Nitrogen loading to the Bassin d'Arcachon coastal lagoon (SW France) was evaluated by studying la... more Nitrogen loading to the Bassin d'Arcachon coastal lagoon (SW France) was evaluated by studying land-use and nitrogen output in its 3001 km2 catchment. At present, the catchment is dominated by forestry (79%), while intensive agriculture occupies 9% of the surface. The N-output of two hydrological subunits, i.e. the Tagon subunit dominated by pine forestry and the Arriou II subunit comprising both forestry and intensive agriculture, were monitored for a seven year period (1996-2002). From these observations it was calculated that forestry contributes on average 1.6 kg total N ha−1 yr−1, which is dominated by organic nitrogen (DON + PON are 70% of N). On an areal basis, intensive agriculture contributes 26 times more than forestry, i.e. 41.6 kg total N ha−1 yr−1, which is mainly in the form of nitrate (65% of N). These data were upscaled to the catchment and the upscaling was validated by comparison to gauged nitrogen throughputs for the catchment of the Leyre river that is the major tributary to the system. Taking into account the other known N sources and the interannual variability in the catchment it was estimated that nitrogen loading to the lagoon was on average 90 kg ha−1 yr−1 (range from 54 to 126 kg ha−1 yr−1). The sandy soils of the catchment have a clear potential for denitrification, but anoxic conditions (waterlogged) and input of organic matter to fuel this process are required. Currently, agricultural practices and spatial planning do not make use of this potential. Nitrogen loading in the Bassin d'Arcachon is reflected by 10-40 μM nitrate concentrations in winter, which became depleted during spring as a result of uptake by vegetation. Short-term uptake experiments showed that the macroalga Monostroma obscurum is well adapted to temperatures between 10 to 20 °C and competitive with respect to the seagrass Zostera noltii when the nitrate concentrations are above 10 μM. Spring conditions with high nitrate and high insolation are therefore favourable for M. obscurum and this species presents a high risk for algal blooming. In contrast, the macroalga Enteromorpha clathrata well adapted to summertime temperatures around 25 °C, forms occasionally blooms in the lagoon. This phenomenon is limited due to the low DIN concentrations in summer.
Nitrogen loading to the Bassin d'Arcachon coastal lagoon (SW France) was evaluated by studying la... more Nitrogen loading to the Bassin d'Arcachon coastal lagoon (SW France) was evaluated by studying land-use and nitrogen output in its 3001 km2 catchment. At present, the catchment is dominated by forestry (79%), while intensive agriculture occupies 9% of the surface. The N-output of two hydrological subunits, i.e. the Tagon subunit dominated by pine forestry and the Arriou II subunit comprising both forestry and intensive agriculture, were monitored for a seven year period (1996-2002). From these observations it was calculated that forestry contributes on average 1.6 kg total N ha−1 yr−1, which is dominated by organic nitrogen (DON + PON are 70% of N). On an areal basis, intensive agriculture contributes 26 times more than forestry, i.e. 41.6 kg total N ha−1 yr−1, which is mainly in the form of nitrate (65% of N). These data were upscaled to the catchment and the upscaling was validated by comparison to gauged nitrogen throughputs for the catchment of the Leyre river that is the major tributary to the system. Taking into account the other known N sources and the interannual variability in the catchment it was estimated that nitrogen loading to the lagoon was on average 90 kg ha−1 yr−1 (range from 54 to 126 kg ha−1 yr−1). The sandy soils of the catchment have a clear potential for denitrification, but anoxic conditions (waterlogged) and input of organic matter to fuel this process are required. Currently, agricultural practices and spatial planning do not make use of this potential. Nitrogen loading in the Bassin d'Arcachon is reflected by 10-40 μM nitrate concentrations in winter, which became depleted during spring as a result of uptake by vegetation. Short-term uptake experiments showed that the macroalga Monostroma obscurum is well adapted to temperatures between 10 to 20 °C and competitive with respect to the seagrass Zostera noltii when the nitrate concentrations are above 10 μM. Spring conditions with high nitrate and high insolation are therefore favourable for M. obscurum and this species presents a high risk for algal blooming. In contrast, the macroalga Enteromorpha clathrata well adapted to summertime temperatures around 25 °C, forms occasionally blooms in the lagoon. This phenomenon is limited due to the low DIN concentrations in summer.
A deterministic one-dimensional reaction diffusion model was constructed to simulate benthic stra... more A deterministic one-dimensional reaction diffusion model was constructed to simulate benthic stratification patterns and population dynamics of cyanobacteria, purple and colorless sulfur bacteria as found in marine microbial mats. The model involves the major biogeochemical processes of the sulfur cycle and includes growth metabolism and their kinetic parameters as described from laboratory experimentation.
ABSTRACT The amino acid composition of organic matrices associated with calcium carbonate precipi... more ABSTRACT The amino acid composition of organic matrices associated with calcium carbonate precipitates in microbialites built by different Phormidium species (cyanobacteria) has been compared for samples recovered in lagoonal settings from two regions of the Southern Tropical Pacific separated by more than 4000 km: New Caledonia (Nouméa lagoon) and French Polynesia (Tikehau atoll).
Nitrogen-fixation (acetylene reduction) rates were measured over an annual cycle in meadows of th... more Nitrogen-fixation (acetylene reduction) rates were measured over an annual cycle in meadows of the seagrass Zostera noltii Hornem in the Bassin d' Arcachon, south-west France, between March 1994 and February 1995, using both slurry and whole-core techniques. Measured rates using the slurry technique consistently overestimated those determined on whole cores, probably due to the release of labile organic carbon sources as a result of root damage during preparation of the slurries. Thus, the whole-core technique may provide a more accurate estimate of in situ activity, since disturbance of physicochemical gradients of oxygen, sulphide, nutrients and the relationship between the plant roots and the rhizosphere microflora is minimised. Rates measured by the whole-core method were 1.8-to 4-fold greater (dependent upon season) in the light than those measured during dark incubations, indicating that organic carbon diffusing from the plant roots during photosynthesis was an important factor in regulating nitrogen fixation in the rhizosphere. Additions of sodium molybdate, a specific inhibitor of sulphate-reducing bacteria (SRB) inhibited acetylenereduction activity by > 80% as measured by both the slurry and whole-core techniques throughout the year, inferring that SRB were the dominant component of the nitrogen-fixing microflora. A mutualistic relation-Communicated by J. P. Thorpe,
Changes in environmental conditions and prokaryote physiology can strongly affect the dynamics of... more Changes in environmental conditions and prokaryote physiology can strongly affect the dynamics of both the lysogenic and lytic bacteriophage replication cycles in aquatic systems. However, it remains unclear whether it is the nature, amplitude or frequency of these changes that alter the phage replication cycles. We performed an annual survey of three Mediterranean lagoons with contrasting levels of chlorophyll a concentration and salinity to explore how these cues and their variability influence either replication cycle. The lytic cycle was always detected and showed seasonal patterns, whereas the lysogenic cycle was often undetected and highly variable. The lytic cycle was influenced by environmental and prokaryotic physiological cues, increasing with concentrations of dissolved organic carbon, chlorophyll a, and the proportion of respiring cells, and decreasing with the proportion of damaged cells. In contrast, lysogeny was not explained by the magnitude of any environmental or physiological parameter, but increased with the amplitude of change in prokaryote physiology. Our study suggests that both cycles are regulated by distinct factors: the lytic cycle is dependent on environmental parameters and host physiology, while lysogeny is dependent on the variability of prokaryote physiology. This could lead to the contrasting patterns observed between both cycles in aquatic systems.
Buffer capacities'' has been defined in ecology as a holistic concept (e.g., Integration of Ecosy... more Buffer capacities'' has been defined in ecology as a holistic concept (e.g., Integration of Ecosystem Theories: A Pattern, second ed. Kluwer, Dordrecht, 1997, 388pp), but we show that it can also be worked out in mechanistic studies. Our mechanistic approach highlights that ''buffering capacities'' can be depleted progressively, and, therefore, we make a distinction between current and potential ''buffering capacities''. We have applied this concept to understand the limited ''local stability'' in seagrass ecosystems and their vulnerability towards structural changes into macro-algal dominated communities. We explored the following processes and studied how they confer buffering capacities to the seagrass ecosystem: (i) net *Corresponding author. Fax: +33-5-5683-5104; : S 0 2 7 8 -4 3 4 3 ( 0 1 ) 0 0 0 4 0 -1 autotrophy is persistent in Zostera noltii meadows where plant assimilation acts as a sink for nutrients, this contrasted with the Ulva system that shifted back and forth between net autotrophy and net heterotrophy; (ii) the Z. noltii ecosystem possesses a certain albeit rather limited capacity to modify the balance between nitrogen fixation and denitrification, i.e., it was found that in situ nitrogen fixation always exceeded denitrification; (iii) the nitrogen demand of organoheterotrophic bacteria in the sediment results in nitrogen retention of N in the sediment and hence a buffer against release of nitrogen compounds from sediments, (iv) habitat diversification in seagrass meadows provides shelter for meiofauna and hence buffering against adverse conditions, (v) sedimentary iron provides a buffer against noxious sulfide (note: bacterial sulfide production is enhanced in anoxic sediment niches by increased organic matter loading). On the other hand, in the coastal system we studied, sedimentary iron appears less important as a redox-coupled buffer system against phosphate loading. This is because most inorganic phosphate is bound to calcium rather than to iron. In addition, our studies have highlighted the importance of plant-microbe interactions in the seagrass meadows. r
The degradation of an Ulva lactuca mat (0.2 kg dw m À2 ) was studied in a controlled flow-through... more The degradation of an Ulva lactuca mat (0.2 kg dw m À2 ) was studied in a controlled flow-through mesocosm for 31 d. Sediment chambers without U. lactuca served as controls. Fluxes of P CO 2 , O 2 , inorganic nitrogen, and urea were determined during the incubation period in addition to sulfate reduction rates, POC and PON content, enumeration of specific bacterial populations and evaluation of the physiological state of the added U. lactuca thalli. After U. lactuca addition to the chambers, there was an immediate increase in the efflux of P CO 2 from 11 to 27 mmol-C m À2 d À1 and a concomitant increase in O 2 uptake from 11 to 23 mmol m À2 d À1 . These effluxes remained elevated throughout the incubation period. In contrast, the NH 4 + efflux increased from 0.1 to 1.8 mmol NH 4 + m À2 d À1 during the first 3 d of incubation, followed by 6 d with a constant efflux rate, after which time it decreased gradually to 0.3 mmol NH 4 + m À2 d À1 by the end of the experiment. In total, NH 4 + accounted for 83% of the total nitrogen efflux after addition of U. lactuca. During the 31 d incubation period there was a continuous colonization of the thalli by bacteria. Sulfate reducers associated with the thalli accounted for 3% of the carbon oxidation on day 31. The molar C:N ratio in mineralization products (the ratio between the efflux of P CO 2 and NH 4 + + NO 2 À + NO 3 À ) increased from 15 mol mol À1 at day 11 after U. lactuca addition to >80 mol mol À1 by the end of the incubation. Since the C:N ratio in the mineralization products was much higher than the original thallus material (8.9 mol mol À1 ) it is probable that a preferential incorporation of NH 4 + into the increasing bacterial biomass occurred. The nitrogen for bacterial growth was most likely obtained from degradation of U. lactuca thalli as there was no stimulation of urea-N turnover in the sediment during incubation. The net increase in bacteria cell number in the 18-mm thick thallus layer was estimated to be 7.6 Â 10 9 to 2.4 Â 10 10 bacterial cells cm À3 . In contrast, the bacterial cell number remained constant in the ÀUlva incubations. #
Nitrogen loading to the Bassin d'Arcachon coastal lagoon (SW France) was evaluated by studying la... more Nitrogen loading to the Bassin d'Arcachon coastal lagoon (SW France) was evaluated by studying land-use and nitrogen output in its 3001 km2 catchment. At present, the catchment is dominated by forestry (79%), while intensive agriculture occupies 9% of the surface. The N-output of two hydrological subunits, i.e. the Tagon subunit dominated by pine forestry and the Arriou II subunit comprising both forestry and intensive agriculture, were monitored for a seven year period (1996-2002). From these observations it was calculated that forestry contributes on average 1.6 kg total N ha−1 yr−1, which is dominated by organic nitrogen (DON + PON are 70% of N). On an areal basis, intensive agriculture contributes 26 times more than forestry, i.e. 41.6 kg total N ha−1 yr−1, which is mainly in the form of nitrate (65% of N). These data were upscaled to the catchment and the upscaling was validated by comparison to gauged nitrogen throughputs for the catchment of the Leyre river that is the major tributary to the system. Taking into account the other known N sources and the interannual variability in the catchment it was estimated that nitrogen loading to the lagoon was on average 90 kg ha−1 yr−1 (range from 54 to 126 kg ha−1 yr−1). The sandy soils of the catchment have a clear potential for denitrification, but anoxic conditions (waterlogged) and input of organic matter to fuel this process are required. Currently, agricultural practices and spatial planning do not make use of this potential. Nitrogen loading in the Bassin d'Arcachon is reflected by 10-40 μM nitrate concentrations in winter, which became depleted during spring as a result of uptake by vegetation. Short-term uptake experiments showed that the macroalga Monostroma obscurum is well adapted to temperatures between 10 to 20 °C and competitive with respect to the seagrass Zostera noltii when the nitrate concentrations are above 10 μM. Spring conditions with high nitrate and high insolation are therefore favourable for M. obscurum and this species presents a high risk for algal blooming. In contrast, the macroalga Enteromorpha clathrata well adapted to summertime temperatures around 25 °C, forms occasionally blooms in the lagoon. This phenomenon is limited due to the low DIN concentrations in summer.
Nitrogen loading to the Bassin d'Arcachon coastal lagoon (SW France) was evaluated by studying la... more Nitrogen loading to the Bassin d'Arcachon coastal lagoon (SW France) was evaluated by studying land-use and nitrogen output in its 3001 km2 catchment. At present, the catchment is dominated by forestry (79%), while intensive agriculture occupies 9% of the surface. The N-output of two hydrological subunits, i.e. the Tagon subunit dominated by pine forestry and the Arriou II subunit comprising both forestry and intensive agriculture, were monitored for a seven year period (1996-2002). From these observations it was calculated that forestry contributes on average 1.6 kg total N ha−1 yr−1, which is dominated by organic nitrogen (DON + PON are 70% of N). On an areal basis, intensive agriculture contributes 26 times more than forestry, i.e. 41.6 kg total N ha−1 yr−1, which is mainly in the form of nitrate (65% of N). These data were upscaled to the catchment and the upscaling was validated by comparison to gauged nitrogen throughputs for the catchment of the Leyre river that is the major tributary to the system. Taking into account the other known N sources and the interannual variability in the catchment it was estimated that nitrogen loading to the lagoon was on average 90 kg ha−1 yr−1 (range from 54 to 126 kg ha−1 yr−1). The sandy soils of the catchment have a clear potential for denitrification, but anoxic conditions (waterlogged) and input of organic matter to fuel this process are required. Currently, agricultural practices and spatial planning do not make use of this potential. Nitrogen loading in the Bassin d'Arcachon is reflected by 10-40 μM nitrate concentrations in winter, which became depleted during spring as a result of uptake by vegetation. Short-term uptake experiments showed that the macroalga Monostroma obscurum is well adapted to temperatures between 10 to 20 °C and competitive with respect to the seagrass Zostera noltii when the nitrate concentrations are above 10 μM. Spring conditions with high nitrate and high insolation are therefore favourable for M. obscurum and this species presents a high risk for algal blooming. In contrast, the macroalga Enteromorpha clathrata well adapted to summertime temperatures around 25 °C, forms occasionally blooms in the lagoon. This phenomenon is limited due to the low DIN concentrations in summer.
A deterministic one-dimensional reaction diffusion model was constructed to simulate benthic stra... more A deterministic one-dimensional reaction diffusion model was constructed to simulate benthic stratification patterns and population dynamics of cyanobacteria, purple and colorless sulfur bacteria as found in marine microbial mats. The model involves the major biogeochemical processes of the sulfur cycle and includes growth metabolism and their kinetic parameters as described from laboratory experimentation.
ABSTRACT The amino acid composition of organic matrices associated with calcium carbonate precipi... more ABSTRACT The amino acid composition of organic matrices associated with calcium carbonate precipitates in microbialites built by different Phormidium species (cyanobacteria) has been compared for samples recovered in lagoonal settings from two regions of the Southern Tropical Pacific separated by more than 4000 km: New Caledonia (Nouméa lagoon) and French Polynesia (Tikehau atoll).
Nitrogen-fixation (acetylene reduction) rates were measured over an annual cycle in meadows of th... more Nitrogen-fixation (acetylene reduction) rates were measured over an annual cycle in meadows of the seagrass Zostera noltii Hornem in the Bassin d' Arcachon, south-west France, between March 1994 and February 1995, using both slurry and whole-core techniques. Measured rates using the slurry technique consistently overestimated those determined on whole cores, probably due to the release of labile organic carbon sources as a result of root damage during preparation of the slurries. Thus, the whole-core technique may provide a more accurate estimate of in situ activity, since disturbance of physicochemical gradients of oxygen, sulphide, nutrients and the relationship between the plant roots and the rhizosphere microflora is minimised. Rates measured by the whole-core method were 1.8-to 4-fold greater (dependent upon season) in the light than those measured during dark incubations, indicating that organic carbon diffusing from the plant roots during photosynthesis was an important factor in regulating nitrogen fixation in the rhizosphere. Additions of sodium molybdate, a specific inhibitor of sulphate-reducing bacteria (SRB) inhibited acetylenereduction activity by > 80% as measured by both the slurry and whole-core techniques throughout the year, inferring that SRB were the dominant component of the nitrogen-fixing microflora. A mutualistic relation-Communicated by J. P. Thorpe,
Changes in environmental conditions and prokaryote physiology can strongly affect the dynamics of... more Changes in environmental conditions and prokaryote physiology can strongly affect the dynamics of both the lysogenic and lytic bacteriophage replication cycles in aquatic systems. However, it remains unclear whether it is the nature, amplitude or frequency of these changes that alter the phage replication cycles. We performed an annual survey of three Mediterranean lagoons with contrasting levels of chlorophyll a concentration and salinity to explore how these cues and their variability influence either replication cycle. The lytic cycle was always detected and showed seasonal patterns, whereas the lysogenic cycle was often undetected and highly variable. The lytic cycle was influenced by environmental and prokaryotic physiological cues, increasing with concentrations of dissolved organic carbon, chlorophyll a, and the proportion of respiring cells, and decreasing with the proportion of damaged cells. In contrast, lysogeny was not explained by the magnitude of any environmental or physiological parameter, but increased with the amplitude of change in prokaryote physiology. Our study suggests that both cycles are regulated by distinct factors: the lytic cycle is dependent on environmental parameters and host physiology, while lysogeny is dependent on the variability of prokaryote physiology. This could lead to the contrasting patterns observed between both cycles in aquatic systems.
Buffer capacities'' has been defined in ecology as a holistic concept (e.g., Integration of Ecosy... more Buffer capacities'' has been defined in ecology as a holistic concept (e.g., Integration of Ecosystem Theories: A Pattern, second ed. Kluwer, Dordrecht, 1997, 388pp), but we show that it can also be worked out in mechanistic studies. Our mechanistic approach highlights that ''buffering capacities'' can be depleted progressively, and, therefore, we make a distinction between current and potential ''buffering capacities''. We have applied this concept to understand the limited ''local stability'' in seagrass ecosystems and their vulnerability towards structural changes into macro-algal dominated communities. We explored the following processes and studied how they confer buffering capacities to the seagrass ecosystem: (i) net *Corresponding author. Fax: +33-5-5683-5104; : S 0 2 7 8 -4 3 4 3 ( 0 1 ) 0 0 0 4 0 -1 autotrophy is persistent in Zostera noltii meadows where plant assimilation acts as a sink for nutrients, this contrasted with the Ulva system that shifted back and forth between net autotrophy and net heterotrophy; (ii) the Z. noltii ecosystem possesses a certain albeit rather limited capacity to modify the balance between nitrogen fixation and denitrification, i.e., it was found that in situ nitrogen fixation always exceeded denitrification; (iii) the nitrogen demand of organoheterotrophic bacteria in the sediment results in nitrogen retention of N in the sediment and hence a buffer against release of nitrogen compounds from sediments, (iv) habitat diversification in seagrass meadows provides shelter for meiofauna and hence buffering against adverse conditions, (v) sedimentary iron provides a buffer against noxious sulfide (note: bacterial sulfide production is enhanced in anoxic sediment niches by increased organic matter loading). On the other hand, in the coastal system we studied, sedimentary iron appears less important as a redox-coupled buffer system against phosphate loading. This is because most inorganic phosphate is bound to calcium rather than to iron. In addition, our studies have highlighted the importance of plant-microbe interactions in the seagrass meadows. r
The degradation of an Ulva lactuca mat (0.2 kg dw m À2 ) was studied in a controlled flow-through... more The degradation of an Ulva lactuca mat (0.2 kg dw m À2 ) was studied in a controlled flow-through mesocosm for 31 d. Sediment chambers without U. lactuca served as controls. Fluxes of P CO 2 , O 2 , inorganic nitrogen, and urea were determined during the incubation period in addition to sulfate reduction rates, POC and PON content, enumeration of specific bacterial populations and evaluation of the physiological state of the added U. lactuca thalli. After U. lactuca addition to the chambers, there was an immediate increase in the efflux of P CO 2 from 11 to 27 mmol-C m À2 d À1 and a concomitant increase in O 2 uptake from 11 to 23 mmol m À2 d À1 . These effluxes remained elevated throughout the incubation period. In contrast, the NH 4 + efflux increased from 0.1 to 1.8 mmol NH 4 + m À2 d À1 during the first 3 d of incubation, followed by 6 d with a constant efflux rate, after which time it decreased gradually to 0.3 mmol NH 4 + m À2 d À1 by the end of the experiment. In total, NH 4 + accounted for 83% of the total nitrogen efflux after addition of U. lactuca. During the 31 d incubation period there was a continuous colonization of the thalli by bacteria. Sulfate reducers associated with the thalli accounted for 3% of the carbon oxidation on day 31. The molar C:N ratio in mineralization products (the ratio between the efflux of P CO 2 and NH 4 + + NO 2 À + NO 3 À ) increased from 15 mol mol À1 at day 11 after U. lactuca addition to >80 mol mol À1 by the end of the incubation. Since the C:N ratio in the mineralization products was much higher than the original thallus material (8.9 mol mol À1 ) it is probable that a preferential incorporation of NH 4 + into the increasing bacterial biomass occurred. The nitrogen for bacterial growth was most likely obtained from degradation of U. lactuca thalli as there was no stimulation of urea-N turnover in the sediment during incubation. The net increase in bacteria cell number in the 18-mm thick thallus layer was estimated to be 7.6 Â 10 9 to 2.4 Â 10 10 bacterial cells cm À3 . In contrast, the bacterial cell number remained constant in the ÀUlva incubations. #
Uploads
Papers by R. Wit