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Mar Biol
DOI 10.1007/s00227-013-2340-3
ORIGINAL PAPER
Eelgrass fairy rings: sulfide as inhibiting agent
Jens Borum · Ane Løvendahl Raun ·
Harald Hasler‑Sheetal · Mia Østergaard Pedersen ·
Ole Pedersen · Marianne Holmer
Received: 14 April 2013 / Accepted: 28 September 2013
© Springer-Verlag Berlin Heidelberg 2013
substantial invasion of sulfide from the sediment. Neither
the clonal growth pattern of eelgrass, sediment burial of
shoots, hydrodynamic forcing nor nutrient limitation could
explain the ring-shaped pattern. We conclude that the most
likely explanation must be found in invasion of eelgrass
shoots by toxic sulfide accumulating in the sediment due
to low iron availability in the carbonate-rich environment.
Introduction
In early summer 2010, Danish media reported on obser-
vations of conspicuous and mysterious ring formations
occurring in shallow water immediately outside the up
to 120-m-high chalk cliffs of the island, Møn, Denmark
(Fig. 1). Suggestions on what could have formed these ring
structures were plenty ranging from World War II bomb
craters to crop circles formed by extraterrestrial beings.
A closer examination revealed that the distinct circles are
formed by the north temperate sea grass, eelgrass (Zostera
marina L.), growing at water depths from 1.5 to 2.5 m. The
rings ranged in diameter from about 1 to 15 m and con-
sisted of narrow fringes of dense eelgrass shoots growing
on sediment accumulated among the shoots (Fig. 1). Out-
side and inside the rings, the bottom consists of large chalk
plates with scattered boulders reflecting the physically
harsh environment created by high wave exposure caused
by a wind fetch of up to 250 km from southeast. Wave
heights may exceed 1 m, and the locality can occasionally
experience strong along-coast currents in addition to wave
forcing. The chalk plates have a patchy growth of ephem-
eral filamentous brown and red algae. The bottom holds
no sediment except from within-chalk crevices and, hence,
does not appear to be a favorable environment for eelgrass
establishment. However, on rare occasions, seedlings have
Abstract Distinct ‘fairy rings’ consisting of narrow
fringes of eelgrass (Zostera marina L.) expand radially
over a bottom of chalk plates outside the calcium carbon-
ate cliffs of the island of Møn, Denmark. We conducted a
survey to evaluate possible explanations for the formation
of the rings and, more specifically, for the apparent die-
off of eelgrass shoots on the inner side of the rings. The
fairy rings were up to 15 m in diameter consisting of 0.3-
to 1-m-wide zones of sea grass shoots at densities of up
to 1,200 shoots m−2 and rooted in an up to 10-cm-thick
sediment layer. On the outer side, shoots expanded over the
bare chalk plates. On the inner side, shoots were smaller,
had lower absolute and specific leaf growth, shoot density
was lower and the sediment eroded leaving the bare chalk
with scattered boulders behind. Sediment organic matter
and nutrients and tissue nutrient contents were not different
among positions. Sediment pools of acid volatile sulfides
and chromium-reducible sulfur increased from outer to the
middle positions of the rings, and so did total sulfur con-
tent of eelgrass tissues, while tissue δ34S isotope ratios,
regardless of position in the fringes, were low reflecting
Communicated by K. Bischof.
J. Borum (*) · A. L. Raun · O. Pedersen
Freshwater Biological Laboratory, Department of Biology,
University of Copenhagen, Universitetsparken 4,
2100 Copenhagen Ø, Denmark
e-mail: JBorum@bio.ku.dk
H. Hasler-Sheetal · M. Ø. Pedersen · M. Holmer
Institute of Biology, University of Southern Denmark,
Campusvej 55, 5230 Odense M, Denmark
O. Pedersen
Institute of Advanced Studies, University of Western Australia,
35 Stirling Highway, Crawley, WA 6009, Australia
Mar Biol
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apparently managed to establish and form clones expand-
ing radially on top of the chalk plates. But the obvious
question we wished to examine was what caused the ring
structures, or more specifically, what could be the agent(s)
responsible for the die-off removing shoots from the center
and inner side of the patches?
Ring-shaped clones are known to be formed by a num-
ber of mushroom species (e.g., Blenis et al. 2004), club-
mosses (Lycopodium spp.; e.g., Stone and Thorp 1971),
Spartina maritima in marshes (Castellanos et al. 1994),
grasses and other clonal vascular plants (e.g., van Rooyen
et al. 2004; Sheffer et al. 2007). Apart from creating major
societal challenges for golfers on putting greens of golf
courses and esthetic disturbances in lawns, the interest in
these ring formations is mainly related to their mysterious
origin. The so-called fairy rings have stimulated fantasy
toward elves and fairies (thereof the name) among the pub-
lic but also among scientists, who have searched for more
rational and mechanistic explanations for their formation.
Explanations vary from poisoning of grasses by cyanide
produced by mushroom mycelia (e.g., Blenis et al. 2004;
Peter 2006) and nutrient depletion (e.g., Stone and Thorp
1971) to interaction with ants or termites (e.g., Albre-
cht et al. 2001). Apart from the interaction with ants and
termites, most studies indicate identified or unidentified
inhibitory agents as causing the die-back centers of the
circles.
Ring- or donut-shaped eelgrass patches with lush fringes
of rapidly growing eelgrass ramets on the outer side of the
patches and partly or fully buried shoots in the center can
be observed in shallow waters of highly exposed shores
with sandy sediments (Fonseca and Kenworthy 1987; RJ
Orth pers. com.). But die-off in these patch centers primar-
ily seems to be caused by sand accumulating within the
patches during storm events with heavy sand movement. At
the Island of Møn, the formations are very different with
low sediment loads and no burying of shoots.
Events of sea grass die-off in beds of eelgrass and tur-
tle grass (Thalassia testudinum) have been related to poor
water column oxygen conditions accompanied by sulfide
invasion from the sediment (e.g., Greve et al. 2003; Ped-
ersen et al. 2004). Hence, we wished to examine whether
the die-off in the centers of the fairy rings at Møn could be
due to sulfide poisoning. Our hypothesis was that sulfides
build up over time in the sediment accumulated within the
eelgrass patches due to sulfate reduction combined with
scarcity of available iron for sulfide immobilization due to
the carbonate-rich environment (Chambers et al. 2001; Hol-
mer et al. 2005a; Holmer 2009). We have characterized the
rings with respect to shoot density, shoot size, leaf growth,
photosynthesis and tissue nutrients at different positions
within the eelgrass fringes, and we present measurements
of sediment characteristics. Finally, we have analyzed sul-
fur contents and δ34S isotope ratios in eelgrass tissues and
sediment to test for invasion of sediment sulfides into the
eelgrass shoots.
Methods
Sampling of eelgrass ramets, sediments and porewater was
conducted in August by SCUBA diving at 1.5–2.5 m depth
about 100 m from the coast outside the chalk cliffs of the
Island of Møn, Denmark (54°57′46′′N, 12°33′12′′E). Five
circular patches ranging in diameter from 2 to 15 m were
selected and marked for all samplings and served as inde-
pendent replicates (n = 5).
Fig. 1 a Photo showing the fairy rings of various sizes and age as
black circles taken from the top of the 120-m-high cliffs (with per-
mission from Jacob Topsøe Johansen). b Photo of the narrow fringes
of eelgrass forming the ring-shaped patches. On the outer side (to
the right), new shoots are colonizing the chalk plates expanding the
periphery of the patch. On the inner side (to the left), shoot density
declines, leaves are shorter and the sediment around the dead black
rhizomes is removed by erosion leaving the chalk plates bare and
open for colonization by red and brown filamentous algae
Mar Biol
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Sea grass analyses
Turfs with intact eelgrass ramets (ramet = a leaf bundle
with rhizome and roots) were dug out of each vegetated
ring with a knife and carefully washed free of sediment
and debris. Sampling was done at three positions within the
0.3–1-m-wide fringes of eelgrass forming the rings. The
three positions represented (1) the outer fourth of the veg-
etation expanding radially out over the hard chalk plates,
(2) the middle of the fringes and (3) the inner fourth of the
rings characterized by smaller and less dense vegetation
with clear signs of die-off. The plant tissues were stored in
plastic bags in a cooler, transported to the laboratory and
processed within 48 h of storage. From each position, 20
ramets were randomly selected for measurements of maxi-
mum leaf and root length (terminal leaf bundles and main
rhizome only). Tissue samples were either dried at 60 °C
for later analysis of tissue nutrient contents or freeze-dried
for analysis of sulfur compounds for 48 h.
In the field, shoot density was counted by SCUBA
divers in 5 quadrates of 0.035 m2 at each position within all
five rings. In addition, 10 leaf bundles were punched with
a syringe needle right above the leaf sheaths to measure
leaf growth (Zieman 1974). After 5 days of growth in the
field, the shoots were harvested and leaf elongation of the
three youngest leaves was measured relative to the refer-
ence mark in the older leaf #4 which has ceased growth.
To test for differences in functioning of the photosynthetic
apparatus, photosynthetic efficiency (Fv/Fm) was measured
2–5 cm above the leaf sheath of the second youngest leaf
in each terminal leaf bundle using a PEA MK2 Hansatech
photosynthetic efficiency analyzer following the procedure
described by Pulido and Borum (2010). Measurements
were conducted in the field in the shoots used for growth
measurements shortly after harvesting.
After leaves had been freeze-dried and grinded, total
nitrogen of leaves was measured on a Carlo Erba EA1108
elemental analyzer and total phosphorus was meas-
ured spectrophotometrically after ignition according to
Andersen (1976). Freeze-dried and homogenized samples
of the different tissues were analyzed for total sulfur (TS),
elemental sulfur (S0) and stable sulfur isotope ratio (δ34S).
TS and δ34S were analyzed by elemental analyzer com-
bustion continuous flow isotope ratio mass spectroscopy
(EA-C-CF-IRMS). S0 was analyzed by HPLC according to
Zopfi et al. (2001).
Sediment analyses
Sediment depth was measured with a meter stick once at
each position within each ring, and sediment cores (i.d.
4.4 cm; depth 6 cm) were sampled at each position within
each of the five ring-shaped patches. Cores taken for
measurements of organic matter, total nitrogen and total
phosphorus were transferred to plastic zipper bags and
stored in a cooler until processed at the laboratory. Organic
content (LOI) was measured as loss on ignition (550 °C
for 2 h) of pre-dried, homogenized sediment. Total nitro-
gen (TN) was measured on a Carlo Erba EA1108 elemental
analyzer, and total phosphorus (TP) was measured accord-
ing to Andersen (1976).
Separate cores (i.d. 2.6 cm, depth 6 cm) were sampled
for measurements of sediment sulfur species at each posi-
tion in the five rings. In the field, the sediment was fixed in
1 M zinc acetate and was, after transport back to the labo-
ratory, frozen until further analysis. Porewater samples for
analysis of ammonium (NH4+), dissolved sulfide (DS) and
reduced iron (Fe2+) were collected in situ without air con-
tact by use of 5-cm-long ceramic lysimeter cups attached
to syringes exposed to under pressure. The lysimeters were
deployed 1 cm below the sediment surface. Porewater was
fixed with 1 M HCl for analysis of Fe2+ and with zinc ace-
tate for analysis of DS. Fe2+ was analyzed using the phen-
anthroline method according to Eaton et al. (1995) and DS
according to Cline (1969). Samples for analysis of NH4+
were immediately added reagents according to Solórzano
(1969) and determined spectrophotometrically within 24 h
after sampling.
Sediment pools of acid volatile sulfides (AVS: H2S and
FeS) and chromium-reducible sulfur (CRS: S0 and FeS2)
were determined on fixed samples using the two-step dis-
tillation technique described by Fossing and Jørgensen
(1989), but using silver nitrate to precipitate sulfides. The
amounts of AVS/CRS were assessed by weighing the pre-
cipitates. The δ34S of these pools was analyzed by EA-C-
CF-IRMS as described above for plant tissues.
The relative contribution of sediment sulfide to total
plant sulfur in leaves, rhizomes and roots was calculated as:
where δ34Swater is the value measured in seawater sulfate,
δ34Stissue is the value of total sulfur in the specific sea grass
tissue and δ34SAVS is the value in the sediment pool of AVS
representing the source of sulfide invasion into tissues
(Frederiksen et al. 2006).
Statistical analyses
All parameters were statistically tested among positions
within the five ring-shaped eelgrass patches (n = 5 unless
otherwise stated) using one-way ANOVA provided homo-
geneity of variance as tested by Bartlett’s test. Tukey’s post
hoc tests were used for pairwise comparisons. If variances
were unequal, the data were analyzed by Kruskal–Wallis
test followed by Dunn’s test for pairwise comparisons. We
Fsulfide =
δ34Stissue −δ34 Swater
δ34SAV S −δ34Swater
Mar Biol
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consistently used a significance level of 0.05. All results are
presented as means ± 95 % confidence limits even if une-
qual variances could indicate non-normality.
Results
Plant characteristics
Average shoot density varied from about 500 to almost
1,200 shoots m−2 and was significantly lower on the inner
side of the rings compared to the outer and middle posi-
tions (Fig. 2a; Tukey’s test). Similarly, individual shoots
were on average smaller and maximum root length shorter
on the inner side (Fig. 2b, c; Tukey’s test). Leaf growth also
varied significantly with position within the rings (Fig. 2d).
Leaf growth was clearly inhibited on the inner side of
the rings compared to shoots from the outer and middle
positions of the rings (Dunn’s test). Shoots were slightly
smaller at the inner position compared to the middle (mean
maximum leaf length was 29 cm at the inner position vs
33 cm at the middle), but leaf growth at the inner position
was only 12 % of growth measured at the middle position
reflecting a markedly lower specific growth rate. Although
growth differed among positions, the photosynthetic effi-
ciency measured as Fv/Fm was not significantly different
(Fig. 2e).
There were no significant differences in eelgrass leaf
carbon, phosphorus and nitrogen contents among positions
(Table 1; one-way ANOVA (leaf P) or Kruskal–Wallis).
Leaf phosphorus content was between 0.10 and 0.11 %
of dry weight and leaf nitrogen content varied from 1.8 to
2.0 % of dry weight among positions.
Sediment characteristics
Sediment accumulated within the eelgrass fringes while
no sediment was deposited outside or inside the rings. The
rhizomes of the outermost shoots grew out over the bare
chalk plates with sediment starting to accumulate a few cm
inside the vegetation front. Sediment thickness increased
from 5.0 ± 2.1 cm measured in the outer fourth of the ring
to 9.0 ± 4.2 and 8.4 ± 4.7 cm in the middle and inner posi-
tions of the rings, respectively. The sediment layer clearly
eroded on the inner side with low shoot density exposing a
thick mat of dead rhizomes.
Fig. 2 a Eelgrass shoot density, b maximum leaf length, c maximum
root length, d leaf growth over 5 days and e photosynthetic efficiency
(Fv/Fm) at different positions within the vegetated fringes of the fairy
rings. All results presented as means ± 95 % CL, n = 5. Letters indi-
cate statistically significant differences
▸
Mar Biol
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The composition of the sediment did not differ signifi-
cantly among positions with respect to organic content,
TP or TN [Table 1; one-way ANOVA or Kruskal–Wallis
(TP)]. The sediment consisted of sand with a low organic
content (0.5–0.6 % LOI) and low contents of TN and TP
(<0.2 μg N mg−1 DW and <0.01 μg P mg−1 DW). Con-
centrations of NH4+ in sediment porewater was moder-
ate (40–45 μM) and did not differ significantly among
positions within the rings (Table 1; one-way ANOVA).
Concentrations of Fe2+ were very low (<5 μM) and did not
differ among positions.
Sulfur content and isotopic composition in sediment
Concentrations of DS were highly variable and ranged
from 1 to 435 μM both within and among positions but
differences were not significant although there seemed to
be decreasing concentrations from outer to inner positions
(Table 2; Kruskal–Wallis). Acid volatile sulfide (AVS: H2S
and FeS) and chromium-reducible sulfur (CRS: S0 and
FeS2) within the sediment followed similar patterns with
low contents at the outer position, increasing at the mid-
dle and slightly decreasing at the inner position (Table 2).
However, only CRS at the outer position was significantly
different from middle and inner (Tukey’s test). The δ34S
of AVS and CRS pools tended to become more negative
from the outer to the inner side of the rings but was not
significantly different among positions (Table 2; one-way
ANOVA).
Sulfur content, isotopic composition and sources of sulfur
in plant tissues
The TS of leaves, rhizomes and roots varied from 73 to
205 μmol g−1 DW and followed sediment sulfur patterns
with relatively low TS-contents at the outer fringes of the
eelgrass rings, high at the middle and low again in the
smaller, remaining shoots at the inner position (Fig. 3).
TS was partly present as elemental sulfur (S0) in roots and
rhizomes. In roots, the S0 content varied between 4.3 and
12.0 μmol g−1 DW corresponding to about 5 % of TS at all
positions. The content of S0 in roots was not significantly
different among positions (Kruskal–Wallis). Small amounts
of S0 were found in rhizomes and none in leaves (data not
shown).
The δ34S value of seawater was 20.9 ± 0.1 ‰. The δ34S
of all eelgrass tissues (Fig. 3) was significantly lower than
that of seawater independent of position (Tukey’s test),
reflecting invasion of sulfide from the sediment. Leaves
had higher δ34S than rhizomes which again had higher val-
ues than roots (Tukey’s test). For all tissue types, the δ34S
tended to be lower at the middle of the rings compared to
the outer fringes, and δ34S again increased significantly in
the surviving shoots at the inner position (Fig. 3). The rela-
tive contribution of sediment sulfides to TS content of the
tissues (Fsulfide) was highest in roots and lowest in leaves
(Fig. 3; Tukey’s test; n = 3–5). Fsulfide mirrored the pat-
tern of δ34S among positions with highest sediment sulfide
contributions at the middle position and lowest in the
Table 1 Organic matter (LOI)/carbon, nitrogen and phosphorus con-
tent of eelgrass leaves and sediment and porewater NH4+ and Fe2+
sampled at different positions within the vegetated fringes of the eel-
grass fairy rings (means ± 95 % CL)
For all parameters, there were no significant differences in contents
among positions (one-way ANOVA or Kruskal–Wallis, p > 0.05)
Plant tissue
Outer Middle Inner
Leaf carbon
(% of DW)
37.4 ± 1.2 38.2 ± 0.3 37.3 ± 2.7
Leaf nitrogen
(% of DW)
1.88 ± 0.14 1.82 ± 0.04 1.96 ± 0.16
Leaf phosphorus
(% of DW)
0.11 ± 0.03 0.10 ± 0.01 0.10 ± 0.02
Sediment
LOI (% of DW) 0.55 ± 0.07 0.54 ± 0.07 0.53 ± 0.10
Total nitrogen
(μg mg−1 DW)
0.16 ± 0.02 0.16 ± 0.03 0.14 ± 0.02
Total phosphorus
(μg mg−1 DW)
0.081 ± 0.013 0.071 ± 0.037 0.079 ± 0.006
Porewater NH4+
(μM)
40 ± 11 42 ± 11 45 ± 6
Porewater Fe2+
(μM)
4.2 ± 2.1 4.5 ± 4.3 2.4 ± 1.9
Table 2 Porewater sulfide and contents and isotopic sulfur composi-
tion of acid volatile sulfides (AVS) and chromium-reducible sulfides
(CRS) in sediments sampled at different positions within the eelgrass
rings (means ± 95 % CL)
Letters indicate statistically significant differences
Outer Middle Inner
Porewater sulfide
(μM)
123 ± 223a92 ± 148a20 ± 23a
AVS (μmol g−1
WW)
1.92 ± 0.95a2.79 ± 0.91a2.65 ± 0.81a
CRS (μmol g−1
WW)
4.15 ± 0.85a7.78 ± 0.85b7.05 ± 2.23b
δ34S in AVS (‰) −12.5 ± 3.2a−14.6 ± 1.9a−15.5 ± 2.3a
δ34S in CRS (‰) −13.2 ± 2.4a−14.3 ± 1.6a−14.5 ± 2.2a
Mar Biol
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remaining shoots at the inner position. At the middle posi-
tion, up to 78 % of the sulfur content in roots appeared to
come from sediment sulfides.
Discussion
The circular shape of the eelgrass patches strongly indicates
that the patches are formed by individual seedlings branch-
ing and spreading out radially over the chalk plates and
between boulders through vegetative rhizome propagation
with dendritic side shoot formation. Modeling of patch for-
mation predicts a nonlinear development of shoot density
and total production of rhizome length (Sintes et al. 2006),
however, with a constant radial expansion determined by
the annual rhizome elongation rate at the outer edge of
the patches (Olesen and Sand-Jensen 1994; Sintes et al.
2006). With annual rhizome elongation rates for Danish
eelgrass patches of 16 cm (mean) and up to 45 cm (Olesen
and Sand-Jensen 1994), the largest eelgrass ring at Møn
of 15 m in diameter corresponded to an age of between 25
and 50 years. With the branching during vegetative growth,
the patch will in its early stage become a diffuse, dendritic
structure later developing into a compact stand filled in by
shoots limited by space (Sintes et al. 2006). Hence, the
clonal growth characteristics by itself cannot explain the
ring shape of the patches.
The ring- or donut-shaped eelgrass patches examined
in this study could potentially have been formed by hydro-
dynamic forcing. Ring-shaped eelgrass patches have been
observed in sandy shallow areas with high currents and
wave actions (Fonseca and Kenworthy 1987). Most often,
sea grass vegetation dampens currents and increases sedi-
mentation within the patches, but under certain circum-
stances, sediment resuspension is higher inside patches
than outside over non-vegetated sea bottom creating ero-
sion of the sediment inside the vegetation (Fonseca and
Kenworthy 1987; Koch 1999) as has also been observed
for band-shaped beds of Zostera noltii exposed to wave
action and tides (van der Heide et al. 2010). However, none
of these mechanisms seem to have been responsible for the
eelgrass fairy rings at the Island of Møn because no sedi-
ment accumulation occurred on the inner side of the rings
and because sediment erosion due to hydrodynamic forcing
would occur on both sides of the fringes of the large rings
and not only on the inner side.
Fig. 3 Plant tissue contents
of total sulfur (TS), isotopic
sulfur composition (δ34S) and
contribution of sediment sulfide
to plant tissue sulfur content
(Fsulfide). Fsulfide was calculated
from isotopic sulfur composi-
tion in tissues, seawater and
sediment pools of reducible sul-
fur (means ± 95 % CL). Letters
indicate statistically significant
differences
Mar Biol
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The lower shoot density, shorter leaves and roots and the
lower leaf growth of eelgrass shoots on the inner side of
the rings suggest that these ramets were either inhibited by
lack of nutrients or by some sort of toxic agent. The nitro-
gen content of the leaves was between 1.82 and 1.96 % of
DW, which is above what is usually considered limiting to
sea grass growth (Duarte 1990). Moreover, sediment total
nitrogen, porewater NH4+ and leaf nitrogen contents were
not significantly different among positions. Similarly, sedi-
ment and leaf phosphorus contents were independent of the
position within the rings. However, leaf phosphorus levels
(~0.1 % of DW) were much lower than the 0.2 % consid-
ered to be non-limiting for sea grasses (Duarte 1990). The
low phosphorus content was likely due to the carbonate-rich
environment known to immobilize available phosphorus
(Jensen et al. 1998). But if phosphorus was indeed limiting
to eelgrass growth, it was so independent of position within
the rings. This was further supported by the lack of differ-
ences in photosynthetic efficiency reflecting a similar status
of the photosynthetic apparatus in shoots independent of
position. Accordingly, tissue nutrient contents could not
explain the occurrence of the less dense, small-sized and
poorly growing shoots on the inner side.
The poorly developed roots and low leaf growth rate of
ramets on the inner side strongly suggest that an inhibiting
agent accumulating within the sediment was responsible for
shoot mortality and low growth. A high oxygen consump-
tion within the sediment could be an alternative explanation
(Sand-Jensen et al. 2005; Raun et al. 2010), but the content
of organic matter in these sediments was very low indeed
(~0.5 % LOI) and not significantly different among posi-
tions. Neither could reduced iron (Fe2+), which is known
to have negative impact on submersed rooted plants else-
where (e.g., Barko and Smart 1983; van Wijck et al. 1992),
provide reasonable explanations for shoot mortality since
concentrations of Fe2+ were also very low (<5 μM), much
lower than known to be toxic to rooted plants (Snowden
and Wheeler 1993). The low Fe2+ concentrations are likely
due to the high calcium carbonate content of the sediments
with limited iron availability (Holmer et al. 2005a).
Sulfide accumulation and invasion into the shoots
seemed to be the most likely inhibitory agent explaining
shoot mortality on the inner side of the rings. Sulfide is
known to be toxic to sea grasses (e.g., Koch and Erskine
2001; Borum et al. 2005; Dooley et al. 2013) and, although
variable and not extremely high for coastal sediments, con-
centrations of dissolved sulfide were relatively high for a
pristine Danish area (Frederiksen et al. 2006) and also for
pristine carbonate-rich sediments (Holmer et al. 2005a).
AVS and CRS clearly accumulated within the sediment
from the outer to the middle and inner side of the rings. The
pool of CRS was, however, low, likely reflecting, as often
seen in carbonate-rich sediments with low sulfide buffer
capacity, that the availability of iron to precipitate sulfides
was low leaving more free sulfides for invasion into eel-
grass (Chambers et al. 2001; Holmer et al. 2005a; Holmer
2009).
Calculated from the isotopic composition of plant tis-
sues and that of seawater and sediment AVS, the sediment
sulfide contribution to total tissue sulfur was high (>50 %
of TS) and increasing from outer to the middle position
suggesting a substantial invasion of reduced sulfur from the
sediment. This is somewhat surprising because water col-
umn hypoxia, which may be a prerequisite for sulfide inva-
sion (Pedersen et al. 2004), would not be expected to be a
frequent event in the relatively pristine waters of this open
coast. However, contrary to the leaves themselves, that can
tolerate relatively high sulfide concentrations (Dooley et al.
2013) as also reflected by the lack of negative impact on
photosynthetic efficiency in this study, dividing cells of sea
grass leaf meristems are greatly impacted by sulfide inva-
sion leading to reduced growth (Garcias-Bonet et al. 2008).
In the present study, sulfide invasion clearly had occurred
as also reflected by the content of elemental sulfur found
inside roots and rhizomes. In addition, the controls of
sulfide invasion in eelgrass are complex (Frederiksen et al.
2006), and the complexity of sulfide invasion and toxic-
ity may be responsible for the fact that the small, remain-
ing shoots on the inner side of the rings had relatively less
sulfide invasion than shoots at the middle and outer posi-
tions but still had much lower specific growth rates.
In conclusion, we find that the clonal growth pattern,
burial by sand, hydrodynamic forcing, nutrient limitation
and high sediment oxygen demand could not be responsi-
ble for the formation of the ring-shaped eelgrass patches
at the Island of Møn. Instead, the most plausible explana-
tion for the die-off of shoots on the inner side of the ring-
shaped patches, and hence for the formation of the peculiar
eelgrass fairy rings, seems to be the gradual accumula-
tion of sediment-derived sulfide in eelgrass shoots of the
expanding patches. Accordingly, the mechanism, that most
likely is responsible for sudden events of massive eelgrass
die-off in eutrophied coastal areas (Pedersen et al. 2004;
Borum et al. 2005; Holmer et al. 2005b), may be a more
or less continuous process going on in the carbonate-rich
sediments forming the eelgrass fairy rings at the Island of
Møn. Apart from donut-shaped sea grass patches (Fon-
seca and Kenworthy 1987), there is to our knowledge only
one earlier scientific report on ring-shaped plant patches
in aquatic ecosystems (Castellanos et al. 1994). In their
paper, Castellanos and co-workers reported on ring-shaped
patches of Spartina maritima and suggested that an uni-
dentified inhibitory agent caused plant die-off in the cent-
ers of patches. Sulfide toxicity could potentially have been
responsible for this observation and also for anecdotal
reports on other ring-shaped plant patches in wetlands. We
Mar Biol
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therefore encourage addressing and testing the potential
role of sulfide inhibition when ring-shaped plant patches
are found in marine waters and wetlands. With the present
exploration of the ring-shaped eelgrass patches, we have
documented that the puzzling fairy rings made by vascu-
lar rooted plants, although rare phenomena, are widespread
and can be found from deserts to fully submersed habitats.
Acknowledgments We would like to thank LVSE Petersen and B.
Kjøller for technical assistance, and we are grateful for the logistic
support provided by Geocenter Møns Klint. We also thank Jacob
Topsøe Johansen for his permission to use the photo of the rings. The
study was funded by the Danish Strategic Science Foundation through
grant DSF 09-063190.
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