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Second-Order Seasonal Variability in Diel Vertical Migration Timing of Euphausiids in a Coastal Inlet

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Second-Order Seasonal Variability in Diel Vertical Migration Timing of Euphausiids in a Coastal Inlet Vol. 480: 39–56, 2013 MARINE ECOLOGY PROGRESS SERIES Published April 22 doi: 10.3354/meps10215 Mar Ecol Prog Ser Second-order seasonal variability in diel vertical migration timing of euphausiids in a coastal inlet Mei Sato1,*, John F. Dower2,1, Eric Kunze3, Richard Dewey1 1School of Earth and Ocean Sciences, University of Victoria, Victoria, British Columbia V8P 5C2, Canada 2Department of Biology, University of Victoria, Victoria, British Columbia V8P 5C2, Canada 3Applied Physics Laboratory, University of Washington, Seattle, Washington 98105-6698, USA ABSTRACT: Variability in the diel vertical migration timing of euphausiids in Saanich Inlet, British Columbia, Canada, is quantified using 2 yr of echosounder data from a cabled observatory. The continuous and high-resolution nature of the observations allows examination of second- order seasonal variability in migration timing relative to civil twilight times. Early dusk ascent and late dawn descent occur during spring–fall, while late dusk ascent and early dawn descent occur during winter. Ascent timing appears to be regulated by (1) light availability at the daytime depth of the euphausiids, which is modulated by phytoplankton bloom shadowing, and (2) euphausiid size-dependent visual predation risk. Because (1) does not apply at dawn, descent timing appears to be regulated by (2). During the pre-spawning period, higher energy demand for reproduction may cause earlier dusk ascent and later dawn descent to maximize energy gain, even with larger body size. Instead of the traditional view of diel vertical migration timing, correlated solely with civil twilight, our data suggest that euphausiids also adapt their migration timing to accommodate changes in environmental cues as well as their growth. KEY WORDS: Diel vertical migration · Euphausiids · Variability · Echosounder · Time-series Resale or republication not permitted without written consent of the publisher INTRODUCTION surface to deep waters (Longhurst & Harrison 1988). However, its global contribution to biogeochemical Many planktonic species migrate away from food- cycles remains unresolved. rich surface waters during the day to avoid visual If diel vertical migration behavior is primarily a predators (Zaret & Suffern 1976). Quantifying the consequence of the conflicting requirements of feed- variability in such diel vertical migration is essential ing and predator avoidance (Bollens & Frost 1989), to understand predator–prey interactions and assess the time at which organisms migrate between deep the least-known component of the biological pump and surface waters should reflect this trade-off. Light (Steinberg et al. 2000). Euphausiids play a key role in has long been known to be the dominant factor pelagic food webs, being a grazer on phytoplankton, controlling the timing of diel vertical migration (For- a predator on microzooplankton, and a key prey item ward 1988). Since migrating zooplankton typically for invertebrate, fish and marine mammals (Mauch- reside at depth during the day, the light intensity that line 1980, Mackas et al. 1997). Variations in migra- they experience can vary with the attenuation and tion timing can influence encounters with both pred- spectral characteristics of light, which, in turn, are ators and food, so they are a key factor for survival. affected by the presence of dissolved and suspended Diel vertical migration also serves as a vector con- material (Hulburt 1945, Tyler 1975). In addition to the necting deep-water and pelagic communities by ambient light level, food availability and predator actively transporting carbon and nutrients from the density are also thought to be factors affecting diel *Email: [email protected] © Inter-Research 2013 · www.int-res.com 40 Mar Ecol Prog Ser 480: 39–56, 2013 vertical migration. According to the hunger-satiation For example, Sato & Jumars (2008) demonstrated the hypothesis (Pearre 2003), diel vertical migration is value of high temporal and spatial sampling resolu- asynchronous within the population; animals leave tion by showing a shift in the dominant rhythm of the surface waters during the night when they are mysid emergence patterns from diel in summer to satiated, and may go back for a second meal before semidiurnal in fall. dawn after digesting their early-night meal (e.g. If diel vertical migration is a trade-off between Simard et al. 1985, Sourisseau et al. 2008). Differ- energy gain and mortality risk (Bollens & Frost 1989), ences in predation risk between and within species then migration timing should not be tied solely to also cause variations in migration timing (e.g. De civil twilight times. Rather, variability in migration Robertis et al. 2000, Tarling et al. 2001). timing due to light intensity, food availability, preda- Although these various factors have been studied tor density and predation risk should be expected. in the lab and over short periods in the field, long- The goal of this study is to examine second-order term in situ observations of diel vertical migrations variability in migration timing relative to civil remain few. Previous studies based on long time- twilight, and to identify the factors responsible for series have shown changes in migration timing due this variability. Here, we present data from a 2-yr to the seasonal shift in daylight length (Tarling 2003, 200-kHz echosounder time-series collected in Saa- Lorke et al. 2004, Jiang et al. 2007) at first order. nich Inlet by the Victoria Experimental Network However, second-order variability (variability in mi- Under the Sea (VENUS) cabled observatory. gration timing relative to civil twilight times) has re - ceived little attention due to low sampling resolution b and short record length. One of the few examples is consideration of variations in migration timing with euphausiid body size (De Robertis et al. 2000). High 45’ 50 sampling resolution and long time-series are essen- Cowichan 100 50 tial for understanding behavior driven by factors River Fraser whose relative importance can change temporally. 50 42’ Satellite Channel River 100 Q 150 a 39’ 50 Saanich 200 Inlet 51°N British Columbia 36’ 03 km Vancouver Island 50° Fraser N River 48°33’N Pacific Canada Q Weather station 49° Ocean USA MEOS buoy VENUS cabled Juan de Fuca 30’ observatory Strait 48° 123°36’W 33’ 30’ 27’ 24’ 0100 km c 47° 0 129°W 127° 125° 123° 121° Fig. 1. (a) Study site on the southeastern tip of Vancouver Is- 50 land, British Columbia, Canada. (b) The Victoria Experimental Network Under the Sea (VENUS) cabled observatory, the 100 Marine Ecosystem Observatories (MEOS) buoy 46134, and the VENUS cabled shore-based weather station (from which insolation data were Depth (m) observatory gathered) in Saanich Inlet. Bold arrows: major sources of fresh- 150 water input from the Cowichan River (winter) and Fraser River (summer). (c) Vertical section, showing change in bathy metry 200 across Saanich Inlet near the VENUS cabled observatory 123°32’W 31’ 30’ 29’ Sato et al.: Diel vertical migration timing of euphausiids 41 MATERIALS AND METHODS the year, constituting 77 to 100% of all euphausiids followed by Thysanoessa spinifera and T. raschii Study site (Bary et al. 1962, Pieper 1971, De Robertis 2001). E. pacifica live 1 to 2 yr (Tanasichuk 1998) with their Data were collected in Saanich Inlet (48° 39.1’ N, main spawning period from early May to mid-July, 123° 29.2’ W), British Columbia, Canada, from June coinciding closely with the periods of higher phyto- 2008 to May 2010 (Fig. 1). Saanich Inlet is a fjord, plankton abundance (Parsons et al. 1967, Heath with a 75 m sill at its mouth and maximum depths 1977). >200 m (Herlinveaux 1962). It is a reverse estuary The gammarid amphipod Orchomene obtusus is with its major supply of freshwater outside the inlet also abundant in Saanich Inlet (Bary et al. 1962, De mouth. The Cowichan River in winter and the Robertis 2001), but resides at 100 to 125 m depth and Fraser River freshet in summer produce a year- does not migrate (De Robertis et al. 2000, De Robertis round stabilizing salinity gradient in the upper 2001). Since the present study focuses on diel vertical water column. Wind and tidal forcing in Saanich migration behavior in the upper 50 m of the water Inlet are generally weak (Gargett et al. 2003). The column, O. obtusus is unlikely to contribute to the estuarine circulation is normally too weak to permit major acoustic scatterers. Low biomass of copepods flushing of deeper waters below the sill, so that a (Bary et al. 1962) and their low target strength (TS) secondary halocline exists at sill depth (Herlinveaux values at 200 kHz due to their small body size 1962). High primary production (~475 g C m−2 yr−1) (Trevorrow 2005) suggest that their contribution to combined with infrequent deep-water replenish- volume backscattering strength (Sv) is also minimal. ment contribute to the formation of deep-water Deca pods, mysids, shrimps, physonectid siphono - anoxia during much of the year (Timothy & Soon phores, gastropods, hydromedusae, cteno phores, 2001, Grundle et al. 2009). There are typically 2 chae tognaths and cephalopods have been observed renewal events per year in Saanich Inlet where in previous studies, but their rare oc currence and low dense oxygenated waters enter the mouth at the density in scattering layers suggest insignificance as sill depth, shoaling the deep anoxic waters upward: acoustic scatterers (Bary et al. 1962, Pieper 1971, (1) deep-water re newal (below the sill depth but Mackie & Mills 1983, De Robertis 2001). Although not reaching the bottom) during spring and (2) bot- the thecosome pteropod Limacina helicina and the tom-water renewal during fall (Anderson & Devol gas-filled pneumatophores of siphonophores are 1973, Manning et al. 2010). The oxycline plays a strong acoustic targets (Stanton et al. 1994), scatter- major role in Saanich Inlet in determining the day- ing model calculations of L. helicina, which are time depth of the scattering layer, posing a physio- <2 mm in diameter, indicated that their contribution logical barrier for euphausiids (Pieper 1971, Devol to echoes is not significant at 445 kHz (De Robertis 1981, Mackie & Mills 1983).
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