Mixing and phytoplankton dynamics in a submarine canyon in the West Antarctic Peninsula

Abstract

Bathymetric depressions (canyons) exist along the West Antarctic Peninsula shelf and have been linked with increased phytoplankton biomass and sustained penguin colonies. However, the physical mechanisms driving this enhanced biomass are not well understood. Using a Slocum glider data set with over 25,000 water column profiles, we evaluate the relationship between mixed layer depth (MLD, estimated using the depth of maximum buoyancy frequency) and phytoplankton vertical distribution. We use the glider deployments in the Palmer Deep region to examine seasonal and across canyon variability. Throughout the season, the ML becomes warmer and saltier, as a result of vertical mixing and advection. Shallow ML and increased stratification due to sea ice melt are linked to higher chlorophyll concentrations. Deeper mixed layers, resulting from increased wind forcing, show decreased chlorophyll, suggesting the importance of light in regulating phytoplankton productivity. Spatial variations were found in the canyon head region where local physical water column properties were associated with different biological responses, reinforcing the importance of local canyon circulation in regulating phytoplankton distribution in the region. While the mechanism initially hypothesized to produce the observed increases in phytoplankton over the canyons was the intrusion of warm, nutrient enriched modified Upper Circumpolar Deep Water (mUCDW), our analysis suggests that ML dynamics are key to increased primary production over submarine canyons in the WAP.

Testing the Canyon Hypothesis: Evaluating light and nutrient controls of phytoplankton growth in penguin foraging hotspots along the West Antarctic Peninsula

“…Relatively high sea surface temperatures resulting from the intrusion of mUCDW have been hypothesized to result in earlier sea ice retreat, increased melting, shallower MLD, and increased chlorophyll (Kavanaugh et al ). The hypothesis that recurrent pulses of mUCDW replenish surface waters with abundant macro‐ and micronutrients required to fuel primary production (Prézelin et al , ) is plausible, yet the results from this and other recent studies (Carvalho et al ; Annett et al ; Bown et al ; Sherrell et al ) do not support this hypothesis. Our analysis reveals that neither mixing deep water with surface water (simulating upwelling of mUCDW) nor iron enrichment stimulated phytoplankton growth, as nutrients were already abundant in the nearshore surface layers.…”

“…This would imply that the biological “hotspots” are not actually sites of extreme phytoplankton blooms in response to local forcings but instead reflect a concentrated amalgamation of biomass from several typical nearshore spring blooms. The authors also found regional differences across the canyon where increased residence times in the Northern region explain the increased chlorophyll concentration found by Carvalho et al (), showing that, given time, phytoplankton do grow locally.…”

“…This evidence counters the Canyon Hypothesis, that local canyon driven upwelling is required to supply this critical micronutrient to the surface layer (Sherrell et al ). Accordingly, previous studies in coastal waters of the WAP have shown that inshore regions, including the three “biological hotspots” of the Canyon Hypothesis, do not show signs of Fe‐limited primary production (Hopkinson et al ; Annett et al ; Carvalho et al ).…”

“…Light is another major factor hypothesized to control primary production in the WAP shelf and other biological hotspots along the Antarctic coast. Both the MLD and water column stability have been widely linked to phytoplankton dynamics (Holm‐Hansen and Mitchell ; Sakshaug et al ; Moline and Prezelin ; Carvalho et al ). Phytoplankton live in a dynamic light environment that is controlled by a combination of incident light, MLD, and the rate of turbulent mixing (Lewis et al ; Cullen and Lewis ).…”

Light Is the Primary Driver of Early Season Phytoplankton Production Along the Western Antarctic Peninsula

“…Combined sea ice cover and MLD largely control mean ML PAR, which is the dominant controlling factor for spring NPP in the WAP (Vernet et al, ; this paper). Further, high ice cover years lead to stable, stratified water columns, while low ice years enable enhanced wind mixing that creates deep MLs (Carvalho et al, ; Schofield et al, ; Smith et al, ; Venables et al, ; Vernet et al, ). Consequently, future changes in sea ice cover will have large implications for NPP in the WAP.…”