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Lakes are globally relevant for socio-economic needs as they provide fresh water sources for water supply and recreational activities, can be sites of critical sustenance fisheries, are often places of great natural beauty, and have cultural importance for many groups. In addition, lakes support diverse biota. The ecosystem balance of these natural waters is linked to the lake's physical processes that drive the transport and mixing of carbon, nutrients, sediments, and oxygen. For much of the year, thermal stratification acts as a barrier to vertical transport. At the same time, the existence of a thermal stratification allows for the occurrence of the upwelling process, which can potentially promote vertical transport and mixing. The upwelling phenomenon can alter the water quality in the littoral zone, and nutrient fluxes have been reported during the active upwelling phase. Conceptual models describe the flow structures to follow either a non-rotational or a fully rotational conceptual model, with the former applied to small lakes and systems of moderate size. Despite recognizing lake hydrodynamics influenced by the Coriolis force in lakes of moderate size, the intrinsic details of the rotational influence during lake upwelling are poorly understood, and the thresholds for the validity of the conceptual models have not been defined to date. This dissertation uses fine-resolution numerical simulations with a lake model to quantify the Coriolis force effects during the active and relaxation phases of lake upwelling and to detail the physical processes observed in lakes that exhibit partial rotational influence in the flow structures. The numerical simulations conclude that lakes that show Coriolis force effects do not follow the binary perspective of upwelling, and rotational flow structures describe lake response to wind forcing during weak stratification. Furthermore, the analysis of the flow and temperature patterns of over 300 numerical simulations of lakes res

Hypoxia and hydrogen sulfide (H2S) intrusion at night contribute to large-scale seagrass mortality events worldwide. Declining water quality has lowered irradiance and enhanced hypoxia in seagrass ecosystems, but linkages between low irradiance and seagrass internal pO2 in situ are not well understood. We examined low irradiance effects on leaf and meristem pO2 dynamics of a dominant tropical seagrass, Thalassia testudinum, using microsensors over multiple diurnal cycles. Further, we determined how O2 dynamics affect H2S intrusion into shoot meristems under low irradiance. Sequential days of low irradiance disrupted internal leaf O2 status in three ways: i) causing a longer lag in morning O2 pressurization, ii) depressing maximum pO2 during the day, and iii) shortening the time where maximum pO2 was sustained into the afternoon. There was a close relationship between leaf and water column pO2 during the day (R2 = 0.93 +/- 0.09); thus, internal O2 dynamics appear more dependent on water column pO2 under low irradiance. The nighttime minimum of leaf and meristem pO2 (1.2 and 0.4 kPa, respectively) were very low and the length of time the meristem sustained hypoxia (< 1.5 kPa pO2) at night was high (9:40 h:min). H2S intrusion into the meristem at night following 24-48 h of shading was persistent, likely the combined effect of limited internal O2 flow without saturating irradiances for photosynthesis, and a breakdown of the sediment oxic microshield. Under low irradiance, more frequent intrusions of H2S to the meristems increases the chances of large-scale seagrass mortality events at night.

Both physical and electrical impacts have been linked to North Sea fisheries activity. This study evaluates how these effects can influence marine ecological functioning by assessing their consequences on benthic pelagic coupling. Experiments were conducted on sediment microcosms taken from 9 North Sea and 2 Eastern Scheldt locations. Samples were subjected to physical disturbances by mechanically stirring the sediment surface or electrical stimulation with exposure to high frequency pulsed bipolar or direct currents. Electrical exposure times of 3 and 120-s were used to simulate in situ exposure times related to sole (Solea solea) and razor clam (Ensis spp.) electric fisheries respectively. Water column oxygen rapidly declined after sediment resuspension, inducing an immediate uptake ranging from 0.55 to 22 mmol oxygen per m−2 of sediment disturbed. Mechanical disturbances released the equivalent of up to 94 and 101 h of natural ammonium and silicate effluxes respectively. Fresh organic material significantly predicted the magnitude of mechanical-induced oxygen, ammonium, phosphate and silicate changes. No biogeochemical effects from bipolar (3 s or 120 s) or 3-s direct current exposures were detected. However, significant changes were induced by 120-s exposures to direct currents due to electrolysis and ionic drift. This lowered the water column pH by 1–1.3 units and caused the appearance of iron oxides on the sediment surface, resulting in the equivalent of 25–28 h of sedimentary phosphate removal. Our findings demonstrate that prolonged (+1 min) exposure to high frequency pulsed direct currents can cause electrochemical effects in the marine environment, with implications for phosphorus cycling. Nevertheless, bi-directional pulsed currents used in flatfish pulse trawling and AC waveforms featured in Ensis electrofishing, seem to severely limit these effects. Mechanical disturbance, on the other hand, causes a much greater effect on benthic pelagic coupling, the

Transformation and retention of nitrogen and other biologically reactive solutes in the hyporheic zones of running water contribute to an essential ecosystem service. However, the synoptic impact of intense agricultural or urban land-uses, elevated nutrient loading, flow alterations, riparian clear-cutting and channelization on the source-sink behavior of solutes in hyporheic zones remains largely uncharacterized and unquantified. Therefore, we studied nutrient dynamics in a hydro-morphologically and chemically modified stream reach using a new monitoring approach allowing the simultaneous measurement of nutrient and water flux through a screened area in the subsurface of rivers (Hyporheic Passive Flux Meter, HPFM). With HPFMs we directly assessed time-integrated lateral hyporheic nitrate fluxes during early spring and mid-summer covering different temperature and discharge regimes.Contrary to our expectations, higher stream discharge coincided with substantially lower hyporheic exchange rates. While in streams featuring a natural morphology, bed-form induced exchange commonly increases with surface flow, the influence of groundwater level was dominant in this reach. Furthermore, in contrast to less impacted environments, where progressive substrate depletion with depths reduces metabolic rates in the subsurface, we identified not the upper, but the intermediate layer of the hyporheic zone as hotspot of nutrient turnover. Overall, the hyporheic zone at the study site functioned partly as nitrate source, partly as a sink. Neither of the commonly used determinants redox state and residence time could explain this source or sink function. Our results give clear evidence to carefully transfer the knowledge of hyporheic zone processes from “natural” systems to anthropologically modified streams.

In this paper, we report on phosphorescence measurements for oxygen dynamics in cells by means of a correlation method, which is an expansion of the fluorescence correlation spectroscopy. The intensity correlation function of the emission excited by a pulsed light source was measured. With changing the pulse timing, both the fluorescence correlation function and the decay time of phosphorescence could be analyzed. This method was applied for the analysis of the oxygen dynamics in HeLa cells stained by Pd(II)-porphine. The decay function consisted of two exponential components, which might be attributed to free and protein-bound forms of Pd(II)-porphine in the cell, respectively. The relative change of the oxygen concentration under normal and uncoupled respiration conditions was also measured. The simplicity of this method is a great advantage in the biological applications. Although the current system we used was limited in the temporal resolution, the method is in principle applicable to faster decay time measurements down to the nano-second range of the fluorescence decay times.

In this paper, we report on phosphorescence measurements for oxygen dynamics in cells by means of a correlation method, which is an expansion of the fluorescence correlation spectroscopy. The intensity correlation function of the emission excited by a pulsed light source was measured. With changing the pulse timing, both the fluorescence correlation function and the decay time of phosphorescence could be analyzed. This method was applied for the analysis of the oxygen dynamics in HeLa cells stained by Pd(II)-porphine. The decay function consisted of two exponential components, which might be attributed to free and protein-bound forms of Pd(II)-porphine in the cell, respectively. The relative change of the oxygen concentration under normal and uncoupled respiration conditions was also measured. The simplicity of this method is a great advantage in the biological applications. Although the current system we used was limited in the temporal resolution, the method is in principle applicable to faster decay time measurements down to the nano-second range of the fluorescence decay times.

In this paper, we report on phosphorescence measurements for oxygen dynamics in cells by means of a correlation method, which is an expansion of the fluorescence correlation spectroscopy. The intensity correlation function of the emission excited by a pulsed light source was measured. With changing the pulse timing, both the fluorescence correlation function and the decay time of phosphorescence could be analyzed. This method was applied for the analysis of the oxygen dynamics in HeLa cells stained by Pd(II)-porphine. The decay function consisted of two exponential components, which might be attributed to free and protein-bound forms of Pd(II)-porphine in the cell, respectively. The relative change of the oxygen concentration under normal and uncoupled respiration conditions was also measured. The simplicity of this method is a great advantage in the biological applications. Although the current system we used was limited in the temporal resolution, the method is in principle applicable to faster decay time measurements down to the nano-second range of the fluorescence decay times.