Changes in Arctic Ocean Primary Production from Kevin R. Arrigo G. L. van Dijken

Transcription

1 Changes in Arctic Ocean Primary Production from Kevin R. Arrigo G. L. van Dijken Department of Environmental Earth System Science Stanford University

2 Background Circulation influenced by both Atlantic and Pacific waters Atlantic (red): Warm, salty, low nutrient, deep Pacific (blue): Cold, fresh, high nutrient, shallow Cold halocline layer restricts influx of nutrients from depth

3 Background Extensive continental shelves (53% of area) - Highly productive Large input of freshwater from rivers Relatively low surface water nutrients High nutrients in deep basins Bathymetry (m)

4 Background Sea ice dynamics are important Maximum ice extent Minimum ice extent

5 Background Sea ice dynamics are important Decreasing trend in summer minimum ice cover ~20% drop

6 Background Long term decline in sea ice attributed to: Increased advection of warm water into Arctic Ocean (Steele and Boyd 1998, Dickson et al. 2000, Maslowski et al. 2001, Shimada et al. 2006) Atmospheric circulation favoring ice advection out of Arctic (Rigor and Wallace 2005, Maslanik et al. 2007, Serreze et al. 2007) Increased Arctic temperatures due to greenhouse warming (Rothrock and Zhang 2005, Lindsay and Zhang 2005) Above processes result in strong ice albedo feedback (Perovich et al. 2007)

7 Given ongoing changes in the Arctic Ocean How has primary production changed in recent years?

8 Background How primary production was calculated Algorithm modified from Southern Ocean (Arrigo et al. 2008, Pabi et al. 2008) Based on remotely sensed SST, Chl a, and sea ice Forced with winds, cloud cover, and solar radiation

9 Background How primary production was calculated Different Chl a algorithms and sensors give different results Empirical OC3 and OC4v4 Chl a algorithms exhibit similar interannual patterns Semi-analytical GSM algorithm, which estimates Chl a, CDOM, and backscattering, gives low values -Chla not well validated

10 Background How primary production was calculated Used merged Chl a product from MODIS and SeaWiFS Validated primary productivity algorithm using in situ Chl a and primary production

11 Background How did primary production in the Arctic vary prior to 2007? Tg C yr -1 from (Pabi et al. 2008) Non-significant increase in annual primary production 1998 within 10% of Sakshaug (2003): 329 Tg C yr -1 Pabi et al. (JGR, 2008)

12 Changes in Arctic Sea Ice Cover

13 Changes in Arctic Sea Ice Cover Summer minimum sea ice cover dropped dramatically in 2007 and

14 Changes in Arctic Sea Ice Cover Difference ( ) Large area of Arctic Ocean was exposed for first time Approximately 1.3 x 10 6 km 2 (area in red) New ice-free pelagic habitat Arrigo et al. (GRL, 2008)

15 Changes in Arctic Productivity How has Arctic primary production changed since 2006? Annual Primary Production Tg C 544 Tg C 480 Tg C 2007 and 2008 are the most productive years on record Between 2006 and 2007, production increased by >15% Only 30% of this increase was due to increased open water habitat in 2007 (Arrigo et al., GRL 2008)

16 Changes in Arctic Productivity 70% of 2007 increase in primary production related to longer growing season

17 Changes in Arctic Productivity Similar pattern in 2008, but not to same degree as in 2007

18 Changes in Arctic Productivity QUESTION: If interannual changes in production are tied to changes in sea ice cover, why was annual primary production in 2008 so much less than in 2007? In 2008, the summer minimum sea ice extent was similar to that in 2007 Shouldn t annual production be similar?

19 Changes in Arctic Productivity It turns out, summer minimum ice cover is not a particularly good predictor of annual primary production Annual production is more highly correlated with annual mean open water area This metric is better because it incorporates changes in length of the open water season

20 Changes in Arctic Productivity Although the summer minimum ice cover in 2007 and 2008 were similar, the length of the open water season was reduced in 2008 Daily production was similar between years More open water in 2007 all year As a result, annual production was greater in 2007 than in 2008

21 Spatial Changes in Arctic Productivity Annual production Mean (Tg C yr -1 ) Largest increase In : Beaufort, Chukchi, East Siberian, Laptev (25-75%)

22 Temporal Changes in Arctic Productivity These 4 sectors also exhibited the largest interannual differences in the timing of the spring bloom over the last 3 years 39 days 27 days 26 days 48 days

23 Temporal Changes in Arctic Productivity Related to changes in the timing of increase in open water area 21 days 31 days How will organisms respond to changes in the timing of the spring bloom? 28 days 40 days

24 Changes in Arctic Productivity Annual primary production increased by 140 Tg C yr -1 between 1998 and 2008 (statistically significant trend) A 40% increase over the last decade Unexpected given presumed nutrient limitation Largest increases on continental shelf

25 Changes in Arctic Productivity If current trends in mean annual open water area continue, annual Arctic primary productivity could increase by 14 Tg C yr -1 each year Are continued increases in primary production sustainable? Would require additional nutrient input Increased shelf-break upwellling? (Carmack and Chapman 2003)

26 Impacts of Changes in Arctic Productivity Increased flux of organic carbon Higher benthic-pelagic coupling? May depend on mechanism for sea ice loss: 1) Atmospheric circulation - Could reduce ice cover while ocean was still cold - Algae bloom in cold water - Reduced grazing losses 2) Ocean warming - Algae bloom in warm water - Increased grazing losses

27 Impacts of Changes in Arctic Productivity Increased flux of organic carbon Atlantic water More denitrification on Arctic shelves? Less nitrogen (more excess phosphorus) entering N. Atlantic? Will nitrogen loss be compensated for by greater N 2 -fixation in Atlantic? Is there enough Fe? Yamamoto-Kawai et al. (2007)

28 Conclusions Changes in Arctic Ocean productivity related to changes in sea ice cover Both the magnitude and timing of production are changing 40% increase in production over last decade represents a weak negative feedback on atmospheric CO 2 concentration and greenhouse warming If further loss of sea ice is accompanied by processes that increase upward flux of nutrients, Arctic productivity could increase even more in the future Many ecological and biogeochemical ramifications