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

Alcia Dur 0 Enric Saiz 0 0 Institut de Ciencies del Mar, CSIC, Departament de Biologia Marina i Oceanografia , Ps. Joan de Borbo s/n, 08039 Barcelona, Catalunya , Spain 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. 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. Introduction Method Cruises Sample analysis Data analysis 2 3 NPC I = 3 24 DT Results Hydrography Abundance and distribution of chaetognaths Trophic ecology Pterosagitta draco Sagitta bipunctata Sagitta enflata Sagitta lyra Sagitta minima ind per m2 ind in 1000 m3 % Stage I % Stage II % Stage III ind per m2 ind in 1000 m3 % Stage I % Stage II % Stage III ind per m2 ind in 1000 m3 % Stage I % Stage II % Stage III ind per m2 ind in 1000 m3 % Stage I % Stage II % Stage III ind per m2 ind in 1000 m3 % Stage I % Stage II % Stage III Sagitta setosa Coastal (n = 7) Shelf (n = 7) Front (n = 7) ind per m2 23.6 10.37 ind in 1000 m3 742 316.1 % Stage I 18 % Stage II 31 % Stage III 52 85.8 25.72 111.5 24.40 1085 292.5 557 122.0 27 22 26 29 47 49 Oceanic (n = 7) Sagitta bipunctata Sagitta decipiens Sagitta enflata Sagitta lyra Sagitta minima ind per m2 ind in 1000 m3 % Stage I % Stage II % Stage III ind per m2 ind in 1000 m3 % Stage I % Stage II % Stage III ind per m2 ind in 1000 m3 % Stage I % Stage II % Stage III ind per m2 ind in 1000 m3 % Stage I % Stage II % Stage III ind per m2 ind in 1000 m3 % Stage I % Stage II % Stage III Coastal (n = 6) Shelf (n = 6) Front (n = 6) Oceanic (n = 6) Sagitta setosa ind per m2 ind in 1000 m3 % Stage I % Stage II % Stage III Fig. 4. Abundance and horizontal distribution of Sagitta setosa in cruise FRONTS-95. Left panel corresponds to the first survey; the right one to the second survey. Crosses indicate that no specimens were caught. Fig. 5. Abundance and horizontal distribution of Sagitta lyra in cruise FRONTS-95. Left panel corresponds to the first survey; the right one to the second survey. Crosses indicate that no specimens were caught. 1995 n FCR SE NPC SE 1996 n FCR SE NPC SE Sagitta bipunctata Day Night Sagitta enflata Sagitta lyra Sagitta setosa n, number of stations. Day Night Day Night Day Night 5 37.1 8.88 12 4.0 0.87 2.2 0.50 9 31.0 5.11 8 28.1 4.59 13 10.7 2.68 11 16.9 3.35 0.04 0.009 0.02 0.006 0.32 0.053 0.30 0.046 0.11 0.027 0.17 0.035 14.1 3.02 35.8 8.68 5.9 2.42 9.7 1.17 16.8 7.32 28.6 12.43 8.2 1.20 18.3 4.32 0.14 0.031 0.36 0.088 0.06 0.024 0.10 0.011 0.18 0.082 0.31 0.129 0.08 0.010 0.19 0.050 FRONTS-95 Species Contents % FRONTS-96 Species Contents % Sagitta bipunctata Unidentified 100 88.6 10.5 Sagitta enflata Sagitta lyra Sagitta setosa Unidentified Copepods Cladocerans Other crustaceans Chaetognaths Unidentified Copepods Euphausiids Other crustaceans Chaetognaths Cephalopod larvae Fish larvae Unidentified Copepods Cladocerans Other crustaceans Chaetognaths 36.3 54.0 4.0 Unidentified stands for prey remains not identified. Sagitta bipunctata Unidentified Copepods Chaetognaths Sagitta enflata Unidentified Copepods Cladocerans Other crustacean Chaetognaths Sagitta lyra Unidentified Copepods Euphausiids Other crustaceans Chaetognaths Sagitta setosa Unidentified Copepods Cladocerans Other crustaceans Chaetognaths Sagitta bipunctata Sagitta bipunctata Centropages typicus Corycaeus Temora stylifera Clauso/Cteno/ Paracalanus Sagitta enflata Sagitta lyra Sagitta setosa Corycaeus Euchaeta Calanus Pleuromamma Temora stylifera Centropages typicus Temora stylifera Microsetella Clausocalanus Clauso/Cteno/ Paracalanus Sagitta enflata Sagitta lyra Sagitta setosa Candacia Euchaeta Calanus Pleuromamma Centropages typicus Temora stylifera Microsetella Euterpina acutifrons Clausocalanus Clauso/Cteno/ Paracalanus 48.8 FRONTS-95 cruise Sagitta bipunctata Sagitta enflata Sagitta lyra Sagitta setosa FRONTS-96 cruise Sagitta bipunctata Sagitta enflata Sagitta lyra Sagitta setosa Length Head Intercept Slope 9.2 0.21 11.0 0.09 14.8 0.19 7.5 0.03 9.6 0.10 8.6 0.15 15.9 0.39 6.9 0.06 0.59 (...truncated)