Macroalgae and phytoplankton standing crops in the central Venice lagoon: Primary production and nutrient balance

Abstract

Biomass measurements carried out in the central part of the Venice lagoon during the season of the highest macroalgal production, at the beginning of June 1987, showed that 85 km 2, corresponding to 65% of the examined area, were covered by remarkable amounts of nitrophile macroalgae. Total macroalgal biomass was 546 900 tons, accounting for 17 679, 1911 and 170 tons of carbon, nitrogen and phosphorus, respectively. The annual net primary production, calculated from the maximum production/biomass (P/B) ratio determined in a hypertrophic area of the central lagoon, was estimated to be 866 160–1154 880 tons (wet wt). Therefore, in Spring-Summer, macroalgae recycled 78–104% and 38–51% of the total annual nitrogen and phosphorus which entered the central lagoon. Phytoplankton standing crop appeared negligible in most of the studied area. High chlorophyll- a concentrations were only measured near the Porto Marghera industrial zone where macroalgae were almost absent; in the area between the islands of S. Angelo and Gidecca, where the water depth was > 1 m; and in the area facing the island of Lido used by the Municipality of Venice as a test area for algal removal experiments. Based on both space and time measurements, nanoplankton accounted for about 80% of the total phytoplankton.