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 half 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. Before and during heatwave exposure, community filtration rates were measured keeping the communities intact, using their host boxes as incubation chambers within the mesocosms. To initialize measurements, chlorophyll spikes were provoked by adding Rhodomonas salina mono culture to the boxes' water columns, after which water samples were taken with a 10-minute interval and immediately analyzed for the relative change in chlorophyll-a concentration using a bbe© AlgaeLabAnalyser (370 – 610 nm). To obtain absolute chlorophyll-a values, additional water samples were taken after 1, 20, and 60 minutes and filtered through glass microfiber filters. Subsequently, 90% acetone solution was yielded from the filters and measured for light absorbance rates at 750, 663, 645, and 630 nm wave lengths using an UVIKON© XS UV/Vis spectrophotometer. Chlorophyll-a concentrations were then calculated from absorbance rates following Jeffrey and Humphrey (1975). In a next step, 10-minute interval values were interpolated by fitting the results to the relative concentration changes previously determined via the AlgaeLabAnalyser. Community filtration rates in µg chlorophyll-a per hour were calculated for each 10-min interval from the respective concentration changes, normalized to per hour and the incubated water volume. Filtration rates differed significantly between species, with higher community filtration rates for C. edule than M. balthica, the latter resembling the bare sand controls. For C. edule, the mild heatwave induced elevated community filtration rates at high chlorophyll-a concentration compared to communities exposed to no heatwave, the same was not observed for the strong heatwave. Due to the methodology applied, the exact number of alive and actively filtering individuals at the times of measurement is unknown, thus results represent community-level responses.
