Macrophyte production in a shallow coastal lagoon. Part II: Coupling with sediment, SPM and tissue carbon, nitrogen and phosphorus concentrations

The spring-summer concentrations of total nitrogen (TN) and total phosphoru s (TP) in the top 0-5 cm of sediment in two areas of the Venice lagoon cove red by seagrasses and seaweeds, respectively, showed opposite trends. Durin g biomass build up and decomposition, the sediment TN concentration increas ed by ca. 45% in an area covered by Ulva, whereas, after an early spring in crease, TN progressively decreased to a concentration close to the winter v alues in an area populated by Zostera. A similar trend was recorded for TP, increasing by ca. 18% in the area Ulva-dominated and decreasing by ca. 14% in that populated by Zostera. The concentration of organic carbon (OC), ov er the same period, increased in the surface sediments of both areas, more in that populated by Zostera (ca. 100%) than in that covered by Ulva (ca. 3 0%). Winter values were then re-established after October. A similar trend was found at 10-15 cm sediment depth although lower nutrient concentrations were present. The mean concentrations of TN and TP in the particulate matt er collected by sediment traps were ca. 3-6 times higher than in the surfac e sediments in both areas but showed less marked seasonal changes. The spri ng-summer concentrations of TN and TP in both Ulva and Zostera tissues decr eased by a factor of 2-4, with nitrogen possibly acting as a limiting facto r. Based on the gross primary production and the tissue nutrient content, t he estimated amounts of C, N, P annually recycled by rhizophyte in the lago on are 200,000-300,000; 11,700-17,500 and 1150-1730 tonnes, respectively. I n contrast, the actual nutrient recycling by Ulva is at least one order of magnitude lower. These amounts are much higher than the nutrient availabili ty via point or diffuse sources as well as by surface sediment fluxes; thus the sea is thought to be a major nutrient supplier for macrophyte growth i n the lagoon of Venice. (C) 1999 Elsevier Science Ltd. AII rights reserved.