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Enviornmental change around Eluvuka

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Enviornmental change around Eluvuka

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INSTITUTE OF APPLIED SCIENCES

THE UNIVERSITY OF THE SOUTH PACIFIC

ENVIRONMENTAL CHANGE AROUND

ELUVUl<A (TREASURE ISLAND),

MAMANUCA GROUP, WESTERN FIJI:

RESULTS OF 1993 CORAL, ALGAL AND

SEDIMENT SURVEYS AND WATER

TESTING

IAS ENVIRONMENTAL REPORT NO. 71

\\1q4

Patricl< D Nunn

Milil<a R Naqasima

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CONTENTS

l. Background

2. Work in 1993

3. Recent sediment movement around Eluvuka

(i) Introduction

(ii) Northwest Eluvuka

(iii) Southeast Eluvuka

(iv) Recommendations

4. Algal growth around Eluvuka

(i) Introduction

(ii) Distribution and identification of algae

(iii) Temporal variations in algal distribution

5. Water sample analysis

(i) Sampling

(ii) ResuIts

(iii) Interpretation

6. Coral growth

(i) Coral growth

(ii) Recommendations

7. Summary of recommendations

Acknowledgments

References

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BACKGROUND

Eeleufvuakboa,utm1o0rekmcomwmesotnolyf

known as Treasure

Vuda Point on Viti

Island,

Levu.

is a sand island built on a circular

It is one of a group of three sand

�'.solapnudlasrlyin

the vicinity, the other two

known as Beachco111ber �sland.

being Kadavu,

Eluvuka and

which

Tai are

is uninhabited, and Tai,

occupied by Treasure and

Beachcomber Island resorts respectively.

The authors were initially contacted by the management of Treasure Island Fiji in A(1gust

1992. The specific object of the first visit in October 1992 was to examine recent coral

growth around the freshwater and sewage pipes and to recommend appropriate action. In

view of the lack of substantial previous work on the island, we looked at the broader

context of recent coral growth around Eluvuka and, in addition took ·water samples for

measurement of faecal coliform levels. The results are presented in Nunn and Naqasima

(I 9qT).

2. WORK IN 1993

In May 1993, several months after we presented our first report, we were again cont.acted

by the management of Treasure Island who sought advice regarding three specific

problems: the increase in the amount of algae visible on the reef and shoreline, the

movement of sand along the island's coast, and the possibility of changing levels of water

quality.

To address these problems, one of us (MN) visited Eluvuka from 7-9 August 1993 to

inspect algal growth, take water samples and measure water clarity. Then both of us

visited Eluvuka from 12-15 November 1991 to take more water samples, measure water

clarity, inspect coral-reef growth around the pipelines, to ascertain the extent of algal

cover and productivity, and to look for and interpret indications of recent sand movement.

The results of our 1993 surveys are described below.

3. RECENT SEDIMENT MOVEMENT AROUND ELUVUKA

3(i) Introduction

In 1992 a map of the island was made (Figure l) and a compass-and-pace sur vey in

Nove111ber 1993 revealed that there had been no substantial changes in the interim despite

the proximity of several tropical cyclones in the intervening periGd.

T11e most serious of these for Eluvuka was Tropical Cyclone JONI, the eye of which

passed close to Eluvuka in Dece111ber 1992. JONI approached Eluvuka, as do most

tropical cyclones, from the northwest and high long-fetch waves caused most damage to

the northwest side of the island. Considerable erosion of this area at this time led to the

construction of retaining walls, built above the high-tide level, to keep the sand

composing the island in place.

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o�·.

"o0

,,-_<'

·.

•

. ,,, ·,( ·-'\\

(

·>-.

ELUVLJKA

EDGE OF

GENERATOn

INTAl<E PIPE

···_··_··_· ... ,.·

r n ES II WATEn

r If' E

3tHJ

" . . . . . . . . ·."'

\\2 �·-�� )-

·.-1/

Figure 1.

Map of Eluvnlrn and Sllrrollncling reefs (from Nllnn and Naqasima,

1993). This map was made in 1992, no significant change was recorded

in November 1993.

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There has been erosion during the last twelve months in the south east of Eluvuka. This

erosion appears not to have been associated with cyclonic conditions but with the

tradewind-driven currents which affect the island at most other times of the year. The

response to this erosion has been the construction of eight (as at November 1993)

shoreline-normal groynes in the vicinity.

The best evidence of sand movement around Eluvuka in the last twelve months is the

changed distribution of exposed beachrock. Beachrock is a rock which forms around

upper tidal limits I 0-15 cm or more beneath the surfaces of medium to high energy

beaches. It is simply beach sand which has become consolidated through the slow

downward percolation of seawater. The cement forms from chemicals precipitated out of

the seawater.

Exposed beachrock 1s no longer actively forming, its exposure signifies that its

unconsolidated cover has been stripped off. On many sand cays, exposed beachrock

ridges out at se<1 testify to migration of the whole island (Nunn, 1994) but the exposed

beachrock on Eluvuka is insufficiently far removed from the modern shoreline for this

explanation to be wholly correct. Most exposed beachrock on Eluvuka is exposed

because its sand cover has been recently or temporarily removed.

The distribution of exposed beachrock in 1992 around Eluvuka and that mapped 111

November 1993 is shown in Figure 2.

3(ii) Nort'hwest Eluvuka

The beachrock at the northwest tip of Eluvuka was mapped in 1992. Its exposure a year

later was much less although the junction of the two ridges at the tip itself was still

visible. These ridges mark old shorelines and testify to the nature of the long-term

migration of the island from northwest to southeast.

The recent covering of part of the beachrock at the northwest tip of the island does not

mean that its long-term migration is ceasing or its direction is changing. It means that

there has been a recent increase in sand supply to the area which has covered the old

beachrock. 1t is clear from the difference in the strike of this beachrock and the trend of

the shoreline that this beachrock did not form beneath the beach in this area.

Evidence for an increase in sand supply is also evident along the island's north coast

where the area of exposed beachrock mapped in 1992 has clearly decreased. It is not

possible to determine the source of this sand precisely nor the timing of beachrock

coverage. It is possible that people have been responsible for redistributing sand along

this coast. Alternatively, it is possible that the sand is that which was eroded from part

of the north coast (where the retaining wall has been built) during Tropical Cyclone JONI

in December 1992. This sand could have been redistributed during the same storm or

circulated in the lagoon for a while and been slowly redistributed along a much larger

area of the shoreline. The latter explanation is favoured.

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There are three reason_s for favouring th!s ~xplana~ion . Firstly, had a large sediment

(sand) body hav~ been 1ntroclucecl to the fnng111g '.eet flat around Eluvuka during Tropical

Cyclone JONI, 1t w~ulcl probably have been noti ced. Secondly, the volume _of material

lost from that part ot the north coast corresponds almost exactly to that deposited up and

down the coast on top of beachrock . Thirdly , conspicuous levels of algal growth during

!993 are probably associated, in part at least, with increased water turbidity: this was

probably associated with erosio1~ _of Eluvuka during JO_NI and ~ther tropical storn~s at t~e

beginning of 1993 . Such cond1t1ons would have persisted until most of the sediment 111

suspension had been removed from th e system by deposition on the island 's shoreline.

Whatever the reason for increased deposition along the north coast, it is clear that it has

taken place and this alone is good news for Eluvuka and similar islands which are often

regarded as highly vulnerable to sedi ment loss rather than gain.'

F r 1992

BEACHROCK

'---

November 1993

100

'----....1

metres

LOW

TIDE

MARK

LANDING

Figure 2.

Distribution of exposed beachrock around Eluvuka. (a) 1992

distribution (from Nunn and Naqasima, 1993), (b) 1993 distribution.

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31jii) Southeast Eluvulrn

The southeast encl of the island is believed to be experiencing long-term accumulation as

the island migrates from northwest to so utheast (Nunn and Naqasima, 1993) yet the most

conspicuous effect in the last 12 month s has been erosion. The rubble bank, mapped in

1992, remains similarly positioned relative to the shoreline yet a bank of exposed

beachrock has appeared to its west (Figure 3). As in our earlier report , we regard the

rubble bank as becoming incorporated into the island. It seems likely that the newly-

exposed beachrock is a response to particularly severe erosion by tradewind-driven waves

during the last twelve month s.

Eight shoreline normal groynes have been constructed recently (Figure 4). These are

each approximately 12 m long and I 111 wide and terminate about 12 m above mean low

water mark (just above mean sea level). Each groyne is separated from the next by a

distance of 10-15 m. Groynes are mad e of rocks ranging from boulder to pebble in size,

many of which have been taken from the rubble bank, covered with plastic-coated fence

material. The upper ends of the groynes have been incorporated into the elevated sand

island itself and have had tables with umbrellas erected at their junctions. These appear

the weakest points of the structures. During the ni ght of November 14th 1993, a double

umbrella collapsed as the result (we were told) of large waves associated with an

unusually high tide.

It is deceptively easy to criticise initiatives that appear not to be working before they have

had time to prove their value. Bearing this in mind and ignorant of any future plans

regarding the development of coastal protection structures in the southeast part of

Eluvuka, we yet feel that so me comment is appropriate .

Groynes are mo st effective along coastlines where most sediment movement and erosion

occurs along (i .e. parallel to) the shore. In southeast Eluvuka, for reasons explained in

our earlier report (Nunn and Naqasima, 1993), most erosion is shore-normal rather than

shore-parallel. The groynes, as they exist at present, are thus serving as foci for erosion

rather than accumulation of sand . It may be that groynes will exacerbate rather than

alleviate erosion in this part of Eluvuka. We suggest extending the groynes seaward and

reshaping them so that they run , at least in part, parallel to the shoreline. This is offered

as a general comment rather than spec i fie advice.

lt seems possible that the sand eroded from on top of the beachrock (now exposed) in

so utheast Eluvuka was gradually put into circulation in the lagoon along the island 's south

coast where it is contributing to the turbidity responsible (in part at least) for the increase

in algal growth and productivity on the reef.

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- - -·---

N'Jvember 1993

100

metres

. ... .

RUBBLE

BANK

--- ~,l'lof\\n

position)

exposed

_..--- beachrock

..·• . . '.

Figure 3.

The eastern extremity of Eluvuka showing the position of the rubble

banks, groynes and the beachrock exposed during 1993.

3(iv) Recommendations

One way of reducing algal growth and productivity is to reduce the amount of sediment in

circulation. On Eluvuka, this can be clone by reducing the amount of sediment lost from

the island's beaches both in large volumes during single events (such along on the

northwest coast) and more slowly over long periods of Lime (such as in the so utheast of

the island). Appropriate shoreline protection structures must acknowledge the dominance

of shore-normal over shore-parallel processes .

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4'.

ALGAL GROWTH AROUND ELUVUKA

4(i) Introduction

The prolific growth of brown algae on the Eluvuka reef flat and fringing reef in the early

months of I 993 caused concern to the Treasure Island Resort management, particularly

because of the amount of algal debris which was being washed up along the island's

beaches. Two visits were made to examine these algae. One of us (MN) visited Eluvuka

from 7-9 August 1993, both of us visited Eluvuka from 12-15 November 1993.

4(ii) Dist,·ilrntion and identification of algae

Four main species of algae were found growing extensively across the reef flat and

fringing reef seaward of the mean low-tide level. Living algae are concentrated in the

middle to outer zo_nes of the reef flat, except locally. The inner reef flat is dominated by

detached masses of algae, particularly mobile at high tide, many of which coalesce and

end up on the beach at the high-tide level.

Few living or detached algal masses were observed on the fringing reef. This reflects the

fact that the preferred substrate for algal growth are sediments of silt and fine-sand grade,

such as are found in back-reef areas .

The main species are Sargassum sp., Padina sp., Colpomenia sinuosa, and Turbanaria sp .

In such species, because so much energy is invested in the reproductive process, there is

very little left over for the production of phenolic compounds that deter the growth of

epiphytic algae. As a result, the plants are smothered by a massive growth of microalgal

epiphytes which cause the plants to break up and drift onto the beach. This is believed to

be the main reason why such detached algae are daily washed up on the beaches of

Eluvuka.

4(iii) Temporal variations in algal distribution

Far fewer algae were observed, both living and detached, during the November v1s1t

compared to that in August 1993. Although all the algal species identified are common in

thi:, area, there are factors which occasionally cause their abundance to increase. Such

algal blooms may be the result of several factors, acting either singly or together.

:a) Increased water movement associated with rough weather conditions. Such

movement would cause the optimal distribution of algae.

(b) Increased supply of nutrients. This might result from pollution by chemicals,

sewage or appropriate discharges from ships. Algal blooms now occur in the

Sigatoka River on Viti Levu because of the increased amount of fertiliser and

pesticides being used on adjacent fields and being washed into the river. Water

sample testing will help evaluate the likelihood of increased nutrients causing algal

blooms around Eluvuka.

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c) Increased ocean-water temperatures. These co uld be associated either with gradual

Jong- term wanning w hi ch, w hil e uncloubl eclly occurring, is thought unlikely to

prod uce algal blooms sudd enly , or with a temporary influx of warm water into the

area. At E luvuka, it is possible that seasonal changes in seawater temperature are

associated with warm-water build -up in Nadi Bay, where most of the water

surrounding E luvuka co mes from (Figure 4). Such build-ups could occur

seasona lly or be beg innin g as the res ult of changes in land use around N adi Bay,

particul arly perhaps th e Denarau developm ent.

L _ _ _ _~

~ S:ivnl n

Tlvua o

MALO L O

9 M :,vinl

MAMANUCA

LAGOON

Figure 4.

The Mamanuca lagoon showing pathways for the movement of

sed im ent respo nsibl e for th e growth or sa nd islands in th e El uvuka

area. The Denanrn development is located around the sout hern tip of

Nadi Bay.

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1-11 e principal clanger of alga l blooms is that they will cause eutrophication. This 1s

aused by the excessive accumulation of algae and aquatic weeds (macrophytes) at the

:ater surface which can block off sunlight to the plants below . Death -of surface algae

and their subsequent decomposition can use up all the available oxygen in the water

leading to a foul taste and smell throughout the water body.

We consider that eutrophication is unlikely to occur around Eluvuka because of its open

situation. Eutrophication is a phenomenon usuall y restricted to confined water bodies

such as lakes and rivers in which water moves slowly. We would, however, advocate

continued monitoring of algal growth around Eluvuka.

5. WATER SAMPLE ANALYSTS

S(i) Sampling

Water samples were taken at the sea surface (0 m) and at depth at the 8 locations (sample

sites 1-8) shown in Figure 5 . These samples are all designed to test whether high nutrient

levels, possibly responsible for algal blooms, are found in the water around the sewage

pipeline. These sa mples were taken both in August and November .

Additional samples (samples 9-14) were taken of the water around the island itself to test

for levels of faecal coliform. These sa mples were taken in both August and November.

SAl.4PLE

. . ·· .. .

/ S IT ES

·.0 ~

SEWAG~ ··

p IP E

'- 0

~ - -,o'o

molr os

i<'igure 5.

Sample sites 1-8 111 relntion to the sewage pipeline (after Nunn and

Naqasima, 1993)

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S(iD Results

f<esults of the A ugust sa mplin g fo r nutri ents are given in Tab le I bel ow .

Tnble l . Nutrient. levels nt. sit.es 1-8 in August 1993

Site (d epth )

N itra te (µ.g/ 1

N OJ)

Nit rite (µ.g/ 1

N 0 2)

Phosph ate

(µ.g PO4/L)

Water

cla r i t y

1 (0 Ill)

< 34

I ( 15 rn )

< 34

2 (0 m)

< 34

2 1. 9

5 Ill

2 1. 9

3 1.0

2 (15 m)

< 34

3 (0 m)

< 34

3 ( 12 m)

< 34

4 (0 m)

45 .4

6 .9

4 (10 Ill)

< 34

5 (0 Ill)

< 34

5 (6 m)

< 34

6 (0 111 )

< 34

25 .5

25 .S

34 .7

43 .8

38.3

2 1. 9

25.5

34.7

4 Ill

5 Ill

3. 5 Ill

1. 5 Ill

6 (deep)

no sampl e poss ib le (water too shall ow)

7 (0 m)

< 34

7 (1 2 Ill)

< 34

8 (0 111)

< 34

8 ( 12 m)

< 34

34 .7

43.8

38.3

34 .7

4 Ill

4.5 Ill

Resul ts of nutrient anal yses at the same si tes 111 November 1993 are show n m

Tab le 2.

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Table 2. Nutrient. levels at. sit.es 1-8 in November 1993

Site (depth) Nitrate (µg/1 Nitrite (µg/1 Phosphate · Water

N03)

NO2)

I (µg PO4/L) I clarity I

1 (0 m)

124

3.5 111

1 (15 m) 120

215

2 (0 111)

150

<37

4 Ill

2 (15 111)

<34

3 (0 111)

<34

<37

3.5 Ill

3 (12 m)

132

<37

4 (0 m)

5 Ill

4 (10 111)

<34

5 (0 Ill)

6 Ill

5 (6 m)

142

6 (0 m)

1. 7 Ill

6 (1.7111)

<34

7 (0 m)

160

7 (12 m) 53

4 Ill

<37

8 (0 m)

<34

8 (12 m) 56

<37

5 Ill

<37

Results of faecal coliform measurements at sites 9-14 in August and November are

given in Table 3.

Table 3. Faernl coliform counts, August. and Novembe1; 1993

Site number

Faecal coli forms

(FC/ I 00 ml) in August

(FC/100 ml) in

1993

I November 1993 I

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Note: World Health Organisation reco111mended guideline for recreational water 1s less

than 350 FC/1001111

5(iii) Jnterpretat ion

The nitrate content of open ocean water is normally in the range 30-300 µg/1. The nitrate

levels recorded in Nove111ber 1993 were considerably greater than those recorded earlier

yet still well within the normal range.

There was very little change in nitrite levels . These remained consistently ·low . This is

normal , given that nitrite levels are usually found in lower concentrations than nitrate.

The dissolved phosphate content of coral reef waters is normally in the range I 0-30 µg/1.

Phosphate levels recorded in November were considerably higher than this and than the

levels recorded during our two earl ier visits . The surface sample at site 2 and all the

samples at sites 3 and 8 have remained consistently low in phosphate content.

Eleven of the sixteen sa111p les taken in November showed phosphate levels higher than

those desirable for coral growlh. Corals growing in high phosphate conditions have a

very low skeletal density and are therefore weak and break very easily. High levels of

phosphate also increase growth of benthic algae which then compete with corals for

space. High phosphate concentrations also red uce water clarity owing to enhanced

phytoplankton growth . Yet water clarity is fairly nor111al at all sites so we suspect that

phosphate levels in these sam ples have been exaggerated, probably because the seawater

was not filtered before testing .

Faecal coliform levels were higher in November but still well below the World Health

Organisation stanclarcls for recreational water.

In ,u111111ary, nutri ent levels were normal in 1993 . There is no reason to suspect leakage

along the sewage out.fall pipe or co ntamination fro111 elsewhere.

6. CORAL GROWTH

6(i) Coral growth

Two dives were made in November 1993 to observe coral growth on the pipelines . The

first dive was along the sewer pipeline fro111 Eluvuka and the second dive was along the

freshwater pipeline between Eluvuka and the Viti Levu mainland.

There was a manifest in crease ·in co ral growth on the sewer pipe compared to the last

survey in October 1992. There were 111ore colonies of Acropora and Pocillopora, the new

colonies 111easuring 10-15 cm in dia111eter. There were a few Acropora colonies

111easuring up to 30 cm in diameter.

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':]"f�e:1•,�c0�ee1e1ycewtodhraeasilnsvapeOnordclyytyoplbisotetclareera1ecn9hn9afo2lnto.goebrToirhnbieunrcgteoaroirasrelgnaosonvuiescsprmageursnsodewyeedftthoratohbcneoorvnteheceearirtfne.reasstbThroehwutecathtwetehrseeipgopihfpitpepeoispfibneecciowenrghaitilspchuownndacsottuhlrenaesosdett

}uspended sections of pipeline may cause stress and eventually rupfure.

The second dive was conducted along the freshwater pipeline between Eluvuka and Tai

(Beachcomber) Island. On a populated stretch of 3.5 111, there were as many as fifteen

colonies of Acropora and Pocillopora measuring 12 cm in diameter. The number of

colonies will have to be monitored closely here, particularly in places where the pipeline

is suspended above the ocean floor rather than resting upon it.

6(ii) Recommendations

There is a healthy fringing reef on the eastern side of Eluvuka. The coral species

diversity is high although invertebrates are not especially abundant. Snorkellers would

benefit from a snorkelling trail identifying the main types of coral by name and form.

Giant clams and the larger holothur ians (sea cucumbers) are among the invertebrates

whic:h could be easily identified for snorkellers.

Nutrient levels and algal abundance should continue to be monitored, every few months if

possible, to confirm any seasonal differences.

7. SUMMARY OF RECOMMENDATIONS

We recommend:

(a) that shoreline protection structures continue to be constructed but that their form

acknowledge the dominance of shore-normal sediment movement over shore

parallel sediment movement.

(b) that beachrock distribution along the Eluvuka coast be closely monitored.

early maps and photographs.

(cl) that water clarity measur ements and nutrient analyses continue to be carried out

along the sewage pipeline (sites 1-8).

(e) that faecal coliforms continue to be measured at sites 9-14.

(f) that algal abundance and species composition be closely monitored.

(g) that reef growth along the various pipelines continue to be monitored.

(h) that snorkelling trails are established for visitors across the Eluvuka fringing reef.