Hans-Georg Hoppe

Changes in biogenic carbon flow in response to sea surface warming

The pelagic ocean harbors one of the largest ecosystems on Earth. It is responsible for approximately half of global primary production, sustains worldwide fisheries, and plays an important role in the global carbon cycle. Ocean warming caused by anthropogenic climate change is already starting to impact the marine biota, with possible consequences for ocean productivity and ecosystem services. Because temperature sensitivities of marine autotrophic and heterotrophic processes differ greatly, ocean warming is expected to cause major shifts in the flow of carbon and energy through the pelagic system. Attempts to integrate such biological responses into marine ecosystem and biogeochemical models suffer from a lack of empirical data. Here, we show, using an indoor-mesocosm approach, that rising temperature accelerates respiratory consumption of organic carbon relative to autotrophic production in a natural plankton community. Increasing temperature by 2–6 °C hence decreased the biological drawdown of dissolved inorganic carbon in the surface layer by up to 31%. Moreover, warming shifted the partitioning between particulate and dissolved organic carbon toward an enhanced accumulation of dissolved compounds. In line with these findings, the loss of organic carbon through sinking was significantly reduced at elevated temperatures. The observed changes in biogenic carbon flow have the potential to reduce the transfer of primary produced organic matter to higher trophic levels, weaken the oceans biological carbon pump, and hence provide a positive feedback to rising atmospheric CO2.

Impact of warming on phyto‐bacterioplankton coupling and bacterial community composition in experimental mesocosms

Global warming is assumed to alter the trophic interactions and carbon flow patterns of aquatic food webs. The impact of temperature on phyto-bacterioplankton coupling and bacterial community composition (BCC) was the focus of the present study, in which an indoor mesocosm experiment with natural plankton communities from the western Baltic Sea was conducted. A 6 °C increase in water temperature resulted, as predicted, in tighter coupling between the diatom-dominated phytoplankton and heterotrophic bacteria, accompanied by a strong increase in carbon flow into bacterioplankton during the phytoplankton bloom phase. Suppressed bacterial development at cold in situ temperatures probably reflected lowered bacterial production and grazing by protists, as the latter were less affected by low temperatures. BCC was strongly influenced by the phytoplankton bloom stage and to a lesser extent by temperature. Under both temperature regimes, Gammaproteobacteria clearly dominated during the phytoplankton peak, with Glaciecola sp. as the single most abundant taxon. However, warming induced the appearance of additional bacterial taxa belonging to Betaproteobacteria and Bacteroidetes. Our results show that warming during an early phytoplankton bloom causes a shift towards a more heterotrophic system, with the appearance of new bacterial taxa suggesting a potential for utilization of a broader substrate spectrum.

Biologie und Ökologie

Der entscheidende Faktor fur die Pflanzen- und Tierwelt der Ostsee ist der Salzgehalt. In dem sehr jungen Brackwassermeer konnte sich kaum eine eigenstandige Flora und Fauna entwickeln. Die meisten Organismen sind uber die Nordsee eingewandert. Im nordlichsten Teil gibt es noch einige eiszeitliche Relikte, die hier von ihrem Hauptverbreitungsgebiet in der Arktis isoliert wurden. Mit abnehmendem Salzgehalt geht die Zahl der marinen Arten immer weiter zuruck. Das wird besonders deutlich in der zentralen Ostsee, wo die Salinitat nur noch zwischen 5 und 12%o liegt. In diesem Bereich konnen sich auch nur wenige Suswasserorganismen behaupten. Fur sie ist der Salzgehalt wiederum zu hoch. Bei den meisten von ihnen stellen 3%o die Grenze dar. So finden sich Suswasserarten vor allem in den Kustenbereichen mit starken Zuflussen, aber auch in der nordlichen Bottenwiek und im ostlichsten Teil des Golfs von Finnland. Die marinen und auch die limnischen Arten stosen in der Ostsee immer irgendwo an ihre Verbreitungsgrenzen, so das sie hier empfindlicher gegenuber naturlichen und anthropogenen Stresfaktoren sind als in den Kerngebieten ihrer Verbreitung.