UPDATING OF SEAGRASS PRIMARY PRODUCTION IN THE LAGOON OF VENICE

Venice Lagoon will shortly be a regulated lagoon with closures of the mouths of communication with the sea through the MoSE gates in the event of waters above the normal high-water level. For this reason, between January 2019 and January 2020, a study was carried out to determine the annual net primary production of the seagrasses Cymodocea nodosa (Ucria) Asherson, Zostera marina Linnaeus and Zostera noltei Hornemann in three lagoon stations where previous data were already available (Ca’ Roman colonized by Z. marina, Santa Maria del Mare by C. nodosa and Petta di Bò by Z. noltei). Seagrasses were sampled monthly (6 sub-samples collected with a 20*25 cm sampler) to detect fresh and dry biomasses of shoots, rhizomes and dead parts, shoot length and number of leaves per shoot. Net shoot production was determined bi-monthly by the leaf-marking technique, whereas rhizomes growth was calculated monthly by the biomass variations. Seagrasses showed different maximum growth period according to their autoecology: C. nodosa, a subtropical species that grows only in the warm season, reached in August a biomass of 7067 g fwt m-2 while in the cold season the biomass was almost completely represented by roots-rhizomes; Z. marina and Z. noltei grew during all the year reaching the highest biomasses respectively in June (4830 g fwt m-2) and in July (3628 g fwt m-2). As regards net primary productions, C. nodosa and Z. marina showed similar values (12554 g m-2 y-1 and 12981 g m-2 y-1), while Z. noltei showed almost halved value (and 6647 g m-2 y-1). Overall, these results are similar or lower than those found in the past, probably because the increase of water turbidity and the reduction of the trophic level which affected the lagoon after the first production measurements carried out between the ‘90s and early 2000s.