Numerous posts on our forums have sought to infer from electromagnetic theory (or just plain intuition) how sunlight ought to interact with sea ice and the water underneath, the issue being the distribution of heat from absorbed light -- a principal driver of the melt season -- along the snow-ice-water column. However those notions do not capture the incredible complexity actually observed in field campaigns. Those in turn are necessarily limited by logistics in areal scope and season.
The N-ICE2015 expedition ran from winter to late spring, whereas almost all previous field data relates to summer (as modestly instrumented unattended winter buoys soon fail or start reporting questionable numbers). N-ICE2015 observed an unprecedented event in late May at 80-81ºN, a very substantial phytoplankton bloom 80 km north of the ice edge. The growth was local, not advected in by Atlantic Waters but perhaps seeded from elsewhere.
The primary species involved, haptophyte algae Phaeocystis pouchetii, is not part of the more familiar ice-underside pennate diatom community and their blooms. Diatoms deplete silicic acid in the upper 50m in building their cell walls (frustules), not observed here.
Being beneath a snow-covered surface ice, the Phaeocystis bloom could not be detected even under leads, much less quantitated, by satellite (segmentation: 1st image) Snow-covered thick ice transmitted only 1% of incident photosynthetically appropriate solar energy to the underlying water column, compared to 20% for refrozen leads created by the frequent ice divergence events in the oft-dodgy ice north of Svalbard. Melt ponds are not applicable light availability in May as snowmelt only began in early Jun
(Provided skies are clear, large blooms in open water do show up well in WorldView. JayW posted about a bloom already in the Aleutians; last August, massive and persistent blooms covered the north Barents.)
The complex mix of species in these blooms must be determined by sampling; they are never monocultures nor do species proportions remain static over the course of a bloom.The one studied here was comprised of 104 taxa of protistic plankton at 5m depth: diatoms, dinoflagellates, ciliates, prasinophytes, cryptophytes, choanoflagellates, flagellates, chlorophytes and euglenozoa, dominated by the prymnesiophyte Phaeocystis.
As the Arctic Ocean enters a new era of predominantly first-year ice, one might expect more sunlight would reach photosynthetic organisms at wavelengths relevant to chlorphyll capabilities, even prior to melt pond season. The resulting growth in the upper ten meters would then effectively capture this light, putting the resultant heat in a more favorable position to bottom-melt the ice than had the sunlight continued on to depth. Here the bloom correlated strongly with shoaling of the pycnocline bringing reduced turbulent mixing which increased residence time in the surface layer and so light available to photosynthetic plankton.
While an upper layer of snow can block transmission, frequent leads in this weaker ice open and refreeze (initially without any snow cover) creating significant areas of reasonably transparent ice becomes available on the lead surfaces. An algae like Phaeocystis, well-adapted to dim and variable light intensity, can then flourish during long spring days despite ice drift and cold temperatures, to the point the bloom
severely depletes available nitrogen to a depth of 50 meters (2nd image below).
The overall nitrogen cycle in the Arctic is very complex (see doi:10.1038/ncomms13145). Ferric iron is not a rate-limiting essential nutrient in the Arctic as it is in the Southern Ocean. Nitrogen depletion by Phaeocystis will diminish melt season diatom blooms that do sink to depth “with far-reaching repercussions on bloom timing and composition, strength of the biological carbon pump and energy flow through Arctic marine food webs”.
Another notion, floated by climato-optimists, is that while
blooms are bad for surface water heating and ice bottom melt, they more than make up it with deep carbon sequestration. In this view, a blue Arctic ocean provides a game-changing offset for atmospheric emissions. However this concept is wishful thinking for Phaeocystis blooms — little fixed carbon dioxide makes it down to seafloor sediment but instead adds to near-surface opacity.
Prior to the RV Lance, an under-ice Phaeocystis bloom in May fell into the ‘unknown unknowns’ category of positive feedbacks contributing to global climate change, whereas ocean-wide blooms in a post-apocalyptic Arctic remain ‘known unknowns’ not considered ripe for inclusion into climate models until mid-century. As the biogeochemistry of the Arctic Ocean is already impossibly complex, it is delusional to think its rapid future change can be modeled in any meaningful way (but see doi: 10.1002/gbc.20055).
Leads in Arctic pack ice enable early phytoplankton blooms below snow-covered sea ice
P Assmy et al Jan 2017
https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5244362/ free full text, summarized above
Effects of an Arctic under-ice bloom on solar radiant heating of the water column
T Taskjelle et al Jan 2017 DOI: 10.1002/2016JC012187 free full text
The deposition of solar energy in the upper Arctic Ocean depends on the composition of the water column. The May 2015 bloom led to significant changes in the inherent optical properties (IOPs) of the upper ocean. Simulations are used to investigate the change in depth-dependent solar heating of the ocean after the onset of the bloom, for wavelengths in the region 350–700 nm. Effects of clouds, sea ice cover, solar zenith angle, as well as the average cosine for scattering of the ocean inclusions are evaluated. An increase in energy absorption in the upper 10 m of about 36% is found under 25 cm ice with 2 cm snow relative to pre-bloom conditions with implications for ice melt and growth in spring. Thicker clouds and lower sun reduce the irradiance available, but lead to an increase in relative absorption.
Altered inherent optical properties and estimates of the underwater light field during an Arctic under-ice bloom of Phaeocystis pouchetii
AK Pavlov JGR Apr 2017 DOI 10.1002/2016JC012471
Absorption and scattering in the upper 20 m of the water column at visible wavebands increased 3x and 10x relative to pre-bloom conditions. Absorption by colored dissolved organic matter 2x. Total absorption by phytoplankton particles increased 10x. The ratio between photosynthetically active radiation and downwelling planar irradiance below sea ice reached 1.85. Our findings could help to improve light parameterizations in primary production models.
Windows in Arctic sea ice: Light transmission and ice algae in a refrozen lead
HM Kauko et al Jun 2017 DOI: 10.1002/2016JG003626
The Arctic Ocean is rapidly changing from thicker multiyear to thinner first-year ice cover with significant consequences for radiative transfer through the ice pack and light availability for algal growth. A thinner, more dynamic ice cover will result in more frequent leads covered by newly formed ice with little snow cover. We studied a refrozen lead 0.27m thick in drifting pack ice, measuring downwelling incident and ice transmitted spectral irradiance, colored dissolved organic matter, particle absorption, ultraviolet-protecting mycosporine-like amino acids and chlorophyll in melted sea ice samples (which are troubled by osmotic shock artifacts). Leads are important for phytoplankton growth by acting like windows into the water column.
The seeding of ice-algal blooms in Arctic pack ice: the multiyear ice seed repository hypothesis
LM Olsen et al Jun 2017 DOI: 10.1002/2016JG003668
The physical properties and ice algal community composition was investigated in the three different ice types during the winter-spring-summer transition. Algae remaining in sea ice surviving the summer melt season are subsequently trapped in the upper layers of the ice column during winter and may function as an algal seed repository.
Once the connectivity in the entire ice column is established as a result of temperature-driven increase in ice porosity during spring, algae in the upper parts of the ice are able to migrate towards the bottom and initiate the ice-algal spring bloom. Furthermore, this algal repository might seed the bloom in younger ice formed in adjacent leads.
This mechanism was studied in detail for the {blue]often dominating ice diatom Nitzschia frigida{/blue].The proposed seeding mechanism may be compromised due to the disappearance of older ice in regime shift toward a seasonally ice-free Arctic Ocean.