Abstract
Considering Alappuzha Mud Bank in the southern Kerala coast as a typical case of biologically productive Mud Banks that form along the southwest coast of India during the Southwest Monsoon (June - September), the present study addresses several pertinent missing links between the physical environment in Mud Banks and their influence on plankton stock. This study showed that very strong coastal upwelling prevails in the entire study domain during the Southwest Monsoon, which manifests itself in the form of significantly cool, hypoxic and nitrate-rich waters surfacing near the coast. The upwelled water persisting throughout the Southwest Monsoon period was found to have fuelled the exceptionally high phytoplankton stock in the entire study area, including the Mud Bank region. Having accepted that Mud Banks are special because of the calm sea surface conditions and relatively high turbidity level in the water column around them, the present study showed that except at points close to the sea bottom, turbidity level in the Alappuzha Mud Bank was below the critical level to inhibit the plankton stock. The suspended sediments that form in the Mud Bank occasionally could be attributed to the disturbance of the bottom fluid muddy layer and their vertical spurts.
Highlights
Mud Bank (Chakara) is a unique and well-known littoral feature that occurs in certain fragmented sections along the Southwest Indian (Kerala) coastline during the Southwest Monsoon (June–September) period[1,2,3,4]
Time series of vertical temperature distribution in the water column showed a remarkable drop in temperature during the Southwest Monsoon period (Fig. 2), which was more pronounced in the subsurface waters as compared to the surface
The water column was vertically stratified during most part of the Early- and PeakMonsoon period, with a clear gradient in temperature between the surface and subsurface waters; the surface water was always warmer than the subsurface waters (Fig. 3)
Summary
Mud Bank (Chakara) is a unique and well-known littoral feature that occurs in certain fragmented sections along the Southwest Indian (Kerala) coastline during the Southwest Monsoon (June–September) period[1,2,3,4]. It is significant to recall here that even with a dozen hypotheses in place to explain the causative mechanism of Alappuzha Mud Bank, a fool-proof reasoning is yet to emerge[2,3,5,8] It is thought-provoking to observe that some of the earlier studies describing the biological manifestations of Alappuzha Mud Bank have challenged our fundamental scientific reasoning[4,6,7,9,10]. Some of these studies describe Mud Banks as highly turbid environments[3,4,5] while on the other, highlight the exceptionally high stock of phytoplankton[9], zooplankton[10] and fish associated with it[6,7]. We presented the following aspects of the Alappuzha Mud Bank to comprehend conclusively the actual biophysical coupling of plankton in the region (a) nutrients, turbidity and PAR levels in the water column (b) the phytoplankton and zooplankton stock in the region when Mud Bank is active (c) the oceanographic mechanism involved in the sustenance of high plankton stock in the Mud Bank region, and (d) turbidity in the Mud Bank and its possible influence on the plankton community
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