G . Ceccherelli et al. J. Exp. Mar. Biol. Ecol. 243 2000 227 –240
229
Several explanations have been given for the loss of seagrass habitat: biological invaders have been evidenced as a great threat. For example, changes to Zostera marina
L. meadows have been associated in the Atlantic with the spread of the invading introduced seagrass Zostera japonica Aschers. and Graebn. Posey, 1988. Furthermore,
Den Hartog 1997 documented that the introduced brown alga Sargassum muticum Yendo Fensholt settles into Zostera marina meadows and interferes with the
regeneration of the bed, and no seagrass germlings were found where the alga was present. Also in the Mediterranean, the successful invasive alga, Caulerpa taxifolia, was
invoked as a further cause of loss of meadows of both Cymodocea nodosa Ucria
` Ascherson Ceccherelli and Cinelli, 1997 and Posidonia oceanica de Villele and
Verlaque, 1995. Overall, although many non-indigenous species have been recognised as negatively interacting with native seagrasses, their contribution to declines is poorly
known. For Caulerpa racemosa, many indications of impact have been seen in Cymodocea nodosa beds with lower shoot densities or unhealthy P
. oceanica meadows displaying yellowish leaves Piazzi, personal observation. However, the negative effect
of this alga on seagrasses has not been experimentally demonstrated. In this study, which represents a preliminary approach, the effect of the native
seagrass Posidonia oceanica on the non-indigenous Caulerpa racemosa is examined: a multifactorial experiment was designed to test the importance of 1 the seagrass canopy
structure and 2 orientation of the edges of meadow on algal performance along the edge and inside the meadow of P
. oceanica and 3 whether patterns of algal growth are consistent at different spatial scales ranging from few centimetres to several metres.
Given that seagrass canopy has been shown to structure the entire understory assem- blages Bell and Westoby, 1986a, 1986b; Orth, 1992; Connolly and Butler, 1996;
Ceccherelli and Cinelli, 1998, manipulation of the density of the seagrass, such as reduced shoot density, can be used to examine the influence of the seagrass on size and
growth of the alga. In this system P
. oceanica provides a subcanopy microhabitat to C. racemosa that is distinctly different from the open substrate because of canopy shading
Ceccherelli and Cinelli, 1999, reduction of water motion Gambi et al., 1989, 1990 and production of secondary metabolites that can allelopathically interfere with
understoried species and grazers Cuny et al., 1995. Furthermore, orientation of seagrass edges was tested as a potential source of variation on C
. racemosa performance since the west edge is seaward and hence more exposed relative to the east one which is
faced to the land; thus, given the geographical position of the coast, algal specimens on east-facing edge may be exposed to a different water flow regime, degree of bending of
the seagrass leaves, photoperiod and light intensity relative to the west-facing one Ceccherelli and Cinelli, 1999.
This study represents a useful basis for further experimental investigations on the factors and mechanisms affecting the performance of this alga in the Mediterranean.
2. Methods
The study site is shallow subtidal seagrass bed composed of patches of Cymodocea nodosa and Posidonia oceanica in 2 m depth, located near Livorno in Antignano Italy
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. Ceccherelli et al. J. Exp. Mar. Biol. Ecol. 243 2000 227 –240
along the Tuscan coasts of the north-western Mediterranean 43 8309 N, 108209 E. In this
area Caulerpa racemosa occurs on sandy and rocky substrate, mixed with C . nodosa and
along the edge of P . oceanica meadows.
This study was carried out for 1 year from July 1997 until August 1998 and consisted in a multifactorial experiment testing the importance of seagrass shoot density
and orientation of edges landward seaward on algal performance: experimental units 25
3 50 cm in size were prepared at the edge of Posidonia oceanica meadow, given that Caulerpa racemosa fragments establish naturally in this position Ceccherelli and
Piazzi, personal observations. Three different shoot densities of P . oceanica, 10, 50 and
100 of natural density, which corresponded to the categories of very sparse 50–150
2 2
2
shoots m , dense 400–700 shoots m and very dense . 700 shoots m meadow
according to Giraud 1977, were obtained by clipping different percentages of shoots of
2
averaged natural density corresponding to 1525 shoots m . Eighteen areas, nine at the west exposed, seaward and nine at the east sheltered, landward position, were
randomly chosen at least 3 m apart and randomly selected to be assigned to each level of density. Three areas were attributed to each of the six combinations of the levels of
shoot density and position 10 E, 10 W, 50 E, 50 W, 100 E and 100 W and in each area three plots were positioned 50 cm apart so that a total of 54 experimental
units were prepared for the whole experiment.
Transplanting of fragments of the alga was undertaken in July 1997: fragments 14.34
60.65 cm, mean6SE in size, n 5 20 were collected from a nearby established meadow at the same depth. Three fragments per unit were fixed to the substratum at the
edge of the seagrass using two staples per fragment within an hour of collection. All fragments were taken from the same area and we assumed that removal of effects of
differing initial sizes was not needed. Performance of Caulerpa racemosa transplanted at the edge of Posidonia oceanica was assessed estimating size of the specimens and
growth both inside and outside the seagrass meadow.
Caulerpa racemosa size blade length was sampled in the field using a plastic calliper in 6 times from the start of the experiment September, October 1997, February,
May, June and August 1998. On each sampling time, one quadrat in the north, centre or south position was randomly chosen within each experimental unit and we measured the
length of two randomly chosen blades within the quadrat. To obtain independence of time, data relative to three sampling times were randomly chosen for analysis out of
those available those used for analysis are indicated in Fig. 1 so that blade size of different quadrats were analysed. Data were analysed by using a five way-ANOVA with
‘position’ two levels, ‘density’ three levels,‘time’ three levels, ‘area’ three levels and ‘plot’ three levels.
Growth of stolons of C . racemosa inside the meadow was assessed on seven dates
September, October, November 1997, February, April, June and August 1998 by measuring in situ stolon elongation in each plot taking the distance between the furthest
stolon apex grown inside the meadow and the edge where fragments were transplanted. To avoid dependence of data among sampling times, data relative to the sampling time
representing the longer-term information available August 1998, last one, were analysed using a 3-way ANOVA with ‘position’ two levels, ‘density’ three levels and
‘area’ three levels as factors.
G . Ceccherelli et al. J. Exp. Mar. Biol. Ecol. 243 2000 227 –240
231
Fig. 1. Temporal variation of mean 6SE Caulerpa racemosa blade length cm at the edge of defoliated
Posidonia oceanica 10, 50 and 100 of shoot density at the esast end west edge of the seagrass meadow n
5 18.
Although patterns of spread of the alga were extremely variable among experimental units, because Caulerpa racemosa elongated in different directions relative to the plot
along the edge of the seagrass as well as outwards the meadow, the density of the alga in the experimental units was estimated measuring the percent cover: on three different
occasions December 1997, April and August 1998 photographs of a 25 3 25 cm
surface in two plots per area were taken in situ and subsequent analysis of projected images through a transparent grid divided in 130 small quadrats was performed Foster
et al., 1991. Percent of small quadrats in which the alga was present was calculated for each sampling time and only data relative to the last August 1998 were analysed, as
above for the growth of stolons inside of the meadow.
Spread of Caulerpa racemosa outside the meadow along the edge of Posidonia oceanica was assessed on four different times August 1997, October 1997, April 1998
2
and August 1998 by measuring in each experimental area the total surface m affected
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by the alga through the spread originating from the three experimental units; in fact, after spring 1998 the three algal patches belonging to each area were mostly
indistinguishable because of edge collapse and thus we decided to measure the total surface coverage per area. As above, data relative to the last sampling time August
1998 were analysed by a two-way ANOVA with ‘position’ two levels and ‘density’ three levels as factors.
For all analyses ‘position’ and ‘density’ were treated as fixed and orthogonal, while ‘area’, ‘time’ and ‘plot’ random, where ‘area’ is nested in the ‘position
3 density’ interaction, ‘plot’ is nested in ‘area’ and ‘time’ is orthogonal. Cochran’s test was used
prior to the analyses to remove heterogeneity of variances when necessary and SNK test to compare levels of significant factors.
3. Results
