Effects of nutrient enrichment and mesoscale eddies on metabolic balance in the subtropical North Pacific Ocean

Abstract

The present study seeks to understand the influence of nutrient loading events on mixed layer community metabolism in the NPSG. Chapter 2 addresses the results of in vitro nutrient enrichment experiments where nutrient-rich deep water was mixed with oligotrophic mixed-layer water to assess the influence of nutrient enrichment on plankton metabolism. Production and respiration were measured based on the in vitro oxygen light/dark bottle method, and samples were taken for nutrient, pigment and flow cytometry analyses. The experiments were sampled daily to monitor chlorophyll concentrations, and community metabolism. The results of these experiments demonstrate that following nutrient enrichment, gross primary production increased significantly while respiration remained relatively constant, indicating that metabolic balance shifted from a state of net heterotrophy to a state of net autotrophy during the course of the experiment. These findings suggest that a nutrient loading event could cause a change in the net metabolic balance of a community. The results from these experiments are presented in Chapter 2. The second part of this thesis (Chapter 3) involved direct observation of nutrient enrichment events, specifically eddies. In winter 2004-2005, the Eddy Flux (E-Flux) project was conducted to examine the effects of eddy dynamics on local biogeochemistry, phytoplankton dynamics and carbon export. Three cruises were proposed to study the spin up, mature and spin down phases of an eddy's "life cycle" (Sweeney et al. 2003). During these cruises, a series of in vitro nutrient enrichment experiments were conducted inside and outside of the eddies. The results were used to calculate a threshold for net autotrophic community metabolism. Rates of water column primary production inside and outside the eddy were examined via the construction of photosynthesis vs. irradiance (P vs. E) curves. Phytoplankton physiological parameters, including the maximum rate of light-saturated photosynthesis (Ps), were determined from the best-fit solution of the data to the Platt et al. equations (1980). After conversion to units of oxygen, Ps was compared with the net threshold for net autotrophic community production, mentioned above, to infer community metabolism in the mixed layer inside and outside of the eddies as well as at the deep chlorophyll maximum layer (DCML) inside the eddies. The results indicate that understanding the influence of mesoscale eddies requires euphotic zone integrated primary production measurements. The experimental design, results and discussion are present in Chapter 3.