The biochemical composition of seston along a salinity gradient were examined in the low-turbidity, temperate, estuarine embayment, Gwangyang Bay in Korea. Seasonal variations in sestonic protein (PRT), carbohydrate (CHO), and lipid (LIP) concentrations were analyzed to assess the effects of physiological status and taxonomic composition of phytoplankton. The concentrations of biochemical compounds displayed a close relationship with chlorophyll a (Chla). PRT:CHO ratios were high (>1.0) in the estuarine channel in warmer months and in whole bay in February, indicating a N-replete condition for phytoplankton growth. High CHO:LIP ratios (>2.5) in the saline deep-bay area during the warmer months (>2.0) emphasized the importance of temperature and photoperiod over nutritional conditions. The low POC:Chla (<200), molar C:N (~7) ratios, and biopolymeric carbon concentrations coupled with high primary productivity indicated a low detrital contribution to the particulate organic matter pool. Diatom dominance throughout the year contributed to consistently high carbohydrate concentrations. Furthermore, generalized additive models highlighted that phytoplankton community (i.e., size) structure may serve as an important descriptor of sestonic biochemical composition. Collectively, our results suggest that physiological and taxonomic features of phytoplankton play prominent roles in determining the biochemical composition of seston, supporting the fact that the ecosystem processes in Gwangyang Bay are largely based on phytoplankton dynamics.

The monthly phytoplankton communities and dietary items of the filter-feeding ark clam (Anadara kagoshimensis) in cultivation were examined in a shallow temperate coastal embayment of Yeoja Bay in Korea, to identify dietary changes in clams associated with phytoplankton community patterns. A self-organizing map (SOM) algorithm was applied to shape the community structures of phytoplankton. Clam δ¹³C and δ¹⁵N values were determined monthly and compared with those of phytoplankton, microphytobenthos, suspended particulate organic matter (SPOM), sedimentary organic matter (sedimentary OM), and Phragmites australis. Our SOM clustered monthly phytoplankton communities, revealing a seasonal shift in the dominance of large-sized diatoms (sporadically together with dinoflagellates), which were detected almost year-round, to small-sized taxa (chlorophytes, prasinophytes, and prymnesiophytes), which were observed in May–June. The δ¹³C and δ¹⁵N measurements revealed that pelagic and benthic diatoms serve as the main contributors to the clam diets. A reduction in their dietary contribution accompanied a considerable increment in the contribution of Phragmites detritus in association with the dominance of small-sized phytoplankton during the late spring. Our results suggest that the dominance of small-sized phytoplankton during the critical spring period of the clam life cycle may decrease the availability of preferred items (i.e., size-related food quality) and lead to dietary changes in the clams in relation to climate forcing in this warming sea.

Monsoon rains trigger pulsed flows from tributaries, which in turn impact the structure of riverine food webs. However, the mechanisms driving food web dynamics in tributaries in response to these pulsed flows are not yet fully understood. We employed carbon (δ13C) and nitrogen (δ15N) stable isotopes of macroinvertebrates and fish to quantify changes in the trophic base and diversity, food chain length, and food web trophic niches before and after the monsoon in two tributaries of the northeast Asian monsoon region. The δ13C and δ15N values of primary basal resources (leaf litter and biofilms) were consistent before and after the monsoon, with a notable increase in δ15N values from forest streams to agricultural channels. Consumer δ13C and δ15N values remained stable over time but exhibited a longitudinal increase due to greater nutritional contributions from local resources. Community isotopic niche metrics were consistent across locations and seasons, while trophic niches diverged between watersheds and closely overlapped seasonally in isotopic space. These results highlight the significant impact of agricultural inputs on downstream channel food webs and demonstrate the limited effect of monsoonal rains on altering the longitudinal trajectory of trophic niches across tributaries.