lecture 8 soft sediment intertidal, estuaries. soft sediment intertidal

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Lecture 8

Lecture 8Soft Sediment Intertidal, Estuaries

Soft Sediment IntertidalSand vs. Mud Bottom BenthosSandMudInterstitial microfauna presentNo interstitial faunaMeiofauna dominated by nematodes, polychaetes, copepodsMeiofauna dominated by nematodes, coppepods, ostracodsMacrofauna dominated by filter feeding bivalvesMacrofauna dominated by deposit feeding polychaetesAbundance and productivity low (thousands/m2)Abundance and productivity high (tens to hundreds of thousnads/m2)Soft Sediment IntertidalZonation not as distinct as on rocky shoresReduced vertical desiccation and temperature stress gradientsOrganisms can burrow to avoid temperature stress and desiccationVariation in larval settlement not as important as it is on hard substrates

4Variation in ZonationTemperateTropicalSupra-littoralamphipodsghost crabsMid-littoralisopodsIntra-littoralMoon snail, large clams, sand dollars5Biogeography in Soft-Bottom SedimentsWidest variation in densities and highest species diversity occurs in tropicsTemperate beaches usually have a high amount of faunal diversity, a greater amount of longer-lived species, and a greater stability of faunal composition than tropical beaches6

7Interspecific Competition in Soft SedimentsFood and space for burrows is limitedBurrowing invertebrates compete for space within sedimentDominant species found at different levels below sediment-water interfaceLittle evidence of competitive exclusion

Soft Sediments - Vertical StratificationExperimentally reduce density of deep-dwelling clams, remaining individuals grow fasterRemoval of shallow dwelling species of bivalves has no effect on growth of deeper-dwelling speciesLikely limiting factor = space

9Saccoglossus bromophenolosusSome burrowing species produce Bromine poisons Discourages settlement of other species (possibly discourages predation also)

Soft Sediments - Competition10Food Supply in Soft-Sediment IntertidalSuspension Feeders Phytoplankton suspended in waterDeposit FeedersMicroalgae and bacteria on sediment surfaceDecomposing organic matterInput can be spatially variable11Patchy occurrence of sea lettuce (Ulva sp.) leads to spatially patchy inputs of particulate organic matterPatchy POM leads to patchy distribution of small polychaetes and mud snails

Food Supply in Soft-Sediment IntertidalFood Supply in Soft-Sediment IntertidalFood supply for deposit feeders is more stable than the food supply for suspension feeders Diatoms and other microalgae that deposit feeders eat are a renewable resourceCan have seasonal blooms of deposit feeders

Movement of OrganismsSwash riders: move up and down to maintain burrowing position in moist sand, as tide rises and falls

14Predation in Soft SubstratesPredation and physical disturbance are likely the main processes responsible for maintaining high variability in distribution of organismsDo not see a lot of competitive exclusion, so predation typically has little effect on species diversity Partial predation is significant in soft substrates15Predation Types of PredatorsSurface predators prey at or near surface; consume whole animals or only parts of their prey Burrowing predators move down tubes and burrows of prey to attack themDigging predators excavate through the sediments to obtain their prey16Differential Effects of PredatorsQuammen (1984) examined effects of birds, crabs, and fishes on tidal flat communitiesCrabs had greatest impact; fishes had least impact; effects of birds were variable and depended on habitat type

Reise (1978) found that smaller predators can have greater effects than larger predators17Predation Seasonal EffectsSeasonal influxes of predators can devastate local soft-sediment communitiesPredators focus on most abundant species

Disturbance and Habitat HeterogeneityDisturbance re-suspends sediments and blasts out organisms1st successional species usually small polychaetesPhysical and biological disturbanceOrganisms can affect habitat heterogeneitySpatial Scales of Disturbance

20Estuaries, Salt Marshes, Seagrasses, and Mangroves


EstuariesEstuary = partially enclosed section of the coast where freshwater from rivers mixes with seawaterWatershed = the surrounding land that provides freshwater input to the estuaryWatersheds

Tampa Bay WatershedMobile Bay WatershedTypes of Estuaries

Estuarine StructureEstuarine structure is controlled by seaward flow of freshwater combined with tidal mixing

27Estuarine StructureSalinity structure of an estuary is determined by:Watershed topographySlope and size of river(s) feeding into main part of estuarySize of main estuary channelTidal flowEstuarine StructureOverall river discharge important to salinity transitions within estuariesStorm events (hurricanes, etc.) can lower salinity throughout estuary Productivity in EstuariesGeologically ephemeral but biologically richNutrients from freshwater sources and nutrients recycled from seabed support high levels of primary production30Estuarine Species and SalinityMarine species can generally tolerate salinity fluctuations as long as salinity stays above 10-15 pptVertical salinity stratification bottom organisms can go farther upstream than planktonic speciesMixed estuaries infaunal species experience less salinity fluctuation than epifaunal species b/c of buffering effect of sediment pore waters

Estuarine Species and SalinityNumber of marine species declines with decreasing salinity, especially in so-called critical salinity range of 3-8 ppt

Two-Phase Life Cycles of Some Estuarine InhabitantsSome species complete their entire life cycles within an estuaryOther species have a two-phase life cycle in the estuary and on the continental shelf

Suspension Feeders in EstuariesRetention time = the average number of days that a phytoplankton cell stays in an estuaryTurnover time = the number of days that it takes for a bivalve population to completely filter the water columnNot all water in estuary may be able to be filtered by bivalves due to:StratificationSpatial heterogeneity of current flow

Suspension Feeders in EstuariesIn well-mixed estuaries, bivalves may be able to greatly reduce phytoplankton densitiesTop-down and Bottom-up Effects in EstuariesIncreased nutrient inputs (bottom-up)High levels of phytoplankton in water column can decrease water clarity Ungrazed phytoplankton dies and sinks to bottom Top-down and Bottom-up Effects in EstuariesLoss of top predators due to overfishing (top-down) can have cascading effects on lower trophic levelsExample:

Threats to EstuariesPollutionShoreline habitat alterationBiological invasions


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