Diel variations of copepod feeding and grazing impact in the high‐nutrient, low‐chlorophyll zone of the equatorial Pacific Ocean (0°; 3°S, 180°)

[1] Diel variations of copepod biomass and feeding were studied at two time series stations in the high-nutrient, low-chlorophyll region of the central equatorial Pacific (0° and 3°S, 180°). During 48-hour studies at each station, samples were taken at 3-hour frequency in the neuston layer (0–1 m) and over the 0–100 m depth range. Feeding rates were assessed through the spectrofluorometric analysis of gut pigment contents. During the cycles, significant variations of biomass and gut pigment contents were found in both the neuston layer and the 0–100 depth strata. However, the amplitude of day-night variations of feeding was larger and more clearly related to the day-night cycle in the neuston layer than in the water column. Maxima in biomass and gut pigment contents did not coincide in either the surface or the water column. The differences in the mean copepod feeding activities at the equator and at 3°S were related to the mean standing stocks of chl a and to the pattern of undegraded pigments, which were different at the surface and in the water column, especially at the equator. In the surface layer, gut pheopigment minima were observed during the day, and maxima at night. This pattern clearly followed the periodicity of diel vertical migration, with intense feeding observed after the main upward ascent around sunset. Conversely, the feeding pattern in the integrated water column was more related to food abundance with a higher feeding during the day and around sunset, especially at the equator. Gut pigment contents in the water column displayed late afternoon maxima and dawn minima, coinciding with the maxima and minima of depth-integrated chlorophyll a (chl a). On the basis of standing stocks of chl a containing particles larger than 3 μm, mean grazing pressure (i.e., specific grazing rates multiplied by copepod dry weights and divided by in situ chl a) varied between 3.0 and 4.0% in the upper 100 m and between 3.5 and 3.8% in the neuston layer. Because of the cyclical nature of feeding activities, short-term variability in grazing pressure estimates were substantial, with individual estimates ranging from a high of 17.5% d−1 to a low of 0.2% d−1 in the neuston layer and from 0.9 to 7.8% d−1 in the 0–100 m water column.