Coral reefs are endangered by constantly rising water temperature due to global warming. This triggers a breakdown of the nutritional symbiosis between cnidarian hosts and their Symbiodiniaceae symbionts, resulting in the loss of the algal partner. In the Symbiodiniaceae exists a high genetic diversity with broad physiological plasticity within and between species, resulting in large thermal tolerance. While these variations have been studied in individual taxa, comprehensive comparative experimental data on numerous species are still rare. In the present study, the photosynthetic performance and tolerance as function of light and temperature of nine Symbiodiniaceae genetic types of four different clades were determined. The data indicate significant differences in the response patterns. Almost all algal isolates exhibited low to moderate light requirements for photosynthesis without photoinhibition, and a photosynthetic efficiency between 20 and 80% in the temperature range 20–34°C, indicating a broad thermal tolerance to temperature fluctuations in tropical regions. The presented data clearly point to a broad photophysiological tolerance and thermal plasticity of genetically different Symbiodiniaceae, which contributes as an important finding to a better understanding of host-symbiont response to an increasing sea surface temperature.

<p>The Elbe Estuary is strongly impacted by anthropogenic activities such as dredging and eutrophication. Together, these cause oxygen minimum zones (OMZ) regularly during summer in the Hamburg Port area, within the tidal freshwater region of the estuary. Over the last years, this OMZ has expanded spatially and temporally. We present an analysis of an extraordinary oxygen minimum event in June 2022, when an all-time lowest oxygen concentration was observed upstream of the Hamburg Port.</p><p>We combine data from six transect cruises (early May -late June, 2022) and monitoring stations (2016-2022), to show the decrease of oxygen, and the increasing number of oxygen minimum events. In June 2022, the OMZ moved upstream due to the collapse of a phytoplankton bloom upstream of the tidal weir. This was accompanied by particularly warm temperatures and low river discharge, providing a glimpse into the potential future changes of central European estuaries under climate change.</p>

<p>Estuaries are important biogeochemical reactors that can remove dissolved inorganic nitrogen (DIN, mostly nitrate) from the water column, but can also generate nitrate via remineralization and subsequent nitrification of organic matter in the water column. To assess this regeneration of nitrate, an important nutrient source for phytoplankton that contributes to eutrophication, various isotope-based laboratory methods are in use.</p> <p>In this study, we compare two commonly used stable-isotope-based techniques to measure nitrification in estuarine water, the isotope dilution method and the addition of 15N-ammonium. Both measure the isotope enrichment in nitrate but have drastically different incubation times. We apply both methods in the estuary of the Elbe River and evaluate the drawbacks and advantages of each method to develop application recommendations.</p> <p>Our results indicate that nitrification measurements using isotope dilution are less variable between stations, but suggest that rates are overestimated at high phytoplankton activity. On the counter side, the addition of 15N-ammonium as a tracer apparently overestimates nitrification in heterotrophic settings, probably because substrate addition stimulates nitrification. The adequate measurement technique must this be carefully chosen depending on the selected study site.</p> <p>Funding information  - This study has been carried out and was financially supported by the BMBF “Blue-Estuaries” project (grant no. 03F0864C)</p>