285
that far into the coastal areas, permitting the co-existence of offshore seagrass
meadows and coral reefs (Fig. 10.5a). If a river catchment includes disturbances in
rural areas from clearing vegetation for grazing and agriculture, the turbid waters
and sediment plume can extend far offshore, resulting in seagrass dieback
(Fig. 10.5b). Mangroves can be largely unaffected by these disturbances, or even
become established on new depositional banks, achieving a net gain in areal extent.
In cases where even the mangrove forests are cleared, slightly turbid water can
extend farther toward reef areas (Fig. 10.5c). Seagrass dieback occurs in turbid
waters and coral damage in slightly turbid water. In the Great Barrier Reef, there has
been a shift from pristine conditions (Fig. 10.5a) to disturbed conditions (Fig. 10.5b,
c) within the last 200 years, since European settlement. Rehabilitation of upstream
ecosystems is considered the only way of restoring downstream marine ecosystems
(Fig. 10.5d). The maintenance of healthy mangrove forests can therefore be seen as
a prerequisite for keeping coral reefs (and seagrass meadows) productive, and thus
they should be rehabilitated or conserved together as a connected seascape.
Mangrove forests can enhance the biomass of coral reef fishes. Mumby et al.
(2004) showed that mangroves in the Caribbean strongly influence the community
structure of fish on neighboring coral reefs, and the biomass of some commercially
important fish is more than doubled when the adult fish habitat is connected to mangroves. More recently, Serafy et al. (2015) pointed out that at a regional scale in the
Caribbean, a greater expanse of mangrove forest generally functions to increase the
densities on neighboring reefs of those fishes that use these shallow, vegetated habitats as nurseries.
10.3.3 Relationships Between Corals and Macroalgal
Communities: Phase Shift
Several reefs around the world have been degraded and shifted from a coraldominated phase to a macroalgae-dominated phase. This phase shift has been
reported in Caribbean reefs and was attributed to increased nutrient loading as a
result of changed land-use and intensive fishing, which reduced the numbers of
herbivorous fish species (Scheffer et al. 2001).
The phase shift toward macroalgae could influence the carbon cycle in the reefs.
For example, Haas et al. (2013) found that macroalgae released more DOC than
hermatypic corals, but the exudates from macroalgae and corals had different
impacts on neighboring ecosystems. Coral exudates increased the net planktonic
microbial community production and enhanced autotrophic benthic microbial community production, thus shifting toward a net autotrophic system. In contrast, macroalgal exudates stimulated heterotrophic organic carbon consumption rates by the
planktonic and benthic microbial community, thus there was an overall shift toward
a microbial community metabolism that was substantially more heterotrophic.
10 Carbon Dynamics in Coral Reefs
that far into the coastal areas, permitting the co-existence of offshore seagrass
meadows and coral reefs (Fig. 10.5a). If a river catchment includes disturbances in
rural areas from clearing vegetation for grazing and agriculture, the turbid waters
and sediment plume can extend far offshore, resulting in seagrass dieback
(Fig. 10.5b). Mangroves can be largely unaffected by these disturbances, or even
become established on new depositional banks, achieving a net gain in areal extent.
In cases where even the mangrove forests are cleared, slightly turbid water can
extend farther toward reef areas (Fig. 10.5c). Seagrass dieback occurs in turbid
waters and coral damage in slightly turbid water. In the Great Barrier Reef, there has
been a shift from pristine conditions (Fig. 10.5a) to disturbed conditions (Fig. 10.5b,
c) within the last 200 years, since European settlement. Rehabilitation of upstream
ecosystems is considered the only way of restoring downstream marine ecosystems
(Fig. 10.5d). The maintenance of healthy mangrove forests can therefore be seen as
a prerequisite for keeping coral reefs (and seagrass meadows) productive, and thus
they should be rehabilitated or conserved together as a connected seascape.
Mangrove forests can enhance the biomass of coral reef fishes. Mumby et al.
(2004) showed that mangroves in the Caribbean strongly influence the community
structure of fish on neighboring coral reefs, and the biomass of some commercially
important fish is more than doubled when the adult fish habitat is connected to mangroves. More recently, Serafy et al. (2015) pointed out that at a regional scale in the
Caribbean, a greater expanse of mangrove forest generally functions to increase the
densities on neighboring reefs of those fishes that use these shallow, vegetated habitats as nurseries.
10.3.3 Relationships Between Corals and Macroalgal
Communities: Phase Shift
Several reefs around the world have been degraded and shifted from a coraldominated phase to a macroalgae-dominated phase. This phase shift has been
reported in Caribbean reefs and was attributed to increased nutrient loading as a
result of changed land-use and intensive fishing, which reduced the numbers of
herbivorous fish species (Scheffer et al. 2001).
The phase shift toward macroalgae could influence the carbon cycle in the reefs.
For example, Haas et al. (2013) found that macroalgae released more DOC than
hermatypic corals, but the exudates from macroalgae and corals had different
impacts on neighboring ecosystems. Coral exudates increased the net planktonic
microbial community production and enhanced autotrophic benthic microbial community production, thus shifting toward a net autotrophic system. In contrast, macroalgal exudates stimulated heterotrophic organic carbon consumption rates by the
planktonic and benthic microbial community, thus there was an overall shift toward
a microbial community metabolism that was substantially more heterotrophic.
10 Carbon Dynamics in Coral Reefs