Structural equation modeling of the influence of environmental factors on summer phytoplankton growth in the Ross Sea

The Ross Sea is a highly productive region of the Southern Ocean, and net phytoplankton growth varies seasonally, ranging from zero to near the temperature-limited maximum. Given that variations in growth can result from a number of factors (such as irradiance and iron concentrations), variability in net growth rate was investigated using structural equation modeling (SEM) and data collected during a January-February 2012 cruise to the Ross Sea. Structural equation modeling indicated that summer growth rates were significantly affected by iron concentrations and particulate organic carbon (POC) levels, the latter which most likely contributed to the seasonal depletion of iron by phytoplankton. Conversely, growth rates did not strongly vary with mixed layer depth (and hence irradiance). SEM indicated that if iron concentrations were increased by 1.0 standard deviation (ca. 0.12 nM), summer growth rates would increase by 0.5 standard deviation (ca. 0.07 day(-1)). Similarly, if POC (a measure of phytoplankton biomass in this region) was increased by 1.0 standard deviation (from 23.0 to 39.4 A mu mol L-1), growth rates would decrease by 0.31 standard deviations (similar to 0.04 day(-1)), which we speculate is likely due to heightened iron limitation via increased total iron uptake. This modeling exercise confirms the dominant role of iron in regulating summer phytoplankton rates over the continental shelf of the Ross Sea.