Isotopic Variation of Macroinvertebrates and Their Sources of Organic Matter Along an Estuarine Gradient

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

“…Those authors used both end-member values to estimate the contribution of each source to the coastal POM pool. In the present study, we observed that δ 13 C POM values fell within the usual ranges of riverine POM (annual mean: −25.7 ± 1.2 ), dominant marsh vegetation (−27.5 ± 0.6 , P. australis), tidal-flat microphytobenthos (−14.8 ± 0.8 ), and eelgrass (−8.6 ± 1.0 , Z. marina) (Kang et al, 2020;Kim et al, 2020). The longitudinal variation in the δ 13 C POM values thus indicates mixed sources with varying proportions of local producers and river-borne POM.…”

“…In addition, microscopic observations showed that centric diatoms, rather than pennate diatoms, constitute the major component of phytoplankton assemblages, indicating against a dominant role for resuspended microphytobenthos in the phytoplankton assemblage of Gwangyang Bay (Baek et al, 2015;Kang et al, 2020). Indeed, while an episodic increase (360-880) in POC:Chla ratios in the deep-bay stations during periods of high precipitation and river discharge (summer and fall 2016) were attributable to the increased importance of non-living detritus, low C:N ratio (3-6) and δ 13 C POM values (−22.5 to −19.8 ) of POM during that period were close to typical values of marine POM in the bay (Kang et al, 2003(Kang et al, , 2020Kim et al, 2020), emphasizing the importance of marine phytoplankton-derived organic matter even in detrital POM.…”

“…Like other estuarine embayment systems, the landscape of Gwangyang Bay has patchy and heterogeneous habitats [e.g., a common reed (Phragmites australis) bed in the estuarine littoral wetland, the intertidal muddy sand flat in the lower estuary is covered by benthic microalgae, eelgrass (Zostera marina) bed is on the fringe of the intertidal flat] and faces both, the river and the open sea (Figure 1; Kim et al, 2020). These ecologically diverse habitats connect directly and indirectly to the main channel of the bay.…”

“…As discussed earlier, several lines of evidence indicate that the increasing trend in δ 13 C POM values along the salinity gradient in Gwangyang Bay should be explained by the δ 13 C values of phytoplankton rather than by various mixtures of local producer and river-borne terrestrial POM (Kim et al, 2020). The spatial variability in the δ 13 C POM values may impede the identification of POM origins (Cloern et al, 2002;Guo et al, 2015).…”

“…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.…”