ALBERT

Description: Rivers represent a transition zone between terrestric and aquatic environments, and between methane rich and methane poor environments. The Elbe River is one of the important rivers draining into the North Sea and with the Elbe potentially high amounts of methane could be imported into the water column of the North Sea. Twelve cruises from October 2010 until June 2013 were conducted from Hamburg towards the Elbe mouth at Cuxhaven. The dynamic of methane concentration in the water column and its consumption via methane oxidation was measured. In addition, physico-chemical parameters were used to estimate their influence on the methanotrophic activity. We observed high methane concentrations at the stations in the area of Hamburg harbor ("inner estuary") and about 10 times lower concentrations in the outer estuary (median of 416 versus 40 nmol/L). The methane oxidation (MOX) rate mirrowed the methane distribution with high values in the inner estuary and low values in the outer estuary (median of 161 versus 10 nmol/L/d respectively) Methane concentrations were significantly influenced by the river hydrology (falling water level) and the trophic state of the water (biological oxygen demand). In contrast to other studies no clear relation to the amount of suspendended particulate matter (SPM) was found. Methane oxidation rates were significantly influenced by methane concentration and to a weaker extent by temperature. Methane oxidation accounted for 41 ± 12% of the total loss of methane in summer/fall, but only for 5 ± 3% of the total loss in winter/spring. We applied a modified box model taking into account the residence times of a water parcel depending on discharge and tidal impact. We observed almost stable methane concentrations in the outer estuary, despite a strong loss of methane through diffusion and oxidation. Thus we postulate that in the outer Elbe estuary a strong additional input of methane is required, which could be provided by the extensive salt marshes near the river mouth.

Description: The zooplankton community in a relatively small and mountain pond was studied during the spring growing season. To investigate which factors operate in the community structure, we explored several physical conditions, such as high inflows, and the biotic dynamics of the main zooplankton groups (i.e., rotifers, cladocerans and copepods).  Two extreme flood events occurred during the investigated period and caused dramatic changes in physical conditions and reduction of the planktonic community abundances. The short period between both high-flow events was enough for the recovery of microplankton, but not for the metazoan zooplankton. Our results are in agreement with the common situation in which high flood events commonly favour rotifers over crustaceans, likely due to rotifer species have great colonization ability and grow faster. However, we found that the dominance of rotifers over crustaceans in our system is evidenced by an extremely, unusual high ratio between their abundances.  We observed that, at the time of the great floods, crustacean abundances as well as rotifer populations notably decreased until near zero values. Although rotifer abundance began declining before high floods, the decrease was particularly notable when the great flood happened.  Our results evidenced that i) dilution rate and temperature were the main drivers which are operating in the structure of the zooplankton community; and ii) no negative biotic interactions were detected between large and small cladocerans and rotifers. Additionally, we found surprisingly that a repeated disturbance caused by high flood events does increase the species diversity of rotifers. Finally, our study also detected some cues which may indicate that diapausing egg bank is also playing an important role in the zooplankton community, favouring the dominance of rotifers; however, this phenomenon deserves further studies.