ISSN 0001-4370, Oceanology, 2010, Vol. 50, No. 4, pp. 531–541. © Pleiades Publishing, Inc., 2010. Original Russian Text © N.M. Pertsova, K.N. Kosobokova, 2010, published in Okeanologiya, 2010, Vol. 50, No. 4, pp. 566–575. 531 INTRODUCTION The multiyear observations of the White Sea zoop- lankton resulted in a significant amount of data on the seasonal dynamics of its abundance, composition, and biomass [21, 41]. Currently, the observed tendencies of the climate change, including the warming of the Arc- tic regions [16], lead us to pay much attention to this dataset to analyze the influence of the environmental factors on the dynamics of the zooplankton popula- tions. It is widely accepted that cold-water species make up 80–90% of the total zooplankton biomass in the White Sea [21, 41]. Among them, Calanus glacialis Jashnov, 1955, is one of the most important for the ecosystem, feeding on the phytoplankton and being the main component of the diet of the herring and other fish species [32, 33]. This arctic copepod prefers low water temperatures close to 0°С. It was supposed that a water temperature increase in the areas inhabited by C. glacialis might lead to crucial changes in its abun- dance and even to the replacement of this species by oth- ers [37]. The last process may result in nonreversible changes in the plankton structure of the White Sea. The multiyear observations performed for the last 50-years period allowed us to analyze the relationships between the temperature dynamics and the C. glacialis population variation. The comparative analysis of the age structure, abundance, and biomass of C. glacialis was performed for the deep areas of the White Sea in different seasons from 1960 through 2007, and C. gla- cialis’s flexibility concerning the interannual tempera- ture fluctuations was assessed. MATERIALS AND METHODS The study was based on the zooplankton samples obtained from the deep areas of the White Sea (the Kandalaksha Bay and Basin) in 1998–2003 at moni- toring stations A, B, and C with depths of about 300 m (Fig. 1). In 1998–1999, the samples were taken in the spring, summer, and autumn periods; in 2000–2001, in the spring; in 2002, in the autumn; and in 2003, in the winter time. In the spring of 2000, the zooplankton was sampled using a stationary grid that covered the whole of Kandalaksha Bay, including its shallow areas (Fig. 1). During these periods, 260 zooplankton sam- ples were taken. Additionally, the samples obtained in the spring, summer, and autumn periods in the deep Kandalaksha Bay in 1960–1962, 1972, 1983–1987, 1990–1992, 1998–1999, and 2001 and at the monitor- ing station (100 m depth) on the border of Kandalak- sha Bay and the Velikaya Salma Strait in 1960–2007 were included into the analysis. In total, more than 500 samples were analyzed. The zooplankton was sampled by means of a Juday net (37 cm diameter; 180 [mu]m mesh size) by stan- Interannual and Seasonal Variation of the Population Structure, Abundance, and Biomass of the Arctic Copepod Calanus glacialis in the White Sea N. M. Pertsova a and K. N. Kosobokova b a Moscow State University, Moscow, Russia b P.P. Shirshov Institute of Oceanology, Russian Academy of Sciences, Moscow, Russia E-mail: xkosobokova@ocean.ru Received October 30, 2008; in final form, November 5, 2009 Abstract—The results of multiyear observations of the seasonal and inter-annual variability of the population structure, abundance, and biomass of the arctic calanoids copepod Calanus glacialis in the White Sea are pre- sented. The spring season represents the most crucial period for the population’s seasonal dynamics. During the spring, the maximal abundance, biomass, and contribution of C. glacialis to the total zooplankton biom- ass is observed. The interannual variability of the abundance is closely related to the timing of the spring warming of the upper water column and the respective shifts of the onset of reproduction and the offspring development. The development of a new generation to the overwintering copepodite stage IV is usually com- pleted three to four weeks later in the cold years compared to the warm ones. Our multiyear observations sug- gest that C. glacialis could be more tolerant of Arctic warming than it is usually believed. The high abundance of the C. glacialis population in the White Sea indicates that this arctic species is able to cope with the seasonal surface warming and should continue to do so, being provided with the cold water “refuge” in the deep sea. DOI: 10.1134/S0001437010040090 MARINE BIOLOGY