Results of <i>χ</i>² and reduced chi-square (χ˜2) goodness-of-fit test for the site investigated in correspondence of the four sampling periods.

<p>The number of samples, used for the test and distanced of 1 m, is n = 200 corresponding to consider from the surface the first 200 m of depth.</p><p>Results of <i>χ</i>² and reduced chi-square (</p><p></p><p></p><p></p><p></p><p><mi>χ</mi><mo>˜</mo></p><mn>2</mn><p></p><p></p><p></p><p></p>) goodness-of-fit test for the site investigated in correspondence of the four sampling periods.<p></p

Magnitude, depth, and width of the DCM, as a function of time.

<p>Red lines indicate theoretical results obtained for a yearly cycle with a time resolution of one month. Green points indicate experimental data acquired in the site analyzed (39° 30′.00 N, 13°30′.00 E) in correspondence of the four sampling periods (VECTOR-TM1, 24 November 2006; VECTOR-TM2, 3 February 2007; VECTOR-TM3, 22 April 2007; VECTOR-TM4, 9 June 2007).</p

Xanthophyll cycle modulation.

<p>Evolution of diatoxanthin (Dt)/chlorophyll (Chl) <i>a</i> (in mol Dt/100 mol Chl <i>a</i>) over the light gradient, in <i>Pseudo-nitzschia multistriata</i> cells experiencing light gradual increases peaking at the PFD of 100, 250, 350, 500 and 650 µmol photons m⁻² s⁻¹, during the 5 h (A), 3 h (C) and 2 h kinetics of light increase (E). Values are means ± SD (<i>n</i> = 3). Relationship between Dt and diadinoxanthin (Dd)/Chl <i>a</i> (in mol pigment/100 mol Chl <i>a</i>), during the 5 h (B), 3 h (D) and 2 h kinetics of light increase (F). In (B) and (F) data measured at PFD ≤350 µmol photons m⁻² s⁻¹ (black dots, <i>n</i> = 39) and ≥500 µmol photons m⁻² s⁻¹ (white dots, <i>n</i> = 6) are discerned. In (D) data measured at PFD ≤250 µmol photons m⁻² s⁻¹ (black dots, <i>n</i> = 33), at 280 and 350 µmol photons m⁻² s⁻¹ (grey dots, <i>n</i> = 6), and at PFD ≥500 µmol photons m⁻² s⁻¹ (white dots, <i>n</i> = 6) are discerned.</p

How diffusivity, thermocline and incident light intensity modulate the dynamics of deep chlorophyll maximum in Tyrrhenian Sea

<p>During the last few years theoretical works have shed new light and proposed new hypotheses on the mechanisms which regulate the spatiotemporal<br>behaviour of phytoplankton communities in marine pelagic ecosystems.<br>Despite this, relevant physical and biological issues, such as effects of the timedependent mixing in the upper layer, competition between species, and dynamics of non-stationary deep chlorophyll maxima, are still open questions. In this work, we<br>analyze the spatio-temporal behaviour of five phytoplankton species in a real marine<br>ecosystem by using a one-dimensional reaction-diffusion-taxis model. The study is performed, taking into account the seasonal variations of environmental variables, such as light intensity, thickness of upper mixed layer and profiles of vertical turbulent diffusivity, obtained starting from experimental findings. Theoretical distributions of phytoplankton cell concentration was converted in chlorophyll concentration, and compared with the experimental profiles measured in a site of the Tyrrhenian Sea at four different times (seasons) of the year, during four different oceanographic cruises. As a result we find a good agreement between theoretical and<br>experimental distributions of chlorophyll concentration. In particular, theoretical results<br>reveal that the seasonal changes of environmental variables play a key role in the phytoplankton distribution and determine the properties of the deep chlorophyll maximum. This study could be extended to other marine ecosystems to predict future changes in the phytoplankton biomass due to global warming, in view of devising strategies to prevent the decline of the primary production and the  consequent decrease of fish species.</p> <p> </p

Profiles of <i>chl a</i> concentration acquired in the sampling site (39° 30′.00 N, 13° 30′.00 E).

<p>Data were collected during four oceanographic cruises: VECTOR-TM1, 24 November 2006 (panel a); VECTOR-TM2, 3 February 2007 (panel b); VECTOR-TM3, 22 April 2007 (panel c); VECTOR-TM4, 9 June 2007 (panel d). The black lines have been obtained by connecting the experimental points corresponding to samples distanced of 1 meter along the water column. The total number of samples measured in the site is <i>n</i> = 196 for VECTOR-TM1, <i>n</i> = 198 for VECTOR-TM2, <i>n</i> = 199 for VECTOR-TM3, and <i>n</i> = 196 for VECTOR-TM4.</p

Theoretical distributions (red line) and experimental profiles (green line) of the total <i>chl a</i> and <i>Dvchl a</i> concentration.

<p>The numerical results, obtained by the five-population model and given as a function of the depth, are compared with the experimental data collected in the sampling site (39° 30′.00N, 13°30′.00E), during the oceanographic surveys: VECTOR-TM1, 24 November 2006 (panel a); VECTOR-TM2, 3 February 2007 (panel b); VECTOR-TM3, 22 April 2007 (panel c); VECTOR-TM4, 9 June 2007 (panel d).</p

Photosynthetic and physiological properties, and photosynthetic pigment content of <i>Pseudo-nitzschia multistriata</i>.

<p>The measurement of photosynthetic and physiological properties was performed on cells in the exponential growth phase, during preacclimation, the day before the experiments started. The growth rate did not change during experiments. _relETR_max, maximal relative electron transport rate (in mol e⁻ g Chl <i>a</i>⁻¹ h⁻¹); Ek, saturation light for photosynthesis (in µmol photons m⁻² s⁻¹); µ, growth rate (in d⁻¹); F_v/F_m, photosystem II maximal photochemical efficiency. Values are means ± SD (<i>n</i> = 9). Chlorophyll <i>a</i> cellular content (Chl <i>a</i>, in 10⁻¹⁶ mol Chl <i>a</i> cell⁻¹) and photosynthetic accessory pigments Chl <i>a</i>⁻¹ content (in mol pigment/100 mol Chl <i>a</i>) measurements were performed during experiments. Fuco, fucoxanthin: Chl <i>c</i>, chlorophyll <i>c</i>₁,₂,₃. Pigment data are means ± SD of the all data set (<i>n</i> = 135).</p

Influence of the kinetics of light increase on the photoprotection modulation.

<p>(A) Evolution of the number of absorbed photons per Chl <i>a</i> integrated over time (integrated absorbed light, Int Abs Light; expressed in mol photons mg Chl <i>a</i>⁻¹) over the light gradient, at the PFD peaks of 100, 250, 350, 500 and 650 µmol photons m⁻² s⁻¹, during the 5 h (white dots), 3 h (black squares) and 2 h kinetics of light increase (black triangles). Induction of the sustained light-acclimated NPQ (NPQ_sl; B) and evolution of the de-epoxidation state (DES = Dt/[Dd+Dt]; C) <i>versus</i> Int Abs Light during the 5 h (white dots), 3 h (black squares) and 2 h kinetics of light increase (black triangles). Values are means ± SD (<i>n</i> = 3).</p

Sustained light-acclimated non-photochemical fluorescence quenching (NPQ_sl).

<p>Induction of NPQ_sl over the light gradient, in <i>Pseudo-nitzschia multistriata</i> cells experiencing light gradual increases peaking at the PFD of 100, 250, 350, 500, and 650 µmol photons m⁻² s⁻¹, during the 5 h (A), 3 h (B) and 2 h kinetics of light increase (C). Black dots are values estimated for the first and second sampling time point, white dots are values estimated for the last sampling time point. Values are means ± SD (<i>n</i> = 3).</p

Carotenoid content of <i>Pseudo-nitzschia multistriata</i> cells.

<p>β-carotene (β-Car), violaxanthin (Vx) and zeaxanthin (Zx)/chlorophyll (Chl) <i>a</i> (in mol pigment/100 mol Chl <i>a</i>) of <i>Pseudo-nitzschia multistriata</i> cells experiencing light gradual increases peaking at the PFD of 100, 250, 350, 500 and 650 µmol photons m⁻² s⁻¹, during the 5 h, 3 h and 2 h kinetics of light increase (see <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0103782#pone-0103782-g001" target="_blank">Fig. 1B−D</a>). Pigment values are means ± SD.</p