Seasonal variation in biomass, morphometric parameters and production of seagrasses in the lagoon of Venice

Abstract

Results of a year long study on the biomass dynamics and production of three seagrass species (i.e. Zostera marina L., Zostera noltii Hornem., and Cymodocea nodosa (Ucria) Asher.) recorded in the lagoon of Venice are presented in this paper. Seagrasses showed different biomass trends and reproductive phenology. Cymodocea nodosa, in spite of its shorter growing period, exhibited the greatest maximum (∼7.6 kg fwt m −2) and mean (∼4.0 kg fwt m −2) standing crops, on an annual basis. The biomass was significantly lower in Z. noltii (maximum: 5.4 kg fwt m −2, mean: 2.6 kg fwt m −2), whereas, Z. marina showed intermediate values (maximum: 6.3 kg fwt m −2, mean: 3.1 kg fwt m −2). On average, the biomass of shoots in Z. marina and Z. noltii was twice as high as that of roots–rhizomes, whereas, the biomasses of Cymodocea roots–rhizomes and shoots were similar. This latter species, of sub-tropical origins, showed a below-ground biomass 3–4 times higher than shoot biomass in winter. The daily length increase (DLI) of shoots and rhizomes of Z. marina, derived from marking techniques, on an annual basis was 3.6±1.2 cm d −1 (maximum: 5.7 cm d −1) and 0.16±0.07 cm d −1 (maximum: 0.28 cm d −1), respectively. The per cent relative growth rate (%RGR) of shoots ranged from 1.1% to 4.5% (mean: 1.87%). The plastochrone interval was 12.4±3.1 days and the mean leaf age 48±16 days. The net production of Z. marina was estimated to be ca. 15 kg fwt m −2 y −1, for above-ground parts, and 5.9 kg fwt m −2 y −1, for roots–rhizomes, accounting for a total carbon production of ca. 1093 g C m −2 y −1. On an annual basis, the net production of Z. marina was estimated to be ca. 0.8 million tonnes, on a fresh weight basis, whereas, the total seagrass production could be higher than 2 million tonnes.