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Over millennia, droughts could not be understood or defined but rather were associated with mystical connotations. To understand this natural hazards, we first needed to understand the laws of physics and then develop plausible explanations of inner workings of the hydrological cycle. Consequently, modeling and predicting droughts was out of the scope of mankind until the end of the last century. In recent studies, it is estimated that this natural hazard has caused billions of dollars in losses since 1900 and that droughts have affected 2.2 billion people worldwide between 1950 and 2014.
For these reasons, droughts have been identified by the IPCC as the trigger of a web of impacts across many sectors leading to land degradation, migration and substantial socio-economic costs. This thesis summarizes a decade of research carried out at the Helmholtz Centre for Environmental Research on the subject of drought monitoring, modeling, and forecasting, from local to continental scales. The overarching objectives of this study, systematically addressed in the twelve previous chapters, are: 1) Create the capability to seamless monitor and predict water fluxes at various spatial resolutions and temporal scales varying from days to centuries; 2) Develop and test a modeling chain for monitoring, forecasting and predicting drought events and related characteristics at national and continental scales; and 3) Develop drought indices and impact indicators that are useful for end-users. Key outputs of this study are: the development of the open source model mHM, the German Drought Monitor System, the proof of concept for an European multi-model for improving water managent from local to continental scales, and the prototype of a crop-yield drought impact model for Germany.
Weakly electric mormyrid fish comprise about 200 species. 15 species of the genus Campylomormyrus have been described. These are very diverse concerning the trunk-like snout and the shape and duration of the electric organ discharge (EOD) and the anatomy of the electric organ. In this dissertation data on the reproduction in captivity of four species and on the ontogeny of the EOD and the EO of three species are presented.
Reproduction of the four species C. compressirostris, C. rhynchophorus, C. tshokwe and C. numenius: Cyclical reproduction was provoked by changing only water conductivity (C): decreasing C led to gonadal recrudescence, an increase induced gonad regression. Data on the reproduction and development of three species are presented (in C. numenius gonad development could only be achieved in males). Agonistic behavior in the C. tshokwe pair forced us to divide the breeding tank; therefore, only ovipositions occurred. However, injection of an artificial GnRH hormone allowed us to obtain ripe eggs and sperm and to perform successful artificial reproduction. All three species (C. compressirostris, C. rhynchophorus, C. tshokwe) are indeterminate fractional spawners. Spawnings/ovipositions occurred during the second half of the night; no parental care was observed; no special spawning substrates were necessary. C. compressirostris successfully spawned in breeding groups, C. rhynchophorus as pair. Spawning intervals ranged from 6 to 66 days in C. rhynchophorus, 10–75 days in C. tshokwe, and 18 days in C. compressirostris (calculated values). Fecundities (eggs per fractional spawning) ranged from 70 to 1570 eggs in C. rhynchophorus, 100–1192 in C. tshokwe, and 38–246 in C. compressirostris. All three species produce yolky, slightly sticky eggs. Egg diameter ranges from 2.3–3.0 mm. Hatching occurred on day 3, feeding started on day 11. Transition from larval to juvenile stage occurred at around 20 mm total length (TL). At this size C. rhynchophorus developed a higher body than the two other species and differences between the species in the melanin pigmentation of the unpaired fins occurred. Between 32 and 35 mm TL the upper and lower jaws developed.
C. compressirostris and C. tamandua are similar in morphology and both produce short EODs of ca. 150-200 μs duration. Both species reproduce easily in captivity. We tried to obtain natural hybrids in two breeding groups, 1) four males of C. compressirostris and three females of C. tamandua and 2) six females of C. compressirostris and four males of C. tamandua. In both combinations several times oviposition occurred, however, we never found fertilized eggs. In subsequent experiments, not described here, we obtained hybrids between these two species by means of artificial reproduction.
Ontogeny of the EOD and the EO: The Campylomormyrus species are very diverse both concerning the shape and the duration of their EODs. There are species with very short EODs, e.g. C. compressirostris duration, a species with an EOD length of about 4-8 ms duration (C. tshokwe) and species with very long EODs of about 25 ms duration (e.g. C. rhynchophorus). Due to the successful breeding of the three species in captivity, we were able to investigate in detail the ontogeny of the EOD. Larvae of the three species C. compressirostris, C. tshokwe and C. rhynchophorus first produce a biphasic larval EOD typical for these small larvae. The first activity of the adult electric organ in the caudal peduncle is a biphasic juvenile EOD. Juvenile C. compressirostris and C. tshokwe start out with a short biphasic EOD of about 160 – 200 μs duration at sizes between 25 mm (C. compressirostris) and 37 mm (C. tshokwe). Adult C. compressirostris show an EOD identical to that of the juvenile. In C. tshokwe, the juvenile EOD changes continuously during development both concerning duration, amplitude increase and shape. 18 cm long C. tshokwe still do not yet produce an EOD typical for the adult fish. Juveniles of C. rhynchophorus produce at 33 mm total length a juvenile biphasic EOD, however, of longer duration (about 640 μs) than the two species mentioned above. This juvenile EOD changes continuously both in form, amplitude increase and duration with growth until the adult EOD waveform appears at about 15 cm body length. In juveniles about seven cm long the triphasic feature of the EOD starts to develop due to the appearance of a second head positive phase. Specific EOD stages are produced in relation to size and not to age. Individual differences in the EOD both concerning shape and duration are very small. The basic anatomy of the electrocytes is very similar in all three species: the main stalk which receives the innervation, is located at the caudal face of the electrocyte. Membrane penetrations of the stalks do not occur. However, there are differences in the fine structure of the electrocytes in the three species. Papillae, proliferations of the membrane, which increase the surface area of the electrocyte and are thought to incrase the EOD-duration, are only found in C. tshokwe and C. rhynchophorus. In these two species in addition, holes develop in the electrocytes during ontogeny. This might also have an impact on EOD duration.