Potential Hydrodynamic Impacts of Offshore Wind Energy on Nantucket Shoals Regional Ecology: An Evaluation from Wind to Whales 

The transition to renewable energy has spurred many efforts to scale up the U.S. portfolio of efficient clean energy resources, including the development of offshore wind farms. The Nantucket Shoals region off the coast of Massachusetts is the first large scale wind farm installation under development in U.S. waters. To ensure Nantucket Shoals region offshore wind energy installations are being planned, constructed, and developed in an environmentally responsible way, the Bureau of Ocean Energy Management (BOEM) asked the National Academies to evaluate the potential for offshore wind farms in the Nantucket Shoals region to affect oceanic physical processes, and, in turn, how those hydrodynamic alterations might affect local to regional ecosystems. Of particular interest to BOEM are the potential effects of hydrodynamic changes on zooplankton productivity and aggregations, which may affect foraging for the critically endangered North Atlantic right whale.

This report found the impacts of offshore wind projects on the North Atlantic right whale and the availability of their prey in the Nantucket Shoals region will likely be difficult to distinguish from the significant impacts of climate change and other influences on the ecosystem. Further study and monitoring of the oceanography and ecology of the Nantucket Shoals region is needed to fully understand the impact of future wind farms. This report recommends the Bureau of Ocean Energy Management, the National Oceanic and Atmospheric Administration, and others should promote observational studies and modeling that will advance understanding of potential hydrodynamic effects and their consequent impacts on ecology in the Nantucket Shoals region during all phases of wind energy development.

Summary

As part of the permitting process required to install and operate offshore wind farms, the Bureau of Ocean Energy Management (BOEM) requires assessment of any potential associated ecosystem impacts. To inform its decisions, BOEM requested that the National Academies evaluate the potential for installation and operation of offshore fixed-bottom wind turbine generators to affect physical processes in the Nantucket Shoals region (such as tidal fronts, waves, and currents), and, in turn, how those hydrodynamic alterations might affect ecosystems. Of particular interest to BOEM are the potential effects on zooplankton productivity and aggregations, which may affect foraging for the critically endangered North Atlantic right whale (Eubalaena glacialis).

The potential effects of wind turbine generators on the ocean can be due to the physical presence of the structures across the water column and to the effects of wind energy extraction on ocean circulation. A single offshore wind turbine can alter local hydrodynamics by interrupting circulation processes through a wake effect and induce turbulence in the water column surrounding and downstream of the turbine supporting structure, the pile. Moving away from single turbine effects and looking at arrays of turbines in a wind farm or at multiple adjacent offshore wind farms, these effects become more complex with implications for both local and regional circulation. Understanding these hydrodynamic effects is essential to develop predictions of the potential impacts of wind farms on the region’s ecosystem, from phytoplankton to marine mammals.

To date, few studies exist to assess the potential hydrodynamic and ecological impacts of offshore wind development, and those that do exist consist of modeling studies with limited observational data developed for wind farms in the North Sea, which have different hydrodynamic and ecosystem characteristics. Based on what is known, the impacts on ecosystems from development and operation of offshore wind may be difficult to distinguish from natural and other anthropogenic variability (including climate change) in the Nantucket Shoals region, where the oceanography and ecology is dynamic and evolving. Targeted observations and studies are critical to understand and quantify the hydrodynamic and ecological effects of offshore wind in the Nantucket Shoals region.

A DYNAMIC AND EVOLVING OCEANOGRAPHIC REGIME

The hydrodynamics of the Nantucket Shoals region are driven by complex interactions among shelf-break processes, seasonal stratification, interannual variability, bottom friction, tides, and flows over complex bathymetry. This complex oceanography is additionally influenced by region-specific processes such as long-term surface densification, onshore midwater intrusions of slope water, warm core rings, onshore displacement of the shelf-break front, and interdecadal variability in circulation.

Major oceanographic changes have occurred in the region since 2000, including warming of surface and bottom temperatures, increased frequency of Gulf Stream warm core rings, and midwater intrusions into the tidally mixed inshore region. Warming water temperature affects onset, decay, and intensity of seasonal stratification. These changes affect the oceanography of the region, but the long-term trends and consequences remain to be determined, particularly because the system is subject to additional changes.

Phytoplankton productivity is primarily controlled by water column stratification and seasonal solar insolation, with a dominant seasonal bloom in autumn/winter associated with a weaker stratification. Zooplankton forage on the phytoplankton produced in these seasonal blooms and most higher-trophic-level species associated with the Nantucket Shoals region feed either directly or indirectly on zooplankton found in the region. High concentrations of zooplankton, including the primary prey of right whales—the copepod Calanus finmarchicus in winter–spring—may account for the great numbers of right whales observed feeding in the Nantucket Shoals region and other areas of high productivity in Southern New England, for example, Cape Cod Bay.

UNDERSTANDING HYDRODYNAMIC EFFECTS

As the wind blows across a turbine or wind farm, wind energy is extracted, thus creating a wind wake behind the turbine and reducing wind-driven circulation in the upper ocean. Additionally, the turbine structure in the water column causes an ocean wake, meaning the water becomes more turbulent behind the turbine. The increased turbulence and decreased wind forcing both affect the structure and movement of the water as it passes the turbine, referred to as the hydrodynamics. Knowledge of the effects of offshore wind turbine structures on hydrodynamics is limited and primarily based on modeling studies in the North Sea that have not been validated by observations. The structure and magnitude of the wind wake at the sea surface is poorly understood, with most of the observational and modeling studies focused on wind speed reductions at the height of the turbine and not at the sea surface. The effect of ocean surface roughness on wind stress reductions at the sea surface is also poorly understood.

At the turbine scale, there are few observations that can be used to verify turbine-scale wake behavior. At the wind farm scale, the potential impacts include reductions in ocean current speeds, stratification, ocean surface wind speed, and deflection of the pycnocline [a boundary layer of water with a large density gradient, separating low-density surface water and higher-density deeper wate]. At the regional scale, perturbations due to offshore wind turbines are difficult to quantify because of the natural processes that drive significant environmental variability across the region.

Given the significant uncertainties in the hydrodynamic response of the wind wake and ocean wake, hydrodynamic effects of turbines are difficult to isolate from natural and other anthropogenic variability (including climate change). Some hydrodynamic observations are available at the regional scale, but there is a scarcity of observations at the turbine and wind farm scales that can be used to quantify hydrodynamic perturbations and provide validation and calibration of hydrodynamic models used to assess effects at these scales.

For a hydrodynamic model to be suitable for the Nantucket Shoals region, the key physical oceanographic processes should be represented. In addition to tide- and wind-driven circulation on the continental shelf, the hydrodynamic models implemented for the Nantucket Shoals region must include the seasonal progression of stratification and capability to simulate interannual variability scenarios as well as the effects of long-term surface densification, onshore advection of warm core rings, and onshore migration of the shelf-break front. Accurate representation of these complex oceanographic processes is difficult because of the open boundaries and atmospheric forcing, as well as the imperfect parameterizations of turbulent mixing and turbine-induced ocean and atmospheric wakes. The hydrodynamic modeling studies funded by BOEM (DHI-MIKE 3 Flexible Mesh [FM], Delft3D FM, Finite-Volume Community Ocean Model [FVCOM]) differ in parameterizations and/or representations of offshore wind turbines and wind energy areas, and therefore produce differences in simulated hydrodynamic wind and ocean wake impacts.

Model inaccuracies and uncertainty propagate through to the predictions and introduce associated uncertainty in assessing hydrodynamic and ecological impacts. Taking a hierarchical approach to model calibration, verification, and validation would reduce error and uncertainty. Such an approach would begin with small-scale idealized simulations and large eddy simulation (LES) models that resolve individual turbines and progress to larger-scale fully dynamic models at the regional scale.

POTENTIAL IMPACTS TO RIGHT WHALE PREY

Right whales feed on small, energy-rich zooplankton and, in particular, copepods such as C. finmarchicus, in New England waters. Successful foraging depends on copepods being found in sufficient densities and at appropriate depths, and as such right whales are sensitive to disturbances of their prey in the water column.

The paucity of observations and uncertainty of the modeled hydrodynamic effects of wind energy installations make potential ecological impacts of offshore wind farms difficult to detect, particularly considering the scale of natural variability as well as other anthropogenic variability of the Nantucket Shoals region’s evolving oceanography and ecology. Though studies exist, the spatial and temporal coverage of studies concentrated at the proposed wind energy lease sites do not adequately capture broad-scale right whale use of the Nantucket Shoals region and potential impacts from offshore wind farms.

Additionally, there are gaps in understanding of foraging by right whales in the Nantucket Shoals region, including the basic question of which zooplankton taxa right whales are feeding on and how this prey changes seasonally. Surveys of zooplankton associated with foraging right whales as well as simultaneous collection of oceanographic data linked to zooplankton variability would improve this understanding. Given the limited state of understanding of the entire system and the changing oceanography and ecology, identification of substantial impacts on zooplankton, and specifically on right whale prey, that may result from wind energy development in the Nantucket Shoals region is difficult to assess.

Right whale distribution and demography has been shown to depend on the distribution and density of zooplankton, in particular the late-stage of the copepod C. finmarchicus, but studies focusing on the link between right whale habitat use and zooplankton in the Nantucket Shoals region are limited. The supply of zooplankton to the Nantucket Shoals region is dependent on regional circulation, but aggregation is presumably dependent on local physical processes and zooplankton behavior.

Overall, there is a lack of robust (coupled physical/biological) models that can effectively incorporate the supply of zooplankton, their behavior, and the physical oceanographic processes that aggregate the zooplankton in the Nantucket Shoals region in sufficient densities for right whale foraging. Given this lack of models, it will be difficult to predict potential effects of wind farms on right whales. Regarding the right whale prey field, there are potential hydrodynamic mechanisms to support each of these three possibilities: (1) turbines could cause an increase in zooplankton productivity and/or aggregation of zooplankton into high-density patches to support right whale foraging and increase right whale use of this habitat; (2) turbines may decrease zooplankton productivity and/or reduce the potential for high-density aggregations, thus potentially reducing foraging opportunities for right whales in the region; or (3) wind farm development may have no appreciable impact on right whale foraging dynamics.

Committee on Evaluation of Hydrodynamic Modeling and Implications for Offshore Wind Development: Nantucket Shoals
Ocean Studies Board
Division on Earth and Life Studies

National Academies Press: 2023 (prepublication copy)

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