Abstract
Posidonia oceanica meadows are ecosystem engineers that play several roles in marine environment maintenance. In this sense, monitoring of the spatial distribution and health status of their meadows is key to make decisions about protecting them against their degradation. With the aim of checking the ability of a simple low-cost acoustic method to acquire information about the state of P. oceanica meadows as ecosystem indicators, ground-truthing and acoustic data were acquired over several of these meadows on the Levantine coast of Spain. A 200 kHz side scan sonar in a vertical configuration was used to automatically estimate shoot density, canopy height and cover of the meadows. The wide athwartship angle of the transducer together with its low cost and user friendliness entail the main advantages of this system and configuration: both improved beam path and detection invariance against boat rolling. The results show that canopy height can be measured acoustically. Furthermore, the accumulated intensity of the echoes from P. oceanica in the first 30 centimeters above the bottom is indirectly related to shoot density and cover, showing a relation that should be studied deeply.
Highlights
The marine phanerogam Posidonia oceanica (Linnaeus) Delile (1813) forms biodiversity hotspots in the Mediterranean Sea, where it is endemic
P. oceanica meadows are complex and structured ecosystems, and some of them have been dated to be older than 6000 years ([1] in [2]), they are experiencing habitat fragmentation [3] and their area is declining in many regions [4]
Following the initiative of these authors, we propose a vertical use of an side scan sonar (SSS) transducer to improve the bottom detection and to maximize the scattering volume of P. oceanica leaves before the first hit of the acoustic pulse on the sea bottom
Summary
The marine phanerogam Posidonia oceanica (Linnaeus) Delile (1813) forms biodiversity hotspots in the Mediterranean Sea, where it is endemic. Considered ecosystem engineers [5,6], their leaves can reach 120 cm length [7] and are grouped in shoots that form rich and broad meadows up to 40 m deep [8]. They support fisheries, carbon sequestration and coastal protection [9]. Several factors contribute to the regression of P. oceanica meadows. These valuable ecosystems are vulnerable to coastal development, especially mechanical damage, turbidity and pollution. The seagrass regression problem is compounded by the fact that their recovery is hampered by their slow rhizome growth and low genetic variability [2,14]
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