Underwater Light Characteristics of Turbid Coral Reefs of the Inner Central Great Barrier Reef

Abstract

Hyper-spectral and multi-spectral light sensors were used to examine the effects of elevated suspended sediment concentration (SSC) on the quantity and quality (spectral changes) of underwater downwelling irradiance in the turbid-zone coral reef communities of the inner, central Great Barrier Reef (GBR). Under elevated SSCs the shorter blue wavelengths were preferentially attenuated which together with attenuation of longer red wavelengths by pure water shifted the peak in the underwater irradiance spectrum ~100 nm to the less photosynthetically useful green-yellow waveband (peaking at ~575 nm). The spectral changes were attributed to mineral and detrital content of the terrestrially-derived coastal sediments as opposed to chromophoric (coloured) dissolved organic matter (CDOM). A simple blue to green (B/G, λ455:555 nm) ratio was shown to be useful in detecting sediment (turbidity) related decreases in underwater light as opposed to those associated with clouds which acted as neutral density filters. From a series of vertical profiles through turbid water, a simple, multiple component empirical optical model was developed that could accurately predict the light reduction and associated spectral changes as a function of SSC and water depth for a turbid-zone coral reef community of the inner GBR. The relationship was used to assess the response of a light sensitive coral, Pocillopora verrucosa in a 28-d exposure laboratory-based exposure study to a daily light integral of 1 or 6 mol quanta m2. PAR with either a broad spectrum or a green-yellow shifted spectrum. Light reduction resulted in a loss of the algal symbionts (zooxanthellae) of the corals (bleaching) and significant reduction in growth and lipid content. The 6 mol quanta m2 d−1 PAR treatment with a green-yellow spectrum also resulted in a reduction in the algal density, Chl a content per cm2, lipids and growth compared to the same PAR daily light integral under a broad spectrum. Turbid zone coral reef communities are naturally light limited and given the frequency of sediment resuspension events that occur, spectral shifts are a common and previously unrecognised circumstance. Dedicated underwater light monitoring programs and further assessment of the spectral shifts by suspended sediments are essential for contextualising and further understanding the risk of enhanced sediment run-off to the inshore turbid water communities.

Inherent Optical Properties based Vulnerability Assessment of Euphotic Zone Compression in peatland influenced Southeast Asian coastal waters

“…The coastal browning has caused a shift in the timing of the spring phytoplankton bloom by up to three weeks in the North Sea (Opdal et al, 2019). Light reduction due to sediment resuspension events are a frequent phenomenon in the coral reef zone of the inner Great Barrier reef (Bessell-Browne et al, 2017;Jones et al, 2021). Sensitive organisms such as corals and crustose coralline algae in the inner Great Barrier reef have been reported to suffer tissue discoloration (bleaching) and partial mortality in just 3-5 days in response to darkness (Yonge and Nicholls, 1931;Titlyanov et al, 2001;Bessell-Browne et al, 2017).…”

“…Terrestrial run-off from modified catchments can additionally enhance the transport of sediments, minerals, and CDOM that could potentially reduce the amount and compress the depth of penetration of solar radiation to the seabed for seagrass and corals. The high attenuation and poor water quality due to the enhanced terrestrial run-off are considered a threat to coastal ecosystems (Fredston-Hermann et al, 2016;Jones et al, 2021). The effects of photic compression will exacerbate the vulnerability of coastal and marine species, habitats, and ecosystems (Gattuso et al, 2006;Brandao et al, 2017;Jones et al, 2021).…”

“…The high attenuation and poor water quality due to the enhanced terrestrial run-off are considered a threat to coastal ecosystems (Fredston-Hermann et al, 2016;Jones et al, 2021). The effects of photic compression will exacerbate the vulnerability of coastal and marine species, habitats, and ecosystems (Gattuso et al, 2006;Brandao et al, 2017;Jones et al, 2021). Vertically compressed coral depth distributions have been reported in Singapore's turbid reef systems located in the central Sunda Shelf Sea (Morgan et al, 2020).…”

The influence of visual pollution on navigation mechanisms in the damselfish (Chrysiptera cyanea)

“…Turbidity has profound effects on light in aquatic systems (Bowers & Binding 2006). The materials which contribute to turbidity are categorised into chromophoric dissolved organic matter (CDOM), phytoplankton and non-algal particulates (NAP) (Jones et al 2021). While water absorbs strongly in the longer red wavelengths (Kirk 2010), phytoplankton absorbs strongly in the shorter blue-green wavelengths (Van Duin et al 2001), CDOM in the shorter blue-ultraviolet wavelengths (Shi et al 2013) and NAP in the shorter blue wavelengths with a tapering absorbance towards the longer red wavelengths (Babin et al 2003).…”

“…The reflection, absorption, polarisation and production of light are integral to behaviours underpinning the survival and reproduction of reef-dwelling organisms (Marshall et al 2019; Losey et al 1999; Kamermans & Hawryshyn 2011), from conspicuous territorial and breeding displays (Dawkins & Guilford 1994) to the use of landmarks for orientation (Braithwaite & Burt de Perera 2006). However, with rising coastal turbidity compromising the quality and spectral distribution of light (Jones et al 2021), constricting photic zones (Cacciapaglia & van Woesik 2016) and restricting benthic irradiances (Anthony et al 2004), visual behaviours are likely to be disrupted. In this study, we utilise a T-maze paradigm to interrogate the influence of turbidity on local navigation in a reef fish.…”