Main factors determining bioerosion patterns on rocky cliffs in a drowned valley estuary in the colombian pacific (eastern tropical pacific)

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.

[email protected]

b. Titular Professor. Department of Biology. Faculty of Natural and Exact Sciences.

Universidad del Valle. Calle 13 #100-00. Cali, Colombia. AA. 25360.

[email protected]

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