hardened runoff drainage channels, vessel wake and turbulence, and dredging of vessel channels and coastal
development projects that generate dust. Alterations to
flows will also influence plant distributions sensitive to,
and dependent on, upstream salinity gradients.
The impacts of fine sediment deposition are further
observed downstream within estuaries of disturbed catchments, indicated by: increased and prolonged turbidity in
estuarine coastal waters with enlarged plumes during
periods of greater runoff; blocking and choking estuarine
waterways with sediments reducing overall channel
depths; depositional gains as measured by increased formation of mudbanks downstream, often colonized by
mangroves; and the unusually rapid formation of mangrove islands and verges at the mouths of estuaries and
surrounding coastal foreshores.
Factor indicators
Changes due to hydrology and sediments are influenced
by a number of factors. Constructions, like roads and sea
walls, and natural features, like beach berms, all alter
water flow and tidal exchange. Tree death is caused mostly
by excessive inundation of breathing roots, erosion of sediments, or deposition of sediments. Mangrove occupation
of downstream mud banks is the result of clearing catchment vegetation, causing soil erosion coupled with alterations to estuarine margins because of changes in
channel hydrology. Raised deposition ridges burying
mangroves are caused by the influence of waves and
hydrological conditions altered by storms and dredging.
Construction of weirs, barrages, and dams reduces freshwater runoff so saline conditions and salt-tolerant mangrove species penetrate further upstream. Comparatively
high turbidity levels in estuarine waters are measured
using automated instruments and by hand.
Change type and corresponding factors
For hydrology and sediments (Figure 2), measurable areas
of disturbance for respective types of change include:
restricted flow with impoundment (Olsen, 1983; Gordon,
1988; Jupiter et al., 2007), estuarine shift from barriers
to flow from upstream (Duke et al., 2003) and root burial
from deposition above mean sea-level (Ellison, 1999,
2009; Duke et al., 2003) depositional gain from deposition
below mean sea-level (Duke and Wolanski, 2001;
Wolanski and Duke, 2002; Duke et al., 2003).
Pollutants
Predominant drivers
Pollutants in tidal wetlands are mostly the indirect consequence of human activities elsewhere. Pollutants are either
excess amounts of naturally occurring substances or foreign and manufactured compounds. Such substances are
grouped as two distinct types, namely, those natural ones
including those that stimulate plant growth like nutrients
and others which inhibit plant growth like toxicants.
Two chief nutrients, nitrogen and phosphorus, are
essential nutrients for plant and animal growth, but they
are derived from two quite different sources. While nitrogen is obtained naturally from the atmosphere by bacteria
and microalgae and phosphorus is derived naturally from
eroded inorganic sediments, this effectively limits their
availability respectively. However, with human development, this has changed dramatically, and there are now
two new chief sources, namely, point source sewage outflows from urban and industrial treatment sites and diffuse
source runoff from agricultural lands and urban-industrial
development. The effects downstream are readily noticeable with unnaturally elevated levels of nutrients. The
effects on tidal wetlands are largely unquantified but
include abundant growth of epiphytic algae and excess
wrack smothering sensitive above-ground breathing roots.
Also washed downstream are chemical agents like agricultural herbicides in catchment runoff. Other toxic
chemicals, notably petroleum hydrocarbons, are added to
estuaries in chronic oil spill incidents in port areas. The
fact that these chemicals are not often, if ever, found in
natural environments indicates that the biota often have
no prior adaptation, or capacity, to respond successfully
to some toxic elements. Another natural pollutant are dust
clouds although these will have a negative effect on canopy health where photosensitive foliage is covered by fine
sediments leading to reduced photosynthetic capacity and
loss of foliage density, with possible dieback.
Oil spills are mostly accidental and unintended, but
while the application can be direct, it is often delayed by
a matter of days. This is significant since delay is usually
substantial where petroleum hydrocarbons degrade rapidly in the open with most toxic fractions lost first. Spilled
oil on reaching mangrove coasts noticeably smothers plant
surfaces resulting in tree death in extreme circumstances.
Herbicides, by contrast, are essentially water soluble and
invisible to see. Their presence is cryptic but their effect
is more obvious, with the notable death of sensitive species. The source of agricultural chemicals is agricultural
lands upstream in the catchment.
Factor indicators
Changes due to pollutants, nutrients, and toxicants are
influenced by a number of factors. Nutrient-laden fertilizers
used on crop lands in surrounding catchment areas plus
sewage outflows and septic seepage increase smothering,
algal growth on mangrove roots. In Eastern Australia, a currently unknown agent, in combination with high nutrient
loads, has led to the rapid decomposition of below-ground
roots, resulting in sediment collapse, subsidence and centre
sinking. This leads to shallow anoxic pools within mangroves and salt marsh vegetation. In other instances, species
specific death of Avicennia trees was correlated with toxic
herbicides in runoff from catchment in coastal areas
(Duke et al. 2005a). Spillage of toxic chemicals used in
local transport causes tree death from toxicity and smothering of breathing surfaces. Dust buildup on foliage notably
affects plant health and survival.
418
MANGROVE COASTS
development projects that generate dust. Alterations to
flows will also influence plant distributions sensitive to,
and dependent on, upstream salinity gradients.
The impacts of fine sediment deposition are further
observed downstream within estuaries of disturbed catchments, indicated by: increased and prolonged turbidity in
estuarine coastal waters with enlarged plumes during
periods of greater runoff; blocking and choking estuarine
waterways with sediments reducing overall channel
depths; depositional gains as measured by increased formation of mudbanks downstream, often colonized by
mangroves; and the unusually rapid formation of mangrove islands and verges at the mouths of estuaries and
surrounding coastal foreshores.
Factor indicators
Changes due to hydrology and sediments are influenced
by a number of factors. Constructions, like roads and sea
walls, and natural features, like beach berms, all alter
water flow and tidal exchange. Tree death is caused mostly
by excessive inundation of breathing roots, erosion of sediments, or deposition of sediments. Mangrove occupation
of downstream mud banks is the result of clearing catchment vegetation, causing soil erosion coupled with alterations to estuarine margins because of changes in
channel hydrology. Raised deposition ridges burying
mangroves are caused by the influence of waves and
hydrological conditions altered by storms and dredging.
Construction of weirs, barrages, and dams reduces freshwater runoff so saline conditions and salt-tolerant mangrove species penetrate further upstream. Comparatively
high turbidity levels in estuarine waters are measured
using automated instruments and by hand.
Change type and corresponding factors
For hydrology and sediments (Figure 2), measurable areas
of disturbance for respective types of change include:
restricted flow with impoundment (Olsen, 1983; Gordon,
1988; Jupiter et al., 2007), estuarine shift from barriers
to flow from upstream (Duke et al., 2003) and root burial
from deposition above mean sea-level (Ellison, 1999,
2009; Duke et al., 2003) depositional gain from deposition
below mean sea-level (Duke and Wolanski, 2001;
Wolanski and Duke, 2002; Duke et al., 2003).
Pollutants
Predominant drivers
Pollutants in tidal wetlands are mostly the indirect consequence of human activities elsewhere. Pollutants are either
excess amounts of naturally occurring substances or foreign and manufactured compounds. Such substances are
grouped as two distinct types, namely, those natural ones
including those that stimulate plant growth like nutrients
and others which inhibit plant growth like toxicants.
Two chief nutrients, nitrogen and phosphorus, are
essential nutrients for plant and animal growth, but they
are derived from two quite different sources. While nitrogen is obtained naturally from the atmosphere by bacteria
and microalgae and phosphorus is derived naturally from
eroded inorganic sediments, this effectively limits their
availability respectively. However, with human development, this has changed dramatically, and there are now
two new chief sources, namely, point source sewage outflows from urban and industrial treatment sites and diffuse
source runoff from agricultural lands and urban-industrial
development. The effects downstream are readily noticeable with unnaturally elevated levels of nutrients. The
effects on tidal wetlands are largely unquantified but
include abundant growth of epiphytic algae and excess
wrack smothering sensitive above-ground breathing roots.
Also washed downstream are chemical agents like agricultural herbicides in catchment runoff. Other toxic
chemicals, notably petroleum hydrocarbons, are added to
estuaries in chronic oil spill incidents in port areas. The
fact that these chemicals are not often, if ever, found in
natural environments indicates that the biota often have
no prior adaptation, or capacity, to respond successfully
to some toxic elements. Another natural pollutant are dust
clouds although these will have a negative effect on canopy health where photosensitive foliage is covered by fine
sediments leading to reduced photosynthetic capacity and
loss of foliage density, with possible dieback.
Oil spills are mostly accidental and unintended, but
while the application can be direct, it is often delayed by
a matter of days. This is significant since delay is usually
substantial where petroleum hydrocarbons degrade rapidly in the open with most toxic fractions lost first. Spilled
oil on reaching mangrove coasts noticeably smothers plant
surfaces resulting in tree death in extreme circumstances.
Herbicides, by contrast, are essentially water soluble and
invisible to see. Their presence is cryptic but their effect
is more obvious, with the notable death of sensitive species. The source of agricultural chemicals is agricultural
lands upstream in the catchment.
Factor indicators
Changes due to pollutants, nutrients, and toxicants are
influenced by a number of factors. Nutrient-laden fertilizers
used on crop lands in surrounding catchment areas plus
sewage outflows and septic seepage increase smothering,
algal growth on mangrove roots. In Eastern Australia, a currently unknown agent, in combination with high nutrient
loads, has led to the rapid decomposition of below-ground
roots, resulting in sediment collapse, subsidence and centre
sinking. This leads to shallow anoxic pools within mangroves and salt marsh vegetation. In other instances, species
specific death of Avicennia trees was correlated with toxic
herbicides in runoff from catchment in coastal areas
(Duke et al. 2005a). Spillage of toxic chemicals used in
local transport causes tree death from toxicity and smothering of breathing surfaces. Dust buildup on foliage notably
affects plant health and survival.
418
MANGROVE COASTS