Pulsed pressure: Fluctuating impacts of multifactorial environmental change on a temperate macroalgal community

Abstract

Global change impacts marine organisms and communities mainly through ocean warming, acidification, deoxygenation, and changes in nutrient inputs and water circulation. To assess the ecological impacts of global change, the effects of multiple interacting environmental drivers, including their fluctuations, should be tested at different levels of biological organization. In an outdoor mesocosm study, we investigated the differential effects of three simulated upwelling events coupled with ocean warming (1-5 C above ambient) on a temperate benthic community in the Western Baltic Sea. Ocean warming, especially in summer when temperatures are close to or above the physiological optimum of many species, is likely to impose thermal stress with species-specific impacts. As the properties of deep water vary seasonally, so will the effects of upwelling. Upwelling of cooler deep water in midsummer may alleviate thermal stress, although this mitigation may be modulated by upwelling-associated shifts in other water-quality parameters such as salinity, nutrients, or late-summer hypoxia. This investigation showed that in the Western Baltic Ocean warming was rather beneficial in early and late summer but detrimental when ambient temperatures were highest in midsummer. The effects of upwelling in the absence of ocean warming were generally weakly beneficial, while this effect tended to vanish with intensifying imposed ocean warming. Hypoxia associated with the late summer upwelling impacted some of the grazer species but did not impact the macroalgae. We conclude that in coastal temperate benthic communities, ocean warming is the predominant stressor that may partially and seasonally be buffered by upwelling.

The Role of Recovery Phases in Mitigating the Negative Impacts of Marine Heatwaves on the Sea Star Asterias rubens

“…Naturally, an interruption of a heatwave is most likely caused by a cold-spell during an upwelling event at which deeper and colder waters are shoaled to the surface (Lehmann and Myrberg, 2008;Wahl et al, 2021). Though, upwelling events in late summer are often hypoxic (Karstensen et al, 2014) and were shown to negatively impact A. rubens (Rühmkorff et al, unpublished data), they can still be beneficial when they interrupt lethal temperature extremes during heatwave events.…”

Role of hydrodynamics in shaping chemical habitats and modulating the responses of coastal benthic systems to ocean global change

“…Especially for perennial species, more attention should be paid to seasonal variations because both environmental baselines (i.e. means) and their distance to specific optima change with seasons (Ricart, Ward, et al, 2021; Wahl et al, 2021). These measurements require new technical developments in autonomous loggers and infrastructure (Pansch & Hiebenthal, 2019), as well as publication of best practice guides to standardize procedures (Lorenzoni et al, 2017).…”

“…We highlight (iii) the importance of these specific chemical habitats in providing refugia against ocean acidification and deoxygenation but also (iv) point out how they can facilitate the resistance of populations or decrease species' fitness, depending on species’ capacities to respond to large abiotic fluctuations. We deliberately do not address in this review indirect hydrodynamic effects on shifts in species’ interactions and interactive impacts of multiple drivers fluctuating asynchronically (see Wahl et al, 2021 for instance).…”

“…Together, biological, geochemical and physical processes govern the complex regulation of local physicochemical parameters at small temporal and spatial scales to produce ‘ocean weather’ that often masks ocean global change trends (Bates et al, 2018; Borges & Gypens, 2010; Chan et al, 2017; Duarte et al, 2013; Johnson et al, 2013; Waldbusser & Salisbury, 2013). Predicting how marine coastal ecosystems will be affected by ocean global changes in the future is, therefore, challenging, in particular because spatially distinct coastal zones may experience different ocean weather conditions (Ghedini et al, 2015; Provost et al, 2017; Wahl et al, 2018, 2021).…”

Global climate change and the Baltic Sea ecosystem: direct and indirect effects on species, communities and ecosystem functioning

“…The timing of temperature stress is however important for the damage experienced by algae. Experiments done with bladderwrack Fucus vesiculosus (in 1500 L tanks in Kiel, in the western Baltic Sea) showed that in the early and late summer warming of 1 to 5 • C above ambient had mostly beneficial effects on bladderwrack, whereas in midsummer a similar warming had negative effects (Wahl et al, 2021). During midsummer, the heatwaves surpassed the physiological tolerance limits of the species, with detrimental effects on growth.…”