Robert C. Wissmar

The Role of Disturbance in Stream Ecology

We define disturbance in stream ecosystems to be: any relatively discrete event in time that is characterized by a frequency, intensity, and severity outside a predictable range, and that disrupts ecosystem, community, or population structure and changes resources or the physical environment. Of the three major hypotheses relating disturbance to lotic community structure, the dynamic equilibrium hypothesis appears to be generally applicable, although specific studies support the intermediate disturbance hypothesis and the equilibrium model. Differences in disturbance frequency between lentic and lotic systems may explain why biotic interactions are more apparent in lakes than in streams. Responses to both natural and anthropogenic disturbances vary regionally, as illustrated by examples from the mid-continent, Pacific northwest, and southeastern United States. Based on a generalized framework of climatic-biogeochemical characteristics, two features are considered to be most significant in choosing streams for comparative studies of disturbance: hydrologic regimes and comparable geomorphology. A method is described for quantifying predictability of the hydrologic regime based on long-term records of monthly maximum and minimum stream flows. Different channel forms (boulder and cobble, alluvial gravelbed, alluvial sandbed) have different responses to hydrologic disturbance from spates. A number of structural and functional components for comparing disturbance effects within regions and across biomes are presented. Experimental approaches to studying disturbance involve spatial-scale considerations, logistic difficulties, and ethical questions. General questions related to disturbance that could be addressed by stream ecologists are proposed.

Disturbance regimes, resilience, and recovery of animal communities and habitats in lotic ecosystems

Disturbance regime is a critical organizing feature of stream communities and ecosystems. The position of a given reach in the river basin and the sediment type within that reach are two key determinants of the frequency and intensity of flow-induced disturbances. We distinguish between predictable and unpredictable events and suggest that predictable discharge events are not disturbances.We relate the dynamics of recovery from disturbance (i.e., resilience) to disturbance regime (i.e., the disturbance history of the site). The most frequently and predictably disturbed sites can be expected to demonstrate the highest resilience.Spatial scale is an important dimension of community structure, dynamics, and recovery from disturbance. We compare the effects on small patches (⩽1 m2) to the effects of large reaches at the river basin level. At small scales, sediment movements and scour are major factors affecting the distribution of populations of aquatic insects or algae. At larger scales, we must deal with channel formation, bank erosion, and interactions with the riparian zone that will affect all taxa and processes.Our understanding of stream ecosystem recovery rests on our grasp of the historical, spatial, and temporal background of contemporary disturbance events.

Organic Carbon: Oxidation and Transport in the Amazon River

Spatial and temporal patterns in the organic carbon load (< 1 millimeter) ofthe Amazon River indicate that oxidation was constant throughout the river at any one time but was much greater at rising water than at high water, whereas transport was constant. The total effective efflux, as the sum ofoxidation plus transport in the river, was about 1014 grams of carbon per year. Estimates for other river systems suggest that global riverine carbon fluxes exceed 1015 grams per year. 1348 on Jauary 8, 2021 http://sce.sciencem agorg/ D ow nladed fom Manaus, Brazil, to Iquitos, Peru, in February to March 1977 (R.J.N. and J.T.B.) at the beginning of the rainy season when water levels were rising about 1 m per week. Transect 2 extended 3400 km, from Iquitos, Peru, to Belem, Brazil, in May to June 1977 (J.E.R., R.C.W., and R.F.S.) during peak flooding when the river inundates terrestrial habitats for several kilometers on each side of the normal channel in upstream reaches to 20 to 100 km in downstream reaches. Chemical measurements included particulate organic carbon < 1 mm (POC), dissolved organic carbon (DOC), the organic matter content of seston, and the respiratory oxidation activity (4). The amount of organic matter present in the fine suspended load averaged about 10 percent during both transects (Table 1) (5). The concentration of POC during the rising-water period was 15 to 20 g m-3 upriver and decreased to 8.2 g m-3 downriver at Manaus, whereas at high water the POC concentration reached an upriver maximum of 3.7 g m-3 and exhibited downstream values of 1 to 2 g m-3. During both rising and high water, the DOC concentration was relatively uniform throughout the river, averaging 4.2 and 6.5 g m-3, respectively. The POC and DOC values appear comparable to those of other investigators at single stations with regard to both wetand dry-season values (6). Respiration Tributaries Tributaries

Plankton Metabolism and Carbon Processes in the Amazon River, Its Tributaries, and Floodplain Waters, Peru‐Brazil, May‐June 1977

A synoptic study of photosynthetic and respiratory activity of plankton communities in different Amazon surface waters indicates that large—scale events such as flooding can have a major impact upon the cycling of carbon and nutrients in these aquatic ecosystems. During high water, the major factors influencing primary production appeared to be nutrient concentrations in the mouthbays and varzea (floodplain) lakes and high levels of suspended matter in the Amazon mainstem. In riverine systems, plankton primary production (PPP) averaged 4.04 mg C°m—3°h—1, and measures of respiration (Re) averaged 0.67 mg C°m—3°h—1. In the more productive varzea lakes and mouthbays, PPP averaged 26.37 mg C°m—3°h—1 and Re averaged 2.30 mg C°n—3°h—1. Bacterial densities, 14 C—acetate rate constants for uptake, and plankton carbon: ATP ratios implied that heterotrophic microbiota were important components of the plankton communities in riverine waters. The importance of terrestrial organics to metabolic activity in all waters was implied by high particulate carbon: nitrogen ratios (20:1). These features were especially evident in riverine surface waters where integrated respiration rates exceeded those of plankton primary production. Riverine respiratory levels may be attributed to several factors: adequate supplies of terrestrial organic carbon, sufficient dissolved nutrient concentrations, increased surface area of suspended matter for microbial attachment and growth, and shading of phytoplankton by suspended matter which reduces photosynthetic activity. Observed supersaturation of Amazon waters by carbon dioxide was similar to observations for other rivers of the world. Shifts of CO2 solute components to CO2 in surface waters of varzea lakes and mouthbays and of some tributaries implied high partial pressures of carbon dioxide (°500—1500 Pa). The primary source of CO2 is most likely decomposing organic matter in planktonic and benthic environments of the rivers, lakes, and flooded terrestrial lowlands. The hypothesis that respiratory input of CO2 balanced by evasion (gas lost to atmosphere) is sufficient to explain high CO2 vapor pressures in the Amazon River appeared true from our calculations but needs further examination. Particular attention should be given to periods of rising water, when planktonic respiration appears to be two orders of magnitude greater than at periods of high water. Subsequent seasonal studies of the Amazon and other large rivers are needed to determine how the plankton community, the chemistry of terrestrially derived organics and their biological oxidation in water, and inorganic factors control CO2 supersaturation and exchange with the atmosphere.

Carbon Flow in Four Lake Ecosystems: A Structural Approach

Direct and indirect carbon fluxes in lakes Marion (British Columbia), Findley (Washington), Wingra (Wisconsin), and Mirror (New Hampshire) are compared, using budgets and input-output analysis. Overall differences in carbon flow between the lakes are shown with cycling indices of .031, .108, .572, and .661, respectively. The results suggest that lake ecosystems may be considered unique aggregatins of similar components.

Biological Responses of Lakes in the Mount St. Helens Blast Zone

Loadings of dissolved organics and suspended particulates from destroyed forests and volcanic debris produced by the 18 May 1980 eruption of Mount St. Helens altered the trophic structure of many blast zone lakes to the extent that anoxic conditions and chemoorganotrophic and chemolithotrophic microorganisms prevailed. High bacterial counts and high adenosine triphosphate concentrations were directly related to enhanced concentrations of dissolved organic carbon, and plankton chlorophyll a was inversely related to light extinction. The recovery of these lakes to the preeruption state appears dependent upon the oxidation of organics and the stabilization of watersheds.

Chemical Changes of Lakes Within the Mount St. Helens Blast Zone

Differences in the dissolved chemistry of lakes devastated by the 18 May 1980 eruption of Mount St. Helens are attributable to location relative to the lateral blast trajectory of the eruption and to the emplacement of mineral deposits. Elemental enrichment ratios of pre- and posteruption measurements for Spirit Lake and comparisons of the chemical concentrations and elemental ratios for lakes inside and outside the blast zone reflect the influences of the dissolution of magmatic and lithic deposits. The pH changes were minor because of buffering by carbonic acid and reactions involving mineral alteration, dissolved organics, and biological processes.

Riparian corridors of Eastern Oregon and Washington: Functions and sustainability along lowland-arid to mountain gradients

Abstract.Riparian corridors of eastern Oregon and Washington, like those in other regions, comprise small portions of river drainages but provide disproportionately important ecosystem functions. However, most riparian and fluvial (streams and rivers) systems have been greatly altered. Degraded ecosystems commonly reflect influences of land-uses (e. g., mining, logging, road construction, fire suppression, livestock grazing), hydro-developments (e.g., dams, irrigation, flood control) and other human actions. Some important consequences include: degradation and fragmentation of habitats, changes in riparian plant associations, isolated fish populations, and altered flow and sediment regimes. This synthesis paper evaluates some major environmental factors that can influence the sustainability of riparian corridors and fluvial systems along lowland-arid to mountain gradients within river drainages of eastern Oregon and Washington. Four tributary rivers of the Columbia River, the Grande Ronde and John Day Rivers in northeastern Oregon, and the Yakima and Methow Rivers on the eastside of the Cascade Mountains in Washington, provide perspectives on environmental conditions. Factors evaluated include: a) dominant riparian plant associations and distributions in relation to differences in precipitation and temperature regimes along elevation gradients; b) ecological and physiochemical functions of riparian and fluvial systems along elevation gradients; c) long-term historical and contemporary cumulative impacts of human actions; and d) management provisions that could restore and sustain ecosystem functions. Ecological functions of riparian and fluvial systems are viewed as being closely coupled because of their dependence on hydrological (surface and sub-surface) and sediment routing regimes. From a river landscape perspective, achieving greater connectivity can be a key objective for analyzing and integrating the management of riparian and fluvial ecosystems. Effective management should include ensuring the delineation of major limiting factors (e.g., erosion, water shortages and temperatures) and identification of streamside and channel networks that link critical habitats at multiple landscape scales (e. g., locations and spacing of refuge habitats for fish and wildlife). Management actions should encourage the connectivity of reaches and habitats and maintenance of riparian and fluvial functions so interactions can occur. Efforts should include renewal of natural flood and sediment routing regimes and the reestablishment of habitats adjacent to ecologically intact habitats.

SOURCES AND INFLUENCES OF ALLOCHTHONOUS INPUTS ON THE PRODUCTIVITY OF A SUBALPINE LAKE

Allochthonous and autochthonous inputs of soluble reactive phosphate, dissolved organic P, particulate P, dissolved inorganic N, total N, dissolved organic C, and particulate organic C were measured seasonally over a 2—yr period in a subalpine lake of a coniferous forest. Such data have not previously been collected for this lake type. Using a new error analysis technique, nutrient budgets were constructed and analyzed for patterns and relative importance of material flow pathways. Of total annual particulate organic C inputs to the water column of 15.9 g/m2, °83% was allochthonous, mainly from fluvial and snowpack inputs in the spring and litter inputs in the fall. Annual allochthonous inputs of soluble reactive P and dissolved inorganic N averaged 0.12 g/m2 and 2.24 g/m2, respectively; spring fluvial sources provided 33%—50% of these annual incomes. Zooplankton excretion of soluble reactive phosphate was comparable to allochthonous inputs of soluble reactive phosphate, and °18% of total inputs of dissolved inorganic N was autochthonous. Particulate losses were assignable to both fluvial and sedimentation processes, whereas °70% of soluble reactive phosphate and dissolved inorganic N incomes were lost through the outflow and the rest to uptake. Allochthonous dissolved organic carbon inputs (68 g/m2) and allochthonous dissolved organic P inputs (0.21 g/m2) were mostly fluvial and °100% were lost fluvially. These data are related to those of other lakes, and patterns and apparent anomalies are discussed. These results suggest that terrestrial inputs to the sediments, and the subsequent cycling of nutrients in the sediments, have a major influence on biological activity in both benthic and water—column environments during most of the growing season.

Habitat Factors Affecting Sockeye Salmon Redd Site Selection in Off-Channel Ponds of a River Floodplain

Abstract Sockeye salmon Oncorhynchus nerka exhibit diverse use of rivers, lake beaches, sloughs, and floodplains for spawning. Their ability to use these habitats has been reduced by loss of freshwater habitat. We investigated redd site selection by sockeye salmon in two off-channel ponds (one naturally created and one man-made) of the Cedar River, Washington, which has lost floodplain habitats as a result of flow regulation, flood control, and urbanization. The purpose of the study was to identify habitat preferences of adult sockeye salmon in off-channel ponds and provide insights for preserving and restoring off-channel areas. We examined the influence of water depth, subsurface flow, substrate, water temperature, bank cover, detrital depth, distance to shore, and woody debris on redd site selection using geographic information systems, logistic regression, and electivity indices. Redds were most frequently constructed in areas with upwelling water, moderate water depths (10–80 cm), and gravel or cobbl...