Oceanography

Physical and Chemical Oceanography

Lisa Eisner, Seth Danielson, and Carol Ladd

Final Report
2012 Preliminary Results
2013 Preliminary Results
2013 Beaufort Canyon Exploration

Our comprehensive oceanographic sampling regime undertaken in 2012 and 2013 will provide real-time verification of our existing conceptual model of circulation in the Chukchi Sea (Figure 1 from Hunt et al. 2013). Additionally, as discrete water sample processing continues, we will be able to provide primary production estimates through chlorophyll analysis, dissolved nutrient levels, and oxygen concentrations throughout the water column over our entire sampling area.

Figure 1. Hunt et al. Chukchi Sea circulation diagram.

Final Report

Final Report to CIAP and BOEM:
Danielson, S.L., Eisner, L., Ladd, C., Mordy, C., Sousa, L., Weingartner, T. J. 2015. A comparison between late summer 2012 and 2013 water masses, macronutrients, and phytoplankton standing crops in the northern Bering and Chukchi Seas. US Dept. of the Interior, Bureau of Ocean Energy Management, Alaska OCS Region. OCS Study BOEM 2011-AK-11-08 a/b. 74 pp. – DRAFT REPORT

Abstract:
Survey data from the northern Bering and Chukchi sea continental shelves in August-September 2012 and 2013 reveal inter-annual differences in the spatial structure of water masses along with statistically significant differences in thermohaline properties, chemical properties, and phytoplankton communities. We find that the near-bottom Bering-Chukchi Summer Water (BCSW) water mass was more saline in 2012 and Alaskan Coastal Waters (ACW) were warmer in 2013. Both of these water masses carried higher nutrient concentrations in 2012, supporting a larger chlorophyll a standing crop biomass that was comprised primarily of small (<10 µm) size class phytoplankton. The location of phytoplankton biomass concentrations and their size compositions reveal linkages between the wind fields, seafloor topography, water masses, and the pelagic production. We speculate that the decrease in salinity and nutrients from 2012 to 2013 may have been related to an observed decrease in net Bering Strait transport from 2011 to 2012 (Woodgate et al., submitted). The horizontal structure of the shelf water masses, including the strength and location of stratification and fronts, respectively, differed in part because of the August regional wind field, which was more energetic in 2012 but was more persistent in direction in 2013. ACW were found all along the coast from Nunivak Island to Point Barrow in 2012, but in response to the persistent wind of 2013 ACW was not found north of Ledyard Bay. Instead, the 2013 NE Chukchi shelf was flooded with cold and fresh waters derived from ice melt waters (MW) that resided above cold and salty Bering-Chukchi Winter Waters (BCWW). Similarly, in the northern Bering Sea, low-salinity coastal waters from western Alaska were driven offshore to a greater extent in 2013, while in 2012 they were found more confined to shore and more prominently extended northward through Bering Strait. The water mass distributions together with the winds and limited surface current data suggest that the NE Chukchi Alaskan Coastal Current (ACC) was shut down for a time in August and September 2013. Our results have implications for the fate of fresh water, heat, and pelagic production on the Bering-Chukchi shelves.

Manuscript:
Danielson, S., Eisner, L., Ladd, C., Weingartner, T., Mordy, C. in press. A comparison between late summer 2012 and 2013 water masses, macronutrients, and phytoplankton standing crops in the northern Bering and Chukchi Seas. Deep Sea Res. II: Arctic Eis Special Issue.

2012 Preliminary Results


Figure 2. Arctic Eis 2012 sea surface temperature in deg C.

Figure 4. Arctic Eis 2012 near-surface sea water salinity in practical salinity units (PSU).

Figure 6. Arctic Eis 2012 surface sea water nitrate concentration (uM/kg).

Figure 8. Arctic Eis 2012 surface sea water silicate concentration (uM/kg).

Figure 10. Arctic Eis 2012 surface sea water phosphate concentration (uM/kg).

Figure 12. Arctic Eis 2012 surface sea water ammonium concentration (uM/kg).

Figure 14. Arctic Eis 2012 water column integrated chlorophyll a concentration (ug/l).



Figure 3. Arctic Eis 2012 near-bottom sea water temperature in deg C.

Figure 5. Arctic Eis 2012 near-bottom sea water salinity in practical salinity units (PSU).

Figure 7. Arctic Eis 2012 bottom sea water nitrate concentration (uM/kg).

Figure 9. Arctic Eis 2012 bottom sea water silicate concentration (uM/kg).

Figure 11. Arctic Eis 2012 bottom sea water phosphate concentration (uM/kg).

Figure 13. Arctic Eis 2012 bottom sea water ammonium concentration (uM/kg).

Click on images to enlarge.

In summer 2012 (August-September) temperature typically decreased and salinity increased from E- W (inshore to offshore), as expected. Riverine inputs contributed to the low salinities and high temperatures seen in Norton Sound and Kotzebue Sound. Sea surface temperature (SST) ranged from ~2 to 11 °C with the highest SST (9 to 11 °C ) near the coast in Kotzebue Sound and Norton Sound but also at the innermost station along 69.5, 70.5 and 71 °N (Figure 2). Bottom temperature ranged from ~ -2 to 10 °C with the highest bottom temperatures seen inshore in the same locations as high SST, and lowest bottom temperatures seen in the northern regions at 71 to 72.5 °N (-2 to 0°C) and offshore (-1 to 2°C) (Figure 3).

Sea surface salinity (SSS) ranged from 19 to 33.4 and was inversely related to SST for regions up to 71 °N. For 71.5 to 72.5 °N, near Hanna Shoal, SST and SSS were positively related with lower salinity seen in colder waters (Figure 4). Bottom salinity ranged from ~ 28.6 to 33.8 and was inversely related to temperature for the whole survey area, with highest salinity (> 33) seen in the lowest temperature water on the northern most transects near Hanna Shoal and in offshore water masses (Figure 5). The water column was stratified with pycnoclines above 20 m, except for stations located close to the Alaska Coast (including parts of Bering Strait).

Surface nitrate was low (< 4 µmol kg-1) at all stations except at offshore locations just south of Bering Strait at 64 to 65 °N (Figure 6). High bottom nitrate (Figure 7) was seen in the colder, higher salinity water masses. Bottom nitrate distributions were similar to surface nitrate distributions, although higher values (> 4 µmol kg-1) were found at most of the offshore stations from 64 to 72.5°N, including Bering Strait (which had values of ~16-48 µmol kg-1) and at some of the northernmost locations (inshore at 71 to 71.5 °N) and NW of Hanna Shoal.

Surface silicate was generally higher inshore than offshore (ranged from 4 to 24 µmol kg-1 inshore and from 0 to 8 offshore), although the highest value (> 36 µmol kg-1) was seen just south of Bering Strait at the station with the highest surface nitrate (Figure 8). Bottom silicate mirrored bottom nitrate distributions (Figure 9).

Chlorophyll a (chla, Figure 14) was highest (6-20 µg L-1) at the surface at the stations just south of Bering Strait (stations with high nitrate and silicate concentrations and low temperature). This location is a common hot spot for primary productivity due to the high surface nutrients characteristic of the Anadyr water mass found in this area. Anadyr water is a cold water mass that originates in the Gulf of Anadyr and flows northward through Bering Strait. Relatively high surface chlorophyll a (> 2 µg L-1) was seen in Norton Sound and offshore in Kotzebue Sound, NW of Nunivak Island and NE of St. Lawrence Island and at an inshore station at 71°N (a station with high SST and low SSS). Relatively high (2- 12 µg L-1) subsurface chla at 20 or 30 m was seen at some offshore locations at 68°N near Herold Shoal, 70 °N and 72 °N, over Hanna Shoal, and at two stations along 71°N. Future analyses will evaluate vertical profiles of fluorescence calibrated with discrete chla samples, to better characterize vertical phytoplankton distributions.

2013 Preliminary Results


Figure 15. Arctic Eis 2013 sea surface temperature in deg C.

Figure 17. Arctic Eis 2012 near-surface sea water salinity in practical salinity units (PSU)

Figure 19. Arctic Eis 2013 surface sea water nitrate concentration (uM/kg).

Figure 21. Arctic Eis 2013 surface sea water silicate concentration (uM/kg).

Figure 23. Arctic Eis 2013 surface sea water phosphate concentration (uM/kg).

Figure 25. Arctic Eis 2013 surface sea water ammonium concentration (uM/kg).

Figure 27. Arctic Eis 2013 water column integrated chlorophyll a concentration (ug/l).



Figure 16. Arctic Eis 2013 bottom sea water temperature in deg C.

Figure 18. Arctic Eis 2012 near-bottom sea water salinity in practical salinity units (PSU)

Figure 20. Arctic Eis 2013 bottom sea water nitrate concentration (uM/kg).

Figure 22. Arctic Eis 2013 bottom sea water silicate concentration (uM/kg).

Figure 24. Arctic Eis 2013 bottom sea water phosphate concentration (uM/kg).

Figure 26. Arctic Eis 2013 bottom sea water ammonium concentration (uM/kg).

Click on images to enlarge.

In summer (August-September) 2013, temperature typically decreased and salinity increased from E- W (inshore to offshore), similar to 2012. Riverine inputs contributed to the low salinities (as low as 19 psu) and high temperatures (10-12 °C) in Norton Sound and Kotzebue Sound. Sea surface temperatures (SST) in 2013 ranged from – 2 to 12 °C with the highest SST near the coast from 61 to 69.5°N (Figure 15). Near-shore SST was warmer in 2013 than in 2012, but warm values did not extend as far north in 2013, suggesting that northward moment of the Alaska Coastal Current (ACC) was constrained in summer 2013. Similar to 2012, bottom temperature ranged from -2 to 10 °C with highest bottom temperatures inshore in the same locations as high SST (Figure 16). The lowest bottom temperatures were observed from 70.5 to 72.5 °N (-2 to -1°C) and in the northern Bering Sea from 61 to 63 °N (-1 to 0°C). Bottom temperatures also suggest that the ACC was constrained in 2013, unlike in 2012, when near-shore bottom T were high (> 8°C) up to 71 °N at the edge of our survey grid.

Sea surface salinity (SSS) ranged from ~19 to 32.5 and was inversely related to SST (similar to 2012) up to ~ 71 °N, south of Hanna Shoal (Figure 17). Near Hanna Shoal, SST and SSS were positively related with lower salinity in cold waters influenced by ice melt. Bottom salinity ranged from ~ 19 to 34.5 and was inversely related to temperature for the whole survey area, with highest salinity (> 33) seen near Hanna Shoal and in offshore stations at 68 °N and 70.5 °N (Figure 18).

Surface nitrate was very low (< 2 µM) at all but three stations - two located south of Bering Strait at 64.5 °N and one at the head of Barrow Canyon at 70.5 °N (Figure 19). High bottom nitrate was seen in the colder, higher salinity water masses (Bering Strait, offshore Kotzebue Sound, northernmost region near Hanna Shoal)(Figure 20). Surface silicate ranged from 0.2 to 30 µM with the highest values observed in Norton Sound (Figure 21). Bottom silicate was highest in Norton Sound (Figure 22), at one station in the Bering Strait region in Anadyr water (characterized by high nutrient input from the Anadyr Current which flows from the Gulf of Anadyr into Bering Strait), and at the same northernmost stations with high bottom nitrate. Surface phosphate was also highest (> 1 µM) at the head of Barrow Canyon, and at one station in Bering Strait (Figure 23). Surface ammonium was very low (< 2 µM) at all stations except for the inner-most station at 70.5 °N at the head of Barrow Canyon (Figure 25). Similar to nitrate and silicate, bottom ammonium and phosphate were highest in the northern most region and offshore Kotzebue Sound. Overall nutrient concentrations appeared higher in 2012 than in 2013 (particularly evident for ammonium).

Integrated chlorophyll a (chla, Figure 27)) was highest (> 100 mg m-2) at stations south of Bering Strait and offshore of Kotzebue Sound (areas with higher nutrient concentrations), similar to 2012. Both these regions are designated biological observatory (DBO) stations with a history of high primary production. The high chla in the Bering Strait location is due to the high surface nutrients in the Anadyr water mass found in this area. Relatively high integrated chlorophyll

(50-100 mg m-2) was also seen offshore at 61.5 °N and inshore at 70 °N. The lowest chla was observed at near-shore stations south of 70 °N and offshore at 70 to71.5 °N. The average integrated chla was lower in 2013 than in 2012 (P < 0.05).

Data for large phytoplankton size fractions (> 10 µm particles) was only available for the Chukchi Sea in 2013. The percent large size phytoplankton (>10 µm/total chla) were highest offshore of Kotzebue Sound and near Hanna Shoal (> 50% large), suggesting that large taxa, such as diatoms or dinoflagellates, may make up a greater portion of the total chla at these locations. The fraction of large phytoplankton near Hanna shoal was lower in 2012 than in 2013, suggesting variations in phytoplankton community composition between years. Future analyses will focus on additional comparisons between 2012 and 2013, such as the depth of the phytoplankton bloom, nutrient ratios and relationship to environmental parameters.

2013 Beaufort Canyon Exploration

Near the ice edge, the freshwater sill was prominent. Overall, the temperatures were very cold (-1.5 °C) below the shallow thermocline or freshwater sill. Above the thermocline temperature was ~ 1-3 °C. In Barrow Canyon (Figure 28), the FastCat instrument revealed a freshwater sill on top of Shelf/Pacific water on top of Atlantic water from the north. This was the location where Age1+ Arctic cod were caught.

Figure 28. Beaufort Canyon Arctic Cod acoustics profile (left) and the FastCat water column profile (right). You can see age class segregation of Arctic cod appears to be associated with water column temperature and salinity characteristics. (Remember, water is most dense at ~4 degrees Celsius; with decreasing temperature thereafter becoming less dense as it starts to crystalize into ice. Hense, the warmer, saltier Atlantic water below the colder, less saline Pacific water!)

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