62
E. Jaramillo and M. Lastra
climate of exposed sandy beaches varies with the morpho dynamic states. For
example, reflective beaches have swashes with short periods, dissipative ones
have longer swash periods, and intermediate beaches show swash periods
intermediate between that of reflective and dissipative beaches (McArdle and
McLachlan 1991). The following Dean's values have been mentioned (e.g.
Short and Wright 1983) for the beach types: ::;;1.0=reflective beaches, 1.0-6.0=
intermediate beaches, and >6.0=dissipative beaches.
Dean's parameter is a useful tool to categorize sandy beaches on microtidal
coasts. However, when tide ranges are greater than 2-3 m, the role of tides in
beach morpho dynamics increases (Masselink and Short 1993). To account for
this, McLachlan et al. (1993) created the beach state index (BSI) where
BSI=log [(Hb x MTR/T x sand fall velocityxET}+I]. MTR is the maximum
tide range and ET is the maximum theoretical equilibrium tide for which the
earth covered in water is 0.8m (McLachlan et al. 1993). Based upon a
comparative study of about 70 beaches, McLachlan et al. (1993) suggested the
following scale for BSI: <0.5=reflective beaches, 0.5-1.0=low to medium
energy intermediate beaches, 1.0-1.5=high energy intermediate-dissipative
beaches, 1.5-2.0=fully dissipative beaches, and >2.0=ultra-dissipative macrotidal beaches.
Beach morpho dynamics have been considered a key factor in the
community structure of the sandy beach macroinfauna around different
zoogeographic regions of the world. A comparative study of sandy beach
macroinfauna communities from all around the coast of South Africa,
Australia and the coast of Oregon showed a linear increase in species richness
and an exponential increase in abundance and biomass of the macroinfauna
from reflective to dissipative conditions (Fig. 3.1, McLachlan 1990). Since the
BSI allows for comparisons of sandy beaches located at different zoogeographical regions which have very large differences in tidal range, McLachlan
et al. (1993, 1996, 1998) subsequently studied the sandy beach macroinfauna
in South Africa, Oregon, Australia, south central Chile, and Oman showing
that the patterns already found also hold globally. However, recent seasonal
studies carried out along the coast line of Chile (from about 19 to 42° S, circa
2500 km) show that these sorts of relationships are indeed quite variable.
Thus, the higher abundance and biomass values are found at intermediate
beaches with Dean's values close to 3 (Fig. 3.2a). These values of abundance
and biomass are usually higher than the values predicted by the worldwide
model given by McLachlan et al. (1996) in which the BSI is used (Fig. 3.2b).
In the first part of this study we compare the community structure of the
sandy beach macroinfauna vs. beach morpho dynamic types on different
coasts. We will explore the role of the most distinctive suspension feeder
along Chilean sandy beaches - the anomuran crab Emerita analoga - in terms
of the observed trend for this coast by comparing unpublished information of
sandy beach surveys carried out along the Chilean coast and Spain (Galicia)
E. Jaramillo and M. Lastra
climate of exposed sandy beaches varies with the morpho dynamic states. For
example, reflective beaches have swashes with short periods, dissipative ones
have longer swash periods, and intermediate beaches show swash periods
intermediate between that of reflective and dissipative beaches (McArdle and
McLachlan 1991). The following Dean's values have been mentioned (e.g.
Short and Wright 1983) for the beach types: ::;;1.0=reflective beaches, 1.0-6.0=
intermediate beaches, and >6.0=dissipative beaches.
Dean's parameter is a useful tool to categorize sandy beaches on microtidal
coasts. However, when tide ranges are greater than 2-3 m, the role of tides in
beach morpho dynamics increases (Masselink and Short 1993). To account for
this, McLachlan et al. (1993) created the beach state index (BSI) where
BSI=log [(Hb x MTR/T x sand fall velocityxET}+I]. MTR is the maximum
tide range and ET is the maximum theoretical equilibrium tide for which the
earth covered in water is 0.8m (McLachlan et al. 1993). Based upon a
comparative study of about 70 beaches, McLachlan et al. (1993) suggested the
following scale for BSI: <0.5=reflective beaches, 0.5-1.0=low to medium
energy intermediate beaches, 1.0-1.5=high energy intermediate-dissipative
beaches, 1.5-2.0=fully dissipative beaches, and >2.0=ultra-dissipative macrotidal beaches.
Beach morpho dynamics have been considered a key factor in the
community structure of the sandy beach macroinfauna around different
zoogeographic regions of the world. A comparative study of sandy beach
macroinfauna communities from all around the coast of South Africa,
Australia and the coast of Oregon showed a linear increase in species richness
and an exponential increase in abundance and biomass of the macroinfauna
from reflective to dissipative conditions (Fig. 3.1, McLachlan 1990). Since the
BSI allows for comparisons of sandy beaches located at different zoogeographical regions which have very large differences in tidal range, McLachlan
et al. (1993, 1996, 1998) subsequently studied the sandy beach macroinfauna
in South Africa, Oregon, Australia, south central Chile, and Oman showing
that the patterns already found also hold globally. However, recent seasonal
studies carried out along the coast line of Chile (from about 19 to 42° S, circa
2500 km) show that these sorts of relationships are indeed quite variable.
Thus, the higher abundance and biomass values are found at intermediate
beaches with Dean's values close to 3 (Fig. 3.2a). These values of abundance
and biomass are usually higher than the values predicted by the worldwide
model given by McLachlan et al. (1996) in which the BSI is used (Fig. 3.2b).
In the first part of this study we compare the community structure of the
sandy beach macroinfauna vs. beach morpho dynamic types on different
coasts. We will explore the role of the most distinctive suspension feeder
along Chilean sandy beaches - the anomuran crab Emerita analoga - in terms
of the observed trend for this coast by comparing unpublished information of
sandy beach surveys carried out along the Chilean coast and Spain (Galicia)