Long term relationship between farming damselfish, predators, competitors and benthic habitat on coral reefs of Moorea Island

Understanding the processes that shape biodiversity is essential for effective environmental management. Across the world’s coral reefs, algal farming damselfish (Stegastes sp.) modify the surrounding benthic community through their creation of algae “farms”. Using a long-term monitoring dataset (2005–2019) from Moorea Island, French Polynesia, we investigated whether the density of dusky damselfish (Stegastes nigricans) is associated with benthic habitat composition, the density of predators and/or competitors, and whether the survey area was inside or outside of a Marine Protected Area (MPA). We found no evidence that benthic cover or number of competitors were associated with dusky damselfish densities, both inside and outside MPAs. In contrast, fluctuations in dusky damselfish densities were negatively associated with the density of predators (e.g. Serranidae, Muraenidae and Scorpaenidae) in the preceding year in non-MPA areas, and both within and outside of MPAs when predator densities were high (2005–2010). These results suggest that healthy predator populations may be important for regulating the abundances of keystone species, such as algal farming damselfish, especially when predator densities are high.

From the Reef to the Ocean: Revealing the Acoustic Range of the Biophony of a Coral Reef (Moorea Island, French Polynesia)

The ability of different marine species to use acoustic cues to locate reefs is known, but the maximal propagation distance of coral reef sounds is still unknown. Using drifting antennas (made of a floater and an autonomous recorder connected to a hydrophone), six transects were realized from the reef crest up to 10 km in the open ocean on Moorea island (French Polynesia). Benthic invertebrates were the major contributors to the ambient noise, producing acoustic mass phenomena (3.5–5.5 kHz) that could propagate at more than 90 km under flat/calm sea conditions and more than 50 km with an average wind regime of 6 knots. However, fish choruses, with frequencies mainly between 200 and 500 Hz would not propagate at distances greater than 2 km. These distances decreased with increasing wind or ship traffic. Using audiograms of different taxa, we estimated that fish post-larvae and invertebrates likely hear the reef at distances up to 0.5 km and some cetaceans would be able to detect reefs up to more than 17 km. These results are an empirically based validation from an example reef and are essential to understanding the effect of soundscape degradation on different zoological groups.

How many fish could be vocal? An estimation from a coral reef (Moorea Island)

A recurrent question arising in fish bioacoustics research concerns the number of vocal fish species that may exist. Although it is not possible to provide a precise globally valid number, an estimation based on recordings already collected at coral reefs (Moorea) and on morphological approaches indicates that approximately half of the fish families of this particular environment has at least one known sound-producing species. In light of this, acoustic behaviour should be fully considered in biology, ecology and management plans as it may provide information on a consistent portion of fish biodiversity. Fish bioacoustics has switched from anecdotal reports to long-term, large-scale monitoring studies, capable of providing high resolution information on fish populations’ composition and dynamics. This information is vital for successful management plans in our quickly changing seas.

Assessment of the Chemical Diversity and Potential Toxicity of Benthic Cyanobacterial Blooms in the Lagoon of Moorea Island (French Polynesia)

In the last decades, an apparent increase in the frequency of benthic cyanobacterial blooms has occurred in coral reefs and tropical lagoons, possibly in part because of global change and anthropogenic activities. In the frame of the survey of marine benthic cyanobacteria proliferating in the lagoon of Moorea Island (French Polynesia), 15 blooms were collected, mainly involving three species—Anabaena sp.1, Lyngbya majuscula and Hydrocoleum majus-B. Their chemical fingerprints, obtained through high performance liquid chromatography combined with UV detection and mass spectrometry (HPLC-UV-MS) analyses, revealed a high extent of species-specificity. The chemical profile of Anabaena sp.1 was characterized by three major cyclic lipopeptides of the laxaphycin family, whereas the one of L. majuscula was characterized by a complex mixture including tiahuramides, trungapeptins and serinol-derived malyngamides. Toxicity screening analyses conducted on these cyanobacterial samples using Artemia salina and mouse neuroblastoma cell-based (CBA-N2a) cytotoxic assays failed to show any toxicity to a degree that would merit risk assessment with regard to public health. However, the apparently increasing presence of blooms of Lyngbya, Hydrocoleum, Anabaena or other benthic cyanobacteria on coral reefs in French Polynesia encourages the implementation of ad hoc monitoring programs for the surveillance of their proliferation and potential assessment of associated hazards.

Diatom phenotypic plasticity: Olifantiella gorandiana epizoic on ‘G5-Manahere’ (Society Archipelago, South Pacific), a case study

First described from the Mascarenes (Rodrigues Island, Indian Ocean), Olifantiella gorandiana Riaux-Gobin was later observed from several other oceanic basins. We here examine the morphology of this taxon from a scraping of a juvenile green turtle Chelonia mydas Linnaeus, named ‘G5-Manahere’, from Moorea Island (Society Archipelago, South Pacific). In this sample, a great range of morphologies, not previously described for the species, were encountered, demonstrating a high degree of polymorphism with regard to valve shape and also stria pattern. These morphological differences appear to be associated with size, as smaller cells become more rounded. The fine structure (e.g., stria density and pattern) may also be associated with changes in shape. These specimens may all belong to the same and unique taxon: Olifantiella gorandiana. This high degree of polymorphism is described, and put into the context of the constraining epizoic conditions. This study permits furthers the description of the fine structure of O. gorandiana, using focused ion beam (FIB) and scanning electron microscopy (SEM) techniques.

THE SUPERSPECTRAL/HYPERSPATIAL WORLDVIEW-3 AS THE LINK BETWEEN SPACEBORNE HYPERSPECTRAL AND AIRBORNE HYPERSPATIAL SENSORS: THE CASE STUDY OF THE COMPLEX TROPICAL COAST

<p><strong>Abstract.</strong> Earth observation of complex scenes, such as coastal fringes, is based on a plethora of optical sensors constrained by trade-offs between spatial, spectral, temporal and radiometric resolution. The spaceborne hyperspectral EO-1 Hyperion sensor (decommissioned in 2017) was able to acquire imagery with 10&amp;thinsp;nm spectral (220 bands) at 30&amp;thinsp;m spatial resolutions over 1424.5&amp;thinsp;km² scenes. Conversely, the widespread unmanned airborne vehicle (UAV) hyperspatial DJI Mavic Pro camera can collect only natural-coloured imagery of 100&amp;thinsp;nm spectral (3 bands) but at 0.1&amp;thinsp;m spatial resolution over &amp;sim;10&amp;thinsp;km² scenes (with a single battery and calm meteo-marine conditions). The spaceborne WorldView-3 (WV3), featured by 60&amp;thinsp;nm spectral (16 bands) at 0.3&amp;thinsp;m spatial resolution (when pansharpened) over 1489.6&amp;thinsp;km² scenes, has the capacity to bridge both sensors. This study aims at testing the spectral and spatial performances of the WV3 to discriminate 10 complex coastal classes, ranging from ocean, reefs and terrestrial vegetation in Moorea Island (French Polynesia). Our findings show that geometrically- and radiometrically-corrected 0.3-m 16-band WV3 bands competed with (30-m) 167-band Hyperion performance for classifying 10 coastal classes with 2-neuron artificial neural network modelling, while being able to segment objects seized by 0.1-m (3-band) UAV. Unifying superspectral and hyperspatial specificities, the WV3 also leverages hypertemporal resolution, that is to say 1-day temporal resolution, rivalling UAV’s one.</p>

MONITORING CORAL GROWTH &ndash; THE DICHOTOMY BETWEEN UNDERWATER PHOTOGRAMMETRY AND GEODETIC CONTROL NETWORK

Creating 3-dimensional (3D) models of underwater scenes has become a common approach for monitoring coral reef changes and its structural complexity. Also in underwater archeology, 3D models are often created using underwater optical imagery. In this paper, we focus on the aspect of detecting small changes in the coral reef using a multi-temporal photogrammetric modelling approach, which requires a high quality control network. We show that the quality of a good geodetic network limits the direct change detection, i.e., without any further registration process. As the photogrammetric accuracy is expected to exceed the geodetic network accuracy by at least one order of magnitude, we suggest to do a fine registration based on a number of signalized points. This work is part of the Moorea Island Digital Ecosystem Avatar (IDEA) project that has been initiated in 2013 by a group of international researchers (https://mooreaidea.ethz.ch/).