The Effect of Deep Oceanic Flushing on Water Properties and Ecosystem Functioning Within Atolls in the British Indian Ocean Territory

Emma V Sheehan,Chloe A Game,Marianne Wootton,Martin J Attrill,Jessica J Meeuwig,David Tickler,David G Johns,Phil Hosegood

doi:10.3389/fmars.2019.00512

Abstract

Within atolls, deep water channels exert significant control over local hydrodynamic conditions; which are important drivers of planktonic distributions. To examine planktonic responses to oceanography, this study tested the effect of proximity and exposure to deep oceanic flushing through these channels on water properties and planktonic assemblages across four atolls (Diego Garcia, Salomon, Egmont and Peros Banhos) in the British Indian Ocean Territory Marine Reserve. As this is the largest, most isolated and sparsely inhabited atoll complex in the world, it provides the perfect experimental conditions to test the effect of oceanic flushing without confounding factors related to anthropogenic development. Results are discussed in the context of ecosystem functioning. A total of 30 planktonic taxa and 19,539 individuals were identified and counted. Abundance was significantly different between atolls and significantly greater within inner regions in all atolls except southeast Egmont. Planktonic assemblage composition significantly differed between atolls and between inner and outer stations; exhibiting higher similarity between outer stations. Within outer stations of Diego Garcia, Peros Banhos and Egmont, evidence suggesting oceanic flushing of cold, saline and dense water was observed, however a longer time series is required to conclusively demonstrate tidal forcing of this water through deep water channels. Planktonic variability between inner and outer atoll regions demonstrates that broad comparisons between oceanic and lagoon regions fail to capture the complex spatial dynamics and hydrodynamic interactions within atolls. Better comprehension of these distributional patterns is imperative to monitor ecosystem health and functioning, particularly due to increasing global anthropogenic pressures related to climate change. The extensive coral bleaching described in this paper highlights this concern.

Highlights

Coral atolls are often characterized by a rich biodiversity of fishes, corals, and coralline algae, highly mobile consumers such as manta rays (Mccauley et al, 2014) and sea turtles (Mendonca and Ehrhart, 1982), and apex predators (Sheppard et al, 2012) such as reef sharks (Tickler et al, 2017)
The habitat in Diego Garcia was dominated by a matrix of sandy mounds created by benthic organisms, covered in trails made by gastropods and hermit crabs (40%), areas of bleached Bommies (22.8%) on sand and coral rubble (26.3%)
The water property and the plankton data support the hypothesis that atolls have regional differences which is likely to be the result of increased isolation of inner stations from oceanic exposure relative to outer regions

Summary

Introduction

Coral atolls are often characterized by a rich biodiversity of fishes, corals, and coralline algae, highly mobile consumers such as manta rays (Mccauley et al, 2014) and sea turtles (Mendonca and Ehrhart, 1982), and apex predators (Sheppard et al, 2012) such as reef sharks (Tickler et al, 2017). The greater productivity within atolls relative to the surrounding oligotrophic tropical waters (Rayner and Drew, 1984; Letessier et al, 2016) likely supports such diverse assemblages As such they have been claimed to be “oases of life in an oceanic desert” (Odum and Odum, 1955; Johannes and Gerber, 1974). Such atoll productivity may be maintained by high nutrient recycling, through microbial food webs and coral symbionts (Hatcher and Frith, 1985; Falter et al, 2004), and geomorphology Geomorphological features such as deep water channels are thought to affect productivity within atolls, by facilitating oceanic exchange between lagoon and oceanic waters; controlled by the tidal forcing (Pugh and Rayner, 1981; Kench, 1998; Chevalier et al, 2017; Green et al, 2018). The nutrient enrichment (Thompson and Golding, 1981; Thomson and Wolanski, 1984; Wolanski et al, 1988) and environmental stability exerted by these deep water channels through water renewal likely plays a key role in ecosystem functioning

Methods

Results

Conclusion