Climate and ocean acidification effects on benthos in Kongsfjorden, Svalbard

Climate change and ocean acidification effects on marine organisms are being studied in Kongsfjorden. In field and lab studies, researchers in the Fram Centre Ocean Acidification Flagship test whether the fjord’s environmental gradients can serve as natural analogues for future, more acidic marine conditions.

Arctic glacial fjords, such as Kongsfjorden, have strong environmental gradients in salinity, sedimentation, primary production, and biogeochemistry. Because of glacial meltwater input, the inner fjord area becomes fresher, and suspended sediments reduce light transmission into the water column, thus restricting primary production. Sediment gradients along the fjord axis exist in Kongsfjorden seasonally, but also are representative of future changes: due to increased run-off from land, light attenuation in the water column has increased in Kongsfjorden during the last two decades, which has resulted in less macroalgae in the deeper part (>15 m depth), where it has become too dark for their photosynthesis and growth.

As part of the Fram Centre Ocean Acidification (OA) Flagship, we started to explore the possibility of using Kongsfjorden’s environmental gradients as natural analogues for investigations on effects of climate change. The concept behind the natural analogue idea is that researchers can use existing gradients to study how organisms respond to various factors in their environment, for instance those related to climate change.

Natural analogues provide an opportunity to study changes in community or population structure and how well species can adapt to a changing environment. Parameters relevant for studying possible future ocean acidification include increases in partial pressure of CO2 (pCO2), and reduced carbonate-ion concentration ([CO32-]) and pH. Measurements have shown reduction in [CO32-]during summer in the inner part of Kongsfjorden due to freshwater input, combined with suspended glacial sediments that block sunlight, reducing the ability of phytoplankton and macroalgae to remove CO2 from the water through photosynthesis. Together, these factors help create an environmental gradient along the length of Kongsfjorden – a natural analogue that can be used to study the effects of OA.

We tested the natural analogue idea in two studies with organisms collected at various sites in Kongsfjorden (see map). The first study examined three populations of the circumpolar Arctic/subarctic amphipod Gammarus setosus in the Kongsfjorden–Krossfjorden area of Svalbard. These were examined both in the field and in the laboratory for signs of physiological stress related to changes in salinity and pCO2.

In field tests, populations subject to low salinity, and generally lower pH, showed reduced metabolic rates and cellular energy allocation (CEA), which may affect their growth and reproduction (see fact box). However, the effect of pCO2 could not be separated from the effect of salinity.

In the lab, both the lower-salinity (Blomstrandhalvøya) and higher-salinity (Ny-Ålesund) populations were exposed to ambient (400 atm) and elevated (1000 atm) pCO2 at two different salinities (23‰ and 30‰), and several physiological and energetic responses were measured. The physiological responses were consistently higher under more acidic conditions (that is, at higher pCO2), and CEA was consistently lower regardless of salinity. Fresher fjord water owing to increased glacial melting may become limiting for G. setosus, especially as temperature increases.

Despite being found naturally at salinities down to 12‰ in Kongsfjorden, the amphipod’s tolerance has been shown to decrease with increasing temperature. Thus, G. setosus may be confined to the colder inner part of glacial fjords, whereas Atlantic species, such as the boreal Gammarus oceanicus, may occupy vacant habitats. Gammarus oceanicus has a wider thermal tolerance and is already more common in the outer part of Kongsfjorden.

The other study investigated summer and winter levels of magnesium carbonate (MgCO3) in the skeleton of Arctic bryozoans. Bryozoans are colonial suspension feeders, often attached to rocks or macroalgae. They occur worldwide at broad depth ranges from abyssal to the intertidal zone. Because of their high abundance and diversity, they are considered important components of Arctic marine ecosystems and significant calcium carbonate producers. Arctic bryozoan colonies often consist of thousands of units (zooids) in most cases composed of calcite (calcium carbonate; CaCO3) with variable amounts of MgCO3. Because the magnesium (Mg) content in biogenic CaCO3 skeletons determines their solubility and mechanical properties, it is important to understand what factors control it within calcified skeletons.

The study tested whether changes in seawater chemistry due to seasonal primary production, and thus changes in carbonate-ion concentration ([CO32-]) and pH, can influence the uptake of bio-available Mg2+ into calcified skeletons of Arctic bryozoans.

If Mg content in skeletons follows seasonal variation and depth-related gradients, this could indicate environmental control of skeletal parameters.

Five bryozoan species were sampled at different depths (50, 100, 150 m) and locations in Kongsfjorden during summer and winter (see map), and the concentration of MgCO3 was determined in their calcified skeletons in the laboratory. No clear differences between summer and winter levels of skeletal MgCO3 were found despite seasonal differences in calcium carbonate saturation in the water. Nor did we detect any depth-related differences in skeletal MgCO3 content.

This suggests that Arctic bryozoans can control skeletal MgCO3 concentrations biologically. Yet the variability in MgCO3 content in skeletons from stations with different seawater parameters suggests that environmental factors can also, to some extent, shape the skeletal chemistry of Arctic bryozoans.

Both studies produced results related to effects of ocean acidification and other climate-related changes in Arctic marine organisms. Salinity appears to have effects on energy allocation in amphipods and we observed some environmentally-induced variation in bryozoans.

Lack of effects of ocean acidification, within an existing environmental range in biogeochemical variables, is also an important finding, which gives hope that these Arctic marine organisms can continue to persist in marine waters with higher pCO2. Field experiments making use of natural analogues appear promising, but must move from simpler gradient-type studies to include more complex spatial-temporal mosaics of environmental drivers.

These need to be backed up by laboratory experiments, as was done in the amphipod study, to further deduce responses to environmental parameters in marine organisms. Such approaches will be critical in determining the tolerance limits of Arctic marine organisms in a higher CO2 world.

Acknowledgements 

These projects were partially funded by the Fram Centre Ocean Acidification Flagship.

Locations in Kongsfjorden and Krossfjorden for collection of the three populations of Gammarus setosus (black dots) and bryozoans during winter (red) and summer (green). Land map from Norwegian Polar Institute, bathymetry from Norwegian Mapping Authority (Figure modified from Brown et al 2020; used with permission).

Framsenteret, 15 April 2021. Full article.

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