Combined effects of fragmentation and herbivory on Posidonia oceanica seagrass ecosystems

Abstract

Summary1. Fragmentation is a major agent for seagrass meadow decline, yet little is known about how it interacts with processes like herbivory, an important functional driver of seagrass meadows. The interaction with external stressors like fragmentation could exacerbate the effects of internal ecosystem drivers like herbivory, with distinct implications for ecosystem management. 2. We used manipulative field experiments to assess these interactive effects in two Posidonia oceanica seagrass meadows. We monitored replicated plots in small and large patches in two meadows suffering fragmentation with and without herbivores (using exclusion cages) to test whether fragment size and herbivory could act together to alter ecosystem functioning. We measured changes in defoliation rates, primary production, canopy height and nutrient content in all plots after 4 months of herbivore exclusion. 3. Our results show that herbivores increased defoliation rates resulting in reduced primary production, nutrient content and canopy structure (canopy height). Patch size (fragment) on its own also reduced primary production, nutrient content and canopy structure. We also observed significant additive interactions between herbivores and fragmentation on canopy structure and production responses. In addition, small patches showed nutrient limitation but were able to accumulate more carbohydrate reserves, probably due to a higher light availability. This may explain why small patches can persist under significant herbivore pressure. 4. Synthesis. While fragmentation has already been identified as an important external agent of seagrass decline, the combination of fragmentation and herbivory can seriously exacerbate structural losses and affect primary production, profoundly compromising the role of seagrasses as habitatforming ecosystems. These interactions between external stressors and internal drivers may result in large unexpected consequences that may flow on to the rest of the ecosystem.

Natural patches in Posidonia oceanica meadows: the seasonal biogeochemical pore water characteristics of two edge types

“…A clear disparity also occurs between the eroded and colonized edge with both a seasonal and bathymetrical variation of leaf biomass with higher disparities at 10 m in June (colonized edge 1415 gDW m −2 ; eroded edge 1133 gDW m −2 ). The the salema Sarpa salpa (Borg et al 2006;Prado et al 2009;Gera et al 2013). In this way, a P. oceanica seascape corresponds to a set of different habitats, i.e.…”

Relating trophic resources to community structure: a predictive index of food availability

“…However, relatively little of this biomass is directly consumed by grazers, owing to various deterrent compounds [38,39]. Only a few herbivores, sometimes reaching high densities, are able to consume the abundant green leaf biomass, most notably sea urchins [40], some isopods [41] and a few fishes [42]. As predicted, modelled food availability does not necessarily correspond to food consumption.…”

Biophysical Interactions in Fragmented Marine Canopies: Fundamental Processes, Consequences, and Upscaling

“…The hydrodynamics of fragmented canopies are more spatially heterogeneous than the hydrodynamics of homogeneous canopies -because of the spatial heterogeneity of the canopies themselves -and this leads to heterogeneity in their stresses and facilitations, e.g., spatial variations in sheltering (e.g., Folkard, 2005) and nutrient supply (e.g., Morris et al, 2008). Fragmentation of canopies also leads to their becoming more vulnerable to external pressures (Gera et al, 2013). Understanding the large-scale impacts these effects have is therefore important for managing the many coastal areas where marine canopies are found, especially those affected by anthropogenic stresses and climate change (El Allaoui et al, 2016).…”

“…Thus, near-field positive effects combined with far-field negative effects create patches (van Wesenbeeck et al, 2008). In other cases, the positive effect of nutrient retention by patches is counterbalanced by reducing light availability for each shoot as patches get larger (Gera et al, 2013).…”