Zooplankton-based delta C-13 and delta N-15 isoscapes from the outer continental shelf and slope in the subtropical western South Atlantic

Genyffer Cibele Troina, Frank Dehairs, Silvina Botta, Juliana Couto Di Tullio, Marc Elskens, Eduardo Resende Secchi

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Abstract

Characterizing the patterns of stable carbon (δ13C) and nitrogen (δ15N) isotopes at the base of the food webs (baseline) is essential in ecological studies for assessing the feeding habits and migration patterns of marine predators. We analysed δ13C and δ15N in zooplankton samples collected in spring and autumn (2012-2015), along the south (SCM) and southeast (NCSM) Brazilian oceanic waters. An increase in δ13C from the shelf break towards the offshore was associated with the influence of continental water input and upwellings along the shelf break, which introduce 13C-depleted inorganic carbon to the surface, where it will be incorporated into the biological system through phytoplankton growth. An opposite trend was observed in δ15N, with higher values along the shelf break related to the intrusion of upwelled deep-water nutrients, contrasting with the oligotrophic offshore waters where N2 fixation takes place, resulting in primary and secondary production that is relatively more depleted in 15N. A latitudinal (north-south) decrease in δ13C along the offshore area coincided with known isotopic patterns in inorganic carbon, which were more 13C-depleted towards higher latitudes. On the other hand, an increase in δ15N was observed towards the south, reflecting the contrast between the N2 fixation in the more oligotrophic waters in the NCSM and the higher nutrient availability in the SCM. Additionally, δ13C was significantly higher in autumn and δ15N was higher in spring. Although inter-annual differences in δ15N were non-significant, δ13C was significantly lower in 2012 than in the later years. This study provides novel information regarding the patterns of baseline δ13C and δ15N in the southwestern Atlantic Ocean, which will be useful for future investigation of the ecology of higher trophic-level organisms occurring in this area.
Original languageEnglish
Article number103235
Number of pages14
JournalDeep-Sea Research Part I
Volume159
Early online date5 Feb 2020
DOIs
Publication statusPublished - May 2020

Bibliographical note

Funding Information:
We thank all the researchers and students who helped collect the data at sea, the crew of FURG's R/V Atlântico Sul and the late Mr. Homero Poujeaux Alvariz. We would also like to thank V.M. Tavano and two anonymous reviewers for their valuable comments and suggestions. This project was mostly funded by Chevron Brasil Upstream Frade Ltda . Additional funding was provided by BG Group, Brasil . The Brazilian Inter-Ministerial Commission for the Resources of the Sea (CIRM) supplied diesel for the ship for all surveys. This article is part of GCT's Ph.D. thesis in Biological Oceanography (Universidade Federal do Rio Grande-FURG in co-tutelage with Vrije Universiteit Brussel, VUB) under the supervision of ERS, SB and FD. The Coordination for the Improvement of Higher Education Personnel (CAPES) provided a doctoral scholarship (Process 88881.135706/2016-01) and a post-doctoral fellowship (88887.314453/2019-00 - PROANTAR) to GCT and the National Council for Technological and Scientific Development (CNPq) provided a research fellowship to ERS (PQ 310597/2018-8). CAPES also provided access to the Portal de Periódicos. We acknowledge financial support from VUB in the framework of the FURG- VUB co-tutelage agreement. This agreement is within the scope of the Capes PrInt Program (public notice 041/2017). This study is a contribution of the Research Group “Ecologia e Conservação da Megafauna Marinha-EcoMega/CNPq” and the Brazilian National Institute of Science and Technology - INCT-Mar COI funded by CNPq Grant Number 610012/2011-8 .

Funding Information:
Moreover, zooplankton ?15N was higher along the shelf break (?+2?), in spring (?+1?, Table 2) and increased towards the south of the study area (Fig. 9). Depletion in the heavy N isotopes observed in zooplankton sampled in the NCSM-OFF subarea (Table 1, Fig. 5) suggests the incorporation of N from a relatively more 15N-depleted food source. The NCSM-OFF area had the highest values for temperature and salinity, being strongly influenced by the oligotrophic tropical water mass during the study period (Lima et al., 2019). Some diazotrophic cyanobacteria, such as those of the genus Trichodesmium, assimilate N2 with very low isotope discrimination, resulting in biomasses with low isotopic ratios close to that of the atmospheric N2 source and lower than for fixed-N species dissolved in surface water (Capone et al., 2005; Mompean et al., 2013). Therefore, lower ?15N values are often found in oligotrophic waters, where N2 fixation can be a major source of nitrogen for phytoplankton. In Brazil, a large abundance of Trichodesmium has been observed in areas under the influence of tropical water, especially in the region north of Cape Santa Marta (Detoni et al., 2015; Lima et al., 2019). A sample of Trichodesmium collected in the NCSM in the spring of 2014 had high ?13C values (?18.4?) and very low ?15N values (?0.4?, unpublished data). The same patterns of 13C-enrichment and 15N-depletion observed in the NCSM-OFF have been associated with the incorporation of fixed-N2 into marine food webs, mainly in oligotrophic waters from tropical and subtropical regions (Carpenter et al., 1997; Mompean et al., 2013). Thus, organic carbon and nitrogen fixed by Trichodesmium could be assimilated by the zooplankton and expressed as both higher ?13C and lower ?15N values in the NCSM-OFF area. Additionally, in the same period as our sampling, higher concentration of dissolved inorganic nitrogen (DIN, sum of the concentrations of nitrate, nitrite and ammonium) were observed towards the south of our study area, in the SCM region (Lima et al., 2019). The south-to-north decrease in zooplankton ?15N values along with increasing ?13C values observed in the offshore area could thus be the result of a higher contribution of Trichodesmium nitrogen fixation due to lower N-nutrient concentrations and higher ?13C-DIC values in the warmer oligotrophic NCSM waters. In contrast, towards the south, the increased influence of nutrient-rich waters providing other sources of nitrogen for phytoplankton growth resulted in relatively higher ?15N values; while lower ?13C-DIC (Gruber et al., 1999) led to lower ?13C values in primary producers, which is a pattern reflected in the zooplankton isotopic composition. Similar ?15N trends have been reported for the North Pacific, where copepods and chaetognaths sampled in the southern California Bight had higher ?15N values than those from the North Pacific Central Gyre (NPCG) (Mullin et al., 1984). The authors associated these results with the importance of N2 fixation in the NPCG, contrasting with the California Bight, where nitrate (NO3-) was the main source of nitrogen for phytoplankton. In waters with higher nutrient concentrations, such as in coastal or upwelling zones, NO3- is the main source of nitrogen for primary producers. Deep-water NO3- ?15N values tend to be constant around +5? (Sigman et al., 2009), while the surface-water NO3- pool becomes more enriched in 15N as growing phytoplankton discriminate against the heavy isotope (Altabet and Francois, 1994). Consequently, in waters where NO3- is the main nutrient source for primary producers, plankton will be characterized by relatively higher ?15N values in comparison with nutrient-poor areas where N2 fixation is the main source of fixed organic carbon and nitrogen (Montoya et al., 2002; Sigman et al., 2009). Such patterns were seen in our study with the latitudinal increase, as well as the shelf-break-to-offshore decrease in ?15N values: while the higher N2-fixation offshore introduced isotopically light N to the system, along the shelf break and towards the south the higher DIN availability (Lima et al., 2019) resulted in relatively higher plankton ?15N values. Further, our environmental data provided support to the seasonal influence of upwelled SACW along the shelf break, where lower SST was registered in spring. Accordingly, the seasonal differences in the nitrogen isotope ratios, with higher mean zooplankton ?15N values in spring and along the shelf break, agree with our hypothesis that upwelling waters contributed to 15N-enriched deep-water nutrients into the photic zone.We thank all the researchers and students who helped collect the data at sea, the crew of FURG's R/V Atl?ntico Sul and the late Mr. Homero Poujeaux Alvariz. We would also like to thank V.M. Tavano and two anonymous reviewers for their valuable comments and suggestions. This project was mostly funded by Chevron Brasil Upstream Frade Ltda. Additional funding was provided by BG Group, Brasil. The Brazilian Inter-Ministerial Commission for the Resources of the Sea (CIRM) supplied diesel for the ship for all surveys. This article is part of GCT's Ph.D. thesis in Biological Oceanography (Universidade Federal do Rio Grande-FURG in co-tutelage with Vrije Universiteit Brussel, VUB) under the supervision of ERS, SB and FD. The Coordination for the Improvement of Higher Education Personnel (CAPES) provided a doctoral scholarship (Process 88881.135706/2016-01) and a post-doctoral fellowship (88887.314453/2019-00 - PROANTAR) to GCT and the National Council for Technological and Scientific Development (CNPq) provided a research fellowship to ERS (PQ 310597/2018-8). CAPES also provided access to the Portal de Peri?dicos. We acknowledge financial support from VUB in the framework of the FURG-VUB co-tutelage agreement. This agreement is within the scope of the Capes PrInt Program (public notice 041/2017). This study is a contribution of the Research Group ?Ecologia e Conserva??o da Megafauna Marinha-EcoMega/CNPq? and the Brazilian National Institute of Science and Technology - INCT-Mar COI funded by CNPq Grant Number 610012/2011-8.

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Keywords

  • Brazil; stable isotopes; marine biogeochemistry; trophic ecology; shelf break; slope

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