bluuebunny/crossref_metadata_2025 huggingface dataset:
The influence of multiple environmental stressors on susceptibility to parasites: An experimental determination with oysters
Authors: Lenihan Hunter S. , Micheli Fiorenza , Shelton Stephen W. , Peterson Charles H.
Published: May 2010

A large ‐scale field experiment was used to test whether exposure to a suite of potential environmental factors (flow speed, temperature, salinity, and low dissolved oxygen) influences the level of parasitic infection of the oyster Crassostrea virginica. The parasite was the protozoan Perkinsus marinus, which has decimated populations of oysters in estuaries of North America. The environmental factors were considered stressors because they influence the physiological condition of either the host or parasite. Between December 1994 and July 1995, flow speed, temperature, salinity, dissolved oxygen concentration (DO), Perkinsus infection, and mortality of oysters were monitored across 24 experimental oyster reefs in the Neuse River estuary, North Carolina. Eight reef treatments were created consisting of an orthogonal combination of three factors: water depth (3 m vs. 6 m deep), reef height (2 m vs. 1 m tall), and position on reef (base vs. crest). Principal component analysis revealed that there was clear separation of environmental factors among reefs and that a majority of the variation (96.2%) among treatments could be explained by two principal component axes (PCs): one (24.3% of variation explained) was formed by flow and the other (71.9% of variation explained) by temperature, salinity, and DO. Oysters with the highest proportion of individuals infected (prevalence), highest intensity of infection, and highest mortality were located at the base of reefs, where flow speeds and food quality were lowest and sedimentation rates highest. However, there was no significant effect of hydrographic conditions on Perkinsus infection or mortality of oysters, despite large differences in mean salinity, DO, and exposure to hypoxia ‐anoxia. Temperature did not vary among treatments. Correlation of disease responses (infection prevalence and intensity, and mortality) with the first two PCs showed that these response variables were significantly (and negatively) correlated with flow only. Oysters in low flow were hypothesized to have the greatest susceptibility to Perkinsus infection because of their poor physiological condition. The restoration of oyster reefs increases reef height and thus flow speeds (by elevating oysters higher above the boundary and by actively influencing flow), thereby reducing the negative effects of disease by restoring reef morphology to its natural size.
Estimating lake‐atmosphere CO2 exchange
Authors: Anderson Dean E. , Striegl Robert G. , Stannard David I. , Michmerhuizen Catherine M. , McConnaughey Ted A. , LaBaugh James W.
Published: May 2010

Lake‐atmosphere CO2 flux was directly measured above a small, woodland lake using the eddy covariance technique and compared with fluxes deduced from changes in measured lake‐water CO2 storage and with flux predictions from boundary‐layer and surface‐renewal models. Over a 3‐yr period, lake‐atmosphere exchanges of CO2 were measured over 5 weeks in spring, summer, and fall. Observed springtime CO2 efflux was large (2.3–2.7 umol m‐2 s‐1) immediately after lake‐thaw. That efflux decreased exponentially with time to less than 0.2 umol m‐2 s−1 within 2 weeks. Substantial interannual variability was found in the magnitudes of springtime efflux, surface water CO2 concentrations, lake CO2 storage, and meteorological conditions. Summertime measurements show a weak diurnal trend with a small average downward flux (−0.17 μmol m‐2 s1) to the lake's surface, while late fall flux was trendless and smaller (−0.0021 μmol m‐2 s−1). Large springtime efflux afforded an opportunity to make direct measurement of lake‐atmosphere fluxes well above the detection limits of eddy covariance instruments, facilitating the testing of different gas flux methodologies and air‐water gas‐transfer models. Although there was an overall agreement in fluxes determined by eddy covariance and those calculated from lake‐water storage change in CO2, agreement was inconsistent between eddy covariance flux measurements and fluxes predicted by boundary‐layer and surface‐renewal models. Comparison of measured and modeled transfer velocities for CO2, along with measured and modeled cumulative CO2 flux, indicates that in most instances the surface‐renewal model underpredicts actual flux. Greater underestimates were found with comparisons involving homogeneous boundary‐layer models. No physical mechanism responsible for the inconsistencies was identified by analyzing coincidentally measured environmental variables.
The role of colloids in tracemetal transport and adsorption behavior in New JerseyPinelands streams
Authors: Ross James M. , Sherrell Robert M.
Published: May 2010

Dissolved and colloidal concentrations of Al, Fe, Mn, Cu, Zn, Cd, and Pb were determined in the Mullica River, New Jersey, and its tributaries on nine occasions between August 1994 and September 1995. Cross‐flow filtration (CFF) at a nominal 10 kDa cutoff was used to separate total dissolved (>0.45 μm) riverine metals into colloidal and truly dissolved fractions. Reproducibility tests on full procedural replicates demonstrated CFF's precision and indicated that temporal or spatial differences exceeding 10–15% in colloidal fraction can be resolved using these techniques. For the whole data set, mean colloidal fractions of 0.45‐ μm filtrates were 65% for Al, 82% for Fe, 29% for Mn, 66% for Cu, 31% for Zn, 46% for Cd, and 88% for Pb. The magnitude of the colloidal fraction followed an elemental order that was largely consistent from site to site and among different sampling dates, despite large variations in total dissolved metal concentrations. The order was very similar to that observed previously in rivers of varying geochemical properties, suggesting that the degree of colloidal association is driven by a metal's affinity for humic materials and is relatively insensitive to other riverine properties such as pH, dissolved organic carbon, and suspended mass. The constancy of this elemental pattern in Pinelands streams suggests that the relative fates of metals during estuarine colloid flocculation may be predictable and temporally stable. Iron behavior was distinct from that of the other elements: colloidal Fe fraction was positively correlated with pH (range, 4.3–7.1) and the colloidal Fe/Al ratio showed clear seasonal variation. The other metals did not exhibit these behaviors, which suggests that their colloidal association was distinct from that of Fe. Colloidal metal/Al ratio for Fe, Pb, and Zn was clearly different from that of >0.45 μm particles; that finding is consistent with the distinct compositions and sources for colloidal and particulate fractions. The distribution coefficient (Kd) for Pb was positively correlated with suspended particulate mass (<10‐fold variation) over time at one representative site. This inverse particle concentration effect runs contrary to recent observations in other freshwater and marine systems and suggests that colloidal and particulate size fractions vary independently over the course of the year.
Odor transport in turbulent flows: Constraints on animal navigation
Authors: Finelli Christopher M. , Pentcheff N. Dean , Zimmer-Faust Richard K. , Wethey David S.
Published: May 2010

Odor plumes are common features of aquatic and terrestrial environments, forming an olfactory landscape through which animals must navigate to locate resources and avoid potential hazards. Time‐averaged concentration profiles suggest that plumes consist of stable gradients in odor that animals may use for orientation. However, the time scales necessary to generate such profiles are much longer than those typically associated with the neural or behavioral components of odor‐mediated search. In contrast, plume measurements made at biologically relevant scales have indicated that turbulent plumes consist of discrete odor filaments separated by clean water. In addition, certain characteristics of individual odor filaments may vary consistently with distance from the odor source, thus providing directional information to a navigating organism. Unfortunately, there is no method to predict the distribution of these putative chemical cues, and our knowledge of odor dispersal is limited to very few laboratory flume studies. Here, we present the results of a field study during which we measured the distributions of the time‐averaged concentration, properties of odor filaments, conditional statistics, and relevant hydrodynamic mixing parameters. Many of the observed odor plume characteristics have similar spatial distributions through a range of hydrodynamic conditions. The high degree of similarity in the distribution of many odor plume characteristics suggests that organisms can rely on any number of metrics to successfully orient in an odor plume. However, the temporal and spatial scales of odor dispersal may constrain the strategies used by navigating organisms and influence the efficiency of odor‐mediated search. These field results should provide the basis for further empirical and theoretical work on chemosensory‐mediated behavior of aquatic animals.
Vertical secondary flows in submersed plant‐like arrays
Authors: Nepf Heidi M. , Koch Evamaria W.Koch
Published: May 2010

Obstructions that protruded from a laboratory test bed into the benthic boundary layer were exposed to gradients in longitudinal velocity (δu/δz) that produced vertical pressure gradients along the surface of the obstruction. These pressure gradients generated vertical secondary flows that may have ecological significance for benthic fauna and aquatic macrophytes. Laboratory experiments demonstrated that secondary flows of up to 15% of the local longitudinal velocity were produced behind individual obstructions within a submersed plant‐like or animal tube array and for conditions like those found in aquatic canopies or colonies. Our observations support theoretical predictions based on a reduced form of the Navier‐Stokes equations, confirming that the ascending flow is controlled by a local balance of vertical pressure gradient, proportional to δu2/δz, and viscous stress. The secondary flows were shown to transport dye from the bottom to a height dictated by the in‐canopy current speed and turbulence intensity. By extension, the ascending flows can potentially contribute to the advection of nutrients from sediments, where they have been regenerated by microbial processes, to areas higher in the canopy, where they can be used by epiphytes and macrophyte leaves. Pressure gradients generated near the stem base also produce pore‐water exfiltration. The coupled pressure‐driven exfiltration and vertical advection have the potential to control nutrient availability in the bed.
Patchy surface stone movement during disturbance in a New Zealand stream and its potential significance for the fauna
Authors: Matthaei Christoph D. , Peacock Kathi A. , Townsend Colin R.
Published: May 2010

A patchy movement of surface stones during disturbance in streams has been proposed from field observations, but few attempts have been made to quantify this phenomenon. If spates produce a mosaic of stable and disturbed areas, the former could serve as refugia for benthic invertebrates. We monitored the stability of surface stones at three geomorphically contrasting sites in a river with an unstable bed during three spates and one large flood. Stones were marked in situ by drilling holes in them or by scraping them with a chisel. For each stone, we determined visible surface area, embeddedness in the substratum, water depth, size of surrounding stones, and presence or absence of large stones upstream. During the first monitoring period, which covered one spate and the large flood, we marked 400 stones at each 20‐m‐long site in a systematic grid with 40 to 60 cm between stones. Stones were relocated after each disturbance, but each stone set was used to monitor both subsequent events. After the large flood, few marked stones were left. Therefore, additional sets of 200 stones were marked for a second monitoring period covering the remaining two spates; these stones were located at the same sites, but the distance between transects was doubled. Patchy bed movement occurred during all four disturbances, especially during the three smaller ones, which moved only 33 to 72% of marked stones. Stability of individual stones was mainly influenced by their size and embeddedness and sometimes by the water depth above the stone. Larger‐scale parameters (e.g., the position of the stone across the transect) were less important. Whole‐site stability differed little among sites. During the three smaller events, many stable surface stones were available as potential invertebrate refugia. In contrast, invertebrate refugia may have been restricted to the hyporheic zone and inundated flood‐plain gravels during the large flood. Because patchy stone movement was observed in a river with an unstable bed, it is likely to be a feature of most rivers. Therefore, small‐scale experiments may be able to simulate the effects of disturbance on the benthic community more effectively than previously thought.
Influence of depth and mixing regime on sedimentation in a small, fluctuating tropical soda lake
Authors: Verschuren Dirk
Published: May 2010

The historical sediment record of Lake Sonachi (Kenya) was used to study the influence of lake depth and mixing regime on patterns of sedimentation in a small, fluctuating tropical soda lake. Lake Sonachi last desiccated completely in the early nineteenth century and has fluctuated between 3− and 18−m lake depth over the past 115 yr. A freeze‐core of offshore sediments describes recent lake history as a succession of meromictic episodes, represented by varved or subannually laminated muds, and holomictic episodes, represented by more coarsely layered muds. Two interbedded horizons of colloidal amorphous silica dated to a period of rising lake level after a prolonged lowstand were deposited by abiogenic, pH‐driven precipitation from the water column and represent the instantaneous sequestering of an estimated 63 mg liter−1 or possibly >50% of the lake's dissolved‐silica reservoir. Changes in offshore sedimentation and inferred bottom dynamics over time indicate that sediment resuspension and focusing in Lake Sonachi occur mostly during infrequent events of deep circulation between the average mud deposition boundary depth at ∼2 m and the chemocline depth at 4–5 m; wind‐driven sediment redistribution across the lake floor is important only at lake depths of ≤3 m. Dry sediment accumulation has varied between 86 and 620 g m−2 yr−1 over the past 175 yr, with no relationship to lake depth but, on average, lower rates during meromixis (199 ± 90 g m−2 yr−1) than during holomixis (349 ± 152 g m‐2 yr‐1). Net organic carbon accumulation offshore varied between 0 and 92 g m‐2 yr‐1, with no significant relationship to either lake depth or mixing regime at the time of deposition. Sedimentary organic carbon content (5.7–26.9%) is negatively correlated with bulk sediment accumulation; 73% of this variation is accounted for by the clastic dilution of sedimented planktonic algal production with low‐organic littoral sediments redeposited offshore.
Rapid and precise determination of dissolved oxygen by spectrophotometry: Evaluation of interference from color and turbidity
Authors: Roland Fabio , Caraco Nina F. , Cole Jonathan J. , del Giorgio Paul
Published: May 2010

Several researchers have proposed spectrophotometric modifications of the Winkler titrimetric method for measuring dissolved oxygen (DO). These modifications, although simple, are not widely used because of concern about accuracy, calibration, and possible sources of interference. Here we show, using natural samples from lakes and rivers as well as samples manipulated in the laboratory, that the spectrophotometric method can provide accurate and very precise measurements of DO over a wide range of concentrations (4 to ∼13 mg O2 liter−1). Further, interference from dissolved organic carbon (color) and turbidity are minor. We propose corrections for both color and turbidity, where necessary, that can be easily incorporated into the measurement design. Because of the speed and simplicity of the spectrophotometric method, it is easy to replicate measurements and thereby increase precision without greatly increasing analytical time. In 10 min of effort, we were able to achieve a coefficient of variation (CV) within one bottle of 0.09%, or 0.8% among different bottles. With n = 7 bottles, one can easily distinguish changes in DO of 0.05 mg liter−1 with this method, which makes it useful for metabolic studies in many environments. To achieve a comparable CV by conventional titration would require about 100 min of effort.
Oxidation of sulfide by Spartina alterniflora roots
Authors: Lee Raymond W. , Kraus David W. , Doeller Jeannette E.
Published: May 2010

Root tips from the marsh grass Spartina alterniflora, collected from areas of high and low pore‐water sulfide, exhibited a substantial capacity to catalyze sulfide oxidation, as determined by closed‐chamber respirometry. A large proportion of this catalysis was apparently nonenzymatic and was higher in roots of plants from the high‐sulfide versus the low‐sulfide site. Activity exhibiting characteristics of enzymatic sulfide oxidation was significantly higher in plants from the low‐sulfide site. Results from elemental analysis of root tissue were consistent with the theory that metals play a role in nonenzymatic catalysis. These results indicate that estuarine plants may detoxify environmental sulfide via sulfide oxidation.
Phosphorus in rainwater: Partitioning inputs and impact on the surface coastal ocean
Authors: Migon Christophe , Sandroni Valérie
Published: May 2010

Phosphate concentrations in rainwater were measured at a Ligurian coastal sampling site (Cap Ferrat, France) from February 1997 to February 1998 to study the impact of wet atmospheric phosphorus (P) input on the surface ocean. Soluble and particulate fractions were differentiated to evaluate the atmospheric supply of bioavailable P. Complexed and reactive phases within the dissolved fraction were also separated. Preliminary results showed a high temporal variability in total concentration (0.05–4.3 μmol liter−1). The factors controlling the partitioning between reactive and complexed components are not clear. However, the partitioning between dissolved and particulate fractions is linked to emission sources. Soil‐derived dust from the Sahara was identified as an important source of atmospheric P, mainly insoluble. Conversely, anthropogenic emissions are sources of soluble P (i.e., basically bioavailable). A significant part of these emissions could originate from incinerators and/or biomass burning. The different wet fluxes are calculated to total 165 μmol m−2 yr−1, and the dissolved and particulate inputs are 95 and 70 umol m−2 yr−1, respectively. Taking into account the respective sol ubility of such inputs, anthropogenic emissions appear to be responsible for relatively high amounts of bioavailable P. Even if the atmosphere is globally a minor source of nutrients (compared with riverine inputs and marine vertical mixing), it might be the only source of P in oligotrophic conditions. For example, assuming that P is a limiting factor in the Mediterranean Sea, the rain event of 19 June 1997 (17 μmol m−2 of bioavailable P) potentially induced a new production of 0.02 g C m−2, which is a significant value in such conditions. Converted into biomass and integrated over a 5‐‐thick water layer, such an atmospheric input represents 0.35–0.45 mg chlorophyll a m−2, an appreciable portion of the total biomass during this period. This observation underlines the major role of the atmosphere during oligotrophic periods in the western Mediterranean.
Inorganic nitrogen acquisition in the seagrass Thalassia testudinum: Development of a whole‐plant nitrogen budget
Authors: Lee Kun-Seop , Dunton Kenneth H
Published: May 2010

Whole‐plant nitrogen (N) uptake experiments were used to quantify the N budget of Thalassia testudinum growing under different sediment nutrient regimes at two locations in the western Gulf of Mexico. At both sites, Corpus Christi Bay (CCB) and lower Laguna Madre (LLM), Texas, concurrent measurements of plant biomass and levels of dissolved inorganic nitrogen (DIN) in the water column and sediments were made over a 12–month period (October 1996–October 1997). Water‐column NH4+ and NO3− + NO2‐ concentrations were not significantly different between study sites (ca. 1.2 µM [NH4+] and 0.7 µM [NO3− + NO ] at both sites), but sediment NH4+ concentrations in CCB (87 µM) were significantly higher than in LLM (26 µM). The higher sediment NH4+ levels at CCB correlated with significantly higher leaf biomass at CCB, but there was no difference in root biomass between study sites. Leaf NH4+ uptake showed clear seasonal variation: Vmax was highest in summer and fall, but Km was highest in winter. Vmax of leaf NO3‐ uptake did not change with season, but Km decreased with increasing incubation temperature. There were no clear differences in leaf NH4+ and NO3− uptake rates between study sites, although leaf NH4+ uptake affinity was higher than that of NO3−. Root NH4+ uptake was variable with season and did not saturate at the experimental NH4+ concentrations at either site (0–300 µM). Based on these measurements, N acquisition was highest during summer and fall and lowest during winter and spring. Roots and leaves contributed nearly equally to total plant N acquisition (root NH4+ = 52%; leaf NH4+ = 38%; and leaf NO3− = 10%) at both sites. Annual N acquisition in CCB was double that of LLM (97.03 and 53.49 g N m−2 yr−1, respectively), but >50% of N uptake was not incorporated into biomass at either site. DIN turnover time ranged from 0.21 to 0.91 d in the water column and from 0.95 to 1.75 d in sediment pore water, indicating the importance of DIN regeneration processes for supporting seagrass production. The similarity in the relative tissue contributions between plants at both sites, despite a significant difference in sediment NH4+ pool sizes, results from the higher fraction of biomass allocated to below‐ground tissues in plants living under low‐sediment N conditions (LLM). In N‐sufficient sediments, overall plant productivity is greater as T. testudinum is able to allocate a greater proportion of its biomass into photosynthetic aboveground tissues.
A new approach to historical reconstruction: Combining descriptive and experimental paleolimnology
Authors: Kerfoot W. Charles , Robbins John A. , Weider Lawrence J.
Published: May 2010

Here we introduce a combined experimental and descriptive approach (termed resurrection ecology) to reconstructing historical perturbations, pointing out how direct tests with sediments and hatched resting eggs complement the traditional descriptive calculation of microfossil fluxes. In the Keweenaw Waterway, a freshwater estuary off Lake Superior, turn‐of‐the‐century copper mining impacted the resident biota. Remain fluxes document that diatom, rhizopod, and Bosmina production all declined during stamp sand discharges but recovered rapidly after World War II, moving above background levels due to developing eutrophication. In addition to biogenic silica, we discovered that bromine flux holds promise as an indicator of diatom production and confirmed that this element is present in several genera. Fluxes of Daphnia resting eggs also increased dramatically since the 1940s, dominated by a hybrid apparently produced from crosses between offshore and interior Waterway species, after channeling promoted greater mixing of water masses. Toxicity studies with sediments and Daphnia clones directly tested recovery of environ ments after cessation of mining activities. The studies document that increased concentrations and fluxes of copper in the Waterway during mining discharges were toxic to invertebrates. Once stamp sand discharges ceased, the biota recovered rapidly due to a combination of decreased copper cycling and organic complexation. Although sedimentation has returned to near‐background conditions and surficial sediments in much of Portage Lake are no longer toxic, eutrophication and faunal exchange with Lake Superior make it unlikely that the original zooplankton community composition will return to the Waterway system.
Rhizosolenia mats: An overlooked source of silica production in the open sea
Authors: Shipe Rebecca F. , Brzezinski Mark A. , Pilskaln Cynthia , Villareal Tracy A.
Published: May 2010

The contribution of Rhizosolenia mats to silica cycling in the central North Pacific and the coupling of mat silicon metabolism and their vertical migration was examined in areas to the west of the Hawaiian Islands (23–28°N and 159–175°W) in 1995 and to the east of Hawaii along 31°N (160–127°W) in 1996. The biogenic silica content of Rhizosolenia mats sampled in 1995 averaged 1.82 ± 1.87 (SD) µmol Si mat−1. Larger mats that averaged 4.56 ± 3.54 (SD) µmol Si mat−1 were observed in 1996. Kinetic experiments indicated that substrate limitation of mat silica production was widespread across the study region, with ambient [Si(OH)4] restricting silica production to 33% of maximum potential rates. Three lines of evidence indicate that silicon metabolism is not tightly coupled to the migration of mats to and from the nutricline. In 1996, mats in surface waters could double their Si content in 0.55 d on average without migrating to the nutricline to obtain Si. However, average doubling times (9.8 d) in 1996 were of the same order as a migration cycle, necessitating significant Si uptake at depth. Si uptake rates did not differ significantly between ascending and descending mats, suggesting that mats ascending from the nutricline had not fulfilled their Si uptake requirements. Finally, small internal pools of Si in ascending mats indicated that if significant amounts of Si were taken up at depth, they were not stored for use in the surface waters. The biomass and silica production rates of mats collected using SCUBA in the upper 20 m were extrapolated to 150 m by using abundances determined using a video plankton recorder (VPR). The results suggest that mats account for about 3% of the standing stock of biogenic silica and 26% of silica production in the upper 150 m. The daily silica production by Rhizosolenia mats (317 µmol Si m−2 d−1) is 50–76% of the total silica production in the Sargasso Sea. This high rate of silica production combined with the wide geographic distribution of mats throughout several mid‐ocean gyres suggests that mats may contribute significantly to global silica production.
Predicting fish distribution in a mesotrophic lake by hydroacoustic survey and artificial neural networks
Authors: Brosse SÉbastien , Lek Sovan , Dauba Francis
Published: May 2010

The present work describes the development and validation of Artificial Neural Networks (ANN) by comparison with classical and more advanced parametric and nonparametric statistical modeling methods such as Multiple Regression (MR), Generalized Additive Models (GAM), and Alternating Conditional Expectations (ACE) to esti‐mate spatial distribution of fish in a mesotrophic reservoir. The modeling approaches were developed and tested using 60 hydroacoustic transects covering the whole lake. Each transect was divided into 100‐m‐long sections, constituting a total of 732 sampling units. For each of them, the relationships between topology, chemical characteristics, and fish abundance were studied. The models had six independent topological (i.e., depth, distance from the bank, slope, and stratum) and chemical (i.e., temperature and dissolved oxygen) variables and one dependent output variable (fish density, FD). The data matrix was divided into two parts. The first contained units where FD was nonnil (i.e., 399 sampling units), and the second contained only cases without fish (i.e., 333 sampling units). Model training and testing procedures were run on the first submatrix after log(FD + 1) transformation. As linear MR results were not satisfactory (r2 = 0.42 in the training set, and r2 = 0.51 in the testing set) compared with ANN (r2 = 0.81 in the training set, and r2 = 0.77 in the testing set), we tried nonlinear transformations of the variables such as logarithmic, lowess (for the GAM), and an optimal nonlinear transformation using the SAS Transreg procedure (for the ACE model), but the determination coefficients remained clearly lower than those obtained using ANN (r2 = 0.60 in the training set for ACE, and r2 = 0.66 in the training set for GAM). The results of a second test on the nil submatrix stressed that, compared with other statistical techniques, ANN and, to a certain extent, GAM models were able to clearly define the potential FDs in samples where no fish were actually found. The model showed, on the basis of the topological and chemical variables taken into account, that the predicted potential FDs in the surface stratum are higher than in the underlying stratum. Finally, on the basis of the sensitivity analyses performed on the ANN and GAM results, we established relationships between FDs and the six environmental variables. Our results exhibit a clear summer habitat preferendum, the fish (predominantly roach) being located mainly in the surface stratum, in the warm shallow littoral areas. These observations led us to discuss the ecological significance of such a fish distribution, which may be due to a trade‐off between feeding, predation avoidance, and endogenous fish requirements.
Oxic/anoxic oscillations and organic carbon mineralization in an estuarine maximum turbidity zone (The Gironde, France)
Authors: Abril GwenaËl , Etcheber Henri , Le Hir Pierre , Bassoullet Philippe , Boutier Bernard , Frankignoulle Michel
Published: May 2010

The study of vertical particle dynamics in the highly turbid Gironde Estuary has shown intense cycles of sedimentation and resuspension at both diurnal and neap‐spring time scales. Fluid mud, with suspended particulate matter (SPM) concentrations between 50 and 500 g liter−1, has been observed during neap tides. Vertical profiles of biogeochemical parameters have been measured in the fluid mud. Anoxic conditions have been detected when SPM concentration exceeded 50 g liter−1 in the upstream and 140 g liter−1 in the downstream parts of the maximum turbidity zone (MTZ). At the downstream part of the MTZ, anoxic fluid mud was partitioned into a denitrification layer (SPM = 140−250 g liter−1), intensively reworked at the tidal time scale, and by an Mn(IV)‐reduction layer (SPM > 250 g liter−1) preferentially reworked at the neap spring time scale. Due to the alternation of sedimentation and resuspension periods, most of the sediment experienced oxic/anoxic oscillations throughout the neap‐spring cycle. Fluid mud resuspension occurred without any observable incidence on the surface‐water oxygenation. An increase in total alkalinity was found in the fluid mud, due to both anaerobic respiration and a carbonate dissolution coupled to aerobic respiratory CO2 generation. This phenomenon significantly affected the inorganic carbon budget of the estuary, increasing the HCO3− input to the coastal ocean and reducing the CO2 flux to the atmosphere. An accumulation of labile dissolved organic carbon observed in the fluid mud suggests that these oscillations result in an acceleration of particulate organic matter (POM) decomposition. In the Gironde MTZ, a net loss of refractory land‐derived POM occurs. This system acts as an efficient oxic/suboxic “fluidized bed reactor,” similar to mobile deltaic muds.
Biological and isotopic changes in coastal waters induced by Hurricane Gordon
Authors: Fogel Marilyn L. , Aguilar Carmen , Cuhel Russell , Hollander David J. , Willey Joan D. , Paerl Hans W.
Published: May 2010

The effects of a major storm event (Hurricane Gordon) on the biogeochemistry of Atlantic coastal and Gulf Stream waters were investigated during a research cruise in November 1994. Prestorm, NH4+, NO3+, and PO4−3 concentrations were consistently well below 1 μM, whereas after the storm, nutrient concentrations were higher in the surface‐water samples: >2 μM, in some instances. Primary and secondary (bacterial) production were stimulated by factors of 5 and 2, respectively, up to 4 d following the storm. Bioassay experiments showed that additions of inorganic N stimulated chlorophyll a (Chl a) concentrations, 14CO2 fixation, and stable isotope fractionations both before and after the storm, but the addition of phosphate had a greater impact in post‐storm experiments. The δ15n of particulate nitrogen (PN) varied from +5 to +1.5‰ before Gordon, then afterward attained a consistent value of +3.0‰. Sedimentary organic δ15n values were similar to water‐column organic N, and the δ15n of dissolved NH4+ from surface sediments (+4.0‰) almost matched the δ15n of water‐column particulates. These results indicate that storm‐generated winds mixed sediments along with dissolved nutrients into surface waters, which supported a rapid increase in water‐column primary production.
Sedimentation of copepod fecal material in the coastal northern Baltic Sea: Where did all the pellets go?
Authors: Viitasalo Markku , Rosenberg Mirja , Heiskanen Anna‐Stiina , Koski Marja
Published: May 2010

We investigated the sedimentation of copepod fecal pellets in three different sea areas representing a sheltered bay, an archipelago area, and the open sea on the southwestern coast of Finland in the northern Baltic Sea. Fecal carbon sedimentation was always <0.05% of the total sedimentation of particulate organic carbon, whereas the fecal carbon production (estimated from copepod abundance, assuming production rate of 10 pellets copepod−1 d−1) contributed to 4–17% of particulate organic carbon sedimentation. Thus, >99% of copepod fecal material was remineralized within the mixed water layer (0–20 m). However, in the area and season dominated by the large calanoid copepod Limnocalanus macrurus (bay station in spring), fecal carbon sedimentation was an order of magnitude higher than at the other two stations. From June onwards, when the bay station was dominated by cyclopoids, the situation changed: the fecal carbon sedimentation remained 30% lower in the bay than in the archipelago, although the fecal carbon production was estimated to be 2 times higher in the bay. Furthermore, pellet fragmentation (percentage of broken pellets of total fecal carbon sedimentation) was highest in spring and autumn at all areas and increased towards the open sea, being 27%, 45%, and 61% at the bay, archipelago, and open sea stations, respectively. This gradation was probably due to more intense turbulence and water column mixing in the open sea, resulting in more efficient loosening and breakup of pellets. The overall contribution of copepod feces to vertical carbon export in the northern Baltic Sea appears to be small, but seasonal and spatial variations in hydrography and mesozooplankton community structure significantly affect the fecal pellet sedimentation rates.
Circulation and larval distribution in internal tidal bore warm fronts
Authors: Pineda Jesús
Published: May 2010

Internal tidal bore warm fronts were observed during the summer of 1996 off the coast of Southern California. Warm bore fronts had concentrating currents resulting from high‐frequency internal motions and from a larger two‐way flow; the two‐way flow featured surface currents onshore and bottom currents offshore. A sharp thermocline depression and high‐frequency, large amplitude internal motions followed the leading edge of the bore, with down‐welling currents on the trailing side of the crest of the nonlinear internal waves and upwelling currents in front of the crest. Warm bores propagated onshore with a propagation speed, c, that ranged from 10.6 to 19.6 cm s−1, while time‐averaged frontal currents, ū, varied from 11.2 to 17.6 cm s−1 in the shallowest bin. In one out of three cases ū > c, which implied that there were faster currents than the rate of advance of the front and which implied that the origin of surface frontal material is behind the front, not in front of it. Three invertebrate larval taxa were found at all sites across fronts, but only two intertidal barnacles, Pollicipes polymerus and Chthamalus spp., were concentrated at the front's surface, while the subtidal bryozoan Membranipora spp. was not. Frontal Pollicipes were more concentrated than were Chthamalus. The frontal downwelling currents observed suggested that concentrated larvae would have to swim upward in order to maintain depth. Pollicipes were abundant on the offshore warm side of the fronts but were absent or rare on the onshore colder side, suggesting that the origin of frontal Pollicipes was behind the front, although an alternative cannot be ruled out conclusively. Chthamalus were more abundant at depth than at the surface at all sites except at the front, where this pattern was reversed. The origin of frontal Chthamalus is uncertain. Membranipora were more abundant on the onshore colder side of the fronts, and abundances were usually higher at depth than at surface. Lack of accumulation in this species may be due to its limited swimming capability.
An optimization‐based model of iron—light—ammonium colimitation of nitrate uptake and phytoplankton growth
Authors: Armstrong Robert A.
Published: May 2010

The ability to model “new” (nitrate‐based) production is crucial to predicting the ocean's role in the global carbon cycle. The ability to model the distribution of high‐nutrient/low‐chlorophyll (HNLC) areas is particularly important in this regard. Here I draw together three elements that appear to be necessary for constructing the required model: (1) iron limitation of algal growth rates as an ultimate cause of the HNLC condition; (2) ammonium inhibition of nitrate uptake and utilization as a proximate mechanism that leads to reduced nitrate use; and (3) the dependence of both processes on algal cell size. In the model, cells are postulated to maximize their growth rates by partitioning scarce iron between nitrogen‐ and carbon‐related demands. The effect of iron limitation is postulated to depend on cell size through surface/volume effects on uptake efficiency; this dependence on cell size in turn affects phytoplankton community structure and community‐level uptake of nitrate. The efficacy of the model is demonstrated by its ability to reproduce community‐level curves of nitrate uptake versus ammonium concentration from both HNLC and non‐HNLC areas. The partition formalism can be incorporated directly into ecosystem models; when implemented in an ecosystem model with multiple size classes, the model should produce HNLC versus non‐HNLC conditions in appropriate locations and for appropriate reasons. In particular, the model's ability to produce iron–light and iron–light–nitrogen colimitation should be useful in understanding and predicting the HNLC condition in parts of the Southern Ocean. With suitable changes to parameter values, the postulated mechanism for iron partitioning may also prove useful in modeling iron and energy partitioning in nitrogen fixers.
Microbial breakdown of Phaeocystis mucopolysaccharides
Authors: Janse Ingmar , van Rijssel Marion , Ottema Alex , Gottschal Jan C.
Published: May 2010

Mucus from the microalga Phaeocystis was partially purified and used as the C source in various stable enrichment cultures wherein the mucus carbohydrates were degraded under oxic as well as anoxic conditions. The breakdown of mucopolysaccharides (initially at a rate of 50% in 11 d at 12°C) markedly slowed after about 15 d, leaving a fraction nondegraded. The carbohydrate composition of the residual fraction did not change during breakdown. It was shown that the incomplete degradation of mucopolysaccharides was not due to inherent resistance to breakdown in parts of the mucus carbohydrates but to the release of inhibitors produced during breakdown. The (potentially high) overall degradability of mucopolysaccharides has important consequences for the fate of C fixed by Phaeocystis. The balance between recycling of Phaeocystis biomass in surface waters and its accumulation, sedimentation, and burial in the sediment will greatly depend on factors such as nutrient availability, presence of inhibiting agents, and composition of the microbial community, rather than on structural impediments inhibiting mucopolysaccharide degradation.
Crevices as refugia for stream diatoms: Effect of crevice size on abraded substrates
Authors: Bergey Elizabeth A.
Published: May 2010

This study explored the mechanisms by which crevices act as refugia for small algae during abrasive disturbances. Four substrates with different crevice features were subjected to three levels of abrasion in a stream‐based experiment. Substrates were sand‐grain caddisfly cases and three glass rod substrates that mimicked caddisfly cases in size and shape, as well as forming a gradient of larger crevices (i.e., smooth with no crevices, sanded with shallow pits, and scratched with deeper grooves). Substrates were subjected to no, low, or high abrasion levels during 18 d of incubation by pulling substrates through a sand‐gravel mixture in floating enclosures that housed the experiment. Diatom density, relative biovolume, location in crevices, detritus accumulation, and filamentous algal density were compared among substrate types and abrasion levels. Across all abrasion levels, diatom and filamentous algal densities were higher on caddisfly cases than on all three glass rod substrates. Although abrasion greatly reduced overall diatom density, diatoms within crevices were largely protected. Crevice size influenced diatom size and composition. The small crevices of sanded rods contained primarily small diatoms of a single species (Achnanthidium minutissimum), whereas the larger crevices of scratched rods and caddisfly cases contained a greater range of diatom size and, consequently, greater species diversity. Detritus accumulated in all crevices, and accumulation increased with level of abrasion. Diatom biovolume decreased and location in crevices increased with the level of abrasion on the less textured smooth and sanded glass rods. Abrasion level had little effect on algal assemblages on caddisfly cases. This study demonstrates that crevices can be important refugia from abrasive disturbances, and that the size of crevices relative to organismal size can influence the protective value of crevices.
Phosphorus availability in the Paraná floodplain lakes (Argentina): Influence of pH andphosphate buffering by fluvial sediments
Authors: Carignan R. , Vaithiyanathan P.
Published: May 2010

The phosphate‐buffering properties and P chemistry of suspended particulate matter (SPM) collected in three geologically contrasting rivers (Bermejo, Paraguay, and Upper Paraná) are compared in order to explain the abundance of dissolved PO4−P in the Paraná floodplain lakes. The rivers show distinctive P‐buffering capacities related to the chemical composition of their SPM. At natural pH, the linear adsorption coefficient of PO4−P (Kd) ranges from 0.25 liter g−1 for the Paraguay River to 1.38 liter g−1 for the Upper Paraná River. Equilibrium PO4−P ranges from 0.17 μmol liter−1 for the Upper Paraná River to 2.92 μmol liter−1 for the Bermejo River. Short‐term (60 h) desorbable PO4−P ranges from 0.23 μmol g−1 for the Paraguay to 0.77 μmol g−1 for the Upper Paraná. These values increase substantially with a small decrease in pH representative of conditions experienced by riverine SPM when it enters the floodplain environment. For the carbonate‐bearing SPM of the Bermejo and Paraguay Rivers, a pH decrease of 1–1.5 units causes a 10‐fold increase in equilibrium PO4−P and a 5− to 10−fold increase in desorbable PO4−P, and it is associated with the release of Ca. In the Upper Paraná, a similar decrease in pH has opposite effects that are consistent with a stronger complexation of PO4−P by hydrous iron oxides at low pH. Compared to the parent riverine SPM, floodplain lake sediments are depleted in Ca‐bound P and enriched in Fe‐bound P. Although Ca‐bound P is often considered biologically unavailable, our observations suggest that in the Paraná floodplain lakes, most of the available P originates from the acid dissolution of Ca‐bound P. Loading of fluvial sediments to the lakes and the acidic dissolution of Ca‐bound P appear sufficient to explain the permanent excess of dissolved PO4−P in these waters.
Phosphorus limitation in Daphnia: Evidence from a long term study of three hypereutrophic Dutch lakes
Authors: DeMott William R. , Gulati Ramesh D.
Published: May 2010

The Loosdrecht lakes comprise three shallow, interconnected hypereutrophic lakes in The Netherlands. A lake restoration project conducted during the 1980s resulted in reduced phosphorus loading. However, no changes in phytoplankton abundance or species composition were noted, although seston carbon:phosphorus (C:P) ratios increased. Filamentous cyanobacteria and detritus continued to dominate the seston. Moreover, planktivorous fish were very abundant. Relationships between zooplankton abundance and seston abundance and stoichiometry (C:P and carbon:nitrogen ratios) were analyzed using data collected during 9 yr of intensive study of the three lakes. Analysis based on annual means shows surprisingly strong and consistent inverse relationships between Daphnia abundance and the seston C:P ratio for two of the three lakes. In these two lakes (Loosdrecht and Vuntus Lakes), the annual mean abundance of Daphnia cucullata ranged from 104 to 0.7 individuals L−1 over a range in mean seston C:P ratios from about 250 to 500 (molar). In the third lake, Breukeleveen, Daphnia abundance was higher for a given seston C:P ratio, especially during the 2 yr following a biomanipulation project in this lake. Analysis of seston C:P ratios and Daphnia abundance in individual samples provides further evidence that high seston C:P ratios constrained Daphnia abundance in all three lakes. In contrast to Daphnia, the abundances of zooplankters with low P requirements, including Bosmina spp. and cyclopoid copepods, show little relationship to seston C:P ratios. These results suggests that the abundance of Daphnia in the Loosdrecht lakes was determined by the variable P content of their diets and consistently strong fish predation.
Diel periodicity in Synechococcus populations and grazing by heterotrophic nanoflagellates: Analysis of food vacuole contents
Authors: Dolan John R. , Šimek Karel
Published: May 2010

The relationship between heterotrophic nanoflagellates (HFLAG) and the autotrophic prokaryote Synechococcus was examined in the bay of Villefranche (NW Mediterranean) in June 1998. We determined Synechococcus concentrations, % of dividing Synechococcus cells, concentrations of HFLAG, and numbers of Synechococcus cells inside HFLAG food vacuoles in samples taken every 2–3 h over 62 h (n = 28). Synechococcus cells in division ranged from 10 to 30%, with peak values in late afternoon‐early evening hours. Numbers of Synechococcus in HFLAG food vacuoles (0.06–0.34 Synechococcus HFLAG−1) were not related to concentrations of total Synechococcus, or nondividing Synechococcus but were negatively related to % of dividing Synechococcus. The data suggest there may be a relationship between vulnerability to predation and average Synechococcus cell size. HFLAG community grazing pressure, estimated from food vacuole content and HFLAG abundances, ranged from about 0.2% Synechococcus stock removed h−1 at midnight to 1%