Gang Li

Colorimetry is a well-established technique for detecting physicochemical parameters in aquatic environments, yet conventional methods are hindered by bulky instrumentation, labor-intensive procedures, high reagent consumption, and lengthy assay times. Microfluidics offers a promising alternative, but existing passive micromixers often face a trade-off among mixing efficiency, pressure drop, and long-term deployability.This study presents a microfluidic colorimetric sensor for in situ nitrite monitoring in aquatic environments, incorporating a synchronous oscillatory spiral micromixer with sinusoidal channel walls to enhance Dean vortex formation while maintaining smooth flow. The micromixer achieves complete mixing at a Reynolds number of 50 with substantially reduced pressure drop. Integrated with an optical absorbance cell, the lab‐on‐a‐chip sensor provides a detection limit of 6.12 nM, a linear range up to 60 µM, and a measurement time of 140 s. Two proposed sensors were deployed in Daya Bay for over one week at two‐hour intervals, producing 164 valid data points. Intercomparison of nitrite concentrations measured by the proposed sensors and a benchtop UV‐2600 spectrophotometer, using concurrently collected grab samples, showed strong agreement, with principal component analysis yielding a near 1:1:1 ratio among the three datasets.Observed nitrite concentrations varied coherently with salinity, tidal level, chlorophyll‐a, and dissolved oxygen, reflecting tide‐driven water exchange and biological activity. With a micromixer footprint under 5 mm², the proposed platform is suitable for chip‐level integration and multi‐parameter analysis, demonstrating a high‐sensitivity, low‐power, high‐temporal‐resolution solution for in situ nutrient monitoring with potential for multiplexed oceanographic applications

Ocean alkalinity enhancement (OAE) is a leading climate mitigation strategy for atmospheric carbon dioxide removal, with theoretical potential to sequester gigatons of CO ₂ annually while counteracting ocean acidification. However, as a deliberate anthropogenic perturbation to the marine carbonate system, the ecological consequences for the microbial communities that underpin marine biogeochemical cycles remain incompletely understood. Alterations in seawater pH and carbonate saturation states could influence microbial assemblages, with implications for ecosystem functioning and stability under global climate change. In this study, we experimentally assessed the responses of marine prokaryotic communities to unequilibrated alkalinity additions (85 to 495 μmol·kg ⁻¹ ) using Mg(OH) ₂ in the subtropical South China Sea. Across both microcosm (55 l) and mesocosm (50,000 l) scales during the wet and dry seasons, a total alkalinity increase of 80 to 427 μmol·kg ⁻¹ substantially elevated pH (8.57 to 8.77) and carbonate saturation states. Despite these pronounced chemical perturbations, the prokaryotic diversity, community structure, and predicted metabolic functions remained remarkably stable. This stability persisted across different experimental scales and contrasting seasonal conditions. Instead, seasonal variability and nutrient availability were the dominant drivers of microbial community shifts, overwhelming the minor effects of alkalinity addition. Our findings align with recent studies from the North Atlantic and the Equatorial Pacific, suggesting that the resilience of prokaryotic communities to moderate, unequilibrated OAE is a robust, globally relevant phenomenon. This study emphasizes that even under relatively intense OAE scenarios, microbial community stability may help sustain ecosystem functioning amid emerging ocean-based carbon dioxide removal interventions, a key finding for developing effective monitoring, reporting, and verification (MRV) frameworks.

The acute rise in temperature due to marine heatwaves has a strong impact on marine phytoplankton. To determine whether these effects depend on ambient temperature and cell size, we acclimated two diatom species, smaller Thalassiosira pseudonana (Hasle and Heimdal, 1970) and larger Thalassiosira rotula (Meunier, 1910), at low (LAT), medium (MAT) and high ambient temperatures (HAT) and examined their physiochemical and transcriptional responses to temperature rise (AT + 6 °C). The specific growth rate (µ) of smaller cells was increased by 32% due to temperature rise at LAT, but decreased by 13% at HAT, with the stimulatory and inhibitory extent being ~50% less than that of larger cells. At LAT, chlorophyll a (Chl a), carotenoid (Car) and carbon (POC) contents were increased in smaller cells due to temperature rise, but were decreased in larger cells; at HAT, Chl a and Car were increased in both smaller and larger cells and POC was increased in only smaller cells. At LAT, temperature rise led to a disproportionate increase in photosynthesis and dark respiration, resulting in an increase in carbon utilization efficiency (CUE) in smaller cells and a decrease in CUE in larger cells; at HAT, there was a decrease in CUE in both the smaller and larger cells, but to a lesser extent in the former than in the latter. Our results also show that smaller cells cope with the acute temperature rise mainly by strengthening their enzyme activity (e.g., the antioxidant system) and conservatively regulating their metabolism, while larger cells mainly regulate their photosynthetic and central carbon metabolism. Moreover, larger cells can outperform their smaller counterparts when the temperature rise occurs at lower ambient temperature.

Human-induced nutrient input into the world's coastal waters is leading to an increase in nutrients and increasing eutrophication. However, how the functioning of aquatic ecosystems responds to these changes is still poorly understood. In this study we report on the long-term changes in nutrient regime and planktonic ecosystem function in the Daya Bay, a typical subtropical semi-enclosed bay that has experienced rapid economic and social development for several decades. Time-series data (from 1991 to 2018) were collected at a mostly quarterly resolution to visualize the long-term changes in dissolved inorganic nutrients and plankton abundances, based on which we constructed a simplified abundance size spectra (SASS) and plankton abundance ratio to describe the functioning of the planktonic ecosystem. The results showed a long-term increase in the system productivity but a decrease in integrated energy transfer efficiency of the planktonic ecosystem with increasing dissolved inorganic nitrogen (DIN) contents. Changes in the nutrient regime and functioning of the planktonic ecosystem were detected at a tipping point or threshold around 2006–2007, the shifts of which were characterized by abrupt changes in the trends of nutrient contents (phosphate, ammonia and nitrite), nutrient ratios (DIN/phosphate and silicate/phosphate), plankton abundance and total plankton biomass. Compared to the nutrient regime, the functioning of the planktonic ecosystem shifted a few years later. Overall, our findings suggest that the pelagic ecosystem in coastal waters such as Daya Bay may change significantly in response to long-term increases in human nutrient inputs. These shifts may have profound implications for fisheries production and ecosystem management in the bay.