An estuary is a semi-enclosed coastal body of water with a free connection to the open sea and within which sea water is diluted by fresh water. Estuaries are geologically ephemeral and can rapidly disappear with a relatively small change in sea level via glaciations (drop of sea level) and deglaciations (rise of sea level). Deposition of sediment and expansion and succession of salt marshes over time both act to fill in estuaries. The specific morphology of an estuarine basin is determined by its origin. Estuaries may originate from (1) the drowning of river valleys or fjords, (2) the formation of barrier beaches that enclose a shallow bay, or (3) tectonic activity (Levinton 1982). The Hudson River Estuary is a drowned river valley which was also partially glacially excised.
The fresh water derived from land drainage tends to float over the denser sea water, but tidal mixing can reduce or obliterate this stratification. Estuaries are highly variable in physical, chemical, and biological properties. The restricted exchange between estuaries and the open sea allows rapid changes in salinity, temperature, nutrients, and sediment load; this variability and low salinity have strong effects on both the composition and the dynamics of the biota.
Water Density Stratification in Estuaries
The upstream-downstream physical structure of estuaries varies in response to the interaction of fresh water flow, friction, and tidal mixing. The salinity structure of an estuary can be categorized as (1) highly stratified, (2) moderately stratified, or (3) vertically homogeneous.
In a highly stratified estuary, freshwater flows over a deeper layer of dense seawater; in the absence of friction, this seawater layer extends upriver to mean sea level. Friction tends to transfer denser seawater into the overlying freshwater layer. Such estuaries only exist where river flow dominates tidal motion. Good examples are the Columbia River estuary and the Mississippi River, where a layer of salt water extends seaward from the estuary’s mouth. In fjords, a freshwater layer may also extend a great distance over a deeper seawater layer with little mixing.
With moderate tidal motion, mixing occurs at all depths, generating a moderately stratified estuary. The vertical mixing causes the salinity of both upper and lower layers to increase seaward, but at any point, the deeper layer salinity exceeds surface salinity. Chesapeake Bay and the openings of its principal rivers are good examples of moderate stratification. The lower Hudson exhibits moderate stratification for parts of the year, and salinity in lower Manhattan increases with depth.
Vigorous tidal mixing homogenizes the vertical salinity gradient and results in a vertically homogeneous estuary. The salinity at any point in the estuary changes rapidly with the state of the tide. At low tide, the salinity is dominated by downstream river flow, whereas at high tide the inflow of seawater dominates the salinity. Vertically mixed estuaries are of necessity shallow, allowing wind and tidal exchange to mix the water column thoroughly. Such vertical mixing is common in broad shallow estuaries such as the lower part of Delaware Bay.
Salinity and Temperature Profiles
Reduced mixing with the open sea permits local river drainage or the precipitation-evaporation balance to affect seawater properties, e.g., salinity, temperature, and density. Salinity is the amount of dissolved salts in grams per 1000 g of seawater. It is expressed as parts per thousand ( or ppt) and ranges from 33 to 40 in the open ocean. Salinity may also be expressed as units, with no measure (e.g., salinity is 30), but the implication is still parts per thousand. Measures of chloride ion concentration (a major constituent) or electrical conductivity are often used as indices of salinity. In small, tidally mixed estuaries, salinity can change from fresh to completely marine in a tidal cycle. In larger estuaries, salinity gradients are more gradual, tidal motion is of lesser importance, and seasonal freshwater input affects salinity structure to a greater degree.
Species richness decreases up-estuary and reaches a minimum at the critical salinity. The critical salinity, of approximately 5 to 8, denotes a pronounced minimum in the number of benthic invertebrate species. This may be due to the decrease in the numbers of freshwater species at this salinity range and the inability of many marine invertebrates to regulate specific ionic concentrations at and below the critical salinity. In daily estuaries, where salinity is tidally regulated, benthic organisms face a great physiological challenge because they experience both fresh and saltwater in a single tidal cycle. In larger estuaries, less influenced by tidal motion, acclimation is possible as salinity changes gradually over the course of the year. Seasonal changes in freshwater input, however, may change salinity at any point in the estuary throughout the year.
Sea water has a much narrower range of temperature than air (-1.9oC to 40oC). In the open ocean, away from the poles, a stratified profile of temperature usually develops as a result of solar heating. A warm surface layer floats upon a denser colder layer. In the open ocean, there is usually an intermediate layer called the thermocline; in this layer, temperature decreases rapidly with depth. In estuaries, tidal mixing generally prevents the formation of a thermocline. In a highly stratified estuary, the deeper saline layer will be much colder than the upper freshwater layer, but in other estuary types the temperature difference decreases as the influence of tidal mixing increases.
Density is usually expressed in grams per cubic centimeter, but can also be expressed as
ΣSTP = (density-1) x 103
ΣSTP refers to the density of water of a given salinity, temperature, and at atmospheric pressure. Density increases with increasing salinity and decreasing temperature. This explains why, within an estuary, the water of low salinity will float above denser, colder, and higher salinity water.
Tidal Motion
Tidal motion is the result of the gravitational interaction of the earth, moon, and sun. Gravitational attraction (force) between two bodies is proportional to the product of the masses of the two bodies, divided by the square of the distance between them. In addition to the gravitational force, there is a centrifugal force caused by the rotation of the earth and moon. At the center of the earth, these forces are in balance. At points on the earth’s surface, these forces are out of balance, such that there is a net gravitational force on the side of the earth facing towards the moon and a net centrifugal force on the side facing away from the moon. This results in high tides on the sides facing towards and away from the moon, with corresponding low tides on those parts of the earth with no net excess of gravitational or centrifugal forces. Because the moon passes over any point on the earth’s surface every 24h and 50m, there are generally two high and two low tides at most locations every day.
When the sun, earth, and moon are aligned, the gravitational force of the sun amplifies that of the moon causing spring tides – tides of maximal vertical range. When the earth, sun, and moon form a right angle, the two bodies cancel each other, and neap tides of minimal vertical range occur. The spatial arrangement of land and sea as well as the depth and size of basins affect the timing of tides as well as tidal heights. Within estuaries, water movement will depend upon tidal action, basin shape, as well as freshwater discharge (Levinton 1982). All estuaries share a basic estuarine flow, where surface water of low salinity moves seaward and is replaced by more saline water moving up the estuary along the bottom.
Seasonal Change in Estuaries
Estuaries are among the most productive of marine environments, although food abundance does fluctuate greatly over space and time. The extraordinary productivity of estuaries is a product of the large amounts of nutrients that enter the estuary seasonally and of the extensive recycling of nutrients between the overlying water and the biologically active sediments.
Fresh water drainage seasonally delivers large amounts of nutrients in dissolved and particulate form; dissolved nutrients also enter estuaries with the deep-water oceanic flow. These nutrient spikes may be the source of phytoplankton blooms in small estuaries. Infrequent storms also deliver sediment and particulate organic matter to estuaries. The microbial community in the sediments decomposes the particulate organic matter and permits the continuous recycling of nutrients between the bottom and overlying water.
Estuaries fertilized by sewage usually export dissolved nutrients and particulate organic matter to shelf waters (Levinton 1982). The 2 largest organic carbon inputs into the Hudson are: runoff from the upper watershed (26.8%) and sewage (23.5%) [Table 1, Moran & Limburg 1986]. The Hudson River estuary transports nutrients to the inner shelf, which in turn supports high phytoplankton and zooplankton production. However, estuaries are inherently variable. It is thus difficult to make generalizations about the overall importance of nutrient export by estuaries. Strong geographic variation exists in the extent of nutrient export due to variation in estuary size and morphology, variation in river input, regional variation in groundwater and precipitation inputs, and seasonal variation in nutrient cycling. The seasonal cycle of primary productivity depends upon temperature, light, and nutrient levels which are in turn affected by various interacting factors, e.g., climatic regimes, basin size and morphology, river input, etc. These factors will determine estuarine variability in both space and time.
Table 1. Organic carbon inputs into the Hudson River estuary [taken from Moran & Limburg 1986].
|
SOURCE |
METRIC TONS | % OF TOTAL |
| Phytoplankton |
36,364 |
14.8 |
| Macrophytes – Emergent |
3,936 |
1.6 |
| Macrophytes – Tidal Flats |
1,428 |
0.6 |
| Upper Watershed > River Mile 152 |
66,024 |
26.8 |
| Lower Watershed < River Mile 152 |
43,254 |
17.6 |
| Sewage |
57,649 |
23.5 |
| Marine |
36,898 |
15.0 |
|
Total |
245,553 |
100 |
Hydrology of the Hudson River
The Hudson River drains a total of 13,390 square miles in northeastern New York, and parts of Vermont, Massachusetts, Connecticut, and New Jersey (Moran & Limburg 1986). The basin contains three drainage areas: the upper Hudson from Mt. Marcy to Troy, the Mohawk from Rome to Troy, and the lower Hudson from Troy to New York Bay. The Hudson and Mohawk drainage basins are fresh water; the lower Hudson is an estuary.
The Hudson River Estuary is a drowned river valley rising only 1.5 miles along 150 miles between New York City and Troy. The estuary is maintained as a shipping channel, and dredged to maintain a minimum depth of 9 -11 m, although portions of the river are much deeper, e.g. 66m at West Point (Moran & Limburg 1986, Cooper et al. 1988). Slightly more than half the estuary is covered by marshes and wooded swamps; the remainder consists of mud flats that are flooded at high tide. Wetlands are in greatest abundance in the upper third of the estuary.
Hudson Estuary: River Volume, Tidal Volume, and Salinity
The Hudson River Estuary is tidally influenced from the Battery to the federal dam at Troy. Mean tidal flow varies from 425,000 cubic feet/second [cfs] (12,040 m3/s) at the Battery to 0 cfs at the federal dam (Cooper et al. 1988). Two high and two low tides occur daily; the average tidal range is 1.4 m. Strong winds from the south and north can push water into or out of the estuary, obscuring the true tidal regime. Mean flood and ebb current velocities are 0.36 and 0.40 m/s, respectively. Tidal flow can be 10 – 100 greater than fresh water flow; fresh water flow varies seasonally (Moran & Limburg 1986). The highest fresh water flows into the estuary occur in spring and fall, associated with snow melt and rains; the lowest input occurs in late summer. Most of the fresh water enters the estuary above Troy; the remainder joins the Hudson from tributaries. A rough approximation of flushing time, based on the ratio between water volume to annual fresh water flow is 0.35 years (126 days).
Salinity Variation Along Estuaries
The estuary can be divided into 4 salinity zones: polyhaline (18.5-30), mesohaline (5-18), oligohaline (0.3-5), and limnetic-fresh water (<0.3). The location of these zones varies seasonally as well as daily depending on tidal and fresh water inputs. During average fresh water flow, salt water intrusion reaches West Point, about 50 miles from the Battery. During conditions of high fresh water runoff (usually in the spring), salt water intrusion can be pushed south, as far as 15 miles from the Battery. The estuary is generally well mixed during low fresh water flow conditions. Only a 10% increase is generally found from top to bottom; under high fresh water input, vertical salinity differences of 20% are observed. A sharp salinity gradient occurs 25 miles north of the Battery indicating distinct fresh and saline layers, but this gradient disappears approximately 41 miles upriver. The channel deepens 40-44 miles upriver; such channel irregularities create water turbulence and promote vertical mixing. Average temperatures within the estuary generally follow mean air temperature; temperatures range from 0oC in January to a July maximum of 27oC. In the spring and summer, temperature decreases towards the Battery as colder saline water enters with tidal flow. This horizontal gradient reverses in late fall and winter because salt water cools to a lesser extent than shallow fresh water.
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