2020 |
Araújo Abrantes de Figueiredo, G. G., et al. "Body size and stable isotope composition of zooplankton in the western tropical Atlantic." J. Mar. Syst.. 212 (2020): 103449.
Résumé: 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 mu m) 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 delta C-13 than zooplankton, indicating a selective use of C-13-rich primary food sources by zooplankton. Particulate organic matter (POM) was more C-13-enriched 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 delta N-15 values between 2.5 and 6.9% (average: 4.0, 4.9 and 4.2% at the shelf, slope and oceanic islands, respectively). Zooplankton delta N-15 and TL increased with body size. The delta N-15 of the 200-500 mu m size fraction was used as baseline for TL estimation. Oceanic areas (average baseline delta N-15 = 5.8% +/- 0.52, n = 14) showed a higher baseline delta N-15 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 delta N-15 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 mu m(-1) with TEF = 2.3 and 0.42 +/- 0.07 TL mu m(-1) with TEF = 3.2.
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Gueroun, S. K. M., et al. "Population dynamics and predatory impact of the alien jellyfish Aurelia solida (Cnidaria, Scyphozoa) in the Bizerte Lagoon (southwestern Mediterranean Sea)." Mediterr. Mar. Sci.. 21.1 (2020): 22–35.
Résumé: Understanding the life cycle strategies and predatory impact of alien jellyfish species is critical to mitigate the impact that these organisms may have on local populations, biodiversity, and ultimately on the functioning of food webs. In the Mediterranean Sea, little is known about the dynamics of alien jellyfish, despite this biodiversity hotspot being one of the most threatened areas by increasing numbers of alien jellyfish. Here, we investigated the population dynamics and predatory impact of a non-indigenous scyphomedusa, Aurelia solida Browne 1905, in the Bizerte Lagoon, Tunisia. The study was based on bimonthly surveys performed over two consecutive years, from November 2012 to August 2014. Field observations showed that the planktonic phase of A. solida occurs from winter to early summer. Prey composition was investigated by means of gut content and field zooplankton analyses. Calanoid copepods, mollusc larvae, and larvaceans represented the main food items of A. solida. To determine the jellyfish feeding rate and their predatory impact on zooplankton populations, the digestion time for zooplankton prey was assessed at three different temperatures: 13, 18, and 23 degrees C in laboratory conditions, corresponding to the average range of temperatures encountered by A. solida in the Bizerte Lagoon. We found that A. solida consumed 0.5-22.5% and 0.02-37.3% of the daily zooplankton standing stock in 2013 and 2014, respectively. These results indicate a non-negligible but restricted seasonal grazing impact on some mesozooplankton groups, explained by the relatively short lifespan of the medusa stage (5-6 months).
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Marsac, F., et al. "Seamount effect on circulation and distribution of ocean taxa in the vicinity of La Perouse, a shallow seamount in the southwestern Indian Ocean." Deep-Sea Res. Part II-Top. Stud. Oceanogr.. 176 (2020): 104806.
Résumé: The La Perouse seamount (60 m depth) has so far been poorly studied despite it being a short distance (160 km) from Reunion Island. As part of the MADRidge project, a multidisciplinary cruise was conducted to evaluate the effect of this shallow seamount on the local hydrology and ecology. Current measurements, temperature and chlorophyll-a profiles, and mesozooplankton and micronekton samples were collected between the summit and 35 km away. Micronekton data were supplemented with stomach content of pelagic top predators as well as fisheries statistics from the domestic longline fleet operating from Reunion. Vertical current profiles revealed distinct patterns between the offshore and seamount-flanked stations, giving evidence of topographical induced flow instabilities, notably on its leeward side (west) relative to the east flank. Distinct patterns in temperature and chlorophyll-a vertical profiles suggest the formation of convergent and divergent circulation cells as a result of the irregular and crescent-like summit topography. Spatial differences in zooplankton abundance were detected with higher biovolumes on the leeward flank. The overall acoustic backscatter for micronekton over the summit was weaker than offshore, but highly concentrated in the upper layer. Albacore tuna and swordfish dominate the longline catch west of Reunion, seemingly in association with a deep (900 m) topographic feature. Yet the largest catch is not directly associated with La Perouse which would be too shallow for top predators to aggregate around in the long term. Enhanced levels of phytoplankton or zooplankton enrichment at La Perouse were not demonstrated in this study, nor was there notable diversity of micronekton species. This might explain the relatively limited importance of this seamount to the tuna fisheries in this region.
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Melo, D. C. M., et al. "Genetic diversity and connectivity of Flaccisagitta enflata (Chaetognatha: Sagittidae) in the tropical Atlantic ocean (northeastern Brazil)." PLoS One. 15.5 (2020): e0231574.
Résumé: The phylogeography of the holoplanktonic chaetognath Flaccisagitta enflata was investigated in the Tropical Western Atlantic (TWA). Considering the cosmopolitan range of this species and the fact that its entire life cycle is planktonic, the central hypothesis of this study is that F. enflata exhibits connectivity due to its high dispersal capacity, forming a panmictic population among the study sites. The evaluated areas included neritic (Port of Recife-PR, and Tamandare – TA) and oceanic (Fernando de Noronha Archipelago-FN, Rocas Atoll-RA, Guara seamount-GS and Saint Peter and Saint Paul's Archipelago-SPSPA) locations of the Brazilian Blue Amazon. We used COI gene sequences as molecular marker. Partial sequences (425 bp) were obtained for 116 specimens and employed to reconstruct the phylogeny, build an haplotype network, evaluate gene flow through a migration model, and estimate diversity indices, population structuring and demographic history. High levels of haplotype diversity (mean: 0.98) and moderate to high levels of nucleotide diversity (mean: 0.023) were observed. The phylogeny and the haplotype network topologies showed some geographic clustering, indicating local structuring in GS and PR. This finding was supported by the AMOVA high global phi st (0.033, significant) and some pairwise phi st comparisons (7 out of 15 were significantly > 0). Significant differences suggested lower levels of connectivity when GS population was compared to those of FN and SPSPA; as well as when TA was compared to FN. These results might be related to particularities of the oceanic dynamics which rules the TWA, sustaining such dissimilarities. Structuring was also observed between PR and all oceanic locations. We hypothesize that the topography of the port inlet, enclosured by a reef barrier, may constrain the water turnover ratio and thus migration rates of F. enflata in the TWA. Accordingly, Migrate-N yielded a four metapopulations model (PR (sic) TA (sic) SPSPA+FN (sic) GS+RA) as the best (highest probability; similar to 0.90) to represent the structuring of F. enflata in the TWA. Therefore, the null hypothesis of one randomly mating population cannot be accepted. The demographic evaluation demonstrated that the neutral hypothesis of stable populations may not be rejected for most of the locations. This work is the start point to broaden the knowledge on the phylogeography and population genetic structure of a numerically dominant species in the Western Atlantic, with key role in the marine trophic web.
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2019 |
Arones, K., et al. "Spatio-temporal trends in zooplankton biomass in the northern Humboldt current system off Peru from 1961-2012." Deep-Sea Res. Part II-Top. Stud. Oceanogr.. 169 (2019): Unsp-104656.
Résumé: Anchovy (Engraulis ringens) is the most important exploited fish species in the Northern Humboldt Current System (NHCS) off Peru. This species, as well as most other pelagic resources, mainly forage on zooplankton. The NHCS is bottom-up controlled at a variety of scales. Therefore, fish biomass is driven by the abundance of their prey. In this context, we studied the spatiotemporal patterns of zooplankton biomass in the NHCS from 1961-2012. Data were collected with Hensen net all along the Peruvian coast. To transform zooplankton biovolume into biomass we used a regression that was calibrated from 145 zooplankton samples collected during four surveys and, for which, precise information was available on both biovolume and wet weight. The regression model was then applied on a time-series encompassing 158 cruises performed by the Peruvian Institute of the Sea (IMARPE) between 1961 and 2012. We observed a clear multidecadal pattern and two regime shifts, in 1973 and 1992. Maximum biomass occurred between 1961 and 1973 (61.5 g m(-2)). The lowest biomass (17.8 g m(-2)) occurred between 1974 and 1992. Finally, the biomass increased after 1993 (26.6 g m(-2)) but without reaching the levels observed before 1973. A seasonal pattern was observed with significantly more biomass in spring than in other seasons. Spatially, zooplankton biomass was higher offshore and in northern and southern Peru. Interestingly, the zooplankton sampling was performed using classic zooplankton net that are well fitted to mesozooplankton and are known to underestimate the macrozooplankton; however, the spatiotemporal patterns we observed are consistent with those of macrozooplankton, in particular euphausiids. This suggests that in the NHCS, when and where macrozooplankton dominates it also dominates the biomass obtained using classic zooplankton net samples. Finally, until now, in the NHCS only time-series on zooplankton biovolume were available. The biomass data we provide are more directly usable in trophic or end-to-end models.
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