Freshwater Ecosystems and Climate Change: Vulnerability and Adaptation

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Freshwater Ecosystems and Climate Change
Vulnerability and Adaptation

• N. LeRoy Poff
• Colorado State University

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Freshwater Ecosystems
Wetlands
Lakes
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Why should we care?

• valuable Ecosystem Goods and Services

(Poff et al. 2002)
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These ecosystem goods and services are being lost
and are under continuing threat
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Why should we care?
Naturally isolated nature of FW ecosystems
combined with natural barriers to gene flow has
facilitated great evolutionary diversification.

• Unrivaled Biodiversity

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Freshwater biodiversity is at disproportionate
risk of extinction
Vulnerable and Imperiled Species
(L. Master, TNC)
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What are the major threats to FW Ecosystems?

• Many interacting factors contribute to global
  biodiversity crisis and FW ecosystem degradation.

Climate change will exacerbate these continuing
threats.
(Dudgeon et al. 2006)
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Growing concern over climate change and FW
Ecosystem Impacts
(Chapter by Allan, Palmer Poff)
http//www.pewclimate.org/global-warming-in-depth/
all_reports/aquatic_ecosystems
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A key point

• Functional, self-sustaining ecosystems are needed
  to provide ecosystem goods services and to
  maintain native biodiversity.

Is this possible during climate change?

• Need to know how FW ecosystems function and how
  sensitive function is to climate change

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Fundamentals of FW ecosystem function that
provide context for vulnerability to climate
change
Vulnerabilities

• Dynamic environments
• Runoff amount, timing, extremes
• Temperature-dependent
• Ectotherms with temperature optima and limits
• Low points on landscape
• Concentrate nutrients and sediment
• Highly spatially structured
• Naturally isolated, with regional and endemic
  species

Hydrologic alteration
Thermal alteration
Pollution, Eutrophication
Fragmentation by dams
Invasive species
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Fundamental properties lead to two key principles
(1) FW ecosystem vulnerability will show regional
variation, reflecting natural differences in
regional climate (runoff and temperature) and
regional species composition.

• Dynamic environments
• Runoff amount, timing, extremes
• Temperature-dependent
• Ectotherms with temperature optima and limits
• Low points on landscape
• Concentrate nutrients and sediment
• Highly spatially structured
• Naturally isolated, with regional and endemic
  species

(2) Fragmentation (natural and human-caused)
constrains the ability of species to move as
local habitat conditions change. (Regional
variation in this as well.)
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Simple conceptual model linking climate change to
FW response
(? Vegetation ET)
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How much will temperature and precipitation
change?

• Depends on CO2 emission levels (IPCC scenarios)
• Regional variation will occur but models vary.

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How do temperature and runoff currently influence
the structure and function of aquatic ecosystems?
Streams Rivers
Lakes
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Climatic controls on LAKE structure and function

• ? Temperature
• Stratification and dissolved oxygen
• Metabolism and decomposition rates
• Ecosystem productivity
• Nuisance algal species
• Thermal habitat and fish species

• ? Runoff
• Lake levels
• Dissolved organic C and water transparency

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Stratification Dissolved Oxygen and Temperature
Summer conditions
Warmer, more O2
Cooler, less O2
Warmer air temperatures reduce volume of
hypolimnion  Productive lakes have less DO in
hypolimnion  Large cold-water fish require
well-oxygenated hypolimnion  Falling lake levels
reduce extent of productive littoral zone
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Evidence for Lake Warming - Reduced Ice Cover
since 1846
Since 1850  Freeze dates delayed an average
of 5.8 days per 100 yr Thaw dates occur on
average 6.5 days per 100 yr earlier
(Magnuson et al., 2000, Science 2891743-1746)
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What about the future? Some projections about
lake warming
10-year average lake temperatures (C) simulated
using the Canadian Climate Center Atmosphere
Ocean General Circulation Model (CGCM1) as input
data. Increased air temperature and reduced ice
cover increase water temperatures.
August control
August 2 x CO2
Summer (June-August) 2 x CO2 minus control
(Hostetler and Small, 1999)
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Water warming will alter fish species composition
in lakes and streams
(Mohseni et al., 2003, Climatic Change 59389-409)
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EUTROPHICATION Excess fertility leading to
excessive algal and plant growth
OPEN-WATER ALGAE
FIXED ALGAE
HIGHER PLANTS
Eutrophication can increase even without added
nutrients, if flushing rates are reduced. More
intense precipitation events, longer inter-rain
durations, and warmer waters are likely to favor
noxious blue-green algae and reduce water quality.
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Climatic controls on STREAM structure and
function

• ? Temperature
• Dissolved oxygen
• Metabolism and decomposition rates
• Ecosystem productivity
• Thermal habitat for species

• ? Runoff
• Natural disturbance regimes
• Baseflow conditions

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• Stream warming and loss of coldwater habitat for
  trout

Present day potential distribution based on 22
isopleth of mean July air temperature.
Future potential distribution based on 3
warming, showing a 49.8 loss of potential
habitat.
(Keleher and Rahel, 1996)
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Stream and river fish affected also
(Mohseni et al., 2003, Climatic Change 59389-409)
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Species expected to migrate to maintain thermal
preferences or tolerances

• Higher latitudes or altitudes
• - Alpine systems may be lost as headwaters retreat

• Migration requires connectivity, but will also
  depend on basin orientation

Many species of fishes in Great Plains streams
near thermal maximum and cannot move northward.
They have no refuge during a period of regional
warming.
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• Anthropogenic fragmentation severely limits
  movement

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Changes in Streamflow
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Streamflow regimea master variable

• Varies over time
• - Day to day, week to week, year to year
• - Inter-annual variation of wet and dry years
• Varies along a rivers length
• Varies regionally with climate

(Poff et al., 1997)
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Streams and rivers differ in natural flow regimes

• Key components that characterize a full flow
  regime and that have ecological importance
• Magnitude of discharge amount
• Frequency of events
• Duration
• Timing - regularity and seasonal predictability
• Rate of change

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Streams and rivers differ in natural flow regimes
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Hydrogeography of natural flow regimes in U.S.

• Offers context for considering regional
  variation in impacts of climate change on pattern
  of natural (background) variation

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Regional Vulnerabilities ?

• Will depend on regional manifestations of climate
  change.
• In general expect more variable and severe
  precipitation events
• Southwestern U.S. particularly vulnerable to flow
  reductions
• Altered timing of peak and ecological impacts
• Lower late-season baseflow and reduced water
  quality / less habitat
• Change environmental template and cause native
  species loss and exotic species spread

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Evidence for current changing flow regimes in
montane western snowmelt rivers
Spring pulse and center of mass of annual flow
(CT) over the period 1948-2002 show earlier onset
(10-30 days) throughout western North
America. Pattern not seen in coastal mountains.
(Stewart et al., 2005, J. Climatology181136-1155.
)
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Ecological implications Loss of riparian
cottonwood forests?

• Gallery forest in arid western river corridors

• Cottonwood forests provide numerous ecosystem
  services
• Nesting habitat for migratory songbirds
• Habitat diversity for terrestrial species.
• Food resources for riparian food webs.
• Aesthetics in arid landscapes

• High flows required to sustain cottonwoods also
  serve to rejuvenate in-channel habitats and
  promote native aquatic diversity.

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• The hydrological and geomorphic requirements of
  cottonwood are well understood
• Establishment Flows
• Flood Magnitude, Timing, Duration,
  Rate-of-change
• Survival flows baseflow

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There is widespread evidence for cottonwood
decline

• Dams alter frequency and timing of high flows
  required by cottonwoods. (An analogue to climate
  change?)

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Truckee River in Nevada
Winter 1977 showing effect of no flooding for
decades geriatric stand
Autumn 1997 after prescribed flooding to recruit
young cottonwoods
(Rood et al. 2004)
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Flood reduction and tamarisk invasion?
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Does flood alteration determine co-existence of
cottonwood and tamarisk?
Implication Human management is a critical
factor in sustaining cottonwood forests. (current
research)
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So, to wrap up General consequences of rapid
climate change for FW Ecosystems

• Climate change will exacerbate other agents of
  global change (land use change, invasive species
  spread, human demand for water) that will
  continue degrade FW ecosystems.
• FW ecosystems will passively react to climate
  change in response to altered environmental
  conditions.
• Ecosystem changes will result in
• Ecological disruptions shifting species
  distributions and food web changes, reductions in
  water quality
• Loss of many ecosystem goods and services now
  provided by FW ecosystems
• Accelerated species extinction
• Surprises!

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How to manage against climate change?

• Are there tipping points?
• These are not obvious from current empirical or
  modeling research
• Dams as analogues of climate change?
• Thermal changes below dams cause species
  extinctions and replacements
• warmwater ? coldwater fishes
• Flow modification causes change in riparian
  structure and function
• Cottonwood and Tamarisk example
• Fragmentation implicated in species declines
• Chinook salmon in Snake River

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How to manage against climate change?

• High uncertainty in exactly how FW ecosystems
  will respond to (equally uncertain)
  manifestations of climate change
• Despite uncertainties we know risks are great and
  consequences of no action is perilous to
  freshwater integrity
• We should aim to maintain functionality of FW
  ecosystems
• Natural process variability
• Sustain natural flow regime dynamics to the
  extent possible, even as these change over time.
• Regional context important regional variation
  in runoff dynamics mean regional differences in
  vulnerabilities to climate change.
• Manage for connectivity
• Severing the linkages between habitats in river
  and stream networks (and associated lakes and
  wetlands) diminishes the ability of species to
  respond fully to climate change.

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Key science challenges?

• Develop a better scientific understanding of
  geographically-distributed risk to FW species and
  ecosystems (research issue)
• Develop science-based conservation priorities
  based on regional risk and likelihood of
  successful interventions
• Develop models that capture fine scale
  hydro-geomorphic and ecological grain (river
  segment scale?) and in a river network context.
  Different from terrestrial systems in this
  regard!
• Develop better methods for quantifying
  ecosystem goods and services and the long-term
  ecological costs and benefits of different
  management options
• New paradigm for water management that
  proactively accommodates ecosystem needs at local
  to regional scales in order to build ecological
  resilience in the face of rapid climate change.

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Thank you and thanks to WWF Fuller Symposium
Some people who have shared ideas John
Matthews, Brian Richter, Julian Olden, David
Strayer, David Allan, Margaret Palmer Supporting
funding agencies USEPA, NSF, USFS, USGS