A comparison of three tidal creeks assessed the effects of the hydrological regime on trophic organization in juvenile fish assemblages of 21 species in a tropical estuary in north-eastern Brazil. There were seven trophic guilds represented spatially. Zooplanktivore and zoobenthivore guilds dominated the lower estuary, whereas omnivores and detritivores dominated the upper estuary. In the rainy season, the zooplanktivore and omnivore guilds were more common throughout the estuary, but in the dry season, zoobenthivores and piscivores occurred throughout. The trophic organization results show that (1) there was a higher complexity in tidal creeks in the upper estuary compared with the first tidal creek in the lower region and (2) trophic linkages increased in the upper estuary, principally the number of omnivore and detritivore species. Spatial variation in trophic structure was primarily associated with differences in the location of the tidal creeks along the estuary, and this variability was partly attributed to fish species richness; the number of species increased towards the upper estuary, and additional species occupied different trophic levels or used additional resources.

Size-based approaches are paramount tools for the study of marine food webs. Here, we investigated the relationship between zooplankton body size, stable isotope composition and trophic level (TL) along a large-scale onshore-offshore gradient in the western tropical Atlantic. Samples were obtained on the Brazilian continental shelf, slope and in oceanic waters (off Fernando de Noronha archipelago and Rocas Atoll) in September and October 2015. Zooplankton was sieved into five size fractions. Zooplankton was dominated by copepods, except for the largest (>2000 μm) size fraction, that showed a high biovolume of chaetognaths, decapods, and fish larvae. Maximum zooplankton abundance and biovolume was found at the continental slope. POM showed consistently lower δ13C than zooplankton, indicating a selective use of 13C-rich primary food sources by zooplankton. Particulate organic matter (POM) was more 13Cenriched in shelf areas (average: −22.8, −23.6 and − 24.3‰ at the shelf, slope and oceanic islands, respectively), probably due to the higher abundance of diatoms nearshore. POM had δ15N values between 2.5 and 6.9‰ (average: 4.0, 4.9 and 4.2‰ at the shelf, slope and oceanic islands, respectively). Zooplankton δ15N and TL increased with body size. The δ15N of the 200-500 μm size fraction was used as baseline for TL estimation. Oceanic areas (average baseline δ15N = 5.8‰ ± 0.52, n = 14) showed a higher baseline δ15N than the shelf (average = 3.9‰ ± 0.69, n = 9) and the slope areas (average = 3.1‰ ± 0.93, n = 9). In spite of differing baselines, the δ15N data produced a consistent pattern of log-linear increase in TL with increasing size, in all areas. The choice of input trophic enrichment factor (TEF) values only slightly changed the log10 (body size) vs TL slopes, but this choice had a considerable effect on the estimates of predator/prey size ratio (PPSR) and predator/prey mass ratio (PPMR). Using a TEF above 2.3 leads to unrealistic PPSR and PPMR estimates. Overall average slope was 0.59 ± 0.08 TL μm−1 with TEF = 2.3 and 0.42 ± 0.07 TL μm−1 with TEF = 3.2. Highlights► Maximum zooplankton abundance and biovolume was found at the continental slope. ► POM showed lower δ 13 C than zooplankton, indicating selective feeding. ► Zooplankton δ 15 N and trophic level increased with body size. ► Using a δ 15 N trophic enrichment factor above 2.3 leads to unrealistic estimates.

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Biogeographic and taxonomic knowledge on planktonic mollusks, specifically on Pterotracheoidea (also known as heteropods or sea elephants) in tropical oceans, is still incomplete. In this paper we report the first record of the heteropod Firoloida desmarestia Lesueur (1817) on waters off the Northeast Brazilian continental slope and Fernando de Noronha Archipelago and extend its geographic distribution in the tropical Atlantic. We provide new detailed digital image and descriptions of this highly fragile and transparent gelatinous species. Samples were taken in the context of the ‘ABRACOS’ (Acoustic along the Brazilian Coast) project using bongo nets. Out of 96 samples analyzed, six specimens were recorded in five samples, three at Fernando de Noronha, and three off the Northeast Brazilian coast. The present study extends the range of distribution of F. desmarestia to 3°S-9°Sin the western the Tropical Atlantic and highlights the importance of detailed studies on large-sized gelatinous plankton biodiversity in tropical oceans. Keywords: Tropical oceanic islands, Continental slope, zooplankton, heteropods, Pterotracheidae.