Climate Change Impacts on Ocean Ecosystems

Over recent decades, marine scientists have detected widespread poleward shifts in species distributions that are consistent with patterns of a warming ocean (Alheit and Hagen, 1997; Holbrook et al., 1997; Mueter and Litzow, 2008; Sagarin et al., 1999; Southward et al., 1995). Marine species can be highly mobile, both as adults and as microscopic young drifting in the plankton (Kinlan and Gaines, 2003). This mobility can lead to larger and faster geographic shifts than in terrestrial ecosystems. For example, two-thirds of the 36 most common bottom-dwelling fish in the North Sea have shifted the geographical center of their range north toward the pole over just 25 years (Perry et al., 2005) (Figure 9.2). Such shifts, if they continue, could move the fish beyond the range of national fisheries. More broadly, because species move at different rates depending on their unique life histories, such shifts could lead to rapid rearrangements of the species composition of some ocean ecosystems (Cheung et al., 2009). The unpredictability of responses by different species is a key barrier to anticipating and adapting to the resulting ecosystem rearrangements.

Given the prominent role of oceans in storing carbon, climate impacts on ocean productivity could also alter their role in the carbon cycle. Overall, oceans contribute roughly half of the globe's net primary productivity (NPP; Field et al., 1998), defined as the net carbon gain by ecosystems over a specific time period, typically annually. Some ocean habitats (polar seas, coastal upwelling systems) may see increased productivity under projected climate change (Arrigo et al., 2008; Bakun, 1990; Behrenfeld et al., 2006; Pabi et al., 2008; Polovina et al., 1995; Snyder et al., 2003). Most of the ocean, however, is permanently stratified with shallow, warm, nutrient-depleted water isolated from cold, nutrient-rich water below. In these seas, warmer surface temperatures generally decrease phytoplankton productivity (Figure 9.3). Given the prominence of these stratified seas, a substantially warmer ocean would "inevitably alter the magnitude and distribution of global ocean net air-sea carbon exchange, fishery yields, and dominant ... biological regimes" (Behrenfeld et al., 2006).

Just as on land, high-latitude marine ecosystems may experience more stress than lower-latitude marine ecosystems, since rates of warming are higher (Gille, 2002; Hansen et al., 2006) and the opportunity for poleward range shifts is limited. Sea ice

FIGURE 9.2 Observed northward shift of marine species in the Bering Sea between the years 1982 and 2006. Length of the yellow bars indicates the distance that the center of a species range has shifted. The average shift among the species examined was approximately 19 miles north of its 1982 location (red line). The northward shift is primarily linked to warming of the Bering Sea during this period. SOURCES: Mueter and Litzow (2008) and USGCRP (2009a).

FIGURE 9.2 Observed northward shift of marine species in the Bering Sea between the years 1982 and 2006. Length of the yellow bars indicates the distance that the center of a species range has shifted. The average shift among the species examined was approximately 19 miles north of its 1982 location (red line). The northward shift is primarily linked to warming of the Bering Sea during this period. SOURCES: Mueter and Litzow (2008) and USGCRP (2009a).

creates critical habitat for a diverse array of marine species, including many mammals and birds (Hunt and Stabeno, 2002). Major declines in sea ice thickness and extent have been observed in the Arctic (see Chapter 6) and are projected for the next few decades (Overland and Stabeno, 2004; USGCRP, 2009a). Ice dynamics, which are highly sensitive to climate, drive dynamics of ocean primary productivity, which in turn has impacts throughout the marine food web in ways that are not clearly understood (Moore and Huntington, 2008; Smetacek and Nicol, 2005). Declines in sea ice can lead to large blooms in phytoplankton (e.g., Arrigo et al., 2008; Pabi et al., 2008) and declines in production from benthic (seafloor) habitats. These changes alter both the food webs of animals that ultimately depend on these different sources of productivity, including humans (Grebmeier et al., 2006; Mueter and Litzow, 2008; USGCRP, 2009a), and the role of high-latitude ocean ecosystems in the carbon cycle. Although the details are highly uncertain, many high-latitude ocean ecosystems appear to be at the threshold of major ecosystem changes (USGCRP, 2009a), especially since climate-induced changes may soon be joined by new human uses and stresses (e.g., oil and mineral exploration, expanded maritime use, and new fisheries in the Arctic) made possible by reductions in sea ice.

Some of the most productive ocean ecosystems are coastal regions where winds push









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FIGURE 9.3 Relationship between changes in sea surface temperature and net primary productivity (NPP) from 1999 to 2004 based on satellite observations. Warmer ocean temperatures typically lead to reduction in the productivity of phytoplankton, which means that they remove less carbon from the atmosphere. SOURCE: Updated from Behrenfeld et al. (2006).

surface waters offshore and draw deep, cold, nutrient-rich waters to the surface (e.g., the west coast of North America).The nutrients fuel plankton blooms that support diverse and abundant food webs and fisheries.These upwelling regions may become even more productive under climate change if forecasts of increasing upwelling and favorable winds hold true (Bakun, 1990). Substantial increases in upwelling, however, can also have catastrophic consequences if the system crosses key thresholds (Chan et al., 2008; Helly and Levin, 2004). Deep ocean waters are typically extremely low in oxygen (hypoxic). Strong upwelling of deep cold waters can pull such hypoxic water onto shallow ocean shelves with devastating impacts on many marine species (Grantham et al., 2004). Hypoxia of coastal waters is more commonly associated with nutrient-laden runoff from land (NRC, 2000; Rabalais and Turner, 2001), but climate-driven changes in winds, ocean temperature, and circulation can cause similar devastation even in areas without runoff from land (Bakun and Weeks, 2004; Chan et al., 2008). The system can rapidly switch from high productivity to "dead zones," where most species cannot live. For example, this transition has recently occurred in summers off the coasts of Oregon and Washington (Chan et al., 2008). Over more than 50 years of observations in the 20th century, hypoxia was rare or absent from these near-shore waters. In the past decade, however, hypoxia has become common and caused major die-offs of coastal species. By 2006, these once highly productive waters were oxygen-depleted along much of the coastline as upwelling winds increased (see Figure 9.4).

In the tropics, warm temperatures pose a "bleaching" threat to corals. Coral reef ecosystems have been compromised by a diverse set of activities including overfishing, damaging fishing practices, eutrophication, and sedimentation, among others (USGCRP, 2009a). On top of these human-caused stresses, recent decades have brought an

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FIGURE 9.4 Hypoxia and anoxia in shallow waters.Values below 0.5 ml/l (left of black vertical line) represent severe hypoxia. Over the latter of half o9t4e 20th century, hypoxia was only found in deep waters. In recent years (red and greea), hypoxia hes extended into waters close to ths surface. SOURCE: Modified from Chan et al. (2008).

increase in widespread bleaching events, where corals eject their symbiotic algae in the face of extreme temperatures (Figure 9.5). In some cases, the bleached corals recover with new symbionts (Lewis and Coffroth, 2004). In other cases, the coral is killed. Periods of mass bleaching have occurred globally since the late 1970s (Glynn, 1991; Hoegh-Guldberg, 1999), with the most severe event in 1998, an El Niño year in which an estimated 16 percent of the world's reef corals died (Wilkinson, 2000). The extent of bleaching varies greatly among species and locations. Some of the variability is tied to the level of other human stresses, which argues for managing reefs for greater resilience to climate change by reducing other stressors (Hughes et al., 2003). The next subsection discusses ocean acidification, which serves as an additional and potentially devastating stressor to corals. Recent models of coral-symbiont dynamics suggest that adaptation could greatly reduce coral bleaching catastrophes if the pace of climate warming is not too rapid (Baskett et al., 2005).

FIGURE 9.5 Photos of corals under normal (top) and acidified (bottom) conditions. The bottom coral lack a protective skeleton (appearing as light yellow in the top panel) and are sometimes called "naked coral." SOURCE: Doney et al. (2009).

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