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

Abstract

In situ diel variations of extracted chlorophyllous pigments, beam attenuation by particles (cp), and in vivo chlorophyll fluorescence (Fiv) 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, cp, and Fiv 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 cp and Fiv. Tchl a reached a maximum around 1500 hours ± 1 hour, clearly before cp (1700 hours) and Fiv (1900 hours). In the 0–30 m layer, diel variations of the integrated chlorophyll concentrations, cp, and Fiv 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, cp increased only during the light period in the upper 30 m, and variations of Fiv were largely opposite to those of extracted Tchl a. Specific phytoplankton growth (μ0) 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.