bluuebunny/crossref_metadata_2025 huggingface dataset:
Evidence for a heterotrophic subtropical northeast Atlantic
Authors: Duarte Carlos M. , Agustí Susana , Arístegui Javier , González Natalia , Anadón Ricardo
Published: June 2010

The mean (±SE) depth‐integrated gross production (P) of 2,600 ± 271 mg O2 m−2 d−1 derived from a compilation of data from nine cruises conducted between 1991–2000 in the subtropical NE Atlantic was found to be significantly lower (t‐test, P = 0.005, N = 33) than the mean (±SE) community respiration (R) of 3,821 ± 276 mg O2 m−2 d−1. Two‐thirds of the stations investigated were heterotrophic, and the P/R ratio of the communities tended to increase as P increased, such that communities where P < 3,000 mg O2 m−2 d−1 tended to be heterotrophic. The tendency for R to exceed P (P/R , 1.0) was statistically significant (Wilcoxon ranked sign test, P < 0.05) in the upper and deep layers of the photic zone, with an overall balance between P and R at intermediate depths. These results provide evidence that the subtropical NE Atlantic is a heterotrophic ecosystem, where planktonic communities respire more organic carbon than they produce, thereby acting as net sources of CO2.
The heterotrophic bacterial response during a mesoscale iron enrichment experiment(IronEx II) in the eastern equatorial Pacific Ocean
Authors: Cochlan William P.
Published: June 2010

The response of the heterotrophic bacterial community to iron addition was determined during the mesoscale iron‐enrichment experiment conducted in the eastern equatorial Pacific during May‐June 1995 (IronEx II). Bacterial abundance and 3H‐leucine incorporation rates were measured for samples collected from the middle of the mixed layer (15 m) over the course of the iron‐induced phytoplankton bloom and its decline. Bacterial abundance and productivity increased 1.7‐ and threefold, respectively, compared to un‐enriched waters. Specific growth rates of heterotrophic bacteria increased three‐ to fourfold. These results demonstrate that iron addition to this high‐nitrate, low‐chlorophyll region affects both autotrophic and heterotrophic microorganisms and that bacterial carbon demand can be potentially met by the fivefold increase in photosynthetic productivity in the mixed layer.
Ultraviolet radiation sensitivity of photosynthesis in phytoplankton from an estuarine environment
Authors: Banaszak Anastazia T. , Neale Patrick J.
Published: June 2010

We have studied temporal variation in the sensitivity of phytoplankton photosynthesis to inhibition by ultraviolet radiation (UV; 280–400 nm) using biological weighting functions (BWFs) that quantify the biological effect of different wavelengths of UV. Variations in irradiance‐dependent BWFs were evaluated for natural phytoplankton assemblages from the Rhode River, a shallow subestuary of the Chesapeake Bay, Maryland, from October 1994 to July 1996. Phytoplankton assemblages were sensitive to UV throughout the year. Rhode River assemblages are inhibited more strongly in the UV‐B (280–320 nm), particularly below 300 nm, but there is a significant influence well into the UV‐A (320–400 nm). There was no inhibition of phytoplankton photosynthesis by photosynthetically available radiation (400–700 nm), but there was significant seasonal variation in the saturated rate of photosynthesis (PsB) and in the light saturation parameter (Es). There was little variation in seasonal average BWFs through the year, but there was considerable variation in BWFs during each season. Individual BWFs varied both in absolute values of the weightings (reciprocal [mW m−2]) and in the spectral shape or relative effect of UV‐B versus UV‐A, which may be due to changes in species composition, light, temperature, and nutrient availability. Comparison of the most sensitive assemblage (spring) with the least sensitive assemblage (winter) indicates that these BWFs are close to the upper and lower bounds in sensitivity for irradiance‐dependent BWFs from all natural and cultured phytoplankton populations. The average, absolute spectral weightings for inhibition of photosynthesis in assemblages from the Rhode River are similar to an average BWF for Antarctic assemblages.
Stable isotope values of lotic invertebrates: Sources of variation, experimental design, and statistical interpretation
Authors: Lancaster Jill , Waldron Susan
Published: June 2010

In a subset of a stream food web, whole‐body isotope values of d13C and d15N were measured for eight populations of lotic invertebrates. Observed isotopic differences among species corresponded broadly to their trophic status, as also revealed by gut content analysis, but with some exceptions. Species within a guild of grazer/scraper mayflies differed significantly in d13C; a predatory caddisfly (Rhyacophila dorsalis) and a collector/gatherer stonefly (Leuctra inermis) had statistically indistinguishable values of d13C and d15N. The variation associated with the mean isotope value of each population was partitioned into the variation among individuals and the variation that arises from analysis by isotope ratio mass spectrometry. For some taxa, within‐population variance was lower than or equal to the variance attributable to the measurement error of the mass spectrometer. The highest but conservative estimate of within‐population variation was a mean coefficient of variation of 11% for d15N in a predator, R. dorsalis. The minimum detectable difference between two populations was negatively associated with the number of replicate samples and the number of individual animals combined in each replicate. The optimum number of replicate samples, therefore, varies depending on the hypotheses of interest.
Transition‐matrix model of bioturbation and radionuclide diagenesis
Authors: Shull David H.
Published: June 2010

Bioturbation rates in muddy sediments are thought to be due primarily to the reworking activities of benthic deposit feeders. However, current mathematical models of bioturbation do not explicitly link rates of particle mixing with realistic biological reworking mechanisms. To address this problem, I present a transition‐matrix model of bioturbation that quantitatively links the reworking activities of individual organisms and community‐level particle‐mixing rates. Solutions to the model are presented for two kinds of tracers; particle‐reactive radionuclides with a constant input flux and conservative tracers added to the sediment as a pulse. The model was used to predict the vertical profiles of excess 234 Th and 210 Pb in the field. The model parameters were determined from benthic community‐structure data. Model predictions were then compared to measured profiles of these tracers. On the basis of this comparison, I inferred that maldanid polychaetes at the study site were collecting sediment at the sediment‐water interface and depositing it at depth. This transport mechanism had a large effect on the predicted tracer profiles. A sensitivity analysis of the model indicated that deposit feeding by the two most abundant species, Mediomastus ambiseta and Nucula annulata, was the most important process determining the burial rate of the tracers. The model results also indicated that the combined effects of deposit feeding and sedimentation were sufficient to determine the vertical distributions of excess 234 Th and 210 Pb at the study site.
Pelagic species size distributions in lakes: Are they discontinuous?
Authors: Havlicek Tanya D. , Carpenter Stephen R.
Published: June 2010

Aquatic ecologists have many models for size distributions of pelagic communities. However, few studies have looked for discontinuities (clumps of similarly sized species or gaps of sizes that contain no or relatively few species) in pelagic community size structure. We investigated size distribution characteristics in aquatic communities by calculating kernel density functions for plankton and fish in 11 lakes in Wisconsin. Size distributions in aquatic communities of these lakes were not smooth. Rather, multiple lump and gap regions were found within each functional group of phytoplankton, zooplankton, and fish. Simulations showed the gaps could not be explained by incomplete censuses of species or by systematic underestimation of intraspecific size variation. In an experimentally enriched lake, before and after comparisons showed lumps were not affected by large additions of P and N, even though biomass and production changed substantially. Lump regions in the two lakes with both food web manipulations and nutrient enrichment were substantially less similar pre‐ versus postenrichment than the reference lake and the lake with only nutrients added, but lump number remained relatively unchanged. Lakes that differed widely in nutrient status, trophic structure, species diversity, and area had similar size distributions. Comparisons of functional groups showed that phytoplankton had more lumps than zooplankton. In these north temperate lakes, size distribution characteristics seem to be conservative properties shaped by common regional ecosystem processes and organism patterns and not by lake‐specific factors.
Bacterial dissolved organic carbon demand in McMurdo Dry Valley lakes, Antarctica
Authors: Takacs Cristina D. , Priscu John C. , McKnight Diane M.
Published: June 2010

The proximal substrate source of planktonic bacteria is dissolved organic carbon (DOC), and the combined sources of DOC (bulk, phytoplankton production, and advected) set an upper limit on how much C is available for bacterial respiration (BR). We compared measurements of bacterial production (BP) and estimates of BR to measurements of what we assumed to be the major DOC inputs for three permanently iceߚcovered lakes in Antarctica: Lakes Fryxell, Hoare, and Bonney. These measured inputs, which included phytoplankton extracellular release (ECR), stream input, and upward diffusion of DOC across the chemocline, sediments, and benthic microbial mats, were three to eight times smaller than planktonic BR, suggesting that a major source of bacterial C was unaccounted for. Despite overestimating DOC and doubling bacterial growth efficiency (BGE), BR in the lakes was 1.25ߚ2 times greater than our estimate of DOC supply. We hypothesize that a major source of organic C for planktonic bacteria in these lakes comes from drawdown of bulk DOC and/or decomposition of particulate material.
Evidence for tight coupling between active bacteria and particulate organic carbonduring seasonal stratification of Lake Michigan
Authors: Van Mooy Benjamin , MacGregor Barbara , Hollander David , Nealson Ken , Stahl David
Published: June 2010

Suspended particulate organic matter (POM) plays a critical role in the planktonic ecology of Lake Michigan during seasonal thermal stratification. We show, based on stable isotope and ribosomal RNA (rRNA) data, that the relationship between suspended POM and active biomass changes as thermal stratification persists. Stable isotope data indicated that sources of suspended POM change between July and October, moving from primary production at a deep chlorophyll layer to recyclingߚbased production in surface waters. Concomitant change in the distribution of active bacterial and eukaryotic biomass was observed as indicated by rRNA abundances. Active bacterial and eukaryotic biomass were highly correlated throughout the year. However, the correlation between suspended POM and active bacterial biomass varied seasonally and reflected the transitions in planktonic ecology. Suspended POM from depths >60 m was primarily of sedimentary origin. The combined application of stable isotope and rRNA analysis of suspended POM indicated a dynamic relationship between the bulk POM reservoir and living planktonic biomass.
Global net community production estimated from the annual cycle of surface water total dissolved inorganic carbon
Authors: Lee Kitack
Published: June 2010

Global net community production is determined, for the first time, from the decrease in salinity (S)ߚnormalized total dissolved inorganic carbon (NCT = CT × 35/S) inventory in the surface mixed layer corrected for changes due to net airߚsea CO2 exchange and diffusive carbon flux from the upper thermocline. Changes in the mixed layer NCT inventory are estimated using a derived annual cycle of NCT and global records of the mixed layer depth. The annual NCT cycle is deduced from regional algorithms relating NCT to sea surface temperature (SST) and nitrate (NO3−), along with global records of seasonal mean SST and NO3−, and from the monthly mean surface partial pressure of CO2 and total alkalinity fields using thermodynamic models. The two methods show similar regional trends and yield global net community production estimates of 6.7 and 8.0 Gt C (1 Gt C = 1 × 1012 kg carbon), respectively. The two global estimates are not significantly different and represent an 8–month period of 1990 (warming period) during which the mixed layer NCT concentration decreases. However, the estimates do not account for net community production during a 4–month cooling period. Ratios of net community production during the warming and cooling periods are estimated from multiyear sediment trap data at the Hawaii Ocean Timeߚseries (228N, 1588W) and Ocean Weather Station P (50°N, 145°W) sites. Global extrapolation of these ratios yields annual rates of net community production of 9.1 ± 2.7 and 10.8 ± 2.7 Gt C yr−1.
Coulometric carbonߚbased respiration rates and estimates of bacterioplankton growth efficiencies in Massachusetts Bay
Authors: Toolan Tara
Published: June 2010

Heterotrophic bacterioplankton production rates have been measured in many aquatic ecosystems over the last two decades, whereas measurements of bacterioplankton respiration rates have been scarce. This paper reports and discusses measurements of carbonߚbased plankton respiration rates made in a coastal ecosystem over an annual cycle. The coulometric technique was used to measure total inorganic carbon (TCO2) production rates in 0.8– µm filtered and unfiltered Massachusetts Bay surface seawater. Bacterioplankton respiration rates, defined as respiration in 0.8– µm filtered seawater, varied from 0.01 to 0.15 mmol C kg−1 h−1 (0.07 ±0.01 µmol C kg−1 h−1) (mean ±1 SE) and accounted for a high proportion (median 70%) of respiration in unfiltered seawater. Microplankton (unfiltered seawater) respiration rates varied from 0.04 to 0.24 µmol C kg−1 h−1 (0.12 ±0.02 µmol C kg−1 h−1). Bacterioplankton growth efficiencies (BGEs), estimated with concurrent measurements of bacterial production and respiration rates, varied from 0 to 69% (median 22%) and were well correlated with specific production rates (r2 = 0.67). To assess carbon flow in aquatic ecosystems, BGEs should be measured when possible because of their high variability. Compared with bacterioplankton specific production rates, bacterioplankton specific respiration rates were relatively constant (0.05 ±0.01 fmol CO2 cell−1 h−1). Given the apparent uncertainty in the values of respiratory quotients (Toolan 1996; Robinson and Williams 1999), carbonߚbased comparisons of microbial respiration and primary production rates will improve evaluations of the role of the coastal ocean in the global carbon cycle.
The influence of particle size on seston deposition in streams
Authors: Thomas Steven A. , Newbold J. Denis , Monaghan Michael T. , Minshall G. Wayne , Georgian Theodore , Cushing Colbert E.
Published: June 2010

We investigated how particle size influences deposition and transport of fine particulate organic matter in streams. Field additions of very fine (VFPOM, 15–52 µm), fine (FPOM, 53–106 µm), and medium (MPOM, 107–250 µm) detritus and live diatoms (Asterionella sp.) were used to quantify the longitudinal loss rate (kP) of each material type and to derive estimates of mean transport distance (SP) and field deposition velocity (vdep). In all experiments, smaller particles deposited more slowly, and thus traveled farther, than larger size classes. Significant differences in kP were detected in four of seven paired FPOM and VFPOM particle additions. vdep estimates were neither equivalent nor closely associated with calculated quiescent water fall velocities (vfall) for all size classes. Variation in SP and vdep of FPOM and VFPOM were strongly correlated across hydrological conditions (r = 0.94 and 0.92, respectively). Variation in vdep was poorly associated with physical attributes of the stream. Transport distances were positively associated with the crossߚsectional area of the transient storage zone (AS) and the uptake length of water (SW) for both size classes. We argue that local hydrological and benthic conditions establish a minimum rate of particle deposition and that departures from this rate due to gravitational forces begin to occur at particle diameters similar to the larger size classes used in this study (50–100 µm).
Large variability in the concentration of mycosporine‐like amino acids among zooplankton from lakes located across an altitude gradient
Authors: Tartarotti Barbara , Laurion Isabelle , Sommaruga Ruben
Published: June 2010

The qualitative and quantitative composition of mycosporine‐like amino acids (MAAs), a family of intracellular UV‐absorbing compounds, were investigated in zooplankton from 15 lakes located in the Central Alps between 913 and 2,485 m above sea level. The lakes differed in their UV water transparency (1% attenuation depth, Z1%, at 320 nm: 1.1–25.6 m) and maximum depth (Zmax: 3–133 m), thus offering the possibility to test the influence of different UV exposure conditions of zooplankton on the concentration of MAAs. Seven distinct MAAs were detected, but shinorine (maximum absorption: 334 nm) was the predominant compound. In the copepods Cyclops abyssorum, C. abyssorum tatricus, and Acanthodiaptomus denticornis, the total MAA concentration ranged from 0.01 to 3.1% of the dry weight. In the rotifers Keratella cochlearis and Polyarthra dolichoptera, MAAs were also found; however, these compounds were undetectable in Asplanchna priodonta as well as in the cladocerans Daphnia hyalina, D. longispina, Bosmina longispina, and Chydorus sphaericus. The total concentration of MAAs in populations of Cyclops spp. and phytoplankton collected simultaneously was not associated (r2 = 0.09, P > 0.05), suggesting a different dynamic in the accumulation of these compounds. The variability in the concentration of MAAs, however, was related with the diffuse attenuation coefficient at 320 nm (r2 = 0.74, P < 0.001) and the fraction of the water column to which 1% of the surface irradiance at 320 nm (Z1% : Zmax) penetrated (r2 = 0.86, P < 0.001). These relationships suggest that the prevailing UV exposure condition in the lakes is a major determinant of the concentration of MAAs found in zooplankton. Our data support the hypothesis that MAAs, together with other photoprotective compounds, play a major role in minimizing the damaging effects of solar UV radiation in zooplankton from transparent lakes.
Calanoid copepods versus cladocerans: Consumer effects on protozoa in lakes of different trophic status
Authors: Burns Carolyn W. , Schallenberg Marc
Published: June 2010

Through their consumption of protozoa, cladocerans and copepods link classical food chains and microbial food webs in aquatic ecosystems. Published results of studies of the effects of these metazooplankton on protozoa in lakes allow few generalizations to be made. To determine if general patterns exist along a trophic gradient, we measured the effects of cladocerans and calanoid copepods on heterotrophic nano‐ flagellates (HNF) and ciliates in four lakes that ranged from ultraoligotrophic to eutrophic using the same methodology. Copepods (Boeckella spp.), and to a lesser extent cladocerans (Daphnia, Ceriodaphnia), had significant negative effects on the growth of protozoa, and the rates at which both groups cleared protozoa from the water were higher in nutrient‐poor conditions than in nutrient‐rich conditions. In two oligotrophic lakes, calanoid copepods ingested HNF at biomass‐specific rates that were 2.2 times higher than those of cladocerans. Rates of ciliate ingestion by copepods, relative to cladocerans (top‐down effects on the ciliate community), increased with lake productivity from 2.5 times higher in an ultraoligotrophic lake to 9.5 times higher in a mesotrophic lake. Our study shows that copepods are more effective consumers of protozoa than cladocerans, particularly in eutrophic conditions.
Detritus as food for grazing fishes on coral reefs
Authors: Crossman David J. , Choat Howard J. , Clements Kendall D. , Hardy Tom , McConochie Jason
Published: June 2010

Algal turf assemblages of the northern Great Barrier Reef, Australia, were sampled to determine the nutritional value of detritus and algae. Samples were collected with a suction apparatus across an exposure gradient from (1) the reef crest at highly exposed outer barrier reefs, (2) the reef crest of moderately exposed midshelf reefs, and (3) the reef slope of sheltered midshelf reefs. The biomass of algae and detritus decreased from sheltered midshelf reefs to moderately exposed midshelf reefs to highly exposed outer barrier reefs. This decrease was significant only for detritus (P < 0.005). Wave energies were calculated across the exposure gradient with the wave model WAMGBR. Detrital mass was inversely correlated with predicted wave energies and fitted a polynomial relationship (P < 0.001) and explained 52.8% of the variation. A similar relationship was also found between algal mass and wave energy (P < 0.001) but only explained 30.0% of the variation. The nutritional value of samples in protein amino acids and starch was assessed. The amino acid composition of detritus and algae was similar and not considered nutritionally different, whereas the concentration of protein amino acids was significantly (P < 0.001) higher in detritus (21.2 ± 2.0 mg g−1) than in algae (11.8 ± 1.0 mg g−1). Starch content was significantly (P < 0.05) higher in algae (7.7 ± 0.9 mg g−1) than in detritus (6.0 ± 1.0 mg g−1). These results demonstrate that detritus is a potentially valuable food source to grazing fishes on coral reefs.
Whole‐lake mineralization of allochthonous and autochthonous organic carbon in a large humic lake (örträsket, N. Sweden)
Authors: Jonsson Anders , Meili Markus , Bergström Ann-Kristin , Jansson Mats
Published: June 2010

Organic carbon mineralization was studied in a large humic lake (Lake Ortrasket) in northern Sweden during a well‐defined summer stratification period following high water flow during snowmelt. Several independent methods including plankton counts, measurements of bacterioplankton and phytoplankton production, stable isotope monitoring, sediment trapping, and mass balance calculations were used. Total organic carbon mineralization showed a summer mean of 0.3 g C m−2 d−1 and was partitioned about equally between water and sediment. In the water column, organic matter was mineralized by bacteria (60%) and protozoan and metazoan zooplankton (30%), as well as by photooxidation (10%). Most of the mineralized organic carbon was of allochthonous origin. Primary production in the lake contributed at most 5% of the total organic carbon input and about 20% of the total organic carbon mineralization. Total carbon mineralization in the epilimnion and metalimnion agreed well with an estimate of CO2 evasion from the stratified lake, while CO2 accumulation in the hypolimnion matched the O2 consumption and resulted in a very negative dδ13C of DIC before autumn overturn (−23‰). Isotopic compositions of DIC and POC confirmed the dominant influence of terrestrial organic input on the cycling of both organic and inorganic carbon in the lake.
Energetics of long internal gravity waves in large lakes
Authors: Antenucci Jason P. , Imberger Jörg
Published: June 2010

An analytical model is used to determine dispersion relations and the ratio of potential to kinetic energy in linear basin‐scale internal waves in lakes affected by the earth's rotation. It is shown that the wave frequency and energy partitioning in elliptic lakes are dependent only on the direction of propagation relative to the earth's rotation, the aspect ratio, the horizontal mode (azimuthal and radial), and the Burger number (Si = ci /Lf, where ci is the nonrotating phase speed, L is a length scale that characterizes the lake dimension, and f is the Coriolis parameter). For the cyclonic (rotating in the same direction as the earth's rotation), lowest radial mode (a Kelvin wave for small Si and a Poincaré wave for large Si), the total potential to kinetic energy ratio was always greater than unity for all azimuthal modes. For all other radial modes (Poincaré waves for all Si), both cyclonic and anticyclonic, the ratio is substantially less than unity, especially as the Burger number decreases. The results demonstrate that basin‐scale Poincaré waves follow the same rotation‐gravity balance as unbounded plane progressive Poincaré waves, in which rotation plays an increasingly important role as the Burger number decreases. The solutions are applied to field experiments conducted in Lake Kinneret (Israel) to determine the dissipation timescale of the basin‐scale internal waves. It is further shown that features of the spatial structure of isopycnal displacement and velocity scales may be inferred from a single station that measures potential energy fluctuations.
Dissolved organic carbon production and consumption in anoxic marine sediments: A pulsed‐tracer experiment
Authors: Hee Carol A. , Pease Tamara K. , Alperin Marc J. , Martens Christopher S.
Published: June 2010

The degradation of phytoplankton and seagrass organic carbon (OC) in sulfate‐reducing (SR) and methaneproducing (MP) sediments was tracked by measuring concentrations of particulate OC (POC), hydrolysis products (HP), fermentation products (FP), and inorganic end products (EP). This experiment used the novel approach of amending sediment with organic substrates having δ13C values unique from that of the OC pool originally present in the sediment. As a result, we could monitor changes in the dynamic dissolved OC pool and gain insight into processes that control the fate of OC. Rates of hydrolysis, fermentation, and terminal metabolism were greater in the phytoplankton‐amended treatments than in the seagrass‐amended treatments during the period of active decomposition. At the end of the incubation, concentrations of HP and FP in amended treatments were not significantly different from those in the controls. An analysis of the δ13C values of HP from the amended treatments indicated that the addition of fresh organic matter stimulated the decomposition of carbon present in the sediment at the time of collection. Hydrolysis of this carbon accounted for >=50% of the total carbon hydrolyzed in the sediment amended with seagrass.
Uptake of dissolved organic selenides by marine phytoplankton
Authors: Baines Stephen B. , Fisher Nicholas S. , Doblin Martina A. , Cutter Gregory A.
Published: June 2010

Se is present in multiple oxidation states in nature, each of which has unique chemical and biological reactivities. As a consequence, the rate of Se incorporation into food webs or its role as either a limiting nutrient or a toxic substance is a function of complex biogeochemistry. In particular, little is understood about the accumulation of dissolved organic selenides by phyto‐ or bacterioplankton. We assessed the bioavailability of dissolved organic selenides to marine and estuarine phytoplankton by presenting various algal species with filtered lysates of the diatom, Thalassiosira pseudonana, grown on media amended with radiolabeled selenite (75Se[IV]). Species known to accumulate selenite effectively also accumulated Se from radiolabeled cell lysates and to approximately the same extent. When exposed to a 4.5‐nM solution of lysate Se, T. pseudonana, Heterocapsa triquetra (Dinophyceae), Tetraselmis levis (Prasinophyceae), Synechococcus bacillus (Cyanobacteria), and Dunaliella tertiolecta (Chlorophyceae) incorporated 42%–53%, 42%, 30%, 32%, and 4% of the dissolved Se, respectively. Se cell contents of T. pseudonana, T. levis, and D. tertiolecta grown in media containing 4.5 nM lysate Se were very similar to Se content of cells grown in 4.5 nM selenite. Our results suggest that recycling of Se(—II) may be more important than previously thought. Consequently, uptake of organic selenides by phytoplankton may need to be considered in models predicting Se incorporation into aquatic food webs, given that it is a significant fraction of dissolved Se in estuarine and oceanic waters.
The roles of predation and postlarval transport in recruitment of the soft shell clam (Mya arenaria)
Authors: Hunt Heather L. , Mullineaux Lauren S.
Published: June 2010

Rates of mortality and transport of newly settled benthic invertebrates can be very high and potentially have profound effects on patterns of recruitment. We used the soft shell clam Mya arenaria as a model organism to compare the magnitude of rates of postlarval transport and predation on juveniles. We hypothesized that there is a critical size below which spatial variation in abundance of juvenile Mya is hydrodynamically controlled and above which abundance is mainly influenced by mortality, particularly due to epibenthic predators. To test this hypothesis, we examined transport and predation of early juvenile Mya in Barnstable Harbor, Massachusetts, USA. A caging experiment demonstrated that exclusion of epibenthic predators had a large impact on the density and size distribution of juvenile Mya within a few weeks of settlement and disproportionately affected juveniles of >2‐mm shell length. Enclosure of the green crab Carcinus maenas changed the size distribution of Mya but did not significantly affect abundance. We attributed the effect of predator exclusion primarily to juvenile green crabs and fish. Current meter measurements suggested that tidal currents were strong enough to cause bedload transport of sediment and Mya. Measurement of gross and net rates of transport of Mya, using bedload traps and pans of defaunated sediment, confirmed that juveniles up to 5‐mm shell length were routinely redistributed by tidal currents, particularly during spring tides. The number of postlarval Mya transported varied among sites and dates and was significantly related to ambient density of Mya and sediment flux. The loss of Mya during the caging experiment was compared to the rate of net flux of Mya due to transport during an intermediate tide. This comparison suggested that the loss due to predation was considerably larger than the flux due to transport for individuals >2 mm, but that rates of the two processes were more similar for individuals <2 mm. These experiments indicate that, while predation is likely the ultimate factor controlling abundance and size of juvenile Mya at this site, transport often redistributes individuals, at least at the scale of a tidal flat (10s to 100s of m).
Ecological interactions of the microparasite Caullerya mesnili and its host Daphnia galeata
Authors: Bittner Kerstin , Rothhaupt Karl-Otto , Ebert Dieter
Published: June 2010

It has been repeatedly suggested that parasites play an important role in the ecology and evolution of Daphnia populations; however, little is known about Daphnia—parasite interactions in lakes with vertebrate predation on Daphnia. Between September 1997 and April 1998, an epidemic of the protist parasite Caullerya mesnili in Daphnia galeata and Daphnia hyalina occurred in Lake Constance (Germany), infecting up to 50% of all individuals. Using laboratory experiments, we investigated the epidemiological interactions between this parasite and its host D. galeata at the individual and the population level. C. mesnili was found to be transmitted directly and horizontally through waterborne infection stages. Transmission of the parasite was dependent on the host density, and all life stages of female D. galeata were susceptible to infection. In a life table experiment at low and high food levels, the life expectancy and fecundity of infected D. galeata were dramatically reduced at both food levels as compared to the uninfected controls. Additionally, we found a significant interaction between infection and food level, indicating a stronger parasitic effect in well‐fed hosts. To test the effects of the parasite at the population level, we compared the size of D. galeata populations infected with C. mesnili with the size of parasite‐free microcosm populations. The population size of infected D. galeata was significantly lower than that of uninfected populations after 4 weeks. In all four infected replicate populations, the parasite drove the host population to extinction.
Trace metal assimilation and release budget in Daphnia magna
Authors: Yu Ri-Qing , Wang Wen-Xiong
Published: June 2010

The assimilation efficiency (AE), efflux rate, and release budget of Cd, Cr(III), Se(IV), and Zn by a freshwater zooplankton Daphnia magna were measured under different food concentrations. The AEs of trace elements by Daphnia on two algal diets (Chlamydomonas reinhardtii and Scenedesmus obliquus) were 30–77% for Cd, 8–44% for Cr, 24–58% for Se, and 7–66% for Zn at food concentrations ranging from 0.136 to 7.50 mg carbon L−1. Metal AEs increased significantly with decreasing food concentrations, with a maximum increase of 5.5 and 4.0X for Zn and Cr, respectively. AEs were generally on the order of Cd > Se > Zn > Cr. Efflux rate constants, determined during 7 d depuration after 8 d of exposure to metals in the dissolved phase or dietary phase, were 0.012–0.216 d−1, with the highest efflux for Zn, followed by Cr > Se > Cd. The relative contribution of different routes of metal loss to the overall metal loss was also quantitatively assessed during the 7‐d depuration period. Metals differed substantially in their routes of release from Daphnia. In general, metal excretion into the dissolved phase was the most important route for metal loss. Molting represented nearly 50–70% and 20–70% of daily metal efflux for Cd and Zn, respectively, following aqueous exposure within the first 4 d but was <20 and <30%, respectively, following food exposure. Release by offspring production contributed substantially to Se efflux by the animals. Up to 44–67% and 16–47% of Se was lost from the animals through reproductive allocation on a daily basis following uptake from the aqueous and dietary phases, respectively. The major routes of Cr efflux were by excretion and feces egestion. Our study suggested that trace metal assimilation and regeneration in Daphnia may play an important role in the biogeochemical fates of metals in lake systems.
Ophiuroid growth within deep‐sea sediment traps: A problem for carbon flux measurements at continental margins
Authors: Lampitt Richard S. , Sumida Paulo Y. G. , Castillo Fernando Pérez‐
Published: June 2010

Large numbers of postlarvae of the bathyal ophiuroid Ophiocten gracilis were collected by two time‐series sediment traps moored on the continental slope of the NE Atlantic for 102 d at 1,000 and 1,400 m deep (469 and 69 mab), respectively. With time, the organisms collected were progressively larger, suggesting that a single cohort was growing within the collection funnel of the traps but above the preservative solution. We believe that specimens from this cohort occasionally lost their grip on the funnel surface and fell into the preservative. This conclusion is supported by (1) the presence of food within the stomachs of postlarvae (these do not feed in their planktonic phase) and (2) the size of specimens (much larger than those previously found in the plankton). The upper trap was well above the benthic nepheloid layer, and suspension of these organisms by ambient currents is highly unlikely. Ophiuroids contributed up to 7% of the total daily flux of particulate organic carbon (POC) in the top trap and almost 30% in the bottom trap, and although they are not normally considered components of downward particle flux, these specimens had been growing on the material entering the traps and so should be included in this portion. Not all of the epifaunal population will have fallen into the collecting cups, but those that fell had requirements for growth and respiration that were a considerable proportion of the daily flux of POC into the traps (up to 16%). If this epifaunal community (a minimum estimate) was distributed evenly within the cone, a high proportion of the settling material would have come into contact with ophiuroids as it slid down the inside walls of the cone. This problem with the sediment trap technique is probably restricted to the continental slope and shelf and will not occur over abyssal depths.
The assimilation of detritus‐bound metals by the marine copepod Acartia spinicauda
Authors: Xu Yan , Wang Wen-Xiong
Published: June 2010

The bioavailability of Cd, Se, and Zn associated with detritus of different biological origins to the marine copepod Acartia spinicauda was examined under laboratory conditions using the radiotracer approach. Three types of detritus, from the macroalga Ulva lactuca, cell debris of the diatoms Thalassiosira weissflogii and Thalassiosira rotula derived after natural decomposition or extraction, and copepod fecal pellets, were examined. The assimilation of metals from the ingested natural phytoplankton assemblage was also experimentally quantified. The assimilation efficiencies (AEs) of metals in copepods fed with different types of detritus were 47‐83% for Cd, 30‐59% for Se, and 41‐75% for Zn. These values were comparable to the AEs measured previously for copepods fed on living algal cells, indicating that metals associated with biogenic particles were directly available to marine copepods. Labeling time had no effect on the metal AEs in copepods fed on detritus derived from U. lactuca. The metal AEs were higher in copepods fed on the freshly prepared diatom debris than on naturally decomposing debris. Zn associated with fecal pellets was assimilated at comparable efficiencies to those associated with macroalgal and microalgal debris, but Cd and Se were assimilated at a lower efficiency when associated with fecal pellets. The AEs of Cd (84‐94%) in copepods fed on the natural phytoplankton assemblage were the highest among all the experiments. There was no obvious effect of the metal distribution in the cytosol of the natural phytoplankton assemblage on the metal AE. The calculated elimination rates of the metals were independent of detritus type and were similar to previous studies. Our study indicated that detritus is a potential food source for marine copepods and the metals associated with the detritus can be efficiently used with a relatively high AE. Detritus may thus play an important role in the overall biogeochemical cycling of metals in the ocean.
Saharan input of phosphate to the oligotrophic water of the open western Mediterranean Sea
Authors: Ridame Céline , Guieu Cécile
Published: June 2010

A Saharan soil, considered as a proxy for Saharan aerosols, was used to perform a series of dissolution experiments: various amounts of Saharan soil were exposed to ultrapure water and seawater for varying lengths of time. The concentration of phosphate released was proportional to the amount of dust introduced. In the case of Saharan events associated with a significant amount of rain, the main dissolution of phosphorus will occur in the air column; for Saharan events associated with only few drops of rainwater, the main dissolution will occur in the surface seawater. Saharan dust represents a source of phosphate to the surface water and may play a role in biological activity, especially during the oligotrophic period. In the western Mediterranean in oligotrophic conditions, biological production is P‐limited, and the atmosphere becomes the main pathway of nutrients to the surface mixed layer. At the scale of the oligotrophic season, the input of “Saharan dissolved inorganic phosporus (DIP)” are negligible compared with the new production integrated over the productive layer. At the event timescale, the production induced by “Saharan DIP” can represent up to 15% of the integrated new production and up to 14% of the total primary production in the mixed surface layer. These inputs of atmospheric DIP would promote fixation of atmospheric N2 that, in return, may enhance the new production in the surface layer.
Quantification, base composition, and fate of extracellular DNA in marine sediments
Authors: Dell’Anno Antonio , Stefano Bompadre , Danovaro Roberto
Published: June 2010

The discovery of high concentrations of DNA in marine sediments unaccounted for by living biomass suggests the presence of a large fraction of extracellular DNA, which might play an important role in gene transfer via natural transformation as well as in phosphorous biogeochemical cycling. But a universally accepted procedure for extracellular DNA extraction is not available yet. In this study, we developed a new nuclease‐based procedure to extract extracellular DNA from marine sediments. Coastal sand and deep‐sea mud samples were collected to test the efficiency of extracellular DNA removal from different sediment types. Extracellular DNA concentrations were quantified at six sediment depths, and changes in base composition were investigated to gather information on extracellular DNA fate. The extraction procedure was highly specific and only extracellular DNA was hydrolyzed after nuclease treatment. Hydrolyzable DNA accounted for ≪10 to ≫70% of the total DNA pool, suggesting that extracellular DNA can only be partially degraded. Base composition changed vertically with depth in the sediment as deoxycytidine content increased and deoxyadenosine decreased with increasing depth. Integrating our results for the top 15 cm of the sediment, we calculate that more than 50% of extracellular DNA was recalcitrant to enzymatic degradation. This finding might explain why DNA accumulates in surface sediments and suggests that DNA might play a nonnegligible role in P biogeochemical cycle.
Ammonium uptake by seagrass epiphytes: Isolation of the effects of water velocity using an isotope label
Authors: Cornelisen Christopher D. , Thomas Florence I. M.
Published: June 2010

In a series of flume experiments, 15N‐labeled ammonium was used to isolate the effects of water velocity on ammonium uptake by epiphytes from that of an assemblage of organisms that included seagrass leaves, epiphytes, and phytoplankton. Rates of NH4+ uptake for epiphytes, seagrass leaves, and the total assemblage were dependent on water velocity. Ammonium uptake rates for epiphytes, normalized to chlorophyll a, increased by an order of magnitude (0.65 to 6.8 × 1028 g N removed [mg Chl a]−1 s−1) over a range of velocity (0.02–0.20 m s−1) and were correlated to uptake by the entire assemblage. The relationship between NH4+ uptake and velocity for the epiphytes was within the range expected for mass transfer limited uptake, which suggests that water flow strongly influences NH4+ uptake by this important component of seagrass communities. Our results demonstrate that isotopically labeled nutrients can be used to isolate the effects of water velocity on rates of nutrient uptake by an individual component of a community and to evaluate how uptake rates for the component compare to those of the community as a whole.
Life history bottlenecks in Diaptomus clavipes induced by phosphorus‐limited algae
Authors: Villar-Argaiz Manuel , Sterner Robert W.
Published: June 2010

Considerable information has accumulated on food quality effects on certain cladocerans; meanwhile, other zooplankton have received comparatively little nutritional study. In particular, little is still known about nutrition of copepods with more complex ontogenetic changes. Recent studies have identified strong ontogenetic differences in phosphorus content of copepods, which have been found to be highest for late naupliar stages. Laboratory experiments were set up to examine the life cycle of a calanoid copepod relative to mineral P limitation on a pure algae diet. We hypothesized that P imbalance would cause nauplii particularly to be affected by algal nutrient status. Developmental rates, growth rates, and fecundity of Diaptomus clavipes under excess (1.0 mg C L−1) P‐deficient and P‐sufficient Scenedesmus obliquus were determined. We found strong differences between diets in survival to maturity: nauplii developed into copepodites and adults that successfully reproduced when fed the P‐sufficient algae but invariably died after molting into copepodite CII when reared on the P‐deficient algae. Differences in developmental rates were small for most life stages of nauplii for both food types but were substantial both for copepodite CI and particularly for CII that lived without molting up to 22 d on P‐deficient food before dying. Surprisingly, copepodites that fed on Pdeficient algae had similar or higher specific somatic growth rates than animals on P‐sufficient algae. Although these experiments demonstrate a strong ontogenetic component to copepod nutrition, and a heretofore undocumented dependence of copepod success on algal P content, the life history bottleneck occurs at a later stage than we hypothesized.
Top‐down control of phytoplankton biomass and community structure in the monsoonal Arabian Sea
Authors: Goericke Ralf
Published: June 2010

During the 1995 monsoons, phytoplankton biomass was low over large areas of the Arabian Sea in spite of often high concentrations of inorganic nutrients, characteristics typical of high‐nutrient, lowchlorophyll (HNLC) areas. The objective of this study, a part of the Arabian Sea U.S. JGOFS program, was to elucidate controls of phytoplankton biomass and community structure in these areas, using primarily pigment‐based methods. Unlike other HNLC areas, phytoplankton biomass and growth rates in the monsoonal Arabian Sea were not likely limited by the availability of trace nutrients but rather controlled by grazers. Evidence for this conclusion is high concentrations of iron, the nutrient likely limiting phytoplankton biomass in other HNLC areas; phytoplankton growth rates clearly not limited by concentrations of inorganic nutrients; blooms during grow‐out experiments in the absence of mesozooplankton; rates of microzooplankton and mesozooplankton grazing that approached rates of primary production; and the absence of blooms in situ in spite of persistently high concentrations of inorganic nutrients. A more detailed look at shifts of phytoplankton community structure with increasing levels of phytoplankton biomass, in situ and during grow‐out experiments, shows that diatoms were the only group of organisms capable of blooming in the absence of mesozooplankton, i.e., their abundance in situ was likely controlled by mesozooplankton grazing. Changes in the abundance of different picoautotroph taxa with increasing levels of phytoplankton biomass suggest that their biomass was controlled within tight bounds by microzooplankton grazing, rather than by the availability of resources. Thus, these results are consistent with the hypothesis that phytoplankton abundance and community structure in the HNLC areas of the monsoonal Arabian Sea are controlled by top‐down forces, grazing, rather than bottom‐up forces, availability of resources.