MACROPHYTE PRODUCTION IN A SHALLOW COASTAL LAGOON .1. COUPLING WITH CHEMICOPHYSICAL PARAMETERS AND NUTRIENT CONCENTRATIONS IN WATERS

Standing crop and production of macroalgae and seagrasses were recorde d, together with the main physical and chemical parameters, including nutrient concentrations of the water column and 5-cm top sediment pore water, the grazing pressure and the settled particulate matter (SPM) i n two areas of the central (Lido station) and southern (Petta di Bo st ation) parts of the Venice lagoon. Then the whole set of data was anal ysed by multivariate analysis. The highest standing crops of Ulva (Lid o station) and Zostera (Petta di Ed station) monitored throughout the year (February 1994-February 1995) were similar to 6.5 and similar to 11.0 kg fwt m(-2), accounting for an annual net production of similar to 20.4 and similar to 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 su ffered by the markedly stratified and light-limited free-floating fron ds 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 or thophosphate concentrations in the water column and sediment porewater were ca 2-3 times higher at the Ulva than at the Zostera station. Con sidering the N:P atomic ratios, nitrogen, during the quick spring-summ er biomass increase, could be temporarily critical for the macrophytic growth, especially at the Lido station. The rates of sediment resuspe nsion and settlement were ca six times higher at the Zostera than at t he Ulva station, mainly because of higher sediment coverage by the lar ge free-floating fronds of Ulva. For the contribution of individual va riables explained by the principal component analysis, it is shown tha t the Ulva decomposition at the Line station and the Zostera productio n at Petta di Bd were the major factors affecting the total variance. (C) 1997 Elsevier Science Ltd.