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Ocean acidification (OA), an alteration of seawater chemistry caused primarily by anthropogenic carbon emissions, is a global issue. However, the local expression of OA can vary widely in nearshore waters around the world. This is due to localized factors such as river input, eutrophication, topography, location (e.g., temperature), and sensitivity of local species. Human impacts from OA also vary depending on societal uses of the ocean and its resources. Managers, policy­makers, and governments need to understand the status and susceptibility of their regions in order to make effective decisions and drive policy. In the early 2000s, scientists recognized the need for a global ocean acidification observing system and called for a coordinated approach to effectively assess global as well as local status with consistent methods. As a result, the Global Ocean Acidification Observing Network (GOA-ON) was formed in 2012 with three goals: (1) to improve understanding of global OA conditions, (2) to improve understanding of ecosystem responses to OA, and (3) to acquire and exchange data and knowledge necessary to optimize modeling of OA and its impacts (Newton et al., 2015; Tilbrook et al., 2019). 

Ocean acidification (OA) is broadly recognized as a major problem for marine ecosystems worldwide, with follow-on effects to the economies of ocean-dependent communities. The urgent need to mitigate and minimize the impacts of OA is a scientific and political priority, as highlighted by the latest Intergovernmental Panel on Climate Change report (IPCC, 2022) and by the inclusion of OA as a target in the United Nations Sustainable Development Goals (SDG). In addition, over 20 years of strong scientific evidence on the impacts of OA provides compelling arguments for urgent CO2 mitigation. Reducing CO2 emissions will require ambitious regulatory and economic instruments, as well as effective systemic changes across governments and societies. It is critical to implement adaptation measures to minimize the impact of OA, among other key environmental stressors, as the mitigation process takes time, and the impacts of OA are already felt globally. Assessing the impacts of solutions and their potential implementations requires information at local scales, considering the variabilities in marine ecosystem responses to OA (e.g., local adaptation, species redundancies). 

Diatoms generate nearly half of marine primary production and are comprised of a diverse array of species that are often morphologically cryptic or difficult to identify using light microscopy. Here, species composition and realized thermal niches of species in the diatom genus Thalassiosira were examined at the site of the Narragansett Bay (NBay) Long-Term Plankton Time Series using a combination of light microscopy (LM), high-throughput sequencing (HTS) of the 18S rDNA V4 region and historical records. Thalassiosira species were identified over 6 years using a combination of LM and DNA sequences. Sixteen Thalassiosira taxa were identified using HTS: nine were newly identified in NBay. Several newly identified species have small cell diameters and are difficult to identify using LM. However, they appeared frequently and thus may play a significant ecological role in NBay, particularly since their realized niches suggest they are eurythermal and able to tolerate the >25 °C temperature range of NBay. Four distinct species assemblages that grouped by season were best explained by surface water temperature. When compared to historical records, we found that the cold-water species Thalassiosira nordenskioeldii has decreased in persistence over time, suggesting that increasing surface water temperature has influenced the ecology of phytoplankton in NBay.