Dancing with the Tides Figure 7. Coherence between fluctuations in chlorophyll, tidal range and SPM on a daily time scale. (A) Time series of daily averaged chlorophyll concentration (green line) and tidal range (blue line) during spring of 2007. (B) Wavelet coherence spectrum of the two time series in panel A. Color coding indicates the coherence of the two time series. Arrows indicate the phase angle between fluctuations of the two time series. See the legend of Fig. 3 for further explanation of wavelet spectra. (C) Relative distribution of phase angles between fluctuations in chlorophyll concentration and tidal range. (D) Time series of daily averaged chlorophyll concentration (green line) and SPM concentration (black line). (E) Wavelet coherence spectrum of the two time series in panel D. (F) Relative distribution of phase angles between fluctuations in chlorophyll concentration and SPM concentration. The phase angle distributions in (C) and (F) are based on the complete time series (2001–2009). doi:10.1371/journal.pone.0049319.g007 Instead, the in-phase fluctuations of both chlorophyll and SPM with the tidal range suggest that a large fraction of the phytoplankton and SPM sinks to the sediment or deeper water layers during calm conditions at neap tide, while they are resuspended by strong tidal mixing during spring tide. Station Warp is part of a large shallow outer estuary of more than 100 km2. Hence, the chlorophyll and SPM fluctuations observed at the time scale of the spring-neap cycle will probably not represent small-scale processes restricted to the local neighborhood of station Warp, but may integrate sedimentation and resuspen- sion over a large area. The 15-day periodicity of the SPM concentration confirms recent spectral analyses of remote sensing data of the North Sea, which indicated spring-neap variations in SPM in the East Anglia plume and Rhine plume [56]. Our interpretation is further supported by McCandliss et al. [24], who investigated the dynamics of suspended particles in coastal waters of the southern North Sea. They observed enhanced particle settling and the formation of a phyto-detrital fluff layer at the sediment surface during neap tide and calm wind conditions. This benthic fluff layer was (partly) resuspended by more turbulent conditions during strong spring tide and storms. Hence, we conclude that the 15-day periodicity in chlorophyll concentration is driven by a similar process of alternating sinking and resuspension as the 6-hour periodicity, integrated over a larger area. Seasonal Variation and the Phytoplankton Spring Bloom Earlier analysis indicated that the spring bloom at our coastal mooring site is triggered by improved light conditions due to a combination of enhanced solar radiation and reduced SPM concentrations in spring [26]. Similar observations have been made in other coastal waters [46,57,58]. Our results support these PLOS ONE | www.plosone.org 10 November 2012 | Volume 7 | Issue 11 | e49319