Abstract
Coastal marine ecosystems are very complex and composed of myriad organisms, including offshore, coastal, and migratory fish occupying diverse trophic positions (TPs) in food webs. The illustration of trophic hierarchy based on the TP and resource utilization of individual organisms remains challenging. In this study, we applied compound-specific isotope analysis of amino acids to estimate the TP and isotopic baseline (i.e. δ15N values of primary resources at the base of food webs) for 13 fish and 1 squid species in a coastal area of Sagami Bay, Japan, where a large diversity in the isotopic baseline is caused by an admixture of ocean currents and artificial nitrogen inputs. Our results indicate that the TP of fish and squid varies between 2.9 and 3.9 (i.e. omnivorous, carnivorous, and tertiary consumers), with low variation within individual species. Moreover, the δ15N values of phenylalanine revealed the diversity of isotopic baselines between and within species. Low values (7.8-10.3‰) and high values (18.6-19.2‰), with a small variation (1σ < 1.0‰), were found in 2 offshore species and 3 coastal species, respectively. In contrast, highly variable values (9.8-19.7‰), with large variation within species (1σ > 1.0‰), were found for the remaining 9 migratory species. These results represent evidence of differential trophic exploitation of habitats between offshore and coastal species, particularly among individuals of migratory species, that were all collected in a single area of Sagami Bay.
Highlights
For the last several decades, marine ecosystems have been increasingly impacted by human activities, including eutrophication of coastal areas by nutrient inputs, ocean acidification by CO2 emissions, in-Publisher: Inter-Research · www.int-res.comMar Ecol Prog Ser 652: 137–144, 2020The characterization of the trophic positions (TPs) of organisms and their use of resources within food webs is useful in understanding changes in marine ecosystems (e.g. Wada et al 1987, Vander Zanden & Rasmussen 1996, Post 2002, Fry 2006).The TPs of organisms have long been estimated by stomach content analysis, based on the direct identification of diets that the organisms feed on
The δ15N values varied from 5.5−22.0 ‰ for phenylalanine (14.0 ± 4.3 ‰, mean ± 1 SD) and from 25.0−43.2 ‰ for glutamic acid (34.4 ± 5.1 ‰) in the studied species (Table S1)
The mean δ15N values of glutamic acid (δ15NGlu) and δ15NPhe values for each of the 13 fish and 1 squid species (Fig. 3) are plotted along the trophoclines that were defined from Eq (1) as having a slope of 1.0 for each TP and an interval of 7.6 ‰ for each trophic transfer in food webs (e.g. Steffan et al 2013, Chikaraishi et al 2014, Bowes & Thorp 2015, Nielsen et al 2015)
Summary
For the last several decades, marine ecosystems have been increasingly impacted by human activities, including eutrophication of coastal areas by nutrient inputs, ocean acidification by CO2 emissions, in-Publisher: Inter-Research · www.int-res.comMar Ecol Prog Ser 652: 137–144, 2020The characterization of the trophic positions (TPs) of organisms and their use of resources within food webs is useful in understanding changes in marine ecosystems (e.g. Wada et al 1987, Vander Zanden & Rasmussen 1996, Post 2002, Fry 2006).The TPs of organisms have long been estimated by stomach content analysis, based on the direct identification of diets that the organisms feed on. This method has several drawbacks, as it provides only a snapshot of feeding, is biased toward detectable prey species, and does not always allow clear identification of digested items To solve these issues, stable isotope analysis of bulk nitrogen in protein-rich tissues of organisms has been used since the 1980s, based on the increase in nitrogen stable isotope ratios (δ15N) between food and consumers (e.g. DeNiro & Epstein 1981, Minagawa & Wada 1986). The use of stable isotopes allows us to see the integrated prey information at the turnover time of the tissue: days to weeks for zooplankton (Tiselius & Fransson 2016) and months to years for fish (Sweeting et al 2005) This bulk isotope method is not always useful for the study of food webs, for coastal marine ecosystems. The large difference in the life span and integration time of the isotopes between phytoplankton and fish makes it difficult to compare the δ15N values between primary producers and consumers, which is further complicated when the latter exploit different habitats (e.g. Cabana & Rasmussen 1996, Fourqurean et al 1997, Vander Zanden et al 1997, O’Reilly et al 2002, Post 2002)
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