Climate Change Is Also Affecting Ocean Ecosystems

Just as on land, ranges of many marine animals have shifted poleward in recent decades (Figure 2.8). The pace of these changes can be faster in the sea because of the high mobility of many marine species. Changes have also been observed in ocean productivity, which measures the photosynthetic activity of organisms at the base of the marine food web. Model projections suggest that some habitats, such as polar seas and areas with coastal upwelling, may see increases in productivity as climate change progresses. The majority of ocean areas, however, are projected to experience declines in productivity as warm, nutrient-poor surface water is increasingly isolated from the colder, nutrient-rich water below. Even in highly productive coastal upwelling systems, it is possible that even stronger upwelling could draw up deeper, low-oxygen (hy-

FIGURE 2.8 Observed northward shift of marine species in the Bering Sea between the years 1982 and 2006. The average shift among the species examined was approximately 19 miles north of its 1982 location (red line). For further details see Figure 9.3. SOURCES: Mueter and Litzow, (2008); USGCRP (2009a).

FIGURE 2.8 Observed northward shift of marine species in the Bering Sea between the years 1982 and 2006. The average shift among the species examined was approximately 19 miles north of its 1982 location (red line). For further details see Figure 9.3. SOURCES: Mueter and Litzow, (2008); USGCRP (2009a).

poxic) water, creating dead zones where few species can survive. Such hypoxic dead zones have recently appeared off the coasts of Oregon and Washington.

Continued losses of sea ice and stronger warming at higher latitudes are expected to drive major habitat alterations in Arctic ecosystems. Ice dynamics plays an important role in ocean productivity, and sea ice is a critical habitat for many species, including birds and mammals. Although the details are uncertain, polar ecosystems are at the threshold of major ecosystem changes due to climate change. Without careful management, these changes may be exacerbated by expanding human uses in polar seas as sea ice continues to decline.

In the tropics, warm temperatures pose a bleaching threat to corals. Corals are animals, but they depend on algae growing in their tissues for much of their nutrition. This tight symbiotic relationship can be disrupted by extreme temperatures, which can cause corals to eject the algae and "bleach." Mass bleaching events, which often lead to coral death, have occurred with increasing frequency over recent decades associated with severe warming events. In the most extreme case, the strong El NiƱo event of 1998, an estimated 16 percent of the world's coral reefs died. Models suggest that the fate of corals under future warming scenarios depends critically on the pace of warming.

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