Short Water Residence Time Research Articles

Flexibility in monomeric Cu,Zn superoxide dismutase detected by limited proteolysis and molecular dynamics simulation

Limited proteolysis by trypsin of monomeric Cu,Zn superoxide dismutase from Escherichia coli induces a specific cleavage of the polypeptide chain at the level of Lys60 located in the S-S subloop of loop 6,5 where, when compared to the eukaryotic enzyme, a seven-residues insertion, completely exposed to the solvent, is observed. This result suggests that this subloop is disordered and flexible, thus enabling binding and adaptation to the active site of the proteolytic enzyme. Indeed, molecular dynamics simulation indicates that the S-S subloop undergoes high fluctuations and that its high flexibility coupled to an high solvent accessibility can explain the specific bond selection of the protease. As a matter of fact, of the possible 14 solvent accessible proteolytic sites only the Lys60 flexible site is cleaved. High flexibility and solvent exposure are confirmed by the short water residence time for the residues corresponding to the cleavage site evaluated by molecular dynamics simulation. These experiments demonstrate that molecular dynamics simulation and limited proteolysis are complementary and unambiguous tools to identify flexible sites in proteins.

Hydrogeologic processes in saline systems: playas, sabkhas, and saline lakes

Pans, playas, sabkhas, salinas, saline lakes, and salt flats are hydrologically similar, varying only in their boundary conditions. Thus, in evaluating geochemical processes in these systems, a generic water and solute mass-balance approach can be utilized. A conceptual model of a coastal sabkha near the Arabian Gulf is used as an example to illustrate the various water and solute fluxes. Analysis of this model suggests that upward flux of ground water from underlying formations could be a major source of solutes in the sabkha, but contribute only a small volume of the water. Local rainfall is the main source of water in the modeled sabkha system with a surprisingly large recharge-to-rainfall ratio of more than 50%. The contribution of seawater to the solute budget depends on the ratio of the width of the supratidal zone to the total width and is generally confined to a narrow zone near the shoreline of a typical coastal sabkha. Because of a short residence time of water, steady-state flow is expected within a short time (<100 years), while steady state for solutes may take much longer (>50,000 years). The solute composition of the brine in a closed saline system depends largely on the original composition of the input water. The high total ion content in the brine limits the efficiency of water–rock interaction and absorption. Because most natural systems are hydrologically open, the chemistry of the brines and the associated evaporite deposits may be significantly different than that predicted for hydrologically closed systems. Seasonal changes in temperature of the unsaturated zone cause precipitation of minerals in saline systems undergoing evaporation. Thus, during the hot dry season months, minerals exhibit retrograde solubility so that gypsum, anhydrite and calcite precipitate. Evaporation near the surface is also a major process that causes mineral precipitation in the upper portion of the unsaturated zone (e.g. halite and carnallite), provided that the relative humidity of the atmosphere is less than the activity of water. The slope of the fresh/brine-water interface in saline lake systems is shallower than in fresh/seawater interface because of the greater density difference between the fresh/brine-water bodies. The interface between sabkha brines and seawater slopes seaward, unlike normal marine–fresh water systems that slope landward. Moreover, the brine/seawater interface does not achieve steady state because it is pushed toward the sea by the sabkha's brine.

Bacterial activity and genetic richness along an estuarine gradient (Rhone River plume, France)

Bacterial diversity and activity were simultaneously investigated by microbial ecological and molecular biological methods along an estuarine gradient from the Rhone River to the Mediterranean Sea. Following a Lagrangian strategy, we sampled plume, frontal and marine layers. The sampled estuarine gradient exhibited large changes both in physico-chemical and in microbiological characteristics. Bacterial abundances and activities showed a more drastic decrease in the low salinity range of the gradient than expected from simple dilution models, indicating that an important fraction of freshwater bacteria disappeared in the mixing area. High specific activities, in particular for leucine, in the marine end-part of the gradient suggested important bacterial protein synthesis, which may be a sign of an active survival strategy for bacterial communities subjected to oligotrophic conditions. Bacterial genetic diversity of the sampled estuarine area, as estimated by the number of DNA-derived denaturing gradient gel electrophoresis (DGGE) bands, was high (13 to 55 bands) compared to that reported in other aquatic ecosystems. This high diversity may be the consequence of the interface position of estuaries. The proportion of active populations was estimated using the ratio of DGGE bands derived from RNA and DNA. This ratio was lower in Rhone water than in marine water, indicating that only a part of the constitutive populations were active, while the activity was distributed within a larger fraction of populations in the marine assemblage. Very few DGGE bands detected in freshwater samples were also detected in the marine end-part of the gradient, suggesting that a very limited number of freshwater bacteria could survive under marine conditions. Detection of these freshwater populations from RNA might indicate that these bacteria were able to synthesize different stress proteins as the result of a survival strategy or that these bacteria were able to maintain metabolic activity under marine conditions. The structure of marine communities was strongly affected by decreasing salinity. However, it seems that the decrease of DNA-derived bands may simply have been the consequence of the mixing of marine and freshwater. No obvious relationship between genetic richness and activity changes was observed. This lack of a relationship may be the consequence of a very short residence time of water in the mixing area studied.

The physical structure and dynamics of a deep, meromictic crater lake (Lac Pavin, France)

The characteristic feature of the physical structure of Lac Pavin is a distinct and permanent chemically induced density increase between about 60 and 70 m depth. This chemocline separates the seasonally mixed mixolimnion from the monimolimnion, which is characterized by elevated temperature and salinity as well as complete anoxia. Previously published box-models of the lake postulated substantial groundwater input at the lake bottom, and consequently a short water residence time in the monimolimnion and high fluxes of dissolved constituents across the chemocline. We present a new view of the physical structure and dynamics of Lac Pavin, which is based on the results of high-resolution CTD profiles, transient and geochemical tracers (tritium, CFCs, helium), and numerical modeling. The CTD profiles indicate the existence of a sublacustrine spring above rather than below the chemocline. A stability analysis of the water column suggests that vertical turbulent mixing in the chemocline is very weak. A numerical one-dimensional lake model is used to reconstruct the evolution of transient tracer distributions over the past 50 years. Model parameters such as vertical diffusivity and size of sublacustrine springs are constrained by comparison with observed tracer data. Whereas the presence of a significant water input to the monimolimnion can clearly be excluded, the input to the mixolimnion – both at the surface and from the indicated sublacustrine spring – cannot be accurately determined. The vertical turbulent diffusivity in the chemocline is well constrained to K ≈ 5×10-8 m2 s-1, about a factor of three below the molecular diffusivity for heat. Assuming thus purely molecular heat transport, the heat flow through the chemocline can be estimated to between 30 and 40 mW m-2. With respect to dissolved constituents, the very weak turbulent diffusive exchange is equivalent to a stagnant monimolimnion with a residence time of nearly 100 years. Based on these results and vertical concentration profiles of dissolved species, diffusive fluxes between monimolimnion and mixolimnion can be calculated. A large excess of helium with a 3He/4He ratio of (9.09 ± 0.01)×10-6 (6.57 Ra) is present in the monimolimnion, clearly indicating a flux of magmatic gases into the monimolimnion. We calculate a flux of 1.0×10-12 mol m-2 s-1 for mantle helium and infer a flux of 1.2×10-7 mol m-2 s-1 (72 t year-1) for magmatic CO2. The monimolimnion appears to be in steady state with respect to these fluxes.

Indicative profundal oligochaete assemblages in selected small Swedish lakes

Fifty-seven small- to middle-size lakes of Sweden (size-range from 5000 specimens ċ m−2). Mesotrophic to moderately eutrophic lakes of the same size categories contain more species than the above extremes, most of them being fairly tolerant to oxygene deficiency such as Tubifex ignotus, a number of species of the genera Aulodrilus and Limnodrilus and a variety of so-called Ponto-Caspian Potamothrix species, most of them having recently invaded south-eastern Sweden. Species richness is moderate to high (4–8 species), while abundance may vary considerably (range from about 300 to about 5000 specimens ċ m−2). In this respect, small- to middle-size lakes do not differ much from larger lakes. For the trophic characterization of lake water the mean concentrations of total-phosphorus (in μg ċ l−1) is generally regarded to be the most appropriate measure. The match between characterizations based upon oligochaete community structure and total-phosphorus concentrations is generally very good for large lakes. The present study shows that this applies fairly well also to the majority of the small- to middle-size lakes selected for this study. This has not been shown before. Obvious deviations from this rule could be caused by a scarcity of reliable chemical data, or considerable seasonal shifts in the chemical composition ot the water, which in its turn may be caused by short water residence time. Littoral elements in the fauna, which are generally less indicative, or effects of local inflows may also be more frequent here than in lakes of larger size.

A comparison of impoundments and natural drainage lakes in the Northeast USA

We classified 235 randomly selected lentic waterbodies (>1 ha) in the Northeast USA as human created, or natural. We compared geographic extent and distribution, morphology and hydrology, trophic state, and fish assemblage metrics of impoundments and natural drainage lakes. We estimated that 46% of the 10 608 (±1695; 95% CI) lentic waterbodies in the region were impoundments or quarries; 68% of Uplands lakes and 26% of Lowlands lakes were natural. Impoundments were smaller, shallower, had shorter water residence times, and were in watersheds with greater human activity than were natural drainage lakes. More than half (55%) of Lowlands impoundments were eutrophic, accounting for 67% of eutrophic or hypereutrophic lentic waterbodies in the Northeast. An estimated 90% of eutrophic lakes and impoundments were <23 ha. Impoundments had greater proportions of fish species and individuals tolerant of human disturbance, and greater proportions of non-native species and individuals than did natural drainage lakes. We discuss some management implications of the differences between impoundments and natural drainage lakes.

Fluxes of 230Th and 231Pa to the deep sea: implications for the interpretation of excess 230Th and 231Pa/ 230Th profiles in sediments

Analysis of samples obtained with deep-sea moored sediment traps deployed at 15 sites representing a wide range of oceanic conditions confirms that the flux of 230Th scavenged to the seafloor remains close to its production rate from the decay of 234U in the overlying water column, and generally validates the use of 230Th as a normalizing tool for paleoflux reconstruction. After correction for trapping efficiency, the flux of 230Th measured in the low flux regions amounts to 90±6% of the production rate, with the notable exceptions of one site near the Arabian Sea upwelling and one site in the Weddell Sea. A 230Th flux equivalent to 120% of the production rate was found in Panama Basin. Similar or more extensive scavenging of 230Th may be occurring at a Pacific margin site off California and south of the Polar Front, but these estimates are obscured by large errors on our trapping-efficiency estimates. In contrast, the flux of 231Pa and the 231Pa/ 230Th ratio can vary strongly with particle flux, following distinct trends in different oceanic basins. In the Atlantic Ocean, 231Pa fluxes and 231Pa/ 230Th are low and not sensitive to particle flux. This is because of the short residence time of deep water in this basin resulting from thermohaline circulation, which prevents the full development of lateral concentration gradients and full expression of boundary scavenging. In the Pacific Ocean, the sensitivity of 231Pa/ 230Th to particle flux is highest, reflecting the longer residence time of deep water. In the southern ocean, 231Pa/ 230Th ratios are invariably high, even when particle fluxes are low, reflecting the predominance of opal, which fractionates minimally between the two radionuclides. Interpretation of 231Pa/ 230Th recorded in sediments is thus complex. In the Atlantic, this ratio is better suited for recording past changes in the strength of thermohaline circulation. In the Pacific, it has the best potential for providing synoptic maps of past changes in particle flux. In the southern ocean, retrieval of information from 231Pa/ 230Th is more difficult and requires a more quantitative understanding of the influence of particle composition.

Forest-tundra water balance signals traced by isotopic enrichment in lakes

Enrichment of oxygen-18 and deuterium in surface waters is shown to be a useful indicator of water balance variations in remote, permafrost regions of northern Canada where hydroclimate monitoring networks are limited. Necessarily, such indicators must be applied with care as isotopic signals in each lake trace water balance and atmospheric conditions integrated over different time-periods and spatial areas, as determined by lake water-residence time and catchment drainage area. Isotopic enrichment in large lakes is found to be relatively stable in time, and, as interpreted in the context of steady-state models, will yield information more representative of regional climatological-scale processes. In contrast, seasonal isotopic enrichment in shallow lakes, which occurs due to extreme seasonality of the northern climate and short water residence times, may be useful for estimating short-term ‘point’ evaporation rates.

A stable carbon and nitrogen isotope study of lake food webs in Canada’s Boreal Plain

1. Stable carbon and nitrogen isotope and fish stomach content analyses were used to investigate food webs in five relatively undisturbed lakes on the Boreal Plain of Canada. Stable isotope analysis was also used to determine the importance of external and internal carbon sources.2. Overlap in the carbon and nitrogen signatures of primary producers made it difficult to determine unambiguously the feeding habits of many invertebrates. However, isotope analysis suggested that external carbon inputs were detectable in the aquatic food chains of the one lake with a short water residence time («1 year). In the other four lakes, with water residence times ≥1 year, autochthonous carbon was the only detectable carbon source in the food webs.3. Food webs in these lakes spanned a range of four to five trophic levels. Both invertebrates and fish appeared to eat a variety of food, often feeding at more than one trophic level.4. With the exception of one lake (SPH20), top predators in these lakes, northern pike (Esox lucius) and fathead minnows (Pimephales promelas), occupied similar trophic positions despite large differences in body size and trophic morphology. In SPH20, where there were two additional fish species, pike occupied a higher trophic position. However, all the top predators in each lake appeared to be omnivores and generalists.5. The prevalence of omnivory and the apparent generalist feeding habits of fish in these lakes suggest that organisms are flexible in their feeding habits and that these food webs will be resilient to disturbance.

The occurrence and behaviour of phosphate fractions in Izmir Bay, Aegean Sea

The area around Izmir Bay (Turkey) is heavily urbanized and receives, therefore, high concentrations of phosphate originating from industrial and municipal inputs. During the surveys between April 1993 and July 1994, the total phosphate concentrations were highest in the Inner Bay (6.45 μM and 5.59 μM in the surface and bottom water, respectively) where very dense anthropogenic pollution occurs. The total phosphate value gradually decreased towards the Outer Bay where 0.70 μM and 1.18 μM were found in the surface and bottom water, respectively. The distribution of dissolved inorganic, dissolved organic and particulate phosphate along the bay reflected some peculiar spatial and temporal patterns. A high percentage of dissolved organic phosphate was observed in the Outer Bay while the particulate phosphate peaked in the Middle Bay where an upwelling mechanism was observed. In the bay, the biological production was strongly correlated with the variations in the dissolved inorganic, organic and particulate phosphate. The high phosphate concentration in particles caused the particulate phosphate formed in the Inner Bay to be transported to the outer parts of the bay because of the short water residence time. On the other hand, Principal Component Analysis showed that about 35% of the variation in all variables measured were related to the total phosphate, total dissolved phosphate, dissolved inorganic phosphate and particulate phosphate together with ammonium and reactive-Fe while about 14% of the variation was related to the dissolved organic phosphate and the number of phytoplankton cells, chl-a, pH and O2.

Geochemistry of the acid Kawah Putih lake, Patuha Volcano, West Java, Indonesia

Kawah Putih is a summit crater of Patuha volcano, West Java, Indonesia, which contains a shallow, ∼300m-wide lake with strongly mineralized acid–sulfate–chloride water. The lake water has a temperature of 26–34°C, pH=<0.5–1.3, Stot=2500–4600ppm and Cl=5300–12 600ppm, and floating sulfur globules with sulfide inclusions are common. Sulfur oxyanion concentrations are unusually high, with S4O62−+S5O62−+S6O62−=2400 – 4200ppm. Subaerial fumaroles (<93°C) on the lake shore have low molar SO2/H2S ratios (<2), which is a favorable condition to produce the observed distribution of sulfur oxyanion species. Sulfur isotope data of dissolved sulfate and native sulfur show a significant 34S fractionation (ΔSO4–Se of ⩾20‰), probably the result of SO2 disproportionation in or below the lake. The lake waters show strong enrichments in 18O and D relative to local meteoric waters, a result of the combined effects of mixing between isotopically heavy fluids of deep origin and meteoric water, and evaporation-induced fractionation at the lake surface. The stable-isotope systematics combined with energy-balance considerations support very rapid fluid cycling through the lake system. Lake levels and element concentrations show strong seasonal fluctuations, indicative of a short water residence time in the lake as well.Thermodynamic modeling of the lake fluids indicates that the lake water is saturated with silica phases, barite, pyrite and various Pb, Sb, Cu, As, Bi-bearing sulfides when sulfur saturation is assumed. Precipitating phases predicted by the model calculations are consistent with the bulk chemistry of the sulfur-rich bottom sediments and their identified mineral phases. Much of the lake water chemistry can be explained by congruent rock dissolution in combination with preferential enrichments from entering fumarolic gases or brines and element removal by precipitating mineral phases, as indicated by a comparison of the fluids, volcanic rocks and lake bed sediment.Flank springs on the mountain at different elevations vary in composition, and are consistent with local rock dissolution as a dominant factor and pH-dependent element mobility. Discharges of warm sulfate- and chloride-rich water at the highest elevation and a near-neutral spring at lower level may contain a small contribution of crater-lake water. The acid fluid-induced processes at Patuha have led to the accumulation of elements that are commonly associated with volcano-hosted epithermal ore deposits. The dispersal of heavy metals and other potentially toxic elements from the volcano via the local drainage system is a matter of serious environmental concern.

Ecological half-lives of 137Cs in fishes from the Savannah River Site.

Ecological half-lives (Te's) were estimated for 137Cs in largemouth bass, sunfishes, and bullheads from two reservoirs and three streams on the Savannah River Site, a nuclear weapons material production facility in South Carolina. Ecological half-life is the time required for a given contaminant concentration to decrease by 50% as a result of physical, chemical, and/or biological processes that remove it from an ecosystem or render it biologically unavailable. Te's were estimated from whole-fish 137Cs concentrations in samples collected during 1972-1996, following radionuclide releases that occurred primarily during the 1960's and early 1970's. Te's ranged from 3.2 to 16.7 y, and all were shorter than expected from the half-life for radioactive decay (Tp = 30.2 y) alone. Fish taxa from the same locations differed in mean 137Cs concentrations (highest in largemouth bass and lowest in sunfishes) but, in most cases, exhibited similar 137Cs Te's. Rates of 137Cs removal in fishes were strongly correlated with rates of 137Cs removal in water. The shortest Te's occurred in the upper portions of the streams. Te's in lower portions of the streams were longer, as were Te's in one of the reservoirs. Te's in the second reservoir, which had a much shorter water residence time, were nearly comparable to those in the upper portions of the streams until 1991. At that time, 137Cs concentrations in fishes began to increase following drainage and refilling of the reservoir, which apparently resuspended 137Cs buried in the sediments.

Application of geochemistry and radioactivity in the hydrogeological investigation of carbonate aquifers (Sierras Blanca and Mijas, southern Spain)

The chemical characteristics, 3H contents and radioactivity of groundwaters from the Sierras Blanca and Mijas (Southern Spain) have been studied in relation to the chemical composition and radioactivity of the aquifer host rocks, and the residence time of the water. The Sierras Blanca and Mijas are made up of calcitic and dolomitic marbles of Triassic age. The groundwaters that drain the calcitic marbles (which outcrop principally in the western Sierra Blanca) have less mineralization, which descreases quickly with recharge (as does the 3H content), and the gross alpha and beta activities are below detection limit. This is due to the short residence time of water inside the aquifers which are conduit flow systems. The waters of the dolomitic marbles (eastern Sierra Blanca and Sierra Mijas) have higher and less variable mineralization and contain greater concentrations of Mg 2+, SiO 2 and SO 2− 4 (ions normally associated with slow flows). The 3H contents are more uniform with time (indicating an older age) and there is detectable natural radioactivity, because the waters have a longer residence time in the aquifers, which are diffuse flow systems.

Spatial Variation among Lakes within Landscapes: Ecological Organization along Lake Chains

Although limnologists have long been interested in regional patterns in lake attributes, only recently have they considered lakes connected and organized across the landscape, rather than as spatially independent entities. Here we explore the spatial organization of lake districts through the concept of landscape position, a concept that considers lakes longitudinally along gradients of geomorphology and hydrology. We analyzed long-term chemical and biological data from nine lake chains (lakes in a series connected through surface or groundwater flow) from seven lake districts of diverse hydrologic and geomorphic settings across North America. Spatial patterns in lake variables driven by landscape position were surprisingly common across lake districts and across a wide range of variables. On the other hand, temporal patterns of lake variables, quantified using synchrony, the degree to which pairs of lakes exhibit similar dynamics through time, related to landscape position only for lake chains with lake water residence times that spanned a wide range and were generally long (close to or greater than 1 year). Highest synchrony of lakes within a lake chain occurred when lakes had short water residence times. Our results from both the spatial and temporal analyses suggest that certain features of the landscape position concept are robust enough to span a wide range of seemingly disparate lake types. The strong spatial patterns observed in this analysis, and some unexplained patterns, suggest the need to further study these scales and to continue to view lake ecosystems spatially, longitudinally, and broadly across the landscape.

Water–Surfactant Contact Studied by19F–1H Heteronuclear Overhauser Effect Spectroscopy

Intermolecular19F–1H cross-relaxation is measured using heteronuclear Overhauser effect NMR spectroscopy (HOESY) in the micellar solution of cesium pentadecafluorooctanoate. The results are analyzed in terms of a weak1H–19F cross-relaxation between the water protons and the fluorines in the fluoroalkyl chain and a strong19F–19F cross-relaxation within the fluoroalkyl chain. The water–surfactant cross-relaxation indicates a water approach to the first CF2segment in the order of 2.0 Å and a short (⪡ns) water residence time. Evidence of fluorine hydration further inside the micelle is presented.

Seasonal variations of dissolved inorganic carbon and δ13C of surface waters: application of a modified gas evolution technique

Seasonal concentrations and δ 13C of dissolved inorganic carbon (DIC) in a river-tributary system in Kalamazoo, southwest Michigan, USA, have been measured using a modified gas evolution technique. The technique makes use of evacuated glass septum tubes pre-loaded with phosphoric acid and a magnetic stir bar. Water samples are injected into these septum tubes in the field, which eliminates problems associated with CO 2 loss/gain during sample storage and transfer to the vacuum line during DIC extraction. Using this technique, a precision of 1% and 0.1‰ can be achieved for DIC concentrations and δ 13C DIC measurements, respectively. As this technique provides reliable measurements of DIC concentrations and carbon isotope ratios, it was used to evaluate the processes that control DIC in the river-tributary system. Results of DIC concentration and δ 13C DIC measurements of water samples from the river-tributary system show that the DIC pool is mostly dominated by groundwater. The DIC concentrations and δ 13C DIC are within the ranges measured for the most isotopically evolved groundwater in this region. Seasonal variations superimposed on the baseline values are attributed to secondary processes such as CO 2 invasion from the atmosphere, enhanced recharge from lakes and biological activities of photosynthesis, respiration, and decay. With the onset of spring, there is a concurrent increase in the DIC concentration and δ 13C DIC of these streams. A simultaneous increase in concentration and 13C enrichment of the riverine DIC pool is consistent with CO 2 invasion and recharge from lakes. During the summer, biological activity is the predominant control on shifts in the DIC pool. Although photosynthesis, respiration and decay occur during this time, decreases in the DIC concentration and increases in the 13C DIC indicates CO 2 removal from the pool by photosynthesis. In the late summer-early fall, photosynthesis declines and respiration and decay cause an increase in the DIC concentration and a decreases in the δ 13C DIC. When biological activity decreases significantly during the late fall and winter months, the river and most of its tributaries approach a baseline DIC concentration and δ 13C DIC similar to that of isotopically fully evolved groundwater in the Kalamazoo area. Although this holds true for tributaries and the main river, the timing and magnitude of shifts from background DIC is different for individual streams. The magnitude of shifts in the DIC concentration and δ 13C DIC is most pronounced in tributary streams because of the short residence time of water in these streams. This study shows that DIC concentration and δ 13C DIC measurements can be successfully used to evaluate the timing and dominance of the major processes that influence DIC in a riverine system.

Local Sulphide Oxidation Drives Carbonate Dissolution in Modern Shallow Marine Carbonates: Evidence from Oxygen and Sulphur Isotope Composition of Pore Water Sulphate

We have investigated sulphur cycling in modem carbonate sediments from the Florida Platform, U.S.A. Relations among pore water chemistry (SO24 , ZCO2, Ca2+/C1 ) and oxygen and sulphur stable isotope composition of SO 2require a direct coupling between sulphur redox cycling and syndepositional carbonate dissolution. Importantly, despite rapid rates of sulphate reduction, organic-rich carbonate sediment pore waters typically maintain SO2-/C1 ratios within 5% of seawater values, suggesting local reoxidation of H2S. Shallow shelf carbonates comprise 93% of the entire Phanerozoic carbonate rock record and about 38% of the modem oceanic carbonate accumulation, so quantifying sulphur and carbon coupling during the early marine diagenesis is vital in deciphering biogeochemical cycling (e.g. Ronov, 1980; Milliman, 1993). In low-Fe carbonates, H2S-O2 reactions can lower pH values and pore water saturation states promoting carbonate dissolution (e.g. Boudreau, 1991). Given that platform carbonate dissolution fluxes are roughly half of gross biogenic carbonate production, sulphur cycling may greatly affect carbonate preservation and accumulation (Walter and Burton, 1990; Walter et al., 1993). Pore water sulphate ~180 and 834S values were markedly shifted from the established seawater values of +9.5%o (SMOW) and +21.0%o (CDT), respectively. Chemical evolution was least in environments where the sediment-pore water system was relatively 'open' to the overlying seawater. The greatest chemical evolution was observed in bioturbated sediments colonized by Thalassia grass. Here, there was a ~lSOso 4 shift of 5%0, var iab le ~348SO 4 values (+17.7 to +23.3%o), and exceptionally high Ca2+/C1 ratios. These shifts in pore water sulphate isotopic compositions exist despite the apparent lack of net sulphate reduction (Fig. 1). The natural system isotope trend is in marked contrast to the closed system isotope evolution observed in sediment incubation experiments (Fig. 1). Importantly, sulphide oxidation is required as an acid source because pore water Ca+2/C1 and ZCO2 values exceed those attributable to the utilization of dissolved oxygen and the small amount of net sulphate reduction. The observed degree of carbonate dissolution (up to 2.3 mM excess Ca 2+) and sulphate isotopic values are best explained by a redox cycle where bacterial sulphate reduction is followed by efficient H2S oxidation resulting in relatively invariant SO24-/C1 ratios. The 02 required to locally oxidize pore water H2S may come from oxygenated seawater advection or from marine grass rhizome systems which enhance oxygen supply rates. A mass balance model demonstrates that the observed 81SOso4 values are consistent with measured sulphate reduction rates and the short pore water residence times. Pore water geochemistry and sediment incubation experiments confirm that sulphur cycling has a profound effect on the early marine diagenesis of shallow platform carbonates. The important diagenetic processes are oxic respiration, bacterial sulphate reduction, sulphide oxidation, and carbonate dissolution. In most sediments, pore water SO2-/C1 ratios are near overlying ocean values, yet in many cases SO24 has been recycled from HzS. The regularity of pore water evolution and sediment environment becomes more apparent when the oxygen and sulphur isotope compositions of SO 2are examined. Diagenetic effects are most

Nitrogen Losses Through Sediment Denitrification in Boston Harbor and Massachusetts Bay

Sediment denitrification is a microbial process that converts dissolved inorganic nitrogen in sediment porewaters to N2 gas, which is subsequently lost to the atmosphere. In coastal waters, it represents a potentially important loss pathway for fixed nitrogen which might otherwise be available to primary producers. Currently, data are lacking to adequately assess the role of denitrification in reducing or remediating the effects of large anthropogenic nitrogen loads to the coastal zone. This study describes the results of 88 individual measurements of denitrification (as a direct flux of N2 gas) in sediment cores taken over a 3-yr period (1991–1994) from six stations in Boston Harbor, nine stations in Massachusetts Bay, and two stations in Cape Cod Bay. The dataset is unique in its extensive spatial and temporal coverage and includes the first direct measurements of denitrification for North Atlantic shelf sediments. Results showed that rates of denitrification were significantly higher in Boston Harbor (mean=54, range<5–206 μmol N2 m−2 h−1) than in Massachusetts Bay (mean=23, range<5–64 μmol N2 m−2 h−1). Highest rates occurred in areas with organic-rich sediments in the harbor, with slower rates observed for low-organic sandy sediments in the harbor and at shallow shelf stations in the bay. Lowest rates were found at the deepest shelf stations, located in Stellwagen Basin in Massachusetts Bay. Observed rates were correlated with temperature, sediment carbon content, and benthic macrofaunal activity. Seasonally, highest denitrification rates occurred in the summer in Boston Harbor and in the spring and fall in Massachusetts Bay, coincident with peak phytoplankton blooms in the overlying water column. Despite the fact that sediment denitrification rates were high relative to rates reported for other East Coast estuaries, denitrification losses accounted for only 8% of the annual total nitrogen load to Boston Harbor, a consequence perhaps, of the short water-residence times (2–10 d) of the harbor.

Elevated and variable values of 13C in speleothems in a British cave system

δ 13C isotope variations in speleothems have been investigated for samples from the British Isles, where plants which use the Hatch-Slack or C 4 photosynthetic pathway are not present. The range of δ 13C expected in speleothem carbonate formed in isotopic equilibrium with soil CO 2 derived from the overlying C 3 vegetation should thus fall in the range −12 to −6‰. Forty-one actively growing speleothem samples from low-discharge sites were analysed from Stump Cross Caverns, Yorkshire, England. Ten percent have δ 13C greater than −6%. In addition, a large range of δ 13C was observed (−8.06 ± 1.38‰, a 1 σ variability of 17%), with adjacent samples having δ 13C differing by a maximum of 4.74‰. Similar findings were obtained from a review of analyses of late Quaternary speleothem samples from the British Isles, with 75% of flowstone samples and 57% of high-flow stalagmite samples exhibiting elevated δ 13C. Three possible processes are proposed as possible causes of elevated δ 13C in speleothems. Firstly, fractionation may occur between the stalactite and stalagmite due to evaporation or degassing. Secondly, degassing of the groundwaters may have occurred within the aquifer before reaching the cave void, allowing release of some CO 2 from the water whilst remaining saturated in calcium. Finally, the elevated δ 13C may be due to short water residence times in the soil, such that equilibrium between soil water and soil CO 2 is not reached. Evidence presented here demonstrates that any one of these mechanisms may be important in the karst areas of the British Isles. Caution is needed before interpreting the δ 13C signal within speleothems in terms of palaeovegetation.

Effects of zebra mussels on food webs: interactions with juvenile bluegill and water residence time

We evaluated how water residence time mediated the impact of zebramussels Dreissena polymorpha and bluegill sunfish Lepomis macrochirus on experimental food webs established in1100-l outdoor mesocosms. Water residence time was manipulated asa surrogate for seston resupply – a critical variable affectinggrowth and survival of suspension-feeding invertebrates. We used a2×2×2 factorial experimental design witheight treatment combinations (3 replicates/treatment) including thepresence or absence of Dreissena (2000 per m2), juvenilebluegill (40 per mesocosm), and short (1100 l per d) or long (220 lper d) water residence time. Measures of seston concentration(chlorophyll a, turbidity and suspended solids) were greaterin the short- compared to long water-residence mesocosms, butintermediate in short water-residence mesocosms containing Dreissena. Abundance of rotifers (Keratella and Polyarthra) was reduced in Dreissena mesocosms and elevatedin short residence time mesocosms. Cladocera abundance, in general,was unaffected by the presence of Dreissena; densities werehigher in short-residence time mesocosms, and reduced in thepresence of Lepomis. The growth of juvenile Lepomiswere unaffected by Dreissena because of abundant benthicfood. The final total mass of Dreissena was significantlygreater in short- than long-residence mesocosms. Impacts of Dreissena on planktonic food webs may not only depend on thedensity of zebra mussels but also on the residence time of thesurrounding water and the resupply of seston.