Effects of climate change on marine primary production in the Arctic

1. Impact of Climate Change on Marine
Primary Production in the Arctic
Malvi Golwala
29/11/2017

2. Introduction
• Primary production or net primary production (NPP) provides an
estimate of the organic material available to fuel the ocean’s food
webs.
• Changing climate due to global warming has many side effects in the
oceans as it does in the environment.
• Phytoplankton/Primary production is a link in carbon cycling between
living and inorganic stocks.
• Everyday, hundred million tons of carbon as CO2 is fixed into organic material
in the ocean by phytoplankton which is then transferred to marine ecosystems
by sinking and grazing.
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3. Introduction
• Phytoplankton grow in well-illuminated upper ocean.
• The basic requirements for phytoplankton growth are nutrients like nitrogen,
phosphorus, silicon and iron
• From nutrient rich deep waters that are upwelled and mixed with upper ocean waters with
an exception in iron that it receives from mineral continental dust as well.
• Satellite ocean colour sensors over the past decade have recorded a
huge change in NPP from 1,930Tg C yr-1 to 190Tg C yr-1.
• Temperature change in the upper ocean brings a change in stratification which
influences nutrient availability for phytoplankton.
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7. What’s going on in the Arctic?
• An yearly average increase of 27.5TgCyr-1 since 2003 and 35TgCyr-1
between 2006 and 2007 was observed in the Arctic.
• 30%: loss of sea ice and decreased minimum sea ice extent; which is due to
increasing temperature.
• 70%: longer phytoplankton growing season.
• Continuity in these trends would bring a 3-fold increase in in
productivity above 1998-2002 level
• Potentially altering marine ecosystem structure and the degree of pelagic-
benthic coupling.
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10. Why is NPP increasing in the Arctic?
• Due to increasing temperatures, sea ice in Arctic is melting, due to
which:
• Sunlight is more available than before
• Increased upwelling of nutrients due to wind influence on surface waters
• More water-to-environment interaction to absorb nutrients
• All of these factors positively influence the growth of phytoplankton
and hence then increase in NPP.

11. Arctic models
• While studying nutrient availability for NPP, a lot of controversies
took place between these models.
• Some overestimated surface nitrate due to incapability of reproducing vertical
mixing dynamics.
• Nitrate could have been overestimated because of insufficient nutrient uptake
by phytoplankton which depends on both temperature and irradiance
• both of which are changing due to climate change.
• There was also an understanding that NPP would reduce in the Arctic
in the future due to low surface nutrients because of regular enhanced
freshening from ice melting.
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12. Arctic models
• Some models that overestimated nitrate and the length of euphotic
zone, NPP should increase.
• There could also be a possibility that simulated NPP was low not due
to limiting factors but because of very little biomass which could have
been because of:
• Excessive loss of carbon from or in the upper water column
• Physiological response of phytoplankton to light showed that
subsurface plankton were acclimated to lower irradiance.
• Photo-acclimation, photosynthetic efficiency and max. chlorophyll.
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13. Plankton Blooms under Arctic Sea Ice
• Observations made on ICESCAPE cruise reported massive phytoplankton
blooms 0.8-1.3m beneath first year thick ice on Chukchi sea.
• ~4-fold greater than in open water; extended for >100 km into the ice pack.
• Biomass was greatest (>1000mgCm−3) near the ice/seawater interface and
was associated with nutrient depletion to depths of 20 to 30 m.
• indicative of phytoplankton, rather than ice algal, growth.
• Species composition of the bloom was distinct from that in the overlying ice
and was dominated by healthy pelagic diatoms.
• genera Chaetoceros, Thalassiosira, and Fragilariopsis.
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15. Plankton Blooms under Arctic Sea Ice
• Phytoplankton biomass in open waters was lower than beneath the ice
and lowest at depths of 20-50m because of nutrient depletion near the
surface.
• The high oxygen (480mmoll−1) and low dissolved inorganic carbon
(2020mmoll −1) relative to the low phytoplankton concentrations
(~150mgCm−3) in these nutrient-depleted waters suggest that they had
recently supported high rates of phytoplankton growth.
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16. Plankton Blooms under Arctic Sea Ice
• The light required by the under-ice bloom had to penetrate the fully
consolidated ice pack to reach the upper ocean.
• The fraction of first-year ice, is much thinner (0.5 to 1.8 m) than the
historically dominant multi-year ice pack (2 to 4 m), especially by a
high surface melt pond fraction (25 to 50%).
• Optical measurements showed that the ice beneath these melt ponds
transmitted 4-fold more incident light (47 to 59%) than adjacent snow
free ice (13 to 18%).
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17. Light Transmission through bare ice and melt
ponds
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18. Depth Integrated Chlorophyll and POC
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19. Plankton Blooms under Arctic Sea Ice
• Similar blooms were reported in the Barents Sea, Beaufort Sea, and
Canadian Arctic Archipelago, suggesting that under-ice blooms are
widespread.
• Thus, current rates of annual NPP on Arctic continental shelves, based
only on open water measurements, may be drastic underestimates,
being 10-fold too low in our study area.
• This solves the controversies by previous model that suggested low
NPP; which was due to lack of nutrients as they were being used by
blooms under ice.
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20. Summary
Fig. 16: Changes in annual NPP between 1998-2012 Fig. 17: Map showing percentage change in annual
NPP between 1998-2012
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22. References
1. Arrigo, K., Dijken, G., Pabi, S. Impact of a shrinking Arctic ice cover on marine primary
production. Geophysical Research Letters. 35 (2008).
2. Behrenfeld, M., O’Malley, R., Siegel, D., McClain, C., Sarmiento, J., et al. Climate-driven
trends in contemporary ocean productivity. Nature. 444, 752-755 (2006).
3. Lee, Y., Matrai, P., et al. Net primary productivity estimates and environmental variables in the
Arctic Ocean: An assessment of coupled physical-biogeochemical models. Journal of
Geophysical Research: Oceans. 121, 8635-8669 (2016).
4. Palmer, Molly., Dijken, G., Mitchell, B., et al. Light and nutrient control of photosynthesis in
natural phytoplankton populations from the Chukchi and Beaufort seas, Arctic Ocean.
Limnology Oceanography. 58, 2185-2205 (2013).
5. Arrigo, K., Perovich, D., Pickart, R., et al. Massive Phytoplankton Blooms Under Arctic Sea
Ice. Science. 336, 1408 (2012).
6. Arrigo, K., Perovich, D., Pickart, R., et al. Phytoplankton blooms beneath the sea ice in the
Chukchi sea. Elsevier. 105, 1-16 (2014).
7. Arrigo, K., Dijken, G. Continued increases in Arctic Ocean primary production. Elsevier. 136,
60-70 (2015).

23. Arctic models
• A research which used 21
regional and global
biogeochemical models
assessed NPP, Zeu
(euphotic layer depth),
sea ice concentration,
nitrate concentration and
mixed layer depths.
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Fig. 10: Log transformed distribution of in situ iNPP down to
100m

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25. Summary
• Early in the season, wintertime nutrient replenishment and regeneration increases
nutrient concentrations throughout the water column, with low light preventing net
photosynthesis. As solar elevation increases throughout the spring, the presence of
extensive sea ice results in little to no light penetration to the surface ocean, and
phytoplankton do not grow
• Once the snow has melted and light beneath the 100% sea ice cover exceeds the
compensation irradiance for phytoplankton net growth, the spring bloom develops
under the sea ice. Melt pond formation enhances light penetration through the ice,
accelerating the time to reach the light threshold necessary for photosynthesis.
This is aided by shade-adaptation by phytoplankton: our data show that low light
availability initially limits P* m, but nutrients are high so phytoplankton can
synthesize additional Chl a, allowing them to absorb more of the available light.
This large investment in light-harvesting machinery increases the photosynthetic
efficiency, growth rate, and accumulation of phytoplankton biomass under the ice.
Nutrients begin to be depleted from the surface waters under the ice pack
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26. Summary
• As the season progresses, UI phytoplankton utilize the enhanced light (both
from increased solar intensity in the summer as well as thinner sea ice) to
extend deeper in the water column, depleting nutrients at increasingly
greater depths. Our data show that phytoplankton may grow to depths of up
to 30 m under the ice as they move deeper to exploit nutrients, and also that
an SCM may also begin to develop once nutrients are exhausted in the
surface layers. The highest rates of P* M are associated with depths where
NO3 is still available and light is sufficient for photosynthesis. The
phytoplankton blooming under the ice are photo-acclimated to the low-light
conditions and maintain comparable levels of P* m, a*, a¯*, and Wm
between the surface and the SCM. Importantly, the phytoplankton
community growing near the surface has high light availability, whereas the
community growing in the subsurface has high nutrient availability
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27. Summary
• Finally, the ice melts and retreats, stratifying the water column and isolating
the nutrient-poor surface waters from nutrient rich waters below. As
described in Arrigo et al. (in press), it may be that in areas where there was
significant phytoplankton production under the ice, no bloom develops in
surface waters of the MIZ because of depleted nutrients, and the SCM in the
MIZ and OW zone could be considered a remnant under-ice bloom. Our
photo-physiological data are consistent with this idea, because it shows that
phytoplankton that had been growing in the low-light UI environment are
already acclimated to the low-light conditions of the OW SCM: the OW
subsurface communities have higher growth rates, Chl a :POC ratios, Wm,
and a*, and lower a¯* than those in the OW surface. The final phase of this
new paradigm described in Arrigo et al. (in press) is that as the OW SCM
becomes progressively deeper throughout the season, shade-acclimated
phytoplankton continue to grow well at depths where both light and
nutrients are available
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