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Soft Sediment Intertidal, Estuaries

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1 Soft Sediment Intertidal, Estuaries
Lecture 8 Soft Sediment Intertidal, Estuaries

2 Soft Sediment Intertidal

3 Sand vs. Mud Bottom Benthos
Interstitial microfauna present No interstitial fauna Meiofauna dominated by nematodes, polychaetes, copepods Meiofauna dominated by nematodes, coppepods, ostracods Macrofauna dominated by filter feeding bivalves Macrofauna dominated by deposit feeding polychaetes Abundance and productivity low (thousands/m2) Abundance and productivity high (tens to hundreds of thousnads/m2)

4 Soft Sediment Intertidal
Zonation not as distinct as on rocky shores Reduced vertical desiccation and temperature stress gradients Organisms can burrow to avoid temperature stress and desiccation Variation in larval settlement not as important as it is on hard substrates

5 Moon snail, large clams, sand dollars
Variation in Zonation Temperate Tropical Supra-littoral amphipods ghost crabs Mid-littoral isopods Intra-littoral Moon snail, large clams, sand dollars

6 Biogeography in Soft-Bottom Sediments
Widest variation in densities and highest species diversity occurs in tropics Temperate 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 beaches


8 Interspecific Competition in Soft Sediments
Food and space for burrows is limited Burrowing invertebrates compete for space within sediment Dominant species found at different levels below sediment-water interface Little evidence of competitive exclusion

9 Soft Sediments - Vertical Stratification
Experimentally reduce density of deep-dwelling clams, remaining individuals grow faster Removal of shallow dwelling species of bivalves has no effect on growth of deeper-dwelling species Likely limiting factor = space

10 Soft Sediments - Competition
Some burrowing species produce Bromine poisons Discourages settlement of other species (possibly discourages predation also) Saccoglossus bromophenolosus

11 Food Supply in Soft-Sediment Intertidal
Suspension Feeders Phytoplankton suspended in water Deposit Feeders Microalgae and bacteria on sediment surface Decomposing organic matter Input can be spatially variable

12 Food Supply in Soft-Sediment Intertidal
Patchy occurrence of sea lettuce (Ulva sp.) leads to spatially patchy inputs of particulate organic matter Patchy POM leads to patchy distribution of small polychaetes and mud snails

13 Food Supply in Soft-Sediment Intertidal
Food 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 resource Can have seasonal “blooms” of deposit feeders

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

15 Predation in Soft Substrates
Predation and physical disturbance are likely the main processes responsible for maintaining high variability in distribution of organisms Do not see a lot of competitive exclusion, so predation typically has little effect on species diversity Partial predation is significant in soft substrates

16 Predation – Types of Predators
Surface 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 them Digging predators – excavate through the sediments to obtain their prey

17 Differential Effects of Predators
Quammen (1984) examined effects of birds, crabs, and fishes on tidal flat communities Crabs 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 predators

18 Predation – Seasonal Effects
Seasonal influxes of predators can devastate local soft-sediment communities Predators focus on most abundant species

19 Disturbance and Habitat Heterogeneity
Disturbance re-suspends sediments and blasts out organisms 1st successional species usually small polychaetes Physical and biological disturbance Organisms can affect habitat heterogeneity

20 Spatial Scales of Disturbance

21 Estuaries, Salt Marshes, Seagrasses, and Mangroves


23 Estuaries

24 Estuaries Estuary = partially enclosed section of the coast where freshwater from rivers mixes with seawater Watershed = the surrounding land that provides freshwater input to the estuary

25 Watersheds Tampa Bay Watershed Mobile Bay Watershed

26 Types of Estuaries

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

28 Estuarine Structure Salinity structure of an estuary is determined by:
Watershed topography Slope and size of river(s) feeding into main part of estuary Size of main estuary channel Tidal flow

29 Estuarine Structure Overall river discharge important to salinity transitions within estuaries Storm events (hurricanes, etc.) can lower salinity throughout estuary

30 Productivity in Estuaries
Geologically ephemeral but biologically rich Nutrients from freshwater sources and nutrients recycled from seabed support high levels of primary production

31 Estuarine Species and Salinity
Marine species can generally tolerate salinity fluctuations as long as salinity stays above ppt Vertical salinity stratification – bottom organisms can go farther upstream than planktonic species Mixed estuaries – infaunal species experience less salinity fluctuation than epifaunal species b/c of buffering effect of sediment pore waters

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

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

34 Suspension Feeders in Estuaries
Retention time = the average number of days that a phytoplankton cell stays in an estuary Turnover time = the number of days that it takes for a bivalve population to completely filter the water column Not all water in estuary may be able to be filtered by bivalves due to: Stratification Spatial heterogeneity of current flow

35 Suspension Feeders in Estuaries
In well-mixed estuaries, bivalves may be able to greatly reduce phytoplankton densities

36 Top-down and Bottom-up Effects in Estuaries
Increased nutrient inputs (bottom-up) High levels of phytoplankton in water column can decrease water clarity Ungrazed phytoplankton dies and sinks to bottom

37 Top-down and Bottom-up Effects in Estuaries
Loss of top predators due to overfishing (top-down) can have cascading effects on lower trophic levels Example:

38 Threats to Estuaries Pollution Shoreline habitat alteration
Biological invasions

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