Epiphytic bryozoans on Neptune grass – a sample-based data set

Abstract Background The seagrass Posidonia oceanica L. Delile, commonly known as Neptune grass, is an endemic species of the Mediterranean Sea. It hosts a distinctive and diverse epiphytic community, dominated by various macroalgal and animal organisms. Mediterranean bryozoans have been extensively studied but quantitative data assessing temporal and spatial variability have rarely been documented. In Lepoint et al. (2014a, b) occurrence and abundance data of epiphytic bryozoan communities on leaves of Posidonia oceanica inhabiting Revellata Bay (Corsica, Mediterranean Sea) were reported and trophic ecology of Electra posidoniae Gautier assessed. New information Here, metadata information is provided on the data set discussed in Lepoint et al. (2014a) and published on the GBIF portal as a sampling-event data set: http://ipt.biodiversity.be/resource?r=ulg_bryozoa&v=1.0). The data set is enriched by data concerning species settled on Posidonia scales (dead petiole of Posidonia leaves, remaining after limb abscission).


Introduction
In the marine environment, the term "epiphyte" is defined as: "all micro-or macroorganism living attached on a plant substrate" (Steel and Bastow Wilson 2003). The seagrass Posidonia oceanica L. Delile, commonly known as Neptune grass, is an endemic species of the Mediterranean Sea that hosts a distinctive and diverse epiphytic community, dominated by various macroalgal and animal organisms (Boero et al. 1985;Mazzella et al. 1989;Peres and Picard 1964;Van Der Ben 1971). Among epiphytic animals fixed on seagrass substrates, cheilostome bryozoans are often the most abundant and diverse taxa (Balata et al. 2007;Lepoint et al. 1999;Nesti et al. 2009;Pardi et al. 2006). Some species, like Electra posidoniae Gautier, are strictly found on Neptune grass leaves (Gautier 1961;Lepoint et al. 2014b;Matricardi et al. 1991). Due to this important contribution, cheilostome bryozoan diversity has received particular attention (Balduzzi et al. 1983;Gautier 1961;Harmelin 1973;Hayward 1975;Kocak et al. 2002;Lepoint et al. 2014a), but quantitative data are often lacking and seasonal variability is less often documented than spatial variability. This spatio-temporal variability was assessed quantitatively in Lepoint et al. (2014a) and the occurrence and abundance data set resulting from this study was made freely available on the GBIF portal as a sample-based data set. Biological and ecological features of the leaves' community are also presented and discussed in Lepoint et al. (2014a). Trophic ecology of Electra posidoniae, an obligate epiphyte of P. oceanica, are discussed in Lepoint et al. (2014b). Scale community data were only published in the GBIF data set but not discussed.

General description of the dataset
The data set is a sample-based data set (n = 103 sampling events), recording occurrence (n = 1234) and abundance of cheilostome Bryozoa (n = 6488 counted colonies) settled as epiphytes on leaves and scales of the seagrass P. oceanica. Sampling encompasses an entire annual cycle (from November 2002 to December 2003) (n = 5 sampling seasons) and six sampling depths (7, 10, 15, 20, 25, 30 m). The data set package is composed of two data files: the former describing sampling events and the latter reporting occurrence and abundance of bryozoan colonies.

Project description
The data were collected during a postdoctoral project (G.L.) entitled: "Nitrogen dynamics and ecology of the epiphyte community in a Posidonia oceanica seagrass bed".
The Posidonia seagrass beds are one of the most important ecosystems in the Mediterranean coastal zone. Seagrass beds are in regression in many areas of the world, while the reasons of such regression are complex and often poorly understood. The epiphyte community constitutes an important component of the seagrass meadow. Variability in epiphyte community composition and/or biomass may sometimes be linked to anthropogenic disturbance and used as monitoring tool (Mabrouk et al. 2013;Martinez-Crego et al. 2010;Piazzi et al. 2004). This argues for more fundamental studies of the seagrass ecosystem functioning and epiphyte biodiversity.
The general objective of this project was to study the dynamics of nitrogen in the epiphyte community of the Posidonia leaves in relation with its ecology and its spatiotemporal structure.
Sampling and experiments were performed in Revellata Bay (Corsica, France), near the STARESO oceanographic station between 2002 and 2004. Samples were taken at a reference site (depth 10 m) followed by our laboratory since the 1970s, as well as along a permanent transect (7 to 35 m depth). At the level of a Posidonia shoot, particular attention was given to the spatio-temporal evolution of the structure of the epiphytic community. Specific composition of sessile fauna (mainly Bryozoa) was established at 7, 10, 15, 20, 25 and 30 m depth (Lepoint et al. 2014a). Temporal dynamics and trophic ecology of the dominant species Electra posidoniae was studied using seasonal sampling and isotopic approach (Lepoint et al. 2014b). Finally, we have measured experimentally the nitrogen uptake by different epiphytic macroalgae components (Lepoint et al. 2007). Complementary information about the epiphytic community of P. oceanica in Revellata Bay may be also found in, for example, Dalla Via et al. 1998;Jacquemart and Demoulin 2006;Lepoint et al. 1999;Michel et al. 2015;Pête et al. 2015.

Study extent
All sampling events (n = 103) and measurements were performed in Revellata Bay (Calvi, Corsica, France), near the marine research centre of STARESO (42°35'N, °43'E) (University of Liège) along the same permanent transects. Sampling encompassed one complete seasonal cycle and the 7 to 30 m depth range occupied by P. oceanica shoots in Revellata Bay (see above).

Sampling description
Seagrass shoots were collected in triplicate in November 2002, March, June, September and November 2003 along the same permanent transects set at 7, 10, 15, 20, 25 and 30 m depth. Each sample is constituted of a single replicate gathering three shoots of P. oceanica.

Quality control
To determine sample size, previous work was performed to determine the number of shoots that accumulates at least the 75% of the leaf epiphyte bryozoan species. For publication on GBIF portal, synonymies were matched against the authoritative, expert-driven World Register of Marine Species (WoRMS) and corrected compared to Lepoint et al. (2014a).

Step description
The shoots were immediately frozen, then conserved in 4% formalin diluted in seawater. Identifications of species settled on leaves and scales (i.e. dead petioles remaining after leaf abscission) under a stereomicroscope (Stemi 2000, Zeiss) were done to the lowest systematic level using keys for Bryozoa (Prenant and Bobin 1966;Zabala and Maluquer 1988), and the works of Balduzzi et al. (1991), Gautier (1961) and Hayward and McKinney (2002). Colonies on scales were not counted and data are presented only as occurrence. All the colonies found on the two sides of leaves were counted. Colonies counted were reported per shoot and per metre square, accounting the average P. oceanica shoot number per metre square, measured at each sampling depth. This parameter was measured monthly using a quadrate with an area of 0.1 m 2 randomly set in the meadow. The shoot density did not show any significant variation during the duration of this study. Bathymetric variability of this parameter in Revellata Bay may be found in Gobert et al. (2003).

Description
Revellata Bay is a part of Calvi Bay and lies in the western Mediterranean, on the northwestern coast of Corsica (42°35'N, 8°45'E). Its western limit is Punta Revellata Cape, and its eastern limit is Punta San Francesco Cape. The STARESO (STAtion de REcherches Sous-marines et Océanographiques) research station (University of Liège) is located on Punta Revellata, at the western border of the bay. Salinity of the water of Calvi Bay is approximately 38‰, and is relatively invariant throughout the year. Temperature of water varies between minima of 12°C (February) and maxima of 26°C (August), with a notable vertical thermal stratification from May to September. Amplitude of tidal variation is weak. Nutrient concentrations (N, P) and particle load in the water column are typically low and characteristic of oligotrophic areas (Lepoint et al. 2004). Posidonia oceanica meadows cover approximately 50% of the area of the bay, and reach depths of nearly 40 m. Meadows show, in most places, a continuous extension, but local erosion ("intermattes") occurs (Abadie et al. 2015). The vast majority of meadows grow on soft bottoms, but they seldom colonize rocky substrates. Meadows of Calvi Bay are relatively dense, and show an important foliar biomass and production despite the oligotrophic character of the area (Gobert et al. 2003).

Description
The dataset includes 54 species of cheilostome Bryozoa, belonging to 25 different families.

Taxa included
A full list of taxa included in this dataset is given in Table 1. is linked to file 1 by event identifiers (eventID column) (Table 3).

Author contributions
GL conceived the sampling design, performed the sampling and performed identification of specimens. LNM and AH formatted and published the dataset. LNM, GL & AH wrote the paper.  Localisation of the colony on the plant (on seagrass leaf or on seagrass scales) organismQuantity number of colonies per square metre organismQuantityType Nature of counted items (colonies for all species) dynamicProperties Remarks/number of ovicellated colonies 2.45.69.03) and the Action de Recherche Concertée 10/533 (French-speaking Community of Belgium). We also warmly thank the Belgian Biodiversity Platform (BEL-SPO, Belgium) that convinced and formed us to valorise and share such data set for biodiversity study purpose. This paper is MARE paper number 325.