Other hard intertidal habitats in estuaries
In addition to rocky shores, there are other intertidal habitats composed of hard surfaces, i.e., not beaches, mudflats,
or mangroves, in estuaries.
Boulder fields
Boulder fields are important intertidal habitats for a wide
range of species. In addition to their upper surfaces, which
support similar assemblages to those on rocky shores,
their undersurfaces provide a unique sheltered habitat.
This is occupied by species that are vulnerable to harsh
environmental conditions, which affect intertidal assemblages during low tides. Thus, they often support similar
species to those found in rock pools.
On open coasts, boulders are vulnerable to overturning
by waves, which tends to reduce the number of species
living on and under them. Boulder fields in estuaries
have more sheltered conditions and can support diverse
assemblages. In areas that are urbanized or popular for recreation, human disturbances may be very large, e.g.,
overturning boulders while foraging for food or bait. They
may also be subject to burial or being smothered by silt,
particularly when rivers flood. How damaging this is to
the animals and plants depends on how long the disturbance lasts before normal conditions reappear.
Built habitats
With increasing coastal urbanization, a common feature of
many estuaries is an ever-increasing expanse of artificial
shoreline (Dugan et al., 2011). This includes walls to prevent erosion, marinas and wharves for shipping and
boating, pylons of bridges and aquaculture facilities,
among others. They create novel intertidal “rocky shores,”
although they may be made of a mix of different materials,
natural and manmade. In some areas, they impinge on or
have replaced soft sedimentary habitat. In others, they
have replaced natural rocky shores. In some part of the
world, they are the only intertidal hard substrata available
in very urbanized estuaries.
These artificial shores can have major impacts on native
intertidal biodiversity. First, they may provide hard substrata where it does not naturally occur, allowing species
to move into new areas, potentially influencing the native
biota. Even when replacing natural rocky shores, artificial
surfaces do not act as surrogates of natural shores and may
not support the same diversity of intertidal biota, even
when created from natural local rock. Rarer species seem
most vulnerable to loss when shores are replaced by seawalls as many of the common species can live on both
types of structure. So, although many artificial structures
seem to support a thriving and diverse intertidal assemblage, one cannot be complacent that these structures
can act as surrogates of natural shores (Chapman and
Underwood, 2011).
Summary
Estuarine rocky intertidal habitats are like similar habitats
on open coasts, which are sheltered from large waves, with
some modifications. Estuaries generally have irregular
pulses of freshwater, which reduces the number of species
that live on estuarine rocky shores. Nevertheless, they are
often occupied by a suite of micro- and macroalgae, sessile habitat-forming animals, herbivores, predators, and
scavengers. Interactions among these species and their
responses to local conditions are complex, occur at multiple spatial scales, and vary through time. This variability,
natural to all intertidal assemblages, makes their responses
to climatic change and other anthropogenic disturbances
difficult to predict. Yet with increasing urbanization in
and around estuarine areas, these assemblages are becoming more vulnerable to disturbances from which they may
not fully recover.
Bibliography
Attrill, M. J., and Rundle, S. D., 2002. Ecotone or ecocline: ecological boundaries in estuaries. Estuarine, Coastal and Shelf
Science, 55, 929–936.
Barnes, R. S. K., and Hughes, R. N., 1999. An Introduction to
Marine Ecology, 3rd edn. London: Wiley-Blackwell.
Bishop, M. J., and Chapman, M. G., 2004. Managerial decisions as
experiments: an opportunity to determine ecological impact of
boat-generated waves on macrobenthic infauna. Estuarine,
Coastal and Shelf Science, 61, 613–622.
Chapman, M. G., and Underwood, A. J., 2011. Evaluation of
ecological engineering of “armoured” shorelines to improve
their value as habitat. Journal of Experimental Marine Biology
and Ecology, 400, 302–311.
Dayton, P. K., 1971. Competition, disturbance, and community
organization: the provision and subsequent utilization of space
in a rocky intertidal community. Ecological Monographs, 41,
351–389.
Dugan, J. E., Airoldi, L., Chapman, M. G., Walker, S. J., and
Schlacher, T., 2011. Estuarine and coastal structures: environmental effects, a focus on shore and nearshore structures. In
Wolanski, E., and McLusky, D. S. (eds.), Treatise on Estuarine
and Coastal Science. Waltham: Academic, pp. 17–41.
Jones, C. G., Lawton, J. H., and Shachak, M., 1994. Organisms as
ecosystem engineers. Oikos, 69, 373–386.
Menge, B. A., and Branch, G. M., 2001. Rocky intertidal communities. In Bertness, M. D., Gaines, S. D., and Hay, M. E. (eds.),
Marine Community Ecology. Sunderland: Sinauer, pp. 221–251.
Stephenson, T. A., and Stephenson, A., 1972. Life Between Tide
Marks on Rocky Shores. San Francisco: W. H. Freeman Company.
Underwood, A. J., and Chapman, M. G., 1996. Scales of spatial
patterns of distribution of intertidal invertebrates. Oecologia,
107, 212–224.
Underwood, A. J., and Chapman, M. G., 2007. Intertidal temperate
rocky shores. In Connell, S. D., and Gillanders, B. M. (eds.),
Marine Ecology. Melbourne: Oxford University Press,
pp. 402–427.
Underwood, A. J., and Fairweather, P. G., 1989. Supply-side
ecology and benthic marine assemblages. Trends in Ecology &
Evolution, 4, 16–20.
Wootton, J. T., 1993. Indirect effects and habitat use in an intertidal
community: interaction chains and interaction modifications.
American Naturalist, 141, 71–89.
510
ROCKY INTERTIDAL SHORES
In addition to rocky shores, there are other intertidal habitats composed of hard surfaces, i.e., not beaches, mudflats,
or mangroves, in estuaries.
Boulder fields
Boulder fields are important intertidal habitats for a wide
range of species. In addition to their upper surfaces, which
support similar assemblages to those on rocky shores,
their undersurfaces provide a unique sheltered habitat.
This is occupied by species that are vulnerable to harsh
environmental conditions, which affect intertidal assemblages during low tides. Thus, they often support similar
species to those found in rock pools.
On open coasts, boulders are vulnerable to overturning
by waves, which tends to reduce the number of species
living on and under them. Boulder fields in estuaries
have more sheltered conditions and can support diverse
assemblages. In areas that are urbanized or popular for recreation, human disturbances may be very large, e.g.,
overturning boulders while foraging for food or bait. They
may also be subject to burial or being smothered by silt,
particularly when rivers flood. How damaging this is to
the animals and plants depends on how long the disturbance lasts before normal conditions reappear.
Built habitats
With increasing coastal urbanization, a common feature of
many estuaries is an ever-increasing expanse of artificial
shoreline (Dugan et al., 2011). This includes walls to prevent erosion, marinas and wharves for shipping and
boating, pylons of bridges and aquaculture facilities,
among others. They create novel intertidal “rocky shores,”
although they may be made of a mix of different materials,
natural and manmade. In some areas, they impinge on or
have replaced soft sedimentary habitat. In others, they
have replaced natural rocky shores. In some part of the
world, they are the only intertidal hard substrata available
in very urbanized estuaries.
These artificial shores can have major impacts on native
intertidal biodiversity. First, they may provide hard substrata where it does not naturally occur, allowing species
to move into new areas, potentially influencing the native
biota. Even when replacing natural rocky shores, artificial
surfaces do not act as surrogates of natural shores and may
not support the same diversity of intertidal biota, even
when created from natural local rock. Rarer species seem
most vulnerable to loss when shores are replaced by seawalls as many of the common species can live on both
types of structure. So, although many artificial structures
seem to support a thriving and diverse intertidal assemblage, one cannot be complacent that these structures
can act as surrogates of natural shores (Chapman and
Underwood, 2011).
Summary
Estuarine rocky intertidal habitats are like similar habitats
on open coasts, which are sheltered from large waves, with
some modifications. Estuaries generally have irregular
pulses of freshwater, which reduces the number of species
that live on estuarine rocky shores. Nevertheless, they are
often occupied by a suite of micro- and macroalgae, sessile habitat-forming animals, herbivores, predators, and
scavengers. Interactions among these species and their
responses to local conditions are complex, occur at multiple spatial scales, and vary through time. This variability,
natural to all intertidal assemblages, makes their responses
to climatic change and other anthropogenic disturbances
difficult to predict. Yet with increasing urbanization in
and around estuarine areas, these assemblages are becoming more vulnerable to disturbances from which they may
not fully recover.
Bibliography
Attrill, M. J., and Rundle, S. D., 2002. Ecotone or ecocline: ecological boundaries in estuaries. Estuarine, Coastal and Shelf
Science, 55, 929–936.
Barnes, R. S. K., and Hughes, R. N., 1999. An Introduction to
Marine Ecology, 3rd edn. London: Wiley-Blackwell.
Bishop, M. J., and Chapman, M. G., 2004. Managerial decisions as
experiments: an opportunity to determine ecological impact of
boat-generated waves on macrobenthic infauna. Estuarine,
Coastal and Shelf Science, 61, 613–622.
Chapman, M. G., and Underwood, A. J., 2011. Evaluation of
ecological engineering of “armoured” shorelines to improve
their value as habitat. Journal of Experimental Marine Biology
and Ecology, 400, 302–311.
Dayton, P. K., 1971. Competition, disturbance, and community
organization: the provision and subsequent utilization of space
in a rocky intertidal community. Ecological Monographs, 41,
351–389.
Dugan, J. E., Airoldi, L., Chapman, M. G., Walker, S. J., and
Schlacher, T., 2011. Estuarine and coastal structures: environmental effects, a focus on shore and nearshore structures. In
Wolanski, E., and McLusky, D. S. (eds.), Treatise on Estuarine
and Coastal Science. Waltham: Academic, pp. 17–41.
Jones, C. G., Lawton, J. H., and Shachak, M., 1994. Organisms as
ecosystem engineers. Oikos, 69, 373–386.
Menge, B. A., and Branch, G. M., 2001. Rocky intertidal communities. In Bertness, M. D., Gaines, S. D., and Hay, M. E. (eds.),
Marine Community Ecology. Sunderland: Sinauer, pp. 221–251.
Stephenson, T. A., and Stephenson, A., 1972. Life Between Tide
Marks on Rocky Shores. San Francisco: W. H. Freeman Company.
Underwood, A. J., and Chapman, M. G., 1996. Scales of spatial
patterns of distribution of intertidal invertebrates. Oecologia,
107, 212–224.
Underwood, A. J., and Chapman, M. G., 2007. Intertidal temperate
rocky shores. In Connell, S. D., and Gillanders, B. M. (eds.),
Marine Ecology. Melbourne: Oxford University Press,
pp. 402–427.
Underwood, A. J., and Fairweather, P. G., 1989. Supply-side
ecology and benthic marine assemblages. Trends in Ecology &
Evolution, 4, 16–20.
Wootton, J. T., 1993. Indirect effects and habitat use in an intertidal
community: interaction chains and interaction modifications.
American Naturalist, 141, 71–89.
510
ROCKY INTERTIDAL SHORES