Changes in Coastal Ecosystem in the NE Ocean
Harrison, Paul J.
Perry, R. Ian
|Source||Proceedings of International Symposium on Long-term Variations in the Coastal Environments and Ecosystems, Sept. 27-28, 2004, Matsuyama, Japan|
|Summary||Regime shifts and El Niño events can be viewed as natural large scale experiments that involve changes in salinity, temperature, nutrients, trace metals, and subsequent shifts in species composition in the ecosystem. The impacts of the 1997-98 El Niño event off the northwest coast of British Columbia revealed distinct changes in nutrients, phytoplankton and zooplankton. During the 1997-98 El Niño, the coastal area where the summer nitrate was < 1 µM, was 250,000 km2 larger than during the 1999 La Niña. Surface nitrate concentrations in 1998 were half of the concentrations in the 1970s. In contrast, during the 1999 La Niña, the mixed layer depth was 20 m deeper which increased the winter nutrient levels. Stratification was very pronounced in 1998 due to the increased temperature and this produced a bottom-up effect of a decrease in the productivity of the plankton communities through decreased nutrient concentrations (i.e. increased N limitation). During the 1997-98 El Niño, there was a lower total zooplankton biomass and shifts in community composition (reduced abundance of endemic boreal and temperate species and an increase in abundance of California species). Although the changes were most extreme in 1998, these changes in the zooplankton biomass and composition had been progressively developing off the BC coast throughout the 1990s. Therefore, even though the El Niño originates in the tropics, the effects are propagated poleward and there was a poleward shift of the west coast marine ecosystem, partially due to the stronger northward current flows. The 1977 regime shift demonstrated the importance of the carrying capacity of the ocean on fish stocks such as the Pacific salmon. Prior to 1977, attempts to rebuild fish stocks failed, but by 1985, catches exceeded historic levels because the 1977 regime shift increased the oceanic carrying capacity. After the probable 1999 regime shift, pink salmon, Pacific halibut, sardine, herring and ocean perch have increased in abundance. One plausible mechanism is that an increase in the oceanic carrying capacity and productivity after a regime shift may increase early marine growth and enable fish to reach a critical size and survive the first ocean winter when there is a high demand on energy stored by the fish.|
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