François Bordeyne1, Aline Migné1, Dominique Davoult1
Article en préparation pour Marine Ecology Progress Series
1 Sorbonne Universités, UPMC Univ Paris 06, CNRS, UMR 7144 AD2M, Station Biologique de Roscoff, 29680 Roscoff, France
Chap. 4 – Métabolisme et structure des communautés durant la succession
Abstract: Small scale disturbances regularly create bare spaces in intertidal communities dominated by canopy-forming brown algae, allowing succession to occur. To date, this process has been widely investigated in a perspective of transition of species from early to late successional stages. In contrast, very few studies have assessed changes in primary production and respiration over successional sequence, while these functions are essential for macroalgal communities. To investigate this issue, we placed several bare granite slabs in the Fucus vesiculosus and F. serratus communities, which are established at different tidal levels. Thus, the influence of emersion gradient on succession was also evaluated. The structure of assemblages settling on the slabs (i.e. density of Fucus spp. and taxonomic diversity) was assessed every three months over a 42 months period, and compared with what was recorded in Fucus communities. Measurements of primary production and respiration of slabs assemblages were carried out every two to five months during emersion periods and were also compared with metabolism of Fucus communities. Succession was highly variable during this survey, and some slabs remained uncolonized by Fucus species. For each tidal level, and regarding the slabs where Fucus settled, the structure of assemblages developed toward Fucus communities but remained still different after 42 months. In contrast, primary production and respiration of slabs assemblages were similar to those of Fucus communities after approximately 24 months, indicating that, due to rapid growth of Fucus, these functions were rapidly filled in. Interestingly, emersion gradient had a weak influence on successional sequence once Fucus is settled, even if in the lowest tidal level F. vesiculosus fulfilled the role of F. serratus.
Keywords: Emersion gradient; Fucus serratus; Fucus vesiculosus; Gross primary production; Recruitment; Succession.
Acknowledgements: The authors thank all the students of the EFEB team, as well as R. Garnier for their help on the field. We also thank S. Coelho for her advices about induction of gamete release for Fucus species. This work benefited from the support of the Brittany Regional Council and the French National Research Agency through the Investments for the Future program IDEALG ANR-10-BTBR.
Chap. 4 – Métabolisme et structure des communautés durant la succession
Introduction
Communities dominated by canopy-forming brown algae (Phaeophyceae) established on intertidal rocky shores are widely distributed around the world (Lüning 1990, Neiva et al. 2016). These communities are of critical importance for coastal areas, as they contribute substantially to nutrient cycling and production of organic matter through photosynthesis (e.g. Tait and Schiel 2010; Tait et al. 2015). This organic matter benefits to a large variety of species inhabiting these communities permanently (Riera et al. 2009) or temporarily (Silva et al. 2010), as well as to other coastal ecosystems (Crawley et al. 2009), through the export of detritus by water movement (Duarte & Cebrian 1996).
Intertidal rocky shores are regularly subjected to disturbances resulting from both physical and biological processes, either natural or man-induced, that may affect community functioning (Sousa 1984). These disturbances, which take part in the large temporal and spatial variability that characterizes rocky shores (e.g. Dethier 1984; Benedetti-Cecchi 2000), can release new spaces in these habitats, depending on their magnitude. Within these bare spaces, successional sequences occur and usually consist in a transition from a community made of a few opportunistic and ephemeral species, tolerant to harsh environmental conditions but characterized by a low diversity in functional traits, to a more complex community, made of numerous species with a more diverse and complementary pool of functional traits (Noël et al. 2009). This process is, however, highly variable in time and space (Jenkins et al. 2005). In particular, successional sequences are expected to vary according to the vertical zonation, which is known as a fundamental driver of community structure and species distribution (Underwood 1980, Raffaelli & Hawkins 1999). According to shore height, species interactions (Bertness et al. 1999) and primary production of benthic assemblages (Migné et al. 2015a) may also greatly vary, influencing therefore their dynamics.
A mosaic of patches of different successional ages may be commonly observed on rocky shores, as a consequence of series of disturbances. These patches, which can strongly differ in species composition (Dethier 1984), may therefore exhibit large discrepancies in term of community functioning, and especially in productivity. Such changes in primary production over successional sequences have rarely been addressed on rocky shores (Martins et al. 2007), while succession has been the focus of intensive research during the past decades (e.g. Connell and Slatyer 1977; Lubchenco 1983; McCook and Chapman 1993; Benedetti-Cecchi 2000). Yet, the
Chap. 4 – Métabolisme et structure des communautés durant la succession
short life expectancy of the majority of organisms involved in the successional sequence of intertidal communities makes them an ideal place to investigate such issues.
Thus, the present study associates diversity and primary production measurements over successional sequences. It was conducted on two widespread intertidal communities dominated by canopy-forming Phaeophyceae, Fucus vesiculosus Linnaeus and F. serratus Linnaeus, respectively. These communities, which are adjacent in the vertical pattern of rocky shores, usually consist of dense and complex assemblages mainly composed of Fucus spp. associated with epibionts, understory and encrusting algae, as well as invertebrates. The successional sequences were investigated in these communities during a period of three years and a half, corresponding approximately to the life span of both F. vesiculosus and F. serratus (Fish & Fish 1989). During this period, we tested the hypotheses that (1) primary production varies over successional sequences according to the identity of settled species, and that (2) both taxonomic diversity and primary production are comparable to those of established communities of the same areas at the end of the survey. Then, we hypothesized that (3) the timing of successional sequences differs according to the shore height, being faster in low shore than in high shore, due to more favorable environmental conditions.