bluuebunny/crossref_metadata_2025 huggingface dataset:
Summer water clarity responses to phosphorus, Daphnia grazing, and internal mixing in Lake Mendota
Authors: Lathrop Richard C. , Carpenter Stephen R. , Robertson Dale M.
Published: May 2010

Linear models were developed for predicting mean Secchi disk depth readings as a measure of water clarity for the summer months in Lake Mendota, Wisconsin. The 20‐yr (1976–1995) data set also included external phosphorus (P) loadings and in‐lake April P concentrations as indices of lake nutrient status, and monthly (28 d) water column stabilities and Daphnia biomasses as indices of lake mixing and algal grazing potentials, respectively. June Secchi depths were mostly controlled by food web dynamics, which dictated whether the larger‐bodied Daphnia pulicaria or the smaller‐bodied D. galeata mendotae dominated during the spring clear‐water phase. May Daphnia biomasses were significantly greater in D. pulicaria‐dominated years than in D. galeata‐dominated years; D. pulicaria‐year biomasses were also greater during the summer months. The model for the midsummer (July–August) months indicated that Secchi depths were inversely related to April P concentrations and positively related to midsummer Daphnia biomasses and lake stabilities. Scenarios for midsummer Secchi depths were tested using the observed minimum and maximum values for each predictor variate. While holding two variates constant, April P, Daphnia biomass, and lake stability each resulted in relatively similar Secchi ranges (0.81, 0.81, and 1.17 m, respectively). Our results suggest that summer water clarity in eutrophic Lake Mendota is controlled by interacting ecosystem processes linked to land use activities, lake food web dynamics, and climate.
Oxidation of ammonia and methane in an alkaline, saline lake
Authors: Joye Samantha B. , Connell Tracy L. , Miller Laurence G. , Oremland Ronald S. , Jellison Robert S.
Published: May 2010

The oxidation of ammonia (NH3) and methane (CH4) was investigated in an alkaline saline lake, Mono Lake, California (U.S.A.). Ammonia oxidation was examined in April and July 1995 by comparing dark 14CO2 fixation rates in the presence or absence of methyl fluoride (MeF), an inhibitor of NH3 oxidation. Ammonia oxidizer‐mediated dark 14CO2 fixation rates were similar in surface (5–7 m) and oxycline (11–15 m) waters, ranging between 70–340 and 89–186 nM d−1, respectively, or 1–7% of primary production by phytoplankton. Ammonia oxidation rates ranged between 580–2,830 nM d−1 in surface waters and 732–1,548 nM d−1 in oxycline waters. Methane oxidation was examined using a 14 CH4 tracer technique in July 1994, April 1995, and July 1995. Methane oxidation rates were consistently higher in July, and rates in oxycline and anaerobic bottom waters (0.5–37 and 7–48 nM d−1, respectively) were 10‐fold higher than those in aerobic surface waters (0.04–3.8 nM d−1). The majority of CH4 oxidation, in terms of integrated activity, occurred within anoxic bottom waters. Water column oxidation reduced the potential lake‐atmosphere CH4 flux by a factor of two to three. Measured oxidation rates and water column concentrations were used to estimate the biological turnover times of NH3 and CH4. The NH3 pool turns over rapidly, on time scales of 0.8 d in surface waters and 10 d within the oxycline, while CH4 is cycled on 103‐d time scales in surface waters and 10−2 time scales within oxycline and bottom waters. Our data suggest an important role for NH3 oxidation in alkaline, saline lakes since the process converts volatile NH3 to soluble NO2−, thereby reducing loss via lake‐atmosphere exchange and maintaining nitrogen in a form that is readily available to phyto‐plankton.
Sedimentation, bioturbation, and Hg uptake in the sediments of the estuary and Gulf of St. Lawrence
Authors: Smith J. N. , Schafer C. T.
Published: May 2010

Sedimentation rates and mixing depths have been estimated from the application of a two‐layer biodiffusion model to 210Pb profiles in sediment cores collected in the Laurentian Trough in the Estuary and Gulf of St. Lawrence. Sedimentation rates decrease exponentially with distance seaward from 0.70 cm yr−1 (0.47 g cm−2 yr−1) near the head of the Trough to 0.04 cm yr−1 (0.03 g cm−2 yr−1) in the Gulf of St. Lawrence. Mercury (Hg) sediment profiles exhibit subsurface maxima that decrease from concentrations of 0.6 ug g−1 near the entrance to the Saguenay Fjord to 0.04 µg g−1 in the central part of the Gulf. Hg release data for a chloralkali plant on the Saguenay River were used as the input function for the biodiffusion model and provided accurate simulation of measured profiles when mixing was permitted to extend into the deep (>10 cm) sediment layer. Estimates of Hg sediment inventories indicate that 100–130 tonnes (t) of Hg have been retained within the sediments of the St. Lawrence Estuary, which, combined with an Hg inventory of 30 t estimated for the sediments of the Saguenay Fjord, approximately balance the 136 t of Hg released from the chloralkali plant since 1947. 137Cs and 239,240Pu sediment‐depth profiles were also simulated by the biodiffusion model using input functions based on atmospheric fallout modified by radionuclide transport through the St. Lawrence River drainage basin.
Phosphorus distribution in three crustacean zooplankton species
Authors: Vrede Tobias , Andersen Tom , Hessen Dag O.
Published: May 2010

The distribution of phosphorus (P) was assessed in homogeneously 33P‐labeled Daphnia magna, Daphnia galeata and Eudiaptomus gracilis. The specific P contents were 1.48, 1.41, and 0.50% of dry weight (DW), respectively. The results support the view of low intraspecific variability in P : DW ratios in crustacean zooplankton. The fraction of P allocated to nucleic acids, phospholipids, and other P compounds was assessed in D. galeata and E. gracilis. In both species, the major pool, 35–69% of the total P content, was associated with nucleic acids. This fraction decreased with both body size (age) of D. galeata and E. gracilis and with the reproductive rate of D. galeata. D. magna and D. galeata revealed similar patterns of P allocation between body, carapace, and eggs. The carapace contained approximately 14% of the total P content. If most P is not reabsorbed from the exoskeleton prior to molting, the molting will result in a substantial P drain both from the individual and, at times, from the entire planktonic system.
Differential response of equatorial Pacific phytoplankton to iron fertilization
Authors: Cavender-Bares Kent K. , Mann Elizabeth L. , Chisholm Sallie W. , Ondrusek Michael E. , Bidigare Robert R.
Published: May 2010

Recent unenclosed iron‐fertilization experiments in the equatorial Pacific Ocean have shown that phytoplankton biomass can be increased substantially by the addition of iron. Analyses of size‐fractionated chlorophyll indicate that much of the increase during the most recent fertilization experiment, IronEx II, occurred in the >10‐um size fraction. We used flow cytometry, combined with taxon‐specific pigment measurements by high‐performance liquid chromatography (HPLC), to analyze the responses of five different groups of phytoplankton: Prochlorococcus, Synechococcus, ultraplankton, nanoplankton, and pennate diatoms. These results are unique in the suite of measurements from the IronEx studies in that they simultaneously examine individual cell properties, which are grazer independent, and population dynamics, which reflect the net result of growth and grazing. Our results show that the overall increase of chlorophyll a (Chl a) in the patch was due in part to increases in chlorophyll content per cell and in part to increases in cell numbers of specific groups. Cellular fluorescence was stimulated by iron addition in all five groups to a qualitatively similar degree and was correlated with taxon specific changes in cellular pigments. In terms of net cell growth, however, these groups responded very differently. The groups that dominated the community before the addition of iron increased at most twofold in cell number; Prochlorococcus actually decreased. In contrast, the initially rare pennate diatoms increased 15‐fold in number by the peak of the iron‐induced bloom. Within 1 week, this differential response led to a dramatic change in the phytoplankton community structure, from one dominated by picoplankton to one dominated by large diatoms. It is not known whether this shift would be sustained over extended periods of fertilization, a response that would ultimately change the structure of the food web.
The effect of variations in irradiance on buoyancy regulation in Microcystis aeruginosa
Authors: Wallace Brett B. , Hamilton David P.
Published: May 2010

A mechanism of buoyancy regulation in cyanobacteria was investigated in a fluctuating light environment typical of the mixed layer in a lake. New measurements of density and cellular carbohydrate content were obtained from a field sample of the cyanobacterium Microcystis aeruginosa. The rate of increase in carbohydrate and density in this cyanobacterium does not instantaneously adjust to an increase in light intensity. There is a time lag, the response time, corresponding to physiological adjustment to the increase in light, before equilibration of the rate of carbohydrate and density increase. Also, the rate of density and carbohydrate increase is nonlinear over the response time. Based on these experimental findings, a new model for buoyancy regulation was developed that includes the response time. This model was applied to two fluctuating light regimes that simulate Langmuir circulation and turbulence. The simulations reveal that the response time is critical in determining if a cell equilibrates its rate of density change to a change in the light that it receives. If the response time is longer than the time scale of the light fluctuation, then the rate of density increase will be less than the optimal equilibrium rate. Models that do not include the response time will not correctly predict the buoyancy of cyanobacteria and their water column position after an episode of mixing.
Site‐specific and ontogenetic variations in nutrition of mussels (Bathymodiolus sp.) from the Lucky Strike hydrothermal vent field, Mid‐ Atlantic Ridge
Authors: Trask Jennifer L. , Van Dover Cindy Lee
Published: May 2010

Lucky Strike mussels (Bathymodiolus sp.) support two metabolically distinct (methanotrophic and thiotrophic)
prokaryotic endosymbionts in their gills. Differences in source inorganic carbon isotope ratios and in carbon fixation
pathways between these two symbionts typically result in organic carbon with distinctive δ13C values. Site‐specific differences in isotopic compositions of host mussels may therefore reflect differences in sulfide and methane availability. Large differences in mean carbon and nitrogen isotopic compositions were observed in adult mussels collected
from two chemically distinct vents at Lucky Strike (Sintra: d13C 5 221.3‰, δ15N 5 –4.5‰; Eiffel Tower: δ13C = –30.7‰, δ15N = –10.5‰). These values are consistent with the hypothesis that Sintra mussels are more dependent on methanotrophy than are Eiffel Tower mussels. Relative abundances of the two types of endosymbionts in mussel gill tissues (Sintra: mean = 15% methanotrophs; Eiffel Tower: mean = 6% methanotrophs) provide further support of this hypothesis, suggesting that Lucky Strike mussels express a nutritional response to environmental variations. Within sites, there were small but significant correlations between isotopic composition and mussel size over the shell lengths of 15–80 mm, but these shifts are so small that they are attributable to factors other than ontogenetic shifts in nutritional strategy. In contrast, large nitrogen isotope differences were observed between larval and adult stages. Based on δ15N values, mussel larvae appear to rely very little on photosynthetically derived organic material. Observations of the demersal nature of mussel larvae and isotopic similarities between Sintra and Eiffel Tower larvae and Sintra adults suggest that the potential of stable isotopes as useful tracers of larval sources within the Lucky Strike vent field should be considered.
Comparative bioaccumulation studies of colloidally complexed and free‐ionic heavy metals in juvenile brown shrimp Penaeus aztecus (Crustacea: Decapoda: Penaeidae)
Authors: Carvalho Roberta A. , Benfield Mark C. , Santschi Peter H.
Published: May 2010

A large fraction of the trace metal pool in estuarine and coastal waters is complexed with colloidal‐sized macromolecular organic matter. Bioaccumulation and bioavailability studies of trace metals have generally utilized free‐ionic dissolved metal ions. Consequently, little is known about the uptake and depuration kinetics of the colloidally bound species. Chemical characteristics such as the relative hydrophobicity of these metal‐colloid complexes may alter their bioavailability by interfering with or enhancing transport across membrane lipid bilayers. We used radiotracers to compare the bioaccumulation and sites of accumulation of colloidally complexed and free‐ionic forms of Ag, Cd, Ba, Fe, Sn, Zn, Co, Hg, and Mn in juvenile brown shrimp Penaeus aztecus. In this paper, we present the results from 14‐d uptake and bioaccumulation studies in which shrimp could take up radiotracers only from the water. Our results indicated that the colloidally complexed forms were bioavailable to shrimp. Uptake kinetics appeared to be similar in the two treatments for most metals, although by the end of the uptake study, the whole‐body activities of Ba, Sn, and Zn were significantly higher in shrimp exposed to free‐ionic metals. While the hepatopancreas appeared to be the site of highest accumulation on a mass‐specific basis for both treatments, the total activity level of radiotracer differed between the two treatments. In the colloidally complexed treatment, the majority of total tracer activity was associated with the hepatopancreas while shrimp exposed to free‐ionic metals exhibited the highest proportion of total activity in the abdomen. The mechanisms underlying these differences remain to be tested.
Pore‐water convection induced by peeper emplacement in saline sediment
Authors: Webster Ian T. , Ford Phillip W. , Grigg Nicola J.
Published: May 2010

“Peepers” (dialysis samplers) rely on exchange across a dialysis membrane between fluid‐filled cells and the surrounding sediment in order to sample the pore water at various depths. In the current study, we show that under some realistic circumstances, where there is a large density difference between the pore water and the cell fluid in permeable sediments, the resulting convective motion in the pore water distorts concentration distributions near the peeper. Peepers withdrawn prematurely in these conditions will contain seriously misrepresentative samples. We present numerical model and laboratory experiment results that show that the equilibration dynamics are complex and that the time required for equilibration may be far in excess of the duration predicted by a simple molecular diffusion model.
Estimates of the nonconsumptive mortality of mesozooplankton by measurement of sedimentation losses
Authors: Gries Thomas , Güde Hans
Published: May 2010

Nonconsumptive mortality of mesozooplankton in Lake Constance was directly estimated by collecting dead zooplankton with sediment traps. Patterns of zooplankton sedimentation observed in the sediment trap reflected the population dynamics in the pelagic zone. The migration of Cyclops vicinus toward the sediment (where they spend their diapause) at the end of May resulted in a pronounced occurrence in the traps (migration rate >106 individuals (ind.) m−2 d−1). Other copepods and Daphnia hyalina had only sedimentation losses of 0.5 and 0.2% of the standing stock per day, respectively, demonstrating the minor role of nonconsumptive mortality for these species. In contrast, nonconsumptive mortality had a high significance on the population dynamics of nonmigrating epilimnetic Daphnia galeata. High sedimentation rates (up to 3 × 103 ind. m−2 d−1); in total, 38% of the total population decline from 16 × 104 to 1 × 104 ind. m−2 within 21 d) in June 1993 were attributed to an unidentified infection. From April to November, losses of D. galeata as a result of non‐consumptive mortality (average of 2.3% of standing stock per day) accounted on average for 23% of the estimated production (10% of the standing stock per day).
Increased recruitment of northeast Pacific barnacles during the 1997 El Niño
Authors: Connolly Sean R. , Roughgarden Joan
Published: May 2010

Recruitment of intertidal barnacles was markedly higher in May–September 1997, just after the apparent onset of El Niño conditions in the waters off California, than over the same period during the previous year. This increase was geographically broad, spanning five degrees of latitude, and was unusually large, relative to interannual differences in barnacle recruitment previously documented for this region. Increased onshore transport associated with El Niño events probably caused high recruitment in 1997. This explanation accords with the findings of previous, smaller scale studies in central and northern California and indicates that oceanographic transport anomalies associated with El Niño can strongly affect benthic communities in the northeast Pacific.
Dispersal of the fairy shrimp Branchinecta coloradensis (Anostraca): Effects of hydroperiod and salamanders
Authors: Bohonak Andrew J. , Whiteman Howard H.
Published: May 2010

The absence of fairy shrimp (Anostraca) from permanent ponds and lakes is hypothesized to be the result of vertebrate predation. However, hatching cues for anostracan diapausing eggs include factors associated with the filling of temporary pond basins, and desiccation often increases the fraction of eggs that hatch. Thus, it is possible that in some species, eggs dispersed to permanent habitats never hatch, and vertebrate predation is not the proximate factor limiting distributions. We experimentally transplanted live egg‐bearing females of the fairy shrimp Branchinecta coloradensis into permanent and temporary ponds in small chambers and allowed the chambers to overwinter in situ. There was no discernible effect of pond drying on hatching success (mean success = 50.9%). We also determined whether metamorphic salamanders (Ambystoma tigrinum nebulosum) could disperse viable fairy shrimp eggs by feeding on B. coloradensis in one pond and defecating in another. Hatching success for a salamander “ingested” treatment was estimated as 0.9%. Results of a third experimental treatment suggested that the eggs being carried by females were not fully mature, so that ingestion resistance might vary throughout the reproductive period of B. coloradensis. By combining these results with data on salamander movement and diet, we estimated salamander‐mediated dispersal rates for fairy shrimp eggs. Metamorphic A. tigrinum nebulosum sp. are likely to be dispersing thousands of B. coloradensis eggs among ponds annually. Because our results demonstrate that dispersal between ponds can occur in large numbers, they support the prevailing hypothesis that vertebrate predators limited to permanent ponds are the proximate mechanism preventing the invasion of fairy shrimp.
Adaptive strategies that reduce predation on Caribbean spiny lobster postlarvae during onshore transport
Authors: Acosta Charles A. , Butler Mark J.
Published: May 2010

Like many marine species with meroplanktonic larvae, the Caribbean spiny lobster (Panulirus argus) has a postlarval stage that moves from the oceanic plankton to inshore nurseries only under specific environmental conditions (i.e., at night, in the surface water layer, on the flood tide, and during new moon), presumably to avoid predation or to enhance onshore transport. Using field and mesocosm experiments, we compared predation on planktonic postlarvae swimming at night near the surface and bottom over coastal habitats along typical offshore–inshore transport paths and determined whether predation rates differed between lunar periods (new moon vs. full moon) and with prey density (i.e., predator encounter rates). We also measured predation on transparent (newly settled) and pigmented (nearing metamorphosis) postlarvae sheltering in coral reef, seagrass, and macroalgal habitats during the day. We measured predation on postlarvae swimming near the surface and bottom along typical offshore–inshore transport paths (i.e., coral reefs, coastal lagoon, and bay) by tethering postlarvae to floats that drifted on the nightly flood tide during new moon. To test the hypothesis that new‐moon transport of postlarvae may have evolved as a means to avoid higher predation under the bright full moon, we repeated the pelagic tethering experiments at the reef and in the bay during full moon. Mortality was highest over coral reefs regardless of lunar phase, but it was lower nearshore, especially in the bay near the surface and during new moon. Predation on benthic, recently settled transparent postlarvae and pigmented postlarvae (nearing metamorphosis) was also higher when tethered on the reef as opposed to vegetated habitats in the lagoon and bay. In experimental mesocosms, planktivorous fish were equally efficient at consuming postlarvae under new‐ and full‐moon conditions when postlarval density was high, as it is in the constricted water column over the reefs. However, when postlarvae were less dense, mortality was significantly lower during new moon. Collectively, these results indicate that several behavioral traits exhibited by postlarval spiny lobsters, including inshore migration during the darkest lunar phase, use of surface waters, and settlement in vegetated habitats, reduce their risk of predation, particularly in the shallow bay. No single behavioral strategy is universally advantageous across all coastal habitats, but combined, they are an effective means to reduce predation across heterogeneous environments that postlarvae must transit during recruitment.
Stable isotope analyses of the pelagic food web in Lake Baikal
Authors: Yoshii Koichi , Melnik Natalia G. , Timoshkin Oleg A. , Bondarenko Nina A. , Anoshko Pavel N. , Yoshioka Takahito , Wada Eitaro
Published: May 2010

Stable isotope ratios of various organisms were analyzed to elucidate food web structure in the pelagic zone of Lake Baikal. The pelagic food web of Lake Baikal is simple and consists of five major ecological groups: phytoplankton (Aulacoseira baicalensis), mesozooplankton (Epischura baicalensis), macrozooplankton amphipod (Macrohectopus branickii), fish (Coregonus autumnalis migratorius and four species of cottoid fishes), and seal (Phoca sibirica). Because of the low diversity and consequently small number of possible diets for each species, we were able to quantitatively estimate the diet composition of each animal with stable isotopes. Our carbon isotope data indicated that pelagic phytoplankton are the primary carbon source of the pelagic food web because δ13C levels of animals were close to those of pelagic phytoplankton. The δ15N levels of animals showed a clear trend of stepwise enrichment with trophic level according to the following equation: δ15N (‰) = 3.3 (Trophic Level – 1) – 3.8. In addition to interspecific food web analysis, important pelagic animals, such as M. branickii, two species of pelagic sculpin, C. autumnalis migratorius, and P. sibirica, were also examined, with emphasis on ontogenic diet changes. δ15N levels of M. branickii and sculpins increased with body length, suggesting a change in feeding habits during growth. We demonstrate that carbon and nitrogen stable isotopes can be successfully applied to elucidate trophic relationships and conclude that the pelagic food web of Lake Baikal has an ideal, isotopically ordered structure.
Recent climatic trends in nearshore water temperatures in the St. Lawrence Great Lakes
Authors: McCormick Michael J. , Fahnenstiel Gary L.
Published: May 2010

In the Great Lakes region, the observational evidence for climatic change has been primarily limited to changes in lake‐ice conditions, with no long‐term trends identified in water temperatures. Seven nearshore water intake sites (Bay City, Michigan; Green Bay, Wisconsin; Sault Ste. Marie, Michigan; St. Joseph, Michigan; Sandusky Bay, Ohio; Put‐In‐Bay, Ohio; and Erie, Pennsylvania) in the Great Lakes were chosen, and their data were examined for any climatic trends. Regression results on the annual mean temperatures showed varying support in favor of a warming trend at five of the seven sites. A new approach facilitated determination of the interannual variability in the timing of the 4°C temperature of maximum density. Two of the three sites with data records extending back to the early part of this century (Sault Ste. Marie and Put‐In‐Bay, respectively) showed a 4‐ and a 6‐h yr−1 rate of increase in the maximum potential duration of summer stratification (DSS). Over the time span of these two data sets, this equates to a 14‐ and 18‐d increase in the potential DSS, respectively. The rate of increase in the duration data was skewed, with most of the increase due to an earlier transition to springlike conditions. Finally, the data do not extend far enough back in time to know if these climatic trends are part of an unresolvable natural cycle or forced by anthropogenic activity.
The importance of siderophores in iron nutrition of heterotrophic marine bacteria
Authors: Granger Julie , Price Neil M.
Published: May 2010

Recent studies demonstrate that dissolved iron in seawater is bound to strong organic complexes that have stability constants comparable to those of microbial iron chelates. We examined iron acquisition by seven strains of heterotrophic marine bacteria from a number of siderophore–iron complexes, including desferrioxamine B (DFB) and marine siderophores partially purified from iron‐limited cultures. Hydroxamate siderophores were detected in the supernatants of four strains, one of which also produced a catechol. All strains transported iron bound to siderophores regardless of whether or not they produced their own, and the majority took up iron bound to DFB. Uptake rates of Fe siderophores were similar among iron‐limited strains and among ligands. Transport of FeDFB by strain Neptune was enhanced 20 times by iron limitation, whereas uptake of unchelated iron (Fe') did not saturate at the highest concentration tested and was not regulated by the iron nutritional status of the cells. The half‐saturation constant for uptake of FeDFB by Neptune was 15 nM, the lowest reported for an Fe siderophore in any microor ganism. Iron uptake by the catechol‐producing strain, LMG1, differed markedly in two respects from the other strains: LMG1 could not take up iron bound to DFB; furthermore, transport of Fe' by iron‐limited LMG1 was 10 times faster than the other strains and was upregulated 46 times compared to Fe‐sufficient cells. Experimental evidence suggests that iron transport by LMG1 may be mediated by surface‐associated catechol siderophores that scavenge inorganic ferric species as well as iron bound to weaker complexes, such as EDTA (ethylenediaminetetraacetic acid). The combined results of the study highlight the importance of siderophores in iron transport by heterotrophic marine bacteria and suggest, by inference, that bacteria may rely on siderophores to acquire iron in situ.
Temperature regulation of nitrate uptake: A novel hypothesis about nitrate uptake and reduction in cool‐water diatoms
Authors: Lomas Michael W. , Glibert Patricia M.
Published: May 2010

Marine diatoms generally form large blooms during periods of cool temperature (<20°C), high NO3− fluxes (>25 μM‐N), and turbulent mixing, but the adaptations that allow diatoms to bloom under these conditions are not well understood. We have conducted both NO3− uptake kinetics and direct short‐term temperature manipulation studies on field diatom‐dominated populations from Chesapeake and Delaware Bays during both spring and fall blooms. Absolute rates of NO3− uptake by a Rhizoseleni‐dominated population did not appear to saturate even at concentrations as high as 180 µM‐N. We observed contrasting patterns of NO2−, NH+, and urea utilization as a function of experimental temperature (ambient ± 9°C). Over the temperature range of 7–25°C, absolute uptake rates of NO3− (ρNO3−) decreased an average of 46% with increasing temperature from 7 to 25°C (nine individual experiments), while ρNH4+ and ρUREA increased with increasing temperature by an average of 179 and 86% (eight individual experiments), respectively. Based on these observations and the nature of the physical environment, we hypothesize that these diatom‐dominated populations were taking up NO3− in excess of nutritional requirements, the reduction of which may serve as a sink for electrons during transient periods of imbalance between light energy harvesting and utilization. We suggest that the increase in non‐nutritional NO3− uptake increases proportionately with the magnitude of the imbalance between light energy harvesting and imbalance. This hypothesis reconciles previous observations of low C:N uptake ratios, high release rates of dissolved organic nitrogen or NO2− by diatom‐dominated assemblages, other observations of nonsaturating NO3− kinetics in field populations, and the apparent "preference" for NO3− by the netplankton size fraction. The two phenomena described here, nonsaturable kinetics and a negative relationship between NO3− uptake and short‐term temperature shifts, have important ecological implications. The hypothesized ability of these diatom‐dominated populations to better modulate the flow of photosynthetic electron energy, via NO3− reduction, in variable environments may provide a competitive advantage to diatoms and could potentially explain why diatoms frequently dominate in regions of cool temperature, high NO3− flux, and turbulent mixing. Also, models of new production may need to incorporate terms for temperature dependence of NO3− uptake. Finally, if a significant fraction of NO3− uptake is regulated by non‐nutritional mechanisms in the cell, and if some fraction of nitrogen reduced by this mechanism is subsequently released in the form of NO2−, NH+, or dissolved organic nitrogen (DON), then estimates of new production based solely on NO3− uptake could be seriously biased.
Experimental test of the effect of ultraviolet‐B radiation in a planktonic community
Authors: Mostajir Behzad , Demers Serge , de Mora Stephen , Belzile Claude , Chanut Jean‐Pierre , Gosselin Michel , Roy Suzanne , Villegas Piedad Zulema , Fauchot Juliette , Bouchard Josée , Bird David , Monfort Patrick , Levasseur Maurice
Published: May 2010

Ultraviolet‐B (UVB, 280–320 nm) radiation is a natural component of sunlight that harms organisms and disturbs natural communities in surface waters. A natural planktonic assemblage of organisms (<240 μm) was studied in a mesocosm experiment for 7 d under varying conditions of UVB radiation: UVB excluded, natural radiation, and UVB enhanced at two different levels. The dynamics of several populations at different trophic levels comprising heterotrophic bacteria (<1 μm), heterotrophic flagellates (2–10 μm), small phytoplankton (<5 μm), large phytoplankton (5–20 μm), and ciliates (15–35 μm) were monitored during the experiment. Enhanced UVB provoked a significant decrease in the number of ciliates (66%) and large phytoplankton (63%) relative to natural UVB con‐ditions. The severe effects of UVB radiation on ciliates and large phytoplankton communities shown here would strongly limit upward transfer of mass and energy. The decline of predator abundance (ciliates) under UVB stress relative to natural conditions resulted in a positive feedback between enhanced UVB radiation and prey abundances, shown by increased abundances of bacteria (49%), heterotrophic flagellates (up to 300%), and small phytoplankton (41%). Similarly, with respect to carbon partitioning, the decrease in ciliate and diatom carbon biomass (64 and 56%, respectively) under enhanced UVB exposure was balanced by an increase in the carbon biomass of hetero‐trophic bacteria (48%), heterotrophic flagellates (126%), and autotrophic flagellates (162%). As a manifestation of enhanced UVB at the community level, the ecosystem develops toward a microbial food web in preference to an herbivorous food web. Thus, enhanced UVB radiation can change the structure and dynamics of the pelagic food web.
Bio‐optical properties of the marine diazotrophic cyanobacteria Trichodesmium spp. I. Absorption and photosynthetic action spectra
Authors: Subramaniam1 Ajiit , Carpenter Edward J. , Karentz Deneb , Falkowski Paul G.
Published: May 2010

The optical absorption, fluorescence excitation and emission, and photosynthetic action spectra were measured in vivo on intact colonies of Trichodesmium from the Caribbean Sea. The optical cross‐sections were dominated by ultraviolet‐A (UVA) absorption, which was a consequence of massive accumulations of mycosporinelike amino acids. The visible region of the spectrum was decomposed into several bands, among which chlorophyll a (Chl a), carotenoids, and individual phycobilipigments could be discerned. There was a clear diel periodicity in the ratio of the optical absorption cross‐sections of phycourobilin (PUB) to phycoerythrobilin (PEB), increasing from around 1.7 at night to 2.1 at midmorning. The diel cycle in PUB/PEB is consistent with a reversible interconversion of the two pigments. The ratio of PUB/PEB was inversely correlated with the transfer of excitation energy to photosystem II (PSII). Light absorbed by PUB was not transferred to PSII with a high efficiency, but rather, a significant fraction was reemitted at 565 nm as fluorescence. These observations suggest that the PUBs and PEBs in Trichodesmium act as a dynamic biophysical energy valve that modify the rate of excitation energy delivered to PSII in response to changes in ambient light regime. The low‐temperature (77 K) fluorescence emission spectra reveal an extremely weak 685‐nm emission signal in relation to that at 730 nm. Based on a simple model, these data suggest that the ratio of PSI/PSII reaction centers in Trichodesmium is about 24:1. Such an extraordinary bias against PSII may help minimize damage to nitrogenase from O2 production in PSII, but it also reduces the photosynthesis‐enhanced growth and makes Trichodesmium virtually undetectable by chlorophyll fluorescence. The unique bio‐optical properties of Trichodesmium can be used to develop algorithms to study its temporal and spatial distributions from remotely sensed information.
Photophysiology of the marine cyanobacterium Prochlorococcus: Ecotypic differences among cultured isolates
Authors: Moore Lisa R. , Chisholm Sallie W.
Published: May 2010

Cultured isolates of Prochlorococcus from the Mediterranean Sea (MED4) and Sargasso Sea (SS120) have been shown to have dramatically different pigment composition and growth rate responses when grown over a range of irradiances. Moreover, analyses of field populations in the North Atlantic have shown that distinct ecotypes can coexist in the same water column. These and other observations have led to the hypothesis that Prochlorococcus is comprised of genetically distinct ecotypes that collectively expand the range of light intensities over which the genus can thrive. In this paper, we explore this hypothesis by comparing the photophysiology of 10 different Prochlorococcus isolates from diverse oceanographic regimes. We found that the 10 isolates could be grouped into two loose clusters based on their growth response to varying light intensity and their chlorophyll b/a2 (Chl b/a2) ratios. Although both groups photoacclimate when grown over a range of light intensities, isolates with distinctly higher Chl b/a2 ratios (high B/A ecotype) reach maximal growth rates at lower irradiances (Ik,g), have high growth efficiencies (αg), and are inhibited in growth at irradiances where isolates with low Chl b/a2 ratios (low B/A ecotype) are growing maximally. High Chl b/a2 ratios resulted in higher spectrally weighted average Chl a2‐specific absorption coefficient (āchl*), Chl a2 specific light‐harvesting efficiency (αchla), and quantum yield (φm) under low growth irradiances for the isolates of the high B/A ecotype relative to the others. The distinction between the high and low B/A ecotypes is supported by molecular phylogenies constructed using the 16S ribosomal ribonucleic acid (rRNA) gene. The physiological differences between the ecotypes most likely result in different relative distributions in a given water column and in fluctuations in their relative abundances as a function of seasonal dynamics and water‐column stability.
Sulfate reduction in surface sediments of the southeast Atlantic continental margin between 15°38'S and 27°57'S (Angola and Namibia)
Authors: Ferdelman Timothy G. , Fossing Henrik , Neumann Kirsten , Schulz Horst D.
Published: May 2010

Sulfate reduction rates in the surface sediments from 17 stations from an along‐slope transect (1,300 m) and from a cross‐slope transect (855–4,766 m) were determined in the continental margin sediments of the Benguela Upwelling system. Profiles at all sites in the upwelling area showed increasing sulfate reduction rates from near zero at the surface to a peak at 2–5 cm (up to 29 nmol cm−3 d−1) and then decreasing exponentially with depth to near background rates at 10–20 cm depth (<2 nmol cm−3 d−1). Depth‐integrated sulfate reduction rates were greatest at 1,300 m and decreased exponentially with water depth. Along the transect following the 1,300‐m isobath, depth‐integrated sulfate reduction rates were highest in the north Cape Basin (1.16 +/− 0.23 mmol m−2 d−1), decreased over the Walvis Ridge (0.67 +/− 0.02 mmol m−2 d−1), and were lowest in the south Angola Basin (0.31 +/− 0.23 mmol m−2 d−1). Depth‐integrated sulfate reduction rates were consistent with the known pattern of coastal upwelling intensities and were also strongly correlated with surface organic carbon concentrations. Sulfate reduction rates, both as a function of depth and in comparison with sediment trap data, indicated that lateral downslope transport of organic carbon occurs. Sulfate reduction was estimated to account for 20–90% of the published rates of total oxygen consumption for the sediments at 1,300 m depth and 3–16% of sediments from 2,000 to 3,000 m depth. Comparison of the sulfate reduction rate profiles with the published diffusive oxygen uptake rates showed that the kinetics of oxygen utilization in the surface sediments are much faster than those for anaerobic organic carbon remineralization, although the underlying cause of the difference was not clear.
Acetogenesis from CO2 in an anoxic marine sediment
Authors: Hoehler Tori M. , Albert Daniel B. , Alperin Marc J. , Martens Christopher S.
Published: May 2010

A combination of radiotracer and pore‐water concentration measurements provide evidence for the occurrence of acetogenesis from CO2 in anoxic marine sediments that are ordinarily dominated by sulfate reduction and methanogenesis. In a month‐long incubation experiment using sediments from Cape Lookout Bight, North Carolina, we measured H2 and acetate concentrations and monitored the incorporation of 14CO2 into 14CH4 and 14C‐acetate. Depletion of pore‐water sulfate resulted in a period of elevated H2 concentrations that made acetogenic CO2 reduction thermodynamically favorable. During this period, 14C‐acetate was produced from 14CO2 at rates comparable to those of methanogenesis or sulfate reduction during their respective periods of dominance in the incubation. Maintenance of elevated but constant H2 concentrations immediately following sulfate depletion likely reflects control by acetogenic bacteria, suggesting they were the dominant consumers of H2 during this period.
Effects of nutrient loading on the carbon balance of coastal wetland sediments
Authors: Morris James T. , Bradley Paul M.
Published: May 2010

Results of a 12‐yr study in an oligotrophic South Carolina salt marsh demonstrate that soil respiration increased by 795 g C m−2 yr−1 and that carbon inventories decreased in sediments fertilized with nitrogen and phosphorus. Fertilized plots became net sources of carbon to the atmosphere, and sediment respiration continues in these plots at an accelerated pace. After 12 yr of treatment, soil macroorganic matter in the top 5 cm of sediment was 475 g C m−2 lower in fertilized plots than in controls, which is equivalent to a constant loss rate of 40 g C m−2 yr−1. It is not known whether soil carbon in fertilized plots has reached a new equilibrium or continues to decline. The increase in soil respiration in the fertilized plots was far greater than the loss of sediment organic matter, which indicates that the increase in soil respiration was largely due to an increase in primary production. Sediment respiration in laboratory incubations also demonstrated positive effects of nutrients. Thus, the results indicate that increased nutrient loading of oligotrophic wetlands can lead to an increased rate of sediment carbon turnover and a net loss of carbon from sediments.
Microwave digestion method for phosphorus determination of fish tissue
Authors: Tanner Danny K. , Leonard Edward N. , Brazner John C.
Published: May 2010

A simple time‐saving method is described for the digestion of fish tissue for spectrophotometric determination of phosphorus. Microwave digestion of fish tissue was compared to the commonly used method of ashing tissue in a muffle furnace. Microwave digestion produced significantly higher phosphorus recoveries and took only half the time to run the same number of samples compared to ashing.
Penetration of ultraviolet radiation in the waters of the estuary and Gulf of St. Lawrence
Authors: Kuhn Penny , Browman Howard , McArthur Bruce , St‐Pierre Jean‐Francois
Published: May 2010

Downwelling ultraviolet (UV) irradiance (290‒400) nm was measured at 1‐nm intervals in the surface waters of the estuary and Gulf of St. Lawrence, Canada, in September 1996 and June and September 1997. Measurements were made with an Optronic Laboratories OL 754‐O‐PMT spectropho‐tometer equipped with a fiber‐optic cable and an underwater sensor. Fluxes greater than the instrument's noise equivalent irradiance (NEI) level of 1 × 10−5 Wm−2 nm−1 just below the sea surface were measured at wavelengths of ≥294 nm. At 310 nm, the diffuse attenuation coefficient (Kd) ranged between 0.7 and 4.5 m−1, with corresponding 10% depth penetrations between 4 and 0.5 m. This represents close to half of the summer mixed‐layer water column in the Gulf of St. Lawrence. The following correlation coefficients were found: 0.81 between dissolved organic carbon (DOC) and Kd, 0.73 between chlorophyll a (Chl a) and Kd, and 0.73 between Chl a and DOC. The spectral variation of Kd suggests that yellow substance, and therefore DOC, is responsible for UV attenuation in these bodies of water.
Atmospheric inputs of dissolved organic nitrogen stimulate estuarine bacteria and phytoplankton
Authors: Seitzinger S. P. , Sanders R. W.
Published: May 2010

Atmospheric deposition is recognized as a potentially large source of inorganic nutrients to many ecosystems. In marine systems, where nitrogen (N) is the nutrient typically limiting phytoplankton growth, rainwater is often a significant source of N. Although a considerable portion of atmospheric N deposition is in the form of organic N, only the inorganic N in rainwater has been considered by most previous studies. Laboratory experiments presented here indicate that dissolved organic nitrogen (DON) from rainwater can stimulate productivity of coastal marine bacteria and phytoplankton. A large percent of rainwater DON was potentially biologically available; 45–75% was rapidly utilized by the microorganisms. The magnitude of the response of bacterial and phytoplankton biomass to equivalent amounts of DON‐N or ammonium‐N was similar. However, the community composition of phytoplankton at the end of the experiment differed in treatments receiving DON and inorganic N. Diatoms and dinoflagellates accounted for >90% of the phytoplankton biomass in treatments receiving rainwater DON. In contrast, small (≪2 μm) monads accounted for >85% of the biomass in the treatments receiving ammonium. The results indicate that DON in rainwater can be an important source of N to ecosystems.
Effects of agriculture, urbanization, and climate on water quality in the northernGreat Plains
Authors: Hall Roland I. , Leavitt Peter R. , Quinlan Roberto , Dixit Aruna S. , Smol John P.
Published: May 2010

The Qu'Appelle Valley drainage system provides water to a third of the population of the Canadian Great Plains, yet is plagued by poor water quality, excess plant growth, and periodic fish kills. Fossil algae (diatoms, pigments) and invertebrates (chironomids) in Pasqua Lake were analyzed by variance partitioning analysis (VPA) to determine the relative importance of climate, resource use, and urbanization as controls of aquatic community composition 1920‐1994. From fossil analyses, we identified three distinct biological assemblages in Pasqua Lake. Prior to agriculture (ca. 1776‐1890), the lake was naturally eutrophic with abundant cyanobacterial carotenoids (myxo‐xanthophyll, aphanizophyll), eutrophic diatoms (Stephanodiscus niagarae, Aulacoseira granulata, Fragilaria capucina/bidens), and anoxia‐tolerant chironomids (Chironomus). Principal components (PCA) and dissimilarity analyses demonstrated that diatom and chironomid communities did not vary significantly (P > 0.05) before European settlement. Communities changed rapidly during early land settlement (ca. 1890‐1930) before forming a distinct assemblage ca. 1930–1960 characterized by elevated algal biomass (inferred as β‐carotene), nuisance cyanobacteria, eutrophic Stephanodiscus hantzschii, and low abundance of deep‐water zoobenthos. Recent fossil assemblages (1977–1994) were variable and indicated water quality had not improved despite 3‐fold reduction in phosphorus from sewage. Comparison of fossil community change and continuous annual records of 83 environmental variables (1890–1994) using VPA captured 71–97% of variance in fossil composition using only 10‐14 significant factors. Resource use (cropland area, livestock biomass) and urbanization (nitrogen in sewage) were stronger determinants of algal and chironomid community change than were climatic factors (temperature, evaporation, river discharge). Landscape analysis of inferred changes in past algal abundance (as β‐carotene; ca. 1780‐1994) indicated that urban impacts declined with distance from point sources and suggested that management strategies will vary with lake position within the catchment.