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16. Mangroves and Seagrasses
N. C. Duke & A. W. D. Larkum
N OVERVIEW
Sandwiched between two of the world’s iconic tropical
ecosystems of coral reefs and rainforests, are two important coastal communities: mangroves and seagrasses
(Fig. 16.1). While corals flourish in shallow warm seas,
and rainforests cover wetter upland regions, mangroves
inhabit the sheltered intertidal margins barely above
mean sea level. Seagrasses occupy depths from intertidal to deeper habitats, depending on the clarity of the
water column. These biota- structured ecosystems play
an important role in coastal processes with highly developed linkages and connectivity between them, and
to the neighbouring communities represented by coral
reefs and rainforests. These relationships appear vital to
the survival of each. For example, while sediment-loving mangroves depend on waters sheltered by coral reef
structures, they in turn protect hypersensitive corals
from excessive sediments and nutrients coming down
from surrounding catchments.
N GENERAL DESCRIPTION AND
ROLE IN ECOSYSTEMS
Mangrove and seagrass communities are characterised
by a small number of widely distributed angiosperm
taxa, having evolved mostly post-Cretaceous (within
the last 60–100 million years). The relatively recent evolution of these communities may explain their comparatively low species diversity today, although this is also
arguably related to the harsh environmental factors
that characterise these communities’ habitats. The relatively rich mangrove and seagrass floras of today are
testament to their adaptive and evolutionary success
for living in and adjacent to the intertidal zone. These
highly specialised plants flourish in soft sediments,
tapping rich estuarine nutrients with their distinctively
vascular root systems. Mangroves further provide significant habitat and structure where their biomass accumulation, while readily seen as contiguous with
adjacent rainforests, is also analogous to that created
by coral reefs.
While important details of relevant phyletic origins
remain lacking, ancestral mangrove and seagrass plant
taxa are known to have reinvaded marine environments
in multiple episodes from the diverse selection of angiosperm lineages. Their evolution appears constrained
by key functional attributes essential for survival in saline and aqueous settings where isotonic extremes, desiccation and hydrologic exposure combine as uniquely
harsh constraints on organisms living in tidal zones and
estuaries. To achieve this, mangroves and seagrasses
share a number of ecophysiological traits and have
evolved mechanisms to cope with life at the land-sea
interface, for example, salt tolerance, translocation of
gases to aerate their roots, and reproductive strategies.
Both plant communities perform important ecosystem
services such as sediment stabilisation, nutrient processing, shoreline protection, and provide habitat and
16. Mangroves and Seagrasses
N. C. Duke & A. W. D. Larkum
N OVERVIEW
Sandwiched between two of the world’s iconic tropical
ecosystems of coral reefs and rainforests, are two important coastal communities: mangroves and seagrasses
(Fig. 16.1). While corals flourish in shallow warm seas,
and rainforests cover wetter upland regions, mangroves
inhabit the sheltered intertidal margins barely above
mean sea level. Seagrasses occupy depths from intertidal to deeper habitats, depending on the clarity of the
water column. These biota- structured ecosystems play
an important role in coastal processes with highly developed linkages and connectivity between them, and
to the neighbouring communities represented by coral
reefs and rainforests. These relationships appear vital to
the survival of each. For example, while sediment-loving mangroves depend on waters sheltered by coral reef
structures, they in turn protect hypersensitive corals
from excessive sediments and nutrients coming down
from surrounding catchments.
N GENERAL DESCRIPTION AND
ROLE IN ECOSYSTEMS
Mangrove and seagrass communities are characterised
by a small number of widely distributed angiosperm
taxa, having evolved mostly post-Cretaceous (within
the last 60–100 million years). The relatively recent evolution of these communities may explain their comparatively low species diversity today, although this is also
arguably related to the harsh environmental factors
that characterise these communities’ habitats. The relatively rich mangrove and seagrass floras of today are
testament to their adaptive and evolutionary success
for living in and adjacent to the intertidal zone. These
highly specialised plants flourish in soft sediments,
tapping rich estuarine nutrients with their distinctively
vascular root systems. Mangroves further provide significant habitat and structure where their biomass accumulation, while readily seen as contiguous with
adjacent rainforests, is also analogous to that created
by coral reefs.
While important details of relevant phyletic origins
remain lacking, ancestral mangrove and seagrass plant
taxa are known to have reinvaded marine environments
in multiple episodes from the diverse selection of angiosperm lineages. Their evolution appears constrained
by key functional attributes essential for survival in saline and aqueous settings where isotonic extremes, desiccation and hydrologic exposure combine as uniquely
harsh constraints on organisms living in tidal zones and
estuaries. To achieve this, mangroves and seagrasses
share a number of ecophysiological traits and have
evolved mechanisms to cope with life at the land-sea
interface, for example, salt tolerance, translocation of
gases to aerate their roots, and reproductive strategies.
Both plant communities perform important ecosystem
services such as sediment stabilisation, nutrient processing, shoreline protection, and provide habitat and