Showing papers in "Limnology and Oceanography in 1996"
TL;DR: In this paper, the authors investigated the bacterial utilization of high-molecular-weight (HMW; > 1 kDa) and low molecular weight (LMW; < 1 kda) dissolved organic C (DOC) in freshwater and marine systems by measuring dissolved oxygen consumption, bacterial abundance, and bacterial production in size-fractionated samples.
Abstract: Bacterial utilization of high-molecular-weight (HMW; > 1 kDa) and low-molecular-weight (LMW; < 1 kDa) dissolved organic C (DOC) was investigated in freshwater and marine systems by measuring dissolved oxygen consumption, bacterial abundance, and bacterial production in size-fractionated samples. Tangentialflow ultrafiltration was used to separate HMW and LMW DOC. More than 80% of the DOC in Amazon River samples was recovered in the HMW fraction, whereas most marine DOC (up to 70%) was of LMW. Bacterial growth efficiencies were consistently higher in the LMW fractions (16-66%) than in the HMW fractions (8-39%), indicating compositional differences in the two size fractions. In all experiments, measured rates of bacterial growth and respiration in HMW incubations were higher than those in LMW incubations. Carbon-normalized bacterial DOC utilization rates were ‘1.4-4-fold greater in the HMW fractions than in the LMW fractions, and a greater proportion (0.7-22.5%) of HMW DOC was utilized per day than LMW DOC (0.5-6.6%). All bacterial growth and respiration measurements indicated that HMW DOC was utilized to a greater extent than LMW DOC in all environments investigated. The traditional model of DOM degradation, stating that LMW compounds are most bioreactive, does not appear to apply to the bulk of natural DOM. Rather, the data and results from independent studies suggest a new conceptual model whereby the bioreactivity of organic matter decreases along a continuum of size (from large to small) and diagenetic state (from fresh to old). This size-reactivity continuum model suggests that the bulk of HMW DOM is more bioreactive and less diagenetically altered than the bulk of LMW DOM.
1,025 citations
TL;DR: Ex- trapolation of these results to the global ocean identifies marine angiosperms, which only contribute 4% of total ocean NPP, as major contributors of the NPP stored and subsequently buried in marine sediments, to lead to estimates of total burial of marine NPP that exceed current estimates by 15-50%.
Abstract: The fate of photosynthetic carbon in marine ecosystems dominated by different types of primary producers was examined by compiling published reports on herbivory, autotrophic respiration, decomposition, carbon storage, and export rates as fractions of net primary production (NPP) in ecosystems dominated by different types of autotrophs (i.e. oceanic and coastal phytoplankton, microphytobenthos, reef algae, macroalgae, seagrasses, marsh plants, and mangroves). A large fraction (>40%) of the NPP of marine ecosystems is decomposed within the system, except for microphytobenthos (decomposition, -25% of NPP). Herbivory tends to be highest for microalgae (planktonic and benthic, >40% of NPP) and macroalgae (33.6 ?4.9% of NPP) and is somewhat less for higher plants. Microphytobenthos export on average a much higher proportion of their NPP than do other microalgal communities, whereas marine macrophytes, except marsh plants, export a substantial proportion (24.3-43.5% on average) of their NPP. The fraction of NPP stored in sediments is 4-fold greater for higher plants (- 10-1 7% of NPP) than for algae (0.4-6% of NPP). On average, -90% of the phytoplankton NPP is used to support local heterotrophic metabolism (i.e. grazed or decomposed). This fraction is even higher in oceanic communities. Mangrove forests, and to a lesser extent seagrass meadows and macroalgal beds, produce organic carbon well in excess of the ecosystem requirements, with excess photosynthetic carbon (i.e. export rate plus storage) in these ecosystems representing -40% of NPP. Ex- trapolation of these results to the global ocean identifies marine angiosperms, which only contribute 4% of total ocean NPP, as major contributors of the NPP stored (30% of total ocean carbon storage) and subsequently buried in marine sediments. Consideration of burial of NPP from marine angiosperms should lead to estimates of total burial of marine NPP that exceed current estimates by 15-50%.
858 citations
TL;DR: A role for oxidative stress in the inhibition of photosynthesis by UV radiation and elevated temperatures is confirmed and the addition of exogenous scavengers of active oxygen improves photosynthetic performance, but not to pre-exposure rates.
Abstract: Elevated temperatures and solar ultraviolet (UV) radiation have been implicated as causes for the loss of symbiotic algae in corals and other invertebrates with photoautotrophic symbionts (i.e. bleaching). Significantly higher cellular concentrations of superoxide radicals and hydrogen peroxide are observed when cultures of Symbiodinium bermudense are exposed to elevated temperatures with and without exposure to UV radiation. This increase in oxidative stress is accompanied by a reduction in the quantum yield of fluorescence for photosystem 2 and protein-specific activities of the carboxylating enzyme, Rubisco. An increase in antioxidant enzyme activities is unable to protect these cells from oxidative stress during exposure to UV radiation and elevated temperatures (3 1OC). The addition of exogenous scavengers of active oxygen, however, improves photosynthetic performance, but not to pre-exposure rates in zooxanthellae exposed to both elevated temperature and UV radiation, confirming a role for oxidative stress in the inhibition of photosynthesis by UV radiation and elevated temperatures. After exposure of zooxanthellae to UV radiation and elevated temperatures, an action spectrum for the inhibition of photosynthesis shows significantly greater wavelengthdependent effects of UV radiation between 290 and 375 nm than for zooxanthellae exposed to UV radiation alone.
581 citations
TL;DR: A period of prolonged warmer, drier-than-normal weather in northwestern Ontario during the 1970s and 1980s resulted in severe forest fires that caused dramatic changes to lake and stream catchments as mentioned in this paper.
Abstract: A period of prolonged warmer, drier-than-normal weather in northwestern Ontario during the 1970s and 1980s resulted in severe forest fires that caused dramatic changes to lake and stream catchments. The changed interactions of weather with catchments and hydrological processes caused unexpected changes in physical, chemical, and biological processes in lakes and streams. Permanent first-order streams became ephemeral. Flows at spring melt were lower, and chemical exports from catchments were reduced. Although catchments burned by forest fire had slightly higher flows and chemical exports than unburned basins in the years following fires, chemical exports generally declined due to lower streamflow. Decreased exports of silica indicated lower rates of weathering. Base cation exports also decreased, as did the ratio of base cations to strong acid anions in streams. Changes in lakes included warmer temperatures, clearer waters, deeper thermoclines and euphotic zones, higher alkalinities, and higher concentrations of base cations and nitrogen, but lower concentrations of dissolved organic C, silica, and P. The increase in alkalinity was caused by increases in the ratio of base cations to strong acid anions, resulting from the interaction of increased water retention, microbial sulfate reduction, and exchanges of cations between water and sediments. Declines in chlorophyll and increases in phytoplankton biomass were observed, but there was no detectable effect on areal phytoplankton production. Summer subthermocline habitats for cold stenotherms were reduced slightly in extent as the results of thermocline deepening and lower hypolimnetic oxygen. There is considerable potential for interaction between climatic change and other human perturbations affecting boreal lakes, including acidification, increased incident UV radiation, eutrophication, and overharvesting.
528 citations
TL;DR: In this paper, the effects of climate warming on the thermal habitat of 57 species of fish of the U.S. were estimated using results for a doubling of atmospheric carbon dioxide that were predicted by the Canadian Climate Center general circulation model.
Abstract: The effects of climate warming on the thermal habitat of 57 species of fish of the U.S. were estimated using results for a doubling of atmospheric carbon dioxide that were predicted by the Canadian Climate Center general circulation model. Baseline water temperature conditions were calculated from data collected at 1,700 U.S. Geological Survey stream monitoring stations across the U.S. Water temperatures after predicted climate change were obtained by multiplying air temperature changes by 0.9, a factor based on several field studies, and adding them to baseline water temperatures at stations in corresponding grid cells. Results indicated that habitat for cold and cool water fish would be reduced by -5O%, and that this effect would be distributed throughout the existing range of these species. Habitat losses were greater among species with smaller initial distributions and in geographic regions with the greatest warming (e.g. the central Midwest). Results for warm water fish habitat were less certain because of the poor state of knowledge regarding their high and low temperature tolerances; however, the habitat of many species of this thermal guild likely will also be substantially reduced by climate warming, whereas the habitat of other species will be increased. The threat of global climate change has stimulated the publication of numerous research papers dealing with the potential effects of surface and groundwater warming on the thermal habitat of freshwater fish (e.g. Coutant 1990; Magnuson et al. 1990; Meisner 1990; Stefan et al. 1995). With few exceptions (e.g. Shuter and Post 1990), most of these studies deal with responses to temperature in single water bodies or small geographic regions, so we decided to attempt an assessment of effects at a much larger scale consisting of the lower 48 states. Most of the studies cited above also dealt with effects of climate change on lake fish or habitat. We expected that using less detailed input data would keep the analysis manageable, but that we would use information compiled for or generated from previous smaller scale studies. These existing data include a quality-assured database of historical surface water temperature maxima at U.S. Geological Survey (USGS) stream monitoring stations and maximum temperature tolerances estimated from a field database for several dozen freshwater species of fish, as reported by Eaton et al. (1995); results of analyses of the relationship between air temperature and stream-water temperatures (Stefan and Preud’hommc 1993; Sinokrot and Stefan 1993); and a
524 citations
TL;DR: In noneutrophic shield lakes, transparency is controlled by the concentration of dissolved organic C (DOC).
Abstract: The depth of the summer mixed layer (E d ) of 21 Canadian Shield lakes in northwestern Ontario was examined in relation to lake size (surface area, A 0 ) and water clarity (extinction coefficient, K d ) for periods ranging from 2 to 23 yr (n = 1,408). The lakes range in A 0 from 4 to 4.9 x 10 5 ha, and midsummer (mid-June through mid-August mean) K d varied from 2.5 m -1 with greatest variation in small lakes that were experimentally eutrophicated or acidified or whose terrestrial basins were burned. Over the full spectrum of lake sizes, A 0 was the primary determinant of E d ; transparency significantly modified this relationship but only in small lakes (A 0 <500 ha). In noneutrophic shield lakes, transparency is controlled by the concentration of dissolved organic C (DOC). Lower DOC concentrations (a likely consequence of 2 x CO 2 climate change) would cause transparency to increase, resulting in 1-2-m-deeper epilimnia in small lakes ; there would be no similar effect in large lakes.
466 citations
TL;DR: In this paper, the authors demonstrate the fast transfer of suspended particles from the boundary layer into the upper strata (z 2 x 10-l l m2) incubated in a laboratory flume.
Abstract: We demonstrate the fast transfer of suspended particles from the boundary layer into the upper strata (z 2 x 10-l l m2) incubated in a laboratory flume. Increased pressure up- and downstream of small mounds (z = 2.5 cm) drove water 5.5 cm into the core, carrying suspended particles (1 pm) to 2.2-cm sediment depth within 10 h. Simultaneously, decreased pressure at the downstream slope of the protrusions drew pore fluid from deeper layers (z 5 10 cm) to the surface. In the sediment, friction reduced the velocity of the particulate tracers, resulting in size fractionation and layers of increased particle concentration. Ripple topography (0.8-2.8 cm high) enhanced interfacial particle (1 pm) flux by a factor 2 .3 when compared to a level control core. The pathways of the particle and solute tracers below a sediment ripple are explained with a source-sink model that describes the pore flow velocity field. Our results suggest that bedform-induced interfacial flows are important for the uptake of particulate organic matter into permeable shelf sediments.
465 citations
TL;DR: In this paper, the authors address the ecological ramifications of variation in hydrologic interaction between streams and alluvial aquifers in catchments with alluvium derived from parent materials of contrasting geologic composition.
Abstract: We address the ecological ramifications of variation in hydrologic interaction between streams and alluvial aquifers in catchments with alluvium derived from parent materials of contrasting geologic composition. We present a conceptual model in which solute retention in streams results from hydrologic retention (increased water residence time resulting from surface-groundwater exchange), biological nutrient cycling, and chemical processes. Solute injection experiments were done in study catchments comprised of sandstonesiltstone (site l), volcanic tuff (site 2), and granite-gneiss (site 3). Distribution of an injected conservative tracer (Br) illustrated that rate and extent of surface-water penetration into the alluvial aquifer increased across study catchments as was predicted from increasing alluvial hydraulic conductivity. Concurrently, groundwater inputs at baseflow represented between 13 and 57% of aboveground discharge at upstream transects, indicating bidirectional hydrologic exchange along the study reaches. N : P ratios in surface water ranged from 4 to 16, suggesting strong biotic demand for inorganic N. Coinjection of NaBr and NaNO, revealed longest nitrate uptake length (S,) at site 1, intermediate S, at site 2, and shortest uptake length at site 3. Modeling of transient hydrologic solute storage revealed that S, correlated with hydraulic storage, suggesting an important role for subsurface processes in total nitrate retention.
455 citations
TL;DR: The organic matter contents of sediments deposited in Lake Erie and Ontario contribute to the record of changes in the lake watersheds and aquatic ecosystems which have resulted from European settlement and cultural eutrophication of these systems over the past two centuries as discussed by the authors.
Abstract: The organic matter contents of sediments deposited in Lakes Erie and Ontario contribute to the record of changes in the lake watersheds and aquatic ecosystems which have resulted from European settlement and cultural eutrophication of these systems over the past two centuries. Compositions of n-alkanes and n-alkanoic acids extracted from lake sediments track the clearing of the original natural forests and the appearance of agriculture in the watershed areas beginning about 1820. Aquatic productivity increased as runoff of soil nutrients increased. Cultural eutrophication in the 1950s is recorded in increases in organic C and in n-C 17 alkane concentrations. Diagenetic effects overprint source changes as shorter chain length and unsaturated lipid components are preferentially removed from the sedimentary record.
423 citations
TL;DR: The interaction between environmental variation and generation overlap (produced by prolonged diapause) results in the maintenance of biotic diversity (both species richness and genetic variation), which forms the foundation for response to future environmental change.
Abstract: The long-lived diapausing eggs of zooplankton constitute an ecological and evolutionary reservoir that can impact the rate and direction of population, community, and ecosystem response to environmental change. Viable diapausing eggs are often extremely abundant and can survive in aquatic sediments for decades or longer. As the mean environment changes, the frequency of extreme conditions will likely increase, and species with prolonged diapause will be able to survive extreme years of no recruitment, whereas species lacking an egg bank will not. An altered environment may change which species have poor recruitment and even, through effects on thermal diapause cues, which species produce an egg bank. The interaction between environmental variation and generation overlap (produced by prolonged diapause) results in the maintenance of biotic diversity (both species richness and genetic variation), which forms the foundation for response to future environmental change.
390 citations
TL;DR: The results indicate that Fe(III) oxide reduction could mediate a considerable amount of organic carbon oxidation and significantly suppress CH, production in freshwater wetlands situated within globally extensive iron-rich tropical and subtropical soil regimes as discussed by the authors.
Abstract: High concentrations (20-75 pmol cm-3) of amorphous Fe(III) oxide were observed in unvegetated surface and Juncus eflusus rhizosphere sediments of a freshwater wetland in the southeastern United States. Incubation experiments demonstrated that microbial Fe(III) oxide reduction suppressed sulfate reduction and methanogenesis in surface scdimcnts and mediated 240% of depth-integrated (O-10 cm) unvegetated sediment carbon metabolism, compared to I 10% for sulfate reduction. In situ CO2 and CH, flux measurements verified that nonmethanogenic pathways accounted for - 50% of unvegetated sediment carbon metabolism. Lower (- 1 O-fold) rates of dark/anaerobic CH, flux from experimental vegetated cores relative to unvegetated controls suggested that methanogenesis was inhibited in the Juncus rhizosphere, in which active Fe(III) oxide reduction was indicated by the presence of low but readily detectable levels of dissolved and solid-phase Fe(II). Fe(III) oxide reduction accounted for 65% of total carbon metabolism in rhizosphere sediment incubations, compared to 22% for methanogenesis. In contrast, methanogenesis dominated carbon metabolism (72% of total) in experimental unvegetatcd sediment cores. The high Fe(III) oxide concentrations and reduction rates observed in unvegetated surface and Juncus rhizosphere sediments were perpetuated by rapid Fe(III) regeneration via oxidation of Fe(II) compounds coupled to 0, input from the overlying water and plant roots, respectively. The results indicate that Fe(III) oxide reduction could mediate a considerable amount of organic carbon oxidation and significantly suppress CH, production in freshwater wetlands situated within globally extensive iron-rich tropical and subtropical soil regimes.
TL;DR: In this article, photo-oxidation of dissolved organic carbon (DOC) to inorganic carbon (DIC) in five oligotrophic south Swedish lakes of different humic content (3.9-19 mg DOC liter-r, O-140 mg Pt liter-*, Secchi depth 7.61.
Abstract: Photo-oxidation of dissolved organic carbon (DOC) to inorganic carbon (DIC) in five oligotrophic south Swedish lakes of different humic content (3.9-19 mg DOC liter-r, O-140 mg Pt liter-*, Secchi depth 7.61.5 m) was investigated. Sterile-filtered (0.2 pm) water was incubated in UV-transparent quartz tubes and in tubes covered with aluminum foil. Samples were incubated at different depths (0,0.20,0.65, and 2.00 m) from sunrise to sunset (0400 to 2200 hours) in July. Inorganic C was measured before and after incubation. Total plankton respiration in unfiltered lake-water (production of DIC) was also measured. At the surface, photo-oxidative DIC production was 86410 mg C m-3 d-l, while plankton community respiration was 10 l-274 mg C m-3 d-l. Photo-oxidation integrated over depth was 44-l 7 1 mg C m-2 d-r, while respiration was 201-547 mg C m-2 d-r to a depth of 2 m and 398-860 mg C m-2 d-* over the depth of the epilimnion. Depth-integrated photo-oxidation was independent of water color or DOC concentration in the lakes. Photooxidation was detected deeper than the penetration of UV-B radiation, indicating that longer wavelengths (UV-A and possibly PAR) are also active. DIC production was linearly related to loss of fluorescence (excitation 355 nm, emission 455 nm) in light-incubated samples. Our study shows that photo-oxidation of DOC may be an important process causing the regularly observed supersaturation of DIC in lakes.
TL;DR: The precision of cytometric counts was 10-fold higher than corresponding epifluorescence bacterial counts, and the sample processing time is decreased by 10-15-fold relative to the epIFluorescence technique.
Abstract: We have developed a protocol for the cytometric enumeration of bacterioplankton in lakes with the nucleic acid stain SYTO 13; our protocol can be used with any standard bench-top flow cytometer equipped with an argon laser with excitation at 488 nm. The protocol performs well in the range of bacterial densities from 10 5 to 10 6 ml -1 . Estimates of bacterial abundance using the cytometric technique ranged from 72 to 141% of the epifluorescence estimates with DAPI staining. The average C.V. of replicate cytometric counts was 2.4%, and the average C.V. for cytometric counts in replicate water samples was 6.7%. The precision of cytometric counts was 10-fold higher than corresponding epifluorescence bacterial counts, and the sample processing time is decreased by 10-15-fold relative to the epifluorescence technique. This cytometric protocol can be applied to samples from a wide variety of lakes and offers an accurate, precise, and fast alternative to standard epifluorescence bacterial counts.
TL;DR: In this article, the results suggest that natural organic Fe(III) chelators exist in significant concentrations and control the dissolved iron concentration in seawater, and that only the Fe(OH),+ species is significant.
Abstract: Iron solubilities of Fe(III) hydroxide in coastal and oceanic waters and in the ultraviolet (UV)-irradiated seawaters over a pH range of 5.7-8.2 at 20°C were determined by a sim:Dle filtration (0.025 pm) involving y-activity measurement of 59Fe. At pH ranges of 5.7-7.2 (coastal water) and of 5.7-7.6 (oceanic water), only the Fe(OH),+ species is significant. The calculated solubility products, log*&,, for coastal and oceanic waters were 4.8-5.0 and 4.4-4.6, respectively. The solubilities within the pH range of 7.8-8.2 are relatively independent of pH and aging time. Solubility in the oceanic water was about one order of magnitude lower than that in the coastal water, and UV irradiation reduced solubility to 40.1 r-I solubility minima (0.15-0.2 nM) occur at a depth of 50-200 m. These results suggest that natural organic Fe(III) chelators exist in significant concentrations and control the dissolved iron concentration in seawaters.
TL;DR: The results point to a general importance of Fe reduction in C oxidation in continental margin sediments as this pool decreased with depth, and the importance of SO4(2-) reduction increased.
Abstract: Rates and oxidative pathways of organic carbon mineralization were determined in sediments at six stations on the shelf and slope off Conception Bay at 36.5”s. The depth distribution of C oxidation rates was determined to 10 cm from accumulation of dissolved inorganic C in l-5-d incubations. Pathways of C oxidation were inferred from the depth distributions of the potential oxidants (0,, N03-, and oxides of Mn and Fe) and from directly determined rates of SOd2- reduction. The study area is characterized by intense seasonal upwelling, and during sampling in late summer the bottom water over the shelf was rich in NO,- and depleted of OZ. Sediments at the four shelf stations were covered by mats of filamentous bacteria of the genera Thioploca and Beggiatoa. Carbon oxidation rates at these sites were extremely high near the sediment surface (> 3 pmol cmm3 d-l) and decreased exponentially with depth. The process was entirely coupled to SOd2reduction. At the two slope stations where bottom-water O2 was > 100 PM, C oxidation rates were lo-fold lower and varied less with depth; C oxidation coupled to the reduction of 02, N03-, and Mn oxides combined to yield an estimated 15% of the total C oxidation between 0 and 10 cm. Carbon oxidation through Fe . reduction contributed a further 12-29% of the depth-integrated rate, while the remainder of C oxidation was through SOd2- reduction. The depth distribution of Fe reduction agreed well with the distribution of poorly crystalline Fe oxides, and as this pool decreased with depth, the importance of SOd2- reduction increased. The results point to a general importance of Fe reduction in C oxidation in continental margin sediments. At the shelf stations, Fe reduction was mainly coupled to oxidation of reduced S. These sediments were generally H,S-free despite high SOd2- reduction rates, and precipitation of Fe sulfides dominated H,S scavenging during the incubations. A large NO,- pool was associated with the Thioploca, and the shelf sediments were thus enriched in N03- relative to the bottom water, with maximum concentrations of 3 pmol cm-3. The NO,- was consumed during our sediment incubations, but no effects on either C or S cycles could be discerned.
TL;DR: In this paper, the authors apply an empirical model to available DOC (dissolved organic C) data to estimate the depths to which 1% of surface UV-B and UV-A radiation penetrate for several major regions of North America.
Abstract: Climate warming in North America is likely to be accompanied by changes in other environmental stresses such as UV-B radiation. We apply an empirical model to available DOC (dissolved organic C) data to estimate the depths to which 1% of surface UV-B and UV-A radiation penetrate for several major regions of North America. UV attenuation depths are also estimated from DOC data collected from treatment and reference basins during the experimental acidification of Little Rock Lake, Wisconsin. In some regions of North America 25% of the lakes have 1% attenuation depths for UV-B radiation on the order of 4 m or more (western and northwestern U.S., Newfoundland). In other regions, 75% of the lakes have 1% attenuation depths for UV-B shallower than 0.5 m (Florida, upper midwestern U.S., northwestern Ontario, Quebec, and Nova Scotia). Attenuation depths for UV-A radiation are -2.5 times as deep as those for UV-B. Experimental acidification approximately doubled the estimated 1% attenuation depths for UV radiation in Little Rock Lake. The strong dependence of 1% attenuation depth on DOC below the l-2 mg liter-l DOC range suggests that UV attenuation in low DOC lakes is highly sensitive to even very small changes in DOC. We conclude that changes in climate, lake hydrology, acid deposition, and other environmental factors that alter DOC concentrations in lakes may be more important than stratospheric ozone depletion in controlling future UV environments in lakes.
TL;DR: It is suggested that cyanobacteria modify Cu chemistry in seawater, creating conditions more favorable for growth, as well as a tight linear correlation between chelator and Cu concentration.
Abstract: Copper speciation in the upper marine water column is dominated b>, strong ligands thought to be of recent biological origin. Cultures of the marine cyanobacteria Synechococcu.? spp., a ubiquitous and important group of phytoplankton highly sensitive to Cu toxicity, were previously shown to produce chelators comparable in strength to those detected in the water column. Here we shoyw that cultures of Synechococcus exposed to toxic concentrations of Cu produce an extracellular ligand with a binding constant comparable to constants for ligands found in the water column. Coordination of Cu by this compound decreases the concentration of free cupric ion (the toxic form) in the culture media to le-rels that do not inhibit growth. A tight linear correlation between chelator and Cu concentration suggests l.hat production of this substance may be regulated by the concentration of free Cu in the media in a feeldback mechanism. Similarly, the concentrations of Cu and Cu-binding ligands in the water column are o?ten closely related. These results suggest that cyanobacteria modify Cu chemistry in seawater, creating conditions more favorable for growth.
TL;DR: A new dynamic model that uses a small number of prescribed parameters to predict the chlorophyll a:carbon ratio and growth rate of phytoplankton in both constant and varying irradiance is presented.
Abstract: We present a new dynamic model that uses a small number of prescribed parameters to predict the chlorophyll a:carbon ratio and growth rate of phytoplankton in both constant and varying irradiance. The model provides a self-contained description of energy and mass fluxes and regulation of partitioning of photosynthate during phytoplankton adaptation to irradiance. The kinetics and steady-state outcomes of photoadaptation are described in terms of changes in the rates of synthesis of three intracellular carbon pools. These pools account for the distribution of cell material between light-harvesting components, the biosynthetic apparatus, and energy storage compounds. Regulation of the flow of recent photosynthate to these pools is controlled by the ratio of realized to potential photosynthetic electron flow at a given instant. The responses of growth rate and Chl a:C to static and dynamic irradiance regimes can be adequately described by specifying four parameters: the initial slope of the photosynthesis-irradiance curve, the maximum growth rate, the maximum Chl a:C observed under light limitation, and the maintenance metabolic rate. The model predictions compared favorably with observations of the diatoms Thalassiosira pseudonana and Phaedactylum tricornutum.
TL;DR: In this paper, the thermal characteristics of four lakes at the North-Temperate Lakes Long-Term Ecological Research site were analyzed and used to validate simulations of lake physics with the dynamic reservoir simulation model.
Abstract: A lo-yr record of the thermal characteristics of four lakes at the North-Temperate Lakes Long-Term Ecological Research site was analyzed and used to validate simulations of lake physics with the dynamic reservoir simulation model. Simulations of cool, warm, and intermediate years were rerun with meteorology from four general circulation models with a doubling of CO,. In all simulations with doubled COZ there is an earlier onset of stratification, increased summer epilimnetic temperature (l-7”C), and an increased intensity and longer duration of stratification. Maximum surface temperatures at times may exceed upper lethal limits of warm and cool water fish in some scenarios. Suitable thermal habitat for cold water, cool water, and warm water fish generally increases in all scenarios after climate change. Changes in the vertical migration of Daphnia, however, are expected to vary depending on the interaction of thermal stratification and fish habitat use. In northern Wisconsin lakes with cold water planktivores, habitat overlap between fish and zooplankton is expected to decrease, while in southern Wisconsin lakes habitat overlap is expected to increase. Although most physical responses of lakes to climate change are consistent among all climate scenarios, biological responses will likely be more variable owing to the complex nature of factors determining ecological interactions in lakes.
TL;DR: By measuring Chl a, zooplankton biomass, nutrient concentrations, and water residence time in 31 rivers in eastern Canada, the following hypotheses were tested: ChlA is positively related to nutrient concentrations and water residency time, and zoopLankton woody biomass is positive related to chl a andWater residence time.
Abstract: By measuring Chl a, zooplankton biomass, nutrient concentrations, and water residence time in 31 rivers in eastern Canada, we tested the following hypotheses: Chl a is positively related to nutrient concentrations and water residence time, and zooplankton biomass is positively related to Chl a and water residence time. Chl a ranged from 1.8 to 27.6 μg liter -1 , total phosphorus (TP) from 7.3 3 to 212.3 μg liter -1 and water residence time from 3.3 to 19.4 d. There was a positive relationship between Chl a and TP (R 2 = 0.76) and no relationship between Chl a and residence time. In contrast, zooplankton biomass was positively related to water residence time (R 2 = 0.33). There was only a weak positive relationship between Chl a and zooplankton biomass. Mean river zooplankton biomass was low (11.3 μg liter -1 dry mass) compared to lakes of similar Chl a concentration.
TL;DR: It is suggested that even low levels of grazing can greatly accelerate DMS production, suggesting that DMSP lyase and DMSP are segregated within these cells and reaction only under conditions that result in cell stress or damage.
Abstract: Emiliania huxleyi clones CCMP 370 and CCMP 373 produced similar amounts of dimethylsulfoniopropionate (DMSP) during axenic exponential growth, averaging 109 mM internal DMSP. Both clones had detectable DMSP lyase activity, as measured by production of dimethyl sulfide (DMS) during in vitro assays of crude cell preparations, but activities and conditions differed considerably between clones. Clone 373 had high activity; clone 370 had low activity and required chloride. For both strains, enzyme activity per cell was constant during exponential growth, but little DMS was produced by healthy cells. Rather, DMS production was activated when cells were subjected to physical or chemical stresses that caused cell lysis. We propose that DMSP lyase and DMSP are segregated within these cells and re-action only under conditions that result in cell stress or damage. Such activation occurs during microzooplankton grazing. When these clones were grazed by the dinoflagellate Oxyrrhis marina, DMS was produced; ungrazed cells, as well as those exposed to grazer exudates and associated bacteria, generated no DMS. Grazing of clone 373 produced much more DMS than grazing of clone 370, consistent with their relative in vitro DMSP lyase activities. DMS was only generated when cells were actually being grazed, indicating that ingested cells were responsible for the DMS formation. We suggest that even low levels of grazing can greatly accelerate DMS production.
TL;DR: The arrival of internal tidal bores generated by breaking internal waves caused dramatic, high-frequency variation in temperature, salinity, water velocities, and concentration of chlorophyll a on Conch Reef, Florida Keys as discussed by the authors.
Abstract: Internal tidal bores generated by breaking internal waves cause dramatic, high-frequency variation in temperature, salinity, water velocities, and concentration of chlorophyll a on Conch Reef, Florida Keys. The arrival of bores on the reef slope is linked to a semidiurnal internal tide and is marked by temperature drops of up to 5.4?C and salinity increases of up to 0.60oo in 1-20 min. These changes are accompanied by the sudden onset of upslope flow 1-15 m above the bottom with speeds of 10-30 cm s-'. Cool, high-salinity water is transported from below the thermocline seaward of the reef and is resident on the reef slope for up to 4 h before it mixes with surface waters and recedes downslope. Compared with ambient surface water, this deep water can contain significantly elevated concentrations of dissolved nitrate. Physical variability produced by this mechanism increases significantly with depth on the reef slope. Analysis of 3-yr temperature records indicates the arrival of internal bores is a consistent feature at this site from May through November, with peak activity in July-September. Pulsed delivery of subthermocline water appears to significantly affect the temperature, nutrient, and particle flux regimes on this coral reef.
TL;DR: In this article, the authors measured chemoautotrophic bacterial nitrification rates in the lower euphotic zone at the North Pacific Time-series Station ALOHA using low-level chemical assays and inhibitor-sensitive radiocarbon uptake experiments.
Abstract: We measured chemoautotrophic bacterial nitrification rates in the lower euphotic zone at the North Pacific Time-series Station ALOHA using low-level chemical assays and inhibitor-sensitive radiocarbon uptake experiments. These measurements were compared with independent nitrification rate estimates based on nitrous oxide distributions, nitrate assimilation rates based on nitrate changes during an in situ incubation, and historical estimates of nitrification and nitrate assimilation from this region. Ammonium oxidation rates ranged from 1.0 to 137.4 hmol m-3 d-l, and nitrite oxidation rates varied from undetectable to 138.0 pmol m-3 d-l. Conservative estimates of depth-integrated euphotic zone nitrification rates from the complete three-cruise data set were 0.69 mmol rnd2 d-l for nitrite oxidation and 1.64 mmol m-2 d-l for ammonium oxidation. The highest nitrification rates were found below the primary nitrite maximum, suggesting a significant contribution of euphotic zone ammonium oxidation to the nitrite pool below but not within this feature. A mass balance of nitrous oxide indicates that this gas is produced within the euphotic zone at a rate of 1.68-7.94 pmol m-2 d-l. The nitrous oxide production rate provides an independent estimate of the total euphotic zone nitrification rate in the range of 0.34-l .59 mmol m-2 d-l. We estimate that the total euphotic zone nitrification rate is 47-142% of the concurrent nitrate assimilation rate, indicating that nitrification in the euphotic zone is an important source of regenerated nitrate. Nitrification, the microbiologically mediated oxidation of ammonium to nitrite and nitrite to nitrate, is a key process in the global nitrogen cycle (Ward 1986). In the marine environment, the vertical flux into the cuphotic zone of nitrate regenerated by nitrification in deep water is considered crucial to ecosystem stability through its support of new primary photosynthetic production (Eppley and Peterson 1979). However, nitrifying bacteria are also active within the euphotic zone, especially within the primary nitrite maximum (Olson 198 1
TL;DR: In this paper, a series of dialysis experiments was performed to study the relative importance of substrate limitation and grazing in controlling the proportion of active cells of coastal marine bacterioplankton.
Abstract: A series of dialysis experiments was performed to study the relative importance of substrate limitation and grazing in controlling the proportion of active cells of coastal marine bacterioplankton. The grazer community was manipulated by filling dialysis bags with unfiltered water and water serially passed through 150-, 40-, and O.p Ducklow and Carlson 1992). This pattern has prompted speculation that bacterial growth and abundance must be tightly regulated across aquatic systems and confined within a relatively narrow range by a combination of resource limitation and grazing (Sanders et al. 1992; Wright 1988). One issue that is seldom incorporated into models of microbial food webs is the varying proportion of metabolically active and inactive cells. In
TL;DR: Enrichment experiments with P and N were conducted in humic Lake Ortrasket in northern Sweden and the composition of the microplankton community showed a dominance by bacterioplankton, followed by mix plankton.
Abstract: Enrichment experiments with P and N were conducted in humic Lake Ortrasket in northern Sweden. The composition of the microplankton community showed a dominance by bacterioplankton, followed by mix ...
TL;DR: With the exception of Cr, AEs of elements increased with gut passage time, implying more efficient digestion-absorption when the element was retained longer in the digestive tract, in waters containing large mussel populations.
Abstract: Assimilation efficiency (AE) is an important physiological parameter in determining trace element influx from food sources into aquatic animals. We used radiotracer techniques to examine the influence of diet [seven species of algae (two diatoms, two chlorophytes, a prasinophyte, and two dinoflagellates) and glass beads] on the assimilation of seven trace elements (Ag, Am, Cd, Co, Cr, Se, Zn) and C in the mussel Mytilus edulis. Trace element assimilation was related to C assimilation and cytoplasmic distribution in the algae and to gut passage time in the mussels. Mussels displayed different C AEs for the different algal diets; the chlorophytes, which had highly refractory cell walls, were the least digestible food. Assimilation of Cd, Se, and Zn was directly correlated with C assimilation; for Am, Ag, and Cr, no relationship with C assimilation was apparent. For each species except the chlorophytes, AEs of all elements significantly correlated with their cytoplasmic distribution within each algal cell. Among all species, AEs of Am, Co, and Se also increased with elemental penetration into the cytoplasm; however, this relationship was not statistically significant for Ag, Cd, or Zn. With the exception of Cr, AEs of elements increased with gut passage time, implying more efficient digestion-absorption when the element was retained longer in the digestive tract. In waters containing large mussel populations, unassimilated particle-reactive elements should be removed from suspension and enriched in biodeposits in sediments, whereas assimilated metals should be enriched in mussel tissues.
TL;DR: In this paper, the concentration-dependent uptake of nitrate and its inhibit ion by ammonium (NH) was surveyed in surface waters of the North Atlantic Ocean and the half-saturation parameter for uptake (KN) ranged 2-3 orders of magnitude, covarying with ambient NO, concentrations.
Abstract: The concentration-dependent uptake of nitrate (NO,) and its inhibit ion by ammonium (NH,) were surveyed in surface waters of the North Atlantic Ocean. Low-level NO, determinations combined with conventional tracer methods provided the first comprehensive data set on NO, utlization kinetics at nanomolar concentrations. Uptake followed saturation kinetics described by the Michaeli s-Menten equation. The half-saturation parameter for uptake (KN) ranged 2-3 orders of magnitude, covarying with ambient NO, concentrations. KN concentrations in oceanic waters averaged -20-30 nM. NH, half-saturation parameters could only be approximated (i.e. KA + A), but observations suggested that KA and K,, were of similar magnitude in oceanic waters. Maximum uptake rates of nitrate, prntN), and ammonium, prncA), covaried, but prncA) almost always exceeded prncN); in oceanic waters, the disparity was an order of magnitude or greater. Most of the variability in A,,(~ and prncA) could be explained by variations in phytoplankton biomass and temperature. The slope of the uptake vs. concentration relationship, CX, was also investigated but was highly variable and could not be related to any of the oceanographic properties observed; cr, was generally greater than ayN. Kinetics analysis showed that NH4 is preferentially utilized over NO3 over the full spectrum of nitrogen concentrations, nanomolar to micromolar. The inhibition of NO3 uptake by NH,, was also parameterized using the Michaelis-Menten expression. The inhibition half-saturation parameter (KJ covaried with KN, but Kj concentrations in oceanic waters (N 4050 nM) always exceeded KW Maximum inhibition (1,) was rarely complete (i.e. I,,., < I), even at 2,000 nM ammonium. Overall, results suggest that nitrogen utilization parameters currently used in ecosystem models of the open ocean should be re-examined. The mechanisms of supply and the kinetics of utilization of dissolved inorganic nitrogen are believed to play a critical role in determining the productivity, size structure, and species succession of phytoplankton communities in much of the world’s oceans. Dugdale (1967) was the first to propose a theoretical framework for nitrogen acquisition by phytoplankton at steady state based on the concentration-dependent uptake of nitrate and ammonium and represented by the well-known Michaelis-Menten equation of enzyme kinetics:
TL;DR: In this article, a coupled physical-biological two-box model was used to investigate potential effects of climate change on seasonal oxygen cycling and hypoxia in river-dominated coastal waters.
Abstract: Projections of general circulation models suggest that freshwater discharge from the Mississippi River to the coastal ocean will increase 20% if atmospheric CO, concentration doubles. This result is likely to affect water column stability, surface productivity, and global oxygen cycling in the northern Gulf of Mexico, which is the site of the largest (up to 16,500 km*) and most severe hypoxic zone (~2 mg 0, liter-l) in the western Atlantic Ocean. We use a coupled physical-biological two-box model to investigate potential effects of climate change on seasonal oxygen cycling and hypoxia in river-dominated coastal waters. The model was developed and calibrated using comprehensive environmental data sets collected on the Mississippi River and in the northern Gulf of Mexico between 1985 and 1993. The relative magnitude of changes in river runoff and severity of hypoxia during the 1993 Mississippi River flooding provide an excellent data set for model verification. Model simulations for a doubled CO, climate predict a 30-60% decrease in summertime subpycnoclinal oxygen content, relative to a 1985-1992 average. Under those conditions, the hypoxic zone in the northern Gulf of Mexico will expand and encompass an area greater than that of summer 1993.
TL;DR: Phytoplankton-mediated isotopic variability must be considered when isotopic data are used to examine biogeochemical and physical processing of organic matter in marine ecosystems, particularly when biosynthesis and loss processes are decoupled in either space or time during bloom conditions.
Abstract: Isotopic fractionation of ammonium (NH,+ ) and nitrate (NO,-) during uptake by phytoplankton was examined in batch culture experiments with the diatom SkeZetonema costatum under nitrogen-enriched conditions (5-100 PM). The fractionation factor (E) for NOJ- uptake by Skeletonema was -9.Of0.7%0 and was concentration-independent. For NH,+, E was more variable and dependent on ambient NH,+ concentration. For NH4+ concentration ranges of 100-50, 50-20, and 20-5 PM, E was -24.6k5.5, -27.2-+ 1.6, and -7.8+3.0%. In these cultures, isotopic fractionation by phytoplankton caused variations in 615N of up to 50?& for NH4+, 1 ~%XI for NO,-, and 25% for particulate N. Similar variability in the 615N of both dissolved inorganic and particulate organic N pools should be expected during phytoplankton blooms in nature. As a result, phytoplankton-mediated isotopic variability must be considered when isotopic data are used to examine biogeochemical and physical processing of organic matter in marine ecosystems, particularly when biosynthesis and loss processes are decoupled in either space or time during bloom conditions.
TL;DR: In this paper, past lake-water salinity from fossil diatom assemblages were used to infer past climatic conditions at Moon Lake, a climatically sensitive site in the northern Great Plains.
Abstract: Estimates of past lake-water salinity from fossil diatom assemblages were used to infer past climatic conditions at Moon Lake, a climatically sensitive site in the northern Great Plains. A good correspondence between diatom-inferred salinity and historical records of mean annual precipitation minus evapotranspiration (P - ET) strongly suggests that the sedimentary record from Moon Lake can be used to reconstruct past climatic conditions. Century-scale analysis of the Holocene diatom record indicates four major hydrological periods: an early Holocene transition from an open freshwater system to a closed saline system by 7300 B.P., which corresponds with a transition from spruce forest to deciduous parkland to prairie and indicates a major shift from wet to dry climate; a mid-Holocene period of high salinity from 7300 to 4700 B.P., indicating low effective moisture (P - ET); a transitional period of high salinity from 4700 to 2200 B.P., characterized by poor diatom preservation; and a late Holocene period of variable lower salinity during the past 2,200 yr, indicating fluctuations in effective moisture.