Under‐Ice Phytoplankton Blooms Inhibited by Spring Convective Mixing in Refreezing Leads

Abstract

Spring phytoplankton growth in polar marine ecosystems is limited by light availability beneath ice‐covered waters, particularly early in the season prior to snowmelt and melt pond formation. Leads of open water increase light transmission to the ice‐covered ocean and are sites of air‐sea exchange. We explore the role of leads in controlling phytoplankton bloom dynamics within the sea ice zone of the Arctic Ocean. Data are presented from spring measurements in the Chukchi Sea during the Study of Under‐ice Blooms In the Chukchi Ecosystem (SUBICE) program in May and June 2014. We observed that fully consolidated sea ice supported modest under‐ice blooms, while waters beneath sea ice with leads had significantly lower phytoplankton biomass, despite high nutrient availability. Through an analysis of hydrographic and biological properties, we attribute this counterintuitive finding to springtime convective mixing in refreezing leads of open water. Our results demonstrate that waters beneath loosely consolidated sea ice (84–95% ice concentration) had weak stratification and were frequently mixed below the critical depth (the depth at which depth‐integrated production balances depth‐integrated respiration). These findings are supported by theoretical model calculations of under‐ice light, primary production, and critical depth at varied lead fractions. The model demonstrates that under‐ice blooms can form even beneath snow‐covered sea ice in the absence of mixing but not in more deeply mixed waters beneath sea ice with refreezing leads. Future estimates of primary production should account for these phytoplankton dynamics in ice‐covered waters.

Characteristics and Transformation of Pacific Winter Water on the Chukchi Sea Shelf in Late Spring

“…As noted in section , the NVWW is critically important to the regional ecosystem because of its high nutrient content, which spurs primary production (Hill & Cota, ; Lowry et al, , ; Pickart et al, ). Our SUBICE cruise revealed that in late spring, the nitrate levels in the upper part of the water column are generally quite high (see also Arrigo et al, ).…”

“…In the Chukchi Sea, leads tend to be 100 m or less in width, several kilometers in length, and oriented perpendicular to the prevailing wind; although the currents can play a role as well (Tschudi et al, ). The leads during this study were generally 50–200 m long (Lowry et al, ).…”

Variations in Rates of Biological Production in the Beaufort Gyre as the Arctic Changes: Rates From 2011 to 2016

“…Recent work has highlighted the importance of massive blooms of phytoplankton under sea ice (Arrigo et al, ; Lowry et al, ). No such blooms were observed during the cruises for this study, perhaps because under ice blooms would have occurred earlier in the season than the cruises occurred.…”

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

“…Mean PAR in the ML was calculated using downwelling incident irradiance, specular reflection, ice concentration, and attenuation by snow, ice, and water. To determine PAR just below the ocean surface ( E 0− ) we calculated a mean incident PAR ( E incident = 582.52 μmol photons m −2 s −1 ) from the mast PAR sensor during the sampling period (30 October–20 November 2014) and reduced this value to account for specular reflection ( r ) at either the snow/ice ( r snow = 0.05 (Kirk, ; Perovich, )) or seawater surface ( r water = 0.356; calculated from ice‐free stations based on the fraction of PAR just below the surface of the water (0 m, extrapolated from CTD measurements beginning at 1 m) relative to mast PAR at the same station): where z is the thickness (m) of snow or ice, f is the fraction (unitless) surface cover of ice or open water at that location obtained using Special Sensor Microwave Imager data, and K (m −1 ) is the diffuse attenuation coefficient for downwelling irradiance, taken here to be K d snow = 21.4 m −1 and K d ice = 1.59 m −1 (Lowry et al, ; Perovich, ; Perovich et al, ). The thickness of snow or ice was either measured at the ice stations (Selz et al, ), or where no measurements were available, the mean depths measured during the cruise were used ( z snow : 0.28 m, z ice : 0.81 m).…”