THE BAY ECOSYSTEM EditorsEditors Salomão Salomão Bandeira Bandeira | José | José Paula Paula Book title: The Maputo Bay Ecosystem.

Editors: Salomão Bandeira José 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 Marine Science Association (WIOMSA) Mizingani Street, House No. 13644/10 P.O. Box 3298, Zanzibar, Tanzania. Website: www.wiomsa.org E-mail: [email protected]

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 | [email protected] Photographers: credits referred in respective legends. Printed by: Guide – Artes Gráficas, Lda. (www.guide.pt) Printed in Portugal 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 1

Chapter 1. AN INTRODUCTION TO THE MAPUTO BAY 3 José Paula and Salomão Bandeira

Chapter 2. GEOGRAPHICAL AND SOCIO-ECONOMIC SETTING OF MAPUTO BAY 11 Armindo da Silva and José Rafael

Case Study 2.1. Maputo Bay’s coastal habitats 21 Maria Adelaide Ferreira and Salomão Bandeira Case Study 2.2. Main economic evaluation of Maputo Bay 25 Simião Nhabinde, Vera Julien and Carlos Bento

Chapter 3. GEOMORPHOLOGY AND EVOLUTION OF MAPUTO BAY 31 Mussa Achimo, João Alberto Mugabe, Fátima Momade and Sylvi Haldorsen

Case Study 3.1. Erosion in Maputo Bay 39 Elidio A. Massuanganhe

Chapter 4. HYDROLOGY AND CIRCULATION OF MAPUTO BAY 45 Sinibaldo Canhanga and João Miguel Dias

Case Study 4.1. Maputo Bay offshore circulation 55 Johan R.E. Lutjeharms† and Michael Roberts Case Study 4.2. Ground water flow in/into Maputo Bay 61 Dinis Juízo

Chapter 5. HUMAN SETTINGS IN MAPUTO BAY 67 Yussuf Adam, Júlio Machele and Omar Saranga

The Maputo Bay Ecosystem XVII Chapter 6. ISLAND: THE CRADLE OF MARINE RESEARCH IN MAPUTO BAY 87 AND 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 99 period 1983-2010 Almeida Guissamulo and Salomão Bandeira Case Study 6.2. Inhaca and Portuguese islands reserves and their history 101 Salomão Bandeira, Tomás Muacanhia, Olavo Deniasse and Gabriel Albano

PART II MAIN HABITATS AND ECOLOGICAL FUNCTIONING 107

Chapter 7. MANGROVES OF MAPUTO BAY 109 José Paula, Célia Macamo and Salomão Bandeira

Case Study 7.1. Incomati mangrove deforestation 127 Celia Macamo, Henriques Baliddy and Salomão Bandeira Case Study 7.2. Saco da Inhaca mangrove vegetation mapping and change detection using very 131 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 135 Adriano Macia, Paula Santana Afonso, José Paula and Rui Paula e Silva Case Study 7.4. Crab recruitment in mangroves of Maputo Bay 141 José Paula and Henrique Queiroga

Chapter 8. SEAGRASS MEADOWS IN MAPUTO BAY 147 Salomão Bandeira, Martin Gullström, Henriques Balidy, Davide Samussone and Damboia Cossa

Case Study 8.1. Zostera capensis – a vulnerable seagrass species 171 Salomão Bandeira Case Study 8.2. Thalassodendron leptocaule – a new species of seagrass from rocky habitats 175 Maria Cristina Duarte, Salomão Bandeira and Maria Romeiras Case Study 8.3. Morphological and physiological plasticity of the seagrass Halodule uninervis at 181 , Mozambique Meredith Muth and Salomão Bandeira

Chapter 9. CORAL REEFS OF MAPUTO BAY 187 Michael Schleyer and Marcos Pereira

XVIII The Maputo Bay Ecosystem Table of Contents

Case Study 9.1. Shrimps in coral reefs and other habitats in the surrounding waters of Inhaca Island 207 Matz Berggren

Chapter 10. MARINE MAMMALS AND OTHER MARINE MEGAFAUNA OF MAPUTO BAY 215 Almeida Guissamulo

Case Study 10.1. Seagrass grazing by : Can habitat conservation help protect the ? 223 Stela Fernando, Salomão Bandeira and Almeida Guissamulo

Chapter 11. MARINE TURTLES IN MAPUTO BAY AND SURROUNDINGS 229 Cristina Louro

Chapter 12. THE TERRESTRIAL ENVIRONMENT ADJACENT TO MAPUTO BAY 239 Salomão Bandeira, Annae Senkoro, Filomena Barbosa, Dalmiro Mualassace and Estrela Figueiredo

Case Study 12.1. Inhaca Island within Maputaland centre of endemism 255 Annae Senkoro, Filomena Barbosa and Salomão Bandeira Case Study 12.2. Uses of plant species from Inhaca Island 259 Filomena Barbosa, Annae Senkoro and Salomão Bandeira Case Study 12.3. The avifauna of Maputo Bay 265 Carlos Bento

PART III FISHERIES OF MAPUTO BAY 275

Chapter 13. SHALLOW-WATER SHRIMP FISHERIES IN MAPUTO BAY 277 Rui Paula e Silva and Zainabo Masquine

Case Study 13.1. Influence of the precipitation and river runoff on the semi-industrial shrimp 285 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 287 Sónia Nordez Case Study 13.3. Distribution and abundance of the shrimp Fanneropenaeus indicus in Maputo Bay 289 António Pegado and Zainabo Masquine Case Study 13.4. By-catch in the artisanal and semi-industrial shrimp trawl fisheries in Maputo Bay 291 Vanda Machava, Adriano Macia and Daniela de Abreu

The Maputo Bay Ecosystem XIX Chapter 14. THE MAGUMBA FISHERY OF MAPUTO BAY 297 Paula Santana Afonso and Zainabo Masquine

Chapter 15. ARTISANAL FISHERIES IN MAPUTO BAY 303 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 321 Alice Inácio Case Study 15.2. Aspects of the reproductive biology of saddle grunt (Pomadasys maculatus) and 325 silver sillago (Sillago sihama) in Maputo Bay Isabel Chaúca Case Study 15.3. Socio-economic aspects of gastropod and bivalve harvest from seagrass beds – 329 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 335 Island Stela Fernando and Salomão Bandeira Case Study 15.5. Recreational and sport fishing in Maputo Bay 341 Marcos Pereira and Rudy Van der Elst

PART IV CROSS CUTTING ISSUES 345

Chapter 16. POLLUTION IN MAPUTO BAY 347 Maria Perpétua Scarlet and Salomão Bandeira

Case Study 16.1. Aerosols in Maputo Bay 373 António Queface Case Study 16.2. Heavy metal contamination of penaeid shrimps from the artisanal and semi- 377 industrial fisheries in Maputo Bay Daniela de Abreu, David Samussone and Maria Perpétua Scarlet

Chapter 17. POTENCIAL CLIMATE CHANGE IMPACTS ON MAPUTO BAY 383 Alberto Mavume, Izidine Pinto and Elídio Massuanganhe

Chapter 18. MANAGEMENT OF MAPUTO BAY 399 Sérgio Rosendo, Louis Celiers and Micas Mechisso

XX The Maputo Bay Ecosystem Table of Contents

Chapter 19. MAPUTO BAY: THE WAY FORWARD 419 José Paula and Salomão Bandeira

The Maputo Bay Ecosystem XXI 4 Hydrology and Circulation of Maputo Bay

Sinibaldo Canhanga and João Miguel Dias

Introduction currents. It is now recognized that although sewage Bays are important coastal systems, and when they discharge into the coastal systems has been for long present estuarine characteristics (as it is the case of one of the most disseminated ways to dispose of Maputo Bay), particular interest in their study arises anthropogenic waste, in some regions little attention with respect to their ecological role. The socio-eco- was paid to the fate of the pollutants in the ocean, nomic importance of bays is also very relevant, and which depends essentially on the local physical con- include vulnerable landscapes, while supporting rec- ditions. The hydrodynamics, as well as the hydrology reational activities and with margins that are gener- of the coastal system will drive the mixture and dilu- ally places of urban and industrial development. In tions processes into the nearby ocean. In the particu- Mozambique for instance, the two major metropoli- lar case of the sewage and stormwater outfalls of the tan areas have grown around the Maputo Bay Maputo metropolitan area they have their openings (Maputo city) and the Sofala Bay (Beira city). cited almost in the superficial waters of the Bay. As an In the case of the Maputo metropolitan area, it is evident consequence of these locations, there seems evident that the population growth during the last to be an increase in pollution in the mud and coastal years, has increased the economic and social impor- waters near these sites (see Chapter 16 - Pollution in tance of this region (see Chapter 5 - Human settings Maputo Bay). This emphasizes the fact that while in Maputo Bay). At the same time, the coastal zone the hydrodynamic (circulation and their causes) and adjacent to this metropolitan area (the Maputo Bay) hydrologic characteristics (inducing density gradients is threatened by sewage discharges (see Chapter 16 and the thermohaline circulation) are not well known, -Pollution in Maputo Bay). According to Abel (1989), the degradation of water quality and the negative whether the sewage discharge seriously effects the effect from the arbitrary dumping of the domestic receiving environment appears to depend upon the sewage into the coastal systems is taking place. The extent to which satisfactory dispersion can be degradation of water quality in the Maputo Bay achieved through the design and location of the out- coastal system has reached a level that might result in fall in relation to local conditions of tides, winds, and human health problems when this water is utilized

The Maputo Bay Ecosystem 45 I . Environmental and Human Setting

for leisure proposes. N There are also evidences of sand bank move-

0 100 200 ments in the Bay, as well as of coastline retreat in km some sectors along the coast (see Chapter 3 - Geo- Pemba Lichinga morphology and Evolution of Maputo Bay). To Cidade understand these phenomena it is crucial to accu- de Nacala rately understand sediment transport, which is Nampula strongly dependent on local hydrodynamics. How- Tete ever, the influence of currents in sediment dynamics is barely known for Maputo Bay, although compre- Quelimane hension of the residual currents may provide impor- tant information concerning the identification and migration of erosion/accretion areas in this coastal Beira system. INDIAN OCEAN The objective of this chapter is therefore to present a revision of the physical characteristic of Malvérnia 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 Maputo describes the energy distribution, and analyzes the existence of non-tidal (residual) currents. Figure 1. Map of Mozambique showing the study area (Maputo General water mass characteristics Bay). and Hydrological characterization

The Maputo Bay (Figure 1) is considered a shallow ent S2 represent almost 50% of the major semidiurnal water system, with depths in most parts less than 10 constituent M2, according to Canhanga and Dias m. According to the analysis of vertical sections of (2005). Silva et al. (2010) have found that in the Bay salinity and temperature (Hoguane, 1994; Canhanga, there is a significant quarter diurnal signal, represent-

2004), the Bay can be considered well mixed, verti- ing approximately 10% of the M2 signal, inducing a cally homogeneous. flood-ebb asymmetry pattern. Examination of the tidal harmonic constants The Bay receives freshwater mainly from five indicates that the tides are semidiurnal and that the rivers (Figure 2): Incomati, to the north, Maputo to amplitudes of the semidiurnal constituents are one or the south, Unbeluzi, Matola and Tembe to the west. two orders of magnitude higher than the amplitude There are also small rivers, marshes, channels and of the major diurnal constituents. Approximately mangroves. The total freshwater from all the above- 90% of the total astronomical tide can be represented mentioned sources is approximately equal to 6 km3 -1 by the two major semidiurnal constituents (M2 – prin- year (Hoguane and Dove, 2000; Canhanga, 2004). cipal lunar, and S2 – principal solar), and the constitu- Analysis of 30 years of the observed river data has

46 The Maputo Bay Ecosystem 4. Hydrology and Circulation of Maputo Bay revealed that the main part of the total freshwater Canhanga, 2004). The average wind intensity from flows during the summer season, and that Incomati winter to summer is approximately constant and cor- and Maputo rivers present the main discharges into responds to about 4.4 m s-1. the Bay compared to the other rivers. In all rivers, the In 2000 and 2001, the Instituto de Investigacão maximum discharges were observed in February; Pesqueira (IIP) carried out salinity and temperature whilst with the exception to the Tembe and Incomati measurements in four sections in Maputo Bay (Fig- Rivers (where the minimum discharges were ure 3). From this study are drawn salinity and tem- observed in August), the minimum discharges were perature sections for the three most representative observed in September. The maximum discharge sections (A, B, and D), (presented in Figure 4), the (260.19 m3 s-1) was observed in the Incomati River, results of which are analysed and discussed. Section whereas the minimum discharge (0.58 m3 s-1) was A is oriented in the west-east direction; section B in observed in the (Hoguane and Dove, the north–south direction, and section D in the 2000; Canhanga and Dias, 2005). northwest–southeast direction. Considering the wind regimes (observed from Based on the analysis of salinity and temperature 1960-1990), during the summer (from November to contours performed by Hoguane (1994), the Bay can March), winds prevails from southeast (SE) and east be divided in two parts (West and East). The western (E); whereas in the winter (from May to August) pre- side, with notable salinity variability, is more influ- vails winds from northeast (NE) (Teles et al., 2001;

Figure 2. The Maputo Bay bathymetry and the location of the main Figure 3. Cross sections and sampling stations locations where rivers that flow to the Bay. The location of Maputo Harbor (MH), vertical profiles were carried out by IIP. Baixo Ribeiro, (BR) and Portuguese Island (PI) stations is also represented (Source: Canhanga and Dias, 2005).

The Maputo Bay Ecosystem 47 I . Environmental and Human Setting enced by fresh water discharge from the rivers, while 32.6 to 33.4 psu in the Bay/ocean boundary. Temper- the eastern side presents low salinity variability. The ature was found to decrease with depth (from 26.2 °C same pattern is shown in the maps in Figure 4. The to 25.6 °C in the centre of the Bay and from 26.2 °C heterogeneity identified in the far ends of Sections A to 25.8 °C in the Bay/ocean boundary). and B and at the centre of Section D, is explained by From these results, it appears that during the the location of these zones in the areas where the summer, horizontal gradients prevail with evidence river influence is notable. In the middle of the Sec- of vertical stratification (higher compared to those tion D, the heterogeneity of salinity is reinforced by observed in 2001) and an increase of salinity and a the existence of a deep channel crossing this area, decrease of temperature with depth. The warm and which increases the difficulty of vertical mixing since rainy characteristics of the summer season in the the turbulence induced by the bottom friction that Southern Hemisphere seem to induce these vertical promotes vertical mixing will probably be restricted patterns. Studies carried out by Sete et al. (2002), and to the deepest zone of the water column. Silva et al. (2010), have revealed also that there is a Analysis of temperature and salinity profiles in tendency for hyper - saline conditions to develop in the stations located in the central part of the Bay, the Bay toward the end of the dry season in response which are influenced by the fresh water, as well as in of increasing heating. the stations located near to the Bay/ocean boundary, Significant flooding occurred in the general area with no influence of fresh water, were well described during 2000, from which we conclude that the verti- by Canhanga (2004). The vertical salinity profiles cal structure of the Maputo Bay can generally be con-

(from the top to bottom), in the central part of the Bay sidered homogeneous under average rivers discharge for the winter season varied from 30.35 psu to 31.3 conditions. However, in the rainy seasons, usually psu, whereas at stations with no influence of fresh during the summer and in the particular situations water salinity along the profile were almost constant where flooding occurs, the Maputo Bay may be char- and equal to 34.5 psu. The temperature profiles in acterized by a slight vertical stratification of salinity the centre of the Bay were almost constant, equal to and temperature, as well as by an intensification of 25.8 °C, while at the Bay-ocean boundary the tem- the horizontal salinity gradients. perature profile were also approximately constant, at These results are in accordance with those found 26.2 °C. These results show that the vertical mixing by Silva (2007), who pointed out that during the dry processes are well established during the winter. The season (winter) the water column was found to be analysis of salinity profiles in the centre of the Bay always fully mixed, with weak horizontal density gra- and in the Bay/Ocean boundary revealed a horizontal dients, and revealing a residual circulation mainly salinity difference of approximately 4.8 psu between induced by tidally-rectified currents. In contrast, dur- the inside waters (influenced by river runoff), and the ing the wet season (summer), according to this author waters from the outside of the Bay (influenced by freshwater buoyancy was observed to induce marked marine processes). horizontal salinity gradients and stratification, more Summer analysis (measured in 2000) of vertical pronounced during neap tides. This stratification is profiles for stations located in the centre of the Bay largely eroded during spring tides, but semi-diurnal and in the Bay/Ocean boundary, for salinity and tem- periodic stratification was still apparent, resulting perature show a vertical variation of salinity ranging from local production plus advection from offshore. from 24 to 30 psu in the centre of the Bay, and from Silva et al. (2010) found that the stratification in

48 The Maputo Bay Ecosystem 4. Hydrology and Circulation of Maputo Bay

0 0 -1 -1

-3 -3

-5 -5

-7 -7

-9 -9

-11 -11

-13 -13

-15 Salinity Section A -15 Temperature (ºC) Section A 25.5 24.0 22.5 21.0 30.0 24.0 6.0 0.0 18.0 12.0 -17 -17 19.5

-19 -19 0 2 4 6 8 10 12 14 16 18 20 22 24 0 2 4 6 8 10 14 16 18 20 22 24 0 0

-2 -2

-4 -4

-6 -6

-8 -8

-10 -10

-12 -12 Salinity Section B Temperature (ºC) Section B 250 220 190 160

-14 34 31 28 25 22 19 16 -14

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

-2 -2

-4 -4

-6 -6

-8 -8

-10 -10

-12 -12

-14 -14

-16 -16

-18 -18 Salinity Section D Temperature (ºC) Section D -20 -20 Depth (m) Depth (m) -22 -22 0 2 4 6 8 10 0 2 4 6 8 10

Distance (km) Distance (km)

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.

The Maputo Bay Ecosystem 49 I . Environmental and Human Setting

Maputo Bay should be mainly ascribed to estuarine Island (PI) stations, and velocity components data circulation. In fact, they reported that the effect of from the PI and Baixo Ribeiro (BR) stations (Can- tidal straining induces vertical mixing of the water hanga, 2004; Canhanga and Dias, 2005). The differ- column, reinforced occasionally by wind stress acting ences between the observed and predicted on the water surface, dominating over the surface amplitudes of the main harmonic constituents M2 heat exchange that induces buoyancy. and S2 were lower than 10 cm, and the phase differ- ences were below 3.37o. Regarding the comparisons The hydrodynamic conditions of tidal currents, and excluding the differences in BR To study the hydrodynamics of the Maputo Bay, station, which were equal to 9.69 cm/s, the differ- mindful of some of its characteristics described in the ences between the observed and predicted currents last section, a vertical integrated bi-dimensional were always below 3 cm/s, whereas the phase differ- hydrodynamic model (SYMSYS2D) (Leendertse and ences were below 3.72o. The results of the calibration Gritton, 1971; Leendertse, 1987) was applied by Can- have revealed that the model reproduces quite well hanga (2004). The model solves the second order the tidal propagation in the Maputo Bay. partial differential equations for depth-averaged fluid flow derived from the full three-dimensional Navier- Tidal Currents Stokes equations. This gives a system consisting of The tidal current evolution was simulated under the one equation for mass continuity and two-force extreme tidal forcing associated with spring tide con- momentum equations. The assumptions made are ditions. By using T_TIDE (Pawlovicz et al., 2002), it that the fluid should be Newtonian and that the den- was possible to synthesize the sea surface elevation sity should be constant. The equations of this model, time series for the Maputo Harbour tide gauge as well as the procedures for simplifications and between the years 1990 and 2000. Subsequently, the assumptions are described in Canhanga (2004), and period corresponding to the occurrence of maximum Canhanga and Dias (2005). In the application of this spring tide was identified, which was numerically model to Maputo Bay, the authors used a computa- simulated to evaluate the tidal evolution along this tional domain (numerical bathymetry) obtained from cycle. These results are presented in Canhanga the nautical charts 644 and 2930, edited by the UK (2004). Figure 5 represents the tidal heights and tidal Hydrographical Office, and from the chart 495 edited currents patterns three hours after the establishment by the Hydrographical Institute of Portugal. The of low tide at the oceanic open boundary. Time series domain comprised a regular grid spaced by 250´250 in the upper left corner represents the tide imposed m, with 350´380 cells on yy and xx directions, respec- in the open boundary and the snapshot instant is rep- tively (Figure 2). In order to improve the computa- resented by the last black point; the white arrows tional efficiency the grid was rotated 20o to the east in represent the tidal currents in the Bay. relative to geographic north. Three hours after the establishment of low tide, The model was calibrated by comparing predic- the tidal current seems to be more intense in the cen- tions and observations for several stations distributed tral part of the Bay. This feature was also observed along the Bay, changing the bottom stress in order to along the entire tidal cycle. optimise the model’s prediction accuracy. In this pro- Canhanga (2004) has shown that at high tide the cedure were used free surface elevation data col- flood is completely established in the Bay, however, lected in the Maputo Harbour (MH) and Portuguese one hour after the occurrence of low tide, ebb cur-

50 The Maputo Bay Ecosystem 4. Hydrology and Circulation of Maputo Bay

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 Figure 5. Sea surface elevation and tidal current (white arrows) numerical model predictions of sea surface elevation pattern, 3 hour after the low tide (Source: Canhanga and Dias, and tidal velocity, and the results for tidal velocity are 2005). shown in Figure 6; the phase differences between rents still occur in the Bay. Analysis of tidal current sea surface elevation and tidal velocity in all Maputo patterns has revealed an intensive flow in the centre Bay for the tidal constituents M2 and S2 are presented of the Bay, with the current intensities attaining val- in Figure 7. Few areas in the Bay have presented -1 ues ranging from 0.8 to 1.0 m s . It is also noted that phase differences equal to 0°, both for M2 and S2. in the zones earlier established as areas of weak tidal This shows that for both constituents, in most parts currents, is observed the new flooding at the end of of the Maputo Bay the energy flow does not attain its the ebb flux of the former cycle. This patterns may maximum value. As the phase differences for both M2 explain the high residence times found for these and S2 constituents are not equal to 90° for almost the zones when compared to the remaining zones in the Bay, residual currents different from zero should Bay (Canhanga, 2004), suggesting that they might be occur, inducing ebb or flood dominant flows, which highly sensitive areas in the eventual case of occur- will determine the long-term properties of transport rence of pollution. inside the Bay.

The phase difference between the tide and tidal Perspectives and future research current, its implication on the residual currents The existence of observed data related to Maputo and the flow of tidal energy Bay is very limited spatial and temporally. In future According to Lewis (1997), in shallow waters, due to works, mooring of physical equipment should be the interaction between the tidal wave during its extended to larger areas and maintained operational propagation and the bottom, the friction forces may for longer periods to ensure near spatial and temporal

The Maputo Bay Ecosystem 51 I . Environmental and Human Setting

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.

52 The Maputo Bay Ecosystem 4. Hydrology and Circulation of Maputo Bay continuity. Conclusion Recognising the new research perspectives in The main objective of this chapter was to describe physical oceanography, and the large amount of data the hydrological conditions of Maputo Bay, as well as required, it is of crucial priority to update the numer- its circulation, both driven by the main dynamic forc- ical bathymetry that is being used to understand the ing. These subjects are very important since in hydrodynamics of Maputo Bay. The models will Maputo Bay diverse sectorial developments (recrea- require new numerical bathymetry that would con- tion, fisheries, etc.) are notable. Consequently, if the siderably improve the overall quality and accuracy of hydrodynamic characteristics of the coastal systems the numerical simulation results. are not well known, any human/anthropogenic inter- Studies carried out previously have shown a vention in these systems may lead to a large-scale glimmer on a non-symmetric pattern of the tidal alteration of natural balances by changing their waves and currents, inducing the residual currents in coastal or bottom topography or by degradation of the Maputo Bay. The nature of these residual cur- water quality. rents and their main forcing, constitute uncertainties The main conclusions from the studies by within the understanding of the physical oceanogra- Hoguane and Dove (2000), Teles et al. (2001), Can- phy of Maputo Bay. Mindful that the long term proc- hanga and Dias (2005) and others were that the tides esses in the marine system, such as erosion and in Maputo Bay are semidiurnal, and the M2 and S2 pollution processes or the birth and migration of spe- constituents represent almost 90% of the total astro- cies in their larval stages, are not explained with the nomical tide. The Bay receives freshwater mainly support of the instantaneous tidal currents, but with from five rivers, and their total freshwater input is the knowledge of the residual currents, it seems cru- approximately equal to 6 km3 year-1. The wind cial to further research as to understand the patterns regimes are mainly characterized from SE and E dur- of residual currents in the Bay. Specifically it would ing the summer, while in winter the winds are mainly be important to clarify to which extent the different from NE; the wind intensity presents a slight yearly driving forces contribute to establish the global pat- variability, with values of about 4.4 ms-1 throughout tern of the residual currents in the Bay. Looking the year. The Maputo Bay can be considered as a sys- ahead at future research in this perspective, the tem vertically homogeneous during the winter. Dur- observation and better knowledge of river run-off, ing the summer, in the rainy season, or when flooding precipitation and wind regimes seems to be decisive occurs, a slight stratification of salinity is observed in to improve our perception of the residual currents the water column, as well as an intensification of the patterns in the Bay. horizontal gradient of salinity. Maputo Bay was shown to include pronounced Through the analysis of results from the bi- horizontal salinity gradients while the water temper- dimensional hydrodynamic numerical model it was ature remains almost horizontally homogeneous, found that tidal currents are higher in the central part which may induce horizontal density gradients of the Bay, with maximum current intensities reach- related to different water masses in the Bay. As such, ing 0.8-1 m s-1. The analysis of the relationship the relation between the density field and the flow between the sea surface elevation and the tidal cur- should be accessed in future research, both in some rent phases has revealed that for the main tidal con- points of the Bay, by using observed data, and over stituents (M2 and S2), the tidal flow energy does not broad spatial scales by using numerical models. attain its maximum value in almost all the Maputo

The Maputo Bay Ecosystem 53 I . Environmental and Human Setting

Bay and that the mean currents along the tidal cycle in order to determine the patterns of these residual are not zero in almost all the Bay. This should re-ori- currents, as well as the influence of different forcing ent future work on circulation studies in Maputo Bay, factors in their establishment.

Bibliography

Abel, P.D., 1989. Water pollution Biology. Ellis Hor- Lewis, R., 1997. Dispersion in estuaries and coastal wood Ltd., Chichester, England, 231 pp. waters. John Wiley & Sons, Ltd, Chichester, UK, Canhanga, S.J.V., 2004. Hydrodynamic modelling of 312 pp. Maputo Bay. MSc thesis, Universidade de Aveiro, Pawlowicz, R; Beardsley, B., Lentz, S., 2002. Classical 134 pp. tidal harmonic analysis including error estimates Canhanga, S.J.V., Dias, J.M., 2005. Tidal characteris- in MATLAB using T_Tide. Computers & Geo- tics of Maputo Bay, Mozambique. Journal of sciences 28, 929-937. Marine Systems 58, 83-97. Sete, C., Ruby, J., Dove, V.F., 2002. Seasonal variation of Hoguane, A.M., 1994. Tidal currents and Oil Spill tides currents salinity and temperature along the Dispersion in Maputo Bay, 16 pp. coast of Mozambique. CENADO, Maputo, 72 pp. Hoguane, A.M., Dove, V.F., 2000. Condições oceano- Silva, J.D.L., 2007. Controls on exchange in a sub- gráficas da Baía de Maputo. Relatório de Estudos tropical tidal embayment, Maputo Bay. PhD the- Ambientais, Instituto de Investigação Pesqueira, sis, University of Bangor, 133 pp. Maputo, Moçambique. Silva, J.D.L., Simpson, J.H., Hoguane, A.M., Har- Leendertse, J.J., 1987. Aspects of SIMSYS2D, a sys- court-Baldwin, J.L., 2010. Buoyancy-Stirring tem for two-dimensional flow computation. interactions in a subtropical embayment: a syn- Report R-3572-USGS. The Rand Corporation, thesis of measurements and model simulations in New York, USA. Maputo Bay, Mozambique. African Journal of Leendertse, J.J., Gritton, E.C., 1971. A water quality Marine Science 32, 95-107. simulation model for well-mixed estuaries and Teles, M., Barata, A., Rosa, T., 2001. Sistemas de coastal seas, vol. II, Computation Procedures, modelos matemáticos para a gestão integrada da R-708-NYC, The Rand Corporation, Santa Mon- Baía de Maputo. Report, Instituto de Investiga- ica, California, 53 pp. ção Pesqueira, vol I/II, 43 pp.

54 The Maputo Bay Ecosystem