Species composition, biomass and net primary production in coastal shallow waters: the Venice lagoon

Three areas displaying the main characteristics of the central Venice lagoon were monitored in 1989-91 to study macroalgal production in relation to different biomass densities and species compositions. Two areas that were covered by dense (0-20 kg m-2, wet wt), monospecific, Ulva rigida populations (> 96%) showed a net biomass production of 7-16 kg m-2 year-1, wet wt, corresponding to 149-358 g C m-2 year-1, and daily biomass yields up to 1500 g m-2, wet wt. A third area, characterized by a biomass of < 3 kg m-2 constituted by five main species, displayed a net biomass production of 6-9 kg m-2 year-1, i. e. 132-196 g C m-2 year The percentage daily biomass increase was low (0-5%) at high biomass densities (> 5 kg m-2, wet wt) and very high (up to 200%) when biomass was < 100 g m-2, wet wt. The ratio of annual biomass production to highest measured biomass (ABP/HMB) was inversely related to the biomass density and ranged from 1.5 to 4.5. On the basis of these ratios and of the standing crops measured by 180 sampling sites in the late summer during the highest-algal-growth period, the annual net biomass production in the central Venice lagoon was estimated to be approximately 1 550 000 tons on a wet-weight basis