Exxon Valdez oil spill [EVOS] legacy: Synthesis of long-term ecosystem responses

Riki Ott, Charles H. “Pete” Peterson & Stanley “Jeep” Rice

Theme: Chronic effects of decade-long contamination of key shoreline habitats and indirect interactions are important

1994 EVOS oil, Prince William Sound, AK

Delayed, chronic, and indirect effects of shoreline oiling/treatment

• Treat EVOS as an ecosystem perturbation

• Capitalize on vast research effort

• Synthesis focused on shoreline habitats

• Contrast of NRDA based on old ecotoxicity risk models vs. field-based sampling

Assumptions about oil toxicology in 1989

•Alaska's Water Quality Standard for PAHs was 10 ppb and provided conservative protection of natural resources

•Oil toxicity declined very rapidly in a matter of days/weeks

•Acute toxicity tests of lab animals adequately predicted risk

•Contact with feathers and fur was the only significant route of injury to birds and mammals

•Oil spill impacts could be assessed on a species-by-species basis with no regard for dependencies within the ecosystem

Contrasting terms

• Acute vs. Chronic exposure (= short- vs. long-term)

• Lethal vs. Sublethal impacts (= mortality vs. growth, reproduction, body condition)

• Immediate vs. Delayed response (= rapid vs. postponed)

• Direct vs. Indirect effect (= A B vs. A C B)

• Trophic cascade vs. Biogenic habitat loss (= change in predator affects its prey, which affects its prey, etc. vs. change in an organism that provides structural living space for other organisms)

Benefits of EVOS field sampling approach

• Employs statistical sampling design• Integrates responses across all

mechanisms• Includes chronic effects on long

time scales• Includes interactions of oil and

other stressors• Includes indirect interactions from

trophic cascades, habitat modifications, etc.

Synthesis of long-term ecosystem

responses

• Shows old assumptions of oil ecotoxicity to be inadequate

• Weathered oil persists

• Weathered oil remains bioavailable in important environments

• Weathered oil (multi-ring PAHs) induces toxicity during chronic exposure

Water column PAH contamination after EVOS

• Low (1-8 ppb) even during weeks 1-5 in 1989

• Essentially below detection by end of summer 1989 using traditional water sampling

• But filter-feeding mussels revealed exposure and bioavailability even into summer 1992 via filtration of contaminated particulates

Persistence of oil

• Asphalts high on shore - biologically inactive

• Biologically available pockets in protected sites for >10 yrs

– Under armor of mussel beds with relatively unweathered oil contaminating mussels at least into summer 1994 at study’s end

– In groundwater of deltas of anadromous streams yrs later

– In sediments among boulders on oiled beaches for yrs

• Transported on particles to shallow subtidal where elevated PAHs persisted until 1995 at study’s end

Is the oil still there?

2001 Survey Results:

91 sites with 9,000 total pits

- 53 sites with oil - 38 sites without oil

100 Miles

Prince WilliamSound

Alaska

Gulf ofAlaska

N

Distribution of oil 12 years later

Upperintertidal

Biologicalzone

(lower intertidal) Oil below sampling grid = YesHow far down = ?

Tidalzone (m)

+ 4.8+ 4.3+ 3.3+ 2.8+ 2.3+ 1.8

< 1m

Surface oil(# of pits)

375658604029

5286991

123117

Subsurface oil(# of pits)

Shoreline treatments

• Intense in 1989, 1990 summers with some extending into 1991

• Invasive including wiping surfaces, pressurized washes with hot and cold water, bioremediation, rock washing, tilling, and berm relocation

• Had major impacts on shoreline habitat, plants and animals

Direct effects on rocky intertidal species• Fucus removal - high and mid

shore• Limpet Tectura persona decline

high on shore• Balanoid barnacle decline• Blue mussel decline• Periwinkle Littorina sitkana

decline• Drilling predator Nucella

lamellosa decline• Main cause - pressurized washing

Indirect effects on rocky intertidal community

• Modest bloom of ephemeral algae in absence of Fucus competition and with low grazing

• Absent nearby canopy, Fucus spore arrival is limited and recruits desiccate high on shore

• Opportunistic barnacle colonizes heavily

• Fucus colonizing barnacle tests is uprooted

• Fucus expansion into low shore inhibits red algae

0

100

0

100

0

100

unoiled

oiled & not cleaned

oiled & cleaned

1990 1991 1992

% c

over

Fuc

usR

ed a

lgae

Possible indirect effects on rocky

intertidal

• Potential induction of unstable cycle in Fucus cover as single-aged colonist plants senesce in synchrony 5 yrs later

• Reduction of biogenic habitat normally provided by Fucus and blue mussels impacts gastropods and smaller invertebrates