Towards understanding the impact of ocean acidification on the shell condition of thecosome pteropods

Open Access
Author:
Oakes, Rosemary Louise
Graduate Program:
Geosciences
Degree:
Doctor of Philosophy
Document Type:
Dissertation
Date of Defense:
May 08, 2017
Committee Members:
  • Timothy Bralower, Dissertation Advisor
  • Timothy Bralower, Committee Chair
  • Michael Allan Arthur, Committee Member
  • Lee Kump, Committee Member
  • Timothy Michael Ryan, Outside Member
  • Matthew S Fantle, Committee Member
Keywords:
  • Pteropods
  • Ocean acidification
  • Micro-CT
  • Nano-SEM
  • Taphonomy
  • Plankton
  • Preservation
  • Biological oceanography
  • Chemical oceanography
  • Climate change
Abstract:
Anthropogenic activities are altering the composition of the atmosphere at unprecedented rates. Carbon dioxide levels reached 400 ppm for the first time since the Pliocene in 2016, and are predicted to be as high as 1000 ppm by the end of the century. These increases alter global temperatures and precipitation patterns, but also impact ocean chemistry, in a process known as ocean acidification. Pteropods are a group of planktonic molluscs at high risk from ocean acidification. They form their shells from aragonite, the more soluble form of calcium carbonate, and reach high abundances in polar and sub-polar latitudes where the effects of ocean acidification are predicted to be the most pronounced. Pteropods play a key role in the marine food chain and carbon cycle, especially at high latitudes, and therefore it is essential that we understand how they will be impacted by ocean acidification. Pteropods are visibly affected by dissolution changing from glassy and transparent when pristine to white and opaque when dissolved. Shell condition has traditionally been assessed qualitatively using light and scanning electron microscopy. Here I develop a quantitative method to assess shell condition using micro-CT scanning (Chapter 1). This technique is applied to assess the inter- and intra-specific variation in pteropod shell properties from five locations. Although the greatest variability in shell thickness occurs between pteropods of different species, within one species, Limacina retroversa, differences in thickness vary with local seawater density, likely a biological response to control buoyancy. This new method is also applied in Chapters 2 and 3, but further work needs to be conducted to expand the number of specimens, locations, and species scanned to establish a robust baseline to which future changes can be compared. Understanding how pteropods will be affected by ocean acidification is inherently difficult as these organisms are very difficult to culture. Incubation experiments are therefore used to test how pteropods respond to predicted future conditions over short periods of time. I analysed the shell condition of pteropods with damaged periostraca incubated under a range of pCO2 settings (Chapter 2). I found that the aragonitic shell only undergoes dissolution when both the periostracum is damaged, and the shell is exposed to undersaturated seawater. This suggests pteropods are more resilient to ocean acidification than previously predicted. The dissolution of pteropod shells in the water column is thought to be controlled by seawater chemistry. However, sediment trap studies have found that there is a significant dissolution occurring above the saturation horizon where water is supersaturated with respect to aragonite. I test the effect of the decomposition of the pteropod body on the condition of the pteropod shell using decay and incubation experiments (Chapter 3). I find that internal shell dissolution, linked to the decomposition of the organic soft tissue, is greater than external dissolution associated with the chemistry of the surrounding seawater. This suggests that there is likely more carbonate dissolution above the lysocline than previously thought, and has implications for the interpretation pteropod shells from sediment traps and the fossil record. Pteropods collected at sea are preserved, and brought back to the lab to study. I test how six different methods of preservation alter pteropod shell condition over a period of 15 months between sample collection and analysis (Chapter 4). I find that most preservation techniques cause some alteration, with the least amount of change occurring when samples are air dried, or preserved in buffered ethanol; the greatest amount of alteration is observed when samples are preserved in formalin. This study enables preservation-related bias to be evaluated in studies of modern and historical pteropod records, therefore opening up a wider range of records for accurate interpretation. In summary, this dissertation has worked to develop the methods needed to assess the response of pteropods to past, and predicted future changes in ocean chemistry. By combining a range of techniques focused at different scale, we can build a more holistic understanding of how and why pteropods are impacted by ocean acidification.