Seasonal dynamics of particulate organic matter and its response to flooding in the Pearl River Estuary, China, revealed by stable isotope (δ¹³C and δ¹⁵N) analyses

Abstract

Nine cruises were conducted on a seasonal basis from 2013 to 2015 to investigate the spatial distribution and seasonal variability of δ13C and δ15N in particulate organic matter (POM), and its response to flooding in the Pearl River Estuary (PRE), south China. Our study reveals highly variable isotope ratios between seasons in this subtropical estuary, following seasonal climatic and hydrological cycles. Wet seasons had more isotopically depleted δ13C values, indicating the dominance of terrestrial and freshwater algae POM, whereas the contribution from marine phytoplankton (16%–59%) was higher during the dry seasons. In contrast, δ15N exhibited a sharp increase (up to 17.6‰) at low salinities (0–5) during high flow seasons. This was consistent with high NO3− concentrations, reflecting phytoplankton and bacteria assimilation of δ15N enriched‐ NO3− as well as notable isotope fractionation during microbial mineralization. There was little annual variability in δ13C over the 2 year period; however, particulate nitrogen (PN) exhibited lower concentrations but more enriched isotope values in 2015 than in 2014. This can be best explained by temperature‐modulated biological processing of particulate organic nitrogen, partially due to different biogeochemical responses during normal (2014) and strong El Niño (2015) years. After flooding in June 2015, terrestrial organic matter and freshwater phytoplankton were the major components of POM within the estuary and shelf areas, whereas marine phytoplankton was the dominant component in the adjacent coastal waters with mid‐salinities (10 < S < 20), as revealed by a phytoplankton bloom (>10 μg L−1) and δ13C‐enriched but δ15N‐depleted POM.

Disentangling protist communities identified from DNA and RNA surveys in the Pearl River–South China Sea Continuum during the wet and dry seasons

“…To measure the chlorophyll a content, water samples of ~200 ml were filtered through GF/F filters (Whatman, Little Chalfont, Buckinghamshire, UK) under low vacuum, and the chlorophyll a was determined under low light using a UV‐8000 spectrophotometer (Metash, Shanghai, China) after extraction with 90% acetone (Parsons, Maita, & Lalli, ). Suspended particulate matter, particulate organic carbon and particulate nitrogen were measured, as described in detail by Ye, Guo, Shi, Jia & Wei (). The concentrations of nutrients including NH 4 , NO 2 , NO 3 and PO 4 were determined using standard methods, as described by Ye, Ni, Xie, Wei & Jia ().…”

Impacts of anthropogenic inputs on the hypoxia and oxygen dynamics in the Pearl River Estuary

“…The organic matters can fuel the sediment oxygen demand when deposited and mineralized in the sediment layers, which has been found to dominate the DO depletions within the bottom waters of the Pearl River Estuary (Yin et al, 2004;Zhang and Li, 2010). In coastal systems the organic matters are often derived from the dead phytoplankton (Green et al, 2006), while in the Pearl River Estuary the organic matters mainly originate from the riverine inputs (Ye et al, 2017;Yu et al, 2010). The distinct characteristics of the organic matter source suggest the importance of studying the impact of riverine organic matters on hypoxia in the Pearl River Estuary.…”

“…In the northern Gulf of Mexico, the marine source serves as the major source of particulate organic carbon (Green et al, 2006), which means increasing the nutrient loading can facilitate the sediment oxygen demand rates by increasing the depositional fluxes of dead phytoplankton that consequently promote the formation of hypoxia. However, in the Pearl River Estuary, the sediment oxygen demand is primarily fueled by the terrestrial POC (Ye et al, 2017;Yu et al, 2010) which is set unchanged in all our nutrient simulations.…”

“…The relative contributions of terrestrial versus marine POC inputs have long been a topic of debate in the Pearl River Estuary. Some studies suggest that terrestrial source dominate in wet seasons due to the high river discharges (Ye et al, 2017;Yu et al, 2010), while others argue that marine inputs can also play an important role (Guo et al, 2015;Su et al, 2017). Previous studies (Guo et al, 2015;Hu et al, 2006;Ye et al, 2017) show that the ratios of terrestrial inputs to marine inputs have distinct spatial and seasonal variabilities in the Pearl River Estuary.…”

Using δ13C to reveal the importance of different water transport pathways in two nested karst basins, Southwest China

“…Each DIC source has a different δ 13 C DIC isotopic signature (from -26‰ to -9‰ for soil organic matter (SOM) (Wang et al, 2017a), -8‰ to -6‰ for atmospheric CO 2 and approximately 0‰ for carbonate rocks (Brunet et al, 2005). Moreover, C isotopes are used to assess riverine C transformation as well as follow the riverine DIC transport into oceans (Zeng et al, 2015Brunet et al, 2005Deirmendjian and Abril, 2018;Hu et al, 2017;Ye et al, 2017). Monitoring changes in spatial and temporal DIC concentrations and δ 13 C DIC values from inland water systems can provide a better understanding of C sources.…”