Abstract This study investigates the temporal and spatial trends of allochthonous organic matter (AOM) contributions to coastal sediments from the boreal Glomma River to the outer Oslofjord, southeastern Norway. Seven sediment cores from the area were analysed, ranging from proximal estuarine sites to distal marine sites. Stable isotope analyses of carbon (δ¹³C) and nitrogen (δ¹⁵N), coupled with C/N ratios, revealed clear gradients in AOM contributions, with proximal sites dominated by terrestrial signatures (-28‰ δ¹³C) and distal sites reflecting marine sources (clustering around − 23‰ δ¹³C). Temporal trends showed increasing sediment accumulation rates (SAR) across all sites, with SAR increasing on average by ~ 1,440 g m⁻² yr⁻¹ after the onset of sustained sediment accumulation, which occurred broadly between the 1960s and 1980s. Terrestrial organic carbon accumulation (C org AR) trends followed those of SAR closely, also increasing over the past century and reaching up to 208 gC m⁻² y⁻¹ at the most proximal site and 68 gC m⁻² y⁻¹ at the most distal site. Both trends were driven by intensified land use changes and climate effects such as increased precipitation and prolonged growing seasons. Notably, the δ¹³C profiles indicated a steady decline of δ¹³C over time, with a vertical range of change of up to 1.52‰ at the proximal site, reflecting enhanced terrestrial inputs into marine sediments. This corresponded to an average increase in allochthonous organic carbon of 0.58% per decade across all cores over the past 100 − 150 years. Variability in sediment characteristics across sites highlighted the importance of riverine inputs in proximal sites, where allochthonous organic carbon constituted 58–89% of total organic carbon, contrasting with near-zero fractions observed at distal sites. This work emphasizes the historic increase in AOM input to the studied coastal area. This has many ecological implications, including coastal darkening – an increase in light attenuation by the dissolved terrestrial organic matter in water column, which limits light availability for phototrophs. If this trend of AOM enrichment continues, it could have significant consequences for the Oslofjord ecosystem, including suppression of phototrophs and subsequent cascading effects on the food web and ecosystem productivity.

Abstract The eastern oyster ( Crassostrea virginica ) is a commercially and ecologically important species across eastern North America and the Gulf of Mexico. In the Mississippi Sound, an estuarine system between the Mississippi River and Mobile Bay, Alabama, USA, oyster reefs have declined substantially over the past two decades due to multiple stressors, which may include increasingly frequent operations of the Bonnet Carré Spillway (BCS). This flood-control structure diverts Mississippi River water away from New Orleans into the western Mississippi Sound, which can create prolonged low-salinity and unsuitable conditions for oyster reefs. Although the Mississippi Department of Marine Resources (MDMR) has monitored reefs locally for decades, researchers have not used these data to evaluate how freshwater inflow anomalies affect oyster mortality. This study used a multi-decadal MDMR dataset to assess how freshwater discharge from BCS operations and co-occurring environmental stressors influenced oyster mortality. Environmental covariates included freshwater discharge from the National Water Model, salinity and temperature from USGS monitoring buoys, precipitation, dissolved oxygen, and BCS operational data. Oyster mortality varied interannually, but mortality rates and complete mortality events were strongly influenced by BCS operations and freshwater inflow volumes. Mortality was also influenced by rainfall, prolonged suboptimal salinity, and elevated water temperatures. These patterns are consistent with freshwater-induced mortality observed elsewhere across the US Gulf Coast. Projected increases in river discharge, rainfall intensity, and BCS operation frequency under climate change highlight the need to incorporate freshwater tolerance into oyster restoration and management strategies in the Mississippi Sound.

Abstract Low-inflow estuaries are highly variable and productive habitats. Organisms in these systems must cope with constantly changing conditions. This study investigates the continuous response of oysters to environmental stimuli in a small, low-inflow, estuary, Los Peñasquitos Lagoon (LPL), in San Diego, California, USA. The oysters were placed on a mooring at near-surface and near-bottom locations and instrumented with noninvasive sensors to measure their shell opening (oyster valve gape), concurrently with physical and chemical measurements (water depth, salinity, temperature, dissolved oxygen, pH, solar radiation, wind velocity, and wave radiation stresses) over 136 days. The time series of each oyster with each variable over tidally averaged and shorter time scales were analyzed using traditional statistical methods. The strongest gape relationships were with salinity and dissolved oxygen (Spearman correlation coefficient 0.6), among the oysters near the water surface. Diurnal phase averaging showed how the surface oysters open and close in phase with insolation, while the near-bottom oysters have a lagged response. Tidal phase averaging further illustrates the distinct gape behavior, with surface oysters closing consistently during low tides, and bottom oysters generally remaining open. A subset of variables that had high Spearman correlations (near surface oxygen and salinity, near-bottom pH, solar irradiance, and near-bottom turbidity) and their 18-hour time lags were used to test a set of multivariate models for predicting surface oyster valve gape. The best performing model ( $$r^2 = 0.75$$ ) was consistent with the correlation analysis, showing salinity and dissolved oxygen to be the most important predictors. Key insights from this study are that oysters in LPL have predictable responses to environmental conditions (particularly salinity and dissolved oxygen) with differing responses to conditions near-surface and near-bottom on tidal and tidally-averaged, and faster than tidal timescales.

Abstract Shifts in phytoplankton communities, from beneficial taxa that fuel food webs to harmful taxa that degrade water quality, are often linked to anthropogenic modifications to estuaries. We analyzed monitoring data from 2000 to 2023 in the heavily modified southern, landward portion of the Sacramento-San Joaquin Delta (California, USA) to determine the status and drivers of phytoplankton abundance and community composition along the riverine to freshwater tidal transition. Diatoms and green algae dominated the phytoplankton community, especially when chlorophyll a (chl- a ) concentrations exceeded 16 µg L − 1 . Common riverine diatoms (e.g., Cyclotella and Aulacoseira ) dominated upstream riverine stations, while taxa associated with tidally influenced conditions (e.g., colonial cyanobacteria, cryptophytes) were more prevalent in western, freshwater tidal stations. The potentially toxigenic cyanobacterium, Microcystis , was present each summer, but was not associated with elevated chl- a or cyanotoxins (microcystins were detected in 12% of samples with a maximum concentration of 3.9 µg L − 1 ). A Bayesian ordinal regression showed that mean prior week water speeds below 0.18 m s − 1 and water temperatures above 22 ˚C were associated with the presence of Microcystis. Random forest modeling to assess factors associated with phytoplankton blooms (chl- a > 16 µg L − 1 ) found that mean prior week turbidity, inflows from the San Joaquin River, region, and mean prior week water temperature were important drivers of blooms. These modeling results emphasize the importance of hydrodynamics in controlling phytoplankton dynamics in lowland rivers and tidal deltas, offering insights for how anthropogenic impacts on flow regimes may affect the base of estuarine food webs.

Abstract Efforts have been underway over 40 years to reduce nutrient pollution of the Chesapeake Bay, USA, in one of the world’s most sustained and studied campaigns to abate coastal eutrophication. Nitrogen and phosphorus loads have been reduced predominantly from advanced wastewater treatment and diminished atmospheric deposition of nitrogen. Reductions from nonpoint sources, including agricultural activities and stormwater, continue to fall short of targets set under legal requirements to meet a Total Maximum Daily Load by 2025. Water quality goals for the estuary have not been attained as quicky as some expected. Is it the case that the required load reductions will never be achieved or that the water quality goals would never be attained even if they were? Improvements in dissolved oxygen and water clarity have been documented consistent with scientific understanding of ecosystem responses, suggesting that improvements will accelerate as further load reductions are realized. Prioritizing reduction of pollution into localized, shallow water habitats would be counterproductive because it ignores the dominant role of recycling of nutrients from distant sources within the estuary as intensified by deep-water hypoxia. Reducing the most consequential loads from nonpoint sources should be addressed systemwide. Required reductions can be implemented by the new 2040 deadline, but require adjusting approaches to include efficacious incentives, targeting of the most significant sources, and alleviating nutrient mass imbalances in areas of intense animal production. Mandatory compliance, technological innovations, new or modified interstate compacts, and climate change adaptations should also be considered in developing and implementing an effective strategy.

Abstract Biogenic mangrove wetlands rely on trapping and burying organic production in soil to pace sea-level rise. However, the fate and role of allochthonous marine wrack in regulating carbon processing and wetland elevation in fringe mangrove forest soils remain unclear. We quantified long-term changes in tidal hydrology and surface soil elevation (2011–2024) across two representative fringe mangrove forest sites (BISC-1, BISC-2) in Biscayne National Park (Florida, USA). To test the influence of marine wrack deposition, we measured the spatiotemporal variation of monthly wrack deposition, litter breakdown rates, soil organic carbon, and stable isotope δ 13 C and δ 15 N content along landward transects (2022–2024). Our results indicate that relative sea-level rise (annual rate ± SE: 8.5 ± 0.4 mm yr − 1 ) was greater than annual rates of surface elevation change at either site (BISC-1 2.1 ± 2.6 mm yr − 1 ; BISC-2 3.9 ± 3.2 mm yr − 1 ). Rhizophora mangle litter breakdown rates (mean ± SE: BISC-1 0.011 ± 0.001 k d − 1 ; BISC-2 0.008 ± 0.001 k d − 1 ) were greater than Thalassia testudinum (BISC-1 0.009 ± 0.001 k d − 1 ; BISC-2 0.005 ± 0.001 k d − 1 ), suggesting that wrack deposition can potentially enhance elevations gains more than mangrove litter. Stable isotope content in soil, mangrove, and seagrass tissue suggested that allochthonous marine wrack contributed between 36.1% ± 0.8% and 53% ± 3.6% to the soil organic carbon content. Our findings emphasize the importance of ecological connectivity in transporting wrack as a marine subsidy that can enhance blue carbon storage and elevation increases by reducing breakdown rates and promoting peat formation in fringe mangrove forests.

Abstract The El Niño –Southern Oscillation (ENSO) profoundly alters climatological and hydrological conditions, reshaping environmental gradients and ecological dynamics in riverine, estuarine, and coastal ecosystems. We hypothesized that El Niño events would promote homogenization of fish assemblages and reduce β-diversity, whereas La Niña conditions would enhance habitat differentiation and increase β-diversity. To test this hypothesis, we investigated variations in fish β-diversity across a freshwater–estuarine–marine gradient in the Patos Lagoon, southern Brazil, under La Niña , El Niño , and neutral climatic conditions. We analyzed total β-diversity and its local and species contribution components in relation to environmental variability associated with ENSO phases. The results revealed a clear pattern of homogenization between the estuarine and marine environments during El Niño , characterized by increased species sharing and reduced taxonomic differentiation. Species sharing between these regions increased from 10 to 13 species during La Niña and neutral conditions, respectively, to 18 species during El Niño . In contrast, La Niña promoted stronger habitat compartmentalization, resulting in higher total β-diversity (0.558) than El Niño (0.538) and neutral conditions (0.528), indicating greater spatial heterogeneity in fish assemblages. These findings support our hypothesis that ENSO-driven changes in environmental gradients shape fish β-diversity, with El Niño promoting biotic homogenization and La Niña enhancing habitat differentiation. As ENSO events are projected to become more frequent and intense, these processes may have important implications for the biodiversity and structural organization of subtropical coastal lagoons and other estuarine ecosystems.