bluuebunny/crossref_metadata_2025 huggingface dataset:
Orientation in complex chemical landscapes: Spatial arrangement of chemical sources influences crayfish food‐finding efficiency in artificial stream
Authors: Keller Troy A. , Tomba Abbie M. , Moore Paul A.
Published: June 2010

Fluid dynamics has been shown to alter ecologically important behaviors of aquatic organisms orienting to distant chemical sources. Because the fluid dynamics and chemical plumes change across hydraulic environments, it is unclear which of these factors influence orientation behavior more. This study examined how alterations in chemical signal structure, through changes in source spatial arrangement, affect chemically mediated search behavior. Microelectrochemical measurements of tracer molecules revealed that source arrangement significantly alters the down‐stream fine‐scale structure of chemical plumes. Flume hydrodynamic characterizations (as measured with laser Doppler velocimetry) also differed among source arrangements; however, differences were minor and existed only at select upstream regions of the flume. Crayfish (Orconectes virilis) found the source faster and spent less time in refuges when sources were separated, compared with sources together. Similar numbers of crayfish found the source regardless of source arrangement. Crayfish searched more efficiently with increased spatial complexity at the source. These results supported the hypothesis that spatial and temporal dynamics of chemicals within plumes contain important information that organisms use during olfactory‐mediated orientation in streams.
Role of colloids and fine particles in the transport of metals in rivers draining carbonate and silicate terrains
Authors: Hill David M. , Aplin Andrew C.
Published: June 2010

We have used cross flow filtration (CFF) to determine the pools of fine particulate (0.1–0.45 µm), colloidal (1,000 Dalton to 0.1 µm), and dissolved (≪1,000 Dalton) metals in seven rivers that are all relatively rich in organic matter but have differing pH, alkalinity, and ionic strength. The metal content of ≪0.45‐µm filtered river water primarily reflects a mixing of two metal pools with differing elemental compositions: a truly dissolved pool and a colloidal pool. Fine particulate metals contribute ≪10% of the total metal load of ≪0.45‐µm filtered water. Colloidal metals generally comprise at least the following percentages of the metal load of ≪0.45‐µm filtered water: ≫50% of Fe, Al, and trace metals; 30% Mn, 25% Ca and Mg; 15% Na and K; and a few percent Si. We believe that these figures are minima because recovery experiments with the CFF apparatus suggest that whilst recoveries of pure metal salt solutions are close to 100%, only 70% of particle‐reactive metals are recovered from metal salt solutions spiked with humic acid. We propose that the metal composition of riverine colloids is primarily controlled by element mobility during weathering. In organic‐rich soils with low acid‐neutralizing capacity, low pH solutions drive the dissolution of aluminosilicates and oxides. Uptake of Al, Fe, and trace metals onto colloidal organic matter helps to maintain low activities of dissolved metals and enhances mineral dissolution. Colloids derived from these soils are thus enriched in Al, Fe, Mn, and trace metals, resulting in high concentrations of these elements in ≪0.45‐µm filtered water. In carbonate‐rich soils, rapid neutralization of rainwater by carbonate minerals restricts the mobilization of aluminosilicates and oxides. Compared to colloids from rivers draining silicate terrains, colloids in carbonate‐rich systems have higher total loadings of metals, are enriched in alkali and alkaline earth metals, but are depleted in Al, Fe, and trace metals.
Zooplankton survival and reproduction responses to damaging UV radiation: A test of reciprocity and photoenzymatic repair
Authors: Grad Gabriella , Williamson Craig E. , Karapelou Danielle M.
Published: June 2010

As stratospheric ozone concentrations are reduced, exposure of organisms to damaging ultraviolet radiation (UVR) reaching the earth increases. Many organisms can repair DNA damaged by UVR through photoenzymatic repair (PER) using the enzyme photolyase, in the presence of photorepair radiation (UV‐A and visible light [PRR]). Biological weighting functions have been used to model UVR damage in organisms in an attempt to predict the short‐term effects of ozone depletion. One assumption of these studies is that reciprocity—satisfied when the effect of a dose is independent of the dose rate—must hold. Here we exposed organisms to damaging UV‐B radiation in both the presence and absence of PRR. This study explicitly tests whether reciprocity holds in two zooplankters with differing PER abilities: Daphnia pulicaria, and Asplanchna girodi. We found significant PER and a failure of reciprocity when comparing Daphnia and Asplanchna survival at different dose rates. However, in a higher dose experiment with Asplanchna, we found no significant PER, and reciprocity held. These experiments show a link between PER and reciprocity. The ability of Daphnia and Asplanchna to produce viable offspring after UVR exposure did not vary with dose rate but did vary with PRR. Reciprocity held in all cases where PRR was provided, but in the absence of PRR all offspring died. This study shows that overall dose, dose rate, the ability to undergo PER, and the presence of PRR are important factors to consider when studying the effects of UVR on organisms.
Rate of growth of isolated bubbles in sediments with a diagenetic source of methane
Authors: Boudreau Bernard P. , Gardiner Bruce S. , Johnson Bruce D.
Published: June 2010

Observation of bubbles in estuarine and coastal sediments indicates that bubbles at or below 10 cm depth grow on seasonal time scales (May‐October). In order to determine the controls on this growth rate, we have constructed a diffusion‐reaction model that accounts for the dynamics of methane formation, its diffusion through pore waters, its incorporation into a bubble, and the consequent growth of the bubble. The model produces an explicit equation for the radius of a growing bubble, R(t), with time using mean parameter values and under the assumption that the mechanics of the sediment response to growth can be neglected: [equation]. where w is the porosity, D is the tortuosity‐corrected diffusivity, cg is the concentration of gas in the bubble, S is the rate of methanogenesis near the bubble, R1 is the half‐separation distance between bubbles (R1 k R), c1 is the ambient CH4 concentration, c0 is the pore‐water CH4 concentration at R, t is time, and R0 is the initial bubble radius, if not zero. The effects of the source S and supersaturation (c1 2 c0), thus, appear as separate contributing terms, and this formula can then be applied even in those cases where apparently c1 ≈ c0. The model is applied to three sediments where bubbles have been previously studied, i.e., Cape Lookout Bight (USA), White Oak River (USA) and Eckernfoörde Bay (Germany). In all three cases, using the site‐specific time‐averaged parameter values, the model predicts seasonal growth rates, consistent with the observations. Furthermore, the source term dominates the rate of growth at the first of these two sites, whereas diffusion from the ambient supersaturation dominates at the German location. Real bubbles may follow a more complicated growth history than predicted by the above equation because of the mechanical properties of sediments; nevertheless, the overall growth times are concordant with ultimate diffusion control. The effects of rectified diffusion, that is, the pumping of gas into a bubble by pressure oscillations, e.g., from waves and tides, were also examined. Existing models for that process suggest that it is negligible, due to the low frequency of these types of oscillations.
Water motion relative to subtidal kelp fronds
Authors: Stevens C. L. , Hurd C. L. , Smith M. J.
Published: June 2010

A study of water velocities near subtidal kelp fronds, relative to the motion of the kelp itself, is described. The dynamic model developed by Utter and Denny (1996) is used to develop a quantitative measure of the relative velocity vector, ur. The numerical model is extended through the addition of a current, use of a larger drag coefficient, and explicit consideration of ur. The model is tested by comparing it with microwave radar observations of a tethered float. The modeling of kelp frond motion uses parameters derived from current meter observations as input data. The results indicate that the relative velocity of seawater to the kelp frond is rarely zero and often is of comparable magnitude to the local Eulerian velocity of the water relative to the seabed. This has ramifications for modeling of both drag and boundary‐layer transport processes. Use of a monochromatic wave field in the modeling in conjunction with examination of the results in the time domain suggests the magnitude of the current is very important in controlling motion at frequencies lower than the wave frequency.
The influence of watershed land use on lake N: P in a predominantly agricultural landscape
Authors: Arbuckle Kelly E. , Downing John A.
Published: June 2010

This study tests the hypothesis that lakes in watersheds dominated by row‐crop agriculture (e.g., maize or soybeans) have systematically higher N: P than lakes in watersheds with large tracts of pasturelands. Current biogeochemical models of eutrophication suggest that agricultural nitrogen and phosphorus fluxes lead to a systematic decline in the N: P of receiving waters. In contrast, different agricultural activities (i.e., row‐cropping vs. animal agriculture) use greatly divergent N and P amendments, and fluxes from agricultural watersheds diverge through a broad range of observed N: P (i.e., sub‐Redfield to .100). Animal agriculture leads to low N: P fluxes and row‐cropping to high N: P. The connection between agricultural watershed land use and lake nutrient stoichiometry was tested in a highly agricultural region of the United States (Iowa) on 113 lakes in watersheds with different amounts of row‐crop (0%–95%) and pastureland (0%–36%). Multiple regression analysis shows that lakes in watersheds with large areas in pasturelands have low N: P, whereas lakes in watersheds dominated by row‐cropping have systematically high N: P. Lakes in watersheds with >30% pasture had the lowest N: P, approaching Redfield levels. N: P was most frequently high (>50 as atoms) in lakes with >90% of their watersheds in row‐crop agriculture. The dynamics of agricultural practice necessitates the inclusion of real‐world differences among agricultural systems in nutrient stoichiometric models. Intensive row‐crop agriculture yields N: P stoichiometry at high levels usually observed in pristine headwaters and open oceans, whereas increased animal agriculture will drive N: P to low levels usually associated with cyanobacterial blooms.
High‐resolution metal gradients measured by in situ DGT/DET deployment in Black Sea sediments using an autonomous benthic lander
Authors: Fones Gary R. , Davison William , Holby Ola , Jorgensen Bo Barker , Thamdrup Bo
Published: June 2010

DET (Diffusive equilibration in thin films) and DGT (diffusive gradients in thin films) have been deployed in situ using an autonomous benthic lander to measure concentrations and induced fluxes of Fe and Mn (DET/DGT) and trace metals (DGT) in pore waters at millimeter spatial resolutions. The newly developed deployment system is described, and based on these first results, its strengths and weaknesses are discussed. Deployments were made in the Western Black Sea in shelf sediments overlain by well‐oxygenated water at a water depth of 77 m. Maxima of the redox‐sensitive metals at 4 and 8 cm deep within the sediment indicated that two zones of reduction dominated the geochemistry. Sharp, but systematic, features were superimposed on this general picture and were well replicated in the profiles of Mn, Co, and Cd, but the sharp features in the Fe profile were offset from those of the others elements by several millimeters. Detection of this functional discrimination between Fe and Mn as regulators of trace metals would not have been possible using more conventional sampling procedures.
Bacterial recolonization of deposit‐feeder egesta: In situ regrowth or immigration?
Authors: Plante Craig J. , Wilde Susan B.
Published: June 2010

The related processes of disturbance, recolonization, and succession can play major roles in structuring communities and in generating spatial heterogeneity. Nonequilibrium processes have been little studied with respect to microbial community dynamics. Here we report experimental efforts to identify the mechanisms of recolonization of egesta employing two intertidal deposit feeders, Balanoglossus aurantiacus and Nereis succinea. Using direct microscopic counts and Biolog plates, we compared the quantitative and qualitative patterns of recovery in egesta isolated from underlying sediments on “latrines” to recolonization in naturally incubated fecal casts. Significant recovery over the 2‐h observation periods was never seen on latrines. Rapid recovery in metabolic potential was observed in the naturally incubated fecal casts of both animal species. Likewise, a rapid numerical recovery was seen in the egesta of N. succinea casts on sediments. In contrast, no numerical recovery was evident in any fecal casts of B. aurantiacus over 2 h, regardless of treatment. For these two species, recolonization appears to be dominated by migration of bacteria from underlying sediments as opposed to repopulation by survivors of ingestion. These findings indicate that renewal of available (digestible) microbial resources to deposit feeders is more rapid than would be predicted if regrowth was the dominant process of recolonization. Furthermore, because recovery time is short relative to the interval between disturbances, deposit feeding is unlikely to play a major role in structuring benthic microbial communities.
Differential effects of iron additions on organic and inorganic carbon production byphytoplankton
Authors: Lam Phoebe J. , Tortell Philippe D. , Morel François M. M.
Published: June 2010

Bottle and mesoscale experiments have demonstrated that iron additions enhance phytoplankton growth and reduce surface pCO2 in highߚnutrient, low‐chlorophyll (HNLC) regions of the world oceans. Here we show that iron additions specifically stimulate organic but not inorganic carbon production in the HNLC Subarctic Pacific. Fiveߚhour 14C labeling experiments performed during incubation of surface water samples demonstrated a large increase in the rate of organic carbon produced but no change in the rate of inorganic carbon production. The same result was obtained on two different dates: one when coccolithophores formed a relatively large proportion of total autotrophic biomass; the other when coccolithophores were less abundant. Together with previous taxonomic observations, our results imply that iron fertilization may be particularly effective in drawing down CO2 in surface waters by stimulating primary production but not calcium carbonate precipitation, which augments CO2.
Eighty years of spatially coherent Austrian lake surface temperatures and their relationship to regional air temperature and the North Atlantic Oscillation
Authors: Livingstone David M. , Dokulil Martin T.
Published: June 2010

Eighty years of monthly mean lake surface temperature (LST) data from eight lakes in the northern perialpine area of Austria show a high degree of coherence among lakes in all seasons and reflect much of the temporal structure of the regional air temperature. Coherence is least in winter because of the distorting effect of varying periods of ice cover. In spring, regional coherence in meteorological driving forces that are essentially uncorrelated with air temperature (e.g., geostrophic wind speed) contribute to the coherence in LST, presumably by partially determining the timing of the onset of stratification. In summer, spatial coherence in LST appears to be related directly (via the radiation balance) and/or indirectly (via air temperature) to largeߚscale variations in highaltitude cloud cover. Correlations of the Austrian LSTs with (1) seasonal indices of the North Atlantic Oscillation (NAO), (2) the timing of spring ice breakߚup in Finland, and (3) air temperatures in northern and western Europe, suggest that from autumn to spring, spatial coherence of LST in central Europe is related to the dominance of the weather by largescale climatic processes occurring over the North Atlantic, whereas in summer the processes responsible are more regional in nature. The influence of the NAO on LST is greatest in lowߚlying lakes in which periods of ice cover are infrequent and short.
Effects of mineral nutrients on the growth of bacterioߚ and phytoplankton in two southern reservoirs
Authors: Chrzanowski Thomas H. , Grover James P.
Published: June 2010

Nutrient limitation of bacterioߚ and phytoplankton was studied simultaneously in two warmߚwater lakes in the southern United StatesߞJoe Pool Lake (JPL) and Eagle Mountain Lake (EML). Lakes were sampled approximately biweekly between March 1998 and December 1999 from a single station. Nutrient limitation was assessed through dilution bioassays in which nitrogen (N, 50 µM above ambient), phosphorus (P, 10 µM above ambient), carbon (C, bacterioplankton only, 83 µM above ambient), and trace nutrients (Tm, phytoplankton only) were supplied. In both lakes, growth of bacterioߚ and phytoplankton was stimulated by nutrient additions. Multiple nutrient limitation was common. P alone and in combination with N and C or Tm most frequently limited growth of both bacterio and phytoplankton in JPL. N alone and in combination with P and C or Tm most frequently limited growth of both bacterioߚ and phytoplankton in EML. Comparison of in situ growth rates to growth rates under potentially nutrient saturating conditions revealed that both bacterioߚ and phytoplankton in both lakes were growing well below maximum potential during warm months but near maximum potential during cooler months. This result was due to a combined effect of low temperature in winter and restricted nutrient availability in summer. Phytoplankton was generally more strongly limited by nutrient availability than was bacterioplankton, but there were occasions when the intensity of limitation shifted between communities.
Calculation of spectral weighting functions for the solar photobleaching ofchromophoric dissolved organic matter in temperate lakes
Authors: Osburn Christopher L. , Zagarese Horacio E. , Morris Donald P. , Hargreaves Bruce R. , Cravero Walter E.
Published: June 2010

The effect of solar radiation on the dissolved absorption coefficient (λCDOM[λ]), which reflects the concentration of chromophoric dissolved organic matter (CDOM), was investigated in several lakes near Bariloche, Argentina and in northeastern Pennsylvania, USA. Samples of 0.2 µm filtered lake water were exposed in quartz tubes to different portions of the solar spectrum using optical cutoff filters to remove parts of the ultraviolet (UV) region of the solar spectrum. Changes in the spectral absorption in these samples and the absorbed incident energy were used to calculate spectral weighting functions (SWFs) for the photobleaching (PB) of CDOM. PB was measured as the loss of αCDOM[λ] (the αCDOM[λ] was averaged from 280 to 500 nm) per unit absorbed energy. CDOM from humic and clear lakes, as well as from a Sphagnum bog and an algal culture, was used in the experiments covering a wide range of carbon sources. We used an iterative, nonlinear optimization method to fit the measured results to a simple exponential function in order to generate each SWF. Comparing individual SWFs calculated for various CDOM sources, we computed a summary SWF from the experiments using epilimnial CDOM from our study lakes. Our summary SWF was able to explain 80‐90% of the observed variance in our exposure experiments, and we were able to predict PB results obtained for other Argentine lakes (mean error 14.5%). Finally, we calculated that the effect of UV‐B radiation on PB was small (<20% of total decrease in the absorption coefficient) compared to UVA and blue light radiation. This suggested that increased UV‐B radiation due to stratospheric ozone depletion would not greatly increase the photobleaching of whole water column CDOM in Patagonian lakes (<10%).
Iron and macronutrients in California coastal upwelling regimes: Implications for diatom blooms
Authors: Bruland Kenneth W. , Rue Eden L. , Smith Geoffrey J.
Published: June 2010

The supply of iron, relative to that of the macronutrients nitrate, phosphate, and silicic acid, plays a critical role in allowing extensive diatom blooms to develop in coastal upwelling regimes. The presence or absence of a broad continental shelf influences the supply of iron. The iron input to central California upwelling waters varies spatially and can be characterized by two end‐member regimes. One end member, which includes Monterey Bay and extending north to Pt. Reyes, is an iron‐replete regime where upwelling occurs over a relatively broad continental shelf that results in waters with high concentrations of dissolved and particulate iron (>10 nM) entrained together with high concentrations of nitrate and silicic acid. In these iron‐replete regions, extensive blooms of large diatoms deplete macronutrient concentrations, which results in correspondingly high chlorophyll a concentrations. The other end member, located to the south of Monterey Bay off the Big Sur coast, is an iron‐deplete regime where upwelling is focused offshore of a narrow continental shelf. Upwelled waters in the Big Sur region are characterized by low dissolved and particulate iron concentrations (<1 nM), together with high concentrations of nitrate and silicic acid. Extremely low iron concentrations, unused nitrate and silicic acid, and a low abundance of large diatoms characterize surface waters in these iron‐deplete regions, and thus represent coastal upwelling, high‐nutrient, low‐chlorophyll systems limited by the micronutrient iron.
Heterotrophic bacterial utilization of nitrogenous and nonnitrogenous substrates, determined from ammonia and oxygen fluxes
Authors: Rodrigues Rubina M. N. V. , Williams Peter J. le B.
Published: June 2010

We describe a simple procedure to allow the broad nature of the organic substrates used for planktonic bacterial growth to be determined. The method analyzes the coupled oxygen and ammonia fluxes in terms of the relative proportions of nitrogenous and nonnitrogenous substrates assimilated by the microheterotrophs. The model uses a stoichiometric equation that requires the knowledge of the respiratory quotient, substrate C/N ratio, cell C/N quota, and bacterial carbon growth yield and the assumption that nitrification was not occurring. We discuss the uncertainties associated with the attribution of values for these constants and illustrate the use and limitations of the approach in the interpretation of field observations on bacterioplankton metabolism in a temperate coastal ecosystem. With the exception of a single observation, we were able to interpret the data using the proposed model. Our findings are that during the spring bloom, nitrogenous substrates made up 40–80% of the total, falling to <20% in the postbloom period. Thus, with essentially routine methods for determining oxygen and ammonia fluxes, we have been able to determine a fundamental aspect of the cycling of organic material by the bacterioplankton.
Mesozooplankton grazing effect on primary production: A global comparative analysis in marine ecosystems
Authors: Calbet Albert
Published: June 2010

A comparative analysis of the importance of mesozooplankton (200–20,000 µm) as grazers of the phytoplanktonic primary production (PP) across a wide spectrum of marine ecosystems revealed mesozooplankton ingestion rates to increase nonlinearly with increasing PP. The slope of the log‐log relationship between ingestion rates and PP was significantly ,1, indicating a decline of relative importance of mesozooplankton grazing with increasing PP. The effect of mesozooplankton on PP (as the percent PP consumed per day) is moderate in most of the studies (mode 6%, mean 22.6%) and decreases exponentially with increasing productivity. Contrary to the common assumption, the size barrier imposed by dominant picoautotrophs does not always result in a lower grazing pressure in unproductive communities (we consider here those with PP < 250 mg C m−2 d−1). Yet, the amount of phytoplanktonic carbon ingested per unit of mesozooplankton biomass is lower in unproductive than in moderate (250 to 1,000 mg C m−2 d−1) and highly productive communities (>1,000 mg C m−2 d−1). This observation, together with the generally low values of daily biomass‐specific ingestions, suggests that alternative food sources (e.g., protozoans) must represent an important component of mesozooplankton diet in unproductive ecosystems. The relationships obtained in the study yield an estimate of 5.5 Gt phytoplanktonic C consumed per year in the global ocean, which represents ~12% of the oceanic PP.
Non‐Redfield C:N ratio of transparent exopolymeric particles in the northwestern Mediterranean Sea
Authors: Mari Xavier , Beauvais Sophie , Lemée, Rodolphe , Pedrotti Maria Luiza
Published: June 2010

The stoichiometric model of Redfield, which describes the elemental composition of marine organic matter, is generally used to link the production of new organic matter to the uptake of nitrate. The Redfield C:N molar ratio of 6.6 is a well‐established value for particulate organic matter produced in surface waters. Yet recent studies have shown that the inorganic C:N uptake ratio during nitrate‐limiting conditions is >14. This non‐Redfield behavior during the production of new organic matter suggests that a large standing stock of organic matter, rich in carbon, should accumulate in the euphotic zone from the spring bloom to late summer. This hypothetical pool of carbon‐rich organic matter and the pool of transparent exopolymeric particles (TEP) exhibit similar seasonal distributions, which suggests that TEP may indeed be this carbon‐rich pool. For this scheme to work, TEP should have a high C:N ratio. TEP distribution from an open‐ocean site in the northwestern Mediterranean Sea (DYFAMED) was monitored, and TEP C:N ratio was measured from TEP produced in the laboratory by bubbling dissolved organic matter collected in the field. We found that the TEP pool increases during the summer season and that the C:N ratio of TEP produced from naturally occurring dissolved organic matter, is well in excess of the Redfield ratio with an overall average C:N molar ratio of ~20. As a result, the production of TEP could be the main pathway of carbon overconsumption required of oligotrophic, nitratelimiting waters, and, thus, TEP may represent an important intermediary pool in the ocean carbon cycle.
Effect of grazing and nutrient supply on periphyton biomass and nutrient stoichiometry in habitats of different productivity
Authors: Hillebrand Helmut , Kahlert Maria
Published: June 2010

The impact of grazing and nutrient supply on epilithic periphyton was investigated in factorial field experiments in four seasons at three Swedish sites of different productivity and herbivore composition (Lake Limmaren, Lake Erken, and Väddö, a low salinity coastal site). Nutrient supply was enhanced by a granulose fertilizer containing nitrogen (N) and phosphorus (P), and grazer density was manipulated by exclusion cages. Algal biomass was increased by nutrient enrichment and reduced by grazer presence, but effects were highly variable between sites and seasons. Generally, grazers had stronger effects on algal biomass than nutrient enrichment, but there was no overriding effect of either grazing or nutrients. This indicated a simultaneous top‐down and bottom‐up control of algal biomass. Taxonomic composition of the periphyton was more affected by grazer presence than by nutrients. Internal nutrient ratios of the algae indicated N limitation at two of the sites. At all sites, the content of N and P in the periphytic assemblage was enhanced by the experimental nutrient enrichment, resulting in decreased C:N and C: P ratios. The presence of herbivores also increased periphytic nutrient content (decreased N: P and C: P ratios) in some experiments, suggesting an increase in algal P due to excretion. The effect strength of grazers and nutrients on periphyton was affected by different abiotic characteristics such as light availability, nutrient concentrations, and temperature. However, single environmental characteristics were not sufficient to explain the relative importance of grazing and nutrients.
Sorptive fractionation of dissolved organic nitrogen and amino acids onto fine sediments within the Amazon Basin
Authors: Aufdenkampe Anthony K. , Hedges John I. , Richey Jeffery E. , Krusche Alex V. , Llerena Carlos A.
Published: June 2010

A consistent observation of river waters in the Amazon Basin and elsewhere is that suspended fine particulate organic matter (FPOM) is compositionally distinct from coexisting dissolved organic matter (DOM). The present article presents experimental results that show that at least some of these compositional patterns are the outcome of selective partitioning of nitrogen‐rich DOM components onto mineral surfaces. Nine laboratory experiments were conducted in which natural DOM from two rivers, one wetland, and two leachates from the Peruvian Amazon were mixed with natural suspended riverine minerals or organic‐free kaolinite. Concentrations of organic carbon, organic nitrogen, and hydrolyzable amino acids were measured in both dissolved and particulate phases before and after mixing. In each of these trials, nitrogen was preferentially taken into the particulate fraction relative to the “parent” DOM, as were total hydrolyzable amino acids with respect to total organic carbon and nitrogen. Amino acid compositional patterns also indicated preferential sorption of basic amino acids, with positively charged nitrogen side chains, to the negatively charged aluminosilicate clay minerals. In short, sorption of natural DOM to minerals reproduced all contrasting organic nitrogen compositional patterns observed in the Amazon Basin. Although previously conjectured from FPOM‐DOM compositional trends from river samples, this is the first direct evidence for preferential uptake of naturally occurring nitrogenous DOM by suspended riverine minerals. Last, nonprotein amino acids, which are commonly used as diagenetic indicators in sediments, preferentially remained dissolved, which suggests that sorptive fractionation may significantly complicate comparisons of FPOM and DOM diagenesis on the basis of interpretation of organic composition.
Bioavailability of natural colloid‐bound iron to marine plankton: Influences of colloidal size and aging
Authors: Chen Min , Wang Wen-Xiong
Published: June 2010

Iron (Fe) is mostly complexed with the organic ligands or colloids that are very abundant in natural seawater. In this study, natural colloids were isolated by ultrafiltration and radiolabeled with 59Fe. The biological uptake of radiolabeled colloid‐bound Fe of different sizes (1–10 kDa and 10 kDa‐0.2 μ) and ages (1 and 15 d) by diatoms (Thalassiosira pseudonana) and copepods (Acartia spinicauda) was then determined. The uptake of radiolabeled colloid‐bound Fe was compared with the uptake of low molecular weight (LMW) complexed Fe (<1 kDa) or that of EDTA‐Fe (at a ratio of 1 : 2 for Fe :EDTA). Our study demonstrates that the colloid‐bound Fe of different sizes and ages was bioavailable to the diatoms. The uptake of colloidal bound Fe was, however, 6–31 times lower than the uptake of LMW Fe, suggesting that colloidal binding reduced Fe bioavailability to diatoms. Fe bound with small colloids (1–10 kDa) was taken up at a higher rate than Fe bound with large colloids (10 kDa‐0.2 μ) at typical colloidal organic carbon concentrations. The uptake of colloid‐bound Fe was also much higher when the Fe had been bound with the colloids for 1 d rather than for 15 d. Differences in the colloidal organic carbon concentration did not appreciably affect the uptake of colloid‐bound Fe by the diatoms. Similarly, copepods accumulated colloid‐bound Fe at a much higher rate when the Fe was associated with small colloids rather than with large colloids. Direct ingestion of colloidal particles by the copepods appeared to be negligible. In both diatoms and copepods, Fe uptake may involve its dissociation from the colloids before being accumulated by the organisms. The study therefore demonstrates that colloid‐bound Fe is less available for marine phytoplankton and that colloidal size and thus the colloidal speciation may be critical in controlling colloid‐bound Fe uptake by aquatic organisms.
Daphnia food limitation in three hypereutrophic Dutch lakes: Evidence for exclusion of large‐bodied species by interfering filaments of cyanobacteria
Authors: DeMott William R. , Gulati Ramesh D. , Van Donk Ellen
Published: June 2010

The Loosdrecht Lakes comprise three shallow, hypereutrophic lakes in The Netherlands. Research conducted over the past two decades suggests that absence of large‐bodied Daphnia from these lakes can be explained by size‐specific effects of both poor food quality and predatory mortality. The phytoplankton is dominated by filamentous cyanobacteria, which interfere with the feeding of large‐bodied Daphnia. Moreover, dense populations of planktivorous fish are expected to feed selectively on larger prey. More recent research, however, suggests that the growth of the small native Daphnia species, Daphnia cucullata, is limited by low dietary phosphorus (P). In this study, we compared the growth and feeding rates of four Daphnia taxa representing a gradient in body size. In each of 10 growth experiments, native D. cucullata feeding in natural seston exhibited improved growth in response to additions of phosphate and the P‐rich cyanobacterium Synechococcus. The two largest species, Daphnia magna and Daphnia galeata, however, failed to grow in natural seston, even when the seston was supplemented with phosphate, Synechococcus, or Scenedesmus. A D. galeata × cucullata hybrid exhibited weak growth in natural seston and no response to the supplements. Feeding experiments with polystyrene beads show that feeding inhibition in natural seston increased markedly with increased Daphnia size. Our results verify that Daphnia differing in size face different food quality constraints. The growth of native D. cucullata (adult size 0.6–0.8 mm) is reduced by P deficiency, whereas larger Daphnia appear to be excluded by interfering filaments of cyanobacteria.
The optics of chromophoric dissolved organic matter (CDOM) in the Greenland Sea: An algorithm for differentiation between marine and terrestrially derived organic matter
Authors: Stedmon C. A. , Markager S.
Published: June 2010

The optics of chromophoric dissolved organic matter (CDOM) in the Greenland Sea were investigated and compared to results from earlier studies in the Southeastern North Sea. Absorption at 375 nm (a375) in the Greenland Sea varied from 0.77 m−1 to the detection limit of our instrument (0.05 m−1), with the highest values found during summer. The spectral slope coefficient (S) ranged from 8.2 to 26.4 µm−1 with the highest values occurring during winter. Seasonal variations in the in situ production and degradation of CDOM where shown to be responsible for the trends seen. A negative correlation between S and a375 was evident in the Greenland Sea and differed noticeably from that found in coastal waters. The differing S‐a375 behavior of CDOM known to be of terrestrial origin allowed the development of an algorithm for the differentiation between marine and terrestrial organic matter. The behavior of marine CDOM was modeled by S = 7.4 + 1.1/a375.
Modeling 50 years of historical temperature profiles in a large central European lake
Authors: Peeters Frank , Livingstone David M. , Goudsmit Gerrit-Hein , Kipfer Rolf , Forster Richard
Published: June 2010

A unique data set of 50 years of monthly temperature profiles from Lake Zurich, a normally ice‐free lake located on the Swiss Plateau, allowed the one‐dimensional numerical k‐Ε lake model “SIMSTRAT” to be calibrated (1948—1957) and validated (1958—1997). Hindcasts of temperature profiles agree excellently with the measured data. Both interannual and intraannual variations in thermal structure are reproduced well during the entire 50‐yr simulation, thus demonstrating the stability and good prognostic qualities of the model. Simulations conducted with raised and lowered air temperatures (Tair) suggest that an increase in Tair will lead to an increase in lake water temperature at all depths. In comparison to the continuous modeling approach taken in this study, the commonly employed discontinuous modeling approach (with no heat carryover during winter) substantially underestimated the degree of long‐term hypolimnetic warming that can be expected to result from an increase in Tair. Thus, whereas the discontinuous approach yields valid predictions for strictly dimictic lakes that are ice‐covered each winter, heat carryover during winter makes a continuous approach necessary in lakes like Lake Zurich that are only facultatively dimictic. The significant degree of hypolimnetic warming found in this study suggests that the response of facultatively dimictic lakes to increases in Tair is likely to differ from that of the strictly dimictic lakes modeled in other investigations. In Lake Zurich, an increase in Tair is predicted to result in more frequent suppression of deeply penetrative winter mixing events, with a potentially negative impact on the lake ecosystem.
A pH plate fluorosensor (optode) for early diagenetic studies of marine sediments
Authors: Hulth Stefan , Aller Robert C. , Engström Pia , Selander Erik
Published: June 2010

Pore water pH distributions are closely coupled to early diagenetic reactions and transport processes in surficial sediments. In this study, an optical plate fluorosensor for rapid two‐dimensional detection of H+ concentration patterns in sediment pore waters was developed. The dual excitation (405/450 nm) single emission (520 nm) pH fluorophore HPTS (8‐hydroxypyrene 1, 3, 6, trisulfonic acid trisodium salt) was immobilized onto transparent, supporting sensor foils. Excitation/emission spectra of immobilized HPTS exhibited pH dependent shifts in seawater standards, and fluorescence excitation ratios were used for two‐wavelength, ratiometric detection of pH distributions in sediment and bottom water close to the sediment—water interface (pH ~ 6.5 to 8.5). Sensor foils were fixed to the inner sides of plastic box corers or glass plate aquaria containing sediment and overlying seawater. Equilibration with pore water occurred in seconds, and fluorescence response (excitation/emission) was scanned nondestructively using a monochrome, integrating CCD camera. The two‐dimensional sensor system has a vertical and horizontal resolution of 56 × 54 µm (5 × 5 pixels) applied over an image of 34 × 26 mm. Reproducibility and accuracy were best when foils were prepared from solutions of similar ionic strength to samples. Optical sensor pH distributions correlated well with high‐resolution, vertical distributions measured simultaneously with a minicombination pH electrode, and two‐dimensional pH patterns demonstrated directly the transport‐reaction heterogeneity associated with microtopography and biogenic structures in surface deposits and overlying waters close to the sediment—water interface. Sensor foils retained calibration characteristics for at least 50 d.
The rates and pathways of carbon oxidation in bioturbated saltmarsh sediments
Authors: Kostka Joel E. , Gribsholt Britta , Petrie Ellen , Dalton Dava , Skelton Hayley , Kristensen Erik
Published: June 2010

This study was carried out to quantify the effects of higher organisms, invertebrate macrofauna, and macrophyte plants on the rates and pathways of microbial respiration coupled to organic matter oxidation in saltmarsh sediments. Sediment geochemistry, rates of microbial metabolism, and the abundance of anaerobic respiratory bacteria were determined at sites differing in the abundance of fiddler crab (Uca pugnax) burrows and vegetation (Spartina alterniflora) coverage. Solid‐phase Fe(III) concentrations were 50 to 100 times higher, and solid sulfide concentrations were eight times lower in bioturbated, vegetated sediments (BVL) as compared to nonbioturbated, unvegetated (NUC) sediments. Integrated sulfate reduction rates were 10 times lower in BVL (2 mmol m−2 d−2) as compared to NUC sediments (20 mmol m−2 d−1). Directly measured Fe(III) reduction rates were high at the BVL site, whereas no Fe(III) reduction was detected at NUC or in killed sediment treatments. Molybdate, a specific inhibitor of sulfate reduction, inhibited 70% of carbon oxidation when added to NUC sediment but showed no effect on Fe(III) reduction or C oxidation in BVL sediments. Counts of Fe(III)‐reducing bacteria (FeRB) were two orders of magnitude higher in BVL sediments (107 cells g−1) in comparison to NUC sediments (105 cells g−1). Fe(III) respiration comprised up to 100% of carbon oxidation in BVL sediments, whereas sulfate reduction was the dominant respiration process (≥70% of C oxidation) at NUC. We provide strong evidence to show that macroorganisms stimulate FeRB to outcompete sulfate‐reducing bacteria in saltmarsh sediments by supplying an abundance of reactive Fe(III) through reoxidation processes.
Photoreceptors in the cnidarian hosts allow symbiotic corals to sense blue moonlight
Authors: Gorbunov Maxim Y. , Falkowski Paul G.
Published: June 2010

In many species of symbiotic corals, spawning occurs synchronously several nights after the full moon. This process is correlated with the level of lunar irradiance, but the mechanism by which these cnidarian/zooxanthellate symbioses can detect such low levels of light remains unknown. Here we report the first biophysical evidence that the host animal exhibits extraordinarily sensitive photoreception in the blue region of the spectrum. Using a high‐resolution laser‐induced signal to detect tentacle scattering, we measured the effect of low irradiance on the contraction of polyps in the corals that normally have their tentacles extended in darkness. Similar to most deep‐sea invertebrates, the action spectra of coral photoreception reveal a maximum sensitivity in the blue, at 480 nm, with a spectral band width (at full‐width half‐maximum) of ca. 110 nm. The spectra closely overlap the maximal transparency of oligotrophic tropical waters, thus optimizing the perception of low light at depth. The detected threshold of photoreception sensitivity is ~1.2 × l015 quanta m−2 s−1 in the blue region. This makes corals capable of sensing the blue portion of lunar irradiance, as evidenced from the recorded slight contractions of polyp tentacles under variations in moonlight intensity.
Seasonality of in situ respiration rate in three temperate benthic suspension feeders
Authors: Coma Rafel
Published: June 2010

Natural respiration rates of suspension feeders in temperate ecosystems are still poorly known. This lack of information constrains our understanding of the functioning and dynamics of benthic marine ecosystems in temperate areas.We examined the in situ seasonal variation in respiration rate of three benthic suspension feeders (a sponge, an ascidian, and a gorgonian) in northwestern Mediterranean sublittoral communities using a recirculating flow respirometry system. The in situ technique is shown to be highly applicable to seasonal studies of the physiological energetics of benthic suspension feeders. Respiration rates of the three species varied two‐ to threefold through the annual cycle, exhibiting a marked seasonal pattern but showing no daily cycle or significant day‐today variability within months. The respiration rate of the sponge and ascidian, active suspension feeders, increased with temperature. The respiration rate of the gorgonian, a passive suspension feeder, did not correlate with temperature. We estimated a Q10 of 1.1, which indicates that respiration rate in this species is not highly dependent on temperature. Synthesis of new tissue of some Mediterranean benthic suspension feeders, such as gorgonians, does not correlate with temperature, which allowed us to isolate the effects of temperature and synthesis of new tissue on respiration rate. Synthesis of new tissue increased respiration rate of the gorgonian by ~40%. The low rate of synthesis of new tissue during summer, together with the contraction of polyps and the low Q10, explains the low respiration rates of the gorgonian observed during the period of highest temperature. These low respiration rates support the hypothesis that energy limitations may underlie summer dormancy in some benthic suspension‐feeding taxa in the Mediterranean.
Distribution, production, and ecophysiology of Picocystis strain ML in Mono Lake, California
Authors: Roesler Collin S. , Culbertson Charles W. , Etheridge Stacey M. , Goericke Ralf , Kiene Ronald P. , Miller Laurence G. , Oremland Ronald S.
Published: June 2010

A recently described unicellular chlorophytic alga isolated from meromictic Mono Lake, California, occupies a niche that spans two environments: the upper oxic mixolimnion and the deeper anoxic and highly reducing monimolimnion. This organism, Picocystis sp. strain ML, accounts for nearly 25% of the primary production during the winter bloom and more than 50% at other times of the year. In incubations, it is heavily grazed by the brine shrimp, Artemia monica. We assessed growth and photosynthetic parameters over broad ranges of irradiance, salinity, and pH and under oxic and anoxic conditions. Picocystis appears to be particularly adapted to low irradiance; we observed an order of magnitude increase in the cellular pigment concentrations, as well as marked increases in cellspecific photosynthetic parameters for cells acclimated to low‐growth irradiance. Growth rates of 0.3–1.5 d−1 were observed over a salinity range of 0–260‰ and a pH range of 4–12, with maximal growth at ~50 mmol photons m−2 s−1, 40‰, and pH 6–10. Growth and oxygenic photosynthesis were observed under anoxic conditions at rates comparable to those measured under oxic conditions. The ability of the organism to acclimate and grow under such a broad range of environmental conditions makes it an important component of the Mono Lake ecosystem and likely contributes to its dominance of the monimolimnion/mixolimnion interface.