Spatial and temporal variability within marine isoscapes: implications for interpreting stable isotope data from marine systems

Analyses of intrinsic biogeochemical markers, such as stable carbon (delta C-13) and nitrogen (delta N-15) isotopes, in animal tissues are used to investigate multiple ecological parameters for marine species. Their successful application relies on a mechanistic understanding of isotopic variations at the base of the food web because those variations influence isotope values in higher trophic level species. To better determine the potential for and mechanisms driving spatial and temporal changes in isotope values from an oceanographically complex nearshore marine system, we (1) constructed fine-scale delta C-13 and delta N-15 isoscapes of the Southern California Bight (SCB) using isotope values from particulate organic matter (POM) collected over 5 seasons from +/- 30 stations and (2) related the isotope data to geographic, seasonal, and oceanographic parameters collected from the same stations via a multimodel procedure and regression analyses. Important variables for predicting the delta C-13 and delta N-15 values from the POM included chlorophyll a, latitude, and season, and longitude, season, nitrate, and oxygen, respectively. We related these variables to seasonal shifts in nutrients most pronounced around localized eddies that concentrate upwelling. The potential for such variability should be considered when interpreting small-scale geographic and/or seasonal patterns in isotope data from species in the SCB and other dynamic coastal waters. However, the overall isotopic variability for the SCB was relatively low, with mean (+/- SD) delta C-13 and delta N-15 values of-22.7 +/- 2.0% and 8.0 +/- 1.5%, respectively, allowing for isotopic categorization of the SCB and comparison with other Eastern Pacific coastal areas to better understand larger-scale animal migration patterns, foraging ecology, and habitat use.