bluuebunny/crossref_metadata_2025 huggingface dataset:
A relationship between submarine groundwater borne nutrients traced by Ra isotopes and the intensity of dinoflagellate red‐tides occurring in the southern sea of Korea
Authors: Lee Yong-Woo , Kim Guebuem , Lim Weol-Ae , Hwang Dong-Woon
Published: January 2011

We measured short‐lived radium isotopes (223Ra and 224Ra), dissolved inorganic and organic nutrients, and photosynthetic pigments during the summers of 2006 and 2007 in the southern sea of Korea, where harmful dinoflagellate blooms occur every year. The Ra tracer measurements reveal that coastal groundwater, rather than other sources previous suggested (i.e., Yangtze River diluted water or Kuroshio currents), is the main source of nutrients that fuel red tides in this region. Although inorganic‐nutrient levels are different for different regions and different years, either dissolved inorganic nitrogen or phosphorus is depleted in the red‐tide region. This depletion is accompanied by highly elevated levels of dissolved organic nutrients, transformed from groundwaterborne dissolved inorganic nutrients either inside Yeoja Bay or in offshore red‐tide areas, thereby creating favorable conditions for the growth of dinoflagellates in competition with diatoms. The intensity of red tides correlates well with the activity of 224Ra (half life = 3.66 d) in seawater over daily or yearly time scales. Because the chemically conservative 224Ra can trace groundwater‐borne nutrients, which are utilized by marine biota in this red‐tide region, the intensity of red tides seems to be related to the amount of nutrient‐enriched groundwater supplied to the offshore red‐tide region.
Iron and silicic acid effects on phytoplankton productivity, diversity, and chemical composition in the central equatorial Pacific Ocean
Authors: Marchetti Adrian , Varela Diana E. , Lance Veronica P. , Lance Veronica P. , Palmucci Matteo , Giordano Mario , Virginia Armbrust E.
Published: January 2011

A microcosm nutrient‐amendment experiment using central equatorial Pacific Ocean (0°, 140°W) mixed‐layer waters was conducted to determine biogeochemical controls on phytoplankton with an emphasis on post‐iron enrichment nutrient uptake dynamics and species composition. The addition of either Fe (termed Fe‐only) or Fe and Si(OH)4 (termed FeSi) to on‐deck incubations resulted in growth primarily of pennate diatoms, with statistically equivalent increases relative to the control in maximum photochemical efficiency, chlorophyll a (Chl a) concentrations, particulate organic carbon and nitrogen concentrations, and dissolved inorganic carbon uptake rates. In contrast, at peak Chl a concentrations, there was a 3.4‐fold higher abundance of large diatoms and a 3.9‐ fold lower abundance of small pennate diatoms in FeSi relative to Fe‐only, which translated into a 3.5‐fold higher Si(OH)4 uptake rate and a 2.1‐fold higher biogenic silica concentration. Fourier transform infrared spectroscopy indicated that relative to cells from Fe‐only, cells from FeSi possessed the lowest protein : carbohydrate ratios, and ratios of lipids, proteins, and carbohydrates relative to silica, consistent with differences in diatom C allocation or increased silicification or both. Our results suggest that after Fe addition, diatom organic matter accumulation rates (i.e., C and N uptake rates) are enhanced but the low, ambient [Si(OH)4] retards cell division rates, resulting in fewer large diatoms with relatively high C and N contents. After the simultaneous addition of Fe and Si(OH)4, enhanced rates of diatom organic matter accumulation and cell division results in more large, heavily silicified diatoms with relatively low C and N contents.
Long‐term patterns of dissolved organic carbon in lakes across eastern Canada: Evidence of a pronounced climate effect
Authors: Zhang Jan , Hudson Jeff , Neal Richard , Sereda Jeff , Clair Thomas , Turner Michael , Jeffries Dean , Dillon Peter , Molot Lewis , Somers Keith , Hesslein Ray
Published: January 2011

We analyzed the 21‐yr dynamics of dissolved organic carbon (DOC) in 55 lakes during ice‐free periods in five regions across eastern Canada in relation to total solar radiation (TSR), precipitation, air temperature, sulfate deposition (SO4), Southern Oscillation Index (SOI), North Atlantic Oscillation, and Pacific Decadal Oscillation (PDO). A synchronous pattern in DOC was found among lakes within each region; however, a synchronous pattern in DOC was not found among regions, except for Kejimkujik and Yarmouth. Long‐term trends of increasing or decreasing DOC concentration were not evident except at the Experimental Lakes Area (ELA), where an increase in DOC correlated with a decrease in summer TSR and an increase in summer precipitation. Annual mean temperature increased at the Nova Scotia and Turkey Lakes Watershed regions (TLW) over the study period, but there was no corresponding change in DOC. TSR and precipitation were important explanatory variables across all regions, except for the TLW. Summer TSR, or annual TSR, had a negative relationship, while summer precipitation had a positive relationship with the temporal DOC pattern in all regions except TLW. TSR and precipitation explained 78%, 49%, and 84% of the variation in the long‐term DOC patterns at Dorset, ELA, and Nova Scotia (NS) regions, respectively. In contrast, the long‐term pattern in DOC at TLW was only weakly related to SOI and PDO.
Denitrification in shallow, sublittoral Gulf of Mexico permeable sediments
Authors: Gihring Thomas M. , Canion Andy , Riggs Ashley , Huettel Markus , Kostk Joel E.
Published: January 2011

We examined nitrogen cycling over a 1‐yr period in shallow, sandy sediments at two contrasting sites near a barrier island in the northeastern Gulf of Mexico, and we provide here the direct determination of dinitrogen gas (N2) production at ambient nitrate concentrations in permeable marine sediments. Nitrogen‐stable isotope tracer techniques were used to quantify N2‐production rates and pathways in sediment cores and slurries. To simulate pore‐water advection, the dominant transport process in the upper layer of the permeable sand beds, intact sediment cores were perfused with aerated seawater. This perfusion increased denitrification rates up to 2‐fold in Apalachicola Bay sands and up to 17‐fold in Gulf of Mexico sublittoral sands, respectively, relative to static cores. Seasonal N2‐production rates were highest in spring and fall. Denitrified nitrate originated almost entirely from benthic nitrification at the exposed Gulf site, whereas water‐column nitrate dominated sedimentary denitrification at the sheltered Bay site. Sediment incubations in stirred chambers were used to determine net fluxes of oxygen (O2), N2, nitrate, and ammonium across the sediment‐water interface during varied degrees of continuous pore‐water exchange. Rates of N2 efflux correlated with rates of pore‐water flow increasing from 0.12 mmol N m‐2 d‐1 under diffusion‐limited transport conditions up to 0.87 mmol N m‐2 d‐1 with pore‐water advection. Mineralized nitrogen was completely converted to N2 gas in Gulf of Mexico sediments. Our results demonstrate that advective pore‐water circulation will accelerate benthic N2 production by coupled nitrification‐ denitrification and that substantial nitrogen loss occurs from coastal permeable sediments.
Phytoplankton distribution and photosynthesis dynamics in the Chubut River estuary (Patagonia, Argentina) throughout tidal cycles
Authors: Walter Helbling E. , Pérez Daniel E. , Medina César D. , Lagunas Marcos G. , Villafañe Virginia E.
Published: January 2011

The aim of this study was to determine the effects of the tidal dynamics and physical forcing on phytoplankton distribution and photosynthesis dynamics in the Chubut River estuary, Argentina. Physical, chemical, and biological variables measured at the surface and at the bottom of the water column (variable between 0.5‐m and 6‐m depth) were made every 30‐45 d during tidal cycles (13 h of duration) from June 2007 to November 2008 (11 tidal cycles). There was a remarkable consistency among different tidal cycles throughout the year, with strong stratification during the flood and almost complete mixing during the period high tide‐ebb‐low tide. Strong stratification during the flood resulted in significant inhibition of photosynthesis of mostly nanoplankton cells at the surface, while microplankton sank out of this upper layer and, thus, were less inhibited. Mixing conditions during the ebb, together with relatively high concentration of dissolved organic matter and particulate material, resulted in partial protection for phytoplankton against solar radiation stress and, therefore, relatively high maximum electron transport rate values were determined under this condition. However, the lowest photoinhibition and higher maximum rate of production (P>max) values occurred at depth during stratified conditions, probably due to relatively low solar radiation in this condition. Tidal dynamics together with physical forcing are key factors that condition the distribution, dynamics, and photoinhibition of phytoplankton in the Chubut River estuary.
Settlement rates of macroalgal algal propagules: Cross‐species comparisons in a turbulent environment
Authors: Taylor David , Delaux Sebastien , Stevens Craig , Nokes Roger , Schiel David
Published: January 2011

The ability of propagules (fertilized eggs) of five species of fucoid algae (Hormosira banksii, Durvillaea antarctica, Cystophora torulosa from New Zealand, and Fucus gardneri and Pelvetiopsis limitata from Oregon, U.S.A.) to settle and attach was tested in a turbulent, stirred tank. The time taken to reach a steady state of settlement numbers varied between species and turbulence intensities. Normalized steady‐state (NSS) settlement numbers showed differences among species. A settlement model, based on principles invoked in the analysis of motion of bed sediments in rivers, was developed. The model indicates that the NSS settlement number depends on two parameters, a propagule Reynolds number and an entrainment function that represents the relative importance of the shear stress experienced by settled propagules and their submerged weight. The inability of this model to collapse the data for all species suggests that the stickiness of the propagules, due to their mucus coatings, plays a significant role in the settlement process. P. limitata (largest propagules) exhibited the least effective attachment to the substratum, whereas F. gardneri (second largest) and D. antarctica (smallest propagules) were the most effective at withstanding hydrodynamic forces that detach propagules. We also model the boundary layer above a flat‐bed, driven by linear water‐waves, using a skin‐friction drag coefficient and show that this study represents the lower end of the shear velocity u* range. However, these experiments capture the main region of variability in long‐term propagule attachment, and indicate that most of these fucoid species will have successful settlement only during calm conditions.
Experimental and modeling evidence of appendicularian‐ciliate interactions
Authors: Lombard Fabien , Eloire Damien , Gobet Angelique , Stemmann Lars , Dolan John R. , Sciandra Antoine , Gorsky Gabriel
Published: January 2011

Interactions between appendicularians and ciliates were observed over the life span of Oikopleura dioica in laboratory cultures and clarified with the use of mathematical modeling and microscopic observations. Complex interactions including competition, parasitism, predation, and histophagy occurred simultaneously, resulting in apparent mutualism. The large ciliate Strombidium sp. entered the inlet filters of appendicularian houses (larger than 500 µm body size) by distorting the mesh. Once inside, Strombidium fed on particles concentrated on the filters. When appendicularians were larger than 900 µm, both the high flow rate in the buccal tube and their esophagus width allowed the "host" appendicularian to capture and ingest ciliates. Thus, ciliates seem to be sequentially competitors, then parasites or commensal in appendicularian houses, and finally prey for appendicularians. Appendicularian rates of somatic growth and reproduction were enhanced when ciliates were ingested. This additional food supply could be essential in oligotrophic environments. Reciprocally, appendicularians support higher ciliate growth rates, allowing ciliates to survive and grow in food‐limited environments. Appendicularians thus modify the size spectrum of the microbial food web both by removing small organisms (0.2‐30 µm) and enhancing the growth of mid‐sized and large ciliates.
A comprehensive study of sterols in marine diatoms (Bacillariophyta): Implications for their use as tracers for diatom productivity
Authors: Rampen Sebastiaan W. , Abbas Ben A. , Schouten Stefan , Sinninghe Damste Jaap S.
Published: January 2011

Diatoms are one of the most important organisms contributing to aquatic primary productivity and their sterols are frequently used as markers for their presence and abundance. In this study, the sterol composition of >100 diatom cultures was analyzed and its distribution was compared to the diatom phylogeny to identify typical diatom biomarkers. Forty‐four sterols were detected, 11 of them being commonly present as major sterols (contributing >10% to the total sterols). 24‐Methylcholesta‐5,24(28)‐dien‐3β‐ol is the most common sterol in diatoms, being present in 67% of all cultures analyzed, followed by the Δ5 sterols, cholest‐5‐en‐3β‐ol (cholesterol), 24‐methylcholest‐5‐en‐3β‐ol, and 24‐ethylcholest‐5‐en‐3β‐ol. 24‐Methylcholesta‐5,22E‐dien‐3β‐ol, previously suggested to be specific for diatoms, was only the fifth most common sterol; this sterol was absent in some of the major diatom groups, and high relative concentrations seem to be restricted to pennate diatoms. No sterols are restricted to specific phylogenetic groups of diatoms. Cluster analyses, however, do reveal distinct sterol distributions: Thalassiosirales typically contain high relative abundances of 24‐methylcholesta‐5,24(28)‐dien‐3β‐ol, high relative abundances of cholesta‐5,22E‐dien‐3β‐ol are typical for Cymatosirales, high relative abundances of 24‐ethylcholesta‐5,22E‐dien‐3β‐ol are characteristic for related Amphora, Amphiprora, and Entomoneis species, and a combination of high relative abundances of 24‐methylcholest‐5‐en‐3β‐ol, 24‐methylcholesta‐5,24(28)‐dien‐3β‐ol, and 24‐ethylcholest‐5‐en‐3β‐ol is typical for Attheya species. High contributions of 24‐methylcholesta‐5,22E‐dien‐3β‐ol (>50% of all sterols) seem to be restricted to pennate diatoms. None of the major sterols found in diatoms can be used as an unambiguous diatom biomarker because all of them have been reported as common sterols in other algal groups.
The time for sex: A biennial life cycle in a marine planktonic diatom
Authors: D'Alelio Domenico , d'Alcalà Maurizio Ribera , Dubroca Laurent , , Diana Sarn , Zingone Adriana , Montresor Marina
Published: January 2011

Whereas it is accepted that the population dynamics of higher organisms is strongly grounded in life history processes, such as the time taken to mature and reproduce, regulation in the abundance of aquatic protists is generally attributed to proximate properties of the environment in which they grow. We used 10 yr of data from the Gulf of Naples to determine the life history of a planktonic diatom, Pseudo‐nitzschia multistriata, through the analysis of cell abundances and cell size patterns. Asexual and sexual phases recurred with remarkable regularity. Maximum abundance occurred annually, between the end of summer and the beginning of autumn, but cohorts of large cells, derived from sexual events, were found only every second year. We developed a model of population growth, based on parameters determined from laboratory cultures, with further tuning using information from natural populations. The model simulated the birth, maturation, and disappearance of age classes like those in natural populations, but only when we imposed seasonal variation in the rate of cell division and timed sexual reproduction during the stationary phase of a bloom. The model predicts that P. multistriata will become locally extinct if sexual reproduction does not occur within 4 yr. Our data and model show that coherent life cycle properties can emerge in natural populations of unicellular organisms, analogous to those in multicellular organisms, as a result of finely tuned regulation of cell division and sexual competence.
Variability in greenhouse gas emissions from permafrost thaw ponds
Authors: Laurion Isabelle , Vincent Warwick F. , MacIntyre Sally , Retamal Leira , Dupont Christiane , Francus Pierre , Pienitz Reinhard
Published: January 2011

Arctic climate change is leading to accelerated melting of permafrost and the mobilization of soil organic carbon pools that have accumulated over thousands of years. Photochemical and microbial transformation will liberate a fraction of this carbon to the atmosphere in the form of CO2 and CH4. We quantified these fluxes in a series of permafrost thaw ponds in the Canadian Subarctic and Arctic and further investigated how optical properties of the carbon pool, the type of microbial assemblages, and light and mixing regimes influenced the rate of gas release. Most ponds were supersaturated in CO2 and all of them in CH4. Gas fluxes as estimated from dissolved gas concentrations using a wind‐based model varied from 220.5 to 114.4 mmol CO2 m‐2 d‐1, with negative fluxes recorded in arctic ponds colonized by benthic microbial mats, and from 0.03 to 5.62 mmol CH4 m‐2 d‐1. From a time series set of measurements in a subarctic pond over 8 d, calculated gas fluxes were on average 40% higher when using a newly derived equation for the gas transfer coefficient developed from eddy covariance measurements. The daily variation in gas fluxes was highly dependent on mixed layer dynamics. At the seasonal timescale, persistent thermal stratification and gas buildup at depth indicated that autumnal overturn is a critically important period for greenhouse gas emissions from subarctic ponds. These results underscore the increasingly important contribution of permafrost thaw ponds to greenhouse gas emissions and the need to account for local and regional variability in their limnological properties for global estimates.
Population connectivity of the Olympia oyster in southern California
Authors: Carson Henry S.
Published: January 2011

The Olympia oyster is a useful model to study the population connectivity of estuarine invertebrates because it incorporates chemical information about the natal site into its shell before dispersal. This information can be accessed in recruits using laser ablation inductively coupled plasma mass spectrometry and compared with predispersal shells from possible source populations. Shelled Ostrea lurida larvae were periodically removed from adults at eight sites in four Southern California estuaries in the summers of 2006 and 2007. The ratios to calcium of copper, barium, lead, and uranium in the shells were used to distinguish among sites in linear discriminant function analyses. Jackknifed assignment accuracy by site was increased to 74% and 82% for 2006 and 2007, respectively, by grouping the eight sites into three regions. Settlement plates at each site in 2007 collected recent O. lurida recruits that were assigned to one of the three regions defined by the broods. San Diego Bay and the North County produced over 80% of the identifiable recruits in 2007, with Mission Bay producing the remaining minority. The majority of recruits to San Diego Bay were self‐recruits, whereas over half of the North County recruits were unidentifiable. Patterns of connectivity varied between recruitment pulses, but a substantial portion of larvae were exchanged among estuaries separated by as many as 75 km of coastline.
Ecosystem and physiological scales of microbial responses to nutrients in a detritus‐based stream: Results of a 5‐year continuous enrichment
Authors: Suberkropp Keller , Gulis Vladislav , Rosemond Amy D. , Benstead Jonathan P.
Published: January 2011

Our study examined the response of leaf detritus‐associated microorganisms (both bacteria and fungi) to a 5‐yr continuous nutrient enrichment of a forested headwater stream. Leaf litter dominates detritus inputs to such streams and, on a system‐wide scale, serves as the key substrate for microbial colonization. We determined physiological responses as microbial biomass and activity expressed per unit mass of leaves and system‐level responses by quantifying leaf litter standing crop monthly and expressing responses per unit area of streambed. Physiological (mass‐specific) trends differed from system‐level (area‐specific) trends. Physiological responses to enrichment were generally positive. With the exception of bacterial biomass, nutrients increased all metrics expressed per unit mass leaf litter in the treatment stream relative to the reference (fungal biomass and production, bacterial production, microbial respiration). This positive physiological response to nutrient enrichment was associated with lower leaf litter standing crop in the treatment stream, resulting in less substrate for microbial colonization. Consequently, during most years on a system‐level scale, only fungal production and microbial respiration were positively affected by nutrients, whereas fungal biomass was negatively affected. Thus, from a whole‐stream perspective, nutrients led to a lower quantity of leaf detritus with greater variation, resulting in net reductions of associated fungal biomass and greater intra‐annual variability in both fungal biomass and respiration. Our results demonstrate profound effects of nutrients on heterotrophic pathways that mediate detritus processing in stream ecosystems. Similar effects on heterotrophic microbes and detrital resources may be a widespread consequence of anthropogenic nutrient enrichment.
The role of effective discharge in the ocean delivery of particulate organic carbon by small, mountainous river systems
Authors: Wheatcroft Robert A. , Goñ i Miguel A. , Hatten Jeff A. , Pasternack Gregory B. , Warrick Jonathan A.
Published: January 2011

Recent research has shown that small, mountainous river systems (SMRS) account for a significant fraction of the global flux of sediment and particulate organic carbon (POC) to the ocean. The enormous number of SMRS precludes intensive studies of the sort conducted on large systems, necessitating development of a conceptual framework that permits cross‐system comparison and scaling up. Herein, we introduce the geomorphic concept of effective discharge to the problem of source‐to‐sink POC transport. This idea recognizes that transport effectiveness is the product of discharge frequency and magnitude, wherein the latter is quantified as a power‐law relationship between discharge and load (the ‚rating curve’). An analytical solution for effective discharge (Qe) identifies two key variables: the standard deviation of the natural logarithm of discharge (Σq), and the rating exponent of constituent i (bi). Data from selected SMRS are used to show that for a given river Qe‐POC , Qe‐sediment, Qe for different POC constituents (e.g., POCfossil vs. POCmodern) differs in predictable ways, and Qe for a particular constituent can vary seasonally. When coupled with the idea that discharge peaks of small rivers may be coincident with specific oceanic conditions (e.g., large waves, wind from a certain direction) that determine dispersal and burial, these findings have potentially important implications for POC fate on continental margins. Future studies of POC transport in SMRS should exploit the conceptual framework provided herein and seek to identify how constituent‐specific effective discharges vary between rivers and respond to perturbations.
Effects of seasonal stratification on turbulent mixing in a hypereutrophic coastal lagoon
Authors: Cousins Mary , Stacey Mark T. , Drake Jeana L.
Published: January 2011

Seasonal variations in vertical turbulent mixing rates, density structure, inorganic nutrient concentrations, and phytoplankton biomass were investigated in a shallow, tidally choked coastal lagoon. The project site, Rodeo Lagoon, is located in the Golden Gate National Recreation Area, California, and has experienced intense blooms of the cyanobacteria Nodularia spumigena and Microcystis aeruginosa in recent years. Monthly measurements collected along a transect of the lagoon from March 2006 to April 2008 show it is strongly stratified by brackish water in winter, when freshwater inputs from the watershed and saltwater inputs from storm surge are both at their highest. The squared buoyancy frequency exceeds 0.5 s‐2 under these conditions. In summer, weaker diurnal temperature stratification is the result of strong light absorption characteristics in this hypereutrophic lagoon. Wind is the dominant driver of turbulent mixing. Although water depths of less than 2.5 m lead to the expectation of rapid vertical mixing, limited fetch and strong density gradients reduce the coupling of wind stress and bottom stress. The vertical turbulent diffusivity is reduced by as much as three orders of magnitude across the pycnocline, and the water column in and below the pycnocline shows active turbulence only intermittently. The annual cycle of salt‐based stratification and accompanying reduction in turbulent exchange of nutrients between the sediments and overlying water column inhibit the flushing of nutrients out of the lagoon and contribute to excessive phytoplankton biomass.
Nanofibrils and other colloidal biopolymers binding trace elements in coastal seawater: Significance for variations in element size distributions
Authors: Stolpe Björn , Hassellöv Martin
Published: January 2011

Colloidal size spectra of Fe, Cu, Ag, La, and Pb were determined by asymmetrical flow field‐flow fractionation coupled to high‐resolution inductively coupled plasma mass spectrometry, in samples from 0.5‐40‐m depth profiles from the Gullmarsfjord on the Swedish west coast, at nine occasions between February 2004 and July 2005. Trace elements were quantified in unfiltered and 0.45‐mm filtered samples, and total organic carbon and transparent exopolymer particles were measured. Atomic force microscopy was used to determine sizes and shapes of colloids. Most Fe was associated with Al‐rich particles that were resuspended from the sediment or derived from a local river. Dissolved Fe and Cu had depth profiles similar to total organic carbon and transparent exopolymer particles, and highest concentrations occurred in the surface and the upper mixed‐water layer, where the in situ biological production and the inputs of water from the Baltic Sea and a local river were high. The colloidal size spectra of the elements, and the atomic force microscopy images, revealed that three to four classes of colloids were present in the samples, differing in size, shape, and element‐binding properties. Small (0.5‐3 nm) spherical macromolecules of chromophoric dissolved organic matter, binding most elements, occurred at all depths and on all sampling occasions. Populations of 3‐7‐nm globular or slightly elongated colloids, binding Cu, Ag, and Pb, and 7‐40‐nm fibrillar colloids, binding Fe and Pb, appeared in the upper mixed‐water layer during late spring and summer, thereby changing the colloidal size spectra of associated elements in June‐July.
No empirical evidence for community‐wide top‐down control of prey fish density and size by fish predators in lakes
Authors: Mehner Thomas
Published: January 2011

I tested relationships between fish predator and prey densities or size across 66 lakes of the European temperate zone, covering mean depths between 1 and 22 m and a phytoplankton biomass range between 1 and 270 mg m‐3 chlorophyll a. I hypothesized that in lakes of comparable phytoplankton biomass or depth, prey fish densities and size were lower at high predator densities or size than in systems with lower piscivory. Accordingly, I also expected a stronger trophic cascade from predatory fish to phytoplankton biomass in high‐piscivory systems. None of the hypotheses were confirmed. Prey abundances and biomasses were consistently higher in high‐piscivory than in low‐piscivory lakes, if the effects of trophic state or lake depth on fish densities were accounted for. Chlorophyll a concentrations in lakes of similar depth did not differ between high and low piscivory, suggesting no difference in the strength of the trophic cascade. The overall size of prey was slightly higher in lakes with larger predators. The weak top‐down influence of predatory fish on lower trophic levels may be attributable to the fact that the numerically dominant predator, perch, has to recruit through smaller ontogenetic stages, which dominate the prey community in most of the lakes. The availability of size refuges for prey and the overall omnivory of predators may contribute to the weak evidence for a trophic cascade.
Upwelling couples chemical and biological dynamics across the littoral and pelagic zones of Lake Tanganyika, East Africa
Authors: Corman J. R. , McIntyre P. B. , Kuboja B. , Mbemba W. , Fink D. , Wheeler C. W. , Gans C. , Michel E. , Flecker A. S.
Published: January 2011

We studied the effects of upwelling on nutrient and phytoplankton dynamics in the pelagic and littoral zones of Lake Tanganyika near Kigoma, Tanzania. During the dry season of 2004, a rise in the thermocline and sudden drop in surface water temperatures indicated a substantial upwelling event. Increases in concentrations of nitrate, soluble reactive phosphorus, and silica in the surface waters occurred simultaneously after the temperature drop. Within days, chlorophyll a concentrations increased and remained elevated, while inorganic nutrient concentrations returned to preupwelling levels and organic nutrient concentrations peaked. We observed parallel temporal patterns of water temperature, nutrient concentrations, and phytoplankton chlorophyll in both the pelagic and the littoral zones, demonstrating that upwelling strongly affects the nearshore ecosystem as well as the pelagic zone. Concurrent records from 12 littoral sites indicated spatial variation in the timing, magnitude, and biological response to upwelling. There was no discernable latitudinal pattern in the timing of upwelling, suggesting that mixing did not result from a progressive wave. Our monitoring, as well as other multiyear studies, suggests that dry‐season upwelling occurs during most years in northern Lake Tanganyika. The observed sensitivity of littoral nutrients and phytoplankton to upwelling suggests that reductions in upwelling due to global climate change could strongly affect the dynamics of the spectacular nearshore ecosystem of Lake Tanganyika, as has been proposed for the pelagic zone.
Double‐diffusive convection in Lake Kivu
Authors: Schmid Martin , Busbridge Myles , Wü est Alfred
Published: January 2011

Double‐diffusive staircases with a total of 230‐350 mixed layers and sharp interfaces were observed in nine microstructure temperature profiles measured during February 2004 in Lake Kivu. The presence of these staircases at depths > 120 m indicates that diapycnal turbulent mixing is weak and vertical diffusive transport is dominated by double diffusion. Contrary to previously investigated natural or laboratory double‐diffusive systems, the dissolved gases CO2 and CH4 contribute significantly to the density stratification, thereby influencing the formation and the structure of the staircases. The density ratio (i.e., the ratio of the stabilizing effect of dissolved substances to the destabilizing effect of temperature) ranges between 2.0 and 4.5 in large sections of the deep waters, implying a high susceptibility to the formation of staircases. The mixed layers (average thickness 0.48 m) are shown to be in a state of active convection. The average thickness of the interfaces (0.18 m) is surprisingly constant and independent of the large‐scale stratification. The vertical heat fluxes correlate well with the temperature steps across the interfaces. Lake Kivu receives inflows from subaquatic springs at several depths that maintain the large‐scale structure of the density stratification and disturb the staircases. In comparison to earlier observations from 1972, the double‐diffusive heat fluxes appear to have been reduced, leading to a heat accumulation in the deep waters. Conversely, the strengthening of the main chemocline indicates an increased discharge of the subaquatic springs that could be responsible for recent changes in the nutrient cycling and methane production in the lake.
Ocean acidification hotspots: Spatiotemporal dynamics of the seawater CO2 system of eastern Pacific coral reefs
Authors: Manzello Derek P.
Published: January 2011

Seawater CO2 system dynamics were assessed from eastern Pacific reef sites in Panamá over 5 consecutive years (2003‐2008) and twice in the Galápagos Islands (2003 and 2009). The seawater CO2 system was highly variable in time and space, but was explained by physical forcing from meteorological (seasonal rainfall) and oceanographic (upwelling, tides) processes interacting with diurnal reef metabolism. Galápagos coral reef communities are naturally exposed to the highest ambient partial pressure of CO2 (pCO2) and lowest aragonite saturation (Ωarag) values documented for any coral reef environment to date. During upwelling in the Galápagos, mean pCO22 and mean Ωarag at five different sites ranged from 53.1 to 73.5 Pa and 2.27 to 2.86, respectively. Values of pCO2 and Ωarag ranged from 21.0 to 48.7 Pa and 2.47 to 4.18, respectively, on the Saboga Reef in the seasonally upwelling Gulf of Panamaá, with the highest pCO2 and lowest Ωarag values occurring during upwelling. The Uva Reef, in the nonupwelling Gulf of Chiriquí of Pacific Panamá, had mean Ωarag values that were always significantly greater than those at the Saboga Reef. Diurnal changes in the seawater CO2 system from reef metabolism on the Uva Reef were magnified at low tide and highly significant differences were measured over depths as shallow as 15 m because of the shallow thermocline that is pervasive throughout the eastern Pacific. These naturally high‐CO2 reefs persist near the Ωarag distributional threshold for coral reefs and are thus expected to be the first and most affected by ocean acidification.
Hydrologic and biotic influences on nitrate removal in a subtropical spring‐fed river
Authors: Heffernan James B. , Cohen Matthew J. , Frazer Thomas K. , Thomas Ray G. , Rayfield Travis J. , Gulley Jason , Martin Jonathan B. , Delfino Joseph J. , Graham Wendy D.
Published: January 2011

We use a long‐term chemical and hydrologic record in combination with longitudinal sampling and high‐frequency nitrate (NO3‐) measurements from in situ sensors to describe temporal and spatial patterns of nitrogen (N) inputs and removal in the spring‐fed Ichetucknee River (Columbia County, Florida) and to determine the hydrological, geomorphic, and biological factors that influence those dynamics. Over a 20‐yr period of record, NO3‐N removal averaged 118 kg N d‐1 (0.77 g N m‐2 d‐1) over the upper 5 km of the Ichetucknee River. Three independent estimates of gross autotrophicNassimilation (from gross primary production, diel NO3‐ variation, and standing biomass) agreed closely but accounted for less than 20% of observed N removal. Longitudinal surveys indicate negligible or negative dissolved organic nitrogen and ammonium (NH4+) production, suggesting that denitrification is the predominant mechanism of N removal in this river. A positive relationship between discharge and the magnitude of NO3‐N removal shows that interactions with the surrounding floodplain exert considerable influence at high flows, and longitudinal NO3‐ patterns indicate that N removal may be influenced by channel morphology. These results suggest a greater role for dissimilatory processes and hydrologic connectivity with hyporheic and floodplain sediments than has been previously recognized in highly productive spring‐fed rivers of north Florida. While hydrologic variation is the primary determinant of variation in NO3‐ removal within the Ichetucknee River, comparison across systems indicates that biotic characteristics can cause significant deviation from predictions based on purely physical models of relationships between river size and N removal.
Contrasting spring and summer phytoplankton dynamics in the nearshore Southern California Bight
Authors: Santoro Alyson E. , Nidzieko Nicholas J. , Dijken Gert L. van , Arrigo Kevin R. , Boehma Alexandria B.
Published: January 2011

We analyzed a 9‐yr record of ocean color data from the Sea‐viewing Wide Field‐of‐view Sensor (SeaWiFS) together with a 4‐yr record of satellite‐derived sea surface temperature (SST) measurements to investigate potential mechanisms influencing nearshore phytoplankton variability in the Southern California Bight. We also compared in situ physicochemical conditions at four shoreline stations during a year with an extensive nearshore dinoflagellate red tide (2005) to those of the following year when there was no extended red tide (2006). SeaWiFS data indicated that in spring, chlorophyll a (Chl a) and nitrate (NO3−) were inversely correlated to SSTs, consistent with an upwelling‐dominated system, but in summer Chl a and NO3− were unrelated to SST. Summer dissolved inorganic nitrogen concentrations were not well correlated between alongshore sampling stations, but showed a significant effect of tide stage (low vs. high). Mean summer nearshore Chl a was positively, linearly related to rainfall in the previous winter. The relationship between summer Chl a and winter precipitation could be due either to direct freshwater inputs during the winter rainy season or to delayed input via groundwater discharge. A first‐order analysis of nutrient delivery from both surface runoff and groundwater discharge showed that they could sustain summer blooms of up to 7540 and 1700 km2, respectively—areas on the order of the study site. However, precipitation‐induced physical changes to the nearshore water column cannot be conclusively ruled out as a potential causative mechanism.
Temporal and spatial features of the thermohydrodynamics of shallow salty lagoons in northern Chile
Authors: Fuente Alberto de la , Ninóo Yarko
Published: January 2011

Salares in the altiplanic region of Chile, Bolivia, and Argentina sustain important ecosystems in small, shallow wetlands, with inflows provided by groundwater and outflows consisting mainly of evaporation. The temporal and spatial thermohydrodynamics of one of these extremely shallow, saline, endorheic wetlands in northern Chile is analyzed on the basis of field observations of water temperature and salinity, and low‐dimensionality models accounting for conservation of heat, mass, and volume. Temperature in the studied system is spatially homogeneous but with strong daily oscillations. Salinity, on the other hand, exhibits longitudinal gradients from the zone of (salty) groundwater upwelling to the final evaporation lagoon. Heat balance in the system is largely controlled by wind, as the evaporative heat flux balances out the net solar radiation. Heat exchange with the bottom sediments damps the daily temperature oscillations by retaining (releasing) heat from (to) the water column. Simulated longitudinal profiles of salinity and flow discharge agree reasonably well with field observations but salinity effects on the rates of both evaporation and salt precipitation need to be taken into account. Laboratory experiments were conducted to develop a simple model for the former effect while the latter effect was accounted for by using existing information. Field measurements provided calibration of the model parameters. The observed longitudinal salinity gradients are correlated with the biological structure of the associated ecosystem to explain, for instance, the existence of specific flamingos' feeding areas in the system.
Effects of biological and physical factors on seasonal oxygen dynamics in a stratified, eutrophic coastal ecosystem
Authors: Rivera Zoraida J. Quiñones , Wissel Björn , Rabalais Nancy N. , Justića Dubravko
Published: January 2011

Using a dual‐budget approach based on oxygen concentrations and stable isotopes, we examined primary production, respiration, and production to respiration ratios (P : R) across the Louisiana continental shelf to determine the relative roles of biological and physical factors in controlling seasonal oxygen dynamics. Regression models for δ18O and P :R indicated that the concentration of algal biomass explained most of the variability of the seasonal oxygen dynamics in surface waters. Physical factors, such as salinity, temperature, water‐column stability, and station depth were only of minor importance. Seasonal oxygen dynamics were more pronounced in bottom waters than in surface waters as hypoxia was common in bottom waters during summer months (May‐October). During cooler winter months (November‐April), oxygen concentrations throughout the water column were close to equilibrium with the atmosphere. The relative importance of benthic respiration to water‐column respiration in bottom waters was more pronounced during summer months when the water column was stratified. A strong physical disturbance caused by Hurricane Claudette during the summer of 2003 resulted in lower surface productivity relative to the calm 2001 and 2002 summertime conditions. Even though estimates of surface primary production and P :R were lower in July 2003 compared to July 2002, concentration of algal biomass (particulate organic carbon, C: N) remained the most important factor for d18O and P :R dynamics. The physical disturbance due to the hurricane, however, diminished the relative importance of benthic respiration, so that the system rather resembled wintertime conditions.
Seasonal variations in the concentrations of methyl and ethyl nitrate in a shallow freshwater lake
Authors: Hughes Claire , Kettle Anthony J. , Unazi Godwin A. , Weston Keith , Jones Matthew R. , Johnson Martin T.
Published: January 2011

Between February and October 2007, a time series of alkyl nitrate concentrations in the water column of a shallow freshwater lake, University of East Anglia Broad, located in southeastern England was carried out to determine whether methyl and ethyl nitrate are present in freshwater systems and to improve understanding of alkyl nitrate production mechanisms in aquatic environments. Concentrations ranged from 4.7 (± 0.5) to 53.7 (± 4.36) pmol L−1 methyl nitrate, and 2.5 (± 0.3) to 11.1 (± 0.4) pmol L−1 ethyl nitrate and were within the range of those measured previously in seawater. Peaks in the concentrations of methyl and ethyl nitrate at 4 m were observed in 9 and 6 of the 18 depth profiles measured, respectively. Gradients in concentrations within the hypolimnion suggest that the alkyl nitrates are produced in the bottom waters or sediments, are transported to the lake in groundwater, or both. Colored dissolved organic matter absorbance data suggests that the penetration of ultraviolet light was limited in the lake, so the deep maxima must be due to a non‐photochemical alkyl nitrate source.
Riverine export and the effects of circulation on dissolved organic carbon in the Hudson Bay system, Canada
Authors: a C. J. Mundy , Gosselin Michel , Starr Michel , Michelc Christine
Published: January 2011

The distribution of dissolved organic carbon (DOC) in Hudson Bay (HB), Foxe Basin (FB), and Hudson Strait (HS) was examined during 01‐14 August 2003. The HB system displayed relatively high DOC concentrations with medians of 109, 90, and 100 mmol L−1 for measurements made in HB, FB, and HS, respectively. Waters were significantly modified as they circulated through the HB system. An influence of marine‐derived DOC was inferred for waters entering the system from northern HS and FB. The presence of a cold‐water layer and elevated DOC concentrations observed in HB along the western coast and at depth was explained through either brine rejection and export of surface DOC to depth during sea ice formation or the decomposition of a settling algal bloom. As waters circulated in HB, an input of terrigenous DOC was the dominant modifying factor. In particular, DOC‐laden rivers in southern HB increased the DOC concentration and then displayed a conservative behavior as water exited the bay along the southern coast of HS. Additionally, the late stages of ice melt observed during this study showed a significant dilution effect on surface DOC concentrations within eastern HB. Input and export of riverine DOC in the HB system was estimated at ~5.5 Tg C yr−1, which is approximately 23% of the annual DOC input from rivers draining directly into the central Arctic Ocean and therefore represents an important contribution of terrigenous carbon to northern seas.
Isotopic niches and trophic levels of myctophid fishes and their predators in the Southern Ocean
Authors: Cherel Yves , Fontaine Camille , Richard Pierre , Labatc Jean-Philippe
Published: January 2011

We report the trophic structure of a myctophid assemblage by measuring the isotopic niches of 14 species living in Kerguelen waters, southern Indian Ocean. Most of the species show distinct isotopic niches that differ by at least one of the two niche axes (δ13C habitat and δ15N trophic position), indicating trophic partitioning within the assemblage. Strong niche segregation occurs within each of the three most common genera of myctophids (Electrona, Gymnoscopelus, and Protomyctophum), illustrating the different mechanisms (habitat and dietary segregation) that allow coexistence of closely related species. Calculated trophic levels (TLs) of myctophids ranged from 3.3 to 4.2, showing that they are secondary and tertiary consumers in the pelagic ecosystem. The positive relationship between TL and standard length of fish points out a structuring effect of size, with larger species (Gymnoscopelus spp.) occupying a higher trophic position than smaller species (Krefftichthys anderssoni and Protomyctophum spp.). Myctophids occupy an intermediate trophic position between macrozooplanktonic crustaceans and seabirds and marine mammals within the pelagic ecosystem. However, the TLs of large myctophids overlap those of crustacean‐eating seabirds [e.g., Eudyptes spp. (crested penguins) and Pachyptila belcheri]. The isotopic niche of myctophids indicates that Aptenodytes patagonicus (king penguin) adults prey upon K. anderssoni when they feed for themselves, thus exemplifying the usefulness of isotopic datasets on potential prey of predators to depict trophic relationships.
Regulation of the nitrogen biogeochemistry of mountain lakes by subsidies of terrestrial dissolved organic matter and the implications for climate studies
Authors: Bunting Lynda , Leavitt Peter R. , Weidman R. Paul , Vinebrookeb Rolf D.
Published: January 2011

Stable isotopes of nitrogen (N) were analyzed in modern sediments of mountain lakes, dissolved organic matter (DOM), and sediment cores spanning the past 12,000 yr to test the hypothesis that spatial and temporal (100‐ 1000 yr) variation in the N content of mountain lakes is regulated by influx of allochthonous DOM. Analysis of spatial patterns in an elevation gradient of 75 mountain lakes revealed that most N was associated with DOM rather than inorganic N, particularly in subalpine lakes (, 1700 m above sea level). Similarly, analysis of N isotope ratios (δ15N) from 22 lakes showed that whole sediments of subalpine sites were significantly more depleted (0.74‰ ± 1.58‰) than were those of alpine lakes above 2200 m (3.04‰ ± 1.21‰), consistent with the depleted δ15N of isolated DOM (~1.3‰). Sedimentary δ15N values of Crowfoot Lake, presently near tree line, also varied greatly during the past 12,000 yr, with enriched values (~4%) during the alpine phases of the lake's history and depleted values (~1%) during the intervening subalpine phase (ca. 10,050‐4160 14C yr before present) when DOM was abundant. In contrast, sedimentary δ15N values remained constant (~2.5%) at Snowflake Lake, an alpine reference site that never experienced a DOM‐rich subalpine phase. These analyses suggest that climate regulates N influx and lake biogeochemistry by changing the subsidies of terrestrial DOM, and warn that future climate change may initially reduce N influx on a decadal scale by reducing hydrologic transfer before increasing N subsidies on a centennial scale by increasing terrestrial production of DOM.