Using dredged sediments to promote SAV expansion

Submerged aquatic vegetation (SAV) are rooted and flowering plants found in shallow marine, coastal, estuarine, and freshwater habitats that provide a variety of critical ecosystem services. In addition to contributing to direct economic benefits such as habitat to support recreationally and commercially important fisheries, SAV also provide multiple social and environmental benefits, such as erosion protection (sediment stabilization), flood risk reduction (wave and current attenuation), nutrient cycling, and carbon sequestration.

Nearly 30% of global SAV coverage has been lost since the late 1800s due to both natural (extreme events) and anthropogenic stressors (excess nutrient and sediment loading) and is expected to be exacerbated by climate change (increased sea level, sea surface temperature, storms) in the future. Furthermore, SAV loss often creates a feedback loop that reinforces habitat loss.

Although SAV can migrate landward in response to loss of habitat at the deeper limits due to insufficient light availability, developed shorelines can restrict this process.

SAV restoration efforts, such as large-scale plantings, to mitigate some of these losses have been expensive and relatively unsuccessful. However, utilizing dredged material to elevate benthic habitats, which apart from unsuitable depths can support SAV, is a Beneficial Use of Dredged Material (BUDM) restoration strategy that has not been widely pursued. The lack of applications of this approach is in part due to regulatory restrictions that are meant to minimize water quality degradation in sensitive habitats.

Historically, dredging could be very destructive to SAV habitats due to direct removal of the plants if within the dredging footprint, which is now generally avoided. Dredging is also associated with temporary negative water quality impacts, such as increased turbidity, but these are typically not greater than natural fluctuations, and only contribute to SAV loss if these conditions are persistent.

Because SAV continue to face a variety of threats, which are only expected to increase, it is necessary to explore novel opportunities to expand SAV coverage. BUDM can benefit SAV by augmenting habitat depths so that formerly too deep areas can support SAV and keep pace with sea level rise. However, case study examples are critical for developing the guidance for BUDM to benefit SAV as well as informing the regulatory agencies so environmental regulations are not unnecessarily restrictive.

A navigation channel was initially dredged through an SAV bed in Wood Island Harbor in 1990. Subsequent SAV surveys in 2016 showed SAV recovery with the channel. SAV was also affected following maintenance dredging in 2020, but surveys from 2021 indicated recovery along slopes of the channel.

Dredged material was placed at 6 placement areas that were fringed by SAV in 1994. SAV was monitored at the placement sites annually to track post-dredging recovery. Although SAV initially declined, full recovery occurred within 5 years.

Placement Site 26B in Barnegat Bay was used 16 times to place material dredged from the Oyster Creek Navigation Channel between 1981 and 2017. Initially an open-water placement site, it evolved into an emergent island that is now fringed by SAV. Historic imagery demonstrates that SAV began colonizing the area before 1995, with significant increases in coverage by 2015.

Since placement at site 26B has been discontinued, Site 6, approximately 1-km west, is now used for open-water placement of material dredged from Oyster Creek Channel. Placement began in 2020 in water too deep to support SAV and recent field surveys indicate that the placement site is >100 m from existing SAV beds.

Dredged material was placed on Swan Island, an island in the Chesapeake Bay to counteract significant erosion, in 2019. Despite efforts to minimize impacts, SAV on the south eastern shoreline of the island was buried. Recent aerial surveys indicate some recovery of the SAV in this area. SAV will continue to be monitored annually.