Thunen Institute For Seefischerei

Hamburg, Germany

Thunen Institute For Seefischerei

Hamburg, Germany
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Nunez-Riboni I.,Thunen Institute For Seefischerei | Akimova A.,Thunen Institute For Seefischerei
Progress in Oceanography | Year: 2017

New 67-year long (1948–2014) gridded time series of salinity in the North Sea at all depths allowed to quantify, spatially resolved, the amount of inter-annual salinity variability explained by each of its driving mechanisms: sea level pressure (SLP), precipitation, river run-off, zonal and meridional winds and currents over the eastern North Atlantic. For the current data, not only annual averages but also their deviations, as measure of turbulence, were considered. Our results summarize and expand the knowledge gathered in the last 50 years about the mechanisms driving inter-annual variability of salinity in the North Sea. Three mechanisms, uncorrelated with each other and acting over separate regions of the North Sea, arise as most important: (1) River run-off from continental Europe explains 50–80% of inter-annual salinity variations at lag 0 in the Southern and German Bights and the Norwegian Trench up to the connection with the North Atlantic, down to the seabed near the coasts and to the deep Norwegian Trench (100 m); (2) Remote variations of salinity in the Rockall Trough explain 70% of salinity variations of the tongue of high salinity in the northwestern North Sea with a lag of one year and down the water column; (3) The Neva discharge explains 60% of salinity changes in Skagerrak and southern Norwegian trench at lag 0. An explanation for this correlation might be the Baltic freshwater outflow being modulated by the Neva discharge through intensification of the estuarine gravitational circulation. We confirmed known relations between river run-off, precipitation over continental Europe, SLP over northern Europe and zonal wind over western Europe. Linked to these changes, we found also changes of meridional wind north of Scotland favoring eastward Ekman transport of salty North Atlantic waters into the North Sea off the Norwegian coast. Excluding this only case, we found no significant correlation between wind-driven currents and North Sea salinity changes. This result supports the notion that the Atlantic inflow into the North Sea is mainly density-driven. Salinity in the region east of Scotland and northern England was alienated from all driving mechanisms tested. An explanation was found in concomitant canceling changes of the intensity of the North Sea circulation and the discharge of the river Tay. © 2017 Elsevier Ltd


Martin B.,University of Hamburg | Eggert A.,Leibniz Institute For Ostseeforschung Warnemunde | Koppelmann R.,University of Hamburg | Diekmann R.,Thunen Institute For Seefischerei | And 2 more authors.
Marine Ecology | Year: 2015

Spatial and temporal distribution patterns of zooplankton are highly variable in the Northern Benguela Upwelling System. We studied the distribution of zooplankton (size class ≥ 0.33 mm) and used field data from four cruises that took place between March 2008 and February 2011, as well as simulation results of a regional ecosystem model. Remotely sensed sea surface temperatures (SST) and surface chlorophyll concentrations were analysed to investigate environmental influences on zooplankton biomass. The Intense Benguela Upwelling Index showed a distinct seasonal signal throughout the years and the highest upwelling peaks in August/September. Even though surface chlorophyll concentrations were very variable throughout the year, the highest concentrations were always detected in September, following the upwelling of nutrient-rich water. In field catches, zooplankton biomass concentration in the upper 200 m was highest above the outer shelf and shelf-break in December 2010 and February 2011, i.e. 6 months after the upwelling peaks. In contrast, zooplankton biomass simulated by the model in the surface water was highest in September. In March/April, biomass maxima were typically measured in the field at intermediate water depths, but the vertical distribution was also affected by extensive oxygen minimum zones. The ecosystem model reproduced this vertical pattern. Although general trends were similar, simulation data of zooplankton standing stocks overestimated the field data by a factor of 3. In upwelling systems, food webs are generally considered to be short and dominated by large cells. However, our field data indicate more small-sized zooplankton organisms above the shelf than offshore. © 2014 Blackwell Verlag GmbH.


Siegel V.,Thunen Institute For Seefischerei | Reiss C.S.,Southwest Fisheries Science Center | Dietrich K.S.,Southwest Fisheries Science Center | Rohardt G.,Alfred Wegener Institute for Polar and Marine Research
Deep-Sea Research Part I: Oceanographic Research Papers | Year: 2013

Net-based data on the abundance, distribution, and demographic patterns of Antarctic krill are quantified from a contemporaneous two ship survey of the Antarctic Peninsula during austral summer 2011. Two survey areas were sampled focussed on Marguerite Bay in the south, and the tip of the Antarctic Peninsula in the north. Data from 177 stations showed that the highest concentrations of krill were found in the southern sampling area. Differences between areas were associated with a few large catches of one year old krill found in anomalously warm and productive waters in Marguerite Bay, and small krill catches in the less-productive, offshore waters in the north. Estimated krill density across the survey area was 3.4krillm-2, and was low compared to the long-term average of 45krillm-2 for the Elephant Island area. Overall recruitment between the two survey regions was similar, but per capita recruitment was about 60% lower than historical mean recruitment levels measured at Elephant Island since the late 1970s. Demographic patterns showed small krill concentrated near the coast, and large krill concentrated offshore on the shelf and slope all along the survey area. The offshore distribution of adult krill was delineated by the warm (~1°C), low salinity (33.8) water at 30m, suggesting that most krill were present shoreward of the southern boundary of Antarctic Circumpolar Current Front. Distributions of larvae indicated that three hotspot areas were important for the production of krill: slope areas outside Marguerite Bay and north of the South Shetland Islands, and near the coast around Antarctic Sound. Successful spawning, as inferred from larval abundance, was roughly coincident with the shelf break and not with inshore waters. Given the rapid changes in climate along the Antarctic Peninsula and the lower per capita recruitment observed in recent years, studies comparing and contrasting production, growth, and recruitment across the Peninsula will be critical to better understand how climate change will impact krill populations and their dependent predators in the Scotia Sea. © 2013 Elsevier Ltd.


Godo O.R.,Norwegian Institute of Marine Research | Reiss C.,Southwest Fisheries Science Center | Siegel V.,Thunen Institute For Seefischerei | Watkins J.L.,Natural Environment Research Council
CCAMLR Science | Year: 2014

The climate-induced changes presently seen in the ecosystems of the Antarctic region require a precautionary approach with respect to the human use of these ecosystems. In particular, resource harvesting requires enough basic knowledge, as well as adequate monitoring, to avoid unintended impacts on the harvested stocks and the associated ecosystem. Due to the vastness and remoteness of the Antarctic region, research vessel capacity is not readily available for conventional coverage of harvested stocks and their ecosystems. This paper describes the potential of using commercial fishing vessels to bridge the gap in research vessel capacity. The various tasks and required instrumentation are presented and discussed. To illustrate this concept a description of a Norwegian krill fishing vessel now under construction is presented. This type of combined fishing and research vessel could make a large amount of important data available for both management, through CCAMLR, and the broader scientific community and thus improve the basis for resource evaluation and management. © 2014, Comm. for the Conserv. of Antartic Marine Living Resources. All rights reserved.


Nunez-Riboni I.,Thunen Institute For Seefischerei | Kristinsson K.,Iceland Marine Research Institute | Bernreuther M.,Thunen Institute For Seefischerei | van Aken H.M.,Netherlands Institute for Sea Research | And 3 more authors.
Deep-Sea Research Part I: Oceanographic Research Papers | Year: 2013

This study provides evidence of the influence of hydrography and large scale ocean circulation on the geographical distribution of beaked redfish (Sebastes mentella) in the Irminger Sea on the interannual time scale, from 1992 to 2011. The results reveal the average relationship of adult pelagic redfish to their physical habitat from 100 to 800. m depth: the most preferred latitude, longitude, depth, temperature and salinity for redfish are approximately 58°N, 41°W, 557. m, 4.5. °C and 34.87, respectively. The redfish habitat corresponds in a temperature-salinity (TS) diagram to a mixing triangle between East Greenland Current Water (EGCW), Labrador Sea Water (LSW) and Irminger Current Water (ICW). The geographical centre of mass of the redfish distribution (as revealed by acoustic fish density) indicates displacements from year to year. Changes in hydrographic conditions were investigated in detail for possible reasons for these displacements. Empirical Orthogonal Analysis reveals that maximum variations of water mass volume on an interannual time-scale in the study region correspond to ICW and LSW changes, while EGCW remains comparatively stable. Indices of redfish geographical centroid, LSW volume, ICW temperature and Subpolar Gyre (SPG) intensity suggest that the geographical redfish displacements are closely related to interannual changes of ICW modulated by the SPG intensity with a lag of 1 or 2 years. In comparison, LSW seems to have no impact on the redfish distribution at the studied depth range. The time lag between ICW and redfish displacements indicates an indirect influence of temperature on redfish. Hence, changes of chlorophyll-a (from satellite imagery), as a proxy for primary production, were used in a first approach to study the role of food availability. The analysis is based on acoustic and trawl data from nine expeditions coordinated by the International Council for the Exploration of the Sea (ICES), around 71,000 hydrographic stations from the Integrated Climate Data Center, World Ocean Database 2009 and Coriolis (among others), 60 years of circulation data from the Max-Planck Institute Ocean Model and 14 years of satellite chlorophyll-a from SeaWiFS, MODIS-Aqua and MERIS. © 2013 Elsevier Ltd.


Cisewski B.,Thunen Institute For Seefischerei | Cisewski B.,Alfred Wegener Institute for Polar and Marine Research | Strass V.H.,Alfred Wegener Institute for Polar and Marine Research
Progress in Oceanography | Year: 2016

The success of any efforts to determine the effects of climate change on marine ecosystems depends on understanding in the first instance the natural variations, which contemporarily occur on the interannual and shorter time scales. Here we present results on the environmental controls of zooplankton distribution patterns and behaviour in the eastern Weddell Sea, Southern Ocean. Zooplankton abundance and vertical migration are derived from the mean volume backscattering strength (MVBS) and the vertical velocity measured by moored acoustic Doppler current profilers (ADCPs), which were deployed simultaneously at 64°S, 66.5°S and 69°S along the Greenwich Meridian from February, 2005, until March, 2008. While these time series span a period of full three years they resolve hourly changes.A highly persistent behavioural pattern found at all three mooring locations is the synchronous diel vertical migration (DVM) of two distinct groups of zooplankton that migrate between a deep residence depth during daytime and a shallow depth during nighttime. The DVM was closely coupled to the astronomical daylight cycles. However, while the DVM was symmetric around local noon, the annual modulation of the DVM was clearly asymmetric around winter solstice or summer solstice, respectively, at all three mooring sites. DVM at our observation sites persisted throughout winter, even at the highest latitude exposed to the polar night. Since the magnitude as well as the relative rate of change of illumination is minimal at this time, we propose that the ultimate causes of DVM separated from the light-mediated proximal cue that coordinates it. In all three years, a marked change in the migration behaviour occurred in late spring (late October/early November), when DVM ceased. The complete suspension of DVM after early November is possibly caused by the combination of two factors: (1) increased availability of food in the surface mixed layer provided by the phytoplankton spring bloom, and (2) vanishing diurnal enhancement of the threat from visually oriented predators when the illumination is quasi-continuous during the polar and subpolar summer.Zooplankton abundance in the water column, estimated as the mean MVBS in the depth range 50-300 m, was highest end of summer and lowest mid to end winter on the average annual cycle. However, zooplankton abundance varied several-fold between years and between locations. Based on satellite and in situ data of chlorophyll and sea ice as well as on hydrographic measurements, the interannual and spatial variations of zooplankton mean abundance can be explained by differences in the magnitude of the phytoplankton spring bloom, which develops during the seasonal sea ice retreat. Whereas the vernal ice melt appears necessary to stimulate the blooming of phytoplankton, it is not the determinator of the blooms magnitude, its areal extent and duration. A possible explanation for the limitation of the phytoplankton bloom in some years is top-down control. We hypothesise that the phytoplankton spring development can be curbed by grazing when the zooplankton had attained high abundance by growth during the preceding summer. © 2016 Elsevier Ltd.


Haraldsson M.,Gothenburg University | Siegel V.,Thunen Institute For Seefischerei
Marine Ecology Progress Series | Year: 2014

Although Thysanoessa macrura is one of the most abundant and widely distributed euphausiid species in the Southern Ocean, knowledge about its seasonal distribution and overwintering strategy remains sparse. We describe the variation in abundance, demography, and distribution of T. macrura during 3 seasons in the Lazarev Sea, located in the south-east Atlantic sector of Antarctica. Adult and larval T. macrura were widely distributed across the Lazarev Sea in all seasons, but with large seasonal variability in the adult population. Mean summer densities (4.9 and 7.0 ind. m-2 in 2005/06 and 2007/08, respectively) were 20-fold higher than densities recorded during autumn (0.4 ind. m-2) and winter (0.2 ind. m-2). Comparison of length-frequency distributions and sex ratios, particularly between shallow (0-200 m) and deep (0-2000 m) samples taken during winter, suggest a seasonal vertical migration. A large proportion of the adult population seems to migrate to greater depth during winter, possibly as an overwintering strategy. Further, the demographic data and a set of growth models suggest that females and males have deviating life history traits. Females grow to larger size but their growth rate is slower, and they may mature later than males. The presence of mature males and females with spermatophores attached during late June to August indicated that spawning had already started during mid-winter conditions. These data, which were uniquely sampled in the same region from summer to winter, provide a new perspective on the seasonal behavior and general life history of T. macrura. © Inter-Research 2014.


Nunez-Riboni I.,Thunen Institute For Seefischerei | Akimova A.,Thunen Institute For Seefischerei
Journal of Marine Systems | Year: 2015

We used a statistical model mainly based on optimal or Gauss-Markov interpolation (OI) to produce monthly maps of in situ hydrography in the North Sea through 66. years (from 1948 to 2013) with a resolution of 0.2°. ×. 0.2°. The in situ hydrography data are approximately 430,000 hydrographic profiles and aggregated thermosalinograph data from various international databases. Duplicates, outliers and vertical density instabilities were removed. Regions with poor OI estimates were replaced with a harmonic reconstruction arising from the most reliable OI estimates. Adjustments for vertical density stability were based on the standards of the World Ocean Atlas. We mapped at 54 depth levels through the water column, focusing on surface and bottom hydrography because this type of map is of particular interest for ecosystem and fisheries research. Average OI temperature and salinity expected errors at the surface are 0.3. °C and 0.1, respectively. OI errors decrease with depth following decrement of signal and noise variances and apparently independent of the data amount (indicating a good data coverage). Alternative error estimates were obtained with the Median Absolute Deviation between our hydrography estimates and time series excluded from the analysis and are on average 0.3. °C and 0.1 salinity units. While our product seems limited for analysis of variability on monthly and seasonal time scales, particularly in the regions of large variability, it is suitable for studies of inter-annual and decadal variability. A comparison with two alternative analyses (KLIWAS and SODA) is discussed. As direct application of our results, we present a new hydrographic climatology of the North Sea at various depths with an improved effective resolution. © 2015 Elsevier B.V.

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