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Impact of climate variability on bottom water biogeochemistry in the context of demersal fisheries of the Northwest Atlantic shelf

The northwest Atlantic supports a number of important fisheries and is also the location
of rapid changes in physical and biogeochemical environment driven by warming
waters and increased atmospheric CO2. The region is highly susceptible to future ocean
acidification (OA) both ecologically and economically because of naturally low pH waters and many economic sectors of the US Northeast rely on potentially sensitive marine shellfish. These
past and potential future changes have been identified in surface waters using satellite products and underway observations, but because subsurface data has been limited until more recently, little is known about how bottom water carbonate chemistry has changed, and may change in the future. Although there has been significant investment in expanding regular monitoring of the region’s bottom water biogeochemistry, limitations in existing datasets make it difficult to evaluate the impact of specific processes such as OA on economically important species. Our proposed study will address these limitations using a data and model driven approach to develop a historical time series of bottom water carbonate chemistry at a spatial resolution that is relevant for the evaluation and management of economically important demersal fisheries in the northwest Atlantic. Summary of work: To address this goal, we will compile historical monitoring data focusing specifically on carbonate chemistry from publicly available databases that will allow us to leverage ongoing sampling programs such as the NOAA EcoMon and ECOA programs, OOI service cruises, and NES-LTER cruises. Using historical data, ocean reanalysis products, and machine learning tools, we will develop high-resolution historical bottom water carbonate chemistry fields for the U.S. Large Marine Ecosystems Northeast region (NELME) region at a horizontal resolution of 1/12 of a degree. We will complete a detailed comparison of observational bottom water biogeochemical data to the historical runs of global climate models from the CMIP5 database that will allow us to develop downscaled historical and future projections from the CMIP5 model ensemble, comparing a business-as-usual (RCP8.5) to a climate policy scenario (RCP4.5). Finally, we will explore spatial variability in bottom water carbonate chemistry to determine how the subsurface carbonate system responds to modes of climate variability.

Broader Impacts and relevance: This project addresses priority areas A and C of the RFP. In
addressing priority A, we will determine the relationship between key climate processes and
spatial and temporal variations in bottom water carbonate chemistry. Subsurface measurements
of carbonate chemistry have been recently identified as a significant monitoring gap limiting our
understanding of acidification on the NOAA LME 7 (NELME). This project leverages recent
investments in adding carbonate system measurements to regular cruises (OOI, NES-LTER,
NOAA EcoMon and ECOA) to expand our understanding of climate processes on bottom water
biogeochemical indicators. This project will also address priority C of the RFP by synthesizing
observations and output from state-of-the-art climate models using machine learning to produce
bottom water biogeochemical fields at a spatial resolution that is relevant for the management of
economically important demersal fisheries in the NELME. The results from this project will determine if the low resolution, 1 degree models with data assimilation using machine learning
will provide bottom water fields that can inform fisheries management decisions. Furthermore,
the relationships developed between key climate indices and spatial variability in carbonate
chemistry will be helpful in developing seasonal forecasting models to aid in fisheries management decisions.

Climate Risk Area: Marine Ecosystems

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