Seasonal Dynamics of Physicochemical Characteristics and Nutrient Enrichment in a Tropical Mangrove Estuary, South-Eastern, Nigeria

Abstract

Mangrove estuaries are dynamic ecosystems influenced by complex interactions between freshwater inflow, tidal exchange, nutrient cycling, and anthropogenic activities. This study investigated the seasonal dynamics of physicochemical characteristics and nutrient enrichment in the Imo River Estuary, South-Eastern Nigeria, from September 2023 to February 2024. Five sampling stations were monitored across wet and dry seasons to evaluate spatial and temporal variability in water quality parameters and nutrient distribution. Results showed significant seasonal variation (p < 0.05) in most physicochemical parameters, while spatial differences were station-dependent. Water temperature increased significantly from 29.3–30.7 °C in the wet season to 31.7–32.0 °C in the dry season across all stations. pH shifted significantly (p < 0.05) from slightly alkaline conditions (7.87–8.23) in the wet season to acidic conditions (5.60–5.78) in the dry season. Total dissolved solids (TDS) exhibited strong spatial and seasonal variation (p < 0.05), reaching a peak of 6852.67 ± 457.07 ppm at Station 1 during the wet season and significantly lower values (e.g., 2513.67 ± 142.80 ppm) at downstream stations. Transparency showed moderate spatial variability during the wet season but no significant seasonal difference (p > 0.05). Nutrient concentrations displayed highly significant seasonal enrichment (p < 0.05), with ammonium (NH₄-N) peaking at 92.24 ± 2.21 mg/L and total nitrogen (TN) reaching 170.03 ± 4.30 mg/L during the dry season, indicating strong eutrophic tendencies. Phosphate and total phosphorus exhibited significant spatial redistribution (p < 0.05), linked to sediment-water interactions and reduced tidal flushing, particularly in the dry season. Silicate concentrations were significantly higher during the wet season (p < 0.05) due to increased riverine input. Chlorophyll-a concentrations ranged from 0.23 ± 0.09 to 1.20 ± 0.37 µg/ml in the wet season and 0.50 ± 0.02 to 0.75 ± 0.08 µg/ml in the dry season, with significant spatial variation (p < 0.05) reflecting localized phytoplankton productivity. Principal Component Analysis (PCA) revealed that nutrients (TN, NH₄-N, PO₄-P), TDS, and transparency were the major drivers of seasonal water quality variability. Although limited to a six-month sampling period and five stations, these findings underscore the dominant role of hydrological seasonality, nutrient loading, and internal biogeochemical processes in regulating water quality in the Imo River Estuary. The study provides essential baseline data for monitoring, ecological assessment, and sustainable management of tropical mangrove estuaries in South-Eastern Nigeria.