Recovery of an Estuarine Ecosystem after the Stopping of

Wastewater Discharges: Macrobenthic Community Characterization

in the Estuary of Oued Souss (Southwestern Morocco)

H. Bergayou

, E. Anajjar

, L. Lefrère

, A. Moukrim

, E. Gorman

and P. Gillet

Laboratory Aquatic Systems: Marine and Continental Environment (AQUAMAR), Team Biology, Ecology and

Development of Marine Resources , Faculty of Sciences, Ibn Zohr University, B.P. 8106, 80000, Agadir, Morocco

Center for Studies and Research on Aquatic Ecosystems, Institute of Applied Ecology, UCO, BP 808, 44, rue Rabelais,

49008 Angers 01, France

pgillet@uco.fr

Keywords: Biomass, Estuary, Macrobenthos, Macrofaunal assemblage, Specific richness, Wastewater.

Abstract: The communities of benthic macroinvertebrates, living in the estuary of Oued Souss (Agadir Bay,

Morocco), were studied in parallel with the changes that this ecosystem had underwent after the

stopping of pollution caused by untreated wastewater discharges. The specific richness was greater

in the year following the end of discharges (22 species in 2003 instead of 14 found during the

pollution period (2001-2002). A similar finding was noted for the dominance of the species.

Indeed, if the dominating species was Hydrobia ulvae during the pollution period, followed by

Hediste diversicolor and Scrobicularia plana in decreasing order, the sequence: H. diversicolor 

H. ulvae  Cerastoderma edule  S. plana, was noted in 2003. The longitudinal distribution of

species living in this site in 2001 and 2002 had wider in 2003 and average biomass, determined by

the study of the ash-free dry weight, had become clearly greater.

1. INTRODUCTION

Estuarine ecosystems are part of most productive

coastal environments but remain the most vulnerable

due to exposure to toxic anthropogenic effluents

transported by rivers from remote and nearby

conurbations, and industrial and agricultural areas.

The estuary of the Oued Souss (30°21'N, 9°35'W),

located in an arid zone and part of the Souss-Massa

National Park in the Ramsar site, constitutes one of

the rarest humid areas in South Western Morocco. It

is an ecosystem of great ecological interest,

particularly for many migratory birds (Dakki et al.,

1995; El Bekkay, 2013; Oubrou & El Bekkay, 2014).

However, the estuary experienced a profound

ecological change. For a long time, it has been

subjected to the discharge of large amounts of sewage

and industrial effluents (Moukrim et al., 2000). And

since November 2002, the establishment of a sewage

treatment plant marked the end of wastewater

discharge in the estuary (Ait Alla et al., 2006).

If the period in which the estuary was receiving

the discharges has been the subject of several studies

(Snoussi, 1988; Id-Halla et al., 1998; Mimouni et al.,

2002; El Hamidi et al., 2002; Gillet et al., 2003;

Bergayou et al., 2005; Ait Alla et al., 2006a; Ait Alla

et al., 2006b; Anajjar et al., 2008; Moukrim et al.,

2008; Bergayou et al., 2008; Bergayou et al., 2009],

investigations after the establishment of the plant are

of interest because they allow not only to compare the

situation of the mouth of the valley before the end of

the wastewater discharges, but also to follow the

evolution of the restoration of the ecosystem.

It is within this framework that our laboratory has

set up a multidisciplinary research program covering

all the components of the ecosystem. Thus, the

research focused on the physico-chemistry of water

and sediment (Lefrère, 2005), heavy metals (Anajjar

330

Bergayou, H., Anajjar, E., Lefrère, L., Moukrim, A., Gorman, E. and Gillet, P.

Recovery of an Estuarine Ecosystem after the Stopping of Wastewater Discharges: Macrobenthic Community Characterization in the Estuary of Oued Souss (Southwestern Morocco).

DOI: 10.5220/0009771803300341

In Proceedings of the 1st International Conference of Computer Science and Renewable Energies (ICCSRE 2018), pages 330-341

ISBN: 978-989-758-431-2

et al., 2008; Moukrim et al., 2008), pesticides

(Agnaou et al., 2014), biomarkers (Ait Alla et al.,

2006a; Idardare, 2005; Bergayou et al., 2009),

population dynamics and the biology of some species

(Ait Alla et al., 2006b; Lefrere, 2005; Bergayou et al.,

2008).

It is within the framework of this research

program that our investigations are being conducted.

The aim is to study the benthic invertebrate

communities of the intertidal zone of the estuary

during the wastewater discharge period (2001, 2002)

and after their termination (2003). The analysis

focused on faunal composition, longitudinal

distribution of species, species richness, abundance,

dominance and biomass. It also made it possible to

define and compare the biocenotic units during the

two periods.

2. MATERIALS AND METHODS

Three campaigns were undertaken during the

summer season. Two of them were carried out while

the estuary was receiving wastewater discharge in

2001 and 2002. The 2003 campaign was carried out

after the cessation of the pollution. To ensure a good

coverage of the ecosystem through our samples, 12

radials along the estuary and part of Oued Souss are

defined. Radials were numbered from 0 to 12 from

the mouth to upstream with radial 7 being the direct

receptacle of wastewater during the discharge period

(Figure 1).

Figure 1. Location of the sampling stations of the

intertidal macrobenthos in Oued Souss estuary

(Agadir Bay, Morocco) during the summer

campaigns in 2001, 2002, and 2003.

Downstream from radial 2, the stations 1bis and

2bis belong to an arm of the Oued. Between radial 2

and radial 6, the mudflat is wide, and the bed of the

Oued shows sandbanks which, very often, are

accessible. This is why we generally chose four

sampling points by radial: two on the north bank (A,

A') and two on the south bank (B, B'). From radial 6

going upstream, the mudflat decreases. Therefore, we

selected only two points of sampling by radial: one on

each bank. A total of 36 and 39 stations were

respectively sampled at low tide during the period

before and after the discharges were terminated. Their

coordinates were recorded using a GPS (MLR brand,

type SP12X). During the last campaign, three

additional stations were selected (0bis A, 0bis B, and

2'). They are the result of marine hydrodynamics that

have made new sandbanks appear. Finally, it should

be noted that after the discharges stopped, there was

a modification of the watercourse that no longer

reached radial 7. Samples were taken using the

quadratic sampling technique (Elliot & Descamps,

1973). For all the stations considered, each sampling

point consists of 4 quadrats of 0.25m x 0.25m (over a

depth of 0.20m), ie a sampling area of 0.25m² per

sample.

The sorting of the different species was carried out

on site or in the laboratory, with a sieve with 1 mm

mesh. The animals were kept in ethanol at 70° and

their determination was carried out using a binocular

magnifier and, in some cases, using a microscope (to

study the morphology of the parapodia of some

Annelids).

As a first step, the inventory of fauna, the spatial

distribution of species, and parameters such as

abundance (number of individuals of a species in a

sample); the dominance (percentage represented by a

species, a class, or an embranchment in a sample) that

make it possible to reconstruct the faunistic

composition are determined. Species richness

(number of species present in the settlement) as well

as biomass were also evaluated. For the last

parameter, we performed a prior decalcification to the

molluscs in a 10% hydrochloric acid bath and a

drying at 80°C of all the individuals during 48 hours

(to measure the dry weight). The animals are then

placed in an oven at 600°C for 2 hours to reduce them

to ashes. The difference between the decalcified dry

weight and the ash quantity corresponds to the dry

weight of the soft masses (ashless), expressed in g/m²

(Bachelet et al., 1981).

In order to define biocenotic units, a hierarchical

ascending classification is performed. For this, we

first calculated the coenotic affinity between the

station communities estimated from the Jaccard

coefficient (Gillet, 1986):

J = Na, b / (Na + Nb - Na, b)

with Na: number of species in sample survey a, Nb:

number of species in sample survey b, Na, b: number

of species common to sample surveys: a and b. The

Recovery of an Estuarine Ecosystem after the Stopping of Wastewater Discharges: Macrobenthic Community Characterization in the

Estuary of Oued Souss (Southwestern Morocco)

331

different values are grouped in a similarity matrix

from which the dendogram is built according to the

algorithm (Lance & Williams, 1967):

dhij = 0.625 dhi + 0.625 dhj - 0.25 dij

This method is highly recommended by various

authors (Legendre & Legendre, 1998)

3. RESULTS

3. 1. Settlement Faunistic Composition

3.1.1. The Ecosystem Receiving Wastewater

Discharges

The macrobenthic fauna shows a similar faunal

composition for both seasons during the period when

the ecosystem was receiving wastewater discharges.

The number of individuals, all species combined,

amounts to 11270 and 9131 for 2001 and 2002,

respectively (Tables 1 and 2).

Table 1. Faunistic composition of the

macrobenthos of the Oued Souss estuary by phylum

during the period of wastewater discharges (in 2001).

2001 Campaign

Total

Abundance

Nemathelminthes

Annelids

2459 21.82

Arenicola marina

Glycera tridactyla

Hediste diversicolor

Pectinaria koreni

2426

0.106

21.526

0.186

Molluscs

8747 77.61

Cerastoderma edule

Hydrobia ulvae

Macoma cumana

Scrobicularia plana

378

6500

1836

3.354

57.675

0.293

16.291

Crustaceans

64 0.57

Bathyporeia sp

Carcinus maenas

Eurydice pulchra

Haustorius arenarius

Urothoe brevicornis

0.035

0,390

0.133

0.009

Molluscan phylum largely dominates the

settlement with 77.6% (in 2001) and 84.1% (in 2002).

The rest of the benthic fauna is divided between the

Annelids (21.82% in 2001 and 12.42% in 2002), the

Crustaceans (0.57% in 2001 and 3.18% in 2002) and

the Nemathelminthes which constitute a minority and

are identified only in 2002 (less than 1%).

The ecosystem is poor in terms of biodiversity

(table1). Only a limited number of species (N = 14)

are encountered. However, these species are

abundant. Thus, the annelid Hediste diversicolor and

the molluscs Hydrobia ulvae and Scrobicularia

plana, alone represent more than 94.5%. In fact, in

descending order, H. ulvae has an abundance of about

6500 and 6730 and densities of up to 7016 ind/m

and

8088 ind/m

respectively for 2001 and 2002; followed

by H. diversicolor with abundances of 2426 and 1112

and densities of up to 1464 ind/m

and 732 ind/m

respectively in 2001 and 2002; then S. plana with

abundances of the order of 1836 and 827 and densities

up to 3780 ind/m

and 1808 ind/m

respectively in

2001 and 2002.

Table 2. Faunistic composition of the macrobenthos

of the Oued Souss estuary by phylum during the

period of wastewater discharges (in 2002).

2002 Campaign

Total

Abundance

Nemathelminthes

0.274

Annelids

1134 12.42

Arenicola marina

Glycera tridactyla

Hediste diversicolor

Pectinaria koreni

1112

0.131

0.110

12.178

Molluscs

7682 84.13

Cerastoderma edule

Hydrobia ulvae

Macoma cumana

Scrobicularia plana

6730

827

0.789

73.70

0.580

9.057

Crustaceans

290 3.18

Bathyporeia sp

Carcinus maenas

Eurydice pulchra

Haustorius arenarius

Urothoe brevicornis

236

0.526

0.066

2.585

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3.1.2. The Ecosystem after Stopping

Wastewater Discharges

The number of individuals harvested is of the order of

15968 (Table 3). There are 22 species belonging to

four phyla. The Molluscan phylum is dominant

(69.6%), followed by that of the Annelids (28.9%),

followed by the Crustacea (1.4%). Dominance by

species shows that it is Hediste diversicolor which

dominates the settlement with an abundance of 4599

individuals and a density which reaches in some

stations 2328 ind/m

, followed by Hydrobia ulvae

with an abundance of 4495 and a density of up to

4200 ind/m

, Cerastoderma edule with an abundance

of 3765 and a density of 6760 ind/m

and

Scrobicularia plana with an abundance of 2795 and a

density of up to 2336 ind/m

Table 3. Faunistic composition of the macrobenthos

of the estuary of Oued Souss by phylum after

cessation of wastewater discharges (2003)

2003 Campaign

Total

abundance

Nemathelminthes

0.100

Annelids

4615

28.9

Nemerte sp

Arenicola marina

Capitella capitata

Glycera tridactyla

Heteromastus filiformis

Hediste diversicolor

Lanice conchylega

Nephtys hombergii

Pectinaria koreni

4599

0.006

0.031

0.006

28.801

0.006

0.031

Molluscs

11113

69.6

Aplysia punctata

Cerastoderma edule

Donax trunculus

Hydrobia ulvae

Macoma cumana

Scrobicularia plana

3765

4495

2795

0.013

23.578

0.132

28.15

0.219

17.504

Crustaceans

224

1.4

Bathyporeia sp 48 0.301

Carcinus maenas

Eurydice pulchra

Gammarus marinus

Haustorius arenarius

Urothoe brevicornis

128

0.106

0.802

0.075

0.063

0.056

These data show that the ecosystem is

experiencing a significant ecological change in

faunistic composition. The number of individuals

harvested is significantly larger compared to the

period when the ecosystem received wastewater.

Although the phylum of molluscs is still dominant,

followed by that of Annelids and Crustaceans, the

composition of macrobenthic fauna and the

abundance of these organisms change during this

campaign carried out after the cessation of discards.

In fact, we notice an enrichment of the

macrobenthic population in new species and if we

consider the dominance of the species in decreasing

order, four species dominate the population, in

descending order: H. diversicolor, H. ulvae, C. edule

and S. plana instead of three species during the

discard period: H. ulvae, H. diversicolor and S. plana.

3. 2. Longitudinal Distribution of Species

and Biocoenotic Units

3.2.1. The Ecosystem Receiving Wastewater

Discharges

Species populating the environment were

divided into four groups of species, depending on

their distance from the sea (Figure 2):

- a first group linked to the mouth of the

estuary (radials 0, 1, 1bis) and not sinking beyond 250

meters. It consists of the following species: Arenicola

marina, Glycera tridactyla, Bathyporeia sp, Eurydice

pulchra, Haustorius arenarius and Urothoe

brevicornis;

- a second group that is distributed on radials

1, 1bis, 2bis and 2. It is Cerastoderma edule and

Macoma cumana;

- a third group corresponding to species with

very wide distribution on the estuary. It is

Scrobicularia plana in radials: 1 to 4, Hediste

diversicolor in radials: 1 to 6 and Hydrobia ulvae in

radials: 0 to 8;

Recovery of an Estuarine Ecosystem after the Stopping of Wastewater Discharges: Macrobenthic Community Characterization in the

Estuary of Oued Souss (Southwestern Morocco)

333

- a fourth group, spotted in 2002 alone, and

represented by the parasitic nematode which is

confined upstream of the estuary, at the level of

radials 7 and 8.

Regarding the stations beyond radial 8, they

constitute an azo-zone.

The study of the coenotic affinity between settlements

of different radials allowed us to separate 2-3 groups

of radials (Figure 3). Thus, in 2001, the dendrogram

distinguishes (Figure 3A): a first group consisting of

radials 0 and 1, 1bis, 2 and 2bis; and a second group

which associates radials 3, 4, 5, 6, 7, and 8. The latter

has two subgroups (3-6 and 7-8). Whereas in 2002,

Figure 3B shows three groups: Group 1 (radials 0, 1,

1bis and 2bis), Group 2 (radials 2, 3, 4, 5 and 6) and

Group 3 (radials 7 and 8).

3.2.2. The Ecosystem after Stopping

Wastewater Discharges

The species composition and distribution, after

discharges stopped, showed a change that was

manifested by a species enrichment, a longitudinal

widening of their distribution area and the

repopulation of radials that were azo during the

discharge period. Four groups are dentified:

- Group 1. (Radials, close to the sea: 0, 0bis, 1). It

is composed of: Arenicola marina, Capitella

capitata, Glycera tridactyla, Heteromastus filiformis,

Lanice conchylega, Nephthys hombergii,

Bathyporeia sp., Gammarus marinus, Haustorius

arenarius, Urothe brevicornis, and finally Eurydice

pulchra which populated also radials 1 bis and 2 bis;

- Group 2. It is constituted by Cerastoderma

edule, Macoma Cumana and Donax trunculus. This

group has a widening of the distribution range of the

e species that compose it, since we note a penetration

of C. edule in the estuary (station 4), as well as M.

cumana which populates radial 2;

- Group 3. It is composed of species that populate

the most radials. This is the case of S. plana (0 bis to

8), H. diversicolor and H. ulvae (radial 0 or 0 bis to

12). To these species, C. maenas (0 bis to 6) can be

included. Species in this group have all expanded

their range in the estuary.

- Group 4. It consists mainly of nematodes having

migrated upstream of the estuary (radials 11 and 12).

In addition, some species are found exclusively in

radial 3: Pectinaria koreni and Aplysia punctata.

In terms of affinity between settlements (Fig. 3C),

we can classify radials into three groups, after

stopping discharges in the estuary:

- Group 1, consisting of settlements from radials:

0, 0bis, 1, 1bis, 2bis;

- Group 2, counting of settlements from radials: 2,

2 ', 3, 4, and 5;

- Group 3, associating the settlements of radials 6,

8 and those of radials which were azo during

discharges: 9, 10, 11, 12.

Figure 2. Longitudinal distribution of intertidal

benthic macrofauna in the mouth of Oued Souss,

Agadir Bay: during the discharge period (2001 and

2002) and after that period (2003).

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Figure 3. Hierarchical ascending classification of

intertidal invertebrate settlements living during the

period of wastewater discharges (A: 2001, B: 2002),

and after (C: 2003) at the Oued estuary Souss (Bay of

Agadir)

3. 3. Species Richness and Biomass,

before and after the End of

Wastewater Discharge

3. 3.1. Species Richness

During the period when the estuary received

wastewater discharge, the number of species recorded

during the harvest was 14. Figure 4 shows the specific

richness for each radial. There is a negative

correlation between this parameter and the distance of

the radial with respect to the sea. After the cessation

of rejections, the number of species harvested is 22.

Thus, one records an enrichment on almost all the

radials of the estuary with the exception of radial 7,

which has become azoic, after the cessation of

discharges. This last observation is surely in relation

with the change of the river at this level of the estuary.

There is a negative correlation between this

parameter and the distance of the radial with respect

to the sea. After the cessation of rejections, the

number of species harvested is 22.

Thus, one records an enrichment on almost all the

radials of the estuary with the exception of radial 7,

which has become azoic, after the cessation of

discharges. This last observation is surely in relation

with the change of the river at this level of the estuary

Figure 4: Variation of the species richness of the

macrobenthos in the Oued Souss estuary by radial,

before (2001 and 2002) and after (2003) the end of

wastewater discharge

3. 3.2. Biomass

Figure 5 shows differences between the results

obtained for the biomass of fauna recorded by radial

during campaigns carried out before and after the

cessation of discharges.

The results show an increase in the biomass at

radials 0 to 2 of the mouth after the cessation of the

pollution, whereas for radials 3 to 6, there is a

decrease in the biomass.

However, the average biomass calculated after

cessation of discharges has increased significantly. It

is 20.46 g / m

(in 2003) compared to 15.54 g / m

15.84 g / m

(in 2001 or 2002 respectively) in the

presence of wastewater discharges

Biomass results by species (all radials combined)

also make it possible to distinguish differences

between the two periods (Table 4).

Thus, during the release period, H. diversicolor has

the largest biomass (43.64 to 55.84%), followed by S.

plana (38 to 35.75%), C. edule (13.32% to 2.53%),

and finally Hydrobia ulvae (1.67 to 2.25%). The

values quoted correspond respectively to the results

Recovery of an Estuarine Ecosystem after the Stopping of Wastewater Discharges: Macrobenthic Community Characterization in the

Estuary of Oued Souss (Southwestern Morocco)

335

of the 2001 and 2002 surveys. Whereas after the

rejection of the discharges, the dominant species in

biomass are in descending order: C. edule which

represents 31.55% of the total biomass, followed by

H. diversicolor (23.35%), S. plana (22.6%), Aplysia

punctata (10.55%) and then C. maenas (10.23%)

Figure 5. Variation of the total biomass of

macrobenthos in the Oued Souss estuary by station

[AFDW in (g/m2)] during wastewater discharges

(2001, 2002) and after (2003).

Table 4. Cumulative biomass by species [AFDW in

(g)] of the intertidal macrobenthos of the Oued Souss

estuary, during the period of wastewater discharges

(2001-2002) and after (2003), all stations being

considered.

4. DISCUSSION

The campaigns that took place while the Oued Souss

estuary was receiving wastewater (2001-2002) made

it possible to draw up the first inventory of the

intertidal zoobenthic fauna in this ecosystem and to

know the structure of this settlement. Regarding

faunal composition, we observe a year-to-year

variation in the total abundance of the individuals

surveyed, without affecting the order in which species

are ranked in relation to their abundance dominance.

In descending order Hydrobia ulvae, followed by

Hediste diversicolor then Scrobicularia plana.

The horizontal distribution shows a succession of

three communities with regards to the coenotic

affinity between the settlements. The first community

occupies radials 0-2 bis (2001), or 0-2 (2002) while

the second includes radials which constitute the

central part of the estuary: 3, 4, 5, 6, then the last

community consists of the species from radials 7 and

8, which are close to the discharges. This latter

community has sometimes a certain affinity with the

community of previous radials (2001) and sometimes

it dissociates completely (2002). In a parallel, study

in which the granulometry and physico-chemistry of

water in the estuary were treated (Gillet et al., 2003).

It has been argued that in the absence of a true hyaline

gradient at the level of the estuary of the Oued Souss,

grain size and the proximity of wastewater discharges

seem to be the factors that most influence the

distribution of species. We have, indeed,

distinguished a first group composed by the radials of

the mouth where the substrate is sandy (coarse sand

to fine for radial 0 to 1a, or sands silted for 2a and 2);

a second group corresponding to the radials where the

sediment becomes much muddier and clayey (radial:

2 or 3 to 6). Finally, a third group of radials 7 and 8,

which corresponds to the zone where the influence of

wastewater discharges is important and where the

surface of the mud flat is very small.

After the cessation of discharges, the total

abundance of the fauna is much greater, the

abundance dominance has undergone modifications

since it is H. diversicolor which becomes the most

abundant, followed by H. ulvae, Cerastoderma edule

then S. plana.

Differences are also observed at the level of the

settlement structure if we continue referring to the

coenotic affinity. Indeed, this parameter allows us to

distinguish three communities: a group at the mouth

(radial: 0 to 2bis), a second group in the central part

of the estuary (radial: 2 'to 5) and a third group which

associates radials upstream (6 and 8) to those which

were azo during discharges (radial: 9 to 12). Radial 7

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is discarded because at high tide, the seawater does

not reach it any more. This change in the course of

water is surely due to the cessation of discharges.

The preexisting species have seen their range

extend at the level of the estuary. This enlargement is

due either to a colonization of these stations by young

specimens of H. diversicolor, from H. ulvae, S. plana,

or C. edule, or to a migration of these species to these

new territories from nearby habitats. Indeed, several

authors (Mettam, 1981; Lewis et al., 2001; Meziane

& Retière; 2001) report that the populations of H.

diversicolor have a great ability to migrate at any

point in their life cycle. With the exception of

Macoma balthica, which is known for its frequent

migrations (Perkins, 1974), molluscs are not very

mobile. Three species call for particular comments:

- H. ulvae is famous not only for the high densities

it reaches, but also for its controversial behavior.

Authors indicate for this species a tidal cycle of

burial, feeding in a pelagic medium (by flotation) then

after falling on the ground, crawling (Newell, 1962;

Anderson, 1971). On the other hand, other authors

think that buoyancy is accidental, and that burial is a

reaction against dehydration or predators (Barnes,

1981). The dispersion of this small gastropod would

then be due only to the planktonic larval stage or

passive transports, by the movements of the water

(rotation of the adults on the substrate). One can thus

think, after the cessation of the discharges, about a

tidal (floating), exploring the upstream stations. Be

that as it may, the life span of this species is rarely

more than a year and a half. It is reported in the

literature that eggs encapsulated by 3 to 5 are

observable from March to December and larval

arrival occurs three weeks later (Muus, 1986). The

expansion of the range of the species could also be

explained by a new recruitment.

- S. plana, meanwhile, can make rare

displacements in the horizontal direction (Hugues,

1970). Six months after the cessation of the

discharges, the longitudinal widening of the range of

the scrobiculars upstream is surely due to the

installation of juveniles especially as this species

presents, at the level of Oued Souss, two recruitment

periods, from February to March and from late spring

to early autumn (Bergayou & Moukrim, 2005).

- C. edule also has a longitudinal widening of its

distribution on the longitudinal plane. As this species

is living and growing in the sand, the cessation of the

flow of wastewater in the estuary and the absence of

another flow of fresh water in the estuary make the

estuary more susceptible to marine hydrodynamics,

hence the deposit of a thin layer of sand and the

installation of hulls a little further upstream (station

4). This hypothesis is based on the findings of a study

at the level of the Canche estuary (northern France)

(Dobroniak, 2000). The author explains that the

settlements have a certain stability over time, but

migrations could be caused by the morpho-

sedimentary upheavals caused, among other things,

by the hydro dynamism and the geographical

modifications of the bed of the Canche, the

distribution of the biocenoses being essentially due to

these physical factors (Dobroniak, 2000). Another

argument is that the hull is fairly sensitive to

pollution, whatever its nature, according to a study in

the Kinneil mudflats at the Forth estuary in Scotland

(Mac Lusky, 1981). In these mud flats, which receive

domestic, chemical or oil effluents, C. edule does not

live in the sediment near the source of pollution, but

is found 1.5 km from the effluents; and its maximum

abundance is between 1.5 and 2.2 km. In the present

study on Oued Souss, this bivalve was found about

two kilometers from the source of pollution during

discharges and its presence in 2003 in stations 3 and

4 is surely a consequence of the cessation of

pollution.

In marine habitats, the phenomenon of restocking

may depend on the free surfaces in question. For

small areas, restocking is rapid and is done through a

large variety of pioneer opportunistic species (Frid,

1989). For large areas, this repopulation can take

months or years (Beukema et al., 1999). However, in

a more recent study of the Cochin estuary of the

tropical monsoon (India) (Rehitha et al., 2017) where

sites that were or were not subject to dredging were

compared, the authors highlighted the dominance of

a single opportunistic benthic taxon that has settled in

sites that have been dredged.

In this study, only the most downstream, marine-

influenced stations experienced enrichment of new

species. Upstream, preexisting species have

expanded their range. Similar results have been

reported on mudflats at Dutch Wadden Sea (Holland)

(Beukema et al., 1999) and at Clonakily Bay, West

Cork, Ireland (Lewis et al., 2001).

4.1 Species Richness

Specific wealth was very low during the discharge

period (14 species). Indeed, the number of species

usually encountered in an estuary varies between 30

and 300 species. By way of comparison, authors have

identified 32 species in the Loire estuary (Marchand,

1972; Robineau, 1986), 66 species in the Gironde

estuary (Bachelet et al., 1981), 86 species in the

Tagus estuary (Portugal) (Calvario, 1984), 264

species in the Bou Regreg estuary (Morocco)

Recovery of an Estuarine Ecosystem after the Stopping of Wastewater Discharges: Macrobenthic Community Characterization in the

Estuary of Oued Souss (Southwestern Morocco)

337

(Elkaim, 1974; Elkaim, 1976); or 52 species in the

same estuary (Cheggour, 1988) after the Sidi

Mohamed Ben Abdellah dam was commissioned in

1974 in the Bou Regreg basin and, in more recent

studies, 37 species for the macrobenthos furniture in

the Smir lagoon in northern Morocco (Chaouti &

Bayed, 2005) and 57 taxa at the Khnifiss lagoon in

southern Morocco (Lefrere et al., 2015). In the case

of the estuary of Oued Souss, the factors explaining

this faunistic poverty are numerous: the freshwater

inputs are very low because of the rare rainfall; this

phenomenon is further accentuated by the installation

of dams on the bed of the Oued, the most important

of which is that of Aoulouz, 150 km upstream; in

addition, the pollutant load is important because of

urban discharges. Moreover, the lesser diversity of

habitats (lack of rocky substrates, absence of sea grass

beds) at the level of the estuary and its geographical

position with a certain number of species which are at

the southern limit of their distribution area explain to

some extent this low specific wealth.

After the discharges, we counted 22 taxa. This

observation puts the index on the negative effect of

wastewater. However, this is a punctual result and

should be taken with caution. Indeed, the specific

wealth, which reflects stability, depends first and

foremost on breaks in the climax of the ecosystem:

maximum in average conditions, it decreases when

disturbances are important and / or frequent (Connell,

1978). Secondly, the biological regulation of

communities can only take place when the resources

of the environment are fully utilized. Thus, when the

environment remains stable for a very long time,

resources become restricted and competition sets the

limits of the number of species. The dynamic

equilibrium is then reached (Huston, 1994; Pickett &

Cadenasso, 1995): the composition of the settlement

varies according to the colonization and results in a

stable global structure determined by the physical

environment and the predation.

4.2 Biomass

The average biomass of the harvested fauna has

varied. Thus, during the discharge period, it was

15.54 g/m² then 15.84 g/m² (in 2001 and 2002

respectively), with three species (H. ulvae, H.

diversicolor and S. plana) which constitute the

majority: 83.33 to 93.85% respectively for the first

two campaigns. These species are typical of Atlantic

estuaries and are known for their tolerance (Gonzales

Oreja & Sais Salinas, 2003).

On the other hand, after the cessation of

discharges, the average biomass was 20.56 g / m², of

which 78.79% is represented by the four dominant

species in abundance: H. diversicolor, S. plana, C.

edule and H. ulvae. In order of dominance in biomass,

the cockles come first with 31.52% of the total

biomass. This is important and can be explained by

the spectacular expansion in terms of numbers

marked by the species.

The other three species represent only 47.25% of

the total biomass. If we consider the two species S.

plana or H. diversicolor, their total abundance marks,

respectively for the two species, an increase of 1.5 to

3.4 times and 1.89 to 4.13 compared to the period of

discharges. This increase is not accompanied by an

increase in the biomass of these two species.

Knowing their diets: mixed for S. plana (detritivore

at low tide and suspensivore at high tide) and

detitivore for H. diversicolor, we can think of a

slimming of individuals in relation to the decline in

the rate of organic matter noted in the sediment after

the cessation of discharges (Ait Alla et al., 2006b).

In a comparative study of H. diversicolor,

populations of the Bou Regreg estuary (Morocco)

(Gillet, 1986) with populations of this species from

the Ythan (Scotland) estuary (Chambers & Milne,

1975), the author (Gillet, 1986) reaches the same

findings by emphasizing that for neighboring

densities. The highest biomasses corresponded to the

populations of the stations close to the sewage outlets,

and thus whose sediment contained a high level of

organic matter.

Still regarding the biomass, our results are

comparable to those obtained in the Gironde estuary

(France) where 90% of the average biomass

(estimated at 10.4 g / m²) consists of the biomasses of

three dominant species (Bachelet et al., 1981). In

comparison, in a study on mud flats at Duch Wadden

Sea (Holland), an average biomass of 26.6 g/m²

(Beukema, 1976) and a biomass of 13.2 g/m² in the

Lynher estuary (England) are reported (Warwick &

Price, 1975). However, it should be noted that this

type of comparison is difficult because the methods

used are not always homogeneous and the

environmental conditions are rarely taken into

account.

5 CONCLUSIONS

This study proposes an inventory of the intertidal

benthic system of the Oued Souss estuary from a

structural point of view. First of all, it allowed us to

acquire a qualitative (species list, specific wealth) and

quantitative (abundance, biomass, species density)

database of intertidal benthic macrofauna. It

represents a reference state. This tool was used to

ICCSRE 2018 - International Conference of Computer Science and Renewable Energies

338

respond to requests for information about the quality

of the environment immediately after the cessation of

wastewater discharges and could serve as a basis for

conducting impact studies later.

However, while the spatial dimension is relatively

well documented, the temporal dimension was not

taken into account in this study. Finally, we suggest a

continuation of the investigations along the estuary

according to a monthly or seasonal monitoring in

order to be able to establish the structure of the

benthic population over time (seasonal and

interannual variations) and to see the new state of

equilibrium, which must be currently reached by the

different zoobenthic populations of the estuary.

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