Ocean Currents Push Mainland Pollution to Remote Islands – Eos

Marine protected areas, set up to conserve marine ecosystems and species, accumulate pollutants swept in from mainland shores by ocean currents.

It would be natural to assume that remote and sparsely inhabited islands in the middle of the ocean are surrounded by crystal clear, clean waters. Unfortunately, such locations can be affected by marine pollution originating from thousands of kilometers away.

Human populations living at or near the coast are the main source of pollution in the oceans, particularly plastic debris. Once at sea, floating, suspended, and dissolved pollutants are carried by ocean currents and can move great distances over months, years, and decades. Robinson et al. introduce the concept of the connectivity footprint to demonstrate how remote oceanic areas are connected to continental coastlines via a flow of pollutants.

Their research focuses on marine protected areas (MPAs), a form of designation used internationally that seeks to conserve important ecosystems and protect threatened marine species, particularly by restricting human activities in the area.

The MPAs around four island groups (all of them British Overseas Territories) were selected for the study: Pitcairn Island in the South Pacific, South Georgia and the Sandwich Islands in the Southern Ocean, Ascension Island in the South Atlantic, and British Indian Ocean Territory (BIOT, also called the Chagos Islands) in the Indian Ocean.

Using a global ocean general circulation model, the researchers released virtual particles from within the borders of the MPAs. The particles were released into the model at fixed time intervals and at a grid of locations (horizontal and vertical) over the period 2000 to 2009. Then, researchers followed the particles backward in time to assess their upstream origins to assess if, where, and when they had previously come into contact with land.

The researchers gathered data on human population density at those coastal locations to use as a proxy measure of the quantity of pollutants released into the ocean. Put together and taking into account seasonal and interannual variability in ocean currents, the data were used to calculate the annual connectivity footprint for each MPA.

The results show that the Pitcairn MPA has very low connectivity to land (0.06%) through ocean currents and faces no discernable risk from pollutants originating on the mainland. The South Georgia MPA has a low (2%) connectivity to land and a low exposure to pollutants. The Ascension MPA has medium connectivity (34%) to land and faces a significant risk. The BIOT MPA has high connectivity (71%) to land and faces high risk.

Marine protected areas represent a concerted effort to conserve valuable marine habitats and biodiversity. This study shows that granting protection from harmful human activities being carried out within the boundaries of a zone does not prevent the waters being polluted from upstream locations often many thousands of kilometers away. The connectivity footprint has the potential to be a useful tool in managing MPAs and in planning new ones. (Earths Future, https://doi.org/10.1002/2016EF000516, 2017)

Jenny Lunn, Contributing Writer