Salinity, temperature and turbidity structure in the Suva lagoon, Fiji.

Author:Singh, Awnesh


Fiji is an island group located in the Pacific Ocean between latitudes 15 S-22[degrees]C S and longitudes 177[degrees]C W-175[degrees]C E. All major economic activities are based on its two main islands of which Viti Levu is the largest in size (10 388 [km.sup.2]) and population (>75% of total population) and hosts the capital city of Suva as shown in Fig. 1. The city of Suva is home to nearly a quarter of the population of Fiji (1) thereby placing a lot of anthropogenic pressure on its lagoon.


The salinity, temperature and turbidity are some important parameters which are continuously changing with the seasons and need to be studied because they are efficient indicators of variations in the lagoon and can transform the marine ecosystem. Several cruises were conducted to collect necessary data for water quality analysis and the data collected were filtered and analyzed.

Physical geography and bathymetry of the Suva lagoon: The city of Suva is perched on a hilly peninsula between Laucala Bay and Suva Harbour, which together comprise the Suva lagoon in the southeast corner of Viti Levu. The Suva Harbour lays on the west of the lagoon and in between its fringing reefs of Nukusaga and Vunivau lays the Suva Passage as shown in Fig. 2. The Nasese channel links the Suva Harbour and Laucala Bay and is bounded by the Suva Peninsula and the Nawanada Reef. To the east lays Laucala Bay.


The Sosoikula and Nukubuco barrier reefs and the Nasigasiga and Ucuisila barrier reefs are broken by the Nukubuco and Nukulau passages respectively. The little islands of Nukubuco and Nukulau form emerged caps on the barrier reef dipping gently into the lagoon, while Makaluva Island is closely exposed to the reef front (3).

The average water depth in Laucala Bay is 9 m deepening to more than 40 m in the Nukubuco and Nukulau passages. Laucala Bay is connected to Suva Harbour by the narrow Nasese channel that is 5-10 m deep. Suva Harbour has an average depth of 15 m with depths of 80-100 m in the Suva Passage (4).

Weather over the Suva lagoon: At Suva, the long term (1971-2000) averaged daily maximum and minimum temperatures generally have a direct correlation with the average total monthly rainfall for each month as can be seen in Fig. 3.


The discrepancy during the month of February may be attributed to the lower number of tropical cyclones affecting Fiji during that month as when compared to January and March. Furthermore, tropical cyclones in January and March were much severe and frequent than those in February and since severe tropical cyclones are more likely to bring higher rainfall, the total rainfall for the month is affected. During the cooler months of May to October, the average total monthly rainfall is lower than when compared to the warmer months of November to April.

Tides: At Suva, as in most other places, there are two tides per day (semidiurnal tides). The tidal amplitudes at Suva due to the [M.sub.2], [S.sub.2], [K.sub.1] and [O.sub.1] tidal constituents are 0.565 m, 0.091 m, 0.094 m and 0.046 m respectively (6). The area occupied by the water in the lagoon varies with the tide with a mean tidal range of about 1.164 m (6). At high tide the area is 6.34 x [10.sup.7] [m.sup.2] and 5.57 x [10.sup.7] [m.sup.2] at low tide with the capacity to hold a mean volume of the lagoon of about 7.77 x [10.sup.8] [m.sup.3] (7). Generally, the rise and fall of water level due to tides acts as a major flushing mechanism for a lagoon (8).

River runoff and sedimentation: The Rewa River is the longest river and the largest fluvial system in Fiji. The river is on the island of Viti Levu and originates in Tomanivi (Mount Victoria), the highest peak in Fiji (maximum height above sea-level is 1323 m) and flows southeast for 145 km before splitting up two-thirds on its way downstream into several tributaries. One of these tributaries is the Vunidawa River which channels 15% of the total discharge of the Rewa River into Laucala Bay while the remainder flows into the Rewa Delta (7). During the dry season, there is a higher percentage of freshwater discharge into the Vunidawa River than into the Rewa Delta. This is because the Rewa Delta is generally closed due to the formation of sand banks and hence a lower percentage of freshwater discharge. However during the wet season, the freshwater discharge into the Rewa Delta increases due to the higher volume of freshwater entering from the Rewa River.

The freshwater input from the Vunidawa River carries a lot of fine sediments and organic particulate matter into the Laucala Bay (9). The Bay area is subject to much sedimentation and has an average depth of 20 meters with a substratum of clayey-silty ooze caused by siliciclastic fluvial input by the Rewa River (10). The Samabula, Nasinu and Vatuwaqa rivers also deposit sediments into Laucala Bay and have a combined runoff of over 3 [m.sup.3] [s.sup.-1] (11). Because of the weak currents in the Bay, these sediments are deposited easily and this causes the dirty waters to stay in the Bay for a long time (12). Sediments in the Laucala Bay area are generally very fine while coarser-grained sediments are found close to the barrier reefs (3). Bay muds are about 25-40 m thick at Suva Harbour (13).


Wind data: The wind data was monitored at the School of Marine Studies at the University of the South Pacific under technical assistance of Research Unit: Characterization and modelling of exchanges in lagoons subject to terrigenous and anthropogenic influences (UR CAMELIA), IRD in Noumea. The wind speed and direction were...

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