Differential routing of ‘new’ nitrogen toward higher trophic levels within the marine food web of the Gulf of Aqaba, Northern Red Sea

Abstract

Mesozooplankton communities in the mesooligotrophic Gulf of Aqaba, Northern Red Sea, were investigated over a 2 years period (2005)(2006)(2007) with emphasis on the trophodynamic relations among different taxonomic groups ranging from primary consumers to carnivorous predators. Based on stable isotope analyses, we present evidence for a strong contribution of 'new' nitrogen mainly derived from the utilization of aerosol nitrate by unicellular cyanobacteria especially during summer stratification and the propagation of exceptionally low d 15 N onto higher trophic levels. In contrast, N 2 -fixation by diazotrophs seemed to play a minor role, while the utilization of deep water nitrate by cyanobacteria and eukaryotic algae might be of importance during winter mixing. Based on 15 N enrichment of consumers, clear differences between exclusively herbivorous organisms (doliolids, appendicularians, pteropods) and those with omnivorous feeding modes were detected. The category of omnivores comprised a large variety of taxons ranging from small meroplanktonic larvae to non-calanoid copepods (harpacticoids, cyclopoids and poecilostomatoids) that together form a diverse and complex community with overlapping feeding modes. In addition, distinct seasonality patterns in d 15 N of copepods were found showing elevated trophic positions during periods of winter mixing, which were most pronounced for non-calanoid copepods. In general, feeding modes of omnivores appeared rather unselective, and relative contributions of heterotrophic protists and degraded material to the diets of non-calanoid copepods are discussed. At elevated trophic positions, four groups of carnivore predators were identified, while calanoid copepods and meroplanktonic predators showing lowest 15 N enrichment within the carnivores. The direct link between 'new' nitrogen utilization by primary producers and the 15

Carbon and nitrogen stable isotope ratios of pelagic zooplankton elucidate ecohydrographic features in the oligotrophic Red Sea

“…Because abundances of Trichodesmium cells were scarce during their study, Aberle et al (2010) inferred that new N with low δ…”

“…There are few broad species-specific studies of pelagic zooplankton using SIA (e.g. Hannides et al, 2009;Koppelmann et al, 2009;Aberle et al, 2010;Fanelli et al, 2011;Couturier et al, 2013;Kürten et al, 2013a). Though poorer in species than the Indian Ocean, the Red Sea planktonic population is diverse and subject to a pronounced north-south biogeographic gradients and seasonal variations (Halim, 1984;Böttger-Schnack, 1995).…”

“…Specific processes include the monsoon-related intrusion of nutrient and plankton-rich Indian Ocean water (Sofianos and Johns, 2007;Raitsos et al 2015), the biological fixation of atmospheric N 2 by diazotrophic Cyanobacteria (e.g. Trichodesmium erythraeum), the deposition of aerosols (Böttger-Schnack, 1995;Chen et al, 2007;Aberle et al, 2010;Wankel et al, 2010), and diazotroph-eukaryote symbiosis (Kimor et al, 1992;Kürten et al, 2014b). N 2 fixation and aerosol deposition may play significant roles in the N budget of the Red Sea considering the large net export of N toward the Arabian Sea Bange et al, 2000;Wankel et al, 2010).…”

Contribution and pathways of diazotroph-derived nitrogen to zooplankton during the VAHINE mesocosm experiment in the oligotrophic New Caledonia lagoon

“…The δ 15 N values of zooplankton reflect the balance between these contrasting N sources, the relative contributions of which can be estimated using a two-part mixing model (Montoya et al, 2002). This modelling approach has been used to demonstrate a significant contribution of diazotroph-derived N (DDN) to particulate matter and zooplankton biomass (Aberle et al, 2010;Landrum et al, 2011;Loick-Wilde et al, 2012;Mompean et al, 2013;Montoya et al, 2002;Sommer et al, 2006;Wannicke et al, 2013), and transfer of DDN beyond zooplankton to micronekton (Hunt et al, 2015). However, despite this measured contribution of DDN, the predominant pathways of DDN into marine food webs are still in question (Wannicke et al, 2013).…”

Transfer of diazotroph-derived nitrogen to the planktonic food web across gradients of N<sub>2</sub> fixation activity and diversity in the Western Tropical South Pacific

“…Regarding higher trophic levels, the low δ15N signature of zooplankton reveals that N 2 fixation can significantly fuel zooplankton growth in high N 2 fixation areas (Aberle et al, 2010;Landrum et al, 2011;Loick-Wilde et al, 2012;Mompeán et al, 2013;Montoya et al, 2002;Sommer et al, 2006;Wannicke et al, 2013). Few studies reports active grazing of Trichodesmium by some specific copepods (Micro-and Macrosetella, O'Neil et al (1996); O'Neil and Roman (1992)).…”