Between 2003 and 2010, the Greenland Ice Sheet and its associated glaciers experienced a mean annual mass loss of 186 Gt, double the rate between 1983 and 2003. Though this mass loss has been linked to global sea-level rise through meltwater discharge, heightened glacial runoff has also been hypothesized to have another important effect: increasing marine primary productivity through nutrient fertilization. This hypothesis was the focus of a recent study published in Nature Communications, which reports that the upwelling of nitrate-rich deep seawater driven by subglacial discharge— not the meltwater itself— is likely the main driver of the increased productivity.
This question about the impact of heightened glacial runoff is important both for academic research on marine ecosystems and for assessing the future of oceans to serve as carbon sinks. The photosynthesis represented by primary productivity is one of the key mechanisms through which carbon dioxide dissolved in seawater can be captured and retained in the oceans.
During the spring, marine primary productivity off the coast of Greenland increases as phytoplankton bloom. Then, in the summer, productivity usually diminishes. Recently, however, there have been summer phytoplankton blooms accounting for up to half of annual primary productivity. The goal of the study was to examine these changes to summer productivity and see how they relate to nutrient availability during the meltwater season.
The researchers first assessed which nutrient deficiency limits summer primary productivity off of Greenland. In most parts of the high-latitude Atlantic, summer primary productivity is limited by iron or nitrate deficiencies. However, in Greenland, few studies had previously examined the nutrient limits to phytoplankton blooms.
The researchers found that iron values were the most positive near the coasts, while offshore values were close to zero. On the other hand, nitrate values were deficient near the coasts and offshore. These results indicate that iron may help trigger the summer blooms while also inhibiting the drawdown of nitrate by plankton, leading the researchers to conclude that the availability of nitrate is likely the constraint on summer primary productivity.
Is heightened glacial runoff supplying more iron and nitrate, contributing to the summer phytoplankton blooms? Iron concentrations from glacial runoff were comparatively low, unlikely to trigger the blooms given the already iron-rich waters, the authors concluded. Furthermore, in Greenland, glacial runoff supplying iron can have a negative impact on primary production. It has this effect by reducing the availability of other nutrients and by creating cloudy sediment plumes from glacial flour composed of fine-grained rock particles created by glaciers grinding over underlying bedrock. These cloudy plumes limit light availability, says lead author Mark Hopwood, who spoke with GlacierHub about the paper. In contrast, he said, nitrate concentrations were found to be even lower than iron ones, only enough to have a very small effect on phytoplankton blooms.
While the meltwater from glacial runoff is unlikely to be the trigger of the summer plankton blooms off Greenland, the researchers determined marine-terminating glaciers to represent another aspect of glacial discharge.
Unlike their land-terminating counterparts, marine-terminating glaciers discharge meltwater through sub-glacial plumes. This discharge, once injected into the water at the glacial grounding line, entraps nutrient-rich deep seawater in a rising plume. This upwelling, if it occurs at the right depth, takes nitrate-rich waters to the photic zone where light is sufficient for photosynthesis, driving the phytoplankton blooms.
The researchers found four scenarios through which plume upwelling affects nutrient delivery near marine-terminating glaciers, with glacial grounding line depth the primary influence on the efficacy of this delivery. Under the first scenario, a nutrient-rich plume is generated by sub-glacial discharge. However, the glacier is too deep, and the plume is unable to reach the photic zone. In the second scenario, the glacier is in the optimum depth zone, and the nutrient-rich deep sea water is upwelled to the photic zone, enhancing the phytoplankton bloom. In the third scenario, the grounding line depth shallows because of glacial retreat. This shallowing limits the amount of seawater entrapped by the sub-glacial discharge. The seawater that is entrapped lacks the nutrients of deeper waters, thereby lessening the positive effects of the upwelling on phytoplankton blooms. In the final scenario, the glacier has retreated inland and no longer ends in the ocean, so no upwelling is generated.
After delineating these four scenarios, the researchers next simulated the plume upwelling effect to find the optimum conditions for peak nitrate flux to be upwelled to the photic zone. According to Hopwood, each fjord-glacier system in Greenland has unique physical characteristics, such as fjord depth and annual discharge volume.
This means that the optimum conditions for each system varies regionally. As a general rule of thumb, shallow glacier grounding line depths below 100 m will likely lead to low productivity, while grounding line depths between 400 and 600 m will likely be linked with high productivity, according to Hopwood. Other factors also affect summer marine productivity including turbidity and the depth of the photic zone. However, the plume upwelling of nutrients appears to be the dominant factor.
The future of marine productivity off Greenland under climate change will be determined by glacier grounding line depths, which may remain as they currently are or migrate into the optimum zone for subglacial discharge, triggering the upwelling of nitrate nutrients. Shallow glacier grounding line systems are likely to have already experienced peak nitrate supplies, while the peak for deeper systems will likely occur in the future if current retreats continue. For the 243 Greenland glaciers that have been mapped for bed topography, 55 percent will retreat onto land in the future, reducing the ice sheet-to-ocean nutrient fluxes driving summertime phytoplankton blooms.
What happens to the plume upwelling of nutrients in Greenland ultimately depends on climate change and subsequent glacier retreats. One subject for future study that could help improve understanding of marine productivity is the influence of icebergs, says Hopwood. The largest icebergs usually extend far below the ocean surface, hypothetically allowing them to “act as miniature nutrient ‘pumps’ as they melt,” Hopwood told GlacierHub. This is similar to what occurs with glaciers on a larger scale. Yet icebergs are more difficult to study and will require interdisciplinary work between both physicists and chemists to examine how icebergs affect the water column and phytoplankton.
Taken together, this research on the effects of different kinds of glaciers on phytoplankton blooms is key to a better understanding of marine ecosystems, helping scientists to assess the ability of the oceans to serve as sinks for the carbon dioxide that we humans continue to release.