Biogeosciences, 13, 6155–6169, 2016 www.biogeosciences.net/13/6155/2016/ doi:10.5194/bg-13-6155-2016 © Author(s) 2016. CC Attribution 3.0 License. Seasonal variation of CaCO3 saturation state in bottom water of a biological hotspot in the Chukchi Sea, Arctic Ocean Michiyo Yamamoto-Kawai1, Takahisa Mifune1, Takashi Kikuchi2, and Shigeto Nishino2 1Tokyo University of Marine Science and Technology, Tokyo, 108-8477, Japan 2Japan Agency for Marine-Earth Science and Technology, Yokosuka, 237-0061, Japan Correspondence to: Michiyo Yamamoto-Kawai ([email protected]) Received: 2 March 2016 – Published in Biogeosciences Discuss.: 18 March 2016 Revised: 15 August 2016 – Accepted: 25 August 2016 – Published: 22 November 2016 Abstract. Distribution of calcium carbonate saturation state tem and may threaten some organisms (e.g., Gattuso and () was observed in the Chukchi Sea in autumn 2012 and Hansson, 2011; Branch et al., 2013). Of particular concern is early summer 2013. in bottom water ranged from 0.3 to 2.0 the impact on calcifying organisms, such as coralline algae, for aragonite and from 0.5 to 3.2 for calcite in 2012. In 2013, pteropods, bivalves, and corals, because acidification lowers in bottom water was 1.1–2.8 for aragonite and 1.7–4.4 for the saturation state () of calcium carbonate (CaCO3/ in sea- calcite. Aragonite and calcite undersaturation was found in water, which affects the ability of these organisms to produce high productivity regions in autumn 2012 but not in early and maintain their shells or skeletons. In fact, a decrease in summer 2013. Comparison with other parameters has indi- of water can cause enhanced mortality of juvenile shell- cated that biological processes – respiration and photosyn- fish (Green et al., 2009; Talmage and Gobler, 2009) and de- thesis – are major factors controlling the regional and tem- creased calcification, growth, development and abundance of poral variability of . From these ship-based observations, calcifiers (Kroeker et al., 2013, and references therein). we have obtained empirical equations to reconstruct from The shallow shelf seas of the Arctic Ocean are known to temperature, salinity and apparent oxygen utilization. Using be especially vulnerable to ocean acidification. Cold water 2-year-round mooring data and these equations, we have re- dissolves more CO2, large freshwater inputs from rivers and constructed seasonal variation of in bottom water in Hope sea ice melt reduce calcium ion concentrations and alkalinity, Valley, a biological hotspot in the southern Chukchi Sea. Es- the buffering capacity of seawater to added CO2 (Salisbury, timated was high in spring and early summer, decreased in 2008; Yamamoto-Kawai et al., 2011), and respiration at the later summer, and remained relatively low in winter. Calcu- bottom of a salt stratified water column accumulates CO2 in lations indicated a possibility that bottom water could have bottom water (Bates et al., 2009). Because of these charac- been undersaturated for aragonite on an intermittent basis teristics, both surface and bottom waters of Arctic shelf seas even in the pre-industrial period, and that anthropogenic CO2 exhibit naturally low compared to other ocean waters (e.g., has extended the period of aragonite undersaturation to more Fabry et al., 2009; Mathis et al., 2015; Yamamoto-Kawai et than 2-fold longer by now. al., 2013). The Chukchi Sea is one of these seas. Recent studies have found bottom waters already undersat- urated with respect to aragonite-type CaCO3 in the Chukchi Sea during summer and autumn (Bates et al., 2009, 2013; 1 Introduction Bates, 2015; Mathis et al., 2014). Bates et al. (2013) re- ported that ∼ 40 % of sampled bottom waters during sum- During the last decade, ocean acidification due to uptake of mertime cruises between 2009 and 2011 were aragonite- anthropogenic carbon dioxide (CO2/ has emerged as an ur- undersaturated. Some bottom waters were also undersatu- gent issue in ocean research (e.g., Raven et al., 2005; Orr et rated with respect to calcite, a less soluble form of CaCO3 al., 2005). Increasing acidity and consequent changes in sea- than aragonite. These studies indicate that benthic commu- water chemistry are expected to impact the marine ecosys- Published by Copernicus Publications on behalf of the European Geosciences Union. 6156 M. Yamamoto-Kawai et al.: Seasonal variation of CaCO3 saturation state or 8 months of the year. Pacific-origin water enters through the Bering Strait and spreads across the Chukchi Sea. The Pa- cific water transports heat and freshwater to the Arctic Ocean during summer months. In winter, atmospheric cooling and brine injection from growing sea ice mixes the whole water column and modifies the water to form “Pacific winter wa- ter”, which is characterized by low temperature and relatively high salinity (∼ 33). In coastal polynyas, hypersaline winter water (S > 34) occasionally forms. Even in summertime, the remnant Pacific winter water is often observed at the bottom of the shelf beneath the warm and fresh upper layer. The Pacific water also transports nutrients to the Chukchi Sea. Nutrient-rich water upwelled onto the Bering shelf is carried northward, promoting very high primary productivity in the sea (e.g., Springer and McRoy, 1993). Using the nutri- ents, spring phytoplankton bloom occurs immediately after the sea ice retreat or even under sea ice (Fujiwara et al., 2016; Lowry et al., 2014). Relatively high primary production con- tinues to autumn (Wang et al., 2005). Because of the shallow Figure 1. Distribution of ar (a, c) and ca (b, d) in bottom wa- bottom depth and the mismatch between seasonal dynamics ter in September–October 2012 (a, b) and July 2013 (c, d). Circled of phytoplankton and zooplankton, a large proportion of pro- stations were undersaturated with CaCO3 minerals. Stations in red duced organic matter is directly delivered to the seabed (e.g., polygon were used in Fig. 3. White arrows indicate mooring sites. Grebmeier et al., 2006). As a result, the high export rate of or- An insert photo shows bivalves collected by a dredge trawl at a sta- ganic matter supports very high benthic biomass that is prey tion marked with a star in July 2013. for higher trophic levels such as diving ducks, seals, whales and walruses (Grebmeier et al., 2006; Fabry et al., 2009). Exported organic matter is remineralized back to nutrients nities in large areas of this shelf sea have been exposed to and CO in shelf bottom water and sediments. This accu- bottom waters that are corrosive for their CaCO shells and 2 3 mulates CO in bottom water and lowers , to the level of skeletons, at least seasonally. However, full seasonal varia- 2 CaCO undersaturation in summer/autumn (e.g., Bates et al., tion of is still unrevealed due to the lack of shipboard ob- 3 2009). Calcifying bivalves (Fig. 1), amphipods, brittle stars servations during ice-covered months. Because many benthic and crabs are dominant species in the benthic community organisms have planktonic larval stages, timing and duration of the Chukchi Sea (Fabry et al., 2009; Blanchard et al., of CaCO undersaturation in bottom water can be critical for 3 2013) and are key components of the ecosystem. Ocean acid- their growth and populations. ification, therefore, could have considerable impacts on the In the present paper, we show results of ship-based ob- ecosystem and biogeochemical cycles of the Chukchi Sea. servations in September–October of 2012 and early July of In addition, because of the shallow bottom depth of ∼ 50 m, 2013 in the Chukchi Sea. In both years, sea ice concentra- vertical mixing induced by wind, tide or atmospheric cool- tion decreased to < 80 % at the end of May and to < 30 % at ing can bring anthropogenic CO into bottom water to which the beginning of June. Therefore, observations were carried 2 benthos are exposed. out about 3 months and 1 month after the ice retreat in 2012 and 2013, respectively. Because of the difference in season as well as interannual variability in hydrographic conditions, distribution of was largely different between two observa- 3 Observation and analysis tions. By comparing with distributions of physical and bio- Hydrographic data were collected in the Chukchi and Bering geochemical parameters, factors controlling will be dis- seas during the cruises of R/V Mirai of the Japan Agency cussed. Based on these results, we attempt to reconstruct for Marine-Earth Science and Technology (JAMSTEC) from seasonal variations of in the bottom water in the south- 13 September to 4 October in 2012 (Kikuchi, 2012) and T/S ern Chukchi Sea by using data from a 2-year-round mooring Oshoro-Maru of Hokkaido University from 3 July to 18 July observation between July 2012 and July 2014. in 2013 (Hirawake et al., 2013). In the present paper, we use data from stations north of 66◦ N with a bottom depth 2 Study area shallower than 70 m. During both cruises, hydrographic casts were performed using a Sea-Bird 9plus CTD to which a The Chukchi Sea is a vast and shallow (∼ 50 m) shelf sea carousel water sampler with Niskin bottles was mounted. north of the Bering Strait (Fig. 1) and is covered by ice for 7 Seawater samples were collected for total alkalinity (TA) and Biogeosciences, 13, 6155–6169, 2016 www.biogeosciences.net/13/6155/2016/ M. Yamamoto-Kawai et al.: Seasonal variation of CaCO3 saturation state 6157 Bottle DO was determined by Winkler titration follow- ing World Ocean Circulation Experiment Hydrographic Pro- gram operations and methods (Dickson, 1996) with preci- sion of 0.12 µmol kg−1. Nutrient samples were analyzed ac- cording to the GO-SHIP Repeat Hydrography Manual (Hy- des et al., 2010) with a precision of < 0.1 µmol kg−1 for ni- trate, nitrite and phosphate, < 0.6 µmol kg−1 for silicate, and < 0.4 µmol kg−1 for ammonium.