Population dynamics and sensitivity to ocean acidification of the euthecosome pteropod Limacina retroversa

Since the industrial revolution, the ocean has taken up anthropogenic CO2 causing a shift in seawater carbonate chemistry resulting in a gradual decrease in surface ocean pH, known as ocean acidification. Thecosome pteropods are major planktonic CaCO3 producers and therefore of biogeochemical importance. Especially at higher latitudes, they can at times occur in huge numbers and consequently are of ecological importance in the marine ecosystems. The temperate-boreal species Limacina retroversa has its main distribution in waters of the North Atlantic and sporadically also occurs in the Arctic. The results presented in this master thesis are part of the KOSMOS study 2015 in Bergen (Norway) performed in temperate waters of the Raunefjord investigating the response of a natural plankton community exposed to ocean acidification conditions during a period of 51 days. Eight mesocosms were deployed in the Raunefjord enclosing a water volume of ~62m3. An ambient pCO2 treatment (250 - 350 μatm) and an elevated pCO2 treatment (~2000 μatm) were applied to four replicate mesocosms, respectively. Vertical net hauls were taken in the mesocosms and the fjord every eight days with an Apstein net to compare the natural population dynamics of L. retroversa between the enclosed populations at ambient and elevated pCO2 conditions as well as with population dynamics in the fjord. The focus was on the veliger life stage, because they dominated the population in both the mesocosms and the Raunefjord. Population dynamics with respect to abundances and standing stock biomass were negatively affected in the elevated pCO2 treatment. No effects of ocean acidification were found on dry weight, size and stage development. Shell dissolution and degradation of L. retroversa exposed to elevated pC02 concentrations were less pronounced compared to previous studies. Although exposed to extremely high pCO2 concentration, a certain proportion was still able to cope with undersaturated water conditions and seemed to be not as sensitive as previously suggested. Indirect food web mediated effects driven by pCO2 on L. retroversa might be possible and still need to be clarified. The increase in abundance of small-sized individuals in the ambient pCO2 treatment and the fjord indicated a spawning phase of L. retroversa in the Raunefjord in June, which was not observed in the high pCO2 treatment. For the first time, a baseline size-weight relationship and standing stock biomass in terms of carbon (TC, TOC, TIC) was established for L. retroversa fjord organisms. Calculations also included some rough estimates on CaCO3 content. These data on population dynamics can serve as valuable information for the inclusion of thecosome petropods in biogeochemical models and to better assess responses related to ocean acidification at the population level. Since L. retroversa is of biogeochemical and ecological importance, declining populations might have negative impacts for biogeochemical fluxes and the pelagic food web. Future responses to ocean acidification of L. retroversa will depend on the capacity to adapt to changes in more acidifying waters.