Conditioning of the seaweed dominated environments by hydrodynamics

Abstract

The development and composition of rocky shores communities are ruled by several abiotic factors including hydrodynamics. An exposure gradient is observed between sheltered shores, where macroalgal canopies are well developed, and exposed shores, where sessile animals (barnacles, mussels) and limpets dominate the substratum. Hydrodynamics include several components such as wave height, orbital velocity, swell or shear stress but also currents with flow velocity. Based either on in situ monitoring or on geographic data, proxies are usually used to assess hydrodynamical profiles, each showing (dis)advantages depending on the spatial and/or temporal scale. We present here a study on the effect of hydrodynamics, evaluated by several proxies, on the extent and structure of macroalgal communities. In that prospect we developed several complementary approaches combining pressure probe measurements, wave height models and geographical proxies. In parallel, we propose a biological sampling method taking into account the macroalgal specific richness, the cover of macroalgae strata, allowing an accurate description of the structure and development of macroalgal communities. The analysis aims at describing which proxy explains the most the variation of these communities. To obtain a larger, latitudinal view of this approach, a collaborative project is in progress in Europe (Galicia, Ireland, Norway). To achieve an overall assessment of hydrodynamical conditioning of seaweed dominated coastal environments, current measurement was initiated in order to compare effects of wave and currents on macroalgal species distribution and cover. Initial results will be shown in the communication.
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Full text

Conditioning of the seaweed dominated
environments by hydrodynamics
Thomas Burela*, Jacques Grallb, Gauthier Schaala, Erwan Ar Galla
a Laboratoire des Sciences de l’Environnement Marin (LEMAR) UMR 6539, Universtié de Brest –Bretagne Occidentale/CNRS/IRD/Ifremer
b Observatoire du Domaine Côtier, UMS 3113
IUEM (Institut Universitaire Européen de la Mer). Rue Dumont d’Urville, 29280 Plouzané, Bretagne, France
*thomas.burel@univ-brest.fr
References
Ar Gall, E. & Le Duff, M. (2014), Estuarine, Coastal and Shelf Science 139, 99-109. Burrows, M.T., Harvey, R., Robb, L. (2008), Marine Ecology Progress Series 353, 1-12. Cefalì, M.E. et al. (2016),
Estuarine, Coastal and Shelf Science 172, 81-92.Guillou, N. & Chapalain, G. (2015). Renewable Energy 83, 942-953. Harley, C.D.G. & Helmuth, B.S.T. (2003). Limnology and Oceanography 48, 1498-
1508. Helmuth, B., Denny, M.W., 2003. Limnology and Oceanography 48, 1338-1345.
Context of the study
An hydrodynamical gradient is observed on rocky shores between sheltered shores, where macroalgal canopies are well
developed, and exposed shores, where fauna covers the substratum. Hydrodynamics include several components such as
wave height but also currents. We present here a study on the effect of hydrodynamics, evaluated by several complementary
in situ or geographic proxies, at different spatial scale, on the extent and structure of macroalgal communities.
Conclusion
According to the spatial scales, adapted tools are needed to apprehend hydrodynamical conditions. Combining different
accurate proxies should lead to a better understanding of both the extension and the structure of macroalgal communities.
Results should lead to the development of a predictive exposure models for macroalgal communities of the N-E Atlantic Ocean.
This work was supported by ISblue project, Interdisciplinary graduate school for the blue planet (ANR-17-EURE-0015) and co-funded by a grant from
the French government under the program "Investissements d'Avenir“. This work was also supported by the Discovery team (LEMAR), and by the French
Phycological Society.
Material and Methods
Undisturbed:
canopy-forming species
Upright profile:
both understory and canopy-
forming species
33 cm
3 ×
in situ recording
of hydrodynamics,
velocity + wave
heights:
Geographical proxies :
4 km
0
Fetch calculation
example
+
macroalgal diversity
+ ecological state
3rd approach : Continent-scale analysis
(3 communities, 4 areas)
Norway
Galicia
Brittany
Ireland
Celtic sea
Norwegian sea
Bay of Biscay
Iberian coast
North sea
400 km
0
Similarity in European
intertidal communities
Study ending in
Summer 2019
- latitude
- tide amplitude
- biogeographic
region
Evaluate large-
scale factors :
1st approach : Site-scale analysis
(6 communities, 3 sites, dist. max ≈ 15km)
2nd approach : Local-scale analysis
(1 community, 4 seasons, 12 sites, dist. max ≈ 100km)
NA
NA
20 km0
Macroalgal ecological state
(Index Ics) :
Poor < 1.00
Average 1.00 –1.20
Good 1.20 –1.40
High > 1.40
in situ wave height
Swan model
wave height
Fetch
Hydrodynamical proxies
(arbitrary units) :
Sites dominated by A. nodosum show a good –high ecological
state, those dominated by F. vesiculosus are poor –average.
Hydrodynamical proxies reveal complementary results.
Ascophyllum nodosum –
Fucus vesiculosus community
Western tip
of Brittany
Good description of wave heights according to topography.
Correspondance between wave heights and the structure of
macroalgal communities (up to 20%of explanation).
Macroalgal ecological state
(index Ics)
in situ wave heights
Upper and Lower limit of the intertidal zone
Low values Intermediate values High values
Porspoder