Award: OCE-1623867

The El Nino Southern Oscillation (ENSO) is the strongest inter-annual climate fluctuation known and it recurs every two to seven years. As such, ENSO provides a nearly unparalleled opportunity to study the impact of climate oscillations on populations, communities and ecosystems. We seized the opportunity provided by the 2015 -2017 ENSO to test hypotheses about how bottom dwelling marine communities and associated reef fish populations would change in response to the extreme oceanographic conditions in the central Galapagos archipelago (Fig. 1). The 2015 2016 El Nino was the strongest recorded since 1997/1999 event. Satellite measured Sea Surface Temperatures (SST) were more than 4.0o C above the 1981-2010 ENSO baseline. Our subtidal temperature record from the Rocas Gordon site (Fig. 1 B, C) indicated that temperature variability has increased over time in the Galapagos subtidal zone, likely representing increased thermal stress on marine organisms. A maximum temperature of 31.2o C occurred at 6 m depth at one of our sites at the end of March 2016. In addition to evaluating the overall impact of the strong El Nino on subtidal marine communities, we tested an overarching idea, the Sponge Reef Hypothesis (SRH), predicting that sponges may increasingly dominate space as corals decline from future climate change and ocean acidification. The research funded by this award enabled us to conduct bi-annual surveys for 2 years (2015-2017) across 12 widely spaced sites (Fig. 1), while our 16-year baseline on species abundance leading up to this event allowed us to place the effects of the 2015/2017 ENSO in context. We found that sponges declined significantly (rather than increased) during the unusually warm El Nino conditions, thus the SRH was not supported. However, we discovered more changes associated with this ENSO than any other we?ve studied since 1999 (Fig. 2). They included 1) exceptionally high grazing on Crustose Coralline Algae (CCA) during the warmest period, 2) a reduction of barnacles due to heavy predation by fish and whelks during El Nino with a dramatic rebound in barnacle populations in January 2017 as productivity returned during La Nina, 3) a similar trend of population decline and increase during La Nina for 3 species of planktivorous reef fish (Fig. 2) and 4) two unprecedented, novel appearances of biological phenomena during the warmest period of the 2016 El Nino: cyanobacterial mats and disintegrating skin disease in reef fish. Our work showed that mats of these potentially noxious cyanobacteria overgrew ecologically important CCA, that they are temperature mediated, persistent and avoided by predators. In contrast, the fish disease was ephemeral, lasting only six months, focused in the region around our Cuatro Hermanos site (Fig. 1) but affecting up to 16 species of reef fish of all feeding types. The disease appears to have been caused by a bacterium Pantoea. It reduced the population densities of ring tailed damselfish, which is functionally important for increasing algal productivity of the sea floor, as well as king angelfish. This appears to be the first record of wildlife disease associated with a climate oscillation in the Galapagos marine ecosystem. Physical and biological data from this project are available at the Biological and Chemical Data Management Office (BCO-DMO) via these links: https://www.bco-dmo.org/dataset/628180/data https://www.bco-dmo.org/dataset/683633 In addition, a general article describing our research fiundings under this award appeared in an article in the June 2017 isssue of National Geographic Magazine entitled: "Life in the balance: a warming planet threatens the Galapagos species that inspired Darwin?s theory of natural selection." with the link https://www.nationalgeographic.com/magazine/2017/06/galapagos-climate-change-impacts-iconic-creatures/ Last Modified: 02/17/2018 Submitted by: Jon D Witman