Research Article
A Checklist of Lekki Lagoon Diatoms
Department of Botany and Microbiology, University of Lagos, Nigeria
D.I. Nwankwo
Department of Marine Sciences, University of Lagos, Nigeria
Diatoms are valuable indicators of environmental conditions, since they respond directly and sensitively to many physical, chemical and biological changes that occur in the aquatic environment. Among unicellular microalgae, diatoms probably represent one of the most diverse groups, with a number of species estimated to be between 10000 and 100000 (Paula et al., 2005) hence, they constitute an ideal group to study its biodiversity. The diversity, abundance and distribution of phytoplankton within any lagoon have a direct correlation with the water quality and consequently the whole community structure. This is due to the fact that phytoplankton forms the base of any aquatic food chain and organic production in the lagoon and coastal ecosystem (Carol and Timothy, 1993). According to Van Dam et al. (1994), the composition of diatom communities reflects an entire complex of ecological parameters at a particular site. Lagoon and coastal ecosystems are the most productive zone of any marine environment due to the high anthropogenic inputs and shallowness of this zone which allows effective light penetration for photosynthesis by phytoplankton. The bulk of local fish production comes from the artisanal sector operating within this zone in Nigeria. Yet, the lagoonal environments are being highly influenced by ecological factors and human actions mainly from refuse and sewage dump, as well as agricultural wastes coming from river discharges and industrial effluents. The Nigerian coastal zone experiences a tropical climate consisting of rainy season (April to October) and dry season (November to March), it is low lying with heights of not more than 3.0 m above sea level and is generally covered by fresh water swamp, mangrove swamp, lagoonal mashes, tidal channels, beach ridges and sand bars. Lorghurst (1964) reported that the Nigerian coastal surface water is uniformly warm (about 28°C) and of low salinity (<32%). The vegetation is also characterized by Mangrove forests, Brackish swamp forests and Rain forests. At present, there is no such checklist for Lekki lagoon diatoms hence this study lists diatoms species of this lagoon and also mentioned the species that is first record to Nigeria coastal waters.
DESCRIPTION OF STUDY AREA
Lekki lagoon (Fig. 1), a large expanse of shallow freshwater extends between Lagos and Ogun States. It covers an area of about 247 km2. A greater part of the lagoon is shallow (<3.0 m), while some areas are up to 6.4 m deep. It lies between longitudes 4°00 E and 4°12 E and latitude 6°25 N and 6°37 N. The lagoon is fed by river Oni in the North eastern part, while rivers Osun and Saga flow into the North western part. Two peaks of rainfall are associated with this lagoon, a major peak in July and a lesser peak in September. There are two peaks of sunshine hours which approximately correspond to the equinoxes. The mainstay of communities that live around this environment is artisanal fishing.
Fig. 1: | Lekki lagoon showing sampling stations |
A motorized boat and a Global Positioning System (GPS) were used during the 24 months sampling period (June 2003-May 2005). The sampling stations were presented in Table 1 and these stations were chosen to reflect differences in biological characteristics which exist in the same body of water. All samples were taking using standard plankton net of 55 μm mesh size towed steadily for 10 min at low speed and preserved in 4% unbuffered formalin in appropriately labeled plastic container.
Table 1: | Average depth (m) and geographical position of sampling stations |
All samples were collected during the hours of daylight to minimize variations due to diurnal migration. To enhance diatom identification sub samples of the original samples were acid-cleaned using nitric acid and investigation was made using Olympus BX51 photomicroscope at the Bowling Green State University, Ohio, USA. Taxanomic keys employed in the identification included Hustedt (1930-1937, 1942, 1971), Patrick and Reimer (1966, 1975), Prescott (1961, 1973, 1982), Cox (1996), Round et al. (1990), Krammer and Lange-Bertatot (1986, 1988, 1991a, b, 2000) and John and Robert (2003).
Community structure analysis: Three indices of statistical analysis were used to obtain estimates of species diversity in the samples analyzed; species richness index (Margalef, 1970), Shannon and Weaver (1963) and species evenness (J) (Pielou, 1975).
A total of 237 diatom taxa belonging to fifty genera were observed in this study (Table 2), two hundred and three were pennates while thirty four are centric forms.
Table 2: | Diatoms checklist with composition abundance (mL-1) and community structure of Lekki lagoon |
*: New record to Lagos lagoon complex, **: New record for Nigeria |
Fig. 2: | Variations between diversity indices at Lekki lagoon |
The pennate diatoms observed include Fragilaria construens, Bachysira follis, Bacillaria paradoxa, Craticula cuspidata, Synedra ulna var. longissima and Tabellaria fenestrata (Araphidineae) while centrals were ably represented by Aulacoseira granulata, A. granulata var. anguitussima, A. granulata var. anguitissima f. curvata, A. granulata var. anguitissima f. spiralis, Cocconeis disculus, C. scutellum and Cyclotella stelligera. Since, Margalefs d-value is influenced by the number of species and individuals, high d values period reflected high species number and relatively low numbers of individuals. The Shannon-Weaver diversity index (H1) is influenced by both number of species and equitability. In Lekki lagoon, higher H1 values observed could be attributed to high j at that period (Fig. 2).
The number of diatoms taxa (237 species) observed in Lekki lagoon is very high compare to other published study from other lagoons and rivers so far. This checklist is the first that will record such large number of diatoms. The pennales recorded higher number of taxa followed by centrals. This is different from the species composition and phytoplankton abundance density found for some diatom Lagos lagoon (Nwankwo, 1990), some coastal waters of Nigeria (Kadiri, 1999) and Bonny River during complex research study concerning ecosystem of Niger Delta (Onuocha, 1985; RPI Report, 1985). It was the bottom dwelling species that are more prevalent and could be as a result of rainfall which introduces flooding thereby mixing up the water, boat navigation since artisanal fishing is the mainstay of the people living around the area or may be due to their possession of raphe with which they adhere to suitable substrate. From the checklist it can be stated that Lekki lagoon is highly diverse floristically with Aulacoseira sp. being abundant species all through the stations.
We acknowledge the Fulbright exchange program for the scholarship awarded to Adesalu T. A., Prof. Rex Lowe of Bowling Green State University Ohio, USA for his assistance and Bowling Green State University Ohio, USA for accepting the placement of Adesalu T.A.