Ocean acidification impacts on nitrogen fixation in the coastal western Mediterranean Sea

The effects of ocean acidification on nitrogen (N2) fixation rates and on the community composition of N2-fixing microbes (diazotrophs) were examined in coastal waters of the North-Western Mediterranean Sea. Nine experimental mesocosm enclosures of ∼50 m3 each were deployed for 20 days during June-July 2012 in the Bay of Calvi, Corsica, France. Three control mesocosms were maintained under ambient conditions of carbonate chemistry. The remainder were manipulated with CO2 saturated seawater to attain target amendments of pCO2 of 550, 650, 750, 850, 1000 and 1250 μatm. Rates of N2 fixation were elevated up to 10 times relative to control rates (2.00 ± 1.21 nmol L-1d-1) when pCO2 concentrations were >1000 μatm and pHT (total scale) < 7.74. Diazotrophic phylotypes commonly found in oligotrophic marine waters, including the Mediterranean, were not present at the onset of the experiment and therefore, the diazotroph community composition was characterised by amplifying partial nifH genes from the mesocosms. The diazotroph community was comprised primarily of cluster III nifH sequences (which include possible anaerobes), and proteobacterial (α and γ) sequences, in addition to small numbers of filamentous (or pseudo-filamentous) cyanobacterial phylotypes. The implication from this study is that there is some potential for elevated N2 fixation rates in the coastal western Mediterranean before the end of this century as a result of increasing ocean acidification. Observations made of variability in the diazotroph community composition could not be correlated with changes in carbon chemistry, which highlights the complexity of the relationship between ocean acidification and these keystone organisms.

Uncertainty estimates of remote sensing reflectance derived from comparison of ocean color satellite data sets

Assigning uncertainty to ocean-color satellite products is a requirement to allow informed use of these data. Here, uncertainty estimates are derived using the comparison on a 12th-degree grid of coincident daily records of the remote-sensing reflectance RRS obtained with the same processing chain from three satellite missions, MERIS, MODIS and SeaWiFS. The approach is spatially resolved and produces σ, the part of the RRS uncertainty budget associated with random effects. The global average of σ decreases with wavelength from approximately 0.7– 0.9 10−3 sr−1 at 412 nm to 0.05–0.1 10−3 sr−1 at the red band, with uncertainties on σ evaluated as 20–30% between 412 and 555 nm, and 30–40% at 670 nm. The distribution of σ shows a restricted spatial variability and small variations with season, which makes the multi-annual global distribution of σ an estimate applicable to all retrievals of the considered missions. The comparison of σ with other uncertainty estimates derived from field data or with the support of algorithms provides a consistent picture. When translated in relative terms, and assuming a relatively low bias, the distribution of σ suggests that the objective of a 5% uncertainty is fulfilled between 412 and 490 nm for oligotrophic waters (chlorophyll-a concentration below 0.1 mg m−3). This study also provides comparison statistics. Spectrally, the mean absolute relative difference between RRS from different missions shows a characteristic U-shape with both ends at blue and red wavelengths inversely related to the amplitude of RRS. On average and for the considered data sets, SeaWiFS RRS tend to be slightly higher than MODIS RRS, which in turn appear higher than MERIS RRS. Biases between mission-specific RRS may exhibit a seasonal dependence, particularly in the subtropical belt.

New Zealand chironomids as proxies for human-induced and natural environmental change: Transfer functionsfor temperature and lake production (chlorophyll a)

The analysis of chironomid taxa and environmental datasets from 46 New Zealand lakes identified temperature (February mean air temperature) and lake production (chlorophyll a (Chl a)) as the main drivers of chironomid distribution. Temperature was the strongest driver of chironomid distribution and consequently produced the most robust inference models. We present two possible temperature transfer functions from this dataset. The most robust model (weighted averaging-partial least squares (WA-PLS), n = 36) was based on a dataset with the most productive (Chl a > 10 lg l)1) lakes removed. This model produced a coefficient of determination (r2 jack) of 0.77, and a root mean squared error of prediction (RMSEPjack) of 1.31C. The Chl a transfer function (partial least squares (PLS), n = 37) was far less reliable, with an r2 jack of 0.49 and an RMSEPjack of 0.46 Log10lg l)1. Both of these transfer functions could be improved by a revision of the taxonomy for the New Zealand chironomid taxa, particularly the genus Chironomus. The Chironomus morphotype was common in high altitude, cool, oligotrophic lakes and lowland, warm, eutrophic lakes. This could reflect the widespread distribution of one eurythermic species, or the collective distribution of a number of different Chironomus species with more limited tolerances. The Chl a transfer function could also be improved by inputting mean Chl a values into the inference model rather than the spot measurements that were available for this study.

The PRIME 1996 cruise: an overview

The UK PRIME cruise, June-July 1996 in the NE Atlantic, consisted of two legs. During the first, detailed chemical and biological observations were made in time-series mode adjacent to the centre of a cold-core eddy in the vicinity of 59 degreesN 20 degreesW using SF6 tracer techniques as the basis for the Lagrangian study. The eddy, which appeared to have been formed the previous winter, remained coherent over the 9 days of the survey and advected only slowly. The phytoplankton community in the eddy was dominated by the coccolithophorid Coccolithus huxleyi. High microzooplankton grazing rates indicated minimal export losses from the surface layer. Significant shifts in many, but not all, of the chemical and biological properties measured were observed over the course of the experiment, especially after the passage of a storm event, which resulted in considerable deepening of the mixed layer followed by a return to fully stratified conditions. The second leg consisted of a transect from 59 degreesN 20 degreesW to 37 degreesN 19 degreesW, with a further Lagrangian time-series study based on a drogue marker initiated at the southern end of the transect. Maximal biological activity was generally encountered in the region between two fronts located at 52.5 degreesN and 48 degreesN, while to the south of 48 degreesN oligotrophic conditions prevailed. At the southern Lagrangian site, a deep chlorophyll maximum was present and high column new production was recorded as a result of the euphotic zone extending below the depth of the nutricline. Microzooplankton grazing rates were lower at this location than at the northern eddy site. The influx of a warm, saline water body into the upper layers during the southern survey led to a major shift in many of the biological and chemical properties being measured. At both the northern and southern Lagrangian sites, the biomass of the mesozooplankton exceeded that of the microzooplankton. (C) 2001 Elsevier Science Ltd. All rights reserved.

Uptake of carbon, nitrogen and phosphorus by phytoplankton along the 20 degrees W meridian in the NE Atlantic between 57.5 degrees N and 37 degrees N

Phytoplankton biomass and rate of production were measured along a transect from 57.54 degreesN to 37.01 degreesN in the northeast Atlantic during July 1996 and at a series of stations over a 7-day period at 37 degreesN 20 degreesW. Surface nutrient concentrations ranged from 4 mu mol l(-1) NO3-, and 0.35 mu mol l(-1) PO43- at 57.54 degreesN to <10 nmol l(-1) NO3- and similar to 10 nmol l(-1) PO43- at 37.01 degreesN. The greatest phytoplankton biomass and production were measured in the vicinity of a frontal system at 50 degreesN, and there was a general decline in total phytoplankton biomass and production to the south of the transect. Production was measured in three size fractions. At the station with the highest chlorophyll concentrations (50.34 degreesN), phytoplankton cells larger than 5 mum dominated the assemblage, accounting for 72% of the chlorophyll concentration (22.9 mg m(-2)) and 51% of primary production (54.1 mmol Cm-2 d(-1)), but picophytoplankton production was also high (43%). At 57 degreesN, carbon fixation by the > 5 mum fraction accounted for 75% of the daily production of 60.75 mmol Cm-2 d(-1). At 37 degreesN, picophytoplankton was the dominant group, accounting for similar to 58% (10 mg m(-2)) of chlorophyll and similar to 64% (46 mmol Cm-2 d(-1)), of primary production. Nitrate, ammonium and phosphate uptake rates also were determined. Although high nitrate uptake rates were measured in the surface water at similar to 50 degreesN, the greatest uptake rates of both depth-integrated nitrate and ammonium were at the south of the transect. At 37 degreesN, a deep euphotic zone was present and light penetrated through the nitracline; total nitrate uptake was enhanced because of assimilation at the base of the euphotic zone. As a consequence, high values of depth-integrated f-ratio were measured in the oligotrophic waters at the south of the transect. Phosphate was predominantly incorporated into the picoplankton fraction, which included heterotrophic and autotrophic components, at all stations and a significant proportion of phosphate uptake occurred in the dark. The C:N:P assimilation ratios were variable throughout the region; phosphate uptake was generally greater than would be expected if nutrient assimilation were in proportion to the Redfield ratio. (C) 2001 Elsevier Science Ltd. All rights reserved.

Comparisons of plankton at 59 degrees N and 37 degrees N during two Lagrangian drift experiments in the North Atlantic in June and July 1996

Data are summarised for two Lagrangian experiments in the North Atlantic in early summer 1996. At 59 degreesN 20 degreesW, plankton dynamics was studied in an SF, tracer release experiment within a mesoscale eddy over a 9-day period. At 37 degreesN 20 degreesW, a second experiment followed a drifting buoy for 7 days. The data obtained in these two experiments have been averaged for 3 depth strata; the euphotic zone, the surface mixed layer (SML), and the seasonal thermocline immediately beneath the surface mixed layer. At 59 degreesN, the euphotic zone was only marginally deeper than the SML, but at 37 degreesN the SML was ca 30 m and the euphotic depth was ca 110 m. At 37 degreesN, nutrient concentrations in the SML were low but significant new production occurred in the thermocline because of light penetration into the nutricline. The particulate organic carbon (POC) concentration of the SML at 59 degreesN was 13-15 mu mol C kg(-1), but at 37 degreesN POC concentrations were 4 mu mol C kg(-1). These POC measurements include biota and detritus. As a way of investigating latitudinal differences in the plankton communities, estimates have been made of the carbon and nitrogen content of phytoplankton, bacterioplankton, microzooplankton and mesozooplankton. At both 59 degreesN and 37 degreesN, phytoplankton was the largest component, accounting for ca 50% of the planktonic biomass in the SML. At 59 degreesN, microzooplankton was 16% of the planktonic carbon, but at 37 degreesN this reduced to 8% of the total. Mesozooplankton was a relatively constant proportion (ca 20%) of the planktonic carbon in the SML at both 59 degreesN and 37 degreesN. Bacterioplankton was 14% of the biomass at 59 degreesN, increasing to 24% in the microbial loop-dominated system at 37 degreesN. Mean carbon fixation rate in the oligotrophic southern station was 24% of that at the north, with more carbon fixation below the SML at 37 degreesN than at 59 degreesN. Respiration rates showed little variation with latitude, and the rates at 37 degreesN were 80% of those at 59 degreesN. Nitrate and ammonium uptake rates were very low in the oligotrophic conditions in the SML at 37 degreesN, but nitrate uptake in the euphotic zone was comparable to that at 59 degreesN. Ammonium uptake by phytoplankton was also significantly greater at 37 degreesN, in both the euphotic zone and thermocline, but uptake in the SML was only 20% of that in the SML at 59 degreesN. (C) 2001 Elsevier Science Ltd. All rights reserved.

Potential for phosphonoacetate utilization by marine bacteria in temperate coastal waters

Phosphonates are organic compounds that contain a C-P bond and are a poorly characterized component of the marine phosphorus cycle. They may represent a potential source of bioavailable phosphorus, particularly in oligotrophic conditions. This study has investigated the distribution of the phnA gene which encodes phosphonoacetate hydrolase, the enzyme that mineralizes phosphonoacetate. Using newly designed degenerate primers targeting the phnA gene we analysed the potential for phosphonoacetate utilization in DNA and cDNA libraries constructed from a phytoplankton bloom in the Western English Channel during July 2006. Total RNA was isolated and reverse transcribed and phosphonoacetate hydrolase (phnA) transcripts were PCR amplified from the cDNA with the degenerate primers, cloned and sequenced. Phylogenetic analysis demonstrated considerable diversity with 14 sequence types yielding five unique phnA protein groups. We also identified 28 phnA homologues in a 454-pyrosequencing metagenomic and metatranscriptomic study from a coastal marine mesocosm, indicating that > 3% of marine bacteria in this study contained phnA. phnA homologues were also present in a metagenomic fosmid library from this experiment. Finally, cultures of four isolates of potential coral pathogens belonging to the Vibrionaceae contained the phnA gene. In the laboratory, these isolates were able to grow with phosphonoacetate as sole P and C source. The fact that the capacity to utilize phosphonoacetate was evident in each of the three coastal environments suggests the potential for widespread utilization of this bioavailable P source.

Multi-scale dynamics on the Eastern Brazilian margin

Este trabalho combina esforços de simulação numérica e de análise de dados para investigar a dinâmica em diversos compartimentos (oceano aberto, plataforma continental e zona costeira-estuarina) e, em multiplas escalas, na Margem Continental Leste Brasileira (MCLB). A circulação de largo e mesoescala espacial e a propagação da maré barotrópica são investigadas através de uma configuração aninhada do modelo numérico ROMS. O estudo da dinâmica regional da Baía de Camamu (CMB) baseia-se na análise de dados locais. A MCLB, localizada a SW do Atlântico Sul entre 8±S e 20±S, possui plataforma estreita, batimetria complexa, e baixa produtividade primária. A sua dinâmica é influenciada pela divergência da Corrente Sul Equatorial (CSE). As simulações refletem as conexões sazonais e espaciais entre a Corrente do Brasil e a Contra Corrente Norte do Brasil , em conexão com a dinâmica da CSE. As simulações revelam atividades vorticais nas proximidades da costa e interações com a dinâmica costeira, cujos padrões são descritos. A validação do modelo em mesoescala é baseada em cálculos de energia cinética turbulenta e em dados históricos de transporte. A CMB, localizada a 13±400S, abriga uma comunidade piscatória tradicional e extenso de manguezal. Situa-se porém sobre uma bacia sedimentar com grande reservas de óleo e gás, estando em tensão permanente de impacto ambiental. Neste trabalho sumarizamos as condições físicas regionais e investigamos sua dinâmica interna, focando sua variabilidade em amostragens realizadas sob condições de seca (Setembro de 2004) e de chuva (Julho de 2005). Finalmente, o modelo numérico ROMS é forçado com o sinal de maré, empregando-se uma configuração simples (com coeficientes de atrito de fundo constantes e condições hidrográficas homogéneas), com o intuito de avaliar sua resposta e investigar a natureza da propagação da maré barotrópica na MCLB, convergindo na CMB. A análise da resposta do modelo à maré basea-se em séries históricas do nível do mar para a MCLB e dados recentes da CMB.

The LOICZ biogeochemical budget approach applied to Ria Formosa lagoon, - Algarve, South Portugal

Dissertação mest., Gestão da Água e da Costa, Universidade do Algarve, 2007

Toxicity risks linked to cyanobacteria blooms in southern Portugal reservoirs

Tese de doutoramento, Ciências e Tecnologias do Ambiente, Escola Superior de Saúde, Faculdade de Ciências e Tecnologia, Universidade do Algarve, 2015

Effects of (p)ppGpp on the progression of the cell cycle of Caulobacter crescentus.

Bacteria must control the progression of their cell cycle in response to nutrient availability. This regulation can be mediated by guanosine tetra- or pentaphosphate [(p)ppGpp], which are synthesized by enzymes of the RelA/SpoT homologue (Rsh) family, particularly under starvation conditions. Here, we study the effects of (p)ppGpp on the cell cycle of Caulobacter crescentus, an oligotrophic bacterium with a dimorphic life cycle. C. crescentus divides asymmetrically, producing a motile swarmer cell that cannot replicate its chromosome and a sessile stalked cell that is replication competent. The swarmer cell rapidly differentiates into a stalked cell in appropriate conditions. An artificial increase in the levels of (p)ppGpp in nonstarved C. crescentus cells was achieved by expressing a truncated relA gene from Escherichia coli, encoding a constitutively active (p)ppGpp synthetase. By combining single-cell microscopy, flow cytometry approaches, and swarming assays, we show that an increase in the intracellular concentration of (p)ppGpp is sufficient to slow down the swarmer-to-stalked cell differentiation process and to delay the initiation of chromosome replication. We also present evidence that the intracellular levels of two master regulators of the cell cycle of C. crescentus, DnaA and CtrA, are modulated in response to (p)ppGpp accumulation, even in the absence of actual starvation. CtrA proteolysis and DnaA synthesis seem indirectly inhibited by (p)ppGpp accumulation. By extending the life span of the motile nonreproductive swarmer cell and thus promoting dispersal and foraging functions over multiplication under starvation conditions, (p)ppGpp may play a central role in the ecological adaptation of C. crescentus to nutritional stresses.

A comparison between diatom inferred ph changes and diatom populations in brownwater and clearwater lakes

The objective of this study was to determine whether clearwater and brownwater lakes differed in their rate of acidification as inferred by subfossil diatoms analyzed in recent downcore sediments. Differences between associations of diatom populations in brownwater and clearwater environments were characterized. Sediment cores were taken from four lakes located north and east of Lake Superior, near Wawa, Ontario. Two of these lakes were humicrich, brownwater lakes ( lakes U1 and CB2). The two other lakes were clearwater lakes ( lakes Xl and CF). The regression of Nygaard log index-alpha for surficial diatom sediments on observed pH ( Inferred pH = 6.57 - 0.82 log index-alpha ), was utilized to infer lake pH in recent sediments of these lakes. Upon analyzing the downcore diatoms, it was discovered that no significant change, in downcore diatom inferred pH, could be detected in the two brownwater lakes. In contrast, the two clearwater lakes showed significant shifts in downcore diatom inferred pH. In one of these lakes, pH had dropped from 5.3 to 4.5, in the top 9.0 cm of the core, while in the second lake, pH had dropped from 5.4 to 5.0 in the top 1.5 cm of the core. These findings suggested that humic substances, found in brownwater lakes, imparted a buffering capacity to these lake waters. In the clearwater lakes, the decrease in pH was very probably a consequence of acid precipitation. The Ambrosia rise ( circa 1890 ) occurred at the same depth in both brownwater lakes ( 11.0 - 12.0 cm). In both clearwater lakes, the Ambrosia rise occurred at a depth of 14.0 - 15.0 cm. This suggested a lower sedimentation rate in the brownwater lakes. pH influenced the total percentage composition of diatom pH indicator groups. Greater numbers of alkaliphilous taxa were found in less acidic lakes ( e.g. Lake Ul ), While greater numbers of acidloving forms were found in highly acidic lakes ( e.g. Lake Xl ). There was a greater abundance of indifferent forms in the brownwater lakes, than in the clearwater lakes. A number of diatom genera and species were found to be associated with either clearwater or brownwater conditions. The centric diatom, ~elosira distans, significantly increased in abundance in the recent sediments of both clearwater lakes. This may be indicating a shift toward a more oligotrophic state within these acidic, clearwater lakes. This study suggested that a pH index based on subfossil diatoms may be a sensitive indicator of changing lake pH. This study also indicated that humic substances may playa more important role, than previously acknowledged, in controlling the pH dynamics of lake waters, and in determining diatom populations.

Diatoms as indicators of lake trophic status

Relationships between surface sediment diatom assemblages and lake trophic status were studied in 50 Canadian Precambrian Shield lakes in the Muskoka-Haliburton and southern Ontario regions. The purpose of this study was to develop mathematical regression models to infer lake trophic status from diatom assemblage data. To achieve this goal, however, additional investigations dealing with the evaluation of lake trophic status and the autecological features of key diatom species were carried out. Because a unifying index and classification for lake trophic status was not available, a new multiple index was developed in this study, by the computation of the physical, chemical and biological data from 85 south Ontario lakes. By using the new trophic parameter, the lake trophic level (TL) was determined: TL = 1.37 In[1 +(TP x Chl-a / SD)], where, TP=total phosphorus, Chl-a=chlorophyll-a and SD=Secchi depth. The boundaries between 7 lake trophic categories (Ultra-oligotrophic lakes: 0-0.24; Oligotrophic lakes: 0.241-1.8; Oligomesotrophic lakes: 1.813.0; Mesotrophic lakes: 3.01-4.20; Mesoeutrophic lakes: 4.21-5.4; Eutrophic lakes: 5.41-10 and Hyper-eutrophic lakes: above 10) were established. The new trophic parameter was more convenient for management of water quality, communication to the public and comparison with other lake trophic status indices than many of the previously published indices because the TL index attempts to Increase understanding of the characteristics of lakes and their comprehensive trophic states. It is more reasonable and clear for a unifying determination of true trophic states of lakes. Diatom specIes autecology analysis was central to this thesis. However, the autecological relationship of diatom species and lake trophic status had not previously been well documented. Based on the investigation of the diatom composition and variety of species abundance in 30 study lakes, the distribution optima of diatom species were determined. These determinations were based on a quantitative method called "weighted average" (Charles 1985). On this basis, the diatom species were classified into five trophic categories (oligotrophic, oligomesotrophic, mesotrophic, mesoeutrophic and eutrophic species groups). The resulting diatom trophic status autecological features were used in the regressIon analysis between diatom assemblages and lake trophic status. When the TL trophic level values of the 30 lakes were regressed against their fi ve corresponding diatom trophic groups, the two mathematical equations for expressing the assumed linear relationship between the diatom assemblages composition were determined by (1) uSIng a single regression technique: Trophic level of lake (TL) = 2.643 - 7.575 log (Index D) (r = 0.88 r2 = 0.77 P = 0.0001; n = 30) Where, Index D = (0% + OM% + M%)/(E% + ME% + M%); 4 (2) uSIng a' multiple regressIon technique: TL=4.285-0.076 0%- 0.055 OM% - 0.026 M% + 0.033 ME% + 0.065 E% (r=0.89, r2=0.792, P=O.OOOl, n=30) There was a significant correlation between measured and diatom inferred trophic levels both by single and multiple regressIon methods (P < 0.0001, n=20), when both models were applied to another 20 test lakes. Their correlation coefficients (r2 ) were also statistically significant (r2 >0.68, n=20). As such, the two transfer function models between diatoms and lake trophic status were validated. The two models obtained as noted above were developed using one group of lakes and then tested using an entirely different group of lakes. This study indicated that diatom assemblages are sensitive to lake trophic status. As indicators of lake trophic status, diatoms are especially useful in situations where no local trophic information is available and in studies of the paleotrophic history of lakes. Diatom autecological information was used to develop a theory assessing water quality and lake trophic status.

The use of diatom assemblages to determine paleotrophic changes in lakes

The purpose of this study was to develop a classifi cation scheme for l ake trophic status based on the relative abundance of l ake sediment diatom trophic indicator species. A total of 600 diatom frustules were counted from the surface sediments of e a ch of 30 lakes selected to repr e seni~ a continuum from u.lt ra-oligotrophic t,o fairly eutrophic but not hype r-' eutrophic conditions. Published autecological information was used to determine the trophic indicator status of each of the s pecies. A quotieht was derived by dividing the s um of all the e utrophic indicator species by the sum of all oligotrophic indicai.-:.or species. Oligo'- mesotrophic. mesotrophic and meso-eutrophic species were added to both the numerator and denomina tor. Five categories of diatom i.nferred trophic status were recognized : ultra-oligotrophic - 0'-0.2:3, oligotrophic::: 0.24-0 . 70, mesotrophic :: 0.'71 -0.99, meso-elxtrophic :: 1. 00-1. '78 and eutrophic:: 1.. 79-2. 43. But only three of these (oligotrophic:: 0-0.69, mesotrophic ::: 0 . 70'-1.69 j and eutrophic:: 1.70-2.50) proved usef ul. The present study of the relationship between diatom inferred trophic status and the literature-derived trophic status of SO lake s (which were purposely chosen to represent a broad spectrum of lakes in Canada) indicated that: 1) Based on diatom species (assemblages ) it is possible to segregate the lakes from which. th",)se diatoms were taken into three basic categories : o ligotrophic, mesotrophic and eutrophic lake types. ~~) It was not possible t,o separate meso-eutrophic and o l igo-mesotrophic lakes f rom mesotrophic l akes as the the degree of overlap betwee n the diat,om species in these lake types was extremely high. 3) Ha d mo r e ul tra-oligotroph,ic lakes been sampled it might have been possible to more a ccurately s eparate them f rom oligotrophic Jakes. 4 ) Had. more humic lakes been sampled in this s tudy I f eel it would have been possible to identify a unique diatom a ssemblage which would h a ve chara cterized t his lake type . Re gression analyses were performed using the aforementioned diatom inferred trophic index as a f unction of 1) log Sec chi transparency (r = - 0.70) 2) total phosphorus (r = 0. 77 ) and 3) chlorophyll-a (r = 0.74). Once e ach of these rel ationships had been established , it was possible to infer paleotrophic (downcore) changes in an oligotrophic lake (Barbara Lake) and in a eutrophic lake (Chemung Lake) . Barbara Llake was dominated by oligotrophic s pecies and remained oligotrophic throughout the 200-·year history r epresented by i t s 32 em long sediment core. Chemung Lake is currently dominated. by eutrophic species but went through a mesotrophic st,age which was associated with a rise in the water level of the lake followi n g dam construction in its watershed in the early 1.900 J ::;. This was followed by its reversion to it,s present day eutrophic stage (dominated by eutrophic species) possibly as a r esult of shallowing process which can be attributed to " silting' up" of the reservoir and the invasion of the l ake by Myriophyllum spjcatum (Eurasian milfoil) i n the 1970's . In addition, nutrient .:r':l.ch run"'offwhich resulted from increased human a.ctivities associated with cottage development along its shores has contribut ed to its eutrophication. There is some evidence that the rat,e o :f its prog ressive eutrophication has declined during the last decade. This was correlated with legislation enacted in the 60's and 70's in Ontario which was aimed at reducing nutrient loading from cottages.

Quantifying diffusion in biofilms : from model hydrogels to living biofilms

Les biofilms sont des communautés de microorganismes incorporés dans une matrice exo-polymérique complexe. Ils sont reconnus pour jouer un rôle important comme barrière de diffusion dans les systèmes environnementaux et la santé humaine, donnant lieu à une résistance accrue aux antibiotiques et aux désinfectants. Comme le transfert de masse dans un biofilm est principalement dû à la diffusion moléculaire, il est primordial de comprendre les principaux paramètres influençant les flux de diffusion. Dans ce travail, nous avons étudié un biofilm de Pseudomonas fluorescens et deux hydrogels modèles (agarose et alginate) pour lesquels l’autodiffusion (mouvement Brownien) et les coefficients de diffusion mutuels ont été quantifiés. La spectroscopie par corrélation de fluorescence a été utilisée pour mesurer les coefficients d'autodiffusion dans une volume confocal de ca. 1 m3 dans les gels ou les biofilms, tandis que les mesures de diffusion mutuelle ont été faites par cellule de diffusion. En outre, la voltamétrie sur microélectrode a été utilisée pour évaluer le potentiel de Donnan des gels afin de déterminer son impact sur la diffusion. Pour l'hydrogel d'agarose, les observations combinées d'une diminution du coefficient d’autodiffusion et de l’augmentation de la diffusion mutuelle pour une force ionique décroissante ont été attribuées au potentiel de Donnan du gel. Des mesures de l'effet Donnan (différence de -30 mV entre des forces ioniques de 10-4 et 10-1 M) et l'accumulation correspondante d’ions dans l'hydrogel (augmentation d’un facteur de 13 par rapport à la solution) ont indiqué que les interactions électrostatiques peuvent fortement influencer le flux de diffusion de cations, même dans un hydrogel faiblement chargé tel que l'agarose. Curieusement, pour un gel plus chargé comme l'alginate de calcium, la variation de la force ionique et du pH n'a donné lieu qu'à de légères variations de la diffusion de sondes chargées dans l'hydrogel. Ces résultats suggèrent qu’en influençant la diffusion du soluté, l'effet direct des cations sur la structure du gel (compression et/ou gonflement induits) était beaucoup plus efficace que l'effet Donnan. De même, pour un biofilm bactérien, les coefficients d'autodiffusion étaient pratiquement constants sur toute une gamme de force ionique (10-4-10-1 M), aussi bien pour des petits solutés chargés négativement ou positivement (le rapport du coefficient d’autodiffusion dans biofilm sur celui dans la solution, Db/Dw ≈ 85 %) que pour des nanoparticules (Db/Dw≈ 50 %), suggérant que l'effet d'obstruction des biofilms l’emporte sur l'effet de charge. Les résultats de cette étude ont montré que parmi les divers facteurs majeurs qui affectent la diffusion dans un biofilm environnemental oligotrophe (exclusion stérique, interactions électrostatiques et hydrophobes), les effets d'obstruction semblent être les plus importants lorsque l'on tente de comprendre la diffusion du soluté. Alors que les effets de charge ne semblaient pas être importants pour l'autodiffusion de substrats chargés dans l'hydrogel d'alginate ou dans le biofilm bactérien, ils ont joué un rôle clé dans la compréhension de la diffusion à travers l’agarose. L’ensemble de ces résultats devraient être très utiles pour l'évaluation de la biodisponibilité des contaminants traces et des nanoparticules dans l'environnement.