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Bleaching-driven reef community shifts drive pulses of increased reef sediment generation

Citation

Perry, Chris; Morgan, Kyle; Lange, Ines; Yarlett, Rob (2020), Bleaching-driven reef community shifts drive pulses of increased reef sediment generation, Dryad, Dataset, https://doi.org/10.5061/dryad.08kprr4zc

Abstract

The ecological impacts of coral bleaching on reef communities are well documented, but resultant impacts upon reef-derived sediment supply are poorly quantified. This is an important knowledge gap because these biogenic sediments underpin shoreline and reef island maintenance. Here we explore the impacts of the 2016 bleaching event on sediment generation by two dominant sediment producers (parrotfish and Halimeda spp.) on southern Maldivian reefs. Our data identifies two pulses of increased sediment generation in the three years since bleaching. The first occurred within ~6 months after bleaching as parrotfish biomass and resultant erosion rates increased, likely in response to enhanced food availability. The second pulse occurred 1-3 years post-bleaching, after further increases in parrotfish biomass and a major (~4-fold) increase in Halimeda spp. abundance. Total estimated sediment generation from these two producers increased from ~0.5 kg CaCO3 m-2 yr-1 (pre-bleaching; 2016) to ~3.7 kg CaCO3 m-2 yr-1 (post-bleaching; 2019), highlighting the strong links between reef ecology and sediment generation. However, the relevance of this sediment for shoreline maintenance likely diverges with each producer group, with parrotfish-derived sediment a more appropriate size-fraction to potentially contribute to local island shorelines.

Methods

Benthic cover data was collected along replicate 10 m transects (n= 5) in January and September 2016, March 2017 and January 2019, from the south-western margins of five uninhabited atoll interior reefs in Gaafu Dhaalu atoll, southern Maldives. All data were collected from sites along the outer reef flat/upper reef front (~2 m depth contour) using the ReefBudget methodology. Recorded groups included scleractinian corals to the genera and morphological level e.g., Acropora branching, Porites massive etc.; crustose coralline algae (CCA); turf algae; fleshy macroalgae; Halimeda spp.; sediment; rubble; and other benthic organisms. All data were collected as a function of the true 3-dimensional surface of the reefs, thus including cover on overhangs and vertical surfaces.

Parrotfish abundance (ind. ha-1) was quantified via underwater visual census (UVC) along eight 30 m x 4 m belt transects in the same region of each reef in each time period, with all surveys completed by the same experienced observer (K.M.M.). Details on parrotfish species, life phase (juvenile, initial and terminal) and total length (in size classes of 10 cm) were recorded for each individual. Parrotfish biomass (kg ha-1) for each species and size class was then calculated using established length-weight relationships and multiplied by fish abundance, following the approach described in: Januchowski-Hartley FA, Graham NA, Wilson SK, Jennings S, Perry CT. (2017) Drivers and predictions of coral reef carbonate budget trajectories. Proc Royal Soc B. 284:20162533 (doi.org/10.1098/rspb.2016.2533)

Usage Notes

Table 1: Mean transect level cover (% cover) data for corals, turf and filamentous algae, and Halimeda from 5 southern Maldives reef sites from Jan 2016, Sept 2016, March 2017 and Jan 2019.

Table 2: Mean transect level (n = 8) data on parrotfish abundance (ind./30 x 4 m belt transect) and biomass (kg ha-1)  from the 5 southern Maldives reef sites from Jan 2016, Sept 2016, March 2017 and Jan 2019

Funding

Leverhulme Trust, Award: RF-2015-152

Fondation Bertarelli

Fondation Bertarelli, Award: Bertarelli Programme in Marine Science