Changes in vertical distribution and aggregative behaviour in response to population density for larval sea urchins (Strongylocentrotus droebachiensis) and sea stars (Asterias rubens)

We examined the effects of the presence of larval conspecifics on larval vertical distribution of four‐arm and six‐arm plutei of Strongylocentrotus droebachiensis and bipinnaria of Asterias rubens, in laboratory experiments, by manipulating population density. Larvae were introduced to experimental columns (10 × 10 × 30 cm) at one of two or three population densities (S. droebachiensis: 0.1 and 10 larvae·ml^?1; A. rubens: 0.1, 1 and 10 larvae·ml^?1). Subsequent changes in vertical distribution were determined from images of the larvae in the columns illuminated by a 532‐nm laser and captured by a high‐resolution camera. Larvae of both species were found higher in the water column in the high than in the low and intermediate population densities. The relationship between vertical swimming velocity and nearest neighbour distance (NND) was measured for four‐arm plutei of S. droebachiensis, and used to determine a range in distances among individuals that may affect potential interactions. The variation in swimming velocity decreased with increasing proximity below a threshold distance among individuals of 10 mm, which was greater than the estimated distance in the high population density. We suggest that the increased larval aggregation near the water surface under high population density is the result of a behavioural response to conspecifics.     

Phytoplankton abundance and contributions to suspended particulate matter in the Ohio,Upper Mississippi and Missouri Rivers

Main channel habitats of the Ohio, Missouri, and Upper Mississippi Rivers were surveyed during the summers of 2004, 2005 and 2006 using a probability-based sampling design to characterize inter-annual and inter-river variation in suspended chlorophyll a (CHLa) and related variables. Large (fivefold) differences in CHLa were observed with highest concentrations in the Upper Mississippi (32.3 ± 1.8 μg L⁻¹), intermediate values in the Missouri (19.7 ± 1.1 μg L⁻¹) and lowest concentrations in the Ohio (6.8 ± 0.5 μg L⁻¹). Inter-annual variation was small in comparison to inter-river differences suggesting that basin-specific factors exert greater control over river-wide CHLa than regional-scale processes influencing climate and discharge. The rivers were characterized by variable but generally low light conditions as indicated by depth-averaged underwater irradiance <4 E m⁻² day⁻¹ and high ratios of channel depth to euphotic depth (>3). Despite poor light conditions, regression analyses revealed that TP was the best single predictor of CHLa (R ² = 0.40), though models incorporating both light and TP performed better (R ² = 0.60). Light and nutrient conditions varied widely within rivers and were inversely related, suggesting that riverine phytoplankton may experience shifts in resource limitation during transport. Inferred grazing and sedimentation losses were large yet CHLa concentrations did not decline downriver indicating that growth and loss processes were closely coupled. The contribution by algae to suspended particulate organic matter in these rivers (mean = 41%) was similar to that of lakes (39%) but lower relative to reservoirs (61%).     

Pacific subtropical cell variability in coupled climate model simulations of the late 19th–20th century

Observational studies of the Pacific basin since the 1950s have demonstrated that a decrease (increase) in tropical Pacific sea surface temperatures (SSTs) is significantly correlated with a spin-up (slow-down) of the Pacific Subtropical Cells (STCs). STCs are shallow wind-driven overturning circulations that provide a pathway by which extratropical atmospheric variability can impact the equatorial Pacific thermocline and, through upwelling in the eastern equatorial Pacific, tropical Pacific SSTs. Recent studies have shown that this observed relationship between SSTs and STCs is absent in coupled climate model simulations of the late 19th–20th centuries. In this paper we investigate what causes this relationship to breakdown and to what extent this limits the models’ ability to simulate observed climate change in the equatorial Pacific since the late 19th century. To provide insight into these questions we first show that the NCAR Community Climate System Model’s simulation of observed climate change since the 1970s has a robust signal in the equatorial Pacific that bears a close resemblance to observations. Strikingly, absent is a robust signal in the equatorial thermocline. Our results suggest that the coupled model may be reproducing the observed local ocean response to changes in forcing but inadequately reproducing the remote STC-forcing of the tropical Pacific due to the underestimate of extratropical winds that force these ocean circulations. These conclusions are found to be valid in five different coupled climate model simulations of the late 19th–20th centuries (CCSM3, GISS EH, GFDL CM2.1, CSIRO-Mk3, and HadCM3).     

Uranium-lead isotope systematics of Mars inferred from the basaltic shergottite QUE 94201

Uranium-lead ratios (commonly represented as ²³⁸U/²⁰⁴Pb = μ) calculated for the sources of martian basalts preserve a record of petrogenetic processes that were active during early planetary differentiation and formation of martian geochemical reservoirs. To better define the range of μ values represented by the source regions of martian basalts, we completed U-Pb elemental and isotopic analyses on whole rock, mineral and leachate fractions from the martian meteorite Queen Alexandra Range 94201 (QUE 94201). The whole rock and silicate mineral fractions have unradiogenic Pb isotopic compositions that define a narrow range (²⁰⁶Pb/²⁰⁴Pb = 11.16-11.61). In contrast, the Pb isotopic compositions of weak HCl leachates are more variable and radiogenic. The intersection of the QUE 94201 data array with terrestrial Pb in ²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb-²⁰⁸Pb/²⁰⁴Pb compositional space is consistent with varying amounts of terrestrial contamination in these fractions. We calculate that only 1-7% contamination is present in the purified silicate mineral and whole rock fractions, whereas the HCl leachates contain up to 86% terrestrial Pb. This terrestrial Pb contamination generated a ²⁰⁶Pb-²⁰⁷Pb array in the QUE fractions that appears to represent an ancient age, which contrasts with a much younger crystallization age of 327 ± 10 Ma derived from Rb-Sr and Sm-Nd isochrons (Borg L. E., Nyquist L. E., Taylor L. A., Wiesmann H. and Shih C. -Y. (1997) Constraints on Martian differentiation processes from Rb-Sr and Sm-Nd isotopic analyses of the basaltic shergottite QUE 94201. Geochim. Cosmochim. Acta61, 4915-4931). Despite the contamination, and accepting 327 ± 10 Ma as the crystallization age, we use the U-Pb data to determine the initial ²⁰⁶Pb/²⁰⁴Pb of QUE 94201 to be 11.086 ± 0.008 and to calculate the μ value of its mantle source to be 1.82 ± 0.01. The μ value calculated for the QUE 94201 source is the lowest determined for any martian basalt source, and, when compared to the highest values determined for martian basalt sources, indicates that μ values in martian source reservoirs vary by at least a factor of two. Additionally, the range of source μ values indicates that the μ value of bulk silicate Mars is approximately three. The amount of variation in the μ values of the mantle sources (μ ∼ 2-4) is greater than can be explained by igneous processes involving silicate phases alone. We suggest the possibility that a small amount of sulfide crystallization may generate greater extents of U-Pb fractionation during formation of the mantle sources of martian basalts.     

Nitrogen loads to New Zealand aquatic receiving environments: comparison with regulatory criteria

ABSTRACT

There is concern about the deteriorating nutrient status of aquatic receiving environments in New Zealand. We estimated the amount by which current nitrogen (N) concentrations and loads exceed criteria in rivers, lakes and estuaries nationally. Criteria corresponded to national ‘bottom-line’ (i.e. minimal) environmental objectives set by government policy. Three metrics were evaluated: (1) degree of compliance describes the current TN loads in receiving environments relative to criteria; (2) catchment N status describes the acceptability of catchment N loads compared to criteria; and (3) excess load indicates the amount by which the N load exceeds the maximum allowable load (kg yr^?1). Non-compliance with N criteria was broadly distributed nationally particularly in low-elevation catchments. Catchments with unacceptable N status constituted at least 31% of New Zealand’s land area, which corresponds to at least 43% of the country’s agricultural land. The national excess load was estimated to be at least 19.1 Gg yr^?1. We are 97.5% confident that estimated excess loads exceed zero for nine of 15 regions and for the nation as a whole. The analyses provide a strategic assessment of where reductions in N emissions are required to achieve the minimal national objectives.     

Importance of organic Fe complexing ligands in the Mississippi River plume

As part of a larger program focused on understanding the biogeochemistry of large river plumes, we participated in two expeditions during 2000 to sample the Mississippi River plume. Surface water samples were collected using a trace metal clean towed fish and analyzed for total dissolved Fe, organic Fe complexing ligands and their associated conditional stability constants. The ligands in the river plume have conditional stability constants (log K′_FeL between 10.5 and 12.3 with an average of 11.2 and standard deviation of 0.6) very similar to ligands found in the open ocean. Comparison of high flow and low flow regimes indicates that variability in flow may be the main cause of the variability in Fe concentrations in the plume. The organic Fe complexing ligands are in greatest excess during a time of higher flow. These ligands are responsible for maintaining very high (5 nM) Fe concentrations throughout the plume. Due to complexation with these organic ligands, the concentration of Fe remains above the Fe-hydroxide solubility level until a salinity above 35 is reached where there appears to be a sink for Fe in the less productive waters. Therefore, Fe is transported a great distance from the river source and is available for biological utilization in the coastal zone.     

Changes in topography and vegetation near gaps in a protective foredune

Data gathered on a foredune on a nourished beach reveal the relationships between topography and plant communities at gaps in the dune crest. Sand inundation at gaps allows plants in some portions of the dune to rejuvenate to earlier successional stages, while an increase in richness occurs in other portions. Ammophila breviligulata can create an initial dune ridge, colonize new areas of bare sand within the dune field, facilitate growth of other species, and provide a natural alternative to fencing in managing gaps. Foredune mobility can be considered a positive factor if beach width and dune volume provide adequate protection.     

Prediction of hydrocarbon recovery from turbidite sandstones with linked-debrite facies: Numerical flow-simulation studies

A series of two-dimensional numerical flow simulations were carried out to investigate the production characteristics of a sheet sandstone bed with a linked-debrite interval. A deterministic geological model was used based on a two-dimensional representation of a bed from the Marnoso Arenacea Formation. The model was 60 km long and 1 m thick and contained three zones, arranged in a vertical facies arrangement typical of many linked-debrite beds: i) a lower, coarse-to-medium grained, clean turbidite sandstone interval; ii) a middle, muddy sandstone, debrite interval; iii) an upper, fine-grained, clean, laminated sandstone interval. Simulation involved only a 3-km long sector of the model, with one injector well and one production well, placed 1-km apart in the middle of the sector model. The simulated sector was moved progressively down the length of the bed, in 1-km steps, sampling different parts of the bed with different facies proportions. The petrophysical properties of the debrite interval were varied to produce different porosity–permeability cases. All other modelling parameters, including the upper and lower interval petrophysics, were kept constant. Results indicate that, in most cases, key production parameters such as cumulative oil production with time and water cut are proportional to the volume of movable oil between the wells. This relationship does not hold, however, for cases with relatively low values of debrite porosity (≤0.15) and permeability (k_h ≤ 100 mD) where the debrite interval accounts for more than 20% of the interwell volume. In these models, production efficiency declines systematically with reducing reservoir quality and increasing debrite percentage, resulting in relatively low oil production and early water breakthrough.     

Wind-induced Kuroshio intrusion into the South China Sea

The Kuroshio flows north along the east coasts of the Philippines and Taiwan. Between these two land masses lies the Luzon Strait which connects the Pacific Ocean to the South China Sea. The Kuroshio usually flows north past this strait, but at times part or all of it flows west through the strait into the South China Sea forming a loop current. It has been suggested that the loop current forms when the northeast monsoon deflects the Kuroshio through the Luzon Strait. In this study, satellite-derived sea-surface temperature images are used to observe the Kuroshio in the Luzon Strait region. Together with wind data from the region, these observations indicate a loop-current development process which is largely determined by an integrated supercritical wind stress parameter. The loop current grows when a four-day average of the local wind-stress component directed to the south exceeds 0.08 Nm^–2. When this average wind-stress component drops below the critical value, the Kuroshio returns to its northward path.