Examining sources of primary production and bottom-up limitations in nearshore ecosystems of the northeast Pacific Ocean using fish based indicators

Abstract

Our ability to forecast the fate of ecosystems and species hinges on an understanding of how biological systems respond to their environment. In this dissertation, natural indicators of diet (stable isotopes) and production (otolith growth increment width) in two common fishes were used to investigate energy pathways and biophysical relationships in nearshore kelp forests spanning two large marine ecosystems with contrasting oceanography, the upwelling system of the California Current and the downwelling system of the Alaska Coastal Current. Stable isotope analysis indicated high proportions of kelp-derived carbon in two common predatory fish, pelagic-feeding Black Rockfish and benthic-feeding Kelp Greenling. Routinely using both kelp and phytoplankton energy channels reflects strong benthic-pelagic coupling in nearshore marine food webs and may confer food web resilience to perturbations in either energy channel. A network of annual otolith growth chronologies were used to test bottom-up limits of production for nearshore systems and later explore other possible correlates of production. Results of hypothesis tests were consistent with bottom-up forcing of nearshore marine ecosystems, with light and nutrients constraining primary production in pelagic food webs and temperature constraining benthic food webs. A separate exploratory analysis indicated that biophysical relationships were common with (1-2 years) and without time lags and suggested that differences in the abundance and quality of prey influenced the growth of fish through bottom-up processes. The findings from the separate exploratory analysis were generally consistent with findings from hypothesis testing. Lagged relationships were consistent with increasing higher nearshore prey abundance during warm conditions in the Alaska Coastal Current and during cool conditions in the California Current in both pelagic and benthic food webs. Relationships without time lags indicated that benthic prey quality increased during warm conditions in both current systems, while the quality of pelagic nearshore prey increased during cool conditions in the California Current and warm conditions in the Alaska Current. Overall, results of this dissertation demonstrate that kelp provides a source of energy to higher trophic level predators and that continued warming will likely have a negative influence at lower latitudes first (e.g., California Current), while production in higher latitudes (e.g., Gulf of Alaska) may initially increase.