Below, above and beyond – seagrass ecosystem functions in a connected coastal landscape

Sammanfattning: Seagrass meadows constitute important habitat for a plethora of organisms, rendering them important habitats for conserving biodiversity. Seagrass meadows also modify the environment by attenuating wave and current energy. As a result of this, seagrass meadows trap organic matter and in addition to their high productivity, are able to sequester carbon that is stored in their underlying sediments. Due to these ecosystem functions, seagrass meadows are proposed as potential nature-based solutions (NbS) for climate change adaptation and mitigation that concomitantly strengthens local biodiversity. However, the magnitude and relative importance of these ecosystem functions are context-dependent, and it is therefore crucial to understand how and why they differ in space and time. Moreover, it is imperative to apply a broad perspective as these functions can affect and are affected by adjacent ecosystems beyond the seagrass meadow itself. In this thesis, I focus on eelgrass (Zostera marina) and its role in coastal biodiversity and carbon cycling across both natural and restored meadows as well as eelgrass exported onto land. I follow a restoration project in an exposed and a sheltered environment and leverage these field trials to try to understand the timescales of biodiversity effects following seagrass restoration. I focus on fauna diversity and investigate colonization patterns. I relate these features to ecosystem functioning by measuring community metabolism and carbon cycling across restored and natural eelgrass meadows. My findings reveal that benthic fauna rapidly recovers following restoration and attains diversity levels comparable to natural reference meadows. Initial colonization seems to be driven by a few, opportunistic species that occur in large densities during early stages. As these species are generalists, their elevated abundance can increase the functional redundancy and resilience of fauna communities during early restoration stages. I show that eelgrass habitat provisioning is important in both natural and restored eelgrass and across land-sea boundaries. Exported eelgrass biomass that ends up on beaches supports a high abundance of terrestrial fauna. However, vascular plant diversity may instead benefit from removal of eelgrass wrack and it is therefore important to apply a differentiated management approach to sandy beaches that considers different communities and interests.While eelgrass restoration is unequivocally beneficial for benthic diversity, net effects on carbon fluxes vary. I found a positive relationship between benthic diversity and fluxes of dissolved inorganic carbon going out of the meadow, highlighting potential tradeoffs between biodiversity and climate change mitigation. Collectively, the findings in this thesis contribute to a better understanding of how seagrass fauna diversity varies in space and time and what can be expected from restoring seagrass meadows. It reveals novel insights into the balance between diversity and community metabolism and shows how exported eelgrass can have dichotomous effects depending on the community of interest. This knowledge can inform coastal managers on relevant aspects to consider when implementing coastal adaptation measures and how eelgrass can be utilized below and above the waterline.