ALBERT

Beschreibung: Analysis of 12,000 electronic still camera images collected with the ARGO II vehicle near the Trans-Atlantic Geotraverse (TAG) active hydrothermal mound, 26°N on the Mid-Atlantic Ridge, has made possible the first quantitative in situ assessment of both fissure orientation and width within the median valley of a slow-spreading ridge. Fissures near the TAG mound are partially rubble-filled extensional fractures that cut lightly sedimented seafloor and in ∼1% of our observations host pillow lavas. Fissure widths range from 0.15 to 3.5 m, with a mean of 0.7 m, and do not vary systematically within the survey area. First-order estimates of crack depth, based on these width measurements and reasonable elastic moduli, indicate that fissures are restricted to depths 〈500 m, with a mean depth of ∼70 m. Fissure-associated eruptives were therefore probably fed by shallow propagating dikes. TAG fissures exhibit a wide range of orientations, with ∼40% deviating by 〉45° from the strike of the ridge axis. The formation of obliquely oriented fissures requires that the local least compressive stress direction varies (at least temporarily) from that predicted by the regional tectonic stress field associated with plate separation. This stress field reorientation may be facilitated by variations in the style of magma emplacement within the rift. The close spatial association of long-term hydrothermal activity, fissure-hosted lava flows, and faults and fissures trending oblique to the spreading axis suggests a causal relationship between the impact of dike intrusion and the maintenance of localized hydrothermal flow.

Beschreibung: In near-shore and coastal margin sediments remineralization of organic carbon is significantly affected by biologically mediated solute exchange caused by burrow-dwelling infauna. Although irrigation rates have been determined for various environments, little is known about their seasonal variations and their coupling to the food-supply or the oxygen level in bottom waters. These aspects have been investigated at two sites in the Kiel Bight by modelling pore water concentrations of Cl, which is a suitable tracer for transport processes. A very similar temporal pattern of irrigation was determined at both sites. In spring and fall the effect of bioirrigation on the pore water concentration of Cl is important at both sites, and a more than two to five fold enhancement of solute exchange, relative to diffusional transport, was calculated. The temporal pattern of bioirrigation correlates with that of the Chl.-a (eq) inventory of the surface sediments. Enhanced irrigation rates follow the settling of plankton blooms in this region. During the summer, when low oxygen levels were observed in bottom waters, overall irrigation rates are low. Furthermore, the relative importance of irrigation processes operating close to the sediment surface increases suggesting an upward movement and migration of burrow-dwelling organisms in response to low O2-concentrations. Because bioirrigation is an important transport process coupling organic carbon flux, remineralization at the seafloor, and redox zonation in the sediment quantifying the seasonal cycle of the irrigation intensity represents a step forward in the dynamic understanding of benthic processes.

Beschreibung: In the Arabian Sea, productivity in the surface waters and particle flux to the deep sea are controlled by monsoonal winds. The flux maxima during the South-West (June–September) and the North-East Monsoon (December–March) are some of the highest particle fluxes recorded with deep-sea sediment traps in the open ocean. Benthic microbial biomass and activities in surface sediments were measured for the first time in March 1995 subsequent to the NE-monsoon and in October 1995 subsequent to the SW-monsoon. These measurements were repeated in April/May 1997 and February/March 1998, at a total of six stations from 1920 to 4420 m water depth. This paper presents a summary on the regional and temporal variability of microbial biomass, production, enzyme activity, degradation of Full-size image (〈1 K)-labeled Synechococcus material as well as sulfate reduction in the northern, western, eastern, central and southern Arabian deep sea. We found a substantial regional variation in microbial biomass and activity, with highest values in the western Arabian Sea (station WAST), decreasing approximately threefold to the south (station SAST). Benthic microbial biomass and activity during the NE-monsoon was as high or higher than subsequent to the SW-monsoon, indicating a very rapid turnover of POC in the surface sediments. This variation in the biomass and activity of the microbial assemblages in the Arabian deep sea can largely be explained by the regional and temporal variation in POC flux. Compared to other abyssal regions, the substantially higher benthic microbial biomasses and activities in the Arabian Sea reflect the extremely high productivity of this tropical basin.

Beschreibung: A coupled ice–ocean model is utilized to investigate the transports of heat, salt and water in the Baltic Sea for the years 1986, 1988, 1993 and 1994. The oceanic component of the coupled system is a three-dimensional baroclinic model of the Baltic Sea including the Belt Sea and the Skagerrak/Kattegat area. The model has a horizontal resolution of ∼5 km and 28 vertical levels specified. The ice model is based on the Hamburg Sea Ice model, with the same horizontal resolution. The coupled system is driven by atmospheric data provided by the Swedish Meteorological and Hydrological Institute (SMHI; Norrköping, Sweden) and river runoff taken from a monthly mean runoff database. The thermohaline variability of the Baltic Sea strongly depends on the fluctuations of the atmospheric forcing conditions. Therefore, high demands on the spatial and temporal resolution of the meteorological forcing are required. Besides heat and radiation fluxes, precipitation and evaporation rates have to be taken into account. From the coupled runs, the different components determining the energy and water cycle of the Baltic Sea are identified and estimates of the water, heat and salt transports are given for the different years. Furthermore, the thermohaline variability is investigated with respect to the relevant forcing mechanisms including atmospheric, as well as fresh water fluxes. Besides the heat and water fluxes of the Baltic Sea and the water mass exchange with the North Sea, internal fluxes of heat, salt and volume between the different subbasins of the Baltic Sea are presented. Sensitivity studies on the variation of the net fresh water flux indicate that uncertainties in precipitation and/or river runoff can have a strong impact on the inflow of highly saline water from the North Sea, thus, influencing the thermohaline circulation of the Baltic Sea.

Beschreibung: The ‘International Intercomparison Exercise of fCO2 Systems’ was carried out in 1996 during the R/V Meteor Cruise 36/1 from Bermuda/UK to Gran Canaria/Spain. Nine groups from six countries (Australia, Denmark, France, Germany, Japan, USA) participated in this exercise, bringing together 15 participants with seven underway fugacity of carbon dioxide (fCO2) systems, one discrete fCO2 system, and two underway pH systems, as well as systems for discrete measurement of total alkalinity and total dissolved inorganic carbon. Here, we compare surface seawater fCO2 measured synchronously by all participating instruments. A common infrastructure (seawater and calibration gas supply), different quality checks (performance of calibration procedures for CO2, temperature measurements) and a common procedure for calculation of final fCO2 were provided to reduce the largest possible amount of controllable sources of error. The results show that under such conditions underway measurements of the fCO2 in surface seawater and overlying air can be made to a high degree of agreement (±1 μatm) with a variety of possible equilibrator and system designs. Also, discrete fCO2 measurements can be made in good agreement (±3 μatm) with underway fCO2 data sets. However, even well-designed systems, which are operated without any obvious sign of malfunction, can show significant differences of the order of 10 μatm. Based on our results, no “best choice” for the type of the equilibrator nor specifics on its dimensions and flow rates of seawater and air can be made in regard to the achievable accuracy of the fCO2 system. Measurements of equilibrator temperature do not seem to be made with the required accuracy resulting in significant errors in fCO2 results. Calculation of fCO2 from high-quality total dissolved inorganic carbon (CT) and total alkalinity (AT) measurements does not yield results comparable in accuracy and precision to fCO2 measurements.

Beschreibung: Recent subsurface float measurements in 27 Mediterranean Water eddies (Meddies) in the Atlantic are grouped together to reveal new information about the pathways of these energetic eddies and how they are often modified and possibly destroyed by collisions with seamounts. Twenty Meddles were tracked in the Iberian Basin west of Portugal, seven in the Canary Basin. During February 1994 14 Meddles were simultaneously observed, 11 of them in the Iberian Basin. Most (69%) of the newly formed Meddles in the Iberian Basin translated southwestward into the vicinity of the Horseshoe Seamounts and probably collided with them. Some Meddles (31%) passed around the northern side of the seamounts and translated southwestward at a typical velocity of 2.0 cm/s into the Canary Basin. Some Meddles observed there were estimated to be up to similar to 5 yr old. Four Meddles in the Canary Basin collided with the Great Meteor Seamounts and three Meddles were inferred to have been destroyed by the collision. Overall an estimated 90% of Meddles collided with major seamounts. The mean time from Meddy formation to a collision with a major seamount was estimated to be around 1.7 yr. Combined with the estimated Meddy formation rate of 17 Meddies/yr from previous work, this suggests that around 29 Meddles co-exist in the North Atlantic. Therefore during February 1994 we observed about half of the population of Meddles

Beschreibung: 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.

Beschreibung: 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.

Beschreibung: Oxygen isotope values of the extant Caribbean coralline sponge Ceratoporella nicholsoni are compared with published temperatures and δ18O of water calculated from salinities. The measured values from aragonitic sponge skeletons have a mean offset of 1.0 ± 0.1‰ from calculated calcite equilibrium values (αaragonite-calcite = 1.0010). This is in good agreement with published values from synthetic aragonite. They further agree with published near-equilibrium oxygen isotope values of temperate and cold water molluscs and foraminifera extrapolated to the temperature range of the coralline sponges. These results and the mode of skeleton formation of Ceratoporella nicholsoni suggest that these sponges precipitate aragonite close to isotopic equilibrium. The temperature dependence of oxygen isotopic fractionation between the aragonite of Ceratoporella nicholsoni and water is only roughly constrained by the available data, due to the narrow temperature range of the Caribbean reef sites. However, as the data suggest oxygen isotopic equilibrium, we can calculate a well constrained temperature equation combining temperate and cold water equilibrium values from molluscs and foraminifera with our sponge data: Full-size image (〈1 K) and Full-size image (〈1 K).

Beschreibung: Eight different sites from 2300 to 4420 m water depth in the Arabian Sea were sampled for a biochemical quantification of phospholipid concentrations in the sediments. This method serves as a measure of microbial biomass in marine sediments comprising all small-sized organisms, including bacteria, fungi, protozoa and metazoa. Phospholipid concentrations can be converted to carbon units as an estimate of total microbial biomass in the sediments. The average phospholipid concentrations in the surface sediments (0–1 cm) of the 4 abyssal sites ranged from 7 nmol cm−3 at the southern site (SAST, 10°N 65°E, 4425 m) to 29 nmol cm−3 at the western site (WAST, 16°N 60°E, 4045 m). The high values detected at the abyssal station WAST exceeded those in the literature for other abyssal sites and were comparable to values from the upper continental slope of the NE-Atlantic and the Arctic. At the four continental slope sites in the Arabian Sea, average phospholipid concentrations ranged from 9 to 53 nmol cm−3 with the maximum values at stations A (2314 m) and D (3142 m) close to the Omani coast. Records of particulate organic carbon flux to the deep sea are available for four of the investigated locations, allowing a test of the hypothesis that the standing stock of benthic microorganisms in the deep sea is controlled by substrate availability, i.e. particle sedimentation. Total microbial biomass in the surface sediments of the Arabian Sea was positively correlated with sedimentation rates, consistent with previous studies of other oceans. The use of the measurement of phospholipid concentrations as a proxy for input of particulate organic matter is discussed.