Biological-physical interactions in Pacific coral reef ecosystems

Abstract

Coral reefs are some of the most diverse and productive marine ecosystems on earth. They are also among the most threatened by human disturbance. On a local scale, many of these systems are subject to over-fishing and land-based pollution, and on a global scale, these systems are impacted by climate change and ocean acidification: human activities clearly influence the structure of coral reef communities. However, before considering anthropogenic influence, it is necessary to consider the influence of local and regional environmental forcings on these ecosystems. In this research, I investigate natural environmental and anthropogenic drivers of benthic community organization in 41 coral reef ecosystems across the Pacific (14.2°S--28.4°N, 144.8°E--155.4°W). These systems have been the focus of a long-term, multi-disciplinary NOAA-led monitoring effort. I present a new methodological approach to spatially constrain environmental forcings at the scale of individual islands and atolls. The results indicate considerable spatial heterogeneity in environmental forcings, namely sea surface temperature, waves, chlorophyll-a concentration (a proxy for phytoplankton biomass) and irradiance. Further examination of long-term (10-year) chlorophyll-a concentrations revealed sustained increased phytoplankton biomass just offshore of reefs compared to surrounding oceanic waters around a majority (91%) of islands and atolls, providing widespread evidence of the occurrence and scale of the "island mass effect". Additionally, significant differences in horizontal gradients in chlorophyll-a between island and atoll systems were observed. Variations in reef area, bathymetric slope, geomorphic type (e.g. atoll versus island), and human population were identified as important drivers of increased phytoplankton biomass, together explaining 77% of the variability observed. In order to investigate biological-physical relationships at a smaller spatial scale, I focused on one oceanic atoll with a history of minimal human influence, Palmyra Atoll (5.8°N, 162.1°W). At intra-island scales, wave forcing and reef geomorphology were important drivers of benthic community organization. Model performance improved when hard coral cover was modeled in distinct morphological groups (encrusting, plating, branching), highlighting the response of coral reef communities to extrinsic physical forcings. Superimposed on natural variations in coral reef benthic community organization are the effects of anthropogenic disturbance.