Impact of Arctic sea-ice retreat on zooplankton foraging behaviour and vertical carbon flux
Impact of Arctic sea-ice retreat on zooplankton foraging behaviour and vertical carbon flux M. Wallace1, A. Brierley1, G. Tarling2, F. Cottier3, J. Be...
Impact of Arctic sea-ice retreat on zooplankton foraging behaviour and vertical carbon flux M. Wallace1, A. Brierley1, G. Tarling2, F. Cottier3, J. Berge4, C. Griffiths3
University of St Andrews 2 British Antarctic Survey 3 Scottish Association for Marine Science 4 University Centre in Svalbard
1
Wallace et al. 2009; Limnology and Oceanography (submitted)
NASA
http://www.ngdc.noaa.gov
Linking Arctic Observatories
Rijpfjorden •North facing
Rijpfjorden [2006-08]
•Polar Waters •Seasonal ice cover •Short, late bloom Kongsfjorden •West facing •Atlantic Water •Ice Free •Long summer bloom
Kongsfjorden [2002-2012] Th10
Diel Vertical Migration (DVM) •
Classical View: Zooplankton feed in shallow waters then retreat to depth to escape visual predation
•
Zooplankton (via DVM) contribute to vertical export of carbon through production of faecal pellets
•
DVM is considered to be light mediated. What is the impact of intense seasonality and sea ice on DVM in the Arctic?
•
No DVM or Synchronised DVM or Asynchronous migration
•
Changes in Arctic sea ice cover are expected to influence zooplankton migration behaviour
•
What are the impacts on carbon flux?
Mooring Data (300 kHz ADCP) Kongsfjorden
Rijpfjorden
ADCP depth
~100 m
~100 m
Bottom depth
~210 m
~216 m
Dep. 1
Jun 06 – Aug 07
Aug 06 – Aug 07
Dep. 2
Aug 07 – Aug 08
Sep 07 – Aug 08
Data from 4th September – 13th August: 49 weeks
Depth (m)
Rijpfjorden 2006-07
Depth (m)
Rijpfjorden 2007-08
Depth (m)
Kongsfjorden 2006-07
Depth (m)
Kongsfjorden 2007-08
Backscatter -100
dB
-60
Depth (m)
Rijpfjorden 2006-07
Depth (m)
Rijpfjorden 2007-08
Depth (m)
Kongsfjorden 2006-07
Depth (m)
Kongsfjorden 2007-08
Backscatter -100
dB
-60
Depth (m)
Rijpfjorden 2006-07
Depth (m)
Rijpfjorden 2007-08
Depth (m)
Kongsfjorden 2006-07
Depth (m)
Kongsfjorden 2007-08
Backscatter -100
dB
-60
Depth (m)
20
RIJPFJORDEN 80
Depth (m)
20
Depth (m)
Rijpfjorden 2007-08
20
80
Kongsfjorden 2006-07
•No clear pattern in the backscatter or v-vel at Rijpfjorden. •Asynchronous migration or migration related to predation pressures.
•Weak DVM signal during periods of the summer
20
80
Rijpfjorden 2007-08
Kongsfjorden 2006-07
KONGSFJORDEN
80
Kongsfjorden 2007-08 Depth (m)
Rijpfjorden 2006-07
Rijpfjorden 2006-07
•Asynchronous migration for the majority of time
Kongsfjorden 2007-08
Behaviour Summary •
The annual pattern of DVM behaviour at the two sites is broadly similar – patterns diverge only in the summer months
•
Strong Classical DVM is a robust feature of each system during Spring and Autumn – irrespective of ice and/or species – with approximately the same switch on/off time.
•
Distinct differences in migration patterns between the two fjords during summer
•
Different oceanographic environments => difference in food availability and biological communities
•
Decrease in sea ice extent on shelves could lead to earlier blooms, change in the species composition and changes in the summertime vertical migration patterns
Implications for Carbon flux
Implications for Carbon flux •Copepods are an important zooplankton species at both fjords •Copepod faecal pellet (FP) sinking contributes to the downward flux of particulate organic carbon (POC) Wexels-Riser et al., 2007 •On the way down, FP matter can be broken down, reingested etc, reducing downward flux of POC •Vertical migration increases flux (animals feed at surface, then sink and release pellets at depth) => Presence/absence/type of vertical migration during the summer (bloom season) is important for POC flux on Arctic shelves
FP Distribution Model
(Figure from G. Tarling, BAS)
FP Distribution Model Parameters: DVM Type: None, Synchronised, Asynchronous Bloom Profile: Feeding where Chl-a > 3 μg/l Satiation Level: Hungry > Full, ingesting/digesting Feeding rate: 1 stomach per 30 min Digestion rate: 1 stomach per 60 min Depth: Determines feeding and swim speed Swim speed: Depends on migration type/depth/time FPC production: Uses an empirical relationship linked to Chl-a Run: 300 animals for 5 days
No migration
DVM
-1 to 3 cm/s
Empty stomach -1 to 1 cm/s
2 to 5 cm/s
-5 to -1 cm/s
If FULL -2 to -1 cm/s
-5 to -3 cm/s
DVM migration
Asynchronous “Foray-Type” Migration Periods of no synchronised migration during midnight sun •
net hauls show no change in vertical distribution of animals over day/night cycle
•
No clear DVM observed in ADCP data but animals are migrating (e.g. Cottier et al., 2006)
Asynchronous migration
-5 to -1 cm/s -1 to 3 cm/s
If FULL -5 to -3 cm/s
-2 to -1 cm/s
If EMPTY 2 to 5 cm/s
Asynchronous migration
Summary •Vertical migration is associated with more FP at depth (increased carbon drawdown) compared with no migration •Asynchronous migration leads to more FP at depth than DVM •Sea ice appears to be important for vertical migration indirectly by influence on food availability and biological community composition •Reduction of sea ice on Arctic shelves will almost certainly lead to increase in POC flux through enhanced productivity, greater abundance of migrators and active DVM processes.
