Stanley V. Gregory

An Ecosystem Perspective of Riparian ZonesFocus on links between land and water

R iparian zones are the interfaces between terrestrial and aquatic ecosystems. As ecotones, they encompass sharp gradients of environmental factors, ecological processes, and plant communities. Riparian zones are not easily delineated but are comprised of mosaics of landforms, communities, and environments within the larger landscape. We propose a conceptual model of riparian tones that integrates the physical processes that shape valleyfloor landscapes, the succession of terrestrial plant communities on these geomorphic surfaces, the formation of habitat, and the production of nutritional resources for aquatic ecosys-

Stream denitrification across biomes and its response to anthropogenic nitrate loading

Anthropogenic addition of bioavailable nitrogen to the biosphere is increasing and terrestrial ecosystems are becoming increasingly nitrogen-saturated, causing more bioavailable nitrogen to enter groundwater and surface waters. Large-scale nitrogen budgets show that an average of about 20–25 per cent of the nitrogen added to the biosphere is exported from rivers to the ocean or inland basins, indicating that substantial sinks for nitrogen must exist in the landscape. Streams and rivers may themselves be important sinks for bioavailable nitrogen owing to their hydrological connections with terrestrial systems, high rates of biological activity, and streambed sediment environments that favour microbial denitrification. Here we present data from nitrogen stable isotope tracer experiments across 72 streams and 8 regions representing several biomes. We show that total biotic uptake and denitrification of nitrate increase with stream nitrate concentration, but that the efficiency of biotic uptake and denitrification declines as concentration increases, reducing the proportion of in-stream nitrate that is removed from transport. Our data suggest that the total uptake of nitrate is related to ecosystem photosynthesis and that denitrification is related to ecosystem respiration. In addition, we use a stream network model to demonstrate that excess nitrate in streams elicits a disproportionate increase in the fraction of nitrate that is exported to receiving waters and reduces the relative role of small versus large streams as nitrate sinks.

A Hierarchical Approach to Classifying Stream Habitat Features

Abstract We propose a hierarchical system of classifying stream habitats based on three increasingly fine descriptions of the morphological and hydraulic properties of channel geomorphic units. We define channel geomorphic units as areas of relatively homogeneous depth and flow that are bounded by sharp gradients in both depth and flow. Differences among these units provide a natural basis for habitat classification that is independent of spatial scale. At the most general level of resolution, we divide channel units into fast- and slow-water categories that approximately correspond to the commonly used terms “riffle” and “pool.” Within the fast-water category, we identify two subcategories of habitats, those that are highly turbulent (falls, cascades, chutes, rapids and riffles) and those with low turbulence (sheets and runs). Slow-water habitats include pools formed by channel scour (eddy pools, trench pools, mid-channel pools, convergence pools, lateral scour pools and plunge pools) and those formed be...

ALTERNATIVE FUTURES FOR THE WILLAMETTE RIVER BASIN, OREGON

Alternative futures analysis can inform community decisions regarding land and water use. We conducted an alternative futures analysis in the Willamette River Basin in western Oregon. Based on detailed input from local stakeholders, three alternative future landscapes for the year 2050 were created and compared to present-day (circa 1990) and historical (pre-EuroAmerican settlement) landscapes. We evaluated the likely effects of these landscape changes on four endpoints: water availability, Willamette River, stream condition, and terrestrial wildlife. All three futures assume a doubling of the 1990 human population by 2050. The Plan Trend 2050 scenario assumes current policies and trends continue. Because Oregon has several conservation-oriented policies in place, landscape changes and projected environmental effects associated with this scenario were surprisingly small (most #10% change relative to 1990). The scenario did, however, engender a debate among stakeholders about the reasonableness of assuming that existing policies would be implemented exactly as written if no further policy actions were taken. The Development 2050 scenario reflects a loosening of current policies, more market-oriented approach, as proposed by some stakeholders. Estimated effects of this scenario include loss of 24% of prime farmland; 39% more wildlife species would lose habitat than gain habitat relative to the 1990 landscape. Projected effects on aquatic biota were less severe, primarily because many of the land use changes involved conversion of agricultural lands into urban or rural development, both of which adversely impact streams. Finally, Conservation 2050 assumes that ecosystem protection and restoration are given higher priority, although still within the bounds of what stakeholders considered plausible. In response, most ecological indi- cators (both terrestrial and aquatic) recovered 20-70% of the losses sustained since EuroAmerican settlement. The one exception is water availability. Water consumed for out- of-stream uses increased under all three future scenarios (by 40-60%), with accompanying decreases in stream flow. Although the conservation measures incorporated into Conser- vation 2050 moderated the increase in consumption, they were not sufficient to reverse the trend. Results from these analyses have been actively discussed by stakeholder groups charged with developing a vision for the basins future and a basin-wide restoration strategy.

Effects of herbivore type and density on taxonomic structure and physiognomy of algal assemblages in laboratory streams

Four densities of a snail (Juga silicula) and a caddisfly (Dicosmoecus gilvipes) were introduced into separate laboratory streams, and their effects on algal biomass and community structure were monitored for 32 d. Tiles in an ungrazed control stream were covered by thick algal mats by day 32, and were composed primarily of Scenedesmus spp., Characium, and a variety of diatoms. Biomass and community structure of algal assemblages in the stream with the lowest density of snails were very similar to those in the control stream. In the other streams with snails, an inverse relationship developed between algal biomass and snail density after day 16. By day 32, the algal assemblages in the streams with high snail densities were dominated by adnate diatoms (e.g., Achnanthes lanceolata), and basal cells and short filaments of Stigeoclorium tenue. In contrast to the streams with snails, algal biomass was relatively low in all streams with caddisflies. The differences in algal biomass and structure between the streams with the lowest and highest densities of caddisflies were much smaller than those between streams with the lowest and highest densities of snails. On day 32, the taxonomic and physiognomic structure of the algal assemblages in all the streams with caddisflies resembled that in the streams with higher densities of snails. Scanning electron micrographs showed that even at the highest densities, neither snails nor caddisflies could completely remove the algal assemblage. It is concluded that grazing can substantially influence algal growth form and assemblage physiognomy in lotic ecosystems.

Productive capacity of periphyton as a determinant of plant-herbivore interactions in streams

To investigate the influence of plant productivity on plant-herbivore inter- actions in stream ecosystems, we varied the productive capacity of algal assemblages by exposing periphyton to three levels of irradiance and two levels of grazing. We studied interactions between algal assemblages (grown from algae obtained from four Oregon streams) and herbivorous snails (Juga silicula) in 15 laboratory streams containing either 250 snails/m2 or no snails. Biomass, production, export, and taxonomic structure of the algal community were measured at intervals throughout the 75-d study. Ingestion rate and assimilation efficiency of snails also were measured on six different dates using dual-isotope labeling, and snail growth was measured at the end of the experiment. Rates of primary production, algal biomass accumulation, and dominance by chloro- phytes generally increased with higher irradiance, although these patterns were modified by herbivores. Ungrazed periphyton at low irradiance (photon flux density: 20 ,umol-m-2- s-1) accumulated little biomass, which was further reduced by grazing snails. At inter- mediate (100 ,umol.m-2 s-) and high (400 ,Lmol.m-2-s-1) irradiance, snails delayed the accumulation of algal biomass but did not affect the final biomass attained. After 43 d, net primary production (NPP) at high irradiance was unaffected by grazing, whereas grazing increased NPP at both low and intermediate irradiance. Algal export increased with both irradiance and the presence of grazers and constituted a significant loss of plant biomass from the streams. Grazing by Juga delayed algal succession and altered algal taxonomic structure and assemblage physiognomy by reducing the relative abundance of erect and non-attached algae, while increasing the abundance of adnate diatoms. Snails grew slowly at low irradiance, due to scant food resources, but had high growth rates at intermediate and high irradiance, probably because food was not limiting. Assim- ilation efficiencies for snails generally varied from 40 to 70% and were highest at low irradiance. At low irradiance, 90% ofbenthic production was harvested by grazers, whereas only 10% accumulated as attached biomass or was exported. At higher irradiances, 85% persisted as attached algae or was exported. In these stream ecosystems, the biomass and production of grazers were influenced by abiotic constraints placed on algal productive capacity (i.e., the ability of a plant assemblage to generate biomass). The structure and metabolism of algal assemblages were affected, in turn, by consumptive demand of herbivores. The productive capacity of periphyton mod- ified the nature and outcome of plant-herbivore interactions. This capacity therefore has important implications for the operation of stream ecosystems.

Transient storage in Appalachian and Cascade mountain streams as related to hydraulic characteristics

Hydraulic characteristics were measured in artificial streams and in 1st- to 5th-order streams in the Appalachian and Cascade mountains. Appalachian Mountain stream sites at Coweeta Hydrologic Laboratory, North Carolina, were on six 1st-order streams and a 1st- through 4th-order gradient of Ball Creek-Coweeta Creek. Cascade Mountain sites were located on constrained and unconstrained reaches of Lookout Creek, a 5th-order stream in H. J. Andrews Experimental Forest, Oregon. At each site, a tracer solution (chloride or rhodamine WT) was released for 30-180 min and then discontinued. At the downstream end of the release site, the resulting rise and fall of the tracer concentration was measured. These data, along with upstream concentration and measured widths and depths, were used in a computer model to estimate several hydraulic parameters including transient storage and lateral inflow. Estimated transient storage zone size (As) ranged from near zero in artificial streams to 2.0 m2 in 5th-order streams. As was largest relative to surface cross-sectional area (A) at 1st-order sites where it averaged 1.2 × A, compared with 0.6 × A and 0.1 × A in unconstrained and constrained 5th-order sites, respectively. Where measured, lateral discharge inputs per metre of stream length ranged from 1.9% of instream discharge in 1st-order streams to 0.05% of instream discharge at 5th-order sites. Our results show that surface water exchange with storage zones is rapid and extensive in steep headwater streams and less extensive but still significant at 3rd- through 5th-order sites. An understanding of relationships between stream morphology, storage zone size, and extent of interactions between surface and subsurface waters will assist comparisons of solute dynamics in physically diverse streams.

Can uptake length in streams be determined by nutrient addition experiments? Results from an interbiome comparison study

Nutrient uptake length is an important parameter for quantifying nutrient cycling in streams. Although nutrient tracer additions are the preferred method for measuring uptake length under ambient nutrient concentrations, short-term nutrient addition experiments have more frequently been used to estimate uptake length in streams. Theoretical analysis of the relationship between uptake length determined by nutrient addition experiments (SW′) and uptake length determined by tracer additions (SW) predicted that SW′ should be consistently longer than SW, and that the overestimate of uptake length by SW′ should be related to the level of nutrient addition above ambient concentrations and the degree of nutrient limitation. To test these predictions, we used data from an interbiome study of NH4+ uptake length in which 15NH4+ tracer and short-term NH4+ addition experiments were performed in 10 streams using a uniform experimental approach. The experimental results largely confirmed the theoretical predictions: SW′ was consistently longer than SW and SW′:SW ratios were directly related to the level of NH4+ addition and to indicators of N limitation. The experimentally derived SW′:SW ratios were used with the theoretical results to infer the N limitation status of each stream. Together, the theoretical and experimental results showed that tracer experiments should be used whenever possible to determine nutrient uptake length in streams. Nutrient addition experiments may be useful for comparing uptake lengths between different streams or different times in the same stream, however, provided that nutrient additions are kept as low as possible and of similar magnitude.

Flood Disturbance in a Forested Mountain Landscape Interactions of land use and floods

R ecent flooding in the Pacific Northwest vividly illustrates the complexity of watershed and ecosystem responses to floods, especially in steep forest landscapes. Flooding involves a sequence of interactions that begins with climatic drivers. These drivers, generally rain and snowmelt, interact with landscape conditions, such as vegetation pattern and topography, to determine the capability of a watershed to deliver water, sediment, and organic material to downstream areas (Figure 1). Land-use practices can affect watershed responses to flooding through the influences of managed vegetation patterns and roads on delivery of water, sediment, and wood to streams. Watershed responses to floods include geophysical processes, such as landslides and channel erosion, and related disturbances of aquatic and riparian organisms and their habitats. We explore these geophysical-ecological interactions using a recent flood in

Plant-Herbivore Interactions in Stream Systems

Ecological theories of herbivory have been derived primarily from terrestrial research because man has long depended on domesticated grazers and has suffered the detrimental consequences of outbreaks of phytophagous insects. Only in recent decades has the amount of information on herbivory in aquatic ecosystems grown sufficiently to stimulate development of general theories of herbivory for these systems.