Photosynthetic production in the central Arctic Ocean during the record sea-ice minimum in 2012

Abstract

The ice-covered central Arctic Ocean is characterized by low primary productivity due to light and nutrient limitations. The recent reduction in ice cover has the potential to substantially increase phytoplankton primary production, but little is yet known about the fate of the ice-associated primary production and of the nutrient supply with increasing warming. This study presents results from the central Arctic Ocean collected during summer 2012, when sea-ice extent reached its lowest ever recorded since the onset of satellite observations. Net primary productivity (NPP) was measured in the water column, sea ice and melt ponds by 14 CO 2 uptake at different irradiances. Photosynthesis vs. irradiance (PI) curves were established in laboratory experiments and used to upscale measured NPP to the deep Eurasian Basin (north of 78 • N) using the irradiance-based Central Arctic Ocean Primary Productivity (CAOPP) model. In addition, new annual production has been calculated from the seasonal nutrient drawdown in the mixed layer since last winter. Results show that ice algae can contribute up to 60 % to primary production in the central Arctic Ocean at the end of the productive season (August-September). The ice-covered water column has lower NPP rates than open water due to light limitation in late summer. As indicated by the nutrient ratios in the euphotic zone, nitrate was limiting primary production in the deep Eurasian Basin close to the Laptev Sea area, while silicate was the main limiting nutrient at the ice margin near the Atlantic inflow. Although sea-ice cover was substantially reduced in 2012, total annual new production in the Eurasian Basin was 17 ± 7 Tg C yr −1 , which is within the range of estimates of previous years. However, when adding the contribution by sub-ice algae, the annual production for the deep Eurasian Basin (north of 78 • N) could double previous estimates for that area with a surplus of 16 Tg C yr −1 . Our data suggest that sub-ice algae are an important component of the productivity in the ice-covered Eurasian Basin of the central Arctic Ocean. It remains an important question whether their contribution to productivity is on the rise with thinning ice, or whether it will decline due to overall sea-ice retreat and be replaced by phytoplankton.

High Latitude Epipelagic and Mesopelagic Scattering Layers—A Reference for Future Arctic Ecosystem Change

“…These factors impact physical characteristics such as stratification (e.g., Korhonen et al, 2013) and nutrient supply and may also affect timing of phytoplankton and ice algae blooms (e.g., Fernández-Méndez et al, 2015). Wassmann and Reigstad (2011) using an alternative scenario approach, elaborated on how these potential changes might impact the future Arctic ecosystem, focusing primarily on the timing, quantity, and quality of primary and secondary producers, but also range shifts, changes in abundance, growth, behavior, and community structure.…”

Fine-Scale Sea Ice Structure Characterized Using Underwater Acoustic Methods

“…Over the annual cycle, once surface sunlight levels rise and light penetration increases as a result of a thinning snow pack, algae begins to grow on the underside of the sea ice and these algae support a complex system of herbivores at the bottom of the food chain [10,11]. It has been estimated that ice algae contribute 20% of primary production in some polar regions, at a time when few other resources are available [12,13]. Fine-scale acoustic data (cm resolution) can broaden our understanding of these complex ecosystems and can allow multidisciplinary questions to be asked (e.g., spanning biology, ecology, physics, etc.)…”

Algal Hot Spots in a Changing Arctic Ocean: Sea-Ice Ridges and the Snow-Ice Interface

“…Before we drifted into the under-ice phytoplankton bloom , the currents could have provided a constant flux of nutrients to the ridge surface-attached communities. Diatoms are able to store nutrients intracellularly without using them for growth immediately (Kamp et al, 2011;Fernández-Méndez et al, 2015). Based on our nutrient and current measurements, before 25 May (pre-bloom) one centimeter water layer moving below the ice provided 1.56 × 10 3 -7.78 × 10 3 mmol N m −2 d −1 and 5.18× 10 3 -2.59 × 10 3 mmol Si m −2 d −1 , which is two orders of magnitude more than the calculated nutrient demand for these communities (15.7 mmol N m −2 d −1 and 38.9 mmol Si m −2 d −1 ) using the method explained in Cota et al (1987) and our own measured ratios and growth rates.…”

“…Current estimates of algal biomass and production in the ice-covered Arctic Ocean generally include phytoplankton and less often sea-ice algae (Gosselin et al, 1997;Sakshaug et al, 2004). Only recent studies have quantified the contribution of other sea-ice related environments, such as melt ponds (Mundy et al, 2011;Lee et al, 2012;Fernández-Méndez et al, 2015), and other more elusive forms of algal accumulations under the ice such as floating algal aggregates (Assmy et al, 2013;Fernández-Méndez et al, 2014).…”