Estuaries

what are estuaries?

a body of water partially surrounded by land where freshwater from river meets with saltwater from ocean

what is an estuary mouth?

where the estuary meets the ocean, typically with higher salinity

what is the estuary head?

where the freshwater river enters the estuary, typically with lower salinity

what can estuaries be classified by?

degree of mixing/stratification

what can the degree of mixing/stratification depend on?

shape/depth of estuary, intensity of river flow at the head, range of tides at the mouth

what are the estuary classifications by origin?

drowned river mouths, fjords, bar-built, tectonic

what are drowned river mouth estuaries?

incursions of seawater into river mouths as water level rises (Chesapeake Bay)

what are fjord estuaries?

steep, glacially eroded u-shaped troughs; 300-400m deep & terminate in a shallow lip (sill) of glacial deposits; common in Norway, Greenland, New Zealand, Alaska & western Canada

what occurs when sills (fjords) are shallow?

little vertical mixing occurs below the sill depth & they can become stagnant

what occur when sills (fjords) are deep?

the bottom waters mix slowly with the adjacent ocean waters

what are bar-built estuaries?

barrier island or barrier split is built parallel to the coast above sea level; shallow & usually have a narrow inlet connecting them to the ocean so tidal action is limited (Chincoteague Bay in Maryland)

what are tectonic estuaries?

coastal indentations formed by faulting & local subsidence; both freshwater & saltwater flow into the indentation & an estuary results (San Francisco Bay)

what occurs in a vertically/well-mixed estuary?

complete mixing of fresh & salt water from the surface to the bottom

what is flushing time?

volume of an estuary divided by the time it takes for a water parcel to leave; estimate of the length of time required for the estuary to exchange its water

what are the different types of flushing time?

rapid & slow exchange

what does estuary flushing time implicate?

water quality

what is the result of rapid exchange?

low flushing time = low residence time = less polluted

what is the result of slow exchange?

high flushing time = high residence time = more polluted

how is flushing time measured?

in days

what are the estuary classifications by stratification?

salt wedge, well-mixed, partially mixed, fjord

what is a salt wedge estuary?

strong river flow, low tidal range, entrainment of saltwater, steep salinity levels

what is a well-mixed estuary?

low river flow, strong tidal mixing, uniform salinity with depth, generally shallow; wouldn’t cross salinity levels with depth

what is a partially-mixed estuary?

strong river flow & strong tides, often deeper, somewhere between well-mixed & salt wedge; cross salinity levels with depth; slightly stratified

what are fjord estuaries?

high river inflow; little tidal mixing; deep, small surface area; steep salinity levels; highly stratified

what is hypoxia?

when a body or tissues do not receive enough oxygen; water bodies have low dissolved oxygen levels

what is the relationship between stratification, mixing, & hypoxia?

stratification (layering of water due to density differences) can limit vertical mixing & prevent oxygen from surface to reach deeper layers

what is anoxia?

absence of deficiency of oxygen; severe hypoxia

how do estuaries link to nutrients/eutrophication?

more river inflow = increase in freshwater & stratification = increase in nutrients & eutrophication = higher population levels

how do estuaries & nutrients/eutrophication impact marine life?

more nutrient input in water = more plankton = more photosynthesis at surface = plankton die & sink = material is remineralized = loss of oxygen

what is residence time?

the amount of time particles stay in an area

how does hypoxia link to stratification, nutrients, wind, depth, temperature, & residence time?

higher temperature, greater depth, higher residence time & less wind = stratification = less oxygen; freshwater input = nutrients = stratification & algae blooms = less oxygen