f

12 downloads 0 Views 2MB Size Report
river, which is one of ~he most conspicuous factors contributing ..... Average monthly wind directions recorded at Malindi Airport. Meteorological Station in the ...
r·:.. .-

*ccr-S C :

i

~6 C'l

(cC

2/

.-

'7 C(

"

",I·

/

.

'.

' ("" iJ i t' ,

j , " \,' ' ' - ; l "

\

",

~~

...

,)_--..)

"

'\

, . " \ ''••;

\'

,~ "-

" ,\.1

I

..

'. I ,,- .'r'/

1'

~"---_.,/_/

......--.. : ' - .

y

........

// ./

.,;1 ....

n

.~

T~e

Impact of River Discharges on a Kenya Coral Reef

Ecosystem - the physical processes

, . Part two:

I

.

.:

Effects on the Malindi - Watamu Coastal Environment

... f . •

J. Wim Giesen (

I

Karin van de Kerkhof

f "

r:

f~'

r I

I'.

Nijmegen, the Netherlands August 1984

..

"

I

r 4"

I I

~

--

\

........

f

.

".

-...

!

!

\ •... ,

Preface

;".

~~ J~. , ,I. ' .._. r:' ~: :,j .:111 B P.. :.;/\ ('

'..

')0

'

This study was carried out on the fringing reefs near Malindi­ Watamu, Kenya, in the period October '82 to May '83 as a part of the 'Watamu Reef Expedition'. Participants of this expedition were students of the Laboratory for Aquatic Ecology and the Department of Human Geography, both of the Catholic University of Nijmegen, the Netherlands. The aim was to quantify and qualify 'coral death', to assess th3 ~\

contribution of certain factors of disturbance to the decline

of these reefs, and to assess the impact of changes in the reef

on the coastal inhabitants. Our part in this study consisted of

an attempt to quantify and qualify the physical characteristics

'of the reef environment. Special attention therein was paid to the increased influx of terrestial silt originating from the Sabaki river, which is one of ~he most conspicuous factors contributing to the disturbance of this marine milieu. The report has been subdivided into two parts: the first deals with 'The Athi (Sabaki) river Basin', and the second with 'Effects on the Malindi - Watamu Coastal Environment'

Wim Giesen Karin van de Kerkhof

l correspondence address: 1

I l

Toernooiveld Nijmegen the Netherlands

I

(

, ,

Laboratory for Aquatic Ecology Faculty of Science Catholic University of Nijmegen

?

.. .lo

Part two:

Effects on the Malindi - \,latamu Coastal Environment

contents page iJreface

o. O. 1

Introduction General Introduction

1•

1•

0.2

Introduction to the Study

4.

0.3

Acknowledgements

5.

1•

Hydrodynamics and Meteorology

7.

1• 1

Indian Ocean Circulation and Currents

7.

1.2

Tides

8.

1.3

\,laves

9.

1.4

Meteorological Data rainfall

9.

1 .4. 1 1.4.2 1.4.3 2. 2.1 2.2 2.3

.' 3. 3.1

winds temperature, etc ••• Sabaki River Discharge Flow Rates Discharged Solids Estuary Morphology Sabaki River Influences on the Marine Environment Coastal Transport of Particulate Matter

9.

11 • 11.

15. 15. 16. 18. 21. 21.

3.1.1

modes of transport

21.

3.1.2

general pattern of Sabaki River transport

22.

3.1.3

suspended ma tter

24.

3. 1 .4

deposited matter

29.

3. 1.5

substrate changes

38.

3.2

Temperature

42.

3.2.1

methods and results

42.

3.2.2

discussion

42.

contents, continued: page 3.3

Light

45.

3. 3. 1

introduction

45.

3.3.2

visibility measurements

45.

3.3.3

absolute light intensities

49.

3.4

-.ialinity

3.4. 1

introduction

3.4.2

measurements and discussion

3.5

Broad Outline of Chemical Composition

56. 56. tj6. 63.

3.5.1

introduction

6.3.

3.5.2

nutrient status

63.

3.5.3 3.5.4

02 content pH changes

66.

3.5.5

minor constituents and possible toxins

67.

67.

4.

Summary and Conclusions

69.

5.

Recommendations

71.

6.

Referencfs

73.

7.

Appendix

80 ..

-

1•

The Imoact of River Discharges on a Kenyan Coral Reef Ecosystem Part two:

Effects on the Malindi - lJatamu Coastal Environment

o.

Introduction

o. 1

general introduction

The

influen~e

of the Sabaki river on the marine environment is

pro b a b.l y Ion 9- s tan din 9 , a sma y b e j u dg e d fro m the s hap e

0

f t :1e

(SUbmerged) coastline depicted in fig. 0.1 below. Both the Sabaki and the Tana river show a typical submerged, fan-shaped delta extending to a depth of about 200m. In the case of the Sabaki this may extend up to about 20 km from the coast.





I

i

I I I I I I I

-...,

KENYA fig. O. 1 3°



The Kenya Coast (adapted from Schroeder et aI, 1974)





50 km , 5°

ISLAND

in m

4,°

---

·2.

further evidence for the recurrent influence of terrigenous material in the area is given by Pleistocene formations visible in many parts of the study area. Schroeder et al (1974) noted definite signs of reef siltation and recovery in such material in the quarry that lies several hundred metres east of the town­ ship of Gede, some twenty-odd kilometres SSW of Malindi. The disturbance caused by the silt-laden Sabaki waters in the Malindi - lJatamu area is a seasonal event, occurring only during the months of the north-east monsoon, from

Novem~er

to March.

Plate 1 below shows the situation typical of these months, with a plume of discoloured waters the Sabaki mouth. Plate 1:

e~tending

into Malindi Bay, from

Malindi Bay, 11 th February 1965 (originally taken at a scale of 1:80,000; here enls~gQg to approximately 1 :150,000)

Courtesy of the Survey uf Kenya

3.

2,

In the course of the past few decades a number of changes have occurred in the Sabaki river discharge. This has given reason to believe that this stream is the prime suspect in the decline of the Malindi - LJatamu reefs, (Green et aI, 1979; Oidham, 1982). Several changes are notable. The flow pattern of the river has become more erratic, with flash-flooding and mere-trickle flows being more recurring phenomenae than in the past (TAROA, in LJatermeyer et aI, 1981). During the wet season the discharge 3 rate may amount to 5000 m3/s, dropping to a~out 20 m /s in the dry season (Delft Hydraulics, 1970). The total amount of soil and other particulate matter discharged into the Indian Ocean has also increased dramatically in the years after 1960. For the years before this date, an annual deposition of about 58,000 tons has been estimated. After this date, nowever, the figures lea~ed, and present (1983) estimates vary between 7.5 and 14.3 million tons per annum (LJatermeyer et aI, 1981 ) • Another change that may have enhanced the effect of the Sabaki river waters in the region south of the mouth is the southerly deflection caused by a recently created sandbar. During the freak floods of late '61 the Sabaki cut through one of its southern channels at the mouth, and a sandbar was subsequently raised in front of it (Oidham, 1982). These changes have not gone unnoticed. Before 1960 a steady erosion of the beaches in Malindi Bay was reported. After this date, however, a constant accretion has taken place, extending the beaches by about 150 - 200 m. Initially this was welcome, as is demonstrated by the statement made by Delft Hydraulics in 1970:

This is a favourable phenomenon, which should be

gratefully accepted ••• ' Unfortunately, the deposition has not stop~ed. As a result beach hotels are no longer beach hotels, due to an ever-increasing distance to the waters, and the extensicn of the jetty in Malindi Bay is outpaced by accumulating sediment, (see plate 2, next page) •

4 ••

Plate 2:

MaLindi Bay Jet.y (photographed by Ron Eijkman of the ~atamu Reef th Expedition, on the 29 March 1983, at low tide)

Due to the minimal water depth, even at high tide most of the jetty is useless for mooring boats. The dark bands ar.e accumulations of mica and organic matter

0.2

introduction to the study

As was mentioned in the preface, this study was carried out as a part of the 'Watamu ·Reef Expedition' , a joint study by ten biology MSc students and four human geography MSc students of the Catholic University of Nijmegen, the Netherlands. The field work of this part of the study was carried out in the period November '82 to April '83. Twenty-nine sampling points (1A- 10C) located between Malindi and Watamu were marked with buoys, along ten transects at right angles to the coastline,

(see fig. 3.5).

5.

At intervals, water samples were collected at two different depths (1m below surface and 1m above the sea-bottom) at all these sites. These samples were filtered to assess the suspended matter content and further composition. The filtrate was fLrther analysed in the Netherlands. At all sites water temperature, depth and visibility were registered periodically. Sea-bottom soil (grab-)samples were taken at sites 1A - 10C, and beach grab samples were taken at sites 1 - 26 (see fig. 3.5). These were analysed in the Netherlands for carbonate, silicate and organic matter content, and visual attributes were recorded. Data were gathered on meteorology (Kenya Meteorological Department,

Nairobi), Sabaki water quality (Water Quality Section of the

Ministry of ~ater Development) and the coastal environment

( a host of reports and articles, for example, those of Isaac

and Isaac, 1968; Delft Hydraulics, 1970; Norconsult,1977).

Aerial photographs obtained from the Survey of Kenya, Nairobi,

were used to assess changes in reef and estuary morphology

(chapters 3.1.5 and 2.3, respectively).

0.3

acknowledgements

Our list of acknowledgements is large, which reflects both the broadness of the study and the limitations of our own knowledge. :r

We are especially indebted to the head of the Laboratory for Aquatic Ecology of the Catholic University of Nijmegen, Prof. Dr. C. den Hartog, and the staf members of this laboratory, Mrs. Lallie Didham, honorary warden of the Malindi Marine National Park, the acting warden, Mr. J. Nyamongo and the staf of the Malindi Marine National Park, and Mr. F.N. Pertet, assistant director of the Research Section of the Conservation and Management Department.

~ildlife

We are further indebted to, in alphabetical order; Mr. Wilfrid Asawa, Provincial Warden of the Wildlife Conservation and Management Department for the Coastal Province. Miss. frances Green, a member of the in Malindi Marine National Park •

.-.z

1979~eopard

Reef Expedition

_

6,

Dr. A. Hillier and Dr. R. Ruwa, staf members of the Kenya Marine and Fisheries Research Institute, Mombasa.

Dr. V. Jaccarini of the Department of Zoology, University of

Nairobi.

Dr. Fred Janssen, Mr.

de Bruin and Mr. M. Zuiderwind of the

~.

N.I.D.Z. in Texel, the Netherlands (Dutch Institute for Marine Research). Mr.

I.M. Kilonzo, Director, and Mr. J.K. Matulu, of the

Quality Section of the Ministry of

~ater

~ater

Development, Nairobi.

Prof. Dr. H.F. Linskens, Head of department, and Mr.

~.

Bogeman,

of the Botany Department of the Catholic University of Nijmegen.

Mr. Mbuvi, Director of Research of the Management Department, Nairobi.

~ildlife

Conservation and

Nedlloyd shipping company Ltd., the Netherlands.

Mr. S.J.M. Njoroge, Assistant -director, and Mr. P. Muna of the

Climatological Section, Kenya Meteorological Department, Nairobi.

Prof. Dr. F.F. Djany, Head of the Geography Department of the

University of Nairobi.

Dr. P. van Dosterom, staf member of the Soil Science Department

of the Agricultural University of

~ageningen,

the Netherlands.

Dr. B. van der Pouw and Dr. J.J. Vleeshouwer, staf members of STIBDKA (Stichting voor

Bodemkartering/Fo~ndation for

Mapping) in Wageningen, the

Soil

Ne~herlands.

Spac Sport Ltd., Nijmegen.

Staf members of the Coast Province water Branch of the Ministry

of Water Development, Mombasa.

Staf members of the Survey of Kenya, Nairobi.

Stichting Nijmeegse Universiteitsfonds (S.N.U.F.)

Mr. S.M. Tsalwa, Director of the Government Chemists Department,

Mombasa.

Werkgroep Studiereizen Dntwikkelingslanden (W.S.O.) in the

Hague

(~orking

group for Study visits to Developing countries).

Dr. M. Wijsman-Best, staf member of the Museum of Natural History,

Leiden, the Netherlands.

6,

-

7.

~rodynamics

Indian Ocean

1.1

and Meteorology

circulation~tem

and currents

As a consequence of the seasonal change in wind conditions there is a marked seasonal variation in the equatorial surface current system in the Indian Ocean (see fig. 1.1 below). During the period of the south east monsoon, in April - No\,ember, the East African coastal current flows northward, with an average velocity of 0.8 - 1.8 mls (Schroeder et al, 1974). In the subsequent months, the period of the north east monsoon, this current still generally flows in the same direction, but is usually weaker, with average velocities of 0.4 - 0.8 m/s. Occasionally the southward flowing countercurrent may extend as far south as Malindi (Isaac and ~

Isaac, 1968).

fig. 1.1

monsoon currents in the Indian Ocean

f

"

r r-------------------------­

r II'

10' I • DI A •

I • DI A •

D c"[ A •

50 'I[

MOISDDI

la'

)

GC [ A •

la'

la'

IE

MOIIGOI

adapted from Schroeder et al, 1974.

Inshore currents may show a different pattern during the north east monsoon, and a southward current generated by the winds may be observed, with a velocity of 0.4-0.5 m/s. Few recordings of currents have been carried out in the waters near Malindi. As far as the authors can gather, only Delft Hydraulics (1970) under­

1IIIIIn

_

8:

took such activities on a few subsequent days in June, 1968, and these have been quoted by most other authors since. Ue will do the same, as the paucity of data leaves no alternative. At Vasco da Gama's Point they recorded a steady north-going current of 0.2 mls average and 0.35 mls maximum. In Malindi Bay, near the Sindbad Hotel, they recorded weak currents without a consis­ tent direction of about 0.1 m/s. Such incidental recordings hardly form a basis for long-term values, however. During the course of our study, for instance, a very strong southward flowing current was at times very noticeable. On certain days it was even very tangible, sweeping the few thrill-seeking bathers hundreds of meters down the coast and making wading into the current impossible. The velocity of such a current may easily exceed 2 m/s. Current veloci ies as given by several authors are given in table 1.1

current velocities (in m/s)

table 1.1

location

of shore

below.

north east monsoon direction

veloci ty

south east monsoon direction veloci ty

northward

0.4-0.8

northward

0.8-1.8

northward

0.52

northward

1.6

northward

2 - 2.5*

northward inshore

*

southward

0.4

northward

southward

0.5**

northward

authors

Schroeder et a1 (1974) Delft Hydraulic (1970) Norconsul t ( 197 Delft Hydarulic (1970) Norconsult (197

maximum values

** estimated average maximum

1.2

tides

The tides off the East African coast are semi-diurnal, i.e. there are two maxima and minima respectively, per lunar day of 24 hours

)

c'

tab 1 e 1.2

rainfall at MalindiAirport Meteorological Station, I

J

F

8.9

11.6

monthly total 1981

5.7

monthly total 1982

2.0

long term average

M

A

M

J

48.8

158.9

307.3

153.3

0.2

274.7

87.2

220.5

0.0

79.2

185. 1

581. 1

J

(in mm)

A

S

93.7

66.3

47.0

122.9

59.5

79.9

158.4

158.3

19.7

0

N

0

77.5

75.6

35.6

48.5

52.5

34.2

17.2

49.3

279.2

85.1

92.2

(1962 - 1983)

yearly totals:

table

long term average - 1084.5 mm;

1982- 1689.6mm.

rainfall at Malindi Airport Meteorological Station,

1

5

10

15

.

coo

• .... N

November 1982

December 1982

1981 - 1003.0 mm ;

....

....

20

25

..

30

.• .

--t

"'}

£; i.\ :>/\

As was stated in the general introduction, modifications in the Sabaki mouth during the late '61 floods have been conjectured by locals and some authors (Delft Hydraulics, 1970; Green et aI, 1982), as a possible enhancer of the silt influx in Malindi Bay. Fig. 2.1 shows the Sabaki estuary morphology on four dates (1960 - 1975), as adapted from aerial photographs obtained from the Survey of Kenya. A number of changes are marked. Firstly, the Sabaki did tend to flow more through southerly channels at the mouth, up to the late 60's. By 1975, however, the most southern channel was apparantly blocked, choked by accumulating sands and clays, and this passageway was limited. Secondly, due to the accretion of material at the mouth, the Sabaki is forming a delta that is slowly-but-surely extending seawards. Perhaps not as slowly as may be suggested, as the progression, estimated from the photographs themselves, amounts to about 320 m for the period 1960 - 1975. Thirdly, the extensive mangroves that were present in 1960 were greatly reduced in cover in the period' 60 - '65. The 1961 floods had obviousl) taken their toll in these communities. By 1975 some limited increment in area is noticeable, but the recovery is slow. The last change that can be noted is that of the river channel. In 1960 the channel is relatively narrow compared to the later situations~ At a point lying about 500 m downstream from the bridge, the width in 1960 was about 80 m; in 1975 this had widened to about 150 m.

*

Tana and Athi River Development Authority

...

------~--------_

:

,

Tr

'


"'l

l! >

..

1

1

I

....... •

1 I

'-0:.

..... ....... _. . ."."..

I I

./

f

I 1

I",

1 I I

I

Deposition

1

.. ""..

I

I

I

:is

ll-

10

,U I

I

I

I I

I I I

I I

I I

I

0.1

I I

12 115

I

I

1

0.01

I

I

I ~, I ~I

I

I

I ~

'/

I 12 ,

lii

I

0.1 0.001

I 1 I

Erosion

I

I

I I

>. I

0

I

1

I

-. '"

I

I I

I

I

10

100

Diameter of grain (ITIJJI)

adapted from Thurman, 1978

Transport may occur along four different modes: traction, saltation, suspension and flotation.

In the case of traction, particles are

~

22.

rolled or dragged along the bottom as bed load.

~ith

saltation,

particles are moved in a series of short, intermittent leaps or bounces from the bottom. Suspended particles are held indefinitely in the water by upward current eddies, which are abundant in a turbulent flow. Particles transported by flotation drift at the waters surface, held by the surface tension, water buoyancy or by air bubbles (Schroeder et al, 1974). Organic matter is usually transported to a great extent by flotation and perhaps suspension, while non-organic soil particles are transported by the first three modes. Coastal sediments may reveal their mode of transport or their genetic environment from their grain-size distribution. Fisk (1983) and Visher (1969) report the following relationship between particle size a~d mode of transport for the Great Barrier Reef (Australia) and the south/south-eastern coast of the U.S.A. respectlvely:

3.1

particle size and coastal mode of transoort

mode of transport

particle size (in mm)

table

Fisk (1983) *

Visher (1969)

traction

> 0.5

> 0.35

saltation

0.125-0.5

0.1-0.35

susp ension

< 0.125

10m). It must be b~gl"'ed in mind, however, that these values hold for the period of the NE - monsoon, which i s war mer t han the subs e que n t S E - m0 n soon (s e e c hap t e r 1. 4. 3 ) • Kinne (1970) reports 27 - 28 °c for this section of the Indian Ocean during the month of February, which is slightly lower, as he does not deal with inshore waters alone. Sea-water temperatures were furthermore found to be closely related to topography and site depth, as is indicated by the fact that the highest water temperature was attained in a shallow pool (33.0 °C, site 58, December '82, 40 cm water), and the lowest temperatures at deeper sites (sites 78, 7C, 88, 8C, on most occasions).

is that no direct influence on water temperature is exerted by the cooler Sabaki waters, as no gradient situation is evident, with increasing distance from the estuary. However, at all sites the general trend is that of lower water temperatures (about 1 °C) in January 1983, as compared to the other dates. As cloud cover in the Malindi area was lower in January than in the other months, differences in irradiation cannot account for this phenomenon. The only A 'prima facie' impression given by fig. 3.13



°c

.'

°c

33

33

32

32

31 30

29 28

:~/\-/-.

,/'-~\",,/

27 2

3

4

5

6

7

30

29

28

~'\\J'" V \, \.~.~:.......,

x7

.......,/

0" .D

27

0-0

1

31

8

9

10

1

2

3

4

5

6

A sites

a

7

9

10

B si tes

DC fig. 3.13

33

sea-surface temperatures*

32 D

December 1982 January 1983

X

March

31

29 28

27

~

'~>L\-~/ x

}-x_x_x

*

1983

measured about 1m below surface

O__ n ~



30

b-

2

3

4

5

6

7

-----------------------_. 8

9

10

(,J

----

, .

r-

~4.

I I

f

:>

i

I

explanation we can offer is that the Sabaki does exert a minor influence on water temperature (a lowering of about 1 °c in the Malindi/Watamu area), but that it is rather diffuse, and that our methods for measuring temperature were too crude to detect possible gradient situations. As was stated before, the NE - monsoon period, during which our measurements were carried out, is characterized by higher air temperatures than the SE - monsoon. This is also displayed in sea water temperatures. McGill (1973) and Kinne (1970) both report temperatures of 25 - 26 °c for this section of the Indian Ocean du r i n g the S E - m0 n soon. Ins h0 r e wa t e r s pro b a b 1 y de v i ate s 1 i g h t 1 Y from this, as the average nightly minima for the months JulySeptember lie below 22 °c (see 1.4.3). The surface temperatures probably range from 23 - 25 °C. Do these temperature fluctuations affect coral growth in any . adverse way?

Kinsman (1964) reports a temperature optimum of

25 - 29 °c for scleractinians, and tolerance limits of 17 - 35 °C. Other authors (Endean, 1976; Thurman, 1978) generally agree with 0

these figures, though a lower tolerance limit of 18 C is usually mentioned. Sea-bottom temperatures, which are of more importance to corals than surface temperatures, of course, are slightly r.

o

lower than surface temperatures (a difference of 0.5 °C, at average, was found; see table

A

in appendix). The average

annual temperature fluctuation in the study area is usually at o n ;1

:'1 :'ol

least 25.5-29.5 °C, for shallower waters «5m), and 24.5-29.0 for deeper waters (> 10m). During the SE - monsoon inshore waters may be several degrees cooler, however, especially if cooler night temperatures, for instancp, coincide with rainfall and

Q

C

low tide. On the whole, water temperatures will remain within or 1n close proximity of the optimum levels for coral growth. Only at specific sites may corals experience a temperature stress: - in shallow pools, where temperatures may rise to at least 33 °C, and probably at deeper sites (78, 7C, 88, 8C) during the SE - monsoon.

45.

3.3

Light

3.3.1

introduction

Sabaki waters may have a dramatic effect on light penetration in the marine environment. During the north-east monsoon a zone of opaque, reddish-brown water moves down the coast, usually to beyond Malindi township, and occasionally extending as far south as Watamu (plate 1 ). Under these circumstances visibility in Malindi Bay may be reduced to almost zero, and in the Malindi Marine National Park it may be troublesome enough to impede reef viewing by tourists and hamper boat navigation. When the influx of muddy terrestial waters is particularly bad, large parts of the reef further south may even be obliterated, and many fisher­ men dare not venture out with their craft under such conditions. Light attenuation in water is caused by a combination of the factors reflection, adsorption and scattering. Reflection is mainly a property of the water surface, whereas turbidity is the product of the scattering and adsorbing properties of the sus­ pended particles. Sabaki water is very turbid due to a high con­ centration of soil particles and detritus. Turbidity is further increased by the occur~ence of algal blooms, which arise due to the influx of relatively nutrient rich riverine waters in the nutrient-poor marine environment (Aleem, 1972; Qasim and Sankaranarayanan, 1972). On a wider scale this latter phenomenon has been noted by Wickstead (1961), who reports of the bottle­ green colour of northern Kenyan waters due to phytoplankton blooms, compared to the clear blue waters elsewhere on the East African coast. 3. :3 .'2 ..

visibility measurements

Light measurements in water are difficult to carry out success­ fUlly, and in a way that allows comparison with other areas and with previous measurements. The main cause of this is that light adsorption in water is a non-lineair phenomenon that depends on the wavelength of light. Red light, for instance, is adsorbed much sooner than green and blue light. A simple method to by­

r

/

!

pass this difficulty is to measure 'visibility' instead of

absolute light intensities. Vlsibility is usually measured with the aid of a so-called 'Secchi-disk', whereby the maximum depth at which this circular, 30 cm diameter, smooth white disk can still be observed is determined (Thurman, 1978). The disadvantages are obvious, as no absolute values are known directly and sub­ jectivity may bias figures. On the other hand, the determination is swift and simple and values may well be duplicated, especially if the readings are carried out by one person. In our study area Secchi-disk measurements could not be carried out directly according to the method outlined above, as in most cases the maximum visibility (in metres) exceeded the depth of the reef waters. A method was thus devised whereby the disk readings were carried out horizontally, at about one metre depth, by a pair of divers. One diver would hold the disk stretched out in front of him/her at about 1 m depth, facing the sun, and the other would swim away backwards, unreeling a tauntly held measuring tape until the disk disappeared from view. The maximum distance was noted, and the procedure was carried out at least once again.

'Visibility' at a given site, at a given moment, was the

average of these measurements. As tidal motions were found to affect turbidity, measuremen~ were carried out during the first hours of tidal ebb. In all, these were carried out at the sites 1A- 10C (fig. 3.5) on three dates. The values found are given in table A

of the appendix. In fig. 3.14

below, 'relative visibility', the values are displayed on a rela­ tive scale, expressed as a fraction of the maximum visibility encountered in the area during our study. This maximum was 24~, noted at site 6C on 7 December 1982. This maximum visibility value of 24m is probably close to the maximum attainable in the area, as McGill (1973) reports a rela­ tive transparency of less than 20 m for the Kenyan coast. A glance at fig.3.14 shows that under most circumstances there is an in­ crease in visibility from transect 1 to 10, i.e. from north to south in the study area. In other words, visibility increases with the distance from the Sabaki mouth, which is hardly suprising • .An exception to this is formed by the earJ.y December '82 situation,

% 100

% 100

x

80



x

80

~

~:~/o_o/:~

60

/ )~_I / ,/.4\/­ U/

60

~/~/1'0 ·

40

x/x,,-x

0

40

°

20

20

1-/0/ x

1

2

3

4

5

6

7

8

9

10

1

2

3

4

5

6

9

10

/ \

'",,-,

.

80

8

8 - si tes

A ­ sites

% 100

7

60

fig. 3.14

Relative visibility (100%=2410

40

/0



5&:7 December 1982



24 &: 25 January 1983

D -

10 &: 11 March 1983

0

20

visibility)

.t:>

...., .

-.J

2

3

...

5

6

-------------------------7

8

9

10

48 ...

. when a maximum visibility seems to be attained in transects 5 and 6. This may indicate that Mida Creek, a tidal inlet between transects 9 and 10, is a source of suspended particles, or of nutrients that stimulate phytoplankton growth. The temporal pattern of terrestial water influx in the area followed that of rainfall in the hinterland (see table 9, part one, The Athi river basin). Heavy rains had fallen throughout the Athi basin in November and in the first half of December, and silt-laden waters had just entered the northern part of the study area prior to our first measurements on the coast in December. This silt spread slowly throughout the area, aided by a steady north-easterly breeze (see table 1.4) that prevailed up to late February. Visibility throughout _ the area was at its lowest at the end of January '83, improving steadily during February. This latter improvement was due to a sharp drop in the discharge of Sabaki waters (and thus' in silt influx), as little or no rain had fallen in the drainage basin of this river during January and the first half of February. In the second half of February, much precipitation was recorded in the hinterland. By the end of the month the effect at the coast (especially in Malindi Bay) was agEin very noticeable, though nat as dramatically as was the case in late November and early December 1982.

Table 3.4 below gives the average visibilities for both the northern (1 - 5) and the southern (6 - 10) transects at the three moments of measurement. table 3.4

average visibilities

transects

date

visibility (in metres)

1 - 5

5 December '82

7.4

1- 5

24 January '83

4.4

1 - 5

10 March '83

6.9

10

7 December '82

17.8

6 - 10

25 January '83

10.9

6 - 10

1 1 March '83

11 .0

6 -

49.

If the January and March situations are compared, one may tenta­ tively state that, whereas

vi~ibility

in the northern transects

S88ms to improve (from 4.4 to 6.9 m), the situation in the southern transects seems to remain stable (10.9 and 11.0 m). This may seem strange when one bears in mind that an influx of silt-laden waters in the northern area occurred in this period. A possible explanation for this is offered by the fact that most of this latter influx had not progressed further south than Malindi Bay. Another explanation is offered by the occurrence of higher current velocities in the north, due to reef and coastline morphology and the occurrence of channels. A comparison of visibility at A,B and C sites per transect reveals no general tendency. Within a zone of up to several kilometres from the shoreline, the turbidity at a given site is more related to the coastline morphology, the presence of channels and the nature of the currents than the distanc~ from the coast. At tran­ sects 1 and 2, for instance, turbidity is higher close to the coast, while at transect 3 the reverse is the case.

absolute light intensities

No direct measurements of absolute light intensities were carried out, but these may be calculated from Secchi-disk visibility readings. A useful account an this subject is given by Weinberg (1976). Light penetrating any medium will undergo attenuation according to the Lambert - Beer law: -k.d

e

(equation 1)

whereby: Id

= light

intensity at depth d

1 0-

= light

intensity just below the water surface (= total light

minus the surface reflection) k

=

d

= depth

attenuation coefficient (for that medium)

50.

The k constant is unfortunately not as constant as one may wish, as it varies with light wavelength and with water turbidity. Equation 1 may be written as: Id /

I 0-

=

e

-k.d

whereby I / 1 - expresses the relative proportion of surface d 0 illumination that reaches depth d. ~einberg

(1976) calculated the following relationship between k

and Secchi-disk values: k

=

2.6

(D

sd

- 0.048

(equation 2)

+ 2.5)

whereby: Dsd

=

Secchi-disk maximum visibility depth

This relationship was found for the spectral band 460 - 510 nm (blue - blue-green light), and is not valid for other regions of the spectrum * • It is furthermore only useful in the D range of sd 5 to 35 metres, where the average error was found by ~einberg to be about 5.5%. For all sites in the study area Dsd (determined according to our own method) and d are known, thus allowing a calculation of I / 1 ­ through a combination of equations 1 and 2. d 0 The percentage of surface illumination received by benthic organisms at a particular site can be determined by calculating I / 1 ­ d 0 for d = (maximum depth at that particular site). In table 3.5 the percentages of surface illumination received by benthic organisms at the various sites, and under various circum­ stances, are given. The variation indicated is the allowance made for variations in depth due to the tidal cycle. No allowance has been made for tidal effects on turbidity, nor have corrections been made for other factors influencing turbidity, or for vari­ ations in water surface reflection. Depths of sites are indicated

'* This

band falls within 9ne of the peaks of the 'action spectrum' of virtually all assimilating organisms, including coral algal symbionts (Bidwell, 1974).

51.

table

3.S

site

5/7

1A 18

2 - 16

.

....e ....-
.

...... '

Dec. 32

OJ

v 10/11

­

~_~_ •. / ~

e

...

.

.

"-._

',,'

/}_4/_2,5 Jan • -.~~:5/7 Dec.

~/"

.... -
-

,

./

................ ,

....

deep water samples

,24/25 Jan.

,,

,'

.

5/7 Dec.

32

0111

Mar.

30

~~~

28, 2

]

4

~,

7

(,

.

.

R

9

10

transect

a a

'J

'a

-

.... .....

>-

_ ......,

34

I

.....c ..-t l1J

I/l

,.'

.... ,

24/25Jan.

_--~

34

Dec.

5/7

,

a 'a

1 U/l 1

Mar.

32

...,>-

.../

.....

/"_?,4/25 Jan. . ., 5/7 Dec .

~""""

.'/

c

...,,;:/"

"'

I/l

JO/11

Mar.

]2

3D

:III

si~~~

2tl,

"

2

3

G

site c

I

7

,

10 13 9 transect

20

I

.'

I

2

3

4

5

6

7

8

.

9

1]

transect

o~

a

fig. 3. 19

I

0 0

salinity

"0

- 34 >.

.....

~ "

I

~_/ , , , ... -,' .... -,""

32

,

,"

she 11 0

I.J

,I

--"ceE;J

,,

30 ~

,,

, ,,

J,. " t'

a.--'il..1

!ite

281 2

3

4

5

6

7

8

9

Oec.

10

transect ~

0 0

0 0

"0

'0

~

3 f,

..,>-

..,

>-

c

....

.~

c

'

-