Surfing the heatwave: Understanding the influence of marine heatwaves in driving community dynamics and species performance in the North Sea

As a consequence of anthropogenic climate change, the frequency, duration and magnitude of extreme weather events are increasing and expected to rise further in the coming decades. Among them, marine heatwaves (MHWs) are driving significant ecological changes worldwide, including mass mortalities, shifts in species distributions, and biodiversity loss. Growing concerns about MHW impacts have led to intensified research efforts to define, categorize, and assess their consequences across different levels of biological organisation. Despite this increasing attention, key knowledge gaps remain. The unpredictable nature of MHWs makes it challenging to quantify their effects on marine communities. Moreover, most studies on climate change focus on rising mean temperatures, often overlooking biological responses to thermal fluctuations, which can differ from those under constant conditions. Research has also largely concentrated on coral reefs and fish, with less attention to plankton, a key component of marine food webs. This thesis quantifies MHW effects on individuals, populations and communities using zooplankton as model organisms. It consists of two main blocks: (1) analysing historical time series data to assess MHW impacts on mesozooplankton in the North Sea and (2) conducting laboratory experiments to evaluate how present and future MHWs affect the survival, growth, and phenology of meroplankton larvae. Findings reveal that MHW impacts on zooplankton communities vary by season, emphasizing the need for season-specific studies. Copepods, a key zooplankton group, are unable to adjust their phenology in response to MHWs occurring before their bloom. The lack of a phenological shift, combined with rising MHW days and temperature increase rates, can lead to faster and more or less intense blooms, with potential repercussions for higher trophic levels. The thesis also shows that MHW effects on species performance differ from those of general warming, as MHW components interact in complex ways. Species resilient to warmer temperature or invasive species may benefit from MHWs, often at the expense of native species. As MHWs continue to grow stronger, more frequent and longer, understanding the link between their components and their ecological and socio-economic consequences will become increasingly crucial.