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The empirical approach of satellite-derived bathymetry provides a straightforward, easily-implemented, and very effective way of estimating shallow-water depths from high-spatial-resolution satellite images. However, this approach has a great challenge that it requires high-quality in-situ depth data, which often do not exist or are not available due to financial and/or technical reasons, as the depth calibration data. To address this problem, this article assesses the feasibility of replacing high-quality in-situ depth data with four kinds of possible sourced depth data in satellite-derived empirical bathymetry. These four kinds of depth data are satellite altimeter depth data from laser altimeter systems on some earth observing satellites; satellite photogrammetric depth data, which are derived from satellite stereo-images with the photogrammetric method; nautical chart depth data, which come from existing electronic or paper charts; and radiative transfer model-based depth data, which are simulated in accordance with a bio-optical radiative transfer model (RTM). Correspondingly, four kinds of empirical bathymetry's model calibration data are referred to as altimeter data, photogrammetric data, chart data, and RTM data, respectively. The objective of this article is to assess the similarities, differences, limitations, and applicability of these four kinds of possible model calibration data, in order to know which of them is most suitable for a certain empirical bathymetry task. WorldView-2 and Sentinel-2A multi-spectral images covering four typical shallow water areas in Hawaii and Xisha Islands and Stumpf's logarithmic band ratio model were used to compare and contrast the bathymetric performance of these four kinds of possible model calibration data. The result showed that all four kinds of possible model calibration data can be used for satellite-derived empirical bathymetry when the high-quality in-situ depth data are not provided, but they will provide different bathymetric accuracy values. According to five evaluation indexes (i.e., bathymetric performance, data availability, data quality, complexity of data generation, and data adaptability), the altimeter data perform best, the chart data perform second best, the RTM data follow with the third performance, and the photogrammetric data perform markedly worse than other three kinds of model calibration data. Such factors as the in-situ property, spatial coverage, co-registration and independence of model calibration data must be taken into consideration as well during a model calibration data selection. [ABSTRACT FROM AUTHOR]

Understanding the biodiversity of an ecosystem is crucial to determine its structure and resistance to climate change. The South Sandwich Islands (SSI) are located in the Scotia Sea (Southern Ocean), within the South Georgia and the South Sandwich Islands Marine Protected Area. However, the biodiversity of the archipelago remains poorly studied, whilst climate change has the potential for wide-ranging impacts in the Antarctic and Subantarctic regions. Here we used predators as biological samplers to study the bathyal communities of SSI. A total of 61 species including fish, cephalopods and crustaceans, were identified from the diet of 13 predatory taxa (11 fish and two cephalopod). Common Subantarctic and Antarctic species were found, with Moroteuthopsis longimana being the species with the highest density (1.74 individuals per stomach at Montagu Island). Eleven fish and one cephalopod species were recorded for the first time at the archipelago. Furthermore, 16 fish species had their bathymetric range increased. Fifteen fish and one crustacean appear to have SSI as the northern or southern limit of their distribution. Community analysis found two major groups at SSI, one in the north and one in the south, with the southern group subdivided into two groups. This separation is related to the environmental conditions at the archipelago that abruptly change at Saunders Islands. Latitude (correlated with sea surface temperature) and sea surface height (proxy for upwelling) both correlated with the dissimilarity between communities. These results suggest that climate change may affect the biodiversity at SSI in the future as warming waters of the Scotia Sea and changes in the upwelling system may favour range extensions of more northerly species into the archipelago. Furthermore, it could lead to local extinctions of some species exclusively found in the southernmost areas of the archipelago. • Biological samplers allow the study of the deep-sea biodiversity of remote locations. • Twelve species were identified for the first time at South Sandwich Islands. • Subantarctic and Antarctic species were identified at the archipelago. • Two communities exist at South Sandwich Islands, one at north and one at south. • Sixteen species have South Sandwich Islands as the limit of their distribution. [ABSTRACT FROM AUTHOR]