Trends and variability in spectral diffuse attenuation of coral reef waters
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Trends and variability in spectral diffuse attenuation of coral reef waters Eric J. Hochberg1
•
Stacy A. Peltier1 • Ste´phane Maritorena2
Received: 23 September 2019 / Accepted: 18 June 2020 Ó Springer-Verlag GmbH Germany, part of Springer Nature 2020
Abstract Knowledge of water clarity is an essential component of coral reef ecology. While qualitative trends are well known, there are currently few published records of spectral—varying as a (relatively) continuous function of wavelength—water optical properties for these systems. The purpose of this study is to quantify trends in the range and variability of the spectral diffuse attenuation coefficient (Kd) in coral reef and adjacent waters. Using a Biospherical PRR-800, 199 vertical profiles of downwelling spectral irradiance were collected across the reefs and nearby optically deepwaters of Bermuda and Hawaii. A single spectral Kd was calculated for each profile. Results reveal water types ranging from clear oceanic to turbid coastal, with Bermuda and Hawaii showing similar patterns. Kd roughly correlates with reef geomorphic zonation, consistent with well-known reef water characteristics: (a) Suspended sediments are ubiquitous on coral reef flats and in lagoons, chiefly comprising calcium carbonate, which is effectively spectrally flat and has the effect of increasing the overall magnitude of Kd; (b) reefs generate large amounts of dissolved organic matter, which becomes apparent in some lagoonal Kd; and (c) the Kd data exhibit little indication of chlorophyll, which is typically very low in the water column above reefs. These patterns appear reproducible when compared with prior data from French
Topic Editor Mark Vermeij & Eric J. Hochberg [email protected] 1
Bermuda Institute of Ocean Sciences, St. George’s GE01, Bermuda
2
Earth Research Institute, University of California Santa Barbara, Santa Barbara, CA 93106-3060, USA
Polynesia. The depth of the 1% light level varies accordingly, approximately 30 m, 45 m, 75 m, and 105 m for lagoon, reef flat, fore reef, and offshore, respectively. The data presented here provide some insight into trends in the spatial context of reef geomorphology and lay the foundation for a more quantitative understanding of reef water clarity and color and their importance to reef ecology. Keywords Spectral diffuse attenuation Coral reef Light ecology Reef geomorphology
Introduction Light is fundamentally important to coral reef ecosystem function (Yentsch et al. 2002; Dubinsky and Falkowski 2011). Photosynthesis by benthic organisms supplies the majority of organic carbon used by plants and animals on a reef (Kinsey 1985) and ultimately determines the biomass and secondary production of reef consumers (Atkinson and Grigg 1984; Grigg et al. 1984; Polovina 1984). Calcification is another essential biochemical process in coral reef ecosystems and has been shown to be strongly correlated with primary production (Kinsey 1985; Gattuso et al. 1999). Therefore, delivery of incident solar radiation to the benthos is
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