Cruising down the ecology of the Negombo lagoon
Negombo Lagoon is the largest semi-enclosed coastal water body near Colombo, with plenty of natural resources, but subject to potential heavy human impacts from the surroundings, and hence deserves our attention. Out of the 85 lagoons and estuaries in the country, the Directory of Asian Wetlands (IUCN 1989) listed Negombo Lagoon as one of the major 41 wetlands of Sri Lanka.
Estuaries are generally located, at the mouth of rivers, deep, perpendicular to the seashore and are permanently open to the sea, whereas lagoons are shallow, parallel to the shore, and their opening to the sea is occasionally obstructed by shifting sand bars, corals and shingle. The water in estuaries and lagoons is brackish, as mixing of seawater (3.5% salt) with freshwater (0 % salt) occurs there. The mouth of the Negombo Lagoon was obstructed periodically in the 19th century, but thereafter, it has remained open. Traditionally, it has been called a lagoon, although, some refer to it as an estuary or estuary-lagoon! The lagoon is about 12 km long, and 3.75 km at its widest point, with an area of 3164 ha. There are two important ecosystems in the lagoon, seagrasses and mangroves, which cover 22 % and 11% of the lagoon respectively. The lagoon is shrinking by about 1 % annually, as a net result of erosion near the mouth and reclamation, sedimentation and accretion at its shores and southern end.
Structure of the lagoon
Muthurajawela Marshes and Negombo Lagoon are two adjacent ecosystems that should be considered as a continuum, as they interact with each other. Extending from Pamunugama to Uswetakeyawa and Wattala, Muthurajawela is the largest saline coastal peat bog in Sri Lanka, covering 3068 ha. In dry weather, the lagoon has a continuous supply of freshwater from Dandugam Oya and Ja-Ela at the southern end of the lagoon. Negombo Lagoon can be considered as a choked lagoon, as there is only one opening to the sea, which is at Duwa.
At the southern end of the lagoon, the Dutch Canal, renamed Hamilton Canal by the British, connects Negombo Lagoon to Colombo, via Kelani River. At the northern end, the Dutch Canal extends from Negombo town to Chilaw Lagoon, Puttalam Lagoon, and as far as Kalpitiya, where the Dutch maintained a strong naval presence. Remnants of a disused Portuguese canal that also connected the Negombo Lagoon to Colombo can be located in the Muthurajawela marshes. The importance given to Negombo Lagoon by the Portuguese, Dutch and the British shows its strategic importance in administration, transport and naval operations.
Much of the shoreline at Pitipana and Duwa regressed landwards from 1889, because of erosion, and the shoreline stabilised from 1939. A sand bar that obstructed the mouth of the lagoon in 1875 also disappeared in due course. The ebb and flow of the tide had caused deep canals in the middle of the lagoon (2-3 metres), used as navigational canals (ala maga), and shallow (less than 0.5 metres) calm areas, giving a mean depth of 1.2 metres for the lagoon. In the calm, low energy areas are found the submerged sea grasses, fringing mangroves and the emerging mud flats. The study of the elevation and vegetation of three human uninhabited islets of the lagoon, Kakaduwa, Mandagas alamba and Kadol nalalle, adjacent to Munnakkara revealed how these islets would have emerged with their vegetation and associated fauna, as sediment accretion progressed.
Succession in vegetation communities were represented by the initial mud flat without any vegetation, to an islet covered by mūlli (Acanthus ilicifolius) as in Kadol nalalle, then to a mixed vegetation type of Kadols-Bariya-Punkanda (Rhizophors spp, Bruguiera spp. Lumnitzera racemosa, Cerips tagal) as in Kakaduwa, and finally to Manda gas (Avicennia marina) dominated vegetation, as in Mandagas alamba.
There are two major ecosystems in the lagoon; terrestrial and aquatic. The terrestrial fauna are found on and in the soil, in the air or associated with roots, stems and flowers of plants. The organisms that live there reduce competition by occupying different habitats and feeding niches. Among the birds are the waders, actively searching for food in mudflats and shallow areas, reaching their food depending on the length and shape of their beaks (e.g. Sandpipers, Curlews, Whimbrel, Redshank). Others wait patiently for their prey, perched on mangrove roots (e.g. Striated heron or kadol koka). The Pied kingfisher hovers in the air, about 10 m from water, and hurtles down on its prey. Cormorants are excellent divers, pursuing their prey underwater. So, the food of these birds tends to be slightly different from each other, reducing competition.
Another example of feeding niche separation among terrestrial insects was the case of two species of bees visiting mangrove flowers. The larger bee, Apis indica gathers nectar from Mannda gas that flowers from March to June, while the smaller bee, Apis florea gathers nectar from Heen kadol which flowers from May to June.
Mangrove fringed Negombo Lagoon with a brush pile (athu kotu). Brush piles attract fish and the owners harvest them periodically.
The Negombo Lagoon provides a diversity of habitats to its faunal communities, and one of these is the unique habitat provided by mangrove prop roots of Kadol. On the roots are the oyster clumps and the associated community of borers, attached (sessile) forms and fish that take shelter among the oysters. In this community were 24 species, comprising molluscs, crustaceans, polychaete worms, a sponge and a goby fish. Among the crustaceans were the tiny crabs, Baruna socialis and Pyseidognathus deianira and juvenile Metapograpsus messor with a beautiful shell mottled with brownish green.
The aquatic ecosystem of the Negombo lagoon reminds us of crabs, prawns and fish. We collected 19 species of crabs from the Negombo Lagoon and mangrove islets. But, the crabs that are good for the table are the Mud crab or kalapu kakuluwa (Scylla serrate) and the Sea crab (Blue swimmers) or sinnakkali (Portunus pelagicus). The adult Mud crabs reside in the lagoon most of their life, but females spawn in the sea. If caught on full moon days, they do not have much meat. A popular explanation is that they go love-making on full moon days and are not interested in smelly baits, thus a few are caught, and the ones caught do not have much meat, because they have been exhausted in love-making. The biological reason for this is that their moulting and reproductive cycles are influenced by the lunar cycle, on which depends tide.
The adult Sea crabs, do not reside in the lagoon, but their juveniles migrate to the lagoon during the first inter monsoon, or in fishermen’s language, korossme kale (season of Lent, March–April). In the lagoon a high proportion of them are males, but sex reversal takes place as they grow to have a balanced adult population of males and females in the sea. Most of the adult sinnakkali in the market are from the sea. In other countries, male and female are separated, and female sinnakkali are more expensive than males. Unlike the Sea crabs, the condition of Mud crabs deteriorates rapidly after death, and dead Mud crabs are not sold in the market. There are also other swimming crabs belonging to Family Portunidae in the lagoon, but they are considered not suitable for eating.
The crabs of the Family Ocypodidae, the fiddler crabs occur on mud flats. Males swing their large colourful claw to attract the females, but often females pay little attention. Those who respond, move together into the secrecy of their burrows. They all disappear into their burrows at the slightest shadow of danger. Another ocypodid, Macrophthalmus depressus is also found on these mudflats, but prefers to remain submerged, with its long stalked eyes above water, scanning the environment, as a submarine periscope.
In the muddy mangrove islets are numerous species of terrestrial crabs, commonly called shit crabs (goo kakuluwo) belonging to the Family Grapsidae. Most of the organisms in the mangrove swamp are burrowers, and they help to recycle nutrients, bringing subsoil to the surface, while feeding. The shape of the crab burrows are unique to the species, as shown in the diagram. Neosermatium smithi make complex burrows, but when the burrows were flooded, they build mud turrets reinforced by a stem of a small tree, so that they could maintain the burrow environment above the ground. Neosermatium malbaricum make T-shaped burrows, so that they can run into either branch of the burrow to avoid danger. Chiromantes darwinensis and Chiromantes indiarum often do not make neat burrows, but find refuge in the crevices of puddles. Even the Mud crab was found hibernating in deep burrows in mangrove islets.
The other burrowers of the terrestrial habitats of the Negombo Lagoon are the molluscs or matti (Geloina coaxans), polychaete worms, and the mud lobsters (Thalassina anomala). In the shallow lagoon are numerous jelly-like cocoons attached to the bottom, and fishermen mistakenly call them prawn eggs (issan bitthara). In fact they are the egg cocoons of the polychaete worm Marphysa borradailei. There is some confusion, as to whether M. borradailei of Pillai in 1958 from Negombo Lagoon is the same species as M. mossambica of Peters in 1854. Mud lobsters are active at night and do not leave the burrow, but throw mud at the mouth creating a big mound.
Twelve species of prawns and shrimps have been recorded from Negombo Lagoon, out of which four species, the Indian banana prawn or kiri issa (Penaeus indicus), Tiger prawn or karavandu issa (Penaeus monodon), Green tiger prawn or kurutu issa (Penaeus semisculcatus) and Kadal shrimp or malissa (Metapenaeus dobsoni) are of commercial importance. Kiri issa, kurutu issa and malissa breed in the sea and their post larvae migrate to the lagoon and stay there till they mature, and then return to the sea. They reach their maximum size in about two years. Two prawn species, Metapeneus elegans and M. moyebi complete their life cycles within the lagoon habitat without going to the sea. A crustacean that may of interest to the naturalist is the snapping shrimp (Alpheus edwardsii) that makes a cracking noise with one of its claws, and may be found on muddy bottom or in association with oysters.
Hundred and forty species of fish species have been recorded in the Negombo Lagoon. More than half of them are migrant species, whose adult stages are in the sea. Juveniles of groupers (kossa), trevalleys (paraw) and snappers (ranna, thambalaya) are some examples of species that migrate to the sea after their juvenile stage in the lagoon. The Asian bass or barramundi (moda) migrate from freshwater to the lagoon mouth or coastal sea to spawn, and the juvenile stages are in the lagoon often on sea grass beds. The chromides (koraliya), mullets (godaya) rabbit fish (orava), silver biddies (thirali) are almost permanent residents of the lagoon, drifting to the sea occasionally.
Although most fish are caught for food, some are collected and sold as aquarium fish, notably ilathi (Scatophagus argus) and kapu handa (Monodactylus argenteus). Besides its fishery value, the two chromides, Green chromide or koraliya (Etroplus suratensis) and the Orange chromide or ran koraliya (Etroplus maculatus) are of great importance in fish behaviour. In both species, the breeding pairs work together in selecting nesting sites, making nests and protecting the young. Green chromide provides total parental care, as the pair does not forage when nesting, while in the Orange chromide, the pair reverses functions between foraging and caring for the young every few minutes. Though commercially worthless as a fish, the mudskipper Periophthalmus is of interest to the evolutionary biologist, and they are found in mangrove habitat, in water, on land or perched on mangrove roots. It has special adaptations to live on land, including its large eyes, fins to skip and perch, the ability to stay on land for long periods and make burrows with mud turrets. The adaptations depict an attempt of organisms to invade land from their ancestral aquatic environment, and are of interest to the evolutionary biologist. P. koelreuteri is reported to make double turrets and P. sobrinus is known to make single turrets, and both types of turrets have been observed in the mangrove islets of Negombo Lagoon.
After our identification of these species it was necessary to confirm them by experts in each group. It was at that time in 1979 that late Prof. George Dunnet, later president of the British Ecological Society, came with his research students from the Aberdeen University to undertake a joint research project at Yala with the staff of Colombo University. He took our collection from the Negombo Lagoon to the British Museum (Natural History) for confirmation. Most of the Negombo Lagoon fauna were confirmed by Dr R. Wingle, Dr R. J. Lincoln, Ms Solene Whybrow and Ms A. Blake of the British Museum. But, confirmation of the grapsids (shit crabs) of Negombo was difficult for them, and they were sent to an expert on grapsids, Dr. R. Serene in Paris, who had studied them when he was in Raffles Museum in Singapore.
The names, that are given on the figure with the shapes of burrows are his identifications, and their names may have been revised since then, by other taxonomists, who often group, split and rename taxa. The British Museum also could not confirm a burrowing anemone that retreats rapidly into the mudflat, and sent it to Dr. J. C. den Hartog of the Rijksmuseum van Natuurlijke Historie in the Netherlands. He found it to be an interesting rare species, and asked for more specimens, but the work could not be followed, as I took up an academic position in the Philippines.
Function of the lagoon
As much as temperature determines the ecology of ecosystems in temperate countries, in tropical countries such as Sri Lanka, rain determines their ecology, in combination with other factors including temperature. The lagoon receives more rain and run-off during the first inter-monsoon (March–April), beginning of southwest monsoon (May–Sept) and the second inter-monsoon (Oct–Nov). In wet weather, run-off from the Muthurajawela catchment flows into the marshes, remains for sometime, filters and then discharges into Negombo Lagoon.
Following heavy rains, runoff and seepage, salinity in the lagoon drops almost to freshwater level in May and again in Oct–Dec. The water temperature is high from March–May (32°C) and low from Dec–Jan (27°C). Being close to the equator, the tidal fluctuation in Sri Lanka is generally low. From the daily measurements of the tide at Munnakkara from 1976–1977, the mean spring tide fluctuation (on full moon and new moon) was found to be 18 cm, and the mean neap tide fluctuation (on quarter moon) was 2 cm. In the wet months, the intermittent rain and run-off from the catchment gradually increases the mean water level by about 28 cm, while experiencing the daily tidal fluctuations from that level. Following heavy rains, the lagoon swells, the water level remains raised for few days or even a week, if the rain persists, and in a day or two the water level drops rapidly. The entire range of fluctuation of the lagoon caused by the tide and runoff would be around 73 cm, whereas Malaysian and Singapore lagoons record 1-2 m fluctuation from the tide alone.
The rain drives the processes in the lagoon, including changes in salinity, water level, flow speed, turbidity, nutrients and primary productivity, and to a lesser extent the water temperature, pH and dissolved oxygen. Combination of factors can have a cumulative effect, as evident from rain and wind on the lagoon, when one begins to shiver in an open boat. Oysters on mangrove roots spawn in March–April in the first inter-monsoon, when the temperature is high (20°–30°C) and salinity is above 1%. But, as the mean water level in the lagoon drops from June to August in the SW monsoon, the juvenile oysters that settled on the upper part of the mangrove roots die in large numbers, as they get exposed. Among the grapsid crabs, rain induces them to moult and females develop eggs in Oct–Nov in the second inter-monsoon and a large number of juvenile crabs occur in mangrove islets in Jan–March.
Sea crabs are also caught in large numbers from the lagoon and the adjacent sea from March–April during the first inter-monsoon. Rain also influences polychaete worms to come to the upper layers of soil, as the water table rises after rains. The burrows of the grapsid crab Neosermatium smithi get filled with water, and the crabs actively build mud turrets upwards. Juvenile grouper or kossa (Epinephelus) collections from the lagoon for export, increases after heavy rains. All the commercially important prawn species (kiri issa, kurutu issa and malissa) spawn twice a year during the rainy season in March–April and again in Sept–Nov. The fisher families who have been operating Stake seine nets (Kattudel) since 1721, also catch a large amount of prawns, fish and crabs after heavy rain, probably due to the strong flow.
( Next week- Part 2)