Potential and realized connectivity of the seagrass Posidonia oceanica and their implication for conservation

Abstract

Aim: Connectivity assessments are crucial to large-scale conservation planning, in particular for establishing and monitoring connected networks of marine protected areas (MPAs). Using biophysical modelling and genetic analyses, we assessed potential and realized connectivity among MPA populations of a benthic foundation species, the Mediterranean endemic seagrass Posidonia oceanica. Location: Adriatic and Ionian seas (central Mediterranean). Methods:We assessed potential and realized connectivity among eight P. oceanica meadows, mostly located in MPAs. Potential connectivity was assessed over a time horizon of 10 years via an individual-based biophysical model whose physical component relies on fine-scale spatio-temporal ocean circulation fields. Genetic assessments of realized connectivity were carried out by means of a set of 14 neutral microsatellite loci, as well as a larger dataset of 19 loci including outlier loci that did not conform to expectations under neutrality. Results: Our findings point out a relatively high potential connectivity through longrange dispersal of floating fruits. Genetic connectivity analyses show a complex scenario with an apparent lower realized connectivity. The P. oceanica meadow within Torre Guaceto MPA (TOG), a well-enforced MPA within our study area, showed one of the highest levels of genotypic richness, indicative of high levels of sexual reproduction and/or recruitment of foreign genotypes. Both biophysical modelling and population genetics indicate that TOG is important to ensure the viability of the species at the local scale, and does likely play a key role as a source of propagules for the whole Adriatic area. Main conclusions: Our results show that realized dispersal does not necessarily match with the potential for dispersal. Still, both genetic and physical connectivity analyses show good agreement in identifying hotspots of connectivity. Such information can guide management of networks of MPAs and advance conservation of marine biodiversity. K E Y W O R D S dispersal, genetic connectivity, Lagrangian, marine protected areas, propagules, seagrass

Dramatic loss of seagrass habitat under projected climate change in the Mediterranean Sea

“…The unsuitable environmental conditions in the Atlantic, along with the oceanographic barrier that the Strait of Gibraltar entails for dispersal between the Atlantic and the Mediterranean Sea for many species (see Burrows et al, ; Patarnello, Volckaert, & Castilho, ), particularly those with buoyant propagules such as Posidonia oceanica , imply that the expansion of this seagrass species outside the Mediterranean will be unlikely. In addition, the realized connectivity among P. oceanica meadows is lower than its potential estimations (Jahnke et al, ). Thus, its endemic condition in the Mediterranean, experiencing faster warming rate than the rest of oceans (Coll et al, ; Jordà et al, ; Templado, ), renders P. oceanica extremely vulnerable to ocean warming, with a substantial extinction risk.…”

“…According to all future scenarios, Cymodocea nodosa would decrease its extension in the two Mediterranean genetic regions, eastern (EM), and western (WM; Figures 2 and 3 between the Atlantic and the Mediterranean Sea for many species (see Burrows et al, 2011;Patarnello, Volckaert, & Castilho, 2007), particularly those with buoyant propagules such as Posidonia oceanica, imply that the expansion of this seagrass species outside the Mediterranean will be unlikely. In addition, the realized connectivity among P. oceanica meadows is lower than its potential estimations (Jahnke et al, 2017). Thus, its endemic condition in the Mediterranean, experiencing faster warming rate than the rest of oceans (Coll et al, 2010;Jordà et al, 2012;Templado, 2014) the disappearance of the current genetically rich populations of P. oceanica.…”

Predicting the cumulative effect of multiple disturbances on seagrass connectivity

“…However, genetic diversity‐based estimates of connectivity reflect the combined outcome of what will potentially be multiple recruitment events over the life of the existing meadow. The potential for dispersal is often predicted to be larger than realised dispersal in marine ecosystems (Burgess et al., ; Jahnke et al., ). In our study, this could translate to an overprediction of connectivity for meadows that are far apart.…”

“…However, the longer lived nature of seagrass propagules (viable plant fragments, fruit or spathes) relative to other marine organisms, such as coral larvae (Thomas et al., ), supports our predictions of dense habitat graphs with long connections. The reasons for discrepancies between estimates of potential functional connectivity and realised connectivity are postdispersal, presettlement and postsettlement processes (Jahnke et al., ). More information on these processes would improve the performance of connectivity studies that rely on biophysical models to measure dispersal and settlement.…”

“…Seagrass fruits and propagules of most species are capable of dispersing long distances via the convective forces of ocean waves and currents (Kendrick et al., ; McMahon et al., ). Long distance dispersal supports meadow connectivity and the natural recolonisation of sites after disturbance events (Kendrick et al., ); however, very few studies have attempted to quantify seagrass dispersal and connectivity at broad spatial scales (Jahnke et al., ; Ruiz‐Montoya, Lowe, & Kendrick, ). Tropical seagrasses occur in some of the world's most threatened coastal regions (Orth et al., ; Waycott et al., ), and there is a critical need to assess the role of connectivity in seagrass replenishment and recovery after disturbance events, the effect of disturbance events on seagrass connectivity and its implications for the conservation of coastal habitats (York et al., ).…”