Physical Drivers of Ecosystem Change Throughout the Tropical Indian Ocean

Abstract

The past decade has seen improved awareness of increasing anthropogenic stress on marine ecosystems and their inhabitants. To understand how best to protect these ecosystems, it is vital to understand the linkages between physical oceanographic processes and ecosystem variability. To date there have been limited multidisciplinary investigations capable of resolving both local oceanographic influences and ecosystem response at tidal and interseasonal time scales. Here we present findings from three multidisciplinary research cruises to Chagos Archipelago, investigating the dynamics at Egmont, a tropical atoll, and Sandes, a shallow steep sloped seamount pinnacle. Our findings are then applied in a case study at D’Arros, an island in the Seychelles Outer Island Group. Turbulent internal waves are shown to generate near bed temperature fluctuations associated with enhanced reef manta ray detections, driving ecosystem response with site specificity at kilometric scales. The depth band which internal waves influence are modulated by the depth of the background stratification. The stratification depth is controlled by basin scale processes, primarily the Indian Ocean Dipole (IOD) and the monsoon. Our results show the 2019 extreme IOD event drove a deep (< 100 m) 10 thermocline, limiting the impacts of internal wave-based cooling leading to deep coral bleaching. At Sandes the deep thermocline acts as a gating condition for internal wave propagation over the summit due to the deeper intersection of the thermocline with the seamount, suggesting that these biodiversity hotspots are not permeant fixtures. At D’Arros, a limited resource fieldwork campaign was able to identify spatial heterogeneity in the internal wave regime, explaining the differences in coral bleaching severity between the north and south which had been previously observed. Our results demonstrate how improved understanding of ecosystem response to dynamical oceanographic processes can drive more effective, resource efficient, conservation strategy by highlighting key biodiversity hotspots for protection.