bluuebunny/crossref_metadata_2025 huggingface dataset:
Mate finding, mating, and population dynamics in a planktonic copepod Oithona davisae: There are too few males
Authors: Kiørboe Thomas
Published: January 2011

Three‐dimensional (3D) video observations of free‐swimming copepods revealed the mechanisms of mate finding in the cyclopoid copepod Oithona davisae (cephalothorax length 0.33 mm). The females are ambush feeders and spend almost all of their time sinking slowly through the water, interrupted by 0.5–1‐mm‐long jumps every few seconds. The sink—jump behavior distributes produced pheromones along an intermittent trail. Males spend about half of their time ambush feeding, and half of their time cruising rapidly (50 body lengths s−1), searching for female signals. On encountering a pheromone patch, the male starts circling rapidly (up to >100 body lengths s−1) around the patch, apparently to find subsequent patches and, eventually, the female. The pursuit may last several tens of seconds, and ends by the male capturing the female or losing the track. Observed reaction distances and male motility patterns were combined with a simple encounter model to estimate a matesearch volume rate of 4.4 L d−1. The males have a rather limited mating capacity, 0.9 females male−1 d−1, even when mate‐encounter rate is not limiting. Field observations of abundances, combined with estimates of mortality and mate‐search and mating capacities, predict that only approximately one‐third of the females should be fertilized. This is consistent with field observations, and suggests that population growth in this species is severely constrained by fertilization limitation. Fertilization limitation may apply more generally to populations of pelagic copepods, and may partly account for the frequent observation that female copepods produce sterile eggs, even during periods of a plentiful food supply.
Convergences and divergences and thin layer formation and maintenance
Authors: Stacey Mark T. , McManus Margaret A. , Steinbuck Jonah V.
Published: January 2011

The formation and maintenance of thin layers in the presence of turbulent diffusion is considered through the development of a formal framework that evaluates the balance between convergence mechanisms and the diffusive effects of turbulence. Turbulent diffusion acts to broaden layers, thus a convergence is required to produce a persistent and stable layer structure. Convergence mechanisms considered here include straining by a sheared velocity profile, organism motility, and particle buoyancy. The balance between each of these convergences and turbulent diffusion results in a scale estimate for the layer thickness that depends on local conditions. Comparison of these layer thickness scales for each of the three mechanisms enables us to evaluate which mechanism is likely to be dominant in particular situations. An example application of the framework is based on observations of thin layers in East Sound, Washington, for which we conclude that either the buoyancy or straining mechanism could contribute to the maintenance of the layer. Finally, the analytic framework itself provides insights into thin layer dynamics, including the prediction of a finite layer lifetime for shear‐driven layers and the effects of mixing events on the convergence mechanisms acting to maintain the layers.
A biodiversity‐inspired approach to aquatic ecosystem modeling
Authors: Bruggeman Jorn , Kooijman Sebastiaan A. L. M.
Published: January 2011

Current aquatic ecosystem models accommodate increasing amounts of physiological detail, but marginalize the role of biodiversity by aggregating multitudes of different species. We propose that at present, understanding of aquatic ecosystems is likely to benefit more from improved descriptions of biodiversity and succession than from incorporation of more realistic physiology. To illustrate how biodiversity can be accounted for, we define the system of infinite diversity (SID), which characterizes ecosystems in the spirit of complex adaptive systems theory as single units adapting to environmental pressure. The SID describes an ecosystem with one generic population model and continuity in species‐characterizing parameters, and acquires rich dynamics by modeling succession as evolution of the parameter value distribution. This is illustrated by a four‐parameter phytoplankton model that minimizes physiological detail, but includes a sophisticated representation of community diversity and interspecific differences. This model captures several well‐known aquatic ecosystem features, including formation of a deep chlorophyll maximum and nutrient‐driven seasonal succession. As such, it integrates theories on changes in species composition in both time and space. We argue that despite a lack of physiological detail, SIDs may ultimately prove a valuable tool for further qualitative and quantitative understanding of ecosystems.
Optimization‐based model of multinutrient uptake kinetics
Authors: Lan Smith S. , Yamanaka Yasuhiro
Published: January 2011

We present a new, optimization‐based model for uptake kinetics of multiple nutrients, which has the same number of parameters (two for each nutrient) as the Michaelis—Menten model. We fit this model and an existing inhibition‐based model to data from chemostat experiments at various flow rates (under extreme limitation by both nitrogen [N] and phosphorus [P]) and compared these models and the Michaelis—Menten model to an independent data set for the same species in a chemostat at various N: P input ratios (at constant flow rate). Our model fit the data well, with a slightly higher square error than the much more complex inhibition model. We also successfully applied our model to a data set for a different species under various degrees of vitamin B12‐ and Plimitation. Our model agrees with measured cell quotas of nonlimiting nutrients when supply ratios differ greatly from the optimal ratio for phytoplankton, whereas the Michaelis—Menten model greatly overestimates the uptake of nonlimiting nutrients at these extreme nutrient supply ratios. The key to our model's success is the optimization of uptake for the limiting nutrient, which results in distinct behavior for limiting versus nonlimiting nutrients, without additional parameters; phytoplankton allocate their internal resources (nitrogen) to optimize uptake of the limiting nutrient, but not in response to changes in ambient nutrient ratios.
Dispersal of barnacle larvae along the central California coast: A modeling study
Authors: Pfeiffer-Herbert Anna S. , McManus Margaret A. , Raimondi Peter T. , Chao Yi , Chai Fei
Published: January 2011

To investigate the biological and physical mechanisms affecting larval dispersal, we embedded a model of Balanus glandula larval development and behavior into physical circulation fields of waters along the central California coast. Physical circulation fields were generated by a three‐dimensional ocean circulation model with a horizontal resolution of 1.5 km and 20 topography‐following layers in the vertical. The ocean circulation model was forced by air—sea fluxes derived from a mesoscale atmospheric model and assimilated temperature and salinity data from the Autonomous Ocean Sampling Network II experiment. An ecosystem model that calculated chlorophyll a (a proxy for larval food concentration) was also coupled to the ocean circulation model. The coupled model of larval development, larval behavior, food concentration, and physical circulation was used to run simulations of larval dispersal. Simulation results predicted a greater return of larvae to the nearshore waters with relaxation circulation patterns than with upwelling. More larvae were supplied to the coast north of Monterey Bay than to the south, and larvae that successfully returned to the nearshore waters generally had limited dispersal distances. These modeling results agree with previous observations of B. glandula population dynamics in central California.
Importance of scale to the relationship between abundance of sardine larvae, stability, and food
Authors: McClatchie Sam , Rogers Paul J. , McLeay Lachie
Published: January 2011

We used a spatially explicit regression model to relate sardine (Sardinops sagax) larval abundance to water column stability, phytoplankton, and zooplankton off South Australia. The distribution of sardine larvae was significantly associated with stability (p < 0.05) and phytoplankton fluorescence or zooplankton displacement volume (p < 0.001) at broad scales. In contrast there was no relationship between sardine larvae, stability, phytoplankton, or zooplankton at medium or fine scales. The relationships are shown to be scale‐dependent. Although the results generally support Lasker’s (1978) “stable ocean” hypothesis, mid‐range rather than high stability appears to be favored, in accord with theoretical expectations at both large and small scales.
Salmon influences on dissolved organic matter in a coastal temperate brownwater stream: An application of fluorescence spectroscopy
Authors: Hood Eran , Fellman Jason , Edwards Rick T.
Published: January 2011

We examined how spawning Pacific salmon (genus Oncorhynchus) affect streamwater concentrations of inorganic nitrogen and phosphorus and dissolved organic matter in Peterson Creek, a stream in southeast Alaska. When spawning salmon were present, concentrations of ammonium (NH4‐N) increased by more than 100 times over prespawning levels and concentrations of soluble reactive phosphorus increased by more than an order of magnitude. In contrast, concentrations of nitrate (NO3‐N) increased by only two to three times during spawning and were not significantly higher than at an upstream reference site with no salmon. During spawning, concentrations of dissolved organic carbon and dissolved organic nitrogen were significantly higher in the spawning reach compared with the upstream reference site. The influx of salmon‐derived dissolved organic matter (DOM) altered the fluorescence index (FI), which has previously been used to distinguish between terrestrial and aquatic sources of DOM, with the FI increasing significantly during the salmon run. Salmon DOM was rich in protein compared with the DOM derived from the terrestrial portion of the watershed. Spawning salmon may be an important source of labile DOM in Peterson Creek.
Variable effects of marine‐derived nutrients on algal production in salmon nursery lakes of Alaska during the past 300 years
Authors: Brock Curtis S. , Leavitt Peter R. , Schindler Daniel E. , Quay Paul D.
Published: January 2011

We measured historical changes in sedimentary δ15N and fossil pigments in four lakes with anadromous semelparous salmon and two reference lakes to quantify the degree to which the flux of marine‐derived nutrients (MDNs as N isotopes) regulate algal production (as pigments). During the past 300 yr, production of the predominant algae (diatoms) was positively correlated (r = 0.42–0.93, p < 0.02) with sedimentary δ15N in nursery lakes of sockeye salmon (Oncorhynchus nerka) but was inversely correlated with sedimentary δ15N (r = 20.71 to 20.73, p < 0.0001) in reference lakes that lacked migratory fishes. Overall, the pigment‐δ15N correlation during the 20th century was strongly correlated with both mean densities of spawning sockeye salmon during 1956–2000 (r = 0.97, p < 0.002) and the fraction of total ecosystem N derived from salmon during 1900‐2000 (r = 0.98, p < 0.001). Together these patterns suggest that the sign of the δ15N‐pigment correlation can be used to distinguish among lakes or periods of time in which algal production is regulated mainly by MDN influx (positive correlation) or other factors (negative correlation). Tests of this hypothesis revealed that the degree to which MDNs regulated algal production in nursery lakes varied greatly since 1700, with significant periods of weak control even in lakes with abundant salmon. Further, when considered at the landscape scale, the importance of MDNs to individual lakes varied substantially through time and in space, with little evidence of synchrony among sites or catchments.
The effects of temperature and predator—prey interactions on the migration behavior and vertical distribution of Mysis relicta
Authors: Boscarino Brent T. , Rudstam Lars G. , Mata Shylene , Gal Gideon , Johannsson Ora E. , Mills Edward L.
Published: January 2011

The extent to which temperature, temperature gradients, predator smell, and prey availability influence the migratory behaviors and vertical distribution of the opossum shrimp, Mysis relicta, was explored through controlled laboratory experiments and comparisons with field distributions of mysids in Lake Ontario. By varying environmental conditions in 2‐m tall experimental columns in a temperature‐controlled room, we determined that mysids prefer temperatures between 6°C and 8°C with limited movement into waters of 12°C or higher. No mysids moved into waters above 16°C in the absence of prey. However, a higher proportion of mysids moved into temperatures of 14°C and 16°C (but not 18°C) when densities of Daphnia pulicaria exceeding 120 L−1 were present at those temperatures. Mysis avoided waters with kairomones from a primary mysid predator, the alewife (Alosa pseudoharengus). The rate of temperature change with depth did not restrict mysid movements. A temperature preference function based on the experimental data was applied to an existing model of mysid vertical distribution. The modified model predicted the depth of maximum mysid density to within 1 m and yielded high percentage overlap index values when compared with published mysid vertical distributions in Lake Ontario. Our approach may be used to model how diurnal, seasonal, and larger climactic changes can impact both the vertical position and feeding ecology of mysids, a keystone species in many deep‐water pelagic food webs.
Radium‐226 accumulation in Florida freshwater mussels
Authors: Brenner Mark , Smoak Joseph M. , Leeper Douglas A. , Streubert Matt , Baker Shirley M.
Published: January 2011

Selected lakes in Hillsborough County, Florida have been hydrologically augmented with groundwater to offset stage declines caused by excessive pumping of the Floridan Aquifer. Augmentation water can be relatively rich in 226Ra (>5 decays per minute [dpm] L−1). We measured 226Ra activities in shells and soft tissues of adult bivalve molluscs (Elliptio cf. buckleyi) from groundwater‐augmented and nonaugmented lakes to assess bioaccumulation of 226Ra by mussels. Mussels from augmented lakes displayed higher 226Ra in both shells and tissues than did mussels from nonaugmented lakes. Within a sample, 226Ra activity in Elliptio tissues was higher than the value measured in shells. Highest activities were found in a composite mussel sample (n = 6) from an augmented lake; soft tissue activity was 619 ± 33 dpm g−1 dry weight and shell activity was 147 ± 7 dpm g−1 g dry weight. Large mussels displayed greater activities in soft tissues and shells than did small mussels. We transplanted animals from a nonaugmented lake into a groundwater‐augmented water body. 226Ra activity in dry tissue rose from 32 ± 1 to 196 ± 2 dpm g−1 within 2 months. When 226Ra‐rich mussels (232 ± 2 dpm g−1) from the augmented lake were transferred to the nonaugmented lake, they showed no significant 226Ra loss over the 69‐ d experiment. Large Elliptio mussels concentrated 226Ra in their soft tissues to levels about 1,000 to 25,000 times concentrations in lake water. Pumping of groundwater in Florida for residential, agricultural, and industrial use contributes dissolved 226Ra to some surface water bodies, where it can be bioaccumulated by bivalve molluscs.
A biochemical explanation for the success of mixotrophy in the flagellate Ochromonas sp.
Authors: Boëchat Iola G. , Weithoff Guntram , Krüger Angela , Gücker Björn , Adrian Rita
Published: January 2011

We report the influence of different nutritional modes—autotrophy, mixotrophy, and heterotrophy—on the fatty acid and sterol composition of the freshwater flagellate Ochromonas sp. and discuss the ecological significance of our results with respect to the resource competition theory (rct). Polyunsaturated fatty acids (PUFAs) are the most efficient biochemical variable distinguishing between nutritional modes of Ochromonas sp. Decreasing concentrations of PUFAs were observed in the order autotrophs, mixotrophs, heterotrophs. In mixotrophs and heterotrophs, concentrations of saturated fatty acids were higher than those of monounsaturated fatty acids and PUFAs as a result of bacterivory. Stigmasterol was the main sterol in Ochromonas sp., regardless of nutritional mode. Mixotrophs showed higher growth rates than heterotrophs, which could not be explained by rct. Heterotrophs, in turn, exhibited higher growth rates than autotrophs, which were cultured under the same light conditions as mixotrophs. Mixotrophs can synthesize PUFAs, which are important for many physiological functions such as membrane permeability and growth. Thus, mixotrophy facilitated efficient growth as well as the ability to synthesize complex and essential biomolecules. These strong synergetic effects are due to the combination of biochemical benefits of heterotrophic and autotrophic metabolic pathways and cannot be predicted by rct.
The effect of long‐term spatiotemporal variations in urbanization‐induced eutrophication on a benthic ecosystem, Osaka Bay, Japan
Authors: Yasuhara Moriaki , Yamazaki Hideo , Tsujimoto Akira , Hirose Kotaro
Published: January 2011

Detailed spatiotemporal patterns of the influence of urbanization‐induced eutrophication on a metazoan benthic community in Osaka Bay were determined using sediment cores and fossil ostracode assemblages from the last 200 yr. Results suggest that total abundance of ostracodes increased in the middle part of the bay as a result of the increase of food supply by eutrophication. Conversely, abundance decreased in the inner bay, likely because of bottom‐water hypoxia by eutrophication. The variation in species composition among sites within the bay may have decreased because of the effect of eutrophication, i.e., the dominance of species that prefer food‐rich environments throughout all sites. These eutrophication‐induced changes occurred around 1900 as a result of Japan’s industrial revolution and around 1960 as a result of rapid urbanization, depending upon location.
Composition and degradation of marine particles with different settling velocities in the northwestern Mediterranean Sea
Authors: Goutx Madeleine , Wakeham Stuart G. , Lee Cindy , Duflos Marie , Guigue Catherine , Liu Zhanfei , Moriceau Brivaëla , Sempére Richard , Tedetti Marc , Xue Jianhong
Published: January 2011

Settling particles were collected from the Ligurian Sea in the northwestern Mediterranean Sea in May 2003 and separated by elutriation into different settling velocity classes (>230, 115–230, 58–115, and <58 m d−1). Particles of the different classes were incubated for 5 d to study their biodegradability. Particulate opal content and organic compound composition (amino acids, pigments, lipids, and carbohydrates) were analyzed initially and at regular time intervals during the incubation period. Most particles (48–67% of total mass) sank at greater than 230 m d−1 and were dominated by large diatom‐derived aggregates produced during the spring bloom period. The initial organic composition and the biological lability of these particles varied with settling velocity. The strong phytoplankton signal was visible in all settling velocity classes, while slower settling particles carried with them a greater zooplankton and bacterial signature. As the different class particles decomposed, their compositions changed and became more similar with time, with a dominance of compounds that suggests a more degraded state: the amino acids χ‐aminobutyric acid and β‐alanine, the pigments pyropheophorbide and pheophytin, the deoxysugars fucose and rhamnose, and lipid metabolites (diglycerides and monoglycerides, alcohols, and free fatty acids). Biogenic opal in the particles dissolved faster in more degraded particles than in fresher particles, suggesting that loss of organic matter may expose opal to dissolution. The coupling of settling velocity and decomposition rate measurements shows quantitatively that slower settling particles are quickly degraded and faster settling particles retain their original biological signal to a greater degree. Greater preservation of faster settling particles testifies to the importance of these particles as food resources for bathypelagic and surface sediment communities.
Biophysicochemical process coupling controls nitrogen use by benthic biofilms
Authors: Arnon Shai , Gray Kimberly A. , Packman Aaron I.
Published: January 2011

A combination of macroscale and microscale observations was used to show that the interplay between hydrodynamic transport, local chemical conditions, and microbial metabolism controls nitrate use by benthic microbial communities. While it is usually assumed that interfacial transport and nutrient removal both increase monotonically with the velocity of the overlying flow, in laboratory flume experiments we observed substantially greater bulk nitrate removal under slower velocities (0.05 and 0.5 cm s−1) than under a faster velocity (5 cm ss−1), with the greatest rate of nitrate removal occurring under the intermediate flow condition. These results demonstrate that hydrodynamic control of solute transport, specifically here the flux of oxygen and nitrate from the water column to the benthic microbial community, causes facultative bacteria to shift between anaerobic and aerobic metabolism under different flow conditions. Aerobic metabolism is promoted by more rapid hydrodynamic transport conditions, while anaerobic metabolism is favored under low‐transport conditions. This type of coupling is expected to regulate microbial activity in all surficial sedimentary environments and must be parameterized in order to forecast long‐term average biochemical transformation rates in rivers and other dynamic aquatic systems.
Eddy correlation flux measurements: The sediment surface area that contributes to the flux
Authors: Berg Peter , Røy Hans , Wiberg Patricia L.
Published: January 2011

We investigated the size and shape of the area on the sediment surface, the so‐called footprint, that contributes to the flux in subaqueous eddy correlation measurements. Tracer tracking simulations were performed for a dissolved conservative tracer released from the sediment surface into a current‐driven flow not affected by density stratifications and surface waves. Simulations revealed that the footprint length (l) can be calculated as l = −2.783 − 158.7h + 159.2h2 − 120.8h log(z0) (all units in m) for eddy correlation measurements heights (h) between 0.05 and 0.3 m above the sediment surface and for sediment surface roughness parameter (z0) values between 7.04 × 10−6 and 0.01 m. The upstream distance (xmax) to the location that contributes the strongest flux signal can likewise be estimated as xmax = −0.09888 − 11.53h + 10.25h2 ‐ 6.650h log (z0). Because vertical turbulent mixing scales with mean current velocity, l and xmax are independent of current velocity. The footprint width (w) can be calculated as w = 6.531h. These expressions were developed for water depths (H) of H > 27h. In the depth interval 6.7h > H > 27h, l can be calculated by multiplying the length, as given above, by the factor 1 + 8.347exp(−0.2453 H/h), whereas xmax is independent of H. For H > 6.7h, the tracer transfer rate over the air‐water interface controls the size and shape of the footprint. All expressions are valid for isotropic turbulence, but as a first‐order estimate, the expressions for l and xmax also hold for anisotropic conditions. In contrast, w scales with √(Ey/Ez), where Ey and Ez are the transverse and the vertical eddy diffusivity, respectively. Finally, we describe how site‐specific values of z0 and levels of anisotropy in a turbulent near‐bottom flow can be extracted directly from eddy correlation measurements.
Extreme food webs: Foraging strategies and diets of scavenging amphipods from the ocean’s deepest 5 kilometers
Authors: Blankenship Lesley E. , Levin Lisa A.
Published: January 2011

We explore hypotheses that alternate foraging strategies, diet, or nutrient partitioning could help explain the success of scavenging Lysianassoids (Amphipoda) in hadal oligotrophic trenches (depths of 6–11 km) by examining the nutritional strategies of four lysianassoid species (Eurythenes gryllus, Scopelocheirus schellenbergi, Hirondellea dubia, and Uristes sp. nov.) collected with baited traps (6.3‐10.8 km) from the oligotrophic Tonga and Kermadec Trenches (southwest Pacific Ocean). Diets and foraging strategies were examined by use of (1) the nascent DNA‐based analysis of hindgut contents, which provides a ‘snapshot’ of recently ingested organisms, and (2) natural abundance isotopic signatures, which reflect the source of nutrition and relative trophic position. The scavenging guild exhibits remarkable trophic plasticity, and each amphipod species employs alternate foraging modes, including detrivory or predation, to supplement necrophagy. The nutritional strategies of some species appear to shift with age, depth, and even between trenches. Thus, there is no single ubiquitous hadal food web; rather it is influenced by depth and overlying surface productivity. Isotopic data suggest that coexisting species partition the dietary items, providing evidence of competition among members of the scavenging guild. The extreme foraging flexibility of scavenging amphipods may ultimately contribute to their success in severely foodlimited hadal ecosystems.
Oxygen as a control on sea floor biological communities and their roles in sedimentary carbon cycling
Authors: Woulds Clare , Cowie Greg L. , Levin Lisa A. , Andersson Johan H. , Middelburg Jack J. , Vandewiele Sandra , Lamont Peter A. , Larkin Kate E. , Gooday Andrew J. , Schumacher Stefanie , Whitcraft Christine , Jeffreys Rachel M. , Schwartz Matthew
Published: January 2011

13C tracer experiments were conducted at sites spanning the steep oxygen, organic matter, and biological community gradients across the Arabian Sea oxygen minimum zone, in order to quantify the role that benthic fauna play in the short‐term processing of organic matter (OM) and to determine how this varies among different environments. Metazoan macrofauna and macrofauna‐sized foraminiferans took up as much as 56 ± 13 mg of added C m−2 (685 mg C m−2 added) over 2–5 d, and at some sites this uptake was similar in magnitude to bacterial uptake and/or total respiration. Bottom‐water dissolved oxygen concentrations exerted a strong control over metazoan macrofaunal OM processing. At oxygen concentrations <7 µmol L−1 (0.16 ml L−1), metazoan macrofauna were able to take advantage of abundant OM and to dominate OM uptake, while OM processing at O2 concentrations of 5.0 µmol L−1 (0.11 ml L−1) was dominated instead by (macrofaunal) foraminiferans. This led us to propose the hypothesis that oxygen controls the relative dominance of metazoan macrofauna and foraminifera in a threshold manner, with the threshold lying between 5 and 7 µmol L−1 (0.11 to 0.16 ml L−1). Large metazoan macrofaunal biomass and high natural concentrations of OM were also associated with rapid processing of fresh OM by the benthic community. Where they were present, the polychaete Linopherus sp. and the calcareous foraminiferan Uvigerina ex gr. semiornata, dominated the uptake of OM above and below, respectively, the proposed threshold concentrations of bottom‐water oxygen.
Early diagenesis and trophic role of extracellular DNA in different benthic ecosystems
Authors: Corinaldesi Cinzia , Dell’Anno Antonio , Danovaro Roberto
Published: January 2011

To provide new insights into the early diagenesis of extracellular deoxyribonucleic acid (DNA) and its relevance in trophodynamic processes, we collected sediment samples from a variety of coastal and deep‐sea sites. Turnover times of extracellular DNA were approximately threefold shorter in coastal benthic systems than in deep‐sea sediments (0.35 yr and 1.2 yr, respectively). This was due to the higher concentrations of bioavailable extracellular DNA in deep‐sea over coastal sediments (19.8 ± 0.6 and 9.8 ± 2.6 mg DNA g−1, respectively), since deoxyribonuclease (DNase) activities did not show significant differences between sampling sites (135.8 ± 72.7 and 62.8 ± 19.0 ng DNA g−1 d−1, in coastal and deep‐sea sediments, respectively). The coastal benthic systems were characterized by DNase activities per prokaryotic cell significantly lower than those of their deep‐sea counterparts (0.12 ± 0.05 and 0.27 ± 0.06 fg DNA degraded cell−1 d−1, respectively), although carbon production per cell was not significantly different (6.9 ± 4.4 and 2.6 ± 0.9 fg C cell−1 d−1, respectively). These results suggest that DNase activity per cell may be more dependent on the amount of bioavailable substrate rather than on prokaryotic metabolism. Extracellular DNA may supply 20% and 47% of the daily prokaryote anabolic requirements for organic phosphorous in both coastal and deep‐sea sediments, respectively. Overall, our data suggest that bioavailable extracellular DNA might represent an important nutrient source for benthic heterotrophic metabolism and open new perspectives for a better understanding of the factors that influence the functioning of benthic systems.
Nitrogen spiraling in streams: Comparisons between stable isotope tracer and nutrient addition experiments
Authors: Earl Stevan R. , Valett H. Maurice , Webster Jackson R.
Published: January 2011

A common method to quantify stream nutrient uptake length is to enrich the stream at an upstream point and monitor the downstream decline in the concentration of the added nutrient. However, increasing the nutrient concentration of the stream alters nutrient uptake, and uptake length quantified using this amendment approach does not necessarily reflect uptake length under ambient conditions. We conducted a series of 15NO3− tracer and 14NO3− nutrient amendment experiments to (1) assess the overestimation of amendment‐derived uptake length in streams spanning a gradient of ambient nitrogen concentration, and (2) evaluate a technique to estimate ambient uptake length using data from multiple amendments conducted at different concentrations. Single amendment‐derived uptake lengths were consistently longer than ambient uptake lengths quantified using the stableisotope tracer. The ratio of single amendment‐derived to ambient uptake length ranged more than sixfold across streams but was not related to ambient stream water nitrogen concentration or amendment concentration. Estimated ambient uptake length calculated from the multiple amendment technique was a better predictor of ambient uptake length than single amendment‐derived uptake length in three of five experiments. However, with this technique, calculated uptake length was negative in two streams. Our results suggest that nitrogen limitation was weak in these two streams and that the multiple amendment technique may be ineffective in streams where the nutrient of study is not limiting.
Biochemical limitation of resting egg production in Daphnia
Authors: Abrusán György , Fink Patrick , Lampert Winfried
Published: January 2011

Many planktonic invertebrates can reproduce by immediately developing subitaneous eggs, which allow fast reproduction, and by resting eggs, which enable the survival of eggs in unfavorable environmental conditions. Since resting eggs can stay viable for decades or even centuries, they are likely to be richer in essential, easily degrading biochemicals than subitaneous eggs. Using the freshwater cladoceran Daphnia pulicaria we test whether the switch between these two qualitatively different reproductive modes is influenced by the availability of essential polyunsaturated fatty acids (PUFAs) in the food. We show that (1) when raised on a PUFA‐poor diet, resting eggs of D. pulicaria contain much more PUFAs than subitaneous eggs, and unlike subitaneous eggs, contain considerable amounts of eicosapentaenoic acid (EPA), which was not present in their diet; (2) supplementation with EPA or using EPA‐rich algae as food results in dramatically increased resting egg production; (3) when resting eggs are induced by starvation, their increased frequency in the presence of EPA‐rich food can be entirely explained by maternal effects. Since dormancy is a ubiquitous phenomenon among invertebrates, our result that its onset can be severely limited by the availability of essential biochemicals may hold for a variety of taxa.
Ecological stoichiometry in benthic food webs: Effect of light and nutrients on periphyton food quantity and quality in lakes
Authors: Qin Peibing , Mayer Christine M. , Schulz Kimberly L. , Ji Xinli , Ritchie Mark E.
Published: January 2011

To quantify the effect of light and nutrients on periphyton food quantity and quality, a series of experiments, consisting of full light and shade treatments, was conducted in 2003 in 11 lakes differing in nutrient status and turbidity. Periphyton chlorophyll a, C : P, and benthic macroinvertebrate biomass were measured on rocks or artificial substrates (glass slides) or both. Periphyton biomass increased and periphyton C: P decreased as total phosphorus increased. Periphyton biomass and C: P were higher in light than in shade. Benthic macroinvertebrate biomass was higher with high periphyton biomass and low periphyton C : P. Both turbidity and nutrient concentrations in natural water bodies are altered by human‐driven and natural processes; therefore we expect periphyton biomass and C: P to increase in lakes as clarity increases and nutrient content decreases, resulting in high‐quantity but low‐quality benthic food.
Effect of salt exclusion from lake ice on seasonal circulation
Authors: Pieters Roger , Lawrence Gregory A.
Published: January 2011

We describe the circulation of Tailings Lake, located 220 km north of Yellowknife, Northwest Territories, Canada. From Jul 2004 to Sep 2007, temperature and conductivity profiles were collected together with data from three moored temperature chains and a meteorological station. The salt content of the lake was elevated (salinity 1 g L−1) relative to surrounding natural waters (~0.1 g L−1). Winter ice cover included 60–80 cm of transparent "black" ice, which excluded up to 99% of the salt from the water as the ice formed. During winter, the salt excluded from the ice was mixed throughout the water column under the ice. In spring, ice melt and snow runoff resulted in a freshwater cap; the density contrast between this fresh epilimnion and the more saline hypolimnion was sufficient to inhibit spring turnover. In summer, the epilimnion was deepened by storms, mixing anoxic water from the hypolimnion into the surface layer. Because spring turnover was suppressed, oxygen replenishment of the hypolimnion occurred only in fall. Although Tailings Lake has been subject to numerous anthropogenic changes, similar effects can be expected in other natural and manmade water bodies with elevated salinity and significant ice cover.
Mercury dynamics and transport in two Adirondack Lakes
Authors: Selvendiran Pranesh , Driscoll Charles T. , Montesdeoca Mario R. , Choi Hyun-Deok , Holsen Thomas M.
Published: January 2011

We investigated the chemistry and fluxes of mercury (Hg) species for two Adirondack lakes with contrasting landscape, physical, and chemical attributes. Samples were collected monthly at the lake inlets and outlets over 2 yr, and evasion of elemental Hg was measured at one lake (Arbutus). Concentrations of total Hg (THg) and methyl Hg (MeHg) in watershed runoff were highly variable throughout the year, increasing markedly during summer months. The proportion of THg as MeHg ranged from <5% at Arbutus Lake up to 10% at Sunday Lake. Concentrations of Hg species increased with increases in dissolved organic carbon concentrations and percentage wetland coverage. The estimated rate of volatilization loss of elemental mercury (Hgo) from Arbutus Lake surface (7.8 µg m−2 yr−1) was comparable to the rate of direct atmospheric Hg deposition to the lake surface (8.6 µg m−2 yr−1). Dry deposition to Arbutus Lake amounted to ca. 26% of total wet THg deposition. Hydraulic residence time (HRT) played a critical role in controlling Hg dynamics in the lakes. Negligible removal of THg inputs was observed at Sunday Lake, which has a short HRT (0.02 yr), compared with approximately 60% removal at Arbutus Lake, which has a longer HRT (0.6 yr). This pattern of increasing net Hg retention in lakes with increasing HRT was consistent with other mass balance studies in the literature. Both Arbutus and Sunday lakes were net sinks of MeHg.
Mineralization pathways in lake sediments with different oxygen and organic carbon supply
Authors: Maerki Martin , Müller Beat , Dinkel Christian , Wehrli Bernhard
Published: January 2011

The intensity and pathways of mineralization of sedimentary organic matter were investigated in eutrophic Lake Zug, Switzerland. In a depth transect (25–180 m) from oxic to anoxic bottom water we recorded in situ sediment pore‐water concentration profiles of O2, NO3−, and NH4+ with a benthic lander system equipped with both oxygen and ion‐selective electrodes. Anaerobic sedimentary mineralization ranged from 13.1 to 34.9 mmol carbon (C) m−2 d−1 and increased linearly with water depth, as determined from the NH4+ flux rates in the anoxic pore water and the molar C: nitrogen (N) ratio of the surface sediment. A parallel increase of the total organic carbon concentration of the sediment was attributed to lateral transfer of resuspended sedimentary matter. Denitrification was estimated from nitrate profiles and contributed only 1.5‐3.2% to the total organic carbon mineralization at any water depth. Aerobic respiration and oxidation of reduced compounds were calculated from O2 microprofiles and pore‐water data of dissolved Mn(II), Fe(II), S(‐II), and CH4. When the O2 concentration exceeded 0.15 mmol L−1 in the sediment overlying water, 41–58%, or 12.4–18.1 mmol C m–2 d−1, was mineralized aerobically, whereas at lower concentrations (<0.04 mmol L−1), >92% of organic carbon was mineralized anaerobically. Total benthic mineralization of organic carbon was 26.9–34.9 mmol C m−2 d−1. A budget including particulate as well as dissolved reduced compounds in the sediment indicated that >95% of the anaerobic mineralization was due to methanogenesis. Oxidation of CH4 consumed 39‐56% of the O2 at the sediment‐water interface. Oxygen exposure times for these sediments were estimated to be on the order of weeks to months. These time spans are too short to change the reactivity spectrum of sedimentary organic matter.
Carbon cycling within epilithic biofilm communities across a nutrient gradient of headwater streams
Authors: Lyon David R. , Ziegler Susan E.
Published: January 2011

Primary production (PP), nutrient uptake, and carbon (C) cycling within epilithic biofilm communities were assessed in four headwater streams in the Arkansas Ozarks, U.S.A., representing a hundred‐fold range in total dissolved nitrogen (TDN) and soluble reactive phosphorus concentrations. Small enclosures containing stream water and rock substrate were incubated for 8 h with 13C‐enriched dissolved inorganic C. 13C was measured in dissolved organic carbon (DOC), bulk biofilm, and individual biofilm phospholipid fatty acids (PLFA) as biomarkers for C uptake into autotrophic and heterotrophic microorganisms. Biofilm %N, chlorophyll content, C‐specific biofilm PP, N‐specific TDN uptake, and relative abundance of algal PLFA increased with the nutrient gradient represented by the four study streams. Total net DOC release as a proportion of PP was greatest in both the lowest and highest nutrient streams, while the source of released DOC varied with the nutrient gradient. In the most nutrient‐rich stream a greater fraction of DOC was comprised of C fixed during the incubation, but in the most nutrient‐poor stream released DOC was mainly derived from older biofilm C. Carbon uptake into PLFA increased with the nutrient gradient for algal but not heterotrophic bacterial biomarkers, indicating lower relative incorporation of new PP by biofilm heterotrophs in higher nutrient streams. Results suggest that tight C flow between biofilm autotrophs and heterotrophs may support biofilm nutrient retention in low‐nutrient streams while nutrient enrichment may disrupt this coupling and release more new PP as DOC with potential effects on nutrient export to downstream ecosystems.
Meiofauna versus macrofauna: Secondary production of invertebrates in a lowland chalk stream
Authors: Tod S. P. , Schmid-Araya J. M.
Published: January 2011

The whole metazoan community inhabiting macrophyte stands and gravel beds of the English chalk stream River Lambourn were sampled for 1 yr. Secondary production estimates for specific taxa of macrofauna and meiofauna, usually at species or genus level, were made using the size‐frequency method. Annual standing biomass and production to biomass ratios were also estimated. Total annual secondary production was as high as 64.99 g m−2 yr−1 in the macrophyte stands while within the gravel beds total annual secondary production was 22.55 g m−2 yr−1. Macrofauna contributed >91% to secondary production in both habitats. A corresponding standing biomass of 11.87 g m−2 in the macrophyte stands was also higher than within gravel beds of 5.13 g m−2. Macrofauna contributed >97% to standing biomass in both habitats. A large number of meiofauna and some macrofauna species displayed annual production‐to‐biomass (P : B) values in both habitats significantly higher than 9 or 10. Despite higher densities and turnover rates within the benthos of the River Lambourn, there was a minimal contribution of the meiofauna to total secondary production dominated by the macrofauna.
Characterization of stable isotope fractionation during methane production in the sediment of a eutrophic lake, Lake Dagow, Germany
Authors: Conrad Ralf , Claus Peter , Casper Peter
Published: January 2011

We measured δ13C of CO2, CH4, and acetate‐methyl in profundal sediment of eutrophic Lake Dagow by incubation experiments in the presence and absence of methanogenic inhibitors chloroform, bromoethane sulfonate (BES), and methyl fluoride, which have different specificities. Methyl fluoride predominantly inhibits acetoclastic methanogenesis and affects hydrogenotrophic methanogenesis relatively little. Optimization of methyl fluoride concentrations resulted in complete inhibition of acetoclastic methanogenesis. Methane was then exclusively produced by hydrogenotrophic methanogenesis and thus allowed determination of the fractionation factors specific for this methanogenic pathway. Acetate, which was then no longer consumed, accumulated and allowed determination of the isotopic signatures of the fermentatively produced acetate. BES and chloroform also inhibited CH4 production and resulted in accumulation of acetate. The fractionation factor for hydrogenotrophic methanogenesis exhibited variability, e.g., it changed with sediment depth. The δ13C of the methyl group of the accumulated acetate was similar to the δ13C of sedimentary organic carbon, while that of the carboxyl group was by about 12% higher. However, the δ13C of the acetate was by about 5% lower in samples with uninhibited compared with inhibited acetoclastic methanogenesis, indicating unusual isotopic fractionation. The isotope data were used for calculation of the relative contribution of hydrogenotrophic vs. acetoclastic methanogenesis to total CH4 production. Contribution of hydrogenotrophic methanogenesis increased with sediment depth from about 35% to 60%, indicating that organic matter was only partially oxidized in deeper sediment layers.
Physical factors influencing the distribution of a top predator in a subtropical oligotrophic estuary
Authors: Heithaus Michael R. , Delius Bryan K. , Wirsing Aaron J. , Dunphy-Daly Meagan M.
Published: January 2011

We used longline fishing to determine the effects of distance from the ocean, season, and short‐term variation in abiotic conditions on the abundance of juvenile bull sharks (Carcharhinus leucas) in an estuary of the Florida Everglades, U.S.A. Logistic regression revealed that young‐of‐the‐year sharks were concentrated at a protected site 20 km upstream and were present in greater abundance when dissolved oxygen (DO) levels were high. For older juvenile sharks (age 1+), DO levels had the greatest influence on catch probabilities followed by distance from the ocean; they were most likely to be caught at sites with >3.5 mg L−1 DO and on the main branch of the river 20 km upstream. Salinity had a relatively small effect on catch rates and there were no seasonal shifts in shark distribution. Our results highlight the importance of considering DO as a possible driver of top predator distributions in estuaries, even in the absence of hypoxia. In Everglades estuaries hydrological drivers that affect DO levels (e.g., groundwater discharge, modification of primary productivity through nutrient fluxes) will be important in determining shark distributions, and the effects of planned ecosystem restoration efforts on bull sharks will not simply be mediated by changing salinity regimes and the location of the oligohaline zone. More generally, variation in DO levels could structure the nature and spatiotemporal pattern of top predator effects in the coastal Everglades, and other tropical and subtropical estuaries, because of interspecific variation in reliance on DO within the top predator guild.
Zooplankton vertical migration and plasticity of pigmentation arising from simultaneous UV and predation threats
Authors: Hylander Samuel , Larsson Niklas , Hansson Lars-Anders
Published: January 2011

We assessed how zooplankton (copepods) handle the simultaneous threats of predators and ultraviolet (UV) radiation and whether they respond with changes in pigmentation, vertical migration, or both. We found weak vertical migration among copepods in response to UV stress, and this response was not apparently influenced by predation risk. Exposure to high levels of UV radiation caused copepods to retain pigments in the absence of a predation threat. When exposed to predation threat, they reduced their pigmentation regardless of UV level. Thus, they ranked predation as a threat more severe than UV radiation. Reducing the protective pigment level in response to predation in a situation in which UV radiation is high may, however, lead to higher mortality.
Ecological speciation in a cyclic parthenogen: Sexual capability of experimental hybrids between Daphnia pulex and Daphnia pulicaria
Authors: Heier Christopher R. , Dudycha Jeffry L.
Published: January 2011

Daphnia pulex and Daphnia pulicaria are cyclically parthenogenetic crustaceans that are distributed widely in North American freshwaters. Hybrids of D. pulex and D. pulicaria are common in the wild, where population genetic analyses have indicated that they are invariably obligately asexual. In this study, we characterize three sexual aspects (sexual egg production, male production, and sexual offspring production) of life history in experimentally bred hybrids to determine the degree of reproductive isolation. In striking contrast to natural hybrids, out of the 53 F1 hybrids we bred, none were obligately asexual. Neither male production nor sexual egg production was limited by hybridization. Our experimental crosses showed that there was little difficulty in generating or backcrossing the F1 generation. In addition, we were able to intercross F1s successfully to produce F2 progeny. We found no evidence that these taxa are reproductively incompatible, despite other reports of apparent adaptive genetic divergence. We conclude that reproductive isolation between D. pulex and D. pulicaria is due to ecological separation, not genetic incompatibility. In light of our results and published data on the biogeography and breeding system of these taxa, neither sterility due to hybridization nor direct inheritance of a sex‐limited meiosis suppressor appears to provide a satisfactory explanation for the asexuality of wild hybrids. We propose an alternate hypothesis in which one parental taxon invades a population of the other, the resulting F1 hybrids cross back to a carrier of a meiosis suppressor, and out of the resulting progeny, a high‐fitness asexual clone eventually displaces the rest of the population.