bluuebunny/crossref_metadata_2025 huggingface dataset:
Seasonal dynamics of benthic O2 uptake in a semienclosed bay: Importance of diffusion and faunal activity
Authors: Glud Ronnie N. , Gundersen Jens K. , Røy Hans , Jørgensen Bo Barker
Published: July 2010

The benthic O2 uptake and the O2 microdistribution in a coastal sediment of Aarhus Bay, Denmark, were investigated during a seasonal study. Measurements were performed in situ by a profiling lander and a flux chamber lander, as well as on recovered sediment cores. The O2 penetration depth, the diffusive O2 uptake, and the volumespecific O2 consumption rate strongly depended on the seasonal changes in bottom water O2 concentration and the sedimentation of organic carbon. The in situ O2 penetration depth varied between 0.5 mm in summer and 4.5 mm in winter. The diffusive O2 uptake varied between 8 and 30 mmol m−2 d−1, whereas the volume‐specific O2 consumption rate varied by a factor of 13. The O2 distribution was very sensitive to environmental controls, and microprofiles obtained in the laboratory tended to overestimate the in situ O2 penetration depths and underestimate the in situ diffusive O2 uptake. Three‐dimensional O2 flux calculations based on in situ microtopographic mapping showed that the actual diffusive exchange rate was ~10% higher than the simple one‐dimensional, microprofilederived diffusive O2 exchange. The total O2 uptake measured in the laboratory showed less distinct seasonal variation, but on the average, it was ~20% higher than the diffusive O2 uptake. The difference reflected the microtopography of the sediment surface and the contribution from benthic macrofauna. In situ total O2uptake was generally twice as high as laboratory rates, reflecting a higher fauna‐related O2 consumption in the larger enclosures incubated in situ. Annually, the in situ three‐dimensional diffusive O2 consumption was 6.2 mol O2 m−2, whereas the additional benthos‐mediated O2 uptake was 3.9 mol O2 m−2. Thus, 40% of the total O2 uptake was due to faunal activity and respiration. The present study demonstrates the importance of realistic faunal representation during sediment incubations in order to obtain correct benthic mineralization rates.
Predicting wave exposure in the rocky intertidal zone: Do bigger waves always lead tolarger forces?
Authors: Helmuth Brian , Denny Mark W.
Published: July 2010

Hydrodynamic forces from breaking waves are among the most important sources of mortality in the rocky intertidal zone. Information about the forces imposed by breaking waves is therefore critical if we are to interpret the mechanical design and physiological performance of wave‐swept organisms in an ecologically and evolutionarily relevant context. Wave theory and engineering experiments predict that the process of wave breaking sets a limit on the maximum force to which organisms can be subjected. Unfortunately, the magnitude of this limit has not been determined on rocky shores. To this end, at a moderately exposed shore in central California, we measured the maximum hydrodynamic forces imposed on organism‐sized benthic objects and related these forces to nearshore significant wave heights. At 146 of 221 microsites, there was a significant and substantial positive correlation between force and wave height, and at 130 of these microsites, force increased nonlinearly toward a statistically defined limit. The magnitude of this limit varied among sites, from 19 to 730 newtons (N). At 37 other sites, there was no significant correlation between surf zone force and wave height, indicating that increased wave height did not translate into increased force at these sites either. At only 16 sites did force increase in proportion to wave height without an apparent upper bound. These results suggest that for most microsites there is indeed a limiting wave height beyond which force is independent of wave height. The magnitude of the limit varies substantially among microsites, and an index of local topography was found to predict little of this variation. Thus, caution must be exercised in any attempt to relate observed variations in ocean"waviness" to the corresponding rates of microsite disturbance in intertidal communities.
Zooplanktivory by a nocturnal coral‐reef fish: Effects of light, flow, and prey density
Authors: Holzman Roi , Genin Amatzia
Published: July 2010

Visual predation by nocturnal zooplanktivorous fish is assumed to be sufficiently limited to allow the evolution of vertical migration as means for predation avoidance by zooplankton. However, in situ measurements of predation rates by nocturnal fish are lacking. Most of our knowledge of this predation is based on stomach contents and laboratory experiments. Our objectives were to measure the in situ rate of zooplanktivory by a common, nocturnal coral‐reef fish, Apogon annularis, and to assess, through laboratory and field experiments, the effects of light, prey density, and flow on the fish’s predation. In situ, the fish selectively fed on large zooplankton (>1 mm) at a rate of 0.12 prey min−1, or 52% efficiency. Predation rates increased linearly with prey density, with no apparent effects of current speed and light intensity. Flume experiments indicated that feeding rates were saturating at a level corresponding to that found at 3 m depth on a moonless night and were negligible at >18 m. Predation rates by A. annularis were two orders of magnitude lower than those of diurnal fishes; however, the difference in carbon gain was largely offset by the greater size of nocturnal prey. Nocturnal vision in A. annularis is sufficiently sensitive to allow a remarkable detectability of large prey in its natural habitat. If this fish is representative of other nocturnal fishes, reconciliation between the potential for substantial nocturnal predation, as is demonstrated in the present study, and the evolution of vertical migration in large zooplankton could be related to the total abundance of those fishes and their depth distribution. Both parameters are as yet poorly known.
Influence of zinc and iron enrichments on phytoplankton growth in the northeastern subarctic Pacific
Authors: Crawford D. W. , Lipsen M. S. , Purdie D. A. , Lohan M. C. , Statham P. J. , Whitney F. A. , Putland J. N. , Johnson W. K. , Sutherland N. , Peterson T. D. , Harrison P. J. , Wong C. S.
Published: July 2010

Near‐surface seawater from the northeastern subarctic Pacific was incubated on deck for 8 d, supplemented with (1) control, no additions (2) +Zn (3) +Fe (4) +Zn+Fe. Concentrations of total Zn and Fe at time zero (t0) and in the control remained at ~0.1–0.2 nmol L−1. In the control, chlorophyll (=0.3 mg m−3), 14C uptake into POC and PIC, and inorganic nutrients all remained relatively constant. Addition of Zn slightly but significantly increased chlorophyll (p = 0.05), decreased phosphate (p = 0.01) and nitrate (p = 0.05), and in P versus E experiments, increased Pm >10‐fold and Pmchl 2–3‐fold. The abundance of small diatoms and coccolithophores was higher in the +Zn treatment compared to the control. The +Fe and +Zn+Fe treatments, compared to the control, both showed >10‐fold increases in chlorophyll and 14C uptake into POC and PIC and complete removal of nitrate (≤0.2 mmol m−3). However, differences were observed in size‐fractionated data; the +Zn+Fe treatment had significantly lower percent chlorophyll in the >20‐ µm fraction (p = 0.01) and a higher percentage in the 0.2–5‐ µm fraction (p = 0.01) than the +Fe treatment. In P versus E experiments, both +Fe treatments increased Pm and α around 100‐fold and Pmchl and αchl by 5–10‐fold compared to the control. The +Fe treatment showed a slightly higher αchl and slightly lower Pmchl than the +Zn+Fe treatment. Abundance of large diatoms, small diatoms, small flagellates, and coccol‐ chl m ithophores all increased substantially (~7–1,000‐fold) in response to Fe addition, whereas dinoflagellate abundance only doubled. The +Zn+Fe treatment had higher abundances of small diatoms and small flagellates than the +Fe treatment. We conclude that Zn additions had limited influence on conventional indices of phytoplankton growth compared to Fe, but that there might be subtle influences of Zn that require further attention.
Effects of drought‐induced acidification on diatom communities in acid‐sensitive Ontario lakes
Authors: Faulkenham Shari E. , Hall Roland I. , Dillon Peter J. , Karst-Riddoch Tammy
Published: July 2010

Chemical recovery of many acidified lakes in North America has been delayed or reversed as a result of interactions between climatic variability (alterations between drought and nondrought periods) and previously deposited acids stored in wetlands, but effects of this wetland‐mediated reacidification phenomenon on aquatic biota remain unknown. We compare changes in diatom assemblages in 200‐yr‐long sediment cores from two lakes with similar basin characteristics but different wetland area (4.4% of catchment at Chub Lake, 0% at Blue Chalk Lake) to evaluate the role of wetland‐mediated interactions among acid deposition and climatic variability on algal communities in acid‐sensitive lakes. Diatom assemblages were significantly more variable in Chub Lake than in Blue Chalk Lake. Variance partitioning analysis of approximately annually resolved sedimentary diatom analyses (1977–1997) identified that unique effects of water chemistry, independent of acid deposition and climatic factors, accounted for the greatest amount of variation (25%) in diatom assemblages at Chub Lake. Acid deposition (24%) and climatic factors (22%) accounted for similar, significant amounts of variation in diatom communities. Complex interactions among all three factors, which are attributable to wetland‐mediated drought‐induced reacidification, explained an additional 10% of the variation in diatoms at Chub Lake but only 1% at Blue Chalk Lake. Droughtrelated reacidification effects on water chemistry might thus cause important effects on algal communities in acid sensitive lakes with modest wetland coverage, but not in lakes without wetlands.
Viability of invertebrate diapausing eggs collected from residual ballast sediment
Authors: Bailey Sarah A. , Duggan Ian C. , van Overdijk Colin D. A. , Jenkins Philip T. , MacIsaac Hugh J.
Published: July 2010

Natural or anthropogenic movement of sediments may be an important vector for the dispersal of invertebrate resting stages between water bodies. Here we record the presence of invertebrate diapausing eggs in residual sediments from transoceanic vessels and explore whether these may pose an invasion risk. Viability of diapausing eggs was explored under light and dark conditions using sediment collected from eleven tanks on nine vessels operating on the Great Lakes. Seventeen cladoceran, copepod, and rotifer taxa were identified. Four of the species hatched have not yet been reported as established in the Great Lakes. Egg viability for individual species varied from 0% to 92%. Exposure to saline water may impact egg viability of some freshwater species. Generally, the proportion of eggs hatched in light and dark treatments did not differ significantly, indicating that light was not required to terminate diapause. As a result, eggs could potentially hatch in dark ballast tanks when immersed in freshwater loaded as ballast during operation on the Great Lakes. Viability of diapausing eggs differed among ballast tanks on a single vessel, indicating that tanks with independent ballast histories have different invasion risks. While additional work is needed to quantify risk, results from this study indicate that vessels entering the Great Lakes with only residual ballast are a potential vector for the introduction of new nonindigenous species during multiport operations.
Predicting herring recruitment from young‐of‐the‐year densities, spawning stock biomass, and climate
Authors: Axenrot Thomas , Hansson Sture
Published: July 2010

Because fish are key organisms in most aquatic ecosystems, we seek to understand what determines their highly variable reproductive success. From our work, it appears that yearclass strength of Baltic Sea herring (Clupea harengus L.) can be predicted from young‐of‐the‐year densities in a small coastal area (hydroacoustic data), a climate index (the North Atlantic Oscillation), and the spawning stock biomass. These three factors explained 93% of the variation in the number of age 2 herring during 1985–2000. By predicting year‐class strength 3 yr before the fish enter the fishery, we provide managers with the opportunity to adjust fishing pressure per upcoming year classes and manage the fishery by multiannual catch quotas.
Elemental composition of single cells of various strains of marine Prochlorococcus and Synechococcus using X‐ray microanalysis
Authors: Heldal M. , Scanlan D. J. , Norland S. , Thingstad F. , Mann N. H.
Published: July 2010

We measured the elemental composition of single cells from six strains of marine Prochlorococcus and two strains of marine Synechococcus by X‐ray microanalysis in the transmission electron microscope (TEM), which allows measurements of all major elements in cells without any fixation or staining. The mean carbon : volume ratios ranged from 136 to 280 fg µm−3 in Prochlorococcus and 138 to 290 fg µm−3 in Synechococcus. Using mean values of elemental content of C:N:O: P : S, we obtained molar ratios of 143–214 : 15–24 : 15–41 : 1 : 0.58–1.64, and 65–293 : 7–36 : 11–36 : 1 : 0.31–1.54 for strains of Prochlorococcus and Synechococcus, respectively. The diffusible ions Mg++ (89–490 mmol L−1) and Na+ (230–660 mmol L−1) dominated in Prochlorococcus. In Synechococcus Na+ (90–480 mmol L−1) dominated. For all samples the range of K+ content was 23–130 mmol L−1. Elemental composition varied widely in relation to strains and growth media. However, C: P and N: P ratios were above the Redfield ratio in all Prochlorococcus strains and one of the Synechococcus strains (WH 8103). This likely reflected the low P content of these cells. This low P requirement would be clearly advantageous in the oligotrophic conditions occupied by these organisms. The high relative carbon content, compared to what has been found for heterotrophic bacteria using similar methods, is suggested to confer a competitive advantage to photosynthetic over heterotrophic bacteria in ecosystems where both functional groups are mineral nutrient limited.
Planktonic carbon cycling and transport in surface waters of the highly urbanized Hudson River estuary
Authors: Taylor Gordon T. , Way Juliette , Scranton Mary I.
Published: July 2010

We examined variations in organic carbon (OC) pools and microplanktonic carbon fluxes at three stations in the Hudson River estuary during 12 cruises between Octobers of 1996 and 1998. Phytoplankton biomass and net primary production varied from 5 to 40 µmol C L−1 and 0.3 to 318.3 mmol C m−2 d−1, respectively. Biomass and production of bacterioplankton in the surface layer commonly exceeded those of phytoplankton, varying from 0.2 to 72 mmol C L−1 and 1.4 to 70 mmol C m−2 d−1, respectively. Median planktonic respiration varied from 275 to 605 mmol CO2 m−2 d−1 between stations along the salinity gradient. Primary production/respiration (P :R) ratios varied temporally and spatially between 0.003 and 6.60, averaging 0.22. Carbon mass balances revealed that under low river discharges (<250 m3 s−1), the estuary processed 2.4‐fold more carbon internally and advected 2.7‐fold less carbon seaward than under higher flows. Annual OC budgets suggest that ~19 x 109 mol C yr−1 of total organic carbon (TOC) entered the estuary, whereas 30 x 109 mol TOC yr−1 was exported seaward, representing a net gain of 11 x 109 mol TOC yr−1 within the estuary. Microplankton processed ~32 x 109 mol C yr−1, of which 5.6 x 109 mol C was attributed to photo‐ and chemoautotrophic (~72%) and heterotrophic (~28%) production; the remainder was respired to CO2 (26.5 x 109 mol CO2 yr−1). The persistent imbalance between carbon oxidation and carbon fixation strongly suggests that the Hudson River estuary usually is net heterotrophic and is a significant source of inorganic carbon to the coastal ocean and atmosphere.
Modeling carbon to nitrogen and carbon to chlorophyll a ratios in the ocean at low latitudes: Evaluation of the role of physiological plasticity
Authors: Lefèvre Nathalie , Taylor Arnold H. , Gilbert Francis J. , Geider Richard J.
Published: July 2010

Simulation modeling provides a means for testing the limits of our quantitative understanding of the factors that control phytoplankton biomass, growth rate, and primary productivity in the sea. We simulated the annual cycles of chlorophyll a (Chl a) concentration, primary productivity, nitrogen export, phytoplankton carbon to nitrogen (C :N) and carbon to Chl a ratios (C : Chl a) using a physiological model of phytoplankton carbon, nitrogen, and Chl a dynamics. The model was embedded within a one‐dimensional physical model of vertical exchanges that included simple mortality and recycling terms. A sensitivity analysis allowed evaluation of the relative effects of changes in phytoplankton physiology, physical forcing, mortality, and nutrient cycling on Chl a distributions and phytoplankton C: N. Critical to the success of the model was the treatment of mortality, which included seasonal (temperature) and depth‐related components, and the treatment of recycling efficiency, which was considered to be a function of the inorganic nitrogen concentration. The subtropical simulation compared favorably with data obtained at the Bermuda Atlantic Time‐series Study (BATS) station. Our results illustrate the utility of physiological data in validation of biogeochemical models. In particular, model predictions of phytoplankton C: Chl a, which ranged from 30 to 170 g C (g Chl a)−1, compared well with direct estimates based on 14C labeling of Chl a. However, predictions of phytoplankton C: N, which ranged from ~5–9 g C (g N)−1, could not be verified because of lack of data. This range of C:N suggests a slight limitation of phytoplankton growth rates by nutrients in surface waters.
Photochemical effects on the interaction of enzymes and dissolved organic matter in natural waters
Authors: Scully Norman M. , Tranvik Lars J. , Cooper William J.
Published: July 2010

Extracellular enzymes such as phosphatase (Pase) and glucosidase (Gase) can be inactivated in natural waters through photochemical processes. In this study, we examined the mechanisms involved in enzyme inactivation. We first explored the possibility of direct photoinactivation. The quantum yield spectrum (ΦΛ) for the direct photoinactivation of Pase increased exponentially with decreasing wavelength, with a much steeper slope relative to other ΦΛ. This, combined with modeled half‐life values for direct photoinactivation in excess of 5 d, indicates that direct photoinactivation of Pase by natural sunlight in lakes is negligible. Nonetheless, photoinactivation of enzymes occurred rapidly in light‐exposed natural waters and suggested an indirect mechanism. The pH of natural waters greatly affected photoinactivation. In acid humic lake water exposed to ultraviolet radiation, the half‐lives of both Pase and Gase were 4 h. The half‐life of these enzymes under the same conditions were twofold higher for lake waters obtained from a limed humic lake (8 h). The higher rate of inactivation in acid water was likely caused by a pH‐mediated increase in Fe photoreduction and enzyme binding. Solutions of Pase with Fe(II) and H2O2 resulted in rapid inactivation (half‐life 7 min at 8.9 µmol L−1 Fe). There was no significant inactivation of controls without H2O2, indicating that enzymes are inactivated through Fe(III)/enzyme binding, which is enabled through the oxidation of Fe(II) by H2O2 to Fe(III). Direct inactivation by reactive oxygen species (ROS) was ruled out by tests with ROS scavengers.
Performance of sinking and nonsinking phytoplankton taxa in a gradient of mixing depths
Authors: Ptacnik Robert , Diehl Sebastian , Berger Stella
Published: July 2010

According to a recent dynamical model, the depth of a well‐mixed water column should have contrasting effects on the abundances of sinking and nonsinking phytoplankton taxa. Because of increasing light limitation, nonsinking taxa should decline monotonically with increasing mixing depth, and because of sinking loss limitation at low mixing depths, sinking taxa should peak at intermediate mixing depths. Along a gradient of mixing depths, the position of this maximum should increase with increasing taxon‐specific sinking velocity and decrease with increasing background turbidity. In two field‐enclosure experiments, we investigated the effects of mixing depth and background turbidity on a variety of sinking and nonsinking phytoplankton taxa. We exposed the natural, 100‐µm screened phytoplankton community of a clear, unproductive, but silica‐rich lake to a gradient of mixing depths (1.5‐ 15 m) during 4–6 weeks. To mimic two different background turbidities, the transparent enclosure walls were surrounded by either white or black foliage. Although diatoms suffered from high sedimentation losses at low mixing depths, they dominated biomass at all mixing depths throughout both experiments. Results were largely in accordance with model predictions. Specific gross growth rates of most common taxa were negatively related to mixing depth. In both experiments, the abundances of most sinking taxa showed a unimodal pattern along the mixing depth gradient, while two of three motile taxa declined monotonically with mixing depth. The depths where these taxa reached their maximal abundances were positively related to taxon‐specific sinking velocity and negatively related to background turbidity.
Does resuspension prevent a shift to a clear state in shallow lakes during reoligotrophication?
Authors: Jeppesen Erik , Jensen Jens Peder , Søndergaard Martin , Hansen Kjeld Sandby , Møller Poul Hald , Rasmussen Helle Utoft , Norby Vibeke , Larsen Søren E.
Published: July 2010

Water managers often debate whether resuspension of sediment with high organic matter and water content accumulated during eutrophication delays improvement of water clarity after reduction of external nutrient loading. Using data from 15 shallow (mean depth <5 m) eutrophic lakes surveyed during 8–12 yr, we show that the reduction in phytoplankton biomass after external loading reductions of phosphorus or changes in the abundance of planktibenthivorous fish was accompanied by a proportional or nearly proportional reduction in detritus and inorganic suspended solids. The reduction occurred irrespective of lake size (0.1–40 km2), extent of phytoplankton biomass reduction (up to 10‐fold), and despite dominance of sediments with high water and organic content. Therefore, we conclude that recovery of shallow lakes after nutrient loading or fish stock reduction is apparently not significantly delayed by resuspension of organic or inorganic matter accumulated in the sediment during eutrophication.
A behavioral cascade: Top‐predator induced behavioral shifts in planktivorous fish and zooplankton
Authors: Romare Pia , Hansson Lars-Anders
Published: July 2010

It is well known that the effects of direct (lethal) predator‐prey interactions propagate through food webs (the trophic cascade). However, in the present study we show for the first time that, parallel to the trophic cascade, there exists a "behavioral cascade" in the sense that behavioral responses, induced by the nonlethal presence of a top predator, are transmitted down the food chain over more than one trophic link. By using a new method, horizontal echo sounding, in an enclosure study in a shallow lake in southern Sweden, we recorded the swimming activity and refuge use of young‐of‐the‐year (0+) roach (Rutilus rutilus) in the presence and the absence of a caged predatory fish. By connecting the predator avoidance behavior of 0+ roach with that of zooplankton throughout summer, we show that species interactions are more dynamic than had been predicted earlier by food web theory. In our study, 0+ roach changed their behavior by becoming less active in early summer and by hiding in a refuge in late summer in the presence of a piscivore, and this caused Daphnia to spend more time feeding in the open water than when piscivores were absent. Thus, we conclude that, to explain patterns of habitat use in natural systems, it is of crucial importance that we recognize the existence of behavioral cascades.
Effects of water color on predation regimes and zooplankton assemblages in freshwater lakes
Authors: Wissel B. , Boeing W. J. , Ramcharan C. W.
Published: July 2010

Northern temperate lakes often have high water color because of high concentrations of dissolved organic carbon (DOC). Altered light, temperature, and oxygen profiles in these brown‐water lakes should reduce the foraging abilities of planktivorous fish and reduce predation on zooplankton and invertebrate predators such as Chaoborus. Additionally, reduced diurnal vertical migration should limit exposure to cold temperatures and increase zooplankton growth rates. We hypothesized that, with increasing water color, Chaoborus would become more important, and this change would be followed by a shift in the zooplankton assemblage toward larger species. To test this hypothesis, we carried out a 2 x 2 x 2 factorial enclosure study to examine the effects of high and low color and the presence and absence of both fish and Chaoborus on zooplankton assemblages. We also analyzed the zooplankton community structure of two lakes with similar morphometry and fish composition but very different water color. Both studies showed that, in highly colored water, Chaoborus was more abundant and the zooplankton community shifted from small species, such as Bosmina and small copepods, to large species, such as Daphnia and Holopedium. Concurrently, the food web structure changed from top‐down control to intraguild predation. Because not only the physical habitat differs between clear‐ and brown‐water lakes, but also the predation regimes and food web structure, we conclude that brown‐water lakes are a distinct lake type.
Compositional and interlake variability of zooplankton affect baseline stable isotope signatures
Authors: Matthews Blake , Mazumder Asit
Published: July 2010

Zooplankton are commonly used to establish a baseline isotopic signature for pelagic production in lakes. Our objective was to evaluate this approach by quantifying among‐lake and within‐lake variability of δ13C and d15N for different taxa of pelagic zooplankton. We measured the δ13C and δ15N of Daphnia, Holopedium, and calanoid copepods from four lakes sampled from June to November 2001 and from eight additional lakes sampled once in midsummer. In the four lakes with temporal sampling, within‐lake differences due to taxonomic grouping accounted for 36.7% of the variance in δ15N and 41.7% of the variance in δ13C. Among all lakes, the δ15N of calanoid copepods was on average 2.55‰ and 2.44‰ higher than Daphnia or Holopedium, respectively, whereas the δ13C of calanoid copepods was 2.19‰ and 2.23‰ lower than Daphnia or Holopedium, respectively. If 15N fractionation is similar among species, the differences in δ15N suggest that calanoid copepods either feed at a higher trophic position in the food web or they have a consistently higher baseline δ15N signature than Daphnia or Holopedium among lakes. Differences in δ13C suggest that zooplankton taxa in the pelagia of lakes have different food sources. We conclude that species composition and feeding behaviors of the zooplankton community should be considered before making among‐lake comparisons of food web structure. We show that Daphnia is a useful isotopic baseline for organisms that rely on primary production in lakes.
Low symbiont diversity in southern Great Barrier Reef corals, relative to those of the Caribbean
Authors: LaJeunesse Todd C. , Loh William K. W. , van Woesik Robert , Hoegh‐Guldberg Ove , Schmidt Gregory W. , Fitt William K.
Published: July 2010

The specific identity of endosymbiotic dinoflagellates (Symbiodinium spp.) from most zooxanthellate corals is unknown. In a survey of symbiotic cnidarians from the southern Great Barrier Reef (GBR), 23 symbiont types were identified from 86 host species representing 40 genera. A majority (>85%) of these symbionts belong to a single phylogenetic clade or subgenus ("C") composed of closely related (as assessed by sequence data from the internal transcribed spacer region and the ribosomal large subunit gene), yet ecologically and physiologically distinct, types. A few prevalent symbiont types, or generalists, dominate the coral community of the southern GBR, whereas many rare and/or specific symbionts, or specialists, are found uniquely within certain host taxa. The comparison of symbiont diversity between southern GBR and Caribbean reefs shows an inverse relationship between coral diversity and symbiont diversity, perhaps as a consequence of more‐rapid diversification of Caribbean symbionts. Among clade C types, generalists C1 and C3 are common to both Caribbean and southern GBR symbiont assemblages, whereas the rest are regionally endemic. Possibly because of environmental changes in the Caribbean after geographic isolation through the Quaternary period, a high proportion of Caribbean fauna associate with symbiont taxa from two other distantly related Symbiodinium clades (A and B) that rarely occur in Pacific hosts. The resilience of Porites spp. and the resistance of Montipora digitata to thermal stress and bleaching are partially explained by their association with a thermally tolerant symbiont type, whereas the indiscriminant widespread bleaching and death among certain Pacific corals, during El Niño Southern Oscillation events, are influenced by associations with symbionts possessing higher sensitivity to thermal stress.
Climatic control of ultraviolet radiation effects on lakes
Authors: Leavitt Peter R. , Cumming B. F. , Smol J. P. , Reasoner M. , Pienitz R. , Hodgson D. A.
Published: July 2010

Ultraviolet radiation (UVR) damages most biota, yet little evidence exists for its long‐term effects on natural ecosystems. We used paleoecological techniques at three low‐elevation lakes to show that algal abundance in lakes was depressed 10‐fold by UVR during the first millennium of lake existence. Likewise, analysis of data from a lake near treeline showed that algal biomass declined 10–25‐fold both early in the lake history and during the last ~4000 yr, when inputs of UVR‐absorbing dissolved organic matter (DOM) declined despite constant nutrient levels since ~10,000 14C yr before the present. This rapid (–1.25% yr−1), sustained (>600 yr) suppression of algal abundance arose from directional climate change that reduced DOM inputs and occurred despite initial reservoirs of photoprotective DOM that are typical of most boreal lakes. Hence, we conclude that many lakes may be vulnerable to order‐of‐magnitude declines in algal abundance arising from future climate‐DOM‐UVR interactions.
Periphyton density influences organochlorine accumulation in rivers
Authors: Berglund Olof
Published: July 2010

To investigate the influence of eutrophication on organochlorine dynamics in lotic systems, we related polychlorinated biphenyl (PCB) concentrations in brown trout (Salmo trutta) to the periphyton density and total phosphorus concentrations (Tot‐P) in 10 Swedish rivers. Tot‐P concentrations in the water ranged from 37 ± 4 to 156 ± 51 µg L−1. Periphyton density in the rivers during the experiment ranged from 6 to 199 mg carbon (C) m−2 and was not related to the Tot‐P concentration. The concentrations of PCB 52, PCB 66/95, PCB 90/101, PCB 132/153/105, PCB 160/138/158, PCB 180, and PCB 194 in brown trout were positively related to both periphyton density and Tot‐P concentration, with periphyton density having the highest degree of correlation, 40–58%, compared to 6–44% for Tot‐P. All seven PCB concentrations in brown trout were significantly, positively related to the areal mass (ng m−2) of the PCB in periphyton (r2 = 0.69–0.86). Contrary to what has been found for lakes, eutrophication and organochlorine accumulations are positively related in rivers. Thus, an increase in primary production should increase the exposure of the local biota but lead to a decrease of the downstream transport of these pollutants to coastal areas and oceans.
Benthic algae control sediment—water column fluxes of organic and inorganic nitrogen compounds in a temperate lagoon
Authors: Tyler Anna Christina , McGlathery Karen J. , Anderson Iris C.
Published: July 2010

Coastal lagoons are a common land‐margin feature worldwide and function as an important filter for nutrients entering from the watershed. The shallow nature of lagoons leads to dominance by benthic autotrophs, which can regulate benthic—pelagic coupling. Here we demonstrate that both microalgae and macroalgae are important in controlling dissolved inorganic as well as organic nitrogen (DIN and DON) fluxes between the sediments and the water column. Fluxes of nitrogen (NH4+, NO3−, DON, urea, and dissolved free and combined amino acids [DFAA, DCAA]) and O2 were measured from October 1998 through August 1999 in sediment cores collected from Hog Island Bay, Virginia. Cores were collected from four sites representing the range of environmental conditions across this shallow lagoon: muddy, high‐nutrient and sandy, low‐nutrient sites that were both dominated by benthic microalgae, and a mid‐lagoon site with fine sands covered by dense macroalgal mats. Sediment—water column DON fluxes were highly variable and comparable in magnitude to DIN fluxes; fluxes of individual compounds (urea, DFAA, DCAA) often proceeded simultaneously in different directions. Where sediment metabolism was net autotrophic because of microalgal activity, TDN (total dissolved nitrogen) fluxes, mostly comprised of DIN, urea, and DFAA, were directed into the sediments. Heterotrophic sediments, including those underlying macroalgal mats, were a net source of TDN, mostly as DIN. Macroalgae intercepted sediment—water column fluxes of DIN, urea, and DFAA, which accounted for 27–75% of calculated N demand. DON uptake was important in satisfying macroalgal N demand seasonally and where DIN concentrations were low. Up to 22% of total N uptake was released to the water column as DCAA. Overall, macroalgae assimilated, transformed, and rereleased to the water column both organic and inorganic N on short (minutes—hours) and long (months) time scales. Microalgae and macroalgae clearly regulate benthic—pelagic coupling and thereby influence transformations and retention of N moving across the land—sea interface.
Benthic primary production and nutrient cycling in sediments with benthic microalgae and transient accumulation of macroalgae
Authors: Dalsgaard Tage
Published: July 2010

Annual rates of sediment denitrification and sediment—water fluxes of oxygen and nutrients were quantified at two shallow locations, Virksund and Ulbjerg, in the Limfjorden, Denmark. The sediment was sandy and colonized mainly by bivalves with a wet weight of 2,508 g m−2 at Virksund and 572 g m−2 at Ulbjerg. A benthic microalgal community was present throughout the year, and for 1–2 months in the summer, floating macroalgae partly covered the sediment. Annual budgets for the sediment both including and excluding the activity of macroalgae were calculated. In the absence of macroalgae, the benthic primary production was highest at Ulbjerg, which was autotrophic on an annual basis, whereas Virksund was heterotrophic. When macroalgae were included, both sites were strongly autotrophic on an annual basis. From 13% to 58% of the NH4+ produced by mineralization was retained in the sediment in the absence of macroalgae, primarily because of the assimilation of NH4+ by the microphytobenthic community. Only 25% and 38% of the total NO3− uptake at Ulbjerg and Virksund, respectively, was denitrified in the absence of macroalgae, whereas in the presence of macroalgae, 12% and 39% of the NO3− uptake was denitrified at those sites. Nitrate uptake associated with benthic primary production limited denitrification through competition for NO3−. The release of NH4+ from the sediment at Virksund was reduced more than 50%, and at Ulbjerg, release of NH4+ changed to uptake when the macroalgae were included in the annual budget. Nutrient uptake by macroalgae competed with all other nutrient‐consuming processes, and the transient occurrence of macroalgae totally changed both the primary productivity and the nutrient budgets for the two sites.
Benthic metabolism and sulfur cycling along an inundation gradient in a tidal Spartina anglica salt marsh
Authors: Gribsholt Britta , Kristensen Erik
Published: July 2010

Central aspects of carbon and sulfur biogeochemistry were studied along a transect extending from an unvegetated mudflat into a Spartina anglica salt marsh. Conditions along the transect differed with respect to tidal elevation, sediment characteristics, vegetation coverage, and benthic macrofauna abundance. Dark sediment O2 uptake and CO2 emission at the highly bioturbated mudflat were low and relatively unaffected by tidal coverage. Sulfate reduction accounted for 30–60% of the daily CO2 emission from the open mudflat sediment. Sediment O2 uptake within the nonbioturbated and vegetated marsh was up to seven times higher during air exposure than during inundation, whereas the difference in CO2 emissions always was less than a factor of 2. The contribution of sulfate reduction to CO2 production was low (<21%) and decreased progressively with tidal elevation as a result of the oxidizing capacity of S. anglica roots in the vegetated marsh. The boundary between the mudflat and the retreating marsh is a unique environment. High near‐surface pore‐water concentrations of dissolved organic carbon (DOC) above the marsh cliff and highly elevated total carbon dioxide (TCO2) pore‐water concentrations at both sides of the cliff during air exposure coincided with extremely high TCO2 emissions and apparent respiratory quotients (up to 14) only below the marsh cliff during inundation. We propose that substantial seepage of DOC‐poor and HCO2−‐rich pore water may have occurred from the elevated marsh to the unvegetated sediment below during low tide followed by massive release of HCO3− during high tide. Accordingly, sulfate reduction accounted for more than the TCO2 release above the marsh cliff, but only for about 40% below the cliff. Mineralization rates and pathways in salt–marsh sediments vary considerably on small spatial and temporal scales and are dependent on inundation frequency as well as the composition and distribution of flora and fauna.
The effect of water velocity on stable carbon and nitrogen isotope signatures of periphyton
Authors: Trudeau Véronique , Rasmussen Joseph B.
Published: July 2010

Water movement affects carbon and nitrogen isotopic signatures of algae, possibly through effects on boundarylayer thickness. We describe a laboratory experiment carried out in artificial streams, which supports this hypothesis. Periphyton δ13C and δ15N signatures were significantly depleted in 13C and 15N when grown at higher water velocity over the range of 5–62 cm s−1, the normal range found in small streams. δ13C signatures ranged between —16.7‰ and —28.1‰ from the lowest to the highest water velocity, respectively. Similarly, δ15N signatures ranged between 7.2‰ and 2.3‰ from the lowest to the highest water velocity, respectively. This pattern was found for algal communities growing on glass (mainly diatoms) as well as for those growing on rock (mainly filamentous green algae). For both C and N, the slopes of the responses were not different between the periphyton communities growing on each substrate, although the effect was statistically weaker for the communities on rocks. The intercept for δ13C was significantly higher for the communities on rocks, but not different for δ15N. Thus, while both isotopes are fractionated to a greater extent as velocity increases, the diatom communities growing on glass appeared to fractionate C isotopes more than the filamentous green algae growing on rock. This relationship between the stable isotopic signatures of aquatic plants and water velocity will hopefully allow a better understanding of the differences in isotopic signatures of fish in different habitats.
Using the hidden isotopic heterogeneity in phyto‐ and zooplankton to unmask disparity in trophic carbon transfer
Authors: Pel Roel , Hoogveld Hans , Floris Virgilio
Published: July 2010

In this study, we show that natural phototrophic populations can be probed individually for their in situ δ13C signature by linking fluorescence‐activated cell sorting and isotope‐ratio mass spectrometry (IRMS) using in‐line pyrolytic methylation. This novel methodology greatly improved the resolution in discriminating and tracing the differential carbon (C) pathways at the base of the pelagic food web in the cyanobacteria‐dominated Lake Loosdrecht (The Netherlands). Our analysis revealed the co‐occurrence of phytoplankton taxa differing by 6–10‰ in δ13C. Predominant micro‐ and mesozooplankton species reflected this difference as the result of preferential grazing and/or selective digestion. Flow cytometric (FCM) retrieval of phytoplankton δ13C signatures, applied in conjunction with 13C‐carbonate labeling, also enabled an assessment of in situ population‐specific growth rates. Diatoms and green algae exhibited up to ninefold higher growth rates than those for cyanobacterial species. The coexistence of phytoplankton populations widely differing in δ13C, standing stock, and turnover time has important implications for the interpretation of C transfer in pelagic food webs. Our approach disclosed a disproportional impact on trophic cascades by numerically minor phototrophs that otherwise would have gone unnoticed. Despite the abundance of cyanobacterial‐derived C, the zooplankton largely rely on eukaryotic algae for growth. Rotifers take a central position in passing on this algal C to the cyclopoid copepod populations in the lake. The bosminid‐dominated cladoceran population uses both the cyanobacterial‐ and algal‐derived C in approximately equal shares.
Seasonal and diel patchiness of a Daphnia population: An acoustic analysis
Authors: Hembre Leif K. , Megard Robert O.
Published: July 2010

Detailed information about the location and extent of zooplankton patches is fundamental to understand how abiotic and biotic forces interact to structure the spatial distribution of zooplankton. We mapped zooplankton patchiness in a Minnesota lake during spring, summer, and autumn with high‐frequency (192‐kHz) single‐beam sonar. Conventional plankton samples of aggregations detected acoustically revealed that Daphnia pulicaria (mean body length 1.6 mm, mean target strength —120 dB) scattered most (~63%) of the sound. Other taxa were smaller (<½ the length of D. pulicaria) and were usually less abundant and therefore scattered much less sound than D. pulicaria. Our acoustic estimates of Daphnia concentrations illustrate extreme patchiness, with concentrations varying by as much as four orders of magnitude over vertical distances of less than 1 m. Seasonal patterns of patchiness were related to predation by rainbow trout and to abiotic factors associated with stratification. Daphnia concentrations were highest from June to October in a deep‐water “refuge zone” where oxygen concentrations were between 3 and 5 mg L−1. These oxygen levels are suitable for Daphnia but are lower than those required by rainbow trout. Heterogeneity in Daphnia concentration along the lake's long axis was highest in May and June, when the population resided primarily in the oxic hypolimnion during the daytime. From July to October, as oxygen concentrations declined in the hypolimnion, the population became more metalimnetic and more uniformly distributed in the horizontal dimension. A diel study of the population in October indicated that the patchiness of population also changed dramatically between day and night. During the day the population aggregated densely in a thin layer (~2 m thick) in the thermocline. After sunset the population dispersed into the epilimnion, where concentrations were ~100,000 m23 less than they were during the day in the thermocline.
Assessing the error in photosynthetic properties determined by fast repetition rate fluorometry
Authors: Laney S. R.
Published: July 2010

Fast repetition rate (FRR) fluorometry is an optical technique for estimating photosynthetic properties of phytoplankton from measurements of variable fluorescence yield. I determined the minimum error in such estimates contributed by inherent instrument biases, improper measurement protocols, and the type of optimization algorithm used to infer photosynthetic variability from changes in variable fluorescence. Many of these errors were nonrandom in origin and would not be reduced with repeated sampling or averaging. Characterization of a commercial FRR fluorometer (FRRF) showed undocumented hardware biases with magnitudes roughly equivalent to those addressed by the current characterization approach. Robust optimization algorithms were less likely to misidentify these biases as representing actual photosynthetic variability. In general, robust algorithms improved the accuracy, precision, and distribution of error in analyses of both simulated and actual variable fluorescence measurements. When reanalyzed with robust algorithms, in situ data from an FRRF indicated different photosynthetic behavior than did the analysis tool used originally. Methods to improve and standardize the collection and analysis of FRR variable fluorescence data are essential for evaluating the strengths and limitations of this powerful, but involved, technique. Although the error minimization procedures described were developed primarily to minimize errors and artifacts with FRR fluorometry, several are generally applicable to any fluorescence yield technique in which a physiological model is used to estimate photosynthetic parameters from variable fluorescence measurements.
Influence of CO2, nitrate, phosphate, and silicate limitation on intracellular dimethylsulfoniopropionate in batch cultures of the coastal diatom Thalassiosira pseudonana
Authors: Bucciarelli Eva , Sunda William G.
Published: July 2010

We measured intracellular dimethyl sulfoniopropionate (DMSP) concentrations in nutrient‐limited batch cultures of the coastal diatom Thalassiosira pseudonana. Under N, P, CO2, and Si growth limitation, intracellular DMSP concentrations decreased to low values during the exponential phase of growth and increased under nutrient limitation. The intracellular DMSP concentration increased exponentially with decreasing growth rate and cellular chlorophyll a, in response to the type and degree of nutrient limitation. For a given growth rate, N‐limited cells showed the greatest increase in cellular DMSP concentrations, CO2 and Si limitation had an intermediate effect, and P limitation caused the smallest increase. The results demonstrate the importance of nutrient limitation in enhancing DMSP concentrations in marine algae. This enhancement may be linked in part to the role of DMSP as a cellular antioxidant and to increased oxidative stress within cells during nutrient limitation. The replacement of DMSP, a sulfur‐containing osmolyte, for N‐containing osmolytes such as proline may also explain at least some of the DMSP increase under N limitation. Our results also point to the possible importance of diatom blooms in global or regional sulfur cycles.