Forage fish such as silversides and minnows are key components of temperate coastal marine ecosystems because they act as predators of zooplankton and prey for larger trophic organisms like striped bass, bluefish or piscivorous birds. The sensitivity of coastal forage fish to ocean acidification was largely unexplored and thus became the focus of this NSF-funded research project. Based on a series of independent experiments, our first discovery was that growth and survival of Inland silverside (Menidia beryllina) embryos and early larvae were significantly reduced in direct response to CO2 concentrations (~ 1,000 µatm) that are predicted in the average open ocean for the end of this century (Fig.1). This spurred on a wide range of experimental approaches; one consisted of novel, multi-year experiments that combined high-frequency pH monitoring in a temperate tidal salt marsh with repetitive sampling of a wild fish population (Atlantic silverside, M. menidia) spawning in there and standardized CO2 exposure experiments on offspring over of two years. This demonstrated for the first time an interannually consistent, seasonal change in offspring CO2 tolerance in a marine organism. The shifting response strikingly coincided with the rapid seasonal acidification pattern typical for this and many other coastal habitats (Fig.2 - primary image). This project further led to a new quantitative genetic approach to determine the evolutionary adaptation potential of fish to high CO2 environments; while demonstrating a viable method to better understand the long-term vulnerabilities of marine organisms to unfolding ocean acidification. In addition, this NSF grant facilitated the measurement and analysis of pH, dissolved oxygen (DO) and in situ pCO2 levels in a wide range of New England bays and estuaries (Fig.3), hence broadening the current understanding of the spatio-temporal variability of these factors. For example, it clearly indicated that in productive coastal habitats, pH and DO covary on tidal, diel, seasonal and interannual time scales (Fig.4), because respiration due to microbial degradation of organic matter necessarily consumes oxygen while producing CO2. Stressfully low pH and oxygen conditions are therefore particularly prevalent in coastal ecosystem that are also affected by eutrophication. Consequently, this project started to explore the additive and synergistically negative effects of co-occurring hypoxia and acidification on the early life stages of coastal organisms such bay scallops and quahogs (Fig.5), or silversides and sheepshead minnows (Fig.6). This NSF project trained three Master students, who successfully graduated from Stony Brook University in 2014. It involved numerous undergraduate and highschool students from diverse backgrounds in field and experimental work, thereby conveying the challenges and opportunities of this research field. The PIÆs used their experiences and findings to actively reach out to stakeholders, members of congress, academic and governmental researchers and the general public – using print and electronic media, lectures, seminar and conference talks. Last Modified: 03/15/2015 Submitted by: Hannes Baumann