Small-scale spatial patterns of Posidonia oceanica
inflorescences: variation in inflorescence frequency and
maturity as a function of depth
Hannah Perlkin and Emily Tucker
Ecology and Evolutionary Biology, University of California, Santa Cruz
Abstract
For most species there is a clear set of advantages resulting from sexual
reproduction. In part because of the longevity of clonal species along with the
combination of clear evidence of asexual reproduction and scant evidence of successful
recruitment of new sexually derived individuals, sexual reproduction has been widely
considered to be relatively or completely unimportant in seagrasses. With increased
monitoring efforts over the last two decades, there has been an increased reporting of
flowering events and a renewed consideration of the role of sexual reproduction in longdistance dispersal and recovery of meadows after disturbance. Posidonia oceanica can
spread vegetatively through plagiotropic (horizontal) and orthotropic (vertical) rhizome
growth. This study was carried out during mid-late October, 2012 in The Bay of Calvi,
Corsica. Inflorescence proportion and maturity were sampled at 10, 20, 30, 40, 50 and 60
feet. The inflorescence proportions encountered during our study are consistent with
previously recorded major flowering events in the Mediterranean. The significant
variation in inflorescence proportion and maturity with depth and light reflects the
relationship between depth and PAR. Among depth, instantaneous light and cumulative
light, the latter was found to be relatively most important. Increased inflorescence
proportion and inflorescence maturity at depths with highest light intensity (10-20 feet)
suggests the importance of photosynthetic accumulation to initiate sexual reproduction.
1. Introduction
For most species there is a clear set of advantages resulting from sexual
reproduction. These include recombination, the production of novel genotypes and
phenotypes and for most species it is the only current means by which genes can be
“passed” on. The life history of many clonal species presents a type of theory-based
tension to this logic (Arnaud-Haond et al. 2010). Clonal species are generally very longlived and capable of substantial non-sexual vegetative “reproduction” by a variety of
processes (e.g. growth, fragmentation, pseudovivipary) (Arnaud-Haond et al. 2012; DiazAlmela 2007; Balestri et al. 2005; Ballestros et al. 2005; Orth et al. 1994). In part because
of the longevity of clonal species combined with clear evidence of asexual reproduction
and scant evidence of successful recruitment of new sexually derived individuals, sexual
reproduction has been widely considered to be relatively or completely unimportant
(Vermaat 2009; Balestri and Cinelli 2003; Duarte et al. 1994; Buia and Mazzella 1991).
This is particularly true for seagrasses, where vegetative propagation has been the main
focus while sexual reproduction has been dismissed as having little importance in
meadow maintenance due to it’s episodic and highly variable nature (Remizowa et al.
2012; Diaz-Almela et al 2006; Gobert et al. 2005; Ballestri and Vallerini 2003; Campey
et al. 2002; Inglis and Smith 1998; Buia and Mazzella 1991).
The primary counter-arguments to the idea that sexual reproduction in clonal species is
unimportant for seagrasses (and other clonal species) are: (1) the observation that
flowering events and seed set is common and often massive, (2) such events are
energetically costly diverting resources away from vegetative growth and, (3) while often
episodic with long intervals between such events, flowering years are numerous relative
to the lifespan of the clone (Arnaud-Haond et al. 2012; Almela-Diaz 2006; Gobert et al.
2005, 2001; Balestri and Vallerini 2003; Duarte et al. 1994). With increased monitoring
efforts over the last two decades, there has been an increased reporting of flowering
events and a renewed consideration of the role of sexual reproduction in long-distance
dispersal and recovery of meadows after disturbance (Balestri and Valerini 2004, 2003;
Buia and Mazzella 1991).
Posidonia oceanica is a particularly long lived, persistent and slow growing species with
high primary productivity that has been shown to create increased complexity to many
marine ecosystems of the Mediterranean (Diaz Almela 2007; Orth 2006; Duarte 2001).
Seagrass productivity, growth and abundance are extremely sensitive to changes in light
availability (Serrano et al. 2011). In P. oceanica specifically, vegetative growth and
colonization are largely dictated by light availability. (Duarte 1991). Variation in its
growth has been attributed to PAR availability, which varies as a function of depth
(Rollon et al. 2003; Duarte 1991; Dennison and Alberte 1985). Light provides sucrose,
energy for P. oceanica as well as carbon, a resource that is accumulated in reserves in
late summer and is utilized in fall in the production of inflorescences leading to seed
production in early spring the following year (Serrano et al. 2011; Vermaat 2009).
Carbon and phosphate accumulated through photosynthesis serve in inflorescence
elaboration in P.oceanica (Gobert et. al. 2005). In particular, carbon is stored in the
rhizomes, where carbohydrate concentrations are highest (Collier et al. 2010).
The resources that are continuously allocated to vegetative growth in P. oceanica are
diverted to sexual reproduction during flowering events. As noted above, such reallocation decreases P. oceanica’s ability to grow suggesting that the maintenance of
sexual reproduction increases the fitness of the individual relative to having only asexual
reproduction. (Balestri and Vallerini 2003).
Because of the noted relationship between PAR and depth, sequestration of resources
occurs more slowly at deeper depths which has provided the basis for the observation that
there is often a one to two month delay in the flowering process in ‘deep’ (over 15 meters
depth) stands of Posidonia (Buia and Mazzella 1991; Remizowa et al. 2011). Rhizome
growth becomes limited with increased depth due to decreased light availability which
could explain this two-month delay as smaller rhizomes (deep) require more time to
accumulate the resources sufficient to allow production of inflorescences (Duarte
1991). Despite the physiological basis for this allocation model and some anecdotal
observations, there has been no detailed assessment of the temporal phenology of P.
oceanica as a function of depth. Our goal was to provide this assessment.
Specifically, the goal of this study was to describe patterns in inflorescence proportion
and maturity as a function of depth under the specific prediction that inflorescence
proportion and maturity decrease with increasing depth. Our study addressed 4
hypotheses: (1.) Inflorescence proportion (IP) and inflorescence maturity (IM) vary
monotonically and negatively as a function of depth (2.) Light varies negatively as a
function of depth (3.) The most parsimonious explanation for the effect of depth on
inflorescence proportion is the attenuation of cumulative PAR with depth.
2. Methods
Species description
Posidonia oceanica, a monoecious perennial marine plant, grows from the waters
surface to depths of 80 meters and has an optimum temperature of 17-20 degrees Celsius
(Balestri and Vallerini 2003; Duarte 2001; den Hartog 1970). It is widely distributed in
the Mediterranean, as well as the North African coast and in the Atlantic along the coasts
of Portugal and Spain (den Hartog 1970). Seagrasses are characterized by their ability to
spread vegetatively as their main mode of propagation and reproduce sexually on rare
occasions, which allows them to persist in near shore (and potentially stressful)
environments (Kendrick et al. 2012).
P. oceanica exhibits a triphase annual growth pattern. A period of minimal
growth from October to January is followed by a period of rapid growth, which persists
throughout spring. At the end of May the growth rate begins to decrease considerably,
continuing through summer until growth stops completely in October (den Hartog 1970).
Anthesis initiation has been observed from July to September (Gobert et al. 2005).
Inflorescences have been observed during these periods of limited growth, between the
months of October and January with the fruits occurring from March to June (Bay 1984;
den Hartog 1970). The four-month time period in which P. oceanica inflorescences can
be observed coincides with an observed two month delay in development between the
shallow (0-15m) and deep (15-28m) stands (Remizowa et al. 2012; Buia and Mazzella
1991).
Life History
P. oceanica is a species with low tolerance to changes in salinity, water
temperature and pollution (den Hartog 1970). In areas with water temperatures reaching
above 20 to 22 degrees high mortality in leaves has been reported (den Hartog 1970). It
inhabits exposed to moderately sheltered areas with coarse sand, good circulation and
well-aerated water (den Hartog 1970). This sensitivity contributes to patchy distribution
in otherwise homogeneous shallow (10 to 20 meter) beds around ports and river mouths
(den Hartog 1970).
P. oceanica can spread through plagiotropic (horizontal) and orthotropic (vertical)
rhizome growth though it’s main mode of spreading is plagiotropic (Balestri et al. 2005).
The main plagiotropic axis performs constant expansion and growth, allowing for
exploration and colonization of new territory, while orthotropic rhizomes and their lateral
branches use local resources, and facilitate light capture by extending leaf blades higher
into the canopy, and escaping if sedimentation is high (Balestri et al. 2005) After a
minimum period of vegetative maturation (two years), the apical meristem of an
orthotropic shoot may produce one or occasionally two inflorescences (Balestri et al.
2005; Balestri and Cinelli 2003). Flowers have also been found on plagiotrophic
rhizomes (Balestri et al. 2005). While the relative importance of the two modes of
spreading are unknown, both play a role in population maintenance of the genus P.
oceanica (Campey et al. 2002).
Study site
This study was carried out during mid-late October, 2012 at the Station de
Recherches et Sous-marines et Océanographiques (STARESO) located in The Bay of
Calvi (Mediterranean Sea, Corsica, France, 42◦ 35’ N, 8◦ 43’ E). Calvi Bay is considered
to host meadows of P. oceanica that are of high ecological (considered as having no
disturbance) status with a sharp lower limit to the meadow and approximately 310.4
shoots 𝑚2 (Lopez y Royo et al. 2010). The Bay of Calvi is characterized by poor
nutrients (Gobert et al. 2005; Bay 1984) and extensive P. oceanica meadows that
continue to surround the entire island of Corsica (Pasqualini et al. 1998). All sampling
was done within 1 km of shore between depths of 10-60 feet.
2.1 Inflorescence proportion and maturity as a function of depth
All sampling was carried out on SCUBA. Surveys were conducted along six
parallel transects. Transects were established at constant depths (10, 20, 30, 40, 50, 60
feet) within and immediately outside of STARESO harbor. Transects were 12-20 meters
in length, depending on geomorphological constraints.
In order to approximate inflorescence proportion (number of inflorescences per
area) a 0.45 𝑚2 quadrat was used to count total rhizomes and inflorescences. Rhizomes
were considered to be individual lateral branches of the orthotropic axes. IP was
quantified within two quadrats at every meter, on either side of the transect. In order to
determine if IP varied as a function of depth an ANOVA (five degrees of freedom) was
run.
In order to qualify inflorescence maturity, four inflorescences were assessed
within each 0.45 𝑚2 quadrat. Inflorescence maturity was assessed within the same
quadrats used to sample IP as described above. IM was qualified for four inflorescences
per quadrat. To maximize randomness, the first two inflorescences encountered from the
top-left and bottom-right quarter of the quadrat were selected. Each inflorescence was
categorized as closed, open or flowering [figure 1]. ANOVA (five degrees of freedom)
was used to determine if relative maturity of inflorescence varied with depth.
2.2
as a
Figure 1.
Inflorescence maturity closed, open and flowering shown (from left
to right). IM was defined as follows: closed (blades closed around
inflorescence), open (blades separated but flowers not protruding)
or flowering (flowers emerging), from left to right.
Light
function of depth
In order to determine instantaneous light intensity (lumens 𝑓𝑡 −2 ) two 50 meter
transects were established, one to the North (42°58’07’’ N, 08°72’49’’ E) and one to the
South (42°57’82’’ N, 08°72’48’’ E) of the parallel transects. Transects were run
perpendicular to shore. A PAR sensor was exposed every ten meters along each transect
for 30 seconds. These measurements were used to graph instantaneous light as a function
of depth.
2.3 Inflorescence proportion and light
Variation in cumulative light intensity between depth classes was determined
using multiple PAR sensors established at depths of 10, 20, 30, 40 and 50 feet. PAR
sensors were deployed for 28 hours (12:00 26 October - 16:00 27 October) and were
fixed atop PVC posts of roughly the same height as the surrounding meadow.
A regression model was used to determine the relative importance of function
depth, instantaneous light, and cumulative light for IP. An AIC model selection
procedure was used to run all reduced model in order to select for the best-fit variable.
3. Results
3.1 Inflorescence proportion and maturity as a function of depth
The results of sampling conducted to determine if inflorescence proportion
decreased with increasing depth strongly supported the hypothesis that IP varies as a
function of depth (figure 2, table 1, p<0.00001). Inflorescence proportion decreased
monotonically with depth.
Figure 2.
This bar graph depicts decreasing IP as
a function of depth.
The results of sampling conducted to determine if open and flowering
inflorescence proportions decreased with increasing depth strongly supported the
hypothesis that IM varies as a function of depth (figure 3, table 1, p<0.00001).
Figure 3.
The bar graph on the left shows the significant decrease in open IP as a
function of depth while the graph on the right shows the same
relationship with flowering IP. The two represent the negative
relationship of IM with depth.
Table 1. Summary of ANOVA analyses for IP and IM as a function of depth.
ANOVA
d.f.
P-value
5
<0.00001
IP
5
<0.00001
IM (open)
5
<0.00001
IM (flowering)
3.2 Light as a function of depth
The results of our sampling conducted to determine if cumulative and
instantaneous light intensity (Lumens 𝑓𝑡 −2 ) decreased with increasing depth supported
the hypothesis that light availability varies as a function of depth (figure 4).
Figure 4.
Both graphs describe the relationship of light as a function of depth.
Both relationships are non-linear and negative. These graphs show that
the relationship between cumulative light (right) and depth is steeper
than instantaneous (left).
3.3 Inflorescence proportion as a function of light
The results of our sampling conducted in order to determine if inflorescence
proportion varied as a function of light strongly supported the hypothesis that
inflorescence proportion varies as a function of light (figure 5: instantaneous light,
p<0.00001; cumulative light p<0.00001). The results of our comparison of IP as a
function of instantaneous light versus cumulative light tested the idea that cumulative
light was the most parsimonious explanation to describe the pattern in IP. The results
were strongly supported by the best-fit of cumulative light (depth 𝑅 2 =0.88; instantaneous
light, 𝑅 2 =0.96; cumulative light, 𝑅 2 = 0.99).
4. Discussion
With our study we were able to capture a clear pattern in inflorescence proportion
with depth, instantaneous light and cumulative light. The significant variation in
inflorescence proportion and maturity with depth and light reflects the relationship
between depth and PAR (Serrano et al. 2011; Rollon et al. 2003; Duarte 1991; Dennison
and Alberte 1985). The high relative importance of cumulative light to describe the
pattern in IP supports the theory that resource accumulation is necessary for inflorescence
induction in P. oceanica (Gobert et al 2005).
Our results finding increased inflorescence proportion and inflorescence maturity
in areas of higher light intensity (10-20 feet) suggests plants have accumulated sufficient
surplus reserves and/or can mature more quickly (Gobert et al 2005). As indicated in
previous studies, P. oceanica is highly sensitive to light availability, which dictates
photosynthetic capacity (Peirano et al. 2011; Ruiz and Romero 2001). Photosynthates
accumulated throughout spring and summer are stored and utilized during fall and winter,
periods of lower PAR availability (Serrano et al. 2011). October, the time of our study,
coincides with the end of resource accumulation and has historically correlated with the
initial stages of inflorescence production (Gobert et al. 2005; Bay 1984; den Hartog
1970). This facilitated our inflorescence maturity observations, as the early stages of
inflorescence development were distinct.
Our short-term, small-scale study contributed to the understanding of the
relative importance of depth and light in determining patterns in sexual reproduction.
Unfortunately, the short time-scale of our study did not allow us to determine if this
variability was due to the well-cited two-month delay between shallow and deep stands
or if it reflects an overall higher proportion of inflorescences in shallower water. Both of
these potentially conflicting explanations have support in literature (two-month delay:
Remizowa et al 2012; Buia and Mazzella 1991; increased IP in shallows: Diaz-Almela
2006; Inglis and Smith 1998;).
The inflorescence proportions encountered during our study are consistent with
those of previously recorded major flowering events across the entire Mediterranean
(Urra et al. 2011). The flowering event we captured was similarly observed Croatia and
the Balearic Islands and thus likely represents a major flowering event (E. Serrão,
personal communication). These major flowering events, often referred to as ‘episodic,’
must be considered in the temporal context of millenary species (Arnaud-Haond et al.
2012).
The massive scale of these events, their relative frequency and the high
energetic cost emphasize the importance of sexual reproduction in P. oceanica (Balestri
and Vallerini 2003). Theoretically a trade-off must occur; the energetic sacrifices of
sexual reproduction must be matched by some increase in fitness (Kendrick et al. 2012).
Benefits of sexual reproduction in clonal seagrass are known to include: reestablishment
of damaged meadow, establishment of new meadows and likely facilitates species
persistence through adverse conditions. Advancements in any of these areas would
benefit meadow monitoring, restoration, and conservation of clonal seagrass meadows
Literature
Arnaud-Haond, S. et al. Implications of extreme life span in clonal organisms: millenary clones in
meadows of the threatened seagrass Posidonia oceanica. PloS one 7, e30454 (2012).
Arnaud-Haond, S., Marbà, N., Diaz-Almela, E., Serrão, E. A. & Duarte, C. M. Comparative Analysis of
Stability—Genetic Diversity in Seagrass (Posidonia oceanica) Meadows Yields Unexpected
Results. Estuaries and Coasts 33, 878–889 (2010).
Balestri, E. Flowering of the seagrass Posidonia oceanica in a north-western Mediterranean coastal area:
temporal and spatial variations. Marine Biology 145, 61–68 (2004).
Balestri, E. & Cinelli, F. Sexual reproductive success in Posidonia oceanica. Aquatic Botany 75, 21–32
(2003).
Balestri, E. & Vallerini, F. Interannual variability in flowering of Posidonia oceanica in the North-Western
Mediterranean Sea, and relationships among shoot age and flowering. Botanica marina 46, 525–
530 (2003).
Balestri, E., Vallerini, F. & Lardicci, C. On the unusual flowering of plagiotropic shoots in the seagrass
Posidonia oceanica. Aquatic Botany 82, 82–88 (2005).
Ballesteros, E. et al. Pseudovivipary, a new form of asexual reproduction in the seagrass Posidonia
oceanica. Botanica Marina 48, 175–177 (2005).
Bay, D. A field study of the growth dynamics and productivity of Posidonia oceanica (L.) delile in Calvi
Bay, Corsica. Aquatic Botany 20, 43–64 (1984).
Buia, M. C. & Mazzella, L. Reproductive phenology of the Mediterranean seagrasses Posidonia oceanica
(L.) Delile, Cymodocea nodosa (Ucria) Aschers., and Zostera noltii Hornem. Aquatic Botany 40,
343–362 (1991).
Campey, M., Kendrick, G. & Walker, D. Interannual and small-scale spatial variability in sexual
reproduction of the seagrasses Posidonia coriacea and Heterozostera tasmanica, southwestern.
Aquatic Botany 74, 287–297 (2002).
Collier, C. J., Prado, P. & Lavery, P. S. Carbon and nitrogen translocation in response to shading of the
seagrass Posidonia sinuosa. Aquatic Botany 93, 47–54 (2010).
den Hartog, C. 1970. The Seagrasses of the World. North Holland Publ, Amsterdam.
Dennison, W. C. & Alberte, R. S. Role of daily light period in the depth distribution of Zostera marina
(eelgrass). Marine Ecology Progress Series 25, 51–61 (1985).
Diaz-Almela, E. et al. Patterns of seagrass (Posidonia oceanica) flowering in the Western Mediterranean.
Marine Biology 148, 723–742 (2006).
Diaz-Almela, E., Marbà, N. & Duarte, C. M. Consequences of Mediterranean warming events in seagrass
(Posidonia oceanica) flowering records. Global Change Biology 13, 224–235 (2007).
Duarte, C. M. et al. Reconstruction of seagrass dynamics: age determinations and associated tools for the
seagrass ecologist. Marine Ecology Progress Series 107, 195–209 (1994).
Gobert, S., Defawe, O., Lepoint, G., Demoulin, V. & Bouquegneau, J. Anthesis effects on Posidonia
oceanica (L.) Delile phenology in the Bay of Calvi (Corsica , Mediterranean Sea). Hydrobiologia
455, 121–125 (2001).
Gobert, S., Lejeune, P., Lepoint, G. & Bouquegneau, J. C, N, P concentrations and requirements of
flowering Posidonia oceanica shoots. Hydrobiologia 533, 253–259 (2005).
Inglis, G. J. & Lincoln Smith, M. P. Synchronous flowering of estuarine seagrass meadows. Aquatic
Botany 60, 37–48 (1998).
Kendrick, G. a. et al. The Central Role of dispersal in the maintenance and persistence of seagrass
populations. BioScience 62, 56–65 (2012).
Lopez y Royo, C., Casazza, G., Pergent-Martini, C. & Pergent, G. A biotic index using the seagrass
Posidonia oceanica (BiPo), to evaluate ecological status of coastal waters. Ecological Indicators
10, 380–389 (2010).
Orth, R. J. et al. A Global Crisis for Seagrass Ecosystems. (2006).
Orth, R. J., Luckenbach, M. & Moore, K. A. Seed dispersal in a marine macrophyte: implications for
colonization and restoration. Ecology 75, 1927–1939 (1994).
Pasqualini, V., Pergent-Martini, C., Clabaut, P. & Pergent, G. Mapping of Posidonia oceanica using Aerial
Photographs and Side Scan Sonar : Application off the Island of Corsica (France). Estuarine,
Coastal and Shelf Science 47, 359–367 (1998).
Remizowa, M. V, Sokoloff, D. D., Calvo, S., Tomasello, A. & Rudall, P. J. Flowers and inflorescences of
the seagrass Posidonia (Posidoniaceae, Alismatales). American Journal of Botany 99, 1592–1608
(2012).
Rollón, R. N., de Ruyter van Steveninck, E. D. & van Vierssen, W. Spatio-temporal variation in sexual
reproduction of the tropical seagrass Enhalus acoroides (L.f.) Royle in Cape Bolinao, NW
Philippines. Aquatic Botany 76, 339–354 (2003).
Ruiz, J. & Romero, J. Effects of in situ experimental shading on the Mediterranean seagrass Posidonia
oceanica. Marine Ecology Progress Series 215, 107–120 (2001).
Serrano, O., Mateo, M. & Renom, P. Seasonal response of Posidonia oceanica to light disturbances. Marine
Ecology Progress Series 423, 29–38 (2011).
Urra, J., Mateo, A., Marina, P., Rueda, J. L. & Garcia Raso, J. E. First records of Posidonia oceanica
flowering at its westernmost distributional limit (Malaga, Alboran Sea). Botanica Marina 54, 101–
104 (2011).
Vermaat, J. E. Linking clonal growth patterns and ecophysiology allows the prediction of meadow-scale
dynamics of seagrass beds. Perspectives in Plant Ecology, Evolution and Systematics 11, 137–155
(2009).