Dancing with the Tides Figure 2. Time series measured in 2007. (A) Water level (red line) and tidal range (blue solid line) at station Sheerness. When tidal data at station Sheerness were missing, we show the tidal range at station K13A (blue dashed line) rescaled to match the tidal range at Sheerness. (B) Chlorophyll concentration (green) and SPM concentration (black). (C) Nitrate concentration (dark purple) and light intensity at 1 m depth (pink). (D) Salinity (blue) and water temperature (red). In (B-D), dots show the hourly averages and lines the daily averages. doi:10.1371/journal.pone.0049319.g002 current speed and the rate of change in chlorophyll concentration. Wavelet coherence analysis also shows significant coherence between the time series of chlorophyll and SPM concentration, which fluctuated in phase at a 6-hour and 12-hour periodicity (Fig. 5E–G). Analysis of Daily-averaged Time Series In Figure 6, we move away from the within-day variability and focus on longer time scales using the daily-averaged time series of 2007. Comparison of transformed data (black line) and untrans- formed data (green line) illustrates that expressing the chlorophyll data as relative rates of change suppresses the amplitude of large peaks such as the spring bloom, and uncovers periodic variation in chlorophyll concentration during other times of the year (Fig. 6A). Wavelet analysis reveals significant 15-day periodicity in chloro- phyll concentrations during March-April and June (Fig. 6B). In May, the significant area is spread across a wider range of periodicities due to the relatively large amplitude (and, hence, large power) of the spring bloom. Daily-averaged SPM and salinity also show significant 15-day periodicity, at least during part of the year (Figs. 6D,F). Daily-averaged temperature does not show a clear peak at any specific time scale in the wavelet power spectrum (Fig. 6H). Analysis of the entire dataset of 2001–2009, summarized in the global wavelet spectra (Figs. 6C, E, G, I), confirms the 2007 results with significant peaks at a 15-day periodicity for chlorophyll, SPM and salinity. Wavelet coherence analysis shows that the chlorophyll concen- tration fluctuated in phase with the tidal range (Fig. 7A–C) and SPM concentration (Fig. 7D–F) during most of the year. In 2007, the spring bloom slowly built up over several spring-neap cycles in March and April, and then took off to high chlorophyll concentrations in early May, when the SPM concentration declined much stronger at neap tide than in the preceding months (Fig. 7D; see also Fig. 2A). Interestingly, during this chlorophyll peak in May, the chlorophyll fluctuations lost coherence with the tidal range and SPM fluctuations at a 15-day periodicity (Figs. 7B,E). In a sense, one might say that the spring bloom ‘‘escaped’’ from the spring-neap tidal cycle. This sequence of events, where a strong decline of the SPM concentration in late April or early May was accompanied by a large spring bloom, was also observed in the years 2001, 2003, 2004 and 2008 (Figs. S2, S3, S4, S5, S6, S7, S8, S9). In all other PLOS ONE | www.plosone.org 5 November 2012 | Volume 7 | Issue 11 | e49319