Dancing with the Tides Figure 6. Wavelet analysis of chlorophyll, SPM, salinity and temperature on a daily time scale. (A) Original time series (green line) and transformed time series (rate of change, black line) of daily averaged chlorophyll concentration in the year 2007. (B-I) Wavelet power spectra (left panels) and global wavelet spectra (right panels) of daily averaged data of (B, C) chlorophyll concentration, (D, E) SPM concentration, (F, G) salinity, and (H, I) 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.g006 variation in convective mixing driven by thermal micro-stratifica- tion during the daytime and surface cooling at night. Van Haren et al. [54] reported similar diurnal variation in chlorophyll fluorescence at the Oystergrounds, in the central North Sea. In addition to chlorophyll fluorescence, they also measured chloro- phyll by HPLC, and found that decreasing chlorophyll concen- trations at 11 m depth matched the increasing chlorophyll concentrations at 23 m depth during the daytime. Hence, van Haren et al. [54] conclude that diurnal convective mixing led to net sedimentation of phytoplankton at daytime and net resuspen- sion at night. However, contrary to the Oystergrounds, station Warp does not show significant 24-hour periodicity of surface temperatures (Fig. 4H), so we expect that the effect of thermal microstratification is relatively small at station Warp. Fluctuations Driven by the Spring-neap Tidal Cycle Superimposed upon the 6-, 12- and 24-hour periodicities the chlorophyll concentration also fluctuated at a significant 15-day periodicity, in phase with the spring-neap tidal cycle. A possible explanation for the 15-day periodicity in phytoplankton abun- dance is that the spring-neap cycle modulated the nutrient supply. This mechanism was described by Sharples et al. [20,55], who found that the upward nitrate flux into the surface layer was much higher during spring tide than during neap tide, and fueled new primary production. However, their measurements were made in a stratified water column of 200 m depth at the shelf edge of the Celtic Sea, where upward mixing of nutrients across the thermocline was driven by an internal tide. In contrast, coastal waters of the southern North Sea are shallow (15 m deep at station Warp) and without thermohaline stratification (Fig. S1). In fact, the nitrate concentration at station Warp was highest during neap tide (Fig. S12), when nutrient-rich freshwater from the river Thames was less intensely mixed with the more saline North Sea water. Because nutrient concentrations fluctuated in anti-phase with the tidal cycle, while chlorophyll fluctuated in phase, it seems unlikely that variation in nutrient availability was the main driver for the phytoplankton fluctuations at the time scale of the spring- neap cycle. PLOS ONE | www.plosone.org 9 November 2012 | Volume 7 | Issue 11 | e49319