Abstract
Abstract. The intratidal, spring–neap and seasonal variations in stratification were examined in the Cochin estuary. The observations established a strong connection with the distribution of chemical and biological properties. The influence of tides and river discharge forcing in water column stability was quantified using potential energy anomaly (PEA) and stratification parameter. Partially mixed (neap) and well-mixed (spring) conditions during low river discharge (dry) period were altered in monsoon by the salt wedge intrusions. The ecological impact of salt wedge propagation on high tides bringing upwelled water to the system was evident from the bottom hypoxic, high chlorophyll a and nutrient-rich conditions. Phosphate and nitrite concentrations were higher at the bottom saline conditions but silicate and nitrate were clearly supplied by river water. However, during ebb tide this front was driven out of the estuary. The periodic advance and retreat of the salt wedge was inevitable in making the system immune from extended hypoxia/anoxia and maintaining the health of the Cochin estuary. For the seasonally varying river flow in the estuary, salt intrusion receded with increasing river flow in monsoon and rebounded with decreasing river flow in dry season. During monsoon, the intense flushing and reduction in salinity field expansion seemed to be responsible for the limited chlorophyll a levels along the surface of the Cochin estuary.
Highlights
The key to understanding stratification and de-stratification processes in different time scales has gained the most attention during the last decades due to its tremendous relevance to the estuarine ecosystem
The system is characterised by its long axis lying parallel to the coastline, with several small islands and interconnected waterways, and it covers an area of about 300 km2. The width of this estuarine system varies from 450 m to 4 km, and the depths range from 15m at the Cochin inlet to 3 m near the head with an average depth of 2.5 m
Since the environmental behaviour and dynamics of the Cochin estuarine system is highly influenced by monsoonal rainfall and the associated runoff, we had ideally chosen both extreme conditions of seasons for data collection: pre-monsoon and monsoon seasons
Summary
The key to understanding stratification and de-stratification processes in different time scales (intra-tidal, spring–neap, seasonal) has gained the most attention during the last decades due to its tremendous relevance to the estuarine ecosystem. An interdisciplinary approach linking the physical phenomena with chemical and biological properties is essential, and for the Cochin estuary this study is first of its kind.The physical processes (stratification, horizontal and vertical advection, flushing, etc.) that govern the ecological parameters have not been rigorously investigated in the region to date. For the efficient implementation of estuarine management plans, an imperative study of the impact of stratified systems on water quality and ecosystem ecology is essential. The objective of the present study is to assess (1) the intratidal and spring–neap variations in stratification of water column in dry and wet seasons, and (2) the horizontal extent of salt intrusion and the relation between salinity and property distributions
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