bluuebunny/crossref_metadata_2025 huggingface dataset:
Estimation of intergenerational drift dispersal distances and mortality risk for aquatic macroinvertebrates
Authors: Humphries Stuart , Ruxton Graeme D.
Published: July 2010

Estimates of the total lifetime dispersal capacity of individuals comprising invertebrate drift in streams and rivers have proved very difficult to determine empirically. Here we use recent data on dispersal in the amphipod Gammarus pulex L. to illustrate a method for estimating the total distance an individual is likely to travel downstream during the period between hatching and its first reproductive episode. For the system we consider, this estimate is on the order of 1.5 km. This method may be useful in future explorations of the ecological relevance of within‐stream displacement and population persistence. Furthermore, we are able for the first time to estimate an upper bound for the mortality risk associated with entering the drift, which, for the G. pulex population under consideration, is less than 1% for individual drift events. We suggest that this risk may not be nearly as high as had previously been thought. These mortality and distance estimates may illustrate a fundamental difference between dispersal in lotic systems and those in other habitats, in that the mean dispersal distance is of much higher ecological relevance than that of rare long‐distance events.
δ13C of fluvial mollusk shells (Rhône River): A proxy for dissolved inorganic carbon?
Authors: Aucour Anne-Marie , Sheppard Simon M. F. , Savoye Régine
Published: July 2010

The relationship between the δ13C of dissolved inorganic carbon (DIC) and modern mollusk aragonite from rivers was calibrated for the purpose of reconstructing DIC paleochemistry from the shell record. The δ13C values of aragonitic bivalves (Dreissena polymorpha, Corbicula fluminea), prosobranch gastropods (Bithynia tentaculata, Theodoxus fluviatilis, Viviparus viviparus), and an air‐breathing pulmonate gastropod (Limnea auricularia) were analyzed from several locations on the Rhône River (—13.7‰ to —6.0‰) and its major tributary, the Saône River (—11.4‰ to —10.2‰). The δ13CDIC varied from —11.5‰ to —7.5‰, and the δ13C of particulate inorganic matter (POM) varied from —31.7‰ to —25.4‰. At a given site, the δ13C of all species except the pulmonate were within 1‰ of each other. Whole‐shell δ13C correlated positively with δ13CDIC, with a slope close to unity. Bioaragonite—DIC fractionations were 0–1.5‰ for bivalves and 0–2.7‰ for gastropods (excluding the pulmonates). Applying these fractionations, bivalves that live in open water are a reliable proxy, monitoring the average δ13CDIC value to within its natural ~2‰ temporal variation within the growth period. For the suspension feeders (bivalves) using POM as a food source, the δ13C of whole shells and bulk POM indicated that the incorporation of carbon derived from respiratory sources lay in the range 10–30%. Fine‐scale analyses of growth increments of C. fluminea could not be related simply to δ13CDIC because metabolic and seasonal variations in δ13CDIC produced similar isotopic fluctuations (≤2.5‰).
Daphnia growth on microcystin‐producing and microcystin‐free Microcystis aeruginosa in different mixtures with the green alga Scenedesmus obliquus
Authors: Lürling Miquel
Published: July 2010

The hypothesis that negative effects of Microcystis on Daphnia growth and reproduction can be explained from the presence of microcystin in the Microcystis cells was tested by comparing the effects on Daphnia life history characteristics of a microcystin‐free mutant strain and microcystin‐containing strain of the cyanobacterium Microcystis aeruginosa PCC7806. To avoid nutritional deficiency, Microcystis was offered to Daphnia alone and in various mixtures with the high‐quality green alga Scenedesmus obliquus. In contrast to expectation, growth of Daphnia on microcystin‐free cells was not much better than growth on microcystin‐containing cells. Because nutritional insufficiency, morphology, and feeding inhibition could not explain the observed effects, the results show that Microcystis must contain substances other than microcystins that are poisonous to Daphnia.
Bioaccumulation of silver‐110m, cobalt‐60, cesium‐137, and manganese‐54 by the freshwater algae Scenedesmus obliquus and Cyclotella meneghiana and by suspended matter collected during a summer bloom event
Authors: Adam Christelle , Garnier-Laplace Jacqueline
Published: July 2010

Laboratory experiments were done to assess 110mAg, 60Co, 137Cs, and 54Mn uptake by two phytoplankton species, the chlorophyte Scenedesmus obliquus and the small diatom Cyclotella meneghiana. Mn and Co were characterized by similar uptake kinetic rates, 20–30 d−1, whatever the algal species, whereas depuration rates were 3–60 d−1. Silver uptake and depuration rates were very high (144–293 d−1). However, Cs accumulation and depuration were very slow, with kinetic constants of 0.6–5 d−1. Mn, Co, and Ag were more strongly accumulated by C. meneghiana than S. obliquus and vice versa for Cs. To evaluate the extrapolation of the kinetic rates fitted for S. obliquus and C. meneghiana to natural conditions, suspended solids were also collected during a bloom event and contaminated. Radionuclide exchange between three distinct compartments among the suspended solids was modeled: the kinetic rates fitted for S. obliquus and C. meneghiana were used to represent chlorophyte and bacillariophyte contamination, whereas kinetic rates describing a third compartment were estimated when possible. A third compartment was evidenced only for Mn and Co, whereas, for Ag, the chlorophyte and bacillariophyte compartments were sufficient to describe the particulate phase. For Cs, algae kinetic rates could not be used, so a single compartment was fitted. These experiments confirm the low affinity of Cs for phytoplankton and the high bioavailability of Ag. In the case of Co and Mn, several processes acting simultaneously govern the contamination of natural suspended solids.
Speciation and partitioning of cadmium and zinc in two contrasting estuaries:The role of hydrophobic organic matter
Authors: Turner Andrew , Le Roux Sophie M. , Millward Geoffrey E.
Published: July 2010

The hydrophobicity and particleߚwater interactions of cadmium (Cd) and zinc (Zn) in an “organicߚpoor“ estuary (Plym, United Kingdom) and an “organicߚrich“ estuary (Beaulieu, United Kingdom) have been investigated using radiotracers. The hydrophobic fraction of dissolved Cd (retained by a reverseߚphase C18 column) was about 50% in the River Beaulieu and less than 10% in the River Plym, and in both cases this fraction decreased rapidly with increasing salinity, such that only a few percent was hydrophobic at salinities above 10. Hydrophobicity of Cd was unaffected by the addition of suspended sediment particles, indicating that hydrophobic and hydrophilic forms adsorb in accordance with their relative abundance or that rapid reequilibration of dissolved Cd species occurs after specific forms have undergone adsorption. In the Plym, a logarithmic, salinityߚdependent reduction in the sedimentߚwater distribution coefficient, KD, and a high proportion of adsorbed Cd bound exchangeably indicate that particleߚwater interactions of Cd in environments of moderate concentrations of organic matter are governed by inorganic processes. In the Beaulieu, more uniform KDs and a significantly lower exchangeability of adsorbed Cd indicate that organic interactions might also be important. In both estuaries, the hydrophobic fraction of dissolved Zn was between about 80% and 100% and displayed an increase with increasing salinity and a reduction in the presence of suspended sediment particles. Throughout the Beaulieu, and in the Plym above salinities of about 4, KD also increased with increasing salinity. These observations were interpreted and modeled in terms of the preferential uptake and salting out of hydrophobic complexes of Zn and indicate that, regardless of the concentration of organic matter, the speciation and uptake of Zn in many estuaries might be strongly influenced by hydrophobic interactions.
Anaerobic N2 production in Arctic sea ice
Authors: Rysgaard Søren , Glud Ronnie Nøhr
Published: July 2010

We quantified anaerobic N2 production through bacterial denitrification and anaerobic NH4+ oxidation (anammox) in first‐year ice from Young Sound (74°N) and in an ice floe off Northeast Greenland (79°N). Bacterial denitrification activity (100–300 nmol N L−1 sea ice d−1) occurred in the lower 0.5 m of the sea ice, which had high concentrations of NO3−, NH4+, and dissolved organic carbon (DOC). Despite sea‐ice algal production in the lower sea‐ice layers, heterotrophic activity resulted in a net O2 consumption of 13 µmol O2 L−1 sea ice d−1 in the lower 0.5‐m ice layers. Together with melting of deoxygenated ice crystals, this led to anoxic conditions in the brine system favoring conditions for anaerobic NO3− reduction. Numbers of anaerobic NO3−‐reducing bacteria in the same ice layers were high (1.1 × 105 cells ml−1 sea ice, corresponding to 1.2 × 106 cells ml−1 brine). Area‐integrated denitrification rates were 10–45 µmol N m−2 sea ice d−1, which corresponds to 7–50% of the sediment activity in the area. Although the proportion of anammox to total N2 production was up to 19% in layers of the ice floe from the Greenland Sea, the integrated rate only accounted for 0–5% of total NO3− reduction at the investigated localities.
Spring phytoplankton bloom dynamics in Norwegian coastal waters: Microbial community succession and diversity
Authors: Larsen Aud , Flaten Gro A. Fonnes , Sandaa Ruth-Anne , Castberg Tonje , Thyrhaug Runar , Erga Svein Rune , Jacquet Stéphan , Bratbak Gunnar
Published: July 2010

Most studies of spring bloom succession in Norwegian waters have employed light microscopy and accounted for species composition of phyto‐ and zooplankton. Flow cytometry and molecular tools enable us to extend such investigations to include smaller organisms like bacterio‐ and virioplankton. Here, we describe succession and diversity of algae, bacteria, and viruses in relation to environmental changes from 15 February to 27 April. The spring succession started with an increase in autotrophic picoeukaryotes and Synechococcus sp. The diatoms bloomed around the middle of March and caused nutrient depletion in the upper part of the water column. Upwelling in the beginning of April gave rise to a second bloom, consisting of diatoms and Phaeocystis pouchetii. Numerically, autotrophic picoeukaryotes and Synechococcus sp. dominated the periods between and after these two major blooms. Heterotrophic bacterial abundance increased throughout the experimental period and reached peak values during and after phytoplankton blooms. These bacteria were succeeded by viruses having low DNA fluorescence, whereas viruses with medium DNA fluorescence bloomed during or after blooms of autotrophic picoeukaryotes. High—DNA fluorescence viruses reached maximum concentrations during and after the diatom and Phaeocystis blooms. The diversity of the bacterial community remained relatively stable, whereas viral diversity varied more and increased after major phytoplankton blooms. Our investigation thus demonstrates how virioplankton are important elements of the total microbial diversity and how they are intimately linked to the rest of the microbial community and possibly act as an internal driving force in spring bloom successions.
Surface avoidance by freshwater zooplankton: Field evidence on the role of ultraviolet radiation
Authors: Alonso Cristina , Rocco Vanina , Barriga Juan Pablo , Battini Miguel Ángel , Zagarese Horacio
Published: July 2010

The avoidance of surface waters by crustacean zooplankton is a common phenomenon both in marine and freshwaters. The contemporary paradigm interprets such behavior as an antipredator strategy. However, the phenomenon has also been reported in predator‐free environments, which suggests that other variables may contribute to this depth‐selection behavior. We investigated the possibility that ultraviolet radiation (UVR) could influence the avoidance of surface waters in three interconnected lakes differing in their transparency. Each lake was sampled 17 or 18 times. In the two most transparent lakes, the percentage of the population occurring in the surface layer decreased during spring and summer, but this pattern was not observed in the less transparent lake. Such differences in the patterns of surface‐water avoidance were unrelated to the abundance and vertical distribution of the dominant planktivores (early stages of galaxiid fishes) as well as to temperature and food availability. They were inversely related, however, to the average UVR levels within the surface layer. A UV avoidance strategy was predicted for the copepod Boeckella gracilipes, on account of its low UV tolerance and lack of photoprotective compounds. Our results support this prediction and strongly suggest that surface avoidance by freshwater Patagonian zooplankton is not just a byproduct benefit of an antipredator behavior but is a direct response to high UVR levels.
In situ pCO2 and O2 measurements in a lake during turnover and stratification: Observations and modeling
Authors: Baehr Matthew M. , DeGrandpre Michael D.
Published: July 2010

Sensors for the partial pressure of CO2 (pCO2) and dissolved O2 (DO) were deployed near the surface and bottom of a freshwater lake (Placid Lake, Montana) during the period from ice cover to seasonal stratification. Sources of variability were examined using one‐dimensional physical and biogeochemical models. Model predictions for pCO2 and DO were compared to further constrain model parameters. A number of transient processes were documented that have not been well characterized in previous studies. The models made it possible to link these short‐term events to specific forcings. We found that (1) 11 d of the 13‐d turnover period occurred under ice through lightdriven convective mixing, (2) phytoplankton biomass increased to its highest seasonal level under ice, (3) weak stratification set up immediately after ice‐out, causing bottom water pCO2 and DO to diverge from surface levels, (4) subsequent diel convective mixing brought bottom pCO2 and DO back toward surface levels, and (5) before stable stratification, vertical entrainment of CO2‐rich water, net production, and air‐water exchange drove 100‐200 µatm daily changes in pCO2, but, because of their counterbalancing effects, surface pCO2 remained >1,000 µatm for nearly 1 month after ice‐out. Upon stable stratification, net production and air‐water exchange overcame pCO2 gains from mixing and heating and reduced pCO2 to near atmospheric levels within 20 d. Net production and gas exchange accounted for ~75% and 25%, respectively, of the decrease in surface pCO2 observed after ice‐out. Diel convection was the dominant mixing process both under ice and after ice‐out and may be an important underrepresented mechanism for CO2 and DO exchange between surface and bottom water.
The continuity and intensity of ultraviolet irradiation affect the kinetics of biosynthesis, accumulation, and conversion of mycosporine‐like amino acids (MAAs) in the coral Stylophora pistillata
Authors: Shick J. Malcolm
Published: July 2010

Colonies of Stylophora pistillata unexposed to ultraviolet radiation (UVR) in laboratory aquaria contained minimal amounts of mycosporine‐like amino acids (MAAs). The concentration of MAAs increased rapidly during exposure to broadband UVR, then stabilized between 14 and 28 d of exposure. The four MAAs known to be produced by diverse zooxanthellae (Symbiodinium spp.) in culture were the first to increase in coral colonies, followed by six additional MAAs. Decreases in these four primary MAAs between days 14 and 28 were balanced by stoichiometric increases in the six secondary MAAs, suggesting a precursor‐product relationship. Discontinuing UV exposure caused rapid declines in the concentrations of primary MAAs, while secondary MAAs continued to increase, likely a manifestation of different rates of turnover of the putative enzymes involved in the de novo biosynthesis of primary MAAs and their conversion to secondary MAAs. The continued production of certain MAAs after UV exposure ceased indicates that UV is not required catalytically but is a signal that induces the enzymes of the biosynthetic pathway. Biochemically inhibiting the biosynthesis of primary MAAs after a pool of these MAAs had accumulated did not affect their conversion to secondary MAAs. Doubling the UV irradiance increased the rate of accumulation, first, of primary MAAs and, later, of secondary MAAs. Under normal UV irradiance, the eventual decline in the accumulation rate of primary MAAs was quantitatively explained by their conversion to secondary MAAs. Under doubled UV irradiance, where MAA concentrations were higher, the deceleration of accumulation of primary MAAs was twice as great as the acceleration in secondary MAAs, suggesting a regulation of the de novo biosynthesis of MAAs, perhaps by their own progressive, concentration‐dependent attenuation of UVR.
Virus production and life strategies in aquatic sediments
Authors: Mei M. L. , Danovaro R.
Published: July 2010

Aquatic sediments host much of the bacterial biomass and biodiversity and play a key role in biogeochemical cycles. However, the potential effect of viral infection remains unknown. We present estimates of virus production in a variety of benthic habitats of the Mediterranean Sea, characterized by different contamination levels and trophic states. Viriobenthos abundance in aquatic sediments ranged 108−109 ml−1 of sediment and was ~20 times higher than virioplankton abundance. Vertical profiles in sediment cores revealed large virus numbers at 1−m depth below the sediment surface. Virus production in both marine and freshwater sediments was 1–2 orders of magnitude higher than typical values reported for virioplankton (0.13–1.60 x 108 virus‐like particles ml−1 sediment h−1), which indicates that virus turnover and infection rates can be higher in aquatic sediments than in the water column. Eutrophic and contaminated sediments displayed the highest virus production rates. Virus production was significantly correlated with the abundance of active bacteria and with bacterial cell production. We estimated an average burst size (BS) of 15–18 in marine and freshwater sediments. Viru‐mediated bacterial mortality was high (on average, ≫40% of bacterial production) and increased from surface to deeper sediment levels, down to 100−cm depth. The fraction of bacteria with lysogenic infection (range, 0.0–1.8%) increased in deeper sediment layers (3.3% at 100−cm depth). Our results suggest that high benthic virus production rates can have a significant effect on benthic bacterial dynamics and indicate that virus production should be included in biogeochemical models of aquatic sediments.
Ecological, morphological, and genetic differentiation of Daphnia (Hyalodaphnia) from the Finnish and Russian subarctic
Authors: Schwenk1 Klaus , Junttila Paavo , Rautio Milla , Bastiansen Finn , Knapp Jürgen , Dove Outi , Billiones Raquel , Streit Bruno
Published: July 2010

We studied genetic differentiation of two subarctic Daphnia species (subgenus Hyalodaphnia; Cladocera: Anomopoda) in relation to ecological and morphological diversification. Daphnia longispina and the recently discovered species Daphnia umbra are genetically differentiated based on mitochondrial 12S rDNA and internal transcribed spacer (ITS) regions. Genetic differentiation of 12S rDNA among the two sister taxa is in the range of differentiation among other Hyalodaphnia species (uncorrected genetic distance = 0.11). Despite frequent interspecific hybridization among Daphnia (Hyalodaphnia) species, we found no interspecific hybrids of D. umbra and D. longispina. D. umbra is for the first time recorded to occur in Northern Finland and Russia (Pechora Delta), and to cooccur in neighboring sympatry with Daphnia longispina across a subarctic region in northern Finland. Species are ecologically differentiated: D. umbra occurred at higher elevations, in larger and deeper water bodies than D. longispina. Species did not differ significantly in levels of ultraviolet‐protective melanin pigmentation but varied with regard to environmental preferences, such as fish predation and levels of total dissolved organic carbon (DOC). These findings argue that ecological differentiation and divergent selection might have caused speciation or at least are responsible for the maintenance of reproductive isolation among subarctic Daphnia (Hyalodaphnia) species.
Climatic forcing and primary productivity in a subalpine lake: Interannual variability as a natural experiment
Authors: Park Sangkyu , Brett Michael T. , Müller-Solger Anke , Goldman Charles R.
Published: July 2010

We analyzed a 42‐yr record of primary productivity in small, subalpine Castle Lake to determine how climatic variability might influence lake primary productivity. A Pacific Decadal Oscillation (PDO) polarity reversal in 1977 significantly affected winter air and summer water temperatures in Castle Lake. The timing of lake ice‐out was explained by spring air temperature and winter total precipitation (r2 = 0.72) and significantly affected water temperature (r2 = 0.74). Primary productivity was negatively correlated with ice‐out date and positively correlated with primary productivity during the previous year (r2 = 0.47). Alternatively, primary productivity was positively correlated with water temperature and primary productivity during the previous year (r2 = 0.49). Ammonium availability immediately after ice‐out was significantly related to primary productivity from the previous and the current year, suggesting that nutrient availability is an important mechanism for the serial correlation. Daphnia and cyanobacteria biomass also increased during warmer years. Our results suggest that variability in air temperature and precipitation from global warming, PDO, and the El Niño Southern Oscillation (ENSO) influence primary productivity and plankton communities in North American dimictic lakes.
Relevant scales in zooplankton ecology: Distribution, feeding, and reproduction of the copepod Acartia hudsonica in response to thin layers of the diatom Skeletonema costatum
Authors: Bochdansky Alexander B. , Bollens Stephen M.
Published: July 2010

We investigated the interaction of the copepod Acartia hudsonica in relation to thin layers of the diatom Skeletonema costatum. Thin layers have recently received much attention, since they are common and persistent features in the water column, often overlooked by traditional sampling methods. Their frequent abundance in coastal oceans and the high biomass associated with them has led to the assumption that they are important grazing sites of calanoid copepods. We employed 2‐m tall tower tanks that allowed us to simulate thin layers. Three variables representative of three time scales were considered: the distribution of copepods in the tanks (time scale of minutes), fecal pellet production as a proxy for ingestion rate (time scale of hours), and egg production rate (time scale of >12 h). A. hudsonica responded significantly but very little to the thin layers in terms of their distribution. Given a choice, there was a slightly higher tendency to swim through a patch of diatoms than to swim around it. Fecal pellet production was slightly lower in the thin‐layer treatments than in the homogeneous controls. Egg production was not influenced by differential distribution of diatoms in the tanks, which indicated that the copepods dealt equally well with patchy food as when the same numbers of cells were available in a homogeneous distribution. Time series experiments showed that ingested carbon is integrated over time scales of >12 h. Therefore, small scale fluctuations of food in space and time do not necessarily translate into small‐scale fluctuations in reproductive output.
Interannual variability in abundance of North Sea jellyfish and links to the North Atlantic Oscillation
Authors: Lynam Christopher P. , Hay Stephen J. , Brierley Andrew S.
Published: July 2010

Pronounced interannual variability in the abundance of medusae of the jellyfish species Aurelia aurita, Cyanea lamarckii, and Cyanea capillata (Phylum Cnidaria, Class Scyphozoa) in the North Sea was evident in data arising from the International Council for the Exploration of the Seas International 0‐group Gadoid Surveys between 1971 and 1986. Possible climatic forcing of jellyfish abundance, via the North Atlantic Oscillation (NAO), was investigated with data on medusae from four areas of the North Sea (east of Scotland, north of Scotland, east of Shetland, and west of northern Denmark). There were significant inverse relationships between medusa abundance and the NAO Index (December‐March) in two regions: west of northern Denmark (A. aurita r2 = 0.70, P = 0.003, n = 10; C. lamarckii r2 = 0.74, P = 0.002, n = 10) and east of Scotland (A. aurita r2 = 0.53, P = 0.008, n = 12). Fluctuations in the abundance of A. aurita and C. lamarckii medusae might be linked to hydroclimatic changes induced through atmospheric effects (as encapsulated in the NAO Index) on wind stress, temperature, and currents. These fundamental hydroclimatic changes alter the timing of spring phytoplankton blooms and zooplankton community composition. Predation by an abundance of medusae on zooplankton and ichthyoplankton could affect the North Sea ecosystem through top‐down and bottom‐up mechanisms. Because the NAO is presently in a high phase, climatic conditions could be serving to depress the abundance of medusae: a future reversal of the NAO might favor jellyfish and weaken the persistence or recovery of fisheries.
Transmission of oxygen concentration fluctuations through the diffusive boundary layer overlying aquatic sediments
Authors: Røy Hans , Huettel Markus , Jørgensen Bo Barker
Published: July 2010

Even though the O2 concentration in the diffusive boundary layer overlying natural aquatic sediments usually varies over time, microprofiles with little or no error bars are abundant in the literature. These temporal concentration fluctuations are not accounted for in the conceptual models used to describe the diffusive boundary layer, and their effects on phenomena investigated through microprofile measurements are unclear. This paper focuses on concentration fluctuations in the diffusive boundary layer above a marine sediment. One microsensor was inserted into a laboratory flume from below and positioned with the sensing tip precisely at the sediment surface. A second microelectrode was deployed from above through the flowing water column and positioned within the diffusive boundary layer directly above the tip of the lower sensor. Time series of the fluctuating O2 concentration at the two points were measured simultaneously with the two sensors and demonstrated a tight coupling of concentration fluctuations across the diffusive boundary layer. A dynamic model accounting only for molecular diffusion plus zero‐order O2 consumption within the sediment explained the observed coupling through the diffusive boundary layer and enabled calculation of instantaneous O2 profiles from the top of the true diffusive boundary layer to the depth of O2 penetration. Model results confirmed that concentration fluctuations were linked from the top of the true diffusive boundary layer down to about 0.5 mm sediment depth. The transient profiles moved back and forth, without losing the general shape derived from the averaged steady state O2 distribution. Apart from this description of the temporal structure of the diffusive boundary layer, the model shows that concentration fluctuations in the layer do not necessarily indicate turbulence within the true diffusive boundary layer. This dynamic description is used to discuss experimental procedures when measuring microprofiles.
Reach‐scale isotope tracer experiment to quantify denitrification and related processes in a nitrate‐rich stream, midcontinent United States
Authors: Böhlke John Karl , Harvey Judson W. , Voytek Mary A.
Published: July 2010

We conducted an in‐stream tracer experiment with Br and 15N‐enriched NO3− to determine the rates of denitrification and related processes in a gaining NO3− ‐rich stream in an agricultural watershed in the upper Mississippi basin in September 2001. We determined reach‐averaged rates of N fluxes and reactions from isotopic analyses of NO3−, NO2−, N2, and suspended particulate N in conjunction with other data in a 1.2‐km reach by using a forward time‐stepping numerical simulation that included groundwater discharge, denitrification, nitrification, assimilation, and air‐water gas exchange with changing temperature. Denitrification was indicated by a systematic downstream increase in the δ15N values of dissolved N2. The reach‐averaged rate of denitrification of surface‐water NO3− indicated by the isotope tracer was approximately 120 ± 20 µmol m−2 h−1 (corresponding to zero‐ and first‐order rate constants of 0.63 µmol L−1 h−1 and 0.009 h−1, respectively). The overall rate of NO3− loss by processes other than denitrification (between 0 and about 200 µmol m−2 h−1) probably was less than the denitrification rate but had a large relative uncertainty because the NO3− load was large and was increasing through the reach. The rates of denitrification and other losses would have been sufficient to reduce the stream NO3− load substantially in the absence of NO3− sources, but the losses were more than offset by nitrification and groundwater NO3− inputs at a combined rate of about 500‐700 µmol m−2 h−1. Despite the importance of denitrification, the overall mass fluxes of N2 were dominated by discharge of denitrified groundwater and air‐water gas exchange in response to changing temperature, whereas the flux of N2 attributed to denitrification was relatively small. The in‐stream isotope tracer experiment provided a sensitive direct reach‐scale measurement of denitrification and related processes in a NO3− ‐rich stream where other mass‐balance methods were not suitable because of insufficient sensitivity or offsetting sources and sinks. Despite the increasing NO3− load in the experimental reach, the isotope tracer data indicate that denitrification was a substantial permanent sink for N leaving this agricultural watershed during low‐flow conditions.
Carbon‐nitrogen coupling and algal‐bacterial interactions during an experimental bloom: Modeling a 13C tracer experiment
Authors: Van den Meersche Karel , Middelburg Jack J. , Soetaert Karline , van Rijswijk Pieter , Boschker Henricus T. S. , Heip Carlo H. R.
Published: July 2010

We tracked flows of carbon and nitrogen during an experimental phytoplankton bloom in a natural estuarine assemblage in Randers Fjord, Denmark. We used 13C‐labeled dissolved inorganic carbon to trace the transfer of carbon from phytoplankton to bacteria. Ecosystem development was followed over a period of 9 d through changes in the stocks of inorganic nutrients, pigments, particulate organic carbon and nitrogen, dissolved organic carbon (DOC), and algal and bacterial polar‐lipid‐derived fatty acids (PLFA). We quantified the incorporation of 13C in phytoplankton and bacterial biomass by carbon isotope analysis of specific PLFA. A dynamic model based on unbalanced algal growth and balanced growth of bacteria and zooplankton adequately reproduced the observations and provided an integral view of carbon and nitrogen dynamics. There were three phases with distinct carbon and nitrogen dynamics. During the first period, nutrients were replete, an algal bloom was observed, and carbon and nitrogen uptake occurred at a constant ratio. Because there was little algal exudation of DOC, transfer of 13C from phytoplankton to bacteria was delayed by 1 d, compared with the labeling of phytoplankton. In the second phase, the exhaustion of dissolved inorganic nitrogen resulted in decoupling of carbon and nitrogen flows caused by unbalanced algal growth and the exudation of carbon‐rich dissolved organic matter by phytoplankton. During the final, nutrient‐depleted phase, carbon and nitrogen cycling were dominated by the microbial loop and there was accumulation of DOC. The main source (60%) of DOC was exudation by phytoplankton growing under nitrogen limitation. Heterotrophic processes were the main source of dissolved organic nitrogen (94%). Most of the carbon exudated by algae was respired by the bacteria and did not pass to higher trophic levels. The dynamic model successfully reproduced the evolution of trophic pathways during the transition from nutrient‐replete to ‐depleted conditions, which indicates that simple models provide a powerful tool to study the response of pelagic ecosystems to external forcings.
The timing of benthic copepod emergence
Authors: Teasdale Michael , Vopel Kay , Thistle David
Published: July 2010

We investigated emergence of marine benthic harpacticoid and cyclopoid copepods from subtidal sediment into the bottom water. Previous studies that used traps in the field found that copepods emerged from the sediment within 2 h of dusk. We examined emergence in 20‐min sampling periods over 13 h from 1200 to 0100 h in a laboratory flume under conditions of constant flow speed. The light intensity was adjusted to field conditions every 20 min. We observed a peak in emergence during the 20‐min sampling period after the onset of darkness, greatly increasing the accuracy with which the timing of emergence is known. This result will help to set the stage for studies of emergence cues.
Effects of increased bedload on algal‐ and detrital‐based stream food webs: Experimental manipulation of sediment and macroconsumers
Authors: Schofield Kate A. , Pringle Catherine M. , Meyer Judy L.
Published: July 2010

Sedimentation poses a significant threat to stream ecosystems throughout the world, and increases in bedload can be especially detrimental to benthic communities. To examine how increased bedload affects algal‐ and detrital based stream communities, we manipulated sediment (via daily sediment addition to 0.25‐m2 areas of stream bottom) and top‐down effects of macroconsumers (fishes and crayfish, via electric exclusion) in two factorial experiments, one using tiles and one using leaf packs as sampling substrates. Sediment addition had significant effects on both algal‐ and detrital‐based stream benthic communities, most notably via the alteration of macroconsumer‐mediated biotic interactions; these effects largely were due to sediment transport across the stream bottom, rather than sediment deposition. In the tile experiment, macroconsumers reduced total insect biomass and biomass of several dominant insect taxa under ambient sediment conditions. Sediment addition eliminated all macroconsumer effects except their reduction of chironomid biomass. In the leaf pack experiment, sediment addition eliminated macroconsumer effects on fungal accumulation rates; in general, however, leaf packs were not as affected by sediment addition as tiles. Direct effects of sediment addition were minimal in both experiments: algal composition was altered on tiles, while dipteran predator biomass tended to decline in leaf packs. These experiments demonstrate that small, environmentally realistic increases in bedload affect benthic communities, primarily via the alteration of macroconsumer effects. Although indirect effects of sedimentation have been examined less frequently than direct biotic responses, this study demonstrates the importance of sediment regime in determining the outcome of fish and crayfish‐mediated species interactions.
Diurnal cycling of glutathione in marine phytoplankton: Field and culture studies
Authors: Dupont Christopher L. , Goepfert Tyler J. , Lo Peter , Wei Liping , Ahner Beth A.
Published: July 2010

Glutathione is an abundant intracellular thiol that is involved in the detoxification of reactive oxygen species generated in the presence of light. We used short‐term continuous cultures of Emiliania huxleyi and Thalassiosira pseudonana to show that a diurnal light cycle causes intracellular glutathione to undergo at least twofold variations, with maximum concentrations occurring during light periods. Cysteine concentrations also appear to vary in E. huxleyi to a similar extent, with the highest concentrations occurring at night. A complementary field study conducted at Salt Pond in Falmouth, Massachusetts, yielded a similar diurnal cycle of glutathione concentrations in particulate samples. This cycling is important to consider when measuring particulate thiols in both culture and field studies and provides additional evidence that glutathione plays an important role as an antioxidant in eukaryotic marine phytoplankton.
Coupling of epipelagic and mesopelagic heterotrophic picoplankton production to phytoplankton biomass in the Antarctic polar frontal region
Authors: Simon Meinhard , Rosenstock Bernd , Zwisler Walter
Published: July 2010

We assessed relationships between phytoplankton standing stock, measured as chlorophyll a (Chl a), primary production (PP), and heterotrophic picoplankton production (HPP), in the epipelagic zone (0–100 m) as well as in the mesopelagic zone (100–1,000 m) in the polar frontal zone of the Atlantic sector of the Southern Ocean in austral summer (late December to January) and fall (March to early May). Integrated epipelagic HPP was positively correlated to integrated PP in summer (data for fall are not available) but not to integrated Chl a. However, integrated mesopelagic HPP was positively correlated to Chl a in summer as well as fall. The mesopelagic fraction of HPP as a percentage of total HPP was also positively correlated to Chl a, whereas the epipelagic fraction of HPP was negatively correlated to it. These results indicate that with increasing phytoplankton standing stock, constituted mainly of highly silicified diatoms, the focus of its consumption by heterotrophic picoplankton shifts from epipelagic to mesopelagic waters. With a growth efficiency of 30%, our HPP data indicate that in both the epipelagic and mesopelagic zone heterotrophic picoplankton consume 20% of PP. Mesopelagic heterotrophic picoplankton consumed around 80% of the sinking flux, measured from depletion of 234Th, which is a lower fraction than that reported from the central and subarctic Pacific. Our analysis indicates that it is important to include mesopelagic HPP in comprehensive assessments of the microbial consumption of PP, phytoplankton biomass, and particulate organic matter in cold oceanic systems with high rates of export production.
Relationship between bacterial community structure, light, and carbon cycling in the eastern subarctic North Pacific
Authors: Van Mooy Benjamin A. S. , Devol Allan H. , Keil Richard G.
Published: July 2010

Biogeochemical controls on the community structure of heterotrophic marine bacteria are not well understood, and these organisms play a critical role in the global carbon cycle. Through terminal restriction fragment length polymorphism (T‐RFLP) analyses of bacterial 16S rRNA genes along a zonal transect in the eastern subarctic North Pacific, variations in community structure were compared to commonly obtained chemical and biological oceanographic measurements. It was found that heterotrophic bacterial community structure was strongly, but independently, related to both phytoplanktonic production and light. Based on computer‐simulated T‐RFLP analyses of database 16S rRNA gene sequences, it appeared that the relationship between bacterial community structure and light observed along the transect was driven by the distribution of α‐proteobacteria with phototrophic capabilities. In addition, experiments were conducted at sea to measure the growth of heterotrophic bacteria in response to amendments of amino acids, protein, N‐acetyl glucosamine, and chitin. Variation in the response of heterotrophic bacteria in the experiments did not correlate significantly with variation in community structure along the transect, which suggests that the ability of the heterotrophic bacterial community to use specific components of the dissolved organic carbon reservoir was not related to its phylogenetic structure as detected by T‐RFLP. However, a possible relationship between community structure and the cycling of chitin is discussed. Overall, the results stress the importance of both heterotrophic and phototrophic metabolisms when considering environmental controls on the structure of bacterial communities in the sea.
Metabolic poise in the North Atlantic Ocean diagnosed from organic matter transports
Authors: Hansell Dennis A. , Ducklow Hugh W. , Macdonald Alison M. , O‐Neil Baringer Molly
Published: July 2010

Recently there has been discussion about the metabolic state of the ocean, with arguments questioning whether the open ocean is net autotrophic or net heterotrophic. Accurately determining the metabolic balance of a marine system depends on fully defining the system being evaluated and on quantifying the inputs and outputs to that system. Here, a net northward transport of dissolved organic carbon (DOC) (across 24.5°N) of 3.3 ± 1.9 Tmol C yr−1 was determined using basin‐wide transport estimates of DOC. This flux, coupled with DOC inputs from the Arctic Ocean (2.2 ± 0.8 Tmol C yr−1), the atmosphere (0.6 ± 0.08 Tmol C yr−1), and rivers (3.1 ± 0.6 Tmol C yr−1), indicates net heterotrophy in the North Atlantic (full depth, 24.5–72°N) of 9.2 ± 2.2 Tmol C yr−1. This rate is small (<2%) compared to autochthonous production (~494 Tmol C yr−1) and consumption (production : respiration of 0.98), indicating that the North Atlantic is essentially metabolically balanced and that autochthonous production is remineralized within the basin. The upper layer of the subtropical gyre has previously been reported to exhibit high rates of net heterotrophy, but our analysis does not support those findings. Instead, allochthonous inputs of organic carbon to the upper subtropical gyre are an order of magnitude less than required by the elevated rates of net heterotrophy reported. We find, too, that net mineralization of allochthonous DOC within the basin could account for 10% of the preindustrial inorganic carbon exported from the basin to the south. Two factors, the import of organic matter and the unique thermohaline circulation pattern of the North Atlantic, are primary in ensuring net heterotrophy in the basin.
Benthic nitrogen fluxes along a depth gradient in a microtidal fjord: The role of denitrification and microphytobenthos
Authors: Sundbäck Kristina , Linares Frank , Larson Fredrik , Wulff Angela , Engelsen Anna
Published: July 2010

In littoral sediments, microphytobenthic (MPB) nitrogen assimilation often exceeds nitrogen removal by denitrification, partly because MPB activity suppresses denitrification. Little is known about the balance between these two processes at sublittoral depths. Benthic pigment composition, light and dark oxygen, and nutrient fluxes (NO3−, NH4+, dissolved organic nitrogen (DON), PO43−, Si(OH)4), as well as denitrification were measured between 1 and 15 m in depth in Gullmar Fjord (Skagerrak) in spring and autumn. The hypothesis was that the assimilation/denitrification ratio would decrease with depth, along with decreasing MPB activity caused by light limitation. MPB photosynthesis occurred along the entire depth gradient, although sediments were net autotrophic only above 5 m. Inorganic nitrogen (DIN) (and silica) flux changed along the depth gradient, the general pattern being sediment uptake at ≤5 m and efflux at ≥10 m depth. DON flux (~50% of total dissolved nitrogen flux) showed a less clear pattern. Two trends regarding DIN fluxes and denitrification—significant light effects and negative correlations with gross primary productivity—showed that MPB activity influenced nitrogen (N) turnover. Although denitrification increased with depth, rates remained low (<0.4 mmol N m−2 d−1), and MPB assimilation (0.2–3.6 mmol N m−2 d−1) exceeded or equaled denitrification. MPB incorporated ~35% of the remineralized N along the depth gradient, whereas denitrification removed ~20%. Thus, the influence of MPB on benthic nitrogen turnover, denitrification included, extends to sublittoral depths. Further, denitrification does not necessarily remove more N in the deeper, heterotrophic part of the photic zone, compared to the littoral, autotrophic zone.
Mechanisms maintaining sympatric distributions of two ladyfish (Elopidae: é) morphs in the Gulf of Mexico and western North Atlantic Ocean
Authors: McBride Richard S. , Horodysky Andrij Z.
Published: July 2010

Two morphs of ladyfish exist in the western North Atlantic and adjacent waters: Elops saurus, a high‐count morph (79‐87 myomeres or vertebrae) in the north, and Elops sp., a low‐count morph (73–78 myomeres or vertebrae) in the south. It has been proposed that these morphs are two allopatric species, but significant questions about their ecology remain. We examined 4,597 specimens and found that E. saurus was distributed principally in the western North Atlantic and Gulf of Mexico, whereas Elops sp. was distributed principally in the Caribbean Sea. Mixing of both morphs occurred along the east coast of the United States and the gulf coast of Mexico. Our results are consistent with the hypothesis that these meristic differences arise from latitudinal differences in temperature between spawning areas (i.e., Jordan󂀙s rule). Dispersal via the Gulf Stream System and associated mesoscale features appear sufficient to explain all areas of sympatric distribution. Elops larvae were found nearly year‐round in Florida estuaries, but E. saurus larvae were dominant in winter and spring collections, and Elops sp. larvae were dominant in summer and autumn. Thus, juvenile Elops sp. experience a shorter growing season and a different suite of estuarine conditions than juvenile E. saurus. The low initial abundance and high mortality of Elops sp. reduce the potential for interbreeding with E. saurus where they are sympatric. Such ecological data uphold the postulation that the two morphs are different species and support the existence of macroscale biotic connectivity between the Caribbean region and North America.
A note on the potential transport of scalars and organisms by surface waves
Authors: Monismith Stephen G. , Fong Derek A.
Published: July 2010

Wave‐induced transport that is not directly reflected in Eulerian current measurements, an effect known as Stokes drift, may play a significant role in the transport of organisms and solutes in the nearshore environment. Two sets of field observations are presented that illustrate the potential importance of wave‐induced transport. Velocities measured near Santa Barbara, California, are used to show that the theoretical Stokes drift can be stronger than measured Eulerian currents. Dye plume measurements made at Duck, North Carolina, in both wavy and nonwavy conditions indicate that wave‐induced scalar transport can be significant. To demonstrate the generality of these results, we present calculations of the Stokes drift for several depths and as functions of wave amplitude and period. Given that cross‐shore velocities typical of the nearshore coastal ocean are generally comparable to or smaller than these computed values of the Stokes drift, we conclude that observations and predictions of nearshore transport should include explicit consideration of wave‐induced transport.
Haunted by the past: Evidence for dormant stage banks of microparasites and epibionts of Daphnia
Authors: Decaestecker Ellen , Lefever Christophe , De Meester Luc , Ebert Dieter
Published: July 2010

Microparasites and epibionts have important implications for the ecology and evolution of their zooplankton host populations. Many parasites and epibionts produce resistant spores that infect new hosts upon intake. We explored the hypothesis that these spores build up dormant stage banks that remain infective for several years (decades). In laboratory experiments, we exposed Daphnia magna to sediments taken from different depths in sediment cores from four different shallow water bodies. All samples analyzed contained infective stages of epibionts, suggesting that dormant stage banks remain infective for decades. Microparasite infections from old sediments were only obtained in one of the four ponds studied. We found mainly the bacterium Pasteuria ramosa but also a yet undescribed microsporidium. We discuss the implications of long‐lasting spore banks for the disease dynamics and coevolution in the Daphnia‐microparasite system.