In marine ecosystems, rising atmospheric CO2 and climate change are associated with concurrent shifts in temperature, circulation, stratification, nutrient input, oxygen content, and ocean acidification, with potentially wideranging biological effects. Population-level shifts are occurring because of physiological intolerance to new environments, altered dispersal patterns, and changes in species interactions. Together with local climate-driven invasion and extinction, these processes result in altered community structure and diversity, including possible emergence of novel ecosystems. Impacts are particularly striking for the poles and the tropics, because of the sensitivity of polar ecosystems to sea-ice retreat and poleward species migrations as well as the sensitivity of coral-algal symbiosis to minor increases in temperature. Midlatitude upwelling systems, like the California Current, exhibit strong linkages between climate and species distributions, phenology, and demography. Aggregated effects may modify energy and material flows as well as biogeochemical cycles, eventually impacting the overall ecosystem functioning and services upon which people and societies depend.

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Climate Change Impacts on Marine Ecosystems

Coral reefs support immense biodiversity and provide important ecosystem services to many millions of people Yet reefs are degrading rapidly in response to numerous anthropogenic drivers In the coming centuries, reefs will run the gauntlet of climate change, and rising temperatures will transform them into new configurations, unlike anything observed previously by humans Returning reefs to past configurations is no longer an option Instead, the global challenge is to steer reefs through the Anthropocene era in a way that maintains their biological functions Successful navigation of this transition will require radical changes in the science, management and governance of coral reefs

/pdf/coral-reefs-in-the-anthropocene-31kb7o272c.pdf

Coral reefs in the Anthropocene

Heatwaves are important climatic extremes in atmospheric and oceanic systems that can have devastating and long-term impacts on ecosystems, with subsequent socioeconomic consequences. Recent prominent marine heatwaves have attracted considerable scientific and public interest. Despite this, a comprehensive assessment of how these ocean temperature extremes have been changing globally is missing. Using a range of ocean temperature data including global records of daily satellite observations, daily in situ measurements and gridded monthly in situ-based data sets, we identify significant increases in marine heatwaves over the past century. We find that from 1925 to 2016, global average marine heatwave frequency and duration increased by 34% and 17%, respectively, resulting in a 54% increase in annual marine heatwave days globally. Importantly, these trends can largely be explained by increases in mean ocean temperatures, suggesting that we can expect further increases in marine heatwave days under continued global warming.

/pdf/longer-and-more-frequent-marine-heatwaves-over-the-past-2qkkojl57f.pdf

Longer and more frequent marine heatwaves over the past century

A hierarchical approach to defining marine heatwaves

The public health, tourism, fisheries, and ecosystem impacts from harmful algal blooms (HABs) have all increased over the past few decades. This has led to heightened scientific and regulatory attention, and the development of many new technologies and approaches for research and management. This, in turn, is leading to significant paradigm shifts with regard to, e.g., our interpretation of the phytoplankton species concept (strain variation), the dogma of their apparent cosmopolitanism, the role of bacteria and zooplankton grazing in HABs, and our approaches to investigating the ecological and genetic basis for the production of toxins and allelochemicals. Increasingly, eutrophication and climate change are viewed and managed as multifactorial environmental stressors that will further challenge managers of coastal resources and those responsible for protecting human health. Here we review HAB science with an eye toward new concepts and approaches, emphasizing, where possible, the unexpected yet promising new directions that research has taken in this diverse field.

https://www.whoi.edu/fileserver.do?id=201484&pt=2&p=28251

Progress in understanding harmful algal blooms: paradigm shifts and new technologies for research, monitoring, and management.

The dynamics of a surface-to-bottom density front on a uniformly sloping continental shelf and the role of density advection in the bottom boundary layer are examined using a three-dimensional, primitive equation numerical model. The front is formed by prescribing a localized freshwater inflow through the coastal boundary. The resulting freshwater plume turns anticyclonically and moves along the coast, generating offshore transport in the bottom boundary layer, which advects freshwater offshore and creates a sharp surface-to-bottom density front with a surface-intensified alongshelf jet over the front. The offshore buoyancy flux in the bottom boundary layer moves the front offshore until it reaches a depth where the vertical shear within the front leads to a reversal in the cross-shelf velocity at the shoreward edge of the front. Consequently, the offshore buoyancy flux in the bottom boundary layer vanishes shoreward of the front. Within the front, a steady balance is established in the bottom bo...

Trapping of a Coastal Density Front by the Bottom Boundary Layer

A two-dimensional numerical model is used to study the response to upwelling- and downwelling-favorable winds on a shelf with a strong pycnocline. During upwelling or downwelling, the pycnocline intersects the surface or bottom, forming a front that moves offshore. The characteristics of the front and of the inner shelf inshore of the front are quite different for upwelling and downwelling. For a constant wind stress the upwelling front moves offshore at roughly a constant rate, while the offshore displacement of the downwelling front scales as t because the thickness of the bottom layer increases as the front moves offshore. The geostrophic alongshelf transport in the front is larger during downwelling than upwelling for the same wind stress magnitude because the geostrophic shear is near the bottom in downwelling as opposed to near the surface in upwelling. During upwelling, weak stratification is maintained over the inner shelf by the onshore flux of denser near-bottom water. This weak stratif...

https://www.whoi.edu/fileserver.do?id=80764&pt=10&p=52174

The Inner Shelf Response to Wind-Driven Upwelling and Downwelling*

The inner continental shelf, which spans water depths of a few meters to tens of meters, is a dynamically defined region that lies between the surf zone (where waves break) and the middle continental shelf (where the along-shelf circulation is usually in geostrophic balance). Many types of forcing that are often neglected over the deeper shelf—such as tides, buoyant plumes, surface gravity waves, and cross-shelf wind stress—drive substantial circulations over the inner shelf. Cross-shelf circulation over the inner shelf has ecological and geophysical consequences: It connects the shore to the open ocean by transporting pollutants, larvae, phytoplankton, nutrients, and sediment. This review of circulation and momentum balances over the inner continental shelf contrasts prior studies, which focused mainly on the roles of alongshelf wind and pressure gradients, with recent understanding of the dominant roles of cross-shelf wind and surface gravity waves.

https://www.whoi.edu/fileserver.do?id=105664&pt=10&p=57953

The Wind- and Wave-Driven Inner-Shelf Circulation

Physical oceanography of the Amazon shelf

Observations from the Oregon, northwest Africa, Peru, and northern California shelves are used to examine the characteristics of the surface boundary layer in coastal regions during the upwelling season. The observations from these four regions yield a consistent picture of the structure of the surface boundary layer. Both CTD and moored observations reveal the presence of surface mixed layers that are typically 0–20 m thick with variability at diurnal and subtidal (periods longer than 36 hours) frequencies. The subtidal surface mixed-layer depth variability scales as u*/(NIf)½, where u* = (τS/ρ0)½ is the shear velocity, NI is the buoyancy frequency below the surface mixed layer, and f is the Coriolis frequency. Surprisingly, this relationship indicates that the subtidal variability of surface mixed-layer depth does not depend strongly on either the surface heat flux or advection of heat, both of which are large in coastal upwelling regions. Within the surface mixed layer the cross-shelf current ...

Steven J. Lentz

Papers

Trapping of a Coastal Density Front by the Bottom Boundary Layer

The Inner Shelf Response to Wind-Driven Upwelling and Downwelling*

The Wind- and Wave-Driven Inner-Shelf Circulation

Physical oceanography of the Amazon shelf

The Surface Boundary Layer in Coastal Upwelling Regions