The influence of coastal dynamics on the upper limit of the Posidonia oceanica meadow

Abstract

The recognized ecological importance of Posidonia oceanica, the most important seagrass of the Mediterranean Sea, makes it crucial to assess the state of health of its meadows, discriminating natural from anthropogenic impacts. In this paper, the hydrodynamic conditions at the upper limit of P. oceanica meadows along the Ligurian coast (NW Mediterranean Sea) were investigated. A relationship between the distance of the upper limit of the meadow from the shoreline and the morphodynamic domain of the beach (i.e. distinctive types of beach produced by the topography, wave climate and sediment composition) was found. A zonation of the state of the shallow portions of the meadows down the submerged beach profile was identified. Zone a, from the shoreline to the breaking limit, is naturally critical for the development of the meadow. Zone b, from the breaking limit to the closure depth, is subjected to natural and human impacts. Zone c, below the closure depth, is little influenced by coastal dynamics. This study quantifies for the first time how much the status of the shallow portions of P. oceanica meadows is dependent on coastal dynamics, which is important for their proper management.

Patterns of wide‐scale substitution within meadows of the seagrass Posidonia oceanica in NW Mediterranean Sea: invaders are stronger than natives

“…PI showed a positive correlation with CI in the deep portions of the meadows, but not in the shallow ones. As CI has been described as an indicator of anthropogenic impacts (Moreno et al, 2001), this result may be interpreted as evidence that patchiness at the deeper limit of the meadows is probably due to human causes, while patchiness in the shallower areas may be more due to natural causes, such as substratum heterogeneity and hydrodynamic regime (Wicks et al, 2009;Vacchi et al, 2010).…”

Is Posidonia oceanica regression a general feature in the Mediterranean Sea?

“…(iii) Part of the recorded regressions could be linked to the natural continuous rise in the sea level since 19,000 BP (Laborel et al, 1994;Morhange et al, 1996;Collina-Girard, 2003;Morhange, 2003;Clark et al, 2009). Also, the presence of dead mattes, beyond the lower limit of P. oceanica meadows could, in some cases, be very old and unrelated to anthropogenic pressures (Boudouresque et al, 2009;Vacchi et al, 2010). Thus, although it seems possible to specify the size of the last regression of P. oceanica meadows, considerable uncertainties weigh as to their future, particularly in the light of the changes brought about by climate change (Pergent et al, 2012).…”

Impact of anthropogenically created canopy gaps on wave attenuation in a Posidonia oceanica seagrass meadow

“…oceanica shoot density was measured following Vacchi et al (2010), by counting shoots within a 40 × 40 cm area subdivided into 4 sub-quadrats of 20 × (Alcoverro et al 1995), on 10 vertical shoots collected at each station (see Table 2). Surveys were repeated during each of 5 field campaigns which captured the seasonal variability of the structural characteristics of each station during (1) (5) summer: 1 July 2013.…”

“…found that the anchoring impacts of large boats modified substrate quality, by increasing the hydrogen sulfide concentration, at a month scale. Vacchi et al (2010) demonstrated the poor condition of P. oceanica meadows in the Ligurian Sea; although this has been attributed to anthropogenic impacts, natural factors, especially wave climate were also implicated. Indeed, in the Ligurian Sea there was a higher degree of P. oceanica meadow patchiness in the shallower zones (to about 14 m depth) in humanimpacted areas compared to meadows in natural areas , highlighting a possible interaction between anthropogenic influences, wave climate and meadow health.…”