Argo floats revealing bimodality of large-scale mid-depth circulation in the North Atlantic

Analysis of Argo float trajectories at 1 000 m and temperature at 950 m in the North Atlantic between November 2003 and January 2005 demonstrates the existence of two different circulation modes with fast transition between them. Each mode has a pair of cyclonic - anticyclonic gyres. The difference is the location of the cyclonic gyre. The cyclonic gyre stretches from southeast to northwest in the first mode and from the southwest to the northeast in the second mode. The observed modes strongly affect the heat and salt transport in the North Atlantic. In particular, the second mode slows down the westward transport of the warm and saline water from the Mediterranean Sea.     

Rapid invasion of Crassostrea gigas into the German Wadden Sea dominated by larval supply

Invasions of non-indigenous species into coastal habitats have been a frequent phenomenon in the last decades, sometimes with significant impact on the receiving ecosystem. However, the understanding of the entire process and especially the relative importance of larval supply and local recruitment remains unclear. In this study, we simulate the invasion of a benthic invertebrate into a previously uncolonised habitat over several years and validate the results with field data. Therefore, we present field data from a monitoring programme revealing the rapid invasion of the oyster Crassostrea gigas into the East Frisian Wadden Sea, North Sea, between 2003 and 2005. The applied model combines a simple, spatially-explicit population dynamics model for the adult stage with a particle tracking model for the larval stage of the life cycle. Simulation results are able to reproduce the large-scale pattern of the field data and indicate a domination of larval supply on the population dynamics in the early stage of the invasion. Though monitoring and simulations suggest a single larval source outside the study area in the west, the population dynamics in the eastern part is only explainable with an additional source within the study area attributed to an unintentional input of juveniles by mussel fishery. High sensitivities to uncertain parameters result in distinct deviations between monitoring and simulations at particular sites. Especially the impact of site-specific variations of the post-settlement mortality underlines the variability of local recruitment conditions and indicates the need for spatially resolved information for exact predictions.     

The role of horizontal impulses of the faulting continental slope in generating the 26 December 2004 tsunami

For a long time, people have believed that the vertical displacement of seafloor due to undersea earthquakes is the primary cause of tsunami genesis. However, seismically-inverted seafloor deformation of the 2004 Sumatra–Andaman earthquake shows that the total vertical displacement is not enough to have generated the powerful Indian Ocean tsunami. Based on the seismically-inverted data and a three-dimensional ocean general circulation model (OGCM), we show that the momentum force, transferred by the horizontal impulses of the faulting continental slope in that earthquake, has accounted for two thirds of the satellite-observed tsunami height and generated kinetic energy 5 times larger than the potential energy due to the vertical displacement. The asymmetric tsunami pattern, recorded by tide-gauges showing leading-elevation waves toward Sri Lanka and leading-depression waves toward Thailand, is best explained by the horizontally-forced mechanism. The same mechanism has also explained the March 2005 Nias earthquake and tsunami data, suggesting that the horizontal motions of faulting have played more important roles in tsunami genesis than previously thought.     

Long-term stability of the fish assemblages in a warm-temperate South African estuary

Little is known about long-term changes in estuarine fish populations and related environmental variations. Fishes in the temporarily open/closed East Kleinemonde Estuary were sampled bi-annually, in summer and winter, using seine and gill nets between December 1994 and July 2005. A total of 18 families, represented by 33 species, were recorded. The 10 most abundant species caught were consistently recorded in catches each year, but CPUE of individual species varied on an annual basis and this can often be related to mouth state. Multivariate analyses of the annual marine fish community identified two distinct groups, with more species recorded during years that succeeded spring (September to November) mouth-opening events than in years following no mouth-opening events in spring. Interannual community stability (IMD) and seriation (IMS) also increased from the years following no opening events in spring to the years that succeeded spring opening events. These results highlight the importance of the timing of mouth opening to the marine fish community in a temporarily open/closed estuary. This study reinforces the importance of long-term studies to understanding community changes in estuaries caused by environmental variations over different time scales.     

Structure and photosynthetic properties of phytoplankton assemblages in a highly dynamic system, the Northern Adriatic Sea

The photosynthetic properties of phytoplankton populations as related to physical–chemical variations on small temporal and spatial scales and to phytoplankton size structure and pigment spectra were investigated in the Northern Adriatic Sea off the Po River delta in late winter 1997. Large diatoms (fucoxanthin) dominated the phytoplankton in the coastal area whereas small phytoflagellates (mainly 19′-hexanoyloxyfucoxanthin, chlorophyll b, 19′-butanoyloxyfucoxanthin) occurred outside the front. The front was defined by the steep gradient in density in the surface layer separating low-salinity coastal waters from the offshore waters.Physical features of the area strongly influenced phytoplankton biomass distributions, composition and size structure. After high volumes of Po River discharge several gyres and meanders occurred in the area off the river delta in February. Decreasing river discharge and the subsequent disappearance of the gyres and the spreading dilution of the river plume was observed in March. The dynamic circulation of February resulted in high photosynthetic capacity of the abundant phytoplankton population (>3.40 mg m⁻³). In March, the slow circulation and an upper low-salinity water layer, segregated from the deeper layers, resulted in lack of renewal of this water mass. The huge phytoplankton biomass, up to 15.77 mg chl a m⁻³, became nutrient depleted and showed low photosynthetic capacity. In February, an exceptionally high P_max^B, 20.11 mg C (mg chl a)⁻¹ h⁻¹ was recorded in the Po River plume area and average P_max^B was three-fold in February as compared to the March recordings, 10.50 mg C (mg chl a)⁻¹ h⁻¹ and 3.22 mg C (mg chl a)⁻¹ h⁻¹, respectively.The extreme variability and values of phytoplankton biomass in the innermost plume area was not always reflected in primary production. Modeling of circulation patterns and water mass resilience in the area will help to predict phytoplankton response and biomass distributions. In the frontal area, despite a considerable variability in environmental conditions, our findings have shown that the phytoplankton assemblages will compensate for nutrient depression and hydrographic constraints, by means of size and taxonomic composition and, as a result, the variability in the photosynthetic capacity was much less pronounced than that observed for other parameters.     

Polar stratospheric clouds from satellite observational data

The spectral aerosol-extinction coefficients (SAECs) obtained from SAGE III measurements are used to study the physical and integral microphysical characteristics of polar stratospheric clouds (PSCs). Different criteria for PSC identification from SAEC measurements are considered and analyzed based on model and field measurements. An intercomparison of them is performed, and the agreement and difference of the results obtained with the use of different criteria are shown. A new criterion is proposed for PSC identification, which is based on the estimate of how close the measured vector of the spectral attenuation coefficient is to a model distribution of the PSC ensemble. On the basis of different criteria, cases of PSCs are isolated from all SAGE III observations (over 30000). All selection criteria lead to a qualitatively and quantitatively similar space-time distribution of the regions of PSC localization. The PSCs observed in the region accessible to SAGE III measurements are localized in the latitudinal zones 65°–80° in the Northern Hemisphere and 45°–60° in the Southern Hemisphere during the winter-spring period. In the Northern Hemisphere, PSCs are observed within the longitudinal zone 120° W–100° E with the maximum frequency of PSC observation in the vicinity of the Greenwich meridian. In the Southern Hemisphere, the region of PSC observation is almost the same in longitude but with a certain shift in the maximum frequency of PSC observation to the west. This maximum is observed in the vicinity of 40°W, and the region of usual PSC observation is the neighborhood of 60° of the maximum’s longitude. The physical parameters of PSCs are estimated: the mean heights of the lower and upper boundaries of PSCs are 19.5 and 21.9 km, respectively, and the mean cloud temperature is 191.8 K. The integral microphysical parameters of PSCs are estimated: the total surface of NAT particles S _NAT = 0.41 μm²/cm³; the total volume of NAT particles V _NAT = 1.1 μm³/cm³; and, for all aerosol and cloud particles together, S is 2.9 ± 1.5 at a standard deviation of 2.7 μm²/cm³ and V is 2.8 ± 1.5 at a standard deviation of 4.2 μm³/cm³. A high frequency of PSC occurrence and high values of S and V in PSCs both for all particles and for NAT particles have been noted in January–February 2005 as compared to the rest of the period of SAGE III measurements for 2002–2005.     

Anoxia over the western continental shelf of India: bacterial indications of intrinsic nitrification feeding denitrification

Studies on the Arabian Sea coastal anoxia have been of immense interest, but despite its ecological significance there is sparse understanding of the microbes involved. Hence, observations were carried out off Goa (15 degrees 30'N, 72 degrees 40'E to 15 degrees 30'N, 72 degrees 59'E) to understand the processes that mediate the changes in various inorganic nitrogen species in the water column during anoxia. Water column chemistry showed a clear distinct oxic environment in the month of April and anoxic condition in October. Our study based on microbial signatures indicated that oxygen deficit appeared as a well-defined nucleus almost 40 km away from the coast during the oxic period (April) and spreads there after to the entire water column synchronizing with the water chemistry. Striking results of net changes in inorganic nitrogen species in nitrification blocked and unblocked experimental systems show that denitrification is the predominant process in the water column consuming available nitrate ( approximately 0.5 microM) to near zero levels within approximately 72 h of incubation. These observations have been supported by concomitant increase in nitrite concentration ( approximately 4 microM). Similar studies on denitrification-blocked incubations, demonstrate the potential of nitrification to feed denitrification. Nitrification could contribute almost 4.5 microM to the total nitrate pool. It was found that the relation between ammonium and total dissolved inorganic nitrogen (DIN) pool (r=0.98, p<0.001, n=122) was significant compared to the latter with nitrite and nitrate. The occurrence of high ammonium under low phosphate conditions corroborates our observations that ammonium does not appear to be locked under low oxygen regimes. It is suggested that ammonium actively produced by detrital breakdown (ammonification) is efficiently consumed through nitrification process. The three processes in concert viz. ammonification, nitrification and denitrification appear to operate in more temporal and spatial proximity than hitherto appreciated in these systems and this gives additional cues on the absence of measurable nitrate at surface waters, which was earlier attributed only to efficient algal uptake. Hence we hypothesize that the alarming nitrous oxide input into the atmosphere could be due to high productivity driven tighter nitrification-denitrification coupling, rather than denitrification driven by extraneous nitrate.     

Influx of Streaming Methane into Anoxic Waters of the Black Sea Basin

Oceanology - Based on long-term data on the spatial distribution and fluxes of streaming (bubbling) methane in the Black Sea, the study considers various types of streaming gas emissions...     

Oceanographic Studies in the Northwest Pacific and Seas of Japan and Okhotsk on Cruise 73 of the R/V Professor Gagarinskiy and Cruise 53 of the R/V Akademik Oparin

Oceanology - During cruise 73 of the R/V Professor Gagarinskiy and cruise 53 of the R/V Akademik Oparin, oceanographic studies of the Sea of Japan and Sea of Okhotsk and the Northwest Pacific were...     

Variability of Ice Conditions of the Bering Sea and Assessment of Their Possible Modeling

Oceanology - Various aspects of the seasonal and interannual variability of the sea ice cover are estimated based on all available Bering Sea ice data from 1960 to 2017. The possibility of...