150
N. Marb `
a, M. Holmer, and E. Gacia
sensitive parts of the seagrasses, such as the meristematic region. The degree of detoxification in Z.
marina was dependent on the reoxidation capacity
of the plants, as shaded plants with reduced photosynthetic activity showed clear signs of sulfide exposure with rotting meristems (Holmer, unpublished
data).
During eutrophication the anaerobic decomposition of organic matter increases, and sulfate reduction may increase further in the seagrass sediments.
The organic enrichment of P. oceanica sediments
observed toward marine fish farms is coupled to
increasing sulfate reduction rates (Holmer, unpublished data). As seagrasses also may suffer from
reduced light conditions in the water column due
to blooms of phytoplankton and increased growth
of epiphytes caused by eutrophication (Borum
and Sand-Jensen, 1996) sulfide stress may add to
reduced performance of the seagrass meadows.
F. Interactions Between Iron
and Sulfur Cycling
Recent studies have investigated the importance of
iron pools in carbonate sediments as a buffer against
sulfide toxicity. Iron additions to T. testudinum growing on carbonate sediments resulted in increased
above-ground biomass and it was suggested that decrease of sulfide levels due to enhanced precipitation
of iron-sulfides, and thus detoxification, improved
plant performance (Chambers et al., 2001). Iron additions have also been made to organic-enriched carbonate sediments with the slow-growing seagrass
P. oceanica (Holmer et al., in press). Here it was
found that, in addition to the improved sediment
conditions, the availability of iron also increased,
and the activity of the iron-demanding alkaline phosphatase enzyme increased. The uptake of phosphate
was stimulated and seagrass growth increased. This
is consistent with previous findings of iron deficiency in seagrasses growing in carbonate sediments
(Duarte et al., 1995).
Iron concentrations are generally higher in finegrained terrigenic sediments, which increase the potential for reoxidation of sulfides by oxidized iron as
well as burial of sulfides as iron-sulfides or pyrites
(FeS 2 ). The reoxidation of sulfide is complex and often involves many different processes, of both chemical and biological origin. Sulfate is the most important end-product of reoxidation and can be utilized
again by sulfate reducing bacteria. Reoxidation is an
important process as >90% of the sulfide production
is considered to be reoxidized (Thode-Andersen and
Jørgensen, 1989), and it may be even higher in seagrass sediments due to the release of oxygen from
the plant roots. The burial of sulfides in marine sediments is thus minor compared to the production,
and is often positively correlated to the sulfate reduction activity. The enhanced sulfate reduction activity found in seagrass sediments is also reflected
in increased burial of reduced sulfides compared to
unvegetated sediments in both terrigenic and carbonate sediments (Holmer and Nielsen, 1997; Holmer
and Laursen, 2002; Holmer et al., 2003). Enhanced
precipitation of sulfides with iron in carbonate sediments, e.g. due to eutrophication, may increase the
iron deficiency of the seagrasses. Pools of sulfides
have also been shown to be positively correlated
with the shoot density and biomass (Holmer and
Nielsen, 1997) and the burial of sulfides thus increases during seagrass colonization. For carbonate
sediments, where the iron availability is limited, this
scenario may lead to iron deficiency and lowered
sulfide buffer capacity. Eventually this may limit the
colonization of the seagrass meadows.
G. Phosphorous Cycling and Interactions
with Iron and Sulfur Cycling
The cycling of phosphorus in seagrass beds has
been examined indirectly by measuring phosphorus contents of seagrass tissues to assess nutrient
limitation (Duarte, 1990; Fourqurean and Zieman,
2002), whereas the dynamic interactions between
nutrient uptake and phosphorus availability in the
water column and sediments remain to be examined. Phosphorus limitation is thought to control
seagrass growth in carbonate sediments, although
it has been shown that the sedimentary pools of
phosphorus can potentially support plant growth for
decades (Jensen et al., 1998). The phosphorus appears to be almost irreversibly bound in organic and
carbonate pools (Jensen et al., 1998), despite a small
fraction of it becoming available in pore waters as
a result of enhanced carbonate dissolution by seagrass beds (Burdige and Zimmerman, 2002; see also
Section II.F, this Chapter). The dissolution of inorganic pools and mineralization of organic pools
is too slow to support maximum seagrass growth
rates, and the pore water pools only represent a small
fraction of the required nutrients (McGlathery et al.,
2001). Fertilization studies have shown that seagrass
growth and patch expansion are enhanced by additions of phosphorus rather than nitrogen to carbonate
N. Marb `
a, M. Holmer, and E. Gacia
sensitive parts of the seagrasses, such as the meristematic region. The degree of detoxification in Z.
marina was dependent on the reoxidation capacity
of the plants, as shaded plants with reduced photosynthetic activity showed clear signs of sulfide exposure with rotting meristems (Holmer, unpublished
data).
During eutrophication the anaerobic decomposition of organic matter increases, and sulfate reduction may increase further in the seagrass sediments.
The organic enrichment of P. oceanica sediments
observed toward marine fish farms is coupled to
increasing sulfate reduction rates (Holmer, unpublished data). As seagrasses also may suffer from
reduced light conditions in the water column due
to blooms of phytoplankton and increased growth
of epiphytes caused by eutrophication (Borum
and Sand-Jensen, 1996) sulfide stress may add to
reduced performance of the seagrass meadows.
F. Interactions Between Iron
and Sulfur Cycling
Recent studies have investigated the importance of
iron pools in carbonate sediments as a buffer against
sulfide toxicity. Iron additions to T. testudinum growing on carbonate sediments resulted in increased
above-ground biomass and it was suggested that decrease of sulfide levels due to enhanced precipitation
of iron-sulfides, and thus detoxification, improved
plant performance (Chambers et al., 2001). Iron additions have also been made to organic-enriched carbonate sediments with the slow-growing seagrass
P. oceanica (Holmer et al., in press). Here it was
found that, in addition to the improved sediment
conditions, the availability of iron also increased,
and the activity of the iron-demanding alkaline phosphatase enzyme increased. The uptake of phosphate
was stimulated and seagrass growth increased. This
is consistent with previous findings of iron deficiency in seagrasses growing in carbonate sediments
(Duarte et al., 1995).
Iron concentrations are generally higher in finegrained terrigenic sediments, which increase the potential for reoxidation of sulfides by oxidized iron as
well as burial of sulfides as iron-sulfides or pyrites
(FeS 2 ). The reoxidation of sulfide is complex and often involves many different processes, of both chemical and biological origin. Sulfate is the most important end-product of reoxidation and can be utilized
again by sulfate reducing bacteria. Reoxidation is an
important process as >90% of the sulfide production
is considered to be reoxidized (Thode-Andersen and
Jørgensen, 1989), and it may be even higher in seagrass sediments due to the release of oxygen from
the plant roots. The burial of sulfides in marine sediments is thus minor compared to the production,
and is often positively correlated to the sulfate reduction activity. The enhanced sulfate reduction activity found in seagrass sediments is also reflected
in increased burial of reduced sulfides compared to
unvegetated sediments in both terrigenic and carbonate sediments (Holmer and Nielsen, 1997; Holmer
and Laursen, 2002; Holmer et al., 2003). Enhanced
precipitation of sulfides with iron in carbonate sediments, e.g. due to eutrophication, may increase the
iron deficiency of the seagrasses. Pools of sulfides
have also been shown to be positively correlated
with the shoot density and biomass (Holmer and
Nielsen, 1997) and the burial of sulfides thus increases during seagrass colonization. For carbonate
sediments, where the iron availability is limited, this
scenario may lead to iron deficiency and lowered
sulfide buffer capacity. Eventually this may limit the
colonization of the seagrass meadows.
G. Phosphorous Cycling and Interactions
with Iron and Sulfur Cycling
The cycling of phosphorus in seagrass beds has
been examined indirectly by measuring phosphorus contents of seagrass tissues to assess nutrient
limitation (Duarte, 1990; Fourqurean and Zieman,
2002), whereas the dynamic interactions between
nutrient uptake and phosphorus availability in the
water column and sediments remain to be examined. Phosphorus limitation is thought to control
seagrass growth in carbonate sediments, although
it has been shown that the sedimentary pools of
phosphorus can potentially support plant growth for
decades (Jensen et al., 1998). The phosphorus appears to be almost irreversibly bound in organic and
carbonate pools (Jensen et al., 1998), despite a small
fraction of it becoming available in pore waters as
a result of enhanced carbonate dissolution by seagrass beds (Burdige and Zimmerman, 2002; see also
Section II.F, this Chapter). The dissolution of inorganic pools and mineralization of organic pools
is too slow to support maximum seagrass growth
rates, and the pore water pools only represent a small
fraction of the required nutrients (McGlathery et al.,
2001). Fertilization studies have shown that seagrass
growth and patch expansion are enhanced by additions of phosphorus rather than nitrogen to carbonate