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To investigate the potential impact of marine heatwaves on the intertidal fauna of the German Wadden Sea, a mesocosm experiment was conducted on the island of Sylt, Germany, from June to August 2022. Organisms of the two common bivalve species Cerastoderma edule and Macoma balthica were collected from the upper intertidal of the local Wadden Sea in May 2022. Organisms were then placed into open boxes filled with sieved sediment (1000 µm) from the sampling area, as single-species communities mimicking in-situ observed densities of 119 individuals m-2 (C. edule) and 357 m-2 (M. balthica) and with initial community wet weights corresponding to approximately 35.7 g m-2 (both species) to ensure comparability. The communities were introduced into outdoor land-based mesocosm tanks equipped with tidal simulation, seawater flow through and temperature control on June 1st, 2022. After an adjustment phase of three weeks, four replicate tanks (and communities) were exposed to a single, mild 15-day heatwave with water temperatures of maximum 2.8°C above ambient, entailing nine days at peak temperature and three days onset/ offset each. Another four replicates were exposed to a strong heatwave of the same design but with maximum 4.4°C above ambient, and four more replicates were kept under ambient conditions for control. Two weeks after the heatwave exposure, the mesocosm experiment was terminated and all communities were again weighed and counted for surviving individuals. No significant treatment effects on community survival or biomass were recorded for either model species. However, average survival was lower following the strong heatwave for C. edule, corresponding to observed community biomass losses. In contrast, survival in M. balthica remained high across all treatments, and community biomasses appeared to have increased marginally over the course of the experiment.
Hourly means of temperature and oxygen as logged in the tanks.
Grazing rates of the isopod Idotea sp. on the macroalga Fucus vesiculosus during upwelling 3 in response to warming and upwelling.
Impact of warming and upwelling on macroalgal growth (expressed as anomaly or z-growth).
Abundances (numbers) of major grazer species in the tanks as assessed by subsamples taken at three time points (before upwelling 2 and 3 and between the two upwelling events).
Photosynthetic performance of macroalgae (assessed by light and dark incubation) during upwelling 3 without or with (+4°C) warming.
Temperature, pH, salinity and oxygen logged and measured in the tanks.
Weekly measures of nutrients (silicate, nitrate, nitrite, phosphate, ammonium) in the tanks.
Microfouling (expressed as cell numbers per cm2) by bacteria and diatoms in response to warming (+0, 2 and 4°C) and upwelling during upwelling event 3.
Control measures of temperature, pH, salinity and oxygen as taken by WTW multimeter daily 3h after sunrise.
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