In June-August 2012, we conducted a 54-day field experiment on the north shore of Moorea, French Polynesia. We manipulated the presence of the vermetid Ceraesignum maximum (formerly Dendropoma maximum) and high sedimentation, yielding four fully-crossed treatments. In response to which, we measured the growth of the reef-building coral massive Porites (a species complex mostly composed of P. lobata and P. lutea, see Veron 1995).
This study was performed in a shallow lagoonal backreef area, where vermetid densities can be quite high (up to >100 per m2: Smalley 1984; Shima et al. 2010). We selected 36 large (approximately 1 meter in diameter), mostly-dead colonies of massive Porites to serve as deployment sites for our experimental corals. These colonies occurred at a similar depth along a 150-m transect that was roughly parallel to the reef crest and perpendicular to prevailing water currents. Because differences in water flow could affect vermetid net casting efficacy and sediment removal rates, we quantified relative flow variation among sites using calibrated clod cards (Jokiel & Morrissey 1993). The flat top of each site was home to an average of 5 ± 1 (SD) vermetids in a 400 cm2 area, ensuring that mucus nets would consistently cover the experimental corals receiving the “+vermetid” treatments. We randomly assigned the four experimental treatments to our sites, from half of which we removed all vermetids to create the “–vermetid” treatments. Our experimental corals were 36 juvenile massive Porites colonies that we collected from a nearby location. Each coral was attached to a small plastic base using marine epoxy, and buoyantly weighed (following Davies 1989). They were then assigned to nine blocks of four corals based on similarity in morphology, and outplanted to each deployment site using u-nails.
For the +sedimentation treatments, approximately 11 grams (dry weight) of sediment of visibly similar grain size was scooped from the nearby seafloor and gently sprinkled 2 cm above the coral every 1-2 days. This yielded 47 sediment applications over 54 days at an average rate of 54.2 ± 21.2 (SD) mg cm2 day-1, which is well within locally observed reef sedimentation rates (see LTER sedimentation rates). During each visit to the site, experimental corals were examined for vermetid net presence or absence. Twenty-eight corals were videotaped in blocks to quantify relative sediment removal rate, determined as: [(Cinitial-Cfinal)/Cinitial] where Cinitial and Cfinal are the coral surface areas that were covered by sediment at the start vs. end of the trial. We used ImageJ software to quantify Cinitial and Cfinal.
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