Description: Low-salinity stress can severely affect the fitness of marine organisms. As desalination has been predicted for many coastal areas with ongoing climate change, it is crucial to gain more insight in mechanisms that constrain salinity acclimation ability. Low-salinity induced depletion of the organic osmolyte pool has been suggested to set a critical boundary in osmoconforming marine invertebrates. Whether inorganic ions also play a persistent role during low-salinity acclimation processes is currently inconclusive. We investigated the salinity tolerance of six marine invertebrate species following a four-week acclimation period around their low-salinity tolerance threshold. The species investigated were Asterias rubens, Mytilus edulis, Littorina littorea, Diadumene lineata, Strongylocentrotus droebachiensis and Psammechinus milliaris. To obtain complete osmolyte budgets of seawater, body fluids and tissues we quantified total osmolality (via osmometer), organic osmolytes (methylamine and free amino acids) via 1H-NMR spectroscopy and inorganic osmolytes (anions and cations) via flame photometry and a novel protocol using ion-chromatography. We further determined the fitness proxies survival, growth and tissue water content. Our data show the importance of the organic and inorganic osmolyte pool during low-salinity acclimation. It also shows the importance of specific compounds in some species. This data can be used in future osmolyte and salinity tolerance research. This type of data is essential to establish reliable physiological limits of species in order to estimate consequences of future salinity changes with ongoing climate change. It can be used to assess the salinity tolerance capacity and to obtain a better understanding of the basic mechanisms that are utilized in a wide range of species. The established cellular inorganic and organic osmolyte profiles can build a foundation for applied cellular physiological research, for example for designing suitable buffers for in vitro assays as these buffers need to incorporate complex organic and inorganic osmolyte changes. Knowledge about cellular and whole-organism biochemistry and physiology is absolutely crucial for characterizing the functions of genes that are under selection by climate change stressors. A quantitative knowledge of cellular osmolyte systems is key to understand the evolution of euryhalinity and to characterize targets of selection during rapid adaptation to ongoing desalination.

Description: From June 2022 to May 2023, seawater temperature and salinity data were measured repeatedly by a hand-held salinity probe (WTW Handheld ProfiLine Multi 3320 with conductivity measuring cell TetraCon 325) at GEOMAR (Kiel, Germany) pier at 54°19'48.8N 10°08'59.6E. The seawater was continuously pumped from 1m depth to a container on the 'Kiel Outdoor Benthocosm' (KOB) platform in which the measurement took place. The sensor was calibrated prior to each measurement.

Description: From January 2022 to May 2023, temperature and salinity (only until March 2023) data were logged with a SEABIRD SBE 37-SMP MicroCAT CT in 2-minutes intervals at GEOMAR (Kiel, Germany) pier at 54°19'48.8N, 10°08'59.6E. The sensor system is mounted to a floating platform to ensure a continuous depth of 1 m at any time. After cleaning and other re-boots of the sensor package, temperature and salinity data tend to deviate from true values. Hence, 60 minutes of data after any re-boot (after sensor servicing with re-deployment, data download or power failure) were deleted. Due to massive fouling the conductivity sensor caused false salinity data during the last two months of this measurement period which were, hence, also deleted from the data set.

Description: From January 2020 to October 2021, seawater temperature and salinity data were measured repeatedly by a hand-held salinity probe (WTW Handheld ProfiLine Multi 3320 with conductivity measuring cell TetraCon 325) at GEOMAR (Kiel, Germany) pier at 54°19'48.8N 10°08'59.6E. The seawater was continuously pumped from 1m depth to a container on the 'Kiel Outdoor Benthocosm' (KOB) platform in which the measurement took place. The sensor was calibrated prior to each measurement.

Description: The plea for using more “realistic,” community‐level, investigations to assess the ecological impacts of global change has recently intensified. Such experiments are typically more complex, longer, more expensive, and harder to interpret than simple organism‐level benchtop experiments. Are they worth the extra effort? Using outdoor mesocosms, we investigated the effects of ocean warming (OW) and acidification (OA), their combination (OAW), and their natural fluctuations on coastal communities of the western Baltic Sea during all four seasons. These communities are dominated by the perennial and canopy‐forming macrophyte Fucus vesiculosus—an important ecosystem engineer Baltic‐wide. We, additionally, assessed the direct response of organisms to temperature and pH in benchtop experiments, and examined how well organism‐level responses can predict community‐level responses to the dominant driver, OW. OW affected the mesocosm communities substantially stronger than acidification. OW provoked structural and functional shifts in the community that differed in strength and direction among seasons. The organism‐level response to OW matched well the community‐level response of a given species only under warm and cold thermal stress, that is, in summer and winter. In other seasons, shifts in biotic interactions masked the direct OW effects. The combination of direct OW effects and OW‐driven shifts of biotic interactions is likely to jeopardize the future of the habitat‐forming macroalga F. vesiculosus in the Baltic Sea. Furthermore, we conclude that seasonal mesocosm experiments are essential for our understanding of global change impact because they take into account the important fluctuations of abiotic and biotic pressures.

Description: From January 2020 to January 2022, temperature, salinity, and oxygen (only July 2020 - April 2021) data were logged with an AANDERAA oxygen sensor 3835 and a SEABIRD SBE 37-SI MicroCAT CT(D) in 10-minutes intervals at GEOMAR (Kiel, Germany) pier at 54°19'48.8N 10°08'59.6E. The sensor system is mounted to a floating platform such that a continuous depth of 1 m is ensured at any time. Oxygen data were corrected for salinity, temperature and depth following the manual for Aanderaa Optodes using the salinity and temperature measurements from the SEABIRD SBE 37-SI MicroCAT CT(D) sensor. After cleaning and other re-boots of the sensor package, temperature, salinity and oxygen data tend to deviate from true values. Hence, 60 minutes of data after any re-boot (after sensor servicing with re-deployment, data download or power failure) were deleted. Two major data gaps (January 2020 – July 2020, April 2021 – June 2021) are due to longer periods when the sensors were serviced in the workshop.

Description: Laboratory experiments were conducted in the climate chambers at GEOMAR Helmholtz Centre for Ocean Research Kiel in the time between March and November 2018. Experiments were designed to study the effect of long-term (1 month) exposure to low salinity in osmoconforming invertebrates. The study organisms (Asterias rubens, Mytilus edulis, Littorina littorea, Diadumene lineata, Strongylocentrotus droebachiensis and Psammechinus milliaris) were collected in Kiel Fjord, Eckernförder Bight or the Kattegat from spring to autumn 2018. Organisms were acclimated to climate chamber conditions for 1 week (under habitat salinity, 14˚C, constant aeration) and then subjected to salinity acclimation for 1-2 weeks until the final salinity treatment level was reached. Then different salinity treatments were maintained for 4 weeks. Water physiochemistry (temperature, salinity, pH, nitrite, nitrate, phosphate) was recorded frequently. After the experiment, samples were taken from seawater and body fluids to measure total osmolality (mosmol/kg) with an osmomat and inorganic ions (mmol/l). No body fluid samples were taken from Diadumene lineata as organisms were too small and volumes too low. Anions were measured with a novel protocol via ion chromatography, cations were measured via flame photometry.

Description: Laboratory experiments were conducted in the climate chambers at GEOMAR Helmholtz Centre for Ocean Research Kiel in the time between March and November 2018. Experiments were designed to study the effect of long-term (1 month) exposure to low salinity in osmoconforming invertebrates. The study organisms (Asterias rubens, Mytilus edulis, Littorina littorea, Diadumene lineata, Strongylocentrotus droebachiensis and Psammechinus milliaris) were collected in Kiel Fjord, Eckernförder Bight or the Kattegat from spring to autumn 2018. Organisms were acclimated to climate chamber conditions for 1 week (under habitat salinity, 14˚C, constant aeration) and then subjected to salinity acclimation for 1-2 weeks until the final salinity treatment level was reached. Then different salinity treatments were maintained for 4 weeks. Water physiochemistry (temperature, salinity, pH, nitrite, nitrate, phosphate) was recorded frequently. Throughout the experiment survival was recorded. Before and after the experiment organism weight and, where feasible, size was measured. Weight data was also used to calculate tissue water content. This dataset comprises the physiological fitness parameters for each species at the respective salinity treatment. Given are data for survival, growth and tissue water content.

Description: Laboratory experiments were conducted in the climate chambers at GEOMAR Helmholtz Centre for Ocean Research Kiel in the time between March and November 2018. Experiments were designed to study the effect of long-term (1 month) exposure to low salinity in osmoconforming invertebrates. The study organisms (Asterias rubens, Mytilus edulis, Littorina littorea, Diadumene lineata, Strongylocentrotus droebachiensis and Psammechinus milliaris) were collected in Kiel Fjord, Eckernförder Bight or the Kattegat from spring to autumn 2018. Organisms were acclimated to climate chamber conditions for 1 week (under habitat salinity, 14˚C, constant aeration) and then subjected to salinity acclimation for 1-2 weeks until the final salinity treatment level was reached. Then different salinity treatments were maintained for 4 weeks. Water physiochemistry (temperature, salinity, pH, nitrite, nitrate, phosphate) was recorded frequently. After the experiment, samples were taken from tissues to measure total osmolality (mosmol/kg) with an osmomat, and inorganic ions (mmol/kg or µmol/g wet mass). Anions were measured with a novel protocol via ion chromatography, cations were measured via flame photometry. Organic osmolytes were measured via 1H-NMR.