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Calanus Propinquus and Calanoides Acutus in the Weddell Sea

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Calanus Propinquus and Calanoides Acutus in the Weddell Sea Polar Biol 32001) 24: 771±784 DOI 10.1007/s003000100283 ORIGINAL PAPER Anna F. Pasternak á Sigrid B. Schnack-Schiel Seasonal feeding patterns of the dominant Antarctic copepods Calanus propinquus and Calanoides acutus in the Weddell Sea Accepted: 13 April 2001 / Published online: 26 June 2001 Ó Springer-Verlag 2001 Abstract The diets and feeding activity patterns of two rigid in Calanoides acutus, and more ¯exible in Calanus dominant Antarctic copepods, Calanus propinquus and propinquus. Calanoides acutus, were studied throughout the seasonal cycle on material from several ``Polarstern'' cruises to the southeastern Weddell Sea. The observed dierences in feeding patterns were closely linked with the pecu- Introduction liarities of the life-cycle strategies and changed with ambient food concentration. Calanoides acutus under- The feeding patterns of the dominant Antarctic plank- went diapause at depth and was never found feeding in ton copepods are closely linked with their life-cycle winter. The winter descent of the population of Calanus strategies 3Atkinson 1998; Pasternak and Schnack- propinquus was less pronounced, as some of the older Schiel, in press). The two most extensively studied large copepodids stayed in the upper layer and contained copepods, Calanoides acutus and Calanus propinquus, food. The period of active feeding in Calanoides acutus follow basically the same strategy and possess similar was shorter than in Calanus propinquus. The former did foraging features. However, even these closely related not start feeding before late in spring. Feeding activity at species dier in patterns of seasonal 3ontogenetic) ver- the main study region 3south of 67°S) was compared tical migrations 3Voronina 1970; Voronina et al. 1978; with that at the northern 360±67°S) stations with higher Marin 1988; Atkinson 1991; Schnack-Schiel and Hagen food abundance, where feeding activity of Calanus 1994, 1995; Atkinson et al. 1997), lipid storage 3Sch- propinquus was high even in winter. Calanoides acutus nack-Schiel et al. 1991; Hagen et al. 1993; Kattner and contained a higher proportion of diatoms and uniden- Hagen 1995), timing of onset/termination of an over- ti®ed mass in the gut, while the Calanus propinquus diet wintering stage and reproduction 3Voronina 1970; included more dino¯agellates, proto- and metazoan Marin 1988; Atkinson 1991; Schnack-Schiel et al. 1991; prey. The proportion of unidenti®ed mass decreased in Pasternak et al. 1994; Ward et al. 1996; Spiridonov and the older developmental stages and from winter towards Kosobokova 1997) and changes in metabolism 3Ikeda autumn; it was higher in the deep layer. Gut contents of and Mitchell 1982; Hirche 1984; Drits et al. 1993; the two species were compared with seasonal distribu- Pasternak et al. 1994). tion of the food items in the environment. The dier- Diets of Antarctic copepods have been studied with ences in feeding of the two species are directly connected gut content analysis 3Voronina and Sukhanova 1976; to a higher degree of carnivory in Calanus propinquus, Hopkins 1987; Hopkins and Torres 1989; Hopkins et al. especially when phytoplankton food was scarce, and 1993; éresland and Ward 1993; Pasternak and Schnack- re¯ect the speci®city in the life-cycle strategies, which is Schiel, in press). Feeding rates have also been measured using gut ¯uorescence 3Huntley and Escritor 1991; Atkinson et al. 1992; Drits et al. 1993; Pasternak et al. 1994). Few grazing experiments have been carried out 3Schnack 1985; Schnack-Schiel et al. 1991; Atkinson A.F. Pasternak 3&) 1994, 1995). These studies provide a general framework P.P. Shirshov Institute of Oceanology RAS, 36 Nakhimov av., 117853 Moscow, Russia of the trophic relations in the copepod community of the E-mail: [email protected] Southern Ocean. The main sources of variability in S.B. Schnack-Schiel feeding patterns of plankton copepods are seasonal and Alfred-Wegener-Institut fuÈ r Polar-und Meeresforschung, spatial changes in food availability. However, basic 27515 Bremerhaven, Germany traits of even the most studied dominant species, in 772 conjunction with ambient food composition and abun- dance, are still lacking. One of the essential points is in Materials and methods how the various species use temporal and spatial vari- ability of available food. Studies on the responses of Sampling copepods to environmental variability would help ex- The study region is shown in Fig. 1. Zooplankton was collected plain an essential trait of the life-cycle strategies ± their during the RV ``Polarstern'' cruises III/3, V/2, V/3, X/3 and X/7. ¯exibility. Stations were grouped according to time of the year, latitude and The present study takes advantage of the large co- depth. As in our previous paper 3Pasternak and Schnack-Schiel, in press), we distinguished ®ve time intervals: mid-winter 3July/Au- pepod collection at the Alfred-Wegener-Institute for gust), late winter/early spring 3October/beginning of November), Polar and Marine Research. Sampling was carried out spring 3mid-November/December), summer 3January/February) during several RV ``Polarstern'' cruises to the eastern and autumn 3April/May). Zooplankton was collected with a mul- Weddell Sea, and covered almost every season of the tiple opening/closing net of 100-lm mesh size, towed vertically. Samples were taken at random times during the day. The oceanic year. stations deeper than 1,000 m and the shallow shelf stations were Our aim was to analyse how feeding patterns of the considered separately. Usually, ®ve successive hauls were taken at two dominant species, Calanoides acutus and Calanus the oceanic stations from 1,000 m to the surface, but in winter propinquus, re¯ect temporal and spatial environmental 1986, only from 600 m to the surface. They were grouped in three variability. The main objectives were to compare: 31) layers: upper 30±100 m), intermediate 3100±500 m) and deep 3500± 1,000 m). Sampling layers at the shelf stations varied according to seasonal changes in diet composition and feeding ac- the bathymetry; only two layers were distinguished, upper 30± tivity of the two species in relation to food availability; 100 m) and deep. Primarily, the stations to the south of 67°Swere 32) feeding in the upper and deep water layers; 33) analysed; however, some more northern stations sampled in winter feeding in oceanic and shelf regions. were used for a comparison of feeding patterns of studied species under milder ambient conditions. Fig. 1 Study region 773 Feeding activity 3Fig. 2B) in winter, and only few specimens of stages IV and V contained food in their guts. In late winter/ Feeding activity of all development stages of Calanus propinquus and Calanoides acutus was estimated both as a percentage of co- early spring, rare individuals of the youngest stages 3CI pepods of a given stage with food in gut, and as a mean length of a and CII) were not feeding, while a low number of the food pellet in the posterior gut, as it has been shown that this CIIIs contained food. However, a relatively high per- satisfactorily re¯ects feeding activity 3Drits 1985; Ayukai and Ni- centage of the older stages and adults had food in their shizawa 1986; Tsuda and Nemoto 1990). These two indices, i.e. guts. In late spring, the percentage of feeding specimens percentage of copepods with full guts, and the mean length of a food pellet, are often closely correlated 3Drits 1985; Pasternak among the younger stages increased, while no dier- 1995). If the number of copepods with food inside was high, the ence was observed in the older stages. A high per- percentage of feeding specimens was obtained from the ®rst 30±50 centage of CVs with food occurred in summer; other taken at random; otherwise the whole sample was examined. older stages exhibited a similar feeding activity as in spring. The peak of feeding activity in almost all stages Diet was reached in autumn, with maximum values in CI to CIV. At the same time, females seemed to stop feeding. Diet composition was obtained through microscopic analysis of gut An increase in feeding activity with the ongoing season contents. Specimens 310±15) of each developmental stage of the 2 species from each sample were dissected if possible. Preferably, was further con®rmed by an increase in the mean pellet a formed pellet from the mid-gut was analysed, as it was easier length 3Fig. 3A). to extract it undamaged and distribute its contents evenly on a The youngest stages 3CI and CII) were found feeding microscopic slide. This also helps to exclude possible impact of only in the upper water layer 3Fig. 2B). Copepodids of post-capture feeding in nets. Food items were identi®ed as far as possible, counted, and their length and width measured, usually stages III±V contained food at all three depth layers, at ´400 magni®cation. Cell volumes were calculated from and there was no considerable dierence in percentage approximations to simple geometrical shapes. Diatoms, ¯agellates, of animals with food between depths. In summer and dino¯agellates, proto- and metazoans were grouped according to autumn, copepodids from stages III±V had a higher their size and shape. The proportion of unidenti®ed mass was percentage of feeding animals in the upper and inter- estimated by subtracting the summarised volumes of all identi®ed food items from the pellet volume. mediate layers. In spring, more females with food were To reduce the number of food items to manageable propor- found in the intermediate layer, while in summer, their tions, we grouped them into four categories: an unidenti®ed mass, feeding activity was most pronounced in the upper diatoms+dino¯agellates+silico¯agellates, and proto- and meta- layer. zoan prey). Further, we separated the identi®able part of the gut content into small diatoms 37±40 lm in the largest dimension), Calanoides acutus.
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