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