Changes in seasonal succession of phytoplankton induced by the storm-surge barrier in the Oosterschelde (S.W. Netherlands)

Abstract

Seasonal succession of the phytoplankton was followed weekly in the eastern and western parts of the Oosterschelde estuary, before (1982–1983) and during (1984–1986) the construction of the storm-surge barrier. Construction of the barrier seriously influenced the hydrodynamics of the estuary. In the eastern part, decreased current velocities led to the emergence of a vertical salinity gradient, to an increasing sedimentation of suspended matter and to a rise in transparency during a prolonged period of the year. The relationship between species composition and biomass on the one hand and environmental variables on the other hand, was analysed in a Canonical Correspondence Analysis, for both compartments separately. The analysis revealed that the phytoplankton assemblage more and more obtained a summer character, extending its growth season both earlier and later in the year. The transition of spring to summer phytoplankton assemblages proceeded parallel to the main light-turbidity gradient. The factor light did not only explain the seasonal pattern, but also the long-term trend from pre-barrier to barrier period. Partial Detrended Correspondence Analysis after correction for the mean seasonal pattern confirmed this trend significantly.