Identifying and assessing evidence for recent shoreline change attributable to uncommonly rapid sea-level rise in Pohnpei, Federated States of Micronesia, Northwest Pacific Ocean

Abstract

Those parts of the northwest Pacific Ocean where sea level has been rising fastest over the past few decades include islands in the Federated States of Micronesia. To understand the possible effects of rapid sea-level rise, coastal surveys were undertaken within Pohnpei State in October 2014. The high volcanic island of Pohnpei was targeted along with 10 reef-edge island groups on its surrounding barrier reef as well as islands on Ant Atoll, 15 km southwest. Evidence of shoreline erosion attributable to sea-level rise is found only in a few places along the main island’s northeast (windward) coast. High rainfall has led to the accumulation of terrestrial sediment along the coast that is covered with mangrove forest 2–3 km broad in places shielding the island’s coast from wave erosion. A different picture is found on reef-edge islands around which erosion over the last few decades can mostly be explained by recent sea-level rise. Islands have disappeared within living memory, others drastically reduced in size in the past decade, while others – their sand cover washed away – are being reduced to a skeletal (boulders anchored by mangrove) state. The coasts of Ant Atoll appear little affected by erosion ascribable to sea-level rise. In summary, fewer effects than might be expected from recent sea-level rise were seen in Pohnpei, largely for reasons of natural coastal resilience or a lack of record, especially for reef-edge islands. The importance of mangrove conservation and an understanding of sediment dynamics on the broad reef-lagoon shelf surrounding the main island is manifest.

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Notes

  1. 1.

    Mostly Rhizophora apiculata and Sonneratia alba.

  2. 2.

    Note that ‘Dekehtik’, which means ‘small island’ in Pohnpeian, is a name given to at least three islands off the main island of Pohnpei; the only Dekehtik referred to in this study is that adjoining Na Island off the southeast Pohnpei coast.

  3. 3.

    The dominance of waves from the northeast may also explain why there are no reef islands along the northern reef barrier of Pohnpei, a conspicuous contrast to the situation in the south (see Figure 1). This observation might also be explained (in part) by differential subsidence of the volcanic edifice from which the island of Pohnpei and its surrounding reef platform rises; yet it is difficult to explain this by the fact that the northwest part of this edifice collapsed in a large-scale landslide event around 8.4 Ma (Spengler 1990) because that is more likely to mean that the northern side of Pohnpei is rising isostatically rather than subsiding faster than its south side.

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Acknowledgments

The Government of the Federated States of Micronesia endorsed the research on which this study is based. PN and RK were funded by the Faculty of Arts and Business at the University of the Sunshine Coast. Kalahngan to our many friends on Pohnpei especially Jerry Martin, Allois Malfitani (Pohnpei Surf Club), and Ertin Poll (Kehpara Island). Petra Nunn helped in numerous ways.

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Nunn, P.D., Kohler, A. & Kumar, R. Identifying and assessing evidence for recent shoreline change attributable to uncommonly rapid sea-level rise in Pohnpei, Federated States of Micronesia, Northwest Pacific Ocean. J Coast Conserv 21, 719–730 (2017). https://doi.org/10.1007/s11852-017-0531-7

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Keywords

  • Islands
  • Erosion
  • Mangroves
  • Coral reef
  • Reef islands
  • Climate change