Surveys and data analyses were undertaken in the framework of several research projects carried out by IBER-BAS in recent years. These include:

Enlargement of the Natura 2000 ecological network within the Bulgarian Black Sea sector. Contract 7976/04.04.2011 between MoEW and IO-BAS.

FP7 - Policy-oriented marine Environmental Research in the Southern EUropean Seas (Perseus); GA 287600.

FP7 - Towards COast to COast NETworks of marine protected areas (from the shore to the high and deep sea), coupled with sea-based wind energy potential (CoCoNET) GA 287844.

Balkan-Mediterranean 2014-2020- Regional cooperation for the transnational ecosystem sustainable development (Reconnect), Transnational Cooperation Programme Interreg V-B, co-funded by the European Union and national funds of the participating countries.

FEMA-MARE - Assessment and Mapping of MARINE Ecosystem Condition and Their Services in Bulgaria. Approved under programme BG03 Biodiversity and Ecosystems, financed by the EEA financial mechanism 2009-2014, Contract No. Д-33-87/27.08.2015.

MSFD National Monitoring Programme 2017.

“LTER - BG: Upgrading of the distributed scientific infrastructure Bulgarian Long-Term Ecosystem Research Network" under agreement D01-405/ 18.12.2020 with the Ministry of Education and Science (MES) of Bulgaria.

''Enlargement of the Natura 2000 ecological network within the Bulgarian Black Sea sector". Contract 7976/04.04.2011 between MoEW and IO-BAS"; FP7 Perseus; FP7 Coconet; BalkanMed Reconnect; FEMA-MARE; MSFD National Monitoring Programme 2017; “LTER - BG: Upgrading of the distributed scientific infrastructure Bulgarian Long-Term Ecosystem Research Network" under agreement D01-405/ 18.12.2020 with the Ministry of Education and Science (MES) of Bulgaria.

The current distribution of Zostera spp. seagrass meadows along the Bulgarian Black Sea coast was studied - from Cape Sivriburun in the north, to the mouth of River Rezovska in the south. Our efforts were focused in the area with most abundant presence of seagrasses - Burgas Bay, but also included sites that provide favourable conditions for the development of seagrass beds.

The mapping of the habitat extent was done using a combination of historical and recent observations of the habitat along the studied coastline following the methodological guidelines of Roelfsema and Phinn (2009) and Roelfsema et al. (2015).

We identified the presence of seagrass meadows in a certain area from the only published historical data from the late 1970s (Petrova-Karajova 1982), as well as from more recent studies and publications (Vassilev et al. 2005, Uzunova 2010, Dahl et al. 2016, Holmer et al. 2016, Karamfilov et al. 2019, Klayn and Karamfilov 2021), communication with local fishermen and divers and personal observations. Once the presence of seagrass meadows in a certain location was confirmed, we acquired the most recent and clear satellite images available in the Google Earth Pro libraries of the area. Georeferenced drone photomosaics were also used in certain locations, which provided more detailed spatial information and allowed us to follow the interannual variation in meadow size and extent. In certain locations, these methods were combined with high-resolution sonar side-scan mosaics, where the borders of the seagrass meadows were outlined, based on the clearly visible differences in texture of vegetated and non-vegetated sediments (Nikolopoulou et al. 2021).

The identified extent of seagrass meadows was verified in-situ by various methods. At a number of locations, scientific divers collected samples in predetermined locations following standard sampling procedures. Destructive samples were collected in order to help assess the impact of local eutrophication gradients on the ecological state of seagrass habitats, where Z. noltei was used as an indicator species (see Karamfilov et al. 2019 for details). This particular evaluation did not aim at obtaining a complete overview of the biometrics and biodiversity of all seagrass species in the studied areas. Whenever feasible, samples from the deeper Z. marina-dominated sections of the seagrass meadows were also collected.

Destructive samples were collected by a team of two SCUBA divers using a circular frame with cutting edges, with diameter of 31.5 cm and an area of 0.78 m². The roots of the plants along the inner edge of the frame were carefully cut to a depth of 10–15 cm, making sure no roots, rhizomes or leaves from outside the sampling area fell into the sample. The plants were uprooted in a manner ensuring better removal of sand and mud, labelled and stored in a coolbox on board and transported to the laboratory where they were stored in a freezer at –20^◦C. The wet weight biomass of leaves (aboveground biomass, AG W.W.) and of roots and rhizomes (belowground, BG W.W.) were measured on an electronic scale. The Leaf Area Index (LAI) of Z. noltei and Z. marina was measured following the methodology described in Short and Coles (2001).

In locations where the collection of destructive samples was not feasible, georeferenced digital photo and video transects were carried out, which allowed us to collect a large number of samples along the whole depth range of distribution of a given seagrass meadow (see Berov et al. 2018 for detailed description of applied methodology). Meadows with large spatial extent (e.g. Sarafovo), where scuba divers could not cover the whole extent of the habitat within a reasonable and safe number of dives, were groundtruthed with a drop camera deployed from a boat along predetermined points and transects (Fig. 2) (see Karamfilov et al. 2019). Images and video files were analysed by an experienced benthic ecologist who noted the presence of various species and identified habitat types in accordance with the EUNIS habitat classification scheme, as well as other attributes, such as sediment type and the coverage of the benthal by different organisms. The groundtruthing and mapping methods applied at each meadow were marked in the attribute table of the database under the column ‘map_method’. The date of the in-situ observation was noted in the column ‘dive_date’ and the date of the satellite image/side-scan used for the mapping is noted in the ‘Sat_date’ column.

Figure 1.  

Zostera spp. seagrass meadows in Burgas Bay.

Figure 2.  

Zostera spp. seagrass meadow in front of Sarafovo, Inner Burgas Bay. In-situ data from drop camera deployments and scuba diving georeferenced photos are overlaid, with dominant species identified and labelled.

For most seagrass meadows, we acquired satellite images that were more recent than the dates of the groundtruthing, allowing us to map their most recent spatial extent. In several cases, we could acquire a series of satellite images from different years, where the visible extent of the mapped meadows changed significantly over the period of a few years. In such locations, we mapped the seagrass meadow extent, based on the latest satellite image available. Thus the calculated seagrass area is valid for the data in the 'Sat_date' column of the attribute table.

Polygons with the actual area of the surveyed seagrass meadows were created manually in ArcMap 10.2. In-situ point data from samples and photo and video observations were overlaid on top of satellite images, photomosaics and side-scan mosaics and the visible boundary between vegetated and non-vegetated sea bottom was outlined (Fig. 2). Special care was taken not to misidentify shallow rocky reefs as seagrass beds, as they were often present in close vicinity of Zostera meadows and had similar colour characteristics on satellite photos (Figs 3, 4). In such locations, the correct mapping of the seagrass meadows requires detailed in-situ surveys, carried out by scuba divers applying georeferenced sampling and photo and video survey techniques that would allow the distinction between areas with segarass coverage from adjacent rocky reefs with macroalgae.

Figure 3.  

Zostera spp. seagrass meadow in front of Ravda with an adjacent rocky reef covered with Gongolaria barbata (Stackhouse) (sensu Cystoseira barbata (Stackhouse)) with an overlay of data from a georeferenced photo scuba survey.

Figure 4.  

Zostera marina Linnaeus on sandy bottoms growing next to a rocky reef covered with Gongolaria barbata (Stackhouse) (sensu Cystoseira barbata (Stackhouse)) in a seagrass meadow in front of Ravda (depth ~ 2 m).

All survey methods and data analysis procedures, applied in this study, were initially tested, verified and approved in the seagrass meadows in Sozopol Bay (eLTER site Sozopol-Black Sea, https://deims.org/04c70bae-b13c-4df5-bbdb-dc2be9e9d411). The area has several seagrass meadows existing in a local pollution and eutrophication gradient with varying water quality, sediment properties and resulting community structure and ecological status (Holmer et al. 2016). The Zostera seagrass meadows in Sozopol Bay have been included in the long-term monitoring programme of this eLTER site since 2020.

A total of 1859 video and photo observations were filmed and analysed and 129 samples were collected and processed (Holmer et al. 2016, Anonymous 2018, Karamfilov et al. 2019) over the whole study period (2010-2020).

Above-ground biomass of Z. marina, sampled in seagrass meadows in Sozopol Bay in the summer season, varied between 28.3 g.m^-2 and 60.8 g.m^-2, while below-ground biomass was in the range between 25.8 g.m^-2 and 66.4 g.m^-2 (dry weight). Z. marina shoot densities were in the range between 295 sh.m^-2 and 690 sh.m^-2, leaf length was in the range betwen 110 mm and 719 mm and leaf area index values were between 4.2 m².m^-2 and 8.1 m².m^-2 .

Above-ground biomass of Z. noltei sampled in seagrass meadows in an eutrophication gradient in Burgas Bay (Karamfilov et al. 2019) in the summer season varied between 21.03 g.m^-2 and 210.25 g.m^-2, while below-ground biomass was in the range between 28.7 g.m^-2 and 165.1 g.m^-2 (dry weight). Z. noltei shoot densities were in the range between 747.1 sh.m^-2 and 2246.5 sh.m^-2, leaf length was in the range betwen 199.3 mm and 413.9 mm and leaf area index values were between 1.9 m².m^-2 and 3.8 m².m^-2.

Тhe total area of the habitat type, classified in the EUNIS habitat types list (2019) as ‘MB548 - Black Sea seagrass meadows on lower infralittoral sands’ in the study area, is 916.9 ha, of which only 17.9 ha are in man-made sheltered environments (harbours) (Fig. 1). The seagrass bed in front of Burgas and Sarafovo represents 52% (411 ha) of the total area of this habitat in Bulgarian Black Sea waters. Other large meadows include those at Pomorie, Sunny Beach, Poda, Otmanli, Sozopol Bay and Sv. Vlas. All seagrass meadows identified by Petrova-Karajova (1982) in 1978-79 were also located during the current survey, despite the increased eutrophication pressures and overall degradation of benthic habitats in the western Black Sea during the 1980s and early 1990s.

Standard procedures for quality control in field sampling and laboratory sample processing were applied.

Our studies covered the majority of the known Zostera seagrass meadows along the whole Bulgarian Black Sea coast, stretching from Cape Sivriburun in the north to Rezovska river mouth in the south.