Cathodically Protected Steel Mesh as an Environmentally Friendly Material for Oyster Reef Restoration
Plastic mesh is widely used in oyster reef restoration efforts worldwide, including the Indian River Lagoon (IRL) estuary in Florida. Plastic degrades overtime, releasing microplastics and nanoplastics particles into the environment and causing harm to the marine ecosystem. Thus, there is an ongoing need to find an alternative eco-friendly material. Cathodically protected steel, a possible alternate substrate for such restoration, involves an electrical current running through steel to create an impressed cathodic system. This results in the formation of chalk on the steel, similar to the limestone (calcium carbonate) base of reefs. A benefit of this process is that the low current stimulates mineral growth and may aid in the shell formation of calcareous organisms, thus increasing the settlement and growth of oysters and other marine organisms. The aim of this thesis was to address the use of steel mats in the IRL for oyster reef restoration. Specifically, a two-part study was designed to determine the impact of cathodically protected steel on oyster growth and benthic organism biodiversity. The first study measured the effect of three current treatments (low, medium and high) on growth and survival rates of the Eastern Oyster Crassostrea virginica. Replicate steel boxes for each of the electrical currents and control plastic boxes house eight triploid oysters (25.6 mm –43.3 mm) per box. Oyster growth was assessed by weekly weight and length measurements , as well as mortality counts. At the end of the 8-week experiment, a significant difference was found for the length growth rate among the steel treatments (p < 0.05).The highest length growth rate of 7.7% was observed with the high current treatment. The high current treatment and the low current treatment boxes had 90% survival rate compared to the medium current treatment and plastic controls which had a 75% survival rate. The second study monitored cathodically protected steel mats and plastic mats over the course of 21-weeks at three testing sites in the IRL. Oyster settlement and biodiversity associated with the mats was conducted with weekly assessments. At the end of the experiment, 70 individual oysters were observed on the steel mats compared to 18 oysters on plastic mats at one of the test sites. The diversity on steel mats had a significant difference (p < 0.05) among test sites demonstrating the importance of ecological and environmental parameters for the success of the steel mats. Overall, the research supported the idea that the steel mats can be used as an environmentally friendly alternative material for oyster reef restoration. The steel and plastic mats had comparable results for oyster growth and recruitment, as well as the biodiversity of sessile and mobile organisms. The chalking on electrical stimulated steel mats created a suitable substrate for the oysters and other hard and soft benthic organisms to grow not only on the dead oyster shells but also directly on the steel mats.