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Ice algae are key contributors to primary production and carbon fixation in the Arctic, and light availability is assumed to limit their growth and productivity. We investigated photo-physiological responses in sea ice algae to increased irradiance during a spring bloom in West Greenland. During a 14 d field experiment, light transmittance through sea ice was manipulated to provide 3 under-ice irradiance regimes: low (0.04), medium (0.08), and high (0.16) transmittances. Chlorophyll a decreased with elevated light availability relative to the control. Maximum dark-adapted photosynthetic efficiency (ΦPSII_max) showed an initially healthy and productive ice algae community (ΦPSII_max > 0.6), with ΦPSII_max decreasing markedly under high-light treatments. This was accompanied by a decrease in the light utilization coefficient (α) and photosynthetic capacity (maximum relative electron transfer rate), and a decrease in the ratio of mono- to polyunsaturated fatty acids. This was partly explained by a corresponding increase of photoprotective pigments (diadinoxanthin and diatoxanthin), and a development of mycosporine-like amino acids as identified from a distinctive spectral absorption peak at 360 nm. After 14 d, in situ fluorescence imaging revealed significant differences in ΦPSII_max between treatments of dark-adapted cells (i.e. those sampled before sunrise and after sunset), during diel cycles, with clear chronic photoinhibition in high and medium treatments. Data demonstrate the high sensitivity of spring-blooming Arctic sea ice algae to elevated irradiance caused by loss of snow cover. The predicted loss of snow cover on landfast ice will negatively impact ice algae, their potential primary production, and nutritional quality for higher trophic levels.

Foraminiferans are very common almost everywhere on the sea bottom and many of them influence the sediment surface in different ways. Ammonia cf. aomoriensis created a rough sediment surface through their construction of sediment coverings (secondary tests) by the use of pseudopods. Their maximal distance moved in our experiment was 14.4 mm (median = 7.4 mm, standard error = 0.3 mm) during 24 h. The maximal sediment area covered was 21.0 mm 2 (median = 9.6 mm 2, standard error = 0.3 mm 2) during 24 h of incubation. Colchicine (5 μ M in seawater) was experimentally used to destroy the pseudopodia and consequently prevent them to form any sediment structures. Their influence on sediment stability was experimentally studied by using the LABEREX equipment where the shear stress exerted on the sediment surface during the resuspension was controlled. Our study showed that sediment incubated with colchicine and consequently removed pseudopods is more stable than sediment incubated with no colchicine. The activity of pseudopodial network will to some extent increase the sediment stability but simultaneously increase the bottom roughness, which in turn decreases the sediment stability by reducing the thickness of the viscous sublayer. This allows more turbulence close to the bottom and consequently resuspends and destabilizes the sediment. Active pseudopodia can ingest food (bacteria) but destroyed pseudopodia are unable to do this. The latter condition will allow bacterial populations to increase in number and produce more EPS (extracellular polymeric substances) that will stabilize the sediment even more.

Over the course of evolution for billions of years, bacteria that are capable of light-driven energy production have occupied every corner of surface Earth where sunlight can reach. Only two general biological systems have evolved in bacteria to be capable of net energy conservation via light harvesting: one is based on the pigment of (bacterio-)chlorophyll and the other is based on proton-pumping rhodopsin. There is emerging genomic evidence that these two rather different systems can coexist in a single bacterium to take advantage of their contrasting characteristics in the number of genes involved, biosynthesis cost, ease of expression control, and efficiency of energy production and thus enhance the capability of exploiting solar energy. Our data provide the first clear-cut evidence that such dual phototrophy potentially exists in glacial bacteria. Further public genome mining suggests this understudied dual phototrophic mechanism is possibly more common than our data alone suggested., Conserving additional energy from sunlight through bacteriochlorophyll (BChl)-based reaction center or proton-pumping rhodopsin is a highly successful life strategy in environmental bacteria. BChl and rhodopsin-based systems display contrasting characteristics in the size of coding operon, cost of biosynthesis, ease of expression control, and efficiency of energy production. This raises an intriguing question of whether a single bacterium has evolved the ability to perform these two types of phototrophy complementarily according to energy needs and environmental conditions. Here, we report four Tardiphaga sp. strains (Alphaproteobacteria) of monophyletic origin isolated from a high Arctic glacier in northeast Greenland (81.566° N, 16.363° W) that are at different evolutionary stages concerning phototrophy. Their >99.8% identical genomes contain footprints of horizontal operon transfer (HOT) of the complete gene clusters encoding BChl- and xanthorhodopsin (XR)-based dual phototrophy. Two strains possess only a complete XR operon, while the other two strains have both a photosynthesis gene cluster and an XR operon in their genomes. All XR operons are heavily surrounded by mobile genetic elements and are located close to a tRNA gene, strongly signaling that a HOT event of the XR operon has occurred recently. Mining public genome databases and our high Arctic glacial and soil metagenomes revealed that phylogenetically diverse bacteria have the metabolic potential of performing BChl- and rhodopsin-based dual phototrophy. Our data provide new insights on how bacteria cope with the harsh and energy-deficient environment in surface glacier, possibly by maximizing the capability of exploiting solar energy.

The study is based on a very extensive data-set of physical, biological, and optical parameters from below the sea ice in the western Amundsen Basin, central Arctic Ocean, in August–September 2012 during the record low sea ice extent. The water column was strongly stratified at all stations related to salinity differences between a surface layer of reduced salinities (34). A nitrate utilization-based budget in the surface layer gave a primary production of 67.5 mg C m− 2 d− 1, which reduced to 3.9 mg C m− 2 d− 1 in August 2012. Amundsen Basin primary production rates are lower than rates determined for other Arctic Ocean deep-water basins, and also lower compared to rates on the shelf. Below ice phytoplankton was well adapted to low light conditions in the Amundsen Basin and the photosynthetic potential was high, but limited by the low nutrient fluxes induced by the strong stratification. Amundsen Basin is foreseen to be ice-free in summer in 3–4 decades, and the question whether primary production will increase when ice-free was resolved with a coupled physical-biogeochemical model. Results showed that production will increase 10 to 14 times from the present 3.9 mg C m− 2 d− 1 to 37.4 and 55.2 mg C m− 2 d− 1 for an ice-free August and July–August, respectively. The study substantiates that both present and future ice-free low production rates were related to the strong stratification, reduced nutrient fluxes, and deep lying nutrient rich waters. Low production rates and strong stratification are discussed in the view of parameters that increase this stratification as higher freshwater run off or reduce stratification as wind.

Internal hydraulic control, which occurs when stratified water masses are forced through an abrupt constriction, plays an enormous role in nature on both large and regional scales with respect to dynamics, circulation, and water mass formation. Despite a growing literature on this subject surprisingly few direct observations have been made that conclusively show the existence of and the circumstances related to internal hydraulic control in nature. In this study we present observations from the Little Belt, Denmark, one of three narrow straits connecting the Baltic Sea and the North Sea. The observations (comprised primarily of along-strait, detailed transects of salinity and temperature; continuous observations of flow velocity, salinity, and temperature at a permanent station; and numerous vertical profiles of salinity, temperature, fluorescence, and flow velocity in various locations) show that internal hydraulic control is a frequently occurring phenomenon in the Little Belt. The observations, which are limited to south-going flows of approximately two-layered water masses, show that internal hydraulic control may take either of two configurations, i.e. the lower or the upper layer being the active, accelerating one. This is connected to the depth of the pycnocline on the upstream side and the topography, which is both deepening and contracting toward the narrow part of the Little Belt. The existence of two possible flow configurations is known from theoretical and laboratory studies, but we believe that this has never been observed in nature and reported before. The water masses formed by the intense mixing, which is tightly connected with the presence of control, may be found far downstream of the point of control. The observations show that these particular water masses are associated with chlorophyll concentrations that are considerably higher than in adjacent water masses, showing that control has a considerable influence on the primary production and hence the ecosystem in the area.

This chapter describes a period of low air temperatures, darkness or very little light, cold ice, minimum brine volumes, and strongly reduced ice permeability. The first section provides an overview of the physical and optical conditions of this winter ice (3.1). Both snowfall and snowdrift are frequent during winter and an overview of the effects of a thick snow cover on sea ice is provided (3.2). Increased colonization of the sea ice bottom by bacteria and ice algae is initiated in late winter around first light, and an example is given (3.3). Once these microorganisms have colonized the ice the important growth limitations of bacteria and algae are considered (3.4), along with how they are able to adapt to these extreme conditions, and their seasonal species succession (3.5). The purposes of EPS and the functioning of different pigments found in ice algae are described (3.6). This is combined with a description of low light conditions and related photosynthesis (3.7), and exemplified with a case study from Station North, NE Greenland (3.8).

This chapter is a description of the field and laboratory methods that are generally applied in sea ice ecological research. The first section describes sampling techniques and handling of sea ice samples (6.1). Calculations of brine and gas volumes are dealt with in (6.2), and how to measure and calculate optical parameters (6.3). Sampling, handling, and calculation of ice bacterial production are described (6.4), and methods for ice algae biomass spatial distributions are dealt with in (6.5). Measuring photosynthesis and ice algae primary production are described (6.6), and the determination of Chl a content is explained (6.7). The recently developed fluorescence imaging of ice algae distributions and derived photosynthetic parameters are described (6.8). This is followed by a description of the IP25, a substance synthesized by ice algae and applied for mapping sea ice and food web studies (6.9).

This chapter describes the period of development of new ice during autumn freeze-up, different ice types, and development of the brine channels (2.1). Optical properties of young and developing sea ice are described in terms of the transition from open water to bare sea ice, that later can be covered by snow (2.2). The incorporation and scavenging of ice algae, bacteria and meiofauna from the water column during ice growth and freeze-up are then described (2.3). Development of platelet ice, a prominent under-ice feature in Antarctica and recently observed in the Arctic, is described (2.4). The final section describes frost flowers and brine skim development, and the dynamics of the bacteria communities in the frost flowers and brine skim on top of newly formed sea ice (2.5).

This chapter describes the structure of the book and the diverse significance and importance of sea ice. The first section explains how and why the book is structured following the seasonal events governing the abiotic and biotic parameters in sea ice (1.1). The second section places sea ice into a physical and geopolitical perspective with the new and developing international borders in the Arctic Ocean (1.2). Sea ice ecosystems are then compared to other ecosystems (1.3), followed by a section describing the important and global ecological features of sea ice (1.4).

This chapter describes one of the most dynamic and ecological important seasons. The onset of ice melt initiates significant changes in the physical properties of the ice (4.1), and the related biotic processes in the spring/summer sea ice are addressed (4.2). Ice algal spring blooms are described and exemplified with two studies from contrasting Arctic sites (4.3). A Case Study 2 has a focus on effects and consequences for ice algae of increased irradiances at the ice bottom in terms of pigments, fatty acids, and MAAs (4.4). Ice algae bloom dynamics are investigated in a combined model and field study (4.5). Melt ponds termed “windows to the ocean” develop on the surface of the ice with increased light transmittance (4.6). Seeding of the ice or water column below with viable ice algae can initiate ice algae or pelagic blooms, and mechanisms are explored in model and field studies (4.7). Case study 3 show that species of meiofauna graze directly on the ice algae, and that the meiofauna establishes an important ecological component in the sea ice (4.8).

This chapter is a description of the role and importance of sea ice and sea ice biota on large scales and in relation to the effects of climate change. The decrease in summer sea ice extent and thickness are evident and described in (5.1). A Case Study 4 based on our observations in the Fram Strait and the Arctic Ocean illustrates some of the consequences and effects of increased inflow of warm Atlantic water (5.2). The question whether more light in an ice-free water column will increase pelagic primary production in the Arctic Ocean, is addressed with a model (5.3). Sea ice plays an important role in the exchange of CO2 between ocean and atmosphere and the sea ice CO2 pump is described in (5.4).

Climate change is leading to harsher resuspension events in shallow coastal environments influencing benthic nutrient fluxes. However, we lack information on the quantitative connection between these fluxes and the physical forces. Two identical experiments that were carried out both in May and August provided novel knowledge on the instantaneous effects of resuspension with known intensity on the benthic dissolved inorganic (phosphate: DIP, ammonium: NH4+, nitrite+nitrate: NOx, silicate, DSi) and organic nutrient (phosphorus: DOP, nitrogen: DON, carbon: DOC) fluxes in the shallow soft bottoms of the archipelago of Gulf of Finland (GoF), Baltic Sea. Resuspension treatments, as 2 times the critical shear stress, induced effluxes of one to two orders of magnitude higher than the diffusive fluxes from the studied oxic bottoms. The presence of oxygen resulted in newly formed iron oxyhydroxides and the subsequent precipitation/adsorption of the redox-dependent nutrients (DIP, DSi, organic nutrients) affecting their fluxes. Resuspension-induced NH4+ and NOx fluxes were associated with the organic content of sediments showing the highest values at the organic rich sites. NH4+ showed the strongest responses to resuspension treatments in August, but NOx at the time of high oxygen concentrations in near-bottom water in May. Foreseen increases in the frequency and intensity of resuspension events due to climate change will most likely enhance the internal nutrient loading of the studied coastal areas. The fluxes presented here, connected to known current velocities, can be utilized in modeling work and to assess and predict the internal nutrient loading following climate change.

Colonization by ice algae at the bottom of the sea ice is important ecologically as these algae are the major carbon source for zooplankton and the Polar food chains from early spring and until ice break-up several months later. At that time, ice algae can seed the water column promoting the pelagic production. We collected a time-series of ice growth, surface roughness, ice algae biomass (Chl-a), photosynthetic performance, primary production, irradiance, species composition, under ice ADCP data, nutrients, and ice algae spatial distribution in Kangerlussuaq, Greenland. A significant increase in biomass, primary production, and photosynthetic performance was observed at the bottom of the first-year sea ice. This colonization was not related to any change in light or nutrient availability, but instead occurred over a few days after which all parameters remained stable and constant. Colonization was concurrent with active ice growth associated with a period of very cold air temperatures as low as minus 35 °C. This new ice growth featured ice crystals and dendrites that significantly increased the surface roughness of the bottom, which facilitated the colonization. The dendrites are about 0.5 mm high and with a similar distance between the crests. The species composition of ice algae was similar in the water below and at the ice bottom, though biomass remained significantly lower in the water, and was dominated by Fragilariopsis oceanica and Navicula vanhoeefi. It is concluded that the colonization of the bottom of the sea ice is solely related to the increase in surface roughness initiated in the growing ice.

Sea level rise and more frequent storm and precipitation events associated with climate change are predicted to increase salinity fluctuations in estuarine and inshore areas, where foundation species such as eelgrass (Zostera marina L.) will be exposed to more frequent salinity changes. Effects of acute hyposalinity exposure on seagrasses remain poorly understood compared to the effects of more prolonged, constant salinity. Here, we examined growth and photo-physiological responses of Z. marina to 5 levels of stable salinity (5, 12 19, 25, 33) and compared effects of prolonged (16 days) versus acute (24 and 48 hours) exposure to hyposalinity (salinity 5 and 12) using fluorescence imaging. We also examined if fluorescence kinetics were affected by age differences across leaves. Growth reached an optimum at salinity 19 and was more affected by hyposalinity than hypersalinity. Rapid reduction from salinity 25 to 5 decreased the maximum quantum yield (Fv/Fm) after just 48 h. In contrast with prolonged exposure, non-photochemical quenching processes were not increased at salinity 5 after 48 h. Young leaves were more susceptible to extreme hyposalinity than older leaves (e.g., lower photosynthetic quantum yield), which emphasizes the importance of considering shoot-scale and within-shoot variations in studies of stress response patterns. Differences between hyposalinity and hypersalinity responses were generally replicated in the literature, but we were not able to detect any differences across studies. Overall, these results suggest that eelgrass is tolerant to large fluctuations in salinity, but sudden extreme reductions may act as a severe co-stressor, and contribute to accumulated stress-exposure effects (chronic or lasting effects).

Here we describe the design, performance and field tests of a lightweight (13.1 kg), low-cost (15.000 USD), and portable remotely operated vehicle (ROV) of dimensions 55 x 43 x 34 cm (L x H x W), with a new optical based positioning system. The ROV is designed for deployments and measurements of the irradiance field at a short distance below sea ice bottom in landfast level sea ice at calm under ice conditions. It is equipped with two cameras (front and rear) for optical positioning based on reference poles with LED lights below the ice. A third upward camera is for guiding during deployment and positioning. The ROV is equipped with spacer poles to maintain a constant distance between ROV with onboard optical sensors and bottom of the ice. All pre-tests of housing, thrusters, optical positioning, and ROV maneuverability were carried out in freshwater basins prior to field trials and tests. These were conducted at Kangerlussuaq, West Greenland on landfast first-year 79-80 cm thick ice with a variable (1-12 cm) snow cover in March 2016. The ROV was easily deployed through a hole (75 x 50 cm) in the ice and easy to maneuver below the ice. Test of positioning system showed an average deviation of 28 +/- 5 cm between optically based position and actual position with an average offset from center line of 16 +/- 5 cm. The ROV was applied for measuring the under-ice irradiance field and results demonstrated a solid negative correlation between snow depth and PAR transmittance. We derived a Normalized Differences Index (NDI) for snow depths: NDIsnow (depth) = [E(610 nm) - E(490 nm)]/[E(610 nm) + E(490 nm)] with minimum attenuation at 490 nm and maximum at 610 nm. It is discussed that the correlations for both PAR transmittance and the NDI with snow depths are due to a combination of a constant distance between optical sensor and ice bottom, and accurate positioning. A test showed that the wakes of thrusters removed parts of the ice algae biomass, but the study demonstrates the applicability of this ROV design for measurements of the under-ice irradiance field below landfast sea ice with a new optical based positioning system.

The frequency of strong and extreme precipitation events and high river discharges is foreseen to increase in southeast Asia in the future. The aims of the study were to evaluate physical and biological impacts of a historical (one-in-36 years) (1980-2015) extreme freshwater inflow in November 2010 into the small tropical Nha Phu estuary. The extreme event impacts were assessed relative to a dataset of samplings (n = 11) through 1.5 year of phytoplankton biomass (chlorophyll a), nutrients, CTD, primary production and suspended particulate matter prior to the extreme event. Results show, that the event significantly decreased salinity, increased chl-a 2-3 times, suspended particulate matter and beam attenuation coefficients by 7 times, and the colored dissolved organic matter absorption coefficient by 3 times. The extreme inflow initiated a phytoplankton bloom fueled by catchment area derived nutrients. The bloom established a considerable carbon production and transport of carbon to the coastal and shelf areas. There was likewise a significant export of suspended particulate matter out of the estuary. Residence time decreased from 4.2 to 3.8 days during the extreme event, and pre-inflow water column conditions were re-established within 4-5 days, shown in a previous study. Annual precipitation in the area was linked to the Pacific ENSO-cycle (El Nino Southern Oscillation) with 2010 as a very strong La Nina year. The entire event accounted for 68% of annual precipitation, 39% of annual discharge, and 53% of annual sediment transport in the Dinh River, which discharges into the Nha Phu estuary. (c) 2017 Elsevier B.V. All rights reserved.

The study is based on CTD and water sampling in the Eurasian Basin of the Arctic Ocean in August–September 2012. The research icebreaker Oden and a helicopter were platforms for the sampling. We here report data for inorganic and organic bound nutrient (NO3 −, NO2 −, NH4 +, PO4 3−, Si(OH)4, DON, DOP) concentrations and chlorophyll a between 10 m and 200 m at 29 stations and down to about 4500 m at 13 stations. The data set also includes below sea ice samples (1-2 m) sampling at 31 stations. A total number of 15 different water masses were identified, and their N:P relationship showed that 13 of these clearly originated from the Atlantic Ocean.

Data on optical properties such as diffuse attenuation coefficient K d(PAR), beam attenuation coefficient (c p) and the optically active constituents (OACs) CDOM, Chl-a and suspended particulate matter were obtained in a Danish temperate coastal plain estuary (56°N) and a Vietnamese tropical ria (12°N) at high discharges. The major difference was the spatial distribution of the optical properties against distance, best described by significant power functions in the ria, compared to significant linear functions in the coastal plain. It was hypothesized that estuarine morphometry could explain this spatial distribution. Partition and multiple regression analyses showed that Chl-a governed K d(PAR) and beam attenuation coefficient in both estuaries. Significant, high correlations were obtained by multiple regression analyses in the estimation of K d(PAR) and beam attenuation coefficients in the two estuaries using OACs as input parameters. It is concluded that there are no large differences in OAC concentrations between the two estuaries. The spatial distributions of OACs and optical properties were significantly different and governed by the estuary morphometry, i.e. a power distribution in the tropical ria and a linear function in the temperate coastal plain estuary, and applicable to similar estuary types.

The aims of the study were to analyse the relations between the physics of a water column and the location of the subsurface chlorophyll maximum (SCM) peaks in a strongly stratified estuary. Could extension and depth location of the SCM be explained by the physical conditions in terms of water column stratification and density interface? Questions were addressed by obtaining data on water column density (CTD), chlorophyll a (Chl a), nutrients, (F v/F m), σPSII and K d(PAR) at 15 positions along a 575 km transect in the Kattegat estuary. Results showed that the estuary was strongly stratified with mixed surface and bottom layers intercepted by a layer where density increased with depth. The SCM occurred only in this density interface, and widths of SCM and density interface were highly correlated. The surface waters were nearly depleted of inorganic nitrogen, phosphate and silicate though with significant higher concentrations in the waters below the interface. The Chl a concentration was comparatively higher in the SCM peak as well as maximum quantum efficiency (F v/F m) and functional cross sectional area (σPSII). The SCM was maintained at very low light levels and by a diapycnal nitrogen flux, with a stratified water column and nutrient depleted surface waters as predecessors. It was concluded that the depth location and vertical extension of the SCM in the estuary were closely linked to the physical structure of the water column in terms of density interface and stratification.

Optical constituents as suspended particulate matter (SPM), chlorophyll (Chl-a), colored dissolved organic matter (CDOM), and grain sizes were obtained on a transect in the arctic fjord-type estuary Kangerlussuaq (66°) in August 2007 along with optical properties. These comprised diffuse attenuation coefficient of downwelling PAR (K d(PAR)), upwelling PAR (K u(PAR)), particle beam attenuation coefficient (c p), and irradiance reflectance R(−0, PAR). PAR is white light between 400 and 700 nm. The estuary receives melt water from the Greenland Inland Ice and stations covered a transect from the very high turbid melt water outlet to clear marine waters. Results showed a strong spatial variation with high values as for suspended matter concentrations, CDOM, diffuse attenuation coefficient K d(PAR), particle beam attenuation coefficients (c p), and reflectance R(−0, PAR) at the melt water outlet. Values of optical constituents and properties decreased with distance from the melt water outlet to a more or less constant level in central and outer part of the estuary. There was a strong correlation between inorganic suspended matter (SPMI) and diffuse attenuation coefficient K d(PAR) (r 2 = 0.92) and also for particle beam attenuation coefficient (c p; r 2 = 0.93). The obtained SPMI specific attenuation—K d * (PAR) = 0.13 m2 g−1 SPMI—and the SPMI specific particle beam attenuation—c p * = 0.72 m2 g−1—coefficients were about two times higher than average literature values. Irradiance reflectance R(−0, PAR) was comparatively high (0.09−0.20) and showed a high (r 2 = 0.80) correlation with K u(PAR). Scattering dominated relative to absorption—b(PAR)/a(PAR) = 12.3. Results strongly indicated that the high values in the optical properties were related to the very fine particle sizes (mean = 2–6 μm) of the suspended sediment. Data and results are discussed and compared to similar studies from both temperate and tropical estuaries.

A laboratory-based diet-shift study using juvenile downfish (Amphiprion frenatus) was performed to determine 1) isotopic turnover rates and 2) trophic fractionation of delta C-13 and delta N-13 in liver and muscle tissues. A brood of A. frenatus was first fed shrimps (Acetes elythraeus) with delta C-13 = -19.2 parts per thousand and delta N-13 = 8.3 parts per thousand for 29 days to provide an isotopic baseline. Subsequently, they were fed muscle filets from a predatory reef fish (Sparus latus) with isotopic signature of delta C-13 = -15.9 parts per thousand and delta N-13 = 11.3 parts per thousand. The results showed significant differences in delta C-13 and delta N-15 between liver and muscle throughout the most of the experiment, indicating different incorporation of isotopes by these two tissues. delta C-13 in muscle tissue was less depleted than liver tissue before the diet shift and more depleted after. The change in isotopic composition was significantly faster for liver than muscle tissue after the diet-shift. A hyperbolic saturation model provided a good fit for predicting the time scale of isotopic turnover. The model showed that liver tissue approaches isotopic saturation 3-4 times faster than muscle tissue. Isotopic fractionations extrapolated from the model output at saturation were remarkably similar for liver and muscle tissues (Delta C-13 of -0.5 and -03 parts per thousand, and Delta N-15 of 2.6 and 2.8 parts per thousand, respectively), which indicates that fractionation is independent of tissue or organ in A. frenatus. Liver tissue therefore appears more useful than muscle tissue as a short-term dietary indicator for A. frenatus and possibly also for other fish species. (C) 2015 Elsevier B.V. All rights reserved.

We have compared 14 different sediment incubation chambers, most of them were used on bottom landers. Measurements of mixing time, pressure gradients at the bottom and Diffusive Boundary Layer thickness (DBL) were used to describe the hydrodynamic properties of the chambers and sediment–water solute fluxes of silicate (34 replicates) and oxygen (23 replicates) during three subsequently repeated incubation experiments on a homogenized, macrofauna-free sediment. The silicate fluxes ranged from 0.24 to 1.01 mmol m−2 day−1 and the oxygen fluxes from 9.3 to 22.6 mmol m−2 day−1. There was no statistically significant correlation between measured fluxes and the chamber design or between measured fluxes and hydrodynamic settings suggesting that type of chamber was not important in these flux measurements. For verification of sediment homogeneity, 61 samples of meiofauna were taken and identified to major taxa. In addition, 13 sediment cores were collected, sectioned into 5–10-mm slices and separated into pore water and solid phase. The pore water profiles of dissolved silicate were used to calculate diffusive fluxes of silicate. These fluxes ranged from 0.63 to 0.87 mmol m−2 day−1. All of the collected sediment parameters indicated that the sediment homogenization process had been satisfactorily accomplished. Hydrodynamic variations inside and between chambers are a reflection of the chamber design and the stirring device. In general, pump stirrers with diffusers give a more even distribution of bottom currents and DBL thicknesses than paddle wheel-type stirrers. Most chambers display no or low static differential pressures when the water is mixed at rates of normal use. Consequently, there is a low risk of creating stirrer induced pressure effects on the measured fluxes. Centrally placed stirrers are preferable to off-center placed stirrers which are more difficult to map and do not seem to give any hydrodynamic advantages. A vertically rotating stirrer gives about five times lower static differential pressures at the same stirring speed as the same stirrer mounted horizontally. If the aim is to simulate or mimic resuspension at high flow velocities, it cannot be satisfactorily done in a chamber using a horizontal (standing) rotating impeller (as is the case for most chambers in use) due to the creation of unnatural conditions, i.e. large static differential pressures and pre-mature resuspension at certain locations in the chamber.

The study is based on a 1 year record (n=46) of the diffuse attenuation coefficient—Kd(PAR) (m−1)—in combination with CTD-casts at one position in Arhus Bay, Kattegat, the North Sea–Baltic Sea estuarine transition. Water samples were collected occasionally for determination of suspended particulate matter (SPM) and chromomorphic dissolved organic matter (CDOM). The Kd(PAR) varied between 0.152 and 0.557 (m−1) with an average of 0.293 (m−1). The depth of the photic zone (1% light level) varied between 8.3 and 15.7 m at maximum and average Kd(PAR). Maximum Kd(PAR) values occurred in periods of high chl-a concentrations. The average CDOM absorption coefficient was 0.232 (m−1) whereas the average chl-a and inorganic SPM concentrations equalled 3.3 (mg m−3) and 4.5 (g m−3). Light attenuation partitioning in percent was: water (9), CDOM (17), SPM (42), and phytoplankton (32) at average conditions. Attenuation by phytoplankton reached up to 74% at high chl-a concentrations whereas SPM light attenuation reached 45% at high SPM concentrations. Results show that light attenuation in the bay is governed by phytoplankton and SPM concentrations whereas CDOM is of less significance. Light absorption in the PAR range—a(PAR)—ranged between 0.262 (m−1) at average conditions and 0.49 (m−1) at maximum phytoplankton concentrations. Scattering was governed by SPM at average conditions whereas phytoplankton scattering was higher than SPM scattering at chl-a concentrations >4.5 (mg m−3).

Vertical particulate matter fluxes were measured by sediment traps at 1.4 m (seabed) and 8 m (pelagic) above the seabed, along with fluorescence (CTD casts), current conditions and salinity, in a semi-enclosed bay in southwest Kattegat covering a close to three-month period. The Kattegat forms the estuarine transitional zone between the high-saline North Sea and low-saline Baltic Sea, as shown by measured maximum and minimum bottom salinities of 32.5 and 17.0. There were no indications of suspended matter transport into the bay that affected the vertical flux. There was a significant and close correlation (r 2 = 0.24) between current shear stress and seabed vertical flux. Average vertical flux was 11.8 g m −2 d −1 at the seabed with a wide range (0.4–57.6 g m −2 d −1 ), which is 6.9 times the average flux of 1.7 g m −2 d −1 at 8 m above the seabed. The flux at this height was generally governed by the production of organic material in the water and showed little variation. The C:N ratio of material collected in the 8 m trap was on average 6.9, close to the Redfield ratio (6.7) and implying flux of fresh material. The average C:N seabed ratio was 8.8. Periods of low C:N seabed ratios close to the Redfield ratio were observed before enhanced organic production in the water column, i.e. spring bloom and sub-surface peak. There was a positive and significant correlation (r 2 = 0.13) between seabed fluxes and C:N ratio. This shows that more decomposed material with a higher C:N ratio is resuspended at higher fluxes. Organic carbon content (percentage) in collected material was high with a maximum of 18% following the spring bloom at the 8 m trap, and an average of 11.4%. Average seabed organic carbon was 8.6%. There was a positive correlation (r 2 = 0.42) between vertical flux and seabed organic carbon content. This shows that the inorganic part of the collected material increases at higher current shear stress. It was concluded that the seabed flux was not affected by changes in the flux related to a spring bloom in terms of amounts, but that the related changes in C:N ratio and organic carbon were discernible, particularly in periods of low current shear stress.

Water and sediment transport were measured and calculated during a two months period on three locations at a 3 km long tidal divide in the Danish Wadden Sea. The water transport was flood dominated in the largest channel, almost neutral in the smaller channel and ebb dominated on the tidal flat. The tidal currents were short and strong during flood tide and long lasting but weak during ebb tide at all three stations. This tidal asymmetry caused the net sand transport across the tidal divide to be flood directed on all three stations. During a storm surge the currents were ebb-directed persistently for 21 hours despite large variations in water level. Grain-size distributions infer that the net sand transport converges to the northeast, indicating a recent north-eastward (flood directed) migration of the tidal divide. The suspended sediment transport during a tidal cycle is generally directed towards the north but on few special occasions the suspended sediment transport is southward directed beca...

Sixteen sediment samples were collected from a square grid (4×4) with a horizontal distance of about 150 m between positions in Århus Bay in the south-west Kattegat (14 to 15 m water depth). Critical shear stress (τc) was measured in all samples and related to sediment parameters: grain-sizes, organic matter, water content, porosity, and chlorophyll-a (chl-a) content, in upper layers. Samples were divided into a low (A) τc group and a high (B) τc group in relation to an erosion rate. A significant (Pc was found between groups A (0·0284 N m−2) and B (0·0380 N m−2). Average chl-a concentrations in groups A (1·4 μg g−1) and B (1·8 μg g−1) were not significantly different (P=0·47) but there was a significant and positive correlation (r2: 0·7, Pc and diatom film abundance. Sediment organic matter and water content were significantly higher in group B compared with A, which contradicts that watery and organic rich sediments generally exhibit low τc. This was explained by the presence of a diatom film cover on the fluff layer that inhibits the action of erosive forces. A fluff layer is characterized by a high water and organic content. The fluff layer was present in the majority of the samples but the highest average chl-a content and a significant (P=0·020) higher abundance of diatom film was observed in group B (high τc). Benthic diatoms were dominated by Haslea crucigeroides, Pleurosigma strigosum, and Bacillaris paxillifer. Spatial variability of sediment parameters was high and variability of a stability/erodibility parameter even exceeded those recorded for highly heterogeneous tidal flats. The occurrence of benthic diatoms at 14–15 m of water depth in the eutrophic Århus Bay was supposedly related to a measured increase in Secci depth in the bay and thereby increased light penetration depth.

The common mud snail Hydrobia ulvae is a widespread and dominant deposit feeder on fine-grained substrata along the European Atlantic coastline. Previous studies have shown that mud snail activities such as grazing, faecal pellet production and mucous production may influence physical properties of the surface sediment layer and thus depositional and erosional processes. To quantify the influence of Hydrobia density on key parameters such as erosion threshold and erosion rate, a short-term laboratory experiment was conducted. Snails were placed on fine-grained sediment at densities of 10000 and 50000 ind m-2 and erosion experiments were carried out one, three and five days after establishment of the sediment beds. Controls without H. ulvae were treated the same way. The presence of H. ulvae significantly increased the erosion rate and decreased the erosion threshold compared to snail-free control plots. The erosion rate was increased by a factor of 2 to 4 when H. ulvae were present and showed stronger influence by the snail than the erosion threshold. Snail density did not affect the erosion threshold, but the erosion rate doubled with an increase in density from 10000 to 50000 ind m-2. The erosion rate was only marginally different after one day but the difference increased over time and the erosion rate was significantly different for all treatments after five days. No significant time dependence was observed for the erosion threshold. The results generally confirm results obtained in situ and differences can be related to different hydrodynamic conditions under field and laboratory conditions.

The Pomeranian Bight (southern Baltic Sea) is a mixing zone between waters of the Baltic Proper and the river Oder, which drains a densely populated and highly industrialised catchment of central Europe. The bight is a nondepositional area, and all material produced in its water column, from erosion of strata at the seafloor and cliffs, and delivered by rivers, is transported near the seafloor to the depositional areas of the Arkona, Bornholm and Gdansk basins. In this contribution, we assess the origin, transformation and mass fluxes of material through the bight based on an integrated field study conducted in the period 1996–1998. The transport mechanism is by wave- and current-induced resuspension and settling cycles, which effectively enrich organic-rich material and associated substances (organic pollutants, heavy metals) in deeper water; the estimated transport time is less than 6 months. The phases in which the material is transported are suspended matter in the water column, a particle- and aggregate-rich benthic boundary layer of 20 m water depth), the fluffy layer is not readily distinguished from the underlying soft, organic-rich sediment and the change in physical and chemical properties is gradual. The organic matter passing through the coastal zone in the southern Baltic is unaffected by biological or chemical modifications in composition. We find no evidence for a preferential removal of nitrogen or phosphorus, even if the speciation of phosphorus changes from biological compounds to minerals. The compositional changes which we see, i.e., in the nitrogen isotopic composition and in trace metal concentrations, are mainly caused by dilution of the river signal. In the case of

The importance of aggregation processes in the benthic boundary layer (BBL) with regards to settling velocities and flux of organic carbon was investigated at four stations on a coastal-depositiona...

Dated sediment cores taken near the head and mouth of a tropical estuary, Nha-Phu/Binh Cang, in south central Viet Nam were analyzed for changes over time in physical, chemical and biological proxies potentially influenced by removal of the mangrove forest lining the estuary. A time-series of satellite images was obtained, which showed that the depletion of the mangrove forest at the head of the estuary was relatively recent. Most of the area was converted into aquaculture ponds, mainly in the late 1990's. The sediment record showed a clear increase in sedimentation rate at the head of the estuary at the time of mangrove deforestation and a change in diatom assemblages in the core from the mouth of the estuary indicating an increase in the water column turbidity of the entire estuary at the time of the mangrove deforestation. The proportion of fine-grained sediment and the δ 13C signal both increased with distance from the head of the estuary while the carbon content decreased. The nitrogen content and the δ 15N signal were more or less constant throughout the estuary. The proportion of fine-grained material and the chemical proxies were more or less stable over time in the core from the mouth while they varied synchronously over time in the core from the head of the estuary. The sediment proxies combined show that mangrove deforestation had large effects on the estuary with regard to both the physical and chemical environment with implications for the biological functioning.

Effects of resuspension on the release of dissolved, redox sensitive elements (Fe, Mn) was studied in cylindrical microcosms. Effects from changing water stirring velocity in sediment pools were evaluated through measurements of pore water profiles of dissolved Mn, Fe and redox potential. Mn was a good natural marker to follow such effects. At current velocities below the threshold velocity for resuspension (37 cm s-1), Mn release rates to overlying water were 100 times higher compared to steady-state values. Pulse increases in Mn concentration were the result of convective currents inside flow chambers. These results were strongly supported by measurements of Eh profiles in the sediment pore water. Furthermore, impacts from increasing stirring velocity were found down to 1.9 cm depth below the resuspended layer of sediment.