Download Pattern of Distribution and Effects on the Seagrass Life History

Epiphytic Community on Posidonia oceanica: Pattern of Distribution
and Effects on the Seagrass Life History
Paloma Lopez
Department of Ecology and Evolutionary Biology, University of California Santa Cruz, Santa Cruz,
CA, USA.
Abstract:
The seagrass, Posidonia oceanica, is essential to the Mediterranean Sea ecosystem. The study
of the relationship between P. oceanica and epiphytes could serve as an important tool to aid
management and conservation of the seagrass. In this study, I examined the distribution of
epiphytes on seagrass across a depth gradient. Furthermore, I determined how epiphyte load
affects flowering events on P. oceanica. Results show that light intensity and epiphytic coverage
decreases as a function of increasing depth. A light threshold for epiphytic growth was derived
at around 90 lum/ft2. Additionally, individuals with higher epiphyte coverage were found to
have lower flowering frequency. These results indicate that epiphyte coverage is driven by light
intensity. Higher levels of epiphyte coverage may also negatively affect the life history of P.
oceanica.
Introduction:
Posidonia oceanica, the only endemic seagrass to the Mediterranean Sea, plays an important
role in ecological processes such as carbon fixation and storage, and is a powerful regulator of
the quality of the water. (Pergent et al., 1994; Mateo et al., 1997; Duarte et al., 2005). The
leaves of this species may attain a maximum age of 300 days (Duarte 1991) and as a result of
their long-life span, old leaves may support a high load of epiphytic growth (Casola et al. 1987,
Romero 1988). High epiphytic load on P. oceanica leaves may have substantial effects on
seagrass survival and growth, such as leaf shading (Hootsmans and Vermaat 1985, Silberstain et
al. 1986) or gas and nutrient exchange with host leaves (McRoy and Goering 1974, Sand-Jensen
1977).
The purpose of this study was to examine the pattern of epiphyte coverage on P. oceanica
leaves as a function of light and depth. To investigate this, I measured epiphyte density and
light intensity as a function of depth to support the hypothesis that both epiphytic load and
light have an inverse relationship with depth. Light intensity as a function of leaf length was
also measured to detect if there is a light threshold for epiphytic growth. Epiphyte load within a
single depth was studied to determine its effect on the seagrass growth and flowering. A
growth experiment was designed to measure if leaves with no epiphytes would increase their
growth rate significantly compared to leaves with their natural epiphytic community present.
Flowering versus non-flowering individuals were compared based on epiphytic load to
determine whether non-flowering plants are associated with higher epiphytic load on
individuals.
Materials and methods:
Species description. Posidonia oceanica is a perennial seagrass endemic to the Mediterranean
Sea. It forms vast meadows from the sea-surface to about 40 meters depth. There is a large
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community of epiphytes that colonize the seagrass’ blades yearlong. Epiphyte colonization is
principally controlled by biological factors such as the host plant growth and the life cycle
of the epiphytes (Casola et al., 1987; Mazzela & Russo, 1989). In turn, biological factors are
affected by environmental conditions such as temperature, light strength, and hydrodynamics,
which change as a function of depth.
Site description. This study was completed at the STARESO field station in Calvi, Corsica, during
October 2012. Sampling was conducted north of the STARESO harbor beginning 50 meters
north of the jetty and ending at about 50 meters north (30° bound) of the starting point.
Transects were carried out by SCUBA parallel to shore between 9 and 15 meters depth within
the Posidonia bed. Leaves were collected over same-depth transects using Ziploc® bags.
Epiphyte index as a function of depth. I calculated epiphyte index by using uniform point
contact along Posidonia blades every centimeter. Leaves were collected from four different
depths: 9 (5 blades), 11.5 (8 blades), 13 (8 blades), and 15 (4 blades) meters. Leaves were
brought to the lab at STARESO and measured using a measuring tape. A ratio of epiphyte point
contact over total point contact was calculated and compared using linear regression analysis
(JMP 10). Greater epiphyte index in shallower depths will support the hypothesis that there is
an inverse relationship between epiphyte coverage and depth.
Amount of light across depth. Instantaneous light measurements were taken by exposing a
PAR sensor for 30 seconds every ten meters along the Northernmost (Site A 42°58’07’’ N,
08°72’49’’ E) and Southernmost (Site C 42°57’82’’ N, 8°72’48’’ E) perpendicular transects)
(Perlkin and Tucker, 2012).
Amount of light along blade. Light was measured along Posidonia leaves at two different
depths: 9 (5 blades) and 15 (3 blades) meters to determine a light threshold for epiphyte
growth along blades at each depth. A PAR sensor was positioned along the blade from bottom
to top with pauses every 10 centimeters. Data was analyzed using analysis of co-variance in
JMP 10 to find relation between amount of light along blade and depth. A light threshold will be
determined to define epiphytic habitat.
Epiphyte effect on growth. 28 Posidonia leaves along a 30 meter transect were scrapped and
cleared of epiphytes using diving knives. This experiment was done at a single depth (9m). Each
of the treated individuals were about one meter apart from each other. A different leaf from
the same individual was cut to the same size as the scrapped one to compare leaf growth with
and without epiphytes. All other leaves from the same individual were trimmed short enough
to distinguish them from the treated leaves. The experiment was monitored after four and
seven days. Results were compared using an ANOVA test. Significantly higher growth on
scrapped individuals will support the hypothesis that presence of epiphytes decreases growing
rates of Posidonia leaves.
Epiphyte effect on flowering. 50 individuals were surveyed along a 50 meter transect. Every
leaf on an individual was categorized into three levels of epiphyte intensity from 1 to 3
indicating low to high epiphyte coverage respectively. An epiphyte index was calculated based
on the total epiphyte coverage over the total number of leaves in an individual. Flowering
events were annotated for each sampled plant. Results were analyzed using a t-test. Higher
number of non-flowering individuals with higher epiphyte coverage will support the hypothesis
that grater epiphyte colonization restricts seagrass development.
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Results:
Epiphyte index as a function of depth. Posidonia leaves showed a higher epiphyte index at 9
meters (mean=0.7) compared to those at 15 meters (mean=0.3). The samples from 11.3 and 13
meters were not as accurate and results show epiphyte index from these samples to be more
widely spread out. However, both depths follow a decreasing pattern of epiphyte coverage
when evaluated along results from 9 and 15 meters (Fig. 1). Statistical analysis strongly support
the hypothesis that epiphyte coverage decreases on Posidonia leaves with increasing depth
(P<0.001, α=0.05)
Epiphyte index By Depth
Figure 1. Epiphyte index as a function of depth. The graph shows a linear decrease of
the epiphyte coverage in Posidonia leaves as a function of depth. The dots represent
each of the replicates per depth.
Amount of light across depth. Figure 2 shows the relationship between light and depth. The
amount of light present in the water column decreases with increasing depth.
Amount of light along blades. The amount of light found along Posidonia blades decreased
linearly with the highest amount of light found at the top of the leave and the lowest at the
bottom. Results from a covariance comparison between 9 and 15 meters show the relationship
between both depths with respect to the amount of light and epiphytic presence (Fig. 3). When
measured from the bottom up, leaves sampled at 15 meters depth had their first epiphytic
growth around the 7th centimeter, which corresponds to the amount of light present in the
region where the first epiphytes were found in leaves at 9 meters depth (1 st centimeter). These
results support the hypothesis that there is a light threshold which limits epiphytic growth
along Posidonia leaves based on amount of light received. From the comparison graph (Fig. 3),
we can infer 90 Lum/ft2 to be the light limit in which epiphytes are adapted to be extant.
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Effect of epiphytes on growth. An analysis of variance test shows no significant results
between scraped and non-scraped leaves (p>0.39, α=0.05). Leaves show negative growth after
4 and 7 days, most likely due to increased herbivore consumption (Fig. 4). There is high
variability seen between results from 4 and 7 days. Conclusions about the effect of epiphyte
cover on growth can therefore not be drawn based on these results.
Effect of epiphytes on flowering. Results show that flowering events are more often associated
with lower epiphytic coverage (measured as epiphyte index, Fig. 5). There were a
disproportionate number of samples that had flowers versus those that did not (9 versus 41
respectively). However, a t-test showed a significant difference between flowering versus nonflowering occurrences in Posidonia (P<0.0387, α=0.05), supporting the hypothesis that nonflowering events occur more often in plants with a higher epiphyte coverage than those plants
with a lower index.
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Epiphyte index by Flowering events
Figure 5. Comparison of flowering events on Posidonia leaves based on epiphyte
index. Flowering events occur more often in individuals with a epiphytic lower index
(P<0.0387, α=0.05).
Conclusion:
According to results in this study, epiphyte coverage and light intensity decrease as a function
of increasing depth. Moreover, there is an identifiable light threshold at which epiphytes are
unable to colonize Posidonia leaves. Thus epiphyte distribution across depth can be attributed
to environmental changes in light. Moreover, individuals with higher epiphyte coverage have
lower flowering occurrences compared to those with lower epiphytes. These results indicate an
adverse effect on the plant’s life-history. Event though results from the growth experiment
were not significant, the experiment showed a different effect of the relationship between
epiphytes and Posidonia. Only the scrapped leaves had evident fish bites, which leads to
speculate that epiphytes provide some level of protection to Posidonia leaves against
herbivores.
The relationship between epiphytes and its host plant is an important interaction to study in
order to determine what disadvantages or benefits each species has over the other. It has been
shown that the epiphytic community on P. oceanica leaves plays an important role in the
energy transfer from plant to higher trophic levels (Chessa et al., 1982). However, results from
this study show that the degree of epiphytic colonization is crucial for the seagrass life history.
Further study of this relationship is needed to determine at which level, the presence of
epiphytes begin to restrain seagrass development.
Studies done by Jupp (1977) and Mendez (1994) concluded that there is an abundant
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development of epiphytic growth in those areas with high levels of fertilizers. Consequently,
changes on epiphyte loading could be used as an indicator of organic pollution levels shifting.
Posidonia oceanica is undergoing significant decline (at least in the Northwestern
Mediterranean; Sánchez-Lizaso et al. 1990, Zavodnik & Jaklin 1990, Marbà et al. 1996), which
may be due in part to increasing turbidity of coastal waters or to increasing epiphyte loading,
both effects being results of increased eutrophication. Studies on epiphyte distribution and on
the factors that control this pattern are important in developing management and conservation
strategies to prevent seagrass decline.
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