Chapter 7 Carbon Flux in Seagrasses
177
that affect food web dynamics and the current threat
of eutrophication to seagrass systems, it is essential that the much more thorough investigations be
carried out.
The role of nutrients is also important because,
with eutrophication, the growth of epiphytes and
phytoplankton are favored at the expense of seagrass production, typically resulting in the loss of
seagrass cover at high nutrient inputs when phytoplankton and epiphytes shade out seagrass systems (Kemp et al., 1983; Borum, 1985. However,
the effects of nutrients on epiphytes and phytoplankton are complex (see Borowitzka et al., Chapter 19 and Walker et al., Chapter 23). Analyzing the isotopic compositions of seagrass residents
under various nutrient regimes may permit identification of differences in the flow of N and C
through the food web, as changes in the nutrient
dynamics may be expected to cause shifts in the
relative contribution of organic matter by various primary producers to higher trophic levels. The isotope
addition experiments magnify differences in the isotopic composition among producers and allow better
resolution of C and N flows from primary producers to consumers (Peterson et al., 1985, 1993). Employing an experimental manipulation to generate
distinct δ
15 N values for seagrass and its epiphytes,
Winning et al. (1999) were the first to use isotope additions. By adding
15 N-enriched potassium nitrate to
mesocosms containing Z. marina and its epiphytes,
they were able to produce significantly changed δ
15 N
values for these two primary producers. This demonstrated the potential of manipulating isotope values
in the field to resolve trophic relationships and such
an approach can actually solve another serious problem that limits the use of δ
15 N as a tracer of organic
matter through simplified food webs in mesocosms
studies (see next three paragraphs).
Fry et al. (1987) observed that the sulfur and carbon isotopic compositions for consumers and their
diets were similar, but the δ
15 N values of consumers
were on average 3.2%0 greater than that of their diet
(as a consequence of excretion of
15 N-depleted nitrogen). In many subsequent studies, this average
value was subtracted from consumers to infer potential diets; however, this practice is ill advised as
the range of variation is very wide (from 0 to 6%0;
Fry et al., 1987). While it is tempting to simplify, a
careful study of seagrass associated food webs based
on stable isotopes requires detailed knowledge of (i)
fractionation phenomena associated with metabolic
assimilation, (ii) seasonal variability in isotopic ratios, and (iii) variability in isotopic ratios between
plant parts (see Vizzini et al., 2003 for the two last
items).
Mutchler et al. (2004) have developed a field
methodology for the isotope addition experiments,
based on the mesocosm work of Winning et al.
(1999). In these experiments,
14 N-labeled (i.e.
15 N
depleted), slow-release fertilizer is used to both simulate eutrophication and generate differential isotopic compositions of H. wrightii and its epiphytes.
After only 20 days of exposure to water column
enrichment, the δ
15 N values of the epiphytes were
significantly different from those of the seagrass
(−78%0 vs. −31%0, respectively, Fig. 11).
Although this methodology was developed to address food web dynamics under eutrophic conditions, the approach could easily be modified to
generate isotopic tracers under ambient nutrient conditions. By actively ‘labeling’ organisms within seagrass beds, one can trace not only the flow of organic
matter to higher trophic levels, but through creative
isotope additions, investigate the degree of movement of ‘labeled’ organisms and assess the extent to
which these organisms facilitate the retention or export of organic matter within and between seagrass
beds and other habitats.
3. Partitioning of Ecosystem
Autotrophic Components
There are a number of components of the seagrass
food web that are often overlooked. For example,
the extent to which food webs depend on locallyderived food resources vs. more transient planktonic resources needs to be quantified. Although
locally-generated production is high in seagrass systems, organic matter flux through the planktonic and
filter-feeding pathways may be significant as well. At
another level, the relative contribution of the resident
producers must be better understood. Until recently,
the role of the sediment microalgae has been largely
neglected (Fry et al., 1987), despite the fact that
sediments within and between seagrass beds constitute a large area in the seagrass landscape (see
Bell et al., Chapter 26). Broadly, the contribution
of sediment microalgae within seagrass beds (∼18–
35%) (see also Sections II.B and II.D.1) may be as
great or greater than the contribution by the seagrass
(∼24%), epiphytes (∼21%), and phytoplankton
177
that affect food web dynamics and the current threat
of eutrophication to seagrass systems, it is essential that the much more thorough investigations be
carried out.
The role of nutrients is also important because,
with eutrophication, the growth of epiphytes and
phytoplankton are favored at the expense of seagrass production, typically resulting in the loss of
seagrass cover at high nutrient inputs when phytoplankton and epiphytes shade out seagrass systems (Kemp et al., 1983; Borum, 1985. However,
the effects of nutrients on epiphytes and phytoplankton are complex (see Borowitzka et al., Chapter 19 and Walker et al., Chapter 23). Analyzing the isotopic compositions of seagrass residents
under various nutrient regimes may permit identification of differences in the flow of N and C
through the food web, as changes in the nutrient
dynamics may be expected to cause shifts in the
relative contribution of organic matter by various primary producers to higher trophic levels. The isotope
addition experiments magnify differences in the isotopic composition among producers and allow better
resolution of C and N flows from primary producers to consumers (Peterson et al., 1985, 1993). Employing an experimental manipulation to generate
distinct δ
15 N values for seagrass and its epiphytes,
Winning et al. (1999) were the first to use isotope additions. By adding
15 N-enriched potassium nitrate to
mesocosms containing Z. marina and its epiphytes,
they were able to produce significantly changed δ
15 N
values for these two primary producers. This demonstrated the potential of manipulating isotope values
in the field to resolve trophic relationships and such
an approach can actually solve another serious problem that limits the use of δ
15 N as a tracer of organic
matter through simplified food webs in mesocosms
studies (see next three paragraphs).
Fry et al. (1987) observed that the sulfur and carbon isotopic compositions for consumers and their
diets were similar, but the δ
15 N values of consumers
were on average 3.2%0 greater than that of their diet
(as a consequence of excretion of
15 N-depleted nitrogen). In many subsequent studies, this average
value was subtracted from consumers to infer potential diets; however, this practice is ill advised as
the range of variation is very wide (from 0 to 6%0;
Fry et al., 1987). While it is tempting to simplify, a
careful study of seagrass associated food webs based
on stable isotopes requires detailed knowledge of (i)
fractionation phenomena associated with metabolic
assimilation, (ii) seasonal variability in isotopic ratios, and (iii) variability in isotopic ratios between
plant parts (see Vizzini et al., 2003 for the two last
items).
Mutchler et al. (2004) have developed a field
methodology for the isotope addition experiments,
based on the mesocosm work of Winning et al.
(1999). In these experiments,
14 N-labeled (i.e.
15 N
depleted), slow-release fertilizer is used to both simulate eutrophication and generate differential isotopic compositions of H. wrightii and its epiphytes.
After only 20 days of exposure to water column
enrichment, the δ
15 N values of the epiphytes were
significantly different from those of the seagrass
(−78%0 vs. −31%0, respectively, Fig. 11).
Although this methodology was developed to address food web dynamics under eutrophic conditions, the approach could easily be modified to
generate isotopic tracers under ambient nutrient conditions. By actively ‘labeling’ organisms within seagrass beds, one can trace not only the flow of organic
matter to higher trophic levels, but through creative
isotope additions, investigate the degree of movement of ‘labeled’ organisms and assess the extent to
which these organisms facilitate the retention or export of organic matter within and between seagrass
beds and other habitats.
3. Partitioning of Ecosystem
Autotrophic Components
There are a number of components of the seagrass
food web that are often overlooked. For example,
the extent to which food webs depend on locallyderived food resources vs. more transient planktonic resources needs to be quantified. Although
locally-generated production is high in seagrass systems, organic matter flux through the planktonic and
filter-feeding pathways may be significant as well. At
another level, the relative contribution of the resident
producers must be better understood. Until recently,
the role of the sediment microalgae has been largely
neglected (Fry et al., 1987), despite the fact that
sediments within and between seagrass beds constitute a large area in the seagrass landscape (see
Bell et al., Chapter 26). Broadly, the contribution
of sediment microalgae within seagrass beds (∼18–
35%) (see also Sections II.B and II.D.1) may be as
great or greater than the contribution by the seagrass
(∼24%), epiphytes (∼21%), and phytoplankton