Connecting production to glacial meltwater

Middelbo, A. B., Sejr, M. K., Arendt, K. E. and Møller, E. F. (2017), Impact of glacial meltwater on spatiotemporal distribution of copepods and their grazing impact in Young Sound NE, Greenland. Limnol. Oceanogr.. doi:10.1002/lno.10633

Marine plankton are sensitive creatures — they require very particular environmental conditions to grow. Scientists around the globe are actively investigating how populations of plankton respond to shifts in their environment. All kinds of things can influence planktonic growth: volcanic eruptions, storm activity, pollutant influx, and changing climate conditions. Basically any shift in a local system can affect what kinds of plankton grow, how quickly they reproduce, and how they interact with each other.

Figure 1 – The box in panel A highlights the study region. The inset in B indicates where the four stations are. Number 1 is closest to the glacier and 4 is in the Greenland sea (Adapted from Middelbo et al., 2017).

Melting glaciers are responsible for quite a few perturbations to nearby coastal ecosystems. The runoff from a glacier increases nutrient levels and turbidity of adjacent ocean waters. The glacial freshwater also freshens and stratifies seas in the vicinity. All of these changes can have a substantial impact on the local plankton populations.

Glaciers do melt naturally during summer months all over the world. But the amount of sea ice that is melting, particularly in high latitudes, is accelerating as the climate warms. That means that more of the glacier is melting and less of it is refreezing on an annual basis. Consequently, there are longer periods of fresh, nutrient-rich, and particulate-clouded water entering the ocean near glaciers. The physical effects of increasing glacial meltwater have been well quantified. The impacts on nearby ecosystems, however, have not been studied in as much detail.

To better understand these impacts, a team of Danish scientists, led by Ane Middelbo, studied a melting event in a Greenland fjord. The Young Sound is on the Northeast coast of Greenland, several degrees of latitude north of the Arctic Circle (Fig. 1). The region is covered in ice for most of the year, with clear waterways for only about 100 days. The area is typically clear of ice from mid-July to October. This open-water duration has, however, been steadily increasing over the past decade.

Middelbo and her team designed a project to assess the effect of freshwater inputs on plankton in the Sound. The group selected four study sites in the fjord at varying distances from the ice sheet. Each station was visited multiple times during two field campaigns in 2014 — one took place just before the ice broke up in the summer and the other sailed right as the sea began to refreeze in the fall.

At each station, they measured a suite of hydrographic parameters in the water column: temperature, salinity, ambient light levels, and turbidity. They also took seawater samples to characterize the phytoplankton assemblage at each site. To look at large zooplankton, the team did vertical net tows in the upper 100 meters of water. Some of the zooplankters were preserved for lab analysis and others were incubated to estimate their grazing rate.

Figure 2 – Sections of hydrographic measurements. The top row was from measurements taken in July and the bottom was taken in October. The x-axis in all the graphs indicates the distance from the glacier and the y-axis shows depth. Notice how the upper part of all the graphs becomes more uniform at the end of the summer (Adapted from Middelbo et al., 2017).

Middelbo’s results indicate that the physical environment in Young Sound is dominated by freshwater runoff during the summer months. The glacial meltwater creates a shallow layer of relatively warm, fresh seawater at the surface that dissipates in the fall (Fig. 2). Right when it forms, the layer is very turbid, blocking sunlight entering the water. These conditions are quite prevalent close to the glacier in the inner part of fjord.

Really small phytoplankton, those less than ten microns in diameter, dominated the turbid waters nearest the glacier in the early summer months. Based on the zooplankton counts and grazing estimates, the group concluded that direct grazing on primary producers was low in the inner fjord. As the summer wore on, smaller phytoplankton dominated the community at all four stations and zooplankton abundance dropped.

Middelbo compared her findings to a running ten-year record of zooplankton abundance in Young Sound. She noted that her group’s biomass estimates are comparable to the historical average. Previous researchers have concluded that the low productivity in the region is due to the short ice-free season. The assumption is that as the open water period gets longer, primary production, zooplankton abundance, and ecosystem productivity will go up. But Middelbo suggests that the freshwater inputs actually reinforce the low productivity, even when the area is clear of ice. More meltwater means an expanding region of fresh, turbid water that will keep growth close to historic levels.

There are many open questions about how these high latitude ecosystems will respond to increasing freshwater input. If the less-sea-ice-more-plankton paradigm is correct, we might expect the Arctic to take up more atmospheric carbon. If not, it might be business as usual but with less ice. Figuring out which scenario is more likely is a critical part of understanding how the future ocean will take up carbon and respond to our ever-changing climate.

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