often nearly all ammonium is taken up by tree roots.
Nitrogen fixation and denitrification rates, in contrast,
are highly variable and, on average, slow processes. The
tight coupling between trees, microbes, and soil nutrient
pools is partly a function of the interactive effects between
forest age and intertidal position. However, the cycling of
P and other elements is poorly known in mangroves.
The need for N in mangrove food webs has attracted
much debate as early notions of animal nutrition pointed
to N in mangrove detritus as the main fuel for food webs
(Heald, 1969). The original paradigm was that microbes
attached to detritus reduced complex indigestible vascular
plant material to simple, more readily digestible forms,
with subsequent microbial enrichment providing sufficient N for adequate nutrition (Ellison and Farnsworth,
2001). The current paradigm derived primarily from work
using stable isotopes is that most mangrove consumers
preferentially assimilate fresh benthic microalgae and
macroalgae, phytoplankton, and algal detritus to meet
their N requirements. Such material is nitrogen-rich and
more readily digestible than mangrove plant detritus.
The discovery that sesarmid and grapsid crabs are foundational ecosystem engineers led to a paradigm shift in recognizing the nutritional importance of algal foods. Crabs
were once a prime example of the notion that mangrove
secondary consumers met their nutritional needs by voraciously eating large amounts of N-poor mangrove litter
and assimilating the associated N-rich microbial biomass.
This early explanation seemed reasonable because crabs
can consume nearly all standing stock of litter in some forests and paste litter fragments onto their burrow walls
facilitating fungal and bacterial colonization, making the
material more palpable and nutritious over time
(Kristensen, 2008). However, mangrove litter is high in
tannins and too poor in nitrogen to sustain adequate
nutrition. In reality, crabs, like most other secondary
consumers, eat a variety of foods to maintain a balanced
diet, obtaining sufficient N from supplemental consumption of animal tissue and meiofauna. Sophisticated feeding
experiments have found that the crabs Episesarma
spp. and Perisesarma spp., as well as penaeid shrimp
larvae, are omnivorous, eating mostly mangrove detritus
and lesser amounts of roots, algae, animal tissue, and
surface microbial biofilms (Nordhaus et al., 2011; Gatune
et al., 2012).
Mangrove-associated fish and zooplankton also have
varied diets but with a preference for algal over detrital
foods. An individual mangrove forest may thus have multiple food webs partitioned by dietary preferences for
algae, detritus, mixed algal/detrital foods, and animal tissues such as carcasses and smaller consumers such as
microzooplankton (Giarrizzo et al., 2011). The nutritional
situation can be even more complicated if seagrasses and
coral reefs are nearby, as fish residents feed mainly on
mangrove-associated foods, but transient fishes actively
forage on seagrass or reef-associated prey items (Vaslet
et al., 2012). The significance of mangrove N (and other
nutrients) thus depends on the location and type of habitat,
relative availability of other primary producers, species
dietary preferences, and universal need to maintain
a balanced diet.
Ecosystem processes: contribution of mangroves
in global coastal ocean
Mangrove forests are structurally and functionally
interlinked with adjacent terrestrial and marine ecosystems and the atmosphere, exchanging biota, air, water,
soils and sediments, nutrients, and other elements, over
space and time. Tides represent an energy subsidy, doing
work by transporting dissolved and particulate materials,
gases, and metabolic by-products between the forest and
other ecosystems. The idea that the fertility of coastal
wetlands such as mangroves via these exchanges supports
primary and secondary production in the adjacent nearshore zone developed in the late 1960s (Heald, 1969),
and this “outwelling” hypothesis generated much subsequent research into the role of mangroves in supporting
coastal production. After decades of research, it is clear
that the amount of material potentially available for export
from mangroves is influenced by such factors as forest
production, tidal range, the ratio of mangrove to watershed
area, local hydrodynamics, amount of rainfall, volume of
water exchange, and the extent of activities of crabs and
other biota (Alongi, 2009).
Most mangroves export nutrients, but some do not.
Globally, mangroves export an average of 28 T g C year
À1
of particulate organic carbon (POC), or about 10–11 %
of particulate terrestrial carbon export to the global
coastal ocean (Alongi, 2014). Mangroves also export large
quantities of dissolved organic carbon (15 T g C year
À1
) or
dissolved inorganic carbon (86 T g C year
À1
) to the coastal
ocean, with most of the latter originating from subsurface
advection of interstitial water within the forest floor. Thus,
nearly 75 % of the total C exported from mangroves originates from respiration by microbes and other mangrove
biota in soils and tidal waters.
A nitrogen mass balance model of the world’s mangroves indicates that (1) 2687 G g N year
À1 is required
to sustain global mangrove NPP; (2) N burial is about
25 % of total N input into an “average” mangrove forest;
(3) about 15 % of total N input to mangrove soils is
denitrified; (4) nitrogen fixation accounts for only about
5 % of total N input although nitrogen fixation on aboveground tree parts and in deep root systems has not been
adequately measured; (5) production of roots and litter
accounts for 40 % and 50 % of mangrove NPP, respectively; (6) tidal losses equate to about 55 % of N input;
(7) denitrification and N 2 O effluxes account for <10 %
of total N losses; and (8) despite proportionally large tidal
losses, the global flux of N in mangroves is roughly in
balance (Alongi, 2013).
The balance of carbon between photosynthetic gains by
autotrophs and respiratory losses from all biota, reflected
in the exchange between ecosystems, atmosphere, and
adjacent ecosystems, is called the net ecosystem
MANGROVES
401
Nitrogen fixation and denitrification rates, in contrast,
are highly variable and, on average, slow processes. The
tight coupling between trees, microbes, and soil nutrient
pools is partly a function of the interactive effects between
forest age and intertidal position. However, the cycling of
P and other elements is poorly known in mangroves.
The need for N in mangrove food webs has attracted
much debate as early notions of animal nutrition pointed
to N in mangrove detritus as the main fuel for food webs
(Heald, 1969). The original paradigm was that microbes
attached to detritus reduced complex indigestible vascular
plant material to simple, more readily digestible forms,
with subsequent microbial enrichment providing sufficient N for adequate nutrition (Ellison and Farnsworth,
2001). The current paradigm derived primarily from work
using stable isotopes is that most mangrove consumers
preferentially assimilate fresh benthic microalgae and
macroalgae, phytoplankton, and algal detritus to meet
their N requirements. Such material is nitrogen-rich and
more readily digestible than mangrove plant detritus.
The discovery that sesarmid and grapsid crabs are foundational ecosystem engineers led to a paradigm shift in recognizing the nutritional importance of algal foods. Crabs
were once a prime example of the notion that mangrove
secondary consumers met their nutritional needs by voraciously eating large amounts of N-poor mangrove litter
and assimilating the associated N-rich microbial biomass.
This early explanation seemed reasonable because crabs
can consume nearly all standing stock of litter in some forests and paste litter fragments onto their burrow walls
facilitating fungal and bacterial colonization, making the
material more palpable and nutritious over time
(Kristensen, 2008). However, mangrove litter is high in
tannins and too poor in nitrogen to sustain adequate
nutrition. In reality, crabs, like most other secondary
consumers, eat a variety of foods to maintain a balanced
diet, obtaining sufficient N from supplemental consumption of animal tissue and meiofauna. Sophisticated feeding
experiments have found that the crabs Episesarma
spp. and Perisesarma spp., as well as penaeid shrimp
larvae, are omnivorous, eating mostly mangrove detritus
and lesser amounts of roots, algae, animal tissue, and
surface microbial biofilms (Nordhaus et al., 2011; Gatune
et al., 2012).
Mangrove-associated fish and zooplankton also have
varied diets but with a preference for algal over detrital
foods. An individual mangrove forest may thus have multiple food webs partitioned by dietary preferences for
algae, detritus, mixed algal/detrital foods, and animal tissues such as carcasses and smaller consumers such as
microzooplankton (Giarrizzo et al., 2011). The nutritional
situation can be even more complicated if seagrasses and
coral reefs are nearby, as fish residents feed mainly on
mangrove-associated foods, but transient fishes actively
forage on seagrass or reef-associated prey items (Vaslet
et al., 2012). The significance of mangrove N (and other
nutrients) thus depends on the location and type of habitat,
relative availability of other primary producers, species
dietary preferences, and universal need to maintain
a balanced diet.
Ecosystem processes: contribution of mangroves
in global coastal ocean
Mangrove forests are structurally and functionally
interlinked with adjacent terrestrial and marine ecosystems and the atmosphere, exchanging biota, air, water,
soils and sediments, nutrients, and other elements, over
space and time. Tides represent an energy subsidy, doing
work by transporting dissolved and particulate materials,
gases, and metabolic by-products between the forest and
other ecosystems. The idea that the fertility of coastal
wetlands such as mangroves via these exchanges supports
primary and secondary production in the adjacent nearshore zone developed in the late 1960s (Heald, 1969),
and this “outwelling” hypothesis generated much subsequent research into the role of mangroves in supporting
coastal production. After decades of research, it is clear
that the amount of material potentially available for export
from mangroves is influenced by such factors as forest
production, tidal range, the ratio of mangrove to watershed
area, local hydrodynamics, amount of rainfall, volume of
water exchange, and the extent of activities of crabs and
other biota (Alongi, 2009).
Most mangroves export nutrients, but some do not.
Globally, mangroves export an average of 28 T g C year
À1
of particulate organic carbon (POC), or about 10–11 %
of particulate terrestrial carbon export to the global
coastal ocean (Alongi, 2014). Mangroves also export large
quantities of dissolved organic carbon (15 T g C year
À1
) or
dissolved inorganic carbon (86 T g C year
À1
) to the coastal
ocean, with most of the latter originating from subsurface
advection of interstitial water within the forest floor. Thus,
nearly 75 % of the total C exported from mangroves originates from respiration by microbes and other mangrove
biota in soils and tidal waters.
A nitrogen mass balance model of the world’s mangroves indicates that (1) 2687 G g N year
À1 is required
to sustain global mangrove NPP; (2) N burial is about
25 % of total N input into an “average” mangrove forest;
(3) about 15 % of total N input to mangrove soils is
denitrified; (4) nitrogen fixation accounts for only about
5 % of total N input although nitrogen fixation on aboveground tree parts and in deep root systems has not been
adequately measured; (5) production of roots and litter
accounts for 40 % and 50 % of mangrove NPP, respectively; (6) tidal losses equate to about 55 % of N input;
(7) denitrification and N 2 O effluxes account for <10 %
of total N losses; and (8) despite proportionally large tidal
losses, the global flux of N in mangroves is roughly in
balance (Alongi, 2013).
The balance of carbon between photosynthetic gains by
autotrophs and respiratory losses from all biota, reflected
in the exchange between ecosystems, atmosphere, and
adjacent ecosystems, is called the net ecosystem
MANGROVES
401