Variability in copepod trophic levels and feeding selectivity based on stable isotope analysis in Gwangyang Bay of the southern coast of the Korean Peninsula

Abstract

Abstract. Trophic preference (i.e., food resources and trophic levels) of different copepod groups was assessed along a salinity gradient in the temperate estuarine Gwangyang Bay of Korea, based on seasonal investigation of taxonomic results in 2015 and stable isotope analysis incorporating multiple linear regression models. The δ13C and δ15N values of copepods in the bay displayed significant spatial heterogeneity as well as seasonal variations, which were indicated by their significant relationships with salinity and temperature, respectively. Both spatial and temporal variations reflected those in isotopic values of food sources. The major calanoid groups (marine calanoids and brackish water calanoids) had a mean trophic level of 2.2 relative to nanoplankton as the basal food source, similar to the bulk copepod assemblage; however, they had dissimilar food sources based on the different δ13C values. Calanoid isotopic values indicated a mixture of different genera including species with high δ15N values (e.g., Labidocera, Sinocalanus, and Tortanus), moderate values (Calanus sinicus, Centropages, Paracalanus, and Acartia), and relatively low δ15N values (Eurytemora pacifica and Pseudodiaptomus). Feeding preferences of different copepods probably explain these seasonal and spatial patterns of the community trophic niche. Bayesian mixing model calculations based on source materials of two size fractions of particulate organic matter (nanoplankton at < 20 µm vs. microplankton at 20–200 µm) indicated that Acartia and Centropages preferred large particles; Paracalanus, Calanus, Eurytemora, and Pseudodiaptomus apparently preferred small particles. Tortanus was typically carnivorous with low selectivity on different copepods. Labidocera preferred marine calanoids Acartia, Centropages, and harpacticoids; on the other hand, Sinocalanus and Corycaeus preferred brackish calanoids Paracalanus and Pseudodiaptomus. Overall, our results depict a simple energy flow of the planktonic food web of Gwangyang Bay: from primary producers (nanoplankton) and a mixture of primary producers and herbivores (microplankton) through omnivores (Acartia, Calanus, Centropages, and Paracalanus) and detritivores (Pseudodiaptomus, Eurytemora, and harpacticoids) to carnivores (Corycaeus, Tortanus, Labidocera, and Sinocalanus).

Dominance of Autochthonous Phytoplankton-Derived Particulate Organic Matter in a Low-Turbidity Temperate Estuarine Embayment, Gwangyang Bay, Korea

“…As indicated by GAMs analysis, seasonal and spatial variation of δ 13 N POM is likely to be caused by changes in the major phytoplankton groups (Chen et al, 2018). Indeed, individual phytoplankton species were related to levels of SPM, POM, total Chla, and phytoplankton productivity, confirming that a shift in phytoplankton community composition may partially or fully explain spatiotemporal variation in POM composition ( Supplementary Figure 1 and Supplementary Table 7).…”

“…Zooplankton grazing pressure (Chen et al, 2018; the authors' unpublished data for grazing rate), the availability of nutrients, and other environmental conditions (Baek et al, 2015) can affect phytoplankton biomass. The size composition (i.e., micro-, nano-, and picophytoplankton) of phytoplankton assemblages may alter their relative contributions to POM quantity and composition.…”

“…Dilution of dissolved inorganic nutrients by freshwater-seawater mixing and their rapid assimilation by phytoplankton lowers nutrient concentrations in the estuarine channel, leading to spatial and seasonal variations (Kwon et al, 2001(Kwon et al, , 2004Jang et al, 2005;Lee and Kang, 2010). Furthermore, because autochthonous POM is of nutritional importance to the bay's food-web base (Kang et al, 2003;Chen et al, 2018;Kim et al, 2020), the spatial and temporal dynamics of biogenic elements and phytoplankton may be crucial to the bay ecosystem. However, considering the entire bay, the dynamics of POM, and associated biogenic elements and phytoplankton, are still poorly understood.…”

Discrete Spawning Aggregations of the Loliginid Squid Doryteuthis gahi Reveal Life-History Interactions of a Dwarf Morphotype at the Center of Its Distribution Range

“…To assess the status of D. gahi within the pelagic food web, we estimated the trophic position for different consumers (e.g., squids, invertebrates, and fish) using the "oneBaseline" model, incorporating uncertainty in the trophic discrimination factor (TDF) for muscle nitrogen (mean ± SD 15 N = 3.4 ± 0.1 ) (Post, 2002), as well as baseline and consumer δ 15 N values through Bayesian inference with tRophicPosition package (Quezada-Romegialli et al, 2018). Assuming that pelagic species of squids, invertebrates, and fish, could be supported mainly by pelagic carbon and nitrogen (Thiel et al, 2007), copepods were used as baseline with a mean trophic position of 2.2 (see Chen et al, 2018). Models were run with 2 chains, 20,000 adapting samplings, and 20,000 iterations.…”

“…The δ 15 N values were 15 N-depleted in E. mucronata (12.2 ± 1.3 ) and brachyuran zoeae (12.5 ), and 15 N-enriched in P. gallapagensis Trophic position (TP) was estimated from D. gahi (mantle tissue) and pelagic consumers. The symbol ( * ) means that baseline consumer and putative trophic position was according to Chen et al (2018).…”