Measurement of suspended particulate matter under sea ice using ADCP and LISST 1 Departement of Ocean Sciences, Inha University, Incheon , South Korea 2 RPS Applied Science Associates, Inc., South Kingstown, RI 02879, USA 3 Scottish Association for Marine Science, Oban PA37 1QA, UK Ho Kyung Ha 1, Yong Hoon Kim 2, Phil Hwang 3 Abstract Using a mooring package comprising an acoustic Doppler current profiler (ADCP) and holographic imaging system, a 1-day ice camp study was performed under the Arctic sea ice in the northern Chukchi Plateau to estimate vertical and temporal variations in total suspended particulate matter (SPM). In early August, the SPM in upper mixed layer (~15 m and above) under sea ice reached up to about 100 mg l-1 even under the offshore regime. Results of both holographic and microscopic analyses showed that dominant constituents of this increased SPM were biogenic rather than lithogenic materials. Due to highest melt and break-up rates of sea ice during the summertime, the export of particulate materials and ice algal communities embedded in the sea ice might significantly contribute to the increase in SPM. This study suggests that combined effects of the increase in ice algal production and the decrease in ice and snow cover and multi-year sea ice extent could create favorable conditions for enhancing the concentration and flux of SPM during summer time. Fig. 1. (a) Map of study area in the Chukchi Plateau, Arctic Ocean. Color represents sea ice concentration on August 7, Contour lines are isobaths (in meters). (b) Progressive vector diagram of sea ice where the mooring package was installed. Black solid dots indicate the GPS location of SIMBA at midnight of each day. Red line indicates the track of sea ice during a one-day measuring campaign. Objectives So far, many studies have been conducted to investigate the variability in sea ice coverage, thickness and motion in response to atmospheric and oceanic forcing conditions. Only a few studies, however, have been carried out on the variability of SPM under Arctic sea ice. This makes it difficult to accurately quantify the concentration and flux of SPM and subsequent effects to the marine system. At this moment, the primary unknowns in the Arctic particle dynamics are the spatial and temporal variations in SPM under sea ice and the role of rapidly-melting summer sea ice as a source of SPM. In order to address these unanswered questions, we carried out a one-day field experiment using a mooring package including holographic and acoustic sensors. Despite the short duration of the experiment, the holographic imaging technology, which has never been deployed under sea ice, provides novel insights into under-ice particle dynamics. Fig. 2. Profiling and time-series data from beneath the ice floe: (a) potential temperature, (b) salinity, (c) potential density, (d) GPS- corrected velocity profiles, (e) current vector at the LISST-Holo sampling level (5.1 m), (f) SPM adcp profiles, (g) SPM adcp at 5.1 m, and (h) flux of SPM adcp. CTD profiles are averages of all casts. The gray and white areas in panel (e) indicate the periods of northward and southward flow, respectively. Thick green line and red dots in panel (g) are low-pass filtered and SPM sam concentration, respectively. One-day Moorings Figure 2 presents the temporal variability of water column structure during a one-day mooring. CTD profiles showed a distinct pycnocline around 10–15 m below the surface. The salinity near the ice-seawater interface was minimum (25.3 psu) influenced by the input of ice meltwater, and gradually increased with depth until it reached 26.2 psu at 10 m, and then sharply increased to 28.0 psu at 15 m. ADCP velocities at the near-surface layer showed reasonable agreement with ice drift motion, and typical inertial motion was well captured in the corrected velocity. It is noted that the measurements are the currents of water mass relative to the moving ice floe. A strong current (> 0.3 m s -1 ) was observed at near-surface layer, being steered by topographic effects of the base of the ice floe. At deeper depths exceeding about 13 m, below the mixed layer, the water movement was relatively slow (< 0.1 m s -1 ). While water was flowing toward the North within the periods of 1–7.5 and 13.5–20 h (see gray areas in Fig. 2e), the current speed exhibited a two-layer structure identified near 12–13 m where the vertical gradient of velocity was highest within the ADCP sensing range (Fig. 2d). Variation in SPM under drifting sea ice SPM adcp profiles converted from acoustic backscatter intensities showed distinct vertical movements (Fig. 2f). During the first 4 h, SPM adcp gradually increased, producing a high- concentration patch near 5 m. SPM adcp reached the maximum of mg l -1 at the uppermost bin (4.4–4.6 m) at 4 h. Abruptly, at 5 h, most of highly-concentrated materials disappeared beyond 4.4 m, probably because it was entrained by the overlying water mass. This upward transport of SPM adcp is also apparent in the upflux signature between 4 and 8 h (Fig. 2h). SPM adcp at the LISST-Holo sampling level (5.1 m) varied in the range of 69.4–94.6 mg l -1. Although water samples taken were limited in number, SPM sam and SPM adcp showed good agreement (Fig. 2g) with correlation r=0.75. The vertical flux was highly variable with time, because it was determined by the competition between upflux by turbulent diffusion and downflux by settling. Vertical velocities were about two orders of magnitude larger than w s LISST (~ 0.2 mm s -1 ). This suggests that the flux of SPM, at least in the under-ice boundary layer, was controlled mainly by under-ice turbulent motion and topographic effects rather than settling. Fig. 3. Near-inertial motion of sea ice floe and acoustic backscatter intensity during the mooring campaign with the camera view from south to north. Thick black line at the surface show the track of ice floe and red arrows represent ice movement. Thin vertical lines are time tick for every hour with numbers representing hours after 00:00 GMT, August 7, Near-inertial motion of drifting sea ice Conclusions  In early August 2011, the SPM concentration under the sea ice reached up to the order of 100 mg l -1, which is several times higher than typical SPM in open ocean environments. The SPM flux was significantly fluctuated upward or downward by the under-ice topographic effects. The export of particulate materials and ice algal communities embedded in the sea ice significantly contributed to the increase in SPM. The advection of increased biomass after phytoplankton bloom might also contribute to the increase in SPM.  Combined effects of the increase in insolation, ice algal production, and the decrease in ice and snow cover and multi-year sea ice extent could create favorable conditions for enhancing the concentration and flux of SPM during the rapidly-melting summer season. With the thinning and retreat trend of Arctic sea ice, it is expected that under-ice seawater will in future receive a still higher rate of discharge of particulate matters from melting sea ice. Acknowledgements This study was supported by KOPRI grants (PM13020) and Inha University Research Grant (INHA-49861). C3