Description: Salt expelled during the formation of ice in polynyas leads to a downward precipitation of brine that causes thermohaline convection and erodes the density stratification of the water column. In this thesis we investigate by means of flux models and satellite data the ability of the Western New Siberian (WNS) flaw polynya to modify the stratification of the water column and to form saline bottom water. The accuracy of existent microwave satellite-based polynya monitoring methods is assessed by a comparison of derived estimates with airborne electromagnetic ice thickness measurements and aerial photographs taken across the polynya. The cross-validation indicates that in the narrow flaw polynyas of the Laptev Sea the coarse resolution of commonly used microwave channel combinations provokes errors through mixed signals at the fast and pack ice edges. Likewise, the accuracy of flux models is tested by comparing model results to ice thickness and ice production estimates derived from high-resolution thermal infrared satellite observations. We find that if a realistic fast ice boundary and parameterization of the collection depth H is used and if the movement of the pack ice edge is prescribed correctly, the model is an appropriate tool for studying polynya dynamics and estimating associated fluxes. Hence, a flux model is used to examine the effect of ice production on the stratification of the water column. The ability of the polynya to form dense shelf bottom water is investigated by adding the brine released during an except ionally strong WNS polynya event in 2004 to the average winter density stratification of the water body. Owing to the strong density stratification and the apparent lack of extreme polynya events in the eastern Laptev Sea, we find the likelihood of convective mixing down to the bottom to be extremely low. We conclude that the recently observed breakdown of the stratification during polynya events is therefore predominantly related to wind- and tidally-driven turbulent mixing.

Description: Die Mündungsgebiete von Elbe und Weser bilden die beiden größten deutschen Ästuare. Sie weisen eine Reihe von hydrologischen, geologischen und klimatischen Gemeinsamkeiten auf. Ziel der Arbeit war es zu untersuchen, ob und wieweit diese Gemeinsamkeiten auch in planktologisch/mikrobiologischer Hinsicht gelten. Hierzu wurden bei niedrigem Oberwasserabfluss im Juni 2005 Oberflächenproben in 10 km Abständen in beiden Ästuaren von ihrem limnischen Bereich bis in die Deutsche Bucht genommen. Untersucht wurden die abiotischen Parameter Temperatur, Salzgehalt, Gesamt- und Feintrübung 〈 2 μm sowie die biologischen Parameter Chlorophyll a und Phäopigmente, Bakterienzahl und bakterielle Biomasseproduktion. Die biologischen Variablen hatten ihr Maximum stets in der limnischen Zone. Hier beliefen sich die Werte in der Elbe auf 10,3 μg l-1 Chlorophyll a (Chl a), 9,5 x 109 l-1 Bakterien (BZ) und eine bakterielle Biomasseproduktion (BBP) von 4,3 μg C l-1 h-1. In der Weser lagen sie bei 22,5 μg l-1 (Chl a), 7,8 x 109 l-1 (BZ) und 4,1 μg C l-1 h-1 (BBP). Ein Minimum wurde im Bereich der oberen Brackwassergrenze mit 5,2 μg l-1 (Chl a), 5,4 x 109 l-1 (BZ) und 1,0 μg C l-1 h-1 (BBP) in der Elbe und mit 3,8 μg l-1 (Chl a), 7,4 x 109 l-1 (BZ) und 1,4 μg C l-1 h-1 (BBP) in der Weser gefunden. An der seewärtigen Grenze der Ästuarregionen trat ein erneutes Maximum auf. Damit stimmten beide Ästuare sowohl in der regionalen Verteilung als auch in der Größe der Parameter weitgehend überein.

Description: Ancient Lake Ohrid is a steep-sided, oligotrophic, karst lake that was tectonically formed most likely within the Pliocene and often referred to as a hotspot of endemic biodiversity. This study aims on tracing significant lake level fluctuations at Lake Ohrid using high-resolution acoustic data in combination with lithological, geochemical, and chronological information from two sediment cores recovered from sub-aquatic terrace levels at ca. 32 and 60m water depth. According to our data, significant lake level fluctuations with prominent lowstands of ca. 60 and 35m below the present water level occurred during Marine Isotope Stage (MIS) 6 and MIS 5, respectively. The effect of these lowstands on biodiversity in most coastal parts of the lake is negligible, due to only small changes in lake surface area, coastline, and habitat. In contrast, biodiversity in shallower areas was more severely affected due to disconnection of today sublacustrine springs from the main water body. Multichannel seismic data from deeper parts of the lake clearly image several clinoform structures stacked on top of each other. These stacked clinoforms indicate significantly lower lake levels prior to MIS 6 and a stepwise rise of water level with intermittent stillstands since its existence as water-filled body, which might have caused enhanced expansion of endemic species within Lake Ohrid.

Description: Precipitation downscaling improves the coarse resolution and poor representation of precipitation in global climate models and helps end users to assess the likely hydrological impacts of climate change. This paper integrates perspectives from meteorologists, climatologists, statisticians, and hydrologists to identify generic end user (in particular, impact modeler) needs and to discuss downscaling capabilities and gaps. End users need a reliable representation of precipitation intensities and temporal and spatial variability, as well as physical consistency, independent of region and season. In addition to presenting dynamical downscaling, we review perfect prognosis statistical downscaling, model output statistics, and weather generators, focusing on recent developments to improve the representation of space-time variability. Furthermore, evaluation techniques to assess downscaling skill are presented. Downscaling adds considerable value to projections from global climate models. Remaining gaps are uncertainties arising from sparse data; representation of extreme summer precipitation, subdaily precipitation, and full precipitation fields on fine scales; capturing changes in small-scale processes and their feedback on large scales; and errors inherited from the driving global climate model.

Description: The scleractinian Lophelia pertusa is a cosmopolitan reef-forming cold-water coral and creates a habitat for a diverse associated community. This ecosystem, however, is threatened by ocean acidification due to declining pH and shoaling of the carbonate saturation horizon resulting in dissolution of coral skeletons. The aragonite saturation state (ΩAr) given in literature refers to the pure mineral but organic compounds within the skeleton might have an impact on dissolution behaviour and thus could influence the solubility product of aragonite. In this study the critical .QAr, under which aragonite starts to dissolve, was determined for dead skeleton framework of L. pertusa through manipulating natural seawater by acidification (by use of HCl and Na2C03) and bubbling with C02-enriched air. Moreover, the organic portion of the skeleton was quantified by determining the ash-free dry weight. Additionally, scanning electron microscopy images were taken after the dissolution experiment to examine visible dissolution effects on the skeleton surface. Measurements revealed an ΩAr range of 0.795-0.971. Combustion of skeleton fragments of the white and red colour variety of L. pertusa presented an organic content of 3.37 and 3.11 % of total weight, respectively. This relatively high organic :fraction within the skeleton could explain the low saturation state obtained which is below the generally stated threshold of ΩAr = 1. Since the dead framework constitutes the major part of a coldwater reef the obtained saturation state might be regarded as the critical ΩAr for an entire L. pertusa reef. Consequently, this low critical saturation state could indicate that Lophelia pertusa reefs are probably more resistant to ocean acidification than thought before resulting in significant implications for future predicting models.

Description: The eddy correlation technique is rapidly becoming an established method for resolving dissolved oxygen fluxes in natural aquatic systems. This direct and noninvasive determination of oxygen fluxes close to the sediment by simultaneously measuring the velocity and the dissolved oxygen fluctuations has considerable advantages compared to traditional methods. This paper describes the measurement principle and analyzes the spatial and temporal scales of those fluctuations as a function of turbulence levels. The magnitudes and spectral structure of the expected fluctuations provide the required sensor specifications and define practical boundary conditions for the eddy correlation instrumentation and its deployment. In addition, data analysis and spectral corrections are proposed for the usual nonideal conditions, such as the time shift between the sensor pair and the limited frequency response of the oxygen sensor. The consistency of the eddy correlation measurements in a riverine reservoir has been confirmed—observing a night–day transition from oxygen respiration to net oxygen production, ranging from −20 to +5 mmol m−2 day−1—by comparing two physically independent, eddy correlation instruments deployed side by side. The natural variability of the fluctuations calls for at least ∼1 h of flux data record to achieve a relative accuracy of better than ∼20%. Although various aspects still need improvement, eddy correlation is seen as a promising and soon-to-be widely applied method in natural waters.

Description: Experiments and models reveal that moderate dispersal rates between local communities can increase diversity by alleviating local competitive exclusion; in contrast, high dispersal rates can decrease diversity by amplifying regional competition. However, hitherto experimental tests on how dispersal affects diversity in the presence and absence of environmental heterogeneity are largely missing, although it is known that environmental heterogeneity influences diversity. For the first time we experimentally show that the interaction between dispersal rate and the presence of an environmental gradient with on-average lower resource availability than the homogeneous control treatment affects diversity. In metacommunities of nine co-occurring species of marine benthic microalgae we factorially manipulated dispersal rate and the presence and absence of a light intensity gradient across local patches to test effects on local, regional, and beta diversity and to compare results to predictions from monoculture experiments. Although species in this experiment did not show resource partitioning along the light gradient as assumed by source–sink models, dispersal limitation maintained diversity in metacommunities with light gradients but not without. Local diversity and evenness were high under low light intensities when dispersal was limited and decreased with both increasing light intensities and dispersal rates. These diversity changes can be explained by the reduction of growth of the regional superior competitor at low light intensities alleviating its competitive strength. Increasing dispersal rate in turn compensated for the superior competitor's slow growth in those local patches with rather unfavorable light conditions and thus led to decreasing diversity and evenness. In contrast, diversity in the metacommunities without a light gradient was constantly low. Here, the superior competitor contributed 90% to total community biomass in all patches. High dominance, however, likely resulted from on-average higher resource availability (i.e., higher light intensities) compared to metacommunities with light gradient and not from patch homogeneity in itself.

Description: Der Nord-Ostsee-Kanal ist ein Brackwasserbiotop, der sich durch einen relativ hohen Salzgehalt in seinem östlichen Teil, einem fast ausgesüßten mittleren Bereich und einem leichten Salzgehaltsanstieg auf den letzten westlichen Kilometern auszeichnet. In seiner westlichen Hälfte ist die Wassertrübung deutlich höher als in der östlichen. Die Bakterienzahl lag nahe der Schleuse Kiel-Holtenau zwischen 3,6 und 7,9 x 10 hoch 9 l hoch -1 Zellen, von denen 0 bis 9,5 % an Partikeln angeheftet waren. In Richtung Westen nahmen die Bakterienzahl und der Prozentsatz der partikelgebundenen Bakterien fast linear zu und erreichten in der Nähe der Schleuse Brunsbüttel mit 11,5 bis 18,0 x 10 hoch 9 l hoch -1 Zellen und 80-94 % partikelgebundenen Bakterien ihre Maxima. Trübungsgehalt, Bakterienzahl und Prozentsatz der partikelgebundenen Bakterien waren hoch signifikant miteinander korreliert. Die Mittelwerte der bakteriellen Aktivität (gemessen als Leucin-Inkorporation), die zwischen 45 pmol l hoch -1 h hoch -1 im Januar und 383 pmol l hoch -1 h hoch -1 im Juli lagen, zeigten den großen Einfluss der sommerlichen Temperaturzunahme. Die maximalen Werte der spezifischen Leucin-Inkorporation erreichten rd. 80 pmol h hoch -1 10 hoch -9 Bakterien. Die Umsatzraten von Acetat, das als Repräsentant für die niedermolekularen gelösten organischen Verbindungen verwendet wurde, beliefen sich auf Werte zwischen 7 und 34,1 % h-1 (Mittelwerte über den gesamten NOK). Die Erneuerungszeit für den Acetat-"Pool" lag demnach zwischen rund 14 h (Januar) und 3 h (April).

Description: This thesis investigates the effects of nutrient limitation in primary producers on higher trophic levels. To this aim the nutrient conditions and their effect on primary consumers in the North Sea were examined. Furthermore experiments were carried out to test the effect of nutrient limitations on tri-trophic food chains in controlled laboratory conditions. Despite the prevailing assumption that the herbivorous primary producers maintain strict homeostasis, thus buffering any nutrient imbalances at the base of the food web and providing their consumers with food of a high and constant quality, their nutrient stoichiometry and fitness was affected by the nutrient content of their food. The nutrient deficiency of the primary producers can thus be passed on through the food web and affect higher trophic levels. In a set of laboratory based experiments the nutrient contents of primary producers was manipulated to change their C:nutrient ratio and thereby their food quality. Primary consumers were reared on these producers and in turn were the food source for a higher consumer. The ctenophore Pleurobrachia pileus was exposed to copepods with manipulated C:P ratios along a gradient. The reaction of the ctenophore to food with a high C:P ratio, generally considered to be food of lower quality, was unexpected. P. pileus was negatively affected by nutrient replete food with a low C:P ratio and displayed higher levels of fitness when feeding on food with a high carbon:P ratio. This ctenophore is therefore more likely to be energy limited than nutrient limited. A possible explanation for this is the high body C:P ratio observed in P.pileus. The results are discussed in light of possible future scenarios in the aquatic environment. Freshly hatched larvae are very vulnerable and the mortality during this stage is particularly high. Therefore the effect of food with different C:N:P ratios was tested on larvae of the European lobster Homarus gammarus. Larvae were exposed to food with varying nutrient content either throughout their development or for the duration of one stage. The results confirm that the youngest larval stages are the most susceptible to a lack of nutrients in their diet, which had strong negative effects on their condition. The effect of P-limitation in particular was observed to change during ontogeny, with older larval stages being less affected. As nutrient-limitations reduce the fitness and reproductive output of primary consumers, this translates into a quantity effect for higher trophic levels by reducing the amount of food available to them. The combined effects of nutrient quality and nutrient quantity on a top predator were investigated in an experimental set-up. The consumer was negatively affected by the lack of nutrients in its diet even at very low nutrient quantities, indicating that quality of the food is more important than the quantity. Disentangling these two effects, the quality and quantity of food, remains a great challenge for future ecological studies in the marine environment. These findings have potentially far-reaching consequences for the trophic interactions and population dynamics of marine organisms.