Hydroacoustic survey of benthic macro-vegetation in the protected offshore area of Cape Martyan (Crimea, Black Sea)

Abstract

Survey of benthic macro-vegetation (BMV) was conducted within the marine part of the nature reserve “Cape Martyan” (the South Coast of Crimea) using hydrobotanical (contact) and hydroacoustic (remote sensing) methods. It clearly showed a fundamental possibility and efficiency of using mobile acoustic equipment based on the SeaCharter 480 DF echo sounder and specialized software WaveLens for detecting and studying general nature of BMV, as well as for identifying BMV boundaries, projective cover, height, etc. The method, including software, was adapted and tested to automatically determine the height of BMV from the sound backscatter profile. In good conformity with data of contact methods, the zonality of BMV distribution along the coastline with a propagation boundary limited by 10–12 m isobaths was shown using acoustic method. The total area of BMV is of about 0.39 km²; the maximum values of the projective cover reach 95–97 % at a depth of 2–3 m, but decrease to 50–70 % at greater depth boundary. It is established that the total stocks of sublittoral BMV reach 1431.3 tones, of which 99.6 % are in the community of Cystoseira. It is noted that cystoseires (Cystoseira barbata (Stackhouse) C. Agardh, 1820 and Cystoseira crinita Duby, 1830) and seagrasses (Zostera marina Linnaeus, 1753 and Zostera noltei Hornemann, 1832), whose communities dominate the vegetation cover of the area, are strong sound scatterers due to high acoustic impedance between air containing in organs and tissues and seawater, which significantly increases the efficiency of detection and description of BMV. At depths of up to 32 m, unidentified bottom acoustic objects in the form of single or group targets up to 2–4 m were recorded at the lower boundary of the BMV; their nature is probably different and is due to the presence of methane seeps or dense accumulations of fish in the studied area. The attention is drawn that conducting a quantitative study of BMV using hydroacoustic methods reduces the negative transforming effect on the object of study, since it does not imply a significant removal of BMV, for example, during mapping. This is especially important in the conditions of nature reserve regime within the specially protected natural areas (SPNA). The study clearly showed the effectiveness of the joint use of contact and distance methods. The results of this study form the basis of the BMV hydroacoustic monitoring; they will be used to map vegetation cover and to determine algae abundance in the protected coastal-marine area near Cape Martyan and other aquatic SPNA.