Estuaries and Salt Marshes (30 March and 1 April)

ESTUARIES
I. Types of estuaries
    A. Semi-enclosed coastal bays in which freshwater and sea water mix

    B. Classification based on climate and geology
        1. coastal plain estuary

        2. tectonic estuary

        3. lagoon (semi-enclosed bay; bar-built estuary)

        4. fjord -- valleys deepened by glaciers and invaded by the sea
 
    C. Classification based on the way salt and freshwater mix
        1. salt wedge estuary (positive estuary; river dominated, stratified) --
            -river water that flows on top of the salt water
            -salt wedge moves back and forth with the tides
        2. homogenous (marine dominated, neutral) --
            -complete mixing occurs or evaporation rates = freshwater inflows;
            -create uniform salinities from top to bottom
        3. evaporite (negative) – high evaporation rate causes surface water to become hypersaline
                and this sinks to the bottom and forms a bottom current
 
        4. intermittent (seasonal)

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 -- water density
            d. tides
            e. the Coriolis effect

        4. varies less in the sediments - slow exchange of interstitial water

    B. Substrate
        1. soft mud dominates -- ecological conditions similar to muddy intertidal
        2. may contain both terrestrial and marine derived materials
            a. Aeolian transport -- wind
            b. Silt from rivers and streams
            c. Suspended material from seawater
        3. Gradient of sediment size (facies) that depends on current strength
        4. Storms and floods can reshape estuaries
    C. Temperature
        1. greater fluctuation in temperature as well

        2. more temperature variation as you move upstream
 
    D. Wave Action and Currents
        1. smaller fetch (distance the wind can blow) than the oceans -- leads to smaller waves
        2. currents from combination of tidal and river flows
        3. flushing time -- time required for a given amount of freshwater to leave the estuary and new freshwater to replace it

    E. 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
    F. Oxygen
        1. generally high due to mixing
        2. will vary in salt and freshwater layers
        3. can be anoxic conditions
                a. especially in summer when vertical stratification is strong
                b. exacerbated by human increases of nutrients that increase oxygen utilization (bacterial decomposition) in the bottom layers
        4. most depleted in the muddy substrates

III. The Biota of Estuaries
    A. Number of estuarine species is less than in adjacent marine and freshwaters
    B. Faunal composition
        1. Marine biota
            a. stenohaline – tolerate little change in salinity
            b. euryhaline – can withstand greater changes in salinity (most tolerate down to 15 psu, some to 5 psu)
        2. brackish water/estuarine species -- adapted to life in intermediate salinities; (5-18 psu)
        3. Few freshwater forms extend into estuaries from rivers (not >5-10 psu)
        4. some species are transitional – passing in and out of the estuary during some of their lifecycle
    
    C. Floral composition
        1. limited in number of large plants
            -low light penetration
        2. common types
            a. seagrasses in lower areas
            b. macroalgae
            c. benthic algae -- diatoms; filamentous cyanobacteria
            d. salt marsh plants dominant -- land plants with high salt tolerances; Spartina (cordgrass), Salicornia (pickle weed -- succulent)
            e. bacteria
    D. Estuarine plankton
        1. reduced number of species
        2. phytoplankton – diatoms, dinoflagellates
        3. zooplankton (copepods, mysids, amphipods)
        4. composition and biomass depends on turbulence, turbidity and flushing rate
IV. Adaptations of estuarine organisms
    A. Morphological Adaptations
        1. similar to those in muddy intertidal and subtidal
        2. plants have aerenchyma that brings oxygen down to roots in anoxic sediments
    B. Physiological Adaptations
        1. salinity changes -- osmosis
            a. animals
                (1) osmoconformers – ability to tolerate fluctuations in salinity without significant tissue damage;
                        osmotic concentration of their internal fluids fluctuates with the external environment
                (2) osmoregulators – moves salts or adjusts ions to maintain a favorable water balance

                (3) some may osmoregulate at low salinities and osmoconform at high salinities
            b. vascular plants – movement of salts
                (1) glands that are salt-secreting (Spartina and some mangroves)
                (2) deposition of salts on leaf surfaces
                (3) shedding leaves loaded with salt
                (4) accumulate water to buffer against loss by osmosis (Salicornia) -- succulence strategy
        2. oxygen depletion in muds – like mud flats
    C. Behavioral Adaptations
        1. burrow deeply -- less salinity fluctuation (also lower predation rates)
        2. migrate to water of specific salinity -- e.g., crabs

V. Ecology of Estuaries
    A. Productivity, organic matter and food sources
        1. algal primary productivity (autochthonous production) is generally low
        2. benthic algae can be an important source of primary production
        3. but estuaries usually have large amounts of organic matter that was produced elsewhere
                (e.g., salt marshes, rivers, open ocean) and transported into the estuary – allochthonous production
        4. few herbivores in estuaries
        5. detritus (and bacteria and fungi on detritus) is the base of the estuarine food web

        6. Two major types of estuaries (really ends of a continuum)
            a. European-type estuaries
                (1) Large mud flats with few large plants;
                    autochthonous production - only benthic diatoms and phytoplankton
                (2) Most of the carbon & energy (C/E) is imported from the sea or river
                (3) Allochthonous-driven – net recipients of energy
            b. American-type estuaries
                (1) Salt marshes surrounding smaller mud flats
                (2) Excess carbon/energy is produced in the estuary and is exported to the sea
                (3) Autochthonous-driven
    B. Food webs –
        1. second trophic level
            a. suspension feeding on planktonic detritus
            b. detritus feeding (deposit feeders) in benthos
        2. detritivores are consumed by invertebrate and vertebrate predators
            a. lots of omnivory
            b. birds

    C. Plankton controls
        1. phytoplankton
            a. high nutrient levels, but often not enough nitrogen relative to other nutrients
            b. reduced light due to high turbidity
            c. can get blooms of some algae – dinoflagellates
        2. zooplankton
            a. limited by low phytoplankton or by currents
            b. consume 50-70% of the phytoplankton production – the rest is eaten by suspension feeders on the bottom

 


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