Some aspects of the biology and sedimentology of laminated algal mats from mannar lagoon,Northwest Ceylon |
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| Authors: | Ananda Gunatilaka |
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| Abstract: | Laminated blue-green algal mats are forming in the intertidal zone of a clastic (non-carbonate), tropical, lagoonal tidal-flat in northwest Ceylon. On the basis of their surface morphology three distinct mat zones are recognised. From the high-water mark seawards these are: (1) a smooth rounded-mat zone with discrete structures; (2) a crinkled- and blistered-mat zone; and (3) a smooth flat-mat zone without any perceptible relief. These mats are formed by the trapping and binding of sediment particles on to a sticky and mucilaginous complex of algal filaments.The morphology and distribution of these mats appear to be controlled by the frequency and duration of their exposure, strength of tidal-current velocities and the rates of bottom sediment movement. The flat zone is characterised by tidal-current velocities of 10–15 cm/sec, greater rates of traction movement and twice daily tidal flooding. The crinkled zone is partly or completely exposed during neap tides, the rate of bottom sediment movement is less than in the flat zone and the tidal-current velocities vary from 6–12 cm/sec. The rounded-mat zone is flooded only during high-water spring tide, with no tidal exchange during the neap tidal phase, but there is a continuous water cover forming a tidal pool. Here the rate of bottom sediment movement is negligible and the tidal-current velocities vary from 1–10 cm/sec. The rounded—elliptical mat forms are elongated in the direction of tidal flow and with decreasing tidal velocities they tend to assume a more rounded shape. All three zones are characterized by laminations produced by the alternation of thick sediment-rich and thin algal-rich layers. They show an accretion rate of 5–15 mm/week. The above features indicate that they are potentially very useful environmental indicators. |
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