THE FLOW STRESSOR-RESPONSE METHOD Jay O’Keeffe, WWF Professor of Freshwater Ecosystems, Department of Environmental Resources

South African rivers Semi-arid Flashy Unpredictable Highly variable Incised channels Few with floodplains No/few commercial fish

DEFINITIONS OF STRESS

There are lots of different types of stress Stresses due to: Flow,Water quality, Temperature,Predation, Competition, etc. etc. In this method, we are dealing initially with stresses caused by low flows. We will also investigate the quite different stresses caused by high flows to see if the method can usefully be extended to deal with these.

OUR DEFINITION

THE CONCEPT OF LOW-FLOW STRESS River processes, functions and components are best adapted to their natural flow regime The flow regime is a main driver of the ecological disturbance regime, which drives much of the biodiversity of a system Natural biodiversity = Ecosystem goods and services, and stability and resilience of ecosystems

BUT People need the water resources By reducing the flow, they increase the low- flow disturbances in the system Natural biodiversity, goods and services, stability and resilience are reduced (its ecological health is reduced) The ecological risk to the system is increased, it becomes an increasingly stressed system We use fish/inverts/veg/sediment processes as indicators of the overall stress in the system

SOMETIMES The low flows are increased (IBT’s, return flows) Natural low-flow disturbances in such systems are decreased, and stress is removed Habitat diversity in time and space is decreased Some species are advantaged to the detriment of others Biodiversity etc is decreased.

Stressors (decreased flow, reduced habitat), cause stress responses (reduced abundance, increased risk, death) Both biotic and abiotic components can be stressed, as can the whole system Natural stress levels will vary from system to system (temporary streams will have much higher natural stress levels than permanent streams) The desirability (or otherwise) of stresses in a system can only be judged in relation to the natural (or reference) stress regime USE FOR THIS METHOD

StressStress  % Duration  Natural (A)  Inc. Flow (C)  Decreased  Flow (C)  Assessing increased and decreased stress

HYDRAULIC ASSUMPTION  The structure and resultant hydraulic complexity of most natural river channels ensures that, when there are areas of fast deep flow, there will also be areas of other hydraulic habitats (eg fast shallow, slow shallow, slow deep and no flow) – and therefore conditions suitable for a variety of species

A Generic Stress Index An index of 0 to 10 where 0 indicates no stress, and 10 the highest level of stress Stressors: Flow-related hydraulics and habitat Biological responses: Reduced abundance (1 to 3), increasing risk to critical life stages (4 to 6), and disappearance of populations (7 to 10) Flow-related hydraulics: Velocity, depth and wetted perimeter Habitat: Quantity and quality (the diversity and connectivity of habitat types)

Site specific discharge (m 3 s -1 ) 1 Stressors 2,3 Flow-related hydraulics 4 Physical habitat Very fast Very deep Very wide WP In excess Very high quality Fast Deep Wide WP Plentiful High quality Fast Deep Wide WP, slightly reduced Critical habitat sufficient Quality slightly reduced Moderate velocity Fairly deep WP slightly/ moderately reduced Reduced critical habitat Reduced critical quality Moderate velocity Some deep areas WP moderately reduced Critical habitat limited Moderate quality Moderate/slow velocity Few deep areas WP moderately/very reduced Critical habitat very reduced Moderate/low quality Moderate/slow velocity No deep areas Narrow WP Critical habitat residual Low quality Slow Shallow Narrow WP No critical habitat Other habitats moderate quality Slow Trickle Very narrow WP Flowing water habitats residual Low quality No flowStanding water habitats only Very low quality No surface waterOnly hyporheic refugia 5 Biological responses of target organism(s) Stress Index AbundanceAquatic Life Stages 8 Persistence 0Very abundantAll 7 healthyYes 1AbundantAll healthyYes 2Slight reduction for 6 sensitive rheophilic spp All healthy in some areas Yes 3Reduction for all 6 rheophilic species All healthy in limited areas Yes 4Further reduction for all rheophilic species All 7 viable in limited areas, critical life- stages of some sensitive rheophilic species at risk Yes 5Limited populations of all rheophilic species Critical life-stages of sensitive rheophilic species at risk or non-viable Yes 6Sensitive rheophilic species rare Critical life-stages of sensitive rheophilic species non-viable, and at risk for some less sensitive species In the short-term 7Most rheophilic species rare All life-stages of sensitive rheophilic species at risk or non- viable Most sensitive rheophilic species disappear 8Remnant populations of some rheophilic species All life-stages of most rheophilic species at risk or non-viable Many rheophilic species disappear 9Mostly pool dwellersAll life-stages of most rheophilic species non- viable Most or all rheophilic species disappear 10Only specialists persist Virtually no development Only specialists persist

The process for applying the FS/R method Select a site, survey and model hydraulic characteristics Specialists apply the stress index to the site, in relation to selected species/groups, to develop stress curves The hydrologist converts the resulting critical stress curve to stress time series for flow scenarios Analyse the stress profile of each scenario in terms of the magnitude, frequency and duration of different stresses Assess the severity of each stress profile in relation to the natural stress profile

AN EXAMPLE OF A SITE SPECIFIC STRESS INDEX FOR INVERTEBRATES

Black = Natural, Red = Recommended, Blue = Present day

Blyde River spell analysis (Stress = 1.5) White = natural, Red = recommended, Blue = Present day

Fish Fish and inverts Fish Rip Veg

OBJECTIVES HIERARCHY Overall objective (Ecological category A to D) General flow objectives Component objectives Target species objectives +Descriptions of conditions for classes below and above the objective category

EXAMPLE OBJECTIVES (e.g. to maintain Category B) Maintain perennial flow Summer flows > winter flows Av. velocity > 0.1 m/s at all times for target fish(stress never > 6) Sufficient depth (30 cm) to allow target fish to feed and breed 85% of time in summer (stress < 1 for 42.5% of time) Av. velocity > 0.3 m/sec for 80% of time, to ensure Trichoptera habitat (stress < 3 for 80% of time) Wetted perimeter in reeds for 60% of summer to provide marginal habitats (stress < 0.8 for 30% of time)

Application of objectives to define ecological category B

Thukela IFR4 Based on dry season flows BB/C

ADVANTAGES OF THE FS-R METHOD Biological responses treated as a continuum rather than as thresholds Analysis includes frequency and duration as well as magnitude Links between flow and biological responses are explicit Analysis of any number of flow regimes is possible and repeatable The natural stress profile provides a reference condition

FURTHER DEVELOPMENTS Apply method to high flows Parallel development of FS-R method for water quality Refinement of stress index Analysis and comparison of stress profiles Regionalised stress/response relationships

REFERENCE: O ’ KEEFFE J H, HUGHES D A AND THARME R E (2002) Linking ecological responses to altered flows, for use in environmental flow assessments: The Flow Stressor-Response method. Proceedings of the International Association of Theoretical and Applied Limnology, 28:84-92.