned streams have a greater terrestrial component in their diet in the fall. Our results indicate that the effects of fire and disturbance on invertebrate communities are difficult to detect 10-years post event. Resilience in the invertebrate community and a flexible diet may be contributing to the resilience of resident trout found throughout our study streams. However, geomorphic changes and habitat alterations caused by massive channel-reorganizing events after wildfire may prevent full invertebrate community recovery for some time after the disturbance.
B43B-04 14:15h
Channel Pattern and the Intermediate Disturbance Hypothesis Predict Biodiversity in River-floodplain Ecosystems
River-floodplain ecosystems are among the most diverse and dynamic environments in the world, yet mechanisms that regulate biodiversity in river corridors are poorly understood. In part, this stems from a lack of integration of geomorphological and biological concepts that link fluvial processes to biological diversity. Here we illustrate how channel pattern predicts biodiversity via the intermediate disturbance hypothesis (IDH). We show that (1) channel pattern predicts disturbance frequency and age diversity of patches, (2) there are distinct life history tradeoffs among colonizing and climax species, and (3) diversity should be highest in channel patterns with intermediate levels of disturbance. We first classify river-floodplains in northwestern USA using geomorphological channel patterns, and show how these patterns predict patch dynamics in river-floodplain systems. We then use space-for-time substitution to illustrate successional patterns of trees and aquatic invertebrates. Finally, we link reach-level patch dynamics to reach-level biodiversity of trees and aquatic invertebrates using the IDH. Patch age diversity is low in straight channels with low movement rates and mostly old surfaces, and low in braided channels with high movement rates and mostly young surfaces. Patch age diversity is highest in channels with intermediate movement rates (meandering and island-braided channels). Vegetation succession drives temporal patterns of biological diversity within individual terrestrial and aquatic patches (alpha diversity). Trees exhibit clear successional trade-offs as patches age, succeeding from hardwood-dominated at the colonizing stage to conifer-dominated at the climax stage. Highest within-patch species richness occurs at an intermediate age. Alpha diversity of aquatic invertebrates follows a similar pattern, probably in response to riparian forest succession and the shifting composition of detrital resources entering river and floodplain channels. We predict that aggregate diversity of trees and aquatic invertebrates in river-floodplain reaches (gamma diversity) follows the pattern of age diversity, with low biodiversity in straight and braided channels. Straight channels should be dominated by climax species, whereas braided channels should be dominated by colonizing species. Meandering and island-braided channels have mixed patch ages, maintaining the coexistence of climax and colonizing species and supporting high species diversity.
B43B-05 14:30h
Influence of wood and forests on fish abundance and richness in a large floodplain river
We investigated the influence of large wood and adjacent floodplain forests on fish assemblages along the 230-km mainstem of the Willamette River, Oregon. Fish were sampled in open reaches using boat electroshocking, beach seining, and backpack electroshocking in paired sites with intact forest and land converted to agriculture or urban use. Fish abundance and richness were statistically significantly greater in reaches with intact forest. We observed that wood abundance in the river was related to density of trees along the floodplain margin and developed an intensive sampling approach for determining fish abundance and richness in accumulations of large wood in the river. Fish were sampled from "wood corrals" and marked by fin clipping. These sites were sampled the following day to determine the abundances of each species by mark-recapture. Nets were placed around similar areas in adjacent habitats without wood. Fish numbers around wood accumulations were more than double those in areas without wood. The number of fish species was greater by an average of four species in sites with wood. These results were used to project the consequence of historical changes in fish abundance and richness in the Willamette River and forecast possible responses to future land use change.
http://willametteexplorer.info
B43B-06 14:45h
Longitudinal Patterns in Benthic Macroinvertebrate Community Structure: A Southern Appalachian Wild and Scenic River Continuum (USA: GA, NC, SC)
As human activities on the landscape continue to alter characteristics of natural systems, it becomes increasingly important to directly measure ecosystem properties. Our objective was to evaluate benthic macroinvertebrate community structure in context of reach position along the longitudinal profile of a wild and scenic river. Trends in taxonomic richness, diversity, and habitat weighted abundance and biomass of benthic macroinvertebrate functional groups were examined in three dominant habitats at four study reaches within the Chattooga River watershed over one year. The continuum began at a first order stream (1:24 000 scale map) and continued into the main channel of the Chattooga River. Macroinvertebrate abundance and biomass was greatest at the first order stream (30,946 ind. m-2; 947 mg AFDM m-2). Gatherers were the most abundant functional group, accounting for 62 - 80 % of reach-scale communities at each study site. Biomass of all functional groups tended to decrease with increasing reach size, except for filterers whose biomass increased. Habitat weighted functional group biomass along the continuum matched some, but not all, predictions of the river continuum concept which postulates that physical and biotic properties of pristine river networks change predictably along a longitudinal profile.