Primary productivity in Arctic sea ice and Ocean

Abstract

Primary productivity in the Arctic Ocean is heavily influenced by sea ice dynamics, which govern light and nutrient availability. Until the last decade, the permanently ice-covered Central Arctic, north of 78ºN, was considered to have low productivity and was often neglected in estimates of primary production in the Arctic. Due to global warming, the Central Arctic is shifting from a perennial to a seasonal ice zone. The reduction in ice cover and thickness leads to an increase in the amount of light that penetrates the ice. In addition, ice melt strengthens stratification that hinders nutrient upwelling, increasing nitrate limitation. Our knowledge about how changes in light and nutrients affect carbon and nutrient budgets in sea-ice algae and phytoplankton is limited, especially in the ice-covered oligotrophic Central Arctic. Understanding the processes that control primary productivity is crucial to predict how the Arctic ecosystem will react to further sea-ice decline. The overall objective of this thesis was to better understand the physical and biological processes that affect primary productivity in the Central Arctic. Specific goals were to investigate i) the role of microbial algal aggregations in carbon and nutrient cycling, ii) the contribution of sea ice to total primary productivity and the impact of sea-ice retreat on annual productivity, and iii) the diversity of nitrogen-fixing bacteria in the Central Arctic. The methods used in this thesis included net primary productivity (NPP) measurements by radioactive carbon (14C) uptake at different irradiances, an irradiance-based model to upscale measured NPP and annual production estimates from the seasonal nutrient drawdown in the mixed layer since last winter. In Chapters I-IV the first specific goal was addressed. Our results showed that sub-ice algal aggregates, such as Melosira arctica filaments, can sink rapidly to the seafloor in early summer when the ice melts contributing to carbon export to the benthos (Chapter I). The aggregates that remain floating below the ice can serve as a food source for ice-associated fauna in late summer (Chapter II). A potential factor controlling the buoyancy of the aggregates was photosynthetic oxygen production (Chapter III). The distribution of aggregates was very patchy and seemed to be governed by ice topography (Chapter IV). In Chapter V the second specific goal was examined. The NPP measurements showed that the relative contribution of sea-ice algae to total NPP was as high as 60% in late summer, when phytoplankton below the ice was light limited. According to the nutrient ratios in the water column, nitrate and silicate were the main limiting nutrients in the Eurasian Basin. Although sea-ice cover was substantially reduced in 2012, total annual new production was similar to estimates of previous years. However, when including the contribution by algal aggregates, the estimated annual production increased. This indicates that sub-ice algae are an important component of the ice-covered Arctic. Nitrogen fixation could increase the amount of nitrate available for new production. Corresponding to the third specific goal, in Chapter IV the nitrogenase coding gene, nifH, was amplified to examine the diversity of nitrogen fixing organisms. A wide diversity of potential nitrogen fixing bacteria was discovered, especially in sea-ice, seawater and algal aggregates. If the observed diversity corresponds to an active community, then Arctic nitrogen fixing bacteria might contribute to alleviate nitrate limitation. In conclusion, the Central Arctic Ocean hosts low but significant productivity in and below the ice and should be included in primary productivity models. As sea-ice retreats further, primary productivity might increase locally due to higher light availability. However, the total amount of carbon fixed will still be constrained by the nutrients available in the surface waters during the productive season. Therefore, understanding the impacts of sea-ice retreat on nutrient replenishment mechanisms is essential to predicting Arctic primary productivity.