The plant world of the sea is made of algae and sea grass. They represent a very significant link in the marine ecosystem. They produce oxygen and serve as shelter and food to many animals. Different types of algae and sea grass develop depending on the conditions in the sea (depth, temperature, salinity, quantity of light).
3 species of sea grass, and 318 species of algae have been established of a total of 638 species in recorded in eastern Adriatic through previous research of the underwater biological diversity of the Nature Park. The dominant ones are red algae with 340 species, followed by 170 species of brown algae, and 118 species of green algae. The composition of plant communities depends on the type of surface on which they develop. Thus, a whole range of algae can be found on shallow stone bottoms, and sea grass is dominant on sandy and muddy bottoms. Different types of Cystoseira calcareous red algae stand out, which is characteristic of the outer open-sea islands.
The most interesting algae of this area are the calcareous red algae (Goniolithon byssoides), widespread in the eastern Mediterranean, but also very rare at the NW border of the Adriatic Sea range. The richest habitat of the algae in the Nature Park is underneath the open southeastern rocks of Dugi Otok and the neighbouring stormy islands.
Dense settlements of the invasive tropical red algae (Womersleyella setacea) ware found during research at sites Grpašćak, Mir, and Mala Prisika. Other common species of algae that can be seen along the coastline of the Nature Park are: peacocks tail (Padina pavonica), forkweed (Dictyota dichotoma), Laurentia (Laurentia obtusa), Cystoseira (Cystoseira sp.), Acetabularia (Acetabularia acetabulum); and sea grass: little Neptune grass (Cymodocea nodosa), and drawf eelgrass (Zostera noltii).
Sea meadows of three strictly protected species of sea grass are most significant in the bay itself: Neptune grass (Posidonia oceanica) and little Neptune grass (Cymodocea nodosa), and to a lesser extent, drawf eelgrass (Zostera noltii). Areas covered in meadows of Neptune grass have been identified through previous research of the underwater, and they spread within the photophyllic zone at a depth of about 5 m to a depth of 20-25 m. Posidonia oceanica, commonly known as “sea grass” is an endemic species of the Mediterranean. Due to various influences such as climate change, invasive species, and many human activities such as anchoring, Neptune grass meadows are highly endangered. In order to determine the state of Neptune grass meadows and other sea grass, and to verify the presence of invasive Caulerpa algae threatening the Neptune grass meadows, the Institution began organizing the programme “Monitoring of Posidonia oceanica meadows,” which will be carried out once a year.