The role of marine heatwaves for biodiversity and ecosystem functioning in coastal waters

The global warming trend is accompanied by unprecedented climate extremes drastically affecting marine ecosystems. Prolonged periods of abnormally warm water in the oceans, known as marine heatwaves (MHWs), have intensified over the past decades and are projected to increase in intensity, duration and frequency until the end of the century. Yet, also current MHW events threaten biodiversity with cascading consequences for ecosystem functioning, structure and services across the globe. The impacts are disproportionally devastating when highly productive coastal areas are affected. Given the crucial importance of coastal areas, the significant knowledge gaps and uncertainties linking cause and effects of extreme temperatures in coastal areas are of concern. These gaps result from insufficient or inconsistent monitoring of these highly dynamic areas and that insights have primarily been gained through field observations or laboratory experimentation. There has further been a focus on structural community changes caused by exceeding thermal tolerances, while functional changes have received much less attention. This thesis addresses these gaps by investigating the effects of MHWs on ecosystem functioning of soft-sediment habitats, which are vital habitats for various ecosystem functions, in the coastal areas of the Baltic Sea. Moreover, the Baltic Sea is also one of the fastest warming seas globally. To evaluate normal and extreme temperature dynamics in a coastal area of southern Finland, and to overcome the limitations of commonly used satellite data products, an in situ time series of over 90 years (every 10 days) and 4 years of high-resolution (every 30 minutes) temperature data were combined. The surface (0-2 m) and sub-surface (about 30 m) layer of the time series data were used (1) to compare and assess the effect when using variable 30-year reference periods, (2) for trend analyses and (3) investigating historic extreme temperatures. The high-resolution data were used to analyse the characteristics of sub-surface MHWs. The results of this assessment served as a baseline to experimentally investigate the effects of realistic, current MHW events on shallow and deeper soft-sediment habitats, as these were demonstrated to have distinctly different climatologies. The seasonal effects of MHWs on macrofaunal community structure and ecosystem functioning, in terms of community metabolism and nutrient cycling and linked macrofaunal behaviour, were investigated by collecting sediment cores in winter, spring, summer and autumn from a deeper, aphotic habitat and exposing them to season-specific MHWs in a laboratory setting. While laboratory experiments are valuable for mechanistic insights, I then also developed a system capable of inducing MHWs in situ to facilitate a better understanding of the realistic effects of extreme temperatures in a natural, complex environment. The surface and sub-surface waters at the southern coast of Finland have drastically warmed since 1927. The warming is especially obvious in the marked increase of abnormally warm events since the 1990’s for both surface and sub-surface layers, based on the reference period 1931-2020. Between 2010-2020, the month with the most warm extremes was September in the surface layer and December for the sub-surface layer, whereas in general the highest increase of warm extremes was observed in the winter months. Based on the high-resolution data, 18 MHWs across seasons with varying intensities and duration were detected in 4 years of monitoring. Overall, the warming trend has caused an upward shift of the climatological baseline, causing nearly every day of the year in the reference period 1991-2020 to be about 1 °C warmer than the respective day in the reference period 1931-1960 or 1961-1990. The experimentally induced (in situ and in the laboratory) strong MHWs did not cause major changes to the community composition across seasons and habitats. However, indications of shifts to smaller organisms during MHWs were detected and behavioural responses during absolute high temperatures, rather than seasonally high temperatures, were observed that could be related to increased burrow maintenance or thermal refuge in deeper sediment layers. MHWs distinctively increased community respiration, rather than oxygen production in the shallow habitat, and boosted nutrient fluxes throughout nearly all seasons and habitats, whereas the effects were season- and habitat-specific. Overall, these complementary studies underscore the critical need for high-quality in situ monitoring to accurately assess the occurrence and impacts of MHWs on coastal ecosystems. The combined approach of extensive monitoring and various experimental techniques indicates that while the benthic macrofaunal communities seem fairly robust, the ecosystem functioning of the entire community can severely change when the ecosystem is experiencing an MHW. Yet, the impact is variable across seasons and habitats, highlighting the importance of considering the highly temporally and spatially heterogenous and complex coastal areas.