Night versus day abundance estimates of zooplankton at two coastal stations in British Columbia, Canada

Zooplankton abundance at 2 coastal stations in southern British Columbia, Canada, was estimated during the day and the night using oblique bongo net hauls. There were no statistically significant differences between night and day abundances when calanoid copepods or euphausiids were grouped by species or life history stage. However, statistically significant differences between night and day abundances were found when species and life history stages were considered separately. The euphausiids had the greatest differences between night and day catches. The maximum night to day (N:D) catch ratios were estimated to be 10.6 for Euphausia pacifica and 4.9 for Thysanoessa spinifera. N:D ratios for E. pacifica showed large interannual and station variability, and varied by an order of magnitude. It is likely that several factors influence these N:D catch ratios, including tidal height, moonlight, and the temporal and spatial patchiness of the euphausiids. The majority of calanoid copepod genera examined had near equal N:D ratios. The highest copepod N:D catch ratios were found for large female copepods (e.g. Metridia pacifica or Eucalanus bungii), and they ranged from 2 to 8. A few smaller calanoid copepod species had abundance estimates that were statistically higher during the day than the night (e.g. N:D = 0.3, Oithona spp.). Overall, we conclude that N:D catch ratios of coastal zooplankton can vary widely within a species over space and time, and that the catch ratios appear to be related to body size: larger animals have higher catches at night. It is recommended that coastal studies using bongo nets to discretely sample copepods or euphausiids should include a 24 h component in the sampling program to quantify differences between night and day abundance estimates.