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Impacts of Greenland Ice Sheet melt on primary productivity and carbon cycling in Greenland coastal ecosystems

Periodic Reporting for period 1 - GrIS-Melt (Impacts of Greenland Ice Sheet melt on primary productivity and carbon cycling in Greenland coastal ecosystems)

Reporting period: 2017-07-01 to 2019-06-30

Anthropogenic climate warming is causing the Greenland Ice Sheet (GrIS) to melt at accelerating rates. This melting contributes to the increasing the freshwater content of the Arctic Ocean. Freshwater run-off from the GrIS influences circulation patterns and stratification of the water column and also introduces large fluxes of allochthonous matter into the Greenland coastal environment, both of which have the potential to influence microbial carbon cycling. Changing patterns of circulation and stratification have the potential to effect water column mixing and thus nutrient supply to primary producers, while introduction of particles and suspended sediments can limit light availability to primary producers and provide increased surface area for bacterial activity. Recent studies also documented that GrIS melt water contains significant amounts of bioavailable organic carbon, which could provide subsidies to microbial communities in the fjord, but the overall impact of this is unknown. Until now, studies have focused on quantifying the flux of allochthonous matter and nutrients in GrIS melt water to the fjords, and merely assuming its impact on carbon cycling. The aim of this study is to actually quantify the downstream impacts of GrIS melt and its constituents on primary production and carbon cycling. Primary producers are the base of the marine food web and support commercial and sustenance fisheries along the Greenland coast. Melting of the GrIS is expected to intensify in the future. Thus, this project proposes a timely effort to determine the impact of GrIS melt water on productivity and carbon cycling in the Greenland coastal ecosystem, the results of which can be used by climate scientist and fisheries managers to predict the future productivity of a locally and globally important ecosystem.
The first major activity of the project took place in August 2017 when I participated in a field campaign to Young Sound. There I set-up a mesocosm experiment to determine the experimental response of adding GrIS melt water to plankton communities from the fjord. Specifically, I was interested in seeing the effects on microbial communities and degradation of the carbon introduced from the GrIS melt water. Initial results indicate an enhanced growth of microbial communities in treatments amended with GrIS melt water. While there was some incorporation of terrestrial carbon, much of the carbon incorporated into microbial biomass was of autochthonous origin. Instead, results suggest that it was actually the formation of flocculated particles from the addition of GrIS meltwater that enhanced bacterial remineralization of organic matter rather than the carbon itself in the melt water.

The second major activity of the project took place in August 2018 when I participated in a field campaign to East Greenland. Along with colleagues I sampled microbial communities to measure primary productivity and bacterial productivity along transects running out fjords. Additionally, I measured stable isotopes of prokaryotic-specific fatty acid biomarkers to determine the contribution of both allochthonous and autochthonous carbon sources to the microbial food web.
Preliminary results indicate gradients of primary production from inner fjord to outer fjord, whereby communities sampled in shelf waters are more productive than those sampled in the inner fjord. Prokaryotic communities in surface water and in the inner fjords have a higher proportion of their production associated to particle attached phases and these communities are more productive overall. Finally, while organic carbon introduced by GrIS melt is highly bioavailable to microbial remineralization, the concentrations of organic carbon in GrIS melt water are an order of magnitude lower than the carbon already available in the fjords, thus organic carbon from GrIS melt water does not appear to subsidize microbial communities in the fjords to a large extent as has previously been suggested in the dominant literature on the topic. Rather, it is likely the particles and subsequent attachment by bacteria rather than subsides of carbon coming in with GrIS melt water are more important to enhance bacterial production in fjords.

Some of the results pertaining to this project have been already been presented in the following peer-reviewed scientific publication and the remaining results will also be exploited in the form of scientific journal publications.

Publication in peer-reviewed open-access journal:

Holding JM, Markager S, Juul-Pedersen T, Paulsen ML, Møller EF, Meire L, Sejr MK (2019) Seasonal and spatial patterns of primary production in a high-latitude fjord affected by Greenland Ice Sheet run-off. Biogeosciences 16:3777–3792.
Results from this project so far, support the conclusions that freshwater induced stratification and light limitation—from introduced particles and suspended sediments—directly introduced by GrIS melt water are the primary influences to primary production within Greenland fjords. Additionally, preliminary results suggest that the organic carbon introduced by GrIS melt is highly bioavailable to microbial remineralization, however, the concentrations of organic carbon in GrIS melt water are an order of magnitude lower than the carbon already available in the fjords, thus we can conclude that organic carbon from GrIS melt water may not subsidize microbial communities in the fjords to a large extent as has previously been suggested in the dominant literature on the topic. Melting of tidewater glaciers attached to the GrIS often comprises mechanisms for upwelling of nutrient-rich water from deep below the limits for phytoplankton production, hence supporting large primary production in some fjords. This is the only mechanism that we can attribute to the enhancement of primary production under increasing GrIS melting scenarios, however as glaciers continue to recede, this mechanism for nutrient renewal to surface waters will become extinct, leaving fjords at the mercy of freshwater stratification that so limits primary productivity, and the cascading effects this may have on the food web and associated industries that rely on it.
Sampling GrIS meltwater run-off in a fjord in Greenland