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Hydrographic Conditions and the Macrozoobenthos of a Coastal Wetland in Ghana

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The prevailing aquatic environmental conditions and their effects on the composition of the benthic macro-invertebrate community were investigated in a few ephemeral pools in coastal Ghana along the Kakum Estuary (about 2 km west of Cape Coast; 5 6' N, o 1 18' W). Water and benthic sediments were sampled every month from five pools in the wetland from July 2009 to February o 2010, and were analysed for six hydrographic parameters as well as the species composition and mean density of benthos. Significant changes occurred in hydrographic conditions during the year. Water temperature, salinity, conductivity, turbidity, dissolved oxygen and pH were relatively low in July 2009 (27.9 C, 0.7‰, 1,503 µS cm , 78 mg L , 3.5 mg L and 6.9 o !1 !1 !1 respectively) but increased progressively to 46.6 C, 5.7 ‰, 12,777 µS cm , 304 mg L , 5.7mg L and 8.62, respectively, in o !1 !1 !1 January 2010. Chironomid larvae (Diptera) and oligochaetes were the only benthic fauna encountered. These organisms were abundant in the wet season (July –September 2009), with mean density of 258 to 1,375 individuals m for diptera, and 81 to 203 -2 individuals m for oligochaetes. Their abundance and density however declined progressively during the dry season (November -2 2009 – January 2010) after which they were not found. The increase in salinity stressed the macrozoobenthic fauna during the dry season leading to their extermination. The ephemeral nature of the environment conceivably resulted in the poor diversity of macro-zoobenthic fauna, and this could limit the development of the fish community.
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International Journal of Ecology and Environmental Sciences 37 (1): 15-22, 2011
© NATIONAL INSTITUTE OF ECOLOGY, NEW DELHI
Hydrographic Conditions and the Macrozoobenthos of a Coastal Wetland in Ghana
I. OKYER E*, J. BLAY AND J. AGGREY-FYNN
Department of Fisheries and Aquatic Sciences, School of Biological Sciences, University of Cape Coast, Ghana.
* Corresponding author; E-mail: okyereisaac@yahoo.com
ABSTRACT
The prevailing aquatic environmental conditions and their effects on the composition of the benthic macro-invertebrate community
were investigated in a few ephemeral pools in coastal Ghana along the Kakum Estuary (about 2 km west of Cape Coast; 5 6' N,
o
1 18' W). Water and benthic sediments were sampled every month from five pools in the wetland from July 2009 to February
o
2010, and were analysed for six hydrographic parameters as well as the species composition and mean density of benthos.
Significant changes occurred in hydrographic conditions during the year. Water temperature, salinity, conductivity, turbidity,
dissolved oxygen and pH were relatively low in July 2009 (27.9 C, 0.7‰, 1,503 µS cm , 78 mg L , 3.5 mg L and 6.9
o!1!1!1
respectively) but increased progressively to 46.6 C, 5.7 ‰, 12,777 µS cm , 304 mg L , 5.7mg L and 8.62, respectively, in
o!1!1!1
January 2010. Chironomid larvae (Diptera) and oligochaetes were the only benthic fauna encountered. These organisms were
abundant in the wet season (July –September 2009), with mean density of 258 to 1,375 individuals m for diptera, and 81 to 203
-2
individuals m for oligochaetes. Their abundance and density however declined progressively during the dry season (November
-2
2009 – January 2010) after which they were not found.
The increase in salinity stressed the macrozoobenthic fauna during the dry season leading to their extermination. The
ephemeral nature of the environment conceivably resulted in the poor diversity of macro-zoobenthic fauna, and this could limit
the development of the fish community.
Key Words: Marsh, Pools, Hydrographic Parameters, Benthic Macroinvertebrates, Composition, Density
INTRODUCTION
Lagoons, estuaries, seasonal and permanent rivers,
streams, lakes, swamps, floodplains, marshes, ponds and
pools are classified as wetland ecosystems under the
Ramsar Classification System for Wetland Type”
(Ramsar Convention Secretariat 2007). These habitats
share common features, the most important being the
occurrence of continuous, seasonal or periodic standing
water or saturated soils.
Macrozoobenthic fauna constitute a very important
community in wetland ecosystems and are of immense
ecological value due to their unique response to
environmental changes. Reports have indicated that the
composition and diversity of macro-zoobenthic commu-
nity is closely linked to aquatic habitat conditions, with
many species serving as biological indicators of pollu-
tion (Lafont et al. 1996, Richardson and Kiffney 2000,
Arslan et al. 2007, Jenderedjian et al. 2007). Records of
macrobenthic fauna are therefore used in assessing the
biodiversity and environmental conditions as well as the
ecological states of wetland systems.
Although the composition, diversity, biomass and
density of benthic macrofauna of some wetland eco-
systems in Ghana have been reported (Blay and Dong-
dem 1996, Gordon 2000, Yankson and Akpabey 2001,
Lamptey and Armah 2008), these investigations focused
on lagoons and estuaries, with no consideration for the
adjoining coastal marshes. There is thus a paucity of
information on the biodiversity of such ecosystems in
Ghana, and the impacts of abiotic factors on their
macroinvertebrate communities. Considering the fact
that wetlands are dynamic systems with varying
hydrological conditions (Acharyya and Mitsch 2001),
and these changes in water quality, periods of anoxia
and accumulating organic matter directly affect the
macroinvertebrate and fish communities (Craft 2000), it
is pertinent to understand the effects of the changing
Okyere et al.: Hydrographic Conditions and Benthic Fauna Int. J. Ecol. Environ. Sci.
16
conditions on the aquatic fauna in a given locality. This
study therefore seeks to provide information on the
hydrographic conditions and the composition and
density of the benthic macroinvertebrate community of
a coastal marsh in the Central Region of Ghana.
MATERIALS AND METHODS
Study Area
The Kakum Estuary wetland is a coastal marshland and
is located about 2 km west of Cape Coast in the Central
Region of Ghana ( 5 6' N, 1 18' W) (Figure 1). The
o o
dominant flora include the saltwater couch Paspalum
vaginatum (Poaceae), the sedge-grass Cyperus arti-
culatus (Cyperaceae) and the bulrush Typha australis
(Typhaceae). The thatch grass Imperata cylindrica
(Poaceae) and coconut trees Cocos nucifera (Palmae)
fringe the band of sandy beach separating the wetland
from the Atlantic Ocean, while the western portion is
bordered by mangroves. A characteristic feature of the
area is flooding during the rainy season and isolated
pools during the dry season. It is noteworthy that the
mean monthly temperature over most parts of Ghana
does not fall below 25 C, with average annual temp-
o
erature of 27 C (Dickson and Benneh 1988). Absolute
o
maxima approach 40 C, especially in the north, with
o
absolute minima descending to about 15 C.
o
Aquatic Environmental Factors
The study was conducted from July 2009 to February
2010, covering parts of the rainy season and the dry
season. Five pools in the wetland were selected and the
surface area of each pool was estimated at the beginning
of the study.The area (A) of the pools was determined
from the measurements of the length and width or the
diameter according to their shape. The maximum depth
of each pool was also determined monthly at the deepest
part using a meter scale.
Samples of benthic fauna and measurement of
hydrographic parameters were undertaken between the
20 and 22 of each month. Temperature, salinity and
th nd
Figure 1. Map of Ghana showing the Kakum Estuary Wetland in Ghana
37: 15-22 Okyere et al.: Hydrographic Conditions and Benthic Fauna 17
conductivity were recorded with aYSI Incorporated
(Model 63) meter. Turbidity was measured with a
turbidimeter (TOA Model TB-1A) and hydrogen ion
concentration (pH) with a portable pH meter (Corning
Incorporated Model 220). Dissolved oxygen (DO)
content of pools was also determined by the modified
Winkler’s titrametric method using a HACH test kit
(Model FF2). Three measurements of each parameter
were taken at different points in each pool on a sampling
date and the average value recorded.
Benthic Macroinvertebrates
Pool sediments were sampled in triplicate with an
Ekman grab (15 × 15 cm) each month for esimating the
benthos. The samples were screened in the field using a
set of sieves (mesh size 4 mm, 2 mm and 0.5 mm), and
the organisms retained in the sieves were preserved in
10% formalin for detailed examination in the laboratory.
Prior to sorting out the organisms, the samples were
dyed with Bengal rose to enhance their visibility. The
macrofauna were identified with the help of manuals
(Yankson and Kendall 2001, Hauer and Lamberti 2006).
Counts of the different taxonomic groups were recorded
for further analysis. The monthly percentage numerical
composition of the different taxa of macrobenthos in the
community was calculated. Due to the small number of
grab samples (n= 3), counts of individuals belonging to
each taxon in a sample were log transformed (Elliot
1977) for computation of the mean density of benthic
organisms (number m ) in each pool.
-2
RESULTS
Physico-chemical Factors
The surface area of the pools were 2680 m (Pool V),
2
779 m (Pools I) 759 m (Pool III) 754 m (Pool IV), and
2 2 2
661 m (Pool II). The depth of the pools decreased
2
steadily from the wet to the dry season months (Figure
2). Although the pools were generally shallow (< 100 cm
deep), Pool V was deeper than the others throughout the
study period. The pools dried between November 2009
and February 2010.
Figure 3 illustrates the monthly changes in the
environmental parameters recorded. Water temperature
increased from 27.9 C in July 2009 to 42.1 C - 43.6 C
o o o
in December 2009. In January 2010, the temperature of
Pool V, the only remaining pool, was 46.6 C. Monthly
o
Figure 2: Changes in the depth of pools in the Kakum Estuary
wetland in Ghana
salinity and conductivity of the pools did not differ
much, with values ranging from 0.7 ‰ to 1.0 ‰ in July
2009, and increasing up to 5.7 ‰ in Pool V in January
2010. Conductivity was relatively low at the beginning
of the study (1,503 to 2,302 µS cm ), but attained
!1
higher levels in December 2009 (11,387 to 11,930 µS
cm ) and January 2010 (12,777 µS cm ). The turbidity
!1!1
also increased from 78- 91 ppm in July 2009 to 258-301
ppm in December 2009, and 304 ppm in January 2010
(Pool V). Dissolved oxygen in Pool V varied between
5.0 mg L and 5.7 mg L during the study period while
!1!1
the DO content of the other pools varied between 2.3 mg
L and 5.0 mg L . The pH was slightly acidic to neutral
!1!1
in July-August 2009 but became alkaline after
September (8.62 in Pool V).
Composition of Benthic Macroinvertebrates
Figure 4 illustrates the monthly composition of the
macrozoobenthos in the five pools. Diptera (comprising
exclusively of chironomid larvae) and Oligochaeta were
the only representative groups in the benthos. In Pool I,
the dipterans varied from 77.08 % to 88.68 % between
July and September 2009, but declined to 9.76 % in
October whereas the oligochaetes increased from 11.3-
22.9 % to 90.24 % during the same period.
In Pool II also, Diptera (70 to 78.6 % of the total
invertebrates) were most abundant during July to
September 2009. Only two invertebrate specimens were
collected in October, one of which was an oligochaete
and the other a dipteran. No organism was found
thereafter till the pool dried in January 2010.
Okyere et al.: Hydrographic Conditions and Benthic Fauna Int. J. Ecol. Environ. Sci.
18
Figure 3. Changes in six environmental parameters of pools in the Kakum Estuary wetland in Ghana
In other Pools also the Diptera were the most
abundant group during July to September 2009. In pool
III, only seven individuals of oligochaetes were recorded
in October 2009 and no benthic organism was found
thereafter. In Pool IV and V, the chironomid larvae
declined steeply in October whereas the abundance of
oligochaetes increased sharply. There were only three
individuals of oligochaetes in November, and only one
survived in December 2009.
Variations in Density of Invertebrate Fauna
Figure 5 illustrates the changes in the densities of both
diptera and oligochaetes. The mean density of dipterans
in pool I ranged from 258 individuals m to 648
!2
individuals m in July – September 2009, after which it
!2
declined to 71 individuals/m in October. Oligochaete
2
density increased from 81-112 individuals m during
!2
July-September 2009 to 476 individuals m in October.
!2
In Pools II and III, the density of both the dipterans
and oligochaetes declined gradually from July to
October, after which they disappeared completely. The
density of the two benthic groups followed nearly the
same pattern also in Pool IV except that the density of
oligochaetes increased slightly in October before dropp-
ing sharply in November. The mean density of dipterans
37: 15-22 Okyere et al.: Hydrographic Conditions and Benthic Fauna 19
Figure 4. Percentage composition of benthic macroinvertebrates in the five pools in the Kakum Estuary wetland (n = number of benthic
invertebrates, + o = 1 – 3 oligochaetes, + d = 1 dipteran)
in Pool V declined steeply from July to October 2009
after which no specimen was found. The density of
oligochaetes varied during the study period.
Okyere et al.: Hydrographic Conditions and Benthic Fauna Int. J. Ecol. Environ. Sci.
20
Figure 5: Monthly mean density of benthic macroinvertebrates in five pools in Kakum Estuary wetland in Ghana
DISCUSSION
The Kakum Estuary exhibits a highly dynamic and
seasonal environment in which dry season pools occur
for about eight months. There is no comparable infor-
mation on the physico-chemical conditions of ephemeral
pools in such coastal wetlands in Ghana. In our study,
the pools started to dry up towards the latter part of the
37: 15-22 Okyere et al.: Hydrographic Conditions and Benthic Fauna 21
study possibly because of the sustained high water
temperature enhancing evaporation. The increase in
salinity and conductivity of the pool waters during the
study period is similar to that reported for floodplain
pools in Africa (Welcomme 1975) and is attributed to
the concentration of dissolved ions with decreasing
water volume (Rosenberg et al. 2001).
It is most likely that the increase in turbidity
observed in all the pools was a result of increasing
phytoplankton production during the dry warmer season
(Howerton 2001). This is reflected in somewhat higher
oxygen concentration during the same period. Similarly,
the relatively low oxygen levels (. 2.5 mg L ) and the
!1
slightly acidic to neutral conditions (pH = 6.5 to 7.5) of
the pools in the wet season may have been caused by
increased rate of decomposition of allochthonous
organic materials. These might as well be attributed to
flows from the Kakum River which reportedly has a low
pH (Bosque-Hamilton et al. 2004).
The oligochaetes and chironomid larvae were
abundant in the wet season but gradually declined in
abundance as the dry season progressed and the pools
reduced in size. This may be attributed partly to the
increasing salinity of the pools. Gordon (2000) and
Lamptey and Armah (2008) reported a similar reduction
in the abundance of macroinvertebrate community in
Muni lagoon and Keta lagoon in Ghana. Bervoets et al.
(1996) and Berezina (2004) reported the upper limit of
salinity for survival of freshwater oligochaetes and
midges as 6‰ to 8‰. In our study, the salinity was >4
‰ during October 2009 to January 2010.
Poor representation of benthic invertebrate fauna in
these pools is somewhat similar to that of the nearby
Fosu lagoon which has only four species of gastropods
and three insect larvae, including chironomids (Blay and
Dongdem 1996).
ACKNOWLEDGEMENTS
We are grateful to the Department of Fisheries and
Aquatic Sciences of the School of Biological Sciences,
University of Cape Coast, Ghana for providing vehicle,
field and laboratory equipment as well as laboratory
space for this research. We also wish to express our
gratitude to the chief technician Mr. Peter Aubyn as well
as the field assistants Mr. John Eshun and Mr. Benjamin
Owusu of School of Biological Sciences, UCC for their
tireless efforts throughout the field and laboratory data
collection.
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\
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Ante esto, analizar y comprender la riqueza de especies y la abundancia de sus componentes, especialmente de aquellos grupos de niveles tróficos que son fuente principal de alimento para especies tróficamente superiores, nos podría ayudar a explicar la dinámica de este ecosistema, permitiendo así proponer estrategias que aseguren su integridad ecológica, como a la vez, su conservación en el tiempo. Por consiguiente, el presente trabajo busca realizar la caracterización de las biocenosis del Humedal de Mantagua, particularmente de las comunidades de macroinvertebrados y de peces que habitan en él. Al comparar los parámetros fisicoquímicos registrados en invierno y primavera se detectaron diferencias significativas en cada una de las zonas, para todos los parámetros analizados, excepto en el pH en la zona del Estuario, el cual aparentemente no presenta diferencias relacionadas a la estacionalidad. La conductividad juega quizás el rol más relevante en relación con los parámetros fisicoquímicas, los cuales moldean la estabilidad del sistema a nivel estacional como espacial. La temperatura y el oxígeno presentan una fuerte variabilidad espacial y estacional, lo cual puede depender de las condiciones ambientales, como la temperatura diaria y el efecto del viento. En cuanto a la macrofauna, en ambas campañas de muestreos, se registró un total de 35 especies pertenecientes a 21 familias, siendo la clase insecta la mejor representada. El Índice biótico ChIBF determinado para Mantagua, tanto en invierno como primavera sugiere que la mayoría de las estaciones quedaran clasificadas como Muy Pobre y Pobre en ambas temporadas. Sin embargo, al comparar las condiciones de invierno y primavera, podemos observar que esta va empeorando, donde de 5 estaciones calificadas como muy mala en invierno, se pasa a 7 en primavera. En cuanto a la ictiofauna en abril 2022, se detectó la mayor cantidad de individuos en la estación 7 ubicada en el límite entre el meandro y el estuario, con 22 individuos de la especie Gambusia holdbrooki y 3 de Cnesterodon decemmaculatus. En julio 2022 se capturaron especímenes de la especie pez mosquito oriental o gambusia (Gambusia holdbrooki) y la especie madrecita o gambusia manchada (Cnesterodon decemmaculatus. En el muestreo de noviembre 2022 se capturó un total de 4 especies de peces, y se registró la presencia de una quinta especie. Las especies capturadas fueron el pez mosquito oriental o gambusia (Gambusia holdbrooki), la madrecita o gambusia manchada (Cnesterodon decemmaculatus), la Pocha (Cheirodon pisciculus) y el Pejerrey de Mar (Odontesthes regia), y se observó la presencia de Carpas en la zona de la estación 6 y 7. De la ictiofauna registrada durante las campañas de abril, julio y octubre, solo dos especies se encuentra listadas en categoría de conservación según el Reglamento de Clasificación de Especies (RCE), Cheirodon pisciculus quien se encuentra Vulnerable y Odontesthes regia que es una especie clasificada en preocupación Menor. Dos talleres se realizaron en el marco de esta consultoría. El primero fue un Taller Ciudadano: Introducción a la identificación de macroinvertebrados bentónicos y peces de humedales costeros el jueves 8 de septiembre en dependencias de la Posada del Parque y el segundo taller se realizó el jueves 10 de noviembre y estuvo enfocado a personas relacionadas con la gobernabilidad municipal de los humedales costeros. Adicionalmente se participó en dos conferencias relacionadas con humedales y que fueron organizadas externamente. La primera de ellas fue el VIII Festival de Bienvenida de Aves Migratorias, realizada el 18, 19 y 20 de noviembre en la Posada del Parque, y la segunda actividad fue el primer encuentro de Biodiversidad y Medio Ambiente de la comuna de Concón, el cual se realizó el sábado 17 de diciembre, donde se dio la charla titulada “Peces en Humedales Costeros de la Región de Valparaíso”. El proyecto, además, permitió la realización de dos tesis de pregrado ya finalizadas y dos recientemente iniciadas durante el 2023. En conclusión, podemos señalar que la conductividad jugo un factor relevante en la caracterización de los componentes de la macrofauna, donde el aumento de este parámetro genero una reducción de la biodiversidad de este componente. En cuanto a los peces este factor ambiental, más la dominancia de especies exóticas genera cambios en la diversidad funcional lo cual contribuye al aumento de la estabilidad ecológica, lo cual ha sido observado en Mantagua, aunque al mismo tiempo pone en riesgo la estabilidad ecológica, ya que en su mayoría son especies introducidas con hábitos alimenticios generales, que pueden depredar a las especies nativas. Por lo tanto, en un futuro, es probable la extirpación o extinción de estas especies nativas por depredación (o competencia por hábitat) por parte de las especies introducidas aumente dentro del humedal, disminuyendo así, la diversidad funcional que existe y aumentando la homogeneización. Ante esta situación, la educación ambiental y el control de especies exóticas en el humedal se plantean como dos importantes estrategias para revertir los fenómenos de perdida de diversidad. En este sentido, podemos señalar que existe un relevante entusiasmo tanto de organizaciones de base como de funcionarios gubernamentales y municipales de conocer estos grupos, por lo que es pertinente continuar con talleres de capacitación a la comunidad que permita mejorar el conocimiento popular de estos grupos, tan relevantes para este tipo de ecosistemas.
... Published by Elsevier B.V. All rights reserved. Devlin, 2005 ;Lee, 2008 ;Okyere et al., 2011 ;Tantikamton et al., 2015 ;Walag and Canencia, 2016 ). ...
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This revised and updated edition of the bestselling Methods in Stream Ecology reflects the latest advances in the technology associated with ecological assessment of streams. In this second edition, all chapters have been updated and modified to reflect the most contemporary protocols covering 6 vital areas of stream ecology: Physical Stream Ecology; Material Transport, Uptake, and Stora Stream B Community Interactions; Ecosystem Processes; and Ecosystem Quality. Each chapter contains basic methods suitable for teaching undergraduate or graduate students and advanced methods for conducting state-of-the-art research. Suitable as a textbook for a course in stream or river ecology, this book is also a critical reference for professional aquatic ecologists, natural resource managers, and for those entering the field of stream ecology who wish to evaluate the condition of streams or their watersheds.
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