SPECIES COMPOSITION, BIOMASS, AND NET PRIMARY PRODUCTION IN SHALLOW COASTAL 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 rel ation to different biomass densities and species compositions. Two are as that were covered by dense (0-20 kg m-2, wet wt), monospecific, Ulv a rigida populations (> 96%) showed a net biomass production of 7-16 k g m-2 year-1, wet wt, corresponding to 149-358 g C m-2 year-1, and dai ly biomass yields up to 1500 g m-2, wet wt. A third area, characterize d by a biomass of < 3 kg m-2 constituted by five main species, display ed 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 bi omass densities (> 5 kg m-2, wet wt) and very high (up to 200%) when b iomass was < 100 g m-2, wet wt. The ratio of annual biomass production to highest measured biomass (ABP/HMB) was inversely related to the bi omass 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 approximat ely 1 550 000 tons on a wet-weight basis.