By: Allison Bailey, Ane Cecilie Kvernvik, Philipp Assmy, Anette Wold, Amalia Keck and Haakon Hop // Norwegian Polar Institute and Clara Hoppe // Alfred Wegener Institute
For the underwater world, the yearly return of the sun melts and fragments the overlying sea ice, letting the sun’s rays penetrate ever deeper into the upper water column.
This kickstarts a plethora of chemical, physical and biological changes. For the miniscule but multitudinous plankton that comprises the bulk of life in the ocean, this is the start of a fast and furious period of growth and physiological change. Copepods just 3 mm long migrate hundreds to thousands of metres from their overwintering habitat at depth to the surface, where they mate and produce young, often doing so in the dark and still starving from the winter. Their food source, the many species of single-celled organisms composing phytoplankton, color the surface waters with thousands of cells per drop of seawater, different species’ populations booming and busting in unison or succession. Changes in seawater salinity, acidity, temperature, and vital nutrients, all of which are important for marine plankton, also follow seasonal cycles.
While significant and largely unstudied, the fine-scale seasonal changes in the Arctic marine ecosystem are also of utmost importance in our quest to understand how climate change and ocean acidification will affect this vulnerable region. While warming and acidification of seawater will affect the physiology of all marine organisms, the severity of their effects depends on many factors which vary by season: an organism’s life stage and physiological state, the presence of their predators, competition from other species, food availability and quality, and the abiotic environment. Therefore, investigations of how climate change and ocean acidification will affect marine organisms must necessarily be conducted throughout the year, to make sure we detect all the responses to environmental change and understand when the greatest responses will occur.
However, investigating the Arctic through all seasons is notoriously difficult due to the extreme weather, natural conditions, remoteness, and distance from infrastructure. Laboratory experiments, which are vital for quantifying how climate change and ocean acidification will affect marine organisms, are also logistically challenging, often restricting experimental studies in the High North to a single point in time. However, the Ny-Ålesund Marine Laboratory in Svalbard provides an excellent site to achieve seasonal monitoring, with year-round access to Kongsfjorden and advanced laboratory facilities to conduct experiments on responses to climate drivers.
In 2019 we conducted a series of nine ocean acidification experiments that covered the entire growing season, from late April to early September, parallel to the initiation of a seasonal monitoring programme in Kongsfjorden, Svalbard, by the Norwegian Polar Institute. Our goal was to determine whether seasonal changes in water chemistry, plankton species composition, food availability or organism physiology would influence how ocean acidification affects energy transfer at the base of the marine food chain: zooplankton grazing on phytoplankton. In each experiment, the natural assemblage of phytoplankton and copepods in the fjord was collected and exposed to increased carbon dioxide partial pressure (pCO2) in the lab, simulating ocean acidification. After four days, we measured the growth of the phytoplankton and the grazing rate of the copepods on the phytoplankton.
Interestingly, we found that the same experiment, repeated at different times throughout the year, provided different results. Early and late in the summer, simulated ocean acidification did not affect copepod grazing. However, during spring, when the phytoplankton was at peak bloom, copepods increased their grazing in response to high pCO2 (ocean acidification). These results are important as background information when interpreting other studies; this shows why there is a potential to make erroneous conclusions and miss detecting important effects if only one-time point is studied.
Our observation that ocean acidification coincides with increased grazing at peak bloom may indicate that there is an energetic cost to life at high pCO2 that copepods are able to compensate by increasing feeding when food is abundant, but not when it is scarce. Alternatively, the effect may be specific to the phytoplankton species that were present, the life stage or physiological status of the copepods, or the chemistry of the seawater at that time.
By examining the biological and chemical data collected from the fjord at the same time, we hope to resolve what drives the variation in ocean acidification effects we have observed, and thus what makes the marine ecosystem in Kongsfjorden more sensitive – or less sensitive – to ocean acidification over time. Seasonal monitoring, combined with repeated experiments, allows us to continue asking important questions about ocean acidification:
- When during the year will marine organisms experience the strongest effects of ocean acidification?
- What point in their life span does this coincide with?
- As the mix of plankton species in the water column shifts throughout the year, which assemblages of species are most robust or most sensitive to ocean acidification?
- How will ocean acidification affect the carbon budgets of the upper ocean, from CO2 outgassing to food for upper trophic levels?
Seasonal monitoring is also important for detecting shifts in the timing of biological events due to climate change
It also constitutes a crucial background for interpreting and comparing studies based on a single data collection point per year. By understanding the variability attributable to seasonality, we can be more assured in our analyses of interannual variation for ecosystem monitoring.
The Arctic and its ecosystems are currently experiencing some of the strongest effects of climate change, and it is vital to predict how Arctic organisms will respond to these changes. The strong seasonality in the Arctic will likely result in temporally variable responses to climate drivers, with important implications for the timing of when marine ecosystems are most vulnerable to climate change and ocean acidification. Our study indicates that even weekly changes in the plankton community through the season affect whether or not an important trophic linkage is affected by simulated ocean acidification. There are likely many more seasonally dependent responses to climate change drivers. The extension of our investigations into the winter period is likely to reveal even more ecologically relevant variations in responses to warming and ocean acidification.
These studies were funded in part by Fram Centre Flagship “Ocean acidification and ecosystems effects in Northern waters” and the Norwegian Polar Institute’s Seasonal Pelagic Monitoring in Kongsfjorden.