High Conductivity Limnology – Estuaries and Saline Lakes


Estuaries
I. What are estuaries?
    A. Definitions
        1. Where freshwater drainages meet the sea
        2. Semi-enclosed coastal bays in which fresh water derived from land drainage and sea water from the ocean mix
        3. Classifications
            · salt wedge 

            · patterns of salt and freshwater (stratified/partially mixed/mixed)
    B. Very productive systems
        1. Allochthonous from rivers and land – detritus is very important
        2. Autochthonous
            a. Phytoplankton
            b. Sea grasses and macroalgae
II. Physical Characteristics of Estuaries
    A. Salinity
        1. single most important characteristic; nowhere is variation in salinity more pronounced than in estuaries
        2. variability is horizontal, vertical and seasonal
        3. causes of variability
            a. amount of freshwater input
            b. evaporation
            c. vertical variability (density differences)
            d. tides are periodic events
    B. Substrate
        1. soft mud
        2. terrestrial and marine derived materials
        3. gradient of sediment size that depends on current strength
        4. storms and floods can reshape estuaries
    C. Wave Action and Currents
        1. currents from combination of tidal and river flows
        2. flushing time

    D. Turbidity --
        1. often high due to resuspension of particles
        2. lowest at mouth of estuary and highest inland and when river flow is large
        3. decreases light penetration -- reduces primary production
    E. Oxygen
        1. generally high due to mixing
        2. will vary in salt and freshwater layers (salinity; temperature, stratification...)
        3. can be anoxic conditions
        4. depleted in the muddy substrates

III. Interesting organisms and their adaptations in estuaries
    A. Number of estuarine species is less than in adjacent marine and freshwater habitats
    B. Faunal composition
        1. Marine and freshwater biota
            a. stenohaline – very narrow salinity tolerance
            b. euryhaline – broad salinity tolerance
                (1)osmotic regulation
                    (a) osmoconformers – osmotic concentration of their internal fluids fluctuates with the external environment

                    (b) osmoregulators

                    (c) some may osmoregulate at low salinities and osmoconform at high salinities
            c. Plankton
                (1)  reduced number of species
                (2) composition and biomass depends on turbulence, turbidity and flushing rate
            d. Benthos --often are lots of anoxic sediments
                (1) get tolerant organisms and organisms that aerate their burrows
                (2) oyster beds
                (3) seagrasses in lower areas
                (4) macroalgae
                (5) benthic algae -- diatoms; filamentous cyanobacteria
        2. brackish water/estuarine species -- adapted to life in intermediate salinities; not in freshwater or seawater (5-18 psu)
        3. some species are transitional – passing in and out of the estuary during some of its lifecycle;
            a. migratory fish
            b. some shrimps
            c. breeding and feeding grounds for many birds, fishes
IV. Human impacts on estuaries
    A. Have historically been treated as wastelands that should be reclaimed for human use --
            housing developments, marinas, seaports, industrial parks, cities, garbage dumps…
        1. Dikes and drainages; fill
        2. 1/3 of all estuaries in the US are completely gone
        3. dredging of navigation channels
    B. Rivers input modifications

        1. damming and diversion often reduces freshwater flow from rivers and affects salinity of estuaries
        2. carry pollutants  -
        3. carry extra nutrients leading to eutrophication (more later in course!)
    C. Exotic species – ports and ballast water (more later in course!)
    D. Overharvesting (fish, shellfish)
    E. Aquaculture

Salt Lakes

I. Distribution
    A. half of the water volume in the world's lakes is fresh and half is saline
    B. global distribution
        1. generally restricted to semiarid and arid regions
       
        2. some in other regions if near underground salt sources or a salt mine
    C. Interesting facts: saline lakes include
        1. the largest (Caspian)
        2. highest (Nan Tso, Tibetan Plateau 4,718 m.a.s.l.)
        3. and lowest (Dead Sea, 400 m.b.s.l.) lakes of the world
    D. less studied

II. Characteristics
    A. salinity
        1. Definition of saline lakes -- salinity greater than or equal to 3,000 mg/L = 3 ppt
            a. often a breakpoint for biota
            b. a point at which certain chemical equilibria change
            c. subsaline lakes >0.5 ppt (500 mg/L) but <3
            d. freshwater <0.5 ppt
        2. Subcategories of saline lakes
            a. hyposaline – 3-20 ppt
            b. mesosaline – 20-50 ppt (oceans are ~33 ppt)
            c. hypersaline - > 50 ppt
        3. records
    B. Freezing point depression
    C. Sometimes high pH
    D. Variety of chemical composition
    E. Extremely high nutrient levels
    F. High, potentially toxic levels of trace metals
    G. Low oxygen

III. Unique fauna and flora
    A. Almost all species in saline lakes are derived from freshwater, not marine species
    B. Decreasing numbers of species phytoplankton, zooplankton and benthos as salinity increases

    C. Species are often cosmopolitan
    D. The few species present are often in high densities and very productive
    E. Some specific organisms and food webs
        1. some tilapias (fish) can live in moderately saline lakes and wetlands
        2. brine shrimp, Artemia - occurs world wide in fishless saline lakes and tolerates high salinities (>300 ppt).


        3. Lake Nakuru (Kenya) - flamingo population
        4. Lake Werowrap, Australia – even simpler food web

IV. Human impacts on salt lakes
    A. Mining of minerals – salt, uranium, lithium, borax, potassium, nitrates, sodium carbonates
    B. Harvesting of phytoplankton -- Spirulina
    C. Harvesting of animals – brine shrimp; sometimes spread to other lakes
    D. Global warming
    E. Case studies -- under threat from diversion of water from rivers for irrigation of crops in the semiarid zone (like Aral Sea)
        1. Great Salt Lake
        2. Dead Sea
        3. Mono Lake, CA – tufa; brine shrimp and brine fly; waterfowl;
            ecosystem, lake level and water management model interact with politics of water management in the Western U.S.

 

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