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

Abstract

The Ross Sea is a highly productive region of the Southern Ocean, and net phytoplankton growth varies seasonally, ranging from zero to near the temperaturelimited 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 lmol L -1 ), growth rates would decrease by 0.31 standard deviations (*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.

Climate change impacts on southern Ross Sea phytoplankton composition, productivity, and export

“…For instance, shipboard incubation experiments found the effects of increased temperature to be generally less than the effects of increased iron concentrations [ Rose et al ., ]. Furthermore, structural equation modeling, which is a multivariate statistical analysis technique for determining direct and indirect causal relationships, in the Ross Sea revealed a weaker relationship between phytoplankton and temperature as compared to MLD or iron concentrations [ Mosby and Smith , ]. Manipulation experiments with Ross Sea phytoplankton demonstrated a response by diatoms to increased temperatures, whereas P. antarctica did not respond to temperature increases except when accompanied by increased iron as well [ Zhu et al ., ].…”

“…Other studies have investigated links between phytoplankton and oceanographic variables. For example, an investigation using structural equation modeling and data from January to February 2012 found summer phytoplankton growth rates in the Ross Sea to be most affected by levels of iron [ Mosby and Smith , ]. Irradiance levels likewise have been found to differentially affect phytoplankton growth in the Ross Sea [ Garcia et al ., ; Mills et al ., ; Feng et al ., ].…”

Environmental Forcings on the Remotely Sensed Phytoplankton Bloom Phenology in the Central Ross Sea Polynya

“…Then, in the present study, the CHL concentrations on all available pixels belonging to the bloom area were spatially averaged and used as the primary data for the determination of CHL phenology. Since there is considerable spatial and temporal variability in the standing stocks of the RS phytoplankton during the austral spring and summer (Mangoni et al, ; Mosby & Smith, ), the determination of bloom area is very difficult. The phytoplankton communities of the central RSP (around the Ross Bank) are dominated alternately by P. antarctica and diatoms, with both exhibiting spatial and temporal variability (Arrigo & van Dijken, ; Coale et al, ; Kaufman et al, ; Sedwick et al, ; Sedwick & DiTullio, ).…”

“…The seasonal phytoplankton blooms associated with the expansion of the RSP (Arrigo et al, ; Arrigo & van Dijken, ) start in late October, when there is a reduction in sea ice, an increase in irradiance, and shoaling of the mixed layer depth (Jones & Smith, ; Smith et al, ). During the austral spring and early summer (November–December), strong offshore winds result in weak stratification (a deep mixed layer depth; Arrigo et al, ; Arrigo & van Dijken, ; Bromwich et al, ; Mangoni et al, ), and a large bloom dominated by Phaeocystis antarctica ( P. antarctica ) develops (Mosby & Smith, ; Smith et al, ). The bloom accounts for approximately 95% of the total phytoplankton bloom in the RSP (Mangoni et al, ).…”

“…The bloom accounts for approximately 95% of the total phytoplankton bloom in the RSP (Mangoni et al, ). The abundance of P. antarctica rapidly declines in late December and early January (Jones & Smith, ; Smith et al, ) over a period of days to weeks (Smith et al, ), and diatoms become more prevalent in the RSP (Mosby & Smith, ; Peloquin & Smith, ; Ryan‐Keogh et al, ; Smith et al, ). This P. antarctica‐ diatom progression of the phytoplankton bloom occurs in response to changes in various environmental conditions (Boyd et al, ), but it is primarily a result of the interaction between irradiance and micronutrients such as iron (Boyd, ; Jones & Smith, ; Peloquin & Smith, ; Ryan‐Keogh et al, ; Sedwick et al, ).…”