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Lecture 8

Lecture 8. Soft Sediment Intertidal, Estuaries. Soft Sediment Intertidal. Sand vs. Mud Bottom Benthos. Soft Sediment Intertidal. Zonation not as distinct as on rocky shores Reduced vertical desiccation and temperature stress gradients

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Lecture 8

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  1. Lecture 8 Soft Sediment Intertidal, Estuaries

  2. Soft Sediment Intertidal

  3. Sand vs. Mud Bottom Benthos

  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. Variation in Zonation

  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

  7. 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

  8. 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

  9. Soft Sediments - Competition • Some burrowing species produce Bromine poisons • Discourages settlement of other species (possibly discourages predation also) Saccoglossusbromophenolosus

  10. 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

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

  12. 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

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

  14. 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

  15. 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

  16. 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

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

  18. Disturbance and Habitat Heterogeneity • Disturbance re-suspends sediments and blasts out organisms • 1stsuccessional species usually small polychaetes • Physical and biological disturbance • Organisms can affect habitat heterogeneity

  19. Spatial Scales of Disturbance

  20. Estuaries, Salt Marshes, Seagrasses, and Mangroves

  21. Estuaries

  22. 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

  23. Watersheds Tampa Bay Watershed Mobile Bay Watershed

  24. Types of Estuaries

  25. Estuarine Structure • Estuarine structure is controlled by seaward flow of freshwater combined with tidal mixing

  26. 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

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

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

  29. Estuarine Species and Salinity • Marine species can generally tolerate salinity fluctuations as long as salinity stays above 10-15 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

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

  31. 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

  32. 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

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

  34. 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

  35. 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:

  36. Threats to Estuaries • Pollution • Shoreline habitat alteration • Biological invasions

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