1:45 PM - 2:00 PM
[ACG42-11] Seasonal variations in nutrient concentrations of low-salinity Pacific Water entering the Canada Basin
Keywords:low-salinity Pacific Water, Nutrients, Seasonal variations
The supply of nitrogenous nutrients by low-salinity Pacific Water (LSPW, S<32) could contribute to primary production in the Canada Basin. However, seasonal variations in nutrient concentrations in LSPW have not been observed because shipboard observations are limited to the summer season. In this study, we investigated concentrations of nitrogenous nutrients in the LSPW throughout the year using a nitrate sensor and a Remote Access Sampler (RAS) at a mooring station in Barrow Canyon (BCE), the gateway to the Canadian Basin for the LSPW.
Instruments were deployed at 42 m depth at the station with a bottom depth of 108 m, from September 12, 2021 to August 28, 2022. Sensor observations include water temperature, salinity, fluorescence (hourly), current velocity, and nitrate (every 3 hours). Discrete water samples were collected with the RAS (every 8 days), stored in sample bags with mercury chloride, and analyzed for nutrients using a Continuous Flow Analysis (CFA) method after the mooring recovery. 25-hr moving average was applied to sensor data to remove tidal effects. In addition, sea ice concentration data at the observation sites was obtained from the Arctic Data archive System (ADS).
We could successfully observe LSPW throughout the year. Its DIN (dissolved inorganic nitrogen: nitrate + nitrite + ammonium) concentration was low in summer, increased in fall, and reached highest value in early winter. After a winter marked by significant fluctuations, DIN concentrations dropped following the spring bloom. The proportion of ammonium in DIN was about 60% in the summer and decreased during the winter. Nitrite concentrations were low throughout the year. This indicate the important contribution of ammonium to the DIN supply, which should be neglected if only nitrate sensor was used.
As an indicator of nutrient supply to the basin, DIN flux at the observation depth was calculated by multiplying DIN concentration by current velocity along the mean flow axis. The flux was the largest in early winter when both concentration and current velocity were high. On the other hand, the flux after January were negative or small due to low velocities despite the high DIN concentrations. Flux in summer showed the second highest value due to high flow velocities, although concentrations were relatively low. In fall, when DIN concentration was the highest, fluxes were small due to frequent occurrence of the flow in the opposite direction.
This study revealed the seasonal variation in DIN concentrations in LSPW for the first time, and that nitrogen supply by LSPW to the Canada Basin mainly occurs in summer and early winter.
Instruments were deployed at 42 m depth at the station with a bottom depth of 108 m, from September 12, 2021 to August 28, 2022. Sensor observations include water temperature, salinity, fluorescence (hourly), current velocity, and nitrate (every 3 hours). Discrete water samples were collected with the RAS (every 8 days), stored in sample bags with mercury chloride, and analyzed for nutrients using a Continuous Flow Analysis (CFA) method after the mooring recovery. 25-hr moving average was applied to sensor data to remove tidal effects. In addition, sea ice concentration data at the observation sites was obtained from the Arctic Data archive System (ADS).
We could successfully observe LSPW throughout the year. Its DIN (dissolved inorganic nitrogen: nitrate + nitrite + ammonium) concentration was low in summer, increased in fall, and reached highest value in early winter. After a winter marked by significant fluctuations, DIN concentrations dropped following the spring bloom. The proportion of ammonium in DIN was about 60% in the summer and decreased during the winter. Nitrite concentrations were low throughout the year. This indicate the important contribution of ammonium to the DIN supply, which should be neglected if only nitrate sensor was used.
As an indicator of nutrient supply to the basin, DIN flux at the observation depth was calculated by multiplying DIN concentration by current velocity along the mean flow axis. The flux was the largest in early winter when both concentration and current velocity were high. On the other hand, the flux after January were negative or small due to low velocities despite the high DIN concentrations. Flux in summer showed the second highest value due to high flow velocities, although concentrations were relatively low. In fall, when DIN concentration was the highest, fluxes were small due to frequent occurrence of the flow in the opposite direction.
This study revealed the seasonal variation in DIN concentrations in LSPW for the first time, and that nitrogen supply by LSPW to the Canada Basin mainly occurs in summer and early winter.