Chapter 4 – Long-Term Change

This chapter discusses the ecological change that occurs over different timescales. Long-term measures of physically important variables are salinity, precipitation, nutrients, stratification, currents, and coastal wind fields are relatively rare. Climate change, whether of natural or anthropogenic origin, brings with it uncertainty and the potential to upset existing economic and social. structures. The barrier to understanding long-term changes in the Gulf of Alaska (GOA) is the complex nonlinear structures that characterize these systems. The space and timescales of physical and biological variables are different from one another and the relation between a physical change and a biological response might be indirect, nonlinear, and involve several other system components. The observations are used in conjunction with models to evaluate model performance and/or to constrain model predictions and hindcasts. Variations have been detected in winds, surface heat fluxes and runoff, water temperature and salinity, mixed-layer depth, nutrient supply, and circulation properties. The temperature of the ocean at a given location is controlled by heat exchange with the atmosphere, mixing with waters of different temperatures, and the advection of heat into or out of a region by ocean currents. The long-term record of salmon abundance comes from two sources — the historical record of the fisheries and geochemical records in sockeye salmon lake sediments. Development of comprehensive life-stage-based models is one step toward understanding the causes of shrimp and crab population changes in the northern Gulf of Alaska.