Bridging physics and biogeochemistry: how will increased freshwater from Greenland affect the ocean?

With the annual discharge of meltwater and ice increasing from the Greenland Ice Sheet in response to anthropogenic climate change, there is an obvious need to assess how this cold freshwater will affect the ocean both in terms of physics, but also biology and chemistry. The potential impact of increasing discharge upon the marine food web is especially important around Greenland given the reliance of the country’s economy on its fisheries.

Studying Greenland’s large marine-terminating glaciers is however easier said than done! Ice conditions in Greenland’s fjords are difficult to predict and often thwart even the best-prepared scientific expeditions from reaching within a few kilometers of the glacier fronts where freshwater discharge first emerges into the ocean. Access in winter is even more challenging and thus most of our water column observations come from the peak of the summer meltwater season in a limited number of fjords.

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Sampling ice and low salinity waters close to Greenland’s glaciers requires the deployment of small boats to maneovre around ice mélange.

However, thanks largely to sustained, on-going monitoring efforts from the Greenland Climate Research Centre at a select number of locations around Greenland, we are now beginning to understand the biogeochemical effects of increased freshwater on the marine environment. As oceanographers, one particularly interesting phenomenon is the pronounced summertime phytoplankton blooms that occur in some, but not all, of Greenland’s fjords. A long-standing hypothesis is that meltwater drives summer phytoplankton blooms and thus increased melting of Greenland’s ice sheet could enhance marine phytoplankton growth. Yet summer phytoplankton blooms around Greenland are very location specific; we see them very regularly in some fjords, yet not at all in others. So why such a contrast?

Recent studies both in Bowdoin fjord (Kanna et al., 2018)in north Greenland and Godthåbsfjord(Meire et al., 2017)in south Greenland have hinted at the answer. Working as close as possible to multiple glacier systems, these studies revealed that the same underlying mechanism drives high summertime productivity. When cold, buoyant meltwater emerges from beneath a marine-terminating glacier, which often lies hundreds of meters below sea-level, it rapidly mixes with deep nutrient-rich seawater. This buoyant mix known as a upwelling plume then continues to rise upwards in the water column, carrying ‘extra’ nutrients from the deep ocean with it. In this way, the relatively small amount of nutrients that go into the ocean from meltwater itself are transformed into a much larger input by adding in all of the nutrients that mixed in from deep seawater. Whilst this processes has been speculated to be a critical mechanism for controlling summertime phytoplankton growth across Greenland, it wasn’t clear to date just how important it is.

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Icebergs are particularly challenging to sample, the ‘fortune favours the brave’ approach preferred by some would likely fall foul of most risk assessments…

By combining field observations from research vessels with a numerical model of the buoyant, rising meltwater in 12 Greenland fjords where we know enough to estimate what the annual pattern of freshwater input looks like (Carroll et al., 2016), it is clear that the magnitude of this nutrient upwelling mechanism is much larger than previously estimated. The upwelled plume of nutrients in just these 12 glacier systems is over 10 times larger than the entire amount of nutrients from surface meltwater that flows into the ocean from all of Greenland. In other words, this deep nutrient ‘pump’ actually provides the largest nutrient input to the ocean associated with the Greenland Ice Sheet (Hopwood et al., 2018). This underlines the importance of inter-disciplinary work in understanding these systems. By combining biogeochemical data with models that describe the dynamics of ice melt and upwelling plumes in fjords, we can create powerful tools for investigating how glacial freshwater perturbs biogeochemical cycles.

As ever, the natural environment is complex, and in this particular case combining physics and chemistry also reveals something else. One particularly critical factor dictating the magnitude of the nutrient flux to the surface is the glacier’s grounding line depth (how deep it lies below the ocean surface). Therefore, in the context of Greenland’s marine-terminating glaciers, we must consider both the changes in grounding line depth and the changes in meltwater volume in order to quantify future shifts in nutrient availability in the future ocean.

It is always incredibly difficult to generalize about Greenland’s fjords because they vary so much in terms of the features that affect residence time; thus each glacier-fjord system is best considered as a unique entity (Straneo and Cenedese, 2015). Yet one thing is very clear; as marine-terminating glaciers retreat inland the potential for the upwelling nutrient pump to sustain high summertime productivity diminishes rapidly. Even with vastly increasing meltwater, or changing freshwater composition, the nutrient pump still collapses as the glacier retreats, shoals, and transitions from a marine-terminating to a land-terminating glacier.

The loss of this nutrient delivery system is bad news for phytoplankton and will be a negative influence on the future productivity of these fjords in the long-term, with potential consequences for Greenland’s fisheries. Yet there is still much to learn about these fjords. Any modeling approach is still constrained by what we don’t know about freshwater discharge around Greenland. The lack of year-round data, of measured seasonal freshwater discharge trends, and the almost complete absence of data from north Greenland leaves major holes in our knowledge of how the Greenland Ice Sheet currently affects ocean biogeochemistry. Thus there is an urgent need to develop new technology and work in more innovative ways to understand these frigid environments at a time when environmental change is proceeding at unprecedented rates.

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RV Sanna, a Greenlandic ship often deployed by the Greenland Institute of Natural Resources, conducting fisheries research in Godthåbsfjord May 2014.Photos courtesy of Thomas Pedersen (@TJuulPedersen)

 

This post was written by Mark James Hopwood, GEOMAR Helmholtz Centre for Ocean Research Kiel, Germany, @markinthelab, and Dustin Carroll, NASA Jet Propulsion Laboratory, USA, @dcarroll_sci

Recommended reading

Carroll, D., Sutherland, D. A., Hudson, B., Moon, T., Catania, G. A., Shroyer, E. L., Nash, J. D., Bartholomaus, T. C., Felikson, D., Stearns, L. A., Noël, B. P. Y. and van den Broeke, M. R.: The impact of glacier geometry on meltwater plume structure and submarine melt in Greenland fjords, Geophys. Res. Lett., 43(18), 9739–9748, doi:10.1002/2016GL070170, 2016.

Hopwood, M. J., Carroll, D., Browning, T. J., Meire, L., Mortensen, J., Krisch, S. and Achterberg, E. P.: Non-linear response of summertime marine productivity to increased meltwater discharge around Greenland, Nat. Commun., 3256, doi:10.1038/s41467-018-05488-8, 2018.

Kanna, N., Sugiyama, S., Ohashi, Y., Sakakibara, D., Fukamachi, Y. and Nomura, D.: Upwelling of macronutrients and dissolved inorganic carbon by a subglacial freshwater driven plume in Bowdoin Fjord, northwestern Greenland, J. Geophys. Res. Biogeosciences, 123, doi:10.1029/2017JG004248, 2018.

Meire, L., Mortensen, J., Meire, P., Juul-Pedersen, T., Sejr, M. K., Rysgaard, S., Nygaard, R., Huybrechts, P. and Meysman, F. J. R.: Marine-terminating glaciers sustain high productivity in Greenland fjords, Glob. Chang. Biol., 23(12), 5344–5357, doi:10.1111/gcb.13801, 2017.

Straneo, F. and Cenedese, C.: The Dynamics of Greenland’s Glacial Fjords and Their Role in Climate, Ann. Rev. Mar. Sci., 7, 89–112, doi:10.1146/annurev-marine-010213-135133, 2015.

 

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