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Types of Estuaries • Drowned river valleys Chapter 12 Estuaries – Most common type of estuary – They were formed by the “drowning” drowning” of low land around the mouths of rivers when sea level rose after the last ice age – Ex: Chesapeake Bay Types of Estuaries • Bar built estuaryestuary– Built by the accumulation of sediments into sand bars or barrier islands – Ex: North Carolina (seen in lower part of the photo to the right near Cape Hatteras) Types of Estuaries • Tectonic estuaries – – Formed as a result of land sinking due to movements of the Earth’ Earth’s crust – Ex: San Francisco Bay Types of Estuaries • Fjords – – Deep channels cut in the coastal zone as a result of retreating glaciers – Ex: Alaska, Norway – Puget sound is a type of glacial fjord Development of Estuaries • Over millions of years canadian glaciers formed and retreated at least 4 times • 20,000 years ago, glacier 1 mile deep • After glaciers was a large lake • Sea levels eventually rose and filled the estuary • http://exhibits.pacsci.org/puget_s ound/graphics/ps_glaciationsm. mov • Physical Characteristics of Estuaries SalinitySalinity– Can vary from 5 – 30 ppt – Salinity varies according to distance from saltwater (tides) or freshwater (river) input – Can also vary as a result of storms – Depth also contributes to salinity – The diagram to the right illustrates that the salinity is not uniform (saltwater is heavier and sinks below freshwater) – this is known as a salt wedge • Estuaries are the best developed in areas where the coastal plain is flat and the continental shelf is wide. i.e. East Coast US • The opposite is true in areas with steep, narrow continental shelves and coastal plains, i.e., Washington, west coast US Dealing with Salinity Changes • Organisms in the estuaries are normally euryhaline (can tolerate a wide variety of salinities) • Some are osmoregulators that have mechanisms for keeping their internal concentration stable • Others are osmoconformers with internal concentrations that vary with their surroundings Osmoregulation in fishes • Ion transport, and urine production Dealing with Salinity Changes • Flowering plants in the estuarine community must either expel excess solutes (such as Spartina and mangrove trees using salt glands) or concentrating solutes in specific tissues (such as in Salicornia, Salicornia, a.k.a. “pickleweed” pickleweed” or glasswort Substrate Water Temperature • Mostly composed of sand or mud • Mud is difficult to move through • Mud or sand can also shift unlike hard substrate which is a challenge for organisms as well • Particle sizes are so small that most areas are actually anoxic (devoid of oxygen) • Like in the intertidal community (and some areas of estuaries are intertidal), temperature can vary greatly. • Temperatures can vary especially at times when water levels are low. Water Clarity Representative Inhabitants in a Mudflat of an Estuary • Water clarity is very poor in estuaries • This is due to suspended sediments and particles from rivers in the water • This reduced clarity makes it more difficult for photoautotrophs in the water • Most primary production is the result of flowering plants in the salt marsh as opposed to algae or plankton Mudflat ecology • Only the upper few mm of the sediment is oxygenated, the rest is anoxic and rich in hydrogen sulfide • Very little hard substrate which is preferred by many organisms • Burrowing organisms can oxygenate their environment and also provide oxygen for other organisms that live in their burrows – Still can have problems when exposed at low tide – anoxia and shorebirds • Seagrasses can provide substrate and also oxygenate the sediment • Species diversity tends to be higher in subtidal sediments that are sand, and lower in mud http://www.youtube.com /watch?v=HK6IdWTodV 4 http://www.youtube.com /watch?v=0PZwkx-YSbs Communities within an Estuary • “Oyster reefs” reefs” are found in many estuaries • These congregations of oysters are the platform on which dozens of other species grow and thrive • The oysters provide the only hard substrate in many estuaries Communities within an Estuary Zostera marina Eel grass • Saltmarsh • The vegetated area of an estuary • Spartina species predominate near the water in most estuaries (Juncus (Juncus or needlerush can be found near the water in low salinity estuaries < 10 ppt) seagrasses Organisms living in eelgrass meadow Eelgrass distribution in puget sound – 33% reduced • • • • • • lack of beds in southern Puget Sound attributed to a combination of high tidal amplitudes and timing of low tides during the summer During low tide events, especially during hot summer middays, desiccation/heat stress limits the upper distribution, at high tides, enough water covers the plants to limit net photosynthesis at depth. the limit of distribution corresponds to the 10.1-foot tidal amplitude isobar. The problem is exacerbated by the fact that the timing of extreme low tides in southern Puget Sound is in midday, when temperatures are the highest. Additional factors affecting eelgrass • Competitors – Introduced brown seaweed Sargassum muticum – the sand dollar (Dendraster excentricus) disturbs substrate – crabs can uproot eelgrass • Excessive nutrients – algal species such as sea lettuce (Ulva spp.) will – overgrow eelgrass – overgrowth by epiphytes on the blades, blocking light, nutrients and gas exchange. • Communities within an Estuary • Mangroves or mangals • Found in tropical and subtropical coastal areas • Replace saltmarshes in these areas normally Facilitation – Several herbivores (the snail Lacuna spp. and the marine isopod Idotea spp.) can control epiphyte density and thereby benefit the underlying eelgrass • Direct herbivory • Storms can bury or kill eelgrass by sand overwash – usually not significant. – Black Brandt, a small seagoose, can eat large quantities Worldwide Distribution of Saltmarshes and Mangrove Forests Communities within an Estuary • Some mangrove tree species possess pneumatophores, or vertical root extensions, that assist the plant with the exchange of gases (pictured at right) Aerial View of a Mangrove