Bacterial growth efficiency in a partly eutrophicated bay of South China Sea: Implication for anthropogenic impacts and potential hypoxia events

Microzooplankton grazing and its key group composition in subtropical eutrophic coast of Southern China: in relation to environmental changes

Microzooplanton play a crucial role in marine ecosystems, as they transfer matter and energy from pico- and nano-phytoplankton to mesozooplankton. In this study, we explored the seasonal variations of microzooplankton grazing derived from dilution experiments in a typical eutrophic coast of Southern China, as well as the abundance and biodiversity of its key group (ciliate), to further understand its function in the subtropical coastal food web associated with potential regulation factors. A total of 29 ciliate species belonging to 18 genera were identified, with the dominating species of Mesodinium rubrum, Strombidium globosaneum and Strombidium conicum. The spatial difference of ciliates abundance was attributed by the changes of temperature and salinity. Phytoplankton growth rate (μ) and microzooplankton grazing rate (m) ranged from 0.03 to 1.36 d^-1 and 0.10 to 1.57 d^-1, respectively, and both μ and m showed the highest values in summer and the lowest in winter. Moreover, microzooplankton grazing pressure on the phytoplankton standing stocks and potential primary production ranged from 10% to 79% and 58% to 471%, respectively. Our results indicated that temperature is the main environmental driving force for the seasonal changes of μ and m, and that the impacts of run-offs from the Pearl River and offshore seawater intrusion from the South China Sea are responsible for the spatial-temporal variations of phytoplankton growth and microzooplankton grazing.

Long-term changes of nutrients and biocenoses indicating the anthropogenic influences on ecosystem in Jiaozhou Bay and Daya Bay, China

Long-term changes of nutrients, plankton and macrobenthos were studied to research the transformation of ecosystem in Jiaozhou Bay and Daya Bay in the past 30 years. Concentrations of dissolved inorganic nitrogen and phosphate increased with significant changes in nutrient compositions and ratios. Concentrations of Chl a slightly decreased in Jiaozhou Bay but increased in Daya Bay. Phytoplankton abundances increased and diatoms were dominant, however, dinoflagellate gradually had the competitive advantage under high N/P and N/Si in the two bays. Zooplankton biomass significantly increased in Jiaozhou Bay, but only increased slightly in Daya Bay over the past years. Polychaetes were dominant in macrobenthos in the bays, indicating their adaptation to the changing benthic environments. The long-time variations of biocenoses and nutrients reflected that the ecological environments have changed under the influence of anthropogenic activities in the two bays.

The benthic fluxes of nutrients and the potential influences of sediment on the eutrophication in Daya Bay, South China

Nutrient concentrations in porewater and their benthic fluxes were investigated in Daya Bay, South China, to study the accumulation and transfer of nutrients at sediment-water interface, as well as the impact of human activities on nutrients. The contributions of sediment to nutrients in water column and the potential influences on eutrophication were also discussed. Nutrients in porewater and overlying water changed in different seasons and areas, which was mainly attributed to human activities, hydrodynamic force and biogeochemical conditions. Mean concentrations of DIN (dissolved inorganic nitrogen), PO₄ and SiO₃ were 70 ± 61, 3.1 ± 4.3, 103 ± 105 μmol/L, and 234 ± 166, 15.6 ± 4.0, 353 ± 48 μmol/L in overlying water and porewater, respectively. Annual mean DIN, PO₄ and SiO₃ fluxes were 330 ± 249, -1.3 ± 16 and 549 ± 301 μmol/(m²d), respectively, indicating that sediment was generally the source of DIN and SiO₃, but was the sink of PO₄. The mean exchange capacities were (7.8 ± 5.5) × 10⁷, (-1.2 ± 34.0) × 10⁵ and (1.2 ± 0.6) × 10⁸ mol/a for DIN, PO₄ and SiO₃, respectively, in Daya Bay.

Is phosphorus a limiting factor to regulate the growth of phytoplankton in Daya Bay, northern South China Sea: a mesocosm experiment

Previous field investigations implied a potential phosphorus (P)-limitation on the growth of phytoplankton in Daya Bay, a mesotrophic bay in the northern South China Sea. Using a total of 15 mesocosms (3 × 3 × 1.5 m, with ~10.8 m³ natural seawater containing phytoplankton assemblages for each), we found P-enrichment caused no obvious effect on phytoplankton (Chl a) growth across 8-day's cultivation in neither winter nor summer, while nitrogen (N)-enrichment greatly increased Chl a in both seasons. N plus P-enrichment further increased Chl a content. The N- or N plus P-enrichments increased the allocation of nano-Chl a but decreased micro-Chl a in most cases, with no obvious effect by P-alone. Coincided with nutrients effect on Chl a content, N- or N plus P-enrichments significantly enhanced the maximum photochemical quantum yield of Photosystem II (F_V/F_M) and maximum relative electron transport rate (rETR_max), but declined the non-photochemical quenching (NPQ), as well as the threshold for light saturation of electron transport (E_K); again, P-enrichment had no significant effect. Moreover, the absorption cross section for PSII photochemistry (σ_PSII) and electron transport efficiency (α) increased due to N- or N plus P-enrichments, indicating the increased nutrients enhance the light utilization efficiency through promoting PSII light harvesting ability, and thus to enhance phytoplankton growth. Our findings indicate that N- or N plus P-enrichments rigorously fuel phytoplankton blooms regardless of N:P ratios, making a note of caution on the expected P-deficiency or P-limitation on the basis of Redfield N:P ratios in Daya Bay.

Response of Bacterial Metabolic Activity to the River Discharge in the Pearl River Estuary: Implication for CO2 Degassing Fluxes

Bacterial production (BP), respiration (BR) and growth efficiency (BGE) were simultaneously determined along an environmental gradient in the Pearl River Estuary (PRE) in the wet season (May 2015) and the dry season (January 2016), in order to examine bacterial responses to the riverine dissolved organic carbon (DOC) in the PRE. The Pearl River discharge delivered labile dissolved organic matters (DOM) with low DOC:DON ratio, resulting in a clear gradient in DOC concentrations and DOC:DON ratios. BP (3.93-144 μg C L^-1 d^-1) was more variable than BR (64.6-567 μg C L^-1 d^-1) in terms of the percentage, along an environmental gradient in the PRE. In response to riverine DOC input, BP and the cell-specific BP increased; in contrast, the cell-specific bacterial respiration declined, likely because labile riverine DOC mitigated energetic cost for cell maintenance. Consequently, an increase in bacterial respiration was less than expected. Our findings implied that the input of highly bioavailable riverine DOC altered the carbon portioning between anabolic and catabolic pathways, consequently decreasing the fraction of DOC that bacterioplankton utilized for bacterial respiration. This might be one of the underlying mechanisms for the low CO₂ degassing in the PRE receiving large amounts of sewage DOC.