The River Continuum Concept (or not?) Stream Classification

Transcription

1 The River Continuum Concept (or not?) Stream Classification Goal: Generalization 2 general types of classification streams (longitudinal) streams (e.g., hydrologic regime classification) Longitudinal Classification What Changes? 1

2 transition zones Zonation (1 & 2 above) Convenient, works in many drainages Can account for discontinuities (e.g., foothill / plains transition) Hard to generalize broadly Continuum maybe more general? Zonation schemes 1) characteristics Width/depth, gradient, temperature, substrate, etc. 2) zonation Fish -- Huet (1949) 4 zones: Trout Grayling Barbel Bream 2

3 Zonation schemes 3) Combination of Physical and Biotic Illies (1961) and Illies & Botsaneanu (1963), a worldwide classification system Kryon = glacier brook (1-5 C) Crenon = spring * = stream (annual T range <20 C) * = river (> 20 C) Illies & Botsaneanu worldwide classification system General Categories Rhithron O 2 always high High gradient flow is fast and turbulent Coarse substrates erosional No plankton Macroinverts are cold stenotherms (lotic forms) cool-water, cold-water fish Potamon Warm stenotherms/ eurytherms Facultative rheophiles Some plankton rhithron potamon über classification Fig. 4.19, Ward 1992 crenon 3

4 The River Continuum Concept (RCC) Vannote et al. (1980) Continuum Idea of gradual change works in many drainages Textbook (Allan textbook, 1995): Bold attempt to construct a single synthetic framework to describe the structure and function of lotic ecosystems from source to mouth. Textbook (Allan & Castillo textbook, 2007): proven to be a resilient encapsulation of the relative roles played by different basal resources along an idealized river systems. Furthermore, the longitudinal distribution of functional feeding groups often, although not invariably, can be shown to be at least approximately in accord with expectations. The River Continuum Concept (RCC) General Idea in physical conditions and energy sources as you go from headwaters to mouth lead to biotic What are the predictable changes? Changes in canopy cover, energy sources, temperature, hydrologic regime What are the biotic adjustments? Biotic responses to available,, physical downstream species replacements and change in functional groups 4

5 Assumptions of RCC Headwater Streams 1) heavily-canopied with vegetation 2) fed 3) coarse substrate 4) 5) nutrient-poor 6) undisturbed (by humans) RCC Patterns/Predictions Break the continuum into segments (zonation?) 1) Headwaters (Orders 1-3) 2) Mid-reaches (Orders 4-6) 3) Large Rivers (Orders 7-12) *What are the main predictions for each of these size domains? 5

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7 Exercise Graph the following (given RCC assumptions): Physico-chemical Factors Substrate Size Bottom Light Diel T Annual Δ T Annual Δ Q Environmental Heterogeneity Energy Sources CPOM/FPOM Nutrient Levels Nutrient Availability (to 1 Producers) P/R Diversity (Richness) Benthos Fish Algae Macrophytes Plankton substrate size bottom light diel Δ T annual Δ T annual Δ Q Environmental Heterogeneity CPOM / FPOM Nutrient Levels Nutrient Availability P / R Benthic Diversity Fish Diversity Algal Diversity Macrophyte Diversity Plankton Stream Order Stream Order Stream Order 7

8 Evidence for RCC predictions? Grubaugh et al. (1996) Fig from text Criticisms of RCC Assumptions Some streams have substantial headwaters Some streams lack sources can 'reset continuum Can import CPOM, coarse substrate, cooler water, etc. Absence of in some canopied headwaters due to zoogeography (e.g., islands) Downstream sources of (floodplains) Historical alterations of large rivers create artifacts beavers floodplain disconnection Allan book: Perhaps the generality of the RCC is a handicap when it is applied to a multitude of specific situations. 8

9 Legacy / Importance of RCC Enduring Insights: 1) Stream organisms "predictably" structured along longitudinal resource gradients, reflecting changes in energy inputs temperature 2) Downstream communities depend on upstream processes Scales of application The very big differences (e.g., Headwaters vs. Mouth) are usually observable (e.g., CPOM vs. FPOM, shredders vs. collectors) But differences among sites along the continuum are often obscure due to local variation Main value, and limitation A conceptual framework for viewing whole river basins, with a focus on linear, upstream-downstream linkages RCC is a theory about the mean state of the system along the longitudinal profile, NOT the variance How to account for variation within continuum? tributary 9

10 How to account for variation within continuum? 1) Transition zones in Continuum Process Domain Concept Montgomery, D.R Process Domains and the River Continuum Concept. J. Am. Water Res. Assoc. 35: ) Tributary interruptions of Continuum Link Discontinuity Concept Rice, S.P., M.T. Greenwood, and C.B. Joyce Tributaries, sediment sources, and the longitudinal organisation of macroinvertebrate fauna along river systems. Can. J. Fish. Aquatic. Sci. 58: Network Dynamics Hypothesis Benda, L., N.L. Poff, D. Miller, T. Dunne, G. Reeves, G. Pess, and M. Pollock The network dynamics hypothesis: how channel networks structure riverine habitats. BioScience 54: Process Domain Concept Geomorphology controls disturbance regime which controls ecological organization Developed for higher gradient, montane streams Prediction: Aquatic communities will be similar in same domain even if stream size varies somewhat A B For example: compare A and B (same domain) versus A and C (same stream size but different domains) C Some evidence supporting, but not well tested. 10

11 Tributaries: Link Discontinuity Concept Tributary are sites along a main channel where, because of the introduction of water and (or), the water volume, bed sediment character, and water quality of the mainstream can change abruptly. Hypothesis: The arrangement of tributaries and related features is an important control on the longitudinal organisation of macroinvertebrate benthos at moderate spatial scales. 4 Lateral sediment sources (LSSs) are marked by arrows and delineate links in the Pine study reach (Rice et al. 2001) 11

12 D 50 Distance downstream (km) Some step-like changes in physicalchemical variables at tributary junctions. Some re-setting of benthic inverts below tributary junctions. Tributaries: Network Dynamics Hypothesis River network is a population of tributaries and their confluences. Tributaries interrupt river continuum inputs of water, sediment and organic material at tributary junctions varies in time depending on disturbance in the trib s watershed Degree of interruption depends on size of tributary relative to mainstem, which in turn depends on network geometry. Temporal cause confluence effects to wax and wane over time. Shape of network, coupled with watershed disturbance regime, influence habitat and therefore will also influence ecological. 12

13 Effects of disturbance (sediment inputs) in a tributary watershed on mainstem river floodplain lower upstream channel, meandering upstream; steeper gradient downstream deposition of woody debris and sediment overall increase in habitat An alluvial fan enlarged following a fire triggers tributary junction effects in the North Fork Boise River (320 km2 drainage area). Junction effects include expanded floodplains, and increased channel meandering and side channels upstream of the fan (from Benda et al Degree of heterogeneity is determined by size of sediment input (and also size of tributary relative to mainstem). Closely-spaced confluences also yield bigger effects. 13

14 Size of tributaries influences confluence effects (bigger in relation to mainstem bigger effect). Therefore, the basin shape is important in predicting the overall influence of confluence effects in the whole system. Comparisons of predictions River Continuum Network Dynamics Conclusion: accounting for tributary influences may help explain longitudinal deviations from the central tendencies predicted by a linear continuum hypothesis. 14