There is a famous Sanskrit shloka (metrical verse or composition) "Aakashat patitam toyam yatha gachchati saagaram.." meaning - all water falling from the clouds reaches the same source i.e. the Ocean. While this is the case, these waters also bring in millions of tons of sediments from the earth and deliver it to the ocean. Observe coastal water during monsoon – especially close to the river mouths – you will find it muddy. Much of this of course, comes as suspension in water, normally called “suspended sediment”. The muddy waters are rich in minerals, dissolved elements, nutrients, humic acids and the pollutants that have drained into the rivers due to anthropogenic activities. Over 30% of the global river discharge of sediments to the seas is from the Asian continent because of mighty rivers like the Ganga-Brahmaputra, Indus (Sindhu), Huang He, Irrawady, Yangtze, etc which have their origin in the Himalayas.

The Ayeyarwady (formerly known as Irrawady) River of Myanmar, one of the muddiest rivers of the world (Fig 1), flows into the northern Andaman Sea and the Gulf of Martaban. The Ayeyarwady is the fifth largest river in the world in terms of suspended sediment discharge. Together with the Salween and Sittang rivers, it deposits annually more than 350 million tons of sediment to the ocean. The scientists of the National Institute of Oceanography, India, in association with Myanmar colleagues, investigated the nature of suspended sediments of this area under the India-Myanmar joint oceanographic program during 21 April – 14 May 2002. 147 surface seawater samples and 48 water column profiles were collected from the Ayeyarwady continental shelf, to assess the suspended sediment concentration (SSC) during this period. This area, like other areas in this region, is characterized by the seasonally reversing Asian monsoons – May to September (SW monsoon) and December to February (NE monsoon). The freshwater and therefore the sediment discharge is highly seasonal with more than 80% of the annual discharge during SW monsoon. A characteristic feature of the Gulf of Martaban is that it has a tide-dominated coastline and therefore called ‘macro-tidal' area, which ranges between 4 to 7 m – the highest at the Elephant Point in the western Gulf of Martaban. The mouths of the Ayeyarwady River, however, is meso-tidal (with a range between 2 to 4m).

The surface SSC values in the northern Andaman Sea during the study period ranged from less than 0.1 mg/l to over 500 mg/l. The unusually, highly turbid zone was noticed mainly in the Gulf of Martaban and extending to about 15 ° N. The turbid front was also found to have a sharp gradient with SSC falling from over 50 mg/l to less than 1 mg/l within a distance of few kilometers on the continental shelf (Fig 2).

The water column profiles indicated an increase in SSC values with depth with a maximum (689 mg/l) in the Gulf of Martaban. These studies clearly showed that the sediment discharged by rivers flows along the bottom of the sea and reaches the deep Andaman Basin via the Martaban Canyon (Fig 3).

Ground truthing of the satellite imageries showed that the imagery of 3 March (Fig. 4) was very close to the ship-borne observations. Minor differences are because satellite images take snapshot measurements while the shipborne data was collected during a stretch of about 30 days. A number of satellite images, spread over all seasons and many years, were examined. Interestingly, it was noticed that suspended sediment concentrations and area covered by the highly turbid zone strongly relates to spring-neap tidal cycles throughout the year – unusual to the conventional understanding that the suspended sediment concentration in coastal seas is mainly due to river runoff. The size of the turbidity zone also varied significantly. During spring tide, when the tidal range was 6.6 m, the turbid zone covered an area more than 45000 km 2 (Fig 5a) – one of the largest perennially turbid zones of the world's oceans. Whereas, during neap tide, with tidal range of 2.98 m, the coverage dropped to about 15000 km 2 (Fig 5b). The edge of the highly turbid zone was found to migrate back-and-forth in synch with every tidal cycle (14 days) by about 150 km!

It is unusual to see such an effect in coastal seas and that too covering such large areas. What are the causes of this perennial feature? Studies of the sediment texture, depth of the water (bathymetry), etc., indicate that the sea floor in the Gulf of Martaban, the surrounding coastal areas and estuaries are covered with silty clays and subject to constant settling and resuspension due to tidal forces . Since tidal forces are generated by the gravitational forces of the moon and sun, the Gulf of Martaban is subject to constant high energy conditions due to the macro-tides. The Gulf is shallow - less than 30 m. In such a situation, the tidal currents mix the waters and bring the resuspended material to the surface. Beyond 30 m water depth the situation is altogether different. The gradient of the sea floor increases sharply and because of deeper waters the tidal forces are unable to resuspend and bring the sediments to the surface. Also, tidal forces become weak with increasing distance from the shore. Interestingly, the zone of maximum turbidity, is not off the mouths of the Ayeyarwady River where the river discharge is the maximum but in the Gulf of Martaban where the tidal currents are strongest!

The resuspension also brings nutrients up in the water from the pore waters of the sediment lying on the sea floor. When the suspended matter concentrations are high, light is not able to penetrate the water column and the nutrients released from the sediment pore waters are unutilized by the ocean algae. Adjacent to the brown muddy waters of the Gulf of Martaban the waters are a distinctly green due to high content of chlorophyll from oceanic algae and diatoms. Just before and after the neap tide, when the turbid area is close to minimum and enough light is penetrating in the seawater, it is speculated that the primary productivity of the Gulf of Martaban will be very high (Fig 6). During this period it is expected that a large fish population should be visiting this area for their food since food supply is abundant and visibility is good. Since tidal range is predictable and can be modeled, the extent of the muddy zone can also be predicted. Near real-time satellite data can be used to demarcate the muddy zone from the highly productive zones. This study therefore provides an important clue for predicting and increasing the fish catch of this area.

The river mouths and deltas of large rivers in the Indian Subcontinent such as Ganga-Brahmaputra, Indus and Narmada are located close to similar macro-tidal areas. Therefore, the scientists feel it important to study similar features in Indian waters having similar conditions. Macrotidal conditions prevail in the Gulf of Khambat (maximum tidal range 11 m), Gulf of Kachchh (7 m) and Ganga-Brahmaputra River mouths (4 m). High suspended matter concentrations are also noticed in the Palk Straights. Worldwide it is observed that some of the best fishing grounds are located adjacent to areas of high suspended and nutrient concentrations. It is indeed good fishing near muddy waters!