Hempel, Gotthilf
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Item-typ:Veröffentlichung, Exploration of structure and trophodynamics of coelobite (cavity-dwelling) communities in Red Sea coral reefs(2002-02-02); ; ; The results of this work contribute to the understanding of the ecology of coral reefs in general. The development of the CaveCam provides the key for a new line of coral reef research allowing to explore and document the labyrinth of framework cavities interlacing coral reefs in a non-destructive way. It yields 1:1 close-up images of specimens up to 4 m inside the reef. Image analysis revealed a rich variety of mainly encrusting organisms. Coralline algae predominated on the walls near the cavity entrances, while darker inner sections of the cavities were colonized by a diverse cryptofauna: filter feeders abounded, notably sponges, which covered up to more than 50% of the substrate. Light is a crucial factor determining the balance between photoautotroph and heterotroph organisms near the cavity entrance, while food supply and competition for space shape the composition of the heterotroph community further away.With the newly developed LightSheet-method the complex morphology of the highly irregular framework cavities was described and reconstructed in 3-D, volume and wall area calculated.The metabolic activity of the coelobite communities is reflected in small-scale gradients in chlorophyll a and oxygen concentrations between cavities and freestream waters over the reef, where stronger gradients appear to be associated with reduced flow and higher coelobite cover. Direct and indirect measurements of water replacement rates within the cavities allowed first quantitative estimates of the bulk filtering effect of the coelobite community. We provide circumstantial evidence, that coelobite filter feeders are indeed causing the observed phytoplankton depletions of chlorophyll a and oxygen. Remineralization of the largely extrinsic organic matter taken up by the coelobite filter feeders may fuel close to 20% of the gross metabolism of the entire reef. A checklist of coelobite organisms in the Red Sea is provided.Dissertation192 125 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Feeding ecology of coral reef sponges(2003-01-17); ; ; Sponges are ubiquitous in coral reefs and in terms of biomass they are often second to corals. Inside coral reef crevices sponges are the dominant organisms, providing up to 60% of the living coelobite cover. We categorized them into 3 distinct groups according to their habitat: obligate coelobites (OC), living exclusively in coral reef crevices; facultative coelobites (FC), occurring both inside crevices and on the outer reef surface; and epi-reefal sponges (ER), dwelling only on the exposed reef surface. In incubation experiments, cryptic sponges released 4 times more total inorganic nitrogen (TIN) (0.51±0.41µmol g AFDM-1 h-1) and 2 times more phosphate (0.07±0.05µmol g AFDM-1 h-1) than ER sponges. 72-91% of TIN released was in the form of ammonia, suggesting that coelobite mineralised nutrients are readily assimilable by algae and zooxanthellae in corals. Comparative in situ measurements of ultraplankton uptake showed that retention efficiency differed between plankton groups: larger eukaryotic algae were retained less efficiently (~60%) than the smaller autotrophic prokaryotes Prochlorococcus and Synechococccus (>90%) with no marked differences between sponge groups. Heterotrophic bacteria were retained most efficiently by OC (83±6%, median±MAD), albeit at 8-fold lower pumping rates. Low volume throughput and high retention efficiency appear as adaptations of OC to the limited supply of plankton and low volume flow in framework crevices. Molecular and histological techniques revealed that ER and FC sponges had only very low or moderate numbers of associated bacteria in their tissue whereas OC sponges harboured high densities. OC and FC community uptake amounted to 0.60±0.36 g C d-1 per projected m2 of reef, equivalent to one sixth of the gross productivity of the entire reef. ER community uptake was more than one order of magnitude lower, compounding the importance of coelobite filter feeders in harnessing pelagic material for the reef benthos.Dissertation335 266 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Ecology, genetic population structure, and molecular phylogeny of fishes on coral reefs in the Gulf of Aqaba and northern Red Sea(2002-04-26); ; ; Aim of this thesis is the study of biogeography and ecology, genetic population structure, and molecular phylogeny of fishes on coral reefs in the Gulf of Aqaba and northern Red Sea. Ecological and genetic pattern are compared on different spatial scales and molecular markers add a temporal scale to study of evolutionary processes.Biogeographic analysis supported the differentiation of the Arabian sub-province from the Indian Ocean, but the affiliation of the Arabian Gulf is not clear.The analysis of the genetic population structure based on the mtDNA control region revealed homogenity between populations of Pterois miles in the Gulf of Aqaba and northern Red Sea and indicate high levels of gene flow.Investigations on interrupted gene flow in the evolutionary history of lionfishes (Scorpaenidae) revealed a differentiation between the Indian Ocean and Western Pacific. Phylogenetic analysis of cyt b and 16S rDNA sequences of the siblings P. miles and P. volitans suggest a divergence time of 2.4-8.3 million years. This coincide with tectonic events and sea level changes in Southeast Asia during the glacial maxima. Additionally, this genetic study gave indications for taxonomic revision of the genera Pterois and Dendrochirus.Ecological studies on the shore fishes off the Jordanian coast showed that fish species richness was positively correlated with hard substrate cover and benthic diversity. Abundance of corallivores was positively linked to live coral cover. The multivariate analysis of the fish community has revealed several associations of fishes in different habitats, such as deep and shallow reef slope.The northern tip of the Gulf of Aqaba and its western shores are particularly subject to human disturbances. Fish abundance at an industrial site was 50% lower than on an undisturbed reef and the trophic community structure was different.Dissertation290 94 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Artisanal Fisheries of Kenya's South Coast: A transdisciplinary case study of a socio-ecological system transition(2010-06-30); ; ; The study addresses the biological and the socio-cultural dimensions of the artisanal fisheries along a 50 km long stretch of the South coast of Kenya. Nine fish landing sites were monitored over two years. Lines, gill nets, traps, and spears were common to all the sites. Beach seine, set nets, cha-cha and ring nets were recently introduced to certain places. Gear selection was not necessarily determined by the efficiency of the gear an indication that artisanal fishery was a social undertaking largely controlled by habit and attitude. The annual catch per unit area was estimated as 14t/km of coastline. The socio-cultural studies in the fishing villages showed strong family bonds. 75% of the fishers were illiterate. Fishing generated one third of total household income in addition to fish used for household consumption. Leadership of the fishing villages was largely in the hands of the elders employing complex effort restrictions based on customs and religious taboos. The study might contribute to an preemptive policy to reconcile the longterm needs and the immediate demands of the local population relying on sustainable fishery resources.Dissertation305 413