The results of surface water physicochemical parameters obtained from Lagos Lagoon showed clearly the impact of land reclamation activities on this waterbody. Spatially, the temperature values ranged from 28.60°C to 36.10°C during the study period with the highest mean temperature observed at Site II (35.97 ± 1.16°C), similar to the work of Oyeleke et al., (2019) who recorded high-temperature ranged of 27°C to 30°C in Lagos Lagoon. The temperature range obtained from this result could be due to the impact of land use or anthropogenic activities within and around this waterbody as well as effect of climatic change which could affects water resources through changes in evaporation, temperature, runoff and rainfall pattern that can affect the mobilization of nutrients, distribution and mobility of pollutants in aquatic ecosystems. The direct and indirect climate change impacts on water quality comprise biological, physical and chemical changes include pathogenic microbes in water, increased water temperature, more stable vertical stratification and less mixing of water of deep-water lakes, and changes in water discharge, affecting water level and retention time and increased nutrient concentrations, water color and decreased oxygen content. The pH range of 6.41–8.69 observed in the study aligns with the report of Emmanuel and Chukwu (2010) who recorded a range of 6.00–9.33 in a freshwater lagoon Lagos. The low pH mean value record at Site II could be due to human activities (dredging, dumping municipal waste, landfilling) at this site, also agrees with the work of Onyena and Okoro (2019) reported that water with little change in pH is generally more conducive to aquatic life. The highest values of turbidity, TSS, BOD, phosphate, nitrate, sodium, magnesium, potassium, lead, chromium, nickel, zinc, and cadmium were observed in Site II could be due to anthropogenic stressors that caused an increase in all these parameters over a period of time as well as an alteration in water hydromorphological, led to the accumulation of organic compounds, and other contaminants (Chen et al., 2013). The high mean values of salinity, TDS, COD, copper, THF, and THUB (15.34 ± 1.29 ppt, 9277 ± 536.42 mg/L, 47.48 ± 9.41 mg/L, 0.12 ± 0.07 mg/L, 1186.25 ± 516.60 cfu/ml and 205.25 ± 46.06% respectively) at Site III and THC, DO, iron and total coliform count were higher in Site IV could be attributed to the run-off from the rivers which receive waste materials from surrounding community and empty it into the Lagoon can contribute to the high concentration of these parameters.
Seasonally, the high mean values of temperature, electrical conductivity, nitrate, and THB (38.10 ± 3.66°C, 61842.83 ± 11777.45 µS/cm, 6.09 ± 3.70 mg/L and 38174.83 ± 79509.21 cfu/ml) observed at Site I during the dry season may be due to water concentration as a result of a reduced influx of fresh water into the Lagoon and/or higher evaporation in this season (Nkwoji and Edokpayi, 2013). The low mean DO concentration recorded at Site II could be due to landfilling materials which are high in organic matter use at this site and can cause an increase in anaerobic organisms’ activities resulting in oxygen depletion during decomposition (Nkwoji et al., 2014). The high mean values of pH, turbidity, BOD, chloride, calcium, and magnesium (7.99 ± 0.05, 10.33 ± 0.40 NTU, 70.80 ± 96.84 mg/L, 11163.33 ± 1667.10 mg/L, 457.57 ± 425.27 mg/L, 0.14 ± 0.10 mg/L) were recorded at Site II during the rainy season might be attributed to high water influx during this season. The mean values of phosphate, calcium, magnesium, potassium, nickel, zinc, cadmium, and THUF were high at Site II during the dry season while a high concentration of ions recorded in the dry season can be governed by the transport of mineral salts leached from the soil into the water during the dry season (Nkwoji and Edokpyi, 2013). The mean concentration of Cr, Ni, Zn, and Cd were higher at Site II during the dry season might be due to the introduction of heavy metals released from petrol chemical products from heavy machines or through other man-made activities at this site. The high concentration of cadmium recorded during the period of this study is one of the most toxic metals that may endanger the growth and development of aquatic organisms (Daive et al., 2010). High BOD recorded during the rainy season could be due to the consumption of oxygen available in the water by the bacteria during the breakdown of organic matter. The high turbidity observed during the rainy season at Site II could be attributed to the high influx of runoffs with high debris, suspended particles, and disturbance at the bottom sediment causing water cloudiness. The mean total hydrocarbon (THC) was higher at Site IV during the rainy season could be a result of petroleum hydrocarbon discharge by ships used for transportation or cooling water discharge into the waterbody by a thermal power station.
The sediment texture results showed a low content of silty clay with average particle composition of % sand dominant in all the stations followed by % silt and % clay which of typical Lagoon sediment. With this typical sediment texture, it provided a large selection of habitats for bottom dwellers or bottom-oriented organisms to burrow into the sediment in unfavorable conditions. Significantly, higher % sand and % silt was recorded in the rainy season compared to the dry season could be attributed to high siltation or/ coarse sediments occurring during the rainy season due to the heavy rainfall from inland water into the Lagoon as well as the low nature of the gradients of the slope of deposition, the higher energy of water movement and coupled with the overland runoffs mostly during the rainy season (Ong et al., 2012). The sediment pH means of 6.87 ± 0.19 was observed at Site II fell within the acidic range could probably due to the decomposition of organic matter which releases organic acidity into the sediment during the process which results in a decrease in the sediment pH concentration as a result of the oxidation of sulphide as a result of the oxidation of the anoxic material, which increases microbial (thiobacteria) activity (Wondim and Mosa, 2015; Reible et al., 2002). Sediment disturbance during land reclamation alters the chemical composition of the sediment, such as an increase in dissolved oxygen (DO), which results in a rise in the redox potential (Eh). The higher organic carbon recorded in the dry season may also be attributed to organic debris arising from the decomposition of mangrove-dropping litter leaves, roots, and dead aquatic plants and animals. The high mean conductivity values recorded in this study might be due to the high concentration of ions while low electrical conductivity values indicate that there is a high presence of silicate materials in the sediments (Swant et al. 2011). The high Pb mean obtained from sediment at Site II was positively related to an increase in temperature, reduced pH, and microbial activity, but seemed to be independent of Pb concentration. Lead reaches the aquatic environment through industrial and municipal discharges, atmosphere deposition, weathering processes in areas of natural Pb mineralization, and in highway runoff. Among aquatic biota, Pb accumulates in organisms like algae, and benthic organisms and is lower in upper trophic predators. Lead is toxic to all phyla of aquatic biota through effects that are modified significantly by various biological and abiotic variables. The exchangeable cations (Ca2+, Mg2+, K+, and Na+), and anions (Cl−) concentrations were higher in the dry season than rainy season except for the sodium which could be due to run-offs from weathering of rock deposits in sediment (Swant et al 2011). The mean concentration of Pb, Cr, Ni, and Cd were relatively high at Site II in the dry season except for Ni which was higher in the rainy season and could be due to petrochemical products used in fueling the heavy equipment at this site that contaminates this habitat. The elemental concentration of sediments not only depends on anthropogenic and lithogenic sources but also on the textural characteristics of organic matter content, mineralogical composition, and depositional environments of sediments (Parizangandi (2008). The highest mean of total heterotrophic fungi, total coliform, total hydrocarbon-utilizing bacteria, and total hydrocarbon-utilizing fungi was recorded at Site II during the dry season and THB was higher at Site III. The results obtained from this present study revealed that the distribution of heterotrophic bacteria and fungi in the study area is influenced by several factors. The highest bacterial density was obtained in the dry season while the highest heterotrophic fungi were higher in the rainy season. The bacterial density is largely dependent upon the amount of utilizable organic matter in the sediments, which in turn is largely controlled by the sedimentation and degradation rates. It is evident that the minimal hydrodynamic circulation during the dry season allows the deposition of organic matter on the bottom sediment. The result confirms the report of Cavallo et al. (1999) that bacterial community in sediment did not vary with seasons. Several studies have revealed that the bacterial community composition in hydrocarbon-polluted sediments tends to comprise mostly bacteria that are specially adapted to use hydrocarbons as carbon sources (Kasai et al., 2002). Microbial degradation such as bacteria, yeast, and fungi are the major ultimate natural mechanism of clean-up the petroleum hydrocarbon pollutants from the environment. The primary effects of petrochemical e.g., diesel fuel toxicity were reductions in species richness, evenness, and phylogenetic diversity, with the resulting community being heavily dominated by a few species.
Benthic macroinvertebrate assemblage showed instability in the abundance across all the stations may be due to the impact of anthropogenic activities, fluctuation of the salinity regime, types of substrata, and abiotic and biotic factors. The disturbed of coastal water as altered the coastal balance, changed the current pattern, waves, quality of the water, alter sediment characteristics, release contaminants, increase nutrients mobility as well as landfilling extra material for reclamation in the waterbody. Gastropoda recorded the highest percentage contribution of 39.12% followed by Polychaeta account 30.34%, while Malacostraca contribution of 2.63% recorded during this study could strongly depend on habitat characteristics and spatial and temporal variability (Gulkekin et al. 2019). Nereis Sp has the highest species occurrence of 89 indiv/m2, followed by Fistubalanus pallidus (72 indiv/m2), and Clibanarius africanus with the least species occurrence (12 indiv/m2). The low abundance of benthic macroinvertebrates recorded could be attribute of the nature of the substratum or other anthropogenic activities which affect their distribution. The low abundance of species observed at Site II in the dry season (99 indiv/m2) compared to the rainy season could be attributed to some ecological imbalance arising from alterations of some important factors governing the distribution of benthic communities such as water quality, substrates type, food availability and the extremely high turbidity at the study could be responsible for the poor macroinvertebrate abundances (Nkwoji et al. 2010). The absence of some species such as Neritina glabrata, Neritina senegalensis and Polydora ciliata maybe due to their sensitivity to heavy pollutants associated with this region of the waterbody. The opportunistic pollution-tolerant macroinvertebrate taxa recorded at these stations are known to thrive in organically polluted areas of an aquatic environment. High occurrence of Tympanotomus sp, Pachymelania aurita, Physa fontinalis, Fistubalanus pallidus, Andara sp and Penaeus notialis recorded in both seasons may be attributed to dominant taxa found associated with grossly polluted with high organic matter, trace metals and petroleum hydrocarbons due to tolerant of low oxygen tensions (Ajao, 1990). Pachymelania aurita was the most abundant at Site II might be due to mode of life indicative of a preference for exposure and short excursion out of the shell for feeding and other activities which are better undertaken on the mudflats rather than in the shallow water. The finding of this work is similar to reports of Nkwoji et al. (2010) that Capitella sp, Nephyts sp, and Leeches sp were the most abundant and can be influenced by the type and organic metals and hydrocarbon content of the sediment. The polychaeta are the most dominant group because they are pollution-tolerant making them a good indicator species for polluted water. Although estuarine macrozoobenthic communities are well known to be dominated by polychaetes and some species of oligochaetes due to their euryhaline capacities (Chainho et al., 2006), studies have shown that their high dominance relative to other organisms due to anthropogenic disturbances that can cause water quality deteriorate thereby engendering a shift in macrofauna composition in favour of the more tolerant taxa. The ecological indices recorded in this study revealed low levels of richness, diversity and evenness of benthic macroinvertebrates in the stations where is high human activities. The low Shannon-Weiner index (H) values recorded at Site II was within the normal range of 0.0–2.53, decreasing with the increasing stress on the aquatic ecosystem (Magurran, 2004). Likewise, the Evenness index which increases with decreasing ecosystem stress was relatively higher at Site II but still far above the ideal metric value of 1 which is indicative that all species of macroinvertebrate population in the system are not proportions distributed. This implies that the species composition was similarly exhibited by the dominance of pollution-tolerant taxa in this waterbody over other sensitive taxa. The biotic index used in this study provided efficient results especially BMWP and ASTP were both successful in separating the stations. Quality evaluations of biotic indices are not always compatible with each other since the value intervals and the number of quality classes of the indices are different. The ASTP evaluated all sampling sites within only three quality classes (moderate, poor, and bad). The correspondence analysis identified macroinvertebrate groups that were more pronounced and closely associated with Sites III, IV, and I than II. The macroinvertebrate assemblages determined by CCA demonstrate good biological characterization of these Sites however, the differences in community structure can be confounded by various environmental variables excluding the ones observed in this study such as the environmental survival ranges for invertebrates, surrounding waterbodies ecosystem, and other environmental stressors, and climate change (White et al. 2010).