Impacts of light limitation on corals and crustose coralline algae

Pia Bessell-Browne,Peta L Clode,Andrew P Negri,Ross Jones,Rebecca Fisher

doi:10.1038/s41598-017-11783-z

Abstract

Turbidity associated with elevated suspended sediment concentrations can significantly reduce underwater light availability. Understanding the consequences for sensitive organisms such as corals and crustose coralline algae (CCA), requires an understanding of tolerance levels and the time course of effects. Adult colonies of Acropora millepora and Pocillopora acuta, juvenile P. acuta, and the CCA Porolithon onkodes were exposed to six light treatments of ~0, 0.02, 0.1, 0.4, 1.1 and 4.3 mol photons m−2 d−1, and their physiological responses were monitored over 30 d. Exposure to very low light (<0.1 mol photons m-2 d-1) caused tissue discoloration (bleaching) in the corals, and discolouration (and partial mortality) of the CCA, yielding 30 d EI10 thresholds (irradiance which results in a 10% change in colour) of 1.2–1.9 mol photons m−2 d−1. Recent monitoring studies during dredging campaigns on a shallow tropical reef, have shown that underwater light levels very close (~500 m away) from a working dredge routinely fall below this value over 30 d periods, but rarely during the pre-dredging baseline phase. Light reduction alone, therefore, constitutes a clear risk to coral reefs from dredging, although at such close proximity other cause-effect pathways, such as sediment deposition and smothering, are likely to also co-occur.

Highlights

A key to the ecological and evolutionary success of scleractinian corals is the formation of a mutualistic symbiosis with endosymbiotic dinoflagellate microalgae (Symbiodinium spp.)[1, 2]
crustose coralline algae (CCA) were dark red at the start of the exposure period and through time this colour intensified in fragments exposed to 0.4 and 1.1 mol photons m−2 d−1
The 30 d EI10 thresholds for bleaching in the corals was 1.2–1.9 mol photons m−2 d−1

Summary

Introduction

A key to the ecological and evolutionary success of scleractinian corals is the formation of a mutualistic symbiosis with endosymbiotic dinoflagellate microalgae (Symbiodinium spp.)[1, 2]. Carbohydrates produced by oxygenic photosynthesis of the algal symbionts and translocated to the coral host provide much of the energy required for maintenance, growth and reproduction[3,4,5] This exchange has enabled the symbiosis to survive and coral reefs to proliferate in oligotrophic environments, the light dependency has placed constraints on phototropic corals, limiting their distribution to comparatively low latitudes (~32° north and south of the equator), and shallow depths (~10% of surface light or 50 m)[6,7,8,9]. Some corals can thrive in highly turbid regions where irradiance is frequently attenuated by elevated suspended sediment concentrations (SSCs); these corals have adapted over extended (ecological) time frames to low-light conditions and are generally limited to shallow depths (

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