Freshwater Inflow Research Articles

Irrigation-driven groundwater depletion in the Ganges-Brahmaputra basin decreases the streamflow in the Bay of Bengal

Ganges and Brahmaputra, two of Asia’s most prominent rivers, have a crucial role in Southeast Asia’s geopolitics and economy and are home to one of the world’s biggest marine ecosystems. Irrigation-driven groundwater depletion and climate change affect the Ganges-Brahmaputra’s hydrology, threatening the stability of the Bay of Bengal. Here, we quantify, using results from a land reanalysis, the impacts of a changing climate and intensive irrigation on the surface water flowing into the Bay of Bengal. The effects of such activities mostly occurring in the Ganges basin, either intensified or lessened depending on the area by the climatic conditions, decrease freshwater flow into the bay by up to 1200 m3/s/year. While the increase in precipitation in the Ganges basin reduces the effects of groundwater depletion on the streamflow, the decrease in precipitation and the snowmelt decline in the Brahmaputra basin exacerbate streamflow reduction due to groundwater depletion at the delta.

Sosialisasi Hukum Tentang Pengaturan Kewenangan Pengelolaan Wilayah Pesisir

Introduction: Coastal areas are areas where land and sea meet, towards land, coastal areas include parts of land, both dry and submerged in water, which are still influenced by sea properties such as tides, sea breezes and the expansion of salt water towards the sea, including parts of the sea that it is still influenced by natural processes that occur on land such as sedimentation and fresh water flows. Management of coastal areas and small islands is carried out through coordinating planning, utilization, supervision and control of coastal and small island resources carried out by the government and regional governments, between land and marine ecosystem sectors, as well as between science and management to improve people's welfare. In this regard, the authority to manage coastal areas is regulated in several statutory regulations which can cause problems regarding which institution has the authority to manage coastal areas, especially in Ambon Bay.Purposes of Devotion: The aim of this community service activity (PKM) is to ensure that the Ambon Bay coastal community, especially the community in Wayame Village, understands the authority of institutions in regulating coastal area management and also to obtain suggestions and input from coastal communities directly which can become study material for the Government. Method of Devotion: Activities are carried out in the form of outreach to the community with the stages of implementing community service in outline starting from: (1) Preliminary Data Collection; (2) Community Service Activities take the form of socialization; (3) Evaluation of Community Service activities.Results of the Devotion: The lack of public understanding regarding the role and function of institutional management of the Ambon Bay coastal area means that it needs to be reviewed so that its duties and roles in carrying out, improving the social and economic life of the community, especially those who live around the Ambon Bay coastal area, can increase. To improve the standard of living of coastal communities, an empowerment program can be launched for fishermen who are the main source of livelihood for communities in coastal areas. Empowerment is carried out through developing alternative livelihoods, increasing access to capital for fishermen, increasing access to technology, both fishing/cultivation technology and processing of fishery products, facilitating fishermen to obtain a decent market.

Zooplankton Bio-indicators Against Changing Hydrological Parameters at Bidyadhari River of Indian Sundarbans

Sundarban Estuarine System is influenced by periodic tidal input and fresh water inflow. It is surrounded by world’s largest mangrove ecosystem and harbour naturally grown fishery. The total system is intersected by a network of many rivers and creeks. Bidyadhari is one of the major rivers of Sundarbans that is surrounded by human habitation on both banks. The dynamic condition of the river due to tidal inflow and fresh water input which is further increased many folds due to human interventions. In the present study, 26 zooplankton taxa have been recorded where 16 taxa were found perennial in all three seasons. 19 species showed its highest density in the monsoon season, 4 in post-monsoon and 3 in pre-monsoon season. Considering all seasons, Pseudodiaptomus serricaudatus was found to be the most abundant species in monsoon when there was a marked increase in temperature, nitrate, phosphate, silicate and a decrease in salinity, DO, TDS, EC and chlorophyll-a concentration. On the other hand, Acrocalanus longicornis was most abundant during the post-monsoon season when pH, EC, DO, chlorophyll-a were found to be highest and temperature was lowest of all seasons. Only 3 of the 26 taxa were found in highest concentration during pre-monsoon season when salinity was found in highest concentration. The Shannon-Weiner Diversity Index, Simpson 1_D Index and Margalef Richness Index of zooplankton diversity were found highest in station 3 (nearest to the sea) during all the three seasons. Pearson Correlation Analysis showed some significant correlation between different hydrological parameters and zooplankton diversity indices. The result of the study indicates the presence of organic pollutants and eutrophication in Bidyadhari river that demands regular monitoring of water quality for maintenance of long-term sustainability of one the largest naturally grown fisheries in the world.

Lovers in strange places: amphibian calling and amplexus detected in tidal mangrove creeks after rainfall

Temporary freshwater lenses can form in saline environments after rainfall, providing essential resources for species including drinking water and dispersal routes. However, there is limited indication that these lenses can also be used for breeding. Herein, we provide evidence of the green and golden bell frog, Litoria aurea, performing breeding activities, including calling and amplexus, in tidal mangrove creeks on Kooragang Island, NSW, Australia. Our findings suggest that these creeks intermittently phase into a low salinity state after the influx of fresh water from rainfall, forming temporary freshwater lenses that can be exploited before the creeks revert to a saline state. These lenses had salinities (1.4 ppt and 4.5 ppt) within the tolerance limit of L. aurea tadpoles, although we are unsure whether oviposition took place and offspring survival to metamorphosis was achieved. It is possible that anthropogenic disturbances to hydrology on Kooragang Island have benefited L. aurea by restricting tidal influences in mangrove creeks, prolonging the duration of freshwater lenses.

Effects of hypo-osmotic stress on osmoregulation, antioxidant response, and energy metabolism in sea cucumber Holothuria moebii under desalination environment

With global climate changing, hypo-salinity events are increasing in frequency and duration because of continuous rainfall and freshwater inflow, which causes reduced cytosolic osmolarity and cellular stress responses in aquatic animals. Sea cucumbers are considered stenohaline because they lack osmoregulatory organs and are vulnerable to salinity fluctuations. In this study, we performed multiple biochemical assays, de novo transcriptomics, and widely targeted metabolomics to comprehensively explore the osmoregulatory mechanisms and physiological responses of sea cucumber Holothuria moebii to hypo-osmotic stress, which is a representative specie that is frequently exposed to hypo-saline intertidal zones. Our results found that H. moebii contracted their ambulacral feet and oral tentacles, and the coelomic fluid ion concentrations were reduced to be consistent with the environment. The microvilli of intestines and respiratory trees underwent degeneration, and the cytoplasm exhibited swelling and vacuolation. Moreover, the Na+, K+, and Cl− concentrations and Na+/K+-ATPase activity were significantly reduced under hypo-osmotic stress. The decrease in protein kinase A activity and increase in 5′-AMP level indicated a significant inhibition of the cAMP signaling pathway to regulate ion concentrations. And small intracellular organic molecules (amino acids, nucleotides and their derivatives) also play crucial roles in osmoregulation through oxidative deamination of glutamate, nucleotide catabolism, and nucleic acid synthesis. Moreover, lysosomes and peroxisomes removed oxidative damage, whereas antioxidant metabolites, such as N-acetyl amino acids and glutathione, were increased to resist oxidative stress. With prolonged hypo-osmotic stress, glycerophospholipid metabolism was enhanced to maintain membrane stability. Furthermore, acyl-CoA-binding protein activity was significantly inhibited, and only a small amount of acylcarnitine was significantly accumulated, which indicated a disruption in energy metabolism. PPAR signaling pathway and choline content were up-regulated to promote fatty acid metabolism under hypo-osmotic stress. Overall, our results provide new insights into the osmoregulatory mechanisms and physiological responses of sea cucumbers to hypo-osmotic stress.

Prediction of slowdown of the Atlantic Meridional Overturning Circulation in coupled model simulations

In coupled perturbed parameter ensemble (PPE) experiments or for development of a single coupled global climate model (GCM) in general, models can exhibit a slowdown in the Atlantic Meridional Overturning Circulation (AMOC) that can result in unrealistically reduced transport of heat and other tracers. Here we propose a method that researchers running PPE experiments can apply to their own PPE to diagnose what controls the AMOC strength in their model and make predictions thereof. As an example, using data from a 25-member coupled PPE experiment performed with HadGEM3-GC3.05, we found four predictors based on surface heat and freshwater fluxes in four critical regions from the initial decade of the spinup phase that could accurately predict the AMOC transport in the later stage of the experiment. The method, to our knowledge, is novel in that it separates the effects of the drivers of AMOC change from the effects of the changed AMOC. The identified drivers are shown to be physically credible in that the PPE members exhibiting AMOC weakening possess some combination of the following characteristics: warmer ocean in the North Atlantic Subpolar Gyre, fresher Arctic and Tropical North Atlantic Oceans and larger runoff from the Amazon and Orinoco Rivers. These characteristics were further traced to regional responses in atmosphere-only experiments. This study suggests promising potential for early stopping rules for parameter perturbations that could end up with an unrealistically weak AMOC, saving valuable computational resources. Some of the four drivers are likely to be relevant to other climate models so this study is of interest to model developers who do not have a PPE.

Development of a modified gas turbine-based sustainable power generation and water treatment system; Economical/environmental considerations and data-driven optimization

Developing environmentally friendly solutions that optimize existing power plants while considering economic factors is crucial for ensuring sustainable energy production and mitigating environmental impact. This study promotes a gas turbine-based power plant design that prioritizes environmental concerns and underscores its versatility. The proposed setup is composed of various components, including a gas turbine, a Rankine cycle (RC), an organic Rankine cycle (ORC), a reverse osmosis (RO) desalination unit, a proton exchange membrane electrolyzer (PEME), and a hydrogen blending module. This system engenders the simultaneous generation of hydrogen, electrical power, and desalinated water. Additionally, the direct utilization of produced hydrogen in the proposed system offers improved technical performance and serves as an effective means to mitigate greenhouse gas emissions. A resilient programming code is formulated to systematically evaluate the system through the lenses of techno-economic and environmental considerations. Incorporating innovative modifications into the existing system resulted in an 8% cost reduction, a 1.2% decrease in carbon dioxide emissions, and a 5% enhancement in exergy efficiency. To attain optimal performance of the system, a data-driven and machine learning methodology is employed, wherein two distinct optimization scenarios are used to define the conditions that yield the utmost system performance. In the first scenario, the optimal values for cost, exergy efficiency, and normalized CO2 emissions have been computed as 39%, 5963 $/h, and 367.4 kg/MWh, respectively. In the subsequent scenario, the optimized system demonstrates the capability to generate a fresh water flow rate of 840.4 kg/s. Concurrently, the cost rate for the system equates to 7054.5 $/h.

Early growth and environmental conditions control partial migration of an estuarine-dependent fish

Partial migration is a widespread phenomenon in animals, whereby multiple groups follow different migration behaviors (i.e. contingents) within a single population. Fishes exhibit particularly high diversity in their early dispersal behaviors; however, whether these represent conditional partial migration behaviors remains unclear. We combined otolith microstructure and chemistry to assess the influence of early life conditions and environmental drivers on juvenile-stage partial migration of anadromous striped bass Morone saxatilis in the Hudson River (USA) in 2 consecutive years with contrasting hydrologic conditions. Time series clustering on otolith strontium (Sr) and barium (Ba) profiles revealed 4 dominant early migration contingents in both years: freshwater residents, oligohaline migrants, small mesohaline migrants, and large mesohaline migrants. In both years, juvenile partial migration appeared to be a conditional strategy linked to a growth-mediated threshold. The propensity to migrate early was related to slower larval growth, whereas freshwater residency and delayed migration were associated with faster larval growth. Differences in hatch dates may have indirectly affected migration contingents by exposing larvae to varying environmental conditions. In the dry year, dispersal timing to mesohaline habitats coincided with high freshwater flow and tidal currents, but not so in the higher-flow year. Recruitment to coastal nurseries outside the Hudson River occurred primarily during the juvenile phase. Early migration contingents could carry over to population dynamics, whereby diverse estuarine nursery habitats contribute differently to recruitment.

Dry Me a River: Ecological Effects of Drought in the Upper San Francisco Estuary

Droughts have major effects on estuaries because freshwater entry is one of the defining features of an estuary, and freshwater flow is an important variable that determines the interannual change in the environment. In the upper San Francisco Estuary (the Sacramento–San Joaquin Delta, Suisun Bay, and Suisun Marsh), the Mediterranean climate includes frequent multi-year droughts. To assess ecosystem responses to droughts in the upper estuary, the Interagency Ecological Program Drought Synthesis Team assembled a set of flow, water quality, chlorophyll, zooplankton, and fish data from 1975 to 2021 to test for differences between multi-year droughts and multi-year wet periods and tested for linear relationships between each variable and the Sacramento Valley Hydrologic Index (see definitions and relationships as outlined in Appendix A). Our models showed droughts decreased Delta outflow, project exports, zooplankton in Suisun Bay, and some fish species. We also found that droughts increased water clarity, salinity, nutrients, chlorophyll in the South Delta, zooplankton in the South Delta, and water residence time. Although our analyses only tested correlations, we hypothesized that most of the food-web responses could be traced to increased residence time, decreased transport rates, or both. However, some responses may have been caused by secondary effects, including shifts in salinity gradients, regional changes in water quality, or differences in top-down effects of increased predation and grazing rates. With increased frequency of droughts in the future, this increasingly low-outflow, warming, clearing estuary—which is invaded by non-native species and has low pelagic fish production—is rapidly becoming the new “normal.”

Effects of Freshwater Inflow during the Rainy Season on the Benthic Polychaete Community in the Geum River Estuary, South Korea

In the estuaries of Korea, the freshwater inflow increases rapidly due to the Changma (Korean summer rainy season). To elucidate the effect of this massive freshwater inflow on the benthic polychaete community, a survey was conducted before, during, and after the rainy season. Comparing the environmental characteristics before and after the rainy season, the salinity and dissolved oxygen decreased, the sand content of sediment was significantly reduced, and silt increased. The number of species decreased sharply, and this change was more considerable at sites closer to the estuary. Loimia sp. and Pseudopotamilla sp., the dominant species before the rainy season, were not found after the rainy season. The massive freshwater inflow during the rainy season has been a tremendous stress on the benthic environment and significantly alters the species composition and distribution of benthic polychaetes.

The impact of rainfall on the sea surface salinity: a mesocosm study

Sea surface salinity may serve as a tracer for freshwater fluxes because it is linked to evaporation and precipitation that force the freshwater balance of the ocean’s surface. The relationship between freshwater fluxes and salinity anomalies in the upper few centimeters remains widely unknown. In a mechanistic approach, we investigated how these anomalies develop by conducting experiments with artificial rain over a large basin. We measured conductivity and temperature at different depths and rain characteristics (intensity, rain temperature, droplet sizes, and velocities). In the absence of turbulence, the rain causes a strong salinity change of up to 6.02 g kg-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document} in 0–4 cm depth. At the highest rain intensity of 56 mm h-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document}, salinity changed thrice as fast as at an intensity of 18 mm h-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document}. At the sea surface microlayer (first millimeter of the surface) the anomalies are always highest and reached a maximum of 14.18 g kg-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document} . With mechanical mixing, salinity changes were less pronounced (maximum SML salinity anomaly: 6.17 g kg-1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-1}$$\\end{document}), and freshwater was mixed fast with the existing seawater body. In general, our study shows that freshwater remains in the upper few centimeters, and even with induced turbulence, are not mixed below 16 cm.

Impacts of extreme river discharge on coastal dynamics and environment: Insights from high-resolution modeling in the German Bight

The intricate relationship between extreme river discharge and its consequential effects on coastal basins’ environment and dynamics remains a subject of profound significance. The impact of extreme river discharge on coastal basin dynamics and environment is a complex phenomenon, particularly relevant in the context of the German Bight. The inflow of freshwater from rivers into the German Bight plays a crucial role in driving the complex thermohaline circulation. Addressing the challenges posed by local hydro-meteorological extremes, compounded by strong wind waves, currents and tides is vital for comprehensive impact assessment. Utilizing the Geesthacht Coupled cOAstal model SysTem (GCOAST) with high-resolution configuration, we investigate the sensitivity of the thermohaline properties to the river forcings. By incorporating river discharge as lateral land forcing in a coupled hydrodynamic and wave model, we conducted five sensitivity experiments. Our findings reveal that the simulated temperatures closely match measurements in all experiments. The salinity, however, is remarkably sensitive to the variation of freshwater from the rivers Elbe and Ems in the German Bight, causing the haline stratification. The statistical evaluation, as demonstrated by the Taylor diagram at the Marnet DB station, underscores the skill of the Mesoscale Hydrologic Model (mHM) in generating the freshwater discharge that drives the thermohaline characteristics of the German Bight, especially during events like the June 2013 flooding. Significantly, the use of climatological runoff proves to be ineffective in simulating stratification during extreme flooding events. In essence, this investigation enhances our understanding of the pivotal role played by high-frequency river freshwater buoyancy. It emerges as a driving force behind salinity fluctuations during extreme floods, providing valuable insights into coastal dynamics within the German Bight.

Abundance and Distribution of Blue Swimmer Crab in Response to Environmental Variation Across Two Contrasting Estuaries

Portunid crabs are an iconic, high value species in NSW, but catches are highly variable in space and time. Substantial variation in biomass is observed in both exploited and unfished populations, and environmental effects on distribution and abundance are an important factor contributing to this variability. Predicting and responding to this variability is a challenge for management and sustainability of exploited populations. We examine spatial and temporal variation in Blue Swimmer Crab (Portunus armatus) populations, over a 2.5 year trapping survey in two temperate estuaries that differ in tidal flow and riverine input. Specifically, monthly catch rates and distribution throughout the estuary are examined alongside variation in temperature and conductivity. In Wallis Lake, the shallower estuary with a restricted entrance, both water temperature and conductivity impacted abundance and distribution of crabs but there was no evidence that pulses of freshwater flow had a major impact. For a 10 °C increase in temperature the population on average shifted ~ 700 m closer to the ocean. Males were consistently located slightly further into the estuary compared to females, but there was no convincing evidence of estuarine egression in response to lower salinity. In Port Stephens, the deeper, tidal estuary, water temperature and conductivity also impacted abundance, but while males were more dispersed than females, the distribution of crabs within the estuary did not appear to be influenced by temperature, conductivity or flow. These results highlight the links between nuanced environmental relationships and estuarine geomorphology for Blue Swimmer Crab.

Controls on the formation of Mn carbonates in mudrocks of the Datangpo Formation, Northern Margin Rift Basin, Yangtze Block

Deglaciation following the Sturtian Ice Age led to widespread manganese carbonate precipitates within marine mudrocks, reflecting unique climatic and oceanographic changes. The Mn carbonates hold economic importance in the Datangpo Formation, Yangtze Block, South China. Although their genesis in the Nanhua Rift Basin is well understood, little is known regarding their formation mechanism in the North Margin Rift Basin (NMRB). With growing interest in Mn ore exploration in this region, this study examines the weathering trend, climate patterns and watermass conditions using a variety of geochemical proxies. The results reveal two phases of climate variation during the deglaciation. Climate during the early phase (Phase I) was unstable, characterized by fluctuating values of Chemical Index of Alteration (CIA = 65 ± 4) and elevated Li isotope (8.8 ± 3.0‰) patterns. The degree of chemical weathering during the later stage (Phase II) was relatively stable (CIA = 65 ± 1, δ7Li = 5.4 ± 2.7‰), but the weathering intensity increased due to the reduced denudation rate, possibly related to a shift in the basin development. Fe speciation data indicate euxinia prevailing much of the section. However, Mo and U enrichment (> 1) and fluctuating ratios of Mo versus total organic content (0–40) suggest that pulses of freshwater influx associated with climate variation during Phase I caused the fluctuation of the chemocline along the continental shelf, facilitating the formation of Mn carbonates through the particulate Fe-Mn-hydroxide shuttle. With the establishment of stabilized climate, basin tectonics, and watermass circulation during Phase II, the accumulation of Mn carbonates in the sediments became less significant. The accumulation of Mn carbonates in the NMRB conforms to the “bathtub ring model”, distinct from the metallogenic model in the NRB, providing valuable guidance for Mn ore prospecting within this area and elsewhere with similar settings.

Lithological features of Lake Bannoe sediments (Southern Urals) as an indicator of environmental and climate changes in the Holocene

Research subject. Bottom sediments of Lake Bannoe (Southern Urals). Aim. Identification of lithologic features of Lake Bannoe sediments, which could reflect sedimentation conditions in the Holocene. Materials and methods. The detailed complex analysis included radiocarbon dating, grain-size analysis, X-ray diffraction analysis, electron microscopy, X-ray fluorescence and isotope analysis, coercive spectrometry and pollen analysis. Results. Radiocarbon dating showed that sedimentation in Lake Bannoe began no later than ~13 thousand years ago. Combination of data from various laboratory studies unraveled four lithological zones and the corresponding stages in the sedimentation history. The grain size, allothigenic particles, carbonate minerals, organic matter and isotopic composition of carbon and oxygen are the most informative indicators. Grain size variations and the ratio between allothigenic and carbonate components reflect changes in the Lake’s depth and clastic material supply, which, in turn, is associated with humidity. Organic matter parameters (TOC, δ13Corg, C/N ratio) can be considered as indicators of climate-sensitive changes in bioproductivity of the sedimentation basin. They also reflect the ratio of exogenous and endogenous organic matter in the sedimentary environment. The isotopic composition of carbon and oxygen (δ13Ccarb, δ18Ocarb) in sedimentary carbonates is an informative indicator of lithological zones and climatic events of the Holocene due to its sensitivity to changes in biomass, temperature fluctuations, and fresh water inflow. The paramagnetic component k_para was used as an indicator of the allothigenic material input into the lake basin for the first time in this region. Conclusions. The granulometric, mineral, and chemical composition, as well as the magnetic properties of Lake Bannoe sediments reflect the history of Lake sedimentation in the Southern Urals, which agrees mainly with the climate stages of the Holocene.

Downstream Nutrient Concentrations Depend on Watershed Inputs More Than Reservoir Releases in a Highly Engineered Watershed

AbstractIn this study, we characterized the impact of regulatory water releases relative to watershed inputs on the quality of receiving waters to identify if and how managed releases could be scheduled to mitigate nutrient export and downstream water quality impairment. We specifically investigated freshwater flow partitioning to the Caloosahatchee River and Estuary (CRE) from a large managed lake, Lake Okeechobee, and the CRE's upstream watershed, the C‐43 basin, in southwest Florida (USA). A water balance was developed to identify dominant freshwater inflow sources (i.e., Lake Okeechobee vs. watershed inputs) over time. From the water balance, analyses of historical trends were performed to detect changes in freshwater inflow contributions to the CRE. Further, seasonal and annual concentration variations and long‐term concentration‐discharge (C‐Q) relationships were analyzed to better understand biogeochemical and hydrological processes in the system in relation to freshwater source. Since 1966, we found the duration and magnitude of flows from the C‐43 basin were higher than those from Lake Okeechobee releases. However, recent increases in the annual water volume and proportion of inflow coming from Lake Okeechobee to the CRE were observed. The C‐Q analysis revealed that nitrate and ammonium concentrations in the CRE were responsive to changes in discharge, while total phosphorus and orthophosphate concentrations were chemostatic. While modifications to the Lake Okeechobee operation schedule could potentially mitigate downstream inorganic nitrogen loading, this potential is limited by complex, seasonal C‐Q relationships and confounding effects from surrounding watersheds.

Surface Heat Fluxes Drive a Two‐Phase Response in Southern Ocean Mode Water Stratification

AbstractSubantarctic mode waters have low stratification and are formed through subduction from thick winter mixed layers in the Southern Ocean. To investigate how surface forcing affects the stratification in mode water formation regions in the Southern Ocean, a set of adjoint sensitivity experiments are conducted. The objective function is the annual‐average stratification over the mode water formation region, which is evaluated from potential temperature and salinity adjoint sensitivity experiments. The analysis of impacts, from the product of sensitivities and forcing variability, identifies the separate effects of the wind stress, heat flux, and freshwater flux, revealing that the dominant control on stratification is from surface heat fluxes, as well as a smaller effect from zonal wind stress. The adjoint sensitivities of stratification to surface heat flux reveal a surprising change in sign over 2 years lead time: surface cooling leads to the expected initial local decrease in stratification, but there is a delayed response leading to an increase in stratification. This delayed response in stratification involves effective atmospheric damping of the surface thermal contribution, so that eventually the oppositely‐signed advective haline contribution dominates. This two‐phase response of stratification is found to hold over mode water formation regions in the South Indian and Southeast Pacific sectors of the Southern Ocean, where there are strong advective flows linked to the Antarctic Circumpolar Current.

Impact of Precipitation on Ocean Responses during a Tropical Cyclone

Abstract Precipitation plays a crucial role in modulating upper-ocean salinity and the formation of the barrier layer, which affects the development of tropical cyclones (TCs). This study performed idealized simulations to investigate the influence of precipitation on the upper ocean. Precipitation acts to suppress the wind-induced sea surface reduction and generates an asymmetric warming response with a rightward bias. There is substantial vertical change with a cooling anomaly in the subsurface, which is about 3 times larger than the surface warming. The mean tropical cyclone heat potential is locally increased, but the net effect across the cyclone footprint is small. The impact of precipitation on the ocean tends to saturate for extreme precipitation, suggesting a nonlinear feedback. A prevailing driver of the model behavior is that the freshwater flux from precipitation strengthens the stratification and increases current shear in the upper ocean, trapping more kinetic energy in the surface layer and subsequently weakening near-inertial waves in the deep ocean. This study highlights the competing roles of TC precipitation and wind. Because the TC category is weaker than category 3, the warming anomaly is caused by reduced vertical mixing, whereas for stronger storms, the advection process is most important.

Unraveling the Physical and Biological Controls of the Global Coastal CO2 Sink

AbstractThe drivers governing the air‐sea CO2 exchange and its variability in the coastal ocean are poorly understood. Using a global ocean biogeochemical model, this study quantifies the influences of thermal changes, oceanic transport, freshwater fluxes, and biological activity on the spatial and seasonal variability of CO2 sources/sinks in the global coastal ocean. We identify five typical coastal behaviors (dominated by biological drawdown, vertical transport, land imprint, intracoastal alongshore currents, and weak CO2 sources and sinks coastal regions) and propose a new processed‐based delineation of the coastal ocean based on the quantification of these controlling processes. We find that the spatiotemporal variability of CO2 sources/sinks is dominated by strong exchanges with the open ocean and intracoastal processes, while continental influences are restricted to hotspot regions. In addition, where thermal changes appear to drive the seasonal CO2 variability, it often results from compensating effects between individual non‐thermal terms, especially biological drawdown and vertical transport.

European summer weather linked to North Atlantic freshwater anomalies in preceding years

Abstract. Amplified Arctic ice loss in recent decades has been linked to the increased occurrence of extreme mid-latitude weather. The underlying mechanisms remain elusive, however. One potential link occurs through the ocean as the loss of sea ice and glacial ice leads to increased freshwater fluxes into the ocean. Thus, in this study, we examine the link between North Atlantic freshwater anomalies and European summer weather. Combining a comprehensive set of observational products, we show that stronger freshwater anomalies are associated with a sharper sea surface temperature front between the subpolar and the subtropical North Atlantic in winter, an increased atmospheric instability above the sea surface temperature front, and a large-scale atmospheric circulation that induces a northward shift in the North Atlantic Current, strengthening the sea surface temperature front. In the following summer, the lower-tropospheric winds are deflected northward along the enhanced sea surface temperature front and the European coastline, forming part of a large-scale atmospheric circulation anomaly that is associated with warmer and drier weather over Europe. The identified statistical links are significant on timescales from years to decades and indicate an enhanced predictability of European summer weather at least a winter in advance, with the exact regions and amplitudes of the warm and dry weather anomalies over Europe being sensitive to the location, strength, and extent of North Atlantic freshwater anomalies in the preceding winter.