Browsing by Author "Pohjalainen, Oskari"

Marine heatwaves (MHWs) are distinct periods of elevated water temperature. They have been shown to affect benthic ecosystems and their frequency is expected to increase as a result of climate change. I studied the effects of MHWs on benthic infauna communities across a vegetation gradient to find possible species- and size-specific as well as habitat dependent changes related to the elevated temperatures. This was done via the newly developed Hotfloor system in which benthic chambers are used to heat the sediment in situ. There were 10 heated chambers and 10 control chambers used across the vegetation gradient. For both treatments, 4 chambers were placed in bare sediment, 3 chambers in sparse vegetation and 3 chambers in dense vegetation. In addition to the chambers, I also sampled the benthic fauna in natural start conditions in the same vegetation categories as a baseline. All sampling was done in situ via scuba diving. The results indicate that vegetation cover protects infauna from MHWs as in the bare and sparse categories the decline in abundance and increase in biomass was clearer than in the dense vegetation. The decreasing abundance results from increased mortality and the increasing average biomass implies that the surviving individuals are of larger size. The decrease in abundance of juveniles and smaller individuals was also clear from the size structures of Macoma balthica which is a key species in the benthic ecosystems of the Baltic Sea. Despite the increased mortality, the species compositions remained relatively similar, and the same species dominated in both the heated and control treatments. This means that the ecosystem functioning was retained after the heatwave, even though MHWs did lead to lower abundances and thus less resilient ecosystems. Further research on the long-term impacts of MHWs is needed to fully understand how these ecosystems can change in the future when MHWs are predicted to become even more severe and frequent.