www.elsevier.com/locate/gloplacha

Global and Planetary Chan

Fluvial fluxes into the Caribbean Sea and their impact on coastal

ecosystems: The Magdalena River, Colombia

Juan D. Restrepo a,*, Paula Zapata b,1, Juan M. Dıaz c,2,

Jaime Garzon-Ferreira b,3, Camilo B. Garcıa d,3

a Departamento de Geologıa, Universidad EAFIT, A.A. 3300, Medellın, Colombiab Instituto de Investigaciones Marinas y Costeras, Invemar, Santa Marta, Colombia

c Instituto Alexander von Humbold, Bogota, Colombiad Universidad Nacional, Departamento de Biologıa, Cecimar/Invemar, A.A.1016, Santa Marta, Colombia

Received 13 November 2004; accepted 9 July 2005

Available online 9 December 2005

Abstract

The Magdalena, a world-class river, in the top ten in terms of sediment load ~150 MT/yr, is the largest river discharging directly

into the Caribbean Sea. Data on water discharge, sediment load, and dissolved load of the Magdalena River is presented as an

initial interpretation of coastal ecosystems changes in relation to water discharge and sediment load from the Magdalena. During

the 1972–1998 yr-period, the Magdalena River has delivered approximately 4022 MT of sediment to the Caribbean coast. The river

reflects high inter-annual variability and delivers large portions of its fluvial discharge and sediment loads in short periods of time.

The analysis of annual deviations from the 27-yr mean sediment load indicates that 59% of the total sediment load variability of the

Magdalena at Calamar could be attributed to flashy peak events. Further analyses of sediment load anomalies suggest that there was

a high discharge period in the Magdalena River between 1985 and 1995 and another one in the Canal del Dique between 1985 and

1992. These increasing trends in sediment load coincide with the overall decline of live coral cover around the Rosario Islands, a

145 km2 coral reef complex in the Caribbean Sea that constitutes a marine protected area. The comparison of live coral: algae ratios

for the 1983–2004 yr-period, also indicates that there has been an associated increase in the percentage of algae cover (i.e., Grande

Island 1983=5%, 2004=59%). Other analyses show that nearly 850 ha of seagrass existing in the Cartagena Bay in the 1930s, only

76 ha remained in 2001, which is less than 8% of the original cover. There has been a mix of multiple stressors (natural and

anthropogenic; local, regional and global; temporal and chronic) affecting the coastal ecosystems in the area, but the effect of the

Magdalena River runoff has been constant and very prolonged (several decades). The impacts of heavy sediment loads and

freshwater discharges from the Canal del Dique to Cartagena Bay have greatly contributed to the partial disappearance of coral

formations and also to a considerable reduction in abundance of seagrass beds in the bay and neighboring areas.

D 2005 Elsevier B.V. All rights reserved.

Keywords: sediment transport; runoff; coastal environment; corals; seagrasses; Magdalena River; Caribbean Sea

0921-8181/$ - s

doi:10.1016/j.gl

* Correspondi

E-mail addr

jgarzon@invem1 Fax: +57 5 42 Fax: +57 1 63 Fax: +57 5 4

ge 50 (2006) 33–49

ee front matter D 2005 Elsevier B.V. All rights reserved.

oplacha.2005.09.002

ng author. Fax: +57 4 2664284.

esses: jdrestre@eafit.edu.co (J.D. Restrepo), pzapata@invemar.org.co (P. Zapata), jmdıaz@humboldt.org.co (J.M. Dıaz),

ar.org.co (J. Garzon-Ferreira), cgarcia@invemar.org.co (C.B. Garcıa).

211377.

086900.

211377.

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–4934

1. Introduction

The flux of sediment to the coastal ocean will con-

tinue to be influenced by human and/or climate change.

Estimating the balance between increasing and decreas-

ing sediment loads is of utmost importance for sound

coastal zone and resource management (Syvitski,

2003). The earliest estimates of the flux of sediment

to the global coastal zone ranged from 13 to 51 Gt/yr

(Syvitski, 2001, 2003). Milliman and Syvitski (1992)

suggested a global estimate of 18 Gt/yr using a data set

of 280 rivers. More recent studies have shown that

rivers discharge more than 35�103 km3 of water and

20 Gt/yr of suspended and dissolved solids into the

coastal zone. Over the past 50 yrs, rivers have dis-

charged between 18 Gt/yr (Farnsworth and Milliman,

2003) and 20–24 Gt/yr of sediment annually to the

Fig. 1. Location of the Magdalena River drainage basin. (A) Map showing th

the Magdalena River. (B) Magdalena River drainage basin, showing the u

Colombia, showing the principal rivers, the main drainage basins, and the hy

and water discharge were measured. The location of the Magdalena delta, th

into the Caribbean Sea, are also shown.

global coastal zone (Ludwig and Probst, 1998; Syvitski,

2003). Northeastern South America, which includes the

Parana, Amazon, Orinoco, and Magdalena rivers (Fig.

1), contributes about 12% of the total sediment load.

These basins combined drain more than half the conti-

nent, 10 of 17�106 km2 (Milliman, 1990).

The focus on rivers draining South America has

been directed toward the three large fluvial systems,

the Amazon (Meade et al., 1979, 1985; Richey et al.,

1986, Richey et al., 1989), the Orinoco (Eisma et al.,

1978; Depetris and Paolini, 1991), and the Parana

(Depetris et al., 1996; Depetris and Gaiero, 1998;

Goniadzki, 1999). However, the sediment flux for the

Magdalena River of Colombia (Fig. 1) is of the same

magnitude as that of these three rivers, all of which

have much larger drainage basins (Bassin, 1976; Milli-

man and Meade, 1983; Javelaud, 1987; Restrepo and

e major basins in South America draining into the Atlantic, including

pper, middle, and lower reaches. (C) Map of the Caribbean coast of

drological stations at Calamar and Santa Helena, where sediment load

e Rosario Islands, and Barbacoas Bay, where the Canal Dique empties

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–49 35

Kjerfve, 2000b, 2004). The Magdalena is a world-class

river due to (1) its contribution of fluxes to the global

budgets (Milliman and Meade, 1983; Milliman, 1990;

Milliman and Syvitski, 1992; Ludwig and Probst,

1998); (2) its sediment yield, the largest on the South

America’s Atlantic seaboard (Restrepo and Kjerfve,

2000a,b); (3) its drainage basin, the largest of any

Andean river; and (4) its most significant contribution

of fluxes source on the Caribbean Sea (Restrepo and

Kjerfve, 2002).

The role of terrigenous sediment in controlling the

occurrence of coral reefs has been globally recognized.

Sediments have inhibitory effects on reef communities

and the sedimentary processes, including various asso-

ciations between substrate type, turbidity and light

availability, affect coral distributions on all scales

from local depth restrictions to broad-scale biogeogra-

phy (Mc. Laughlin et al., 2003; Wolanski et al., 2003).

According to Mc. Laughlin et al. (2003), the greater

impact of fluvial fluxes on coral reef ecosystems is

likely to occur at the scale ranging from small to

medium-sized drainage basins, where land conversion

and deforestation increase runoff and erosion, to the

detriment of near-shore reef environments. Changes at

this scale are poorly understood in the Caribbean basins

and have not been addressed for the Magdalena River,

the largest contributor of water and sediment discharges

into the Caribbean Sea (LOICZ, 2002).

Scientists and decision-makers have been debating

for the past 10 yrs as to whether and to what extent the

fluxes from the Magdalena River have impacted the

adjacent coastal ecosystems, including the Rosario

Islands, a 145 km2 coral reef complex in the Caribbean

Sea that constitutes a marine protected area. Some

studies (i.e., Dıaz et al., 2000; Barrios, 2000) have

shown that the current cover of live coral in El Rosario

Islands is only 33%. This is attributed to many stres-

sors, including (1) freshwater-induced bleaching; (2)

sediment and nutrient loading enhancing the ability of

macro algae to compete with corals for benthic sub-

strate; (3) increased anthropogenic contamination from

urban areas (sewage and toxic substances); and (4)

tourism and overexploitation of coral structures for

construction activities in the region (Garzon-Ferreira

and Kielman, 1993; Dıaz et al., 1996; Garzon-Ferreira

et al., 2001).

It is our objective to synthesize data on water dis-

charge, sediment load, and dissolved load of the Mag-

dalena River, the principal fluvial system discharging

into the Caribbean Sea. We also present an initial

interpretation of reef cover changes in relation to

water discharge and sediment load from the Magdalena

and show as well some implications and impacts on

seagrass ecosystems. This first exercise, assessing pos-

sible causes of coastal ecosystem deterioration (i.e.,

coral reef mortality), necessarily addresses the net com-

bined effects of fluvial discharge-related stressors.

Therefore, the preliminary analysis presented here

may be further used in future studies to identify

approaches to separating and defining sediment-specific

controls. Finally, we make comparisons with other

major fluvial systems draining into the Atlantic Ocean

and elsewhere.

2. The Magdalena River system

Caribbean Colombia is principally drained by the

Magdalena and Sinu rivers, and also receives the Atrato

drainage from west of the Cordilleras (Fig. 1). The

Magdalena River is the largest river system of Colom-

bia with a length of 1612 km and originates at the

Magdalena lagoon at an elevation of 3685 m. The

drainage basin area covers 257,438 km2, 24% of the

Colombia territory, and occupies a considerable portion

of the Colombian Andes (Fig. 1). The basin is charac-

terized by high tectonic activity, hillslopes commonly

exceeding 458, landslides, steep gradients, high relief

tributary basins (71% of the catchment area corre-

sponds to elevations N1000 m), and moderate precipi-

tation with an average rainfall of 2050 mm yr�1. The

basin is formed by 151 sub-catchments from which 42

are second-order watersheds. The river’s upper reaches

are 565 km long, i.e. 37% of the river’s total length, and

extend over a catchment area of 55,140 km2, compris-

ing 22% of total catchment area. The middle reaches

are more than 540 km, comprising 35% of the river’s

total length, and with a catchment area of 105,000 km2,

or 40% of the total drainage basin area. The main

tributaries are the Cauca (the second largest river in

Colombia), Sogamoso, San Jorge, and Cesar rivers. The

Magdalena River’s lower reaches are 430 km long,

comprising 28% of the river’s total length, with a

catchment area of 96,240 km2, or 38% of the total

catchment area. There are numerous lakes, such as

the Mompox tectonic depression, a low floodplain

with an area of 800 km2, and no large tributaries in

the lower reaches (Fig. 1) (Table 1).

The Magdalena River discharges into the southwest-

ern Caribbean and forms a 1690 km2 triangular delta

(Coleman, 1976). The delta plain consists of alluvial

plains, marginal lagoon systems, and beach ridges

(Vernette, 1985). The receiving basin is characterized

by sedimentation, slumping, and compressional tecton-

ics that cause the presence of mud diapirism in the delta

Table 1

River length (RL), drainage basin area (AD), and average annual precipitation (r), water discharge (Q), runoff (Df), runoff coefficient (Df/r),

sediment load (Qs), and suspended sediment concentration (SSC) for the upper, mid and lower basin

Basin RL AD r Df Df/r Q Qs SSC

(km) (�103 km2) (mm yr�1) (mm yr�1) (–) (m3 s�1) (�106 t yr�1) (kg m�3)

Upper basin 565 55441 1535 900 0.60 1390 51.2 1.07F0.64

Mid basin 1110 161300 2185 1260 0.58 4230 81.5 0.60F0.25

Lower basin (Calamara) 1540 266540 1630 700 0.43 7100 144.2 0.6F50.78

a The most downstream gauging station, Calamar, is located 112 km from the Magdalena delta (Fig. 1).

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–4936

front (Shepard et al., 1968; Kolla and Buffler, 1984;

Vernette et al., 1992). The present delta mouth empties

into an offshore canyon with a steep slope (408) (She-pard, 1973). Wave processes extensively rework the

shoreline. Average wave power is 206 ergs s�1 m�1

of coastline (Coleman and Wright, 1975; Coleman,

1976). The delta front experiences a microtidal range

less than 0.5 m (Kjerfve, 1981; Martınez and Molina,

1992). Strong littoral currents predominantly flow to-

ward the west and are the result of open ocean swells,

generated by NE trade winds (Lorin et al., 1973). The

Magdalena is assumed to be the dominant source of

beach sand for the northern Caribbean coast of Colom-

bia. The low sediment flux during the low discharge

season, combined with the wave-dominated coastline,

result in sand bar extension across the river mouth. In

addition, the delta of the Magdalena River is located in

a very arid zone, with an annual water deficit of 1031

mm yr�1. The delta comprises the largest estuarine

lagoon complex in Colombia known as Cienaga Grande

de Santa Marta (Fig. 1), a lagoonal complex surrounded

almost entirely by mangrove forests.

3. Material and methods

We have obtained daily water and suspended sedi-

ment load data in the Magdalena River from the down-

stream station at Calamar, which is located 112 km

upstream from the Caribbean (Fig. 1). Data were

obtained from the Institute for Hydrology, Meteorology

and Environmental Studies (Instituto de Hidrologıa,

Meteoreologıa y Estudios Ambientales or IDEAM,

Colombia) (IDEAM, 2003).

Water discharge data for the period 1942–2002 were

based on daily stage readings for the 60-yr record and

converted to discharges via the established rating curve

(Buchanan and Somers, 1969; Jansen et al., 1979).

Sediment load estimates at Calamar were based on

the measured sediment concentrations done by the

IDEAM between 1972 and 1998, cross-multiplied

with water discharge (Colby, 1956). Regression of sed-

iment load on water discharge for the 55 measurement

occasions yielded a relationship, which was used to

estimate daily sediment loads 1972–1998.

Sediment load estimates in the Canal del Dique at

Santa Helena, a 114-km-long man-made channel from

the Magdalena River at Calamar to Cartagena Bay (Fig.

1), were based on the measured sediment concentra-

tions done by IDEAM during the 1984–1998 yr-period,

cross-multiplied with water discharge. Bed load trans-

port is not included in the analysis since this contribu-

tion to total load is less than 15% in the Magdalena

River (IDEAM, 2001).

The concentrations of major dissolved constituents

and mass transport rates for major Caribbean rivers of

Colombia, including the Magdalena River and the

Canal del Dique (Fig. 1), were based on averages

calculated from monthly samples from 1990–1993

(IDEAM, 1995). Nutrient fluxes of phosphate (PO43�)

and nitrate (NO3�) were based on averages calculated

from monthly samples covering the 3-yr period 1998–

2000 (INVEMAR, 2000, 2003).

To identify anomalies and relate trends of sediment

load to trends of coastal ecosystem composition (e.g.

structure of coral reef communities in the Rosario

Islands), we plotted the cumulative sum of the normal-

ized sediment load (Qs) and fitted a curve. The plots,

which summarize the variation of sediment flux in the

Magdalena River and the Canal del Dique, were de-

veloped in 5 steps: (1) normalization of the original

daily series by subtracting the inter-annual mean and

dividing by the inter-annual standard deviation; (2)

calculation of the monthly averages of sediment load;

(3) plot the F1r and F2r confidence band around the

mean; (4) plot the cumulative normalized sediment

load; and (5) smoothing time series data based on a

4th-order polynomial fit. This method is useful in

discovering certain traits in a time series, such as

long-term trend and inter-annual components (Shum-

way and Stoffer, 2000).

To understand how much variability in discharge is

due to episodic events (e.g., the rivers deliver a large

proportion of their fluvial discharge in short periods of

time), we plotted annual deviations from the inter-an-

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–49 37

nual mean sediment load in the Magdalena River and

the Canal del Dique.

Also, we have plotted a double mass plot of cumu-

lative suspended sediment load versus cumulative an-

nual water discharge in the Magdalena River, in order

to compare the trend of sediment flux relative to that of

water discharge. If both sediment flux and flow evi-

dence similar trends, the slope of the double mass plot

will not change, but if the sediment load increases or

decreases at a greater rate than the annual water dis-

charge, the double mass plot will evidence a departure

from its original slope (Walling and Fang, 2003). The

plots show if trends of sediment load are due to changes

in flow or some human induced factors.

Study sites of reef development and community struc-

ture around the Rosario Islands were selected based on

previous descriptions and data availability. These areas

were at Pavitos, Baru, Grande, Rosario, and Tesoro

Islands (Fig. 2). Community composition and coral and

algal cover datasets were gathered from several studies

made during the last two decades (Ramırez et al., 1985;

Sarmiento et al., 1989; Universidad Jorge Tadeo Lozano-

Inderena, 1989; Garzon-Ferreira and Kielman, 1993;

Fig. 2. Satellite images showing the muddy plumes of the Magdalena River i

Cartagena bays (B). The plume of the Magdalena spans more than 90 km

Cartagena bays are made visible by their high turbidity. The locations of

Rosario Islands at Pavitos (Pa), Baru (Ba), Grande (Ig), Rosario (Ro), and Te

in this figure legend, the reader is referred to the web version of this article

Navas et al., 1992; INVEMAR, 2002, 2003) and a

current survey carried out by the Institute of Marine

and Coastal Research of Colombia, INVEMAR (P.

Zapata and C. Garcıa, unpublished data). Comparisons

between data sets collected as part of the ongoing INVE-

MAR survey and those collected prior to the 1990s

enable an assessment of changes that have taken place

at four of the reef sites. We use algal cover as a proxy

measure of reef health.

4. Results

4.1. Water discharge, sediment flux and dissolved load

The mean annual water discharge of the Magdalena

River is 7200m3 s�1 (Fig. 3A) with amean low discharge

of 4068 m3 s�1 in March and a mean high discharge of

10,287 m3 s�1 in November. The annual volume of

water discharged into the Caribbean Sea is 228 km3.

The Magdalena River displays a strong seasonal

signal of discharge and sediment load variability due

to the El Nino–La Nina cycle. The water discharge at

the Calamar station (Fig. 1) and the smoothed monthly

n the Caribbean Sea (A) and the Canal del Dique in the Barbacoas and

in length along the Caribbean coast. The plumes at Barbacoas and

study sites of reef development and community structure around the

soro (Te) are also shown. (For interpretation of the references to colour

.)

Fig. 3. (A) River discharge data for the Magdalena River 1942–2002 at Calamar; (B) demeaned water discharge of the Magdalena River at Calamar

(thin line) and low-frequency pass filer with zero-phase (bold line) of the Southern Oscillation Index (SOI) (National Oceanic and Atmospheric

Administration (NOAA), 2003, database on the Internet at http://ftp.ncep.noaa.gov/pub/cpc/wd52dg/data/indices).

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–4938

values of the Southern Oscillation Index (SOI), which

is defined as the difference in atmospheric sea level

pressure between Tahiti and Darwin (Glantz, 1997),

show very good coherence for the 60-yr period 1942–

2002. Peak flows usually exceed 12,000 m3 s�1 during

La Nina years and low discharges of 2000–3000 m3 s�1

are observed during El Nino years (Fig. 3B). Mean

annual discharges during El Nino and La Nina years

are 5512 m3 s�1 and 8747m3 s�1, respectively.

During the 27 yrs of monitoring, the Magdalena

River has delivered approximately 4022�106 t of sed-

iment to the Caribbean coast. The 27-yr mean sediment

load is 143.9�106 t yr�1 and is approached or

exceeded in 15 yrs. In 6 of the 27 yrs, the river

delivered less than 110�106 t yr�1, whereas in 14

yrs the load exceeded 146�106 t yr�1 (Fig. 4A).

This interannual mean of sediment load is equal to

86% of the total sediment load of all Colombian rivers

draining into the Caribbean. The main tributary, the

Cauca River, contributes 38% of the total Magdalena

sediment load. The estimate of the sediment load for the

Magdalena River implies a sediment yield of 560 t

km�2 yr�1 for the 257,438 km2 upstream basin.

The Canal del Dique annual water discharge and

sediment load at the Santa Helena station (Fig. 1) has

a mean of 397 m3 s�1 and 5.9�106 t yr�1, respectively.

Discharges as high as of 800 m3 s�1 and sediment loads

as high as 31�103 t day�1 often occur during Novem-

ber. During the 15 yrs of monitoring, the Canal del

Dique has discharged approximately 89�106 t of sed-

iment to Barbacoas Bay (Fig. 1). The mean annual

sediment load of 5.9�106 t yr�1 was reached or

exceeded in 11 different years. In 3 of the 15 yrs, the

Canal delivered less than 6�106 t yr�1, whereas in 4

yrs the load exceeded 7.3�106 t yr�1 (Fig. 4B).

The Magdalena River reflects high interannual vari-

ability and delivers large portions of its fluvial discharge

and sediment loads in relatively short periods of time.

The analysis of annual deviations from the 27-yr mean

sediment load indicates that 59% of the total sediment

load variability of the Magdalena at Calamar could be

attributed to flashy peak events. The sediment load

experienced 16 deviations from the interannual-year

mean (Fig. 5A). The smaller Canal del Dique experi-

enced 7 yrs, or 50% of the total sediment load variabil-

ity, in which the annual sediment load exceeded 50% of

the mean (Fig. 5B).

The lower course of the Magdalena River has large

flood plains known as the Momposina basin (Fig. 1), a

tectonic depression (or bnatural damQ) of 800 km2 in

which much of the sediment load is stored for long time

periods. The Momposina basin concentrates around

80% of the total number of bCienagasQ, depressionsin the Cretaceous–Tertiary bedrock with stagnant or

river-dependent bodies of water that accumulate sedi-

ments. Van der Hammen (1986) drilled 9 boreholes,

and dated 25 peat-rich horizons intercalated with clay,

recording Holocene fluctuations in the flooding inten-

Fig. 4. Annual suspended sediment load for two stations, Calamar (A) and Santa Helena (B), within the lower reach of the Magdalena River. The

mean annual loads are indicated (dashed line). The location of each hydrological station is shown in Fig. 1C.

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–49 39

sity of the Magdalena basin. An estimated sedimenta-

tion rate of 3.0 mm yr�1 was inferred for the last 150014C yr BP. Our sediment transport estimates between

1972 and 1998 indicate that 14% of the Magdalena

sediment load, approximately 21�106 t yr�1, is

trapped in the Momposina basin with a sedimentation

rate of 2.0 mm yr�1. The sedimentation rate of 3.0 mm

Fig. 5. Annual deviations from the interannual mean sediment load of the Ma

(B). The location of each hydrological station is shown in Fig. 1C.

yr�1 obtained by Van der Hammen (1986) differs from

our sedimentation rate by only 33% and suggests that

the Magdalena sediment load has been nearly constant

during the last 1500 yrs BP.

The fate of sediment derived from episodic hyper-

pycnal events in the Momposina basin at present is

difficult to ascertain and has not yet been addressed.

gdalena River at Calamar (A) and the Canal del Dique at Santa Helena

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–4940

How long does it stay in the Momposina basin, how

does it mobilize and where does it go? The effect of such

episodic sediment discharges to the shoreline, benthos,

water column and the coastal zone must be very great.

The concentrations of major dissolved constituents

and mass transport rates for major Caribbean rivers of

Colombia, including the larger Magdalena, the Canal

del Dique, Sinu, and Atrato (Fig. 1), are shown in Table

2. Estimates are based on averages calculated from

monthly samples from 1990 to 1993 (IDEAM, 1995).

Ca2+, Mg2+ are the dominant ions (Table 2), indicating

that the water corresponds to the rock-dominated type.

The hydrological regime of rivers is a major regu-

lator of their chemical composition. For each chemical

element or TDS value, concentrations and fluxes are

discharge-dependent (Meybeck, 1996, 2001a,b). The

estimates of dissolved materials exported to the Carib-

bean basins are controlled mainly by water discharge.

Thus, the Magdalena transports 30�106 t yr�1 of

dissolved materials into the Caribbean (Table 2). The

specific transport rate is highest in the Sinu basin, 167

t km�2 yr�1, followed by that of the Magdalena with

117 t km�2 yr�1.

4.2. Impacts on coastal ecosystems

The Rosario Islands (Fig. 1) are located about 25 km

SW of the city of Cartagena and occupy a tropical

Table 2

Basic hydrochemical data and dissolved solutes of major Caribbean

rivers of Colombia for the 1990–1993 period

River

Parameter Caribbean Colombia

Magdalena Dique Sinu Atrato

pH 7.1 7.1 7.0 5.9

Na+ (mg l�1) 4.6 3.9 4.6 1.1

K+ (mg l�1) 1.7 2.0 1.6 1.6

Mg2+ (mg l�1) 11.7 11.6 22.0 1.5

Ca2+ (mg l�1) 36.2 30.1 34.0 4.4

Cl� (mg l�1) 9.0 7.2 9.4 2.8

SO42� (mg l�1) 6.0 8.5 8.4 0.2

Total ALKAL (–) 60.8 59.7 62.5 18.9

SiO2 (mg l�1) . . . . . . . . . . . .

TDS (mg l�1) 131 123 142.3 30.5

Transport TSS (�106 t yr�1) 144 4.8 6.1 11.3

Transport TDS (�106 t yr�1) 30.0 1.6 1.7 1.1

Transport TSS/TDS 4.8 3.0 3.6 10.3

Net CO2 (mg l�1) 1.6 7.9 18.1 4.6

Solute values are expressed as discharge-weighted mean (modified

from Restrepo and Kjerfve, 2004).

TSS=total suspended solids; TDS=total dissolved solids; (. . .)=noavailable data.

Source: IDEAM (1995).

climatic zone influenced by NE trade winds. This coral-

line complex is one of the most important reef areas of

the Colombian coast because it is characterized by coral

growth up to 50 m in depth and high biodiversity with

approximately 53 species of stony corals (Dıaz et al.,

2000). Also, these coralline islands have supported a

growing tourism industry for more than 30 yrs. Although

the coral reefs and associated marine environments of the

archipelago have been under governmental protection

since 1985, enforcement has been very limited and the

coastal environment has been impacted by human activ-

ities. The muddy plumes of the Magdalena River and the

Canal del Dique have largely affected water quality and

both the entrained mud and reduced salinity may have

caused acute damage to the coral reefs (Fig. 2).

The comparison of live coral: algae ratios indicates

that significant changes are taking place at Pavitos,

Baru, Grande, and Rosario (Fig. 2), where there have

been marked reductions in live coral cover (i.e., Grande

Island 1983=95%, 2004=41%) and an associated in-

crease in the percentage of algae cover (1983=5%,

2004=59%) (Fig. 6). Indeed, the most abundant con-

stituent of the shallow reef in the north side of Grande

Island at present is dead, in situ coral, most of which is

covered by filamentous algae (Figs. 6 and 7) and much

of which shows evidence of intense bioerosion (mainly

by the sponge Cliona sp.) (Cendales et al., 2002).

Clearly, in the analyzed period (1983–2004), major

changes in reef structure and cover have occurred at

many sites around the Rosario Islands.

Average community composition analysis in the

Rosario Islands also suggests that natural disturbances

have largely impacted coral reefs (Fig. 7). The 1982–

1983 ENSO event largely affected the reefs and many

bleaching events on stony corals were reported (i.e.,

Alvarado et al., 1986; Solano et al., 1993). Other

bleaching events also occurred during the 1986–1987

and 1989–1990 yr-periods. At the same time, as oc-

curred in most coralline areas of the wider Caribbean, it

seems that white band disease strongly affected the

populations of two of the main reef-builders (Acropora

palmata and Acropora cervicornis) in the Rosario

Islands and many other reef areas of Colombia (Gar-

zon-Ferreira, 1997; Garzon-Ferreira et al., 2001; Gar-

zon-Ferreira and Dıaz, 2002). At present, the formerly

extensive Acropora spp. formations at the islands are

nearly 100% dead and reduced to algae-covered rubble

(Cendales et al., 2002). In addition, algal cover in the

area increased up to 75% soon after the 1982–1984

massive mortality of Diadema antillarum along the

Caribbean (Lessios et al., 1984; Liddell and Ohlhorst,

1986).

Fig. 7. Annual surveys of community structure around the Rosario

Islands at Grande (A) and Tesoro (B). The specific location of each

study site is shown in Fig. 2.

Fig. 6. Comparison of live coral: algae ratios at different locations

around the Rosario Islands, Caribbean coast of Colombia, including

Pavitos (A), Baru (B), Grande (C), Rosario (D), and Tesoro (E). The

specific location of each study site is shown in Fig. 2.

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–49 41

Fig. 8 shows the anomalies of sediment load above

or below the inter-annual average. The result is a syn-

thetic index with a zero mean and cumulative values of

the normalized monthly averages (z-scores) of sediment

discharge. It also shows the overall covertures of live

and dead coral between 1983 and 2004 for the whole

archipelago. There has been a relatively high discharge

period between 1985 and 1995 in the Magdalena River

(Fig. 8A). The Canal del Dique also evidences an

increasing trend in sediment load between 1985 and

1992 (Fig. 8B). In addition, there was a marked reduc-

tion in live coral cover between 1983 and 1990 (Fig.

8C), a period that coincides with the increased sediment

flux from the Canal del Dique into Barbacoas Bay

(Figs. 2 and 8B). Although Fig. 8 is only a rough

estimation of the relationship between fluvial fluxes

and coastal ecosystem health, it confirms that water

and sediment discharges from the Canal del Dique

and the Magdalena River appear to be one of the

stressors impacting the coral communities in the

Rosario Islands.

As shown above, suspended sediment loads from the

Canal del Dique to Barbacoas Bay have impacted the

coral reef formations of El Rosario Islands (Fig. 8). Not

less important have been the impacts of heavy sediment

loads and freshwater discharges from the Canal del

Dique to Cartagena Bay, which have greatly contribut-

ed not only to the total disappearance of coral forma-

tions but also to a considerable reduction of seagrass

bed abundance in the bay and neighboring areas.

Fig. 8. Anomalies of sediment load (Qs) for two stations, Calamar (A) and Santa Helena (B), showing the interannual trend by using a 4-degree

polynomial fit. The average cover (%) of live and dead coral for the Rosario Islands is shown in C. This regional mean was obtained by averaging

the covertures of the five study sites around the Rosario Islands between 1983 and 2004 (Figs. 2 and 6).

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–4942

A retrospective survey of seagrass distribution and

abundance in the Cartagena Bay has shown that this

critical habitat has almost disappeared from the area in

the course of the last six decades. The study, based on a

sequential comparative analysis of series of panchro-

matic aerial photographs and satellite imagery taken

between 1935 and 2001, allowed a decadal reconstruc-

tion of the spatial distribution of seagrasses in the area.

From nearly 850 ha of seagrass existing in the bay in

the 1930s, only 76 ha remained in 2001, which is less

than 8% of the original cover. The loss rate of seagrass

within the bay exhibited an inverse exponential pattern,

whereas in the neighboring, more exposed areas the

tendency was linear (Fig. 9). The loss rate within the

bay was particularly high in the 1940s and 1950s (about

42 ha yr�1), so that by 1957, more than 60% of the

seagrasses existing in 1935 were already eradicated, the

great majority from the southeastern sector of the bay,

where the Canal del Dique empties into it.

Fig. 10 shows the timing of the major human

induced events that have caused environmental deg-

radation. Since the 1930s, the government of Colom-

bia has dredged and reopened the Canal del Dique

(Figs. 1 and 2). Because of increased sedimentation

in Cartagena Bay between the 1940s and 1960s, new

channels were constructed from El Dique to Barba-

coas Bay. This clearly suggests that the near-disap-

pearance of seagrass beds in the bay and surrounding

areas was probably encouraged by the reopening of

the Canal del Dique early in the 1930s. In addition,

there was a drastic decrease in seagrasses in the

1950s (Fig. 9), an event clearly associated with the

introduction of important amounts of turbid freshwa-

ter and sediments into the Cartagena and Barbacoas

bays (Fig. 10). The reduction of seagrass has been

accompanied evidently by changes in the structure of

the animal community, which has become apparent

through the disappearance of benthic suspension feed-

Fig. 9. Plot showing percentage of seagrass cover in the Cartagena Bay and surrounding areas for the 1945–2001 yr-period (modified from Dıaz and

Gomez, 2003).

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–49 43

ing invertebrates which dominated the animal com-

munity some decades earlier.

4.3. Further comparisons

Our data indicates that the Magdalena River contri-

butes approximately 10% of the total sediment load

discharged from the east coast of South America. The

Magdalena River has the highest sediment yield of the

large rivers along the Caribbean and Atlantic coasts of

South America. Its yield is almost three times greater

than the yield of the Amazon, 190 t km�2 yr�1, Ori-

noco, 150 t km�2 yr�1, Negro (Argentina), 140 t km�2

yr�1 (Milliman and Syvitski, 1992), and much greater

than the yield of the Parana, 30 t km�2 yr�1 (Milliman

and Syvitski, 1992; Goniadzki, 1999), Uruguay, 45 t

Fig. 10. Timing of the major human induced events that have caused env

seagrass beds of the Cartagena Bay and neighboring areas.

km�2 yr�1, and Sao Francisco, 10 t km�2 yr�1 (Milli-

man and Syvitski, 1992) (Table 4). The dissolved load

for the Magdalena, 30�106 t yr�1 (Table 3), is of the

same magnitude as the Orinoco (30.5�106 t yr�1;

Depetris and Paolini, 1991), ten times lower than that

of the Amazon (259�106 t yr�1; Meybeck, 1976), and

similar to that of the Parana River (38.3�106 t yr�1;

Depetris, 1976; Depetris and Paolini, 1991) (Table 4).

In Colombia, pristine fluvial systems like those

draining the Pacific basins have much less PO43� and

NO3� loads when compared to the Caribbean rivers.

The Magdalena and Atrato rivers are by far the Colom-

bian systems which contribute the highest P and N

fluxes to the sea, with total phosphate and nitrate fluxes

up to 186�103 t yr�1 and 47�103 t yr�1, respectively

(Table 3). There are many causes responsible for these

ironmental degradation in the coral reefs of the Rosario Islands and

Table 3

Nutrient fluxes of phosphate (PO43�) and nitrate (NO3

�) in non-pristine

fluvial systems of the Caribbean basins of Colombia

River Water

discharge

Total nitrate

(NO3�)

Total phosphate

(PO4�3)

(km3 yr�1) (�103 t yr�1) (�103 t yr�1)

Caribbean

Magdalena 228 186 47

Dique 9.4 12 3.0

Sinu 11.8 1.5 0.07

Leon (Uraba Gulf) 2.1 2.5 0.7

Atrato (Uraba Gulf) 81 58 2.4

Turbo (Uraba Gulf) 12 0.1 0.003

Nutrient values are based on averages calculated from monthly sam-

ples covering 3-yr period 1998–2000 (modified from Restrepo and

Kjerfve, 2004).

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–4944

high nutrient loads, including massive sewage collec-

tion in cities and towns for NH4+ and PO4

3�, mainly in

the Magdalena basin, and also the fertilization of ba-

nana plantations in the lower course of the Atrato River.

The Magdalena River fits well into the global river

chemistry classification developed by Gibbs (1970),

with Ca2+ and HCO3� dominating the ionic composi-

tion. Also, values of dissolved solutes are in the range

of the most common natural concentration (MCNC)

found in most rivers. This classification was proposed

by Meybeck and Helmer (1989) to replace the baverageworld riverQ, which is greatly influenced by a few

rivers of extreme concentrations. Thus, MCNC

is simply the median value of the distribution of con-

centrations found in pristine major rivers, weighted

by the river discharge. The ionic natural composition

of the Caribbean rivers of Colombia, with respect to

Ca2+NMg2+NNa+NK+ and HCO3�NSO4

� is similar to

the MCNC of other world rivers (cf. Meybeck, 1996).

Table 4

Drainage basin, water discharge, sediment and dissolved loads, calculated

Depetris, 1976; Meybeck, 1976; Milliman and Meade, 1983; Depetris and P

and Kjerfve, 2000a,b, 2004)

River Basin area Water discharge Sedim

(�106 km2) (km3 yr�1) (�106

R. Amazon (Brazil) 6.15 6300 1200

R. Orinoco (Venezuela) 0.99 1100 150

R. Parana (Argentina) 2.60 470 79

R. Magdalena (Colombia) 0.25 228 144

R. Atrato (Colombia) .035 81 11

R. Uruguay (Uruguay) 0.24 253 11

R. Negro (Argentina) 0.10 30 13

R. S. Fran (Brazil) 0.64 97 6

R. San Juan (Colombia) 0.014 82 16

R. Patıa (Colombia) 0.014 10 14

R. Chira (Peru) 0.020 5 20

(. . .)=no available data.

5. Discussion

In the Magdalena basin tributaries, many natural

factors may be responsible for the high sediment

loads. Most of the tributary basins are small (220 km2

to 1400 km2), with narrow alluvial plain surrounded by

steep slopes, frequently steeper than 358, and with

limited deposition/storage within the drainage basin.

These catchments are also characterized by high rates

of precipitation (2000 mm yr�1) with strong patchy

storms."

Besides the above natural factors controlling sedi-

ment transport, the large-scale changes in land use

practices and resource exploitation in the Andes section

are particularly significant for the Magdalena basin and

have altered the sediment flux. Ongoing trends in the

drainage basin include (1) escalating population densi-

ties along the basins and at the river mouths. Eighty

percent of the population of Colombia lives in the

Magdalena watershed, corresponding to a density of

120 inhabitants/km2 (Restrepo and Kjerfve, 2004),

which is very high when compared to 0.24 inhabi-

tants/km2 in the Amazon basin as a whole (Serruya

and Pollingher, 1984; Depetris and Paolini, 1991); (2)

accelerating upland erosion rates due to increasing

deforestation and mining and poor agricultural prac-

tices. A recent examination of spatial and temporal

variability of sediment discharges in the Magdalena

drainage basin indicates that a large portion of the

drainage basin (68%) shows an increasing trend in

sediment load. The extent of erosion within the catch-

ment has increased over the last 10–20 yrs (Restrepo et

al., in press). Also, the percentage of forest cover in the

basin was estimated to have declined from 46% in 1970

to 27% in 1990, with an annual deforestation rate of

yields, and receiving basin for some rivers of South America (from

aolini, 1991; Milliman and Syvitski, 1992; Goniadzki, 1999; Restrepo

ent load Sediment yield Dissolved load TDS Receiving

t yr�1) (t km�2 yr�1) (�106 t yr�1) basin

190 290 N. Atlantic

150 30 N. Atlantic

30 38 S. Atlantic

560 30 Caribbean

315 1.0 Caribbean

45 6 S. Atlantic

140 . . . S. Atlantic

10 . . . S. Atlantic

1150 . . . N. Pacific

972 0.8 N. Pacific

1000 . . . S. Pacific

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–49 45

1.9%, or 234�103 ha yr�1. A recent estimate indicates

that between 1990 and 1996, total forest cover declined

by 15%, or 3.8�103 ha, an annual average loss of

2.4%. This deforestation rate in the Magdalena River

basin is considered to be among the highest in the world

(Restrepo and Syvitski, in press); and (3) as a result of

poor agricultural practices and deforestation, river-in-

duced impacts have produced distortion of natural

hydrographs and altered material transport.

To confirm if trends of sediment load are due to

changes in flow or some human-induced factors, the

double mass plot of cumulative suspended sediment

load versus cumulative annual water discharge for the

Magdalena River shows the trend of sediment flux

relative to that of water discharge (Fig. 11). The anal-

ysis suggests that the increase in sediment flux for the

Magdalena commenced in the mid-1980s and its pro-

gressive steepening further indicates that the impact of

land use change and intensification is increasing.

Recent studies (e.g. Hughes et al., 2003; Pandolfi et

al., 2003; Gardner et al., 2003) claim that coral reefs are

highly degraded throughout the world, and that there

are no pristine reefs left anywhere. Some countries have

seen 50% of their coral reefs destroyed by human

activities in the past 15 yrs. The coral reefs of the

Caribbean Sea and portions of Southeast Asia have

suffered the greatest rates of degradation and are

expected to continue to be threatened (Gardner et al.,

2003). In addition, field studies have provided large

information showing that sedimentation, nutrient en-

richment and turbidity can degrade coral reefs at local

Fig. 11. Double mass plot of cumulative suspended sediment load versus cu

This figure shows the trend of the sediment flux relative to that of water di

scales (Fabricius, 2005). Many assessments on the

impacts of terrestrial runoff on the ecology of coral

reefs have been documented in many places, including

Japan (Shimoda et al., 1998), Philippines (Hodgson and

Walton Smith, 1993), Indonesia (Edinger et al., 1998),

Great Barrier Reef Fabricius and De’ath (2004), and

Costa Rica (Cortes and Risk, 1985).

Ecosystem change is a complex process subject to

both natural and human induced factors. The cumula-

tive effects of anthropogenic disturbances superim-

posed on natural disturbances make recovery less

likely and, in some cases, result in stable states domi-

nated by algae (Figs. 6 and 7). According to Fabricius

(2005), as nutrients increase, coral reef communities

change from dominance of nutrient-recycling symbiotic

organisms such as corals to increasing proportions of

macroalgae (on eastern continental margins naturally

exposed to river runoff). The prevalence of macroalgae

adds strong evidence to the conclusion that terrestrial

runoff can limit or increase macroalgal biomass, and

that they can have a negative effects on reef develop-

ment (e.g., Birkeland, 1987). Thus, dead corals are

colonized by filamentous or fleshy algae after the oc-

currence of natural and/or human-induced disturbances.

The in situ character of the dead coral framework in the

Rosario Islands suggests that mortality cannot be linked

only to natural disturbances (Figs. 6 and 8). High

sediment and freshwater inputs into the Barbacoas

and Cartagena bays (Figs. 2 and 8) create additional

stress (both at ongoing background levels and, occa-

sionally, at dramatic levels), which may periodically

mulative annual water discharge for the Magdalena River at Calamar.

scharge.

J.D. Restrepo et al. / Global and Planetary Change 50 (2006) 33–4946

push local environmental parameters beyond the thresh-

olds for coral survival.

An important factor that has been identified to de-

termine the risk of degradation is the level of exposure

(concentration and duration) to terrestrial flux of a reef

system. This exposure is spatially controlled by the

downstream distance between a reef and the major

sources of discharge. Also, the mean annual load

from the source and dilution processes have strong

effects on coral degradation (West and Van Woesik,

2001; Fabricius, 2005). The muddy plumes of the

Magdalena River and the Canal del Dique, which

have largely affected water quality in the Rosario

Islands, have been constant and very prolonged (several

decades) and therefore their impact could be one of the

most significant (Fig. 8).

The best way to measure the influence of terrestrial

runoff and sediment load on coral reefs and seagrasses

is to analyze data of suspended sediments in waters

overlying these ecosystems. Other methods include the

analysis of accumulated sediments in sediment traps

deployed in specific places. Research focusing on the

environmental conditions in Barbacoas Bay and Baru

Island (Figs. 1 and 2) (Alvarado, 2001), indicates that

levels of suspended sediments during high river dis-

charge periods range from ~70 mg l�1 to b370 mg l�1.

These levels are much higher than the normal value of

~5–10 mg l�1, which has been quoted for fringing reefs

(Larcombe et al., 1995; Gilmour, 1999). In addition,

many fringing reefs are frequently exposed to high

levels of re-suspended sediments resulting from tidal

and wave movements. Under these natural conditions,

re-suspension of sediments may frequently exceed 20

mg l�1 and reach levels N200 mg l�1 during times high

energy swells (Kleypas, 1996). Suspended sediment

concentrations resulting from terrestrial fluvial dis-

charge have been reported to fall between 10 and

N200 mg l�1 (Miller and Cruise, 1995; Kleypas,

1996; Gilmour, 1999). The quoted environmental sur-

vey in the Rosario Islands (Alvarado, 2001) suggests

that coral reef formations in the area are exposed to

increased levels of turbidity and sedimentation.

6. Conclusions

Although live coral cover remains relatively high in

some sites of Rosario such as Tesoro Island (Figs. 6 and

7), a no-take area of the natural park and located

relatively away from the impact of the Canal del

Dique (Fig. 2), the immediate future of the reefs, par-

ticularly at Grande Island, gives cause for concern.

Therefore, reef communities in the archipelago have

been under threat for many years and are showing

clear signs of stress and decline, including a consider-

able loss of living coral cover and overgrowth by algae

(Figs. 6 and 8). The disturbing consequences on trophic

links need close scrutiny but it can be postulated that

current trophic structure has been distorted. Clearly,

there has been a mix of multiple stressors (natural and

anthropogenic; local, regional and global; temporal and

chronic) affecting the coral reefs in the area. In order to

provide direct evidence of impacts of sedimentation,

national coastal surveys should measure and analyze

water-quality parameters in undisturbed and impacted

places of the Rosario Islands.

The Magdalena River deserves international atten-

tion. It is the largest fluvial system discharging directly

into the Caribbean Sea. Thus, the synthesis and prelim-

inary analysis presented in this article are just first steps

toward understanding the natural and anthropogenic

factors that have produced the observed patterns of

water discharge, sediment load, and biochemical fluxes

of the Magdalena River into the Caribbean Sea, and to

relating these processes to the impact on the natural

aspects of coastal ecosystems.

Our preliminary results allow us to formulate a

hypothesis that, with increasing human activities on

the Magdalena catchment (Fig. 11), the area of damage

has already grown much larger than the natural state

area and will continue to increase in both size and

intensity unless human influences are curtailed. Further

studies should develop models to enable quantification

of the effect of various scenarios for control of land-use

activities anywhere. These investigations should also

offer decision-makers and the public a science-based

tool to decide what activities should be allowed, and

how they should be controlled, on drainage basins and

at sea, in order to produce a desired state of health for

coral reefs and other coastal ecosystems.

Acknowledgements

This study was done with support from the Instituto

Colombiano para el Desarrollo de la Ciencia y Tecno-

logıa bFrancisco Jose de CaldasQ—COLCIENCIAS,

Grant COLCIENCIAS 121609-12105 (Proyecto Rıo

Magdalena), and Universidad EAFIT—Departamento

de Geologıa.

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