Volume 65, Issue 4 - Special issue: Marine botany in the Eastern Mediterranean Sea; Issue editors: Christos Katsaros, Sotiris Orfanidis and Frithjof C. Küpper

This article provides a topical review of East Mediterranean phycology and seagrass biology, with a special focus on the outcomes of the multi-year project “Brown algal biodiversity and ecology in the Eastern Mediterranean Sea” supported by the TOTAL Foundation. Following a general overview of East Mediterranean seaweed biogeography and taxonomy, the review covers seagrasses, deep-water (circalittoral) macroalgal diversity, pathogens and algal defence, human impacts and biotic indices, alien species as well as the papers of this special issue.

This study contains an updated checklist and new collections of marine algae from the coasts of the Aegean Sea and the Levantine Sea of Turkey. A total of 573 marine algal taxa at specific and infraspecific levels (137 brown algae, 318 red algae, and 118 green algae) are reported, 32 of which are regarded as endangered or threatened macroalgae (four green algae, nine red algae, and 19 brown algae). Of these, 36 taxa (10 brown algae, 17 red algae, nine green algae) are recorded as alien on the Aegean and Levantine coasts. In the present study, the red alga Palisada maris-rubri is reported for the first time from the eastern Mediterranean Sea for Turkey. In Iskenderun Bay, species were sampled from 1 m depth on a volcanic rock in May 2016.

Various species of marine micro-filamentous green algae (<5 mm) remain overlooked globally. They are difficult to identify in field collections due to their diminutive size and their cryptic morphology. During this study, algal cultures combined with DNA barcoding helped to overcome these challenges. Both substratum samples and macrophytic fragments of Posidonia oceanica , Dictyopteris sp. and Halopteris sp. were incubated in laboratory culture. Germlings of Ulvella endostraca and Blastophysa rhizopus grew from the incubated substratum. Blastophysa rhizopus is a new record for the Maltese islands, as is U. endostraca that had only been reported previously from New Zealand. In situ , germlings of both species were also observed to grow as epiphytes, on Dictyopteris sp. in the case of U. endostraca , and on P. oceanica in the case of B. rhizopus . This study employed the in vitro culture of algal germlings, as well as morphological and molecular analyses. DNA data and subsequent phylogenetic analyses of tuf A and rbc L sequences supported the separation of U. endostraca from other closely related congeners that have been previously reported from the Maltese islands. Moreover, this study includes a comparison of the micro-filamentous algae identified from Malta with those from other areas of the Mediterranean Sea.

The Gongolaria barbata forest inhabiting Šćuza lagoon on the southern Istrian coast (northern Adriatic) represents the last significant refuge for this species among the shallow waters of the Istrian Coast. The lagoon is characterised by extreme winter and summer temperatures. Moreover, the absence of certain grazers could be one of the main factors favouring the development and growth of G. barbata forest. Two morphologically distinct forms of G. barbata inhabit the lagoon: attached and detached. Abundance and morphological characteristics of each form were analysed in randomly sampled thalli. The wet weight, thallus length, number of cauloids and total cauloid length were significantly higher in the detached than in the attached form. Although the main cauloid was highly branched in both forms, its total length was up to three times higher in the detached than in the attached form. Our study suggests that this population of G. barbata can thrive in the extreme conditions found in the lagoon, while also exhibiting two distinct forms. Furthermore, this large population could serve as a donor for future restoration attempts on the western Istrian Coast where a decline of G. barbata has been confirmed in recent years.

The photoperiod alone or in combination with temperature may affect macroalgal growth, reproduction, and development. This study aimed to investigate the responses of Ericaria barbatula , a perennial fucalean species collected from Kavala Gulf, Greece, to photoperiod and temperature. Maximum quantum yield ( F v / F m ), wet biomass ( g ), relative growth rate (day −1 ), and reproduction (receptacles and zygote formation) were monitored in two laboratory experiments, the first one in two consecutive phases (A and B), that lasted at least seven months. While in Experiment 1, phase A, specimens were cultivated under long-day (LD) and short-day (SD) conditions at a constant temperature, in Experiment 1, phase B, they were cultivated at a seasonally changing temperature. The results indicated that E. barbatula reproduced in both phases under SD conditions and grew better in winter to early spring under seasonally changing temperatures. The cultivation under night-break (NB) daylength and seasonally changing temperatures (Experiment 2) indicated that E. barbatula is a facultative (quantitative) SD plant, a result that needs further examination. However, the induction of reproductive organs under SD conditions has been confirmed. It secures the release of zygotes and the germlings’ growth in the field from April to June/July under non-stressful Mediterranean Sea temperatures.

The loss of Mediterranean macroalgal populations dominated by Cystoseira sensu lato is driven by a multitude of pressures. In the eastern Mediterranean Sea, climate change and the establishment of Lessepsian herbivore species have further intensified the ongoing decline of these canopy-forming algae. Knowledge of the reproductive phenology, embryology and growth of Cystoseira species is the first step towards successful reforestation measures. Gongolaria montagnei is one of the most common canopy-forming algal species with a wide horizontal and vertical distribution along the Greek coasts. Mature receptacles were collected from Saronikos Gulf (Aegean Sea), and germlings were cultured in mesocosms for 34 days, reaching up to 1.30 mm in length. Divisions and early developmental stages of G. montagnei are described and possible implications for future restoration efforts are discussed. A new approach to growth rate modeling, based on surface measurements of embryos and germlings is presented.

Combined use of optical (satellite imagery) and acoustic (sidescan sonar) remote sensing techniques, as well as in situ methodologies (visual census; SCUBA diving, Towed Underwater Cameras, and Remotely Operated Vehicles) was employed to map the spatial distribution of seagrass habitats in the coastal waters of the Greek territory. Seagrass meadows were recorded over approximately 70% of the Greek coastline (Eastern Ionian, Aegean and Levantine Seas), and their surface area exceeded 2749.07 km 2 . Posidonia oceanica is clearly the dominant seagrass species in Greek Seas, covering the vast majority of seabed at depths between the shoreline and 25–30 m (or deeper in insular areas), followed by the species Cymodocea nodosa , Zostera noltei , and Halophila stipulacea , that are not widely distributed but are locally abundant in specific areas. Habitat suitability in terms of the of the seabed that is available for the growth of seagrass meadows (i.e., the spatial extent of coastal areas between the shoreline and the isobath of 20 m) and the seawater clarity conditions are highlighted as the critical factors for the formation of well-structured and extensive meadows. The results of this study are of great importance and usefulness for the effective management and conservation of valuable marine ecosystems and are given in the form of an openly accessible shapefile.

Cymodocea nodosa is a common seagrass species of shallow and sheltered Mediterranean waters, where extreme flushing of plumes can occur during excessive rainfall. Cymodocea nodosa shoots were sampled from two habitats of Kavala Gulf, one nearly pristine (less stressed, Vrasidas) and another highly stressed (Nea Karvali), to study if flooding might negatively affect seagrass habitats. Photosynthetic performance of shoots from the pristine habitat acclimated better than shoots from highly stressed conditions simulating a flooding event. Indeed, a significant ( p  < 0.01) interaction between habitat and flooding on photochemical energy harvesting (Δ F /F m ′) values was found, with lower Δ F / F m ′ values in the pristine habitat under control conditions. Furthermore, based on relative electron transport rate (rETR) curves reconstructed from fluorescence-versus-irradiance data, shoots from the pristine habitat performed better after 18 days of treatment to flooding. On the other hand, shoots from highly stressed habitats grew faster than pristine ones, but their growth decreased similarly under flooding conditions. The implications for management and conservation priorities for this phenotypically plastic seagrass in the Mediterranean are discussed.