Deep–Sea Research I 162 (2020) 103324 Contents lists available at ScienceDirect Deep-Sea Research Part I journal homepage: http://www.elsevier.com/locate/dsri Zooplankton diel vertical migration during Antarctic summer John A. Conroy a,*, Deborah K. Steinberg a, Patricia S. Thibodeau a,1, Oscar Schofield b a Virginia Institute of Marine Science, William & Mary, Gloucester Point, VA, 23062, USA b Center for Ocean Observing Leadership, Department of Marine and Coastal Sciences, School of Environmental and Biological Sciences, Rutgers University, New Brunswick, NJ, 08901, USA ARTICLE INFO ABSTRACT Keywords: Zooplankton diel vertical migration (DVM) during summer in the polar oceans is presumed to be dampened due Southern Ocean to near continuous daylight. We analyzed zooplankton diel vertical distribution patterns in a wide range of taxa Mesopelagic zone along the Western Antarctic Peninsula (WAP) to assess if DVM occurs, and if so, what environmental controls Copepod modulate DVM in the austral summer. Zooplankton were collected during January and February in paired day- Krill night, depth-stratifiedtows through the mesopelagic zone along the WAP from 2009-2017, as well as in day and Salp Pteropod night epipelagic net tows from 1993-2017. The copepod Metridia gerlachei, salp Salpa thompsoni, pteropod Limacina helicina antarctica, and ostracods consistently conducted DVM between the mesopelagic and epipelagic zones. Migration distance for M. gerlachei and ostracods decreased as photoperiod increased from 17 to 22 h daylight. The copepods Calanoides acutus and Rhincalanus gigas, as well as euphausiids Thysanoessa macrura and Euphausia crystallorophias, conducted shallow (mostly within the epipelagic zone) DVMs into the upper 50 m at night. Rhincalanus gigas, T. macrura, and L. h. antarctica DVM behavior was modulated by chlorophyll a con­ centration, mixed layer depth, and depth of the subsurface chlorophyll a maximum, respectively. Carnivorous and detritivorous taxa – including the calanoid copepod Paraeuchaeta antarctica, ostracods, chaetognaths, and Tomopteris spp. polychaetes – as well as seasonally migrating copepods, were most abundant in the mesopelagic zone regardless of the diel cycle. Paraeuchaeta antarctica underwent reverse DVM within the top 100 m. The impacts of Antarctic zooplankton summer DVM and the resident mesopelagic assemblage on carbon export should be better quantified. 1. Introduction (Blachowiak-Samolyk et al., 2006). DVM studies in the polar oceans show seasonal variability in Many zooplankton and fishesthroughout the world’s oceans undergo behavior, with DVM magnitude changing in relation to photoperiod. diel vertical migration (DVM), feeding in productive surface waters at During moored Acoustic Doppler Current Profiler deployments in the night and seeking refuge from visual predators at mesopelagic depths Ross, Lazarev, and Weddell Seas, DVM continued through Antarctic during the daytime (Hays, 2003). A global estimate suggests ~50% of winter but ceased during the period of extended daylight from sound-scattering mesopelagic biomass performs DVM (Klevjer et al., November to February (Cisewski et al., 2010; Cisewski and Strass, 2016; � 2016). Diverse zooplankton taxa independently evolved DVM behavior, Picco et al., 2017). Persistent winter DVM occurs as far north as 77 N as it optimizes the adaptive balance between feeding and predator (Hobbs et al., 2018), although zooplankton DVM is restricted to evasion (Zaret and Suffern, 1976; Stich and Lampert, 1981; Gliwicz, small-scale (6 to 8 m) migrations within the upper 30 m during Arctic 1986; Hays, 2003). While predator avoidance is the accepted evolu­ winter (Ludvigsen et al., 2018). Therefore, the apparent pause of DVM tionary driver for DVM, shifts in downwelling irradiance at sunrise and during Antarctic summer may in fact be due to seasonal changes in DVM sunset are the dominant proximate cues for this behavior (Ringelberg amplitude as some species undertake shallower migrations during and Van Gool, 2003; Cohen and Forward, 2009). Thus, it was assumed summer that go undetected by conventional sampling (Flores et al., that DVM is restricted in polar regions and may cease altogether in 2014; Daase et al., 2016). Asynchronous migrations throughout the diel mid-winter and summer during 24-h darkness and light, respectively cycle may also explain why acoustic records do not detect summer DVM * Corresponding author. E-mail address: [email protected] (J.A. Conroy). 1 Current address: University of Rhode Island, Graduate School of Oceanography, Narragansett, RI 02882 USA. https://doi.org/10.1016/j.dsr.2020.103324 Received 20 January 2020; Received in revised form 1 June 2020; Accepted 3 June 2020 Available online 5 June 2020 0967-0637/© 2020 Elsevier Ltd. All rights reserved. J.A. Conroy et al. Deep-Sea Research Part I 162 (2020) 103324 in the Southern Ocean (Cottier et al., 2006). DVM behavior (Marrari et al., 2011). Additionally, there have not been In addition to photoperiod, other environmental conditions are likely any comprehensive studies resolving polar DVM variability over inter­ to influence the amplitude of zooplankton DVM during polar summer. annual timescales. Phytoplankton blooms may halt DVM as zooplankton remain in surface We analyzed zooplankton diel vertical distribution patterns along waters to feed (Cisewski et al., 2010; Cisewski and Strass, 2016). The the WAP during mid-summer as part of the Palmer Antarctica Long- depth of the subsurface chlorophyll a maximum can also influence Term Ecological Research (PAL LTER) program. Zooplankton were zooplankton DVM, as observed with Arctic copepods (La et al., 2015a). sampled at discrete depth intervals through the epi- and mesopelagic Similarly, vertically migrating Arctic zooplankton concentrate just zones in paired day and night net tows using a MOCNESS (2009–2017) below the mixed layer (Berge et al., 2014). In the southern California to assess taxon-specificzooplankton vertical distribution. Day and night Current, increased light attenuation results in a decreased amplitude of epipelagic net tows (1993–2017) throughout the PAL LTER sampling copepod DVM (Ohman and Romagnan, 2016). The interaction between region provided additional information on DVM behavior. We examined light conditions and phytoplankton distribution is thus likely to be a key environmental controls (e.g., photoperiod, mixed layer depth) on DVM driver of Antarctic zooplankton vertical distribution. amplitude in taxa showing clear DVM. Our results show diverse Few studies address polar mesopelagic zooplankton composition and zooplankton DVM modes, depth distributions, and responses to phyto­ taxon-specific variability in DVM behavior. Arctic zooplankton demon­ plankton biomass and vertical distribution, all of which can affect strate asynchronous DVM patterns, which are explained by variation in zooplankton-mediated carbon export during Antarctic summer. feeding ecology, predation risk, and seasonal migration behavior (Fortier et al., 2001; Cottier et al., 2006; Falk-Petersen et al., 2008). 2. Materials and methods Prior studies assessing zooplankton vertical distribution along the Western Antarctic Peninsula (WAP) have either focused on specific 2.1. Study region taxonomic groups (e.g., Nordhausen, 1994a; Lopez and Huntley, 1995) � or lacked the comparable day and night sampling necessary to assess The PAL LTER study region ranges from Anvers Island (64.77 S, Fig. 1. PAL LTER study area along the Western Antarctic Peninsula. ‘North,’ ‘South,’ and ‘Far South’ regions are indicated. Circles indicate epipelagic sampling stations (1993–2017). Stars indicate paired day-night MOCNESS sampling locations on the shelf (2009–2017). Cross indicates paired day-night MOCNESS sampling station on the slope (2017). Shading indicates bathymetry. An: Anvers Island; Ad: Adelaide Island; MB: Marguerite Bay; Ch: Charcot Island. 2 J.A. Conroy et al. Deep-Sea Research Part I 162 (2020) 103324 � � � 64.05 W) in the north to Charcot Island (69.45 S, 75.15 W) in the south, 9023 m3 (range 1715–71929 m3), calculated using a General Oceanics extending from the WAP coast to the continental slope (Ducklow et al., flow meter suspended in the net opening. Ship speed was 2–2.5 knots 2012) (Fig. 1). The PAL LTER research grid is composed of sampling while towing, and typical tow duration was 30–35 min. lines running perpendicular to the Peninsula every 100 km, and stan­ Epipelagic samples were designated night tows when the sun was dard grid stations within each line are separated by 20 km (Waters and below the horizon, accounting for atmospheric refraction (calculated � Smith, 1992). From 1993-2008, the sampling plan included all stations solar elevation � À 0.833 at the start of the tow) (Atkinson et al., 2008; on grid lines 600 to 200. In more recent years, the study area has Steinberg et al., 2015). Sample size varied by taxon (day n ¼ 966–1071 expanded to include lines 100, 000, and À 100, with sampling resolution and night n ¼ 181–198), as not all taxa were identifiedduring shipboard reduced to three stations per line. As in previous studies, three lat­ processing throughout the time series. itudinal sub-regions were designated to represent hydrographic, sea ice, and ecological gradients (Martinson et al., 2008; Stammerjohn et al., 2.3. Taxonomic composition 2008; Steinberg et al., 2015) (Fig. 1), with regional boundaries along sampling grid lines as follows: ‘North’ (lines 600 to 400), ‘South’: (lines 2.3.1. Discrete depth samples 300 and 200),