Macrophyte production in a shallow coastal lagoon. Part I: Coupling with chemico-physical parameters and nutrient concentrations in waters

Abstract

Standing crop and production of macroalgae and seagrasses were recorded, together with the main physical and chemical parameters, including nutrient concentrations of the water column and 5-cm top sediment porewater, the grazing pressure and the settled paniculate matter (SPM) in two areas of the central (Lido station) and southern (Petta di Bò station) parts of the Venice lagoon. Then the whole set of data was analysed by multivariate analysis. The highest standing crops of Ulva (Lido station) and Zostera (Petta di Bò station) monitored throughout the year (February 1994–February 1995) were ~ 6.5 and ~ 11.0 kg fwt m −2, accounting for an annual net production of ~ 20.4 and ~ 20.9 kg fwt m −2, respectively. The estimated gross production of Zostera was, however, ca 35–55% lower than that of Ulva because of the higher decomposition rate and grazing pressure suffered by the markedly stratified and light-limited free-floating fronds of the macroalga. At the Lido station, the overall grazing pressure accounted for ca 65% of the net Ulva production, but it was found to exceed the total production in the July–August period. Ammonium and orthophosphate concentrations in the water column and sediment porewater were ca 2–3 times higher at the Ulva than at the Zostera station. Considering the N:P atomic ratios, nitrogen, during the quick spring-summer biomass increase, could be temporarily critical for the macrophytic growth, especially at the Lido station. The rates of sediment resuspension and settlement were ca six times higher at the Zostera than at the Ulva station, mainly because of higher sediment coverage by the large free-floating fronds of Ulva. For the contribution of individual variables explained by the principal component analysis, it is shown that the Ulva decomposition at the Lido station and the Zostera production at Petta di Bò were the major factors affecting the total variance.