Alain Aminot

Fluorometric determination of ammonia in sea and estuarine waters by direct segmented flow analysis

Abstract A direct automated method for routine determination of ammonia in sea and estuarine waters has been developed using segmented flow analysis. The method, based on the reaction of ammonia with orthophtaldialdehyde (OPA) and sulfite, is easy to use, sensitive and highly reproducible. The combination of reagents makes it highly selective, thus eliminating the need for the gas diffusion cell commonly used to remove amino acid interference. Results on kinetics, on reagent composition and light effects and on pH and salinity changes are presented. The salt-effect does not exceed 3% in the 0–35 salinity range. Interference from primary amines is −1 ); specific recommendations are provided accordingly. Repeatability of 0.01 μmol l −1 was achieved in marine samples with ammonia concentrations ranging from 0.5 to 5 μmol l −1 . Using on-line dilution, ammonia determination can be performed up to 250 μmol l −1 and salt effect becomes negligible. Comparison of the present OPA-method with the indophenol blue method yielded fully comparable results.

Resistance of a coastal ecosystem to increasing eutrophic conditions: the Bay of Brest (France), a semi-enclosed zone of Western Europe

Abstract The Bay of Brest is a semi-enclosed coastal ecosystem receiving high nutrients loading from freshwater inputs. In order to analyse the response of phytoplankton stocks to increasing eutrophic conditions, a survey of the annual cycle of hydrographic properties, nutrients and chlorophyll a concentrations, and carbon uptake rates was performed at four stations in 1993. This database has been compared to earlier measurements performed during several comparable surveys within the last 20 years. As compared to the seventies, a doubled nitrate loading is now entering this ecosystem, which is related to increased agricultural activities on the drainage basins, while the geographical origin of the nitrate input has been modified. As a result of these anthropogenic modifications, summer averaged Si/N stoichiometric balance has decreased during the two last decades but, contrary to what has been observed in other coastal ecosystems, phytoplankton stocks have not increased. Several ecological factors have hindered eutrophication: the high hydrodynamic mixing with adjacent marine waters, caused by the macrotidal regime, induces important nutrients losses, temperature and mostly light limit primary production while Si and P high recycling maintain nitrogen limitation in this ecosystem. Conjunction of these non-anthropogenic factors explains the global stability of phytoplankton stocks.

Concept and determination of exchangeable phosphate in aquatic sediments

Exchangeable phosphate represents a reservoir of bioavailable phosphorus, since it can be rapidly released into a water body when the soluble phosphate concentration decreases. In the absence of a clear definition we first propose to precisely define exchangeable phosphate with reference to phosphate released in extreme conditions of solid dilution. A survey of the literature indicates that a variety of methods have been developed to provide its determination. The theoretical approach behind the corresponding release experiments is presented to support an evaluation of these methods with respect to the definition given. It appears that most are not based on the rigorous application of thermodynamic principles. Therefore, we have presented an infinite dilution extrapolation (IDE) approach, both rigorous and simple, enabling reliable comparison to be made. The method is based on extraction in natural water or a soluble substitute. The effect of pH was studied. Experimental conditions for use of the described method have been developed and various side applications are shown such as comparison of the extracting power of extractants. Results of application to estuarine sediments are briefly presented.

A flow injection-fluorometric method for the determination of ammonium in fresh and saline waters with a view to in situ analyses.

A version of the orthophthaldialdehyde-fluorescence ammonium determination for flow injection analysis (FIA) is presented here, with a view to its use for in situ, low-power consumption systems. Thus, the reaction temperature was limited to 30 degrees C and FIA was used in stop-flow mode (3 min stop). The calibration is linear up to 50 mumol l-1, but concentrations up to 100 mumol l-1 can be measured. Repeatability is around 1% in the range of 0.5-4 mumol l-1 and the detection limit is about 0.03 mumol l-1. Over the salinity range of 5-35 (seawater practical salinity scale) the salt effect is almost negligible (within +/- 2%); and below salinity of 5 it increases to a maximum of -9% in fresh water compared to seawater. Hydrogen carbonate, dissolved oxygen and turbidity (either suspended sediments or phytoplankton cells) have almost no adverse effect in a wide range of concentrations, covering most natural water conditions. Relative interference of primary amines is negligible and mercury (a common sample preservative) does not depress the signal up to 20 mg l-1 Hg II. Sulfide, that may be present in areas with anoxic waters, depresses the signal only slightly and linearly (-9% at 100 mumol l-1 S2-). The method appears to be convenient for the determination of ammonium in most coastal, estuarine and fresh waters. Sample throughput is 9 h-1. The performance of the method can be improved, either by increasing the reaction time (low throughput) or, if enough energy is available, by increasing the reaction temperature (non-stop-flow mode, high throughput). Combining FIA and fluorometry appears to be interesting for in situ determination (submersible devices) of dissolved compounds in environments with variable salinity and turbidity (especially coastal and estuarine waters).

Fate of natural and anthropogenic dissolved organic carbon in the macrotidal Elorn estuary (France)

Abstract Dissolved organic carbon (DOC) was studied, during two consecutive years, in the Elorn estuary (France) to assess its inputs and behaviour, and the possible processes, especially bacterial degradation, which may influence its fate in the estuary. Except in the case of sudden floods, riverine DOC concentration varied slightly (1.76–2.73 mg l −1 ) during the period of study and is apparently not related to river flow variations. In seawater, marked seasonal variations were observed with a minimum concentration (0.77 mg l −1 ) in March and maximum concentration in mid-summer (1.38 mg l −1 ). Because of industrial inputs (∼200 t yr −1 ), in the upper estuary, DOC concentrations are erratic and higher than those encountered in the river. The net flux of DOC to the Bay of Brest amounts to ∼600 t yr −1 ). The contribution of primary production to the DOC pool in the Bay of Brest is almost equivalent to the total input from the major land sources. It appeared that none of the endogenic additions or removal processes affected significantly the concentration of DOC during estuarine transport; this confirms the conservative behaviour. The difference between the net and gross fluxes are in good agreement with anthropogenic inputs. First-order degradation kinetic constants range from 0.006 to 0.011 day −1 for k 1 and from 0.0013 to 0.0036 day −1 for k 2 . Constants usually increase with decreasing salinity. Only one degradation step (one constant) is found for salinity above 27, whatever the season. Refractory DOC, remaining after 4–5 months incubation, accounts for up to 90% of DOC.

The 1994 QUASIMEME Laboratory Performance Study: Nutrients in seawater and standard solutions

Abstract The 1994 QUASIMEME (nutrients) Laboratory-Performance Study (LPS) is presented. The years programme consisted of two periods, December-May and June-November. (These are referred to as Round 2 and Round 3, respectively, the previous 1993 LPS being Round 1.) In each period, participating laboratories were sent two types of sample material, that is seawater samples and a standard solution containing a mixture of nutrient salts. The standard solution required dilution (by participants) and multiple analyses over several weeks in order to assess long-term repeatability. In Round 2, a few laboratories, identified as poor performers in Round 1 (1993), undertook a ‘learning programme’ in which they received, unknowingly, samples identical to those analysed in Round 1. The other laboratories received samples different from those used in Round 1. In Round 3, all laboratories received the same package of samples, different from those used in Round 1 and Round 2. The results clearly indicate that general all-round improvements in data quality have been achieved.

Reference material for nutrients in seawater: stability of nitrate, nitrite, ammonia and phosphate in autoclaved samples

Reference material is increasingly used as a basis for quality control and quality assurance in general. Results of autoclaving applied to produce reference material for nutrients in seawater are presented and discussed. When preliminary experiments had shown satisfactory behavior of nitrate, nitrite and ammonia when autoclaved together in seawater samples, a 27 month experiment was undertaken with these three nutrients and phosphate at three concentration levels. Autoclaving was therefore used to stabilize the samples for the ICES 5th intercomparison exercise and checked over a 19 month period. Nitrogenous nutrients exhibited the greatest inter-sample homogeneity. Among them, nitrate and nitrite were extremely stable throughout the whole experiment with overall standard deviations lower than 0.3% (range 5–50 μmol l−1) and 0.8% (range 0.5–5 μmol l−1), respectively. For these two nutrients, autoclaving appears to be a prime method for production of reference material in seawater. Ammonia exhibited a slight increase over time, up to 0.05–0.07 μmol l−1 per year, attributed to air contamination when samples were insufficiently protected from the laboratory atmosphere. Phosphate increased by 0.02–0.07 μmol l−1 per year due to leaching from the container glass. Despite these slight adverse effects, autoclaving may also be used for ammonia and phosphate with shorter periods of storage under adequate, controlled conditions. The phosphate problem, not pertaining to the stabilization process itself, can probably be solved by using a more inert container.

Autoclaved sea water as a reference material for the determination of nitrate and phosphate in sea water

Abstract A method for the preparation of a stable and reliable reference material for nutrients in sea water was developed, using autoclaving as a stabilization process. Attention was focused on nitrate and phosphate and the effect of autoclaving on their concentration was studied. The precipitation and redissolution of phosphate salt was overcome with an appropriate preacidification of the water to be autoclaved. Homogeneity and stability were checked for up to 14 months together with any possible adverse effect from light or cap contact with the sample. The reproducibility of the concentrations throughout the bulk of samples and as a function of time is greater than the currently stated precision of the determination for either nutrient. Such samples satisfy the requirements to be met by a reference material for nitrate and phosphate in sea water.

Model compounds for the determination of organic and total phosphorus dissolved in natural waters

Abstract The choice of model compounds for checking the recovery of dissolved organic and total dissolved phosphorus in natural waters is discussed. Ten compounds were compared for their recovery in fresh and seawater. Four different oxidative procedures were used: acidic and alkaline persulfate oxidation, continuous flow UV irradiation and high-temperature combustion (HTC). Certain compounds have been shown to decompose when heated for moisture removal. Due to impurities and moisture, HTC is therefore suggested as a reference method for model compounds, assuming 100% recovery of organic phosphorus as phosphate. Full recovery is to be expected for the compounds in fresh water, but not for seawater. Matrix effects vary according to the oxidative method, with recoveries as low as 50% or less for certain compounds. It is therefore recommended that several model compounds should be used, at least one labile (phosphoenolpyruvate, glycerophosphate or riboflavin-5-phosphate) and one refractory (aminoethylphosphonic acid, phytic acid or phosphorylcholine chloride). The alkaline persulfate method can be recommended for marine waters.

An automated photo-oxidation method for the determination of dissolved organic phosphorus in marine and fresh water

A segmented flow automated method with on-line photo-oxidation for the determination of dissolved organic phosphorus + soluble reactive phosphorus (DOP + SRP) in seawater and fresh water is described here. A low-power lamp was used for a compact, easy-to-handle and low-ozone-producing manifold. The influence of seawater matrix components was studied in detail using natural seawater and salt solutions spiked with DOP model compounds. Bromide was found to be the most inhibitory species in seawater. The work shows that most salt solutions referred to as artificial seawater are not satisfactory model matrices to test the actual seawater matrix effect. Since DOP recovered in undiluted seawater samples was about half that obtained in fresh water samples, the described method includes a 5- to 6-fold dilution of seawater samples. This simple procedure overcomes matrix effects and provides satisfactory DOP recovery. No pH effect was found in the 6-9 range corresponding to most natural waters. The standard deviation was 0.007 mu mol l(-1) and the limit of detection 0.02 mu mol l(-1). Linearity was tested up to 5 mu mol l(-1) of DOP, i.e. far above naturally occurring values. A throughput of 20 samples per hour is easily achieved