Seasonal shift in trophic status induced by bacterial metabolic activity in tropical mangrove-dominated estuaries of Southwest India

Mangroves are highly productive ecosystems that supply carbon and nutrients to the coastal food web from autochthonous and allochthonous sources. The fluctuations in the quantity of these sources influence bacterial metabolic activity which is, in turn, directly linked to the trophic food web. Our study, reports the inter-annual variability of nutrients and dissolved organic carbon in addition to their impact on phytoplankton and bacterial biomass, respiration and production in the Mandovi and Zuari estuaries of southwest India. Radio-labeled substrates, NaH¹⁴CO₃, and titrated thymidine (³H-TdR) were employed to quantify primary productivity (PP) and bacterial productivity (BP), respectively. Our findings revealed that community respiration exceeded PP indicating a state of heterotrophy. Similarly, bacterial respiration (BR) exceeded BP suggesting that carbon is predominantly utilized for respiratory rather than assimilatory action. Analysis of BP/PP (Mandovi 0.48 to 4.46; Zuari 1.72 to 3.81) and PP/BR (Mandovi 0.02 to 1.42; Zuari 0.05 to 1.00) ratios showed considerable seasonal variability. Lower bacterial growth efficiency observed in Mandovi (average 30.35%) than Zuari (average 38.38%) suggests greater productivity in Zuari. Uncoupling of phytoplankton and bacteria during the monsoon and post-monsoon seasons implied heterotrophic conditions in both Mandovi and Zuari. Principal component analysis indicated distinct temporal variations in salinity, dissolved inorganic nutrients and dissolved organic carbon which had a significant influence on the trophic status of the mangrove and estuarine waters This study highlights the critical role of allochthonous organic matter in driving coastal heterotrophy and shaping the seasonal trophic dynamics in estuarine ecosystems.