Assessment of trophic change and its probable impact on tropical estuarine environment (the Kodungallur-Azhikode estuary, India)

The cumulative effects of global change, including climate change, increased population density and domestic waste disposal, effluent discharges from industrial processes, agriculture and aquaculture will likely continue and increases the process of eutrophication in estuarine environments. Eutrophication is one of the leading causes of degraded water quality, water column hypoxia/anoxia, harmful algal bloom (HAB) and loss of habitat and species diversity in the estuarine environment. The present study attempts to characterize the trophic condition of coastal estuary using a simple tool; trophic index (TRIX) based on a linear combination of the log of four state variables with supplementary index Efficiency Coefficient (Eff. Coeff.) as a discriminating tool. Numerically, the index TRIX is scaled from 0 to10, covering a wide range of trophic conditions from oligotrophic to eutrophic. Study area Kodungallur-Azhikode Estuary (KAE) was comparatively shallow in nature with average depth of 3.6 ± 0.2 m. Dissolve oxygen regime in the water column was ranged from 4.7 ± 1.3 mgL ^-1 in Station I to 5.9 ± 1.4 mgL ^-1 in Station IV. The average nitrate-nitrogen (NO ₃-N) of KAE water was 470 mg m ^-3; values ranged from Av. 364. 4 mg m ^-3 at Station II to Av. 626.6 mg m ^-3at Station VII. The mean ammonium-nitrogen (NH ₄ ⁺-N) varied from 54.1 mg m ^-3 at Station VII to 101 mg m ^-3 at Station III. The average Chl-a for the seven stations of KAE was 6.42 ± 3.91 mg m ^-3. Comparisons over different spatial and temporal scales in the KAE and study observed that, estuary experiencing high productivity by the influence of high degree of eutrophication; an annual average of 6.91 TRIX was noticed in the KAE and seasonal highest was observed during pre monsoon period (7.15) and lowest during post monsoon period (6. 51). In the spatial scale station V showed high value 7.37 and comparatively low values in the station VI (6. 93) and station VII (6.96) and which indicates eutrophication was predominant in land cover area with comparatively high water residence time. Eff. Coeff. values in the KAE ranges from -2.74 during monsoon period to the lowest of -1.98 in pre monsoon period. Present study revealed that trophic state of the estuary under severe stress and the restriction of autochthonous and allochthonous nutrient loading should be keystone in mitigate from eutrophication process.