How Corroding Sea Structures Can Provide Vital Habitats For Marine Life

(MENAFN- The Conversation) Rust can be incredibly annoying if it appears on your new bicycle or car, but if you are a free-floating baby marine animal (larvae), it could be your dream home.

When metal ends up in the sea, two things happen. The metal will rust (or corrode). Then it will become biofouled – that means it gets covered with marine slime, with seaweed and marine animals, such as barnacles and sea squirts, attaching onto the surfaces.

Biofouling and corrosion are fundamentally linked. Corroded surfaces are more likely to be biofouled and biofouling worsens corrosion . That's why rich and diverse ecosystems often develop around shipwrecks and offshore renewable energy structures, such as wind turbines.

Corrosion happens when a metal structure's chemical elements (usually in the form of charged“ions”) react with a chemical element in seawater. What we call rust is iron reacting with oxygen and the brown colour is iron oxide (a molecule containing iron and oxygen).

Seawater corrosion of metal releases metal ions into the water. In high concentrations, some are potentially toxic to marine life. For example, copper can prevent juvenile barnacles from developing hard calcium-rich outer shells . Luckily, the potentially toxic components (such as heavy metals like mercury and lead) only appear in very low concentrations in most structural metals . In fact, the presence of the corroding structure will usually create an environmental benefit.

Biofouling and corrosion on a welded sample of nickel aluminium bronze after it has been submerged for 18 months in seawater off the coast of Plymouth, UK. Tamsin Dobson, CC BY-NC-ND

We are both marine scientists fascinated by how corroded structures affect larval dispersal and species distribution. While one of us (Tamsin Dobson) researches the effects of marine corrosion and biofouling on marine engineering applications, the other (Molly James) is a marine ecosystems modeller exploring both larval dispersal and pollutant pathways.

Dobson's research showed that biofouling organisms can worsen corrosion . Larger biofouling organisms (such as barnacles and sea squirts) will attach to the surface using special cement or glue that they secrete.

Underneath the organism, the amount of oxygen starts to reduce as the organisms continue to respire (consuming food and oxygen to release energy and carbon dioxide). Because that oxygen cannot be replenished from the surrounding seawater, metal chlorides react with hydrogen in the water, producing hydrochloric acid. This acid is highly corrosive. There are also many biofouling bacteria that play a role in corroding metals.

Marine life can more easily attach to the rough surfaces of corroded metal compared to new, smooth, polished metal. Think about climbing a cliff – it's much easier when there are lots of craggy hand and foot holds to cling to. The crevices provided by corrosion also protect biofouling organisms from surrounding seawater currents. As corrosion develops further, the roughness provides bigger crevices for those organisms to grow in.

When marine biofouling creatures attached to corroded marine structures reproduce or spawn, their tiny babies (larvae) are released into the seawater and carried by ocean currents. Eventually, they may settle on other marine structures, creating a web of connected habitats. The more corroded marine structures in an area, the more potential new homes for the marine larvae to attach to and grow on.

Habitat hotspots

James's recent research used computer models to show how ocean currents and wind patterns act like highways, carrying larvae between the structures, helping to establish vibrant and interconnected marine communities. The existing structures in the North Sea have unintentionally created five distinct communities of marine life – larvae released from one of the North Sea structures will remain in the community that the structure is within.

The same modelling demonstrates that marine larvae float on seawater currents and tides, spreading out in some areas and coming together in other areas known as“hotspots”. These hotspots are the perfect places for building artificial reefs or establishing protected zones where fishing practices or underwater developments are limited.

By providing suitable habitats (like a patch of corroded metal) in hotspots, these areas could enhance the survival of marine biofouling organisms, giving them a safe place to settle and grow into adults. In turn, this provides more food for young marine animals that feed on the biofouling organisms and their larvae, therefore improving ocean health and building the resilience of the marine ecosystem.

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