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Bo Riemann
Director Danish BNI
NERI
Aarhus University
Extensive experience in ecological and ecotoxicological studies on freshwater and marine environments. Aquatic ecology with special reference to production and decomposition of organic matter in pelagic communities.
For additional info: Selected publications: Effects of nutrient and light regimes on the pigment composition of 12 species of marine phytoplankton, representing eight algal classes, were examined in batch cultures. During exponential growth, significant differences in pigment ratios caused by light were found for several pigments in six species. Cellular contents of lutein increased with increasing irradiance in Pyramimonas disomata and Brachiomonas sp. indicating a photoprotective role of this pigment. Nutrient regimes had a significant effect only on fucoxanthin:Chl a in Ditylum brightwellii. During stationary growth phases, fewer pigment to Chl a ratios were significantly affected by irradiance, but significant effects from nutrient-limitation were found on the ratio of most marker pigments to Chl a. In four cryptophytes, the alloxanthin:Chl a ratio increased from 1.3-fold up to 9-fold from exponential growth phase to N-limited stationary phase, illustrating different stability of the marker pigment to Chl a ratio within the same phytoplankton group and even within the same genus. Pigment ratios obtained from exponential and stationary growth phases and from low and high light conditions were applied to pigment data from natural phytoplankton populations from two Danish fjords using the matrix factorization program CHEMTAX. Comparison of calculated phytoplankton compositions with direct phytoplankton counts showed the best agreement for exponential growth phase ratios from medium or low light intensities. Discrepancies between community structure described by microscopy and by using pigment analysis are discussed. Søndergaard, M., Williams, P.LeB., Cauwet, G., Riemann, B., Robinson, C., Terzik, S., Worm, J. (2000) Net accumulation and flux of DOC and DON in marine plankton communities. Limnol. & Oceanogr. 45: 1097-1111 Marine mesocosms were manipulated with inorganic nutrients over a period of 22 d to investigate organic carbon partitioning under a variety of nutrient regimes. The chemical analyses and biotic measurements included inorganic nutrients, pigment signatures, particulate and dissolved organic species, bacterial production, and community respiration. The biodegradability of dissolved organic carbon (DOC) was investigated with in vitro decomposition experiments. The net particulate organic carbon (POC) production was 50% of the total organic production during the initial 6 d of nutrient-replete growth and during a major diatom bloom. In all other situations the carbon partitioning was strongly in favor of DOC, which accounted for 82 to 111% of the total production. The production of new DOC preceded new DON by about 1 week. Thus, the new dissolved organic matter (DOM) initially had an infinite C: N ratio, which fell to 11–20 when DON started to accumulate. The highest C: N ratio was measured during a nutrient-replete diatom bloom. Dissolved polysaccharides accounted for 50 to 70% of the new DOC, and the lowest relative amount was produced during a diatom bloom. The chemical analyses unequivocally demonstrated that carbon partitioning in favor of carbon-rich DOM can take place during an active diatom bloom and not only during the decay of a bloom. The DOC-producing mechanisms cannot be fully identified. However, during the different growth phases the DOC production varied, as did the speciation of DOM with respect to the C: N ratios. When net production of dissolved organic nitrogen (DON) was detected after 11 d, the DON production accounted for 25 to 50% of the daily added and assimilated inorganic nitrogen. The measurements of community respiration made it possible to calculate the maximum carbon recycling by bacteria and bacterial net DOC assimilation. These calculations showed the estimates of carbon partitioning to be very sensitive to bacterial growth yield values and the factors used to convert leucine and thymidine isotope incorporation to bacterial production. Decomposition experiments showed that at least 35% of the new DOC was biodegradable over 10–12 d and that inorganic nutrients only marginally affected use. The calculated turnover times of new DOC were between 15 and 25 d. The semilabile nature of new DOC with respect to microbial attack is suggested as the main reason for the medium-term accumulation of new DOC. Baretta-Bekker, J.G., Baretta, J.W., Hansen, A.S. & Riemann, B. (1998) An improved model of carbon,and nutrient dynamics in the microbial foodweb in marine enclosures. Aquat.Microb. Ecol.14: 91-108 A description of an improved dynamic simulation model of a marine enclosure is given. New features in the model are the inclusion of picoalgae and mixotrophs; the ability of bacteria to take up dissolved inorganic nutrients directly; and, for the phytoplankton functional groups, the inclusion of luxury uptake and the decoupling of the nutrient uptake dynamics from carbon-assimilation dynamics. This last feature implies dynamically variable phosphorus/carbon and nitrogen/carbon ratios. The model was calibrated with experimental results from enclosure experiments carried out in Knebel Vig, a shallow microtidal land-locked fjord in Denmark, and verified with results from enclosure experiments in Hylsfjord, a deep and salinity-stratified Norwegian fjord. Both observations and model simulations showed dominance of a microbial food web in control enclosures with low productivity. In N- and P-enriched enclosures a classical food web developed, while an intermediate system was found in N-, P- and Si-enriched enclosures. Mixotrophic flagellates were most important in the nutrient-limited control enclosures where they accounted for 49% of the pigmented biomass and about 48% of the primary production. Lumping the mixotrophs in the simulation model with either the autotrophic or the heterotrophic functional groups reduced total primary production by 74%. Model-derived, time-averaged phosphorus budgets suggested that bacteria competed with algae for orthophosphate in the control enclosure, but not in the enclosure to which N and P had been added, where bacteria functioned as net mineralisers of phosphate. In the N, P and Si enclosure, bacteria took up only 10% of the amount of orthophosphate taken up by the primary producers, passing most of the organic phosphorus on to their grazers, the heterotrophic nanoflagellates, and mineralising only a small fraction directly. Inclusion of luxury nutrient uptake affected the simulation of the nutrient-enriched enclosures, while the decoupling of carbon and nutrient dynamics affected the simulation of the control enclosure. Without these 2 processes it was not possible to simulate the carbon and nutrient dynamics in the different enclosures adequately with the same parameterisation.
Abundance and bacterivory of mixotrophic flagellates were examined in a vertical profile during 1 wk in June 1992 in the Bay of Aarhus, Denmark. A stable pycnocline separated an upper water mass with low salinity, low inorganic nutrient concentration (< 0.1 umol l-1) and low bacterial abundance (<106 ml-1) from a bottom water mass with higher salinity, inorganic nutrient concentration, and bacterial abundance (>106 ml-1). In the upper layer, bacterivorous, pigmented flagellates (mixotrophs) accounted for 49% of the pigmented biomass. In addition to their function as primary producers, mixotrophic flagellates were responsible for 86% of the entire flagellate bacterivory. The abundance of bacterial food particles (<106 ml-1) was probably not sufficient to sustain growth of most bacterivorous, colourless flagellates, and the nutrient-depleted water body prevented the strict phototrophs from dominating the environment. Below the pycnocline, nutrients were present, bacterial abundance exceeded 106 ml-1, and mixotrophic flagellates made up only 9% of the pigmented biomass and accounted for 19% of the flagellate bacterivory. |
