23 Coastal Oceanographybrosenhe/Oceanography/18_Coastal... · 2010-11-23 · 1 Coastal Oceanography...
Transcript of 23 Coastal Oceanographybrosenhe/Oceanography/18_Coastal... · 2010-11-23 · 1 Coastal Oceanography...
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Coastal Oceanography
Coastal Oceanography
• 95% of ocean life is in coastal waters (320 km from shore)
• Estuaries and wetlands are among most productive ecosystems on Earth
• Major shipping routes, oil and gas exploration, recreational activities take place in these waters
• Oceanography of coastal waters is markedly different than in open ocean.
Coastal Waters
• Generally considered to be marine waters over the continental shelf– Adjoining continents and island– Area dependent on margin type
• Subduction or active margin, coastal waters are a narrow band
• Passive margins, coastal waters cover a larger area
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Salinity and Temperature
• Shallower waters can be mixed rather thoroughly
• The Ekman layer can extend to the bottom seasonally and episodically
• Salinity – isohaline• Temperature – isothermal
Salinity
• River input decreases the salinity– Isohaline at lower salinity than adjacent open
ocean– Conversely, strong haloclines persist where
mixing is weaker– Evaporative latitudes can causecoastal hypersalinity
Coastal Salinity
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Temperature
• In low latitudes, coastal waters can become hyperthermal– No cooler water below to be upwelled– Reflection of the sun off of the bottom
sediments• In high latitudes and in winter, waters do
not get as cold as open ocean water before freezing
Coastal Temperature
Coastal Geostrophy
• Geostrophic currents move around the gyre centers
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Coastal Geostrophy
• Coastal geostrophic currents are created by wind and runoff
Coastal Geostrophy
• Winds can push fresh water runoff against the coast
• As fresh water continues to run off, water flows “downhill” on sloped surface
• Coriolis deflects this flow
Coastal Geostrophic Currents
• Ephemeral –– Depend on wind and amount of runoff
• They can disappear periodically, seasonally
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Estuaries
• Estuary – partially enclosed body of water in which freshwater runoff dilutes salty ocean water– River mouths– Bays, inlets, gulfs– Lagoons
Origin of Estuaries
• Generally, result of sea level rise– Coastal plain estuary
• Rising sea fills existing river valley – “drowned river valley”
Origin of Estuaries
• Fjord estuary– Drowned glaciated valley– Deeper, steeper due to glacial erosion– Moraine deposits at mouth
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Origin of Estuaries
• Bar-built estuary– Created by formation of sandbar deposit– Shallow, barrier island lagoons/bays
Origin of Estuaries
• Tectonic estuary– Formed by tectonic processes
• Faulting• Folding
Estuarine Mixing of Water Masses
• Stratified– Little to no mixing of freshwater into saltwater
• Partially mixed– Moderate mixing of salt and fresh water
• Fully mixed– Isohaline zones
• Salt-wedge– In regions of high runoff
• Inverse– In hypersaline estuaries
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Estuarine Salinity
U.S. Estuaries
• Columbia River mouth – Washington Oregon border
• Chesapeake Bay estuary – drowned Susquehanna River mouth
• Mississippi River – drowned river mouth• Florida Bay – modified bar mouth
Columbia River Estuary
• Salt wedge estuary• Balance between strong tides and high
flow, with limited mixing
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Columbia River Estuary
• Extensive dams provide irrigation and drinking water, electricity, and flood control
• Dams also have caused fairly extensive environmental changes– Salmon run– Nutrient delivery to coastal waters– Sparsely populated watershed
Chesapeake Bay Estuary• Stratified drowned river mouth estuary• Densely populated surroundings• Highly seasonal stratification• Asymmetric freshwater sources
Chesapeake Bay Estuary
• Coriolis effect of water masses
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Chesapeake Bay Estuary
• Strong stratification during spring freshwater flow prevents oxygen from getting to deep water
Chesapeake Bay Estuary
• Coverage of anoxic waters and length of time of anoxic events has increased since the 1950’s. Why?
Lagoons
• Bar-built estuaries– Typical zonation
• Freshwater zone• Brackish zone• Saltwater zone
– Seasonality• Evaporative seasons – seawater flows in, salinity
increases• Rainy seasons – water flows out, runoff decreases
salinity
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Laguna Madre
• North of Rio Grande River, between Texas and Mexico
• Semi-arid, hypersaline• Little tidal exchange• Inverse estuary
Local Estuaries• Lake Pontchartrain
– Rigolet’s Pass attachment to ocean– Bogue Falaya River– Mississippi River
• Mississippi River– Salt-wedge– Bottom below sea level
• 250 feet
– Oak Street submarine coffer dam
Coastal Wetlands
• Coastal wetlands– Salt marsh – Mangrove wetlands
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Mangrove Wetlands
• Important nurseries for larval species• Buffer to coastal erosion• Use of nitrogen and phosphorous
compounds that cause eutrophication
http://academic.evergreen.edu/g/grossmaz/PANKRAGT/
Semi-enclosed Seas
• Mediterranean Sea – sea half-encircled (medi-) by land (-terra)
Mediterranean Sea
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Mediterranean Desiccation• Sea level decreased 150m
during the last glacial maximum
• Only a low flow of Atlantic water entered the Mediterranean
• Other periods, evaporation has far exceeded inflow, drying out the Mediterranean completely and leaving thick salt deposits
http://www2.uca.es/huesped/anasim_gibraltar/english.html
Black Sea Flood
• Noah’s flood?
http://content.answers.com/main/content/wp/en/thumb/1/1b/300px-Black-sea-hist.png
Red Sea• Silled basin like the Mediterranean Sea
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Key Concepts
• Salinity and temperature controls in coastal waters
• Geostrophic currents in coastal waters• Estuaries
– Origins– Types
• Semi-enclosed seas and regional oceanography