Cathodically protected steel as an alternative to plastic for oyster restoration mats

https://doi.org/10.1016/j.ecoleng.2021.106210Get rights and content

Highlights

  • Electricity was supplied to steel mesh which caused mineral accretion development on oyster mats.

  • Oyster growth and mineral accretion were a function of environmental and ecological conditions.

  • Steel oyster mats can promote oyster growth at a rate greater than or equivalent to plastic mats.

  • Steel mats are an environmentally friendly alternative to the traditionally used plastic mats.

Abstract

Oyster populations in many coastal areas have decreased as a result of overharvesting and habitat degradation. In order to help restore oyster populations and natural water filtration, many restoration efforts utilize plastic mesh to reseed oyster reefs. However, plastics do not break down or mineralize in seawater, instead they break down into smaller and smaller pieces eventually becoming what is termed microplastics. One alternative may be the use of a cathodically protected steel which develops a mineral accretion layer and enhances calcareous marine growth. In order to test the efficacy of this material for oyster restoration and its ability to promote oyster growth, a field experiment was designed to compare it to traditional plastic mats at three locations. At all sites, the steel mats were able to promote the recruitment of oysters at a rate equal to or greater than the plastic mats. The amount of mineral accretion and the total number of oysters present on the mats were dependent on the environmental and ecological conditions of the test site. The steel mats were easy to work with, provided sufficient oyster settlement, and gain in weight over time. This makes them an attractive alternative to plastic oyster restoration mats, with implications for creating artificial reefs or living shorelines.

Introduction

Over the past several years the health of the Indian River Lagoon (IRL), an estuary located along Florida's east coast, has undergone a serious decline. The system has seen an increase in the presence of total suspended solids, as well as high nutrient concentrations, which in turn have fueled large scale algal blooms, such as those observed in 2012 by the brown tide pelagophyte Aureoumbra lagunensis (Gobler et al., 2013). Both the rise in turbidity and algal cell density has had a negative impact on the IRL system, blocking sunlight from reaching the benthos and leading to a decline in seagrass beds and fauna mortality (Gobler et al., 2013; Morris and Virnstein, 2004).

Oyster populations throughout the IRL have also decreased as a result of overharvesting, habitat degradation, and low salinity (Wilson et al., 2005; Garvis et al., 2015). Oyster reefs are known for providing many benefits to coastal ecosystems, one of which is their ability to filter large volumes of water (Ehrich and Harris, 2015). This process removes suspended particulates from the water column, including harmful algal species, and improves water clarity. The lack of oysters in the IRL is also thought to contribute to the frequent and persistent algal blooms. Like many estuaries worldwide, there are ongoing efforts in the IRL to restore the oyster populations to their natural abundance or to a state where they can provide sufficient ecosystem services. Currently oyster restoration efforts in the IRL utilize aquaculture plastic mesh to construct oyster mats and bags (e.g. Garvis et al., 2015). However, plastics do not break down or mineralize in seawater, instead they break down into smaller and smaller pieces eventually becoming what is termed “microplastics” (Hidalgo-Ruz et al., 2012). These small plastics are commonly mistaken as a food item for marine organisms ranging in size from plankton to whales. Plastics can also leach out chemicals into the ocean, as well as, adsorb pollutants (Ashton et al., 2010). It is now believed that these plastic pollutants may get passed up the food chain (Mattsson et al., 2017).

As oyster restoration efforts continue to comprise a large portion of estuarine management plans, there is a need to find environmentally friendly alternatives. Thus, different materials have been or are currently under investigation such as crab traps coated with concrete (Hernández et al., 2018), oyster castles (Hernández et al., 2018), Biodegradable Elements for Starting Ecosystems (BESE) (Herbert et al., 2018), burlap ribbon (Soucy, 2020), basalt (Soucy, 2020), and coconut coir (Soucy, 2020). One such possibility may be the use of a mild steel as the base material for oyster mats. This concept is based on prior research that has used cathodically protected steel to develop mineral accretion and enhance calcareous marine growth to form reefs (Hilbertz, 1979). Electricity is supplied to steel to prevent corrosion and cause a rise in the local pH. This causes calcium and magnesium ions to combine with bicarbonate and hydroxide ions and precipitate as CaCO3 (aragonite) or Mg(OH)2 (brucite) on the steel surface (mineral accretion) (Borell et al., 2010). The resulting calcium carbonate which is formed through seawater electrolysis is similar to reef substratum or limestone, and is thus an environmentally friendly approach commonly used in coral reef rehabilitation (Borell et al., 2010, Goreau and Hilbertz, 2005, Romatzki, 2014). It allows for increased survival and growth through reinforced substrate stabilization (Hilbertz and Goreau, 1996). Research with coral transplants has found they are quickly cemented onto the steel, facilitating firm attachment to the substrate. Additionally, the coral has enhanced skeletal growth rates due to an increase in available electrons (Hilbertz and Goreau, 1996).

Similar benefits have been reported for oysters where placing the organisms in a cathodic field increased survival rate and growth (Berger et al., 2012; Shorr et al., 2012). The few published studies on mineral accretion for oyster restoration have utilized substrates such as: rebar (Piazza et al., 2009), a steel structure fixed to a dock piling (Latchere et al., 2016), and a steel helix-shaped structure (Shorr et al., 2012). While each of these field experiments have shown the potential for mineral accretion to enhance the biological performance of oysters, there still remain many uncertainities (Koster, 2017). Especially how would the mineral accretion process be translated to successfully assist in restoration processes that utilize structures other than rebar and wire, and how the electrical measurements driven via a solar panel should be implemented for successful oyster recruitment and growth in estuarine settings.

This study was designed to determine the efficacy of mineral accretion using oyster mat structures, which are commonly used along the east coast of Florida, with the process driven by the use of a solar panel instead of DC power sources reported in previous studies. Steel mesh oyster mats were attached to an impressed current cathodic protection system to develop mineral accretion. The objective was to determine if mineral accretion mats were as effective at promoting the growth of oysters as plastic oyster mats, as well as to record changes in mineral accretion at three different locations in the IRL. It is believed the creation of the mineral accretion structures will enhance oyster growth, which in turn provides increased filtration and improvement to local water quality. The structures will also create habitat for organisms associated with oyster reefs, such as juvenile fish, barnacles, crabs, shrimp, mussels, tunicates, snails, and algae (Barber et al., 2010; Weaver et al., 2018).

Section snippets

Field preparation and deployment

A flattened expanded steel mesh with 0.64 cm openings was selected for this study, as that it is often the mesh size used for the plastic oyster restoration mats (Garvis et al., 2015). The steel mesh was cut into 45.7 × 45.7 cm replicates, and affixed with 36 dead and dried oyster shells (Fig. 1) (Wall, 2004). Oyster shells were attached to steel mats using stainless steel screws, washers, and nuts. Undamaged oyster shells (ranging in length from 6.5 to 9.5 cm) were attached perpendicular to

Results

Each of the three test sites had slightly different water quality conditions. Over the period of immersion, Port Canaveral had an average salinity of 34.7 ± 1.7 ppt, with an average temperature of 28.7 ± 1.6 °C and pH 8.1 ± 0.06. The Grant site had an average salinity of 20.5 ± 8.6 ppt, with an average temperature of 31.8 ± 2.2 °C and pH 8.04 ± 0.1. The Melbourne Beach site had an average salinity of 23.5 ± 6.8 ppt, with an average temperature of 30.8 ± 2.2 °C and pH 8.2 ± 0.2. There were

Discussion

Over the course of the three-month immersion period, mineral accretion and oyster growth were observed at all of the test sites. The steel mats were effective at promoting the growth of oysters, but the overall recruitment was dependent on the test site and each varied based on environmental and ecological conditions. At the Melbourne Beach location, there was a significantly greater presence of oysters on the steel compared to the plastic. At Port Canaveral and Grant, oyster growth at times

Conclusions

In addition to this work being of great interest to scientists and engineers working in coastal ecosystems, there is a growing pressure from the public to use plastic alternatives, such as mineral accretion, for restoration work. It is believed this method may enable a permanent oyster reef structure to become established by the deposition and chalks, promoting the growth of calcareous marine organisms (e.g. oysters, barnacles, tubeworms, mussels) and providing a firm substrate for the growth

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was funded by a grant from the Space Coast Office of Tourism - 2018/19 Lagoon Grant, Brevard County, Florida, USA. The authors would like to acknowledge the support of the Environmentally Friendly Endangered Lands Program, especially Nichole Perna, and Chris Potzar from Rib City at Grant Station, for their support and providing dock space for deployments. Thank you to Dr. Travis Hunsucker for assistance performing analysis in MATLAB.

References (42)

  • S.K. Garvis

    Quantifying the impacts of oyster reef restoration on oyster coverage, wave dissipation and seagrass recruitment in Mosquito Lagoon, Florida. Master’s Thesis

    (2012)
  • S. Garvis et al.

    Formation, movement, and restoration of dead intertidal oyster reefs in Canaveral National Seashore and Mosquito Lagoon, Florida

    J. Shellfish Res.

    (2015)
  • T.J. Goreau

    Marine electrolysis for building materials and environmental restoration

  • T.J. Goreau et al.

    Marine ecosystem restoration: cost and benefits for coral reefs

    World Resour. Rev.

    (2005)
  • R.E. Grizzle et al.

    Historical changes in intertidal oyster (Crassostera virginica) reefs in a Florida lagoon potentially related to boating activities

    J. Shellfish Res.

    (2002)
  • K. Hall et al.

    A laboratory study of reef growth by electro-deposition

    JCR.

    (2005)
  • J.M. Harding et al.

    Fish species in relation to restored oyster reefs, Piankatank River, Virginia

    Bull. Mar. Sci.

    (1999)
  • D. Herbert et al.

    Mitigating erosional effect induced by boatk wakes with living shorelines

    Sustainability

    (2018)
  • A.B. Hernández et al.

    Restoring the eastern oyster: how much progress has been made in 53 years?

    Front. Ecol. Environ.

    (2018)
  • V. Hidalgo-Ruz et al.

    Microplastics in the marine environment: a review of the methods used for identification and quantification

    Environ. Sci. Technol.

    (2012)
  • W.H. Hilbertz

    Electrodeposition of minerals in sea water: experiments and applications

    IEEE J. Ocean Eng.

    (1979)
  • Cited by (5)

    View full text