Diel dynamics of chlorophylls in high‐nutrient, low‐chlorophyll waters of the equatorial Pacific (180°): Interactions of growth, grazing, physiological responses, and mixing

In situ diel variations of extracted chlorophyllous pigments, beam attenuation by particles ( c p ), and in vivo chlorophyll fluorescence ( F iv ) were investigated during a 5‐day time series in high‐nutrient, low‐chlorophyll waters of the equatorial Pacific (date line = 180°). Samples were taken hourly at 10 depths in the upper 100 m during the first 48 hours, then sampling frequency decreased to 3 hours. In the 30–70 m layer the integrated chlorophyll concentrations, c p , and F iv increased during the light period, but the minima and, especially, maxima were not fully synchronized. The lowest values of total chlorophyll a (Tchl a = chlorophyll a + divinyl‐chlorophyll a ) occurred around 5–6 hours, slightly (0–2 hours) before that of c p and F iv . Tchl a reached a maximum around 1500 hours ± 1 hour, clearly before c p (1700 hours) and F iv (1900 hours). In the 0–30 m layer, diel variations of the integrated chlorophyll concentrations, c p , and F iv were clearly out of phase. They showed a nocturnal increase in Tchl a , starting around midnight and peaking in early morning (0900 hours). In contrast, c p increased only during the light period in the upper 30 m, and variations of F iv were largely opposite to those of extracted Tchl a . Specific phytoplankton growth (μ 0 ) and grazing loss (g) rates were estimated from diel variations in the 30–70 m layer and compared to independent rate estimates from experimental incubations. These results are discussed in the context of physical processes and physiological responses of the cells to the daily photocycle.