Acute effects of back-to-back hurricanes on the underwater light regime of a coral reef

Abstract

Some of the best-known disturbances affecting coral reefs are storms, yet their impacts on light are poorly known. Here, we describe the underwater light on a reef off St. John, US Virgin Islands (18°18′37.04N, 63°43′23.17W), during two hurricanes and multiple tropical waves that occurred between 17 August 2017 and 30 November 2017. Photosynthetically active radiation (PAR) was recorded at 19-m depth and at the surface, and rainfall was measured as a cause of turbidity affecting underwater light. Hurricanes Irma and Maria reduced maximum daily underwater photosynthetic photon flux density (PPFD) to < 14 µmol photons m−2 s−1 (< 0.001% of surface PPFD) and, from 1 September to 30 November, were associated with rainfall that was higher (107 cm) than has been recorded over this period since 1972. These reductions in underwater PPFD are unprecedented since at least 2014, and the corresponding values of the diffuse attenuation coefficients (Kd-PAR 0.268–0.426) are among the lowest recorded on a coral reef. Our study reveals the capacity for hurricanes to render a photic habitat temporarily aphotic, and over 69 days (and relative to 2016), to contribute to a 20% reduction in summed, daily integrated underwater PPFD. Large reductions in underwater PPFD have negative implications for photosynthetic taxa, but through the turbidity that reduces underwater light, they create opportunities for population growth by suspension feeding invertebrates.