Studies on the spatial variability of phytoplankton physiology and biomass in the Oregon upwelling system Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/2514nn89p

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  • The Oregon upwelling system is a region of high biomass and primary productivity as well as strong mesoscale variability. In order to examine the interaction of physical forcing and ecosystem dynamics, four 3-week sampling cruises were conducted in the Oregon upwelling system as part of the Northeast Pacific Global Ocean Ecosystem Dynamics (NEP GLOBEC). During each of the four cruises, which took place in June and August of 2000 and 2002, a series of crossshelf transects was completed in the region between 41.5° N to 45° N and -124° W to -126.5° W. Sea surface temperature, salinity, phytoplankton fluorescence, and Fᵥ/Fₘ, the theoretical maximum quantum yield of photosynthesis, were measured continuously from an underway flow-through system with an intake at 5 m depth. During all four cruises, temperature increased offshore, and salinity was high nearshore and low offshore, corresponding to upwelling conditions. Overall trends in Fᵥ/Fₘ were similar to patterns defined by temperature and salinity. Fᵥ/Fₘ was high near the coast and decreased further offshore, suggesting that phytoplankton were healthiest in recently upwelled water. Closer examination of individual transects revealed additional, small-scale variability in all parameters. Decorrelation analysis of 25-km transect sections indicated that this variability eccurred over 3 km on average. However, the scales of variability of Fᵥ/Fₘ were slightly shorter than those of temperature, salinity, or phytoplankton fluorescence. Overall, there were no trends relating the short scale variability of Fᵥ/Fₘ to that of any other parameter within a given transect, suggesting that short-scale variability in Fᵥ/Fₘ is not driven by temperature, salinity, or phytoplankton biomass, but by some other parameter or combination of parameters. Additional comparisons between ph ytopl ankton fluorescence and Fᵥ/Fₘ show high Fᵥ/Fₘ associated with high phytoplankton biomass and variable Fᵥ/Fₘ when phytoplankton biomass is low. This pattern is also reflected in comparisons between nutrient levels and Fᵥ/Fₘ, with Fᵥ/Fₘ high when levels of available nitrogen, phosphorus, and silica are high, and variable when nutrient levels are low, supporting the theory that nutrient limitation affects Fᵥ/Fₘ. These results suggest that both ecosystem dynamics and physical forcing drive variability in biomass distribution and primary productivity in the Oregon upwelling system. However, they do not provide the means to determine which forcing mechanism is dominant.
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