Current connections: how far away coastlines influence marine reserves

Source: Robinson, J., et al. “Far‐field connectivity of the UK’s four largest marine protected areas: Four of a kind?.” Earth’s Future. (2017) doi: 10.1002/2016EF000516

Parks of the ocean

Marine Protected Areas (MPA’s) are a lot like National Parks and Forests, but for the oceans. They are designated regions of the sea that are designed to help protect habitats and biodiversity, restore ecosystems, improve or restore fisheries, maintain spawning stock and to improve nearby fishing grounds. To achieve these goals, MPA’s either outlaw all  fishing within a region, or the fishing is managed sustainably. As of 2016, there are more than 13,650 MPAs around the globe, covering 5.1% of the total area of the world’s oceans.

Official map of Marine Protected Areas as of December 2016. Adapted from Protected Planet.

MPA’s have been a very effective way to manage fishing and preserve biodiversity in the ocean, however there are big challenges with understanding their effectiveness. While humans can define the borders of an MPA, there is no physical boundary and the oceans are strongly connected, so what happens in the waters surrounding MPA’s is hugely important to their success. Changes in ocean conditions can cause species to migrate to a new location and ocean currents can carry materials and small animals in and out of MPA’s.

The connection of an MPA with other parts of the ocean is known as its connectivity. High resolution models of ocean circulation can be used to map out the places that are connected most strongly to a particular MPA, and estimate the time it takes for currents to connect one location and another. This technique is often used to see what happens downstream of MPA’s. For example, larvae in an MPA can be swept outside the boundary, seeding species in other areas. What goes in to MPA’s is just as important as what comes out, but there has been little research into the potential negative impacts of upstream areas on MPA’s. Pollution, including plastics, can be carried into MPA’s by ocean currents, potentially carrying harmful pollutants from coastal cities to remote MPA’s.


Finding connections

A group of researchers from Southampton, UK set out to investigate how four large MPA’s scattered over the globe are connected to the surrounding oceans and coasts. This connectivity could then be applied to figure out how much the MPA’s are being affected by pollution from densely populated coastlines. They looked at Pitcairn MPA, a region of small islands in a remote part of the Southeast Pacific; South Georgia MPA, a string of islands in one of the most biologically productive parts of the Southern Ocean; Ascension MPA, a volcanic island in the tropical Atlantic Ocean and the British Indian Ocean Territory (BIOT) MPA, a group of coral islands in the Indian Ocean.

To tackle the problem, they turned to a state-of-the-art model of surface ocean currents. Models are useful for this purpose because scientists can run many experiments quickly and track current flow backwards in time. The research team released virtual drifters in the model, which are carried by the surface currents, similar to a piece of driftwood or plastic being carried along by currents in the real ocean. They released the drifters inside the boundaries of each MPA once a month, and followed them backwards in time for one year, to identify the area of the oceans that are connected to each MPA.

Coast to coast

When the researchers mapped out the connectivity of each MPA, the first striking result was that all of the MPA’s apart from Pitcairn were connected to coasts on timescales less than a year. In particular, the majority of the virtual drifters that ended up in the BIOT MPA originated in a coastal area, about a third of the drifters that reached Ascension MPA were from coastlines and only 2% of South Georgia MPA drifters started out near land. This suggests that human activities could be having a large influence on the BIOT MPA, while the Pitcairn MPA and South Georgia MPA are practically unaffected by human activities on land.

Figure 2 from Robinson et al. 2017. Colors show the regions of the world ocean connected to each MPA, colored by the time taken for drifters to travel from a given location to within the MPA. The gray scale on land shows population density, with higher densities in darker colors.

To assess this influence in more detail, the researchers combined the estimated connectivity from ocean circulation with population density along coasts, assuming that larger populations are usually associated with higher levels of plastic pollution entering the ocean. What they found was striking: not only was BIOT highly connected to land, the regions it connected to had very high population density, particularly along the coast of India. This finding shows that even though the BIOT mpa is remote and contains mostly pristine coral reefs, it could potentially be strongly influenced by plastic pollution from densely populated coastlines around the Indian Ocean.


A tool for the future

This work focused on how coastal population density could influence remote MPA’s, but there are many other factors that could pose a risk to MPA’s including shipping lanes, oil rigs and fishing grounds; all of which could be examined with this creative combination of computational models and external data. The connectivity framework provides a way to look at how likely these many different risks are to influence a particular MPA.

While this study focused on only four MPA’s, the researchers showed that the role of ocean currents in connecting MPA’s to other parts of the ocean can be very different for different MPA’s, and should be taken into account for formation and management of current and future MPA’s. This is a prime example of how the increasing availability of high-resolution, realistic ocean models and new analysis tools can be used to directly influence protection and management of our oceans.

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