Dancing with the Tides Figure 4. Wavelet analysis of chlorophyll, SPM, salinity and temperature on an hourly time scale. Wavelet power spectra (left panels) and global wavelet spectra (right panels) using hourly averaged data of (A, B) chlorophyll concentration, (C, D) SPM concentration, (E, F) salinity, and (G, H) temperature. The wavelet power spectra show only the year 2007; the global wavelet spectra are based on the complete time series (2001– 2009). See the legend of Fig. 3 for an explanation of wavelet power spectra. doi:10.1371/journal.pone.0049319.g004 Instead, we hypothesize that the 6-hour periodicity and in-phase dynamics of chlorophyll and SPM are explained by alternating sinking and vertical mixing of phytoplankton and SPM driven by the tidal cycle. Phytoplankton and SPM both have a particulate nature. Hence, surface concentrations of phytoplankton and SPM will decrease simultaneously due to sedimentation during weak turbulent mixing (i.e., at each high and each low tide slack). Conversely, phytoplankton and SPM will increase simultaneously by resuspension during periods of intense tidal mixing generated by the high current speeds between each high and low tide. This explanation is supported by the in-phase fluctuations of the chlorophyll dynamics and tidal current speed (Fig. 5). In fact, we found a minor phase delay of 45 min between fluctuations of tidal current speed and of chlorophyll concentration, which might reflect the time it takes for high tidal current speeds to mix sedimented phytoplankton cells upwards to the water surface. Resuspension of benthic diatoms by enhanced vertical mixing has also been observed by many other studies, and is often held responsible for much of the chlorophyll variability in shallow estuaries and intertidal areas [23,39,40]. Indeed, several of the diatom species dominant at our mooring site can be classified as tychoplankton (e.g. Cylindrotheca, Paralia, Plagiogrammopsis), which combine a benthic and pelagic life style as they commonly settle on the sediment during weak mixing but are resuspended during intense tidal mixing [41–43]. Sinking rates of single diatom cells range from near zero to 10 m d21 [44,45]. Models of coastal phytoplankton using such sinking rates do not predict significant 6-hour periodicity in surface chlorophyll concentrations [46,47]. This suggests that higher sinking rates are required to explain the observed 6-hour variation in surface chlorophyll concentration. Sinking rates of diatom aggregates are often much higher than for single cells, ranging from 50 to 150 m d21 [48–50]. Many of the dominant diatom species observed at station Warp are known to form aggregates with other diatoms and sediment particles, through the production of extracellular polymeric substances and the entanglement of diatom spines [51–53]. Aggregate formation is therefore likely to play a role in the 6-hour periodicity of the chlorophyll concentration. Salinity, nitrate and temperature are not subject to sedimenta- tion and resuspension by tidal mixing. Indeed, salinity, nitrate and temperature showed only minor 6-hour periodicity, but fluctuated PLOS ONE | www.plosone.org 7 November 2012 | Volume 7 | Issue 11 | e49319