estuaries Flashcards
How is an Estuary Defined?
drowned river valley where river meets ocean. filled with brackish sea water
what are the patterns of circulation and salinity in estuaries?
CIrculation:
- river flow inputs
salt wedge estuaries: river flow dominates, little tidal mixing–>saline ocean water enters estuary as a wedge beneath freshwater
- highly stratified estuary: river flow still dominates, but tidal currents cause more mixing of saline upward: surface layer becomes progressively more saline
-well mixed estuaries; dominated by tidal currents; no salinity gradient
What is the influence of a sill on an estuary?
A sill can act as a barrier to tidal flow, causing the water level on the upstream side of the sill to be higher than on the downstream side, leading to differences in salinity and circulation
What mechanisms are responsible for the formation of a sill?
Deposition of sediments by rivers or waves
What is the role of river runoff in the chemistry of an estuary?
- introduction of nutrients, sediments, organic matter
-causes increased growth of primary producers
What is the Halocline?
A vertical gradient in salinity
What is the chemocline?
The boundary between the oxygenated and unoxygenated layers
Residence time calculation?
Reservoir size/flow rate
How does the water residence time affect the chemistry of estuary waters?
Longer residences times can lead to accumulation of nutrients and sediments, and higher levels of primary productivity
What mechanisms may affect the depth of the chemocline in the Black Sea?
- incursions of very saline mediterranean water into the deeper anoxic layers
- interannual and decadal changes in the fresh riverine input to the surface layer
- changes in coastal currents due to storms
internal waves in the chemocline
-changes in both plankton productivity and species distribution due to changes in salinity
What is the difference between conservative and non-conservative mixing?
conservative mixing: mixing of water with no change in the concentrations of dissolved species,
Non-conservative mixing: mixing that results in changes in the concentrations of dissolved species
What is the flushing time of an estuary and how is it calculated?
The time it takes for the entire volume of fresh water in the estuary to be replaced
t=total volume of fresh water in an estuary/rate of rate of river discharge in estuary
What is the flushing time for a vertically well-mixed estuary?
1-10 days
How does flushing time influence net primary productivity?
Shorter flushing times can lead to higher levels of net primary productivity by increasing the availability of nutrients for primary producers
Why is Cl- used as the x-axis in figure 7.3 in the text?
Cl- represents the salinity gradient along the length of the estuary
What are the two groups of dissolved, inorganic, species in estuary water and how are they defined?
Conservative ions: have constant concentrations regardless of mixing
Non-conservative ions: which can change in concentration due to mixing or biological processes
What estuary processes removed dissolved species from the water?
Flocculation, sedimentation, microbial activity
What is the importance of flocculation in estuary waters?
flocculation is the process by which suspended particles aggregate and settle out of the water column.
How does salinity affect flocculation?
salinity controls the ionic strength of the water, which affects the stability and mobility of suspended particles
What are the possible fates of ‘flocs’ in estuaries?
settling, resuspension, erosion, and transport out of the estuary
How do organic materials on clay surfaces affect flocculation?
organic materials on clay surfaces provide a surface for particle attachment and increasing particle stability
What is the role of collision in the flocculation process?
collision between suspended particles can lead to flocculation by bringing particles into close proximity and promoting particle aggregation
What is the importance of Fe-hydroxide minerals and the impact of their dissolution on estuary water?
they absorb and release dissolved species, and their dissolution can affect water chemistry
What are the mechanisms controlling the concentrations of the major dissolved species in estuary water?
conservative and non-conservative mixing and sediment-water interactions
What are the biogenic nutrients and what is their behaviour in estuaries?
essential elements, such as nitrogen and phosphorus, that are required for the growth of primary produces
What are the net primary productivity of open ocean and estuaries? Why are they so different from one another?
Open ocean: low net primary productivity due to the limited availability of nutrients
Estuaries: high net primary productivity due to the input of nutrients from land
What is a nutrient trap?
an area in which nutrients are retained, often due to sedimentation or adsorption onto particles
What are the internal and external nutrient sources in estuaries?
Internal: decomposition of organic matter, resuspension of sediments, nutrient regeneration from benthic organisms.
External: river inflow, atmospheric deposition, ocean inputs
What processes permanently remove dissolved nutrients from estuaries? which are the most important?
sedimentation, burial, denitrification and uptake by primary producers
What is the redfield ratio and why is it important?
atomic ratio of carbon, nitrogen and phosphorus in phytoplankton and marine organic matter (106:16:1).
it is important because it indicates the relative availability of these nutrients for primary production
What is the definition of a limiting nutrient?
a nutrient in short supply relative to the requirements of primary producers, which limits their growth
What is the common limiting nutrient in estuaries and why is it the limiting nutrient?
Nitrogen is commonly the limiting nutrient in estuaries
1. ratio of n:p in a number of rivers, especially polluted rivers, is lower than that of estuarine plankton, so there is excess P left over in the estuary upon consumption of all N
2. denitrification (reduction of NO3 to N2 and N2O) causes nutrient nitrogen to be selectively lost from estuary
3. N regenerated much slower than P after sedimentation
What is an example of seasonal changes in the limiting nutrient and why does this occur?
changes in river flow and nutrient inputs
What does the term ‘reactive’ mean when it is applied to elements (e.g., N and P)?
reactive: ability of elements to participate in biogeochemical cycles
What is ‘cultural eutrophication’ and why is it important?
human caused eutrophication: excessive growth of algae
Why does enhanced phytoplankton growth lead to oxygen depletion?
decomposition of dead phytoplankton takes up oxygen
What is the effect of suspended particles on net primary productivity?
What is the difference between hypoxia and anoxia?
hypoxia: low dissolved oxygen levels
anoxia: complete absence of dissolved oxygen
How do changes in annual precipitation affect the nitrate loading of an estuary (e.g., the Mississippi-Missouri river system)?
changes amount and timing of river flow–> affects nutrient inputs
How is global change expected to affect the oxygen concentration in estuary waters?
increases water stratification and nutrient input, aiding in hypoxia,
What is ‘biochemical oxygen demand’ and why is it important?
the amount of oxygen consumed by microbial decomposition of O.M in water.
- high BOD levels can lead to oxygen depletion and hypoxia
How can hypoxia and anoxia be ameliorated in estuaries?
- reducing nutrient inputs
- restoring wetlands
- improving wastewater treatment
What are dinoflagellates and why are they important?
a group of important primary producing unicellular algae that can cause algal blooms
WHy is most sediment deposited in deltas, estuaries and coastal marine environments?
these areas are where rivers meet the ocean and sediment transport processes change
How are heavy trace metals removed from the water in estuaries?
adsorption, uptake by benthic organisms
How does the type of estuary affect the transport of sediments from the river to the ocean?
What role do floods play in the transport of sediments to and through estuaries?
increases sediment supply and alters the flow regime
What is the importance of fecal pellets in estuaries?
they contribute to the nutrient cycling, productivity and food webs of the estuarine ecosystem
Where are most sediments trapped?
intertidal zone where tidal currents and wave action are reduced
- these zones have a high rate of sediment deposition
What are antiestuaries and where can they be found?
saline coastal body of water where seawater flows into estuary in surface layer
What are the mechanisms that create an antiestuary?
tectonic uplift, arid climate, restricted water exchange with the ocean
What minerals form as the water in an antiestuary evaporates and what species do they remove from the water?
Halite (NaCl), gypsum (CaSO4*2H2O), and anhydrite (CaSO4) can form.
- removes ions of Na, Cl, Ca, SO4 from water
How do evaporite rocks differ from the minerals expected to form from evaporating seawater?
What is a barred basin?
a basin separated from the open ocean by a narrow barrier, such as a sandbar or a reef. Barrier restricts water exchange and creates a shallow brackish environment
What is the first mineral to form during seawater evaporation in a barred basin?
Aragonite (CaCO3), which is supersaturated in seawater and can precipitate rapidly
What can cause a lateral variation in the mineralogy of evaporite deposits?
differences in the composition and concentration of the brines that precipitated the minerals, as well as by changes in the environmental conditions, such as temperatuer and salinity
What is a sabhka?
a type of coastal salt flat that forms in arid and semiarid regions.
- characterized by a flat, hard crust of evaporite minerals, such as gypsum and halite, often covered by a thin layer of windblown sand.
