Individual Biovolume of Some Dominant Copepod Species in Coastal Waters off Buenos Aires Province, Argentine Sea

Copepods are key components in the marine communities because of their important role in the transfer of matter and energy from primary producers to higher trophic levels and in the export of organic matter from the euphotic to deeper layers of the oceans (CALBET et al., 2000). Because of their role as prey for fishes at different stages of development, knowledge of zooplankton abundance and biomass in spatial and temporal scales remains a key element of the marine ecosystem approaches (IRIGOIEN et al., 2009). In fisheries science, accurate estimations of abundance, biomass and production of the different components of the food webs are necessary for the construction and implementation of ecosystem models (CHRISTENSEN; PAULY, 1992). Paracalanus parvus, Ctenocalanus vanus, Calanoides carinatus and Oithona nana are dominant copepod species (50-100 %) in the coastal waters of the Argentine Sea (RAMÍREZ, 1981; VIÑAS et al., 2002). These copepods play an important role in the pelagic food web as the main prey item for larvae (CIECHOMSKI; WEISS, 1974; VIÑAS; RAMÍREZ, 1996) and juveniles and adults of anchovy (ANGELESCU, 1982; PÁJARO, 2002). Thus, an accurate estimation of their biomass and productivity is necessary to quantify the transfer of matter and energy across the planktonic food webs. So far, there is only one regional work in which the individual biomass of copepods has been estimated (FERNÁNDEZ ARÁOZ, 1991) in the Argentine Sea, but early copepodite stages were not included because of the mesh size (≥ 220 μm) employed. Our aim was to estimate the individual biomass of all the stages of the above-mentioned copepods by the geometric method and to establish, for each species, significant regression models predicting biovolume from some linear body dimension. Volumetric methods, such as the one employed in the present study, are the only choice if samples are also to be used for taxonomic purposes (POSTEL et al., 2000) and the geometric approach is the only suitable in the case of small-sized zooplankton (OMORI; IKEDA, 1984). The conversion of our results into another biomass proxy from the literature may easily be made. In fact, body wet weight can be derived from measurements of body biovolume by applying a factor of 1 for specific gravity (OMORI and IKEDA, 1984). Dry weight can be obtained by multiplying the wet weight by 0.20 and the carbon content can be considered as 40 % of the dry weight (POSTEL et al., 2000). Samples were obtained on October 18 and November 11 at the permanent coastal station EPEA (38o28’S – 57o41’W), with a Babybongo net (0.18 m diameter) provided with 220 μm and 67 μm meshes. The smallest mesh size was selected in order to retain all the stages of the dominant copepod species Paracalanus parvus, Oithona nana, Ctenocalanus vanus and Calanoides carinatus. Samples were fixed in 4% formaldehyde immediately after collection. A minimum of 30 females and males of each species were measured. The number of copepodite stages measured was variable (Table 1). Measurements were made under a microscope for the smaller species Oithona nana, Paracalanus parvus and Ctenocalanus vanus, and under a stereoscopic microscope for Calanoides carinatus. In both cases calibrated eye-piece graticules were used in which 1 division equalled 5.88 μm and 18 μm, respectively. Two persons performed the measurements working no more than 3 hours a day each in order to avoid fatigue as a source of error in the determinations. Prosome length, width and height, as well as urosome length and width, were measured, in order to apply the model of CHOJNACKI; HUSSEIN (1983) slightly modified (antenna and leg volumes excluded) by FERNÁNDEZ ARÁOZ (1991), in which: