1. Between the Tides Chapter 11
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2. Intertidal Zone
The intertidal zone is the area between the mean low tide and mean high tide
By contrast, the subtidal zone is the area that is always submerged
The intertidal zone will be exposed during low tide

3. Intertidal Substrates
Substrate - type of bottom
Intertidal zone substrate can be rocky or soft bottom
Soft bottoms can vary from sand to silt or a mixture (mud)
Rocky substrate can vary as to the type of rock and the slope angle

4. Intertidal Zone Organisms
Epifauna – organisms that live on the surface of the substrate (ex: mud snails on soft bottoms, barnacles on hard substrates)
Infauna – organisms that live in the substrate (ex: clams burrowing in soft bottoms)
Meiofauna – organisms that are so small that they live between the grains of soft substrate
These organisms can be sessile or motile

5. Challenges Associated with the Intertidal
Due to the exposure to air, organisms face a variety of challenges, including:
Desiccation (water loss)
Temperature changes (can be extreme)
Salinity changes (can be extreme)
Interrupted feeding
Wave action and tides
Oxygen availability and build-up of CO2 at low tide
Limited space

6. Desiccation
When exposed, organisms must deal with potential water loss
Water loss is more pronounced on hot, dry, or windy days
Organisms can deal with the potential water loss by hiding or “clamming up”

7. Desiccation
Hiding may involve moving to a tide pool or an area with more moisture (motile organisms)
Hiding may also mean that some organisms only live in areas where moisture will remain when the tide is out (such as crevices in rocks or shallow spots in soft bottoms)

8. Desiccation
“Clamming up” consists of closing shells or otherwise walling yourself off from the environment to conserve moisture
+ moisture can be conserved
- no exchange of gases or feeding occurs

9. Temperature
The wide range of temperatures that must be tolerated by organisms in the intertidal can be severe
Some organisms have mechanisms to help stay cool

10. Animals hide under algal mats

11. White shells reflect radiation back away from animals

12. Changing Salinity
Salinity can change dramatically due to temperatures or weather.
Normal ocean salinity is ~ ppt.
Estuarine salinity normally varies between 5 ppt and 30 ppt.
Organisms in the intertidal are normally euryhaline (eury = broad; haline = salt)
subtidal organisms are normally stenohaline (steno = narrow)

13. Changing Salinity
Salinity may begin at /00 and climb dramatically during the day due to water loss when the intertidal is exposed.
Salinity may also drop during the day due to a sudden influx of freshwater provided by a passing thunderstorm.
A salinity change of possibly 20 0/00 or more combined with a temperature change of 40 degrees or more may occur

14. Interrupted Feeding If an organisms “clams up,” feeding will stop
Feeding will also cease for filter feeders when exposed to air
A few organisms such as snails can still feed during low tide by scraping algae off surfaces
For communities with semidiurnal tides, an animal could spend nearly half of their day un-submerged and not feeding.

15. Wave Action Wave exposure can be a problem for intertidal organisms
Rocky shores are often exposed to significant wave action

18. Oxygen Availability
Oxygen can be exhausted if an organism “clams up” during low tide
It can also be exhausted in tide pools if high density of organism, particularly algae at night (no photosynthesis)
Carbon dioxide can also build up
Some organisms have evolved the ability to exchange gases in both air and water
This is infauna, but O2 related

19. Limited Space In some intertidal communities, space may be limited
This is particularly true in the rocky intertidal, where surface area is limited
Space can also be limited in soft bottomed communities, where many organisms select hard substrates such as oyster reefs to prevent being washed away

20. Zonation in the Intertidal
Zonation in the intertidal consists of upper intertidal (most exposed) to middle and lower intertidal (least exposed).

21. Zonation in the Intertidal
Competition will be greater in the lower intertidal because it is the least extreme of the zones: exposed to a lesser degree than the upper intertidal
The lower intertidal always has more species for the same reason (although species will vary greatly by location)

22. Competition
Some species are better competitors than others and will exclude others if the community is left undisturbed
If a habitat is disturbed, organisms move in and are excluded in a predictable pattern known as ecological succession
The climax community (end result) will eventually form when the community is undisturbed for a long period of time

23. Predation
Predation by keystone predators such as sea stars strongly affect occurrence and density of animals in the middle intertidal (use of removal and caging experiments: see Box 11.1: Transplantation, Removal, and Caging Experiments)

24. Competition Research Predator exclusion cage experiments
on New England rocky intertidal –
Work by Dr. Griffin, U of So Carolina
Researching barnacle competition in
New Zealand. - work by Rahul Demello,
Victoria University of Wellington

26. Zonation
Biological interactions (competition for space, predation, grazing) are often important factors in determining lower limit of rocky intertidal organisms; physical factors (desiccation) for determining upper limit

27. Substrate in the Soft Bottom Intertidal
Sediment size can vary between sand, silt, clay, or a mixture (mud)
The smaller the sediment size, the less oxygen in the water filling spaces

28. Substrate in the Soft Bottomed Intertidal
The sediment size can be small so that all except the top few inches is anoxic (without oxygen).
Infauna must maintain a connection with the oxygenated zone via tubes, siphons, or extended burrows if they are located in the anoxic zone
Anaerobic bacteria that break down organic matter thrive in the anoxic zone
Most animals are infauna: adaptations to dig or burrow (see Figs and 11.30)
Poor epifauna

29. Soft Bottom Intertidal Organisms
The diagram to the right shows the diversity of infauna and epifauna that may be found in this community.

30. Soft Bottom Food Webs
Detritus (broken down organic particles) and producers such as photoautotrophs and chemoautotrophs form the basis for the food web in this community.

31. Rocky Intertidal Food Webs
The food web in the rocky intertidal is based on autotrophs such as seagrasses, algae and diatoms.
Top carnivores in any community that have the ability to change community composition significantly are known as keystone predators.