Editors Salomão Bandeira | José Paula

THE MAPUTO BAYECOSYSTEMEditors Salomão Bandeira | José Paula

Book title:The Maputo Bay Ecosystem.

Editors:Salomão BandeiraJosé Paula

Assistant Editor:Célia Macamo

Book citation:Bandeira, S. and Paula, J. (eds.). 2014. The Maputo Bay Ecosystem. WIOMSA, Zanzibar Town, 427 pp.

Chapter citation example:Schleyer, M. and Pereira, M., 2014. Coral Reefs of Maputo Bay. In: Bandeira, S. and Paula, J. (eds.), The Maputo Bay Ecosystem. WIOMSA, Zanzibar Town, pp. 187-206.

ISBN: 978-9987-9559-3-0© 2014 by Western Indian Ocean Marine Science Association (WIOMSA)Mizingani Street, House No. 13644/10P.O. Box 3298, Zanzibar, Tanzania.Website: www.wiomsa.orgE-mail: secretary@wiomsa.org

All rights of this publication are reserved to WIOMSA, editors and authors of the respective chapters. No part of this publication may be reproduced, stored in a retrieval system, or transmitted in any form or by any means, electronic, mechanical, photocopying, recording or otherwise, without the prior permission of the editors and WIOMSA. The material can be used for scientific, educational and informational purposes with the previous permission of the editors and WIOMSA.

This publication is made possible by the generous support of Sida (Swedish International Development Cooperation Agency) through the Western Indian Ocean Marine Science Association (WIOMSA). The contents do not necessarily reflect the views of Sida.

Design: Marco Nunes Correia | designer of comunication and scientific illustrator | marconunescorreia@gmail.comPhotographers: credits referred in respective legends.Printed by: Guide – Artes Gráficas, Lda. (www.guide.pt)Printed in Portugal

T h e M a p u t o B a y E c o s y s t e m XVII

TABLE OF CONTENTS

Foreword by the Rector of UEM

Foreword by the President of WIOMSA

Acknowledgements

List of contributors

PART I ENVIRONMENTAL AND HUMAN SETTING

Chapter 1. AN INTRODUCTION TO THE MAPUTO BAY

José Paula and Salomão Bandeira

Chapter 2. GEOGRAPHICAL AND SOCIO-ECONOMIC SETTING OF MAPUTO BAY

Armindo da Silva and José Rafael

Case Study 2.1. Maputo Bay’s coastal habitats

Maria Adelaide Ferreira and Salomão Bandeira

Case Study 2.2. Main economic evaluation of Maputo Bay

Simião Nhabinde, Vera Julien and Carlos Bento

Chapter 3. GEOMORPHOLOGY AND EVOLUTION OF MAPUTO BAY

Mussa Achimo, João Alberto Mugabe, Fátima Momade and Sylvi Haldorsen

Case Study 3.1. Erosion in Maputo Bay

Elidio A. Massuanganhe

Chapter 4. HYDROLOGY AND CIRCULATION OF MAPUTO BAY

Sinibaldo Canhanga and João Miguel Dias

Case Study 4.1. Maputo Bay offshore circulation

Johan R.E. Lutjeharms† and Michael Roberts

Case Study 4.2. Ground water flow in/into Maputo Bay

Dinis Juízo

Chapter 5. HUMAN SETTINGS IN MAPUTO BAY

Yussuf Adam, Júlio Machele and Omar Saranga

1

3

11

21

25

31

39

45

55

61

67

XVIII T h e M a p u t o B a y E c o s y s t e m

Chapter 6. INHACA ISLAND: THE CRADLE OF MARINE RESEARCH IN MAPUTO BAY

AND MOZAMBIQUE

Salomão Bandeira, Lars Hernroth and Vando da Silva

Case Study 6.1. The role of SIDA/SAREC on research development in Maputo Bay during the

period 1983-2010

Almeida Guissamulo and Salomão Bandeira

Case Study 6.2. Inhaca and Portuguese islands reserves and their history

Salomão Bandeira, Tomás Muacanhia, Olavo Deniasse and Gabriel Albano

PART IIMAIN HABITATS AND ECOLOGICAL FUNCTIONING

Chapter 7. MANGROVES OF MAPUTO BAY

José Paula, Célia Macamo and Salomão Bandeira

Case Study 7.1. Incomati mangrove deforestation

Celia Macamo, Henriques Baliddy and Salomão Bandeira

Case Study 7.2. Saco da Inhaca mangrove vegetation mapping and change detection using very

high resolution satellite imagery and historic aerial photography

Griet Neukermans and Nico Koedam

Case Study 7.3. The mud crab Scylla serrata (Forskål) in Maputo Bay, Mozambique

Adriano Macia, Paula Santana Afonso, José Paula and Rui Paula e Silva

Case Study 7.4. Crab recruitment in mangroves of Maputo Bay

José Paula and Henrique Queiroga

Chapter 8. SEAGRASS MEADOWS IN MAPUTO BAY

Salomão Bandeira, Martin Gullström, Henriques Balidy, Davide Samussone and Damboia Cossa

Case Study 8.1. Zostera capensis – a vulnerable seagrass species

Salomão Bandeira

Case Study 8.2. Thalassodendron leptocaule – a new species of seagrass from rocky habitats

Maria Cristina Duarte, Salomão Bandeira and Maria Romeiras

Case Study 8.3. Morphological and physiological plasticity of the seagrass Halodule uninervis at

Inhaca Island, Mozambique

Meredith Muth and Salomão Bandeira

Chapter 9. CORAL REEFS OF MAPUTO BAY

Michael Schleyer and Marcos Pereira

87

99

101

107

109

127

131

135

141

147

171

175

181

187

XIXT h e M a p u t o B a y E c o s y s t e m

Table of Contents

Case Study 9.1. Shrimps in coral reefs and other habitats in the surrounding waters of Inhaca Island

Matz Berggren

Chapter 10. MARINE MAMMALS AND OTHER MARINE MEGAFAUNA OF MAPUTO BAY

Almeida Guissamulo

Case Study 10.1. Seagrass grazing by dugongs: Can habitat conservation help protect the dugong?

Stela Fernando, Salomão Bandeira and Almeida Guissamulo

Chapter 11. MARINE TURTLES IN MAPUTO BAY AND SURROUNDINGS

Cristina Louro

Chapter 12. THE TERRESTRIAL ENVIRONMENT ADJACENT TO MAPUTO BAY

Salomão Bandeira, Annae Senkoro, Filomena Barbosa, Dalmiro Mualassace and Estrela Figueiredo

Case Study 12.1. Inhaca Island within Maputaland centre of endemism

Annae Senkoro, Filomena Barbosa and Salomão Bandeira

Case Study 12.2. Uses of plant species from Inhaca Island

Filomena Barbosa, Annae Senkoro and Salomão Bandeira

Case Study 12.3. The avifauna of Maputo Bay

Carlos Bento

PART IIIFISHERIES OF MAPUTO BAY

Chapter 13. SHALLOW-WATER SHRIMP FISHERIES IN MAPUTO BAY

Rui Paula e Silva and Zainabo Masquine

Case Study 13.1. Influence of the precipitation and river runoff on the semi-industrial shrimp

catches in Maputo Bay

Carlos Bacaimane and Rui Paula e Silva

Case Study 13.2. Influence of estuarine flow rates on the artisanal shrimp catches in Maputo Bay

Sónia Nordez

Case Study 13.3. Distribution and abundance of the shrimp Fanneropenaeus indicus in Maputo Bay

António Pegado and Zainabo Masquine

Case Study 13.4. By-catch in the artisanal and semi-industrial shrimp trawl fisheries in Maputo Bay

Vanda Machava, Adriano Macia and Daniela de Abreu

207

215

223

229

239

255

259

265

275

277

285

287

289

291

XX T h e M a p u t o B a y E c o s y s t e m

Chapter 14. THE MAGUMBA FISHERY OF MAPUTO BAY

Paula Santana Afonso and Zainabo Masquine

Chapter 15. ARTISANAL FISHERIES IN MAPUTO BAY

Alice Inácio, Eunice Leong, Kélvin Samucidine, Zainabo Masquine and José Paula

Case Study 15.1. Biology and current status of the Otolithes ruber population in Maputo Bay

Alice Inácio

Case Study 15.2. Aspects of the reproductive biology of saddle grunt (Pomadasys maculatus) and

silver sillago (Sillago sihama) in Maputo Bay

Isabel Chaúca

Case Study 15.3. Socio-economic aspects of gastropod and bivalve harvest from seagrass beds –

comparison between urban (disturbed) and rural (undisturbed) areas

Elisa Inguane Vicente and Salomão Bandeira

Case Study 15.4. The sea urchin Tripneustes gratilla: insight to an important food resource at Inhaca

Island

Stela Fernando and Salomão Bandeira

Case Study 15.5. Recreational and sport fishing in Maputo Bay

Marcos Pereira and Rudy Van der Elst

PART IV

CROSS CUTTING ISSUES

Chapter 16. POLLUTION IN MAPUTO BAY

Maria Perpétua Scarlet and Salomão Bandeira

Case Study 16.1. Aerosols in Maputo Bay

António Queface

Case Study 16.2. Heavy metal contamination of penaeid shrimps from the artisanal and semi-

industrial fisheries in Maputo Bay

Daniela de Abreu, David Samussone and Maria Perpétua Scarlet

Chapter 17. POTENCIAL CLIMATE CHANGE IMPACTS ON MAPUTO BAY

Alberto Mavume, Izidine Pinto and Elídio Massuanganhe

Chapter 18. MANAGEMENT OF MAPUTO BAY

Sérgio Rosendo, Louis Celiers and Micas Mechisso

297

303

321

325

329

335

341

345

347

373

377

383

399

XXIT h e M a p u t o B a y E c o s y s t e m

Table of Contents

Chapter 19. MAPUTO BAY: THE WAY FORWARD

José Paula and Salomão Bandeira

419

T h e M a p u t o B a y E c o s y s t e m 4545

Sinibaldo Canhanga and João Miguel Dias

Hydrology and Circulation of Maputo Bay

4

IntroductionBays are important coastal systems, and when they

present estuarine characteristics (as it is the case of

Maputo Bay), particular interest in their study arises

with respect to their ecological role. The socio-eco-

nomic importance of bays is also very relevant, and

include vulnerable landscapes, while supporting rec-

reational activities and with margins that are gener-

ally places of urban and industrial development. In

Mozambique for instance, the two major metropoli-

tan areas have grown around the Maputo Bay

(Maputo city) and the Sofala Bay (Beira city).

In the case of the Maputo metropolitan area, it is

evident that the population growth during the last

years, has increased the economic and social impor-

tance of this region (see Chapter 5 - Human settings

in Maputo Bay). At the same time, the coastal zone

adjacent to this metropolitan area (the Maputo Bay)

is threatened by sewage discharges (see Chapter 16

-Pollution in Maputo Bay). According to Abel (1989),

whether the sewage discharge seriously effects the

receiving environment appears to depend upon the

extent to which satisfactory dispersion can be

achieved through the design and location of the out-

fall in relation to local conditions of tides, winds, and

currents. It is now recognized that although sewage

discharge into the coastal systems has been for long

one of the most disseminated ways to dispose of

anthropogenic waste, in some regions little attention

was paid to the fate of the pollutants in the ocean,

which depends essentially on the local physical con-

ditions. The hydrodynamics, as well as the hydrology

of the coastal system will drive the mixture and dilu-

tions processes into the nearby ocean. In the particu-

lar case of the sewage and stormwater outfalls of the

Maputo metropolitan area they have their openings

cited almost in the superficial waters of the Bay. As an

evident consequence of these locations, there seems

to be an increase in pollution in the mud and coastal

waters near these sites (see Chapter 16 - Pollution in

Maputo Bay). This emphasizes the fact that while

the hydrodynamic (circulation and their causes) and

hydrologic characteristics (inducing density gradients

and the thermohaline circulation) are not well known,

the degradation of water quality and the negative

effect from the arbitrary dumping of the domestic

sewage into the coastal systems is taking place. The

degradation of water quality in the Maputo Bay

coastal system has reached a level that might result in

human health problems when this water is utilized

46 T h e M a p u t o B a y E c o s y s t e m

I . Environmental and Human Setting

for leisure proposes.

There are also evidences of sand bank move-

ments in the Bay, as well as of coastline retreat in

some sectors along the coast (see Chapter 3 - Geo-

morphology and Evolution of Maputo Bay). To

understand these phenomena it is crucial to accu-

rately understand sediment transport, which is

strongly dependent on local hydrodynamics. How-

ever, the influence of currents in sediment dynamics

is barely known for Maputo Bay, although compre-

hension of the residual currents may provide impor-

tant information concerning the identification and

migration of erosion/accretion areas in this coastal

system.

The objective of this chapter is therefore to

present a revision of the physical characteristic of

Maputo Bay, by describing its hydrodynamic and

hydrological conditions. In this frame, is presented

the tidal circulation typical of the Bay based on the

analysis of the results of a previous hydrodynamic

numerical model application. This chapter also

describes the energy distribution, and analyzes the

existence of non-tidal (residual) currents.

General water mass characteristicsand Hydrological characterizationThe Maputo Bay (Figure 1) is considered a shallow

water system, with depths in most parts less than 10

m. According to the analysis of vertical sections of

salinity and temperature (Hoguane, 1994; Canhanga,

2004), the Bay can be considered well mixed, verti-

cally homogeneous.

Examination of the tidal harmonic constants

indicates that the tides are semidiurnal and that the

amplitudes of the semidiurnal constituents are one or

two orders of magnitude higher than the amplitude

of the major diurnal constituents. Approximately

90% of the total astronomical tide can be represented

by the two major semidiurnal constituents (M2 – prin-

cipal lunar, and S2 – principal solar), and the constitu-

ent S2 represent almost 50% of the major semidiurnal

constituent M2, according to Canhanga and Dias

(2005). Silva et al. (2010) have found that in the Bay

there is a significant quarter diurnal signal, represent-

ing approximately 10% of the M2 signal, inducing a

flood-ebb asymmetry pattern.

The Bay receives freshwater mainly from five

rivers (Figure 2): Incomati, to the north, Maputo to

the south, Unbeluzi, Matola and Tembe to the west.

There are also small rivers, marshes, channels and

mangroves. The total freshwater from all the above-

mentioned sources is approximately equal to 6 km3

year-1 (Hoguane and Dove, 2000; Canhanga, 2004).

Analysis of 30 years of the observed river data has

Figure 1. Map of Mozambique showing the study area (Maputo Bay).

km

0 100 200

N

Malvérnia

Lichinga

Beira

Maputo

Tete

INDIANOCEAN

Quelimane

Nampula

Pemba

Cidadede Nacala

47T h e M a p u t o B a y E c o s y s t e m

4. Hydrology and Circulation of Maputo Bay

revealed that the main part of the total freshwater

flows during the summer season, and that Incomati

and Maputo rivers present the main discharges into

the Bay compared to the other rivers. In all rivers, the

maximum discharges were observed in February;

whilst with the exception to the Tembe and Incomati

Rivers (where the minimum discharges were

observed in August), the minimum discharges were

observed in September. The maximum discharge

(260.19 m3 s-1) was observed in the Incomati River,

whereas the minimum discharge (0.58 m3 s-1) was

observed in the Matola River (Hoguane and Dove,

2000; Canhanga and Dias, 2005).

Considering the wind regimes (observed from

1960-1990), during the summer (from November to

March), winds prevails from southeast (SE) and east

(E); whereas in the winter (from May to August) pre-

vails winds from northeast (NE) (Teles et al., 2001;

Canhanga, 2004). The average wind intensity from

winter to summer is approximately constant and cor-

responds to about 4.4 m s-1.

In 2000 and 2001, the Instituto de Investigacão

Pesqueira (IIP) carried out salinity and temperature

measurements in four sections in Maputo Bay (Fig-

ure 3). From this study are drawn salinity and tem-

perature sections for the three most representative

sections (A, B, and D), (presented in Figure 4), the

results of which are analysed and discussed. Section

A is oriented in the west-east direction; section B in

the north–south direction, and section D in the

northwest–southeast direction.

Based on the analysis of salinity and temperature

contours performed by Hoguane (1994), the Bay can

be divided in two parts (West and East). The western

side, with notable salinity variability, is more influ-

Figure 2. The Maputo Bay bathymetry and the location of the main rivers that flow to the Bay. The location of Maputo Harbor (MH), Baixo Ribeiro, (BR) and Portuguese Island (PI) stations is also represented (Source: Canhanga and Dias, 2005).

Figure 3. Cross sections and sampling stations locations where vertical profiles were carried out by IIP.

48 T h e M a p u t o B a y E c o s y s t e m

I . Environmental and Human Setting

enced by fresh water discharge from the rivers, while

the eastern side presents low salinity variability. The

same pattern is shown in the maps in Figure 4. The

heterogeneity identified in the far ends of Sections A

and B and at the centre of Section D, is explained by

the location of these zones in the areas where the

river influence is notable. In the middle of the Sec-

tion D, the heterogeneity of salinity is reinforced by

the existence of a deep channel crossing this area,

which increases the difficulty of vertical mixing since

the turbulence induced by the bottom friction that

promotes vertical mixing will probably be restricted

to the deepest zone of the water column.

Analysis of temperature and salinity profiles in

the stations located in the central part of the Bay,

which are influenced by the fresh water, as well as in

the stations located near to the Bay/ocean boundary,

with no influence of fresh water, were well described

by Canhanga (2004). The vertical salinity profiles

(from the top to bottom), in the central part of the Bay

for the winter season varied from 30.35 psu to 31.3

psu, whereas at stations with no influence of fresh

water salinity along the profile were almost constant

and equal to 34.5 psu. The temperature profiles in

the centre of the Bay were almost constant, equal to

25.8 °C, while at the Bay-ocean boundary the tem-

perature profile were also approximately constant, at

26.2 °C. These results show that the vertical mixing

processes are well established during the winter. The

analysis of salinity profiles in the centre of the Bay

and in the Bay/Ocean boundary revealed a horizontal

salinity difference of approximately 4.8 psu between

the inside waters (influenced by river runoff), and the

waters from the outside of the Bay (influenced by

marine processes).

Summer analysis (measured in 2000) of vertical

profiles for stations located in the centre of the Bay

and in the Bay/Ocean boundary, for salinity and tem-

perature show a vertical variation of salinity ranging

from 24 to 30 psu in the centre of the Bay, and from

32.6 to 33.4 psu in the Bay/ocean boundary. Temper-

ature was found to decrease with depth (from 26.2 °C

to 25.6 °C in the centre of the Bay and from 26.2 °C

to 25.8 °C in the Bay/ocean boundary).

From these results, it appears that during the

summer, horizontal gradients prevail with evidence

of vertical stratification (higher compared to those

observed in 2001) and an increase of salinity and a

decrease of temperature with depth. The warm and

rainy characteristics of the summer season in the

Southern Hemisphere seem to induce these vertical

patterns. Studies carried out by Sete et al. (2002), and

Silva et al. (2010), have revealed also that there is a

tendency for hyper - saline conditions to develop in

the Bay toward the end of the dry season in response

of increasing heating.

Significant flooding occurred in the general area

during 2000, from which we conclude that the verti-

cal structure of the Maputo Bay can generally be con-

sidered homogeneous under average rivers discharge

conditions. However, in the rainy seasons, usually

during the summer and in the particular situations

where flooding occurs, the Maputo Bay may be char-

acterized by a slight vertical stratification of salinity

and temperature, as well as by an intensification of

the horizontal salinity gradients.

These results are in accordance with those found

by Silva (2007), who pointed out that during the dry

season (winter) the water column was found to be

always fully mixed, with weak horizontal density gra-

dients, and revealing a residual circulation mainly

induced by tidally-rectified currents. In contrast, dur-

ing the wet season (summer), according to this author

freshwater buoyancy was observed to induce marked

horizontal salinity gradients and stratification, more

pronounced during neap tides. This stratification is

largely eroded during spring tides, but semi-diurnal

periodic stratification was still apparent, resulting

from local production plus advection from offshore.

Silva et al. (2010) found that the stratification in

49T h e M a p u t o B a y E c o s y s t e m

4. Hydrology and Circulation of Maputo Bay

Figure 4. Salinity and temperature cross sections derived from the measurements performed in 2000, in the stations represented in Figure 3 during January, in sections B and D, and in April, in section A. The directions of the sections are as the following: section B – from North to South; section D – from Southeast to Northwest, section A – from West to East.

Dep

th (m

)

Distance (km) Distance (km)

Dep

th (m

)

12

0-1

-3

-5

-7

-9

-11

-13

-15

-17

-190 2 4 6 8 10 14 16 18 20 22 24

0

-2

-4

-6

-8

-10

-12

-14

0

-2

-4

-6

-8

-10

-12

-14

0

-2

-4

-6

-8

-10

-12

-14

-16

-18

-20

-220 2 4 6 8 10

0

-2

-4

-6

-8

-10

-12

-14

-16

-18

-20

-220 2 4 6 8 10

0 2 4 6 8 10 120 2 4 6 8 10 12

30.0

24.0

18.0

12.0

6.0

0.0

34 31 28 25 22 19 16

160

190

220

250

25.5

24.0

22.5

21.0

19.5

0-1

-3

-5

-7

-9

-11

-13

-15

-17

-190 2 4 6 8 10 14 16 18 20 22 24

Salinity Section A Temperature (ºC) Section A

Salinity Section B Temperature (ºC) Section B

Salinity Section D Temperature (ºC) Section D

50 T h e M a p u t o B a y E c o s y s t e m

I . Environmental and Human Setting

Maputo Bay should be mainly ascribed to estuarine

circulation. In fact, they reported that the effect of

tidal straining induces vertical mixing of the water

column, reinforced occasionally by wind stress acting

on the water surface, dominating over the surface

heat exchange that induces buoyancy.

The hydrodynamic conditionsTo study the hydrodynamics of the Maputo Bay,

mindful of some of its characteristics described in the

last section, a vertical integrated bi-dimensional

hydrodynamic model (SYMSYS2D) (Leendertse and

Gritton, 1971; Leendertse, 1987) was applied by Can-

hanga (2004). The model solves the second order

partial differential equations for depth-averaged fluid

flow derived from the full three-dimensional Navier-

Stokes equations. This gives a system consisting of

one equation for mass continuity and two-force

momentum equations. The assumptions made are

that the fluid should be Newtonian and that the den-

sity should be constant. The equations of this model,

as well as the procedures for simplifications and

assumptions are described in Canhanga (2004), and

Canhanga and Dias (2005). In the application of this

model to Maputo Bay, the authors used a computa-

tional domain (numerical bathymetry) obtained from

the nautical charts 644 and 2930, edited by the UK

Hydrographical Office, and from the chart 495 edited

by the Hydrographical Institute of Portugal. The

domain comprised a regular grid spaced by 250´250

m, with 350´380 cells on yy and xx directions, respec-

tively (Figure 2). In order to improve the computa-

tional efficiency the grid was rotated 20o to the east in

relative to geographic north.

The model was calibrated by comparing predic-

tions and observations for several stations distributed

along the Bay, changing the bottom stress in order to

optimise the model’s prediction accuracy. In this pro-

cedure were used free surface elevation data col-

lected in the Maputo Harbour (MH) and Portuguese

Island (PI) stations, and velocity components data

from the PI and Baixo Ribeiro (BR) stations (Can-

hanga, 2004; Canhanga and Dias, 2005). The differ-

ences between the observed and predicted

amplitudes of the main harmonic constituents M2

and S2 were lower than 10 cm, and the phase differ-

ences were below 3.37o. Regarding the comparisons

of tidal currents, and excluding the differences in BR

station, which were equal to 9.69 cm/s, the differ-

ences between the observed and predicted currents

were always below 3 cm/s, whereas the phase differ-

ences were below 3.72o. The results of the calibration

have revealed that the model reproduces quite well

the tidal propagation in the Maputo Bay.

Tidal CurrentsThe tidal current evolution was simulated under the

extreme tidal forcing associated with spring tide con-

ditions. By using T_TIDE (Pawlovicz et al., 2002), it

was possible to synthesize the sea surface elevation

time series for the Maputo Harbour tide gauge

between the years 1990 and 2000. Subsequently, the

period corresponding to the occurrence of maximum

spring tide was identified, which was numerically

simulated to evaluate the tidal evolution along this

cycle. These results are presented in Canhanga

(2004). Figure 5 represents the tidal heights and tidal

currents patterns three hours after the establishment

of low tide at the oceanic open boundary. Time series

in the upper left corner represents the tide imposed

in the open boundary and the snapshot instant is rep-

resented by the last black point; the white arrows

represent the tidal currents in the Bay.

Three hours after the establishment of low tide,

the tidal current seems to be more intense in the cen-

tral part of the Bay. This feature was also observed

along the entire tidal cycle.

Canhanga (2004) has shown that at high tide the

flood is completely established in the Bay, however,

one hour after the occurrence of low tide, ebb cur-

51T h e M a p u t o B a y E c o s y s t e m

4. Hydrology and Circulation of Maputo Bay

rents still occur in the Bay. Analysis of tidal current

patterns has revealed an intensive flow in the centre

of the Bay, with the current intensities attaining val-

ues ranging from 0.8 to 1.0 m s-1. It is also noted that

in the zones earlier established as areas of weak tidal

currents, is observed the new flooding at the end of

the ebb flux of the former cycle. This patterns may

explain the high residence times found for these

zones when compared to the remaining zones in the

Bay (Canhanga, 2004), suggesting that they might be

highly sensitive areas in the eventual case of occur-

rence of pollution.

The phase difference between the tide and tidal current, its implication on the residual currents and the flow of tidal energyAccording to Lewis (1997), in shallow waters, due to

the interaction between the tidal wave during its

propagation and the bottom, the friction forces may

contribute to distort the wave shape. In this case the

sea surface elevation, as well as the current associated

with the tidal wave propagation may lose their sym-

metric shape. This distortion will induce a phase

shift between the tides and currents and therefore

the residual flow will be ebb or flood dominant.

One other relevant aspect concerning the analy-

sis of the phase difference between the tidal wave

and the tidal currents is related to the mean tidal

energy flow (flow in a cross section), which is propor-

tional to the cosine of the difference between the

tide and tidal current phases. When the mean energy

flow is maximal, the wave is progressive (the phase

difference is equal to zero) and the flow becomes

minimum for steady waves (the phase difference is

equal to 90°).

The current phase of the main constituents M2

and S2 was computed for the entire Bay by using the

numerical model predictions of sea surface elevation

and tidal velocity, and the results for tidal velocity are

shown in Figure 6; the phase differences between

sea surface elevation and tidal velocity in all Maputo

Bay for the tidal constituents M2 and S2 are presented

in Figure 7. Few areas in the Bay have presented

phase differences equal to 0°, both for M2 and S2.

This shows that for both constituents, in most parts

of the Maputo Bay the energy flow does not attain its

maximum value. As the phase differences for both M2

and S2 constituents are not equal to 90° for almost the

Bay, residual currents different from zero should

occur, inducing ebb or flood dominant flows, which

will determine the long-term properties of transport

inside the Bay.

Perspectives and future researchThe existence of observed data related to Maputo

Bay is very limited spatial and temporally. In future

works, mooring of physical equipment should be

extended to larger areas and maintained operational

for longer periods to ensure near spatial and temporal

Figure 5. Sea surface elevation and tidal current (white arrows) pattern, 3 hour after the low tide (Source: Canhanga and Dias, 2005).

52 T h e M a p u t o B a y E c o s y s t e m

I . Environmental and Human Setting

52

Figure 6. Tidal currents phase for S2 (left) and M2 (right) constituents (Source: Canhanga and Dias, 2005).

Figure 7. Phase differences between the tide and the tidal currents for S2 (left) and M2 (right) constituents.

53T h e M a p u t o B a y E c o s y s t e m

4. Hydrology and Circulation of Maputo Bay

continuity.

Recognising the new research perspectives in

physical oceanography, and the large amount of data

required, it is of crucial priority to update the numer-

ical bathymetry that is being used to understand the

hydrodynamics of Maputo Bay. The models will

require new numerical bathymetry that would con-

siderably improve the overall quality and accuracy of

the numerical simulation results.

Studies carried out previously have shown a

glimmer on a non-symmetric pattern of the tidal

waves and currents, inducing the residual currents in

the Maputo Bay. The nature of these residual cur-

rents and their main forcing, constitute uncertainties

within the understanding of the physical oceanogra-

phy of Maputo Bay. Mindful that the long term proc-

esses in the marine system, such as erosion and

pollution processes or the birth and migration of spe-

cies in their larval stages, are not explained with the

support of the instantaneous tidal currents, but with

the knowledge of the residual currents, it seems cru-

cial to further research as to understand the patterns

of residual currents in the Bay. Specifically it would

be important to clarify to which extent the different

driving forces contribute to establish the global pat-

tern of the residual currents in the Bay. Looking

ahead at future research in this perspective, the

observation and better knowledge of river run-off,

precipitation and wind regimes seems to be decisive

to improve our perception of the residual currents

patterns in the Bay.

Maputo Bay was shown to include pronounced

horizontal salinity gradients while the water temper-

ature remains almost horizontally homogeneous,

which may induce horizontal density gradients

related to different water masses in the Bay. As such,

the relation between the density field and the flow

should be accessed in future research, both in some

points of the Bay, by using observed data, and over

broad spatial scales by using numerical models.

ConclusionThe main objective of this chapter was to describe

the hydrological conditions of Maputo Bay, as well as

its circulation, both driven by the main dynamic forc-

ing. These subjects are very important since in

Maputo Bay diverse sectorial developments (recrea-

tion, fisheries, etc.) are notable. Consequently, if the

hydrodynamic characteristics of the coastal systems

are not well known, any human/anthropogenic inter-

vention in these systems may lead to a large-scale

alteration of natural balances by changing their

coastal or bottom topography or by degradation of

water quality.

The main conclusions from the studies by

Hoguane and Dove (2000), Teles et al. (2001), Can-

hanga and Dias (2005) and others were that the tides

in Maputo Bay are semidiurnal, and the M2 and S2

constituents represent almost 90% of the total astro-

nomical tide. The Bay receives freshwater mainly

from five rivers, and their total freshwater input is

approximately equal to 6 km3 year-1. The wind

regimes are mainly characterized from SE and E dur-

ing the summer, while in winter the winds are mainly

from NE; the wind intensity presents a slight yearly

variability, with values of about 4.4 ms-1 throughout

the year. The Maputo Bay can be considered as a sys-

tem vertically homogeneous during the winter. Dur-

ing the summer, in the rainy season, or when flooding

occurs, a slight stratification of salinity is observed in

the water column, as well as an intensification of the

horizontal gradient of salinity.

Through the analysis of results from the bi-

dimensional hydrodynamic numerical model it was

found that tidal currents are higher in the central part

of the Bay, with maximum current intensities reach-

ing 0.8-1 m s-1. The analysis of the relationship

between the sea surface elevation and the tidal cur-

rent phases has revealed that for the main tidal con-

stituents (M2 and S2), the tidal flow energy does not

attain its maximum value in almost all the Maputo

54 T h e M a p u t o B a y E c o s y s t e m

I . Environmental and Human Setting

Bay and that the mean currents along the tidal cycle

are not zero in almost all the Bay. This should re-ori-

ent future work on circulation studies in Maputo Bay,

in order to determine the patterns of these residual

currents, as well as the influence of different forcing

factors in their establishment.

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