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

Abstract

The spring-summer concentrations of total nitrogen (TN) and total phosphorus (TP) in the top 0–5 cm of sediment in two areas of the Venice lagoon covered by seagrasses and seaweeds, respectively, showed opposite trends. During biomass build up and decomposition, the sediment TN concentration increased by ca. 45% in an area covered by Ulva, whereas, after an early spring increase, TN progressively decreased to a concentration close to the winter values 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), over 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. 30%). 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 matter collected by sediment traps were ca. 3–6 times higher than in the surface sediments in both areas but showed less marked seasonal changes. The spring-summer concentrations of TN and TP in both Ulva and Zostera tissues decreased by a factor of 2–4, with nitrogen possibly acting as a limiting factor. Based on the gross primary production and the tissue nutrient content, the estimated amounts of C, N, P annually recycled by rhizophyte in the lagoon are 200,000–300,000; 11,700–17,500 and 1150–1730 tonnes, respectively. In contrast, the actual nutrient recycling by Ulva is at least one order of magnitude lower. These amounts are much higher than the nutrient availability 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 in the lagoon of Venice.