In situ biomass production of Gracilariaceae and Ulva rigida: The Venice Lagoon as study case.

The growth and biomass production of the most abundant Gracilariaceae: Gracilariopsis longissima (S.G. Gmelin) M. Steentoft, L.M. Irvine et W.F. Farnham, Gracilaria gracilis (Stackhouse) M. Steentoft, L.M. Irvine et W.F.Farnham and Gracilaria vermiculophylla (Ohmi) Papenfuss, and of Ulva rigida C. Agardh were studied during 1 year in two areas of the Venice Lagoon, namely Tresse and San Giuliano, that were characterized by different trophic levels and water turbidity. Both growth and macroalgal production were monitored every 7–11 days in cubical cages placed on the bottom within the natural biomass. Environmental parameters, nutrient concentrations in the water column and surface sediments, and sedimentation rates were also determined. The biomass production of the two stations was very different and strongly influenced by the water turbidity and sedimentation rates. In both areas the availability of nutrients were high, but Tresse, where waters were more transparent, was more productive. In that area, the annual production of Gracilariaceae ranged between 21.9 and 28.2 kg fresh weight (fw) m−2 and G. longissima was the most productive species. Ulva production was within the same range (25.8 kg fw m−2). The mean relative growth rates on an annual basis were between 1.87 and 2.41% day−1. In contrast, in the turbid area (San Giuliano), the annual production of Gracilariaceae was much lower, ranging from 9.0 to 12.8 kg fw m−2 and that of Ulva was 15.2 kg fw m−2.