Freshwater-saltwater Transition Research Articles

Deltaic and Estuarine Controls on Mackenzie River Solute Fluxes to the Arctic Ocean

River discharge has an important influence on the chemistry of Arctic shelf seas and surface waters. In this study, we examined the changes in nutrient, trace metal, dissolved organic matter, dissolved inorganic carbon, alkalinity, and radium isotope distributions across the Mackenzie River delta and estuary in June 2016, and estimated trace element and isotopes fluxes from the Mackenzie River to the Western Arctic Ocean. While inorganic nutrient, dissolved inorganic carbon, alkalinity, trace metal, and radium levels remained fairly constant through the delta, dissolved organic matter concentrations were 2–3.5 times higher at the mouth of the river, reflecting inputs from the delta. In the estuarine mixing zone, radium isotope ratios indicated that both desorption from suspended sediments and benthic inputs affected solute concentrations. Similarly, most of the measured trace metals (Ba, Co, Cu, Mn, Ni) exhibited non-conservative addition in the freshwater-saltwater transition zone, while Pb was removed and U was mixed conservatively. Inorganic nutrients were affected by both addition and removal processes in the estuary. Dissolved inorganic carbon and alkalinity exhibited net removal, likely due to a combination of biological uptake, gas exchange, CaCO3 precipitation, and surface ion exchange processes. Approximately 45% and 60% of dissolved organic carbon and nitrogen, respectively, were removed rapidly at low salinities, in contrast with previous studies suggesting conservative mixing through Arctic estuaries. This study highlights the need to take deltaic and estuarine processes into account when determining the flux of riverine solutes to the coastal ocean.

Phytoplankton, dissolved oxygen and nutrient patterns along a eutrophic river-estuary continuum: Observation and modeling

Transport and fate of phytoplankton blooms and excessive nutrients along salinity and turbidity gradients of a river-estuary continuum could determine when and where impaired water quality occurs. However, the general spatiotemporal patterns, underlying mechanisms and their implication for water quality management are not well understood. This study reveals typical seasonal variations and longitudinal patterns of phytoplankton, dissolved oxygen (DO) and nutrients (C, N, and P) in the lower St. Johns River estuary in Florida based on 23 years of data and a model which spans 3 years. Evident declines in freshwater phytoplankton and DO concentrations were observed in the freshwater-saltwater transition zone and the estuarine turbidity maxima along the river-estuary continuum. Observations show that most cyanobacteria blooms originating from upstream lakes collapsed in the freshwater-saltwater transition zone where salinity was greater than 1 ppt, but data analysis and model simulation both indicate salinity stress was not the sole reason, other factors such as changes in hydrodynamics and river morphology also contributed to the bloom crashes. Inorganic nutrients (ammonium, nitrate, and phosphate) exhibited inverse longitudinal patterns with phytoplankton. Due to algal uptake, summer concentrations of inorganic nutrients were low in the freshwater, but substantially elevated in the marine reach as a result of large point source inputs and nutrient regeneration from organic detritus. However, because of strong river-ocean mixing, the dramatic increase in nutrients did not promote a phytoplankton bloom in the local marine reach. The nutrients were eventually transported into coastal waters or oceans where they fueled phytoplankton blooms. Our findings highlight that strategies for nutrient reduction and phytoplankton bloom management should be developed beyond local reaches and across a river-estuary-ocean continuum, exploring the possibility that freshwater phytoplankton blooms and excessive nutrients may be transported to downstream estuaries, coastal waters and even oceans that are vulnerable to poor water quality.

Comparison of Migratory and Resident Populations of Brown Trout Reveals Candidate Genes for Migration Tendency.

Candidate genes associated with migration have been identified in multiple taxa: including salmonids, many of whom perform migrations requiring a series of physiological changes associated with the freshwater–saltwater transition. We screened over 5,500 SNPs for signatures of selection related to migratory behavior of brown trout Salmo trutta by focusing on ten differentially migrating freshwater populations from two watersheds (the Koutajoki and the Oulujoki). We found eight outlier SNPs potentially associated with migratory versus resident life history using multiple (≥3) outlier detection approaches. Comparison of three migratory versus resident population pairs in the Koutajoki watershed revealed seven outlier SNPs, of which three mapped close to genes ZNF665-like, GRM4-like, and PCDH8-like that have been previously associated with migration and smoltification in salmonids. Two outlier SNPs mapped to genes involved in mucus secretion (ST3GAL1-like) and osmoregulation (C14orf37-like). The last two strongly supported outlier SNPs mapped to thermally induced genes (FNTA1-like, FAM134C-like). Within the Oulujoki, the only consistent outlier SNP mapped close to a gene (EZH2) that is associated with compensatory growth in fasted trout. Our results suggest that a relatively small yet common set of genes responsible for physiological functions associated with resident and migratory life histories is evolutionarily conserved.

地質時間にわたる淡塩漸移帯の形成過程と形態変化に関する数値解析的検討 （その２） 動的境界条件下における検討

地質時間にわたる淡塩漸移帯の形成過程と形態変化に関する数値解析的検討 （その２） 動的境界条件下における検討

The St. Lawrence River, Canada-USA: the Need for an Ecosystem-Level Understanding of Large Rivers.

Although large rivers are important wildlife habitats and a valuable resource for potable water, transport, energy generation and waste disposal, they have received relatively little attention by limnologists. Successful management of these environments will require consideration of their unique properties as living systems. Using examples from the St. Lawrence River, we identify environmental pressures on large rivers that would greatly benefit from an integrated `whole ecosystem' approach towards their understanding and management: hydraulic control, channel modification, contaminant discharge (industrial carrying capacity), eutrophication, climate change and community shifts, including the invasion of exotic species. The downstream reach of such environments, in particular the freshwater-saltwater transition zone (FSTZ), is a critical ecotone for the entire river system and is highly sensitive to each of these anthropogenic effects. The FSTZ integrates upstream and downstream processes, is one of the most biologically productive sections of the river, and is a prime site for monitoring fluvial and estuarine health.

Numerical simulation of the influence of management alternatives of a projected reservoir on a small alluvial aquifer affected by seawater intrusion (Almuñécar, Spain)

The present study concerns the application of a numerical approach to describe the influence of anthropogenic modifications in surface flows (operation of a projected reservoir) on the freshwater-seawater relationships in a downstream coastal aquifer which has seasonal seawater intrusion problems (River Verde alluvial aquifer, Almunecar, southern Spain). A steady-state finite element solution to the partial differential equation governing the regional motion of a phreatic surface and the resulting sharp interface between fresh water and salt water was used to predict the regional behavior of the River Verde aquifer under actual surface flow conditions. The present model approximates, with simple triangular elements, the regional behavior of a coastal aquifer under appropriate sinks, sources, Neumann and open boundary conditions. A steady-state solution to this numerical approach has been shown to precisely calculate freshwater heads, saltwater thicknesses, and freshwater discharges along steeply sloping coasts. Hence, the adequate treatment and interpretation of the hydrogeological data which are available for the River Verde aquifer have been of main concern in satisfactorily applying the proposed numerical model. Present simulated conditions consider steady-state yearly averaged amounts of external supplies of fresh water in order to determine the influences of the projected Otivar reservoir on the further behavior of the River Verde coastal aquifer. When recharges occur at the coastline, essentially because of freshwater deficits due to groundwater overexploitation, a hypothesis of mixing for the freshwater-saltwater transition zone is made in order to still allow the model to continue calculating groundwater heads under the sea level, and, as a consequence, the resulting seawater intrusion and recharges of saltwater from the sea. Simulations show that a considerable advance in seawater intrusion would be expected in the coastal aquifer if current rates of groundwater pumping continue and a significant part of the runoff from the River Verde is channeled from the Otivar reservoir for irrigation purposes.

Photosynthetic and bacterial production gradients in a larval fish nursery:the St. Lawrence River transition zone

Phytoplankton biomass, primary production and bacterial abundance and production were measured across the freshwater-saltwater transition zone of the St. Lawrence River [Canada) during the seasonal period of maximum concentrations of fish larvae and macrozooplankton. The estuarine front was characterized by steep gradients in b~olog~cal s well as physical properties. Maximum turbidity and high phytoplankton biomass occurred in the well-mixed, low salinity (0.2 to 4 psu) region immediately upstream of the salt wedge. Peak zooplankton and ichthyoplankton biomass occurred within and slightly downstream of this frontal region The hypothesis that lower food chain processes were controlled exclusively by allochthonous carbon and bacterial heterotrophy was not supported. Photosynthetic rates per unit chlorophyll a [chl a) remained high across the freshwater-saltwater transition and the low light penetration was offset by a shallow mean depth of mixing. Bacterial concentrations and activity remained relatively constant across the transition, while chl a declined sharply downstream of the front, consistent with grazing losses. Photosynthesis contributed 34 to 66% of the total production (bacteria + phytoplankton). Freshwater phytoplankton advected from upstream contributed another 20 to 30%. These first-order estimates underscore the combined importance of photosynthesis plus bacterial processes within the downstream food web of large river ecosystems.

Shaly-Sand Log Analysis in an Environmental Application: ABSTRACT

Abstract A procedure to determine ground-water quality from analysis of electromagnetic induction logs was developed to detect and assess the distribution of saltwater intrusion into the alluvial Great Bend Prairie aquifer. The cross correlation between the induction and natural gamma-ray logs from a freshwater well was optimized (based on empirical statistics) to reduce shaly-sand effects — the variations on the induction log due primarily to the presence of clay minerals. The correction method obviates determination of variations of aquifer porosity or cation-exchange capacity for detailed log analysis of porewater conductance. The method was applied to sets of logs collected from a network of monitoring wells in the aquifer. The corrected induction logs (units of mS/m) were calibrated to chloride concentration (mg/L) based on a highly significant linear-regression relationship (R=0.99) between the chlorinity of well-water samples and the median values of the corrected log for the screened intervals of the wells sampled. The derived freshwater-saltwater transition zone profiles were processed by curve-fitting techniques to develop objective, quantitative estimates of key characteristics such as the saturated thicknesses of the usably fresh and unusable salt water (at the 500 mg/L chloride interface) and the determination of aquifer dispersivity. The curve-fit method also allowed incomplete transition zone profiles to be completed to the base of the aquifer by extrapolation. The transition zone characteristics provide a database for regionalization of saltwater distribution and for monitoring changes of the distribution at both local and regional scales.

Photoadaptation to intermittent light across the St. Lawrence Estuary freshwater-saltwater transition zone

Photoadaptation to intermittent light across the St. Lawrence Estuary freshwater-saltwater transition zone

The saltwater-freshwater interface in the Tertiary limestone aquifer, southeast Atlantic outer-continental shelf of the U.S.A.

Abstract Hydrologic testing in an offshore oil well abandoned by Tenneco, Inc., determined the position of the saltwater-freshwater interface in Tertiary limestones underlying the Florida-Georgia continental shelf of the U.S.A. Previous drilling (JOIDES and U.S.G.S. AMCOR projects) established the existence of freshwater far offshore in this area. At the Tenneco well 55 mi. (∼88 km) east of Fernandina Beach, Florida, drill-stem tests made in the interval 1050–1070 ft. (320–326 m) below sea level in the Ocala Limestone recovered a sample with a chloride concentration of 7000 mg l −1 . Formation water probably is slightly fresher. Pressure-head measurements indicated equivalent freshwater heads of 24–29 ft. (7.3–8.8 m) above sea level. At the coast (Fernandina Beach), a relatively thin transition zone separating freshwater and saltwater occurs at a depth of 2100 ft. (640 m) below sea level. Fifty-five miles (∼88 km) offshore, at the Tenneco well, the base of freshwater is ∼1100 ft. (∼335 m) below sea level. The difference in approximate depth to the freshwater-saltwater transition at these two locations suggests an interface with a very slight landward slope. Assuming the Hubbert interface equation applies here (because the interface and therefore freshwater flow lines are nearly horizontal) the equilibrium depth to the interface should be 40 times the freshwater head above sea level. Using present-day freshwater heads along the coast in the Hubbert equation results in depths to the interface of less than the observed 2100 ft. (640 m). Substituting predevelopment heads in the equation yields depths greater than 2100 ft. (640 m). Thus the interface appears to be in a transient position between the position that would be compatible with present-day heads and the position that would be compatible with predevelopment heads. This implies that some movement of the interface from the predevelopment position has occurred during the past hundred years. The implied movement is incompatible with the hypothesis that the freshwater occurring far offshore in this area is trapped water remaining since the Pleistocene Epoch.