Abstract
In estuaries, the sources of organic matter, its composition, and its functional role in the coastal ecosystem food web are all influenced by dominant estuarine processes. We investigated seasonal variations in the quantity and composition of dissolved inorganic nutrients, suspended particulate matter (SPM), and phytoplankton across a low-turbidity estuarine–coastal continuum in Gwangyang Bay, South Korea. The narrow estuarine channel has a sharp salinity gradient (from 0 to around 30) and low SPM concentrations (<20 mg L−1). The channel carries river discharge, with a large quantity of nutrients (NO3 and SiO2), into the bay. The especially low SPM concentrations in the estuary (range: 2.0–29.1 mg L−1), and generous nutrient source, create an exceptionally low-turbidity condition in the water column and high primary productivity with no conspicuous spatial or seasonal trends. Gradually increasing 13C values, low POC:Chla (<200), and low molar C:N (4–8) in the SPM, together with higher Chla concentrations at low SPM locations along the longitudinal transect, suggest that local production of phytoplankton has a strong influence on the distributional patterns of particulate organic matter. Short water-residence times, within the bay system, impede the accumulation of diverse-origin organic matter. The community composition of phytoplankton, distinguished by their size group, also plays a role in characterizing SPM quantity and composition. The results indicate that autochthonous pelagic production governs biogeochemical features in the low-turbidity estuarine embayment, contrasting with a prevalence of allochthonous organic matter, and its active processing, in highly turbid and/or longer residence-time systems.
Highlights
Estuarine processes play a pivotal role in structuring the seasonal and spatial patterns of biogeochemical cycles in the coastal zone where riverine and marine waters interact (Bianchi, 2013; Cloern et al, 2017)
The seasonal variation in water temperature was typical of the temperate zone, with a significant difference (Kruskal–Wallis test, df = 3, P < 0.001; Dunn’s pairwise test, P < 0.001) between summer and winter (7.9◦C), but no remarkable spatial variations
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)
Summary
Estuarine processes play a pivotal role in structuring the seasonal and spatial patterns of biogeochemical cycles in the coastal zone where riverine and marine waters interact (Bianchi, 2013; Cloern et al, 2017). Organic matter goes through more active processing in high-turbidity estuaries with longer residence times (Wang et al, 2004; Wu et al, 2007) In these estuaries, extensive mixing with, and dilution by, estuarine particles mask the contribution of the various organic matter sources and produces uniform characteristics along the salinity gradient. SPM concentrations influence light penetration, affecting primary production and the resultant particulate matter distribution along the estuarine–marine continuum (Cloern et al, 2017) These estuarine processes govern the delivery of nutrients to coastal waters, regulating the level of productivity in the sea (Edmond et al, 1981; Cloern and Jassby, 2012)
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