Freshwater Surface Layer Research Articles

Surface wave height distributions and rogue wave probabilities on two-layer fluids

Rogue waves are individual ocean surface waves much larger than the prevailing wave field. In high sea states they can be a danger to marine operations. Rogue waves are well studied for unstratified water. However, on a two-layer fluid the surface wave spectra are modified due to triad interactions with interfacial waves. Such triad interactions are not possible in homogeneous water. We find the modified surface spectrum reduces rogue waves. These findings are relevant for the changing polar oceans where prolonged ice melt is expected to facilitate open water with higher sea states while also creating a surface layer of freshwater overlaying salt water, as in our idealized model calculations.

The Intercontinental phylogeography of neustonic daphniids

Organisms that live at the freshwater surface layer (the neuston) occupy a high energy habitat that is threatened by human activities. Daphniids of the genera Scapholeberis and Megafenestra are adapted to the neuston but are poorly studied for biogeography and diversity. Here we assess the global phylogeography of neustonic daphniids. We obtained 402 new multigene sequences from the 12S rRNA, 16S rRNA, and tRNA (val) regions of the mitochondrial genomes of daphniids from 186 global sites. We assessed the intercontinental origins and boundaries of mitochondrial lineages and the relative rates of evolution in neustonic and planktonic daphniids. We identified 17 divergent lineages in the neustonic daphniids that were associated with biogeographic regions. Six of these lineages had intercontinental ranges – four of these were Transberingian. Patagonian populations of Scapholeberis rammneri were monophyletic and nested within a closely related clade of western North American haplotypes, suggesting an introduction from the Western Nearctic to South America. The Eastern Palearctic was more diverse than other regions, containing eight of the major lineages detected in the Scapholeberinae. The Genus Scapholeberis had high levels of divergence compared to non-neustonic daphniids. Neustonic daphniids have more divergent biogeographic lineages than previously appreciated.

Molecular and chemical response signatures illustrate species differences of developing neural progenitor cells from human, mouse and rat

The Ediacaran Schisto-Calcaire Group is a ∼1300 m-thick succession belonging to the West Congo Supergroup in Central Africa. In the Comba Basin, it consists of three carbonate-dominated units defined as formations (SCI to SCIII) that are unconformably overlain by clastic deposits (Mpioka Group) interpreted as a molassic formation associated with the Panafrican Orogen. The underlying Upper Tillite and Cap Carbonate (SCIa) units, considered as markers of the Snowball Earth event were studied in three sections. We investigated the carbonates of the Schisto-Calcaire Group by defining new microfacies (MF1-MF7) and we performed C and O isotopic analyses in order to constraint the depositional and diagenetic events directly after the Marinoan interval. Stratigraphic variations of the stable isotopes are important in the series with lighter δ18O values (>1.5‰) than those of the Neoproterozoic ocean in the SCIc unit. According to regional stratigraphy a temperature effect can be dismissed and a freshwater surface layer is the origin of such negative δ18O values in this unit. The negative δ13C anomaly (−3.5‰ on average) of the Cap Carbonate is similarly to the δ18O values (−6.4‰ on average) in the range of the marine domain during postglacial sea level rise. The sample suite as a whole (SCII and SCIII formations) displays heavier δ18O and δ13C than those of the lower part (SCI unit) of the Schisto-Calcaire Group. The comparison with the Lower Congo (Democratic Republic of Congo) and Nyanga (Gabon) basins shows that the meteoric flushing in SCIc unit of the Schisto-Calcaire Group was regional and not local, and could be derived from a climatic evolution. Although an overall overprint is present, our isotopic relationships argue against overall diagenetic resetting of primary compositions and suggest that with careful examination combined with detailed petrographic analysis general depositional and diagenetic controls can be discerned in oxygen and carbon isotopic data in the Schisto-Calcaire Group.

Paleoenvironments, δ13C and δ18O signatures in the Neoproterozoic carbonates of the Comba Basin, Republic of Congo: Implications for regional correlations and Marinoan event

The Ediacaran Schisto-Calcaire Group is a ∼1300 m-thick succession belonging to the West Congo Supergroup in Central Africa. In the Comba Basin, it consists of three carbonate-dominated units defined as formations (SCI to SCIII) that are unconformably overlain by clastic deposits (Mpioka Group) interpreted as a molassic formation associated with the Panafrican Orogen. The underlying Upper Tillite and Cap Carbonate (SCIa) units, considered as markers of the Snowball Earth event were studied in three sections. We investigated the carbonates of the Schisto-Calcaire Group by defining new microfacies (MF1-MF7) and we performed C and O isotopic analyses in order to constraint the depositional and diagenetic events directly after the Marinoan interval. Stratigraphic variations of the stable isotopes are important in the series with lighter δ18O values (>1.5‰) than those of the Neoproterozoic ocean in the SCIc unit. According to regional stratigraphy a temperature effect can be dismissed and a freshwater surface layer is the origin of such negative δ18O values in this unit. The negative δ13C anomaly (−3.5‰ on average) of the Cap Carbonate is similarly to the δ18O values (−6.4‰ on average) in the range of the marine domain during postglacial sea level rise. The sample suite as a whole (SCII and SCIII formations) displays heavier δ18O and δ13C than those of the lower part (SCI unit) of the Schisto-Calcaire Group. The comparison with the Lower Congo (Democratic Republic of Congo) and Nyanga (Gabon) basins shows that the meteoric flushing in SCIc unit of the Schisto-Calcaire Group was regional and not local, and could be derived from a climatic evolution. Although an overall overprint is present, our isotopic relationships argue against overall diagenetic resetting of primary compositions and suggest that with careful examination combined with detailed petrographic analysis general depositional and diagenetic controls can be discerned in oxygen and carbon isotopic data in the Schisto-Calcaire Group.

‘Snorkel’ lice barrier technology reduced two co- occurring parasites, the salmon louse (Lepeophtheirus salmonis) and the amoebic gill disease causing agent (Neoparamoeba perurans), in commercial salmon sea-cages

Diverse chemical-free parasite controls are gaining status in Atlantic salmon sea-cage farming. Yet, the intricacies of their use at commercial scale, including effects on co-occurring parasites, are seldom reported. A new salmon lice prevention method involves installing a deep net roof and ‘snorkel’ lice barrier in cages to shelter salmon from free-living infective larvae which concentrate at shallow depths, and allows salmon to jump and re- inflate their buoyancy-regulating swim bladder by swallowing air. We document use of snorkel cages (10m deep barrier) in commercial farms, where their effects on salmon lice levels, amoebic gill disease (AGD)-related gill scores, the cage environment, fish welfare and farm management practices were compared to standard cages. During an autumn–winter study involving only snorkel cages, high AGD-related gill scores were observed to decline when freshwater was pumped into snorkels, creating a freshwater surface layer for salmon to enter for self-treatment. In a spring–summer study incorporating snorkel and standard cages, snorkel cages were found to reduce new lice infestations by 84%. The deployment of snorkels and intermittent oxygen depletion detected within them in the spring–summer study did not alter fish welfare parameters. Overall, the results suggest snorkel technology has a place in the toolkit of commercial salmon sea-cage farmers co-managing salmon lice and amoebic gill disease outbreaks − two principal parasite issues facing the industry.

Satellite-measured interannual variability of turbid river plumes off central-southern Chile: Spatial patterns and the influence of climate variability

Ocean color imagery from MODIS (Moderate Resolution Imaging Spectroradiometer) onboard the Aqua platform is used to characterize the interannual variability of turbid river plumes off central-southern Chile. Emphasis is placed on the influence of climate fluctuations, namely El Niño Southern Oscillation (ENSO), the Pacific Decadal Oscillation (PDO), and the Antarctic Oscillation (AAO). Additional satellite data on wind, boat-based hydrographic profiles, and regional climate indices are used to identify the influence of climate variability on the generation of anomalous turbid river plumes. The evolution of salinity at a coastal station on the 90m isobath between the Itata and Biobío Rivers shows a freshwater surface layer with salinity <32.5 and 5–10m thick during major plume events in 2002, 2005 and 2006. Surface salinity minima are correlated with peaks in turbidy from the normalized water leaving radiance at 555nm (nLw(555)), both representing turbid river plumes. EOF analysis reveals that major turbid plume events occurred primarily during warm phases of the ENSO and PDO, and negative phases of the AAO, when storm tracks are further north. Anomalously large turbid plumes extend long distances offshore (∼70–80km), and individual plumes coalesce into a continuous plume along the coast that covers the entire continental shelf. Season-specific correlation analyses reveal an increased influence of the AAO on river plumes south of Punta Lavapié in spring-summer (negative correlation). North of this major cape, ENSO and PDO indices have a dominant influence on plumes with positive correlations with the nLw(555) signal in winter (and negative in summer). We discuss the biogeochemical implications of plume events and the importance of long-term and high-resolution ocean color observations for studying the temporal evolution of river plumes.

Marine conditions in central Sweden during the early Preboreal as inferred from a stable oxygen isotope gradient

AbstractThe marine benthic fauna and the δ18Oc of foraminifers and ostracods from six sites situated on a west–east transect through central Sweden have been analysed in order to estimate the palaeosalinity and palaeocirculation in this shallow‐marine environment. The measurements have been undertaken on material from the early Preboreal, when the Baltic Basin was in contact with the North Sea through straits in central Sweden. The δ18Oc values have a more negative value towards the east, indicating decreasing salinity. This was the result of limited possibilities for marine water to penetrate into the Baltic Basin and the mixing with freshwater from the melting Fennoscandian ice‐sheet. Four water masses existed in the area: a surface layer of freshwater, marine water from the North Sea, brackish–marine intermediate water on the Swedish west coast and brackish Yoldia Sea water in the Baltic Basin. The chronology is based on radiocarbon dates of marine fossils and, at one site, on the occurrence of the Vedde Ash (10 400–10 300 14C yr BP). This is the first record from marine settings in Sweden. Copyright © 2001 John Wiley &amp; Sons, Ltd.

Feeding ecology of marine fish larvae across the Great Whale River plume in seasonally ice-covered southeastern Hudson Bay

In ice-covered southeastern Hudson Bay (northern Quebec, Canada), the foraging of first- feeding Arctic cod Boreogadus saida and sand lance Ammodytes sp. was adversely affected by the plume of the Great Whale River. Before the freshet, manne fish larvae and their potential prey were marginally more abundant offshore where porous sea ice supported the development of ice algae than inshore where freshwater ice prevented algal growth. Larval fish foraging under the ice appeared limited by prey availability in the diluted (S < 5 %o), 5 m thick, surface layer and by light availability in the underlying marine waters. Arctic cod larvae which avoided the freshwater surface layer did not feed. The more euryhaline sand lance were present in the surface layer and fed to some limited extent until the freshet when further light attenuation by the turbid waters of the expanding plume completely halted their foraging activity, Feeding resumed in sand lance and started in Arctic cod at the ice break- up when the fragmentation of the ice cover and the vertical mixing of the plume allowed light to penetrate at depth. An anthropogenic reduction of the Great Whale River discharge in spring would generally improve local feeding conditions for manne fish larvae that occur under the ice. The impacts of such a reduction on the productivity of the coastal zone in summer remain to be assessed.

Nutrients and phytoplankton in the Gulf of Lions, northwestern Mediterranean

The mostly oligotrophic character of the Mediterranean Sea is altered drastically in areas receiving the outflow from large rivers. The Gulf of Lions, receiving discharges from the Rhoˆne River, has nutrient and phytoplankton concentration much higher than the adjacent open northwestern Mediterranean Sea. A surface layer of freshwater, with thickness that varies with the meteorological conditions between 2 and 40 m, overlies the deeper open seawater; this is advected onto the shelf and influences an area that covers the eastern half of the Gulf of Lions. Most of the waters affected by the river discharges show property relationships indicating conservative behaviour, with very little or no loss of nutrients through phytoplankton uptake, particularly in winter. Phytoplankton populations in winter are sparse, with maximum densities just above and below the boundary between the fresh- and seawater. Diatoms are the main group of organisms, although dinoflagellates, coceolithophorids and cyanobacteria are abundant. Small heterotrophs (cilliates, tintinnids, etc.) are also abundant and are positively correlated with the diatoms. A water balance model, linking the river discharge to the advective fluxes of water and nutrients, is proposed. The primary productivity supported by such fluxes is estimated.

Mercury distribution in the krka river Estuary (Eastern Adriatic Coast)

The investigation of the distributions of total and organic mercury was carried out in the water, sediments and mussels of the Krka River Estuary and the Kornati Archipelago (eastern Adriatic coast) during 1985–1987. The mercury content of the silt/clay sediment fraction (< 75 μm) was analyzed to reduce the influence of irregular sediment structure. The results obtained suggest that both the Krka River and the Kornati Archipelago are unpolluted by mercury. A slight increase in the mercury concentrations in the central part of the estuary points to the possibility of pollution by anthropogenic sources. The distribution of the concentration of reactive mercury (0.3–3.0 ng dm −3)in the unfiltered water followed the distribution pattern obtained for sediment. The portion of reactive versus total mercury in water was considerably lower in the freshwater surface layer (30–50%) than in the deeper saline water layer ( > 80% ) of the stratified Krka River Estuary. Mercury accumulation was observed at the interface of these two layers (at the halocline). The concentrations of mercury in mussels ( Mytilus galloprovincialis, Link.) (10–50 μg kg −1 wet wt.) were significantly affected by internal biological factors, and their distribution differed from the pattern observed for sediments and water. Despite the fact that organic mercury has been detected in sediments (≈0.5% of total) and the edible part of the mussels (13–65%), it has not for a certainty been established in water. A tendency to consider the Mediterranean as an area of higher ‘natural mercury levels’ has been challenged by the fact that the concentrations obtained are in the same range as those obtained for other unpolluted marine areas throughout the world.

Hydrographic observations and mixing processes in the New York Bight, 1975–1977

A series of 15 hydrographic cruises in the New York Bight over 1975–1977 is described and analyzed. The cruises cover all seasons but focus primarily on spring and summer. Temperature and salinity data show wide seasonal and interannual variability. Each of the three regions—inner bight, midshelf, and outer shelf slope—has distinctive properties and dynamics.The inner bight properties are affected primarily by river flow and wind‐driven flows up the shelf valley which often splits the freshwater surface layer into two plumes, one east and one west of the valley head. The “cold pool” temperature structure on the midshelf is different each year. The water with the lowest recorded temperature (2.58°C) was on the bottom after the thermocline formation in May 1977.Mixing models of the inner bight confirmed previous estimates of residence time as 6.8 days and yielded Kr = 5 × 106 cm2·s−1 with an advective transport component of 1 cm·s−1 to the southwest. A model of the mixing of the lower layer cold pool on the midshelf with onshore, offshore, and upper layer waters yields Kx = 7 × 106 and Kz = 0.1 cm2·s−1 as the on‐offshore horizontal and vertical diffusion coefficients.