Muddy Coast Dynamics and Resource Management

287 Pages·2000·7.217 MB·3-294\287

by  B.W. Flemming, M.T Delafontaine and G. Liebezeit (Eds.)

PREFACE
Muddy coasts are land-sea transitional environments commonly found along low-
energy shorelines which either receive large annual supplies of muddy sediments, or
where unconsolidated muddy deposits are being eroded by wave action. Muddy
coasts are found in all kinds of climates and under any tidal conditions. Accordingly,
their geographic distribution ranges from low tropical to high sub-arctic latitudes
and from microtidal to macrotidal coastal settings. The most conspicuous examples
are the vast mangrove swamps of the tropics and the extensive salt marshes fringing
the shores of estuaries and back-barrier lagoons of mid-latitudinal coasts.
Muddy coastal environments harbour highly variable and fragile ecosystems
which, for the most part, are still poorly understood. Today these ecosystems are not
only threatened by the growing economic interests of man (e.g., tourism, fisheries,
aquaculture, land reclamation) but also by the prospect of an accelerating sea-level
rise in the wake of global warming. While the detrimental effects of the former are
increasingly becoming evident, those of the latter are still largely unknown.
In order to provide an up-to-date review of the state of the art in muddy coast
research, and to identify gaps in our knowledge, both in a scientific and geographic
sense, and to define priorities for future research, an international conference entitled
"Muddy Coasts 97" was convened in Wilhelmshaven, Germany, in September .7991
The conference was co-sponsored by the Senckenberg Natural History Society
(Frankfurt), the Terramare Research Centre (Wilhelmshaven), the Federal Ministry of
Science and Technology (Berlin), the Deutsche Forschungsgemeinschaft (Bonn), and
last but least the Scientific Committee on Oceanic Research (SCOR) under the able
participation of Working Group .601
The book "Muddy Coast Dynamics and Resource Management" forms part of the
proceedings and has been edited by the conference organisers. It presents 12 regional
case-studies from different parts of the world, including the southern Baltic Sea of
Germany (6), the German Wadden Sea (6), the Wash in the U.K. (1), Portugal (1), the
U. .S A. (1), Cameroon (1), Tanzania (1), Korea (1), and China (3). The studies deal
with hydrodynamics and suspended particulate matter in bays and back-barrier tidal
basins, erosion, deposition, and sediment budgets on tidal flats, primary production,
nutrient fluxes and mineralisation in shallow lagoons (Bodden), sediment
geochemistry of salt marshes and Holocene marine deposits, impacts of sea-level rise
and land reclamation, and resource management of muddy coasts. The book is
designated as a companion volume to the proceedings of the SCOR Working Group
601 published under the the title "Muddy Coasts of the World: Processes, Deposits
and Function" edited by Terry Healy (New Zealand) and Ying Wang (China).
The editors wish to express their sincerest gratitude to the numerous unnamed
referees who have contributed substantially to the high standard of the contributions.
gruB ,gnimmelF euqinoM ,eniatnofaleD dna dreG tiezebeiL
,nevahsmlehliW August 0002
CONTRIBUTORS (current addresses)
M.O. Andreae .J.H Black
xaM Planck Institute for Chemistry Institut f6r Okologie
.O.P xoB 0603 ti~tisrevinU Greifswald
02055 Mainz Schwedenhagen 6
Germany 56581 Kloster
Germany
C.A. Angwe
Research Centre for Fisheries and M.I. Ca~ador
Oceanography Instituto de Oceanografia
BMP 77 Limbe Departamento de Biologia Vegetal
South-West Province Universidade de Lisboa
Cameroon 0071 Lisboa
Portugal
I. Austen
Mittelstr. 62 K.-S. Choi
90752 Kronprinzenkoog Department of Oceanography
Germany Seoul National University
Seoul 247-151
H.-D. Babenzien
Korea
Institut r~if eigolok6ressi~weG
und Binnenfischerei M. Collins
Alte Fischerh~tte 2 Department of Oceanography
57761 Neuglobsow Southampton Oceanography Centre
Germany University of Southampton
41OS 3ZH Southampton
H.W. Bange
.K.U
xaM Planck Institute for Chemistry
P.O. xoB 0603 .S Dahlke
02055 Mainz Institut fiir Okologie
Germany ti~tisrevinU Greifswald
Schwedenhagen 6
A. Bartholomii
56581 Kloster
Senckenberg Institute
Germany
Schleusenstr. a93
28362 Wilhelmshaven M.T. Delafontaine
Germany
Senckenberg Institute
Schleusenstr. a93
.S Berghoff
28362 Wilhelmshaven
Department of Biology
Germany
Rostock University
Freiligrathstr. 7/8 B.W. Hemming
15081 Rostock Senckenberg Institute
Germany Schleusenstr. a93
28362 Wilhelmshaven
Germany
iix
C.E. Gabche Y.-F. Liu
Research Centre for Fisheries Department of Geography
and Oceanography Peking University
PMB 77 Limbe Beijing 178001
South-West Province P.R. China
Cameroon
M.I. Madureira
S. Gerbersdorf IPIMAR
Institut ri.if Okologie Av. Brasflia
Universit~it Greifswald 0041 Lisboa
Schwedenhagen 6 Portugal
56581 Kloster
S. Mai
Germany
Eifelstr. 64
M.-K. Han 92506 Frankfurt-Schwarnheim
Department of Geography Germany
Peking University
A.J. Mehta
Beijing 178001
Coastal and Oceanographic
P.R. China
Engineering Department
X. Ke University of Florida
Department of Urban P.O. Box 095611
and Resources Science Gainesville, LF 11623
Nanjing University U.S.A.
22 Hankou Road
. Meyercordt
Nanjing 390012
Institut ftir Okologie
P.R. China
Universit/it Greifswald
B.-K. Khim Schwedenhagen 6
Polar Research Center 56581 Kloster
Ocean Research and Germany
Development Institute
L.-A. Meyer-Reil
P.O. xoB 92
Institut fiir Okologie
Ansan 006524
Universit/it Greifswald
Korea
Schwedenhagen 6
M. Kb'ster 56581 Kloster
Institut ffir Okologie Germany
Universit~it Greifswald
N. Mimura
Schwedenhagen 6
Department of Urban
56581 Kloster
System Engineering
Germany
Ibaraki University
Hitachi 613
Japan
iiix
O.U. Mwaipopo P. Santamarina Cuneo
Institute of Marine Sciences Senckenberg Institute
University of Dar Es Salaam Schleusenstr. 39a
P.O. xoB 866 26382 Wilhelmshaven
Germany
Zanzibar
Tanzania
G. Schlungbaum
N. Nyandwi Department of Biology
Institute of Marine Sciences Rostock University
University of Dar Es Salaam Freiligrathstr. 7/8
P.O. Box 866 15081 Rostock
Zanzibar Germany
Tanzania
.U Selig
T.M. Parchure Department of Biology
Coastal and Hydraulics Laboratory Rostock University
U.S. Army Engineer Waterways Freiligrathstr. 7/8
Experiment Station 15081 Rostock
Vicksburg, MS 39180 Germany
U.S.A.
I. Stodian
Y.-A. Park Institut ftir Okologie
Department of Oceanography Universit/it Greifswald
Seoul National University Schwedenhagen 6
Seoul 151-742 18565 Kloster
Korea Germany
R. Ramesh .C Vale
Max Planck Institute for Chemistry IPIMAR
P.O. xoB 3060 Av. Brasflia
55020 Mainz 1400 Lisboa
Germany Portugal
S. Rapsomanikis A. Voigt
Max Planck Institute for Chemistry Institut ri~f Gew~isser6kologie
P.O. xoB 3060 und Binnenfischerei
55020 Mainz Alte Fischerh~itte 2
Germany 16775 Neuglobsow
Germany
.T Rieling
Institut fihr Okologie I. Wang
Universit/it Greifswald National Marine Data
Schwedenhagen 6 and Information Service
56581 Kloster State Oceanic Administration
Germany 39 Liuwei Road
Tianjin 171003
P.R. China
vix
.Y Wang X. Zou
State Pilot Laboratory of Coast State Pilot Laboratory of Coast
and Island Exploitation and Island Exploitation
Nanjing University Nanjing University
22 Hankou Road 22 Hankou Road
Nanjing 210093 Nanjing 210093
P.R. China P.R. China
c. Wolff
Institut ftir Okologie
Universit/it Greifswald
Schwedenhagen 6
18565 Kloster
Germany
L. Wu
Department of Geography
Peking University
Beijing 100871
P.R. China
T.J. Youmbi
Research Centre for Fisheries
and Oceanography
PMB 77 Limbe
South-West Province
Cameroon
.J Zhang
National Marine Data
and Information Service
State Oceanic Administration
39 Liuwei Road
Tianjin 300171
P.R. China
D. Zhu
State Pilot Laboratory of Coast
and Island Exploitation
Nanjing University
22 Hankou Road
Nanjing 210093
P.R. China
Muddy tsaoC scimanyD dna ecruoseR ,tnemeganaM
.B .W ,gnimmelF .M .T eniatnofaleD dna .G tiezebeiL ).sde(
(cid:14)9 0002 reiveslE ecneicS B.V. llA rights .devreser
Hydrodynamics of Chwaka Bay, a shallow mangrove-fringed tropical
embayment, Tanzania
N. Nyandwi* and O. U. Mwaipopo
University of Dar sE Salaam, Institute of Marine Sciences, .P .O Box 668, Zanzibar, Tanzania
ABSTRACT
Time-series data of currents, sea levels and temperatures from Chwaka Bay,
Zanzibar were analysed with the view of understanding the water circulation of the
bay. The analyses show that there is a tidal asymmetry in the bay, with peak ebb tidal
currents in the deep channels 54( cm s being stronger than flood tidal currents
53( cm s-l), and ebb periods 7( hours) being longer than flood periods 5( hours). The
velocity and time asymmetry as well as the asymmetry in the current direction are
controlled by the morphological variations of the tidal basin. It was found that, as the
water flows from the inner bay during the ebbing tide, it first drains towards the main
tidal creek which leads to concentrated but delayed flows. The temperature variations
in the inner part of the bay are predominantly diurnal, whereas at the mouth of the bay
they are semi-diurnal. There is a general temperature gradient between the inner bay
and the mouth, the highest temperatures being recorded in the inner bay (30.14~
This indicates high residence times of the bay waters, presumably resulting from
entrapment.
.1 INTRODUCTION
The hydrology of many tropical, mangrove-lined bays are characterized by salinity
gradients even in areas without visible river supply, and by the entrapment of water in
the mangrove forests (e.g., Wolanski et al. 1980; Wolanski 1989). Similarly, spatial and
temporal variations in tidal current velocities are commonly observed. Thus, in Coral
Creek, Australia, peak current velocities are generally higher than 1 m s in the tidal
creek, whereas they hardly exceed 0.07 m s 1- in the mangroves (Wolanski et al. .)0891
Indeed, a tidal velocity asymmetry was actually reproduced in a numerical model
using the Coral Creek data. Furthermore, it was observed that human activities such as
land reclamation and the felling of mangrove trees tend to reduce the magnitude and
asymmetry of the tidal currents (e.g., Wolanski .)2991
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Salinity variations are usually observed between the inner and outer parts of bays
and creeks. Several factors which may produce salinity gradients have been identified,
including groundwater infiltration, evapotranspiration, and surface freshwater influx
(e.g., Wolanski et al. 1980; Mazda et al. 1990; Ridd et al. 1990). Dilution by freshwater
influx into mangrove areas usually produces a pronounced salinity gradient between
the bay and the mangroves. Groundwater infiltration, which commonly occurs along
the landward reaches of tidal creeks, can have a similar effect. It is also thought to be an
important flushing mechanism of salts left behind by evapotranspiration (Wolanski &
Gardiner .)1891
The only exception to the above rules are associated with the conditions in hot and
dry environments where evapotranspiration may cause an increase in salinity
landwards of mangrove creeks (Wolanski et al. 1980; Ridd et al. 1990; Wattayakorn et
al. .)0991 A landward increase in salinity under such circumstances can be attributed to
the extraction of freshwater from seawater by mangroves (Wolanski & Gardiner 1981).
The saline water resulting from evapotranspiration may thus induce an inverse
estuarine circulation (Wolanski 1992). Another factor which may affect the circulation
pattern is the trapping of water in mangrove ecosystems (Okubo 1973), the amount of
trapped water appearing to determine general flushing rates (Wolanski .)2991
In the case of Chwaka Bay, a mangrove-lined embayment along the east coast of
Zanzibar Island, Tanzania (Fig. 1A), the existence of a velocity asymmetry was
observed but not verified because data on current variations in the tidal creeks and
mangrove areas were lacking at the time (Wolanski 1989). Similarly, water entrapment
and groundwater infiltration have been suggested as possible factors contributing to
the offshore decrease in water temperature and the increase in salinity in the bay
(Wolanski 1989). The exchange of water between Chwaka Bay and the open sea is not
well understood, and there is no information on the heat budget of the area. The
collection of data on temperature distribution and temporal variation would therefore
be an important first step towards establishing a local heat budget. At the same time, a
better knowledge of current patterns in the bay would not only contribute towards a
better understanding of nutrient dynamics, waste dispersal, and water quality in
general, but could also help explain why muds accumulating in the mangrove forests
are never flushed out to impair coral reef growth at the mouth of the bay.
2. STUDY AREA
Chwaka Bay is located on the east coast of Unguja Island (Zanzibar) which is
situated off the East African coast centred around ~6 and 93 ~ 30'E (Fig. 1B). It is a
shallow embayment with an area of approximately 50 km 2 at high water springs
(HWS). Its mouth is barred by a living offshore coral reef. A dead reef lines the
southern landward end wl~ch is fringed by a 1 to 3-km-wide mangrove forest. The
bathymetry of the bay was first studied by Wolanski ,)9891( using a portable echo
sounder from a small boat operating along east-west transects in the bay and the
mangrove creek. Water depths relative to mean sea level (MSL) are mostly less than
Hydrodynamics of akawhC Bay, Tanzania
5 m along the eastern side of the bay. There are several tidal creeks in the open water of
the bay, some of which connect to the mangrove creeks in the south.
1 " o lo o (cid:12)9 o (cid:12)9 ~o ee o6 ~ eo oa, .eo oo eo e oo ooeOoo eeeeeo~176176
i~ ""-':" ~"~ ": :: :--:- :: :Y..;/~9 ' ": "~';'~ ~--" ~- : ~ : :~
"-- -/~ _ .~ J ~ ~ ~ /
- _ ~= ..F'~ 1PEMBA~I$~/ ..~! seaward limit of seagras$
:;~ ii
..'., '~- ........... "5- ....
c..,k,~ (cid:12)9 2
.~:,~ "'~
~.:.
~6 ..~ 01 ajuJ~aR
.&
~Zanz ibar~' creek
~_.~l Chwakal ;"4
ii ~ ~~ ,.! o~-,.,-. .,
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Figure .1 A: Location of the study area off the African east coast. :B Position of
measurement stations in Chwaka Bay. Current meters and tide gauges were deployed
at stations I and ,2 whereas a tide gauge only was deployed at station .3
The water movement in Chwaka Bay is controlled mainly by tidal motions.
According to tidal records from the harbours of Dar Es Salaam and Zanzibar, the tide
in this part of the Indian Ocean is semi-diurnal, being dominated by the component
M 2
(e.g., Lwiza & Bigendako 1988). Older measurements indicate that peak ebb currents
are stronger than peak flood currents, suggesting a tidal asymmetry in the bay
(Wolanski 1989). The mean spring tidal range in the bay is 3.2 m.
The main ecosystems in the bay include mangrove swamps, coral reefs and seagrass
meadows. There are large intertidal areas which have recently attracted seaweed
farming. Although there is no obvious freshwater supply to the bay, salinity
measurements in Mapopwe creek showed values of 29.5-35%o (Wolanski ,)9891
suggesting some freshwater input to the mangrove swamps. Since no surface runoff
exists, freshwater can only be supplied by groundwater seepage. This type of
freshwater input was, in fact, suggested by Mazda et al. (1990).
Being part of the East African region, Chwaka Bay is subject to two alternating
seasons, the south-eastern (SE) and the north-eastern (NE) monsoons. The former
6 iwdnayN dna opopiawM
begins in April and ends in October, whereas the latter begins in November and ends
in March. During the ES monsoon the winds blow predominantly from the south-east,
being accompanied by heavy rains and thunderstorms. Heavy rains are particularly
common between March and June. During the NE monsoon the winds blow mainly
from the north-east. This season includes the 'short rains' between October and
November. Normally the area is cool with occasional light rains between June and
September. From December to March the weather is relatively hot and dry with only
very occasional rain. Meteorological data from Tanzania indicate that the mean values
for rain and evaporation along the coast are 021 mm month ~- and 4 mm day ,~-
respectively.
The East African Coastal Current (EACC) flows northwards throughout the year but
differs markedly between the two monsoon seasons (Newell 1959). Thus, during the ES
monsoon surface current velocities reach 4 knots 2( m ,)~-s being amplified by the trade
winds of the Indian Ocean. During the NE monsoon, by contrast, the EACC still flows
to the north but its speed is reduced to about 5.0 knots (0.25 m s )~- due to the
domination of north-easterly winds (Newell 1959).
3. MATERIALS AND METHODS
Water fluxes and circulation patterns in Chwaka Bay were measured with self-
recording current meters and tide gauges equipped with temperature sensors. The
temperature data were used to study the heat flux in both Mapopwe creek and the bay.
Sea-level data obtained from the tide gauges were used to show the temperature and
flow variations in the course of a tidal cycle.
Three tide gauges of the type Micro-Tide, and two Sensordata SD6000 recording
current meters were deployed in Chwaka Bay for one month in August-September
1992. The Sensordata SD6000 is a compact vector averaging current meter with
memory capacity for up to 6000 combined data sets of current speed, direction and
water temperature. The tide gauges have a memory capacity of 200 MB, and can
measure and record combined data sets of pressure (water level) and temperature.
A tide gauge and a current meter were deployed at the entrance of Mapopwe creek,
a mangrove creek in the south-western part of the bay (station .)1 A similar set was
deployed in the middle of the bay (station 2), whereas a third tide gauge was located at
the mouth of the bay (station .)3 Three reference points (Security House at Chwaka
village, Ras Juja and Ras Michamwe; .fc Fig. )B1 were used to determine the exact
positions of the instruments by means of triangulation.
The tide gauges and recording current meters were programmed to measure and
record at 10-minute intervals. The tide gauges essentially recorded without
interruption over the whole sampling period of about one month. The current metres
at stations 1 and 2 experienced short interruptions when their propellers were fouled
by seaweed. A set of manually operated gelatine pendulum current meters (Haamer
1974; Cederl6f et al. 1995) were deployed from a boat at a number of different stations
within the bay during peak tidal flow in order to compile a map of spatial current
scimanydordyH of akawhC Bay, ainaznaT
speed and direction patterns. Measurements with the pendulum current meters were
also undertaken on several occasions between 1992 and 1994, particularly at times of
maximum ebb or flood currents (i.e. approximately 3 hours after high and low tide,
respectively).
.4 RESULTS
4.1. Flow patterns
The temporal patterns of the tidal currents at stations 1 and 2 are illustrated in the
time series of Fig. ,2 whereas the spatial patterns within the bay are shown in Fig. .3
From Fig. 2 it is observed that the maximum ebb currents (positive values) at station 1
are stronger than the maximum flood currents, and the ebb phase is longer (about
7 hours) than the flood phase (about 5 hours), indicating both velocity and time
asymmetry. At station ,2 however, there is no time asymmetry, and the velocity
asymmetry was found to be less, with the peak flood velocities being slightly higher
than the ebb velocities. The flow directions in Fig. 3 suggest an asymmetry in the tidal
current direction, especially on the west bank where the flood current flows
southwards whereas the ebb current flows about north-north-eastwards.
--03 ,t A
'011 current velocity
It
,'I A 1
IU| tidal elevation ~/
~ J , !! J II
- ~ I ! I I
111 | ItIl | J I I ' I , 1 , 1~ },: I I
' /' I ~i /' | !i ' ,,
'
I !
I I ! ! IlU |ill I~ t I I ! ! I i
_.,,.> ,'i ; 0~
~ID ~ i
~ "Ji i
J
/ I I I
/ ! I I
>.c /j~ I I
1-
I
I
0-~ i
Stati0n 1
-30
i
2f 22' .......... ' ...... 3"2 ' .... 24' 25
A u g u s t 1992
Figure 2A. Time-series plot showing the temporal pattern of the current velocity and
the tidal elevation at station 1 in the period 21-25 August 1992. Negative values
indicate the flood period, positive values the ebb period.