3D assessment of a coral reef at Lalo Atoll reveals varying responses of habitat metrics following a catastrophic hurricane

Extreme disturbances such as hurricanes can cause reductions in coral cover and three-dimensional (3D) structural complexity of coral reefs. We examined changes in structural complexity utilizing 3D reconstruction of a coral-reef site before and after Hurricane Walaka passed through Lalo of the Northwestern Hawaiian Islands. This event resulted in complete destruction of the coral-reef habitat, with dramatic changes in benthic cover from pre-hurricane tabulate coral to post-hurricane rubble. Rugosity and mean slope decreased after the hurricane, while structural complexity, captured by vector ruggedness measure (VRM), showed resolution-specific responses. This metric captured the structural complexity of rubble at a high raster resolution of 1 cm and that of tabulate coral at lower resolutions, resulting in decreases in mean VRM values at 2- and 4-cm resolutions but an increase at 1-cm resolution. Variability in profile and planform curvature was reduced after the hurricane due to a disappearance of extreme curvature values created by the tabulate coral after the hurricane. This study highlights the varying responses of habitat complexity metrics to the complete destruction of a coral reef and provides us with insights into how choices of habitat complexity metrics can affect quantitative assessments of 3D habitat structure.

MONITORING CHANGES IN CORAL REEF HABITAT COVER ON BERALAS PASIR ISLAND USING SPOT 4 AND SPOT 7 IMAGERY FROM 2011 AND 2018

Beralas Pasir is part of the Regional Marine Conservation Area (KKLD), which was established by the Bintan Regency Government with Bintan Regent Decree No. 261 / VIII / 2007. Water tourism activities undertaken by tourists on the island have had an impact on the condition of the coral reefs, as have other factors, such as bauxite, granite and land sand mining activities around the island. This research aims to determine changes in the coral reef habitat cover and the condition of the coral reefs around Beralas Pasir Island with a remote sensing function, using SPOT 4 imagery acquired on June 1, 2011 and SPOT 7 imagery from April 5, 2020. Data collection of environmental parameters related to the coral reefs was also made. The image processing used the Lyzenga algorithm to simplify the image classification process. The percentage of coral live cover around the island ranges from 26% -53%; this has experienced a significant change, from 67,560 hectares in 2011 to 38,338 hectares in 2018, a total decrease in the area of 29,222 hectares. Some of the natural factors found in the research which have caused damage to the reefs were Drupella snails, the abundance of Caulerpa racemosaalgae, and sea urchins. The majority of the coral reef types consist of Non-Acropora: Coral Massive, Coral, Coral Foliose, Coral Encrusting, Acropora: Acropora Tabulate, Acropora Encrusting, and Acropora Digitate

Shark and ray community structure in a turbid, nearshore coral reef habitat

The coastline of northern Australia may be a refuge for elasmobranchs threatened or extirpated from much of their former ranges across South-East Asia. In this study, we used baited remote underwater video stations to survey the abundance, size and assemblage of elasmobranchs in the Cobourg Marine Park in northern Australia. Two sites were sampled inside the park and one site was sampled outside the park, covering two management zones: open and partially protected. During the austral summer, 85 individuals of 12 species of elasmobranch were observed over 12 days. No significant differences were observed among sites in either size or abundance for common species. Videos were dominated by Carcharhinus melanopterus (relative abundance±s.d., 0.29±0.90h–1), Nebrius ferrugineus (0.03±0.24h–1) and Urogymnus granulatus (0.08±0.28h–1), which comprised >81% of all individuals. Environmental variables had no measurable effect on the abundance of elasmobranchs. The abundance and diversity of elasmobranchs in Cobourg Marine Park is high compared with other similar turbid, inshore areas of northern Australia. The large number of juveniles observed in our surveys also suggests the possibility of a nursery area.

A comparison of prokaryote communities inhabiting sponges, bacterial mats, sediment and seawater in Southeast Asian coral reefs

In the present study, we used Illumina sequencing to explore the prokaryote communities of 17 demosponge species and how they compare with bacterial mat, sediment and seawater samples (all sampled from coral reef habitat in Taiwan and Thailand). The studied sponge species formed three clusters. OTU richness and evenness were by far highest in the sediment and bacterial mat biotopes. There were pronounced differences in OTU richness and evenness among clusters and also considerable variation among certain host species within clusters. Additionally, the relative abundance of some prokaryotic taxa also differed among clusters with Poribacteria, for example, being recorded in all sponge species, but with very low relative abundances in species of two of the three clusters. This sponge-associated phylum was, however, recorded at relatively high mean abundance in bacterial mat samples, which also housed relatively high abundances of actinobacterial and Chloroflexi members. Our results support the HMA status of the species Aaptos lobata, Hyrtios erectus, Pseudoceratina purpurea and Xestospongia testudinaria, which clustered together and LMA status of the species Acanthella cavernosa, Echinodictyum asperum, Jaspis splendens, Ptilocaulis spiculifer, Stylissa carteri and Suberites diversicolor, which also clustered together. Other species (Agelas cavernosa, Agelas nemoechinata, Acanthostylotella cornuta, Paratetilla sp., Hymeniacidon sp. and Haliclona cymaeformis) deviated somewhat from the typical HMA/LMA dichotomy and formed a strongly supported cluster.

Object-Based Mapping of Coral Reef Habitats Using Planet Dove Satellites

High spatial resolution benthic habitat information is essential for coral reef protection and coastal environmental management. Satellite-based shallow benthic composition mapping offers a more efficient approach than traditional field measurements, especially given the advancements in high spatial and temporal resolution satellite imagery. The Planet Dove satellite constellation now has more than 150 instruments in orbit that offer daily coverage at high spatial resolution (3.7 m). The Dove constellation provides regularly updated imagery that can minimize cloud in tropical oceans where dense cloud cover persists. Daily image acquisition also provides an opportunity to detect time-sensitive changes in shallow benthic habitats following coral bleaching events, storms, and other disturbances. We developed an object-based coral reef habitat mapping approach for Dove and similar multispectral satellites that provides bathymetry estimation, bottom reflectance retrieval, and object-based classification to identify different benthic compositions in shallow coastal environments. We tested our approach in three study sites in the Dominican Republic using 18 Dove images. Benthic composition classification results were validated by field measurements (overall accuracy = 82%). Bathymetry and bottom reflectance significantly contributed to identifying benthic habitat classes with similar surface reflectance. This new object-based approach can be effectively applied to map and manage coral reef habitats.