Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

Ecological Quality Index based on phytoplankton in

the lower Magdalena River basin, Northern Colombia

René A. Rojas-Luna1*; https://orcid.org/0000-0003-0990-7808

Diana Pardo-Castañeda1; https://orcid.org/0000-0003-3554-0907

1. Grupo de investigación en Agua–GIATA, Aseguramiento de la Calidad de Agua, Triple A S.A. E.S.P., Departamento del

Atlántico, Barranquilla, Colombia; reneandres132806@gmail.com (*Correspondence), dianapardo21@hotmail.com

Received 20-IX-2024. Corrected 04-II-2025. Accepted 10-V-2025.

ABSTRACT

Introduction: The Magdalena River in Colombia has a high anthropic intervention, and this high load of pol-

lutants that enter mainly in the middle and upper basin of its course are becoming increasingly noticeable in the

lower basin.

Objective: This study evaluated the environmental condition of a section of the Magdalena River in the lower

basin as it passes through the Department of Atlántico, using the Ecological Quality Index (EQI) based on

phytoplankton.

Methods: Six sampling events were carried out between 2021 and 2022, in order to cover contrasting moments

of the hydrological regime, at six monitoring stations on the Western bank of the river. Physicochemical and

phytoplankton variables were sampled to establish reference categories based on the EQI.

Results: Conductivity was determined as the variable that explained the greatest variation in the environmental

gradient, generating reference ranges to establish categories of poor, fair and good ecological quality. 77 phyto-

plankton morphospecies were recorded, of which five were sensitive to the physicochemical conditions of the

Magdalena River, with high optimum values and low tolerance limits: Pinnularia sp2, Diatoma sp3, Navicula sp1,

Closterium limneticum, C. rostratum.

Conclusions: In general, the Magdalena River in the Atlántico showed a fair ecological quality condition, which

deteriorated even more during the dry periods of the year. The EQI showed a relationship with the proportion of

diatom groups and the abundance of phytoplankton, obtaining better quality values when diatoms predominate

over the cyanobacteria group and when the abundance of the phytoplankton is relatively low.

Keywords: algae; bioindicators; ecosystem condition; freshwater; water quality.

RESUMEN

Índice de Calidad Ecológica con base en el fitoplancton en

la cuenca baja del río Magdalena, Norte de Colombia

Introducción: El río Magdalena en Colombia presenta una alta intervención antrópica, y esa carga de contami-

nantes que ingresan en la cuenca media y alta de su recorrido se hace cada vez más notoria en la cuenca baja.

Objetivo: Este estudio evaluó la condición ambiental de un tramo del río Magdalena en la cuenca baja en su

paso por el departamento del Atlántico, mediante el uso del Índice de Calidad Ecológica (EQI) con base en el

fitoplancton.

Métodos: Se realizaron seis eventos de muestreo entre 2021 y 2022, con el fin de abarcar momentos contrastantes

del régimen hidrológico, en seis estaciones de monitoreo sobre la ribera occidental del río. Se tomaron muestras

de variables fisicoquímicas y de fitoplancton, para establecer categorías de referencias basadas en el EQI.

https://doi.org/10.15517/rev.biol.trop..v73i1.61984

AQUATIC ECOLOGY

2Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

INTRODUCTION

The Magdalena basin is the econom-

ic center of development in Colombia with

approximately 80 % of the country’s population

settlement, with a generation of 80 % of the

national gross domestic product. Likewise, its

hydrographic region generates 70 % of hydro-

power production, 70 % of agricultural crops,

90 % of coffee and 50 % of freshwater fisher-

ies (Restrepo et al., 2020). The growth of the

Colombian population and its economic devel-

opment in the Magdalena River basin has gen-

erated multiple pressures on the fluvial network

and its natural environments, which is reflected

in the environmental degradation of the basin

(Jiménez-Segura & Lasso, 2020).

The main water bodies in the different

municipalities of the Department of Atlántico,

most of which are connected to or influenced

by the Magdalena River in its lower basin,

reveal a pronounced deterioration in water

quality because of research carried out in the

last two decades (Oyaga-Martínez, 2013). For

this reason, it is pertinent to monitor the envi-

ronmental and hydrobiological conditions of

the swampy bodies and the main channel of

the river. These wetlands provide ecosystem

services and contribute to the social, economic

and cultural development of the region, to

understand how anthropic activities affect the

conditions of rivers and aquatic biota, to assess

the impacts and contribute to the management

and conservation of these ecosystems (Gutiér-

rez-Fonseca & Ramírez, 2016).

One way to generate information on the

environmental status of aquatic ecosystems is

through the use of the Ecological Quality Index

(EQI) proposed by Haase and Nolte (2008) and

Chalar et al. (2011), which allows defining an

environmental gradient according to the physi-

cal and chemical particularities of each system

and, in turn, determine optimal and tolerance

values for each organism, without conditioning

the group and system to which they belong,

according to this gradient (Flórez-Córdoba,

2020; Hernández et al., 2020), positioning the

EQI as an integral and robust alternative to

biotic indices mostly used in the country, such

as the Biological Monitoring Working Party-

Colombia (BMWP-Col) (Roldán, 2003), which

has some weaknesses (Forero et al., 2014).

The design and implementation of moni-

toring tools are fundamental for the establish-

ment of management plans and the definition

of restoration objectives in South American

rivers (Castillejo et al., 2024). EQI uses infor-

mation from both environmental variables and

biological groups with roles in bioindication

functions to evaluate the ecological quality of

an ecosystem. Ecological quality defined by

its structure and function, and assessed by

physical, chemical, hydromorphological and

biological indicators (European Commission,

2000; Salinas-Camarillo et al., 2020). Indicator

organisms, due to their life history traits, pro-

vide information that physicochemical analyses

alone do not detect.

In the case of phytoplankton, these organ-

isms respond rapidly to environmental changes

Resultados: La conductividad fue determinada como la variable que explicó la mayor variación del gradiente

ambiental, generando rangos de referencia para establecer categorías de deficiente, regular y buena calidad

ecológica. Se registraron 77 morfoespecies de fitoplancton, de las cuales cinco fueron sensibles a las condiciones

fisicoquímicas del río Magdalena, con altos valores óptimos y bajos límites de tolerancia: Pinnularia sp2, Diatoma

sp3, Navicula sp1, Closterium limneticum, C. rostratum.

Conclusiones: En general, el río Magdalena en el Atlántico mostró una condición de calidad ecológica regular,

que se deteriora aún más en los períodos secos del año. El EQI mostró relación con la proporción de grupos de

diatomeas y la abundancia del fitoplancton, obteniendo mejores valores de calidad cuando las diatomeas predo-

minan por encima del grupo de las cianobacterias y cuando la abundancia del fitoplancton es relativamente baja.

Palabras clave: algas; bioindicadores; condición ecosistémica; agua dulce; calidad del agua.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

due to their short life cycle. These changes alter

the structure of their communities and have

repercussions on another group of organisms

in the system, especially because of their impor-

tant role in maintaining the food web (Winder

& Sommer, 2012) as primary producers. This

is related to changes in the proportion, com-

position and structure of community taxa due

to variations in water conditions (Castillejo

et al., 2024). Therefore, microalgae have been

widely used as bioindicators of water quality, as

they are an immediate reflection of changes in

aquatic ecosystems due to their high sensitivity

to variations (Bellinger & Sigee, 2010; Heinrich

et al., 2019; Hemraj et al., 2017).

Similarly, EQI has been applied in other

regions of Colombia using aquatic macroin-

vertebrates and periphytic algae as bioindicator

organisms (Forero et al., 2014; Hernández et al.,

2018, Hernández et al., 2020). This index would

allow responding to the need for a robust indi-

cator to evaluate the quality of the ecosystem

and provide an estimate of the quality of water

used for drinking water treatment at different

points in the department of Atlántico through

the combination of physical, chemical and

hydrobiological variables. For this reason, this

study is proposed on the left riverbank of the

Magdalena River in its lower basin, the main

source of water supply in the Department of

Atlántico, in order to evaluate its environmental

condition by means of the Ecological Quality

Index based on phytoplankton.

MATERIAL AND METHODS

Study area: The Department of Atlántico

is part of the last section of the Magdalena River

along its left riverbank in Northern Colombia; it

includes an area that goes from Canal del Dique

(in the South) to the mouth of the Magdalena

at Bocas de Ceniza (to the North), near Barran-

quilla. A warm thermal floor predominates in

the Atlántico, with temperatures that generally

vary between 24 °C to 28 °C but can exceed 34

°C at some times of the year (García-Alzate et

al., 2016; Instituto de Hidrología, Meteorología

y Estudios Ambientales [IDEAM], 2023).

Six monitoring stations were established in

the Magdalena River in different municipali-

ties of the department: Barranquilla-Las Flores

(S1), Barranquilla-Aqueduct (S2), Sabana-

grande (S3), Ponedera (S4), this taking into

account the location of the different water

catchments for the treatment plants operated

by the Sociedad de Acueducto, Alcantarillado

y Aseo de Barranquilla (Triple A S.A. E.S.P.),

Suan (S5) in the water catchment for supplying

this municipality and finally, Canal del Dique

(S6) in order to approach the greatest spatial

variation along the riverbank (Fig. 1).

The Magdalena River is characterized by

a bimodal hydrological regime of rains and

droughts, with moments of water level transi-

tion from one season to another (Rojas-Luna

& Pardo-Castañeda, 2024). Sampling was con-

ditioned to climatic periods: The moment of

rising waters occurs with the first rains that

increase the river level (M1: May 2021), high

waters when the river reaches its maximum

levels (M3: October 2021 and M6: June 2022),

the period of falling waters when the river level

begins to decrease due to the absence of pre-

cipitation (M2: August 2021 and M4: January

2022) and finally, low waters during the dry

season (M5: March 2022).

Sampling phase: Six bimonthly samplings

were conducted between the months of May

2021 to June 2022, in order to cover the differ-

ent moments of the hydrological regime using

the water level of the Magdalena River in Cala-

mar–Atlántico (2021-2022) taken from Cor-

poración Autónoma Regional del Río Grande

de la Magdalena (Cormagdalena) and IDEAM;

and include the subregions of the Magdalena

River and the Canal del Dique through the

stations, established as areas of environmental

interest under the criteria of Strategic Envi-

ronmental Systems, together with the Coastal

subregion (Avendaño-Maldonado et al., 2021;

Gobernación del Atlántico, 2012).

Variables such as pH with a WTW 3110

pH meter, dissolved oxygen with a WTW

3205 oximeter, conductivity with a WTW 3110

conductivity meter, water temperature, relative

4Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

humidity and environmental temperature with

a UNI-T thermohydrometer were measured in

situ. In addition, water samples were taken in

a two-liter bottle, which was cooled with ice

for subsequent analysis of alkalinity, acidity,

nitrites (NO2-), nitrates (NO3-), sulfates (SO42-

), total suspended solids (TSS), total hardness,

water turbidity and phosphates (PO43-); and a

sample in a 500 ml bottle that was preserved

with H2SO4 until maintaining pH less than

2.0 and refrigerated at a temperature less than

or equal to 6 °C for testing total nitrogen (N),

ammonium (NH4+), total phosphorus (P) and

chemical oxygen demand (COD) following the

standards described by American Public Health

Association (APHA, 2023), in the Quality Con-

trol laboratory of Triple A S.A. E.S.P.

Similarly, surface water (30 l) was filtered

using a 23 µm diameter phytoplankton capture

mesh. The sample was taken for S1, S3, S4 and

S5 directly on the river, for S2 it was taken in

the lagoon (natural pre-sediment of the Barran-

quilla aqueduct treatment system) and finally,

in S6 it was taken on an artificial arm of the

river (Canal del Dique). The volume of filtered

samples was reduced to 120 ml and stored in

amber glass bottles, fixed with Lugol’s solution,

labeled with the field specifications (place, date

and station) and transported to Triple A S.A.

E.S.P.’s Quality Control laboratory for identifi-

cation and counting.

Laboratory phase: Acidity, alkalinity,

ammonium, nitrites, total suspended solids,

sulfates, phosphates and total hardness were

analyzed using the methodology proposed by

APHA (2023). In addition, COD, nitrates, total

phosphorus and total nitrogen were measured

using Spectroquant kits from Merck.

Phytoplankton were observed using a

Sedgewick-Rafter chamber with a chamber

volume of 1 ml and observed under an inverted

Fig. 1. Location of the six monitoring stations in the lower Magdalena River basin, Department of Atlántico, Colombia.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

optical microscope at 200X magnification for

counting and 400X to 1 000X magnification

for taxonomic identification (Rojas-Luna &

Pardo-Castañeda, 2024). The sample inside the

chamber was completely analyzed and its stan-

dardization and representativeness was evalu-

ated by rarefaction curves by fields. Absolute

abundance of the species was used in the EQI

analysis. Identification was made using taxo-

nomic keys and research material in Bicudo

and Menezes (2006), van Vuuren et al. (2006),

Bellinger and Sigee (2010), Ministerio de Medio

Ambiente y Medio Rural y Marino (2011),

Moreno et al. (2012), Montoya-Moreno et al.

(2013), and Oliva-Martínez et al. (2014), down

to the lowest possible taxonomic level. For the

revision of taxonomic names and synonymies,

the online database AlgaeBase was consulted.

Data analysis: For the use of the index,

the methodology proposed by Chalar et al.

(2011), Forero et al. (2014) and Hernández et

al. (2018) was used as a basis, and the meth-

od was executed using R statistical software

(Flórez-Córdoba, 2020).

Initially, the matrices of physicochemi-

cal variables and abundance of organisms

were standardized based on the method of

Guisande-González et al. (2006). A Spearman

correlation was performed between physico-

chemical variables to observe redundancy and

variance inflation factors (VIF) to analyze mul-

ticollinearity (VIF > 20: Strong collinearity) to

reduce the dimensionality of the index with the

exclusion of variables that were explained by

others. A Detrended Correspondence Analysis

(DCA) was used to evaluate the length of the

environmental gradient and to obtain the statis-

tical basis for the application of the appropriate

ordination model; a Detrended Redundancy

Analysis (RDA) is used when the length of the

environmental gradient is less than 2.5 and a

Canonical Correspondence Analysis (CCA)

when it is greater or equal to 2.5, in order to

determine the environmental gradient from the

abundances of the species with respect to the

physicochemical variables of the environment.

The significance of the model was estimated

by means of the Monte Carlo permutation test.

As recorded in the DCA, the multivariate

analysis for this study was a CCA, considering

the length of the environmental gradient (2.63

standard deviation). Then, the values of the

first axis obtained from the ordination analysis

were rescaled between 1 and 10 from a linear

regression, being the lowest score scaled to

ten and the highest was given a value of one.

Optimal and tolerance scores were calculated

for each organism through a Weighted Aver-

ages Analysis (WA), considering the abundance

of the species and the rescaled values; which

allowed identifying sensitive taxa of environ-

mental quality by their sensitivity, with high

optimal and low tolerance values.

Subsequently, the EQI values were calcu-

lated for the monitoring stations in each of the

samplings following the formula described by

Haase and Nolte (2008). To give robustness

to the calculation of the index, a Spearman

correlation was made between the EQI values

and the scores of the first axis of the multivari-

ate analysis. Another correlation was used to

determine the physicochemical indicator vari-

able that models the composition of the organ-

isms, taking into account the EQI values and

the physicochemical parameters with a ratio

of R ≥ 0.5 or R ≤ -0.5 (Ministerio de Ambiente

y Desarrollo Sostenible, 2018). A Clustering

Analysis using Ward’s method and Euclidean

distance was used in order to establish three

groups that represented three quality categories

on a scale of poor, fair and good. The defini-

tion of the ranges for the indicator variable

and the EQI, was made using the minimum

and maximum values of each group, or the

medians if the first ones were superimposed

(Forero et al., 2014).

Finally, a Spearman correlation was used

to determine the degree of association between

EQI values and some parameters involved in

the taxonomic and ecological composition of

phytoplankton such as river level, total abun-

dance and proportion of the main phytoplank-

ton groups (diatoms, green algae, cyanobacteria

and euglenids).

6Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

RESULTS

Phytoplankton community composition:

43 species and 34 morphospecies distributed in

five phylum, eight classes and 36 families were

recorded; however, only 56 taxa were includ-

ed in the analysis considering taxa that were

present in more than one sampling station.

The highest species richness was represented

by the phylum of diatoms (Bacillariophyta:

44.2 %), followed by green algae (Chlorophy-

ta: 20.8 %), blue-green algae (Cyanobacte-

ria: 14.3 %), euglenids (Euglenozoa: 11.7 %)

and finally, charophytes (Charophyta: 9.1 %).

However, it is the Cyanobacteria (47 %) that

contributes most to the relative abundance of

the phytoplankton community, followed by

the Bacillariophyta (38.3 %). Dominant species

include Oscillatoria tenuis Agardh ex Gomont

1892 (31.4 %), O. princeps Vaucher ex Gomont

1892 (10.5 %), Melosira varians Agardh 1827

(5.7 %), Ankistrodesmus sp. (5.3 %), Aulaco-

seira granulata (Ehrenberg) Simonsen 1979

(4.2 %), and finally, Raphidiopsis raciborskii

(Wołoszyńska) Aguilera et al. 2018 (3.9 %) and

Pseudanabaena sp. (3.9 %).

Physicochemical variables: Of the 15

environmental variables taken in the study, 10

were included in the model. Spearman cor-

relations showed that alkalinity (R = 0.65) and

sulfates (R = 0.70) had a high association with

conductivity; likewise, pH (R = 0.66), water

temperature (R = 0.67) and TSS (R = 0.85) had

a high correlation with turbidity. These vari-

ables whose behavior was explained by others

were excluded from the analysis. The use of

the variables selected from the correlations was

supported by the VIFs less than 20, which indi-

cates that there is no collinearity between them.

Environmental gradient: The first two

axes of the CCA obtained the highest eigenval-

ue (0.25 and 0.21, respectively) and explained

36 % of the total variation in the data. The

Monte Carlo test showed that the model is

highly significant (p < 0.001). The first axis had

a high positive correlation with nitrogenous

nutrients such as total nitrogen (0.86) and

nitrates (0.75) and a slight correlation with

conductivity (0.52); meanwhile, the second axis

was negatively associated with dissolved oxygen

(-0.74) and slightly with other nutrients such as

ammonium (0.50) and total phosphorus (0.50).

Thus, the first axis was mainly associated

with nutrients, such as total nitrogen, which

was positively and significantly correlated (R

≥ 0.6 and p < 0.05) with dissolved oxygen and

nitrates, which in turn were positively associ-

ated with turbidity, sulfates and conductivity.

This axis showed a gradient of high nutrient

values in relation to the hydrological regime,

mainly to the period of falling water. Similarly,

for the second axis, dissolved oxygen was also

significantly and positively correlated with pH

and sulfates, while total phosphorus and phos-

phates were not correlated with other variables.

This axis was influenced by phosphorus nutri-

ent contributions from activities and discharges

along the riverbank, which constantly impact

the stations.

The sample scores obtained by the stations

in each monitoring in the CCA were the basis

with statistical and environmental support for

the re-scaling necessary in the calculation of

the index. The rescaled values between 1 (most

impacted stations) and 10 (least impacted sta-

tions) were obtained from the equation y =

-2.0371x + 6.7865, where the value of x was

replaced by the sample score of each station

per sampling.

Optimal and tolerance scores of the

organisms: The WA analysis showed a model

with adequate statistical performance based

on a high coefficient of determination (R2 =

0.96), low root mean square error of prediction

(RMSEP = 0.42) and low mean (Avg = 0.00)

and maximum (Max = 0.54) biases.

Of the 56 taxa considered for the analysis,

38 of these recorded optimal values higher than

6.1 (a value associated with good ecological

quality). Of these 38 taxa only five obtained

low tolerance values (Toli ≤ 1.0), which is

reflected in a high sensitivity to variations in

the physicochemical conditions of the water

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

and confers them characteristics as sensitive

organisms in the ecosystem (Fig. 2). Three of

the five taxa belong to the group of diatoms or

Bacillariophyta (Navicula sp1, Diatoma sp3 and

Pinnularia sp2), and the other two belong to the

same genus Closterium within the Charophyta

(C. limneticum Lemmermann 1899 and C. ros-

tratum Ehrenberg ex Ralfs 1848).

Ecological Quality Index: The EQI values

in the West bank of the Magdalena River using

phytoplankton as a biological model were in

a range between 5.5 and 7.5 (Table 1). These

scores have a high and significant correlation

with the sample scores of the first axis of the

CCA (R = -0.78, p < 0.01). Indicating that

the index explains most of the environmental

variation through the relationship of phyto-

plankton with the physicochemical variables

of the water. Likewise, Spearman’s correlation

showed that EQI has an inverse and significant

relationship with conductivity, representing an

indicator variable of the ecological quality of

the ecosystem (R = -0.54, p < 0.01). Conduc-

tivity is a conservative measure, considered a

key parameter in the limnology of neotropical

Fig. 2. Optimum (Op) and tolerance (Tol) values of phytoplankton species associated with the environmental gradient of the

Magdalena River. RL-Op: Reference limit of optimum values, RL-Tol: Reference limit of tolerance values taken from Forero

et al. (2014).

Table 1

Descriptive statistics of EQI scores and conductivity values, with their respective threshold values for ecological quality

categories.

Statistics Group 1 Group 2 Group 3

Conductivity (mg/l) EQI Conductivity (mg/l) EQI Conductivity (mg/l) EQI

Average 173 6 156 7 125 7

Minimum 162 6 151 6 117 7

Maximum 197 7 160 8 139 7

Median 169 6 156 7 124 7

Standard deviation 11 0.3 3 0.5 6 0.2

Index Group 1 Group 2 Group 3

Ecological quality Poor Fair Good

Conductivity (mg/L) ≥ 161 140-160 ≤ 139

EQI ≤ 5.9 6-7 ≥ 7.1

8Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

water bodies, which provides information on

the metabolic dynamics of aquatic ecosystems;

it is also related to the nutrients that contrib-

ute ions to the system, to primary productiv-

ity (phytoplankton), to pollution contributions

and to the hydrological regime.

The cluster analysis showed three groups,

where the first group represented stations

and monitoring sites with the highest average

conductivity values associated with high con-

centrations of nitrogenous nutrients (nitrates,

total nitrogen and ammonium), sulfates, high

COD values indicating organic pollutants and

high turbidity in the water. The third group

recorded the opposite behavior to group one,

with the lowest average values of conductivity,

nitrogen nutrients, sulfates, COD, turbidity, as

well as TSS in water and alkalinity. Finally, the

second group obtained the sites and periods

with average values intermediate between the

two previous groups (Fig. 3). The results show

that the behavior of the EQI values follow an

environmental gradient from waters with a

tendency to a high concentration of ions and

nutrients to waters with lower ionic dynamics

and less turbidity.

On average, the six stations monitored

in the Magdalena River on its bank over the

Department of Atlántico are in a regular eco-

logical condition, being the most impacted

the Barranquilla-Aqueduct station with the

lowest EQI value. This reflects that the munici-

palities located in the lower part of the basin

are exposed to frequent environmental impacts

and anthropic pressure due to the development

of activities near the bank, such as industrial

activity, wastewater discharge, agriculture, ani-

mal husbandry, deforestation and connection

with highly eutrophicated swampy bodies, even

though the river has resilient characteristics in

its dynamics.

The EQI showed variation with respect

to the hydrological regime, registering values

in poor ecological condition in M5 associated

with low waters with little rainfall where the

river level decreases and the concentration

of anthropic and natural impacts increases,

which is reflected in the predominance of

Fig. 3. Grouping of stations and monitoring considering EQI scores and conductivity values in the Magdalena River. S1:

Barranquilla-Las Flores, S2: Barranquilla-Aqueduct, S3: Sabanagrande, S4: Ponedera, S5: Suan, S6: Canal del Dique. M1:

Monitoring 1, M2: Monitoring 2, M3: Monitoring 3, M4: Monitoring 4, M5: Monitoring 5, M6: Monitoring 6.

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

cyanobacteria and increase in the general abun-

dance of phytoplankton. By contrast, the EQI

showed a good ecological condition in M1

corresponding to the period of the first rains

of the year, which provide a process of mixing

of the ionic conditions of the river, increasing

its flow and turbidity, which impacts the life

cycle of microalgae. However, once the rainy

period is prolonged, the ecological condition of

the ecosystem tends to decrease, mainly asso-

ciated with discharges of solid pollutants and

wastewater through the streams and intercon-

nections with urban centers and swampy bod-

ies characteristic of the Atlántico department’s

riverbanks (Fig. 4).

The EQI values had a high correlation

with the relative abundance of diatoms (R =

0.70) so that when this group predominates

in the phytoplankton community the index

values tend to be higher; the opposite case is

registered with cyanobacteria (R = -0.33) where

their predominance causes a decrease in EQI

values in relation to their ability to compete for

resources preventing other groups of algae to

proliferate, generating ecological imbalances

that are reflected in water quality. This reflects

the importance of the composition and struc-

ture of the biological group under study, so that

once the EQI has been calculated, the indicator

variable plays a key role in subsequent routine

monitoring. Therefore, in this case, conductiv-

ity may register high values but if the phyto-

plankton diversity in terms of balance between

the main groups (diatoms and cyanobacteria)

is not altered, it may be related to a punc-

tual change of the physicochemical variable

(erosion or sedimentation on the riverbanks,

stream discharges, etc.) and not a prolonged

alteration of the water quality. In addition,

another important factor in relation to the EQI

Fig. 4. Ecological quality of the monitoring stations in relation to the hydrological regime of the Magdalena River, Atlántico,

Colombia. S1: Barranquilla-Las Flores, S2: Barranquilla-Aqueduct, S3: Sabanagrande, S4: Ponedera, S5: Suan, S6: Canal del

Dique.

10 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

is the absolute abundance recorded for the phy-

toplankton community (R = -0.57); therefore,

there may be a dominance of cyanobacteria

with respect to the other groups of algae, but

if the total abundance recorded is low, it is not

reflected in a deterioration of the ecological

quality of the ecosystem.

DISCUSSION

Conductivity was the main variable that

determined the variation of the environmen-

tal gradient in the evaluated section of the

Magdalena River. This variable was positively

and significantly correlated with sulfates and

alkalinity: The former are considered the most

important anions in water after carbonates

because algae can take advantage of their avail-

ability in the environment through their pro-

toplasm, which is necessary to produce key

proteins in the life cycle of phytoplankton.

Furthermore, alkalinity is a measure related to

carbonate and bicarbonate ions that influence

the functioning and metabolism of water bod-

ies through photosynthesis and respiration pro-

cesses (Roldán & Ramírez, 2022). In addition

to knowing biological and geochemical fac-

tors of aquatic ecosystems, conductivity allows

detecting sources of anthropic contamination

through the drastic increase in concentrations

related to activities carried out on the riverbank

such as agriculture, industrial and domestic

pollution, or by natural components such as the

hydrological regime or the La Niña or El Niño

phenomena (Restrepo et al., 2020; Roldán &

Ramírez, 2022).

Similarly, the first and second axis of the

CCA showed a correspondence of nitrogenous

and phosphorous nutrients with the environ-

mental gradient, which supports the dynamics

of eutrophication related to periods of rainfall

and drought, as well as anthropic discharges.

This presence of nitrogen compounds such

as nitrate in large quantities can be a risk fac-

tor for the fauna associated with the aquatic

ecosystem, as well as for the use of water for

human consumption (IDEAM, 2023). Likewise,

the increase of phosphorus compounds can

accelerate eutrophication, causing prolifera-

tions of generally harmful algae that generate

a decrease in the concentration of dissolved

oxygen, causing mortality of aquatic fauna or

even terrestrial animals distributed along the

riverbanks (Gutiérrez-Moreno & De la Parra-

Guerra, 2020).

Considering the optimum and tolerance

values, it was possible to identify sensitive

organisms that model the environmental gradi-

ent in the waters of the lower Magdalena River

basin. Three species of diatoms and two charo-

phytes were identified: Pinnularia sp2, a typical

taxon of oligotrophic and low ion concentra-

tion freshwater (da Silva et al., 2016), together

with other Bacillariophyta such as Navicula

sp1 and Diatoma sp3, are robust indicators of

good environmental status. Their sensitivity

to changes in nutrient concentration and con-

ductivity, in addition to their fundamental

role in the food web, reflects the health of the

ecosystem (Srivastava et al., 2016). However,

the ecological particularities of diatoms must

be considered, because these genera may have

species sensitive to eutrophication, as well as

species tolerant to eutrophic conditions (Lobo

et al., 2015; Roa & Pinilla-Agudelo, 2017). Like-

wise, Closterium is a group of algae sensitive

to water quality, including C. limneticum and

C. rostratum, useful in indicating the trophic

state of aquatic ecosystems; they are considered

bioindicators of oligotrophic and oligotrophic-

mesotrophic systems, generally with low abun-

dance in water bodies with eutrophic and/

or toxic characteristics (Gutiérrez et al., 2017;

Wang et al., 2018).

The EQI showed a deterioration in the

environmental quality of the stations moni-

tored in Atlántico, particularly highlighting S2,

mainly related to an increase in sedimentation

near the municipality of Soledad, the discharge

of water with physicochemical characteristics

different from those of the river such as those of

the El Salao’ and El Platanal streams, which are

misused for wastewater discharge in areas with

historical social problems such as land invasion

(Corporación Autónoma Regional del Atlán-

tico , 2023). In addition to these problems, there

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

is the poor disposal of solid waste in the depart-

ment of Atlántico, mainly related to the lack

of environmental awareness among citizens.

These problems also increased due to the high

rainfall caused by the La Niña phenomenon

in 2021 and 2022, which caused the river to

flood the swampy bodies with low-flow water

that drained into the river and deteriorated the

ecological quality of the riverbank.

Good ecological quality was recorded at

the time of rising water when the concentra-

tion of total suspended solids, water turbid-

ity and flow rate increased. In addition, the

concentration of nitrogenous and phosphorus

nutrients, essential for algal growth, decreases

during the same period (Rojas-Luna & Pardo-

Castañeda, 2024). This decrease in nutrients

prevents organisms such as cyanobacteria from

establishing on the aquatic surface (Chorus &

Zessner, 2021). Because the Magdalena River is

characterized by its high sediment production

in South America, with oligotrophic waters in

the rainy season, which limits the proliferation

of cyanobacteria or euglenids, and favors the

predominance of algal groups such as diatoms,

which have morphological structures that allow

them to establish in these conditions (Hein-

rich et al., 2019; Martínez & Donato, 2003;

Restrepo et al., 2020).

The EQI is a tool that allows diagnosing

the environmental status of an aquatic ecosys-

tem through reference categories established

from a robust statistical analysis (Hernández

et al., 2018, Hernández et al., 2020). However,

although EQI considers the abundance of taxa

there are other life history factors of species that

need to be taken into account. As for example,

the proportion of phytoplankton groups was

important in explaining the variation of EQI

values, generally relating values associated with

good ecological quality with a predominance of

the diatom group over cyanophytes and eugle-

nophytes; in contrast, poor quality values are

represented by the dominance of cyanophytes.

According to the classification proposed by

Kruk et al. (2010), diatoms belong to the group

of non-flagellate organisms with silica exo-

skeletons that rarely have substantial negative

effects on water quality. Most cyanophytes

belong to the group of algae with long filaments

and aerotopes, a structure that, together with a

high surface-volume ratio, allows them to have

a low sinking rate, which facilitates their access

to resources. They also have a high tolerance

to conditions of low nutrient concentration

and sunlight incidence, which leads them to

remain even when conditions are not optimal.

The dominance of this group of microalgae can

generate large impacts on water quality related

to high abundance and the potential to form

toxic blooms.

Currently, the Ecological Quality Index

is recommended as a monitoring strategy for

aquatic ecosystems in the Water Resource Man-

agement Plan of Minambiente. It is a quantita-

tive tool that provides physicochemical and

biological information on water bodies and is

a robust alternative to the indexes normally

used in bioindication such as the BMWP-Col

(Roldán, 2003). However, it is necessary to

implement new factors in the study of the EQI:

the hydrological regime influenced by the La

Niña and El Niño phenomena, the life history

traits of the taxa under study, the balance in the

relative abundance of these taxa in the com-

munity, the historical changes in the physico-

chemical condition of the rivers, the strong and

growing anthropic and socio-natural threats

to which the water resources are exposed,

among others, are variables that should be

included in the analysis of the ecological status

of the aquatic ecosystems. The latter taking

into account that the reference categories are

dynamic, and the EQI should be flexible to

increase or decrease the number of categories

as needed.

Ethical statement: The authors declare

that they all agree with this publication and

made significant contributions; that there is no

conflict of interest of any kind; and that we fol-

lowed all pertinent ethical and legal procedures

and requirements. All financial sources are fully

and clearly stated in the acknowledgments sec-

tion. A signed document has been filed in the

journal archives.

12 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

ACKNOWLEDGMENTS

To the submanagement of drinking water

of Sociedad de Acueducto, Alcantarillado y

Aseo de Barranquilla Triple A S.A. E.S.P. for the

space and resources to carry out the research.

REFERENCES

American Public Health Association. (2023). Standard

Methods for the examination of water and wastewater.

APHA Press.

Avendaño-Maldonado, L. J., Camargo-Alarcón, M. A.,

Borja-Acuña, R., & Chacó-Pacheco, J. J. (2021).

Mammals of the department of Atlántico, Colom-

bia. Biota Colombiana, 22(2), 108–126. https://doi.

org/10.21068/c2021.v22n02a06

Bellinger, E. G., & Sigee, D. C. (2010). Freshwater algae:

Identification and use as bioindicators. Wiley. https://

doi.org/10.1002/9780470689554

Bicudo, C. E., & Menezes, M. (2006). Gêneros de algas de

águas continentais do Brasil: chave para identificação

e descrições. RiMA.

Castillejo, P., Ortiz, S., Jijón, G., Lobo, E. A., Heinrich, C.,

Ballesteros, I., & Rios-Touma, B. (2024). Response of

macroinvertebrate and epilithic diatom communities

to pollution gradients in Ecuadorian Andean Rivers.

Hydrobiologia, 851, 431–446. https://doi.org/10.1007/

s10750-023-05276-6

Chalar, G., Arocena, R., Pacheco, J. P., & Fabián, D.

(2011). Trophic assessment of streams in Uruguay:

A trophic state index for benthic invertebrates (TSI-

BI). Ecological Indicators, 11(2), 362–369. https://doi.

org/10.1016/j.ecolind.2010.06.004

Chorus, I., & Zessner, M. (2021). Assessing and con-

trolling the risk of cyanobacterial blooms. In I.

Chorus, & M. Welker (Eds.), Toxic cyanobacteria

in water (pp. 433–503). CRC Press. https://doi.

org/10.1201/9781003081449-7

Corporación Autónoma Regional del Atlántico. (2023).

Plan de Ordenamiento y Manejo de la cuenca del

complejo de humedales de la vertiente occidental del

Río Magdalena en el Departamento del Atlántico.

CRA Press.

da Silva, W. J., Ruwer, D., Noguera, I., & Dunck, B. (2016).

The gennus Pinnularia (Bacillariophyta, Pinnula-

riaceae) from Lago dos Tigres, Britania, Goiás, Bra-

zil. Biota Neotropica, 16(1), e20150028. https://doi.

org/10.1590/1676-0611-BN-2015-0028

European Commission. (2000). Directive 2000/60/EC of

the European Parliament and of the Council esta-

blishing a framework for community action in the

field of water policy. Official Journal of the European

Communities, 327, 1–73.

Flórez-Córdoba, E. (2020). Implementation of a free use

model for the application and support in the interpre-

tation of the ecological quality index [Unpublished

Master’s thesis]. Universidad de Antioquia, Colombia.

Forero, L. C., Longo, M., Ramírez R., J. J., & Chalar, G.

(2014). Ecological quality index based on aqua-

tic macroinvertebrates for the Negro River basin

(ICERN-MAE), Colombia. Revista de Biología Tro-

pical, 62(Supplement 2), S233–S247. https://doi.

org/10.15517/rbt.v62i0.15790

García-Alzate, C., Gutiérrez, L. C., & De la Parra, A. (2016).

El Guájaro reservoir: Environmental diagnosis and

rehabilitation strategies. In M. Alvarado (Ed.), South

of the Atlantico (pp. 155–157). Fundación Promigas.

Gobernación del Atlántico. (2012). Plan departamental de

gestión del riesgo del Atlántico. Unidad Nacional para

la Gestión del Riesgo (UNGRD), Programa de Nacio-

nes Unidas para el Desarrollo Colombia (PNUD).

Guisande-González, C., Barreiro-Felpeto, A., Maneiro-

Estraviz, I., Riveiro-Alarcón, I., Vergara-Castaño, A.,

& Vaamonde-Liste, A. (2006). Data treatment. Edicio-

nes Díaz de Santos.

Gutiérrez-Fonseca, P., & Ramírez, A. (2016). Evaluación

de la calidad ecológica de los ríos en Puerto Rico:

Principales amenazas y herramientas de evaluación.

Hidrobiológica, 26(3), 433–441.

Gutiérrez, G., Dávalos, A., & Herrera, L. (2017). Characteri-

zation of green algae and cyanobacteria of the wetland

Ecoparque Lago de Las Garzas [Unpublished Degree

thesis]. Universidad Icesi Colombia.

Gutiérrez-Moreno, L. C., & De la Parra-Guerra, A. C.

(2020). Contamination in the Magdalena River basin

(Colombia) and its relationship with fish. In L. Jimé-

nez-Segura, & C. A. Lasso (Eds.), XIX Fishes of the

Magdalena River basin, Colombia: Diversity, conserva-

tion and sustainable use (pp. 239–263). Serie Editorial

Recursos Hidrobiológicos y Pesqueros Continentales

de Colombia, Instituto de Investigación de Recursos

Biológicos Alexander von Humboldt.

Haase, R., & Nolte, U. (2008). The invertebrate species

index (ISI) for streams in southeast Queensland, Aus-

tralia. Ecological Indicators, 8(5), 599–613. https://doi.

org/10.1016/j.ecolind.2007.08.005

Heinrich, C. G., Palacios-Peñaranda, M. L., Peña-Salaman-

ca, E., Schuch, M., & Lobo, E. A. (2019). Epilithic

diatom flora in Cali River hydrographical basin,

Colombia. Rodriguésia, 70, e02062017. https://doi.

org/10.1590/2175-7860201970041

Hemraj, D. A., Hossain, M. A., Ye, Q., Qin, J. G., & Leterme,

S. C. (2017). Plankton bioindicators of environmental

conditions in coastal lagoons. Estuarine, Coastal and

Revista de Biología Tropical, ISSN: 2215-2075, Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

identification and quantification of cyanobacteria and

eukaryotic microalgae. Reduca, 5, 110–125.

Oliva-Martínez, M. G., Godínez-Ortega, J. L., & Zuñiga-

Ramos, C. A. (2014). Biodiversity of phytoplank-

ton of inland waters in Mexico. Revista Mexicana

de Biodiversidad, 85, 54–61. https://doi.org/10.7550/

rmb.32706

Oyaga-Martínez, R. F. (2013). Environmental realities

of the water bodies of the department of Atlán-

tico, Colombia. Ingeniare, 14, 43–62. https://doi.

org/10.18041/1909-2458/ingeniare.14.611

Restrepo, J. D., Cárdenas-Rozo, A., Paniagua-Arroyave, J. F.,

& Jiménez-Segura, L. (2020). Physical aspects of the

Magdalena River basin, Colombia: Geology, hydro-

logy, sediments, connectivity, aquatic ecosystems and

implications for biota. In L. Jiménez-Segura, & C. A.

Lasso (Eds.), XIX. Fishes of the Magdalena River basin,

Colombia: Diversity, conservation and sustainable use

(pp. 41–83). Serie Editorial Recursos Hidrobiológicos

y Pesqueros Continentales de Colombia, Instituto de

Investigación de Recursos Biológicos Alexander von

Humboldt.

Roa, D. C., & Pinilla-Agudelo, G. A. (2017). Índice de dia-

tomeas perifíticas para evaluar la calidad del agua en

los humedales distritales bogotanos (IDHB). In G. G.

Monroy, & G. A. Pinilla (Eds.), Estudios ecológicos en

humedales de Bogotá. Aplicaciones para su evaluación,

seguimiento y manejo (pp. 270–284). Universidad

Nacional de Colombia.

Rojas-Luna, R. A., & Pardo-Castañeda, D. (2024). Diver-

sidad del fitoplancton en la cuenca baja del río

Magdalena con relación a su régimen hidrológico.

Caldasia, 46(1), 113–129. https://doi.org/10.15446/

caldasia.v46n1.100385

Roldán, G. (2003). Bioindication of water quality in Colom-

bia. Proposal for the use of the BMWP/Col. Editorial

University of Antioquia.

Roldán, G., & Ramírez, J. (2022). Fundamentals of Neotropi-

cal limnology. Colombian Academy of Exact, Physical

and Natural Sciences.

Salinas-Camarillo, V. H., Carmona-Jiménez, J., & Lobo,

E. A. (2021). Development of the diatom ecologi-

cal quality index (DEQI) for peri-urban mountain

streams in the Basin of Mexico. Environmental Science

and Pollution Research, 28, 14555–14575. https://doi.

org/10.1007/s11356-020-11604-3

Srivastava, P., Verma, J., Grover, S., & Sardar, A. (2016). On

the importance of diatoms as ecological indicators in

river ecosystems: A review. Indian Journal of Plant

Sciences, 5(1), 70–86.

van Vuuren, S. J., Taylor, J., van Ginkel, C., & Gerber, A.

(2006). Easy identification of the most common fres-

hwater algae. North-West University and Department

of Water Affairs and Forestry.

Shelf Science, 184, 102–114. https://doi.org/10.1016/j.

ecss.2016.10.045

Hernández, A. E., Sepúlveda, S. R., Grajales, V. H., Aguirre,

R. N., Vélez, M. F., Caicedo, Q. O., Mesa, J., Cortés,

F., & Montoya, M. Y. (2018). Ecological quality index

using periphytic algae in San Juan River (Antioquia-

Colombia). Revista MVZ Cordoba, 23, 6998–7012.

https://doi.org/10.21897/RMVZ.1423

Hernández, E., Agudelo, D. M., Flórez, E., Sepúlveda, R.,

Zabala, A. M., Vargas, L., Parra, M., Ramírez, G.,

Aguirre, N., & Vélez, F. (2020). Ecological quality

index and water management in three ecosystems

of Colombia. Revista de Biología Tropical, 68(4),

1337–1345. https://doi.org/10.15517/rbt.v68i4.41522

Instituto de Hidrología, Meteorología y Estudios Ambien-

tales. (2023). Estudio Nacional del Agua 2022. Minis-

terio de Ambiente y Desarrollo Sostenible, Colombia.

Jiménez-Segura, L., & Lasso, C. A. (2020). XIX. Fishes of

the Magdalena River basin, Colombia: Diversity, con-

servation and sustainable use. Serie Editorial Recur-

sos Hidrobiológicos y Pesqueros Continentales de

Colombia.

Kruk, C., Huszar, V. M., Peeters, E. T. H. M., Boni-

lla, S., Costa, L., Lürling, M., Reynolds, C. S., &

Scheffer, M. (2010). A morphological classifica-

tion capturing functional variation in phytoplank-

ton. Freshwater Biology, 55(3), 614–627. https://doi.

org/10.1111/j.1365-2427.2009.02298.x

Lobo, E. A., Schuch, M., Heinrich, C. G., da Costa, A. B.,

Düpont, A., Wetzel, C. E., & Ector, L. (2015). Deve-

lopment of the trophic water quality index (TWQI)

for subtropical temperate Brazilian lotic systems.

Environmental Monitoring and Assessment, 187, 354.

https://doi.org/10.1007/s10661-015-4586-3

Martínez, L. F., & Donato, J. (2003). Efectos del caudal sobre

la colonización de algas en un río de alta montaña tro-

pical (Boyacá, Colombia). Caldasia, 25(2), 337–354.

Ministerio de Ambiente y Desarrollo Sostenible. (2018).

Guía para el ordenamiento del recurso hídrico conti-

nental superficial. Dirección de Gestión Integral del

Recurso Hídrico, Colombia.

Ministerio de Medio Ambiente y Medio Rural y Marino.

(2011). Catálogo de cianobacterias planctónicas poten-

cialmente tóxicas de las aguas continentales españolas.

Ministerio de Medio Ambiente y Medio Rural y

Marino, Colombia.

Montoya-Moreno, Y., Sala, S., Vouilloud, A., Aguirre, N.,

& Plata-Díaz, Y. (2013). Lista de las diatomeas de

ambientes continentales de Colombia. Biota Colom-

biana, 14(2), 13–78.

Moreno, J. R., Medina, C. D., & Albarracín, V. H. (2012).

Ecological and methodological aspects of sampling,

14 Revista de Biología Tropical, ISSN: 2215-2075 Vol. 73: e61984, enero-diciembre 2025 (Publicado May. 22, 2025)

Wang, H., Ebenezer, V., & Ki, J. S. (2018). Photosynthetic

and biochemical responses of the freshwater green

algae Closterium ehrenbergii Meneghini (Conjugato-

phyceae) exposed to the metal coppers and its impli-

cation for toxicity testing. Journal of Microbiology, 56,

426–434. https://doi.org/10.1007/s12275-018-8081-8

Winder, M., & Sommer, U. (2012). Phytoplankton respon-

se to a changing climate. Hydrobiologia, 698, 5–16.

https://doi.org/10.1007/s10750-012-1149-2