Water Column Temperature Research Articles

Derivation of total diffuse attenuation coefficient from water column temperature data and meteorological water surface fluxes: A simple management tool

The total diffuse attenuation coefficient of Photosynthetically Available Radiation (Kd (PAR)) is derived by optimising the solution of a surface layer model to match temperature profiles measured with a precision thermistor chain; a non‐linear least‐squares Levenberg‐Marquardt scheme is applied to optimize Kd (PAR). The method was validated in Lake Kinneret (Israel) over 10 days in summer to early winter 2001, Valle de Bravo Reservoir (Mexico) on 1 July 2004 and Winam Gulf (Kenya) on 1 May and 12 August 2005. The method was applied for cases where the horizontal pressure gradient in the surface layer was negligible, the photic depth was deeper than the diurnal surface layer and the increment in temperature in photic region was considerably larger than the thermistor's precision. In all these tests, the errors of the estimated values of Kd (PAR) were mostly within 10% of the actual direct measurements. Investigations of the sensitivity of the model revealed that the accuracy of the method depended primarily on the precision and accuracy of the thermistors as well as the vertical resolution of the temperature measurements in the surface layer; all of these determine the rate of convergence of the optimisation scheme.

Population Structures of Two Genotypes of Vibrio vulnificus in Oysters ( Crassostrea virginica ) and Seawater

Vibrio vulnificus biotype 1 strains can be classified into two genotypes based on the PCR analysis of variations in the virulence-correlated gene (vcg). Genotype has been correlated with human infection for 90% of isolates from human cases having the vcgC sequence type and 87% of environmental strains having the vcgE variant. In this study we examined the dynamics of V. vulnificus populations and the distribution of the two genotypes recovered from oysters and surrounding estuarine wasters. Analysis of 880 isolates recovered from oysters showed a disparity in the ratio of the two genotypes, with those of the vcgE (E) genotype accounting for 84.4% of the population. In contrast, 292 isolates recovered from the waters surrounding the oyster sites revealed an almost equal distribution of the two genotypes. The levels of vcgC (C genotype) strains from both sources increased as a percentage of the population as water temperatures increased, while no culturable V. vulnificus cells were recovered from December through February. Our results suggest that there is a selective advantage for strains of the E genotype within oysters while survival of the C genotype strains may be favored by increased water column temperatures. These data suggest that the low incidence of infections may be due to the comparatively rare consumption of an oyster that contains a greater number of V. vulnificus vcgC genotype strains than of vcgE genotype strains. Levels of the two genotypes as well as seasonal dynamics within both oyster tissue and the surrounding waters may aid in identifying risk factors associated with human infection.

The Effects of Harvesting and Hauling on the Blood Physiology and Fillet Quality of Market‐Size Channel Catfish

AbstractCommercially produced, food‐size channel catfish Ictalurus punctatus that were harvested using routine procedures had significant elevations in plasma cortisol, glucose, and lactate compared with preharvest fish captured from open ponds. Blood samples were taken from 10 fish in each of two treatments (liquid oxygen diffuser‐ and paddlewheel‐aerated holding socks) and four sampling periods (before harvest, after socking, before loading, and before unloading) during each of 10 harvest events. Water column temperatures averaged approximately 27°C during the study. Ten fillet samples were taken from fish held overnight in paddlewheel‐ and liquid oxygen‐aerated holding socks during some harvest events. Mean plasma cortisol levels averaged 3.02 ng/mL before harvest, 84.38 ng/mL immediately after socking, 135.22 ng/mL after 18 h in the sock (i.e., before loading onto the truck), and 128.54 ng/mL after 2–3 h on the truck (i.e., before unloading). Mean plasma glucose levels averaged 34.7 mg/dL before harvest, 95.41 mg/dL after socking, 134.53 mg/dL before loading, and 134.86 mg/dL before unloading. Mean plasma lactate levels averaged 2.43 mmol/L before harvest, 13.24 mmol/L after socking, 11.53 mmol/L before loading, and 7.88 mmol/L before unloading. These results should not be considered maximal until additional studies are conducted with average water temperatures at their highest. There were significant changes within treatment over 7 d of refrigerated storage for fillet pH, color, and drip loss; however, there were no consistent trends between treatments. Results indicate that routine capture and hauling procedures are stressful to commercially produced, food‐size channel catfish under the examined conditions and that using liquid oxygen for aerating fish in holding socks does not significantly improve fillet quality or reduce stress levels.

Spatial and temporal variability of microplankton and detritus, and their export to the shelf sediments in the upwelling area off Concepción, Chile (∼36°S), during 2002–2005

Seasonal variations in diversity and biomass of diatoms, tintinnids, and dinoflagellates and the contribution of microplankton and faecal material to the vertical flux of particulates were investigated at one time series station T (station 18) between 2002 and 2005 and at a grid of stations during November 2004 in the coastal and oceanic area off Concepción (36°S), Chile. The variations were analysed in relation to water column temperature, dissolved oxygen, nutrient concentration, offshore Ekman transport, and chlorophyll- a concentration. Abundance was estimated as cell numbers per litre and biomass in terms of biovolume and carbon units. A sharp decrease with depth was observed in the abundance of both phytoplankton and microzooplankton during the whole annual cycle; over 70% of their abundance was concentrated in the upper 10 m of the water column. Also, a clear seasonality in microplankton distribution was observed at station T, with maxima for diatoms, tintinnids, and dinoflagellates every summer (centred on January) from 2002 to 2005. On the grid of stations, the maximum integrated (0–50 m) micro-phytoplankton abundances (>1 × 10 9 cells m −2) occurred at the coastal stations, an area directly influenced by upwelling. A similar spatial distribution was observed for the integrated (0–200 m) faecal carbon (with values up to 632 mg C m −2). Tintinnids were distributed in all the first 300 miles from the coast and dinoflagellates were more abundant in oceanic waters. At station T, the average POC export production (below 50 m depth) was 16.6% (SD = 17%; range 2–67%; n = 16). The biological-mediated fluxes of carbon between the upper productive layer and the sediments of the continental shelf off Concepción depend upon key groups of phytoplankton ( Thalassiosira spp., Chaetoceros spp.) and zooplankton (euphausiids) through the export of either cells or faecal material, respectively.

A novel approach to dissolution correction of Mg/Ca–based paleothermometry in the tropical Pacific

Mg/Ca in planktonic foraminifers carries two main signals: calcification temperature and postdepositional test dissolution. Shell dissolution thus distorts water temperature reconstructions made with Mg/Ca in foraminifers. This problem could be resolved by quantifying the impact of carbonate dissolution on Mg/Ca with an independent, temperature‐insensitive deep‐sea calcite dissolution proxy, such as the Globorotalia menardii fragmentation index (MFI). To test the validity of this approach, we measured Mg/Ca in the tests of several planktonic foraminifers and MFI in core tops collected over a wide geographic region of the tropical Pacific and covering a wide range of deep‐sea calcite dissolution and seawater temperature. We confirm that Mg/Ca from different species have different susceptibility to temperature and dissolution. Mg/Ca in surface‐dwelling Globigerina bulloides is controlled by calcification temperature and is largely unaffected by carbonate dissolution estimated from MFI. In contrast, Mg/Ca in deeper dwelling G. menardii is minimally sensitive to temperature and dominantly affected by dissolution. Mg/Ca in Neogloboquadrina dutertrei and Pulleniatina obliquiloculata are significantly affected by both temperature and dissolution, and MFI can be effectively used to correct temperature estimates from these species for calcite dissolution. Additional variables besides temperature and dissolution appear to control Mg/Ca in Globorotalia tumida, and their identification is a prerequisite for interpreting elemental shell composition in this species. Combining down‐core measurements of Mg/Ca in multiple foraminifer species with MFI provides a powerful tool for reconstructing past changes in the upper water column temperature structure in the tropical Pacific.

Conceptual model of seagrass die-off in Florida Bay: Links to biogeochemical processes

Seagrasses are recognized as important plant communities in coastal estuaries and lagoons across both tropical and temperate climes; thus, large-scale seagrass die-off events worldwide are of general concern. In Florida Bay, at the southern terminus of the Florida peninsula, seagrass die-off events up to 4000 ha have been reported and smaller scale mortality events are noted annually. In the present study, we examined several hypothesized causative factors (high temperature, hypersalinity, sulfide toxicity) of seagrass ( Thalassia testudinum) mortality in Florida Bay. To test sulfide effects, in situ sulfide production was stimulated by applying a labile carbon source (glucose) to sulfate reducers in the sediment at five sites across the bay (northeastern, northcentral, and southwestern basins). During the one year study, high temperature (32–36 °C) and salinity (> 50 psu) were recorded in the bay associated with a regional drought. We also experienced major seagrass die-off events at two of our southwestern bay sites. These field conditions provided an excellent opportunity to closely examine cause–effect relationships among stressors and die-off events in the field, and verify results of our previous mesocosm experiments. Even though glucose amendments stimulated porewater sulfides in bay sediments (4–8 mmol L − 1 ), no significant differences in biomass, short shoot density or final growth rates were found between control and glucose plots. In addition, the highest growth rates and shoot densities were concomitant with maximum water column salinity (> 50 psu) and temperature (32–36 °C), when porewater sulfides were also in the millimolar range. Large-scale seagrass mortality events, encompassing ∼ 50% of the entire meadow at one site, occurred at southwestern bay sites when plants were down regulating (slower growth and shoot density), probably in response to shorter day length and lower temperature (30–34 to 23–26 °C) from October, 2004 to January, 2005. Sulfate reduction rates (SRR) were also 2-fold higher in the southwestern (214–488 nmol cm − 3 d − 1 ) versus northcentral and northeastern (97–240 nmol cm − 3 d − 1 ) bay sites, possibly limited by labile carbon, which we found to stimulate SRR 3-fold in northeastern and northcentral bay sites (461–708 nmol cm − 3 d − 1 ) and 4-fold at southwestern bay sites (1211–2036 nmol cm − 3 d − 1 ). Based on a synthesis of the field data reported herein, our mesocosm experiments to date, and contributions by others, we present a conceptual model of seagrass die-off in Florida Bay outlining a cascade of stressors, stimulated by P enrichment, which leads to high O 2 consumption in the system triggering a seagrass die-off event.

Effects of current–bed form induced fluid flow on the thermal regime of sediments

Heat transport associated with current–bed form fluid exchange affects biogeochemical and ecological process in a variety of aquatic settings. We simulate turbulent flow over dunes, induced Darcy flow in sediments, and sediment heat transport forced by a diel variation of water column temperature. Heat conduction dominates when the water column current cannot induce rapid fluid exchange. Heat advection becomes important with increasing sediment permeability and/or current velocity, resulting in a complicated space‐time temperature pattern. Weak temperature variations are found beneath dune crests, where water upwells out of the sediments; strong variations are located below dune stoss faces, where water downwells. The relative dominance of heat conduction versus advection is captured by a global Peclet number that depends on dune geometry, and hydrodynamic and thermal properties, including the period of thermal forcing. Thermally induced fluid viscosity variations cause periodic but nonsymmetric changes in fluid flux through the sediments, but do not significantly influence temperature patterns.

Alpine Stream Temperature Response to Storm Events

Abstract Despite continued interest in meteorological influences on the thermal variability of river systems, there are few detailed studies of stream temperature dynamics during storm events. This paper reports high-resolution (15 min) water column and streambed temperature data for storm events of contrasting magnitude, duration, and intensity for three streams (draining glacier, snow, and groundwater sources) across an alpine river system during summers 2002 and 2003. The results demonstrate clear spatial and temporal differences in water column and streambed thermal responses to precipitation events and streamflow peaks. Analysis of all storms across the three sites showed a decrease in water column temperature for 75% of events, with significant negative relationships between stream temperature and precipitation magnitude, precipitation intensity, and stream discharge peaks. Temperature decreases of 10.4°C were recorded, but temperature increases were less marked at up to 2.3°C. Temperature response to precipitation was dampened with increasing depth into the streambed at all sites. Spatial and temporal differences in thermal response to storm events were controlled by precipitation and stream discharge peak characteristics (above) plus antecedent basin conditions, which together determine the nature and rapidity of hydrological response. In this steep alpine basin, stream temperature variability appears to be enhanced by quick routing of precipitation to the river channel (i.e., direct precipitation/channel interception, rapid surface flow over impermeable bedrock/thin alpine soils, and subsurface flow through highly weathered scree slopes). This research highlights the need for integrated hydrometeorological research of precipitation event–hydrological response–stream temperature interactions to advance understanding of runoff generation processes driving event-scale thermal dynamics in alpine and other river systems.

Compositions, sources and depositional environments of organic matter from the Middle Jurassic clays of Poland

The comprehensive biomarker characteristics from previously undescribed Middle Jurassic clays of Poland are presented. The molecular composition of the organic matter (OM) derived from clays of Aalenian to Callovian age has not changed significantly through time. High relative concentrations of many biomarkers typical for terrestrial material suggest a distinct dominance of OM derived from land plants. Increasing concentrations of C 29-diaster-13(17)-enes towards the northern part of the basin indicate an increase in terrestrial input. This terrestrial material would have originated from the enhanced transport of organic matter from land situated at the northern bank of the basin, i.e., the Fennoscandian Shield. The organic matter was deposited in an oxic to suboxic environment, as indicated by relatively low concentrations of C 33–C 35 homohopanes, moderate to high Pr/Ph ratio values, an absence of compounds characteristic for anoxia and water column stratification, such as isorenieratane, aryl isoprenoids and gammacerane, as well as common benthic fauna and burrows. δ 18O measurements from calcitic rostra of belemnites suggest that the mean value of the Middle Jurassic sea-water temperature of the Polish Basin was 13.1 °C. It is suggested that this mirrored the temperature of the lower water column because belemnites are considered here to be necto-benthic. The organic matter from the Middle Jurassic basin of Poland is immature. This is clearly indicated by a large concentration of biomarkers with the biogenic configurations, such as ββ-hopanes, hop-13(18)-enes, hop-17(21)-enes, diasterenes and sterenes. The identification of preserved, unaltered biomolecules like ferruginol, 6,7-dehydroferruginol and sugiol in Protopodocarpoxylon wood samples from these sediments present particularly strong evidence for the presence of immature OM in the Middle Jurassic sediments. Moreover, the occurrence of these polar diterpenoids is important due to the fact that they are definitely the oldest known natural products detected in geological samples.

Distribution and seasonal dynamics of Boroecia maxima (Ostracoda: Halocypridinae) in the Arctic basin and adjacent Atlantic waters

Based on materials obtained during Soviet and Russian expeditions to the Arctic basin and adjacent Atlantic waters during the period from 1929–1993, as well as data from the literature, we examined the distribution range of the common deep-sea ostracod Boroecia maxima. This species occurs throughout the pelagic water column (down to 3000 m depth and below) and prefers low water temperatures (surface Arctic and intermediate water masses). During the year, at all depths, the ratio between age groups of B. maxima remains virtually unchanged. Breeding apparently continues year-round. During the polar night, B. maxima makes no migration into warm Atlantic water, where wintering predatory zooplankton occur in large numbers, and thus avoids grazing pressure.

Modeling of Temperature Distributions in Mount Hope Bay Due to Thermal Discharges from the Brayton Point Station

Brayton Point Station is a 1600-MW electrical generating station located on Brayton Point, in Somerset, MA. The Station draws water from Mount Hope Bay at the Taunton and Lee Rivers for cooling purposes, and discharges the water back into the Bay, through a discharge canal. Mount Hope Bay is a shallow estuary located on the boundary between Rhode Island and Massachusetts. In connection with the renewal of the permit authorizing the withdrawal and discharge of cooling water, a series of studies on Mount Hope Bay were initiated by the owners of Brayton Point Station. These studies included both field and computer modeling components. A hydrothermal model capable of simulating the effects of Brayton Point Station on the Mount Hope Bay waters under a variety of operating scenarios was calibrated using the observed data. Additional cases were run to evaluate the effects of reduced discharges of heated effluent incorporating a cooling tower (enhanced multi mode operation) as well as the case of no discharge. Model results indicated that the temporal temperature variations occur over tidal to annual time scales. Seasonal variations were most discernible in the shallow upper reaches of the Bay, showing warmer than average temperatures during summer and cooler during winter. The calibrated hydrothermal model was also used to estimate the bottom area and water column volume coverage versus temperatures, which helps to quantify the effects of station heat load on the biological functions of winter flounder in Mount Hope Bay.

Patterns of diel vertical migration of zooplankton in acoustic Doppler velocity and backscatter data on the Newfoundland Shelf

Backscatter data from moored acoustic Doppler current profilers (ADCPs) are analysed to quantify the diel vertical migration patterns of zooplankton on the Newfoundland Shelf, Canada. Data from 11 moorings provide long time series (~100 days each) for in-depth statistical analysis. For one deployment, dry weight measurements of zooplankton are used to calibrate the acoustic backscatter. Quantification methods are developed and applied to the backscatter and vertical velocity time series to determine the characteristics of the observed diel migration. We show that the migration responds to changes in light intensity and water column temperature structure. We have sufficient spatial data to show high correlation of migration characteristics over spatial scales of up to tens of kilometres.

Characteristics of egg production of the planktonic copepod, Calanus finmarchicus, on Georges Bank: 1994–1999

Abstract We present here a synthesis of observations of egg production rates (EP) of the planktonic copepod, Calanus finmarchicus, carried out during process cruises of the US GLOBEC Northwest Atlantic/Georges Bank program between January and June 1995, 1997 and 1999. Female C. finmarchicus produced eggs at relatively high rates in at least some regions of Georges Bank during all months between January and June. Median, monthly EP varied between 24 eggs female −1 d −1 in January to 50 eggs female −1 d −1 in April–June; the highest mean EP was 86 eggs female −1 d −1 . Mean egg diameter was negatively related to ambient mean water-column temperature (0–100 m or bottom), decreasing from 149 to 142 μm between January and June. Direct measurements of body C or N or prosome length–mass relationships were used to determine mass-specific egg production rates. The relationships between estimates of chlorophyll a standing stock (mg chl. a m −2 ) and both C- and N-specific rates (% d −1 ) are reasonably well ( r 2 = 0.42 ) described by Ivlev curves. It is likely that chlorophyll standing stock serves as a proxy of both phytoplankton and microzooplankton food concentrations available to adult females. Chlorophyll standing stocks were below the critical concentration (at which EP is 95% of the calculated maximum) at approximately 55% of stations occupied over the study period, indicating frequent food limitation to varying extent. There were periods (e.g., over at least 6 d in April, 1997 on the southern flank) during which food limitation was severe. There was no detectable influence of mean water-column temperature on mass-specific EP. Hatching success varied between 50% and 95% without any seasonal trend. Our qualitative observations suggest the possibility that a significant proportion of hatching nauplii incubations were non-viable, meriting further study.

Variability of chlorophyll a and photosynthetic parameters in a nutrient-saturated tropical great lake

Diurnal measurements of Secchi depth, light attenuation, thermal structure, photosynthetic irradiance (PE) parameters, and chlorophyll a (Chl a) were performed at weekly intervals in three inshore bays in Lake Victoria, Uganda. The only statistically significant diurnal patterns observed were a decline in PE parameters normalized to Chl a and a decline in gross integral phytoplankton production. On a weekly timescale, Chl a was positively correlated to changes in the mean water-column temperature (TWC) in each bay. Meteorologic data in one of these bays suggest that the synchronous increases and decreases in Chl a and TWC are related to the extent of advective exchange with deeper areas of the lake. Analysis of all data from this study, as well as available historic data, reveals that the optical properties of Lake Victoria covary with the concentration of Chl a. On weekly timescales, the PE parameters αB and PBM covary, and both parameters generally decline as Chl a increases, a pattern consistent with historic data from Lake Victoria. Empirical relations are developed that relate optical properties and PE parameters to Chl a. These relations provide a mathematical representation of the limnologic changes that Lake Victoria has experienced through eutrophication and can be used to predict these parameters over larger spatial and temporal scales and facilitate estimates of whole-lake primary production.

Ocean warming and freshening in the northern Gulf of Alaska

Water column temperatures on the shelf in the northern Gulf of Alaska have increased more than 0.8°C and vertical density stratification has increased since 1970 near Seward, Alaska throughout the 250 m depth. This high latitude marine system has low water temperatures, high rates of precipitation, glacial melting, high wind speeds and high rates of biological productivity. A more than 300 km alongshore shift (locally westward) of isotherms is suggested. The observations are consistent with a conceptual ocean‐atmosphere circulation model that employs coastal freshwater discharge, glacial ablation and wind forcing. Positive regional feedback mechanisms accelerate the discharge and poleward heat flux, leading to even higher temperatures, increased ocean stratification and increased storminess. This warming and ocean freshening will have significant impacts on the atmosphere and marine ecosystems of the Northeast Pacific, Bering Sea, Arctic Ocean and quite possibly global ocean circulation.

Temporal and spatial distribution and production of dissolved gaseous mercury in the Bay St. François wetland, in the St. Lawrence River, Quebec, Canada

Wetlands are valued for their high biodiversity and for their ecosystem services. However, we still do have a poor understanding of their role in the redox transformation of contaminants such as mercury, particularly in fluvial settings. Seasonal and spatial variations in dissolved gaseous mercury (DGM) distribution and production were studied in the Bay St. François, a wetland in the St. Pierre Lake, a fluvial lake of the St. Lawrence River, in Quebec, Canada. A high spatial resolution for DGM, with samples taken every 10-cm depth, was used in field measurements. Through a series of parallel field and incubation experiments, we assessed the main factors determining Hg(0) transformations as a function of depth, seasons, and presence/absence of macrophyte beds. Besides light penetration in the water column and water temperature, iron and dissolved organic carbon likely stimulated Hg(II) reduction. Inversely, chloride favored Hg(0) oxidation. Macrophytes and associated epiphytes appeared to be important sites of adsorption/absorption of Hg(II) and likely of DGM. It seems however that the effects of macrophytes were restricted to immediately adjacent waters. Near the bottom, under anoxic conditions, the reduction of Hg(II) was highly promoted. In addition, sediments and decomposing macrophytes seemed to release DGM and/or reducible Hg to bottom waters. Overall, differences in DGM between surface and bottom waters tended to be more accentuated than observed differences in DGM between macrophyte beds and sites devoid of plant.

An experimental demonstration of blind ocean acoustic tomography

Despite the advantages clearly demonstrated by ocean acoustic tomography (OAT) when compared to other ocean monitoring techniques, it suffers from several technical-related drawbacks. One is the requirement for rather expensive equipment to be maintained and operated at several locations in order to obtain sufficient source–receiver propagation paths to cover a given ocean volume. This paper presents the preliminary feasibility tests of a concept that uses ships of opportunity as sound sources for OAT. The approach adopted in this paper views the tomographic problem as a global inversion that includes determining both the emitted signal and the environmental parameters, which is a similar problem to that seen in blind channel identification and was therefore termed blind ocean acoustic tomography (BOAT). BOAT was tested on a data set acquired in October 2000 in a shallow-water area off the west coast of Portugal, including both active and passive (ship noise) data. Successful results show that BOAT is able to estimate detailed water column temperature profiles coherent with independent measurements in intervals where the uncontrolled source signal (ship noise) presents a sufficient bandwidth and signal-to-noise ratio, which clearly define the limitations of the presented method.

Temperature Measurement and Stratification in Facultative Waste Stabilisation Ponds in the UK Climate

Temperature measurements were taken at three pilot-scale facultative ponds located at Esholt wastewater treatment works in West Yorkshire, UK over two one-year periods. The measurements were taken at different depths using a technology called Thermochron iButton. The iButton readings were compared with temperature measurements taken by a YSI sonde probe and maximum/minimum thermometers: they were found to be within 1 .C. In the temperature range 3.18 .C the mean weekly and monthly air temperatures were found to be good predictors for the mean weekly and monthly pond water-column temperatures. The level of stratification in the ponds under UK climatic conditions was assessed; the data revealed that the ponds remained thermally stratified for significant periods during the year despite their shallow depth.

High resolution evidence for linkages between NW European ice sheet instability and Atlantic Meridional Overturning Circulation

Published studies show that ice rafted debris (IRD) deposition preceding Heinrich (H) events H1 and H2 in the NE Atlantic was derived from the NW European ice sheets (NWEIS), possibly offering clues about ice sheet sensitivity and stability, and the mechanisms that caused periodic collapse of the Laurentide Ice Sheet (LIS). We present detailed lithological and geochemical records, including radiogenic isotope fingerprinting, of IRD deposits from core MD01-2461, proximal to the last glacial British Ice Sheet (BIS), demonstrating persistent instability of the BIS, with significant destabilisation occurring 1.5–1.9 kyr prior to both H1 and H2, dated at 16.9 and 24.1 kyr BP, respectively, in the NE Atlantic. Paired Mg/Ca and δ 18O data from the surface dwelling Globigerina bulloides and subsurface dwelling Neogloboquadrina pachyderma sinistral are used to determine late-glacial variability of temperature, salinity and stratification of the upper water column. A picture emerges that the BIS was in a continuing state of readjustment and never fully reached steady state. Increased sea surface temperatures appear to have triggered the episode of NWEIS instability preceding H1. It seems most probable that the so-called ‘precursor’ events were not linked to the H events. However, if response to a common thermal forcing is considered, an increased response time of the LIS, up to ~2 kyr longer than the NWEIS, may be inferred. Negative salinity excursions of up to 2.6 indicate significant incursions of melt water associated with peaks in NWEIS instability. Decreased surface density led to a more stable stratification of the upper water column and is associated with reduced ventilation of intermediate waters, recorded in depleted epibenthic δ 13C ( Cibicidoides wuellerstorfi). We suggest that instability and meltwater forcing of the NWEIS temporarily weakened Atlantic Meridional Overturning Circulation, allowing transient advance of southern-sourced waters to this site, prior to H events 1 and 2.

Thermal variability and stream flow permanency in an alpine river system

AbstractDespite the importance of thermal conditions in influencing biodiversity of alpine river systems, knowledge of year round stream temperature variability is very limited. This paper advances understanding of alpine stream temperature dynamics using hourly resolution data collected over two consecutive years at five sites within a glacierized basin in the French Pyrénées. The potential utility of temperature for understanding river flow patterns at ungauged sites (most notably during winter) is explored. The results indicated marked heterogeneity in water column temperatures; groundwater streams were typically warmer and more thermally stable than those draining snow and ice. Based upon stream temperature patterns, it appears possible to differentiate between river flow conditions including: free‐flowing, surface freezing, dewatering and snow cover. Notably, groundwater‐fed streams appeared to exhibit greater flow permanency than meltwater‐fed streams, the latter freezing for extended periods. These new insights into long‐term alpine stream thermal conditions have major implications for understanding the strategies adopted by benthic macroinvertebrate taxa when overwintering, particularly where streams freeze. Copyright © 2006 John Wiley & Sons, Ltd.