North Pacific Marine Science Organization

PICES Press Vol. 22, No. 2

OSM Session on “Identifying multiple pressures and system responses inNorth Pacific marine ecosystems”
by Ian Perry
Marine ecosystems of the North Pacific, both coastal andoffshore, are impacted by multiple pressures, such asincreased temperature, change in iron supply, harmful algal
bloom events, invasive species, hypoxia/eutrophication andocean acidification. These multiple pressures can act
synergistically to change ecosystem structure, function anddynamics in unexpected ways that differ from single
pressure responses. It is also likely that pressures andresponses will vary geographically. A key objective of thePICES FUTURE science program is the identification andcharacterization of these pressures to facilitate comparative
studies of North Pacific ecosystem responses to multiple
stressors and how these systems might change in the future.
This session had two primary objectives: 1) identify key
stressors and pressures on North Pacific marine ecosystems,
including comparisons as to how these stressors/pressures
may differ in importance in different systems and how they
may be changing in time; and 2) identify ecosystem
responses to these multiple stressors and pressures.
Objective 2 includes understanding how natural and humanperturbations may cascade through ecosystems, andwhether there may be amplifiers or buffers which modify
the effects of perturbations on marine systems. The overall
goal of this session was to contribute to the work of PICES
Working Group 28 on Developing Ecosystem Indicators toCharacterize Ecosystem Responses to Multiple Stressors
and to obtain an overview of the pressures being experienced
by North Pacific marine ecosystems and their impacts on
the marine ecosystems of the North Pacific.

Literature analyses of multiple stressors usually list
between 25 to 50 multiple stressors (Working Group 28 has
been working with an integrated list of about 20 stressors
for its comparative studies). Several presentations by
Working Group 28 members (Takahashi et al., Martone etal., Kulik, Samhouri et al., Zador and Renner, Perry et al.)provided descriptions of multiple stressors in North Pacificmarine ecosystems. The presentation by Perry et al.
concluded that the scientific community is beginning tounderstand issues of sensitivity and exposure of habitats tomultiple stressors (Fig. 3), but there is also consensus that alot of questions remain. Early analyses from Working
Group 28 suggest that there are more stressors, and greater
impacts, in coastal than offshore areas. However,comparative studies also suggest there may be a shorter listof important stressors at regional scales. In analysis ofscenarios of cumulative impacts along the coast of British
Columbia, Canada, Clarke-Murray et al. found climatechange impacts overwhelmed all other stressors.

In total, 15 papers were presented in session S1, plus one
by Isabelle Rombouts in a plenary session (Fig. 1). All
presentations demonstrated that multiple stressors arecommon, and that single stressors are rare (e.g., Fig. 2).

Fig. 1 Plenary speaker, Dr. Isabelle Rombouts addressing the audience.

Fig. 2 Example of multiple and cumulative stressors along an ecological gradient from freshwater to marine systems. From Won et al.

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PICES Press Vol. 22, No. 2

North Pacific Marine Science Organization
larger fishes than on smaller fishes, causing the ecosystemsize structure to shift towards smaller sizes. Their study,
based on two ecosystem models, indicated that impacts
from bottom-up stressors could range from moderate(–20%) to severe (–60%) depending on changes inphytoplankton. Del Raye and Weng identified a need forphysiological models that use aerobic scope for activity tounderstand interactions between temperature and O2 atdiscrete pCO2.
Based on the presentations and discussions, the session
reached the following conclusions: Ecosystem responses to multiple stressors are nonuniform: a suite of indicators is best to capture adiversity of ecosystem responses.
 Because a diversity of ecosystem responses is expected,
it is essential to clarify which types of ecosystemchanges matter to a pre-specified group of people.
 Interactions between multiple stressors more often
appear to be non-additive (synergistic or antagonistic);
there is the need to understand how predicted ecosystem
responses vary with different assumptions about
interactions between stressors (noting, however, that
there is no substitute for data).
 Climate and fishing provide good examples of how
interactions between stressors can act non-additively insome cases and additively in others to change thedynamics of exploited fish populations.

Fig. 3 Example of a risk plot (Exposure by Sensitivity) of multiple
stressors (20 stressors by 22 habitats) for the Strait of Georgia,
Canada. Color coding represents degrees of inferred relative risk.
Horizontal and vertical bars represent uncertainties derived
across multiple experts. From Perry et al.

Several presentations discussed options for developing
ecosystem indicators to characterise ecosystem responses
to multiple stressors. Boldt et al. outlined a number ofrequirements for such indicators. These include the need todefine strategic goals and ecological or management
objectives for these indicators, and the need for a suite ofintegrative indicators that would cover key components andgradients at the appropriate spatial scales. It was also
recognised that mechanistic approaches can give insights
into how pressures are likely to interact and how impacts
may become observable. The synthesis of indicator statusacross multiple trophic levels may reveal broad-scalechanges in the environment that may have importantbiological and management implications. For example,
upper trophic level organisms such as seabirds and halibut
may serve as integrative indicators that can provide nearreal time cues of environmental state (Zador and Rennerpresentation).

Different approaches may be needed for situations withdifferent degrees of complexity. For example, data-driven
evaluations are obviously to be preferred for situations
where data are available (in space, time, and types ofvariables). Expert opinion may be necessary when thefocus is on broad spatial scales, although care should betaken to verify these opinions with data or other experts
when possible.
Acknowledgements
I would like to thank the other S1 co-convenors, Vladimir
Kulik, Rebecca Martone, Jameal Sambouri and Motomitsu
Takahashi for their contributions in organizing and chairing
this session.

Multiple stressors might interact in additive, synergistic, orantagonistic ways. An analysis of interaction type from 171
studies that manipulated 2 or more stressors found that
26% identified additive interactions, which are mostcommonly used in model studies of stressor interactions,but that 36% and 38% of the studies identified synergistic
or antagonistic interactions, respectively (Crain et al. 2008,
Ecology Letters). Examples presented during this sessionincluded the paper by Jung, who concluded that intensive
fishing activities by Korean trawlers could have aggravatedthe potential resilience of the filefish stock, causing it tocollapse when the climate changed; and the paper by
Polovina and Woodworth-Jefcoats, who concluded that
top-down responses in the Central North Pacific ecosystemmeans that fishing and potentially bottom-up climate
impacts are likely to have stronger negative impacts on theSummer 2014

Dr. Ian Perry (Ian.Perry@dfo-mpo.gc.ca) is a research scientistwith Fisheries and Oceans Canada at the Pacific Biological
Station in Nanaimo, British Columbia. His research includes thestructure and function of marine ecosystems and the developmentof ecosystem approaches to management of these systems.

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