Cross-Shelf Variation Among Juvenile and Adult Coral Assemblages on Australia’s Great Barrier Reef

Michelle J Jonker,Patricia Menéndez,Angus A Thompson,Kate Osborne

doi:10.3390/d11060085

Michelle J Jonker, Patricia Menéndez + Show 2 more

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https://doi.org/10.3390/d11060085

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Abstract

Coral reefs are under increasing pressure from a variety of stressors, highlighting the need for information about the status of coral reef communities including the distribution, abundance and composition of juvenile and adult coral assemblages. This information is currently limited for the Great Barrier Reef (GBR) and is necessary for understanding the impacts of disturbances and the system’s potential for recovery. This study reports juvenile and adult hard coral abundance and composition from 122 reefs on the GBR during a period of limited acute disturbance. The data represent baseline observations for juvenile hard coral assemblages spanning the longitudinal cross-shelf gradient of the GBR and 12 degrees of latitude and augment reported distribution of adult coral assemblages over the same scale with inclusion of additional reefs. Juvenile and adult coral assemblages reflected broad differences imposed by the gradient of environmental conditions across the GBR. The mean density of juvenile hard corals was lower in the inshore reefs (5.51 m2) than at either the mid-shelf (11.8 m2) or outer shelf reefs (11.2 m2). The composition of juvenile and adult coral assemblages covaried overall, although there were different relationships between these two life stages across the continental shelf and among community types. Dissimilarity between juvenile and adult coral assemblages was greater on inshore and outer shelf reefs than on reefs in the mid-shelf, although, there were differences in community types both within these shelf positions and those that spanned mid- and outer shelf reefs. Dissimilarity was greatest for Inshore branching Acropora and high for Southern Acropora communities, although very high coral cover and very low juvenile densities at these reefs precluded interpretation beyond the clear competitive dominance of Acropora on those reefs. Dissimilarity was also high between juvenile and adult coral assemblages of Turbid inshore communities suggesting water quality pressures, along with synergistic effects of other stressors, pose ongoing selective pressures beyond the juvenile stage. Conversely, relatively low dissimilarity between juvenile and adult coral assemblages on mid-shelf and lower latitude outer shelf reefs suggests pressures beyond those influencing settlement and early post-settlement survival were having less influence on the composition of adult coral assemblages.

Highlights

Scleractinian corals are the primary habitat architects of coral reefs and provide much of the three-dimensional structure that is used by the myriad of organisms in these highly diverse ecosystems [1,2,3,4]
The LTMP surveys reefs were in three positions across the continental shelf: Inshore were defined as reefs shoreward of the shipping channel that runs the length of the Great Barrier Reef (GBR) and separates the main GBR reef matrix from the mostly island fringing reefs situated in more coastal settings, mid-shelf were defined as reefs in the main GBR matrix seaward of the shipping channel but exclusive of those exposed to the open Coral
This study reported significant spatial heterogeneity of juvenile densities, juvenile taxonomic composition and the relationship between juvenile and adult coral assemblages across the continental shelf and among community types on the GBR

Summary

Introduction

Scleractinian corals (hereafter coral) are the primary habitat architects of coral reefs and provide much of the three-dimensional structure that is used by the myriad of organisms in these highly diverse ecosystems [1,2,3,4]. In addition to high diversity, coral reefs exhibit large variability in the composition of coral assemblages at a range of spatial scales [5,6]. Coral replenishment can occur from regrowth of coral fragments [8,9,10,11], but on most reefs replenishment is primarily driven by the settlement of sexually produced larvae and their subsequent survival and growth, i.e., recruitment [10,11,12,13]. Coral larvae selectively settle into habitats suitable for their subsequent survival and growth [21,22,23,24] The culmination of larval dispersal, pre- and post-settlement selective pressures is reflected in the abundance and composition of juvenile coral assemblages (i.e., corals from ~5–50 mm diameter) [13,14,15,16,17,18,19,20,21,22,23].

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