At the 17th Ronald C. Baird Sea Grant Science Symposium Session #3: Sediment and Sound, Monique LaFrance Bartley (Marine Ecologist, National Park Service in the Ocean and Coastal Resources Branch) explains how offshore wind turbines alter the seafloor and how this, in turn, affects the associated marine communities.

LaFrance Bartley led a study to monitor changes within the benthic environment surrounding three of the turbines at the Block Island Wind Farm over the course of a 3 year period. The benthos plays an important role in the ecosystem by its 1) creation of habitat for a diverse range of species, 2) providing of feeding, spawning and nursery grounds to fish and shellfish and 3) role in sediment stabilization, nutrient recycling and water quality regulation. For these reasons, the benthos is of key interest to a large number of stakeholders.

The study investigated multiple aspects of the benthos over a course of 3 years. One goal of the study was to determine whether or not there had been changes to the sediment characteristics and benthic macrofaunal communities. Macrofauna are organisms that live within or on top of the sediment and are greater than 1 mm, typically being smaller (e.g. amphipods and other crustaceans, polychaete worms, small bivalves) but also including larger species (e.g. mussels, clams, sea stars). The monitoring included looking for changes in species richness, diversity, biomass, abundance and community composition. These observations were made by collecting grab samples during each of the 3 years using both research vessels and divers, as well as video and photographs.

Samples that were collected approximately 10m-70m from the perimeter of the turbines generally exhibited similar sediment characteristics to samples collected within the control areas throughout the duration of the study. In regards to the macrofauna, years 1 and 2 showed similar findings with comparable dominant species abundances and spatial distribution, both across turbines and control areas. However, year 3 showed high abundance of juvenile blue mussels, as well as patches, or “hot spots”, of mature blue mussels in the area around Turbine 1. This finding is believed to be the first documented presence of mussels in the area and is strongly linked to the presence of colonized turbine structures. Substantial changes were also seen by year 3 for the areas underneath the turbine jacket structures. These areas transitioned to fine grain, organic-rich sediment dominated by mature blue mussels and barnacles. 

With respect to the epifauna (organisms attached to a surface, i.e. the turbine structures, such as mussels, anemones, hydroids, coral), the data indicate that though some differences exist throughout the course of the monitoring process, the overall dominant species continued to be blue mussels. Mobile predators and scavengers, such as sea stars, crabs, and moon snails are also present, with notable increases in abundance from year 2 to year 3. LaFrance notes that the observations of fish species and abundance indicate that the turbine structures are in fact acting as an artificial reef.

So, what does this mean? These findings show that the areas underneath the turbine structures are in the process of or have already transitioned into a completely different habitat type characterized by mussels and finer sediment with high organic content. This increase in mussels also correlates with the substantial increase in the abundance of larger mobile predators or scavengers including crabs, sea stars and moon snails. From an ecological perspective, this new habitat is functionally different from the original habitat. Whereas the habitat was previously characterized by polychaete worms (mix of burrowing and tube-building, deposit and filter feeding), it is now dominated by blue mussels (sessile filter-feeding bivalves). Further, the data indicate that the effects of these habitat changes are expanding beyond the area beneath the turbine structures, especially at Turbine 1. Here, high abundances of juvenile blue mussels were documented, as were patches of mature mussel aggregations associated with finer sediment, higher organic content, and alterations in macrofaunal community composition. These changes are strongly associated with the presence of the BIWF structures, which provide surface for colonization by epifauna, including blue mussels. It is clear that changes are still taking place and that further study is needed to fully understand the implications of the Block Island Wind Farm. “It is important to work with resource users to come to an understanding about what the changes mean and how we might want to move forward with future offshore wind farms,” said LaFrance Bartley.