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Chapter 11
Between
the Tides
Copyright © 2016 McGraw-Hill Education. All rights reserved. No reproduction or distribution without the prior written consent of McGraw-Hill Education.
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
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
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
Challenges Associated with the
Intertidal
• Due to the exposure to air, organisms face a
variety of challenges, including:
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–
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
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”
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)
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
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
Animals hide
under algal mats
White shells
reflect
radiation back
away from
animals
Changing Salinity
• Salinity can change dramatically due to
•
•
•
•
temperatures or weather.
Normal ocean salinity is ~ 33-35 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)
Changing Salinity
• Salinity may begin at 20-25 0/00 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
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 unsubmerged and not feeding.
Wave Action
• Wave exposure can be a problem for
intertidal organisms
• Rocky shores are often exposed to
significant wave action
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
https://www.youtube.com/watch?v=smkT9sR3fyY
This is infauna, but O2 related
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
Zonation in the Intertidal
• Zonation in the
intertidal consists of
upper intertidal
(most exposed) to
middle and lower
intertidal (least
exposed).
http://home.earthlink.net/~huskertomkat/
tide.html
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)
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
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)
Competition Research
Researching barnacle competition in
New Zealand. - work by Rahul Demello,
Victoria University of Wellington
Predator exclusion cage experiments
on New England rocky intertidal –
Work by Dr. Griffin, U of So Carolina
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
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
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. 11.29 and 11.30)
Poor epifauna
Soft Bottom Intertidal Organisms
• The
diagram to
the right
shows the
diversity of
infauna and
epifauna
that may be
found in
this
community.
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.
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.