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PeerJ
2020 Jan 01;8:e10315. doi: 10.7717/peerj.10315.
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High ecological resilience of the sea fan Gorgonia ventalina during two severe hurricanes.
Edmunds PJ
.
Abstract
Since about the turn of the millennium, octocorals have been increasing in abundance on Caribbean reefs. The mechanisms underlying this trend have not been resolved, but the emergent species assemblage appears to be more resilient than the scleractinians they are replacing. The sea fan Gorgonia ventalina is an iconic species in the contemporary octocoral fauna, and here its population dynamics are described from St. John, US Virgin Islands, from 2013 to 2019. Mean densities of G. ventalina at Yawzi Point (9-m depth) varied from 1.4-1.5 colonies m-2, and their mean heights from 24-30 cm; nearby at Tektite (14-m depth), they varied from 0.6-0.8 colonies m-2 and from 25-33 cm. These reefs were impacted by two Category 5 hurricanes in 2017, but neither the density of G. ventalina, the density of their recruits (< 5-cm tall), nor the height of colonies, differed among years, although growth was depressed after the hurricanes. Nevertheless, at Tektite, colony height trended upwards over time, in part because colonies 10.1-20 cm tall were reduced in abundance after the hurricanes. These trends were sustained without density-associated effects mediating recruitment or self-thinning of adults. The dynamics of G. ventalina over seven years reveals the high resilience of this species that will contribute to the persistence of octocorals as a dominant state on Caribbean reefs.
Figure 1. Study sites at Yawzi Point (9-m depth) (A) and Tektite (14-m depth) (B) in 2016.Pictures (A, B) show high abundance of Gorgonia ventalina, with representative colonies below: (C) symmetric fan with no mortality, (D) asymmetric fan with small amounts of mortality, and (E) ragged fan with extensive partial mortality. Ragged fans accounted for 10–12% of fans over the 7-year study, but by year, they accounted for 0% (Yawzi Point in 2015) to 24% (Tektite in 2018) of fans. Photo credits: P.J. Edmunds.
Figure 2. Height and density of Gorgonia ventalina at Yawzi Point (A) and Tektite (B).Mean ± SE shown with n = 42–97 for height, and n = 3 for density. Vertical dashed line shows the impacts of Hurricanes Irma and Maria.
Figure 3. Percentage contribution of colonies of Gorgonia ventalina by size class to the populations at Yawzi Point (A) and Tektite (B) from 2013–2019.n = 54–97 colonies y−1 at Yawzi Point, and n = 42–60 colonies y−1 at Tektite. Vertical dashed line shows the impacts of Hurricanes Irma and Maria.
Figure 4. Mean growth rate (±SE) of Gorgonia ventalina at Yawzi Point (n= 30–63) and Tektite (n= 23–31) from 2013–2019.Values based on colonies that were measured in two consecutive years and either did not change in height, or increased in height (i.e., increments of ≥ 0 mm y−1). Vertical dashed line shows the impacts of Hurricanes Irma and Maria.
Figure 5. Scatter plots showing features of Gorgonia ventalina populations at Yawzi Point and Tektite as tests for density dependent effects.(A) Height versus density as a test for self-thinning, and (B) density of recruits (≤ 5-cm tall) versus the density of larger colonies (> 5-cm tall) as a test for density dependent (DD) recruitment. Points represent one datum y−1 from 2013–2019. Neither height versus density (A), nor density of recruits versus density of larger colonies (B), are significantly associated (p ≥ 0.050).
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