[A-OS13] Marine ecosystems and biogeochemical cycles: theory, observation and modeling

[AOS13-04] Seasonality of sea ice and phytoplankton productivity in the Southern Ocean near Antarctic Peninsula

*Aymeric Pierre Marie Servettaz¹, Yuta Isaji¹, Chisato Yoshikawa¹, Nanako O. Ogawa¹, Francisco J. Jiménez-Espejo², Naohiko Ohkouchi¹ (1.Japan Agency for Marine-Earth Science and Technology, 2.Instituto Andaluz de Ciencias de la Tierra, University of Grenada)

Keywords:Southern Ocean, Sea ice, Nutrient, Nitrate, Iron

Sea-ice formation and melting modify the salinity and density of Southern Ocean surface waters, controlling the stability of the water column. Surface melt layer was shown to promote phytoplanktonic blooms, because the community is maintained in the euphotic zone, in an area of the ocean where light is one of the limiting factors to phytoplankton growth. When the light limitation is relieved by the retreating of sea-ice, other limiting factors such as nutrient availability may play a part in controlling the phytoplankton growth, especially when the pool of available nutrient is reduced by a shoaling of the mixed layer depth. Therefore, how sea-ice melting affects productivity is not straightforward.
In an ongoing research project using a sedimentary core retrieved from the Bransfield Strait (61°S, 55°W), we found that chlorophyll a (and its derivatives) concentration and its stable nitrogen isotope ratio show temporal fluctuations during the past 2000 years. We hypothesize that these variations are linked with seasonal dynamics of sea-ice, which affects primary production and nutrient availability in the modern ocean near Antarctic Peninsula. We compared satellite-derived estimates of sea-ice coverage available year-round, to chlorophyll and macronutrients concentrations (nitrate, phosphate, and silicate) in near-surface bottle samples taken during various cruises (Glodapv2 and Geotraces databases).
Sea-ice coverage and amplitude are weakly but significantly correlated with surface chlorophyll and macronutrient concentrations in the Antarctic Peninsula, with greater sea-ice coverage during austral winter generally followed by higher chlorophyll concentration during austral summer. By dividing the ocean area based on the duration of winter sea-ice coverage, we found that nitrate decline starts later in the season in areas where sea ice retreats later, but reaches lower concentration in areas where the duration of sea-ice coverage is longer, suggesting stronger biological uptake. In heavily sea-ice covered ocean (more than 6 months), the nitrate declines to similarly low values, not entirely depleted, which indicates that phytoplankton growth is not limited only by macronutrients. Although the data are unavailable in the focused area, iron in other parts of the Southern Ocean shows complete depletion in the summer, confirming the key role of iron in the phytoplanktonic productivity. Nutrient resupply from the subsurface water starts in autumn, when light becomes weaker and gradual increase in surface density allows for deeper mixing to resume. Even if the nutrient pool is not entirely depleted, the apparent nutrient availability covaries with sea-ice seasonality, which may explain variations of chlorophyll records in sediments.