Distribution and trophic ecology of chaetognaths in the western Mediterranean in relation to an inshore–offshore gradient

Journal of Plankton Research Vol.22 no.2 pp.339–361, 2000 Distribution and trophic ecology of chaetognaths in the western Mediterranean in relation to an inshore–offshore gradient Abstract. This study examines the distribution patterns and feeding ecology of chaetognaths in the Catalan Sea in relation to mesoscale features along an inshore–offshore gradient. The study was conducted during two different periods of the year: late spring of 1995 and late summer of 1996. The two periods differed in hydrographic conditions and mesoscale processes, which affected the distribution patterns of the different species of chaetognaths found. The diet of the chaetognaths was mainly composed of copepods and differed between species. Prey size was not always strongly related to chaetognath size and for certain species, there was an overlap in prey size spectrum. Trophic niche breadth (on a ratio scale) appeared to be constant with growth. Ingestion rates and predation pressure by chaetognaths did not follow a clear trend related to the mesoscale features in the area, such as the presence of a density front. The impact of chaetognaths on copepod standing stock appeared to be extremely low (<1%), but it became more relevant when the species and prey size specificity of the chaetognaths was taken into account. Introduction The role of predation as a decisive factor determining the structure of the marine planktonic food webs was not emphasized until quite recently (Verity and Smetacek, 1996). Within carnivorous zooplankton, chaetognaths play a major role both in their biomass contribution [up to 30% of the biomass of copepods in the global oceans (Reeve, 1970)], and also in their impact on zooplankton communities as one of the main predators of copepods (Pearre, 1980; Stuart and Verheye, 1991). Predator–prey interactions are largely determined by the characteristics of the prey and among these characteristics, prey size has proved to be particularly relevant [(Pearre, 1974) and references therein]. The study of foraging patterns in relation to prey size appears necessary in order to establish patterns of food selection, trophic niche and, eventually, food limitation and competence. Although these topics have been addressed in marine fish larvae (Pearre, 1986; Pepin and Penney, 1997), previous work on chaetognaths has concentrated largely on prey selection and predator–prey relationships (Pearre, 1980; Kehayias et al., 1996) and less so on aspects such as trophic niche breadth and niche overlap. Chaetognaths have another characteristic that makes them particularly interesting from the oceanographic point of view. They have proved to be good indicators of water masses (Pierrot-Bults, 1982) and consequently, appear to be especially suitable for the study of the effects of physical processes acting at the mesoscale on the dynamics and variability of zooplankton populations. It is less known, however, how physical processes at the mesoscale, which can affect the distribution and abundance of zooplankton populations, reflect processes at smaller scales, such as feeding and reproductive performance, as well as predation pressure and, eventually, growth of the zooplankton populations. © Oxford University Press 2000 339 Alícia Duró and Enric Saiz Institut de Ciències del Mar, CSIC, Departament de Biologia Marina i Oceanografia, Ps. Joan de Borbó s/n, 08039 Barcelona, Catalunya, Spain A.Duró and E.Saiz Method Cruises Sampling was conducted during two cruises in the Catalan Sea on board the B/O García del Cid: FRONTS-95 (18–23 June 1995) and FRONTS-96 (16–21 September 1996). The area surveyed in the second cruise was located further to the north than in the first cruise (Figure 1). In each cruise a grid of three transects perpendicular to the shoreline, with four stations each, was sampled. The stations were located, respectively, in coastal waters, shelf waters, at the frontal system and in oceanic waters (Figures 2a and 3a). The station at the frontal system was located at the shelfbreak in FRONTS95, while it moved offshore in FRONTS-96. In each cruise the grid of stations was surveyed two times consecutively, each survey taking about 3 days. The three transects and both surveys for the respective coastal, shelf, frontal and oceanic stations, were pooled as replicates within a cruise when we attempted to determine statistical differences along the transects. At each station, routine CTD casts (Neil Brown) were conducted for temperature and salinity profiles. Chaetognaths were sampled with a BONGO net (60 cm diameter) fitted with 500 µm netting. The net tows were performed obliquely from 200 m depth to the surface at the frontal and oceanic stations, and from 10 m 340 Mesoscale singularities, such as density fronts and eddies might affect not only the distribution and abundance of zooplankton, but also modify the encounter rates between predator and prey due to the presence of associated small-scale turbulence [(Kiørboe, 1997) and references therein]. The objective of this study was twofold: first, we addressed the distribution of chaetognaths in the northwestern Mediterranean in relation to mesoscale physical processes occurring across an inshore–offshore gradient; second, we determined the trophic ecology of co-existing chaetognath species, and attempted to determine their feeding impact on other zooplankters along the mentioned gradient. The study was carried out in the Catalan Sea (northwestern Mediterranean) in June 1995 and September 1996. In the northwestern Mediterranean, there is a semi-permanent density front associated with the Liguro–Provençal–Catalan current, that flows SW along the shelfbreak. This area is characterized by high variability at the mesoscale (Font et al., 1988). The cruises corresponded, respectively, to the periods of stratification and hydrographic transition in the western Mediterranean, and we expected to find differences in the types and intensity of mesoscale physical processes between both periods. Previous studies in the area had shown that some groups of zooplankton tended to concentrate in the vicinity of the hydrographic front, while their distribution was more heterogeneous in coastal waters (Boucher, 1984; Sabatés et al., 1989; Saiz et al., 1992; Sabatés and Olivar, 1996). The accumulation of zooplankton in the frontal area, either active or passive, might lead to the development of characteristic zooplankton communities more stable and of higher productivity than in the surrounding waters. Trophic ecology of chaetognaths above the bottom to the surface when shallower. During the tow, the ship was kept at 2 knots and the towing speed was 20 m min–1. The volume filtered during the tow was estimated by means of a flow meter. Samples were preserved in 5% formalin buffered with borax. Sample analysis All chaetognaths present in the samples were counted and identified to the species level. Their sexual developmental stage was determined accord (...truncated)