Coral carpets- a novel ecological engineering tool aimed at constructing coral communities on soft sand bottoms

Abstract

Hard bottom substrates that are the hallmark property of the coral reefs can be expanded by transforming sedimentary areas into engineered reefs. Testing this rationale, we developed here two preset coral-carpet prototypes (‘coral sods’; 6 units, each 7.5 m2), one containing branching species and one with both coral forms (mixed-units) that were transplanted on a soft bottom area, containing 10 coral species (five branching, five massive; 354 ramets from 30 genotypes and 181 whole-colonies). Ramets from branching genotypes were distributed in central and peripheral coral-sod locations, whereas all massive colonies were put in central locations. The ‘coral sod’ carpets were set-up at the Eilat, Red Sea nursery, held in place for 2–8 months, then transplanted into a highly hostile soft-bottom area (15 m depth) and monitored for up to 17 months. Results revealed high survival rates for corals in the peripheral/center positions (80–100%; most combinations). A fast increase in ecological volume (EV) was recorded during 447 residency days, up to 112% in EV for Acropora variabilis mixed-units in the peripheral position. There was no statistically significant difference between treatments and sod's locations, except for the Pocillopora. damicornis branching/mixed treatment. At day 514, corals' aerial coverage has increased from 50% at onset to 67.3% for the branching units and to 61.5% in the mixed-units. Larvae released from transplanted Stylophora pistillata corals outperformed those of natal colonies in the two following reproductive seasons, significantly higher in the first reproductive season when compared to the second. Coral recruitment (day 485) was up to 4 times higher in the centers compared to the peripheries, in both coral-sod types. Using gypsum sticks dissipation, we recorded reduced water velocities at lateral and uppermost colonial architectures and in the coral sod centers as compared to peripheries. These results reveal the first steps for improved ‘coral sod’ units aiming to shape sandy tropical areas into flourishing coral reef sites.