Abstract Accelerated eutrophication and climate change are driving freshwater cyanobacterial blooms worldwide. Cyanobacteria–bacteria interactions shape phycosphere nutrient cycling and bloom dynamics, but their temporal shifts during bloom progression remain poorly understood. Using metatranscriptomics, we investigated the interactions between Microcystis and phycospheric bacteria throughout a bloom season in Lake Taihu, a heavily eutrophic lake in eastern China. During the early bloom, Microcystis and phycospheric bacteria co‐expressed genes associated with phosphate metabolism and oxidative stress response, whereas genes involved in nitrogen and iron acquisition were co‐expressed during the late bloom, suggesting potential nutrient competition. Carbohydrate metabolism‐related interactions persisted across bloom stages. As the bloom progressed, the dominant interacting taxa shifted from Hyphomicrobiales to Burkholderiales . We further assessed the generality of these patterns using metatranscriptomic datasets spanning multiple years and lakes. Phosphorus‐related interactions were consistently enriched during early blooms, whereas nitrogen competition‐related patterns predominated during late blooms; oxidative stress response‐related interactions, however, were more variable. Together, these findings reveal stage‐dependent shifts in phycospheric interactions that may contribute to bloom development and decline. The recurrence of these patterns across spatial and temporal scales suggests that shifts in phycospheric interactions are a general feature of bloom progression and may help predict bloom responses to environmental change.
Abstract Marine heatwaves (MHWs) are increasingly invoked in explanations of coastal algal blooms, yet shared seasonality can confound apparent MHW–bloom coincidence. We assembled a source‐resolved catalogue of 48 algal bloom records and a satellite‐derived regional daily sea surface temperature series for Sansha Bay, China, to evaluate bloom‐record timing against MHW exposure during 2005–2020. Bloom records peaked in May–June, and regional MHW days peaked in July–September. The weighted circular seasonal centers were separated by 2.73 months, and the monthly overlap coefficient was 0.29. Event‐level classification identified 9 MHW‐related bloom records and 39 records without event‐period or pre‐start MHW exposure. Under the same‐calendar‐month null model, the observed count of 9 fell within the timing‐conditioned distribution centered at 10.84 records (95% interval, 6–16; one‐sided exact p = 0.799). The same‐year‐month, duration, source, and exposure‐window analyses placed the observed count within their respective timing‐conditioned distributions. Seasonal timing therefore provides the background needed to interpret MHW–bloom coincidence in Sansha Bay. This framework separates temporal co‐occurrence from enrichment above the expected timing of a historical bloom‐record catalogue.
Abstract Small hydroelectric power plants, such as run‐of‐river dams (RORs), are increasingly promoted as low‐impact renewable energy sources, yet their disturbance on mercury (Hg) cycling remains poorly evaluated in impact assessments. Recent work suggests that ROR impacts may be underestimated, as transient Hg increases were observed in fish following impoundment, and co‐occurring disturbances including forest fires and logging may further alter carbon and Hg fluxes. This study investigates how RORs (impounded in 1959 and 2008), forest fires (in 2010), and logging (up to 2020) influence Hg dynamics along a regulated river (Quebec, Canada). In summer 2023, we sampled six river sectors representing various combinations of natural and anthropogenic disturbances. Methylmercury (MeHg) concentrations in secondary consumers (macroinvertebrates) in sectors combining dams and watershed disturbances remained up to 2.3‐times those of the reference sector, compared to 1.5‐times in single‐disturbance sectors. Despite a 34% decline in fish MeHg in the dam and fire impacted sector between 2018 and 2023, cyprinids downstream of dams still exhibited up to 2‐times higher contamination than undammed sectors. Fish MeHg was strongly predicted by depleted carbon isotope (δ 13 C) values ( R 2 marginal = 0.72), associated with decomposition processes. Hydrogen isotope (δ 2 H) values contributed to MeHg variability in models including end‐members, suggesting that reliance on autochthonous sources contributes to sustained MeHg bioaccumulation, particularly when allochthonous inputs occur. These findings indicate that RORs may have enhanced ecological Hg impacts when combined with watershed disturbances. Improved, flexible planning will be essential (e.g., logging) under climate change scenarios where forest fires are expected to rise.
Although transitions from dry-to-flowing conditions in non-perennial rivers are recognized as biogeochemical hot moments, the drivers modulating these hot moments remain poorly understood. We identify two main drivers of importance for biogeochemical processes during flow resumption: the dry-to-flow mode, referring to how flow is re-established, and whether resumption leads to sediment transport. Using experimental flumes, we examined how instant and gradual surface flows (the latter preceded by 6-h inundation), with varying sediment transport rates, affect streambed biogeochemistry. Over 24 h, we measured carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) fluxes and porewater concentrations, and analyzed the temporal dynamics of CO2 fluxes and dissolved oxygen. Additionally, we added isotope-labeled nitrate to trace denitrification through changes in nitrogen gas isotope ratios (delta 15N-N2) and determined the uptake and release rates of nitrate () and ammonium (). We found that dry-to-flow mode, irrespective of sediment transport, influenced dissolved oxygen, CO2 fluxes, nutrient dynamics, and changes in delta 15N-N2, but did not affect CH4 or N2O fluxes. Interestingly, both dry-to-flow modes produced nearly synchronized CO2 flux peaks upon water entry to the flumes, regardless of sediment transport rate. Yet, transport was positively associated with dissolved oxygen concentrations and net release and negatively related to changes in delta 15N-N2, CH4, and N2O fluxes. Our findings highlight how the mode of flow resumption and sediment transport dynamics can shape biogeochemical hot moments in rivers and underscore the need to consider the characteristics of hydrological transitions when estimating non-perennial river contributions to global carbon and nitrogen budgets.
Abstract Coastal vegetation is increasingly recognized for enhancing coastal resilience against climate change. Studies show that alternating vegetated and unvegetated patches can effectively dissipate wave energy, while emphasizing the need to better characterize wave diffraction in such configurations. In this study, we develop a semi‐analytical framework that combines vegetation‐induced wave attenuation with diffraction for regions partially covered by submerged vegetation and evaluate it using experiments. The model extends classical semi‐infinite breakwater diffraction theory by incorporating Dalrymple‐type dissipation due to an array of vegetation elements and an empirical lateral scaling that accounts for the porous nature of the vegetation. Under assumptions of spatially uniform vegetation within the vegetated region, laterally uniform incident waves, and linear wave dynamics, the local velocity potential is expressed as the incident potential multiplied by a modified diffraction coefficient. Twenty‐one experiments were conducted in a shallow wave basin under varying wave heights, periods, and water depths for semi‐infinite and periodically arranged vegetation patches. Comparisons between analytical predictions and over 15,000 wave‐height measurements show that the model captures the dominant attenuation–diffraction patterns under the tested conditions, with a typical normalized RMSE of approximately 13%. The analysis clarifies how to estimate the wave decay coefficient in laterally non‐uniform wave fields and demonstrates that, for the geometries tested, Dalrymple's formulation remains applicable when the wave decay coefficient is based on locally attenuated waves. This coupled diffraction–attenuation framework provides a practical tool for analyzing hydrodynamics in partially vegetated regions and for guiding the arrangement of vegetated patches in nature‐based coastal‐protection systems.
