bluuebunny/crossref_metadata_2025 huggingface dataset:
Growth rates of Antarctic krill, Euphausia superba: Comparison of the instantaneous growth rate method with nitrogen and phosphorus stoichiometry
Authors: Arnold Katharine H. , Shreeve Rachael S. , Atkinson Angus , Clarke Andrew
Published: July 2010

Zooplankton growth rates are hard to measure directly, and proxy measurements are desirable to encompass the variety of species and scales of interest. The growth rate hypothesis of stoichiometric theory states that a negative relationship exists between nitrogen : phosphorus (N : P) stoichiometry and growth rate, driven by cellular ribosomal ribonucleic acid (rRNA) content. Despite the wealth of literature on the growth rate hypothesis, there exist no practical demonstrations of its use in the marine literature. We thus investigated whether this hypothesis could be the basis of a technique to estimate growth rates of Euphausia superba by comparing, for the same individual krill, elemental stoichiometry and growth rates derived from the instantaneous growth rate (IGR) method. These growth rates were the first IGR measurements from South Georgia; from within a restricted area over the course of just 1 month, these rates were highly variable, from negative to near maximum rates recorded for the species. Although there were significant differences in N: P ratio and phosphorus content between individuals and schools, there was no relationship between N: P ratio and growth rate when data were grouped by school. Thus, our data do not support the predictions of the growth rate hypothesis at an intraspecific scale. However, when all data were pooled, the mean values of growth rate and N: P ratio did fit the interspecific relationship established previously for freshwater zooplankton. We suggest that krill maintain the biochemical machinery for high growth potential and maintain high growth in summer to take advantage of short‐term fluctuations in food.
Circulation and turbulent exchange characteristics during the thermal bar in Lake Ontario
Authors: Rao Yerubandi R. , Skafel Michael G. , Charlton Murray N.
Published: July 2010

We made observations from vertical moorings of thermistors and current meters in Lake Ontario to study the spring thermal bar circulation, its effect in inhibiting the horizontal mixing, and spatial and temporal variability of vertical mixing during this period. Thermal bar progression rates were compared with two analytical models; both produced overestimates during the early part of the thermal bar and underestimates during the late spring. In a zone between the shore and the thermal bar, currents were influenced by the local wind stress and flowed in a counterclockwise direction. The mean cross‐shore flows in the nearshore zone were reduced during the thermal bar period. The alongshore horizontal exchange coefficients were higher than cross‐shore exchange coefficients. During the thermal bar period, the magnitude of both alongshore and cross‐shore exchange coefficients decreased when the bar was still within middepth (<40 m); when it was further offshore, the thermal bar did not have any significant effect on alongshore exchange coefficients, but cross‐shore exchanges decreased marginally. The vertical exchange coefficients were high in the water column because of convective mixing during the thermal bar period. During the evolution of spring thermal bar, the vertical mixing decreased considerably in the nearshore regions under stable stratification, whereas the high vertical mixing levels continued in the offshore region for a longer period.
Temporal variation of light availability in coastal benthic habitats: Effects of clouds, turbidity, and tides
Authors: Anthony Kenneth R. N. , Ridd Peter V. , Orpin Alan R. , Larcombe Piers , Lough Janice
Published: July 2010

We analyzed the contributions of clouds, turbidity, and tides to variations in irradiance and predicted benthic primary productivity on a coastal coral reef over a period of 2 yr (2001–2002). At 1.5 m below lowest astronomical tide (3.8–m tidal range), attenuation by suspended solids (turbidity) accounted for 74–79% of the total annual variation in irradiance, clouds for 14–17%, and tides for 7–10%. With increasing depth, the contribution from turbidity to irradiance variation increased asymptotically toward 95%. Fourier (spectral) analyses indicated that the benthic irradiance regime followed strong 8‐week periodicities and weaker 2–4‐week periodicities. The 8‐week cycle was driven primarily by turbidity and secondarily by clouds and matches the periodicity of the intraseasonal Madden‐Julian atmospheric oscillation. The weaker 3–4‐week irradiance cycle was driven by turbidity; the 2‐week cycle was driven by tides and, to a lesser extent, clouds. Comparisons of the benthic irradiance pattern with predictions of physiologically optimal irradiance levels (parameter Ek) for the coral Turbinaria mesenterina suggested that corals at the site alternate between states of potential light limitation and light stress, with a 2–8‐week periodicity caused mainly by variations in turbidity. The effect of external sources of light reduction, such as episodic runoff events, on the energetics of benthic primary producers is likely to vary critically with the timing of such events.
Tidal dynamics of dissolved and particulate matter and bacteria in a tidal flat ecosystem in spring and fall
Authors: Grossart Hans-Peter , Brinkhoff Thorsten , Martens Torben , Duerselen Claus , Liebezeit Gerd , Simon Meinhard
Published: July 2010

Tidal flat ecosystems are highly dynamic environments with tremendous short‐term and long‐term changes in physical, chemical, and biological properties. We measured salinity, temperature, oxygen, dry weight (dry wt), inorganic (PIC) and organic carbon (POC) fractions of suspended particulate matter (SPM), and chlorophyll a (Chl a); counted numbers of free‐living and particle‐associated bacteria; and measured dissolved amino acids and carbohydrates, and ectoenzymatic activities throughout tidal cycles in the Spiekeroog tidal flat system of the German Wadden Sea in November 1999 and May 2000. Bacterial production was measured only in May 2000. In November, high sediment resuspension, as indicated by increased amounts of SPM, PIC, and POC, mainly controlled bacterial dynamics and led to enhanced numbers of aggregate‐associated bacteria. In contrast, in May, phytoplankton‐related processes mainly influenced bacterial dynamics. Increased concentrations of oxygen, Chl a, and dissolved combined neutral monosaccharides resulted in high numbers of free‐living bacteria, elevated β‐glucosidase activities, and high bacterial production rates. The introduction of foreign water masses, either from the open North Sea or adjacent tidal basins, as reflected by different signatures of SPM and the mole percent composition of particulate combined amino acids, appeared to be a regular event in this highly dynamic system and rapidly changed physical, chemical, and biological processes.
Why the limiting nutrient differs between temperate coastal seas and freshwater lakes: A matter of salt
Authors: Blomqvist Sven , Gunnars Anneli , Elmgren Ragnar
Published: July 2010

Comparison of bottom‐water chemistry in the marine‐limnic habitat gradient shows greater phosphorus availability in marine waters, primarily because of enhanced iron sequestration by sulfide. In the oxidative hydrolysis of iron and the concomitant precipitation of phosphate, a minimum of two iron atoms are needed to precipitate one phosphate molecule (Fe : P = 2). However, dissolved Fe : P < 2 predominates in anoxic marine waters, therefore leaving some phosphate in solution after oxygenation because of a shortage of dissolved iron for phosphate coprecipitation by iron oxyhydroxide. In contrast, anoxic bottom waters in most freshwater lakes show Fe : P > 2, allowing almost complete phosphate removal on oxygenation. This difference is a consequence of the high sulfate content of sea salt, and a main reason why nitrogen normally limits net primary production in temperate coastal waters, in contrast to the predominant phosphorus limitation of near‐neutral lakes.
A model for lead, zinc, and copper in lakes
Authors: Rippey Brian , Rose Catherine L. , Douglas Richard W.
Published: July 2010

We present a budget for lead, zinc, and copper in Lough Neagh, Northern Ireland, and use the results from this and seven other lakes in Europe and the United States to develop two simple, generic models for heavy metals in lake water. The models, based on retention coefficient and sedimentation coefficient, are assessed by comparing the modeled and measured steady‐state lake concentrations. The results show that deposition from the atmosphere is an important source of contamination of the lakes by Pb (56.3% of total inputs) and Zn (34.8%), but not Cu (8.3%). The mean retention coefficient in five of the lakes is 0.70 (RSD = 29%), 0.72 (24%), and 0.24 (133%) for Pb, Zn, and Cu, respectively; mean total residence time is 0.21 (RSD = 64%), 0.36 (61%), and 0.77 (97%) yr; and mean sedimentation residence time 0.34 (RSD = 73%), 0.91 (99%), and 0.05 (95%) yr. The goodness of fit of the two models are similar, with root mean square error of prediction of ±0.14, 1.9, and 1.6 µg L−1 for Pb, Zn, and Cu, respectively. The magnitude of the errors in relation to the ranges of concentration suggests that the models for Pb and Zn can be used as fate models for these metals in lake water.
Taxonomic variability of phosphorus stress in Sargasso Sea phytoplankton
Authors: Lomas Michael W. , Swain Ashley , Shelton Ryan , Ammerman James W.
Published: July 2010

Low inorganic phosphorus (SRP) concentrations and high inorganic nitrogen to phosphorus ratios suggest that phytoplankton production in the northwestern Sargasso Sea may be controlled to some extent by the availability of phosphorus. Phosphorus stress in marine phytoplankton was qualitatively assessed by using a single‐cell enzyme‐linked fluorescent (ELF) assay for the enzyme alkaline phosphatase, which is induced at low SRP concentrations. During the highly stratified summer period, ~30% of the observed autotrophic eukaryotes in the surface waters were ELF‐labeled, whereas in the well‐mixed fall period, nearly 70% of the observed autotrophic eukaryotes in the surface waters were ELF‐labeled. During the summer, autotrophic flagellates displayed signifi‐ cantly higher ELF‐labeling than did both diatoms and dinoflagellates, and this labeling did not vary with depth, whereas in the fall, autotrophic flagellates and diatoms displayed statistically similar and decreasing percentages of ELF‐labeled cells as a function of depth. This assay allowed for rapid assessment of the in situ physiological condition of individual autotrophic phytoplankton in the Sargasso Sea. By using this assay, we were able to identify taxonomic and potential seasonal variability of phosphorus stress within the autotrophic phytoplankton community.
Pore‐water advection and solute fluxes in permeable marine sediments (I): Calibration and performance of the novel benthic chamber system Sandy
Authors: Janssen F. , Faerber P. , Huettel M. , Meyer V. , Witte U.
Published: July 2010

This contribution introduces the benthic chamber system Sandy that was developed for studying in situ solute fluxes in permeable sandy sediments where advective pore‐water transport can dominate solute exchange across the sediment‐water interface. The Sandy system can be deployed at the seafloor, where it autonomously performs measurements of sediment‐water solute fluxes. The innovative features include an insertion mechanism that permits gentle and deep penetration of the chamber into hard consolidated sands with minimum disturbance, and an adjustable stirrer that generates a rotationally symmetric pressure gradient between the center and the circumference of the enclosed sediment surface. In contrast to similar systems, Sandy takes advective pore‐water exchange into account. Through adjustment of the stirring rate, defined pressure gradients can be established that are similar in shape and magnitude to natural pressure gradients that develop at topographical structures of current‐exposed sediment surfaces. Solute fluxes measured in the chamber at a specific “advective” stirrer setting can be compared with those obtained at reduced stirring, where pressure gradients are absent and solute exchange therefore is restricted to diffusion and bioirrigation. Rates of pore‐water exchange that were determined by means of tracer experiments at different stirrer settings in natural coarse‐grained North Sea deposits demonstrate the feasibility of this approach. This permits, for the first time, an evaluation of the contribution of advective exchange to the total interfacial solute flux in situ and makes Sandy a valuable tool to study advection‐related processes in permeable shelf sediments.
Comparison of size‐dependent carbon, nitrate, and silicic acid uptake rates in high‐ and low‐iron waters
Authors: Franck Valerie M. , Smith Geoffrey J. , Bruland Kenneth W. , Brzezinski Mark A.
Published: July 2010

We compare the relative contribution of large phytoplankton (>5 and >10 µm) to uptake rates of carbon (C), nitrate (NO3−), and dissolved silicon (Si) and uptake ratios of Si :NO and Si :C in Monterey Bay, California (a high‐iron region) with three low‐iron regions in the eastern tropical Pacific: the Costa Rica upwelling dome, Humboldt current, and Peru upwelling. We also demonstrate the effect of iron enrichment on the above parameters in the latter three regions. In Monterey Bay waters, the >5‐µm size fraction accounted on average for the majority of particulate organic C and was responsible for 92% of total C uptake, 81% of total NO3− uptake, and 98% of total 23 Si uptake. In contrast, in low‐iron eastern tropical Pacific waters, the >5‐µm size fraction accounted for less than half of the particulate organic C and was responsible for a substantially smaller proportion of total C and NO3− uptake: 27—43% for C and 34—59% for NO3−. Iron enrichment experiments in the eastern tropical Pacific resulted in much higher NO3−, C, and Si uptake rates, although increases were restricted to cells in the >5‐µm size fraction. Si:NO and Si:C uptake ratios decreased after iron addition at most locations, and decreases were a direct result of lower Si:NO3− and Si:C uptake ratios in the >5‐µm size fraction. Our results suggest that iron availability is a major factor regulating primary production, new production, Si uptake, and Si:NO3− and Si :C uptake ratios in the larger phytoplankton size classes in high‐nitrate, low‐chlorophyll (HNLC) regions in the eastern tropical Pacific.
Diel variations in the photosynthetic parameters of Prochlorococcus strain PCC 9511: Combined effects of light and cell cycle
Authors: Bruyant F. , Babin M. , Genty B. , Prasil O. , Behrenfeld M. J. , Claustre H. , Bricaud A. , Garczarek L. , Holtzendorff J. , Koblizek M. , Dousova H. , Partensky F.
Published: July 2010

We examined the mechanisms related to the diel variations in the parameters of the relationship between the rate of carbon fixation of phytoplankton and irradiance (P vs. E curve). Our goal was to understand what determines the phase of these variations relative to that of the light cycle. We grew the marine prokaryote Prochlorococcus in an axenic cyclostat culture system under a light–dark cycle that mimicked natural conditions at sea surface and followed changes in cell physiology with a 2‐h resolution. Individual cells divide mostly in phase with each other, once a day at the beginning of the dark period. The quantum yields of chlorophyll fluorescence, the maximum quantum yield of carbon fixation (φCmax) and the maximum rate of carbon fixation (PBmax) exhibited diel variations over about factors of 2, 4, and 4, respectively, with maxima at the beginning of the light period. The morning drop in φCmax and the quantum yield of fluorescence, which was accompanied by only a small decrease (<15%) of photochemial efficiency of PSII (Fv/Fm), suggests regulation by light and preceded the drop in PBmax by 4 h. The decrease in PBmax during the day matched a decrease in the transcription level of Rubisco. The quantum yield of fluorescence, φCmax, and PBmax increased again during the dark period, but this recovery was slowed at the time of cell division. Our results suggest that the pattern of diel variations in the photosynthetic parameters is determined both by photoacclimation and the cell‐division cycle.
A general kinetic model for iron acquisition by eukaryotic phytoplankton
Authors: Shaked Yeala , Kustka Adam B. , Morel François M. M.
Published: July 2010

The conventional model of iron uptake in marine eukaryotic phytoplankton—the Fe′ model—suggests a dependency of uptake rate on the concentration of unchelated iron species (Fe′), and not the concentration of total iron or iron chelated with organic ligands. However, iron in seawater is bound by strong organic ligands that buffer such low Fe′ concentrations that they should not support phytoplankton growth. Studies that show uptake and extracellular reduction of siderophore‐bound iron by diatoms and provide indications that the iron uptake system of phytoplankton may be similar to that of yeast in which extracellular reduction is a prerequisite for uptake, call for revisions of the Fe′ model. In this paper we propose a new model for iron uptake by diatoms in which extracellular reduction of all Fe species is a necessary step for iron acquisition. Experiments verifying the predictions of the model are presented. In particular we show data supporting the fact that Fe(II) is formed as an intermediate during Fe uptake from all experimental media, including those buffered by Fe(III)EDTA. This model reconciles the standing Fe′ model with new data and concepts on reduction of iron chelates and provides a convenient framework for designing and interpreting iron uptake experiments in a variety of natural and artificial media.
Complex interactions between the zebra mussel, Dreissena polymorpha, and the harmful phytoplankter, Microcystis aeruginosa
Authors: Sarnelle Orlando , Wilson Alan E. , Hamilton Stephen K. , Knoll Lesley B. , Raikow David F.
Published: July 2010

We report a reversal in the sign of the herbivore‐phytoplankton interaction between the zebra mussel (Dreissena polymorpha) and Microcystis aeruginosa, a harmful planktonic cyanobacterium. A pair of large‐scale manipulations of mussel density in the same lake in consecutive years showed that when phosphorus concentrations were very low (mean total phosphorus [TP] ~ 3 µg L−1), the effect of Dreissena on the biomass of M. aeruginosa was monotonically negative across the full range of sustainable mussel densities. When the enclosures were fertilized with phosphorus (mean TP ~ 9 µg L−1), there was a monotonically positive effect of Dreissena on M. aeruginosa across the same mussel gradient. These contrasting results indicate that D. polymorpha feeds on M. aeruginosa, as shown in previous laboratory feeding experiments, but that the positive effects of D. polymorpha on M. aeruginosa can be larger than the negative effects of consumption. A sign reversal in the interaction between these two species is congruent with highly variable patterns in the response of M. aeruginosa to D. polymorpha invasion across lake and river systems in North America.
The relationship between species richness and taxonomic distinctness in freshwater organisms
Authors: Heino Jani , Soininen Janne , Lappalainen Jyrki , Virtanen Risto
Published: July 2010

Taxonomic distinctness is a newer biodiversity measure that emphasizes the average taxonomic relatedness between species in a community. We examined whether species richness (SR) and taxonomic distinctness (TD) were significantly related and whether they showed similar environmental relationships in regional data sets for various groups of freshwater organisms, ranging from lake mollusks and fishes to stream diatoms and invertebrates. We found that the relationship between SR and TD varied widely, ranging from significantly negative through nonsignificant to significantly positive. In general, SR and TD were related to different environmental gradients, although the particular environmental variables accounting for biodiversity patterns varied among data sets and, more importantly, even between different data sets for the same organism groups. SR and TD can provide complementary views of the variability of biodiversity. These findings thus underline the importance of considering a set of different measures in the assessment of community‐level biodiversity, as well as considering this variability when determining anthropogenic effects in freshwater ecosystems.
Effects of atmospheric nitrogen deposition on nutrient limitation and phytoplankton biomass in unproductive Swedish lakes
Authors: Bergström Ann-Kristin , Blomqvist Peter , Jansson Mats
Published: July 2010

We used chemical data (3,907 lakes) and phytoplankton biomass (chlorophyll a) data (225 lakes) from Swedish lake monitoring programs to assess the effects of atmospheric nitrogen (N) deposition on nutrient limitation and phytoplankton biomass in unproductive Swedish lakes. There was a clear north‐south gradient of increasing lake concentrations of dissolved inorganic nitrogen, which was related to the pattern of atmospheric N input. On the basis of positive relationships between total phosphorus (P) concentrations and phytoplankton biomass we conclude that lakes in areas of enhanced N deposition are mainly P limited during summer. This relationship was not detected in lakes in pristine areas with low N deposition, which, together with experimental evidence from the literature, suggest possible N limitation. During summer, lakes in high N‐deposition areas had clearly higher phytoplankton biomass relative to the total phosphorus concentrations compared to lakes in low N‐deposition areas. Thus, in Swedish unproductive lakes, high atmospheric N input is reflected by increased lake concentrations of dissolved inorganic nitrogen and, possibly, by a shift from natural N limitation of phytoplankton to P limitation. Our results also reveal that increased N input has caused a eutrophication with higher phytoplankton biomass as the result.
Nitrogen as the limiting nutrient for eutrophication in coastal marine ecosystems: Evolving views over three decades
Authors: Howarth Robert W. , Marino Roxanne
Published: July 2010

The first special volume of Limnology and Oceanography, published in 1972, focused on whether phosphorus (P) or carbon (C) is the major agent causing eutrophication in aquatic ecosystems. Only slight mention was made that estuaries may behave differently from lakes and that nitrogen (N) may cause eutrophication in estuaries. In the following decade, an understanding of eutrophication in estuaries proceeded in relative isolation from the community of scientists studying lakes. National water quality policy in the United States was directed almost solely toward P control for both lakes and estuaries, and similarly, European nations tended to focus on P control in lakes. Although bioassay data indicated N control of eutrophication in estuaries as early as the 1970s, this body of knowledge was treated with skepticism by many freshwater scientists and water‐quality managers, because bioassay data in lakes often did not properly indicate the importance of P relative to C in those ecosystems. Hence, the bioassay data in estuaries had little influence on water‐quality management. Over the past two decades, a strong consensus has evolved among the scientific community that N is the primary cause of eutrophication in many coastal ecosystems. The development of this consensus was based in part on data from whole‐ecosystem studies and on a growing body of evidence that presented convincing mechanistic reasons why the controls of eutrophication in lakes and coastal marine ecosystems may differ. Even though N is probably the major cause of eutrophication in most coastal systems in the temperate zone, optimal management of coastal eutrophication suggests controlling both N and P, in part because P can limit primary production in some systems. In addition, excess P in estuaries can interact with the availability of N and silica (Si) to adversely affect ecological structure. Reduction of P to upstream freshwater ecosystems can also benefit coastal marine ecosystems through mechanisms such as increased Si fluxes.
Long‐term trends in eutrophication and nutrients in the coastal zone
Authors: Clarke A. L. , Weckström K. , Conley D. J. , Anderson N. J. , Adser F. , Andrén E. , de Jonge V. N. , Ellegaard M. , Juggins S. , Kauppila P. , Korhola A. , Reuss N. , Telford R. J. , Vaalgamaa S.
Published: July 2010

We used high‐resolution paleoecological records of environmental change to study the rate and magnitude of eutrophication over the last century in two contrasting coastal ecosystems. A multiproxy approach using geochemical and biological indicators and diatom‐based transfer functions provides a long‐term perspective on changes in nutrient concentrations and the corresponding biological and sedimentary responses. In Roskilde Fjord, Denmark, total nitrogen (TN) increased 85% during the last century, with the most rapid increase occurring after the 1950s, corresponding to the postwar increase in N fertilizer use. In Laajalahti Bay, an urban embayment near Helsinki, Finland, total dissolved nitrogen (TDN) increased with growing wastewater inputs and decreased with the remedial actions taken to reduce these discharges. These changes are small relative to the order of magnitude increases in nutrient loading that have occurred in northwestern Europe, where the dissolved inorganic nitrogen (DIN) load has increased more than threefold in certain areas.
Anthropogenic and climatic influences on the eutrophication of large estuarine ecosystems
Authors: Paerl Hans W. , Valdes Lexia M. , Peierls Benjamin L. , Adolf Jason E. , Harding Lawrence Jr. W.
Published: July 2010

We examined the effects of anthropogenic and climatic perturbations on nutrient—phytoplankton interactions and eutrophication in the waters of the largest estuarine systems in the U.S.A., the Chesapeake Bay (CB), Maryland/ Virginia, and the Neuse River Estuary/Pamlico Sound (NRE/PS) system, North Carolina. Both systems have experienced large post‐World War II increases in nitrogen (N) and phosphorus (P) loading, and nutrient reductions have been initiated to alleviate symptoms of eutrophication. However, ecosystem‐level effects of these nutrient reductions are strongly affected by hydrologic variability, including severe droughts and a recent increase in Atlantic hurricane activity. Phytoplankton community responses to these hydrologic perturbations, including storm surges and floods, were examined and when possible, compared for these systems. In both systems, the resulting variability in water residence time strongly influenced seasonal and longer‐term patterns of phytoplankton biomass and community composition. Fast‐growing diatoms were favored during years of high discharge and short residence time in CB, whereas this effect was not observed during high discharge conditions in the longer residence time NRE/ PS. In the NRE/PS, all phytoplankton groups except summer cyanobacterial populations showed decreased abundance during elevated flow years when compared to low flow years. Although hurricanes affected the CB less frequently than the NRE/PS, they nonetheless influenced floral composition in both systems. Seasonally, hydrologic perturbations, including droughts, floods, and storm‐related deep mixing events, overwhelmed nutrient controls on floral composition. This underscores the difficulty in predicting seasonal and longer‐term phytoplankton production and compositional responses to nutrient input reductions aimed at controlling eutrophication of large estuarine ecosystems.
Comprehensive trend analysis of nutrients and related variables in a large eutrophic estuary: A decadal study of anthropogenic and climatic influences
Authors: Burkholder JoAnn M. , Dickey David A. , Kinder Carol A. , Reed Robert E. , Mallin Michael A. , McIver Matthew R. , Cahoon Lawrence B. , Melia Greg , Brownie Cavell , Smith Joy , Deamer Nora , Springer Jeffrey , Glasgow Howard B. , Toms David
Published: July 2010

We used a decadal data set, with weekly to biweekly sampling in April—October and monthly sampling in November—March, to characterize climatic (hurricane‐level storms, a sustained 3‐yr drought) and anthropogenic influences on N and P concentrations and loadings to a large eutrophic, poorly flushed estuary, the Neuse Estuary of the Albemarle—Pamlico Estuarine System. Mass volume transport data were obtained with cross‐estuary transect flow measurements taken near the entrance to the estuary. Although trends were minimally influenced by hurricanes, analyses were significantly affected by the sustained drought near the end of the study. As examples, decreasing trends in total N (TN), total P (TP), and bottom‐water dissolved oxygen concentrations, and in TN loadings were significant considering all data, but these trends were not significant when the sustained drought was excluded from analysis. In addition, the trend in TN loading was especially sensitive to the initial sampling period. NH4+ concentrations dramatically increased (overall by ~500%) as a persistent trend regardless of attempts to control for climatic events. An increasing trend in NH4+ also was documented in an adjacent, rapidly flushed Coastal Plain estuary, the Cape Fear. The NH4+ data suggest a regional‐scale effect of high inputs from inadequately controlled, increasing nonpoint sources. The fragility of TN loading trends, the striking increase in NH4+ concentrations, and the lack of management emphasis on controlling nonpoint sources such as “new” industrialized swine production collectively do not support recent reports of achievement of a 30% reduction in TN loading to the Neuse. Nonpoint sources remain a critical target for reduction to alleviate the negative effects of cultural eutrophication in this system, as in many estuaries throughout the world.
A comparative study of responses in plankton food web structure and function in contrasting European coastal waters exposed to experimental nutrient addition
Authors: Olsen Y. , Agustí S. , Andersen T. , Duarte C. M. , Gasol J. M. , Gismervik I. , Heiskanen A.-S. , Hoell E. , Kuuppo P. , Lignell R. , Reinertsen H. , Sommer U. , Stibor H. , Tamminen T. , Vadstein O. , Vaqué D. , Vidal M.
Published: July 2010

We quantify, compare, and generalize responses of experimental nutrient loadings (LN) on planktonic community structure and function in coastal waters. Data were derived from three mesocosm experiments undertaken in Baltic (BAL), Mediterranean (MED), and Norwegian (NOR) coastal waters. A planktonic model with seven functional compartments and 30–32 different carbon flows fit to all three experiments was used as a framework for flow‐rate estimation and comparison. Flows were estimated on the basis of time series of measured biomass, some measured flows, and inverse modeling. Biomass and gross uptake rate of carbon of most groups increased linearly with increasing LN in the nutrient input range of 0–1 µmol N L−1 d−1 at all locations. The fate of the gross primary production (GPP) was similar in all systems. Autotrophic biomass varied by two orders of magnitude among locations, with the lowest biomass and response to nutrient addition in MED waters. The variation of GPP among sites was less than one order of magnitude. Mesozooplankton dominated by doliolids (Tunicata), but not those dominated by copepods, presumably exerted efficient control of the autotrophic biomass, thereby buffering responses of autotrophs to high nutrient input. Among the many factors that can modify the responses of autotrophs to nutrients, the time scale over which the enrichment is made and the precise mode of nutrient enrichment are important. We suggest a general concept that may contribute to a scientific basis for understanding and managing coastal eutrophication.
Water, salt, and nutrient exchanges in San Francisco Bay
Authors: Smith Stephen V. , Hollibaugh James T.
Published: July 2010

We constructed water, salt, and nutrient budgets for San Francisco Bay and used them to analyze the net biogeochemical performance of the bay. The bay was subdivided into three sectors, North, Central, and South Bay, with the Central Bay serving as a proxy for the oceanic end member. Separate budgets were constructed for the wet (October—March) and dry (April—October) seasons of each year for 6 yr (1990–1995). This period of record contained 2 yr of above normal runoff (1993, 1995) and 4 yr of below average runoff. Effluent from sewage treatment plants accounts for approximately 50% of the nutrient loading to the bay in winter and 80% of the summer loading. Both arms of the bay were apparently net heterotrophic during the winter, with this signal being strongest during the wet winters of 1993 and 1995. We conclude that overall the bay is slightly net autotrophic (production of new organic matter in the bay by plant growth exceeds respiratory demands); however, this varies seasonally (strongest in summer) and is complicated by the possibility of significant abiotic P adsorption in the North Bay.
Reconstructing the development of eutrophication in Louisiana salt marshes
Authors: Parsons Michael L. , Dortch Quay , Eugene Turner R. , Rabalais Nancy R.
Published: July 2010

We collected sediment cores from three salt marsh ponds in coastal Louisiana to test the usefulness of proxies of eutrophication. One‐centimeter increments of 210Pb‐ and 137Cs‐dated sediment were analyzed for diatoms, pigments (phaeophytin and chlorophyll a), biogenic silica, percentage organic matter, percentage carbon, and percentage nitrogen. Both sediment chlorophyll a and a diatom‐based trophic index (TI) were significantly and positively correlated with riverine or local nutrient indices. Two diatom species, Amphora copulata Giffen and Navicula yarrensis Grunow, were significantly and negatively correlated with riverine and local nutrient indices. These results suggest that these variables can be used as potential indicators of trophic status. Results from a complete‐linkage cluster analysis on the diatom assemblage data demonstrated that the sediment cores could be split into three time periods: early 1900s (pre‐1930s/1940s), mid‐1900s (1930s/40s to 1960s/1970s), and late 1900s (1960s/70s to 1990s). Examination of the sediment chlorophyll a and TI data over these time periods, coupled with an analysis of variance of nutrient inputs between the time periods, suggests that nutrient loading increased dramatically from the mid‐ 1960s to the mid‐1970s. This study demonstrates that (1) a retrospective analysis of sediment cores can be conducted in highly variable salt marsh ponds and (2) these salt marsh environments are already affected by the higher nutrient loads from both riverine and local processes occurring over the last 50 years. Additional nutrient loading, e.g., from river diversion projects for the lower Mississippi River, may exacerbate eutrophication already evident in the marsh environment.
Effects of eutrophication, grazing, and algal blooms on rocky shores
Authors: Worm Boris , Lotze Heike K.
Published: July 2010

Eutrophication can profoundly change rocky shore communities. These changes often cause the replacement of perennial, canopy‐forming algae such as Fucus spp. with annual, bloom‐forming algae such as Enteromorpha spp. Grazing, however, can counteract eutrophication by eliminating the annual algae’s susceptible recruits. We examine these generalizations across large scales. We use replicated “bioassay” experiments to compare the effects of eutrophication and grazing across four paired control versus eutrophied sites in the Northwest Atlantic and four eutrophied sites in the Baltic Sea in spring and summer. At each site, annual algal recruitment and grazing pressure were estimated using tiles seeded with Enteromorpha intestinalis propagules. Tiles were exposed for 3 weeks with grazers excluded or allowed access. Productivity of E. intestinalis recruits was strongly related to eutrophication (10‐fold increase) and grazing (80% decrease) and was weakly related to season. While the absolute grazing rate increased in a linear fashion with algal productivity, the relative grazing rate remained surprisingly constant (;80%). Comparative field surveys showed that perennial algae decreased by 30–60%, while annual algae, filter feeders, and grazers increased across a gradient of eutrophication. As eutrophication increased from control to eutrophied to point source sites, rocky shore communities became increasingly dominated by single species of annual algae or filter feeders, and community diversity declined consistently by 24–46%. We conclude that grazers are important controllers of algal blooms but that, ultimately, they cannot override the effects of increasing eutrophication on rocky shore community structure and biodiversity.
A new numerical technique for tracking chemical species in a multi‐source, coastal ecosystem, applied to nitrogen causing Ulva blooms in the Bay of Brest (France)
Authors: Ménesguen Alain , Cugier Philippe , Leblond Isabelle
Published: July 2010

A new numerical technique is presented that allows the tracking of any chemical element from any source in a simulated foodweb, for instance assessing the proportion of these sources (river loadings, sea entrances, point sources) in the algal diet for the limiting nutrient. An application is shown for nitrogen in an Ulva bloom occurring in a shallow embayment connected with a strongly tidally stirred ecosystem with various sources of inorganic nitrogen, the Bay of Brest, Brittany, France. In a first step, a biogeochemical three‐dimensional model was developed to simulate growth and erosion—transport—deposition of free‐floating ulvae; this model was able to converge on a realistic distribution of Ulva deposits after a few months, even though it was initialized with a strongly unrealistic distribution of settled ulvae. In a second step, and unfortunately for recovery plans, the tracking technique, applied in this model to all the nitrogen sources entering the bay, revealed that the small, nitrate‐polluted rivers flowing directly into the eutrophicated area had a negligible effect, whereas more distant but stronger sources, a big river and a big urban sewage plant, even after dilution, accounted for about 50% and 20%, respectively, of the algal nitrogen content during summer. Despite its high N flux, open ocean contributes only 15% to Ulva growth. The suppression of only one of the main nitrogen sources would not significantly decrease the Ulva bloom, because of the high nitrogen surplus present in the site. The remaining sources would still saturate the needs of the maximum Ulva biomass the site is able to produce. The tracking technique, however, shows that the N turnover in Ulva is only 4 months. Thus, improvements would occur within a year following large N reductions.
Eutrophication and trophic state in rivers and streams
Authors: Dodds Walter K.
Published: July 2010

Many natural streams are net heterotrophic, so I propose that trophic state be divided into autotrophic and heterotrophic state. This division allows consideration of the influence of external carbon sources as well as nutrients such as nitrogen and phosphorus. Empirical results suggest that phosphorus and nitrogen are the most important nutrients regulating autotrophic state in flowing waters and that benthic algal biomass is positively correlated to gross primary production in streams. Reference (minimally influenced by human activities) nutrient concentrations and correlations of nutrients with algal biomass are used to characterize reference distributions of stream autotrophic state. Only when reference nutrient concentrations are in the upper one third of those expected in the United States, is maximum benthic chlorophyll projected to exceed 100 mg m−2 (a concentration commonly used to indicate nuisance levels) >30% of the time. Average reference nutrient concentrations lead to sestonic chlorophyll concentrations above those considered typical of eutrophic lakes (>8 mg m−3) less than half the time. Preliminary analysis suggests that autotrophic state is variable in small pristine streams because it is influenced by canopy cover (light), but heterotrophic state is less variable because it can be based on allochthonous or autochthonous production. Nitrogen and phosphorus enrichment can influence both heterotrophic and autotrophic state, and these effects could cascade to animal communities. Stoichiometry should be considered because carbon, nitrogen, and phosphorus are all involved in trophic state. The proposed definition of trophic state offers a starting conceptual framework for such considerations.
Long‐term changes in the areal hypolimnetic oxygen deficit (AHOD) of Onondaga Lake: Evidence of sediment feedback
Authors: Matthews David A. , Effler Steven W.
Published: July 2010

Long‐term trends in the rate of depletion of hypolimnetic dissolved oxygen (DO) are documented for ionically enriched hypereutrophic Onondaga Lake, New York, for the 1978–2002 interval. Depletion rates, represented as areal hypolimnetic oxygen deficits (AHOD, g m−2 d−1), are calculated on the basis of weekly DO profiles of 1‐m resolution and estimates of coincident inputs of DO from overlying layers driven by vertical mixing. Vertical mixing inputs of DO are important in this system, representing from 15% to 37% (mean 25%) of AHOD. Interannual variations in hypolimnetic temperatures have comparatively minor (±6%) effects on AHOD. AHOD decreased 49%, from an average of 2.12 g m−2 d−1 for the 1978–1986 interval, to an average of 1.08 g m−2 d−1 for the 1997–2002 interval. This decrease was driven by an abrupt decrease in the deposition of particulate organic carbon into the hypolimnion starting in 1987. The magnitude of the decrease in AHOD closes reasonably well with decreases in both primary production in the trophogenic zone and organic carbon deposition to the tropholytic zone. The time course of the decrease in AHOD is consistent with localization of oxygen‐demanding processes within the lake sediments, reflecting the progression of sediment diagenesis.
Historic low‐level phosphorus enrichment in the Great Lakes inferred from biogenic silica accumulation in sediments
Authors: Schelske Claire L. , Stoermer Eugene F. , Kenney William F.
Published: July 2010

Sedimentary biogenic silica (BSi) accumulation was used in conjunction with a hypothetical model of BSi accumulation to show that BSi is a sensitive proxy for low‐level phosphorus enrichment in the Great Lakes. We hypothesize that historic nutrient‐driven changes in diatom production altered silica biogeochemistry and induced biologically mediated silica depletion (BMSD) and that a record of the underlying mechanism, enhanced diatom production and BSi sedimentation stimulated by anthropogenic phosphorus enrichment, is preserved in the sediment record. Paleolimnological results support three hypotheses based on this model. First, BSi accumulation increased in Lake Superior and Lake Huron at total phosphorus (TP) concentrations (4 and 5 µg TP L−1 or 0.13 and 0.16 µmol L−1, respectively) too small to induce BMSD and with changes in TP concentration too small to be detected by routine water‐column sampling. Second, a peak in BSi accumulation in Lake Michigan resulted from epilimnetic silica depletion that developed rapidly in the 1950s and 1960s when TP averaged 8 mg L−1 (0.26 µmol L−1). In addition, epilimnetic silica depletion in the late 1800s was inferred from BSi accumulation in Lake Erie and Lake Ontario when the TP concentration was <10 µg L−1 (0.32 µmol L−1). Third, a secondary peak in BSi accumulation in the 1950s and 1960s signaled water‐column silica depletion in Lake Ontario and the eastern basin of Lake Erie that developed as TP concentration increased to 27 µg L−1 (0.87 µmol L−1). Ratios of NAIP :TP, BSi :TP, and BSi : NAIP also provide sensitive proxies for phosphorus enrichment. BSi accumulation is a sensitive proxy for phosphorus enrichment because BSi production by diatoms integrates silica utilization over an annual cycle, silica is recycled slowly (on annual time scales) compared with phosphorus, and sedimented BSi is focused into depositional zones.
Changes in accumulation of organic matter and stable carbon and nitrogen isotopes in sediments of two Slovenian mountain lakes (Lake Ledvica and Lake Planina), induced by eutrophication changes
Authors: Vreca Polona , Muri Gregor
Published: July 2010

We measured accumulation rates of organic carbon (OC) and total nitrogen (TN) and stable carbon (δ13Corg) and nitrogen isotopes (δ15N) in the sediments of two mountain lakes (Lake Ledvica and Lake Planina, northwest Slovenia). Marked variations of these parameters were observed in both sediments. OC accumulation rates ranged from 4 to 23 g m−2 yr−1 in Lake Ledvica, whereas in Lake Planina they were one order of magnitude higher and increased substantially in the upper sediments. δ13Corg and δ15N decreased upward in Lake Ledvica, from ‐25.5‰ to ‐28.4‰, and from +2.1‰ to ‐3.4‰, respectively. In contrast, substantial variations in the δ13Corg profile, ranging from ‐30.9‰ to ‐37.4‰, but a quite uniform δ15N profile of approximately +1.8‰, was observed in Lake Planina. Elemental and isotopic changes of bulk sedimentary organic matter in the lakes were related to changes in the past trophic state of the lakes and their watersheds, inferred by natural development of the lake ecosystems, anthropogenic activities, as well as earthquakes and forest fires. Observed changes and differences in the δ13Corg and δ15N records could be attributed to global changes in isotopic composition of atmospheric CO2 and nitrate in atmospheric deposition, as well as to differences in organic matter, dissolved inorganic carbon, and dissolved inorganic nitrogen sources at various altitudes. Although Lakes Ledvica and Planina are remote mountain lakes, the sediment records showed severe eutrophication in Lake Planina, whereas in Lake Ledvica eutrophication is still moderate, thus reflecting a high sensitivity of these ecosystems to any external inputs.
Control by fiddler crabs (Uca vocans) and plant roots (Avicennia marina) on carbon, iron, and sulfur biogeochemistry in mangrove sediment
Authors: Kristensen Erik , Alongi Daniel M.
Published: July 2010

The influence of mangrove saplings (Avicennia marina) and fiddler crabs (Uca vocans) on carbon, iron, and sulfur biogeochemistry in mangrove sediment was studied using outdoor mesocosms with and without plants (21 m−2) and crabs (68 m−2). Saplings grew more leaves and pneumatophores in the presence of crabs. Dense microalgal mats lead to two to six times higher benthic production and about two times higher benthic respiration in the absence of crabs. Particle mixing by crabs increased the reactive oxidized iron (Fe(III)) in the upper 2 cm of the sediment, whereas oxygen leaching by roots maintained the deeper rhizosphere oxidized and enriched in Fe(III). The highest microbial activity, measured as carbon dioxide production and iron reduction, occurred within the upper 2 cm of ungrazed sediment and was fueled by the large near‐surface biomass of microalgae. Leaching of dissolved organic carbon (DOC) from roots stimulated bulk sulfate reduction and caused an upward cascading reduction of the sediment as indicated by low Fe(III) and high Fe(II) between 2‐cm and 6‐cm depth. The effect DOC was also evident as increased microbial abundance at all depths in the sediment. Fe(III) was the most important electron acceptor for microbial carbon oxidation in ungrazed sediment (63–70%), whereas sulfate reduction was more important in grazed sediment (36‐44%), particularly in the presence of plants. Aerobic respiration always accounted for <20%. Fiddler crabs and roots of A. marina have complementary effects on the biogeochemistry of mangrove sediment. Their association seems to be mutually beneficial with respect to growth and food availability.