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Bioerosion is an important process that destroys coastal rocks in the tropics. However, the rates at which this process occurs, the organisms involved, and the dynamics of rocky cliffs in tropical latitudes have been less studied than in temperate and subtropical latitudes. To contribute to the knowledge of the bioerosion process in rocky cliffs on thePacific coast of Colombia (Eastern Tropical Pacific) we compared: 1) boring volume, 2) grain size distribution of the rocks, and 3) rock porosity, across three tidal zones of two cliffs with different wave exposure; these factors were related to the bioeroding community found. We observed that cliffs that were not exposed to wave action (IC, internal cliffs) exhibited high percentages of clays in their grain size composition, and agreater porosity (47.62%) and perforation (15.86%) than exposed cliffs (EC). However, IC also exhibited less diversity and abundance of bioeroding species (22 species and 314 individuals, respectively) compared to the values found in EC (41.11%, 14.34%, 32 and 491, respectively). The most abundant bioeroders were Petrolisthes zacae in IC and Pachygrapsus transversus in EC. Our findings show that the tidal zone is the common factor controlling bioerosion on both cliffs; in addition to the abundance of bioeroders on IC and the number of bioeroding species on EC. The integration of geology, sedimentology, and biology allows us to obtain a more comprehensive view of the patterns and trends in the process of bioerosion.
Citation preview
Main factors determining bioerosion patterns on rocky cliffs in a drownedvalley estuary in the colombian pacific (eastern tropical pacific)
Alba Marina Cobo-Viveros, Jaime Ricardo Cantera-Kintz
PII: S0169-555X(15)00329-3DOI: doi: 10.1016/j.geomorph.2015.05.036Reference: GEOMOR 5250
To appear in: Geomorphology
Received date: 6 August 2014Revised date: 8 May 2015Accepted date: 10 May 2015
Please cite this article as: Cobo-Viveros, Alba Marina, Cantera-Kintz, Jaime Ricardo,Main factors determining bioerosion patterns on rocky clis in a drowned valley es-tuary in the colombian pacic (eastern tropical pacic), Geomorphology (2015), doi:10.1016/j.geomorph.2015.05.036
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MAIN FACTORS DETERMINING BIOEROSION PATTERNS ON ROCKY
CLIFFS IN A DROWNED VALLEY ESTUARY IN THE COLOMBIAN
PACIFIC (EASTERN TROPICAL PACIFIC)
Alba Marina Cobo-Viverosa and Jaime Ricardo Cantera-Kintz
a,b
a. Research Group in Estuaries and Mangroves - Ecomanglares. Department of Biology.
Faculty of Natural and Exact Sciences. Universidad del Valle. Calle 13 #100-00. Cali,
Colombia. AA. 25360. Phone number: (+57) 2 3212100 ext. 2824.
b. Titular Professor. Department of Biology. Faculty of Natural and Exact Sciences.
Universidad del Valle. Calle 13 #100-00. Cali, Colombia. AA. 25360.
Corresponding author: Alba Marina Cobo Viveros1
1 Present postal address: Instituto de Investigacins Marias. Ra Eduardo Cabello 6. 36208 Vigo
(Pontevedra). Spain. [email protected] Present cell phone: (+34) 622 078 341.
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HIGHLIGHTS
We measured perforation volumes on two rocky cliffs in the Colombian Pacific
coast.
Interior cliffs (not exposed to wave action) presented more porosity and
perforation volumes.
Exterior cliffs (exposed to wave action) had higher bioeroder diversity and
abundance.
Lower tidal zones showed higher abundance of bioeroders than the other zones.
Boring bivalves were less abundant compared to boring crustaceans.
ABSTRACT
Bioerosion is an important process that destroys coastal rocks in the tropics. However,
the rates at which this process occurs, the organisms involved, and the dynamics of
rocky cliffs in tropical latitudes have been less studied than in temperate and subtropical
latitudes. To contribute to the knowledge of the bioerosion process in rocky cliffs on the
Pacific coast of Colombia (Eastern Tropical Pacific) we compared: 1) boring volume, 2)
grain size distribution of the rocks, and 3) rock porosity, across three tidal zones of two
cliffs with different wave exposure; these factors were related to the bioeroding
community found. We observed that cliffs that were not exposed to wave action (IC,
internal cliffs) exhibited high percentages of clays in their grain size composition, and a
greater porosity (47.62%) and perforation (15.86%) than exposed cliffs (EC). However,
IC also exhibited less diversity and abundance of bioeroding species (22 species and
314 individuals, respectively) compared to the values found in EC (41.11%, 14.34%, 32
and 491, respectively). The most abundant bioeroders were Petrolisthes zacae in IC and
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Pachygrapsus transversus in EC. Our findings show that the tidal zone is the common
factor controlling bioerosion on both cliffs; in addition to the abundance of bioeroders
on IC and the number of bioeroding species on EC. The integration of geology,
sedimentology, and biology allows us to obtain a more comprehensive view of the
patterns and trends in the process of bioerosion.
KEYWORDS
Bioerosion, Grain size distribution of rocks, Boring volume, Rock porosity, Bioeroding
fauna, Rocky cliffs, Eastern Tropical Pacific.
1. INTRODUCTION
Bioerosion is an important process that destroys coastal rocks in the tropics (Trenhaile,
1987); it occurs through the biological breakdown and removal of hard substrates by
surface abrasion and boring. During surface abrasion, endolithic and grazing organisms
(e.g. molluscs, echinoderms, fish, and some crustaceans) rasp, bite and scrape away a
thin layer of rock (Trudgill, 1985; Trenhaile, 1987), produce particulate detritus
(Torunski, 1979), and obtain nutrition from endolithic algae. During boring, perforating
organisms (e.g. endolithic bacteria, algae, fungi, and lichens; sponges, sipunculans,
polychaetes, bivalves, crustaceans, and echinoderms) directly remove rock material and
weaken the remaining rock, making it more vulnerable to mechanical wave erosion and
weathering (Trenhaile, 2005). As a consequence of these two processes, rocks collapse
and decompose (Hutchings, 1986; Ricaurte et al., 1995; Cantera et al., 1998), generating
new substrates, changing cliff structure, and enriching the surrounding ecosystems with
sediments and rocks from the fallen material, thus modifying the biological community.
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Not all marine organisms destroy the underlying rock; some can also protect it from
incoming waves and physicochemical attack by forming organic crusts in the lower
intertidal and upper subtidal zones (rhodophytes: Lithothanium, Lithophyllum;
chlorophytes: Halimeda; sublittoral brown algae; barnacles: Chthamalus, Balanus;
limpets: Patella, Lottia, Fissurella, Siphonaria, Crepidula) (Trenhaile, 1987). However,
determining the scale used to identify the role played by organisms involved in
bioerosion can be difficult (Forns et al., 2006): sometimes it is clear that an individual
acts as a bioeroder or as an occupant nestler of a previously existing hole (while
modifying it), but this is not always easy to ascertain. For example: encrusting
organisms that have an important role in protecting the rock surface from physical
erosion (Focke, 1977) sometimes also act as bioeroders; they can take away some rock
when removed from the cliff (e.g. barnacles), or they can weaken what they are
supposedly protecting by chemical or other processes (e.g. micro and macroalgae)
(Naylor and Viles, 2002).
Cliffs are also destroyed by mechanical and chemical means (McLean, 1974). Wave
erosion is considered the dominant mechanical erosional agent in many parts of the
world (Trenhaile, 1987); it occurs through steady wave action, generation of high shock
pressures (Trenhaile and Kanyaya, 2007; Bezerra et al., 2011), or the abrasion from
sweeping, rolling, or dragging of rocks and sand (Trenhaile, 1987). Chemical
weathering is the result of a series of chemical reactions (Trenhaile, 1987) that modify
the rock carbonate chemical equilibrium when working together (Trudgill, 1985). Some
conceptual models of erosion on rocky coasts highlight the importance of the wave
force/rock resistance relationship and leave aside that of biological agents (Sunamura,
1994) but the effects of chemical, mechanical, and biological erosion can be synergistic
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(Hutchings, 1986). The reconceptualization of Naylor et al. (2012) demonstrated that
most of the geomorphologic processes are affected by organisms and included
biological agents as important reducers of the resisting force of the rock.
There has been a growing interest in rock coast geomorphology in temperate and sub-
tropical latitudes (Naylor et al., 2010), but bioerosion rates and dynamics of rocky cliffs
in tropical latitudes have been less studied (Moses, 2013). Some publications integrate
biological, geological and sedimentary variables (Fischer, 1981a, 1981b; Cantera et al.,
1998), and quantify cliff retreat (Ricaurte et al., 1995; Cantera et al., 1998) and
bioerosion rates of several organisms (Rasmussen and Frankenberg, 1990; Toro-Farmer
et al., 2004; Herrera-Escalante et al., 2005; Asgaard and Bromley, 2008; Lozano-Corts
et al., 2011). Cantera et al. (1998) measured erosion rates and studied the biodiversity,
zonation, and types of cavities made by perforating fauna in two rocky cliffs in
Buenaventura bay (Pacific coast of Colombia). Additionally, there are some works that
studied perforations by crustaceans and bivalves (Cantera and Blanco-Libreros, 1995;
Ricaurte et al., 1995), and that quantified the erosion rate of sea urchins in rocky cliffs
(Lozano-Corts et al., 2011). However, little work has been done on bioerosion of rocky
cliffs in Colombia, in spite of the impact it can have on nearby human settlements living
on top of the cliffs or near them. This process requires further study (Correa and
Gonzalez, 2000).
The present study contributes to the knowledge of the grain size distribution and boring
volumes of two rocky cliffs in a drowned valley estuary in the Colombian Pacific
(Eastern Tropical Pacific). As intertidal bioerosion cannot be understood without
biological processes (Trudgill, 1985), boring volumes are used as a quantitative
bioerosion indicator relative to wave exposure, tidal zone, and the bioeroding
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community found. Combining the effects of several bioerosion variables can indicate
the areas that erode more quickly, compared to studying the effects of each variable
separately.
2. MATERIALS AND METHODS
2.1. Study site
Buenaventura Bay is an ancient drowned valley estuary located in the Pacific coast of
Colombia, between 348N 354N and 7705W 7720W (Fig. 1). It is located in
one of the most humid places of the world: the average annual precipitation rate is more
than 7000 mm/year, which makes chemical erosion very high. Buenaventura Bay has a
tropical hot and humid rainforest climate: the mean annual temperature is 26.2C, and
the mean relative humidity is 89%. The rainy season occurs between August and
November. Waves can reach heights of 2 m outside the bay, but they are rapidly
reduced to 0.9 m near the entrance due to energy dissipation related to floor friction.
The bay undergoes forcing by semi-diurnal tides with a meso-macrotidal range of 4 m
(Cantera and Blanco, 2001).
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Fig. 1. Geographical location of the cliffs studied. Right: South America, showing the
location of the Pacific coast of Colombia (Middle). Left. Buenaventura Bay, showing
the locations of IC (unexposed cliffs) and EC (exposed cliffs).
The north side of Buenaventura Bay is characterized by vertical to sub-vertical cliffs
that range from 10-20 m in height, cut into horizontal to sub-horizontal Tertiary
sandstones, shales and mudstones (Correa and Morton, 2010). The tops of the cliffs are
covered with dense vegetation, as occurs in most humid tropical regions (Trenhaile,
1987). The cliffs located on this side of the bay are composed by the Raposo and
Mayorqun geological formations (of sedimentary origin) from the Superior and Median
Tertiary (Galvis and Mojica, 1993; Martnez, 1993); these formations consist of shale,
mudstones, and dark gray siltstones organized in layers that vary from a few centimeters
to 2 m thick. Coarse sediments (sandstone, slabs and clusters) are also present,
randomly arranged between the strata (Cantera et al., 1998). Two cliffs located on this
side of the bay were chosen (Fig 1): one on the external zone (EC, located 0.5 km from
the entrance and exposed to wave action) and the other on the internal zone (IC, located
15.4 km from the entrance of the bay and not exposed to substantial wave action).
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Sedimentary deposits of continental origin reach Buenaventura Bay through the rivers
flowing into it; these sediments particularly affect cliffs in IC so that the rocks forming
these cliffs are softer than the ones forming EC.
2.2. Sampling.
Nine blocks of approximately 15 cm x 15 cm x 15 cm were extracted from each cliff,
using a chisel: three from the supralittoral, three from the upper intertidal, and three
from the lower intertidal. These three tidal zones were distinguished based on the
characteristic algae, perforations, and tidal coverage: the high zone (Supralittoral or
splash zone) is covered by patches of the algae Cladophora albida, Cladophora
herpestica (or a mix of both), and Bostrychia tenella; it is poorly bored and is only
covered by the tide during spring tides, but other than that it only receives the splash
from waves that break in the inferior tidal levels. The upper intertidal zone (or Superior
Mesolittoral) is covered by Bostrychia radicans, with patches of Cladophoropsis sp.
and Boodleopsis verticillata; it is slightly perforated and it stays submerged for a longer
period of time than the high zone. The lower intertidal zone (or Inferior Mesolittoral)
can present coverage by B. radicans; it is the most perforated zone, it stays submerged
for longer periods of time than the other two zones, and it is sometimes separated from
the upper intertidal zone in this locality by a stratum of volcanic, hard rock covered by
oysters and barnacles.
After the blocks were extracted from the cliffs, they were submerged in a mixture of
water, alcohol, and clove oil in order to collect all the benthic fauna within the rock
(which was preserved in 70% alcohol); partial desalinization occurred in this process.
The fauna collected inside the blocks were identified using taxonomic keys for each
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group (Haig, 1960; Olsson, 1961; Keen, 1971; Fauchald, 1977; Brusca, 1980; Brusca
and Iverson, 1985; Froidefond, 1985; Williams, 1986; Kim and Abele, 1988; Abele and
Kim, 1989; Ros and Ramos, 1990; Poore, 1994; Hilbig, 1997; Cantera et al., 1998).
The organisms were marked and preserved in 80% alcohol.
The blocks of rock were cored into smaller blocks of approximately 10 cm x 10 cm x 10
cm. To determine the perforation volume, the rocks were first saturated in water
(submersion time depended on the characteristics of each block), taken out, and the
liquid remaining inside the perforations was extracted. Afterwards, the rocks were
weighed in air (with an electronic balance of precision 0.1 g) and in water (Annex B),
calculating their volume from the weight difference in both environments. The volume
of rock (VR), including perforation volume (due to bioerosion) and porosity volume (due
to the incipient rock porosity), was found using equation 1:
(1)
where WR is the weight of rock + rock pores in the air, WsR is the weight of rock + rock
pores in the water, and water is the water density.
The volume of rock without perforations (VT) was found by filling the boreholes with
modeling clay, sealing the rock with paraffin wax, weighing the block in air and water,
and applying equation 2.
(2)
WRC is the weight of the sealed rock in air, and WsRC is the weight of the sealed rock in
water.
The volume of rock due to perforations (VP) was calculated from the difference between
VT and VR (equation 3).
(3)
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2.3. Specific gravity
The specific gravity (Gs) of any substance is the unitary weight of the material divided
by the unitary weight of distilled water at 4C. This variable is used in the hydrometer
analysis (Bowles, 1981a) to obtain the void ratio of a soil or ground, and to predict its
unitary weight (equation 4). The methodology is described in Bowles (1981a).
(4)
Wsol is the weight of the solids, Wfw is the weight of the flask + water, and Wfws is the
weight of the flask + water + solids.
2.4. Hydrometer analysis
This analysis estimates the particle size distribution of soils that contain a considerable
amount of particles between 0.075 and 0.001 mm (clay and silt). The Stokes Law
(equation 5) estimates the falling rate of spheres in a fluid (v, in cm/s) from the specific
weight of the spheres ( , in
g/cm3), the specific weight of the fluid ( , usually water), the absolute viscosity or fluid
dynamic (, in dynes x seg/cm2) and the sphere diameter (D, in cm) (Bowles, 1981a;
Das, 2001).
(5)
This equation is valid for particle diameters between 0.0002 and 0.2 mm. Temperature
was taken into account, since the specific weight and viscosity of water depend on this
variable.
The hydrometer analysis was performed on the rocks from the cliffs and on a control
solution prepared with 125 ml of 4% sodium hexametaphosphate (NaPO3, a dispersing
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agent which neutralizes the charges on the smallest grain sizes that often have negative
charge) and sufficient distilled water to produce 1000 ml. The hydrometer was put into
the control cylinder to record zero and meniscus corrections; the temperature was
measured as well.
To prepare the sample for the hydrometer analysis, the blocks of rock were crumbled
and dried at 110C for 24 h, after which they were macerated and passed through the
#200 sieve to obtain a 50 g sample. 125 ml of 4% NaPO3 were added to the sample and
the resulting mixture was left standing for 24 h. Afterwards the mix was transferred to a
dispersion (or malt mixer) cup and water was added until the cup was about two-thirds
full. The contents were mixed for a minute, and then the mixture was carefully
transferred to a 1000 ml sedimentation cylinder. Any soil left in the dispersion cup was
rinsed using a plastic squeeze bottle and the remains were poured into the sedimentation
cylinder. Next, water was added until the 1000 ml level and the mixture was agitated
again for 1 min to homogenize the material within the column; this was done by placing
the palm of the hand over the open end and turning the cylinder upside down and back.
Finally, the sedimentation cylinder was set on a table and a ASTM 152H hydrometer
was inserted; readings were taken at the time intervals t = 0.5 min, 1 min, 1.5 min, 2
min, 2.5 min, 3 min, 3.5 min, 4 min, 8 min, 16 min, 30 min, 60 min, 120 min, and 240
min, or until the reading became constant. Readings were always taken at the upper
level of the meniscus because suspended soil water solution makes the system opaque.
Temperature was also recorded at each time interval.
After all the readings were taken, a series of corrections were performed due to zero,
meniscus and temperature (Bowles, 1981b). The zero correction (Cz) is applied to the
actual hydrometer reading depending on the hydrometers zero reading in the control
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cylinder: if this reading is below the water meniscus, Cz will be positive; if it is above, it
will be negative; and if the reading is at the meniscus, Cz will be zero (Bowles, 1981a).
The meniscus correction (Cm) is the difference between the upper level of meniscus and
water level of the control cylinder (Bowles, 1981a). The temperature correction (CT) is
done when the temperature of the soil suspension is not 20C (hydrometers are
generally calibrated at this temperature); if it is above, the hydrometer reading will be
less and CT will be positive (and vice versa) (Bowles, 1981a). CT was determined from
Table A1 in the Annex.
The corrected hydrometer reading (Rc) was calculated from the actual reading (Rreal) as
follows:
(6)
The percent finer (P, percentage of particles that go through the sieve) was calculated as
follows
(7)
where a is a correction factor used whenever the Gs of soils is different from 2.65 (the
Gs at which the 152H hydrometer was calibrated). It was determined from Table A2 in
the Annex using Gs. Ws is the weight of the soil sample (in grams).
The equivalent particle diameter (D, in mm) was calculated using the following
formula:
(8)
where the factor K is a function of temperature, Gs and water viscosity; for the known
Gs of the soil, K was obtained from Table A3 in the Annex. L is the effective
hydrometer depth L (in cm) obtained from Table A4 in the Annex for the meniscus
corrected reading; and t is the time interval (in minutes).
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Grain size distribution curves (D) were plotted versus the percent finer (P) on a semi-
logarithmic plot.
2.5. Statistical analysis
The percentage of perforation and porosity volumes were determined and compared
between tidal zones of each cliff with a one-way ANOVA, and between both cliffs with
a two-way ANOVA. Homogeneity of variances was tested for using Levenes test.
Bioeroder abundance and number of bioeroding species between tidal zones was
compared with a one-way ANOVA, and between cliffs with a two-way ANOVA,
because normal distribution and homogeneity of variances are not critical to perform an
ANOVA when sample sizes are equal (Hammer and Harper, 2008). If the ANOVA
showed significant inequality of means, the post-hoc Tukey-Kramer pairwise
comparison was used (Hammer et al., 2001).
The percentage of biodegraded volume was related to the abundance and richness of
eroding fauna, tidal zone, and percentage of natural porosity of the rock using a simple
correlation analysis (Zar, 2010). The correlation coefficients were compared with a one-
way ANOVA.
Finally, to determine if the combined effect of all factors on the bioerosion process was
higher than the effect of each factor taken separately, a multiple regression analysis of
perforated volume, richness and abundance of bioeroding fauna, tidal zone, and volume
of natural porosity of the rock was performed.
3. RESULTS
3.1 Composition of cliff sediments
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Both cliffs are composed mainly of shale. In the hydrometer analysis, both cliffs
showed a high percentage of fine particles in their grain size distribution, but unexposed
cliffs (IC) showed finer particles than exposed cliffs (EC) (Table 1, Fig. 2A). In fact, we
found particles smaller than 2m (clays) in IC but not in EC (Table 1, Fig. 2B).
Table 1. Grain size composition (percentage) of unexposed (IC) and exposed cliffs
(EC) in the Pacific coast of Colombia.
Particle Diameter IC EC
Fine sands < 75m 100% 100%
< 50m 80-92% 62-86%
Silts < 20m 40-51% 28-45%
< 10m 11-30% 8-15%
Clays < 2m 9-11% 0%
One curve from the low (L) and another from the middle (M) tidal zone in EC (blocks
L1 and M1, Fig. 2B) differed from the rest of the cliffs grain size distribution. They
represent an inclusion of hard rock that occurs along these kinds of cliffs, with a thicker
composition than the rest of the analyzed samples: only 40% of the particles exhibited
diameters
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Fig. 2. Grain size distribution of A. Unexposed (IC) and B. Exposed cliffs (EC) on the
Pacific coast of Colombia. The legend on the right represents each tidal zone: low (L),
middle (M), and high (H).
3.2. Perforation and Porosity volumes
More perforation volume (produced by bioerosion of the rock) was found in the low and
high tidal zones of IC, but the middle tidal zone was more densely perforated in EC
(Table 2). Significant differences were found in the perforation volume between tidal
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zones in EC (p = 0.001) due to dissimilarities between the high and low (p = 0.004) and
the high and middle tidal zones (p = 0.002). Significant differences were also found
when we compared the perforation volume of both cliffs (p = 0.002), between the low
tidal zone in IC and the high tidal zone in EC (p = 0.012), and between the high and
middle tidal zones in EC (p = 0.017). Significant differences were not found in
perforations between wave exposures (p = 0.666) or an interaction between wave
exposure and tidal zones (p = 0.126).
Significant differences in the porosity volume (incipient rock porosity) were found
between wave exposures (p = 0.016) because IC showed more porosity than EC in all
tidal zones; these differences were due to dissimilarities between the high tidal zone in
IC and the low tidal zone in EC (p = 0.039). There were no significant differences in
porosity between tidal zones (p = 0.146), nor an interaction between wave exposure and
tidal zone (p = 0.336).
Table 2. Percentage of perforations due to bioerosion (relative to total volume) and
percentage of porosity (relative to volume of solids + volume of pores) found for the
three tidal zones of unexposed cliffs (IC) and exposed cliffs (EC) in the Pacific coast of
Colombia. Richness and abundance of bioeroding species of the studied cliffs is also
shown.
Cliffs Perforations
(%)
Porosity
(%)
Abundance
(Individuals)
Richness
(Number
of species)
IC
Low 25.08 46.39 183 18
Middle 14.37 47.04 99 13
High 8.14 49.41 32 11
Average 15.86 47.62 Total 314 22
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EC
Low 21.02 34.15 295 25
Middle 23.93 41.20 123 14
High 0.30 45.65 73 4
Average 15.08 40.33 Total 491 32
3.3. Bioeroding fauna
We found 314 individuals belonging to 22 macrobioeroding species in IC (Table 2).
More bioeroder species and abundance of grazers and borers were found in the low tidal
zone (58.3%). Petrolisthes zacae was the most abundant species in this cliff (41.1%;
Fig. 3A) because it appeared in great numbers in the low and middle tidal zones. The
amphipod Chelorchestia sp. was the most abundant in the high tidal zone (relative
abundance of 37.5%). Significant differences were found in the abundance of
bioeroders between the high and low tidal zones (p = 0.004) but not in the number of
bioeroding species (p = 0.086).
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Fig. 3. Abundance of bioeroding species in the low, middle and high tidal zones of A.
Unexposed cliffs (IC) and B. Exposed cliffs (EC) in the Pacific coast of Colombia. The
category Others groups bioeroding fauna with total abundances of less than 10
individuals.
A total of 491 individuals belonging to 32 bioeroding species were found in EC,
concentrated in the low zone (25 bioeroder species and 60% of total cliff abundance).
Pachygrapsus transversus, Alpheus javieri and Upogebia tenuipollex were the most
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abundant species in this cliff (relative abundances of 18.1%, 14.5%, and 12.4%,
respectively; Fig. 3B). However, Petrolisthes armatus, P. transversus and Ligia
baudiniana were the most abundant for the low, middle, and high tidal zones (48, 38,
and 59 individuals, respectively). Significant differences were found in the number of
bioeroding species between the high and low tidal zones (p = 0.029), but not in the
abundance of bioeroders (p = 0.104).
When both cliffs were compared, significant differences were found in the abundance of
bioeroders between the high zone in IC and the low zone in EC (p = 0.018). We also
found significant differences in the number of species between the low and high tidal
zones in EC (p = 0.011). However, there were no significant differences in the
abundance or richness of bioeroders between wave exposures (p = 0.149 and p = 0.876,
respectively), nor in the interaction between wave exposure and tidal zone (p = 0.621
and p = 282, respectively).
3.4. Relationship between cliff composition and bioeroding fauna
The perforation volume in both cliffs was negatively correlated to tidal zone (IC: r = -
0.686, p = 0.041, Fig. 4A; EC: r = -0.782, p = 0.022, Fig. 4C) but positively correlated
with the abundance of bioeroders in IC (r = 0.74, p = 0.023, Fig. 4B), and with the
richness of bioeroders in EC (r = 0.725 p = 0.042, Fig. 4D).
The combination of factors did not indicate a statistically significant effect on the
percentage of perforations found for any of the cliffs. However, the R2 values indicate
that 63.5% (83.9%) of the total variation of perforations in IC (EC) is explained by the
regression (Table 3). R2 values are higher in EC than in IC, indicating that all variables
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chosen for the analysis influence the percentage of perforations found in EC, but not in
IC.
Fig. 4. p-value, correlation (r) and determination coefficients (r2) found for significant
linear correlations in unexposed cliffs (IC; A-B) and exposed cliffs (EC; C-D) on the
Pacific coast of Colombia. Biodegraded volume was negatively correlated with tidal
zone in both cliffs (A and C), and positively correlated with abundance of bioeroders in
IC (B) and with richness of bioeroders in EC (D).
Table 3. Multiple regression analysis comparing volumes of perforation with tidal zone,
volume of natural porosity of the rocks, richness and abundance of bioeroding fauna in
unexposed (IC) and exposed cliffs (EC). Multiple correlation coefficient (R), multiple
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determination coefficient (R2), and adjusted coefficient of determination are shown
(Ra2), as well as results of ANOVA for the multiple regression data.
Statistics IC EC
Multiple R 0.797 0.916
Multiple R2 0.635 0.839
Adjusted Ra2 0.271 0.625
F F(4, 4, 0.95) = 1.746 F(4, 3, 0.95) = 3.922
p 0.301 0.145
4. DISCUSSION
4.1. Sediment composition of the cliffs
Our findings confirm the fine sediment composition of the rocks forming the cliffs on
the Pacific Coast of Colombia, which is mainly due to differences in particle
sedimentation rates during cliff formation, and to the energy of the deposition
environment. The sedimentary rocks forming the cliffs are composed of ancient mud
and silt and were produced by accumulation of sediments from the river flow.
Unexposed cliffs (IC) were formed by sediments from rivers flowing into Buenaventura
Bay, which is why they have clays only in the higher and middle zones and few hard
substrata inclusions (as seen by the higher percentage of small grain sizes). The
presence of coarse particles in the rocks between layers of fine sediments in the low and
middle tidal zones of exposed cliffs (EC; blocks L1 and M1 in Fig. 3B) is due to the
Raposo Mayorqun formation, characterized by the presence of coarse sediments
(sandstone, slabs and clusters) randomly arranged between the strata (Cantera et al.,
1998). These coarse sediments result from the consolidation of sediments from the river
flow that subsequently changed the grain size composition of the original Mayorqun
formation.
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It is important to recognize that different agents of bioerosion may operate in distinct
zones across shore platforms (Naylor et al., 2012): for example, biological weathering
and erosion-enhancing agents are typically found in morphologically lower, moister
positions of shore platforms (our lower intertidal zones) (Naylor et al., 2012), whereas
chemical/physical weathering agents are likely to be relatively more important in drier,
morphologically high points where wetting/drying and swelling/contraction are more
common (our supratidal zones) (Gmez-Pujol and Forns, 2009). Wave action is
weaker in the tropics than in high latitudes, and younger limestones or shales are
physically much weaker (Trenhaile, 1987). EC receives a more constant and higher
wave action than IC, which can affect its porosity by removing smaller particles from
the cliff in the first stages of erosion, causing abrasion on the bigger particles that are
left. However, even though EC is more exposed to waves and we expected this cliff to
be more perforated, it seems that porosity volumes play a more important role in
determining perforation volumes. Trudgill (1985) established that increased porosity
decreases rock resistance to erosion compared to that of well-cemented rocks with few
joints; so the increased porosity in IC makes this cliff more susceptible to erosion, in
spite of it being less exposed to waves.
In addition, although waves perform the erosive work, it is the tidally modulated
distribution of wave energy that determines where this work is performed (Trenhaile,
1978). The lower tidal zones of both cliffs stay submerged longer than the other zones
as a result of the semi-diurnal tidal cycle of Buenaventura bay; this benefits the
bioerosion community inhabiting the lower zones of both cliffs, enhancing the higher
perforation volume exhibited by them.
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Bioerosion (in some cases) is greatest in sheltered sites because wave shock and mobile
sedimentary particles driven by waves and currents can prevent colonization of exposed
areas by some organisms (Trenhaile, 1987). Cliffs in IC are exposed to higher river
discharges and a low wave impact; this allows a film to be formed on the surface of
these cliffs, protecting them from rain and growth of microbioeroders (e.g. algae).
Hutchings et al. (2005) found a similar effect on coral bioerosion. In cliffs in IC, this
film also hinders grazer colonization and increases the presence of bioeroding larvae in
all tidal zones in IC that otherwise would be eaten by grazers feeding on the algae.
4.2. Bioeroding fauna
The difference in species richness between tidal heights of the cliffs is exposed sites (as
EC) have better humidity and shelter conditions that allow higher species richness in the
lower tidal zones, especially of species representative of the lower tidal zones
(Petrolisthes armatus, A. javieri, U. tenuipollex). On the other hand, species typical of
the high tidal zone (Chelorchestia sp., Petrolisthes zacae, P. transversus) dominate
sheltered sites (as IC) (Palmer et al., 2003).
The boring habit is well developed in four pelecypod families: Pholadidae and
Petricolidae (mainly mechanical borers) and Gastrochaenidae and Mytilidae (largely
chemical borers that require a calcareous substrate) (Yonge, 1955; Trenhaile, 1987).
Drilling Mytilidae species have been reported as responsible for the greatest amount of
perforations in hard rocks (Cantera et al., 1998), while species of Petricolidae (Ansell,
1970) and Pholadidae have been for soft rocks (Warme and Marshall, 1969; Pinn et al.,
2005, 2008). In this study, we found Cyrtopleura crucigera (Pholadidae) in both cliffs,
while Sphenia fragilis (Myidae) and Pholadidea tubifera (Pholadidea) were found in
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EC only. These findings differ from results previously found for the area, where C.
crucigera, S, fragilis and Pholadidea sp.1 were found in IC (Cantera et al., 1998). Three
species of bivalves were found in EC that were reported as borers by Keen (1971):
Cryptomya californica (Myidae), Ensitellops hertleini (Basterotiidae), and Barnea
subtruncata (Pholadidae). The mytilid species Brachidontes playasensis was found in
low abundances in the low zone of cliffs in IC, where it could be playing an important
role in protecting the rock surface from physical erosion.
Crustaceans, on the other hand, have been reported as borers of wood (Davidson and de
Rivera, 2012), sandstones (Cade et al., 2001), and basalts (Fischer, 1981b). Upogebia
tenuipollex and Alpheus javieri are boring decapods in cliffs of the Colombian Pacific
coast (Ricaurte et al., 1995). They were found in great numbers in EC but not in IC,
where they were outnumbered by Alpheus villus, Upogebia spinigera, and Upogebia
burkenroadi. These three species may be taking an active part in the bioerosion process
in cliffs that are less exposed to wave action
Several worms are also active borers in calcareous and non-calcareous substrates
(Trenhaile, 1987). Hutchings and Peyrot-Clausade (2002) recognize polychaetes and
sipunculans as dominant groups of macro-boring organisms in newly available dead
coral substrate, facilitating the subsequent recruitment of other boring organisms such
as sponges and bivalves. For the Pacific coast of Colombia, Cantera et al. (1998) found
Polydora sp. in cliffs in EC. However, although we found three species of polychaetes
inhabiting the rock burrows in EC (Nereis sp., Lysidice sp. and Syllis sp.) and two in IC
Nereis sp. and Neanthes sp.), we did not find Polydora within our samples. Of the
genera found, Lysidice has been found to be burrowing inside Porites colonies
(Hutchings, 2008), so they could also be boring into rocky cliffs. We only found one
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individual of the sipunculid genus Phascolosoma in a rock burrow in the lower tidal
zone of EC (data not shown), contrary to previous findings that highlighted their
importance on the bioerosion process in cliffs of the Pacific coast of Colombia (Cantera
et al., 1998). Our evidence indicates that molluscs and crustaceans are more important
than polychaetes and sipunculans in the bioerosion process in the cliffs that were
studied.
4.3. Contribution of bioeroding fauna to perforation volumes
Previous authors found that a greater biological contribution to erosion rates occurs on
sheltered shores compared to exposed ones (Trudgill, 1976; Spencer and Viles, 2002;
Moura et al., 2012). This coincides with our findings, in which IC presented better
conditions for the establishment of bioeroder communities, which in turn was reflected
in a higher volume of perforation found compared to EC.
Naylor et al. (2012) highlighted the importance of the biological role in the removal of
rocky masses and the erosion of rocky coasts (which had been previously neglected).
The direct erosional role of grazing organisms is of particular significance on tropical
and warm temperate limestone coasts, where wave attack may be fairly weak
(Trenhaile, 1987). Grazing organisms always contribute directly to the erosion of rock
surfaces, and their presence in IC can explain the higher porosity found here, despite the
high percentage of small particle composition; macro- and micro-grazers facilitate the
penetration of microflora into the substrate and indirectly weaken and increase rock
porosity (Trenhaile, 1987; Naylor et al., 2012). Rock borers play a direct and indirect
role in the disintegration of rocky substrates, particularly in the lower portions of the
intertidal zone. Boring directly removes some rock material, but the rest is left
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susceptible to breakdown by wave action and other destructive mechanisms (Trenhaile,
1987). Borers also enhance the rock environment for algal colonization, and increase
the area of rock surface exposed to other physical and chemical processes (McLean,
1974). The indirect role of rock borers may be of greater significance to the destruction
of coastal rocks than is the direct removal of material.
4.4. Bioerosion and Habitat Heterogeneity
The higher number of perforations (providing refuge) and the time a cliffs tidal zone
remains submerged, determined the higher richness and abundance of bioeroders
inhabiting the burrows of the cliffs lower tidal zones. The tide determines how long a
substrate is underwater or exposed (subject to desiccation) (Trenhaile, 1987), which
partly depends on the tides being diurnal, semi-diurnal, or mixed (Johnson and Sparrow,
1961). This permits less desiccation, and changes in temperature and salinity (Palmer et
al., 2003) in the cliffs lower zones.
Biodiversity, in terms of number of species, is higher when suitable microhabitats for
vagile species are present in addition to those available for sessile species. Bivalve and
crustacean burrows provide more shelter for vagile species than irregularities in the
naturally occurring substratum (such as crevices), and thus enhance the abundance and
diversity of intertidal species low on the shore (Pinn et al., 2008). The use of crevices as
shelter was seen in both cliffs by the presence of eight species of fish during low tide:
Pisodonophis daspilotus, Cerdale ionthas, Gobulus hancocki and Erotelis armiger in
IC; and Clarkichthys bilineatus, Cerdale paludicola, Microdesmus dipus and
Pythonichthys asodes in EC. The fact that both cliffs are located near mangrove zones,
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which are known to act as nurseries, explains why Grapsidae, Porcellanidae and
Ocypodidae crustacean megalops were found in these sites.
4.5. Further studies on tropical rocky cliffs
Sea cliff erosion in the tropics is an understudied subject and there is a dearth of
information on erosion rates and dynamics. Although it is a difficult task, the
understanding of the relative contribution of wave impact and abrasion to total erosion
rates through field measurements requires further study (Moses, 2013). Furthermore, the
effects of climate change on erosion rates need to be more thoroughly studied, because
the predicted increase of storm activity and/or intensity, sea-level rise and the
interaction of both could contribute significantly to erosion (Phillips and Jones, 2006).
To assess and predict the impacts of climate change, the understanding of bioerosion
dynamics needs to be expanded to harder igneous and sedimentary rocks because
studies have been largely limited to recent and relatively weak beach rock and reef
limestone (Moses, 2013; Moses et al., 2014).
Another item that could have important consequences particularly for rocky cliffs in
the Colombian Pacific is the modification of seawater chemistry by organisms
inhabiting the burrows. pH reduction during night hours (as an imbalance between
photosynthesis during the day and respiration during the night) would increase the
solubility of calcium carbonate in the rocks and facilitate their degradation. This process
needs to be further studied.
5. CONCLUSIONS
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Bioerosion is a process in which biological, geological, and geomorphological factors
interact. For the Pacific coast of Colombia, the abundance of bioeroders (biological
factor) and tidal zone (physical factor) were the most influential eroding factors on cliffs
sheltered from wave action. On the other hand, tidal zone and richness of bioeroders
(also a biological factor) were the most important in determining erosive volumes in
cliffs exposed to wave action. Rock composition in IC presented smaller grain sizes
than EC, resulting in more porous and perforated rocks. The highest abundance of
bioeroding organisms was found in the lower tidal zones of both cliffs because this zone
stays under water for a longer period of time, providing vital conditions for the fauna
that takes refuge inside the cliffs during low tide. Boring bivalves were less abundant in
this study compared to that of boring crustaceans. We suggest that the importance of
crustaceans in the bioerosion process needs to be highlighted because it has always been
given a secondary role. In addition to Alpheus javieri and Upogebia tenuipollex
(previously reported as borers of the cliffs on the Colombian Pacific coast), we
recommend that Alpheus villus, Upogebia spinigera and Upogebia burkenroadi should
be considered as active boring species in cliffs of IC. We also include Cryptomya
californica, Ensitellops hertleini, and Barnea subtruncata as active borers for cliffs in
EC.
6. ACKNOWLEDGMENTS
This project was supported and funded by the Universidad del Valle, Biology
Department, Marine Biology Section, and by internal funding of the Research
Vicerectory of the Universidad del Valle. We thank Philip A. Silverstone-Sopkin and
Amparo Viveros for correcting the manuscript, Humberto Maya from the Biological
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Station of Universidad del Valle in Buenaventura, M. Cuellar, S. Cobo-Viveros, A.I.
Vsquez, V. Izquierdo, F. Vejarano and the locals from Piangita who helped during
different times on the extraction of blocks from the cliffs. Biologists J.F. Lazarus, L.A.
Lpez de Mesa, E. Rubio, L. Herrera from Ecomanglares research group, and B.
Valencia helped during the flora and fauna identification process. C. Manrique and N.
Durn from the Civil Engineering School at Universidad del Valle helped on the
sedimentology processing of the rocks. E. Londoo was very helpful on statistical
inquiries. Finally, thanks to R. Neira who helped in the organization of field trips and
experimental design.
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FIGURE LEGENDS
Fig. 1. Geographical location of the cliffs studied. Right: South America, showing the
location of the Pacific coast of Colombia (Middle). Left. Buenaventura Bay, showing
the locations of IC (unexposed cliffs) and EC (exposed cliffs).
Fig. 2. Grain size distribution of A. Unexposed (IC) and B. Exposed cliffs (EC) on the
Pacific coast of Colombia. The legend on the right represents each tidal zone: low (L),
middle (M), and high (H).
Fig. 3. Abundance of bioeroding species in the low, middle and high tidal zones of A.
Unexposed cliffs (IC) and B. Exposed cliffs (EC) in the Pacific coast of Colombia. The
category Others groups bioeroding fauna with total abundances of less than 10
individuals.
Fig. 4. p-value, correlation (r) and determination coefficients (r2) found for significant
linear correlations in unexposed cliffs (IC; A-B) and exposed cliffs (EC; C-D) on the
Pacific coast of Colombia. Biodegraded volume was negatively correlated with tidal
zone in both cliffs (A and C), and positively correlated with abundance of bioeroders in
IC (B) and with richness of bioeroders in EC (D).
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TABLE HEADINGS
Table 1. Grain size composition (percentage) of unexposed (IC) and exposed cliffs
(EC) in the Pacific coast of Colombia.
Table 2. Percentage of perforations due to bioerosion (relative to total volume) and
percentage of porosity (relative to volume of solids + volume of pores) found for the
three tidal zones of unexposed cliffs (IC) and exposed cliffs (EC) in the Pacific coast of
Colombia. Richness and abundance of bioeroding species of the studied cliffs is also
shown.
Table 3. Multiple regression analysis comparing volumes of perforation with tidal zone,
volume of natural porosity of the rocks, richness and abundance of bioeroding fauna in
unexposed (IC) and exposed cliffs (EC). Multiple correlation coefficient (R), multiple
determination coefficient (R2), and adjusted coefficient of determination are shown
(Ra2), as well as results of ANOVA for the multiple regression data.
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Table A1. Temperature Correction Factor (CT) applied to the actual hydrometer
readings. Source: Bowles (1981a).
Temperature
C CT
15 -1.10
16 -0.90
17 -0.70
18 -0.50
19 -0.30
20 0.00
21 +0.20
22 +0.40
23 +0.70
24 +1.00
25 +1.30
26 +1.65
27 +2.00
28 +2.50
29 +3.05
30 +3.80
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Table A2. Correction factor a determined for unit weight of solids (specific gravity).
Source: Bowles (1981a).
Unitary weight soil solids
(g/cm3)
Correction factor
a
2.85 0.96
2.80 0.97
2.75 0.98
2.70 0.99
2.65 1.00
2.60 1.01
2.55 1.02
2.50 1.04
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Table A3. Values of K used in Equation 7 for several combinations of unitary weights and temperatures to compute the particle diameter in
the Hydrometer Analysis. Source: Bowles (1981a).
Temperature
(C)
Gs
2.45 2.50 2.55 2.60 2.65 2.70 2.75 2.80 2.85
16 0.01510 0.01505 0.01481 0.01457 0.01435 0.01414 0.03940 0.01374 0.01356
17 0.01511 0.01490 0.01460 0.01440 0.01420 0.01400 0.01380 0.01360 0.01338
18 0.01492 0.01470 0.01440 0.01420 0.01400 0.01380 0.01360 0.01340 0.01321
19 0.01474 0.01450 0.01430 0.01400 0.01380 0.01360 0.01360 0.01340 0.01305
20 0.01456 0.01430 0.01410 0.01390 0.01370 0.01340 0.01330 0.01310 0.01289
21 0.01438 0.01410 0.01390 0.01370 0.01350 0.01330 0.01310 0.01290 0.01273
22 0.01421 0.01400 0.01370 0.01350 0.01330 0.01310 0.01290 0.01280 0.01258
23 0.01404 0.01380 0.01360 0.01340 0.01320 0.01300 0.01280 0.01260 0.01243
24 0.01388 0.01370 0.01340 0.01320 0.01300 0.01280 0.01260 0.01250 0.01229
25 0.01372 0.01350 0.01330 0.01310 0.01290 0.01270 0.01250 0.01230 0.01215
26 0.01357 0.01330 0.01310 0.01290 0.01270 0.01250 0.01240 0.01220 0.01201
27 0.01342 0.01320 0.01300 0.01280 0.01260 0.01240 0.01220 0.01200 0.01188
28 0.01327 0.01300 0.01280 0.01260 0.01240 0.01230 0.01210 0.01190 0.01175
29 0.01312 0.01290 0.01270 0.01250 0.01230 0.01210 0.01200 0.01180 0.01162
30 0.01298 0.01280 0.01260 0.01240 0.01220 0.01200 0.01180 0.01170 0.01149
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Table A4. Values of L (effective hydrometer depth, in cm) for use in Stokes formula to
determine particle diameters using an ATSM 152H hydrometer. Source: Bowles
(1981a).
Original hydrometer reading
(only corrected for meniscus)
Effective depth
L (cm)
Original hydrometer reading
(only corrected for meniscus)
Effective depth
L (cm)
0
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
16.3
16.1
16.0
15.8
15.6
15.5
15.3
15.2
15.0
14.8
14.7
14.5
14.3
14.2
14.0
13.8
13.7
13.5
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
11.2
11.1
10.9
10.7
10.6
10.4
10.2
10.1
9.9
9.7
9.6
9.4
9.2
9.1
8.9
8.8
8.6
8.4
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18
19
20
21
22
23
24
25
26
27
28
29
30
13.6
13.2
13.0
12.9
12.7
12.5
12.4
12.2
12.0
11.9
11.7
11.5
11.4
49
50
51
52
53
54
55
56
57
58
59
60
8.3
8.1
7.9
7.8
7.6
7.4
7.3
7.1
7.0
6.8
6.6
6.5
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Annex A
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Annex B