Onset Of Summer Stratification Research Articles

Uncoupled seasonal variability of transparent exopolymer and Coomassie stainable particles in coastal Mediterranean waters

Transparent exopolymer particles (TEP) and Coomassie stainable particles (CSP) are gel-like particles, ubiquitous in the ocean, that affect important biogeochemical processes including organic carbon cycling by planktonic food webs. Despite much research on both groups of particles (especially TEP) over many years, whether they exist as distinctly stainable fractions of the same particles or as independent particles, each with different driving factors, remains unclear. To address this question, we examined the temporal dynamics of TEP and CSP over 2 complete seasonal cycles at 2 coastal sites in the Northwestern Mediterranean Sea, the Blanes Bay Microbial Observatory (BBMO) and the L’Estartit Oceanographic Station (EOS), as well as their spatial distribution along a coast-to-offshore transect. Biological, chemical, and physical variables were measured in parallel. Surface concentrations (mean ± standard deviation [SD]) of TEP were 36.7 ± 21.5 µg Xanthan Gum (XG) eq L–1 at BBMO and 36.6 ± 28.3 µg XG eq L–1 at EOS; for CSP, they were 11.9 ± 6.1 µg BSA eq L–1 at BBMO and 13.0 ± 5.9 µg BSA eq L–1 at EOS. Seasonal variability was more evident at EOS, where surface TEP and CSP concentrations peaked in summer and spring, respectively, and less predictable at the shore-most station, BBMO. Vertical distributions between surface and 80 m, monitored at EOS, showed highest TEP concentrations within the surface mixed layer during the stratification period, whereas CSP concentrations were highest before the onset of summer stratification. Phytoplankton were the main drivers of TEP and CSP distributions, although nutrient limitation and saturating irradiance also appeared to play important roles. The dynamics and distribution of TEP and CSP were uncoupled both in the coastal sites and along the transect, suggesting that they are different types of particles produced and consumed differently in response to environmental variability.

Sensitivity of physical lake processes to climate change within a large Precambrian Shield catchment

AbstractPotential future changes in lake physical processes (e.g. stratification and freezing) can be assessed through exploring their sensitivity to climate change, and assessing the current vulnerability of different lake types to plausible changes in meteorological drivers. This study quantifies the impacts of climate change and sensitivity of lake physical processes within a large (5100 km2) Precambrian Shield catchment in south‐central Ontario. Historic regional relationships are established between climate drivers, lake morphology, and lake physical changes through generalized linear modelling (GLM), and are used to quantify likely changes in timing of ice phenology and lake stratification across 72 lakes under a range of future climate models and scenarios.In response to projections of increased temperature (ensemble mean of +3.3 °C), both earlier ice‐off and onset of summer stratification were projected, with later ice‐on and fall turnover compared to the baseline. Process sensitivity to climate change varied by lake type; shallower lakes with a smaller volume (less than 15 m deep and less than 0.05 km3) were more sensitive to processes associated with lake heating (stratification onset and ice‐off), and deeper lakes with a larger surface area (greater than 30 m deep and greater than 1000 ha) were more sensitive to processes associated with lake cooling (fall turnover and ice‐on). These results indicate that whereas small lakes are vulnerable to climate warming because of changes that occur in spring and summer, larger lakes are particularly sensitive during the fall. The findings suggest that lake morphology and associated sensitivity should be considered in the development of sustainable lake management strategies. Copyright © 2016 John Wiley & Sons, Ltd.

Spatiotemporal redox dynamics in a freshwater lake sediment under alternating oxygen availabilities: combined analyses of dissolved and particulate electron acceptors

Environmental contextAt sediment surfaces, the availability of oxygen is controlled by its downward transport from the water surface and its consumption in microbial metabolism. Microorganisms can also consume substances other than oxygen to dispose of the surplus charge that is generated during microbial metabolism. We investigate the complex dynamics of these other substances when the oxygen availability fluctuates, and thereby contribute to the mechanistic understanding of oxygen-consuming processes in aquatic environments. AbstractBenthic mineralisation in lakes largely controls the availability of oxygen in the water column above the sediment. In stratified lakes with anoxic hypolimnetic waters, mineralisation proceeds by anaerobic respiration using terminal electron acceptors (TEAs) other than O2. In past work, hypolimnetic oxygen consumption has been estimated from vertical concentration profiles of redox-active dissolved species in the water column and the underlying sediment. Electron transfer to and from particulate mineral and organic phases in the sediments was, however, not accounted for, mainly because of methodological constraints. In this work we use an electrochemical approach, mediated electrochemical analysis, to directly quantify changes in the redox states of particulate geochemical phases in a lake sediment. In mesocosm incubations, sediments were subjected to shifting oxygen availability similar to conditions during and after lake overturn events. The temporal redox dynamics of both dissolved and particulate phases in sediments were monitored at a high spatial resolution. We used a combination of experimental and modelling approaches to couple the observed changes in the redox state of dissolved and particulate species in the sediment to the oxygen turnover in the overlying water column. For the studied freshwater sediment, the amount of O2 consumed during the re-oxidation of these phases in the top 21mm of the sediment after switching from hypoxic to oxic conditions corresponded to ~50% of the total sediment oxygen consumption that was estimated from in-lake measurements after the onset of summer stratification. We found that solid phases in the sediments play a more profound role in electron accepting processes than previously considered. Based on these results, we propose that the herein presented analytical method offers the possibility to constrain parameters in theoretical models that simulate benthic redox dynamics including the electron transfer to and from geochemical phases in the sediments.

Circulation on the central Bering Sea shelf, July 2008 to July 2010

We examine the July 2008 to July 2010 circulation over the central Bering Sea shelf using measurements at eight instrumented moorings, hindcast winds and numerical model results. At sub‐tidal time scales, the vertically integrated equations of motion show that the cross‐shelf balance is primarily geostrophic. The along‐shelf balance is also mainly geostrophic, but local accelerations, wind stress and bottom friction account for 10–40% of the momentum balance, depending on season and water depth. The shelf exhibits highly variable flow with small water column average vector mean speeds (<5 cm s−1). Mean/peak speeds in summer (3–6 cm s−1/10–30 cm s−1) are smaller than in winter and fall (6–12 cm s−1/30–70 cm s−1). Low frequency flows (<1/4 cpd) are horizontally coherent over distances exceeding 200 km. Vertical coherence varies seasonally, degrading with the onset of summer stratification. Because effects of heating and freezing are enhanced in shallow waters, warm summers increase the cross‐shelf density gradient and thus enhance northward transport; cold winters with increased ice production and brine rejection increase the (now reversed) cross‐shelf density gradient and enhance southward transport. Although the baroclinic velocity is large enough to influence seasonal transports, wind‐forced Ekman dynamics are primarily responsible for flow variations. The system changes from strong northward flow (with coastal convergence) to strong southward flow (with coastal divergence) for northerly and easterly winds, respectively. Under northerly and northwesterly winds, nutrient‐rich waters flow toward the central shelf from the north and northwest, replacing dilute coastal waters that are carried south and west.

Nitrite dynamics in the open ocean—clues from seasonal and diurnal variations

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 453:11-26 (2012) - DOI: https://doi.org/10.3354/meps09525 Nitrite dynamics in the open ocean—clues from seasonal and diurnal variations Efrat Meeder1,2,*, Katherine R. M. Mackey3, Adina Paytan3, Yeala Shaked1,2, David Iluz4,5, Noga Stambler4, Tanya Rivlin2, Anton F. Post2, Boaz Lazar1,2 1Ferdy and Nadine Hermann Institute of Earth Sciences, Hebrew University, Edmond J. Safra Campus, Jerusalem 91904, Israel 2Interuniversity Institute for Marine Sciences Coral Beach, PO Box 469, 88103 Eilat, Israel 3Institute of Marine Science, University of California Santa Cruz, Santa Cruz, California 95064, USA 4Mina & Everard Goodman Faculty of Life Sciences, and 5Department of Geography and Environment, Bar-Ilan University, Ramat-Gan 52900, Israel *Email: efrat_m@cc.huji.ac.il ABSTRACT: Two alternative mechanisms are suggested for nitrite accumulation in the oxygenated oligotrophic water column: (1) excretion by phytoplankton or (2) microbial oxidation of ammonium (nitrification). This study assessed the role of these 2 mechanisms, based on seasonal and high-resolution diurnal depth profiles of the dissolved inorganic nitrogen (DIN) species (nitrite, ammonium, nitrate) and chlorophyll a in the Gulf of Aqaba, Red Sea. Both mechanisms operated in the water column, but in different seasons; nitrification was the prime process responsible for nitrite accumulation during the stratified summer season and phytoplankton nitrite excretion operated during winter mixing. At the onset of summer stratification two N peaks developed below the photic zone, an ammonium maximum (AM) and below it the primary nitrite maximum (PNM). Both peaks were located at a depth range where phytoplankton are thought to be inactive and not excreting nitrite. During summer stratification, the water column deep chlorophyll maximum (DCM), AM, PNM and the nitracline were ordered by a downward increase in N oxidation state similar to the temporal order of the N-species during nitrification. This similarity, together with the diurnal stability of the PNM and its co-existence with oscillating chlorophyll profiles above the DCM, is consistent with nitrification as the key process forming the PNM. We suggest that transport and reaction control the vertical order and separation of N-species in the water column. The ratios between the rate constants for ammonification, ammonium oxidation, nitrite oxidation and nitrate assimilation were estimated by a simple box model to be 1:3:1.5:0.15, respectively. These field estimates are similar to the ratios between the rate constants measured in laboratory experiments. KEY WORDS: Nitrite · Nitrification · Nitrite excretion · Primary nitrite maximum · Ammonium maximum · Gulf of Aqaba · Red Sea Full text in pdf format Supplementary material PreviousNextCite this article as: Meeder E, Mackey KRM, Paytan A, Shaked Y and others (2012) Nitrite dynamics in the open ocean—clues from seasonal and diurnal variations. Mar Ecol Prog Ser 453:11-26. https://doi.org/10.3354/meps09525 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 453. Online publication date: May 07, 2012 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2012 Inter-Research.

Differential Interactions of Two Introduced Piscivorous Salmonids with a Native Cyprinid in Lentic Systems: Implications for Conservation of Roundtail Chub

AbstractThe effects of multiple nonnative piscivore species on native prey species in lentic systems are poorly understood. We studied the relative predation risks posed by two piscivorous salmonids (brown trout Salmo trutta and lake trout Salvelinus namaycush) to endemic roundtail chub Gila robusta in two lakes within the upper Colorado River basin. Gill nets were set in various habitat types to study habitat use by the three species before and after the onset of summer stratification. Roundtail chub and brown trout were mainly associated with shallow‐water habitats, and this association did not change with thermal period. By contrast, lake trout habitat use changed with thermal period as fish moved from shallow areas in the spring to deepwater habitat after stratification. These habitat use patterns indicate that roundtail chub are more susceptible to predation by brown trout than by lake trout because both roundtail chub and brown trout occupy the littoral zone for a prolonged period. Diet data indicated that brown trout consumed littoral fish species (i.e., cyprinids), whereas lake trout primarily consumed opossum shrimp Mysis spp. and dipterans (true flies). Brown trout consumed proportionally more fish than did lake trout, began feeding on fish at smaller total lengths, and increased fish consumption during the period of thermal stratification. An important consideration for efforts to conserve lentic roundtail chub populations is the prevention of future introductions of littoral predators.

Increased sediment-water interface bacterial [3H]-L-serine uptake and biomass production in a eutrophic reservoir during summer stratification

Bacterial consortia at sediment-water interfaces (SWIs) conduct metabolic processes that affect lake and reservoir organic matter cycling and nutrient composition. Seasonal reservoir dynamics such as stratification and overturn influence SWI bacterial activity and bacterial biomass production. We measured SWI bacterial uptake rates of tritium-labeled L-serine (Ser) to assess SWI bacterial activity and production in a monomictic, eutrophic reservoir. Total microbial Ser uptake (Ser tot ) and uptake in the protein fraction (Ser pro ) were measured seasonally. Ser pro was approximately 40 percent of Ser tot which demonstrated that Ser was incorporated into cellular material other than protein. Highest bacterial Ser tot and Ser pro occurred during the onset of summer stratification (Jun 2005) followed by late-season stratification (Oct 2005). Lower Ser tot and Ser pro were observed after the onset of autumnal overturn (Oct 2004) and during winter mixing (Feb 2005). Higher SWI temperature, lower dissolved oxygen, and decreasing redox potential, which are a function of summer stratification, were associated with higher Ser tot and Ser pro . Estimates of SWI bacterial production and growth rates indicated that highest production and fastest community generation times also occurred at onset of summer stratification and during late-season stratification. These data suggest summer stratification, and not autumnal overturn and winter mixing, is responsible for higher SWI bacterial metabolism, even under anoxic conditions during which bacteria utilize less energetically favored electron acceptors. We conclude that stratification is an important event that increases SWI bacterial metabolism, which results in increased carbon mineralization and nutrient cycling from sediments to the water column that affects water quality and reservoir trophic status.

Response of the microbial food web to manipulation of nutrients and grazers in the oligotrophic Gulf of Aqaba and northern Red Sea

The control mechanisms within the pelagic microbial food web of the oligotrophic Gulf of Aqaba and the northern Red Sea were investigated in the spring of 1999. Nutrient conditions and potential grazer impact were manipulated in a series of dilution experiments. Ambient nutrient concentrations and autotrophic biomass were very low (0.23–1.21 µmol NO3 l−1, 0.06–0.98 µmol NH4 l−1, 1.08–1.17 µmol Si l−1, 0.08–0.12 µmol P l−1, 0.15–0.36 µg chlorophyll a l−1). The planktonic community was characterized by low abundances [3.0–5.5×105 heterotrophic bacteria ml−1, 0.58–7.2×103 ultraphytoplankton 8 µm l−1, 0.93–5.4×102 microzooplankton l−1] and dominated by small forms (0.2–8 µm). Dinoflagellates and oligotrichous ciliates were the most common groups in initial samples among the phytoplankton >8 µm and microzooplankton, respectively. Results show that bottom-up and top-down control mechanisms operated simultaneously. Small organisms were vulnerable to grazing, with maximum grazing rates of 1.1 day−1 on heterotrophic bacteria and 1.3 day−1 on ultraphytoplankton. In contrast, algae >8 µm showed stronger signs of nutrient limitation, especially when the final assemblages were dominated by diatoms. Synechococcus sp. were not grazed and only showed moderate to no response to nutrient additions. The high spatial and temporal variation of our results indicates that the composition of the planktonic community determines the prevailing control mechanisms. It further implies that, at this transitional time of the year (onset of summer stratification), the populations fluctuate about an equilibrium between growth and grazing.

Discharge of methylmercury-enriched hypolimnetic water from a stratified reservoir

A study conducted from July 1995 to June 1996 examining spatial and temporal distribution of mercury (Hg) at the Caballo Reservoir, New Mexico, revealed that the highest levels of methylmercury (MMHg) occurred in both the inlet and the Rio Grande upstream of the reservoir. As a result, a second study was designed to identify possible sources of the elevated levels of MMHg, and to determine if water discharged from the Elephant Butte Reservoir upstream could be a primary source. In July 1996, as anoxia began to develop in the hypolimnion of the Elephant Butte Reservoir, surface water MMHg concentrations were below the MDL of 0.018 ng/l while water discharged into the tailrace was 0.149 ng/l MMHg. By September 1996, when the anoxic hypolimnion spanned 60% of the total reservoir depth, surface water MMHg was still below the MDL, while discharge water had increased to 1.144 ng/l MMHg. Following reservoir turnover in November 1996, surface water increased to 0.264 ng/l MMHg while discharge water decreased to 0.420 ng/l MMHg. By January 1997, MMHg in the tailrace decreased to pre-stratification levels, and both surface water and discharge water reached similar MMHg levels until the onset of summer stratification in July 1997. This trend was repeated the following year when MMHg concentrations in the tailrace increased from 0.190 ng/l in August 1997 to 1.240 ng/l in September 1997. In addition, vertical profile sampling of the reservoir from August 1997 to September 1997 showed a buildup of MMHg in the anoxic hypolimnion which coincided with increasing levels of MMHg discharged into the tailrace. During the course of this study the single largest contribution of MMHg to the river below the reservoir was from water released through the dam during the fall months of the year.

Activity of fecal coliform bacteria measured by 4-methylumbelliferyl-ß-D-glucuronide substrate in the northern Adriatic Sea with special reference to marine snow

Activity of fecal coliform bacteria measured by 4-methylumbelliferyl-ß-D-glucuronide substrate in the northern Adriatic Sea with special reference to marine snow

The Population Ecology of Iron Bacteria (Genus Ochrobium) in a Stratified Eutrophic Lake

Bacteria assigned to the genus Ochrobium have not yet been isolated in pure culture, but have been implicated in the deposition of ferric iron. These organisms have been observed in a number of lakes but the conditions under which active growth occurred have not been defined. A population of Ochrobium sp. developed in the anoxic hypolimnion of Esthwaite Water, a eutrophic lake in the English Lake District. Direct counts and experimental evidence obtained with this population suggested that the organism was capable of anaerobic growth. Inhibition of growth by chloramphenicol provided further evidence of its prokaryotic organization. The population overwintered in the sediment and at the onset of summer stratification, with deoxygenation of the hypolimnion, migrated into the water column. There was insufficient evidence to implicate Ochrobium sp. in ferric iron deposition.