Population fluctuations are often driven by an interplay between intrinsic population processes and extrinsic environmental forcing. To investigate this interplay, we analyzed fluctuations in coastal phytoplankton concentration in relation to the tidal cycle. Time series of chlorophyll fluorescence, suspended particulate matter (SPM), salinity and temperature were obtained from an automated measuring platform in the southern North Sea, covering 9 years of data at a resolution of 12 to 30 minutes. Wavelet analysis showed that chlorophyll fluctuations were dominated by periodicities of 6 hours 12 min, 12 hours 25 min, 24 hours and 15 days, which correspond to the typical periodicities of tidal current speeds, the semidiurnal tidal cycle, the day-night cycle, and the spring-neap tidal cycle, respectively. During most of the year, chlorophyll and SPM fluctuated in phase with tidal current speed, indicative of alternating periods of sinking and vertical mixing of algal cells and SPM driven by the tidal cycle. Spring blooms slowly built up over several spring-neap tidal cycles, and subsequently expanded in late spring when a strong decline of the SPM concentration during neap tide enabled a temporary “escape” of the chlorophyll concentration from the tidal mixing regime. Our results demonstrate that the tidal cycle is a major determinant of phytoplankton fluctuations at several different time scales. These findings imply that high-resolution monitoring programs are essential to capture the natural variability of phytoplankton in coastal waters.

This article synthesises a series of studies concerned with physical, chemical and biological processes involved in sediment dynamics (sedimentation, erosion and mixing) of the Molenplaat tidal flat in the Westerschelde (SW Netherlands). Total sediment accretion rate on the flat (sand to muddy sand) was estimated to be ~2 cm yr -1 , based on 210 Pb and 137 Cs profiles. 7 Be showed maximum activity in the surface sediments during summer, reflecting accretion of fine silt at this time of year, and total vertical mixing of sediment to be in the order of 50 cm 2 yr -1 . The extent to which different physical and biological processes (tidal currents, air exposure, bio-stabilisation, biodeposition and bioturbation) contributed towards sediment dynamics was estimated. A sediment transport model based on physical factors estimated sedimentation rates of 1.2 cm yr -1 , but did not account for tidal or seasonal variation in suspended particulate matter (SPM), wind or effects of spring-neap tidal cycles. When the model was run with an increased critical bed shear stress due to the microphytobenthos, net sedimentation rates increased 2-fold. These higher rates were in closer agreement with the rates derived from the depth profiles of radionuclides for the central region of the tidal flat (2.0 to 2.4 cm yr -1 ). Therefore a significant part of the sedimentation rate (~50%) may be explained by spatial-temporal changes in biological processes, including 'bio-stabilisation' by microphytobenthos, together with the enhanced biodeposition of silt by suspension feeders, and offset by processes of 'bio-destabilisation' by grazers and bioturbators. In the centre of the tidal flat there was a shift from high sediment stability in spring-summer 1996 to low sediment stability in autumn 1997, quantified by a significant reduction in critical erosion velocity of 0.12 to 0.15 m s -1 , and accompanied by a 30-to 50-fold increase in sediment erosion rate. The change was associated with a shift from a tidal flat dominated by benthic diatoms and a low biomass of bioturbating clams (Macoma balthica), to a more erodable sediment with a lower microphytobenthos density and a higher biomass of M. balthica. Vertical mixing of sediment and organic matter, studied using a variety of tracers, was rapid and enhanced by advective water flow at sandy sites and by burrowing polychaetes and bivalves at silty sites. The sediment dynamics and biogeochemistry of tidal flats is dependent on the complex interactions between physical, chemical and biological processes/properties, which include tidal currents, river flows, storm waves, air exposure, dehydration in summer, ice-scour in winter, sediment properties (grain size and composition, organic content, nutrient content, redox balance), stabilisation by biota (algal biofilms, mussel beds, salt marsh) and destabilisation by biota (bioturbating bivalves, scouring around clumps of animals and plants). To date, there have been very few multi-disciplinary studies providing an integrated view of the sedim...

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