Stable isotope patterns in micronekton from the Mozambique Channel

Abstract

International audienceWe measured the stable carbon (δ13C) and nitrogen (δ15N) isotopic composition of tissues of micronektonic organisms (fishes, squids, crustaceans and gelatinous organisms) collected in the Mozambique Channel during two scientific cruises in 2008 and 2009. The oceanic circulation in the Mozambique Channel is dominated by mesoscale cyclonic and anticyclonic eddies which play a key role in biological processes of less-productive deep-sea ecosystems. We investigated the potential impact of mesoscale features on the δ13C and δ15N values of 32 taxa of micronekton. Fishes, squids, crustaceans and gelatinous organisms encompassed a wide range of isotopic niches, with large over laps among species. Our results showed that mesoscale features did not really influence the isotopic signatures of the sampled organisms, although cyclonic eddies can occasionally impact the nitrogen signatures of micronekton. We show that δ13C values were intermediate between standard off shore and nearshore signatures, suggesting that pelagic production in the Mozambique Channel could be partly supported by the transport and export of inorganic and organic particles from the Mozambican coast toward the offshore area. Trophic levels calculated from δ15N values ranged from 2.6 to 4.2, showing that micronekton taxa can be tertiary consumers in the Mozambique Channel. Our findings evidenced clusters of micronektonic organisms according to their δ15N or δ13C isotopic signatures, but variations in stable isotope values reflect a complex set of embedded processes linked to physical mesoscale dynamics (rotational dynamics of eddies) and basic biology and ecology of micronektonic organisms (vertical habitat, migration pattern, dietary habits, body length) that are discussed with regard to the stable isotope method based on time-integrated assimilated food

Stable isotopic composition of anguilliform leptocephali and other food web components from west of the Mascarene Plateau

“…The present study and the previous studies that have used bulk stable isotopes to begin to evaluate the feeding ecology of leptocephali (Otake et al, 2003 ;Kimura and Tsukamoto, 2006;Miyazaki et al, 2011) or have included a few leptocephali in larger studies (Waite et al, 2007;Ménard et al 2014) have suggested that leptocephali have a low trophic position that is consistent 20 with feeding on POM, but knowledge about the nitrogen isotopic composition of the primary producers in a particular habitat is required to be able to estimate the trophic positions of consumers such as leptocephali (Post, 2002). Therefore, perhaps the best confirmation of a low trophic position of leptocephali was obtained using δ 15 N of amino acids (glutamic acid and phenylalanine) of small A. japonica leptocephali from the NEC (Miller et al, 2013b).…”

“…These were representative of the range of leptocephali between 20 and 275 mm in size, but did not include the smallest sizes of larvae. Although the stable isotope signatures of a few species of leptocephali have been reported before directly (Otake et al, 1993;Kimura and Tsukamoto, 2006;Miyazaki et al, 2011) or incidentally (Waite et al, 2007;Ménard et al 2014), this is the first study to examine the isotopic signatures of many taxa of leptocephali in comparison to other mesozoplankton, cephalopods, fish, and POM components of the food web in a particular oceanic area. The δ 13 C signatures of the 12 taxa of leptocephali were mostly overlapping, but two moreseparated ranges of δ 15 N signatures were observed among the 50 leptocephalus specimens that were examined, which included species with higher values (Group 1: 9 taxa) and those with lower values (Group 2: Ariosoma, Avocettina, Nemichthydae) of δ 15 N. A previous study that analyzed the isotopic signatures of the Japanese eel, Anguilla japonica, and Ariosoma leptocephali in relation to POM in the North Equatorial Current (NEC) region of the western North Pacific (WNP) found a very similar separation of about 3‰ between the δ 15 N values of anguillid larvae and the Ariosoma 15 larvae (Miyazaki et al, 2011).…”

“…Otake et al (1993) found a high average δ 15 N value for 6 Conger myriaster leptocephali from nearshore areas of Japan (11‰), which is similar to the values of the Conger leptocephali in the present study that were within Group 1. Ménard et al (2014) included 10 unidentified leptocephali (170-260 mm) in their stable isotope study of micronekton from the Mozambique Channel on the other side of Madagascar that had mean δ 15 N and δ 13 C values of 5.8 ± 1.5 and -20.6 ± 0.57, respectively. Considering the size of their leptocephali, these were likely all Group 2 species, and their isotopic signatures directly overlap with our δ 15 N and δ 13 C values of Nemichthys and Ariosoma.…”

The key role of the Northern Mozambique Channel for Indian Ocean tropical tuna fisheries

“…Such mechanisms of exchange between the shelf and open water have been suggested to support the production of micronekton organisms, i.e. small fish, cephalopods, and crustaceans which are the main prey of tuna (Roger 1994;Ménard et al 2014) and other top predators such as seabirds (Weimerskirch et al 2004), whale sharks (Sequeira et al 2012), and cetaceans (Laran et al 2017). The seasonal NMC productivity appears strongly dependent on mesoscale activity which has been shown to intensify during the 1990s-2000s and to be affected by large-scale Indian Ocean variability (Palastanga et al 2006;Backeberg et al 2012).…”

“…Dissolved oxygen and temperature are key drivers constraining tuna physiological abilities and vertical behaviour (Graham and Dickson 2004). Bigeye tuna is tolerant to pronounced oxygen and temperature deficiencies, and commonly forages beyond the scattering layer on mesopelagic preys such as myctophids and squid S. oualaniensis and Ornithoteuthis volatilis (Potier et al 2008a;Ménard et al 2014). Yellowfin and skipjack tunas prefer the oxygenated and warmer waters of the epipelagic layer (Graham and Dickson 2004).…”