Award: OCE-1536165

Concurrent ocean warming, acidification, and oxygen loss are symptoms of man-made marine climate change, but how marine organisms and their populations will react to these changes requires urgent investigation. Our project examined the climate sensitivity of an important model organism, the Atlantic silverside (Menidia menidia), a small, abundant forage fish in nearshore waters along the US east coast. Over 5 years, we conducted a comprehensive set of experiments, some as short as two weeks, some spanning more than a year. We systematically examined how this fish was affected by more acidic, meaning higher CO2 conditions ? either alone or in combination with elevated temperature or lower oxygen levels. To conduct many of these experiments, we engineered and built the ?Automated Larval Fish Rearing System (ALFiRS)? at the Rankin Seawater Lab of the University of Connecticut. It has a 3 x 3 design of nine independent rearing units, each with computer-control over their CO2, O2, and temperature levels. The units can even mimic diel or tidal CO2 x O2 fluctuations, which are very common in nearshore environments. To represent wild silverside populations, we started each experiment with embryos that were collected from a large number of adults collected by beach seine at a local field site (Image 1). In the laboratory, we focused on silverside traits that are known to determine Darwinian fitness, which is the success in contributing offspring to the next generation. This included measuring survival, length and weight growth, metabolism, sex ratios, fecundity and many other traits. Like most organisms and especially fish, we found that silversides are most sensitive to predicted ocean acidification at their early life stages, meaning as unhatched embryos and very young larvae. The responses are highly variable, which is why we employed serial experimentation (n=20) to robustly quantify silverside early life CO2 sensitivity. This revealed an average 10% reduction in embryo survival under future CO2 conditions and a curious seasonal change in offspring CO2 sensitivity that is likely an adaptation of this species to seasonal changes in their productive nearshore habitat (Image 2). We found complex interactions between temperature and CO2, where CO2 effects were often more pronounced at low or very high temperatures, while largely absent at optimal temperatures for this species. We found similar interactions with low oxygen conditions, which is important because high CO2 and low O2 conditions already co-occur seasonally in many coastal areas. These fluctuations are predicted to become more extreme in the future. Our research showed, however, that diel and tidal CO2 x O2 fluctuations appear to be beneficial to silversides and mitigated most effects seen under static conditions. We also conducted a number of novel, long-term rearing experiments on silversides, demonstrating that juveniles grow up shorter under high CO2 conditions, while the fecundity of lab-matured females was largely unaffected by acidification. The findings of this project led to the development of the ?Ocean Variability Hypothesis? that emphasizes the role of natural CO2 variability in the environment for a species? CO2 sensitivity ? a contribution and attempt to synthesize the complexity of empirical data from a rapidly expanding field (Image 3). In addition to its core foci, the project unearthed novel evidence for sex-selective survival of juvenile silversides in the wild (Image 4), quantified patterns in CO2 and O2 fluctuations in 16 estuaries across the US (Image 5), and contributed review and perspective articles to advance the scientific field. Nearly all published datasets are citable and publicly available from NSF?s BCO-DMO data portal as well as from other data repositories. The project made a substantial, positive impact on the lives and careers of more than 20 people involved in it over 5 years. It supported the thesis research of three Master students, one PhD student, and one post-doctoral researcher. It mentored three consecutive NSF-REU students and their research projects and inspired over 15 undergraduate students working or volunteering for the project (Image 6). The project succeeded in capacity building for hands-on skills like fish sampling and fish husbandry, for conducting accurate water chemistry measurements and effectively analyzing, visualizing, and publishing research data. Apart from peer-reviewed publications and conference presentations, we brought our findings to the general public via community college lectures, open-house demonstrations, blogs, brochures and on social media. Funding from this NSF-project has greatly helped with upkeeping and updating the Rankin Seawater Lab as a valuable and shared resource for the university community. Last Modified: 12/08/2020 Submitted by: Hannes Baumann