Antarctic sea ice algae: in situ response to simulated ocean acidification

Chair: Vonda Cummings

Vonda Cummings (1)*, Andrew Lohrer (2), Neill Barr (1), Simon Thrush (3)

1 National Institute of Water and Atmospheric Research, Wellington, New Zealand
2 National Institute of Water and Atmospheric Research, Hamilton, New Zealand
3 University of Auckland, Auckland, New Zealand

Background
Sea-ice algae form highly concentrated films on the underside of Antarctic sea ice in spring and summer. Productivity by these algae is critical to the local marine food web, particularly in shallow coastal regions, and is also a major contributor to carbonate saturation state, yet their response to ocean acidification is unknown.

Methods
We investigate how the functioning and dynamics of coastal sea ice ecosystems might respond in a modified environment through an experiment conducted in situ in McMurdo Sound, Ross Sea, Antarctica in the early austral summer of 2014. We installed purpose built under-ice chambers (each 140 l) on the underside of first year sea ice, and delivered seawater to these chambers at pH levels experienced now, and predicted in the coming decades (i.e., 8.00, 7.90, 7.75, 7.60). Via regular sampling of chamber inflow and outflow we monitored pH, under-ice algal productivity and nutrient utilisation over 14 d. The effects on composition and characteristics of these intact, natural sea ice communities (incl. algae, bacteria, chlorophyll a) were assessed at the end of the experiment.

Findings

  • Under ice algal primary productivity was enhanced at low pH, as shown by greater rates of CO2 uptake and oxygen evolution, and by higher concentrations of chlorophyll a in the lowest pH treatment (7.60) at the end of the experiment. No changes in algal species composition were detected in conjunction with pH treatment during this time frame. The algae in the lowered pH treatments considerably elevated seawater pH relative to the inflow water.

Conclusions
The results of this in situ experiment indicate that ice algal primary productivity increases under ocean acidification, and illustrates their potential role in mediation of CO2 uptake. They also show the potential influence on other ecosystem components (e.g., through effects on nutrient cycling).