Dancing with the Tides: Fluctuations of Coastal Phytoplankton Orchestrated by Different Oscillatory Modes of the Tidal Cycle Anouk N. Blauw1,2*, Elisa Beninca` 1, Remi W. P. M. Laane1,2, Naomi Greenwood3, Jef Huisman1 1 Aquatic Microbiology, Institute for Biodiversity and Ecosystem Dynamics, University of Amsterdam, Amsterdam, The Netherlands, 2 Marine and Coastal Systems, Deltares, Delft, The Netherlands, 3 Marine Observations Systems, Centre for Environment, Fisheries and Aquaculture Science (CEFAS), Lowestoft, Suffolk, United Kingdom Abstract 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. Citation: Blauw AN, Beninca` E, Laane RWPM, Greenwood N, Huisman J (2012) Dancing with the Tides: Fluctuations of Coastal Phytoplankton Orchestrated by Different Oscillatory Modes of the Tidal Cycle. PLoS ONE 7(11): e49319. doi:10.1371/journal.pone.0049319 Editor: Martin Krkosek, University of Otago, New Zealand Received May 22, 2012; Accepted October 7, 2012; Published November 14, 2012 Copyright: 2012 Blauw et al. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited. Funding: This study is part of the ZKO project ‘‘Integrated monitoring of carrying capacity in coastal waters’’, subsidized by the Netherlands Organisation of Scientific Research (NWO). This study was further supported by the project ‘‘Operational water quality forecasting’’ within the Strategic Research program of Deltares carried out for the Centre for Water Management (Waterdienst) of Rijkswaterstaat. The funders had no role in study design, data collection and analysis, decision to publish, or preparation of the manuscript. Competing Interests: The authors have declared that no competing interests exist. * E-mail: anouk.blauw@deltares.nl Introduction What drives fluctuations in population abundances? In the 1920s the famous zoologist Charles Elton argued that population fluctuations of many birds and mammals are most likely due to climatic fluctuations [1]. Shortly thereafter, however, mathemat- ical models of Lotka [2] and Volterra [3] and laboratory experiments by Gause [4] demonstrated that species interactions can also generate population fluctuations, even in the absence of external forcing. Since that time, one of the key challenges for ecologists has been to disentangle the complex interplay between intrinsic population dynamics and environmentally-driven varia- tion [5–7]. This interplay between intrinsic population processes and external forcing is exemplified by the plankton of freshwater and marine ecosystems. Theory and experiments have shown that plankton communities can display striking fluctuations, and even chaos, under constant conditions without external forcing [8–12]. Such non-equilibrium dynamics can limit the predictability of plankton abundances. For instance, Beninca` et al. [12] estimated that the predictability of species fluctuations in an experimental plankton community was limited to a time horizon of only 15–30 days. In addition, plankton communities are also very sensitive to variation in environmental conditions [13–17]. Therefore, a major question is how environmental forcing interacts with the intrinsic population fluctuations in plankton communities. Environmental forcing by the tidal cycle is an important driver of phytoplankton variability in coastal waters [18–21]. The tidal cycle is characterized by periodic fluctuations at several time scales. Systems with a semidiurnal tide, like the North Sea, show horizontal displacement of water masses with a periodicity of 12 hours and 25 min. This horizontal motion generates maxima in tidal current speeds and turbulent mixing with a periodicity of 6 hours and 12 min. Other important tidal components include the spring-neap tidal cycle with a periodicity of 15 days and the apogee-perigee cycle with a periodicity of 28 days. The latter cycle is caused by the moon’s elliptic orbit, which enhances the tidal range during perigee (when the moon is closest to Earth) and reduces it during apogee (when the moon is farthest from Earth) [22]. In addition to the tidal cycle, coastal phytoplankton will also be exposed to other environmental variation in, e.g., solar irradiance, temperature and wind mixing. We hypothesize that these different sources of phytoplankton variability can be distinguished by investigating the time scales of phytoplankton fluctuations. For instance, phytoplankton fluctua- tions with a periodicity of 6 hours 12 min indicate an alternation PLOS ONE | www.plosone.org 1 November 2012 | Volume 7 | Issue 11 | e49319