Modern northern fur seals are abruptly weaned at 4 mo. If northern fur seals begin to ingest solid food shortly after weaning and if recently weaned animals consume similar prey types as 1- and 2-yr-old juveniles, it takes approximately 8 mo for

the δ15N signal of weaning to be completely diluted by bone collagen turnover. Bone collagen δ15N values of these seals do not fully reflect those of their fish and cephalopod prey until animals are approximately 12-mo-old. For retrospective studies that use bone collagen to examine the timing and rate of weaning (abrupt vs. gradual), quantitative comparisons within and among species are possible if isotopic turnover rates and errors associated with see more age determination Lapatinib supplier are carefully considered (Newsome et al. 2007a). The isotopic composition of consumers in marine systems is ultimately set by the isotopic composition of the food and water the animal ingests. These inputs can show strong spatial isotopic gradients, consequently isotopic data can be used to study habitat preference (i.e., pelagic vs. benthic, nearshore vs. offshore vs. estuarine), movement among habitats, and migration patterns at an ocean basin scale. Here, we briefly discuss the factors that create isotopic gradients in marine systems. We focus on carbon and nitrogen isotopes,

and only briefly mention oxygen isotopes, which have primarily been used in paleontological studies. We then provide examples within two regions, the eastern North Pacific Ocean and Bering Sea. Through decades of experiments and field collections, oceanographers have come to understand the physicochemical and biological factors that are responsible Sitaxentan for the gradients in primary producer carbon isotope values. At the most general level, higher δ13C values are associated with rapid growth and lower values are associated with slow growth (Goericke and Fry 1994, Popp et al. 1998). Within oceanic

basins, therefore, primary producer (and particulate organic matter or POM) δ13C values track productivity, with higher values found in productive nearshore regions, such as upwelling zones, in comparison to less productive offshore regions. Because of the preferential uptake of 12C by plants during photosynthesis, nutrient-driven blooms in upwelling zones increase the δ13C of aqueous CO2 by a few per mil as they draw down its concentration. Low aqueous [CO2] can itself lead to lower isotopic fractionation during photosynthesis (and therefore higher plankton or macroalgae δ13C values). In offshore regions, especially in temperate and equatorial regions where the water column is strongly stratified, low nutrient levels lead to low growth rates, so these factors are less important and δ13C values are lower. The gradient in δ13C values between primary producers in nearshore vs.