Effects of detached seaweed and predation on structure and function of polar soft-bottom communities

Abstract

Polar regions are projected to experience faster and stronger impacts of climate change than the rest of the planet. It is crucial to investigate how climate change drivers may affect polar ecosystems, potentially causing pressures on biodiversity and ecosystem functioning. Specifically, seaweed is considered the primary beneficiaries of the loss of sea ice in polar seas, as some species have increased in bathymetric extent, density and biomass. However, the loss of protective sea ice and increased storm frequency has increased detached seaweed in polar coastal systems. Thus, detached seaweed could represent a connecting vector between rocky and sedimentary habitats in polar regions. Nevertheless, research on the effects of detached seaweed on benthic communities is still limited. Some studies suggest that detached seaweed may provide a food supplement, modify biotic interactions and disturb the benthos by mechanical action on the sedimentary substrate. This thesis aims to investigate the mechanisms by which biotic drivers modify the structure and functioning of coastal sedimentary communities at the poles. In Publication I, I investigated the effects of detached seaweed on the structural and functional traits of species assemblages in soft-bottom intertidal habitats. For this, manipulative experiments based on simulation of detached seaweed patches were conducted for comparative purposes between intertidal sites in the Arctic. I found that patches with seaweed compared to unmanipulated areas reduced the abundance of individuals at both sites and the biomass at one of the intertidal sites. In addition, detached seaweed strongly modified the structure and functioning of the soft-bottom species assemblages at both intertidal sites. Thus, I suggest that the detected effects are possibly the result of seaweed-driven changes in environmental conditions and physical disturbances as underlying processes. Publication II assessed the magnitude and direction of the effects of predation on the diversity and structure of Arctic benthic communities. The effects of predation on the benthic communities studied were contrasted with the predictions of the environmental stress model (ESM), which predicts that predation as a community regulating factor decreases at higher levels of environmental stress. To this end, a caging experiment was conducted in which consumers were excluded from plots in two intertidal sedimentary sites in the Arctic. Unmanipulated areas were used as treatment controls, and partial cages were used to estimate cage artefacts. In both sites, predator exclusion changed the species composition of the communities but did not affect univariate ecological responses. This study suggests that consumers have limited effects on the structure and functioning of Arctic intertidal communities, thus providing empirical support for ESM under high environmental stress. In Manuscript II. Detached seaweed accumulated on the seabed could provide food and shelter for epibenthic predators, increasing the density of consumers and indirectly causing more predation pressure on the benthic community. Manipulative field experiments were conducted to assess the combined effect of detached seaweed and predation on the dynamics and structure of the subtidal soft-bottom benthic community at the poles. Seaweed plots, predator exclusion cages and a combination of seaweed patches and exclusion cages were installed at these sites. In the Arctic, I discovered that seaweed patches with access to predators and seaweed patches without access to predators reduced species diversity and abundance of individuals compared to unmanipulated areas. Also, seaweed plots with access to predators in Antarctica and seaweed plots without predators decreased species richness compared to unmanaged areas. Furthermore, the results indicate that predation seems to play a minor role in subtidal soft-bottom benthic communities. Based on the results, I suggest that detached seaweed themselves affect the dynamics and structure of soft-bottom benthic communities at both poles. Overall, the results highlight the importance of research on the effects of biotic drivers on coastal marine ecosystems and of using manipulative field experiments to move towards predictive ecology, even more so in polar regions highly affected by global warming.