The salinity of seawater in the open ocean is remarkably constant at about 35 ppt. Salinity in an estuary varies according to one's location in the estuary, the daily tides, and the volume of fresh water flowing into the estuary. In estuaries, salinity levels are generally highest near the mouth of a river where the ocean water enters, and lowest upstream where freshwater flows in.
Actual salinities vary throughout the tidal cycle, however. Salinity levels in estuaries typically decline in the spring when snowmelt and rain increase the freshwater flow from streams and groundwater. Salinity levels usually rise during the summer when higher temperatures increase levels of evaporation in the estuary.
Estuarine organisms have different tolerances and responses to salinity changes. Many bottom-dwelling animals, like oysters and crabs, can tolerate some change in salinity, but salinities outside an acceptable range will negatively affect their growth and reproduction, and ultimately, their survival. Salinity also affects chemical conditions within the estuary, particularly levels of dissolved oxygen in the water.
The amount of oxygen that can dissolve in water, or solubility, decreases as salinity increases. The solubility of oxygen in seawater is about 20 percent less than it is in fresh water at the same temperature.
The degree to which fresh water and saltwater mix in an estuary is measured using isohalines. Isohalines are areas in the water that have equal salt concentrations, or salinities. In estuaries, salinity levels are generally highest near the mouth of a river where the ocean water enters, and lowest upstream where fresh water flows in. To determine isohalines, scientists measure the water's salinity at various depths in different parts of the estuary.
They record these salinity measurements as individual data points. Contour lines are drawn connecting data points that have the same salinity measurements. These contour lines showing the boundaries of areas of equal salinity, or isohalines, are then plotted onto a map of the estuary.
The shape of the isohalines tells scientists about the type of water circulation in that estuary. To survive, fish, crabs, oysters and other aquatic animals must have sufficient levels of dissolved oxygen DO in the water. Oxygen enters the water through two natural processes: 1 diffusion from the atmosphere and 2 photosynthesis by aquatic plants. The mixing of surface waters by wind and waves increases the rate at which oxygen from the air can be dissolved or absorbed into the water.
DO levels are influenced by temperature and salinity. DO levels in an estuary also vary seasonally, with the lowest levels occurring during the late summer months when temperatures are highest. Bacteria, fungi, and other decomposer organisms reduce DO levels in estuaries because they consume oxygen while breaking down organic matter.
Oxygen depletion may occur in estuaries when many plants die and decompose, or when wastewater with large amounts of organic material enters the estuary. In some estuaries, large nutrient inputs, typically from sewage, stimulate algal blooms.
When the algae die, they begin to decompose. The process of decomposition depletes the surrounding water of oxygen and, in severe cases, leads to hypoxic very low oxygen conditions that kill aquatic animals.
Coastal plain estuaries are usually shallow, and since there is a lot of sediment input from the rivers, there are often a number of depositional features associated with them such as spits and barrier islands. The presence of sand bars, spits, and barrier islands can lead to bar-built estuaries , where a barrier is created between the mainland and the ocean.
The water that remains inside the sand bar is cut off from complete mixing with the ocean, and receives freshwater input from the mainland, creating estuarine conditions Figure Fjords are estuaries formed in deep, U-shaped basins that were carved out by advancing glaciers. When the glaciers melted and retreated, sea level rose and filled these troughs, creating deep, steep-walled fjords Figure Fjords are common in Norway, Alaska, Canada, and New Zealand, where there are mountainous coastlines once covered by glaciers.
Tectonic estuaries are the result of tectonic movements, where faulting causes some sections of the crust to subside, and those lower elevation sections then get flooded with seawater.
San Francisco Bay is an example of a tectonic estuary Figure Estuaries are also classified based on their salinity and mixing patterns. The amount of mixing of fresh and salt water in an estuary depends on the rate at which fresh water enters the head of the estuary from river input, and the amount of seawater that enters the estuary mouth as a result of tidal movements. The input of fresh water is reflected in the flushing time of the estuary.
This refers to the time it would take for the in-flowing fresh water to completely replace all the fresh water currently in the estuary. Estuaries and their surrounding wetlands are bodies of water usually found where rivers meet the sea. Estuaries are home to unique plant and animal communities that have adapted to brackish water—a mixture of fresh water draining from the land and salty seawater.
In fresh water the concentration of salts, or salinity, is nearly zero. The salinity of water in the ocean averages about 35 parts per thousand ppt. The mixture of seawater and fresh water in estuaries is called brackish water and its salinity can range from 0.
The salinity of estuarine water varies from estuary to estuary, and can change from one day to the next depending on the tides, weather, or other factors. Estuaries are transitional areas that straddle the land and the sea, as well as freshwater and saltwater habitats. Figure 4. Therefore, the concentrations of these nutrients are not determined by mixing rates, but rather by algal uptake rates.
Most aquatic organisms function optimally within a narrow range of salinity see example in Figure 5. When salinity changes to above or below this range, an organism may lose the ability to regulate its internal ion concentration. Consequently, shifting salinity distributions can affect the distributions of macrobenthos 8 as well as those of rooted vegetation e.
The nature of the longitudinal salinity gradient and the position of certain isohalines is an important factor in the successful recruitment of larval and juvenile fish 10 Salinity is also an important control on the types of pathogenic organisms and invasive species that can occur in a coastal waterway , on the types of species that can occur in algal blooms 12 13 , and on the activity of nitrifying and denitrifying bacteria As a general rule, widely-varying salinity regimes tend to select for a low-abundance and low- diversity suite of species, which are adapted to a broad range of ionic concentrations e.
Figure 5. Note that in this example, salinity was calculated from the sum of the weights of the major ions e. More metals may also enter solution as salinity and water hardness increases because calcium and magnesium ions compete for binding sites on clay -organic particle surfaces, and this can interfere with the complexation and adsorption of metals However, increasing salinity usually causes a reduction in dissolved metal concentrations because the clay -organic particles form flocs with high settling rates which remove the attached metals from the water column.
Flocculation occurring as salinity increases along an estuary can lead to settling of suspended particles and clarification of the water column. The accepted method for determining the salinity of marine waters is by measuring the conductivity EC and temperature of the waters, then calculating the value of salinity using standard equations. A depth determination is also required for greater accuracy in calculation of salinities in deeper water.
Salinity is calculated using the ratio of the conductivity of the marine water to the conductivity of a standard solution of pure water and potassium chloride KCl at 15oC and one standard atmosphere in pressure, and in which the mass fraction of the KCl is This ratio is called the K15 value , and is used to calculate salinity on the Practical Salinity Scale As a ratio of conductivities has no units, the calculated salinity is also dimensionless.
A salinity of The main advantage of using PSU is that marine waters from around the world can be compared on a common scale. Strictly speaking, this comparison is only robust in open marine waters. There can be problems with the Practical Salinity Scale in estuaries because the ion composition of freshwater inflow is often different than that of seawater 18 , and the conductivity measurement is influenced by ion composition.
One should therefore exercise caution in how the acquired data and calculated salinities are used. For example, serious problems may occur if these data are used for highly accurate density calculations.
When abundant freshwater inflow is apparent, one might consider measuring salinity from the sum of the weights of the suite of major, minor ions, and trace constituents. Suites of major ions e. In practice the ratio of the waters and KCl solution are not measured. Instead, calibrated sondes and probe measure the conductivities, temperatures and depths.
These values are used to calculate the salinity in accordance with the Practical Salinity Scale Some sondes do not provide a PSU calculation.
A seawater equation of state calculator is available on the web, and can be used in such cases. These instruments are often referred to as a CTD which is short for conductivity , temperature and depth depth is derived from measurement of pressure. Some points to consider when making salinity determinations with sondes:.
The data is held by the collecting agencies federal, state and local government, community groups and environmental consultants. The seawater equation of state calculator allows you to compute PSU from water temperature , conductivity and water depth or pressure measurement i. We have already upgraded the design of the site and we will be working to update the content over the coming months. Unfortunately many pages may not be where they used to be because this is such a major upgrade.
Skip to content. What is salinity? Salinity regimes There are four main characteristic types of salinity distributions in coastal waterways: stratified; partially mixed and fully mixed and inverse Figure 2. Stratified conditions Stratified coastal waterways are characterised by a distinct increase in salinity with water depth Figure 2A. Partially mixed conditions In partially mixed coastal waterways, tidal currents generate turbulence which promotes vertical mixing Figure 2B.
Fully mixed conditions Fully mixed conditions occur in coastal waterways in cases where tide, river or wave energy produces enough turbulence to mix the water column Figure 2C. Inverse estuary High evaporation rates in the presence of low freshwater inflow can lead to hyper- salinity in tidal embayments and wide shallow estuaries.
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