First insights into coral recruit and juvenile abundances at remote Aldabra Atoll, Seychelles

Abstract

Coral recruitment and successive growth are essential for post-disturbance reef recovery. As coral recruit and juvenile abundances vary across locations and under different environmental regimes, their assessment at remote, undisturbed reefs improves our understanding of early life stage dynamics of corals. Here, we first explored changes in coral juvenile abundance across three locations (lagoon, seaward west and east) at remote Aldabra Atoll (Seychelles) between 2015 and 2019, which spanned the 2015/16 global coral bleaching event. Secondly, we measured variation in coral recruit abundance on settlement tiles from two sites (lagoon, seaward reef) during August 2018–August 2019. Juvenile abundance decreased from 14.1 ± 1.2 to 7.4 ± 0.5 colonies m-2 (mean ± SE) during 2015–2016 and increased to 22.4 ± 1.2 colonies m-2 during 2016–2019. Whilst juvenile abundance increased two- to three-fold at the lagoonal and seaward western sites during 2016–2018 (from 7.7–8.3 to 17.3–24.7 colonies m-2), increases at the seaward eastern sites occurred later (2018–2019; from 5.8–6.9 to 16.6–24.1 colonies m-2). The composition of coral recruits on settlement tiles was dominated by Pocilloporidae (64–92% of all recruits), and recruit abundance was 7- to 47-fold higher inside than outside the lagoon. Recruit abundance was highest in October–December 2018 (2164 ± 453 recruits m-2) and lowest in June–August 2019 (240 ± 98 recruits m-2). As Acroporid recruit abundance corresponded to this trend, the results suggest that broadcast spawning occurred during October–December, when water temperature increased from 26 to 29°C. This study provides the first published record on coral recruit abundance in the Seychelles Outer Islands, indicates a rapid (2–3 years) increase of juvenile corals following a bleaching event, and provides crucial baseline data for future research on reef resilience and connectivity within the region.

Extremely high but localized pulses of coral recruitment in the southwestern lagoon of New Caledonia and implications for conservation

“…In our case, we hypothesize that lower circulation in the southwestern lagoon, linked to periods of weak south-easterly trade-winds and occasional west breezes during the stormy wet season, favors larval retention near the coast, particularly within areas of high residence time such as the Maa Bay (Jacquet et al 2006, Ouillon et al 2010, Cuif et al 2014, where the coral recruitment records were found. In contrast, the lower recruitment rates that we recorded on the outer-reef slope, most notably for Acroporidae, may be linked to higher exposure to wave and swell actions and the oceanic surface currents that characterize this habitat (Ouillon et al 2010), which may reduce the settlement of coral larvae of numerous coral species, as suggested for other coral reefs worldwide (Hata et al 2017, Koester et al 2021. Ob viously, other hypotheses could be proposed to ex plain the recruitment peak of Acroporidae in 2014, such as a sporadic year of higher fecundity and lower pre-and/or post-settlement mortality.…”

“…On the outer slope, recruit abundances were, on average, lower at 12 and 18 m compared to 6 m depth. Such reef scale and depth variation in recruitment patterns may result from differential larval delivery and settlement preferences, or early post-settlement mortality caused by variation in physical conditions (sedimentation, hydrodynamics, light) as well as different forms of biotic interaction (Mundy & Babcock 1998, Birrell et al 2008, Penin et al 2010, Gouezo et al 2020, Whitman et al 2020, Koester et al 2021, Shlesinger & Loya 2021, Thomson et al 2021.…”

Recovery trends of reef carbonate budgets at remote coral atolls 6 years post‐bleaching

“…This means that reproduction and larval recruitment is increasingly taking place and survival of juveniles appears high. Similarly, high recruitment levels were found in the remote Aldabra Atoll by 2019, with abundances of coral recruits and juveniles exceeding prebleaching numbers (Koester et al 2021). Another difference to consider when comparing size-frequency studies is time after disturbance, as Dietzel et al (2020) evaluated changes in communities directly after back-to-back bleaching events without time for new recruitment.…”

High-frequency variability dominates potential connectivity between remote coral reefs

“…Although we found no significant effect of the IOD on potential connectivity in SECoW, it is possible that realised dispersal is nevertheless suppressed during IOD events due to the increased larval mortality rate. On the other hand, IOD events are generally most strongly expressed in the west Indian Ocean during October to November, which only partially overlaps with the general spawning season in the region (Mangubhai and Harrison, 2009;Koester et al, 2021;Baird et al, 2021). In addition to interannual temperature variability, temperature also varies over shorter timeframes and spatial scales, from meso-to frontal.…”

“…Since the autocorrelation timescale 1 is less than 10 days for almost all reefs in SECoW (the 99 th percentile is 9.9 days), we can approximate 1-day spawning events separated by 10 days as being independent. We thereby generated a synthetic multi-centennial source strength time-series by imagining a coral species that spawns one day per year, and extracting the source strength at each reef site every 10 days between December to February (within the general spawning period for broadcasting corals in the southwest Indian Ocean (Mangubhai and Harrison, 2008;Mangubhai, 2009;Koester et al, 2021;Baird et al, 2021)) from 1993-2019. This resulted in a time-series spanning 243 (9 × 27) spawning events (or years).…”