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  • Other Sources  (24)
  • Articles (OceanRep)  (24)
  • Inter Research  (21)
  • American Meteorological Society
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  • 2000-2004  (24)
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  • 2000  (17)
  • 1
    Publication Date: 2018-05-09
    Description: Grazing experiments were conducted with natural mesozooplankton from Kiel Bight, Germany, using radioactive labelled phytoplankton cultures and seston size fractions. The results of experiments using phytoplankton cultures indicated that bivalve veligers performed highest clearance of particles within a size range of 4.7 to 6.3 µm, whereas optimum particle size for copepods was 15 µm. The results of experiments using labelled natural seston size fractions identified bivalve veligers and appendicularians as those responsible for the removal of particles within the smallest size class (〈2 µm). Seston size fractions larger than 5 µm were mainly cleared by copepods and nauplii. As particle size increased, the contribution of copepod clearance to total zooplankton clearance within size classes increased from 57% (〈5 µm size class) to more than 81% (30 to 100 µm size class). When the nauplii clearance rates were included, the total copepod clearance accounted for 90 to 97.6% of the total volume cleared of particles bigger than 10 µm. Despite low abundances of bivalve veligers and appendicularians in Kiel Bight at the time of the experiment, we calculated that approximately 10 and 8.5%, respectively, of the carbon ingested by total mesozooplankton was due to veliger and appendicularian grazing. The importance of bivalve veligers might be seen in their grazing on seston particles that escape predation by copepods and on the amount of energy that is therefore directed from the water column to the benthos when larvae settle.
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  • 2
    Publication Date: 2018-05-28
    Description: Filamentous sulfur-oxidizing bacteria and geochemical parameters of sediments at the Makran accretionary wedge in the northeastern Arabian Sea off Pakistan were studied. The upper continental slope between 350 and 850 m water depth, which is in the center of the oxygen-minimum zone, is characterized by numerous sites of small-scale seeps of methane- and sulfide-charged porewater. White bacterial mats with diameters 〈1 m were discovered at the surface of these sites using a photo-TV sled. Seep sediments, as well as non-seep sediments, in the vicinity were characterized by the occurrence of the bacterium Thioploca in near-surface layers between 0 and 13 cm depth. Thioploca bundles were up to 20 mm in length and contained up to 20 filaments of varying diameters, between 3 and 75 µm. Up to 169 ind. cm-2 were counted. Maximum numbers occurred in the top 9 cm of sediment, which contained very low concentrations of soluble sulfide (〈0.2 µM) and high amounts of elemental sulfur (up to 10 µmol cm-3). Moderate sulfate reduction activity (between 20 and 190 nmol cm-3 d-1) was detected in the top 10 cm of these sediments, resulting in a gradual downcore decrease of sulfate concentrations. CO2 fixation rates had distinct maxima at the sediment surface and declined to background values below 5 cm depth. The nutritional implications of the distinct morphology of Thioploca and of the geochemical setting are discussed and compared to other sites containing Thioploca communities.
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  • 3
    Publication Date: 2018-05-08
    Description: Spatial and temporal variability in environmental factors can exert major influences on survival and growth of living organisms. However, in many key areas of fisheries science (e.g. growth, survival and recruitment determination), environmental heterogeneity is usually ignored because of insufficient environmental or fisheries data or lack of evidence that such heterogeneity impacts response variables. For the eastern Baltic Sea (ICES Subdivisions 25 to 32), we evaluated spatial and temporal differences in conditions affecting the survival of cod Gadus morhua L. eggs at survival on four distinct spawning sites within the assessment area. We intercalibrated ways of quantifying the volume of water ('reproductive volume') at each site where salinity, oxygen and temperature conditions permitted successful egg development. We have developed and compared a time series (1952 to 1996) of reproductive volumes among the areas to identify spatial differences. The results of 2 independent volume-estimation methods are comparable, indicating that highly significant differences exist among the sites, and that the westernmost spawning ground, Bornholm Basin, has on average the highest reproductive volume and the lowest variability among the 4 sites. These findings may be useful in evaluating how spatial and temporal variability in environmental conditions affect egg hatching success and possibly recruitment in the Baltic stock.
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  • 4
    Publication Date: 2018-05-28
    Description: Natural marine bacteria populations collected from nearshore waters produce different types of siderophores depending on the degree of iron limitation. These siderophores can facilitate iron uptake in the marine diatom Phaeodactylum tricornutum. Water samples from 15 stations along the Italian coast of the northwest Adriatic Sea were collected and filter fractionated (3.0, 0.8 and 0.2 µm). Siderophore production in the fractions was determined using cross-feeding experiments with siderophore-auxotrophic bacteria. At most stations sampled, bacteria collected in the 3.0 and 0.8 µm filters produced siderophores which stimulated growth in Morganella morganii, the indicator strain for α-keto/ α-hydroxy acids. The results suggest that MGF (ŒMorganella-Growth Factor¹) production is common among filamentous and appendaged bacteria or strains associated with particles. Natural bacteria populations grown in iron-deficient media stimulated growth of all the indicator strains in the cross-feeding tests. Examples of known MGF which supply iron to M. morganii were tested for their ability to act as iron source for the marine diatom P. tricornutum. Iron uptake from 55Fe-MGFs was measured in P. tricornutum cells grown in Fe-sufficient and Fe-deficient media. Unchelated iron (55FeCl3 ) and 55FeEDTA were used as controls. The uptake of iron from the 55Fe-MGF and 55FeCl3 by Fe-deficient cells was higher (109 to 150 pgFe mg-1) than from 55FeEDTA (34 pgFe mg-1). Similarly, Fe-sufficient P. tricornutum took up iron from the 55Fe-MGF and 55FeCl3 to the same extent (~50 pgFe mg-1) while minimal uptake (8 pgFe mg-1) was measured from FeEDTA. In growth experiments where iron-deficient diatom cells were incubated in media containing different sources of iron, e.g. FeCl3, Fe-MGF and FeEDTA, a greater increase in number was observed in cells supplied with Fe-MGF. Further experiments also show that the uptake of Fe from MGF was enhanced by light and that a reduction step was involved in the uptake process. MGF also promoted the uptake of colloidal ferrihydrites. This study gives further evidence that siderophores produced by bacteria can be utilized by phytoplankton as an iron source. We therefore suggest that these substances play an important role in increasing the availability of iron to phytoplankton in coastal waters and thus are major factors defining the chemistry of iron in the marine environment.
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  • 5
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    Inter Research
    In:  Marine Ecology Progress Series, 211 . pp. 261-274.
    Publication Date: 2015-02-09
    Description: Marine angiosperms, or seagrasses, continue to be a major focus of marine biologists because of their important ecological role in many coastal ecosystems. Seagrass population biology could benefit from a population genetic perspective because genetic data enable the extraction of useful demographic information such as isolation and gene flow between demes. Moreover, population genetic processes may contribute to the growing ecological risks of local population extinction. Progress in seagrass genetics is partly driven by novel genetic markers which detect variation at the DNA level and overcome the limited polymorphism of allozymes. Key results of studies in the past decade, mostly using RAPD and microsatellites, were (1) considerable genetic and genotypic (clonal) diversity is present in several species in contrast to earlier notions of low polymorphism detected at allozyme loci, and (2) genetic differentiation among populations seems to be the rule despite earlier reports of genetic uniformity. Pronounced genetic structure was detected between populations of 4 species examined thus far (Posidonia oceanica, P. australis, Zostera marina, Thalassia testudinum). The FST estimates varied widely and ranged from 0.01 to 0.623 across studies and species. Genetic differentiation at a systematic range of scales was only studied in eelgrass Zostera marina, where it was positively correlated with geographic distance. The high polymorphism of RAPD or microsatellite markers will allow the augmention of indirect estimates of gene flow by methods detecting individual immigration events through paternity analysis or assignment tests. Important conservation related issues such as the level of inbreeding and the effective population size have also been obtained from genetic marker data, but results are too scarce at the moment to allow generalizations. In Zostera marina and Posidonia australis, several population genetic attributes such as clonal diversity, mating system and effective population size varied among populations within species, highlighting that there is no Œtypical¹ population. An important gap in our knowledge is whether the effects of natural population fragmentation and patchiness enhance the genetic isolation of populations due to anthropogenic disturbances. It is also unclear whether genetic differentiation displayed at marker loci are correlated with fitness-related plant traits, and whether genetic or genotypic diversity is important for medium- to long-term meadow persistence. An assessment of the genetic and genotypic diversity at marker loci should be combined with experiments on the ecological plasticity and reaction norms of genotypes composing the populations in question. This way, the role of genetic diversity for seagrass population maintenance and growth in the face of changing environmental conditions can be evaluated.
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  • 6
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    Inter Research
    In:  Marine Ecology Progress Series, 196 . pp. 269-277.
    Publication Date: 2018-05-09
    Description: Throughout the last 2 decades a shift from a cod- to a sprat-dominated system occurred in the upper trophic levels of the Central Baltic Sea. This was caused by a decline in the cod stock, due to recruitment failure and high fishing intensity, resulting in a decrease in predation pressure on sprat. Concurrently with the lowest cod stock size on record, sprat reached biomass values of above 2 × 106 t in 1992, being relatively stable afterwards. Besides predation mortality through cod and in recent years also an increasing fishing pressure, cannibalism on eggs may be a compensatory process limiting the reproductive success of sprat and hence contributing to the population regulation in the Central Baltic. Based on sprat stomach sampling on 21 cruises between March 1988 and July 1996 cannibalism on sprat eggs was investigated in the Bornholm Basin, one of the main spawning areas of Central Baltic sprat. Using a model of gastric evacuation to estimate daily food intake rates and a Virtual Population/Extended Survivor Analysis for computing predator population sizes, egg cannibalism rates were estimated. These were compared to egg abundance data from ichthyoplankton surveys and to preliminary estimates of seasonal egg productions. The study revealed significant interannual differences in the intensity of sprat egg cannibalism with considerable predation in 1990 to 1992 (〉15% of the egg abundance during peak spawning and 〉60% of the seasonal production) and a reduction in most recent years (〈16% of the corresponding abundance and production). As a possible reason for these differences a combination of changes in the vertical overlap of predator/prey and variability in the food environment were identified. Shortcomings of the applied methods and the possible impact of cannibalism on the reproductive success and population development of sprat in the Central Baltic Sea are discussed.
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  • 7
    Publication Date: 2018-05-08
    Description: Nanoplankton and picoplankton abundance and community grazing on picoplankton were determined in summer and autumn at several stations in a productive coastal environment (Georges Bank, NW Atlantic Ocean) and in an oligotrophic oceanic ecosystem (Sargasso Sea). Ranges of heterotrophic nanoplankton (HNAN) abundance were 1.2 to 3.6 x 103 cells ml-1 on Georges Bank, and 2.2 to 6.8 x 102 cells ml-1 in the Sargasso Sea. Ranges of phototrophic nanoplankton (PNAN) abundance in these ecosystems were 1.9 to 6.0 x 103 and 1.3 to 4.7 x 102, respectively. Mixotrophic nanoplankton (MNAN), operationally defined here as chloroplast-bearing nanoplankton that ingested fluorescent tracers, comprised an average of 12 to 17% of PNAN in surface waters in both environments during August and October. Mixotrophs at specific stations constituted as much as 38% of total PNAN abundance on Georges Bank and 30% in the Sargasso Sea. Mixotrophs represented up to 39% of the total phagotrophic nanoplankton abundance (MNAN/[MNAN + HNAN]). Community grazing impact was estimated from the disappearance of fluorescent prey surrogates (fluorescently labeled bacteria, FLB; cyanobacteria, FLC; and 〈\3 µm algae, FLA). Absolute grazing rates (total picoplankton cells removed d-1) on Georges Bank exceeded those in the Sargasso Sea due to the greater abundances of predators and prey. However, there was overlap in the specific grazing losses at the 2 sites (ranges = 0.08 to 0.38 d-1 in the coastal ocean and 0.05 to 0.24 d-1 in the oligotrophic ocean). Rates of bacterivory were in approximate balance with rates of bacterial production (3H-thymidine uptake), but production exceeded bacterivory on Georges Bank during the summer cruise. These data are among the first documenting the impact of grazing on picoplankton in these environments, and they are consistent with the prediction that nanoplanktonic protists are major predators of picoplankton. While the proportion of phototrophs that are phagotrophic was highly variable, our study indicates that algal mixotrophy is widespread in the marine environment, occurring in both coastal and oligotrophic sites, and should be considered quantitatively in microbial food web investigations.
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  • 8
    Publication Date: 2018-05-28
    Description: Nitrogen fixation rates and related plankton parameters were determined at 8 stations in the Baltic proper and Mecklenburg Bay in 1997 and 1998. Nitrogen fixation was measured with 15N-tracer method in unenriched samples. Measurable nitrogen fixation rates were found from July to October, with highest rates in August when heterocystous cyanobacteria formed blooms. Nitrogen fixation rates measured during a moderate bloom in 1998 were related to biomass of heterocystous and coccoid (non-heterocystous) cyanobacteria and primary production. The size fraction 〈10 µm contributed significantly to total nitrogen fixation both during day and night. It is discussed whether this may be due to small, non-heterocystous cyanobacteria, which were abundant in summer and autumn. They may separate photosynthesis and nitrogen fixation temporally (day and night) and may be especially responsible for the high nitrogen fixation rates observed in the dark. As the fraction of pico- and nanoplankton was not considered in earlier studies, a new budget of nitrogen fixation in the Baltic proper has been estimated. On average, daily nitrogen fixation rates of 2.5 mmol N m-2 d-1 (in July/August 1997/1998) and mean annual nitrogen fixation of 125 mmol N m-2 yr-1 were estimated for the Baltic proper. The high variability is discussed. For summer 1998, a budget of the nitrogen cycle at the main station in the Gotland Sea is give
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  • 9
    Publication Date: 2018-05-28
    Description: Small cetaceans are susceptible to incidental mortality in the various forms of gillnet fisheries throughout their range. Research conducted since 1994 has shown that acoustic alarms (pingers) emitting high-frequency pulsed sounds effectively reduce the number of harbor porpoise Phocoena phocoena casualties in sink gillnets. However, the mechanisms behind the effects of pingers were still not understood. Until now, advantages and risks associated with their widespread use could not be evaluated. Here we present the results of 2 field experiments: (1) theodolite-tracking of harbor porpoises exposed to a single PICE-pinger in Clayoquot Sound, Vancouver Island, Canada and (2) herring Clupea harengus capture rates in surface gillnets equipped with and without acoustic alarms (Dukane Netmark 1000, Lien, PICE) in the Baltic Sea herring fishery at Rügen Island, Germany. Our results show that harbor porpoises do not seem to react to an experimental net in their foraging area (n = 172 groups, median group size = 2 porpoises). Porpoise distance from the mid-point of the net was distributed around a median of only 150 m (range 4 to 987 m). A net equipped with an acoustic alarm, however, was avoided (n = 44 groups) within audible range (distance distribution median = 530 m, range 130 to 1140 m). The porpoises were thus effectively excluded from the ensonified area. Herring, one of the main prey species of harbor porpoises, were not affected by the acoustic alarms tested (n = 25407 fish captured). The advantages and risks of using acoustic alarms to mitigate by-catch are discussed.
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  • 10
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    Inter Research
    In:  Marine Ecology Progress Series, 197 . pp. 19-25.
    Publication Date: 2018-05-09
    Description: This study tested whether the extreme scarcity of larger nanophytoplankton and microphytoplankton in the Gulf of Aqaba and in the open northern Red Sea is caused by nutrient limitation or by selective removal by grazers. Samples of near surface phytoplankton were incubated on board under a fully factorial combination of release from grazing pressure and release from nutrient stress. Release from grazing pressure by different size classes was obtained by sieving through 100, 20, and 10 µm size mesh screens. Release from nutrient stress was obtained by enrichment of Si alone and a full enrichment by N, P, Si and trace elements. Growth rates of most phytoplankton taxa showed a strong, positive response to the full nutrient enrichment and a weaker, but significant response to grazer exclusion. Several diatom taxa showed a weak positive response to Si enrichment. Thus, bottom-up control of medium-sized algae appears to be more important than top-down control.
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