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The extended concept of littoral active zone considering
soft sediment shores as social-ecological systems, and an
application to Brittany (North-Western France)
Lucia Fanini, Christophe Piscart, Enzo Pranzini, Christian Kerbiriou, Isabelle
Le Viol, Julien Pétillon
To cite this version:
Lucia Fanini, Christophe Piscart, Enzo Pranzini, Christian Kerbiriou, Isabelle Le Viol, et al.. The
extended concept of littoral active zone considering soft sediment shores as social-ecological systems,
and an application to Brittany (North-Western France). Estuarine, Coastal and Shelf Science, Elsevier,
2021, 250, pp.107148. �10.1016/j.ecss.2020.107148�. �hal-03102669�
1
Title: 1
The extended concept of Littoral Active Zone considering soft sediment shores as social-ecological 2
systems, and an application to Brittany (North-Western France) 3
4
Authors: 5
Lucia Fanini
1*
, Christophe Piscart
2,3
, Enzo Pranzini
4
, Christian Kerbiriou
5
, Isabelle Le Viol
5,
, Julien 6
Pétillon
2
7
8
1
Hellenic Centre for Marine Research, Institute of Marine Biology, Biotechnology and Aquaculture 9
(IMBBC), Heraklion, Greece 10
2
Université de Rennes, UMR CNRS 6553 ECOBIO, F-35000, Rennes, France 11
3
CNRS,LTSER "Zone Atelier Armorique,", Rennes, France, 12
4
Dipartimento di Scienze della Terra, Università di Firenze, 50121 Firenze, Italy 13
5
Centre d'Ecologie et des Sciences de la Conservation (CESCO), Muséum, national d'Histoire 14
naturelle (MNHN), Centre National de la Recherche Scientifique (CNRS), Sorbonne Université, 15
station marine, Concarneau, France 16
*Corresponding author. Email : lucia@hcmr.gr 17
18
Abstract: 19
This paper applies and extends the concept of Littoral Active Zone (LAZ) of sandy beaches as a 20
relevant dimension to observe and manage this socio ecological system.The LAZ is a dynamic zone 21
where exchanges across land and sea occur, and this concept was initially proposed on a 22
geomorphological background only. However, to achieve full relevance and timely address 23
management choices tied to the functioning of soft sediment shores, it became appropriate to 24
extend its consideration to both the ecological and social templates co-existing on the same 25
physical unit. Current paradigms around the concept of LAZ were used as a background to 26
organize information from different disciplines and extend the concept to different soft sediment 27
shores (mostly sandy beaches and salt marshes), towards a global, integrated relevance of the 28
concept. 29
As a test for this approach, we 1) structured information on the biophysical and social templates 30
around the concept of LAZ, 2) extended it to salt marshes and 3) applied it to the case of the 31
Breton coast for a practical test on information organization. Such an exercise highlighted key 32
characteristics of the LAZ using a multidisciplinary approach, but also gaps to be filled when 33
Accepted Manuscript
2
targeting research, perception, communication, sustainable use and management of the LAZ as a 34
functional unit. 35
36
1.Study approach 37
The background information (paragraphs 2-5) was structured by gathering current paradigms and 38
definitions around soft sediment –when concepts are still debated, versions presented are backed 39
by references and related empirical data. The call for extension of the concept of sandy beach, 40
opening options for landscape scale considerations, is here answered by considering salt marshes 41
(motivations detailed in paragraph 6), hence the use of “soft sediment shores”. Human-modified 42
beaches are included in the background (paragraph 7), assuming that this is a most common 43
condition, in a context where sandy shores are affected by human-driven phenomena –though to 44
a different extent- at global scale. The social template of the Littoral Active Zone is described on 45
such background, and defined physically in space and time to reach the depiction of its boundaries 46
(paragraph 8). We finally provided detailed information for the case of Brittany (paragraph 9), 47
utilizing the standard structure of a social-ecological system as two templates (the social and 48
ecological one), inter-connected by adaptive management and ecosystem services. To this aim, 49
information comprehends not only scientific papers but also grey literature and references to 50
ongoing projects. Conclusions are an open invitation to consider the LAZ and its diverse 51
components as the proper dimension to approach and manage systems connecting land and sea. 52
53
2. Current paradigms on sandy shores: patterns and processes across the Littoral Active Zone. 54
Sandy shores are ecotonal systems connecting land and sea. Their definition is slightly different 55
depending on the discipline considered. For instance, in geomorphology the “beach” goes from 56
the depth of closure to the upper limit of the runup under extreme waves, and the dune is 57
excluded from the sandy shore definition sensu stricto (Komar, 1989).In ecology, sandy shores 58
consist of three entities along the land-sea axis: surf zones, beaches and dunes. Each one of them 59
is characterized by distinct organism assemblages, tightly related to the physical constraints of the 60
microhabitats they inhabit (McLachlan and Defeo 2017 and references within). A unifying concept 61
is that the different zones along the land-sea continuum are connected by a constant exchange of 62
energy and material, including boulders, pebbles, gravel, sand or silt (see Wentworth, 1922 for 63
sediment size categories).Hence the concept of Littoral Active Zone (LAZ) (Tinley, 1985) was 64
proposed to define a soft-sediment coastal system, with boundaries identified by the effects of 65
the waves (seaward) and aeolian (landward) transportation of sediment. Sediment is in fact being 66
Accepted Manuscript
3
exchanged across the geomorphic system of the LAZ, in a dynamic process. This led to the use of 67
sediment budget (Komar, 1989) for sandy shores management purposes: the concept of LAZ 68
referred to the littoral geomorphology and proposed the management of a unit including both 69
marine beach/surf zone and terrestrial dune ecosystems. However, such a conceptual framework 70
has not been fully applied to the biotic component of the system, where multi-level exchanges (of 71
energy, material and nutrients) occur along the same unit (after McLachlan and Defeo, 2017): 72
• Primary producers (surf phytoplankton, seagrasses, benthic microflora, pioneer and dune 73
vegetation); 74
• Primary consumers (dominated by bacteria, peracaridans, annelids, mollusks, insects); 75
• Predators (dominated by birds, coleopterans, spiders, crabs) 76
• “Temporary” i.e. only spending within the LAZ a key stage of their life-cycle: planktonic 77
larvae and juvenile stages; nesting or migrating birds; nesting turtles. 78
While research on sandy shores provided a solid background with respect to morphodynamics, 79
current knowledge related to biotic components indicates that they are 1) mostly physically 80
driven and 2) generally known in terms of patterns, but with a gap in knowledge concerning 81
the processes shaping the patterns measured (Defeo and McLachlan 2013). 82
83
3. Across-shore dimension. 84
Along the LAZ, at a dimension defined as “mesoscale” in ecology of sandy shores (i.e. “within 85
beaches”), both water-breathing and air-breathing organisms can be found, following zonations 86
which fluctuate in space and time linked to habitat availability (McLachlan and Jaramillo, 1995). 87
Indeed, the beach is the one entity of the LAZ in which both water-breathers and air-breathers co-88
occur (Defeo and McLachlan, 2005), while surf zones are exclusively inhabited by water-breathers 89
and dunes by air-breathers. Dugan (2013) proposed the concept of “envelopes” for the beach 90
entity, i.e. spatial units affected by temporal units varying daily-to-yearly. This explained 91
macrofaunal patterns better than the mere “beach width” variable. Scapini et al. (2019) remarked 92
on the relevance of behavior in the macrofauna zonation patterns, with plasticity as the main 93
adaptation to recover and maintain a suitable habitat. By extending the analysis to the whole LAZ, 94
the characteristic mobility of substrate is the main biotic driver that shapes microhabitats and to 95
which the fauna adjusts their behavioural responses. 96
97
4. Along-shore dimension. 98
Accepted Manuscript
4
Each LAZ across-shore relates to an along-shore unit (perpendicular to the land-sea axis), which 99
can be called the physiographic unit, sedimentary cell, or beach units. Along-shore dimension is 100
defined by natural headlands or man-made structures overpassing the depth of closure, i.e. the 101
depth to which the cross-shore sedimentary exchange is insignificant and which is identified 102
where bathymetric profiles repeated over time are connected (closed) (Hallermaier, 1977). By 103
considering the dune as a part of the Active Zone, units can also overpass headlands (e.g. locations 104
in which transgressive dunes have climbed escarpments and passed the headlands, Pye and Blott, 105
2020). Embayments and pocket beaches, with boundaries generating turbulence and rip currents 106
at their edges, are easy to identify as units. Within units however, as e.g. in the case of extended 107
beaches, exposure and longshore currents may affect the sand mobility (Komar, 1989); hence a 108
beach can hence include a continuum from deposition to erosion, confusing the identification of 109
units along-shore (Carter, 1988). Also mixed conditions such as sparse rocks present on sand 110
substrates, affect the flow of energy and material within units. Dynamics become more relevant in 111
the case of wrack inputs ashore, driving the spatio-temporal patterns of stranding. In ecology, 112
given the large amount of studies focusing on sand-only conditions, conditions of periodical inputs 113
of stranded material are often overlooked. However, they are important in terms of dynamics 114
backing the biotic patterns measured ashore. In general, beach fauna patterns present land-like 115
characteristics (McLachlan and Defeo, 2017 but also see Defeo and McLachlan, 2011), with single 116
beaches (“ecological mesoscale”) remaining an essential unit for analysis. 117
118
In summary, the paradigm of sandy beaches as physically-controlled environments, when applied 119
to the mobility of the substrate as LAZ characteristic, highlights the following features as keys for 120
resident biota: 121
1) Pore space for the development of rich interstitial fauna dominated by bacteria, 122
protozoans and meiofauna. The interstitial system processes organic materials flushed into 123
the sand and returning the inorganic nutrients to the sea (Pearse et al., 1942). 124
2) Incohesive substrate as a driver of burrowing mechanisms of macrofauna. “Dealing with 125
substrate” defines essential traits of organisms inhabiting the surf zone (Gibson et al., 126
2006). On the supralittoral area, substrate and non-substrate modifiers can be found, 127
whose presence largely depends on substrate granulometry and related water retention, 128
as well as on allochthonous inputs (e.g. seagrasses mats) (Pérès and Picard, 1958, Sassa 129
and Yang, 2019). 130
Accepted Manuscript
