Diel variation of zooplankton distributions in Hawaiian waters favors horizontal diel migration by midwater micronekton

Abstract

Micronekton in deep-scattering layers around the Hawaiian Islands undergo diel migrations with both vertical and horizontal components. We sought to determine whether resource availability provides an adaptive explanation for this migration. We simultaneously measured the spatio-temporal patterns of micronekton, using acoustics and imaging optics, and of their potential zooplankton prey, using net tows, acoustics, and optics. Zooplankton biomass, density, and total abundance were higher at night than prior to sunset at nearshore sites, whereas relatively little diel variation was observed offshore. All measures of zooplankton availability were 5 to 6 times higher nearshore than offshore during nighttime hours when migrating micronekton species were nearshore. There was no significant nearshore-offshore gradient in zooplankton prior to sunset, leading to 2 possible explanations for the day-night patterns in zooplankton: benthic emergence and vertical migration coupled with horizontal motion. Analysis of taxonomic patterns from net tows did not support the benthic emergence hypothesis. All 3 zooplankton assessment techniques supported the conclusion that zooplankton distribution could favor horizontal migration by micronekton given the pressures for micronketon to be in deep water during daylight to avoid predators. Recently published work has shown that small animals (2 to 10 cm in length) in scattering layers comprised of micronekton travel distances of at least 11 km roundtrip each night, often against currents, to obtain these increased food resources. The length and likely cost of the journey provides some insight about the importance of the potential feeding gains.

A critical scale in plankton aggregations across coastal ecosystems

“…[]) and Oahu, Hawaii (methods in Benoit‐Bird et al . []). On the shelf and slope of New Jersey, fluorescence data were collected along with single‐frequency acoustic scattering (tuned for the identification of zooplankton) as part of the Shallow Water 2006 experiment (detailed in Shroyer et al .…”

Bottom-up regulation of a pelagic community through spatial aggregations

“…Ship-based sampling with a downwardlooking acoustics package and a high-resolution profiler instrumented with optical, acoustical and hydrographic sensors [5,7,8] was conducted close to continuously sampling instruments moored at the 25 m isobath off leeward Oahu over three, 24 h periods, as well as during four other days and nights dispersed over the study period. A moored autonomous profiler collected hydrographic and chlorophyll fluorescence data every half-hour between the bottom and the surface with less than 1 cm vertical resolution [7].…”

“…While some researchers have argued that environmental heterogeneity must be considered in assessing the relative roles of ecological forces [1,4], a widespread and fundamental ecosystem characteristic, patchiness, or the spatial variability in biomass at relatively small scales, has been difficult to incorporate in studies of these controls. We examined the relationships among four trophic levels that are found in distinct, extreme spatial aggregations [5,6] to examine the relative importance of biomass and patchiness in the regulation of a pelagic marine food web.…”

Using active acoustics to compare lunar effects on predator–prey behavior in two marine mammal species

“…Both of these areas are known as daytime aggregation sites for spinner dolphins (Lammers 2004) and active nighttime feeding areas (BenoitBird & Au 2003b). Data used in this study were incidentally collected from studies sampling mesopelagic micronekton (see Benoit-Bird et al 2008.…”