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15
Sea/Air Interaction
Shigalla Mahongo
Opposite page: Cyclone Favio entering the Mozambique Channel on 20 February 2007. © Nasa/Jeff Schmaltz/Goddard Space Flight Center.
Introduction
The atmosphere and the ocean form a coupled system,
constantly exchanging mass (in the form of water, gas,
spray, bubbles and particles) and energy at the interface
between the seawater and air. This energy exchange
occurs in the form of momentum (through wind stress)
and heat. In other words, the atmosphere forces the ocean
through exchange of momentum, net surface heat flux
and freshwater flux. The exchanges at the sea/air interface are irregular, taking place at rates which are largely
induced by the dynamics at the surface. The exchanges
affect the biological, chemical and the physical properties
of the ocean thus influencing its biogeochemical processes, weather and climate. Heat loss from the ocean to
the atmosphere plays a vital role of regulating heat balance as well as moisture and energy budgets of the atmosphere. The mean climate of the Earth over long time
scales is therefore partly shaped by the convergence/
divergence of the oceanic heat exchanges, which act as
sources and sinks of heat for the atmosphere (Lee and
others, 2010).
An understanding of the extent to which the sea and
the air influence each other is about large scale sea-air
interactions. The biogeochemical interaction between
the sea and the air that involve gas and chemical exchanges
are important to life processes. This interaction is sustained by the mixing of the surface by wind and waves to
About half the world’s oxygen is produced by phytoplankton in the sea (Falkowski 2012), which are at the
base of the marine food web. The phytoplankton, through
the photosynthesis process, also extract carbon dioxide
(CO2), a greenhouse gas that contributes significantly to
current global warming (Ciais and others, 2013). The
oceans therefore act as major sinks for atmospheric CO2.
With the exception of the Indian Ocean, where the phytoplankton levels have remained relatively stable since
the 1950s, the levels in the other oceans have generally
declined by about 40 per cent (Boyce and others, 2010).
Whereas photosynthesis is one of the major biogeochemical processes which take the CO2 from the atmosphere to the ocean, there are other biogeochemical
processes which eventually lead to the removal of CO2
from the sea. The dissolved CO2 may either react with
the sea water to form carbonic acid or with carbonates
already in the water to form bicarbonates. Either of the
two processes removes dissolved CO2 from the sea water.
Many marine plants and animals use the bicarbonate to
form calcium carbonate skeletons (or shells). If the sea/air
interaction processes remained unchanged, there would
be a permanent balance between the concentrations of
CO2 in the atmosphere and the ocean. However, the levels of CO2 in the atmosphere have been rising, so more of
the gas is dissolving in the ocean and which is no longer
able to absorb the increased concentration of CO2 in the
atmosphere without changes to the acidity levels (Singh
keep a balance between the ocean and the atmosphere.
and others, 2014).
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The exchanges that involve more reactive gases
such as dimethyl sulphide can alter cloud formation and
hence albedo (Bigg and others, 2003). Particulate matter
containing elements such as iron derived from continental land masses would tend to alter ocean primary productivity with impacts on other biogeochemical
exchanges that might multiply. An iron deficit in sea
water could be one of the major limiting factors for phytoplankton growth. However, wind borne dust from the
deserts such as the Sahara plays a significant role in
replenishing this important element in the sea (Bigg and
others, 2003).
Every atmospheric gas is in equilibrium with its
component that is dissolved in sea water, with dissolved
oxygen and CO2 being among the most important gases.
Dissolved oxygen is critically important for respiration
of aquatic animals, which releases energy from carbohydrates, releasing CO2 and water as by-products. As for
Exchange of momentum through wind stress
The exchange of momentum between the atmosphere
and the ocean, through wind stress, is the primary driver
of ocean circulation, particularly the surface currents.
Wind stress is a measure of momentum transferred from
the atmosphere to the ocean (Collins and others, 2013).
However, momentum exchange is complicated by the
stratification or stability of the atmospheric boundary
layer, the wave field near the surface, and a host of other
processes. At the global level, there is evidence that the
zonal mean wind stress at the sea-air interface has
increased in strength since the early 1980s (Rhein and
others, 2013).
In the WIO region, the trend of surface-wind stress
pattern over the period 1966-2007 showed an enhanced
convergence around 15°S due to anomalous north-westerly winds from the equator (Nidheesh and others, 2013).
These wind changes resulted in a large negative wind
dissolved CO2 this gas is very important for marine
plants, which use dissolved CO2 to manufacture carbohydrates through photosynthesis, releasing oxygen as a
by-product (Karleskint and others, 2012). The main
objective of this chapter is to review the status of the
Western Indian Ocean (WIO) regional sea/air interactions and their associated environmental, economic and
social implications.
stress curl anomaly around 10°S, dynamically consistent
with the decrease in total steric sea level anomaly (SSLA)
observed in the southwestern Indian Ocean region. The
trend of wind stress in the western Equatorial Indian
Ocean (WEIO) and Southwest Indian Ocean (SWIO)
sub-regions with monthly NCEP data for the period 1982
to 1994 (May through September), showed very high values over the sub-region.
Changes in atmospheric fluxes
and concentration levels of oxygen
and carbon dioxide
Changes in Atmospheric fluxes
The turbulent fluxes of heat, water and momentum
through the sea surface constitute the principal coupling
between the ocean and the atmosphere. The fluxes play
an important role in driving both the ocean and atmospheric circulations, thereby redressing the heat imbalance. The air–sea fluxes also influence temperature and
humidity in the atmosphere and, hence, the hydrological
cycle (Rhein and others, 2013). Consequently, the
exchange and transporting processes of these fluxes are
essential components of global climate. The WIO region
is strongly affected by external forcing, leading to interannual climatic variability such as the El Niño Southern
Oscillation (ENSO) and Indian Ocean Dipole (IOD), as
well as seasonal climatic variability such as the monsoon
Heat flux
Information on sea-air heat fluxes over the WIO is generally scarce due to the relative paucity of data, making it
difficult for one to make conclusive judgements on the
general trends. At the global scale, the accuracy of the
observations are also currently insufficient to permit
direct assessment of changes in heat flux (Rhein and others, 2013). However, the net flux of heat at the sea surface
in the tropical oceans, including the WIO region, is characterized by net gain due to incoming shortwave solar
radiation, and net loss due to evaporation and heat fluxes
(Jayakrishnan and Babu, 2013).
A study carried out in the east coast of Zanzibar
Island, at Chwaka Bay, over a short period in January/
February 1996 indicated a net gain to the bay of 275.5 W/
m2 due to incoming shortwave solar radiation (Mahongo
1997). The net heat loss was due to the sum of the fluxes
of evaporation (157.8 W/m2), long-wave back radiation
(38.1 W/m2) and sensible heat (17.8 W/m2), respectively
circulation (Manyilizu and others, 2014).
(Mahongo 1999, Mahongo 2000). The net heat gain to the
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15. Sea/air interaction
sea was therefore 61.8 W/m2 which could be accounted for
by advection flux to the offshore (Mahongo 1997). Similar quantities of evaporation and sensible heat fluxes in
the offshore around the same latitude and season are
slightly lower, being estimated at 92 W/m2 and 11 W/m2
(Ramesh Kumar and Gangadhara Rao 1989).
Over recent decades, trends have been recorded in
many of the parameters that affect heat transfer at the
sea-air interface. These include: definitive increase in sea
surface temperature (SST) (Roxy and others, 2014), in
surface air temperature (Vincent and others, 2011) and in
wind speed (Mahongo and others, 2012, Shaghude and
others, 2013). For instance, during the period 1901-2012,
the tropical WIO, which is generally cool, has experienced
an anomalous warming of 1.2 °C in summer SST, at a rate
that is greater than recorded in any tropical ocean region
(Roxy and others, 2014). The observed ocean warming
could potentially change the monsoon circulation and
Empirical Orthogonal Function (EOF) modes accounted
for between 24.27 and 20.94 per cent of the variance, representing basin-scale cooling/warming (EOF-1) and Indian
Ocean Dipole (EOF-2) events, respectively. The observed
trends in ocean heat content may therefore lead to rising
sea levels and significant stress to some marine ecosystems.
precipitation patterns over the WIO region with impacts
on the marine food webs that would multiply due to
reductions in the productive diatoms which are the basis
of the food chain. This will result in relatively less energy
available to support high-level marine vertebrates such as
fish and marine mammals.
trends in ocean surface salinity is important because
changes in salinity affect circulation, water column stratification as well as changes in regional sea level (Rhein and
others, 2013). For instance, historical salinity measurements in the WIO show strengthening of the South Equatorial Current (SEC) in the 1950–1975 interval compared
with the early 2000s due to excess of evaporation over precipitation (Zinke and others, 2005). This in turn affects biological productivity, the capacity of the ocean to store heat
and carbon and, therefore, the carbon cycle.
Fresh water exchange
The ocean salinity, which is a proxy indicator of freshwater
fluxes to the oceans (Bingham and others, 2012), increased
from 1987 to 2002 in the upper thermocline of the WIO
along 32°S (Álvarez and others, 2011), reversing the freshening trend which previously occurred from 1962-1987
(Bindoff and McDougall 2000). After the recent advent of
the use of Argo floats, tracking of surface ocean salinity
achieved near-global coverage in 2004. The data showed an
increasing trend of ocean salinity in the WIO region
between 2004 and 2013. Diagnosis and understanding of
Upper Ocean Heat Content
The ocean’s large mass and high heat capacity allow it to
store huge amounts of energy. However, due to increasing
concentrations of greenhouse gases, heat radiated from the
earth’s surface does not escape freely into space. Most of
the excess heat is therefore stored in the upper ocean, leading to the rising of the upper ocean heat content (Levitus
and others, 2009). During the last four decades (19712010), estimates from global ocean temperature measurements indicate that the upper ocean (0 to 700 m depth) has
absorbed about 137 TW of heat (1 TW = 1012 watts), equivalent to 93 per cent of the combined heat stored by warmed
air, sea, land and melted ice (Rhein and others, 2013).
In the Indian Ocean, in-situ and satellite observations,
ocean-atmosphere re-analysis products and re-constructed
datasets show basin-scale decadal warming trends in the
upper ocean heat content for the period 1955 to 2008,
which were attributed to anthropogenic forcing (Levitus
and others, 2009). More recently, Chu (2011) used monthly
synoptic temperature and salinity datasets for the Indian
Ocean between 1990 and 2009 to study the upper ocean
Changes in concentration levels of oxygen
and carbon dioxide
Available records from the Indian Ocean (including the
WIO region) between 1962 and 2002 indicate that oxygen
concentration at a mean latitudinal section of 32oS has
undergone two major changes. The first change involved a
pronounced decreasing trend in concentration levels
between 1962 and 1987 (Bindoff and McDougall 2000).
The second phase occurred between 1987 and 2002, where
the concentration levels reversed to reveal an increasing
trend (McDonagh and others, 2005). Bindoff and McDougall (2000) attributed the decrease during the first phase to
the slowing down of the subtropical gyre circulation in the
south Indian Ocean, implying that the gyre circulation may
have accelerated during the second phase.
Changes in the concentration levels of oxygen may
have important implications for marine ecosystems and
heat content. The results indicated that the first two
socio-economic livelihoods of coastal communities. In view
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of this, Körtzinger and others (2006) have proposed oxygen
to be used as one of the climate change indicators. Furthermore, measurements of oxygen generally have relatively
high precision and accuracy, making oxygen suitable for
being used as a target tracer on large‐scale observing programs for detection of decadal changes (Gruber and others,
2007).
In the upper thermocline, subtropical, subsurface
water of the Indian Ocean along 20oS (which includes the
southwestern Indian Ocean), anthropogenic CO2 storage
over an 8-year period (between 1995–2003/2004) is
reported to have increased at an average rate of 7.1 mol/m2
(Murata and others, 2010). The observed change is almost
two times higher than that reported during the previous
decade (between 1978 and 1995), which was 13.6 mol/m2
(Sabine and others, 1999).
The world’s oceans play a significant role as sinks for
anthropogenic carbon, sequestrating roughly one-third of
play an important role in establishing the upper air flow, in
particular the Somali Jet, which has a considerable impact
on WIO weather patterns (Slingo and others, 2005).
The SE trade winds (April-October) originate from the
semi-permanent South Indian Ocean anticyclone (Mascarene High). Conversely, the NE trades (NovemberMarch) originate from the semi-permanent high pressure
system centred in the Arabian Gulf (Arabian High), also
related to pressure build-up over Siberia (Siberian High).
The shifting of the ITCZ northwards and then southwards
gives the coast of East Africa its marked biannual rainy seasons with the long rains in March-April-May and the short
rains in October-November-December. Due to the effect
of the Coriolis force, the NE winds veer to the northwest in
the south of the equator, whereas the SE winds veer to the
west in the north of the equator. An important wind-driven
feature in WIO is the upwelling phenomena off the coast
of Somalia. During the SE monsoon season, an upwelling
the cumulative human CO2 emitted from the atmosphere
over the industrial period (Khatiwala and others, 2013).
Other studies note that the oceanic anthropogenic carbon
inventory has increased between the period spanning from
1994 and 2010 (Christensen and others, 2013). Although
the carbon sequestration concept has recently triggered
global concerns, very few studies have been undertaken in
the WIO region. Among the outstanding studies on carbon
sequestration concept conducted in the WIO region
include Sengul and others (2007), Alemayehu and others
(2014) and Jones and others (2014).
of cold water mass is established in this area, characterized
by lowering of SST to about 22oC on average (Mafimbo
and Reason 2010).
Over the last three decades, both the mean and maximum speeds of the monsoon winds have generally
strengthened in some parts of the region such as in Tanzania (Mahongo and others, 2012). While these changes could
be attributed to the global climate changes, they could also
be related to natural decadal cycles of the climate system,
including the 22-year Hale solar cycle (Mahongo 2014).
Therefore, a longer time series dataset is needed to ascertain whether the increasing trend will persist. Dunne and
others (2012) have found no evidence of changes in the
wind regime in the region (at Diego Garcia) during the
recent past. A global assessment by the Intergovernmental
Panel on Climate Change (IPCC) also noted a low confidence in changes of surface wind speeds due to uncertainties in datasets and measures used (Hartmann and others,
2013).
Meteorological phenomena of the Western
Indian Ocean
Monsoon winds
The WIO exhibits more pronounced seasonal wind reversals than the rest of the Indian Ocean and is an important
region of air–sea interaction (Benny 2002). Two types of
seasonal wind systems have been documented, namely the
northeast (NE) and southeast (SE) trade winds, leading to
the NE and SE monsoon seasons. Both trade wind systems
are influenced by seasonal shifts in the Inter-Tropical Convergence Zone (ITCZ). The most important factor that
determines the generation of the monsoon seasonal wind
pattern is the geographical orientation of the Indian Ocean.
It is bounded to the north by the Asian continent, which
largely influences the meteorology of the region. The other
factor is the presence of the East African Highlands, which
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Tropical cyclones
As part of planet’s Warm Pool, tropical cyclones are an
important feature of the meteorology of the WIO region,
particularly over the southwest Indian Ocean. Cyclone
activity in the WIO region is strongly influenced by anomalously warm SSTs in the tropical South Indian Ocean,
which is a critical factor in their formation. Most of the
cyclones originate from the east of Madagascar (50o–100oE,
5o– 15oS) and some from the Mozambique Channel during
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Austral summer (Ho and others, 2006). The tropical cyclone
season extends from October to May, and about 11-12
cyclones (tropical storms and hurricanes) occur annually
(Bowditch 2002). In each season, about four cyclones reach
hurricane intensity. The islands of Comoros, Mauritius and
Madagascar lie within the region of tropical cyclone activity, while Réunion, Mayotte, Comoros, and Mozambique
are also prone to direct landfall (Figure 15.1). On rare occasions, some parts of southern Tanzania and South Africa
can also be affected.
Webster and others (2005) observed an increase in the
annual frequency of cyclones in the South Indian Ocean
within the period 1970 and 2004. The number of intense
tropical cyclones also increased from 36 during 1980-1993
to 56 during 1994-2007, parallel to a simultaneous but
smaller decrease in the number of tropical storms (Mavume
BOX 15.1.
and others, 2009). Globally however, the current datasets
do not indicate any significant trends in tropical cyclone
frequency over the past century (Christensen and others,
2013). It also remains uncertain whether any reported
long-term increases in tropical cyclone frequency are
robust, after accounting for past changes in observing capabilities (Christensen and others, 2013). Incidentally, no
significant trends were observed in the annual numbers of
tropical cyclones in the South Indian Ocean between the
period 1981–1982 to 2006–2007 (Kuleshov and others,
2010).
Apparently, one of the challenges in tracking changes
in the frequency and intensity of cyclones is that the record
of past events is heterogeneous. This is due to changes in
observational capabilities and how cyclones have been
measured and recorded. It is thus difficult to draw firm con-
Tropical Cyclone Gafilo and its impact in Madagascar
unusually large and violent, with wind speeds of about 250
km/h, and gusts of up to 330 km/h. Gafilo was the deadliest
and most destructive cyclone of the 2003/2004 cyclone season.
Gafilo originated from south of Diego Garcia, and intensified
into a moderate tropical storm on 3 March 2004. Gafilo
became a tropical cyclone on 4 March 2004, and ultimately
intensified into a very intense tropical cyclone on 6 March
2004, prior to making landfall over Madagascar early on 7
March 2004. After crossing Madagascar, Gafilo emerged into
the Mozambique Channel and made landfall over Madagascar again on 9 March 2004. After a three-day loop overland,
the system arrived back over the southern Indian Ocean on
13 March 2004, and transitioned into a subtropical depression on 14 March 2004.
The cyclone caused a massive destruction of property and
85 per cent of the city of Antalaha was destroyed. The
cyclone claimed 237 lives, with 181 missing persons and 879
injured, and caused a property loss of about US$ 250 million
Cyclone Gafilo approaching Madagascar on 6 March 2004. © Nasa/Jeff
Schmaltz/Goddard Space Flight Center.
in Madagascar. More than 304 000 people were displaced by
The Tropical Cyclone Gafilo, which struck the northeast coast
were flooded, resulting in major crop losses. Total rainfall for
of Madagascar early on the morning of 7 March 2004, was
the period 3–10 March 2004 reached values of up to 500 mm
the most intense and devastating tropical cyclone ever
in an area from the central Mozambique Channel eastward
recorded in the Southwest Indian Ocean. The cyclone was
along the northwest coastline of Madagascar.
the storm and more than 6 000 hectares of agricultural land
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clusions on the observed trends prior to the satellite era
and in ocean basins such as the South Indian Ocean (Christensen and others, 2013). According to Christensen and
others (2013), tropical cyclone numbers are unlikely to
increase, but cyclone maximum intensity is likely to
increase on the global average, meaning increased maximum precipitation and winds.
Monsoon rains
Coastal rainfall in the western Indian Ocean is mostly seasonal, with the heaviest and most prolonged rains occurring
during the months of March to June. Most parts of the
region experience annual rainfall of between 1 000 to 2 000
mm, but local variations are common (Ngusaru 2002). Generally, the effect of rainfall and river runoff on salinity is
restricted to narrow coastal fringes.
The trends in precipitation indices in the western
Indian Ocean are generally weak and show less spatial
coherence. Vincent and others (2011) found a significant
decrease in the total annual rainfall during the period 19612008. Their results also indicate some increase in consecutive dry days, no change in daily intensity and consecutive
wet days, and a decrease in extreme precipitation events.
Weak correlations were found between precipitation indices and surface air temperature. The IPCC regional climate projections for the 21st century indicate an increase
in the annual mean rainfall over East Africa (Collins and
others, 2013).
During 1901-2012, the tropical western Indian Ocean
(50-65oE, 5oS-10oN) experienced anomalous warming of
1.2oC in austral summer SSTs, surpassing that of the Indian
Ocean warm pool which increased by only about 0.7oC
(Roxy and others, 2014). The increase in temperature of the
generally cool WIO against the rest of the tropical warm pool
region affects zonal SST gradients, and could potentially
change the monsoon circulation pattern, with impacts on
rainfall patterns as well as changes on the marine food webs
in the region. This is due to the fact that warming causes the
air over the ocean to expand and lower the atmospheric
pressure consequently unsettling the wind pattern which, in
turn, may lead to the changes on monsoon flow pattern.
Besides anthropogenic global warming, the natural longterm warming trend of the Indian Ocean during the period
1901-2012 was influenced by the asymmetry in the El Niño
Southern Oscillation (ENSO) teleconnection, whereby El
Niño events induced anomalous warming over the WIO and
La Niña events failed to do the reverse (Roxy and others,
2014). The period between 1950 and 2012 was characterized
by strong and frequent occurrences of positive El Niño
events (Roxy and others, 2014), consistent with the period
1960-2004 (Ihara and others, 2008). The warming in the
region has also been attributed to positive SST skewness
associated with ENSO, as the frequency of El Niño events
have increased during recent decades (Roxy and others,
2014).
Environmental, economic and social
implications of trends in meteorological
phenomena
Sea Surface Temperature (SST)
The WIO region’s meteorology is dominated by the seasonal reversal of the monsoon wind systems, leading to the
largest annual variations in SSTs found in any of the tropical
oceans. On the inter-annual timescale, the SST in the WIO
region is primarily influenced by the El Niño Southern
Oscillation and the Indian Ocean Dipole (IOD). Manyilizu
and others (2014), using NCEP Reanalysis data stretching
from 1980 to 2007, observed these two signals in the region
that were prominent at periods of 5 and 2.7 years, respectively. According to the IPCC Fifth Assessment Report
(AR5), the WIO region’s SST trend over the period 1950–
2009, computed using monthly SST data extracted from the
Hadley Centre HadISST1.1 data set (Rayner and others,
2003), indicate an increase of 0.60°C (Hoegh-Guldberg and
others, 2014). In the Somali Current surface waters, the same
dataset indicated an increase of 0.26°Cover the period
Abrupt cyclogenesis is known to increase the risk of marine
environmental hazards and coastline erosion particularly for
island states such as Mauritius, Reunion and Madagascar
(Chang-Seng and Jury 2010). The cyclones may bring severe
flooding to coastal areas of Africa, as exemplified in Mozambique in February 2000 (Box 15.1). In some parts of the
region, cyclones had been associated with heavy swells
which created significant rises in sea levels that affected
coastal infrastructure such as roads and settlements, as well
as beach stability (Ragoonaden 1997). Considering the vulnerability of the coastal communities of the WIO region to
tropical cyclone activity, especially the island states and lowlying areas of Mozambique, developing better monitoring
and forecast tools for such sporadic weather events should
between 1982 and 2006 (Hoegh-Guldberg and others, 2014).
be given high priority.
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15. Sea/air interaction
Environmental, economic
and social implications of trends
in ocean acidification
Anthropogenic ocean acidification
Ocean acidification is a relatively new field of research,
with most of the research conducted to-date being
short-term and laboratory based. Acidification is a direct
consequence of increased atmospheric CO2 emissions
to the atmosphere. A proportion of the emitted gas stays
in the atmosphere as greenhouse gas, while some of it
leaves the atmosphere to either become sequestered in
trees and plants or become absorbed in the oceans.
Ocean acidification alters the chemical speciation and
biogeochemical cycles of many elements and compounds in sea water, creating repercussions throughout
marine food chains. The chemical changes caused by
the uptake of CO2 cause many calcifying species to
exhibit reduced calcification and growth rates, and an
increase in carbon fixation rates in some photosynthetic
organisms. Acidification is also known to increase erosion of carbonate rocks, degradation of coral reef habi-
Ocean acidification has only recently been recognized as a
global threat, with potentially adverse environmental, economic and social implications. Being a complex phenomenon, isolating it from other factors affecting the ocean such
as surface warming and coastal pollution is very challenging. Furthermore, it takes time to observe developments
that impact the environment, economy and human society.
The adverse effects of rising acidity include an alteration
of the health of many marine species such as plankton,
molluscs, and other shellfish (Table 15.1). In particular, corals can be very sensitive to increased acidity, which may
lead to alteration to the reef-fish habitat.
Most of the coastal communities in the region are
small-scale artisanal fishers who are highly dependent
tats and alteration of the otoliths of pantropical fish
species with implications for sensory function (Bignami
and others, 2013).
There is a financial barrier in pursuing research on
ocean acidification in developing countries, including the
WIO region, due to the high costs associated with the
nature of the research. Currently, a few monitoring studies on seawater pH are undertaken, but no long-term
observational studies have yet been embarked on
(Sumaila and others, 2014). Recently, however, some initiatives have begun to address these shortcomings
through collaboration with international research institutions by studying some aspects of ocean acidification. In
Tanzania for instance, the UK Ocean Acidification
Research Programme is currently investigating a variety
of geological records of a newly recovered borehole
through marine sediments to study the response of the
carbon cycle during the rapid onset of the Paleocene /
Eocene thermal maximum (PETM) using new computer
models (Aze and others, 2014).
In Kenya, the International Atomic Energy Agency
(IAEA) is collaborating with CORDIO-East Africa
through the Coordinated Research Project (CRP) to:
identify and describe pathways of impact of ocean acidification, improve understanding of the vulnerability of
regions and markets to ocean acidification, and quantify
economic impacts of biological effects of ocean acidification to assist natural resource management and policy
upon fishing for their livelihoods (Kimani and others,
2009). Changes to harvest could therefore be a threat to
food security. Aquaculture in the region is increasing and
has promising potential. A shift toward new production
methods and cultured species may provide benefits to
household livelihoods and small and medium enterprise
development. More information is therefore needed on
carbon chemistry and fisheries in the region.
Although the impacts of long-term changes in ocean
acidification on marine organisms and their ecosystems are
much less certain, due to the fact that the physiological
attributes of marine ecosystems is highly variable, the effects
among organisms will generally also be variable. Considering the generation spans of the different species, it is possible that the impact of acidification and the degree to which
species adapt to their changing environment can take years
or decades to observe. In the WIO region, no observational
or direct experimental data or studies on trends in ocean
acidification impacts have been undertaken. However, areas
such as the monsoon-induced upwelling zones as well as the
coastal and estuarine waters in the region are natural hotspots of special concern. These areas support lucrative fisheries, but the upwelled waters with high CO2 content make
them particularly sensitive to increased ocean acidification.
In view of the on-going changes in global climate and
the associated alterations in ocean acidification, coastal
communities in the WIO region need to have a clear
understanding of the possible scale of potential impacts
decisions on regional and local scales.
based on assessment of exposure, sensitivity and adaptive
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capacity. Case studies need to be undertaken on the environmental, economic and social impacts on ecosystems
and aquatic organisms, especially for the species most
30
40
50
vulnerable to ocean acidification. Comprehensive risk
assessments should also be designed and implemented to
prioritize adaptive responses.
60
70
80
-30
-30
-20
-20
-10
-10
2004-2014 Cyclones
40
30
50
60
70
80
Figure 15.1. WIO Tropical Cyclone Trackline (2004-2014).
Table 15.1. Summary of the anticipated future effects of ocean acidification on different groups of marine organisms, mostly based on experimental
studies from around the world. Note that none of the data is from this region. Table adopted from Sumaila and others (2014).
Group
Main acidification impacts
Warm water corals
A relatively well-studied group. The great majority of experiments show that increasing seawater CO2 decreases
adult coral calcification and growth, and suppresses larval metabolism and metamorphosis. Although most
warm water coral reefs will remain in saturated waters by 2100, saturation levels are predicted to decline rapidly
and substantially; thus coral calcification is unlikely to keep up with natural bioerosion. Interactions with other
climatic and anthropogenic pressures give cause for concern.
Molluscs
Significant effects on growth, immune response and larval survival of some, although with high inter-specific.
Pteropods seem particularly sensitive and are a key component of high latitude food webs. Molluscs are important in aquaculture, and provide a small yet significant protein contribution to human diet.
Echinoderms
Juvenile life stages, egg fertilization and early development can be highly vulnerable, resulting in much reduced survival Adult echinoderms may increase growth and calcification; such responses are, however, highly
species specific.
200
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15. Sea/air interaction
Group
Main acidification impacts
Crustaceans
The relative insensitivity of crustaceans to ocean acidification has been ascribed to well-developed ion transport regulation and high biogenic content of their exoskeletons. Nevertheless, spider crabs show a narrowing
of their range of thermal tolerance by ~2˚C under high CO2 conditions.
Foraminifera
Shell weight sensitive to CO32- decrease in the laboratory with field evidence for recent shell-thinning.
Fish
Adult marine fish are generally tolerant of high CO2 conditions. Responses by juveniles and larvae include diminished olfactory ability, affecting predator detection and homing ability in coral reef fish and enhanced otolith
growth in sea bass.
Coralline algae
Meta-analysis showed significant reductions in photosynthesis and growth due to ocean acidification treatments. Elevated temperatures (+3°C) may greatly increase negative impacts. Field data at natural CO2 vents
show sensitivity of epibiont coralline algae.
Non-calcified
macro-algae; sea grasses
Both groups show capability for increased growth. At a natural CO2 enrichment site, sea grass production was
highest at mean pH of 7.6.
Coccolitho-phores
Nearly all studies have shown reduced calcification in higher CO2 seawater. However, the opposite effect has
also been reported, and ocean acidification impacts on coccolithophore photosynthesis and growth are equivocal, even within the same species. This variability may be due to the use of different strains and/or experimental conditions.
Bacteria
Most cyanobacteria (including Trichodesmium, a nitrogen-fixer) show enhanced photosynthesis and growth
under increased CO2 and decreased pH conditions. Heterotrophic bacteria show a range of responses with
potential biogeochemical significance, including decreased nitrification and increased production of transparent exopolymer particles (affecting aggregation of other biogenic material and its sinking rate). Adaptation by
bacteria to a high CO2 world may be more rapid than by other groups.
Figure15.2. Floods in Mozambique in January 2013. © Phil Hay/UN/World Bank.
We s te r n I n d ia n O ce a n
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IV . Assessment of major ecosystem services from the marine environment
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