The final step in the development of the PWS CEM was to assess the strength of the direct effect of each natural or anthropogenic stressor on each VEC if that stressor occurred at the magnitude and spatial extent that are appropriate to the PWS ecosystems or plausibly could occur in the future ( and ). We assigned levels of High (H), Medium (M), Low (L), or No (-) to these direct cause-effect linkages, following the criteria discussed previously. However, indirect effects clearly are also very important to the ecosystem, where an effect on one VEC is propagated via the trophic structure to effects on one or more other VECs. To capture the pathways for those indirect effects, the trophodynamic model () illustrates how a direct stress-effect relationship between a stressor and a VEC can cascade through the ecosystem to cause indirect effects on other VECs higher in the food web. For example, while a change in nutrients has a major influence on the productivity of phytoplankton, there is no direct effect on planktivores. However, the indirect effect on planktivores is very large: since the plankton producers are the food base of the planktivores, if plankton are depleted, then planktivores are as well. Thus, we integrated the trophodynamical model into the CEM as a new protocol for this class of CEMs (i.e., the risk-based, stress-response construct) to capture both the functionally important ecosystem attributes and the indirect effects of stressors on VECs.
The strength of relationships between the natural stressors and the VEC categories in PWS.
The strength of relationships between the anthropogenic stressors and the VEC categories in PWS.
We recognize that a combination of multiple anthropogenic and/or natural stressors might result in synergistic effects on the ecosystem. For example, a population or biological process might be marginally stressed by the influence of a single stressor. If, however, an additional stressor is added concurrently, the combined effects of both might have more significant consequences on the population. Thus, the effects of stressors might not be simply additive but combine nonlinearly. Any such synergistic effects by necessity are not captured in the matrices or the graphical conceptual models. Nevertheless, we believe the major direct and indirect effects of natural and anthropogenic stressors on the PWS ecosystem are represented in the conceptual models.
By describing the full suite of direct and indirect linkages from drivers to stressors to ecological effects, one can ascertain the relative importance of natural and anthropogenic processes in shaping the condition of the PWS ecosystem. These relationships are organized by driver category and captured graphically in and . These CEMs are constructed with the upper tier (rectangles) representing natural or anthropogenic drivers. These lead to the environmental stressors (ovals) shown at the second tier in these figures. Note that for clarity of presentation, each individual human activity is not shown, with the exception of an oil spill and its clean-up activities; however, more complex and detailed CEMs could readily be developed based on the information in the tables.
Figure 3. The graphical Conceptual Ecosystem Model (CEM) for Prince William Sound for the natural drivers. The information in the figure derives from and . The top tier (rectangular boxes) indicates the specific natural driver for the CEM. (more ...)
Figure 4. The graphical Conceptual Ecosystem Model (CEM) for Prince William Sound for the anthropogenic drivers. The information in the figure derives from and . The top tier (rectangular boxes) indicates the specific anthropogenic driver (more ...)
Using the same CEM, these causal relationships could also be graphically represented by focusing on a particular stressor (i.e., showing all the drivers leading to a particular stressor and all the VECs it affects) or on a particular VEC (i.e., showing all the causal drivers and stressors that put a selected VEC at-risk). In general, for a selected process/driver, we show the resulting dominant stressors in bold red and high-level stressors in blue. Medium-strength relationships are shown in normal black font, and low or no relationships are shown in background.
Since these relationships are aggregated at the driver level, differences at the subcategory level of drivers or processes cannot be shown; within a category in the graphical model, we show the relationship for the highest of the subcategories. Also, to keep each graphic more manageable, we do not show the specific pathways of stressors to effects, other than indicating which effects are direct (normal font) and which are indirect (italicized font); however, those specific pathways can be discerned from the matrices ( and ) and could readily be shown in a more-detailed CEM.
Selecting the level of aggregation or detail to represent in a CEM is always challenging and relates to the targeted audience for the CEM. For the purposes of this CEM, we chose a middle-level of aggregation, aimed at both scientists and at decision-makers. The CEMs are sufficiently detailed to identify the major components of the PWS ecosystem, the types of stressors impinging on the ecosystem, and the direct and indirect linkages among these, and thereby to demonstrate to the scientific community that the CEMs are comprehensive and capture the important elements of the ecosystem. But the CEMs are not so detailed that the big picture of the relative importance of the various factors is lost, obscuring messages relevant to decision-makers. By accompanying the graphical CEMS with the somewhat more detailed matrices and, in particular, the much greater detailed discussion derived from the literature on how the PWS ecosystem works, this CEM should be useful fora variety of audiences, and those desiring more in-depth understanding are led to information sources that can provide that detail. The CEM could be disaggregated to illustrate more-detailed linkages for a particular activity or process, or could show the details of each specific causal pathways by which a particular stressor causes a particular response in a VEC. A next step in development for a CEM aimed at multiple audiences like this would be to create a set of nested CEMs that could cover more of the aggregation-disaggregation continuum, similar to the nested or hierarchical approach used in the CLEARS conceptual models of coastal Louisiana ecosystems (http://www.clear.lsu.edu/conceptualecologicalmodels
As is clear from the previous discussion based on the literature, climate processes have a dominating role in determining the condition of the PWS ecosystem, especially by influencing the salinity, temperature, nutrient, and sediment regimes (see ). Climate processes also have an important role in generating the stressors of physical disturbance, disease (e.g., viral hemorrhagic septicemia, VHS), and introduced species (e.g., species entering the system from warmer climates in response to global climate change). Nutrients especially control pelagic primary productivity, and nutrients in turn are significantly affected by winds and the salinity and temperature regimes through mechanisms such as upwelling, circulation, and vertical mixing, as discussed previously. Nutrients do not have a direct effect on secondary producers and other animals, but because of cascading effects, there is a dominant indirect effect throughout the pelagic trophic structure. Nutrients have an important role in affecting benthic primary productivity, but not as intensely as the control over the phytoplankton producers; consequently the right side of the trophodynamic model (benthic VECs) is shown in blue rather than the red of the pelagic system. Again, effects on benthic macrophytes lead to indirect effects on the rest of the benthic-based trophic structure. Suspended sediments affect the ecosystem significantly, through both reduction in sunlight available to the primary producers and impacts on filter feeders.
Physical/chemical oceanographic processes play a similar dominating role (), particularly as mediated through the nutrient regime (and hence salinity and mixing regime). As we have seen, the climatic and oceanographic processes are themselves tightly coupled, so are quite similar. Because of this coupling of climate and physical/chemical oceanographic processes and the importance of each to the structure and functioning of the PWS ecosystem, the regional physical processes of the North Pacific, including the GOA, operating over seasonal to interdecadal time scales, dominate the condition of the PWS ecosystem.
Watershed/geomorphological processes () also have a dominating role for PWS, through runoff effects on salinity and suspended sediments, but less controlling than the climate and oceanographic drivers. The greatest impact of geomorphological processes on PWS during the past century was the 1964 Great Alaska Earthquake, discussed in detail earlier. The earthquake instantaneously altered virtually all coastal habitats throughout PWS, with profound effects on the coastal forests, migratory-bird nesting grounds, salmon-spawning waters and gravels, and shellfish habitats and populations of the Sound. During this phase, the structural and functional components of the PWS ecosystem were forced to realign to the new physical conditions. However, over a period of time, the coastal habitats and associated communities became reestablished, and the overall PWS ecosystem had essentially recovered after a period of years to a decade or more.
() have an important but much more modest role in shaping the ecosystem, primarily through effects on nutrients and atmospheric deposition of chemical contaminants. These stressors directly or indirectly affect all of the pelagic and benthic trophic systems. Similarly, biological processes
() have an important but not dominating role affecting all of the ecosystem. Harmful algal blooms (HAB) and VHS have a particularly important role on the biota. Introduced species
is a difficult category to characterize because the vast majority of naturally occurring introduced species fail to become established, but in rare instances can change the system fundamentally (cf., Mooney et al. 2005
Only two anthropogenic drivers have a dominant role in the PWS ecosystem: resource harvesting, particularly over-exploitation of fish populations, and the immediate aftermath of a major oil (or potentially a chemical) spill, clearly illustrated by EVOS and its clean-up activities. Other human activities and associated stressors have a relatively minor role in determining the condition of the ecosystem compared to natural processes.
() primarily affects habitat alteration (e.g.,
replacement of coastal habitat with human settlements) and nutrients (e.g.,
release of nutrients from surface runoff in developed areas). These more modest stressors affect primary productivity of both benthic and pelagic producers, but the level of cascading effects through the ecosystem is much less than, for example, effects on primary productivity from oceanographic processes because the contribution of nutrients from this pathway is very small compared to natural sources. Development also leads to chemical contamination (e.g.,
from processing facilities), oiling (e.g.,
leaks from storage tanks), and solid wastes (e.g., Page et al. 1999
(), such as commercial fisheries and hatcheries, has a dominant role in affecting many marine ecosystems throughout the world (Jackson et al. 2001
; Hilborn et al. 2003
), and PWS is no exception. In this case resource harvesting directly affects the forage and large fish components and indirectly affects the marine birds and mammals that feed on them. For example, overfishing in the North Pacific may be a significant contributor to the reduction of some marine mammal populations like the Steller sea lion, as discussed previously (NRC 2003
; Hobson et al. 2004
; Miller et al. 2005
). Other harvesting historically had a dominant role, particularly the direct effect of harvesting marine mammals (e.g.,
sea otters, whales) by commercial or subsistence harvesters. Resource harvesting also affects biological competition (e.g.,
release of genetically homogeneous hatchery-reared fish) (Hilborn and Eggers 2000
) and physical disturbance (e.g.,
effects on benthic habitat from trawling) (Hilborn et al. 2003
). Harvesting may significantly enhance disease, for example, the suspected role of herring confinement in the spawn-on-kelp fishery in enhancing the incidence of VHS (Marty et al. 1998
; Hershberger et al. 1999
). Harvesting also impacts a number of important benthic invertebrate species of commercial and subsistence interest such as king, Tanner, and Dungeness crabs, shrimp, and bivalves. All of the commercially harvested crab and shrimp species have experienced regulatory closures or limited harvests in the last 20-30 years (Berceli and Trowbridge 2006
; Berceli et al. 2008
). While overharvesting is thought to be involved, other factors such as environmental changes may also impact population levels or recovery failures.
(), while a source of chemical contamination, noise, habitat alteration, and so on, is presently at too small a scale to cause dominant-, high-, or even medium-level stressors; thus this figure shows no relationships attaining the level of importance seen for many of the natural and other anthropogenicdrivers. There is the potential for this conclusion to change, however, as recreational tourism in Alaska has increased rapidly in recent years. As one indicator, Colt (2001)
reported that the number of summer visitors (May-September) arriving by air into Alaska increased between 1989 and 1998 from 150,000 to 450,000, and the number arriving by cruise ships doubled to ~600,000. After the highway tunnel to Whittier was completed in 2000, the number of visitors to PWS increased dramatically, and NWF (2003)
stated that 1.4 million people are expected to access PWS via Whittier by 2015. The EVOS Trustees reported 1.2 million visitors to PWS in 2001, double the number in 1989; the number of sportsfishers increased by 65% from 1989 to 1997 (EVOSTC 2005
). If these trends continue, concomitant increases in anthropogenic stressors on the system can be expected.
The immediate aftermath of an oil spill of the magnitude of EVOS is an anthropogenic driver that has a truly dominant role in shaping the PWS ecosystem (). In addition to the four stressors and their immediate catastrophic effects, discussed previously, EVOS adversely affected harvesting through the complete shut-down of the salmon fishery. In essence, much of the entire western PWS ecosystem was fundamentally adversely affected by EVOS, particularly by the direct oiling stressor and perhaps by toxicological stressors. Thus, this anthropogenic driver rose to a level of importance during the months and few years after the oil spill that is comparable to the climate/oceanographic drivers. While there may have been some indirect effects on VECs via the trophic structure, the immediate effects were predominately direct.
By contrast, the normal oil spill-related stressors prior to or well after a major spill () are substantially below levels that could cause even medium-level effects on the PWS VECs. While routine oil releases occur from boats and shipping, many petroleum storage facilities were breached by the 1964 earthquake (Kvenvolden et al. 1993
) and continue to release PAHs into the PWS environment (Page et al. 1995
), and remnant sources of EVOS oil remain in subsurface sediments (Short et al. 2006
; Michel et al. 2006
; Taylor and Reimer 2007
; Harwell et al. 2010
), the magnitude of the oiling, chemical contamination, and physical disturbance stressors caused by these sources are extremely low relative to their magnitude soon after the spill and relative to the various natural drivers/stressors associated with climate, oceanographic, and geomorphological processes, discussed above. In essence, other than in the months to few years after a major spill, oil spill-related stressors and effects are quite lost in the noise of natural variability in the climatic and physical oceanographic processes of the GOA and PWS and their associated effects on the biota.