Effects of organic nitrogen components from terrestrial input on the phytoplankton community in Jiaozhou Bay

Estuarine hydrodynamic processes driving the molecular changes of terrestrial dissolved organic nitrogen: From mixing to biological modification

Estuaries receive substantial amounts of terrestrial dissolved organic nitrogen (tDON), which will be transported from the freshwater to the oceanic terminus through vigorous exchange processes. However, the intricate migration and transformation dynamics of tDON during this transportation, particularly at a molecular level, remain constrained. To address this knowledge gap, Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) was used for the analysis of DON molecular composition in the Pearl River Estuary (PRE), a river-dominated estuarine system influenced by intensified anthropogenic activities in southern China. The results showed a pronounced spatial-temporal variation in DON concentration in the study area. At the molecular level, tDON exhibited reduced unsaturation and aromaticity, coupled with an elevated abundance of DON compounds containing one‑nitrogen atom (1 N-DON, 53.17 %) and compounds containing carbon, hydrogen, oxygen, nitrogen, and sulfur (CHONS) (27.46 %). It was evident that lignin was depleted while more oxygenated tannin compounds were generated in the freshwater-seawater mixing zone. This transformation is attributed to heightened biological activities, likely influenced by the priming effect of terrestrial nutrient inputs. In summer, the prevailing plume combined with biological activities in the strong mixing area and outer estuary increased the abundance of 3 N-DON molecules and a concurrent rise in the abundance of DON compounds containing only carbon, hydrogen, oxygen, and nitrogen (CHON), DON compounds containing carbon, hydrogen, oxygen, nitrogen, sulfur, and phosphorus (CHONSP), and CHONS. This trend also underscores the expanding role of marine plankton and microbes in the utilization of DON compounds containing carbon, hydrogen, oxygen, nitrogen, and phosphorus (CHONP). These findings provide details of tDON transformation processes at the molecular level in a river-dominated estuary and underline the estuarine hydrodynamics involved in transporting and altering DON within the estuary.

Diatom-dinoflagellate succession in the Bohai Sea: The role of N/P ratios and dissolved organic nitrogen components

Dissolved organic nitrogen (DON) is a pivotal component of total dissolved nitrogen pools, serving as a crucial nitrogen source for phytoplankton. This study investigated the impact of nitrogen-to-phosphorus (N/P) ratios and different DON components (hydrophilic vs hydrophobic DON) on diatom-dinoflagellate succession through field culture experiments. Results showed that dinoflagellates have a competitive advantage under high N/P ratios and phosphorus limitation, regardless of DON or DIN treatments. Hydrophilic DON exhibits greater bioavailability than hydrophobic DON (40.6% vs. 21.7 %), resulting in increased algal biomass and diatoms dominance in the community. Additionally, DON was categorized into labile and refractory components (LDON and RDON) based on bioavailability. LDON primarily consists of protein-like components that can be readily consumed by algae, whereas RDON is primarily composed of humic-like components that are less accessible to algae. Diatoms and dinoflagellates exhibited differential responses to LDON and RDON, with diatoms thriving in high LDON environments, while dinoflagellates gained a competitive advantage when RDON was the predominant nitrogen source. Furthermore, a significant negative correlation was observed between bioavailable nitrogen concentration (BAN: DIN + LDON) and the ratio of dinoflagellates to diatoms (p<0.05). In conclusion, our study highlights the role of LDON in promoting diatom dominance, whereas environments dominated by RDON foster dinoflagellate success. These findings enhance our comprehension of diatom-dinoflagellate succession dynamics.

Effect of dissolved organic nutrients on the bloom of Prorocentrum donghaiense in the East China Sea coastal waters

Prorocentrum donghaiense blooms occur annually in the East China Sea coastal waters, degrading ecosystem functions and impeding economic development. Dissolved organic nitrogen and phosphorus (DON and DOP) are the main components in the marine nutrient pools and are closely related to harmful algal blooms. From April to June 2019, a survey was conducted along the East China Sea coast (Sansha and Lianjiang counties) to investigate the relationship between dissolved organic nutrients and P. donghaiense bloom. Our findings showed that dinoflagellates dominated the phytoplankton community, and dissolved organic nutrients were the major factors influencing community structure during the P. donghaiense bloom. Redundancy analysis indicated that P. donghaiense abundance was primarily affected by DON in the Sansha area while it was primarily affected by DON and DOP in the Lianjiang area. Correlation analysis also confirmed a strong positive correlation between dissolved organic nutrients and P. donghaiense abundance both in the Sansha and Lianjiang coastal areas (p < 0.001). Furthermore, a culture experiment was carried out during the bloom to further investigate the effect of dissolved organic nutrients on the phytoplankton community structure. After 10 days of culture, dinoflagellates' relative abundance decreased from 97.1% to 28.2% in the inorganic treatment, whereas dinoflagellates continued to dominate the phytoplankton community in the organic treatment (76.9%). As a result, we propose that dissolved organic nutrients are responsible for the P. donghaiense bloom outbreak and promote the phytoplankton community shift from diatoms to dinoflagellates.