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The ocean plays a major role in controlling atmospheric carbon at decadal to millennial timescales, with benthic carbon representing the only geologic‐scale storage of oceanic carbon. Despite its importance, detailed benthic ocean observations are limited and representation of the benthic carbon cycle in ocean and Earth system models (ESMs) is mostly empirical with little prognostic capacity, which hinders our ability to properly understand the long‐term evolution of the carbon cycle and climate change‐related feedbacks. The Benthic Ecosystem and Carbon Synthesis (BECS) working group, with the support of the US Ocean Carbon & Biogeochemistry Program (OCB), identified key challenges limiting our understanding of benthic systems, opportunities to act on these challenges, and pathways to increase the representation of these systems in global modeling and observational efforts. We propose a set of priorities to advance mechanistic understanding and better quantify the importance of the benthos: (a) implementing a model intercomparison exercise with existing benthic models to support future model development, (b) data synthesis to inform both model parameterizations and future observations, (c) increased deployment of platforms and technologies in support of in situ benthic monitoring (e.g., from benchtop to field mesocosm), and (d) global coordination of a benthic observing program (“GEOSed”) to fill large regional data gaps and evaluate the mechanistic understanding of benthic processes acquired throughout the previous steps. Addressing these priorities will help inform solutions to both global and regional resource management and climate adaptation strategies. 

Climate change is causing decreases in pH and dissolved oxygen (DO) in coastal ecosystems. Canopy-forming giant kelp can locally increase DO and pH through photosynthesis, with the most pronounced effect expected in surface waters where the bulk of kelp biomass resides. However, limited observations are available from waters in canopies and measurements at depth show limited potential of giant kelp to ameliorate chemical conditions. We quantified spatiotemporal variability of surface biogeochemistry and assessed the role of biological and physical drivers in pH and DO modification at two locations differing in hydrodynamics inside and outside of two kelp forests in Monterey Bay, California in summer 2019. pH, DO, dissolved inorganic carbon (DIC), and temperature were measured at and near the surface, in conjunction with physical parameters (currents and pressure), nutrients, and metrics of phytoplankton and kelp biological processes. DO and pH were highest, with lower DIC, at the surface inside kelp forests. However, differences inside vs. outside of kelp forests were small (DO 6–8%, pH 0.05 higher in kelp). The kelp forest with lower significant wave height and slower currents had greater modification of surface biogeochemistry as indicated by larger diel variation and slightly higher mean DO and pH, despite lower kelp growth rates. Differences between kelp forests and offshore areas were not driven by nutrients or phytoplankton. Although kelp had clear effects on biogeochemistry, which were modulated by hydrodynamics, the small magnitude and spatial extent of the effect limits the potential of kelp forests to mitigate acidification and hypoxia. 

Surface-canopy forming kelps provide the foundation for ecosystems that are ecologically, culturally, and economically important. However, these kelp forests are naturally dynamic systems that are also threatened by a range of global and local pressures. As a result, there is a need for tools that enable managers to reliably track changes in their distribution, abundance, and health in a timely manner. Remote sensing data availability has increased dramatically in recent years and this data represents a valuable tool for monitoring surface-canopy forming kelps. However, the choice of remote sensing data and analytic approach must be properly matched to management objectives and tailored to the physical and biological characteristics of the region of interest. This review identifies remote sensing datasets and analyses best suited to address different management needs and environmental settings using case studies from the west coast of North America. We highlight the importance of integrating different datasets and approaches to facilitate comparisons across regions and promote coordination of management strategies. 

Abstract Kelp forests are among the world's most productive marine ecosystems, and they have the potential to locally ameliorate ocean acidification (OA). In order to understand the contribution of kelp metabolism to local biogeochemistry, we must first quantify the natural variability and the relative contributions of physical and biological drivers to biogeochemical changes in space and time. We deployed an extensive instrument array in Monterey Bay, CA, inside and outside of a kelp forest to assess the degree to which giant kelp (Macrocystis pyrifera) locally ameliorates present‐day acidic conditions which we expect to be exacerbated by OA. Temperature, pH, and O₂variability occurred at semidiurnal, diurnal (tidal and diel), and longer upwelling event periods. Mean conditions were driven by offshore wind forcing and the delivery of upwelled water via nearshore internal bores. While near‐surface pH and O₂were similar inside and outside the kelp forest, surface pH was elevated inside the kelp compared to outside, suggesting that the kelp canopy locally increased surface pH. We observed the greatest acidification stress deeper in the water column where pCO₂reached levels as high as 1,300 μatm and aragonite undersaturation (Ω_Ar < 1) occurred on several occasions. At this site, kelp canopy modification of seawater properties, and thus any ameliorating effect against acidification, is greatest in a narrow band of surface water. The spatial disconnect between stress exposure at depth and reduction of acidification stress at the surface warrants further assessment of utilizing kelp forests as provisioners of local OA mitigation.