Phytoplankton occurrence and dynamics in rivers are mainly shaped by hydrophysical conditions and nutrient availability. Phytoplankton main structuring factors have been poorly studied in West African rivers, and this study was undertaken to identify these conditions in two tropical rivers that vary in size and human impact. For this, environmental variables and phytoplankton monthly samples were collected from the middle reaches of Asu and Cross rivers during an 18 months survey from March 2005-July 2006. Phytoplankton biomass (F=11.87, p=0.003), Shannon-Weiner diversity and species richness (F=5.93, p=0.003) showed significant seasonality in Asu but not in Cross River. Data was analyzed with Canonical correspondence analysis (CCA) and showed environmental differences between the two rivers, nitrate in Asu River (5.1-15.5mg/L) was significantly higher than Cross River (0.03-1.7mg/L), while PO4 (0.2-0.9mg/L) was significantly lower in Asu River compared to Cross River (0.03-2.6mg/L) (p<0.05). Eutrophic factors (NO3) determined primarily phytoplankton dynamics in Asu River, especially during the dry season, whereas hydrophysical factors (depth, transparency and temperature) shaped phytoplankton in Cross River. Taxa indicative of an eutrophic condition, such as Euglena, Chlorella, Chlorococcus, Ceratium, Peridinium, Anabaena, Aphanizomenon, Closterium, Scenedesmus and Pediastrum spp., were frequently encountered in the shallow impounded Asu River, while riverine species, such as Frustulia rhomboids, Gyrosigma sp., Opephora martyr and Surirella splendida dominated Cross River. A succession pattern was observed in the functional groups identified: Na/MP→TB→P (rainy→dry season) was observed in Asu River, whereas MP/D predominated in Cross River for both seasons. We concluded that, if nutrients predominate hydrophysical factors in shaping phytoplankton during dry season (half of the year) then, they are as important as hydrophysical factors structuring phytoplankton during rainy season (the other half).
Key words: West Africa, phytoplankton, eutrophication, functional group, Cross River, Asu River.
Resumen
La existencia del fitoplancton y la dinámica de los ríos están principalmente determinados por condiciones hidrofísica y disponibilidad de nutrientes. Los principales factores de estructuración del fitoplancton han sido poco estudiados en los ríos de Africa Occidental, y este estudio fue realizado para identificar estas condiciones en dos ríos tropicales que varían en tamaño e impacto humano. Para ello, variables ambientales y muestras ambientales mensuales de fitoplancton se obtuvieron de la parte media de los ríos Asu y Cross durante un estudio de 18 meses, de Marzo-2005 a Julio-2006. La biomasa del fitoplancton (F=11.87, p=0.003), el índice de diversidad de Shannon-Weiner y la riqueza de especies (F=5.93, p=0.003), mostraron estacionalidad significativa en Asu pero no el río Cross. Los datos fueron analizados con el análisis de correspondencia canónica (CCA) y mostró diferencias ambientales entre los dos ríos, el nitrato en el río Asu (5.1-15.5mg/L) fue significativamente mayor que en el río Cross (0.03-1.7mg/L), mientras que PO4 (0.2-0.9mg/L) fue significativamente menor en el río Asu en comparación al río Cross (0.03-2.6mg/L) (p<0.05). Los factores eutróficos (NO3) determinaron principalmente la dinámica del fitplancton en el río Asu, especialmente durante la estación seca, mientras que los factores hidrofísicos (profunidad, transparencia y temperatura) conformaron el fitoplancton en el río Cross. Taxones indicadores de una condición eutrófica, como Euglena, Chlorella, Chlorococcus, Ceratium, Peridinium, Anabaena, Aphanizomenon, Closterium, Scenedesmus y Pediastrum spp fueron frecuentemente encontradas en las aguas poco profundas del río Asu, mientras que las especies fluviales, como Frustulia rhomboids, Gyrosigma sp., Opephora martyr y Surirella splendida dominaron el río Cross. Un patrón de sucesión se observó en los grupos funcionales, identificados: Na/MP→TB→P (Estacion lluviosa → estación seca), fue observado en el río Asu, mientras que MP/D predominó en el río Cross para ambas estaciones. Se concluyó que, si los nutrientes predominan los factores hidrofísicos en la conformación del fitoplancton durante la estación seca (la mitad del año), entonces, son tan importantes como los factores hidrofísicos estructurales del fitoplancton durante la temporada de lluvias (la otra mitad).
Palabras clave: África occidental, fitoplancton, eutrofización, grupo funcional, río Cross, río Asu.
West Africa is located in the tropical region with well defined dry and rainy seasons. Therefore limnological features of rivers are extremely variable between seasons and between small and large rivers, where the factors regulating phytoplankton species composition, size and dynamics show similar variability (Salmaso & Braioni, 2008). High water velocities and turbidity during rainy season limit the phytoplankton chances to transform light and nutrients into algal biomass (Søballe & Kimmel, 1987; Lewis, 1988; Reynolds & Glaister, 1993). Nevertheless, during the dry season, when water velocity attenuates, ]]>
et al., 2011). The interaction between regulatory physical, chemical and biological factors to structure phytoplankton is modified by anthropogenic alterations of rivers such as dam constructions and re-channelization (Soares, Huszar & Roland, 2007). This trend seemingly encourages high development in river middle reaches, due to reduced flux and increased innocula from riparian shallow floodplain lakes and/or back waters (Köhler, 1994; ]]>
Worldwide, phytoplankton are less studied in rivers compared to lakes and reservoirs (Soares et al., 2007), especially in West Africa where there is sparse studies. Available information (Holden & Green, 1960; Egborge, 1973; Nwadiaro & Ezefill, 1986; Chindah & Pudo, 1991; Chindah & ]]>
Studies have shown that unlike in lakes and reservoirs where zooplankton grazing and nutrients are the dominant limiting factors, phytoplankton in lotic environments are directly regulated by hydrophysical factors (Billen, Garnier & Hanset, 1994; Sabater et al., 2008; Perbiche-Neves, ]]>
et al., 1994; Köhler, 1994; Soares et al., 2007). Nutrients are considered to play only subordinate role in determining algal biomass ]]>
Most rivers around the world are ]]>
In this study, we applied both taxonomical and functional group approaches to evaluate (i) species composition, biomass, diversity and seasonal dynamic of the phytoplankton community, and (ii) their dependence on hydrophysical factors and nutrient concentrations in order to identify the main structuring forces in two rivers differing in size and nutrient level. ]]>
3 and PO4) become increasingly important forcing factors to phytoplankton in shallow compared to large rivers.
Materials and Methods
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Study area: The Cross and Asu rivers are located in the South-Eastern part of Nigeria. Cross River system lies approximately between longitude 3°30’ E and 10°00’ E and latitude 4°N and 8°N. The river basin covers an area of 54 000km2 with 14 000km2 in the Cameroon and 39 500km2 in Southern Nigeria. Mean annual discharge at Obubra is 995m3/s with minimum and maximum values of 80m3/s (February) and 3 300m3/s ]]>
Sample collection: Samples were collected from four sites within the middle reaches of the Cross River, Itigidi (CR1), Ozizza (CR2), Ndibe (CR3) and Unwana (CR4) (Fig 1). Two sites were sampled from Asu River, AR1 (middle reaches of the river) and AR2 (reservoir). Surface water samples were collected using modified Von Dorn water sampler in a monthly basis between March 2005 ]]>
4) and nitrate (NO3) were determined in the laboratory according to the methods of APHA (1992) using atomic ]]>
Phytoplankton samples were collected concurrently with environmental data from the different sites, fixed separately in 5% buffered formalin and then taken to the laboratory for identification. Phytoplankton identification to species ]]>
3/L) was then obtained by multiplication of density of each species by the average volume of its cells (Hillebrand, Dürselen, Kirschtel & Pollingher, 1999). Specific biomass was ]]>
3. Species contributing ≥5% to total biomass (Padisák et al., 2003) were sorted into functional groups using the guide of Reynolds et al. (2002) and Padisák, Crossetti & Naselli-Flores (2009). Species richness (SR) was estimated as the number of taxa in a sample and diversity (H’) was calculated using Shannon-Weiner index (Shannon & Weaver, 1963).
Seasonal and spatial difference in environmental and phytoplankton data were tested using a 2-way analysis of variance (ANOVA) and Duncan multiply range test used for post hoc analysis. The relationship between environmental data and the biomass of phytoplankton functional groups was analyzed through canonical correspondence analysis (CCA; Ter Braak, 1986). The null hypothesis of ]]>
Results
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Temperature varied significantly between seasons (p<0.05) from 25.1-31.1°C during the rainy season to 26.7-34.5°C during the dry season. Transparency and water depth varied significantly between seasons in both rivers and between sites in Cross River only. The highest depth (26.7 m) was recorded for site 2 (Cross River) in July 2006. The mean monthly variations in rainfall and depth are shown in Fig. 2. Dissolved oxygen varied ]]>
Table 1).
In Asu River, NO3 was significantly higher during the dry season while variation in PO4 was not. ]]>
3 was not significantly variable between seasons in Cross River but PO4 was significantly higher during dry season (p<0.5). Nitrate values in Asu River (5.1-15.5mg/L) were higher than those of Cross River (0.03-1.7mg/L) while PO4 values were significantly lower in Asu River (0.2-0.9mg/L) than in Cross River (0.03-2.6mg/L). ]]>
A total of 184 taxa from eight Divisions (Chlorophyta, Bacillariophyta, Euglenophyta, Dinophyta, Chrysophyta, Xanthophyta, Cryptophyta and Cyanobacteria) were encountered in the rivers. Chlorophyta was the most abundant and diverse taxa in Asu River (56 taxa, 52.8%) while Bacillariophyta was the most abundant group in Cross River (39 taxa, 42.3%). Species richness was higher in the latter river (106 taxa) compared to the former (92 taxa), only 14 species were common to both rivers.
In Asu River (AR1), Chlorophyta biomass significantly decreased in rainy from dry season values (F=11.3, p H’=0.004) while Bacillariophyta increased significantly (F=14.0, p=0.004) within the same period, a contrary pattern was observed in AR2 (reservoir). But in Cross River, Chlorophyta and Cyanobacteria biomass increased significantly in rainy season (F=186.6, p<0.0001) while Bacillariophyta decreased significantly from dry ]]>
Fig. 3).
Total biomass (F=11.87, p=0.003), Shannon-Weiner diversity (F=5.93, p=0.003) and species richness (F=5.93, p=0.003) varied significantly between seasons in Asu River, however, only species richness exhibited significant variability between sites (F=9.52, p=0.004) in the river. Peak values in species ]]>
Fig. 4). In Cross River, total biomass, Shannon-Weiner diversity and species richness neither exhibited significant seasonal nor spatial variability, though values were higher during the dry compared to rainy season. Peak biomass (>10mg/L) was attained in all sites in the Cross River in February 2006 (Fig. 4f). The lowest species richness, ]]>
Twenty-one (21) functional groups (FG) were identified in both rivers, 15 from Cross and 17 from Asu (including the reservoir), the frequently observed groups are shown in Table 2. During the rainy season, pennate diatoms and desmids from ]]>
Closterium sp.), Na (Cosmarium sp. and Tabellaria sp.), MP (Navicula spp.) and TB (Nitzschia sp.) accounted for more than 95% biomass in Asu River (Fig. 5), while Na (Cosmarium sp.), P (Closterium sp.) and MP (
Frustulia sp.) were dominant during the dry season. In the reservoir, P (Closterium sp.) and H1 (Anabaena sp.) were dominant during the dry season, while the former and S1 (Lyngbya sp.) predominated during the rainy season. In Cross River, MP (Frustulia rhomboides and Gyrosigma sp.), ]]>
Opephora martyr) and TB (Surirella splendida) were predominant during the dry and rainy season. Functional group succession pattern was Na/MP→TB→P in Asu River and P→S1 (rainy→dry season) in the reservoir. In Cross River, MP and D were alternately dominant during dry and rainy season.
Canonical correspondence analysis (CCA) showed that the first two axes accounted for 48.8% of plankton-environment association (Fig. 6). Nitrate (-0.96), TDS (-0.75), conductivity (-0.72), depth (0.69), pH (-0.42) and PO4 (0.35) explained variability in axis 1. Dissolved oxygen (0.73) and temperature (0.40) explained most of the variation in axis 2. Monte Carlo test performed along with CCA showed that the first axis was significant (eigenvalue=0.87, p=0.001), this axis was mainly related to eutrophication factors (high NO3), while axis 2 was attributed to hydrophysical factors (temperature, dissolved oxygen and transparency). Thus, explaining occurrence of diverse taxa indicative of eutrophication such as Euglena sp. (W1), Chlorella sp. (K), Chlorococcus sp., Ceratium sp. and Peridinium sp. (Lo), Anabaena sp. and Aphanizomenon sp. (H1), Closterium sp. (P) and Scenedesmus sp. and Pediastrum sp. (J) in the reservoir and Asu River compared to the less eutrophic Cross River.
]]>
Discussion
Asu and Cross rivers are under the same climatic influence (temperature and rainfall), however, they differ in size and human impact. Impoundment of Asu River modified seasonal flow and water level fluctuations, which appeared to impact the water quality. The river and reservoir increasingly become ]]>
3, conductivity, TDS and increased oxygen demand were observed during the dry periods. Reservoirs are well reported in literature to modify the downstream water quality of rivers (Soares et al., 2007). The opening and closing of reservoir’s gates (for water level regulation) substantially influence water quality and hydrology downstream the river and consequently, could influence the seasonal development of phytoplankton in rivers where dams are constructed. The NO3 (Asu) and PO4 ]]>
et al. 2007) and Po River (Tavernini, Pierobon & Viaroli, 2011).
Peak phytoplankton ]]>
et al., 2007; Perbiche-Neves et al., 2011). The reasons for such elevation in biomass are more directly attributed to efficient utilization of light and nutrient and reduction in algal wash-out (Bukaveckas et al. ]]>
et al., 2011). ]]>
The phytoplankton of Asu River, thus appear to be influenced by the reservoir, which is similar to previous observations (Reynolds & Descy, 1996) on impounded rivers. Increasing importance of Cyanobacteria in reservoirs especially during periods of low precipitation as observed in this study is a common phenomenon (Padisák, 1997; Marinho & Huszar, 2002). The dominant Cyanobacteria in the reservoir, Anabaena sp. could have ]]>
The collapse of the phytoplankton community during the rainy season occurred during the period of high ]]>
et al., 2007). Therefore, the decline in biomass is not limited to low algal productivity but also due to diminution in large-sized species. Hence, during the rainy season, both rivers supported algal growth and selected for a few disturbance and shade tolerant, opportunistic diatoms and desmids belonging to functional groups MP and TB. An analogous observation was also made by Istvánovics, Honti, Vöros & Kozma (2010) and Okogwu & Ugwumba (2012). ]]>
We identified several environmental differences between the two rivers, which include the seasonal water level fluctuation pattern, nutrient availability, transparency, dissolved oxygen, conductivity and total dissolved solids. Such environmental differences are expected to induce differences in phytoplankton community structure and dynamics between the rivers. It is therefore not surprising that there were obvious differences in the dominant phylogenic and functional groups in the rivers. ]]>
et al., 1994; Tavernini et al., 2011), which explains the dominance of MP such as Opephora martyr, Gyrosigma sp. and
Surirella splendida in Cross River. Rocky shorelines are absent in Asu River and the high nutrient value tend to support the growth of eutrophic species such Chlorococcus sp., Ceratium sp. and Peridinium sp. (Lo), Cyclotella sp. (C), Anabaena (H1) and Lyngbya sp. (S1). Differing hydrogeomorphic ]]>
et al., 2010; Tavernini et al., 2011).
It may be arguably correct to assume that hydrologic and climatic factors were the most important ]]>
et al., 2011). However, the importance of eutrophic factors in shaping phytoplankton tends to increase in relevance in the shallow and more eutrophic Asu River, and during dry season as shown by CCA. The increasing relevance of eutrophic factors in Asu River explains the appearance of numerous species (Phacus sp. (W1), Botryococcus sp. (F),
Volvox sp. (G), Rhodomonas sp. (X2) and Chlorella sp. (K)) that commonly occur in shallow eutrophic lakes (Okogwu & Ugwumba, 2009) in the river but not in the Cross River.
We therefore conclude that nutrients are simply as important as hydrologic and climatic factors in ]]>
Acknowledgment
The authors are grateful to the students of Applied Biology, Ebonyi State University for their assistance in sample collection. ]]>
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*Correspondencia a: 1Okechukwu Idumah Okogwu:Applied Biology Department, Ebonyi State University, PMB 53, Abakaliki, Ebonyi State, Nigeria;okeyokogwu@yahoo.com 2Alex O. Ugwumba:Department of Zoology, University of Ibadan, Ibadan, Oyo State, Nigeria; adiaha4me@yahoo.co.nz
Received 24-IX-2012. Corrected 23-III-2013. Accepted 24-IV-2013.