Investigation into the spatial occurrence of wet season phytoplankton and zooplankton in Lagos lagoon, Nigeria was carried out in October, 2008 in 12 stations. A total of 36 species of phytoplankton from 21 genera, 20 zooplankton species from 17 genera and 10 juvenile forms were recorded for the study. The results showed that freshwater conditions within the lagoon were due to high freshwater inflow from adjoining waterways and reduced tidal incursion from the sea. The wet season spectrum of the lagoon was dominated by diatoms for the phytoplankton and copepods for the zooplankton. The presence and array of juvenile forms in the wet season suggest that the lagoon remain a sanctuary, nursery and breeding grounds for aquatic species Crustaceans dominated the spectrum of the juvenile forms.

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Investigations were carried out on spatial and seasonal composition and distributions of phytoplankton in some parts of the Lagos Lagoon. Phytoplankton samples were collected from twelve stations during the wet (July, 2008) and dry (March, 2009) seasons. A typical freshwater condition prevailed in the study area during wet season with salinity of 0 parts per thousand (o / oo) at every station, whereas during the dry season, relatively high salinity levels were recorded from all the stations with decreasing trend toward the river mouth. The two seasons had the diatoms (Bacillariophytes) of the Orders Centrales and Pennales as the dominant phytoplankton taxa. A total of 2896 phytoplankton belonging to twenty-seven species, eight orders and four classes were recorded during wet season while the dry season had a total of 1550 phytoplankton, which cut across sixteen species, three orders and two classes. Wet and dry seasons respectively had Aulacoseira granulata and Coscinodiscus sps. as the dominant phytoplankton taxa. Higher dominance (0.33) and Margalef diversity (3.26) but lower Shanon-Wiener diversity (1.94) and evenness of distribution (0.26) indices were recorded during the wet than dry seasons. The findings show the influence of seasonality on phytoplankton composition and distributions in the Lagos Lagoon and suggest relatively stress-free environmental conditions within the study area.

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ABSTRACT:The Lagos lagoon is a coastal lagoon and hence, is highly impacted by pollutants from anthropogenic sources. The greater percentage of these pollutants is organic in origin. Hydrochemistry and benthic fauna assemblage of five anthropogenically impacted sites of the Lagos lagoon, Nigeria were studied from May to October, 2016 to investigate the effects of organic pollution on them. Water and composite benthic samples were collected monthly at each study station and analysed in the laboratory following standard procedures. The analysis of variance (ANOVA) indicated no significant difference (p > 0.05) in values of the water parameters measured among the study stations. Higher pH values were recorded at Carter Bridge sampling station and this could be attributed the introduction of carbonates into the water through the washing of cement bags. Salinity of the water was generally low due to the season of the sampling. Introduction of large volumes of biodegradable wastes both reduced the dissolved oxygen level and increased the total suspended solids in the waters of critical sites. A total of 243 benthic macrofauna individuals comprising three Phyla, four Classes, four Orders, eight Families, eight Genera and eight Species were sampled. The gastropods, Pachymelania aurita and Tympanotonus fuscatus dominated the benthic macrofauna assemblage of the study area accounting for 40.3% and 19.3% respectively. The relatively low abundance of the filterfeeding bivalves and the highly sensitive arthropod species in the study area may be indicative of organic pollution of the area. ©JASEM https://dx.doi.org/10.4314/jasem.v21i2.2

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The water chemistry and macrozoobenthos assemblage of ten study stations in the Lagos lagoon were studied from December 2016 to May 2017 to assess the impacts of sediment mining on the water quality and the biological indices of the macrozoobenthos. Water and composite benthic samples were collected monthly at each study station and analysed in the laboratory following standard procedures. Except temperature, pH, TSS, salinity and conductivity showed no significant (P>0.05) difference, while the other parameters were not significantly (P<0.05) different among the stations. The sediment grain size analysis of the study area indicated the dominance of sand in sediment. This could be as a result of the dredging of the study area as Lagos lagoon is originally known to have muddy substratum. A total of 1,237 organisms belonging to 3 phyla, 4 classes, 10 families and 10 Species were recorded during the study period. Analysis of benthic community structure of the study area reveals a community dominated by mollusks, with the Bivalve, Aloides trigona contributing 54% and the gastropod, Pachymelania aurita contributing 33% of the total benthic fauna assemblage during the period of study. The fluctuations in the physicochemical parameters, sediments, and the composition, abundance and diversity of the macrobenthic fauna of the study area were largely influenced by the anthropogenic activities. In particular, stations with pronounced sediment mining activities recorded highly turbid water, changed substratum type and defaunisation.