Dissolution kinetics of biogenic silica collected from the water column and sediments of three Southern California borderland basins

Understanding biogenic silica (bSi) dissolution kinetics in margin environments is important in assessing the global silicon cycle, a cycle closely linked to the global carbon cycle. This understanding is also essential to answer the question of whether bSi content in marine sediment is a valid indicator of productivity in the overlying surface ocean. In this study, plankton tow, sediment trap, and sediment samples were collected at sites in three Southern California borderland basins. Batch dissolution experiments with plankton tow and sediment trap materials (conducted in the laboratory at 22 °C) showed linear dissolution kinetics, from which mean dissolution rate constants of 0.05 d^? 1 for plankton tow samples and 0.07 d^? 1 for sediment trap samples could be calculated. The dissolution rate constants for both types of samples showed seasonal variability but not the same seasonal patterns. Faster dissolution was observed with sediment trap samples collected at 800 m than at 550 m. With sediment multi-core samples, non-linear dissolution kinetics was observed, which complicates the direct comparison of dissolution rates. Nonetheless, dissolution appeared to be slower for the sediments samples than for samples collected from the water column and to decrease with depth in the sediments. Rate constants for surface sediment (0–0.5 cm) were at least 3–5 times less, and sediments at depths > 2 cm had rate constants at least 6–13 times less than those for material sinking to the sediment surface at these sites. Dissolution experiments conducted with Santa Barbara Basin surface sediment samples amended with dissolved aluminum (Al) and San Pedro Basin trap samples amended with enriched detrital materials (obtained by leaching bSi from sediment samples) suggested that dissolution was inhibited by Al and that the sediments from the different basins varied in the extent of Al release.     

Uptake of palytoxin‐like compounds by shellfish fed Ostreopsis siamensis (Dinophyceae)

Abstract

Greenshell? mussels (Perna canaliculus Gmelin), scallops (Pecten novaezealandiae Reeve), and Pacific oysters (Crassostrea gigas Thunberg) were fed with a New Zealand strain of mass cultured Ostreopsis siamensis Schmidt (for 27 and 84 h and with 1.5 × 10⁶ or 8.6 × 10⁶ cells, respectively) under laboratory conditions. The microalgal cells contained 0.3 pg palytoxin equivalents cell^–1 (as determined by the haemolysis neutralisation assay (HNA) of Bignami (1993)) and extracts of these cells were toxic to mice after intraperitoneal injection. No palytoxin‐like material was detected either in the hepatopancreas or the muscle and roe of mussels fed O. siamensis. Oysters contained detectable amounts of toxin in hepatopancreas muscle, and roe while higher concentrations were present in the hepatopancreas of scallops. Extracts of control shellfish (tested biotoxin free and not fed O. siamensis) were toxic to mice, and there was no definitive evidence that feeding shellfish with O. siamensis at the levels employed in the present experiment increased the toxicity of shellfish tissue extracts to mice.     

A possible effect of salinity fluctuation on abundance of benthic vegetation and associated fauna in Northeastern Florida Bay

In southern Florida, a vast network of canals and water control structures mediate freshwater discharge into the coastal zone. Management protocol for one such canal network (C-111) is being modified in part to try to improve habitat for estuarine fish and wading birds in northeastern Florida Bay, an estuarine part of Everglades National Park. Changes in canal management could alter the spatial and temporal salinity regime in the estuary. To better predict the effect of such changes on estuarine habitat, abundances of submersed vegetation and benthic animals were sampled repeatedly at 12 stations that differed in salinity. A variety of other parameters were also measured (nutrients, light, temperature, oxygen, sediment characteristics, and others). Mean salinity among stations ranged from 11.4‰ to 33.1‰. Densities of benthic plants and animals differed among stations by several orders of magnitude. The standard deviation of salinity was the best environmental correlate with mean plant biomass and benthic animal density: less biota occurred at stations with greater fluctuations in salinity. The two stations with the least plant biomass also had the highest mean water temperatures. In a stepwise multiple regression analysis, standard deviation of salinity accounted for 59% of the variation in the logarithm of mean plant biomass among stations. For every 3‰ increase in the standard deviation, total benthic plant biomass decreased by an order of magnitude. Mean water temperature accounted for only 14% of the variation, and mean salinity was not included for lack of significance. At stations with widely fluctuating salinities, not only was biomass low, but species dominance also frequently changed. Severe fluctuation in salinity may have prevented abundant benthos by causing physiological stress that reduced growth and survival. Salinity may not have remained within the range of tolerance of any one plant species for long enough to allow the development of a substantially vegetated benthic community. Hence, gaining control over salinity fluctuation may be the key to estuarine habitat improvement through canal management in southern Florida.     

Hα spectropolarimetry of the Herbig Ae star AB Aurigae

We present spectropolarimetric observations, obtained at H α , of the Herbig Ae star AB Aurigae. Changes in linear polarization across the H α line probe structure in the immediate circumstellar environment of the central star, down to scales of the order of one to a few stellar radii. In the case of AB Aurigae the observed polarimetric signature is complex. After applying a correction for foreground continuum polarization, we find that there is a linear-polarized H α emission component intrinsic to the source. Rotation of the angle of polarization through the emission-line profile suggests scattering in a rotating circumstellar disc. The magnetic accretor model commonly applied to T Tauri stars shows promise of explaining these data.     

Holocene environmental history in high‐Arctic North Greenland revealed by a combined biomarker and macrofossil approach

In this study, we use a combined biomarker and macrofossil approach to reconstruct the Holocene climate history recorded in Trifna Sø, Skallingen area, eastern North Greenland. Chronological information is derived from comparison of lithological, biogeochemical and macrofossil characteristics with a well‐dated record from nearby Lille Sneha Sø. Following local deglaciation around c. 8 cal. ka BP, the local peak warmth occurred between c. 7.4 and 6.2 cal. ka BP as indicated by maximum macrofossil abundances of warmth‐demanding plants (Salix arctica and Dryas integrifolia) and invertebrates (Daphnia pulex and Chironomidae). Warm conditions were dominated by terrestrial organic matter (OM) sedimentation as implied by the alkane‐based P_aq ratio, but increased aquatic productivity is indicated when temperature was highest around 6.5 cal. ka BP. The n‐C₂₉/n‐C₃₁ alkane ratio shows that vegetation in the catchment was dominated by shrubs after deglaciation, but shifted towards relatively more grassy/herbaceous vegetation during peak warmth. After 5.4 cal. ka BP, the disappearance of warmth‐demanding plant and invertebrate macrofossils indicates cooling in the Skallingen area. This cooling was characterized by a significant shift towards dominance of aquatic OM sedimentation in Trifna Sø as implied by high P_aq ratios. Cooling was also associated with a shift in vegetation type from dwarf‐shrub heaths towards relatively more herbaceous vegetation in the catchment, stronger erosion and more oligotrophic conditions in the lake. Our data show that mean air temperatures inferred using branched glycerol dialkyl glycerol tetraethers (brGDGTs) do not seem to accurately reflect the local climatic history. Irrespective of calibration, methylation of branched tetraethers (MBT) palaeothermometry cannot be reconciled with the macrofossil evidence and seems to be biased by either changing brGDGT sources (in situ vs. soil‐derived) or changing species assemblages and/or an unknown physiological response to changing environmental conditions at high latitude.     

Influences of the unsaturated,saturated, and riparian zones on the transport of nitrate near the Merced River,California, USA

Transport and transformation of nitrate was evaluated along a 1-km groundwater transect from an almond orchard to the Merced River, California, USA, within an irrigated agricultural setting. As indicated by measurements of pore-water nitrate and modeling using the root zone water quality model, about 63% of the applied nitrogen was transported through a 6.5-m unsaturated zone. Transport times from recharge locations to the edge of a riparian zone ranged from approximately 6 months to greater than 100 years. This allowed for partial denitrification in horizons having mildly reducing conditions, and essentially no denitrification in horizons with oxidizing conditions. Transport times across a 50–100-m-wide riparian zone of less than a year to over 6 years and more strongly reducing conditions resulted in greater rates of denitrification. Isotopic measurements and concentrations of excess N₂ in water were indicative of denitrification with the highest rates below the Merced River. Discharge of water and nitrate into the river was dependent on gradients driven by irrigation or river stage. The results suggest that the assimilative capacity for nitrate of the groundwater system, and particularly the riverbed, is limiting the nitrate load to the Merced River in the study area.     

Spectroscopic constraints on outflows from BN-type objects

New high-quality high spectral resolution observations of the HI line emission from massive young stellar objects are described and discussed. It is proposed that two distinct physical components contribute to the observed emission. One of these is an optically-thick high-velocity stellar wind, the other a more slowly moving optically-thin volume of gas that may, in the case of S106IR at least, be caused by mass loading of the stellar wind. This decomposition is shown to resolve a long-standing problem regarding the relative widths of high and low opacity lines.     

Seasonal observations of surface waters in two Gulf of Maine estuary-plume systems: Relationships between watershed attributes,optical measurements and surface pCO2

The partial pressure of carbon dioxide (pCO₂) in estuary-plume systems is related to the internal processes of net organic metabolism and physical mixing, but is also strongly influenced by biogeochemical inputs from the land and ocean. Surface layer pCO₂, stimulated fluorescence of chlorophyll (f-chl) and colored organic matter (f-com), and beam attenuation at 660 nm (c-660) were measured during three seasonal surveys of the Kennebec (ME) and Merrimack (MA) estuary-plume systems. These estuaries are both supplied by large New England Rivers and separated by less than 150 km, but significant differences were often observed in the distributions of surface pCO₂ and optical variables. High pCO₂ concentrations were generally associated with high f-com, while lower pCO₂ concentrations were associated with high f-chl and c-660. Using simple regression models, optical measurements were used to estimate chlorophyll and total organic carbon concentrations. Results suggest that labile riverine carbon is responsible for sustaining supersaturated pCO₂ conditions and that phytoplankton productivity, likely driven by inputs of riverine dissolved inorganic nitrogen, is responsible for pCO₂ undersaturation. Although optical variables are often related to surface pCO₂, the results suggest that efforts to retrieve pCO₂ in complex waters using optical data may be enhanced using a site-specific, multivariate approach.