the atoll of minikoi. 45 



Section 4. The Lagoon. 



The distribution of the fauna in the lagoon of an atoll is dependent on the currents, 

 which brinw food and water to the various organisms. To ascertain their force and character 

 I anchored for five nights in different situations, taking serial observations on the direc- 

 tions and rates of the current, temperature and surface fauna'. For rate of current I used 

 Messrs Negretti and Zambra's current meter-. 



To summarise my observations, the tide sets in mainly over the south and west reefs 

 of the atoll, carrying with it weed and dirt from the reefs together with a certain amount 

 of plankton, but in any situation relatively little as compared to the quantity which enters 

 by the north passage. At first the tidal effects were felt in the latter channel, but about 

 half-tide, when the boulder zone began to be covered, a reversal of this current was 

 experienced; a previous current inwards of 1 to 1|^ knots gave place an hour before high 

 tide to an outward current of ^ knot, increasing to 1 knot at high tide. With the ebb 

 a slight inward set over the reef still continued, and was felt until the boulder zone 

 began to be uncovered. The curi'ent in the north passage increased up to 4 knots at a 

 depth of 1 fathom, the surface nets catching a mixture mainly of weed and mud with 

 some few Crustacean larvae. Away from the immediate neighbourhood of reefs I never 

 recorded a current in the lagoon of more than 1 knot, but incidentally I proved that the 

 water is to some extent changed by admixture in every part e.Kcept perhaps immediately 

 opposite the lighthouse. The advent of the water from without was shown by a slight 

 change (lowering) in the temperature and the specific gravity of the lagoon-water, and in 

 the amval of the true sea-plankton. The latter was mainly noticeable in its Medusae, 

 Diphyids and Sagittae ; Appendicularia and Salps were also found. Crustacean constituents 

 were not relatively numerous. Minikoi is a small atoll with a relatively very large reef 

 surface as compared with the lagoon. The fact that there should be a thorough circulation 

 of water is hence of great importance and perhaps accounts for the richness (in quantity, 

 not in species) of the fauna, which the lagoon contains ^ 



The shore of Minikoi as far north as the village slopes gradually into a broad sand- 

 flat, but beyond this a level reef commences and continues up to the north point. This 

 reef varies up to 300 yards in breadth, and no part of it is exposed even at low-tide 

 springs. It slopes absolutely from the base of the beach, and has about 3 feet of water 

 at its edge, where it drops steeply into 2 fathoms of water. Masses of conglomerate, 

 such as are found in the beach, may occur on its surface for some distance, gradually 

 further out decreasing in size. At Boni-Kodi the reef is 216 yards broad from the base 

 of the beach, and pinnacles extend 170 yards out. Its appearance in this position bears 

 a close comparison with the reef outside the island, in that it is very obviously a solid 



' The situations were opposite points, where the reef is attention should be paid to its bearings to avoid friction 



low near Wiringili and Ragaudi Islands, opposite the channel of every sort. 



of Neru-Magu, the centre of the lagoon, and just inside the ^ My observations were made in the south-west monsoon, 

 north passage. The last was not very satisfactory, as the From general considerations I cannot see why there should 

 night was stormy, and my boat was twice filled with water not be nearly as good a circulation of water in the north- 

 by breaking seas. east. The main island would be a barrier to some extent, 



- Vide p. 22. For slow currents, such as I wished to but the currents in the north passage would be probably 



record, a lighter and larger form of instrument is desirable. profoundly modified and stronger inwards, changes which 



The screw should have three or four blades, and great could not fail to be advantageous. 



