Posidonia oceanica Meadow in Erimitis – Final Report

Biology

Posidonia oceanica, is a species of seagrass (Magnoliophyta) endemic to the Mediterranean Sea. Also known as Neptune Grass, it is one of the most common species of seagrass in the region, along with Cymodocea nodosa, and Zostera marina (Traganos et al. 2018). The length of its leaves can reach up to 1.2 meters, the density of the meadows up to 10,000 per square meter while it is observed in the infra‐ littoral zone on rocky to sandy bottoms where it can root. In Greece, Neptune Grass is present along the majority of the mainland coasts and the Greek islands. In the Northern Aegean Sea its meadows can extend down to 25 meters, while in the South Aegean Sea to 35 meters, depending on many factors but primarily water clarity. In the Ionian Sea, a highly oligotrophic area, the meadow can reach depths of 45 m depth (Traganos et al. 2018).

Importance

The endemic species Posidonia oceanica is the most important seagrass in the Mediterranean Sea (Boudouresque et al. 2006) in fact along with the Coralligenous habitats is the most important Mediterranean marine ecosystem (Giakoumi et al. 2013). The role of Posidonia oceanica meadows in marine coastal environments is often correctly compared to that of the forest in terrestrial environments, as they constitute the basis of the richness of coastal waters in the Mediterranean Sea. By producing enormous quantities of vegetal biomass, the meadows form the basis of many food webs (McRoy and McMillan, 1977). This primary production is comparable to or greater than that of other high ‐ production environments, whether terrestrial or oceanic (Fergusson et al. 1980). In addition, P. oceanica meadows constitute a spawning ground, a nursery or a permanent habitat for a lot of species (over 400 different plant species and several thousand animal species populate the meadows of which many commercially important species; ), making these underwater meadows a unique biodiversity hotspot ( . Furthermore, P. oceanica is considered a “ecosystem engineer” as it stabilises the sediment with its roots and changes the hydrodynamic status of the sublittoral zone and protects from erosion (Pergent et al., 2012). Besides, it serves as a purifier as it improves the water quality by reducing particle loads (Hemminga and Duarte, 2000). Moreover, it serves as long‐term carbon storage (Pergent et al. 2012). Finally, their rhizomes concentrate radioactive, synthetic chemicals and heavy metals, recording the environmental levels of such persistent contaminants. Hence the importance, Posidonia oceanica is also used as a ‘biological quality element’ in the long‐term monitoring programmes of the Water Framework Directive (WFD 2000/60/EC) as an indicator for assessing the ecological status of coastal water bodies.

Protection status

Posidonia oceanica meadows are a protected habitat in the EU through a variety of Conventions, Directives and Regulations, at union or state level (See Table 1). Under most frameworks, fishing with dynamic gears is prohibited, in order to prevent physical damage to the meadows (Pergent et al. 2016). Yet more protection legally binding measures should exist as more than half percent of the threats is associated with other human activities apart from fishing, such as coastal development and the impacts it might induce to these precious ecosystems.

Although it is a long‐living plant with an extended Area of Occupancy (AOO) and Extent of Occurrence (EOO) and hence listed as Least Concern by the latest IUCN assessment (Pergent et al. 2016), its extremely slow growth rate makes it highly vulnerable to external disturbances, thus making it very difficult to recover if degraded. In the Mediterranean basin, there has been a decrease in P. oceanica, up to 10% in the last 100 years, but recent analysis of its coverage shows an even steeper decrease of 34% of the area covered by it, in the last 50 years (Boudouresque et al. 2009). The main threat to these meadows is habitat degradation by human activities, such as: Water pollution, Construction of coastal infrastructure, Modification of marine currents (hydrography), Fishing, Invasive species, and shipping (Fig.2) (Boudouresque et al. 2012). In Greece, conservation actions are limited in local projects and in the two Greek MPAs (Zakynthos and North Sporades islands) (Pasqualini et al.2005; Ladakis et al. 2003). In Corfu Island, information about the spatial extent of the seagrass meadows is poor with only two areas been model mapped: Between Othonoi island and Mathraki and from Perama to Ag. Ioannis, whereas there is no available information about their health status (EUNIS Marine Habitat Classification, 2019).

Fig 2: Grouped threats to Posidonia oceanica beds as reported by Mediterranean EU Member States under the EU Habitats Directive ‐ eps

Study area

Erimitis peninsula is located in the Straits of Corfu, a narrow body of water between the coasts of Albania and Greece (Northeast Corfu), that separates the two countries. The channel is a passage from the Adriatic Sea on the north to the Ionian Sea. It is worth mentioning that the mainland area across Erimitis shores is the Butrinti National Park, an UNESCO World Heritage Site. According to the Butrinti National Park report in 2010, the P. oceanica meadows there cover 374.8 hectares, comprising 3.98% of the area and hosting a variety of fish species and marine megafauna (Zotaj, 2010). The region of Erimitis includes seven beaches that are intact from human disturbances from the land, as the beaches can only be approached by trails or from the sea. The study area in the context of this project has a total surface of 25.4 hectares with a maximum depth of 43 meters and is characterized by an extended rocky shore, with isolated small sandy beaches (Fig. 2). The bottom of the study area is a mixture of rocks, gravel and sand.

Methodology

Posidonia oceanica mapping
In order to develop a methodology specific for the area and inspect the whole extent of the meadow in the study area, two preliminary visits were conducted. In total, five dives were conducted, with two divers swimming along the meadow borders, with a buoy attached and the line stretched in order to always be vertical to the sea surface. The boat was following the buoy and the principal investigator of the team was marking the edges of the meadow by taking points in a Garmin eTrex® 22x GPS, with an accuracy of 3 meters.

The GPS points were then inserted to ArcGIS software and the points were overlaid with bathymetric isobath lines, retrieved from Navionics. The points were then merged to compile a polygon and minor corrections were made. Additional routes were made for mapping patches where P. oceanica was absent.

Fig. 3: The study area within the Erimitis Region (red line) with bathymetric isobath lines (in meters)

GPS coordinates were retrieved for the centre of each patch and their area was measured by the divers. These data were also overlaid with the total range polygon of the meadow borders and were excluded from the total mapped meadows.

Biodiversity monitoring

For this work, visual census surveys were conducted with the purpose of creating a marine species list for the study area. A screening of the area was conducted prior to the selection of the transects, and promising sites above, not only, the P. oceanica meadow, but also rocky and sandy substrate were

Fig.4: Diver measuring the transect (top). Schematic representation of the methodology used for the biodiversity surveys (bottom)

selected. The dimensions of each transect were 25 x 5 m. Strip transect sampling is widely applied for marine community studies (Samoilys & Carlos, 2000) The divers/researchers were moving along a 25‐m

diving line, defining the centreline of each strip, and recording all marine species within 2.5 m from the centreline in each direction (Fig.4).

Results

P. oceanica mapping

The total area covered with P. oceanica meadows is presented in Fig. 5. The mapping output was a polygon of a total 15.7465 hectares, covering 61.84% of the study area (Fig.3), although it is worth mentioning that the meadows expanded well beyond the northern and southern borders of the study area. The shallowest borders started at 0.2m and the deepest borders reached depths of 42m, in the central part of the study area, albeit most edges reached the 20 meter bathymetric contour, likely due to limited light availability.

Fig. 5: Posidonia oceanica meadows within the study area (green)

As expected, the meadows’ upper layer near the lake (Fig. 5) started from greater depths than neighbouring meadows, probably due to the lower salinity levels caused by underground water exchange between the lake and the sea.
The length of the leaves varied from 26 cm (in the upper layer of the meadow) to 70cm (in the deeper layer of the meadow). Both vertical and horizontal rhizomes were observed in all the locations of the biodiversity surveys, suggesting that the meadows are growing and expanding (Larkum et al. 2006). The sediment varied in the study area with the southern part being mostly covered with gravels, until the middle part and up to the lake with mixture of sand and small gravels, while the northern part was covered with bigger gravels and rocks. Inside the meadows three big (~1m2) rocks were recorded disrupting the extensive meadow.

Biodiversity monitoring

The biodiversity surveys resulted in the observation of 82 different species, belonging in 10 different Phylum. More specifically, 43 species of fish (Actinopterygii) were observed, along with 10 species of plants (Chlorophyta and Tracheophyta) and 29 other animal species (Table 2). The species list created was crossed with the list of a previously available survey of the same area, conducted in June 2020 (Papadopoulou, 2020). The final list (presented below) includes 107 species. The Mediterranean Conservation Status of each species was extracted from the IUCN ‘Red List of Threatened Species’. The vast majority falls under the Least Concern and Not Evaluated categories. Notable exceptions are: Serranus cabrilla (Endangered), Sciaena umbra (Vulnerable) and Epinephelus marginatus (Near Threatened).

Although, pristine environments do not exist today especially in coastal areas due to anthropogenic pressures (Montefalcone et al. 2019), the study area was almost intact by anthropization. During the 5‐ day fieldwork, (2 days preliminary visit in May and 3 days of surveying in October 2021) only the following activities were recorded in the area:

1. 1)  Fishing with set gill nets and spearfishing,
2. 2)  Recreational activities (swimming and hiking),
3. 3)  Sailing boats and speed boats

Other than the above the area was intact with a completely natural scenery. Meadows are known to be affected by several disturbances as mentioned in the introduction of this study, yet the case was different for the study area. Species compotition was high and the surveys yielded most of the species associated with P.oceanica meadows. It is worth mentioning that no alien species where observed, unlike most parts of the Greek seas, adding to the assumption that the meadow is healthy and intact. The meadow was in a very good condition with little to no fragmented meadows in the most of its area of extend. The upper limit of the meadow was extremely close to shoreline starting from 0.2m depth, in which case is an indication of good health (Montefalcone et al. 2010). Although seagrass meadows may be naturally fragmented by waves, currents and colonization processes into patches of different size and form (Pace et al. 2017), the monitored meadow seemed compact with only 4 patches without P. oceanica in the southern bay of the study area. Furthermore, the density of the leaves was high, throughout most of the meadow.

To sum up, the study area mostly consists of a healthy and undisturbed meadow, hosting a high number of different species. Since P.oceanica meadows are characterized as one of the most productive ecosystems in the Mediterranean, it becomes apparent that the protection of Erimitis bay and the management of its threats and disturbances should be highly prioritized. Last but not least, more research is needed in order to assess the situation of the broader and mostly understudied area, since the meadows expand well beyond the location of Erimitis bay.

Bibliography

● Boudouresque, C. F., Mayot, N., & Pergent, G. 2006. The outstanding traits of the functioning of the Posidonia oceanica seagrass ecosystem. Biol Mar Medit, 13(4), 109‐113.

● Larkum, A. W., Orth, R. J., & Duarte, C. M. (2006). Seagrasses: biology, ecologyand conservation. Phycologia, 45(5), 5.