bluuebunny/crossref_metadata_2025 huggingface dataset:
New aspects of migratory behavior of phytoplankton in stratified waters: Effects of halocline strength and light on Tetraselmis sp. (Prasinophyceae) in an artificial water column
Authors: Erga Svein Rune , Dybwad Maren , Frette Øyvind , Lotsberg Jon Kåre , Aursland Kjetil
Published: July 2010

There is very little information in the literature about how phytoplankton flagellates respond to rapid changes in salinity (e.g., haloclines of different strength). Here we present such data obtained from experiments with Tetraselmis sp. (Prasinophyceae) in a specially designed artificial water column. The experiments were performed with surface salinities varying from 27.4 to 33.4‰, whereas bottom salinities were essentially constant (34.0–34.6‰). Cells were introduced near the bottom. The first stage of the ascent was an accumulation of cells in the lower part of the halocline. The second was a transit to the upper part of the halocline after a variable time lag, and the third stage was a further ascent to the surface layer, again after a variable time lag. Our results reveal a strong positive phototaxis of the swimming cells under continuous surface irradiances in the range 17–144 µmol quanta m−2 s−1. Maximum swimming speeds were found to be 0.9 m h−1. Increasing halocline strength (△S = 0.7, 3.0, and 6.6‰), resulted in reduced swimming activity, but the cells managed to pass through after an adaptation period. Under a 14 : 10 light : dark (LD) cycle, the cell concentrations of the bottom layer decreased gradually for each cycle, and cells accumulated in the surface layer during the light period. During the dark period, cell concentrations were also increasing in the halocline for each cycle. A downward migration started about 2 h before the light period ended and was slower than the ascent of cells from the halocline. It therefore seems that positive phototaxis was stronger than the positive geotaxis as a driving force of cell motility. We conclude that strong haloclines can prevent phytoplankton flagellates in the surface layer from reaching the nutrient‐rich deeper layer during the night and therefore play an important regulating role in the bloom dynamics of phytoplankton.
Mysis relicta in a eutrophic lake: Consequences of obligatory habitat shifts
Authors: Horppila Jukka , Liljendahl-Nurminen Anne , Malinen Tommi , Salonen Mikko , Tuomaala Antti , Uusitalo Laura , Vinni Mika
Published: July 2010

Low numbers of mysids in eutrophic lakes have usually been related to their poor tolerance to low oxygen. In eutrophic Lake Hiidenvesi, we studied the possibility that water quality indirectly affects mysids by forcing them to habitats where they are vulnerable to fish predation. Mysis relicta was absent from basins that did not stratify (water depth< 10 m). In the deep basin, as the summer progressed, M. relicta was concentrated in the deepest areas and moved upward in the water column as water temperature increased and dissolved oxygen decreased. Between mid‐August and mid‐September, only a thin water layer was habitable; the rest had either intolerably low oxygen concentration or too high of a temperature. M. relicta abundance decreased from 2.72 X 108 individuals on 3 June to 3.72 × 106 individuals on 19 October, with the sharpest decrease taking place in August. At the same time, the vertical distribution of smelts shifted downward and the percentage of mysids in the stomach contents of smelts increased steeply. The consumption rate by smelts was high enough to explain the collapse of the M. relicta population; during 18 August–2 September, smelts consumed 5.6 times the mysid production. The results suggested that unfavorable physicochemical conditions did not directly cause the drastic reduction of the population abundance of M. relicta but indirectly facilitated predation by smelts, which was the final cause of the collapse. The high predation rate by smelts was probably accelerated by the emergence of chaoborid larvae, which were important food items for smelts in early summer.
Properties of intertidal marsh sediment mobilized by rainfall
Authors: Torres Raymond , Mwamba Mwasi J. , Goñi Miguel A.
Published: July 2010

We conducted sprinkler irrigation experiments on a low‐tide marsh to investigate the effects of rainfall on the redistribution of organic matter (OM) and nutrients in the intertidal zone. We irrigated 1‐ × 2‐m plots at highmarsh, low‐marsh, and channel bank sites and flood‐irrigated 1‐ × 3‐m plots in the high marsh and low marsh. We measured particulate density, OM content, organic carbon (OC) content, and nitrogen (N) content and calculated OM particulate density and atomic C:N ratios. The content of OM, OC, and N in rainfall‐mobilized sediment was consistently higher than in the substrate. C:N ratios ranged from 13 to 15 and were consistently lower than the sediment substrate (16–21). These observations indicate that rainfall can mobilize N preferentially over OC; hence, rainfall events deplete the marsh substrate of OM, OC, and nutrients. Despite variability in the density of mobilized particulates, the calculated OM densities and the C:N ratios remained unchanged during the irrigation experiments. These results indicate that rainfall runoff processes preferentially and consistently mobilize OM‐rich particulates with low C:N ratios, characteristic of mixed algal or vascular plant sources. The short‐term OC fluxes during these rainfall experiments represent 3–20% of annual primary productivity.
Variability of particulate seawater properties related to bottom mixed layer‐associated internal waves in shallow water on a time scale of hours
Authors: Turnewitsch Robert , Graf Gerhard
Published: July 2010

To obtain information on relations between internal wave‐controlled hydrographic variability in bottom mixed layers (BMLs) and particle‐associated BML parameters, depth distributions of density, relative turbidity, relative chlorophyll a (Chl a), relative phycoerythrin fluorescence (Phyco), total particulate matter (TPM), particulate carbon (PC), and particulate nitrogen (PN) were obtained at a shallow‐water site in the southwestern Baltic Sea in July 2001 in the course of a temporally highly resolved 1‐d time series. The density distribution showed evidence for a BML and undulations of BML thickness probably caused by near‐inertial internal waves. Relative pigment fluorescence in the BML and in the layer above the BML was inversely coupled and related to the wave. Dense transient PC‐ and PN‐depleted TPM clouds, having the highest TPM loads in the BML, occurred on leading wave faces. Leading wave faces and wave backs are suggested to be locations of preferred vertical exchange between adjacent compartments (surface sediment, BML, layer above BML). Different processes control relative turbidity in the BML (decoupled from Chl a) and in the layer above the BML (coupled to Chl a). Most of the time, spatial variability of the Chl a/Phyco ratio was higher within the BML than in the layer above the BML. The Brunt—Väisälä frequency within the layer above the BML was positively and negatively related to the thickness of the BML and the layer above the BML, respectively. The dataset indicates that the BML itself and in coaction with passing internal waves effectively controls the communication between interior water column and sediment.
Phytoplankton contribution to sestonic mass and elemental ratios in lakes: Implications for zooplankton nutrition
Authors: Hessen Dag O. , Andersen Tom , Brettum Pål , Faafeng Bjørn A.
Published: July 2010

Phytoplankton carbon and particulate organic carbon (POC), nitrogen (PON), and phosphorus (POP) (POC :PON: POP) were analyzed in 109 temperate lakes covering a wide span in productivity and other key parameters. Seasonal means of total POC (four samples) ranged from 206 to 7160 µg C L−1, with a grand mean of 960 µg C L−1, whereas estimated phytoplankton C ranged 12 to 1,770 µg C L−1, with a mean of 217 mg C L−1. Sestonic C: P ratios ranged from 59 to 553 (atom : atom), with a mean of 207. The elemental contributions from phytoplankton and other sestonic compartments (mainly detritus) were analyzed with a simple regression model, in which autochthonous and allochthonous components were separated. Model‐derived estimates for N: P ratios of phytoplankton and allochthonous seston compartments were nearly equal (15.4 ± 2.5 and 16.0 ± 2.0) and were not significantly different from the Redfield N: P ratio (16). The estimated C: P ratio of allochthonous detritus was 2.7 times higher than that for phytoplankton (123 ± 15), which again was not significantly different from the Redfield C: P ratio (106). Altogether, this indicates that sestonic components of autochthonous origin should be closer to Redfield proportions in eutrophic than in oligotrophic lakes. It also indicates that major contributions of allochthonous detrital C in oligotrophic lake seston may explain deviations from the Redfield ratio and calls for caution when interpreting elemental ratios in algae versus total seston. The regression model indicates that live phytoplankton cells rarely exceed 40% of total POC, yet it suggests that a major fraction of detritus is derived from autotrophs. This close link between live and dead cells could explain why total seston apparently carries the stoichiometric and biochemical footprints from the phytoplankton. Judged from algal biomass alone, Daphnia would face severe food limitation in a majority of lakes, while if we were to include total seston, Daphnia would be above threshold food levels in all lakes. Likewise, the effect of food quality limitation related to C: P ratios will turn out differently if total seston or only the phytoplankton fraction is considered.
Specificity of crowding response that induces sexuality in the rotifer Brachionus
Authors: Gilbert John J.
Published: July 2010

Crowding induced the production of mictic (male‐producing) females in Brachionus calyciflorus from two North American strains and an Australian strain. The specificity of this response to crowding was tested by culturing single individuals of a North American clone in three treatment conditions: a small volume (high density), a large volume (low density), and a large volume with a high density of an Australian clone. The results were consistent and clear in six experiments using different combinations of clones. Crowding low‐density individuals of the North American strains with the Australian strain failed to induce them to produce mictic females. The mictic‐female response in this treatment was similar to that in the low‐density treatment, and both of these responses were significantly lower than that in the high‐density treatment. Since the mictic‐female response to crowding in Brachionus is mediated by a chemical produced by the rotifers themselves, the chemical inducers produced by the Australian and North American strains must be different. Taxonomically specific responses to crowding should increase fitness by assuring that sexual reproduction in the heterogonic life cycle coincides with a high population density of individuals able to mate with one another and, thus, when the production of fertilized resting eggs can be maximized. This would be especially important in plankton communities with diverse rotifer assemblages and multiple congeneric species. Otherwise, a low‐density population of a species could be induced to initiate bisexual reproduction by populations of other species, curtailing its potential for population growth via female parthenogenesis and limiting its production of resting eggs in the future.
Effects of small‐scale turbulence on copepods: The case of Oithona davisae
Authors: Saiz Enric , Calbet Albert , Broglio Elisabetta
Published: July 2010

We report the effects of small‐scale turbulence on the feeding rates of the marine copepod Oithona davisae. Laboratory experiments were conducted under a range of turbulence dissipation rates between 10−4 and 101 cm2 s−3. Net enhancements of feeding were observed only at the lowest, whereas negative net effects appeared only at the highest, turbulence intensities. These results contrast with expectations from an encounter‐based model for this copepod species that predicted positive feeding enhancements at all turbulence intensities. This disagreement suggests the presence of detrimental effects at moderate and high turbulence intensities, very likely driven by either a lower mechanosensor perception capability or lower capture success. In comparison to other ambush copepods, O. davisae appears much more sensitive to the presence of turbulence, which might be the result of its strict ambush behavior, whereas copepods like Acartia tonsa or Centropages typicus, which can switch into different feeding modes, appear to benefit more from turbulence. The response of O. davisae feeding to turbulence in our experiments agrees with recent field observations on changes in the vertical distribution of Oithona as a function of wind‐driven turbulence events. Hence, O. davisae seems to choose those depths where small‐scale turbulence favors feeding.
Microscale distribution of zooplankton in relation to turbulent diffusion
Authors: Maar Marie , Nielsen Torkel Gissel , Stips Adolf , Visser AndréW.
Published: July 2010

Microscale vertical distribution of proto‐and mesozooplankton in relation to turbulent diffusion was investigated using a high‐resolution sampler (HSR) during cruises in the Skagerrak, Denmark, and the Northern Aegean, Greece. A strong pycnocline and a deep chlorophyll maximum (DCM) characterized both areas. The phytoplankton biomass in the Skagerrak was much higher and dominated by netplankton compared with the Aegean, where picoplankton dominated. Deployments of the HRS and measurements of microscale turbulence were made simultaneously in the mixed surface layer, the pycnocline, and the DCM and revealed a large range (0.2‐250 cm2 s−1) in turbulent diffusion. In general, turbulent diffusion was highest in the surface layer. The variability of the plankton organisms within the 3‐m strata sampled by the HRS was quantified by their coefficient of variation (CV = SD/mean) and analyzed with respect to turbulent diffusion. We hypothesized that the variability of the vertical distribution would be independent of turbulence up to a threshold where dispersion overwhelms the swimming ability of the organism and variability decreases. The hypothesis was confirmed by our results; for the weak swimmers, ciliates, Ceratium spp., and nauplii, there was a significant decrease in CV with increasing turbulence, whereas the variability of the stronger swimmer represented by copepodites was uncorrelated with turbulent diffusion. This underscores the potential of these organisms in locating and exploiting food patches in the water column.
Primary productivity of reef‐building crustose coralline algae
Authors: Chisholm John R. M.
Published: July 2010

The primary productivity of four species of crustose coralline algae was measured as a function of depth (0–18m) and irradiance on samples collected from and growing upon the windward coral reef at Lizard Island, northern Great Barrier Reef, Australia. Significantly higher productivities were measured in the field than in the laboratory. Maximum gross oxygen production in situ varied from 12.8 to 22.8 mmol m−2 h−1; dark respiration consumed between 2.7 and 4.5 mmol O2 m−2 h−1. Integration of photosynthesis‐irradiance models with half sine curve approximations of whole‐day irradiance yielded estimated in situ net productivities of 15–132 mmol O2 m−2 d−1. When multiplied by previously determined photosynthetic quotients, in situ net carbon fixation was estimated to vary from 0.2 to 1.3 g m−2 d−1. Multiplying these rates by measured surface relief factors of 3.1 for the reef crest and 5.0 for the windward slope yielded estimated contributions to reef organic production of ~0.9–5 g C (net) planar m−2 d−1 over the depth interval 0–18 m, given 100% cover. These data suggest that crustose coralline algae make a larger contribution to organic production on coral reefs than has been thought to this time. A curvilinear model is presented that enables their primary productivity to be estimated from measurements of in situ irradiance at the solar zenith.
Episodic nutrient transport to Florida coral reefs
Authors: Leichter James J. , Stewart Hannah L. , Miller Steven L.
Published: July 2010

This study documents the changes in nutrient fluxes associated with internal tidal bores arriving on Florida Keys coral reefs and points to biological use of subthermocline nitrate brought onshore by this mechanism. Internal bores on Conch Reef, Florida Keys, are associated with concentrations of 1.0–4.0 µmol L−1 nitrate (NO3− ) and 0.1–0.3 µmol L−1 soluble reactive phosphate (SRP) and onshore flow velocities of 0.1–0.3 m s−1. The arrival of internal bores causes 10–40 fold increases in nutrient concentrations and 1–2 orders of magnitude increases in nutrient flux relative to ambient, nonbore conditions. The magnitude and duration of cool‐water nutrient transport events increases significantly with increasing depth on reef slopes. In June 2001, the gradient of increased exposure to subsurface water with depth corresponded to increased percentage of N and δ15N and decreased C:N ratio in a common benthic macroalga, Codium isthmocladum. Internal tidal bores are widespread throughout the Florida Keys reef tract, with cool‐water episodes influencing reefs up to 10%–25% of the time during summer months and with significant variability among years. Estimated inputs of nitrogen and phosphorus by internal tidal bores to Florida Keys reef slopes are as much as 40‐fold larger than published estimates of inputs to near‐shore waters from waste water and storm water runoff. Internal tidal upwelling represents an important, previously underestimated, episodic source of nutrients on the Florida Keys reef tract. In order to assess nutrient availability in this system accurately it is essential to understand natural sources of high‐frequency variability.
From Greenland to green lakes: Cultural eutrophication and the loss of benthic pathways in lakes
Authors: Vadeboncoeur Yvonne , Jeppesen Erik , Zanden M. Jake Vander , Schierup Hans‐Henrik , Christoffersen Kirsten , Lodge David M.
Published: July 2010

Benthic community responses to lake eutrophication are poorly understood relative to pelagic responses. We compared phytoplankton and periphyton productivity along a eutrophication gradient in Greenland, U.S., and Danish lakes. Phytoplankton productivity increased along the phosphorus gradient (total phosphorus [TP] = 2–430 mg m−3), but whole‐lake benthic algal productivity decreased, substantially depressing increases in primary productivity at the whole‐lake scale. In shallow, oligotrophic Greenland lakes, periphyton was responsible for 80–98% of primary production, whereas in Danish lakes with TP > 100 mg m−3, phytoplankton were responsible for nearly 100% of primary production. Benthic contributions ranged from 5 to 80% depending on morphometry and littoral habitat composition in lakes with intermediate phosphorus concentrations. Thus, eutrophication was characterized by a switch from benthic to pelagic dominance of primary productivity. Carbon stable isotope analysis showed that the redistribution of primary production entailed a similar shift from periphyton to phytoplankton in the diets of zoobenthos. Benthic and pelagic habitats were energetically linked through food web interactions, but eutrophication eroded the benthic primary production pathway.
Deep‐water renewal in Lake Issyk‐Kul driven by differential cooling
Authors: Peeters Frank , Finger David , Hofer Markus , Brennwald Matthias , Livingstone David M. , Kipfer Rolf
Published: July 2010

During two field campaigns on Lake Issyk‐Kul in March and August 2001, 179 high‐resolution conductivity‐temperature‐depth profiles were measured, along with profiles of dissolved oxygen and light transmission. On the basis of this extensive data set, we investigated the large‐scale vertical advective processes responsible for deepwater renewal in Lake Issyk‐Kul. At some locations sampled in March, variable peak structures in the tracer profiles were observed that indicate horizontally localized intrusions. These intrusions have their origin in density plumesthat propagate along the bottom of the channels in the eastern shelf region. The cold dense water at the bottom of the channels is most likely generated by differential cooling of the water in the shallow shelf regions during winter. During summer, vertical advective processes appear to be limited to the upper 200 m, where upwelling in the open water leads to a doming structure. The August data support the results of earlier investigations that suggested that dynamic forcing by a basinwide gyre is responsible for the upwelling in the open water.
Complex effects of winter warming on the physicochemical characteristics of a deep lake
Authors: Straile Dietmar , Jöhnk Klaus , Henno Rossknecht
Published: July 2010

Recent winter warming over Central Europe associated with a positive phase of the North Atlantic Oscillation (NAO) strongly influenced the thermal and water column stability properties of deep Lake Constance (zmean = 101 m). Volumetrically weighted average water temperatures have increased since the 1960s by an average of 0.017°C yr−1, and its interannual variability was strongly related to the variability in winter air temperature and the NAO winter index (NAOW). The influence of NAOW on water temperature was more persistent than its influence on air temperature. The seasonal persistence of the NAOW signal increased with water depth. Deep‐water temperatures were related to the NAOW from one spring mixing period to the next. This caused a time lag of 1 yr in the response of deep‐water winter temperatures to the NAOW. Reduced winter cooling during high‐NAOW years resulted in the persistence of small temperature gradients that possibly resisted complete mixing. This, in turn, resulted in less upward mixing of nutrients (total phosphorus and total silica), which accumulated in the hypolimnion during the previous stratification period. A second effect of incomplete mixing was the lack of the replenishment of deepwater oxygen during high‐NAOW years. Hence, besides its strong impact on the thermal regime, climate variability influenced both the causes (nutrient supply for phytoplankton growth) and symptoms (the degree of hypolimnetic oxygen deficiency) of trophic changes in Lake Constance.
Bottom‐up controls on bacterial production in tropical lowland rivers
Authors: Castillo María M. , Kling George W. , David Allan J.
Published: July 2010

The importance of organic carbon and inorganic nutrients in controlling bacterial production was investigated in two tropical lowland rivers draining undisturbed forested catchments. Glucose (C), ammonium (N), phosphate (P), leaf leachate, and algal leachate were added alone or combined to water collected from one clear‐water (Cataniapo) and one black‐water (Autana) river of the Middle Orinoco basin, and bacterial production (BP) was measured at 0, 8, 24, and 36 h of incubation. The rivers have low pH (3.8–5.9) and conductivity (6.3–9.1 µS cm−1) and abundant nitrogen (total N, 273–314 µg L−1) compared to phosphorus (total P, 3.6–5.5 µg L−1). BP was significantly stimulated by additions of P, CP, NP, and CNP in both river waters. N or C alone or combined did not stimulate BP, suggesting that P rather than carbon is the primary limiting nutrient in these rivers. Higher responses to CNP and CP amendments (3–7 times over controls), compared to P and NP (1.3–4 times over controls), indicate that carbon is an important secondary constraint to bacterial production. Responses to NP and P were more frequent in the Autana than in the Cataniapo, suggesting that bacteria in the clear‐water river were less resource limited than those in the black‐water river. Responses to added leaf leachate, which had high concentrations of P and dissolved organic carbon (DOC), were generally greater than responses to added algal leachate. Seasonal patterns in response to nutrient addition suggest that the size of the labile fraction of DOC may increase during periods of low and rising water.
Local‐ and regional‐scale effects of wave exposure, thermal stress, and absolute versus effective shore level on patterns of intertidal zonation
Authors: Harley Christopher D. G. , Helmuth Brian S. T.
Published: July 2010

Rocky intertidal ecosystems are characterized by marked zonation patterns in which species replace one another along the vertical gradient of emersion time. Yet, we still do not fully understand the reasons that zonation patterns are variable in space and time. Here, we use effective shore level (ESL), a metric that incorporates the modifying influence of wave splash, to describe the relationship between uninterrupted emersion time and the zonation patterns of two ecologically important species: the mussel Mytilus californianus and the barnacle Balanus glandula. At the local scale (10s to 100s of meters) on Tatoosh Island, the upper limits of both species are closely related to ESL, regardless of substrate aspect or maximum temperature. At larger spatial scales (10s to 100s of kilometers), the upper limit of Balanus is related to ESL at cool sites but not at hotter sites. Thus, although ESL explains most of the local‐scale variation in zonation at a cool site, other factors (temperature, desiccation) likely become important as spatial scale increases to incorporate warmer sites. Our results emphasize that an understanding of where and when specific ecological factors are limiting is crucial for our ability to explain and predict large‐scale biological patterns in space and time.
Accumulation of particles in propagating fronts
Authors: Helfrich Karl R. , Pineda Jesús
Published: July 2010

Field observations imply that accumulation and advection of invertebrate larvae and other plankton by propagating fronts provides an efficient mechanism for the cross‐shelf transport and subsequent coastal settlement of larvae and plankton. Gravity currents propagating into a heavier, uniform‐density fluid are known to have near‐surface velocities u that exceed the propagation speed c of the gravity current nose. It is shown by simple dam‐break laboratory experiments and through corresponding numerical solutions of two‐dimensional stratified flow and Lagrangian particle advection that this characteristic of gravity currents can lead to accumulation in the gravity current nose of particles that originate well behind the gravity current head. However, efficient particle accumulation and transport with the speed c only occurs for particles with sufficient vertical buoyancy or vertical swimming capability. Weakly buoyant or neutral particles are swept down and back away from the gravity current head by the circulation in this region. A dam break into an ambient fluid with stratification (two‐layered) could result in the transport of some particles that are initially trapped in the forepart of the leading disturbance, an undular bore. However, this situation does not result in significant accumulation of particles at the leading edge of the disturbance. The numerical solutions show that particles that originate in the dense fluid ahead of the gravity current are much less likely to be accumulated at the gravity current nose. A simple scaling criterion for the minimum vertical buoyancy velocity for efficient accumulation and transport of buoyant particles based on the gravity current and particle characteristics is developed and compares favorably with the numerical results. The implications of these results for transport of larvae in coastal environments are discussed.
Zinc‐bicarbonate colimitation of Emiliania huxleyi
Authors: Buitenhuis Erik T. , Timmermans Klaas R. , de Baar Hein J. W.
Published: July 2010

In analogy to the Fe hypothesis, the Zn hypothesis states that Zn may limit primary production in some regions of the world oceans and therefore influence the global carbon cycle. The proposed mechanism is via carbon limitation due to a lack of the cofactor Zn in carbonic anhydrase. In the current conceptual model for the use of inorganic carbon by E. huxleyi, carbonic anhydrase in the chloroplast generates CO2 from HCO3− at the site where CO2 is fixed by ribulose bisphosphate carboxylase oxygenase (Rubisco). The H+ that is required in this reaction comes from calcification. From this it can be expected that carbonic anhydrase affects the use of HCO3− in photo‐synthesis. First, we grew E. huxleyi under Zn2+ limitation. The K1/2 for growth of E. huxleyi is 19 ± 8 pmol L−1 Zn2+ with a minimum requirement of 9 ± 3 pmol L−1. Additions of both ethylenediaminetetraacetic acid (EDTA) and ZnCl2 show that EDTA is not detrimental to E. huxleyi up to a concentration of 200 µmol L−1. Then we grew E. huxleyi under Zn2+‐HCO3− colimitation to test the conceptual model outlined above. The results were partly inconsistent with the model. Contrary to what was expected from the conceptual model, the efficiency of CO2 use decreased when both Zn2+ and HCO3− concentrations were low, even though the experiment was conducted at a constant high concentration of CO2. This shows that Zn2+, and possibly carbonic anhydrase activity, are needed for CO2 fixation also. In accordance with the model, we found that Zn2+ affects the efficiency of HCO3− use by E. huxleyi. Since the lowest Zn2+ concentration in the Northeast Pacific is ~0.4 pmol L−1, Zn limitation of E. huxleyi growth may indeed occur.
Advective pore‐water exchange driven by surface gravity waves and its ecological implications
Authors: Precht Elimar , Huettel Markus
Published: July 2010

The effects of surface gravity waves on pore‐water release from permeable sediment (k = 1.3–1.8 × 10−11 m2) in shallow water were studied in a wave tank. Our tracer experiments demonstrated that shallow‐water waves can increase fluid exchange between sandy sediment and overlying water 50‐fold, relative to the exchange by molecular diffusion. The main driving force for this increased exchange are the pressure gradients generated by the interaction of oscillating boundary flows and sediment wave ripples. These gradients produce a pore‐water flow field, with a regular pattern of intrusion and release zones, that migrates with ripple propagation. The ensuing topography‐related filtering rates in the wave tank ranged from 60 to 590 L m−2 d−1 and exceeded the solute exchange rates caused by hydrostatic wave pumping (38 L m−2 d−1) and initial molecular diffusion (corresponding to 10–12 L m−2 d−1). Wave‐induced filtration is ecologically relevant because permeable sandy sediments are very abundant on the continental margins and can be converted into effective filter systems, which suggests that these sediments are sites for rapid mineralization and recycling. We propose that the wave influenced continental shelf may be subdivided into two zones: a shallow zone (water depth < wavelength/2), where wave orbital motion at the sea floor creates ripples and causes topography related advective filtering; and a deeper zone (wavelength/2 < water depth < wavelength), where wave pumping enhances interfacial exchange by hydrostatic pressure oscillations.
On the use of lipid biomarkers in marine food web analyses: An experimental case study on the Antarctic krill, Euphausia superba
Authors: Stübing Dorothea , Hagen Wilhelm , Schmidt Katrin
Published: July 2010

The application of marker fatty acids to trace the feeding habits of Euphausia superba (krill) has produced contradictory results. We examined the effects of various diets on the fatty acid composition of larval, juvenile, and adult E. superba collected in April 1999 in the southwest Lazarev Sea and in April 2001 in the Bellingshausen Sea. Specimens were fed four different diets (mixed phytoplankton, mixed ice algae, the ice diatom Fragilariopsis cylindrus, and mixed copepod assemblages) or starved for up to 44 d. Total lipid content, lipid classes, and fatty acid composition showed very little variation in juvenile and adult krill with the different feeding regimes. Furcilia lipids were much more strongly influenced by the fatty acid signatures of their food. No stage‐specific food preferences were detected in the larvae, and spatial patterns were mirrored by all furcilia stages. Comparison of the fatty acid profiles of the offered food with those of the subsequently excreted feces indicated preferential assimilation of polyunsaturated fatty acids by E. superba .
Processing watershed‐derived nitrogen in a well‐flushed New England estuary
Authors: Tobias Craig R. , Cieri Matthew , Peterson Bruce J. , Deegan Linda A. , Vallino Joseph , Hughes Jeffrey
Published: July 2010

Isotopically labeled nitrate (15NO3−) was added continuously to the Rowley estuary, Massachusetts, for 22 d to assess the transport, uptake, and cycling of terrestrially derived nitrogen during a period of high river discharge and low phytoplankton activity. Isotopic enrichment of the 3.5‐km tidal prism (150,000 m3) was achieved for the 3 weeks and allowed us to construct a nitrogen mass balance model for the upper estuary. Mean δ15NO3− in the estuary ranged from 300‰ to 600‰, and approximately 75%–80% of the 15N was exported conservatively as 15NO3− to the coastal ocean. Essentially all of the 20%–25% of the 15N processed in the estuary occurred in the benthos and was evenly split between direct denitrification and autotrophic assimilation. The lack of water‐column 15N uptake was attributed to low phytoplankton stocks and short water residence times (1.2–1.4 d). Uptake of watercolumn NO3− by benthic autotrophs (enriched in excess of 100‰) was a function of NO3− concentration and satisfied up to 15% and 25% of the total nitrogen demand for benthic microalgae and macroalgae, respectively. Approximately 10% of tracer assimilated by benthic autotrophs was mineralized and released back to the water column as 15NH4+. By the end of the study, 15N storage in sediments and marsh macrophytes accounted for 50%–70% of the 15N assimilated in the estuary. These compartments may sequester watershed‐derived nitrogen in the estuary for time scales of months to years.
The role of the picoeukaryote Aureococcus anophagefferens in cycling of marine high—molecular weight dissolved organic nitrogen
Authors: Berg Gry Mine BergGry Mine , Repeta Daniel J. , LaRoche Julie
Published: July 2010

Environmental evidence suggests that Aureococcus anophagefferens (Pelagophyceae), a eukaryotic picoplankton that blooms in coastal seawaters, can outcompete other organisms because of its ability to use abundant dissolved organic nitrogen (DON). To test this hypothesis, we isolated A. anophagefferens in axenic culture and monitored its growth on high‐molecular weight (HMW) DON collected from sediment pore waters, a putative source for DON in bays where blooms occur. HMW DON originating from pore water had a substantially higher protein content than surface seawater DON. We found that A. anophagefferens could deplete 25–36% of the available nitrogen in cultures with HMW DON as the sole source of nitrogen and that this corresponded well with the protein fraction in pore‐water HMW DON. High rates of cell surface peptide hydrolysis and no detectable N‐acetyl polysaccharide hydrolysis, together with the high percentage of hydrolyzable amino acids compared to hydrolyzable aminosugars present in the HMW DON, pointed to the protein fraction as the more likely source of nitrogen used for growth. Whether or not nitrogen scavenging from protein is a common mechanism in phytoplankton is at present unknown but needs to be investigated.
A test of the role of polyunsaturated fatty acids in phytoplankton food quality for Daphnia using liposome supplementation
Authors: Ravet Joseph L. , Brett Michael T. , Müller-Navarra Dörthe C.
Published: July 2010

We conducted a series of experiments using the herbivorous zooplankter Daphnia pulex to investigate the nutritional importance of dietary polyunsaturated fatty acids (PUFAs). Daphnia were fed three different diets: (1) a PUFA‐deficient cyanophyte mixture, (2) a cyanophyte mixture with fatty acid (FA) amendments, or (3) a PUFA‐rich cryptophyte mixture. We devised a novel method using customized phosphatidylcholine liposomes to deliver specific FA amendments in a pure and bioavailable form that closely mimics their form in natural diets. We added FA‐impregnated liposomes to the cyanophyte mixtures at levels equivalent to the observed differences in FA concentrations between the cyanophyte and cryptophyte mixtures. Liposome control amendments (without FAs added) had no effect on Daphnia growth. Eicosapentaenoic acid (EPA)‐impregnated liposome amendments accounted for 30% of Daphnia somatic growth‐rate differences and 38% of clutch‐size differences between the cyanophyte and cryptophyte diets. Liposome supplementation of a FA mixture, which included a saturated fatty acid and the four ω3 PUFAs most prevalent in cryptophytes but rare in cyanophytes, accounted for 59% of Daphnia somatic growthrate differences and 47% of clutch‐size differences between the cyanophyte and cryptophyte diets. Our study suggests that phytoplankton ω3 PUFA, and especially EPA, content plays an important direct role in herbivorous zooplankton nutrition.
The seasonal evolution of wind/internal wave resonance in Lake Kinneret
Authors: Antenucci Jason P. , Imberger Jörg
Published: July 2010

Data from a thermistor chain and wind sensor collected over an annual stratification cycle in Lake Kinneret (Israel) during 2000 were used to investigate the seasonal evolution of wind/internal wave resonance. Internal wave periods determined from an analytical model were compared with observations, and we show that resonance during 2000 occurred during three distinct times of the year—at the onset of stratification (March), during the heating phase (June), and during the cooling phase (November). In all cases, resonance was between the wind and the dominant radial, azimuthal, and vertical mode‐one cyclonic (Kelvin) wave previously observed in Lake Kinneret.
A modern isotope record of changes in water and carbon budgets in a groundwater‐fed lake: Blue Lake, South Australia
Authors: Herczeg A. L. , Leaney F. W. , Dighton J. C. , Lamontagne S. , Schiff S. L. , Telfer A. L. , English M. C.
Published: July 2010

Groundwater is often a major component of lake water and chemical balances. Alteration in catchment subsurface water balances through land‐use change and pumping can affect lake biogeochemical cycles through changes in groundwater flow rates. We present a modern (~200 yr) sediment isotope record ( δ18Ocarb, δ13Ccarb, and δ13Corg) of changes to the water and carbon budget of Blue Lake, which is situated in karstic limestone in South Australia and is mostly fed by groundwater. The 3.5‰ decrease in δ18Ocarb and δ13Ccarb deposited in lake sediments since ~1850 reflects higher rates of groundwater through‐flow due to rainfall variability, land‐use change, and increased rates of pumping over the past 40 yr. Calculated water residence times, based on the isotopic data, were 23 ± 2 yr before 1850 and decreased by the late twentieth century to 8 ± 2 yr. Concomitant dissolved inorganic carbon (DIC) residence times were 3.8 and 2.2 yr, respectively. Although burial rates of CaCO3 have increased by about threefold over the past 40 yr, there is no evidence that substantial changes in organic carbon deposition has occurred. The shorter DIC residence time is largely due to increased input and higher CO2 evasion rates. About 88% of Ccarb and ~96% of Corg generated within the water column is remineralized, but the isotopic composition is essentially unaltered in the sediments. The measured variations in δ13Ccarb and δ13Corg of sediments are thought to reflect changes to DIC residence time and increased CO2(aq) concentrations rather than changes in lake productivity.
Climate‐related changes in recruitment of the bivalve Macoma balthica
Authors: Philippart Catharina J. M. , van Aken Hendrik M. , Beukema Jan J. , Bos Oscar G. , Cadée Gerhard C. , Dekker Rob
Published: July 2010

Population dynamics of common intertidal bivalves (Cerastoderma edule, Macoma balthica, Mya arenaria, Mytilus edulis) are strongly related to seawater temperatures. In northwestern European estuaries, series of mild winters followed by low bivalve recruit densities lead to small adult stocks. In this study, we examine temperature‐induced effects on reproductive output (eggs m−2), onset of spawning (day of the year), and the juvenile instantaneous mortality rate (per day) of M. balthica. Data analysis was based on an extensive long‐term data set (1973–2001) originating from the western Wadden Sea. Our results strongly suggest that rising seawater temperatures affect recruitment by a decrease in reproductive output and by spring advancement of bivalve spawning. Apparently, global warming upsets the evolved reproductive strategy of this marine bivalve to tune its reproduction to the most optimal environmental conditions for the first vulnerable life stages, most importantly the match/mismatch of time of spawning with that of the phytoplankton bloom and the settlement of juvenile shrimps on the tidal flats. It is hypothesized that the observed density‐dependent mortality of juvenile bivalves may act via competition for food, a behavioral response of shrimp to low spat densities, or be the result of the response of age and size at metamorphosis of marine bivalves to resource variability. It is to be expected that prolonged periods of lowered bivalve recruitment and stocks will lead to a reformulation of estuarine food webs and possibly a reduction of the resilience of the system to additional disturbances, such as the depletion and disturbance by shellfish fisheries.
Bacterial endosymbionts in Asellus aquaticus (Isopoda) and Gammarus pulex (Amphipoda) and their contribution to digestion
Authors: Zimmer Martin , Bartholmé Silvia
Published: July 2010

We demonstrate for the first time the presence of bacterial endosymbionts in the midgut glands (hepatopancreas) of the freshwater detritivore Asellus aquaticus (Isopoda), whereas the hepatopancreas of another crustacean detritivore, Gammarus pulex (Amphipoda), which coexists with the former species, was devoid of such bacteria. We detected both phenol oxidase and cellulase activity in hepatopancreatic extracts from both detritivores, which suggests that both of these enzymes are produced in the midgut glands of both species. After treatment with antibiotics, both the number of hepatopancreatic bacteria and enzymatic activity were reduced in the isopod hepatopancreas, but antibiotics had no effect on enzyme activity in the amphipod hepatopancreas. Feeding on microbially inactivated leaf litter did not affect enzyme activity in hepatopancreatic extracts from A. asellus, but increased cellulase activity was seen in the hepatopancreas of G. pulex. These results (1) confirm the hypothesized enzymatic adaptation of crustacean freshwater detritivores to their food sources of terrestrial origin, (2) demonstrate that the isopod A. aquaticus, like its terrestrial relatives, contains endosymbiotic bacteria that contribute to digestive processes, and (3) show interspecific differences between these coexisting crustacean detritivores in terms of enzyme origin and their dependence on microbiota.
Do we need complex mechanistic photoacclimation models for phytoplankton?
Authors: Flynn Kevin J.
Published: July 2010

The outputs of simple models relating phytoplankton growth and chlorophyll a (Chl a) to irradiance and nutrient (nitrogen [N] and/or iron [Fe]) availability are compared with those of complex mechanistic models. Mechanistic models, which are significantly more expensive in computational terms, are required for a proper description of high‐resolution dynamics of light acclimation with or without changes in nutrient status (for example, with diurnal light—dark periodicity or Fe fertilization). However, for instances in which such detailed descriptions of growth and Chl : carbon (C) are not required, there appears to be no justification for using mechanistic models to simulate nutrient—light interaction. Multinutrient models based on modified quota—type models, coupled with a simple photosynthesis—irradiance growth rate equation and an empirically derived Chl a :C relationship linking irradiance to growth rate, should be adequate for most oceanographic modeling scenarios.
Bioavailability of iron to Trichodesmium colonies in the western subtropical Atlantic Ocean
Authors: Achilles Katherine M. , Church Thomas M. , Wilhelm Steven W. , Luther George III. W. , Hutchins David A.
Published: July 2010

Trichodesmium provides new nitrogen (N) to marine surface waters via N2 fixation, a process that requires a substantial amount of iron (Fe). Organic ligands in seawater that bind Fe could either increase or reduce the bioavailability of Fe. Electrochemical techniques indicate that these naturally occurring ligands have Fe‐binding constants similar to those of siderophores and porphyrins, suggesting that these chelators play an important role in determining the bioavailability of Fe to cyanobacteria. We conducted Fe uptake experiments using model ligands labeled with 55Fe to compare the bioavailability of inorganic Fe(III), porphyrin‐bound Fe(III), and siderophorebound Fe(III) to field‐collected Trichodesmium colonies. Inorganic Fe(III) and siderophore‐bound Fe(III) were more bioavailable to Trichodesmium colonies than was porphyrin‐bound Fe(III). Furthermore, the bioavailability of the siderophore‐bound Fe(III) can be characterized by the functional groups of the siderophore. The dihydroxamate siderophore and an uncharacterized ligand from a cultured Synechococcus sp. increased the bioavailability of Fe compared to the trihydroxamate siderophores. Except for experiments with desferrioxamine B, dark incubations resulted in lower Fe uptake rates for all treatments, relative to parallel lighted incubations. This suggests that light enhances the photochemical dissociation of most of the ligand complexes or that light energy is required for the active transport of Fe complexed to the model ligands. The Fe uptake rate of Trichodesmium colonies also differed slightly on the basis of colony morphology, with higher uptake rates with “puffs” than “tufts.” These experiments show that Trichodesmium colonies are capable of discriminating between Fe bound to different organic complexes.
Biological factors regulating the chemical speciation of Cu, Zn, and Mn under different nutrient regimes in a marine mesocosm experiment
Authors: Muller François L. L. , Jacquet Stéphan , Wilson William H.
Published: July 2010

Previous mesocosm experiments and field studies have indicated a central role of major nutrients (e.g., N, P, and Si) in regulating phytoplankton abundance and species composition and influencing the rate of uptake of bioactive metals by phytoplankton. We monitored the changes in bioactive metals (Cu, Zn, and Mn) concentration and speciation that occurred over the course of a bloom of the coccolithophore Emiliana huxleyi under three different nutrient treatments. In each treatment, the electrochemically labile fraction of Cu decreased from 1.1 to 0.3 nmol L−1 during the formation of the E. huxleyi bloom. This decrease was likely due to the active release of organic ligands by phytoplankton, which shut off as soon as the level of labile Cu reached 0.3 nmol L−1 ([Cu2+] = 0.02 nmol L−1). Organic Zn‐binding ligands showed elevated concentrations in bag 1 (the N‐ and P‐replete bag) only. The release of these ligands coincided with a 15‐fold increase in the concentration of dead E. huxleyi cells in bag 1 but only a threefold increase in the P‐ or N‐depleted bags. This suggests that the Zn‐binding ligands may have originated from dead or decaying E. huxleyi cells. In both the P‐ and N‐limited bags, labile Mn concentrations showed a succession of two peaks and troughs over the 2‐week period; these were synchronized—but inversely correlated—with bacterial abundance. Labile Mn showed weaker fluctuations and no clear synchronization with bacterial abundance in the N‐ and P‐replete bag. We hypothesize that the passive release of organic Zn‐binding ligands under the nutrient‐replete conditions may have helped restrict the detrimental effect of Zn2+ on Mn uptake by phytoplankton cells, thus diverting part of the dissolved Mn away from sequestration or oxidation by bacteria.