Abstract
AbstractGlobal 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.
Highlights
Experimental set-up and design The experiment on community-level impacts of warming and intermittent upwelling events ran from the beginning of May to mid-September 2018 in the Kiel Outdoor Benthocosms (KOB) facility
The three ecologically most relevant abiotic parameters, temperature, salinity and oxygen, fluctuate in time and may reach stressful levels synchronously or sequentially. This leads to a stress regime that we have termed “pulsed pressure.”
Whether a stress is considered pulse or pressure depends on its duration relative to an organism’s lifespan
Summary
Ecological realism requires considering the combined effects of numerous baseline-shifting drivers These can be of mechanical, physical, chemical or biological nature. The most relevant drivers for coastal marine organisms may be the predicted changes in temperature, current and wave regimes, stratification, nutrient loads, oxygen levels, salinity, fishing, constructions, and chemical, noise and light pollution (Bindoff et al 2019). Primary production, stratification, and salinity all vary throughout the year with different frequency, periodicity, and amplitude, we can expect the resulting abiotic characteristics of the upwelled water to differ from one upwelling event to the other These parameters are predicted to further shift with global change over the coming decades. If the sensitivities of different ontogenetic stages and species to a given environmental setting differ, changes in the nature and strength of biological interactions may change, and lead to compositional shifts in the community
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