ESTUARY - derived from Latin “aestuarium”

• Romans, over 2000 years ago - “aestus” - undulating motion or ebb and flow of sea

• Pritchard, 1967 - “An estuary is a semi-enclosed coastal body of water which has a free connection with the open sea and within which sea water is measurably diluted with fresh water derived from land drainage.”

• Many non-tidal seas like Mediterranean, where salt and fresh mix; also, many semi-arid regions do not receive fresh for long periods (definition by Fairbridge, 1980)

• Large estuarine regions found in low-relief coastal regions - Europe and east coast of N. A. - not Pacific coast

• All estuaries are <5000 years old since sea-level peaked to its present level following the last ice age

• Holocene - Age of Estuaries

• Why is there currently a major thrust to understand estuaries?

• Major populations reside on many of the estuaries around the world. Even some of the early civilizations developed along estuaries (Nile, Tigris, Euphrates, etc.)

• 1/3 of U.S. population lives and works close to estuaries; 7 of the 10 largest metropolitan areas border estuarine areas (N.Y., Tokyo, London, Shanghai, Buenos Aires, Osaka, L.A.)

Immense Societal Importance:

1) Areas of high productivity, unlike open ocean - both primary and secondary

2) High primary and secondary productivity lead to increases in commercial fisheries. Estimated that 60-80% of all commercial fishes depend on estuaries for part or all of their life cycle; resurgence after wetlands “hype” on east coast.

3) Sediment transport – satellite images, color differences indicative of suspended sediment loads – high to ocean from large rivers (Amazon, Nile, and Mississippi)

• Margalef 1968 - study of ecosystems at the level in which whole organisms can be considered as elements of interaction, among themselves, or within a loosely organized matrix - geological, chemical, physical and biological processes

Vertical Gradients:

Tides: intertidal zone - floods periodically

• mangroves, salt marshes, mussel and oyster beds, sand and mud flats

• organisms have specialized adaptations in these dynamic zones

Light:

• euphotic zone ~100 m in ocean - lighted

• aphotic zone - no light

• varies with suspended load, photosynthetic activity, etc.

Oxygen:

• estuarine water column usually aerobic

• sediments usually anoxic just a few centimeters below the surface -- effects of bioturbation can alter the RPD layer

• anoxic zone largely controlled organic loading, oxygen consumption and physical mixing

• hypoxia (<2 mg/L) – major problems in Chesapeake Bay and Louisiana coast

Longitudinal Section:

• salinity and oxygen

• oxygen and salinity tend to decrease when moving from the mouth inland regions of estuaries

•Circulation - 3 general types -

• gravitational

• wind

• tidal

Classical - 2-layer flow model

• Most turbid water occurs in oligohaline regions (1-5 ppt) - “turbidity maximum” – flocculation occurs where river and sea mix

Estuarine Food Webs:

• Odum, 1971 –

• Transfer of food energy plants (diatom -> copoepod -> fish)

• Trophic - interchangeable with food (food-chain/food web)

• Estuarine food webs tend to be more complex in relation to open ocean - because of the diversity of resources

• grazing food web - herbivores - starts with live plants

• organic detritus - decomposing organic matter (plant and animal)

• detrital food web - based on detritus (detritivores) - very important in estuaries; water column and particularly sediments

• primary producers - diatoms (phytoplankton)

• secondary consumers - (zooplankton)

• oysters and mussels - filter feeders

•*Benthic-pelagic coupling - important in shallow regions

• Also nutrient regeneration and efflux from sediments

•Energy-Flow Diagrams

• systems ecology

• biotic and abiotic factors

Odum & Copeland, 1972 - Ecosystem is a balance between energies that build structure and order.

Estuarine Ecosystems:

• Energy sources:

1) mechanical energy of moving water

2) sunlight

3) organic and inorganic “fuels” imported into estuaries

• Stresses:

1) stress due to energy diverted from system

2) microscale random disorder (2nd law of thermodynamics)

3) forced losses

Estuarine Focal Point:

• Classic view of why estuaries have high productivity

1) 3 types of 1° productivity (marshgrass, benthic algae, phytoplankton), insures maximum utilization of light

2) ebb and flow movements from tides

3) abundant supply of nutrients

4) rapid regeneration and conservation of nutrients due to activity of microorganisms

Estuarine Geomorphology:

• Many shifts in coastlines over the past few million years. Since last glacial period (15,000 ybp, Wisconsin phase) water rose by 120 m ca. 5000 years ago (Holocene marine transgression)

• Pritchard, 1952 - First physiographic description of estuaries - 5000 ybp all estuaries were formed due to sea level changes (eustatic) - interglacial period ~10,000+/-2000. 3000 ybp to present we have current position in sea level. Eustatic sea level rise is about 2 mm/ yr on U.S. west coast and 1.6 mm/yr on east coast. It is about 8 mm/yr on Gulf coast due to compaction effects.

• During the lowest sea level estuaries occupied the border of shelves

• Any rise in sea level moves estuaries further landward

• River discharges during thaw resulted in high sediment loading to sea - steep sides of canyons - no deposition - thus formed abyssal cones (Mississippi)

• Evidence for estuaries in geologic record from Precambrian on is not well-documented due to variety of facies and variability pf systems, and limited regional expanse

Geomorphic classification:

• Estuaries divided into 6 distinct groups:

1) coastal plain - Thames, UK, Yanghe-Tse, China

2) Ria (former river valleys) - Spain, Argentina

3) lagoons (barrier-built): a. choked (Brazil); b. restricted (Pamlico); c. leaky (Miss. Sound)

4a) fjords: (Sweden, Norway, Chile); 4b) fjard (Sweden, Scotland)

5) tectonically built (San Francisco)

6) delta-front (Mississippi)

Ia. Coastal Plain

• formed from flooded river valleys - from melting glaciers

• seldom deeper than 20 m

• Chesapeake Bay 25 km long, avg 25 km wide

•Ib. Coastal Plain Salt Marsh Estuary - sediment infilling from river; sedimentation much more extensive in salt marsh

• Carolinas to Florida - narrow tidal inlets, exchange into estuary proper - drainage canals

II. Ria

• High relief, mountainous - Spain, Argentina

III. Lagoons

• larger fraction of open water compared to coastal plain estuaries

• lagoons oriented parallel to coast

• uniformly shallow, <2 m deep

• wind dominated

•Lagoons cont.

• Pleistocene interglacial period

• shoreline stabilized 6 m above raised ridge system parallel to coast, usually sand

• or coral reef - last glacial period much erosion on the high areas then - As sea level rose, areas behind the ridges flooded (i.e., Gulf of Mexico – Texas barrier islands

IVa. Fjords:

• origin by glaciation

• scouring by glaciers of river valleys

• sill at seaward edge - deposits of silt not yet scoured

• Fjords: bar on sill ~ 10-90 m, inner fjord 800 m, U-shaped from scouring (spectacular near tectonic plate margins - subducted coasts

• Scandinavia, Alaska, Chile

IVb. Fjard: Swedish; Firths: Scottish -- lower relief than fjords

• usually in area with more extensive continental shelf, not subducted coasts

• not over-deepened basin - much shallower, due to isostatic rebound of continent

• rebound as high as 15 m/1000years, still occurring

V. Tectonic

• tectonically active coasts - San Francisco, flask-shape - Ria - backed by low plains

VI. Delta-Front

• inter-connected embayments

•Sabine Lake, Texas - Chenier Plain near mouth of estuary

• Cheniers - shell-debris ridges set in a marshy coastal plain, many have tree-line above marsh, cheniers furthest inland mark maximum inland stand

• downward draft up to 3 m high

• High sediment supply mud-flats prograde seaward

• When sediment supply decreases waves and longshore currents erode mudflat sediment into chenier ridges

• Alluviation from Sabine and Neches rivers filled the entrenched valley prograding and closing the southern segment of the valley

When examining biofacies and lithofacies:

• oxidized Pleistocene lying beneath fine sand

• recent fluviatile (silt)

• fining upward sequence

Weathering and Erosion

• 40,000 km3 of water run off Earth’s continents

• formation of soils and sediments carries salts, suspended materials, and DOC

• feldspars most abundant group of minerals in lithosphere

• weathering of Na-Feldspar will produce kaolinite

NaAlSi3O8 + CO2 + H2O ---> Al2Si2O5(OH)

Sedimentation:

• Estuarine Facies - A facies is a body of rock with specified characteristics. In sedimentary rocks - basis of bedding, composition, texture, fossils (lithofacies/biofacies) - carbonate, clastic, siliclastic

• Transgressive - subsidence and rise of sea level more important than terrigenous sediment - starved of sediment

• Regressive - subsidence and sea level less important - terrigenous sediment supply high, progradation and increase in proportion of continental facies

Sedimentation Controls in Estuaries:

1) rate and type of sediment supply

2) intensity of flow regime (tides, currents)

3) sea level fluctuations

4) climate

5) animal-sediment relations

6) chemical factors

Estuarine Mixing: water mass diluted or redistributed. A) Advective mixing - longer time scale - dyes, radionuclides, and salinity (most common tracers)

1) conservative constituent - concentration not altered by biogeochemical processes, changes primarily via mixing (i.e., Na, Cl)

2) in most estuaries salt derived from ocean source (35 ppt)

3) easy and inexpensive to use salinity as marker of mixing processes

B) Dispersive mixing - scattering of particulates and dissolved parcels of water in the estuary, from:

1) tidal sloshing – avg. flux of particles by oscillating tidal currents - dominant longitudinal mixing process

2) shear effects - results from different velocities of parallel currents

3) eddy (turbulent) diffusion - random scattering of water and particles by random molecular or eddy motions

4) molecular diffusion - always several orders of magnitude less than turbulent (eddy) diffusion

5) tidal trapping - water temporarily trapped within shoreline indentation

Estuarine Circulation:

• residual water movement, short-term effects averaged out

• residence time - function of circulation patterns - dM/dt = J(in) - J(out)

• dM = particle in question

• time-avg currents - or net currents, tidal currents, tidal residuals, nontidal flows

• Energy: solar heating, gravitational attraction between the moon and sun, on the one hand, and the ocean waters on the other. Solar heating differentials also cause wind, rainfall, etc.

• Estuarine circulation also driven by wind

Summary:

A) gravitational circulation (freshwater runoff)

B) tidal circulation

C) wind-driven

Gravitational Circulation - induced by density and elevation differences between fresh water runoff and salt water. Net seaward flow and bottom landward flow; this is the “classical” estuarine gravitational circulation

Pritchard, 1956 and Pritchard & Kent, 1956 (J. Mar. Res. 15:33-42, 81-91) - gravitational circulation is driven by temperature as well. Flow stronger along right side in estuaryin northern hemisphere -- Coriolis Effect (wide estuaries like Delaware Bay); wind may cause 3 layer circulation in shallow regions

Tidal Circulation:

• In absence of density gradients and wind stress - driven by tidal circulation

• Bay of Fundy - shallow depth and large tidal range (12m - largest range in world); sense of rotation also related to coastal geometry

• Current variability also related to meteorological or wind influence in Texas Estuary (Copeland et al, 1968. Tex. J. Sci. 20:196-199)

Circulation Modes:

1) classical circulation - surface outflow, bottom inflow

2) reverse circulation - surface inflow, bottom outflow

3) 3-layer circulation

4) reverse 3-layer circulation

5) discharge circulation - outflow at all depths

6) storage circulation - inflow at all depths

Far-Field Effects - Long waves! Periods of 2-15 days (0.1-1m/s); wave length of 1000 km

1) Kelvin waves, continental shelf waves

2) Coastal trapped waves

Forces Causing Mixing:

1) tidal forcing

2) wind stress

3) wave motions

4) river runoff