Seasonal and vertical variations in emergence behaviors of Neomysis americana

Abstract

Emergence patterns were observed in the macrotidal Damariscotta River estuary in Maine in fall 2005 and summer 2006. High temporal and vertical resolution was achieved with two multifrequency echo sounders sampling from 265 kHz to 3 MHz. Time-series waterfall plots and spectral analysis revealed that emergence patterns observed at 265-420 kHz (frequencies that detect animals larger than mesozooplankton) followed combinations of diel and tidal rhythms that differed with both depth in the water column and season. The dominant emergent species at the study site, Neomysis americana, showed nocturnal emergence in summer. Toward the end of its emergence season in fall, however, its dominant rhythm shifted to semidiurnal (12.4-h period). The timing of major emergence in the fall coincided with low slack tides near the surface but with peak flood speed near the bottom, providing plausible mechanisms of retention and selective tidal stream transport within the estuary. Our results confirm earlier suggestions from net sampling that emergence in this species is best conceived as a broadening of the vertical spread of the population distribution rather than a migration of the bulk of the population far off the bottom: Most individuals spent most of their time near the bottom, even during emergence. Prior acoustic work that did not resolve the lowermost water column reported emergent mysids to overwhelm the holoplankton in biovolume, but it, too, seriously underestimated mysid abundance because it failed to resolve this high concentration of animals in the lowermost water column. In the mesozooplankton size category, emergence reached surface waters only during low slack tides and systematically avoided times of the fastest currents. This study highlights the importance of high temporal and spatial sampling resolution in detecting and understanding components of emergence and points away from simple models of diel vertical migration toward state-dependent, individual-based models of habitat utilization. © 2008, by the American Society of Limnology and Oceanography, Inc.