bluuebunny/crossref_metadata_2025 huggingface dataset:
Quantification of macrobenthic effects on diagenesis using a multicomponent inversemodel in salt marsh sediments
Authors: Furukawa Yoko , Smith April C. , Kostka Joel E. , Watkins Janet , Alexander Clark R.
Published: July 2010

Using a multicomponent inverse model, we quantified the rates of organic matter (OM) remineralization and the relative importance of major terminal electron acceptors [Fe(III)–(oxy)hydroxides and SO4∐3] in salt marsh sediments with varying degrees of bioturbation and vegetation at Skidaway Island, Georgia. The model determined the rates of OM diagenesis by seeking simultaneous agreement between measured and model‐calculated depth‐concentration profiles of multiple major redox species while using the biological transport parameters determined from direct field observations. The OM degradation rates are found to be greater and penetrate deeper in sediments with vegetation or bioturbation than in sediments with limited macrobenthic components, in which organic matter degradation is restricted to the immediate vicinity of water‐sediment interface. The results also confirmed previous observations that Fe(III)–(oxy)hydroxides are much more important than sulfate as terminal electron acceptors in the heavily bioturbated station and provided the numerical values to the depth‐dependent relative importance. Solid‐phase Fe is recycled as a terminal electron acceptor >=30 times in the bioturbated station, because biological mixing repeatedly moves reduced Fe(II) back into the aerobic environment, where it is reoxidized to form Fe(III)–(oxy)hydroxides and is reused as a terminal electron acceptor. Vegetation appears to have little influence on net solute transport, but it stimulates microbial activities and significantly enhances the remineralization of OM at depths.
Element stoichiometries of individual plankton cells collected during the Southern Ocean Iron Experiment (SOFeX)
Authors: Twining Benjamin S. , Baines Stephen B. , Fisher Nicholas S.
Published: July 2010

During the Southern Ocean Iron Experiment (SOFeX), we analyzed Si, P, S, Mn, Ni, and Zn in individual diatoms, autotrophic flagellates, and heterotrophic flagellates with synchrotron‐based X‐ray fluorescence (SXRF) and calculated cellular C from measurements of cell size. Element stoichiometries for the different types of protists (normalized to either C, S, or P) were generally in good agreement with prior bulk analyses of natural assemblages but also revealed previously undocumented differences in elemental composition among cell types. Flagellated cells contained 39% more P than diatoms, which in turn contained 79% more Mn, 3‐fold more Ni, and 2.6‐fold more Zn than flagellates. Heterotroph cells contained approximately 40% more P and twice as much Zn as autotrophs. Manganese and Ni stoichiometries were negatively related to cell volume, while larger cells contained more Zn per mole C. Iron fertilization resulted in an approximate doubling of Mn, Ni, and Zn quotas and smaller increases in cellular P, but the timing of the stoichiometric changes varied between the two patches. Silicon contents of diatoms dropped approximately 40% after the first Fe addition but returned to prefertilization levels as cellular P doubled following the second addition, resulting in 40% lower Si : P ratios in the fertilized waters. The mean elemental stoichiometries calculated for all cells analyzed were comparable with previously published extended Redfield ratios for mixed plankton assemblages, but the observed differences between diatoms and flagellates and between autotrophs and heterotrophs indicate that valuable information is lost when all types of co‐occurring plankton are grouped together for analysis.
Growth rates, half‐saturation constants, and silicate, nitrate, and phosphate depletion in relation to iron availability of four large, open‐ocean diatoms from the Southern Ocean
Authors: Timmermans Klaas R. , van der Wagt Bas , de Baar Hein J. W.
Published: July 2010

Four large, open‐ocean diatoms from the Southern Ocean (Actinocyclus sp., Thalassiosira sp., Fragilariopsis kerguelensis, and Corethron pennatum) were grown in natural (low iron) Southern ocean seawater with increasing Fe concentrations. With increasing dissolved iron (Fediss) concentrations, the growth rates increased three‐ to sixfold. The species with the smallest cells had the highest growth rates. The half‐saturation constants (Km) for growth were low (0.19−1.14 nmol L−1 Fediss), and close to the ambient Fediss concentrations of 0.2 nmol L−1. The range in Km with respect to Fediss also varied with the size of the diatoms: the smallest species had the lowest Km and the largest species had the highest Km. As Fediss concentrations decreased, silicate consumption per cell increased, but nitrate consumption per cell decreased. Phosphate consumption per cell varied without clear relation to the dissolved iron concentrations. The differences in nutrient consumption per cell resulted in marked differences in elemental depletion ratios in relation to Fedissconcentrations, with the depletion ratios being most affected by iron limitation in the largest cells. These experimental findings are in agreement with previous laboratory and field studies, showing the relatively high requirements of large diatoms for Fe. The size‐dependent response of the diatoms with respect to nutrient depletion is a good illustration of the effects of Fe on silicate, nitrate, and phosphate metabolism.
Bloom formation in heterocystic nitrogen‐fixing cyanobacteria: The dependence on colony size and zooplankton grazing
Authors: Chan Francis , Pace Michael L. , Howarth Robert W. , Marino Roxanne M.
Published: July 2010

The success of filamentous nitrogen (N)‐fixing cyanobacteria in productive, transiently N‐limited freshwaters reflects, in large part, their ability to produce and sustain the activities of specialized N‐fixing heterocyst cells. Heterocyst production is variable, and the responses of planktonic cyanobacterial blooms to N limitation differ markedly among systems. Temporal variations in cyanobacteria colony size may determine both heterocyst production and sensitivity to top‐down control by zooplankton grazers. We promoted the development of cyanobacterial blooms through phosphorus additions and trophic manipulations in freshwater ponds, to test the role of colony size structure in regulating N‐fixer bloom development. The in situ growth and heterocyst production of Anabaena spp. were strongly linked to variations in colonial filament size. Anabaena spp. initially recruited to the water column as short and poorly heterocysted filaments, exhibiting low (mean = 0.09 d−1) rates of population growth. The growth rate increased by more than fourfold (mean = 0.39 d−1) with the onset of colony elongation and heterocyst production, which resulted in rapid seasonal build‐ups of cyanobacterial cells (>105 cells ml−1). Size‐dependent growth was also important in determining the outcome of zooplankton‐cyanobacteria interactions. In microcosm experiments, zooplankton consumers directly grazed on Anabaena spp. colonies, reduced the mean filament size, and reduced the efficiency with which heterocysts fixed nitrogen. These results suggest that colony size is a fundamental mechanism that link cellular physiological constraints to variations in trophic controls and the responses of aquatic systems to N limitation.
Quantitative detection of chlorophyll in cyanobacterial blooms by satellite remote sensing
Authors: Kutser Tiit
Published: July 2010

The extent of cyanobacterial blooms has been mapped using different satellite sensors from weather satellites to synthetic aperture radars. Quantitative detection of chlorophyll in cyanobacterial blooms by remote sensing, however, has been less successful. The first civilian hyperspectral sensor in space, Hyperion, acquired an image of cyanobacterial bloom in the western part of the Gulf of Finland on 14 July 2002. A chlorophyll concentration map was produced from this image using a spectral library that was created by running a bio‐optical model with variable concentrations of chlorophyll. The results show that chlorophyll concentrations in the bloom area were much higher than reported by conventional water‐monitoring programs, ships‐of‐opportunity, and satellite measurements. The reason why both in situ and satellite methods underestimate the amount of phytoplankton during cyanobacterial blooms is vertical and horizontal distribution of cyanobacteria, because cyanobacteria can regulate their buoyancy and are not uniformly distributed within the top mixed layer of water column in calm weather conditions. Aggregations of cyanobacteria form dense subsurface blooms and surface scums during extensive blooms. This study demonstrates that it is difficult to collect representative water samples from research vessels using standard methods because ships and water samplers destroy the natural distribution of cyanobacteria in the sampling process. Flowthrough systems take water samples from the depths at which the concentration of cyanobacteria is not correlated with the amount of phytoplankton that remote sensing instruments detect. The chlorophyll estimation accuracy in cyanobacterial blooms by many satellites is limited because of spatial resolution, as significant changes in chlorophyll concentration occur even at a smaller spatial scale than 30 m.
Methane along the western Mexican margin
Authors: Sansone Francis J. , Graham Andrew W. , Berelson William M.
Published: July 2010

We investigated the processes controlling the water‐column and sediment methane distributions at 14 stations along the western Mexican margin, in and around the Gulf of California. Stations were grouped into two categories: coastal basins and open margins. Diffusive methane fluxes from the sediment at all sites, as estimated from sediment methane gradients, were 0.24–5.5 µmol m−2 d−1, with the highest fluxes observed on the Pacific margin of Baja California at both basin and open‐margin sites. These high rates occur despite the lack of significant terrestrial input to these sediments, reflecting the importance of upwelling‐induced productivity. Methane concentrations in the upper water column were supersaturated with respect to the present atmosphere at all sites, with sea‐air fluxes of methane of 0.5–5.9 µmol m−2 d−1. Four of the open‐margin sites had seafloor depths extending below the oxygen minimum zone (~400–800 m) and contained low methane concentrations below the subsurface methane maximum. The remaining margin site was shallow (593 m), with a seafloor that intersected the oxygen minimum zone, and had elevated methane concentrations throughout the water column; this indicates that such sediments may be a significant source of methane to the eastern tropical North Pacific (ETNP). The seawater within silled basin sites also had supersaturated methane concentrations, reflecting the anoxic conditions within the basins. However, methane levels were low at the sill depth, indicating that the silled basins were unlikely to be significant sources of methane to the ETNP. We observed an inverse relationship between methane concentration and δ13C‐CH4 value in the basin waters, consistent with biological aerobic oxidation of methane being released from the sediment; an apparent kinetic isotopic fractionation factor of 1.0100–0.0038 was calculated for this process. Isotopically heavy methane resulting from similar oxidation of seafloor‐derived methane may be the source of large pools of heavy methane previously observed offshore in the ETNP.
Volcanic heat flux and short‐term holomixis during the summer stratification period in a crater lake
Authors: Shikano Shuichi , Kikuchi Eisuke , Takagi Shigeto , Doi Hideyuki
Published: July 2010

We sampled Lake Katanuma from 1998 to 2002 at weekly or biweekly intervals, except in winter. This dimictic volcanic lake has a pH of 2.0–2.2. In summer, volcanic heat at the lake bottom results in a small water temperature difference of 3–7°C between the epilimnion and hypolimnion. During the April‐A00B0tification period in 1998, 1999, and 2002, the entire water column mixed for 1‐8 d, when air temperature declined 4–8°C from the epilimnion temperature. During these mixing events, anoxic, hydrogen sulfide‐rich water from the hypolimnion spread over the entire lake. In contrast, distinct short‐term turnover did not occur during the stratification period in 2000 or 2001. However, the early onset of autumn turnover in August 2000 and 2001 caused anoxic conditions with negative Eh values to persist for ~2 weeks throughout the entire water column. Volcanic heat flux from the lake bottom fluctuated from 29.0 to 35.8 W m−2 from year to year. The frequency and duration of short‐term holomixis thus depended on interannual variations in volcanic heat flux and weather.
Clearance of aquatic hyphomycete spores by a benthic suspension feeder
Authors: Bärlocher Felix , Brendelberger Heinz
Published: July 2010

We placed individual Elliptio complanata in aerated suspensions of conidia (asexual spores) of aquatic hyphomycetes (Fungi). Mussels actively ingested conidia at an initial rate of over 20,000 h−1. The conidia of the seven most common species were removed at different rates, but there was no consistent correlation between conidial size or shape (tetraradiate, sigmoid, or clove shaped) and removal by E. complanata. At the maximum clearance rate, animals ingested the equivalent of 159 ng conidial biomass h−1, corresponding to 4.7 µg d−1 g−1 animal dry mass. At natural densities, E. complanata may be able to eliminate a considerable proportion of the conidial production, causing a shift in the proportions of species in the remaining conidia.
Effect of net avoidance on estimates of diel vertical migration
Authors: Ianson Debby , Jackson George A. , Angel Martin V. , Lampitt Richard S. , Burd Adrian B.
Published: July 2010

Diel vertical migration (DVM) has been hypothesized to actively transport organic material out of the euphotic layer, thus forming a novel part of the “biological pump.” However, quantifying DVM is made difficult by observational limitations. Conventionally, the difference between night and day biomass from net trawls in the surface has been assumed to be a consequence of species that have migrated up from their deep daytime depths. However, some of this difference might be an artifact of visual net avoidance. Here, we present a method that facilitates quantification of zooplankton that are migrating, those that are not migrating, and those able to avoid net capture. The algorithm is applied to an extensive data set gathered in the Azores Front region. Results indicate that DVM, and thus active carbon transport, calculated in the traditional manner would overestimate the true value by ~50%.
Persistence of aquatic refugia between flow pulses in a dryland river system(Cooper Creek, Australia)
Authors: Hamilton Stephen K. , Bunn Stuart E. , Thoms Martin C. , Marshall Jonathan C.
Published: July 2010

In many dryland rivers with intermittent flow, relatively deep segments of the river channel serve as refugia for aquatic life during protracted intervals between flows. Semipermanent channel segments are known as waterholes in the semiarid Cooper Creek system of western Queensland. Fractional water loss by evaporation was estimated in 15 Cooper waterholes from the increase in conservative ion (Na+ and Cl−) concentrations and independently from evaporative fractionation of oxygen and hydrogen isotopes in water. The major solute chemistry and isotope results indicated that evaporative water loss controlled the water levels between flows and that the surface waters were effectively isolated from underlying groundwater that had a distinctive chemical and isotopic composition. Fractional water loss rates combined with stage‐volume relationships for each basin showed a mean evaporative loss rate of 2.1 m yr−1; during that time (April‐October 2002), pan evaporation averaged 2.5 m yr−1. Site‐specific extrapolation of those estimated evaporative loss rates indicated that the waterholes would dry to 10% of their bankfull volumes in 6‐23 months, although those estimates were based on sampling in 2002, when pan evaporation rates were 18% higher than the long‐term mean. These persistence times show the importance of occasional, irregular flow pulses in sustaining these aquatic refugia; the desiccation of waterholes could become more common if future water withdrawals reduce the frequency and intensity of river flows to the point where they occur less often than annually.
Pore‐water advection and solute fluxes in permeable marine sediments (II): Benthic respiration at three sandy sites with different permeabilities (German Bight, North Sea)
Authors: Janssen F. , Huettel M. , Witte U.
Published: July 2010

This contribution presents total oxygen uptake (TOU) rates and nutrient fluxes of organically poor permeable shelf sands of the German Bight. Measurements have been made in situ with the novel autonomous benthic chamber system Sandy under controlled conditions of advective pore‐water exchange. Average oxygen consumption rates of 31.3 ± 18.2 mmol m−2 d−1 measured in this study were relatively high as compared with rates reported from shelf sediments with much higher organic contents. TOU of highly permeable medium and coarse grained sands was substantially enhanced in the presence of advection. This indicates that advective oxygen supply contributed significantly to respiration in these sediments and that advection has to be considered when assessing oxygen consumption and organic matter mineralization in shelf areas. In fine‐grained, less permeable sands, no effect of advection could be measured. A lower advective oxygen supply in these sediments is in agreement with a release of ammonium instead of nitrate and a shallower oxygen penetration depth. Scaled up to the entire German Bight, the results imply that in 40% of the area an effect of advection on benthic oxygen uptake and other advectionrelated processes can be largely excluded, while in the remaining 60% significant pore‐water advection potentially takes place. However, because permeabilities of the sediments investigated in this study were widely spaced, a significant effect on oxygen supply was only verified for highly permeable sands that are likely to cover approximately 3% of the area.
Does the red tide dinoflagellate Karenia brevis use allelopathy to outcompete other phytoplankton?
Authors: Kubanek Julia , Hicks Melissa K. , Naar Jerome , Villareal Tracy A.
Published: July 2010

Monospecific blooms of phytoplankton can disrupt pelagic communities and negatively affect human health and economies. Interspecific competition may play an important role in promoting blooms, and so we tested (1) whether the outcome of competition between the red tide dinoflagellate Karenia brevis (ex Gymnodinium breve) and 12 cooccurring phytoplankters could be explained by allelopathic effects of compounds released by K. brevis and (2) whether waterborne, lipophilic molecules, including brevetoxins, are involved. Nine of 12 phytoplankton species were suppressed when grown with live K. brevis at bloom concentrations. K. brevis extracellular filtrates or lipophilic extracts of filtrates inhibited six of these nine species, indicating allelopathy. However, these inhibitory effects were weaker than those experienced by competitors exposed to live K. brevis. Brevetoxins at ecologically reasonable waterborne concentrations accounted for the modest inhibition by K. brevis of only one competitor, Skeletonema costatum. The addition of brevetoxins also caused significant autoinhibition, reducing the maximum concentration of K. brevis. Allelopathy is one mechanism by which K. brevis appears to exhibit competitive advantage over some sympatric phytoplankters, although unidentified compounds other than brevetoxins must be involved, in most cases. K. brevis was also susceptible to competitive exclusion by several species, including Odontella aurita and Prorocentrum minimum, known to thrive during K. brevis blooms. Although field experiments are required to assess whether allelopathy plays a fundamental role in bloom dynamics, our results indicate that allelopathy occurs widely but with species‐specific consequences.
Factors controlling δ13C values of sedimentary carbon in hypertrophic Baldeggersee, Switzerland, and implications for interpreting isotope excursions in lake sedimentary records
Authors: Teranes Jane L. , Bernasconi Stefano M.
Published: July 2010

We use stable carbon isotope values (δ13C) in sedimented organic matter and carbonate as proxy indicators of productivity changes in a highly eutrophic to hypertrophic lake. Stable isotope data from a seasonally sampled sediment core recovered from Baldeggersee, Switzerland were compared with direct and inferred total phosphorus (P) concentrations. Carbon isotope values of sedimentary organic matter (δ13Corg) in both seasonal varves and over the past 100 yr are not linearly related to total P concentrations, nor do they mirror observed trends in carbonate isotope values (δ13CCaCO3). Baldeggersee δ13Corg values are influenced by both variations in the relative inputs of eukaryotic biomass, which becomes enriched in 13C with increasing primary productivity, and the contribution of microbial biomass produced in the expanding anoxic bottom waters, which is typically very depleted in 13C. We also examined the fractionation within the organic matter‐CO2–CaCO3 system, calculated as εtotal organic carbon(TOC) = 103{[(δ13CCaCO3 + 1,000)/(δ13Corg + 1,000)] − 1}. Thresholds within the measured εTOC values represent Baldeggersee CaCO3 trophic status more accurately than δ13Corg or δ13CCaCO3 values alone. In alkaline lakes with endogenic carbonate precipitates, εTOC values can facilitate accurate interpretation of values and trends in organic carbon isotope data, and can help to better assess trophic history and lake remediation efforts in lake systems that have been heavily affected by cultural eutrophication.
Diel pattern with abrupt crepuscular changes of zooplankton over a coral reef
Authors: Yahel Ruthy , Yahel Gitai , Berman Tal , Jaffe Jules S. , Genin Amatzia
Published: July 2010

We studied zooplankton abundance and emergence patterns over coral reefs in the Gulf of Aqaba using highresolution acoustics (multibeam sonar, FTV, 1.6 MHz, and WH600 ADCP, 614 KHz), emergence traps, and an underwater pump, supplemented with field observations on feeding behavior of zooplanktivorous fish. The zooplanktonic community over coral reefs is a complex and highly dynamic mixture of pelagic plankton advected into the reef, larvae and eggs released by benthic animals, and demersal plankton. At sunset (0 ±4 min, mean 695% confidence interval) acoustic back‐scattering intensity and zooplankton biomass started to increase rapidly. In contrast with this ascent, the predawn decline began much earlier during the dark (82 ±5 min before sunrise) and terminated before sunrise. This light‐dependent diel behavior was highly consistent throughout the year, regardless of seasonal and environmental changes. Smaller zooplankters (500–700 µm) ascended first and demersal zooplankton accounted for most of the increase in zooplankton soon after sunset. Surprisingly, the emergence of zooplankton occurred while large schools of diurnal zooplanktivorous fish were still foraging in the water column. However, at that time, prey‐capture efficiency by the fish is greatly reduced and corals had not yet expanded their tentacles, producing an optimal window of opportunity of low risk for emergence.
Bacterial freshwater species successfully immigrate to the brackish water environment in the northern Baltic
Authors: Kisand Veljo. , Andersson Nina. , Wikner Johan.
Published: July 2010

We studied the distribution and seasonal dynamics of five species from the genus Flavobacterium and one species from the genus Marinomonas over the course of a year along a northern Baltic Sea river‐marine transect. All of the species had been previously demonstrated as important consumers of riverine dissolved organic carbon. Quantitative DNA–DNA hybridization data showed that two of the Flavobacterium spp. and the Marinomonas sp. had highest abundance in the river water (maximum 20,000 cells ml−1), with maximum relative abundance of 0.5–2.5% of the bacterial community. These species declined in abundance from the river to the estuary and the offshore site. Abundance and dynamics in the estuarine environment suggested successful immigration of freshwater bacteria, accompanied by growth in the brackish water environment. Two of the three abundant species showed high cell numbers also during late autumn to early spring in the estuary, indicating a selective advantage when riverine dissolved organic carbon was the main carbon source. The remaining three species showed more episodic abundance close to the detection limit of the method, providing weaker evidence of occurrence in the freshwater environment. Some bacterioplankton consuming riverine organic carbon in the brackish water environment in the northern Baltic are therefore freshwater species, with a selective advantage during winter.
Nitrate : phosphate ratios and Emiliania huxleyi blooms
Authors: Lessard Evelyn J. , Merico Agostino , Tyrrell Toby
Published: July 2010

It has been hypothesized that phosphate limitation, classically indicated by NO3 : PO4 ratios >16, is one of the critical factors allowing the coccolithophorid Emiliania huxleyi to bloom. This hypothesis is based on physiological studies showing that E. huxleyi has an exceptionally high affinity for orthophosphate and is able to use organic phosphate. Indeed, E. huxleyi has been found to bloom at high NO3 : PO4 ratios in some mesocosm studies and in the oceanic northeast North Atlantic. Recent E. huxleyi blooms on the southeastern Bering Sea shelf, however, occurred under low NO3 : PO4 conditions, which is indicative of nitrogen rather than phosphorus stress. A review of field studies of blooms where nitrate and phosphate were measured indicates that NO3 : PO4 was in fact frequently low. A survey of most of the areas of the world ocean where satellite‐detected E. huxleyi blooms occur also shows that NO3 : PO4 ratios are generally low. These observations suggest that E. huxleyi is able to exploit situations where either phosphorus or nitrogen is limiting to competing species. They also indicate that attention should be directed to examining organic nitrogen, organic phosphorus, and ammonium during E. huxleyi blooms to better understand the role macronutrients play in these blooms.
Eutrophication of freshwater and marine ecosystems
Authors: Smith Val H. , Joye Samantha B. , Howarth Robert W.
Published: July 2010

Initial understanding of the links between nutrients and aquatic productivity originated in Europe in the early 1900s, and our knowledge base has expanded greatly during the past 40 yr. This explosion of eutrophication‐related research has made it unequivocally clear that a comprehensive strategy to prevent excessive amounts of nitrogen and phosphorus from entering our waterways is needed to protect our lakes, rivers, and coasts from water quality deterioration. However, despite these very significant advances, cultural eutrophication remains one of the foremost problems for protecting our valuable surface water resources. The papers in this special issue provide a valuable cross section and synthesis of our current understanding of both freshwater and marine eutrophication science. They also serve to identify gaps in our knowledge and will help to guide future research.
Recent advances in the understanding and management of eutrophication
Authors: Schindler D. W.
Published: July 2010

Major advances in the scientific understanding and management of eutrophication have been made since the late 1960s. The control of point sources of phosphorus reduced algal blooms in many lakes. Diffuse nutrient sources from land use changes and urbanization in the catchments of lakes have proved possible to control but require many years of restoration efforts. The importance of water residence time to eutrophication has been recognized. Changes in aquatic communities contribute to eutrophication via the trophic cascade, nutrient stoichiometry, and transport of nutrients from benthic to pelagic regions. Overexploitation of piscivorous fishes appears to be a particularly common amplifier of eutrophication. Internal nutrient loading can be controlled by reducing external loading, although the full response of lakes may take decades. In the years ahead, climate warming will aggravate eutrophication in lakes receiving point sources of nutrients, as a result of increasing water residence times. Decreased silica supplies from dwindling inflows may increasingly favor the replacement of diatoms by nitrogen‐fixing Cyanobacteria. Increases in transport of nitrogen by rivers to estuaries and coastal oceans have followed increased use of nitrogen in agriculture and increasing emissions to the atmosphere. Our understanding of eutrophication and its management has evolved from simple control of nutrient sources to recognition that it is often a cumulative effects problem that will require protection and restoration of many features of a lake's community and its catchment.
Coastal eutrophication and trend reversal: A Danish case study
Authors: Carstensen Jacob , Conley Daniel J. , Andersen Jesper H. , Ærtebjerg Gunni
Published: July 2010

In the past 2 decades significant measures have been taken to reduce nitrogen and phosphorus discharges from Denmark by 50% and 80%, respectively. These nutrient reduction targets now appear within reach after several consecutive reduction measures are fully implemented, particularly toward diffuse discharges, and reduced nutrient concentrations are beginning to be observed in estuaries and the Danish straits. Phosphorus concentrations have declined by 22% to 57% from the early 1990s, mainly owing to improved treatment of urban and industrial wastewater. Changes in nitrogen concentrations, following reduction measures toward diffuse sources, were more recent and partly masked by large interannual variations in freshwater discharge. The response in marine nitrogen concentrations was delayed relative to the decline in riverine concentrations, most likely owing to large internal loading from the sediments. Two consecutive dry years appeared to be the triggering mechanism for nitrogen concentrations to decline. In the last 5 yr, nitrogen levels in estuaries and coastal waters have decreased up to 44% when interannual variations in freshwater discharge were accounted for. These first signs of environmental recovery were most pronounced in estuaries and coastal waters but also were apparent in open waters of the Kattegat, the Sound, and the Belt Sea. This case study is the first to document significant decreases in nutrient concentrations on a large regional scale resulting from an active management strategy to reduce nutrients from both diffuse and point sources.
Long‐term change in dissolved inorganic nutrients in the heterotrophic Scheldt estuary (Belgium, The Netherlands)
Authors: Soetaert Karline , Middelburg Jack J. , Heip Carlo , Meire Patrick , Van Damme Stefan , Maris Tom
Published: July 2010

We investigated long‐term trends (1965–2002) in dissolved inorganic nutrients in the tidal part of the Scheldt estuary (Belgium, The Netherlands). Annually averaged concentrations of dissolved silicate (DSi), dissolved inorganic nitrogen (DIN), and phosphate (DIP) increased significantly until the mid‐1970s, after which they declined linearly at rates of 0.6, 2.9, and 0.3 µmol L‐ yr‐, respectively. This co‐occurred with a deterioration followed by a restoration of water column oxic conditions. Because of the differences in the reduction rate of DSi (1.2% yr‐), DIN (1.7% yr‐), and DIP (5.4% yr21), the N: P and Si : P ratios more than doubled from 1980 to 2002. The Si :N ratio varied from 0.2 to 0.4 and was positively correlated with river discharge. The part downstream from the confluence of the main rivers was a net sink for DSi during the entire period but evolved from a net sink to a net source for DIP, while the reverse was true for DIN. This differential behavior of the estuary with respect to DIN and DIP strongly buffered the altered loadings to the upper estuary. The input of oxygen‐consuming substances at the head of the estuary triggered a sequence of oxidation reactions. In the early 1970s, high loadings of ammonium and organic matter caused oxygen depletion and intense water‐column denitrification in the upstream part and intense nitrification downstream, with a nitrate maximum succeeding a nitrite peak. With oxic conditions improving and the input of ammonium decreasing, water‐column denitrification declined, the nitrification front migrated upstream, and the estuary evolved from a net producer of nitrite to a net consumer. Now, at the beginning of the 21st century, nitrate behaves almost conservatively over the entire estuary.
Cultural eutrophication in the Choptank and Patuxent estuaries of Chesapeake Bay
Authors: Fisher T. R. , Hagy J. III. D. , Boynton W. R. , Williams M. R.
Published: July 2010

The Choptank and Patuxent tributaries of Chesapeake Bay have become eutrophic over the last 50–100 years. Systematic monitoring of nutrient inputs began in ~1970, and there have been 2–5‐fold increases in nitrogen (N) and phosphorus (P) inputs during 1970–2004 due to sewage discharges, fertilizer applications, atmospheric deposition, and changes in land use. Hydrochemical modeling and land‐use yield coefficients suggest that current input rates are 4–20 times higher for N and P than under forested conditions existing 350 yr ago. Sewage is a major cause of increased nutrients in the Patuxent; agricultural inputs dominate in the Choptank. These loading increases have caused three major water‐quality problems: (1) increased nutrients, phytoplankton, and turbidity; (2) decreased submerged grasses due to higher turbidity and epiphyton shading; and (3) bottom‐water hypoxia due to respiration of excess organic matter. Oxygen in the Patuxent is consistently <3 mg L−1 in bottom waters in summer, whereas oxygen in Choptank bottom waters has been decreasing for the last 20 yr and is now approaching 3 mg L−1 in wet years. The low N: P of sewage inputs to the Patuxent results in an N‐limited, P‐saturated system, whereas the Choptank is primarily limited by N, but with P limitation of phytoplankton during spring river flows. Insufficient action has been taken to improve the water and habitat quality of these estuaries, although reduced eutrophication in dry years suggests that both estuaries will respond to significant decreases in nutrients.
Nitrogen fixation and dissimilatory nitrate reduction to ammonium (DNRA) support nitrogen dynamics in Texas estuaries
Authors: Gardner Wayne S. , McCarthy Mark J. , An Soonmo , Sobolev Dmitri , Sell Karen S. , Brock David
Published: July 2010

We conducted continuous‐flow experiments on intact sediment cores from Laguna Madre, Sabine Lake, East Matagorda Bay, and Nueces Estuary to evaluate internal nitrogen (N) sources, sinks, and retention mechanisms in Texas estuaries having different salinities. Mean ammonium (NH4+) flux ranged from slight uptake (negative values) to NH4+ production rates of about 300 µmol m−2 h−1 (units used for all N rates) and increased with salinity (p = 0.10). Net nitrate (NO3−) flux (–20 to 32) and net N2 flux (–70 to 100) did not relate to salinity. Mean net N2 flux was positive but near zero, indicating that N2 sources and sinks are nearly balanced. Total denitrification, N fixation, and potential dissimilatory NO3− reduction to NH4+ (DNRA) rates were estimated after inflow water was enriched with 15NO3− (100 µmol L−1). Total denitrification rates ranged from 0 to 90 versus N fixation rates ranging from 0 to 97. Potential DNRA, measured conservatively as 15H accumulation, ranged from 0 to 80 and related significantly to salinity (p < 0.01). Increases in total NH4+ release after 15NO3− additions were higher but closely related (r = 0.9998) to 15NH4+ accumulation, implying exchange reactions of DNRA‐regenerated 15N4+ with sediment‐bound 14NH4+. The fate of NO2 was related to salinity, perhaps via sulfide effects on DNRA. Potential DNRA was high in southeastern Corpus Christi Bay in August during hypoxia when the sulfide transition zone was near the sediment surface. Nitrogen fixation and DNRA are important mechanisms that add and retain available N in Texas estuaries.
Periphyton responses to eutrophication in the Florida Everglades: Cross‐system patterns of structural and compositional change
Authors: Gaiser Evelyn E. , Childers Daniel L. , Jones Ronald D. , Richards Jennifer H. , Scinto Leonard J. , Trexler Joel C.
Published: July 2010

We examined periphyton along transects in five Everglades marshes and related compositional and functional aspects to phosphorus (P) gradients caused by enriched inflows. Results were compared to those of a P‐addition experiment in a pristine Everglades marsh. While the water total P (TP) concentration was not related to P load in the marshes or experiment, the concentration of TP in periphyton was strongly correlated with the distance from the P source. Increased P concentration in periphyton was associated with a loss of biomass, particularly of the calcifying mat‐forming matrix, regardless of the growth form of the periphyton (epiphytic, floating, or epilithic). Diatom species composition was also strongly related to P availability, but the TP optima of many species varied among marshes. Enriched periphyton communities were found 14 km downstream of P inputs to one marsh that has been receiving enhanced P loads for decades, where other studies using different biotic indicators show negligible change in the same marsh. Although recovery trajectories are unknown, periphyton indicators should serve as excellent metrics for the progression or amelioration of P‐related effects in the Everglades.
The sensitivity of minimum oxygen concentrations in a fjord to changes in biotic and abiotic external forcing
Authors: Erlandsson Carina P. , Stigebrandt Anders , Arneborg Lars
Published: July 2010

We investigated the possible biotic and abiotic causes of the observed long‐term decrease of oxygen minimum concentrations in the deep water of Gullmar Fjord. Physical factors explained about 40% of the decreased minimum oxygen concentrations since the 1950s. The North Atlantic Oscillation and more regional climate variations account for an important part of this change. The effect of the climate variations was mainly on the timing of the renewal of the basin water. The consumption rate of oxygen in the basin water has increased by 50% since the 1950s. This biotic effect explained about 60% of the decrease in minimum oxygen concentrations. The likely dominating source of oxygen‐consuming matter was (remote) production in the Skagerrak, suggesting that there has been a long‐term increase of particulate organic carbon in the Skagerrak water.
Factors contributing to hypoxia in rivers, lakes, and streams
Authors: Mallin Michael A. , Johnson Virginia L. , Ensign Scott H. , MacPherson Tara A.
Published: July 2010

We investigated physical, chemical, and biological variables contributing to biochemical oxygen demand (BOD) in 17 North Carolina lotic and lentic water bodies affected by mild to severe hypoxia. Phytoplankton production created the dominant reservoir of labile carbon driving BOD, and subsequent hypoxia, in a Piedmont river subject to algal blooms, three urban streams, a set of anthropogenically affected tidal creeks, and two urban lakes. Autotrophic phytoplankton production contributed to the BOD load in some rural streams. Autochthonous heterotrophic processes, stimulated primarily by phosphorus and secondarily by nitrogen loading, were the major influences on BOD in two large black water rivers and some rural black water streams. Inputs of biochemical oxygen‐demanding materials from storm water runoff contribute to BOD in some urban and rural streams and black water rivers. We suggest that reductions of hypoxia can be better achieved by a system‐specific approach based on an array of factors that potentially influence BOD, including both autochthonous and allochthonous variables. In some circumstances targeting the nutrient(s) stimulating phytoplankton blooms will suffice to reduce hypoxia, but in other situations targeting nutrient(s) limiting bacterial production will be necessary. Reduction of non—point source inputs of biochemical oxygen‐demanding materials derived from urbanization or other land disrupting activities will be critical in some cases.
Microbial production, utilization, and enzymatic degradation of organic matter in the upper trophogenic layer in the pelagial zone of lakes along a eutrophication gradient
Authors: Chróst Ryszard J. , Siuda Waldemar
Published: July 2010

The major aim of this study was to evaluate the relationships between the rates of microbial activities (phytoplankton primary production, bacterial secondary production, bacterial utilization of organic matter, enzymatic activities, protozoan grazing on bacteria), bacterial numbers, and dissolved organic carbon concentrations and the trophic state index (TSI) of lakes in the upper trophogenic water layer in the pelagial zone along the trophic gradient (from oligo/mesotrophy to hypereutrophy) in 19 lakes of the Mazurian Lake District (northeastern Poland). Multiple regression analysis (analysis of variance—ANOVA) on all collected data and the TSI along eutrophication gradient showed that all studied microbial processes and parameters were very tightly coupled to the trophic conditions of the studied lakes. All studied microbial processes involved in utilization and enzymatic degradation of organic matter were strongly positively dependent on the intensity and rates of photosynthetic organic matter production and exudation that markedly increased along the eutrophication gradient of lakes. Vmax of alkaline phosphatase, aminopeptidase, and nonspecific esterase showed significant correlation with the TSI of the studied lakes. Protozoans removed a significant portion of bacterial production, i.e., from ~20% to 75‐85% of newly produced bacterial biomass was simultaneously consumed by protozoans along the eutrophication gradient. These observations suggest that the importance of protozoan grazing on bacteria on regulation of bacterial production depends on lake productivity. The general working hypothesis that the intensity of microbial processes of organic matter can be tightly coupled to increasing eutrophication was proven in these studies.
Analysis of the seawater CO2 system in the barrier reef‐lagoon system of Palau using total alkalinity‐dissolved inorganic carbon diagrams
Authors: Watanabe Atsushi , Kayanne Hajime , Hata Hiroshi , Kudo Setsuko , Nozaki Ken , Kato Ken , Negishi Akira , Ikeda Yutaka , Yamano Hiroya
Published: July 2010

We studied the seawater carbon dioxide (CO2) system in the Palau coral reef by measuring total alkalinity (TA) and total dissolved inorganic carbon (DIC). Variation in the CO2 system on the reef flat and in the lagoon was analyzed by TA‐DIC diagrams, taking into accounts the differing residence times of seawater. CO2 in the offshore water was relatively stable in space and time, but on the reef flat it was subject to rapid (about 3 h) and substantial changes due to photosynthesis and calcification during the day and due to respiration and calcification at night. Water flowed into the lagoon where decomposition of organic matter and continuing calcification occurred over relatively long residence times (~30 d). Despite the spatial and temporal variations, the center of the lagoon had relatively constant TA and DIC values similar to the mean values for the entire lagoon. A long‐term 30–40% decrease in reef productivity and calcification has occurred over the last decade, primarily a result of degradation of the reef environment following a major coral reef bleaching event in 1998. This is reflected in decreases in the differences in TA and DIC between offshore lagoon waters and those in center of the lagoon.
Cycling of calcite in hard water lakes of different trophic states
Authors: MÜller Beat , Wang Ying , Wehrli Bernhard
Published: July 2010

Based on oxygen (O2), pH, calcium (Ca2+), and carbonate (CO32‐) sediment pore‐water concentration profiles, we compare the benthic dissolution of autochthonous calcite in oligotrophic Lake Lucerne and eutrophic Lake Sempach. Despite their difference in trophic state, the two lakes have similar benthic O2 regimes because L. Sempach is artificially oxygenated. This peculiarity enabled the study of the direct effect of trophic state on benthic calcite dissolution. Although areal benthic O2 fluxes did not differ, O2 penetrated deeper into the sediment of the oligotrophic L. Lucerne, where most organic matter (OM, 96%) was degraded aerobically, whereas in L. Sempach, aerobic and anaerobic decomposition contributed about equal amounts to the biogenic carbon dioxide production. Release rates of bicarbonate from L. Sempach sediments exceeded those of L. Lucerne nearly twofold. The HCO3− : Ca2+ ratio of benthic fluxes was 3.7 in L. Lucerne compared to 10.3 in L. Sempach, suggesting that in L. Sempach most of the released HCO3− did not originate from calcite dissolution. Furthermore, benthic calcite dissolution in L. Lucerne exceeded that in L. Sempach by two‐fold, despite lower pH values in the pore water of L. Sempach. Differences in benthic microbial decomposition of OM and redissolution of calcite crystals are explained by the longer residence time of OM and calcite in the oxic sediment layer of the oligotrophic lake and the larger weight‐specific surface area of its smaller autochthonous calcite crystals. We suggest that trophic state and O2 supply to the sediment are key parameters controlling the cycling of calcite and organic carbon in lakes.
Photo‐oxidation of dissolved organic matter in river water and its effect on trace element speciation
Authors: Shiller Alan M. , Duan Shuiwang , van Erp Peter , Bianchi Thomas S.
Published: July 2010

To investigate the effect of photodegradation of fluvial dissolved organic matter (DOM) on dissolved trace element distributions, we performed a 3‐week incubation of water from the lower Pearl River (Mississippi). The experiment was performed in natural light (with dark controls) and examined both the changes in DOM and changes in physical‐chemical speciation of a suite of trace metals. During the incubation, dissolved organic carbon (DOC) decreased in the light by about 20%, whereas ultraviolet light absorbance decreased by nearly 40%; dark controls showed no significant change in DOC. For the trace elements, a variety of behaviors were observed. Some elements (alkali and alkaline earth metals, Mo, Mn, Cd, and Zn) showed no change in concentration or speciation. A number of elements, however, did show significant changes in the light. For example, there was a significant, continuous decrease in dissolved (<0.02‐µm) Fe in the light samples during the experiment. This and other speciation results indicate that organically complexed Fe was released during photo‐oxidation of the lowmolecular‐ weight DOM; this was followed by subsequent precipitation of the released Fe as additional colloidal FeOOH. Other elements (Ce, Cu, Cr, Pb, V, and U) also showed decreases in the dissolved (<0.02‐µm) fraction with time. Some of these elements, as well as Co and Ni, also showed decreases in their retention by an anion exchange column, likewise implying a decrease in their organically complexed forms.