Ocean acidification, caused by the absorption of around 25% of carbon dioxide emissions by the world’s oceans, is resulting in lower carbon ion concentrations and a decrease in oceanic pH levels. This phenomenon is affecting certain marine organisms, such as pteropods, which rely on carbon ion to develop and sustain their shells. Pteropods are important components of the marine ecosystem, but due to their tiny size and poor long-term survival in captivity, much remains unknown about their life cycles and population dynamics.
A team of marine scientists has recently investigated the life cycles, abundance, and seasonal variability of shelled sea butterflies in the north-east Scotia Sea, an area that is experiencing some of the fastest climate change in the Southern Ocean. The study’s corresponding author, Dr. Clara Manno of the British Antarctic Survey, stated that the decline of pteropod populations in the Antarctic Ocean could have far-reaching consequences for the food web and carbon cycle. By gaining a better understanding of the life cycle of these keystone organisms, researchers may be able to improve their predictions of how ocean acidification will impact the Antarctic ecosystem.
Population stability essential for species survival
To investigate the life cycle, abundance, and seasonal variability of shelled sea butterflies in the north-east Scotia Sea, the scientists collected samples using a sediment trap, which is a sampling device moored at a depth of 400 meters. Due to the tiny size of sea butterflies and their poor long-term survival in captivity, it is impossible to observe their full life cycle in a laboratory setting. Therefore, the researchers had to piece together information about their spawning, growth rate, and population structure using sediment trap samples, allowing them to successfully reconstruct their life cycle over a year.
The study focused on two dominant species of sea butterflies: Limacina rangii and Limacina retroversa. The scientists observed contrasting life cycles between these two species, which may lead to different vulnerabilities to changing oceans. L. rangii, a polar species, can be found as both adults and juveniles during the winter months. In contrast, L. retroversa, a subpolar species, only appears as adults during the winter.
During the coldest season, the ocean water is more acidic than during other times of the year due to cooler temperatures increasing CO2 dissolution in the ocean. The researchers noted that the life stages of sea butterflies present during this period are more exposed and vulnerable to increased levels of ocean acidification.
Interestingly, the fact that L. rangii adults and juveniles coexist over winter may provide them with a survival advantage. If one cohort is vulnerable, the overall population stability is not at risk. However, with L. retroversa, if one cohort is removed, the whole population may be vulnerable. The researchers suggest that these findings highlight the importance of understanding the different life cycles and vulnerabilities of marine organisms in predicting the impacts of ocean acidification on the marine ecosystem.
Prolonged exposure is a survival challenge
The researchers acknowledged that while the two sea butterfly species are affected differently, both are likely to experience adverse impacts if exposed to unfavorable conditions for prolonged periods. As ocean acidification events become more intense and longer-lasting, they may coincide with spawning events in the spring, putting the most vulnerable life stage, the larvae, at greater risk and potentially endangering future populations.
To better understand the potential consequences of such scenarios in the north-east Scotia Sea, the research team plans to continue studying the sea butterflies in this region. “Our next step will be to analyze multiyear sediment trap samples to identify potential inter-annual variability in the life cycle associated with environmental change,” explained Dr. Jessie Gardner of the British Antarctic Survey, the lead author of the study. By doing so, the researchers hope to gain further insights into the impact of ocean acidification on these keystone organisms and the wider Antarctic ecosystem.