bluuebunny/crossref_metadata_2025 huggingface dataset:
Food web structure and planktonic predator‐prey relationships in two eutrophic European lakes: Stability constraints on carbon fluxes
Authors: Laws Edward A.
Published: November 2010

The food webs in two eutrophic European lakes were analyzed with respect to their stability characteristics. The food web during the summer in Lake Nesjøvatn (Norway) appeared to be stable; its response time to small perturbations was ~12 d. In Lake Kastoria (Greece), the summer food web was almost neutrally stable, with a response time of ~160 d. The major difference in the two systems was the species composition of the phytoplankton community, which was dominated by unicellular cryptophytes in Lake Nesjøvatn and by toxic filamentous and colonial cyanobacteria in Lake Kastoria. The macrozooplankton in both lakes showed a distinct inclination toward carnivory versus herbivory, and the heterotrophic flagellates showed a preference for small phytoplankton versus heterotrophic bacteria as a food source. The requirement for stability of the food webs constrains the grazing rates of predators to values substantially less than those that would maximize the energy flux through the system. This is especially true when a predator preferentially consumes one of several prey species.
Growth rate peaks at intermediate cell size in marine photosynthetic picoeukaryotes
Authors: Bec BÉatrice , Collos Yves , Vaquer AndrÉ , Mouillot David , Souchu Philippe
Published: November 2010

We have performed an in situ test of Raven's prediction that there is a reversal of the relationship between cell size and maximum achievable growth rate in unicellular algae at the low end of size classes. In a natural population of marine phytoplankton, including the smallest picoeukaryote known to date, and under both nutrient sufficiency and deficiency, we find a maximum in growth rate (4.8 and 3.3 divisions d−1, respectively) in the 2–3–µm size class represented by coccoid Chlorella‐like cells, with lower growth rates in both higher and lower size classes. This 2–3–µm size class is also the most robust under nutrient deficiency, reducing its growth rate by 14% only relative to nutrient‐sufficient conditions, versus 50‐60% for the lowermost and uppermost size classes, respectively.
Cyanobacterial surface blooms formed by Aphanizomenon sp. and Nodularia spumigena in the Baltic Sea: Small‐scale fluxes, pH, and oxygen microenvironments
Authors: Ploug Helle
Published: November 2010

Summer blooms of filamentous cyanobacteria, mainly Aphanizomenon sp. and Nodularia spumigena, are characteristic for the Baltic Sea, where they accumulate at the sea surface in calm weather. The chemical microenvironment, and thus the actual growth conditions within these cyanobacterial surface blooms of the Baltic Sea, are largely unknown. Using microsensors, it is shown that photosynthesis is substantial within these millimeter‐thin cyanobacterial layers accumulating at the air‐water interface. Net oxygen fluxes at the air‐sea interface were on average 2.7‐fold higher than those at the aggregate‐water interface beneath the layers. Net photosynthesis at light saturation was 1.7‐2.4 mmol O2 m−2 h−1. Dark respiration varied between 0.21 ± 0.12 mmol O2 m−2 h−1 and 0.50 ± 0.30 mmol O2 m−2 h−1. There is a tight coupling of O2‐producing and O2‐consuming processes within aggregates of these large, heterocystous, nitrogen‐fixing cyanobacteria and their associated heterotrophic microbial community. It is suggested that the close association of autotrophic and heterotrophic organisms and processes creates a pH microenvironment that is favorable for iron uptake for the cyanobacteria, which in turn may release surplus nitrogen to the heterotrophic community. The pH varied between 7.4 and 9.0 between darkness and saturating light intensities. Decaying aggregates were anoxic up to 12 h. The volumetric oxygen consumption rate in an anoxic aggregate was 1.2 µmol O2 cm−3 h−1. During the initial 12 h of decay, aggregate sinking velocity increased 10‐fold, concurrent with a decrease in volumetric oxygen consumption to 0.67 µmol O2 cm−3 h−1.
In situ fluxes of dissolved inorganic nitrogen from the sponge community on Conch Reef, Key Largo, Florida
Authors: Southwell Melissa W. , Weisz Jeremy B. , Martens Christopher S. , Lindquist Niels
Published: November 2010

We measured ammonium and nitrate plus nitrite fluxes from 14 common sponge species on a Florida Keys reef (Conch Reef) using a combination of incubation experiments and an in situ method that requires no manipulation of the sponge. On a 600‐m2 section of Conch Reef, species‐specific biomass for all nonencrusting sponges was measured. The biomass data combined with species‐specific dissolved inorganic nitrogen (DIN) flux rates yielded the benthic DIN flux from 14 species, and allowed us to extrapolate these data to the total nonencrusting sponge community. The species for which we measured DIN fluxes represented 85% of the nonencrusting sponge biomass in the study area and released a combined 480 ± 93 µmol m−2 h−1 of nitrate plus nitrite, and 57 ± 73 µmol m−2 h−1 of ammonium. Approximately 73% of the measured DIN flux was produced by Xestospongia muta, a massive barrel sponge. Of the 14 species studied, 10 hosted active nitrifying communities, and 8 hosted photosynthetic microbial associates. However, the presence of these microbial communities had no apparent effect on the magnitude of the total DIN flux. We estimate that the DIN flux for the entire nonencrusting sponge community is 640 ± 130 µmol m−2 h−1.
Mass coral spawning: A natural large‐scale nutrient addition experiment
Authors: Eyre Bradley D. , Glud Ronnie N. , Patten Nicole
Published: November 2010

A mass coral spawning event on the Heron Island reef flat in 2005 provided a unique opportunity to examine the response of a coral reef ecosystem to a large episodic nutrient addition. A post‐major spawning phytoplankton bloom resulted in only a small drawdown of dissolved inorganic phosphorus (DIP minimum = 0.37 mmol L−1), compared with almost complete removal of dissolved inorganic nitrogen (DIN) (minimum NO3− = 0.01 mmol L−1; NH4+ = 0.11 mmol L−1), suggesting that pelagic primary production is potentially N limited on the timescale of this study. DIN, DIP, dissolved organic nitrogen (DON), and dissolved organic phosphorus were used in the production of biomass, and mass balance calculations highlighted the importance of organic forms of N and P for benthic and pelagic production in tropical coral reef environments characterized by low inorganic N and P. The input of N and P via the deposition of coral spawn and associated phytodetritus resulted in large changes to N cycling in the sediments, but only small changes to P cycling, because of the buffering capacity provided by the large pool of bioavailable P. It is most likely that this large pool of bioavailable P in the sediments drives potential N limitation of benthic coral reef communities. For example, there was sufficient bioavailable P stored in the top 10 cm of the sediment column to sustain the prespawning rates of benthic production for over 200 d. Most of the change in benthic N cycling occurred via DON and N2 pathways, driven by changes in the quantity and quality of organic matter deposited and decomposed post‐major spawning. The heterotrophic and autotrophic microbial communities within the coral reef sands were able to rapidly (6 to 7 d) process the large episodic load of N and P provided by coral mass spawning.
Sources and transport of terrestrial organic matter in Vancouver Island fjords and the Vancouver‐Washington Margin: A multiproxy approach using d13Corg, lignin phenols, and the ether lipid BIT index
Authors: Walsh Elizabeth M. , Ingalls Anitra E. , Keil Richard G.
Published: November 2010

The Branched and Isoprenoid Tetraether (BIT) index (a ratio of marine and terrestrial glycerol dialkyl glycerol tetraether [GDGT] membrane lipids) was recently introduced as a proxy for the fraction of fluvially derived organic carbon (OC) buried in marine sediments. We investigate the source and distribution of terrestrial OC in marine sediments in a large spatial data set that encompasses the Washington‐Vancouver Island Margin, Vancouver Island fjords, and Puget Sound with the goal of comparing the BIT index to other traditional measures of terrestrial OC (δ13C of OC [δ13Corg] and lignin phenols). δ13Corg values and lignin concentrations indicate that the fjords are a significant trap for undegraded, wood‐derived terrestrial material, with surface sediments containing up to 76% terrestrial OC. These proxies also show significant terrestrial OC contributions to margin sediments (up to 54%), with most likely an angiosperm source from the Columbia River. In contrast to δ13Corg values and lignin concentrations, BIT index values indicate extremely low terrestrial carbon throughout the study area (BIT index values range from 0 to 0.28) and are uncorrelated with the other proxies. Though the BIT index did not correlate with other proxies, margin GDGT concentrations varied offshore similarly to other terrestrially derived OC and are well‐correlated with other proxies. We suggest that the BIT index is best interpreted as a proxy for soil‐ or peat‐derived OC. The majority of terrestrially derived OC delivered to sediment in this area has a lignin‐rich, nonsoil source. The disparity between the BIT index and other proxies implies that the BIT index significantly underestimates terrestrial relative to marine OC input to sediments in areas in which riverine OC input is not soil or peat derived.
Seasonal and ontogenetic variation in diel vertical migration of Chaoborus flavicans and its effect on depth‐selection behavior of other zooplankton
Authors: Lagergren Ragnar , Leberfinger Karolina , Stenson Jan A. E.
Published: November 2010

We analyzed diel vertical migration (DVM) of zooplankton in June, July, and September in Lake Kärnsjön, Sweden. In this lake, the density of the invertebrate predator Chaoborus flavicans was high, and they performed normal DVM. However, the migration pattern of Chaoborus differed between its four larval instars. Larger instars were found at greater mean depths during the day than were the smaller instars. In both June and July, very few observations of the fourth instar were made at a depth of 1 m during the night. Hence, in June, when only the fourth instar was present, the upper 1‐m layer was a Chaoborus‐free refuge for other zooplankton during the night. This was not the case in July, when the smaller instars (1‐3) hunt at that depth, or in September, when instar four was present there. Crustacean zooplankton showed great flexibility in their DVM behavior. In September, both Bosmina and small Daphnia tended to perform reverse DVM (i.e., they resided closer to the surface during the day). However, no migration was observed in July. In June, most species performed normal DVM by crowding in the Chaoborus‐free stratum close to the surface during the night. Both this pattern of normal DVM of zooplankton in June and the reverse DVM in September indicate that invertebrate predation has a strong effect on the zooplankton community in this fish‐rich lake, showing that ontogenetic changes in migration pattern of an invertebrate predator may influence the seasonal variation in the depth‐selection behavior of other zooplankton.
Vertical mixing and weak stratification over zebra mussel colonies in western Lake Erie
Authors: Boegman L. , Loewen M. R. , Hamblin P. F. , Culver D. A.
Published: November 2010

Zebra mussels (Dreissena polymorpha) are an invasive species that have been implicated in the reduction of algae stocks in the near‐shore environment of western Lake Erie. To determine their basin‐wide effects, we applied a two‐dimensional hydrodynamic and water‐quality model for 1994. The model accurately reproduced lake‐wide hydrodynamics and water quality. When modeled as true benthic organisms (resting on the bottom), the dreissenids grazed 53% of the western basin May through September net algal growth. This grazing resulted in a ~0.1‐mg L‐1 reduction in the pelagic algae concentration relative to the case without dreissenids. In comparison, dreissenids grazed 77% western basin net algal growth when the lake was modeled as a fully mixed water column. We found that the biomass grazed was governed by a balance between the timescales of vertical wind‐induced mixing and benthic grazing. During calm conditions, weak diurnal stratification (~1°C between surface and bottom waters) was sufficient to suppress vertical mixing, when the mean daily wind speed 4 m above the lake surface (U4) was ~6 m s‐1 . These conditions allowed a concentration boundary layer ~1 m thick to form, accounting for the reduced grazing effect relative to the fully mixed case. Entrainment of the concentration boundary layer occurred for U4>6 m s‐1 (associated with the lake's characteristic 10‐d storm cycle) facilitating algae supply to the benthos. We formulated the mean daily biomass grazed in terms of the dreissenid areal pumping rate (a) and U4 and found that because typically U4 is ~6 m s‐1 , the western basin is weakly stratified thermally and a concentration boundary layer forms when U4<3α or α>2 m3 m‐2 d‐1 . The dynamics of both wind‐mixing and thermal stratification must, therefore, be considered in mixing models applied to shallow weakly stratified lake basins.
Modeling the growth response of Cladophora in a Laurentian Great Lake to the exotic invader Dreissena and to lake warming
Authors: Malkin Sairah Y. , Guildford Stephanie J. , Hecky Robert E.
Published: November 2010

A Cladophora growth model (CGM) is calibrated and validated here to simulate attached and sloughed Cladophora biomass in daily time‐steps in an urbanized location of Lake Ontario, using two years of collected input data and independent measurements of Cladophora biomass. The CGM is used to hindcast Cladophora growth using multiplicative factors of seasonal minimal tissue phosphorus concentrations (QP) and seasonal mean nearshore light attenuation (KdPAR) of the early 1970s and 1980s relative to modern data. The possible effects of climate on growth are also forecast using additive temperature increases. Cladophora QP in Lake Ontario has declined in parallel with decreasing pelagic P concentrations, resulting in reduced Cladophora biomass at all depths in the euphotic zone. KdPAR has also declined, most strongly since the mid‐1990s, following Dreissena mussel invasion, driving an increase in biomass between 3.5‐ and 10‐m depth. Combining these effects, the CGM predicts that biomass along shorelines today is lower in Lake Ontario than in the 1980s. However, any increases in QP in this post‐dreissenid‐mussel period will result in greater Cladophora proliferation than in previous decades due to increased nearshore water clarity. Cladophora QP, while still currently lower than in the early 1980s, may be rising due to P supply to the littoral zone by the invasive mussels. The surface water temperature of Lake Ontario indicates warming of 0.96°C decade‐1 from 1980 to 2006. With increasing surface water temperatures, the CGM predicts an earlier spring growth but only a marginal increase in peak Cladophora biomass.
An empirical evaluation of the nutrient‐color paradigm for lakes
Authors: Webster Katherine E. , Soranno Patricia A. , Cheruvelil Kendra Spence , Bremigan Mary T. , Downing John A. , Vaux Peter D. , Asplund Timothy R. , Bacon Linda C. , Connor Jody
Published: November 2010

We analyzed a diverse set of 1,646 north‐temperate lakes to evaluate the nutrient‐color paradigm that integrates total phosphorus (TP) and colored dissolved organic carbon to define lake trophic status. Our objectives were to quantify the combined influence of TP and color (Col) on lake trophic status, to determine if TP and Col had similar relationships with hydrogeomorphic (HGM) variables, and to examine how TP and Col affected the balance of heterotrophic and autotrophic processes. For the latter we examined the Col to chlorophyll a ratio (Col : Chl a), an index of allochthonous contributions of carbon to pelagic consumers, and deviations of lake pCO2 from atmospheric, an index of net heterotrophy. Both Col and TP had strong effects on Chl a (positive) and Secchi transparency (negative), suggesting that ignoring Col would lead to misinterpretation of these widely used trophic status indicators. Lakes with high TP and Col tended to be shallower with large catchment to lake area ratios. Negative correlations with water retention time (WRT) were stronger for Col than for TP. Both TP and Col were related to forest and wetland land cover, although the direction of the relationships were opposite. Only 29% of the lakes had relatively high allochthony according to their Col : Chl a ratios; these were predominately high color, oligotrophic or mesotrophic lakes with short WRT. Over 90% of a subset of 682 lakes were net heterotrophic, with pCO2 exceeding atmospheric levels. The positive relationship between pCO2 and Col : Chl a suggests that only in very heterotrophic systems was the transfer of allochthonous carbon to pelagic consumers appreciable. Our results provide strong empirical support for the nutrient‐color paradigm and highlight its importance both for management applications and for expanding our understanding of how lakes are influenced by terrestrial subsidies of carbon and nutrients.
Increased tolerance to ultraviolet radiation (UVR) and cotolerance to cadmium in UVR‐acclimatized freshwater periphyton
Authors: Navarro E. , Robinson C. T. , Behra R.
Published: November 2010

We studied the long‐term acclimatization of freshwater periphyton communities exposed to low and high ultraviolet radiation (UVR) intensities that simulate UVR doses received by lowland and high‐mountain streams of central Europe. To assess changes induced by UVR, we compared the community structure (species and biomass), function (photosynthetic yield), and tolerance to UVR and cadmium of periphyton growing in microcosms (artificial channels). On the basis of the rationale behind the pollution‐induced community tolerance concept, we expected that an increase in UVR tolerance would be through the replacement of more sensitive taxa by more tolerant taxa. After 38 d of exposure, periphyton in the high‐UVR treatment was dominated by Cyanobacteria, whereas diatoms dominated periphyton in the low‐UVR treatment. Concomitantly, the high‐UVR community increased its tolerance to UVR and showed cotolerance to cadmium (Cd). Structural changes contributing to this increased tolerance included an increase in UVR‐absorbing compounds, and the formation of cell aggregates that increased self shading. Induction of antioxidant enzymes after UVR and Cd exposure might be involved as defense mechanisms against oxidative stress. These changes reduced the exposure and effects of UVR, resulting in the protection of photosynthesis (high‐UVR photosynthetic yield was unaffected). A fivefold reduction in chlorophyll a in the high‐UVR treatment suggested that acclimatization had high metabolic costs. Additional experiments showed that even though biomass accrual offered some protection against UVR and Cd, the community changes experienced by the high‐UVR community contributed the most to UVR tolerance. Periphyton exposed to high UVR may experience simultaneous positive (tolerance to UVR and cotolerance to Cd) and negative effects (biomass reduction can increase accessibility by toxicants).
Scale‐dependent carbon:nitrogen:phosphorus seston stoichiometry in marine and freshwaters
Authors: Sterner Robert W. , Andersen Tom , Elser James J. , Hessen Dag O. , Hood James M. , McCauley Edward , Urabe Jotaro
Published: November 2010

The classical Redfield ratio of carbon106 : nitrogen16 : phosphorus1 is a cornerstone of biogeochemistry. With the use of >2,000 observations of the chemistry of particulate matter from small and large lakes, as well as near and off‐shore marine environments, we found that the best model to describe seston stoichiometry depended on the scale of analysis. We also found that there were better estimates for seston chemistry than the classical ratio for all habitats, whether freshwater or marine. Across the entire data set, a constant proportionality of C166:N20 : P1 (±error) described the data, which implies higher C sequestration per unit of N and P in surface waters than given in the classical ratio. At a regional scale, however, C: P and C:N often declined with increasing seston abundance, rejecting a constant ratio model. Within both freshwater and marine habitats, higher seston abundance is often associated with lower C: P and C:N ratios (higher nutrient content). The difference in appropriateness of the constant ratio model with respect to the entire data compared with subsets of the data indicates a scale dependence in stoichiometric relationships in seston C:N: P ratios. Given these consistent shifts in seston chemistry with particle abundance, the narrower variation in seston chemistry associated with marine seston chemistry could occur because of a reduced range of particulate nutrient concentration. For all but the largest scales, the classical Redfield model of biogeochemical cycling should be replaced with a more general power function model.
Growth rates of six coccolithophorid strains as a function of temperature
Authors: Buitenhuis Erik T. , Pangerc Tanja , Franklin Daniel J. , Le Quéré Corinne , Malin Gill
Published: November 2010

We determined growth rates of six coccolithophorid strains (five species) as a function of temperature. We grew four strains (three species: Emiliania huxleyi, Gephyrocapsa oceanica, and two strains of Calcidiscus leptoporus) at six temperatures between 6°C and 25°C, Coccolithus braarudii at four temperatures, and Syracosphaera pulchra at two temperatures. The growth rates were to a large extent consistent with the biogeographical distributions of these species. C. braarudii grows relatively fast at low temperatures, the two strains of C. leptoporus have temperature optima of 12°C and 20°C, E. huxleyi has an optimum at 20°C, and the growth rate of G. oceanica (and S. pulchra) increases up to the highest tested temperature of 25°C. This shows that maximum growth rate is an important factor in controlling distribution in the ocean, but it is not the only one.
Could krill schools significantly bias 234Th‐based carbon flux models?
Authors: Rodriguez y Baena Alessia M. , Fowler Scott W. , Warnau Michel
Published: November 2010

To test the recent hypothesis that krill schools and their molting can hinder the accuracy of 234Th‐based particle flux models under specific conditions, radiotracer experiments were conducted with northern krill, Meganyctiphanes norvegica, to examine how 234Th uptake and release kinetics may affect residual 234Th concentrations in seawater. Krill rapidly took up 234Th from seawater, reaching a steady‐state concentration factor of ~180 in less than 1 week, and strongly retained the accumulated radionuclide until molting occurred. The nine molting events observed during the 18‐d experimental period indicated that ~50% of krill whole‐body 234Th activity was associated with the cast exoskeleton, and that a corresponding amount was rapidly adsorbed postecdysis on the newly formed cuticle. In regions characterized by the occurrence of high planktonic crustacean biomass, failure to link the presence of organisms with their effects on 234Th dynamics in the water column may induce significant biases (in some circumstances, up to one order of magnitude) in 234Th‐derived particulate organic carbon flux estimations or sediment trap efficiency calculations based on this method. However, because of the patchy distribution of dense aggregations of krill, the overall likelihood of such dramatic modeling artifacts is estimated to be relatively low (≤5% per sampling time).
Element content of Pseudomonas fluorescens varies with growth rate and temperature: A replicated chemostat study addressing ecological stoichiometry
Authors: Chrzanowski Thomas H. , Grover James P.
Published: November 2010

Ecological stoichiometry is emerging as a central organizing framework upon which our perceptions of aquatic trophic dynamics are being reshaped. The microbial component of aquatic systems is crucial to overall nutrient dynamics, yet little data are available addressing the ecological stoichiometry of microorganisms. Pseudomonas fluorescens, a commonly encountered bacterium, was used as a model organism to investigate the relationships among temperature, growth rate, and element stoichiometry. P. fluorescens was grown in chemostats at low dilution rates (ranging between 0.03 and 0.13 h‐1) and realistic environmental temperatures (ranging between 14°C and 28°C). Cells accumulated elements as an interactive function of temperature and growth rate. The highest element concentrations corresponded to cells growing slowly under low temperatures and to cells growing rapidly under warm conditions. Additionally, small cells had higher concentrations of elements than did large cells. Element ratios (C : N, C: P, and N: P) varied more as a function of growth rate than of temperature. The same dissolved resource pool could conceivably yield bacteria of differing element content simply as an interactive function of growth rate and temperature.
Typhoons initiate predictable change in aquatic bacterial communities
Authors: Jones Stuart E. , Chiu Chih-Yu , Kratz Timothy K. , Wu Jiunn-Tzong , Shade Ashley , McMahon Katherine D.
Published: November 2010

We explored patterns of change in bacterioplankton and phytoplankton community composition in response to typhoons in a subtropical, subalpine freshwater humic lake in Taiwan. Thermal profiles with depth, nearsurface dissolved oxygen, and meteorological conditions were measured at high temporal resolution using an instrumented buoy. We collected samples across a time series spanning two typhoon seasons and assessed bacterial community composition at each time point using a fingerprinting technique targeting the internal transcribed spacer region of the ribosomal RNA operon. Wind and rain associated with the typhoons mixed the otherwise stably stratified water column. After each of six typhoon‐induced mixing events, the bacterial community composition was reset to similar conditions, revealing a surprisingly deterministic pattern of recovery that was reminiscent of secondary succession following large infrequent disturbances in terrestrial ecosystems. Distinct bacterial assemblages were associated with epilimnion and hypolimnion samples collected more than 80 d after a mixing event. These assemblages were also distinct from the communities observed in the generally unstratified water column within 7 d after a typhoon. Our results were surprising since bacterial communities are not expected to respond in such predictable ways because of their immense complexity and diversity. In contrast, phytoplankton communities did not recover in a predictable way after typhoons.
A bush, not a tree: The extraordinary diversity of cold‐water basal foraminiferans extends to warm‐water environments
Authors: Habura Andrea , Goldstein Susan T. , Broderick Sarah , Bowser Samuel S.
Published: November 2010

Foraminiferal species distribution and richness have traditionally been assessed through examination of the shells (tests) built by most of the described species, but molecular surveys indicate that this approach likely underestimates true foraminiferal diversity. We use a series of targeted DNA‐based surveys and focused morphological searches to document the diversity of allogromiid foraminiferans, which have less distinctive tests, at selected locations along the U.S. eastern seaboard. These sites, although well studied by previous foraminiferal researchers, have not been reported to host large numbers or diverse assemblages of allogromiids. Both survey methods revealed the presence of many new allogromiid taxa and documented striking differences in the allogromiid assemblages between the three locations. The assemblage at a given site as determined by morphological criteria also differed substantially from that determined by molecular surveys. Combined with information from other locales, our data strongly suggest that allogromiid diversity is underestimated; indeed, allogromiids may actually dominate many benthic foraminiferal assemblages. This finding has significant implications for benthic trophodynamics and for the reconstruction of paleoenvironments because the morphological inconspicuousness and taphonomic fragility of allogromiid foraminiferans may lead to underestimates of total foraminiferal diversity and abundance.
Intrinsic growth and microzooplankton grazing on toxigenic Pseudo‐nitzschia spp. diatoms from the coastal northeast Pacific
Authors: Olson M. Brady , Lessard Evelyn J. , Cochlan Lessard , Trainer Vera L.
Published: November 2010

We investigated the population ecology of toxigenic diatoms within the genus Pseudo‐nitzschia on the Pacific Northwest coast during 2003, 2004, and 2005. Pseudo‐nitzschia spp. were widespread and abundant across the region, and the maximum density reached at our experimental stations was ~7 × 106 cells L‐1. Pseudo‐nitzschia spp. biomass did not correlate with total phytoplankton biomass, indicating that the growth response and/or mortality rate of Pseudo‐nitzschia spp. were dissimilar to other phytoplankton. In dilution experiments across wide‐ranging ocean conditions, Pseudo‐nitzschia spp. intrinsic growth rates were moderate to high (mean = 0.97 d−1, range 0.24 d−1 to 2.30 d−1, n = 36), and they were consistently higher than the corresponding growth rates of the aggregate <5−mm and >5‐mm chlorophyll a (Chl a) communities. Pseudo‐nitzschia spp. growth was predicted by irradiance and temperature but not ambient nitrate concentration, whereas both Chl a size fractions showed dependence upon nitrate for growth. Microzooplankton grazing rates on Pseudo‐nitzschia spp. were moderate (mean = 0.32 d‐1, range 0.00 d‐1 to 1.70 d‐1, n = 36); they were nearly always lower than corresponding Pseudo‐nitzschia spp. intrinsic growth, significantly lower than the grazing rates on the <5‐mm Chl a size fraction, and comparable to grazing rates on the >5‐mm Chl a size fraction. Our results show that the strong competitive fitness of Pseudo‐nitzschia spp. results more from expression of characteristics that enable sustained high growth during variable and unfavorable conditions than from intrinsic adaptations that reduce grazing mortality.
Long‐term photochemical and microbial decomposition of wetland‐derived dissolved organic matter with alteration of 13C:12C mass ratio
Authors: Vähätalo Anssi V. , Wetzel Robert G.
Published: November 2010

We investigated the long‐term photochemical and microbial decomposition of biologically recalcitrant humic‐like dissolved organic matter (DOM) leached from a vascular wetland plant, the common rush (Juncus effusus). Although the leachate would have been characterized as biologically recalcitrant by short‐term (<14 d) bioassays, microbes decomposed 51% of its organic carbon in 898 d with a first‐order biological decomposition coefficient of 0.0008 d‐1 in darkness. Solar radiation exposure accelerated the decomposition of leachate. Under 459‐d exposure to surface solar radiation, up to 90% of organic carbon was mineralized. During the exposure, the photochemical reactions preferentially mineralized the 12C fraction of organic carbon and enriched the 13C of organic carbon by 6% in the residual leachate. Solar radiation also decomposed nearly completely (up to 99.7%) chromo‐ and fluorophores of DOM. A 439‐d bioassay following the solar radiation exposure resulted in up to 97.3% mineralization of organic carbon. Solar radiation together with microbial metabolism can completely mineralize at least some forms of wetland‐derived DOM in surface waters with sufficiently long residence times.
Geochemical controls on anaerobic organic matter decomposition in a northern peatland
Authors: Beer Julia , Lee Kern , Whiticar Michael , Blodau Christian
Published: November 2010

The decomposition of deep peat deposits controls the long‐term carbon balance of peatlands but is poorly understood with respect to rates and controls. To rectify this deficiency, we estimated in situ dissolved inorganic carbon (DIC) and methane (CH4) production rates from a beaver pond to a central bog dome and related them to organic matter properties, Gibbs free energies of respiration, and δ13C values of DIC and CH4. DIC and CH4 production decreased from maxima of ~10 nmol cm‐3 d‐1 near the water table to values <0.1 nmol cm‐3 d‐1 at depths >1 m, and there was little differentiation among sites. Deeper into the peat, we measured an accumulation of DIC, CH4, and dissolved organic matter (DOM) enriched in aromatic and phenolic moieties, which resulted from the slowness of diffusive vertical pore‐water movement. Lack of transport may have slowed decomposition in two ways: (1) Aromatic and phenolic DOM moieties accumulated, while the release of carbohydrate‐rich DOM from peat was apparently impeded. (2) The accumulation of DIC and CH4 reduced Gibbs free energy of acetoclastic methanogenesis toward a critical threshold value of ‐25 to ‐20 kJ mol‐1 CH4. Hydrogenotrophic methanogenesis was energetically more favorable and generally dominated according to an isotopic fractionation between CO2 and CH4 of 1.053 to 1.076, but it was apparently impeded by some other factor. We conclude that lateral homogeneity and slowness of decomposition in geologically sealed deep peat deposits are assisted by a lack of solute transport, which facilitates the formation of deep peat deposits over millennia.
Temporal variations in organic carbon species and fluxes from the Chena River, Alaska
Authors: Cai Yihua , Guo Laodong , Douglas Thomas A.
Published: November 2010

Water samples were collected biweekly from the Chena River, Alaska, during 2005‐2006 for analysis of dissolved organic carbon (DOC), dissolved inorganic carbon (DIC), total dissolved carbohydrate (TCHO), including monosaccharide (MCHO) and polysaccharide (PCHO), particulate organic carbon (POC) and its isotopic compositions, and Si(OH)4. Carbon species exhibit strong temporal variations with elevated DOC, POC, and TCHO but depleted DIC and Si(OH)4 during the spring freshet and decreased DOC, POC, and TCHO but elevated DIC and Si(OH)4 concentrations under winter ice. Organic matter is mostly derived from surface soil leaching, whereas DIC and Si(OH)4 are associated with groundwater and mineral layer leaching. On average, DIC was the predominant carbon species, accounting for 77% ± 13% of the total carbon pool, whereas DOC and POC comprised 19% ± 10% and 4% ± 4%, respectively. However, DOC became the dominant carbon species during the spring freshet. TCHO comprised 15% ± 4% of DOC with higher CHO:DOC ratios during spring runoff and summer. Within the TCHO pool, MCHO was the predominant CHO component (89% ± 10%), leaving 11% ± 10% as PCHO. The particulate organic matter source during the summer drought season was mostly autochthonous, with low POC: chlorophyll a and C:N but high POC: suspended particulate matter ratios and depleted δ13C values. The annual yields of DOC, DIC, and POC from the basin are 133 ± 8, 361 ± 7, and 27 ± 4 x 103 mol C km‐2, corresponding to an export flux of 6.9 ± 0.4, 18.8 ± 0.4, and 1.4 ± 0.2 x 108 mol C yr‐1, respectively. Anticipated changes in hydrological and biogeochemical cycles in high‐latitude watersheds undergoing climate warming will likely be reflected in the chemical and phase speciation of carbon and other elements.
Patterns of functional biodiversity and function‐environment relationships in lake littoral macroinvertebrates
Authors: Heino Jani
Published: November 2010

I examined variability in the abundances of functional groups, functional diversity measures, and functional structure of littoral macroinvertebrate communities in relation to the environmental features of boreal lakes. The most important environmental variables shaping variation in the abundances of functional groups and functional structure were lake surface area, macrophyte cover, total phosphorous, and water hardness. The same environmental variables (i.e., lake surface area, macrophyte cover) accounted for variability in functional richness and functional diversity, while functional evenness was related to different environmental variables (i.e., hardness, color). Lake surface area and macrophyte cover comprised the most important axes of habitat templets shaping the functional trait structure and biodiversity in boreal lakes: lake area mirrors habitat differences between smaller and larger lakes, and macrophyte cover portrays the effects of habitat structural complexity on functional biodiversity. Functional biodiversity measures were also strongly correlated to species‐level measures, and the correlation between similarity in functional and taxonomic structure was strong. Functional and taxonomic measures of macroinvertebrate communities thus provide rather similar information about littoral communities and ecosystem functioning.
The influence of the Atlantic and regional climate variability on the long‐term changes in gelatinous carnivore populations in the northwestern Mediterranean
Authors: Molinero Juan Carlos , Casini Michele , Buecher Emmanuelle
Published: November 2010

We investigated potential connections between climate and the population size of gelatinous carnivores in the northwestern Mediterranean. The interannual variability of large‐ and meso‐scale climate factors was related to gelatinous carnivore dynamics by means of a multivariate regression model. Gelatinous carnivore population dynamics integrate the climate‐related changes in the northwestern Mediterranean with close correlations between climate and the population size of the Calycophoran siphonophores Chelophyes appendiculata and Abylopsis tetragona and the Hydromedusae Rhopalonema velatum and Solmundella bitentaculata. We show that the sensitivity of particular gelatinous carnivore species to environmental forcing relates to their seasonal appearance and peak of abundance. Our results advocate the occurrence of short time windows, during which gelatinous carnivores appear more sensitive to environmental conditions, and favorable conditions may therefore substantially enhance their annual peaks. Furthermore, there were threshold values from which climate effects on gelatinous carnivores become noticeable, indicating that the climate‐gelatinous carnivore relationship intensifies according to the strength of climate forcing. The possibility of using the North Atlantic climate variability for assessing and predicting interannual abundance changes of these organisms in the northwestern Mediterranean is considered.
Selection by a deposit‐feeding polychaete, Polygordius jouinae, for sands with relatively high organic content
Authors: Ramey Patricia A. , Bodnar Erica
Published: November 2010

Field observations and flume experiments investigated whether subsurface migration and selection by adult Polygordius jouinae for sediment enriched with organic flocs could help explain small‐scale patchiness of this deposit‐feeding polychaete in rippled, sandy sediments on the inner continental shelf off New Jersey. Paired ripple crest‐trough samples taken from stations located kilometers apart in several months in 1994, 1995, and in May 2006 found higher but more variable densities of P. jouinae in troughs compared with crests. Sedimentary particulate organic matter was also patchily distributed at similar spatial scales. In a racetrack flume under realistic flow (shear velocity = 0.32 cm s‐1) and flatbed conditions, arrays of alternating, fresh ambient sediment (including natural levels of particulate organic material) and freshly sieved sediment (reduced in particulate organic content) showed significant subsurface movement of P. jouinae to sediment patches containing higher amounts of particulate organic matter in 48 h. Subsequent experiments showed that locating organic patches was not the consequence of a directed search. However, worms that located richer patches remained there and fed. Rate of movement in sediments indicated that P. jouniae could potentially travel the wavelength of a typical ripple (14‐30 cm) at the Long‐term Ecosystem Observatory in 15‐m water depth in 35‐75 min. Thus in a dynamic environment where food concentrations are low and patchy, the affinity of P. jouinae for particulate organic matter and its undirected, high rate of subsurface movement is a plausible mechanism to account for the similar spatial distributions of P. jouinae and its food resources in continental shelf sediments.
Body temperature during low tide alters the feeding performance of a top intertidal predator
Authors: Pincebourde Sylvain , Sanford Eric , Helmuth Brian
Published: November 2010

Although the direct effects of climate change on species distribution and abundance have become increasingly apparent, considerably less is known about the potential for thermal variations to influence community structure indirectly through altered species interactions. We examined how the low tide body temperature of the rocky intertidal sea star Pisaster ochraceus affected the rate at which this keystone species fed on the mussel Mytilus californianus. Patterns of intertidal distribution and sea star body temperatures were recorded in the field at Bodega Bay, California, in 2006 and 2007. Under controlled laboratory conditions, we separately examined the roles of short‐term (acute) and long‐term (chronic) exposures to high aerial body temperatures in driving patterns of feeding and growth. Upper limits to thermal tolerance were also quantified in the laboratory. Integrating field data on the vertical distribution, microhabitat, and body temperature of sea stars indicated that individuals typically experienced relatively high body temperatures over a series of short periods of time (acute exposures of a few days) and that they escaped from situations where they would have experienced such temperatures for longer periods (chronic exposure). Although sea stars never approached their upper thermal limit (35°C), laboratory trials demonstrated that chronic exposure to realistic peak aerial body temperatures (≥23°C) negatively affected both feeding and growth rates, whereas acute exposure had positive consequences on feeding rates but no effects on growth. Variations in aerial body temperature, and more specifically the frequency of exposure to high body temperatures, can influence interaction strength in this important predator‐prey system.
Transport mechanism and paleoclimatic significance of terrigenous silt deposited in varved sediments of an African rift lake
Authors: Johnson Thomas C. , McCave I. N.
Published: November 2010

We analyzed a varved sequence of sediment from a 350‐m depth in the north basin of Lake Malawi, East Africa, for the size distribution of the sortable silt fraction (10‐64 µm). Mean size of the sortable silt (mean SS) varies measurably in sediments spanning the past 650 yr and covaries with the mass accumulation rate of terrigenous silt and clay (TMAR) over much of the interval. Most of the silt and clay is delivered to the offshore basin in benthic nepheloid plumes of unknown duration and frequency. TMAR appears to be related to annual rainfall (which is related to the North Atlantic Oscillation) because it roughly tracks the historical record of lake level that extends back to 1860. Mean SS should be related to density or thickness of underflow, thus related to resuspension intensity or river flood loading. It also tracks lake level and regional wind strength as determined by National Center for Environmental Prediction (NCEP) Reanalysis.
Relationship of predation potential to mortality of Calanus finmarchicus on Georges Bank, northwest Atlantic
Authors: Ohman Mark D. , Durbin Edward G. , Runge Jeffrey A. , Sullivan Barbara K. , Field David B.
Published: November 2010

Based on 29 cruises on Georges Bank between January and June, conducted as part of the U.S. Global Ocean Ecosystem Dynamics program, we describe seasonal and spatial variability of early life history mortality for the planktonic copepod Calanus finmarchicus and relate mortality to an index of predation potential from a suite of suspension‐feeding predators. Emphasis is placed on the earliest life history phase wherein peak mortality occurs. Instantaneous mortality rates for a combined egg‐through‐nauplius‐3 stage show a recurrent seasonal pattern of a modest elevation in January followed by a seasonal peak in May of all 5 study yr. Spatial differences exist in mortality rates, with a higher probability of mortality on the southern flank in winter and a pronounced seasonal maximum on the bank crest in May. Three hypotheses associated, respectively, with egg hatching success, advection, and invertebrate predation were evaluated to account for the seasonal and spatial mortality patterns. Variations in egg‐hatching success are inconsistent with the observed seasonal patterns of loss. Off‐bank advection, particularly on the southern flank, appears to be the dominant loss term in January. Apart from this winter period, egg mortality is associated with predation potential. Comparison of an index of daily rates of egg loss with an index of consumption rates of eggs by suspended hydroids, hydromedusae, and five species of planktonic copepods show general agreement between mortality and predation in both seasonal variation and spatial patterns. Of the predator taxa, late copepodid and adult C. finmarchicus, supplemented by planktonic hydroids, appear to have the largest quantitative predation effect.
Seasonal and vertical variations in emergence behaviors of Neomysis americana
Authors: Sato1 Mei , Jumars Peter A.
Published: November 2010

Emergence patterns were observed in the macrotidal Damariscotta River estuary in Maine in fall 2005 and summer 2006. High temporal and vertical resolution was achieved with two multifrequency echo sounders sampling from 265 kHz to 3 MHz. Time‐series waterfall plots and spectral analysis revealed that emergence patterns observed at 265‐420 kHz (frequencies that detect animals larger than mesozooplankton) followed combinations of diel and tidal rhythms that differed with both depth in the water column and season. The dominant emergent species at the study site, Neomysis americana, showed nocturnal emergence in summer. Toward the end of its emergence season in fall, however, its dominant rhythm shifted to semidiurnal (12.4‐h period). The timing of major emergence in the fall coincided with low slack tides near the surface but with peak flood speed near the bottom, providing plausible mechanisms of retention and selective tidal stream transport within the estuary. Our results confirm earlier suggestions from net sampling that emergence in this species is best conceived as a broadening of the vertical spread of the population distribution rather than a migration of the bulk of the population far off the bottom: Most individuals spent most of their time near the bottom, even during emergence. Prior acoustic work that did not resolve the lowermost water column reported emergent mysids to overwhelm the holoplankton in biovolume, but it, too, seriously underestimated mysid abundance because it failed to resolve this high concentration of animals in the lowermost water column. In the mesozooplankton size category, emergence reached surface waters only during low slack tides and systematically avoided times of the fastest currents. This study highlights the importance of high temporal and spatial sampling resolution in detecting and understanding components of emergence and points away from simple models of diel vertical migration toward state‐dependent, individual‐based models of habitat utilization.