ARTICLE IN PRESS Deep-Sea Research II ] (]]]]) ]]]–]]] Contents lists available at ScienceDirect Deep-Sea Research II journal homepage: www.elsevier.com/locate/dsr2 Distribution and egg production of Pseudocalanus species in the Chukchi Sea Russell R. Hopcroft a,Ã, Ksenia N. Kosobokova b a Institute of Marine Science, University of Alaska Fairbanks, PO Box 757220, Fairbanks, AK 99775-7220, USA b PP Shirshov Institute of Oceanology, Russian Academy of Sciences, Nakhimovski Prospekt, 36, Moscow 117997, Russia article info abstract The distribution and egg production of Pseudocalanus species was studied in the Chukchi Sea Keywords: during August 2004. Three species were identified throughout the study area, Pseudocalanus Pseudocalanus minutus, Pseudocalanus acuspes and Pseudocalanus newmani. The more subpolar/temperate Egg production P. newmani-dominated stations where water was warmer and likely influenced by Alaska coastal Arctic Ocean waters, and it penetrated northward into the south-eastern corner of Herald Valley. The Arctic P. acuspes Chukchi Sea dominated the colder waters. The number of eggs produced by individuals and populations differed among species, primarily because of differences in body size, but mass-specific egg production was similar among species, and was on the order of 15% per day. Future increases in the prevalence of P. newmani within the Chukchi Sea might be anticipated, and would result in a decreased average size of the copepod community, with potential implication for higher trophic levels. & 2009 Elsevier Ltd. All rights reserved. 1. Introduction those of the central Arctic Ocean, and its marginal seas (Weingartner et al., 1998; Pickart et al., 2009). The water masses Copepods of the genus Pseudocalanus are one of the most entering the region through the Bering Strait, tend to each have dominant constituents of the zooplankton community of the unique assemblages and quantities of zooplankton (Springer et al., Chukchi Sea, both in terms of numbers and biomass (Hopcroft 1989; Coyle et al., 1996; Hopcroft et al., in press) and slowly et al., in press). Pseudocalanus are common throughout coastal acquire more Arctic faunal characters as they are transformed/ waters from temperate to polar ecosystems, where they are modified moving northward (e.g. Smith and Schnack-Scheil, 1990; thought to play an important role in the trophic ecology of these Lane et al., 2008). Of the Pseudocalanus species reported for this systems due to their high rate of production relative to many region, P. newmani and P. mimus are more subpolar to temperate larger crustacean zooplankton (e.g. Corkett and McLaren, 1978). in distribution, with some inshore/offshore preferences (e.g. Napp The genus Pseudocalanus is now recognized to contain a complex et al., 2005). They are likely seasonal imports to the region, and of seven species (Frost, 1989), each with some differences in size, this raises the possibility that Pseudocalanus species could be reproductive output, and phenology (McLaren et al., 1989a; tracers of water mass origin as well as the magnitude of advection Bucklin et al., 1998; Napp et al., 2005). Five of these species are within this region. recorded at present in the Chukchi Sea, Pseudocalanus minutus, The advection of subpolar/temperate Pseudocalanus species Pseudocalanus acuspes, Pseudocalanus major, Pseudocalanus new- into a community of more polar affinity raises interesting mani, and to a lesser extent Pseudocalanus mimus (Frost, 1989). questions about their short-term and long-term viability, and However, their occurrence is seldom reported as more than their competitive success in the Chukchi ecosystem. Ideally one Pseudocalanus spp. due to the difficulty involved in separating would explore the somatic growth rates of Pseudocalanus in their adults by species, and the near impossibility of routinely habitat (e.g. Liu and Hopcroft, 2008), but our inability to separate separating juveniles (e.g. Smith and Vidal, 1986; Lane et al., juveniles of these species precludes such an approach. A more 2008) by other than molecular techniques (e.g. Bucklin et al., 1998, practical approach for exploring species success would be to look 2003). at the reproductive output of these egg-carrying species. Differ- The Chukchi Sea represents a complex gateway into the Arctic, ences in clutch size are known to occur between Pseudocalanus due to the interplay of several distinct water masses of Pacific species (e.g. Napp et al., 2005), as well as differences in egg and origin (i.e. Alaska Coastal, Bering Shelf, and Anadyr Water) with body size (Frost, 1989; McLaren et al., 1989a, b; Napp et al., 2005). Furthermore, even when co-occurring, the more polar species tend to produce only one generation per year, while the more subpolar/temperate species have multiple generations per year à Corresponding author. E-mail addresses: [email protected] (R.R. Hopcroft), (McLaren et al., 1989a). The net effect of these reproductive [email protected] (K.N. Kosobokova). attributes is unknown, but there is clearly potential for differences 0967-0645/$ - see front matter & 2009 Elsevier Ltd. All rights reserved. doi:10.1016/j.dsr2.2009.08.004 Please cite this article as: Hopcroft, R.R., Kosobokova, K.N., Distribution and egg production of Pseudocalanus species in the Chukchi.... Deep-Sea Research II (2009), doi:10.1016/j.dsr2.2009.08.004 ARTICLE IN PRESS 2 R.R. Hopcroft, K.N. Kosobokova / Deep-Sea Research II ] (]]]]) ]]]–]]] in both numerical and weight-specific egg production rates diameter and eggs per clutch determined with a computer- among the different species. To address these questions, we assisted measuring system (Roff and Hopcroft, 1986). Mean clutch determined the distribution of Pseudocalanus species adults size was calculated for experiments with at least 4 clutches within the Chukchi Sea and explored their species-specific produced. Egg production rate (EPR) was calculated as (Mean patterns of egg production. clutch size)  (Fraction of females extruding a clutch)/the incuba- tion time in days (Runge and Roff, 2000) for all cases where more then 10 females of a species were present. In practice, egg 2. Methods production declined precipitously after the second day, so analysis was limited to 48 h. Female dry-mass (DW, in mg) was predicted The Russian American Long-term Census of the Arctic from prosome length (PL, in mm) using a relationship established (RUSALCA) survey consisted of 22 stations along three transects for a mixture of P. newmani and P. mimus in the Gulf of Alaska in the southern Chukchi Sea between Alaska and Russia, plus (log10 DW ¼7.62+2.85 log10 PL: Liu and Hopcroft, 2008). Egg four more northern high-speed transects across the Herald Valley mass was predicted from diameter, assuming a density of À3 (Fig. 1). Non-quantitative 200 mm mesh ring nets were taken at 0.14 ng C mm (Kiørboe and Sabatini, 1994). Egg carbon was alternate stations on the three southern transects, and at the ends converted to ash-free dry-mass assuming carbon as 40% of AFDW. plus midpoint of the upper transects in Herald Valley, for Specific egg production rate (SEP) was calculated for each female reproductive experiments at 21 stations. Nets were slowly using the ratio of her predicted weight to that predicted for her hauled vertically in the upper 25 m of the water column and eggs. catches were diluted. Despite the relative gentleness of collection, Standardization of rates for temperature differences proved egg sacs were dislodged from most Pseudocalanus females that problematic due the large differences in temperature across the may have been carrying eggs, leaving only a few females (usually experimental domain, but restriction to a single incubator placed o5%, always o10%) in the samples with partial remnants of at an intermediate temperature. If we presume there is physio- clutches. Sorting of female Pseudocalanus (genus level only) was logical ‘‘inertia’’ in egg carriers to the multi-day process of usually completed within 2–3 h of capture; further sorting to preparing eggs to be laid (i.e. 11 days at 0 1C, 3 days at 12 1C; species level was not possible on live specimens. Single sac-less McLaren et al., 1989b), then correction should be relative to Pseudocalanus females (N ¼ 30–100) were transferred with a wide ambient temperature. If metabolic processes respond rapidly to bore pipette into 70 ml polystyrene tissue culture flasks, pre-filled change in temperature, then correction should be relative to the with 100 mm prescreened water from the mixed layer (e.g. Napp incubation temperature. In all likelihood, the appropriate correc- et al., 2005). Incubations were performed within 1–2 1C of the tion will be somewhere in between these possibilities. SEP was mixedlayer temperature on a low-light photocycle comparable to standardized from both possible reference temperatures to 5 1C ambient. In the southern domain, temperature was held at 3–6 1C, (the intermediate temperature observed over the study area) then lowered to 1–2 1C in the northern, Herald Valley domain. using a Q10 of 2.7 for food-satiated juvenile broadcast spawners Females were checked every 24 hours over 3 days; females (Hirst and Bunker, 2003). Concurrent estimates of the physical producing egg sacs before the conclusion of the experiment were environment and chlorophyll were provided by other investiga- removed and preserved individually. All non-producing females tors (Lee et al., 2007; Pickart et al., 2009). were preserved together at the conclusion of each experiment. Finally, if a large percentage of the female Pseudocalanus were For calculating egg production rates, females removed and reproductively inactive, the exclusion of individuals with sac preserved during and at the conclusion of the experiments were remnant may introduce bias in our egg production rates. Based on identified to species (Frost, 1989), with prosome length, egg the proportion of females that produced clutches in our experi- ments (see below) and the known hatching rates of Pseudocalanus (McLaren et al., 1989b), most females appear to have been reproductively active.