Norman Göbeler’s research while affiliated with University of Helsinki and other places

The magnitude and frequency of marine heatwaves are increasing and predicted to intensify, but our ability to understand the real‐world effects on vital benthic ecosystems is lagging behind. Prior insights into the impacts of marine heatwaves are often derived from observational or laboratory studies. Observational studies may not fully disentangle the complexities of potential compound events and typically focus on severe, often lethal marine heatwaves. Laboratory studies, on the contrary, while valuable for understanding specific mechanisms, often use artificial setups and can introduce unnatural disturbances that do not reflect real‐world scenarios. To investigate sublethal temperature effects of marine heatwaves in a natural benthic habitat, we developed a novel approach for inducing elevated water temperatures in situ over several days. The system utilizes domestic underfloor heating technology combined with custom‐made benthic chambers. We placed 10 chambers for 15 days in a bare‐sediment habitat at 2.5 m depth and heated five chambers to 5°C above ambient water temperatures in summer for 6 days, followed by a period of 7 days at ambient temperatures. Incubations during day and night were performed during the experiment to assess changes in ecosystem functioning (solute fluxes) and sediment cores were collected at the end of the experiment to assess the effects of a realistic marine heatwave on benthic community structure. The results indicate that while the benthic community structure remained similar between the treatments, except for a size shift of Marenzelleria spp. towards smaller individuals in the heated treatment, elevated temperatures caused a significant increase in community respiration and amplified the magnitude of either efflux or influx of nutrients (NH4⁺‐N, PO4³⁻‐P and Si). Primary production during daytime incubations remained mostly unaffected by the heatwave treatment, contributing to the concept of heterotrophy being more influenced by increased temperatures than autotrophy. This study confirms the suitability of the novel system for examining the impact of temperature on benthic habitats in situ and demonstrates its potential for the investigation of complex habitats and communities, which are essential for our understanding of the ecosystem‐level effects of climate change. Read the free Plain Language Summary for this article on the Journal blog.

The frequency of abnormally warm water events is increasing not only in surface waters, but also in subsurface layers, with major impacts on benthic ecosystems. Previous insights on heatwave effects have been obtained through field observations or manipulative laboratory experiments. Here, we introduce a system capable of inducing elevated water temperatures in benthic habitats in situ over several days. The system consists of a commercially available electric boiler, usually applied in domestic underfloor heating, and custom‐designed benthic acrylic glass chambers connected to individual thermostats. Furthermore, the chambers are semi‐open, allowing constant water exchange, maintaining otherwise near‐natural conditions, including oxygen concentrations, while the temperature is elevated. The water exchange can be stopped to facilitate incubations measuring changes in benthic fluxes. We conducted a 15‐d trial study in July 2021 on a bare‐sediment habitat at 2.5 m depth, exposing five chambers to water temperatures 5°C above ambient temperatures for 6 d and comparing with five control chambers. In this assessment, we demonstrate that the temperature control and stability were reliable while maintaining natural oxygen conditions. The modular character of the system permits adaptations for various benthic habitats, facilitating the investigation of elevated temperatures in situ for future climate change scenarios.

The increasing frequency and intensity of marine heatwaves (MHWs) observed worldwide entails changes in the structure and functioning of ecological communities. While severe and extreme heatwaves often have more destructive effects, the more subtle effects of moderate and strong heatwaves may nevertheless affect ecosystem functioning through complex, context-dependent linkages between different processes. Here we conducted a laboratory experiment to study the effects of repeated short-term, strong MHWs on macrofauna bioturbation and associated solute fluxes as a measure of ecosystem functioning using natural soft-sediment communities from the Baltic Sea. Our results showed changes in both bioturbation and biogeochemical cycling of nutrients following short-term, strong heatwaves, which seemed to contribute to an enhanced degradation of organic matter in the seafloor and an enhanced exchange of solutes across the sediment-water interface as well as increased sediment oxygen consumption. Following changes in these processes, the relative contribution of macrofauna and the environmental context to ecosystem functioning was altered. Our results highlight the potential of even shorter-term, strong MHWs of having system-wide impacts due to changes in the mechanistic process of bioturbation underpinning the biogeochemical cycling of nutrients. This study also highlights the need to measure a wide range of variables for a comprehensive understanding of the changes in functioning under disturbances, such as MHWs.

Global marine heatwave assessments often rely on satellite-derived sea surface temperature. However, these data have low accuracy in coastal areas, are unable to measure sub-surface temperatures and have only been available since the 1980s. Here, we analyse 90 years of in situ surface and bottom (30 m) water temperature data from a Finnish coastal monitoring site. Water temperatures were significantly higher between 1991–2020 than 1931–1960 and 1961–1990. We find strong differences between satellite-derived and in situ temperatures, with in situ temperatures being lower in autumn and winter and higher in spring. Measurements at the seafloor indicate marine heatwaves occurred during all seasons between 2016 and 2020, with intensities and durations exceeding previous records. Since the 1990s, we find an upward shift of the baseline temperature and increasingly frequent occurrence of temperatures previously considered as an extreme. Our findings highlight the importance of long-term in situ data and choice of climatological reference periods for assessing change.

The salinization of freshwaters is a global threat to aquatic biodiversity. We quantified variation in chloride (Cl−) tolerance of 19 freshwater zooplankton species in four countries to answer three questions: (1) How much variation in Cl− tolerance is present among populations? (2) What factors predict intraspecific variation in Cl− tolerance? (3) Must we account for intraspecific variation to accurately predict community Cl− tolerance? We conducted field mesocosm experiments at 16 sites and compiled acute LC50s from published laboratory studies. We found high variation in LC50s for Cl− tolerance in multiple species, which, in the experiment, was only explained by zooplankton community composition. Variation in species‐LC50 was high enough that at 45% of lakes, community response was not predictable based on species tolerances measured at other sites. This suggests that water quality guidelines should be based on multiple populations and communities to account for large intraspecific variation in Cl− tolerance.

Significance The salinity of freshwater ecosystems is increasing worldwide. Given that most freshwater organisms have no recent evolutionary history with high salinity, we expect them to have a low tolerance to elevated salinity caused by road deicing salts, agricultural practices, mining operations, and climate change. Leveraging the results from a network of experiments conducted across North America and Europe, we showed that salt pollution triggers a massive loss of important zooplankton taxa, which led to increased phytoplankton biomass at many study sites. We conclude that current water quality guidelines established by governments in North America and Europe do not adequately protect lake food webs, indicating an immediate need to establish guidelines where they do not exist and to reassess existing guidelines.

Human-induced salinization increasingly threatens inland waters; yet we know little about the multifaceted response of lake communities to salt contamination. By conducting a coordinated mesocosm experiment of lake salinization across 16 sites in North America and Europe, we quantified the response of zooplankton abundance and (taxonomic and functional) community structure to a broad gradient of environmentally relevant chloride concentrations, ranging from 4 to ca. 1400 mg Cl/L. We found that crustaceans were distinctly more sensitive to elevated chloride than rotifers; yet, rotifers did not show compensatory abundance increases in response to crustacean declines. For crustaceans, our among-site comparisons indicate: (1) highly consistent decreases in abundance and taxon richness with salinity; (2) widespread chloride sensitivity across major taxonomic groups (Cladocera, Cyclopoida, and Calanoida); and (3) weaker loss of functional than taxonomic diversity. Overall, our study demonstrates that aggregate properties of zooplankton communities can be adversely affected at chloride concentrations relevant to anthropogenic salinization in lakes.

Sea cucumbers are bottom dwelling invertebrates, which are mostly found on subtropical and tropical sea grass beds, sandy reef flats, or reef slopes. Although constantly exposed to fouling communities in these habitats, many species are surprisingly free of invertebrate epibionts and microfouling algae such as diatoms. In our study, we investigated the anti-fouling (AF) activities of different crude extracts of tropical Indo-Pacific sea cucumber species against the fouling diatom Cylindrotheca closterium. Nine sea cucumber species from three genera (i.e., Holothuria, Bohadschia, Actinopyga) were selected and extracted to assess their AF activities. To verify whether the sea cucumber characteristic triterpene glycosides were responsible for the observed potent AF activities, we tested purified fractions enriched in saponins isolated from Bohadschia argus, representing one of the most active anti-fouling extracts. Saponins were quantified by vanillin-sulfuric acid colorimetric assays and identified by LC-MS and LC-MS/MS analyses. We were able to demonstrate that AF activities in sea cucumber extracts were species-specific, and growth inhibition as well as attachment of the diatom to surfaces is dependent on the saponin concentration (i.e., Actinopyga contained the highest quantities), as well as on the molecular composition and structure of the present saponins (i.e., Bivittoside D derivative was the most bioactive compound). In conclusion, the here performed AF assay represents a promising and fast method for selecting the most promising bioactive organism as well as for identifying novel compounds with potent AF activities for the discovery of potentially novel pharmacologically active natural products.