MATERIALS AND METHODS

Description of the study area: The study was carried out at Lagos Lagoon from January-October, 2017. Lagos Lagoon is in the South-Western region of Nigeria and it lies between longitudes 3°20” and 3°40” E and latitudes 6°15” and 6° 40” N" (Fig. 1) and is generally between 0.5-2m deep. The analysis was carried out at the biological oceanography laboratory of the Nigerian Institute for Oceanography and Marine Research, Lagos.

Fig. 1:

Map showing sampling locations in Lagos lagoon

Collection and analysis of water samples: Surface water samples were collected from 9 stations of Lagos Lagoons namely: NIOMR Jetty, Atlas Cove, Bonny Camp, Parkview, Lekki, Ajah, Sagbo koji, Apapa I and Apapa II, with 1dm³ water samples from all the sampling stations and stored in 1L water bottles and analysed in the laboratory for pH, conductivity, salinity and turbidity using a multi-meter water checker (Horiba U12). Another water sample was collected separately in a 250 mL dissolved oxygen bottle and fixed with Winkler solution at each station. This was followed by dissolved oxygen estimation using Iodometric Winkler’s method. Air and surface water temperature were measured in situ using mercury-in-glass thermometers. The alkalinity of the water samples was determined by titrating dilute Hydrochloric acid against 50 mL of the water sample using methyl orange as an indicator.

Collection and investigation of plankton samples: Plankton samples were collected from each station with a 55 μm mesh size standard plankton net held against the current of the subsiding tide for 10 min. The net was then hauled in and the sample was transferred to a 250 mL well labelled plastic container with a screw cap and preserved with 4% unbuffered formalin and stored in the laboratory before microscopic analysis. In the laboratory, five drops (using a dropper) of the concentrated sample (10 mL) was investigated at different magnifications (50, 100 and 400×) using a Wild II binocular microscope with calibrated eyepiece and the average recorded. The identification and estimation of phytoplankton abundance were done according to the method described by Emmanuel and Onyema¹⁴.

Plankton community structure analysis: Diversity (total number of phytoplankton taxa) and total abundance of phytoplankton for every station were determined using data from identification and counting. The composition, distribution and abundance of phytoplankton of the entire study area were determined for each season by pooling the data for all the stations.

Data analysis: The phytoplankton data obtained were pooled and analysed empirically and ecologically. Diversity indices: Margalef species richness (d) diversity, Equitability, Berger Parker dominance index and Shannon Wiener diversity (H) of the entire study area were computed for each season as described by Ogbeibu¹⁵ using a ‘Past’ software package¹⁶.

RESULTS

The result of the physico-chemical parameters is presented as shown in Table 1. The mean temperature ranged between 28.9°C and 31.4°C during the period of sampling, with the highest temperature recorded in April and the lowest temperature recorded in July. Dissolved oxygen was highest at the NIOMR jetty-station1 with 7.39 mg L^–1 and lowest at Apapa II with 4.35 mg L^–1. Turbidity ranged between 18.25-41.5 NTU during the sampling period. Salinity ranged between 12.5 and 24 ppt while pH had a range of 7.9-8.2. BOD had the highest value of 5.44 mg L^–1 and the lowest value of 3.16 mg L^–1 while the Nitrate level was low throughout the sampling period, ranging between 0.14 and 0.36 mg L^–1. Sulphate value recorded 652 mg L^–1 as the highest value and 199.33 mg L^–1 as the lowest value. Phosphate levels also ranged between 7.11 and 12.25 mg L^–1 and silicate values ranged between 9.67 and 20.33 mg L^–1.

Table 2 shows the spatial phytoplankton compositions in some selected parts of Lagos Lagoons, consisting of six classes (Cyanophyta, Bacillariophyta, Dinophyta, Euglenophyta, Charophyta and Chlorophyta) and sixty-eight different species (Table 2).

Fig. 2:

Total percentage composition of taxa-species in each sampling station

Biological characteristics
Phytoplankton community structure: The species composition, distribution and diversity of the phytoplankton communities are presented in Table 3. A total of 68 species were recorded during the studies. Bacillariophyceae had 49 species, representing 72.1% of the phytoplankton community, Cyanophyceae had 13 species representing 19.1%, Dinophyta had 2 species representing 2.9%, Euglenophyceae recorded two species also, accounting for 2.9%, Chlorophyta recorded one species, representing 1.5% and Charophyceae recorded also one specie, representing 1.5%. Bacillariophyta was the most diverse class, followed by Cyanophyceae, then Dinophyceae and Euglenophyceae, contributing equally to the phytoplankton community, followed by Chlorophyceae and Charophyceae.

Table 4 shows the part of Lagos Lagoons under study consisting of six different phylum of phytoplankton, their numerical abundance per station and total numerical abundance in the study. The numerical abundance and variation of the phytoplankton in part of Lagos Lagoon during the period of study are shown in Table 4.

The number of species found in each station had station one recording the highest percentage of phytoplankton communities in the sampling period (Fig. 2). Station eight recorded the lowest percentage number of different species of phytoplankton communities (Fig. 2) which invariably would be as a result of low dissolved oxygen in that station and thereby not enough oxygen to support life. Station five recorded the highest percentage of total phytoplankton abundance, considering number of species and individual phytoplankton abundance (Fig. 3), while station nine recorded the lowest percentage of individual phytoplankton abundance.

Fig. 3:

Percentage composition of species and individual abundance of phytoplankton in Lagos Lagoons

DISCUSSION

Temperature plays an important role in controlling the abundance of phytoplankton as it influences the lives of aquatic organisms and the physico-chemical parameters of the water body¹⁷. Temperature showed variation between seasons with the maximum recorded in Ajah during the sampling period at 31.4°C , while 28.9°C was the lowest recorded temperature at NIOMR JETTY, this could be attributed to the rainfall during that period.

Dissolved oxygen on average was highest in station 1 (NIOMR Jetty 7.39 mg L^–1) and lowest in station 9 (APAPA II 4.35mg L^–1) during the sampling period. This value in APAPA II is lower compared to the National Environment Standards and Regulations Agency (NESREA) limits of 5.0 mg L^–1 for surface water. The low dissolved oxygen and high biological oxygen demand values recorded in APAPA ll are likely pointers to pollution stress the area is exposed to. Station 9 (Apapa ll) has in its surrounding environment a lot of industrial and municipal activities. The consequence generated waste and effluents are discharged into the surrounding water body, thus the decrease in dissolved oxygen. This agrees with the observation of Adesalu and Kunrunmi¹⁸ who worked on Tomaro and Ajegunle creek and suggested that low dissolved oxygen in that study site could be due to the water body receiving effluents containing oxygen-demanding substances, alongside domestic sewage. An inverse relationship was observed between phytoplankton and dissolved oxygen in station 9 (Apapa ll) and the dominant species in this station was Microcystis aeruginosa.

Turbidity was highest in station 6 (AJAH- 41.5 NTU) and lowest in station 3 (BONNY CAMP-18.25 NTU). The water samples were observed to be more turbid during the rainy season and the influx of particulate matters brought into the lagoon by surface run-off and flood must have caused the higher turbidity during this period and this agrees with the findings that re-suspension of dissolved materials due to surface run-off caused by the rains increases turbidity¹⁹.

Biochemical oxygen demand refers to the amount of oxygen used by microorganisms in the aerobic oxidation of organic matter, BOD is widely used to indicate the organic strength of the water. In the present study, a higher value of BOD was reported as 5.44 mg L^–1 at station 7 SAGBO KOJI and the lowest at station 5 (LEKKI). The recorded high BOD values at station 7 (SAGBO KOJI), maybe due to the influx of organic sewage from anthropogenic activities, wastewater discharges and/or agricultural activities, inviting the presence of several microbes in the water body and accelerating metabolic activities with the increase in the concentration of organic matter and this is in total agreement with Khalid et al.²⁰. Moreover, BOD levels between 1-2 mg L^–1 or less signifies clean water, 4-7 mg L^–1 denotes slightly or moderately polluted water and more than 8 mg L^–1 denotes severe pollution²¹.

Relatively low values of nitrate were observed from this study and the concentration of nitrate varied averagely between 0.14-0.36 mg L^–1, with the highest at station 4 (PARK VIEW) and lowest at station 7 (SAGBO KOJI). A similar observation of low nitrate level has been recorded²².

Sulphate enters into the water body from the catchment area through surface runoff. Generally, sulphate concentration was observed to be high during the study. In the study area, the minimum amount of sulphate was recorded in station 6 (AJAH 199.33 mg L^–1), while station 2 (ATLAS COVE 652 mg L^–1) recorded the highest amount of sulphate. The concentration of sulphate recorded in the study could be attributed to saline water intrusion and the absence of thiosulphate bacteria, that uses sulphate. Municipal drainage and industrial effluent could also contribute to the increased sulphate level recorded. However, the highest phosphate concentration was recorded in station 6 (Ajah- 12 mg L^–1) and the lowest phosphate concentration was recorded in station 1 (NIOMR Jetty- 7,11 mg L^–1).

Reactive silicate value was highest in station 9 (APAPA II) and lowest in station 1(NIOMR JETTY). The high silicate value recorded in the study site would have been as a result of an influx of land drainage that is rich in silicate into the sea, during the rains. The low silicate value observed in this study could be attributed to its uptake by phytoplankton for their biological activities. This can be seen in the higher phytoplankton biomass and recorded in station 1 (NIOMR JETTY), in comparison to a lower phytoplankton occurrence in station 9 (APAPA II) and this observation was in contrast with the work of Effiong and others²³ who observed reactive silicate to be less in months with high phytoplankton biomass.

Relating to the number of species found in each station, station one (1) recorded the highest percentage of phytoplankton communities in the sampling period (Fig. 2), while station (8) recorded the lowest. This might have been as a result of the higher dissolved oxygen which is a pointer to increased photosynthensis rate by the phytoplankton in this station. This agrees with the findings of (Abolude et al., 2012)²⁴, that a water body with a higher dissolved oxygen will support more life. Station eight (8) record of lowest percentage number of different species of phytoplankton communities would be as a result of low dissolved oxygen in that station and thereby no enough oxygen to support life. In terms of the number of specie and individual phytoplankton abundance, station five (5) recorded the highest percentage of total phytoplankton abundance present (Fig. 3), while station nine (9) recorded the lowest. The higher specie abundance recorded in station five (5) could be as a result of increased temperature, especially with the presence of the blue green algae which tolerate high temperature range²⁵. The lowest percentage of specie abundance recorded in station nine (9) would have been as a result of the low dissolved oxygen to support aquatic life.

Phytoplankton plays an important role as producers in the aquatic ecosystem and are useful indicators of water quality. The phytoplankton community in this study was characterized by six classes and sixty-eight species. Their contribution in terms of species abundance are thus, Bacillariophyceae>Cyanophyceae>Dinophyceae and Euglenophyceae>Chlorophyceae and Charophyceae. Bacillariophyceae dominated the sampling period, with Coscinodiscus being the most abundant species and this in station 4 (park view). The abundance of Bacillariophyta in terms of species in this study could be attributed to the fact that it was observed their population increased with increased silica levels and other nutrients like phosphate and nitrate²⁶. A similar observation of Bacillariophyceae as the dominant species in the Imo river have been recorded²⁷. The predominance of diatoms in the study could be attributed to alkaline pH and high levels of the nutrient.

Cyanophyceae one of the major groups of phytoplankton is mostly confined to the freshwater zones. Twelve species of Cyanophyceae were recorded during the sampling period, with Microcystis aeruginosa being the most abundant species among them. The numerical dominance of Microcystis aeruginosa has been reported earlier in the South-Western part of Nigeria as being prevalent in freshwater conditions²⁸. The relative abundance of Cyanophyceae in the study could be attributed to the alkaline condition, nutrient-rich freshwater discharge, high temperature and turbidity as a result of suspended solid that will enhance their growth¹³.

The class Dinophyceae was represented by two species Ceratium furca and Ceratium fusus. Their distribution is influenced by pH, salinity and temperature, as most of them are found in areas with marine influence²⁹. Dinophyceae was few in abundance in this study. This could be as a result of their oligotrophic nature and their competitive rate with diatoms³⁰. Euglenophyceae also was represented by two species in the study Euglena spirogyra and Trachelomonas volvocina. Representatives of this class inhabit both freshwater basins and marine waters and they are known to thrive in waters rich in organic matter that is also exposed to anthropogenic eutrophication³¹.

The class Chlorophyceae was represented by one species Scenedesmus brasiliensis. Their very low occurrence in the study area could be attributed to the influence of salinity as its occurrence was only in station 5 (Lekki), with a reduced salinity value. High turbidity value in the station, is also a limiting factor to its thriving and occurrence. Charophyceae was represented by one species Elakatothrix gelatinosa. Their low number of occurrences could be a result of increased turbidity which can rip them off with the strong tide and high nutrient concentration³².

The importance of species diversity indices in phytoplankton studies is their usage in the assessment of pollution and productivity of a water body. Examining the diversity in the range of polluted and unpolluted ecosystems, the Shannon-Weiner diversity index proposed that a diversity index greater than (>4) is clean water, between 3-4 is mildly polluted water and less than 2 is heavily polluted water³³. However, the highest values of the Shannon-Wiener Index of 2.794 was recorded in station 1 (NIOMR JETTY), while the lowest value of 0.832 was recorded in station 5 (LEKKI). The Shannon-Wiener diversity index in this study ranged between 0.832-2.794 and is, therefore, implies that the water body is in between moderately polluted to highly polluted. Station 5 (LEKKI) with the lowest value of the Shannon-Weiner index also recorded the highest species density of Microcystis aeruginosa, which could contribute also to the pollution status of the station. Margalef’s indices have been used to determine the phytoplankton species richness and the index states that the index values greater than 3.0 signify clean water conditions, values less than 1.0 indicates severe pollution and intermediate values indicate moderate pollution³⁴. Based on the overall values obtained in this study, some sections of the study area is being threatened by pollution as the Margalef value ranges between 1.13-5.54.

The equitability index is a measure of the evenness with which individuals are divided among the taxa. It reflects the equitable abundance of various species throughout the study period. The value lies between 0 and 1, 1 representing complete evenness. The closer the value to 1, the more even the population of phytoplankton species that makes up the community. In the present study, equitability ranged between 0.3-0.91 indicating that the phytoplankton was not evenly distributed, due to the dominance of two classes, Bacillariophyta and Cyanophyta. It was only in station 9 (APAPA II) and station I (NIOMR JETTY) that there was even distribution of the phytoplankton. It ranged between 0.3 in station 5 (LEKKI) and 0.913 in station nine (APAPA II).

CONCLUSION

This study showed the occurrence and diversity of phytoplankton are influenced by environmental variables within the Lagos Lagoon. Nutrients are discharged into the water body via anthropogenic activities and surface run-off. These factors alongside the presence of phytoplankton indicator species reveal the pollution level of the study area. The study revealed that the diatoms were more diverse in terms of phytoplankton species but the blue-green algae dominated in terms of phytoplankton density. Their abundance, alongside the occurrence of pollution indicator species such as Nitzschia, Navicula, Euglena, Spirogyra contributed adversely to the water quality. The occurrence of some of these indicator species also shows that activities that could pose a threat to the aquatic ecosystem like the indiscriminate discharge of municipal waste, discharge of untreated sewage and industrial effluents are ongoing and should be monitored so that both aquatic and human life will not be at risk.

SIGNIFICANCE STATEMENT

This study discovers the level of contamination and pollution threat of aquatic life in some parts of Lagos Lagoon. This novelty work has clearly defined the significance of species diversity indices in phytoplankton studies in the evaluation of water body productivity and pollution assessment. This study will help many other researchers to unearth the critical area of Lagos Lagoons that is under threat of pollution and possibly propose a way forward to mitigate it thus creating a new frontier.