SEASONAL VARIABILITY AND CHARACTERIZATION OF THE DISSOLVED ORGANIC POOL (CDOM/FDOM) AT AN OFFSHORE STATION (NW MEDITERRANEAN SEA)

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Optical measurements of CDOM

Samples water for absorption and fluorescence of CDOM were taken directly from the Niskin bottle in glass flasks of 250 ml, previously washed and precombusted (24 h at 450 0C) to avoid any contamination. The samples were filtered by gravity onto Whatman GF/F filters (porosity 0.7 μm), preserved in the dark and frozen at -20 0C until analysis (Hancke et al., 2014). FDOM samples were analyzed no later than three months after collection, following the methodology described by Nieto-Cid et al. (2006). CDOM absorption was measured in 10 cm quartz cuvettes using a Varian Cary UV-VIS spectrophotometer equipped with a 10 cm quartz cell. Absorbance was performed between 250 and 750 at a constant room temperature of 200C. Milli-Q water was used as blank. The residual backscattering (colloidal material, fine size particle fractions present in the sample) was corrected by subtracting the mean absorbance calculated in the spectral range 600-750 nm. The absorption coefficient (aCDOM(λ) in m-1), was calculated as: aCDOM (λ)=2.303A(λ250-700)/ L Where Abs (λ) is the absorbance at wavelength λ, and l is the optical path length in m and 2.303 is the factor that transforms natural logarithms to decimal logarithms. The spectral slope was calculated over wavelength range (S250-500 and S275-295) using linear regressions of the natural log-transformed aCDOM(λ) according to Nelson et al. (2004) and Helms et al. (2008).

Physical drivers and annual variability of the measured Parameters

In Figure 2 we show the monthly average of the different environmental parameters monitored from 2000 to 2012 together with the discrete values obtained during the period detailed in this study (February 2013 to April 2014). Temperature followed a clear annual cycle: minimum values occurred in February (9.4 and 12.7 ºC for 2013 and 2014, respectively) and the maxima (24 ºC) were registered in August (Fig. 2a). Salinity ranged from 34.28 to 38.5 and showed two minima, the first one in early spring (March 2013) and the second one in early winter (January 2014) (Fig. 2b). These minima (34.7 and 34.28, respectively) were exceptionally low with regard to the average for the last 15 years (Fig. 2b), and coincided with FDOM maxima (Fig. 3c).
Inorganic nutrient concentrations were always low in summer and high in winter and spring. Nitrate concentrations ranged from 0.2 μmol L-1 to 9.2 μmol L-1 (Fig. 2c), with values > 5 μmol L-1 in early-mid winter (January, 2013 and March, 2014). Ammonium concentration ranged between 0.01 μmol L-1 and 0.64 μmol L-1 (Fig. 2d) showing remarkably low values in late spring and autumn 2013 (< 0.1 μmol L-1). Phosphate concentration was low (<0.2 μmol L-1) throughout the time series (Fig. 2e). Silicate presented two maxima in March 2013 and January 2014 (9.9 μmol L-1 and 11 μmol L-1 respectively) (Fig. 2f) coinciding with the salinity minima and with high nitrate values. The N:P (mol:mol) ratio of inorganic dissolved fraction varied from 1 to 130, with values below 16 typically occurring in the summer. Total chlorophyll (Chl a) ranged from 0.05 μg L-1 to 4.39 μg L-1 (Fig. 3a). During the period studied we found two peaks (February, 2013 and January, 2014) with concentrations of 4 μg L-1 and 4.39 μg L-1, respectively. The winter chlorophyll peak in 2013 followed a NH4 + maximum, while the 2014 peak came after a salinity minimum. By contrast, during summer and spring, Chl a was rather low, reaching values of < 0.6 μg L-1.

Temporal variability in the Bay of Banyuls-sur-mer

The Bay of Banyuls-sur-mer was characterized by a well-marked seasonal variability, as previously described by other authors (Marty et al., 2002, Estournel et al., 2003, Grémare et al., 2003). The annual changes on temperature and wind intensity drive the formation and the erosion of the thermocline, this together with the seasonal dynamics of river discharges determined the temporal changes of the biogeochemical variables examined. Two sharp decreases of salinity were observed
in the winter (March 11, 2013 and January, 13, 2014). Coinciding with the salinity minimum of 2013 we observed high water discharges from the Têt and Tech Rivers (≈ 40 m3s-1 each), and also an exceptionally high level of the Baullaury river (around 200 m3s-1). Nutrient dynamics was also strongly influenced by these discharges, the nitrate and silicate peaks coincided with the salinity minima and these variables were significantly negative correlated with salinity (R2 = -0.75, p-value < 0.05).
Two maxima of chlorophyll were observed during the study period (25 February 2013 and 29 January 2014). The 2014 maximum occurred after a minimum of salinity; the rapid increase of chlorophyll could be thus the response to the nutrient enrichment produced by freshwater discharge (Fig. 3a). In contrast, the 2013 peak in February 25 occurred just before the salinity drop and coincided with a relative maximum of NH4 +. This ammonia peak could have been originated from a sediment resuspension process caused by the strong wind observed those days (19 m s-1). High waves and swell have also been reported as a cause for sediment resuspension at the SOLA station (Guizien et al., 2007). In fact, coinciding with this chlorophyll peak we found a relative minimum of temperature. Other authors related these peaks to the development of convective mixing (Béthoux & Prieur, 1983) that typically occurs in winter (Marty et al., 2002). Regardless of the cause of the increase of ammonia, this increase could have induced the phytoplankton bloom in March 2013 where diatoms reached also high abundances (2.0 to 4.0 ·104 cell L-1, data not shown). We compare our results with climatological and environmental data collected from 2000 to 2012 in the same station. We found that our results, in general, varied within the range of the values obtained in the past 15 years, there were, however, two salinity outliers. The annual salinity minima found in our study were also the minima for the last 15 years time series. Considering that these minima are associated to high nutrient concentration, it would be of interest to follow those phenomena in the future to check if they are sporadic events or correspond to a shift in meteorological trends.
A proper understanding of such variability will help to better simulate future scenarios and to predict biological production in coastal areas. It is also noticeable that the ammonia concentration showed values always below the mean of the past 15 years, while the phosphate values tended to be above the average. This change of tendency in nutrient dynamics may be due to local processes as it did not occurred in other close areas of the NW Mediterranean (BBMO, http://www.icm.csic.es/bio/projects/icmicrobis/bbmo/).

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Temporal mismatch between chlorophyll and organic matter

In order to evaluate the importance of phototrophic components in relation to the total biomass we calculate the proportion of particulate organic carbon respect to the chlorophyll concentration (POC/Chla), which can be considered a proxy for estimating the degree of heterotrophy in a system, at least for a comparative usage in the time series. In Figure 7 we plotted the POC/Chla ratio together with the ratio of total inorganic nitrogen respect to phosphate concentration (N:P, mol:mol). A clear seasonality of both variables can be observed with high values of POC/Chla in summer. The quotients DOC/Chla and (POC+DOC)/Chla exhibited the same pattern (data not shown). Altogether indicating a higher degree of heterotrophy in summer as has been found in other Mediterranean stations (e.g. Alonso-Sáez et al., 2008).
These maxima in summer coincide with the minima of N:P values. The higher proportion of particulate organic carbon in relation to chlorophyll in summer could be due, in part, to the use of non-limiting substrates by the osmotrophs to increase in cell size, this mechanism has been proposed by several authors, e.g. Malits et al. (2004) for bacteria and Thingstad et al. (2005) for osmotrophs in general.

Table of contents :

REMERCIEMENTS
PREFACE
GLOSSARY OF RELEVENT TERMS
CHAPTER I. INTRODUCTION AND OBJECTIVES OF THESIS
1. INTRODUCTION
1.1. Marine dissolved organic matter in the ocean
1.2. Marine dissolved organic matter in the carbon cycle
2. OPTICALLY ACTIVE FRACTIONS OF DOM
2.1. Fundamentals of absorption and fluorescence of DOM
2.2 . Absorption of CDOM
2.3. Fluorescence of DOM
3. DISTRIBUTION, SINKS AND SOURCES OF CDOM
3.1. Distribution in the ocean
3.2. Sinks and sources of CDOM
4. STUDY AREA (MEDITERRANEAN SEA)
4.1. General context of the Mediterranean Sea
4.2 Nutrients inputs into the Mediterranean Sea
4.3. Stations studied
5. OBJECTIVES OF THESIS
REFERENCES
CHAPTER II. COUPLED AND MISMATCHED TEMPORAL PATTERNS OF ORGANIC AND INORGANIC NUTRIENTS IN A NW MEDITERRANEAN COASTAL STATION
ABSTRACT
1. INTRODUCTION
2. MATERIAL AND METHODS
2.1. Site and sample collection
2.2. Chemical and biological analyses
2.3. Optical measurements of CDOM
3. RESULTS
3.1. Physical drivers and annual variability of the measured Parameters
3.2. DOM colored fractions
3.3. Optical indexes
3.4. Variability trends
4. DISCUSSION
4.1. Temporal variability in the Bay of Banyuls-sur-mer
4.2. Variability at different time scales
4.3. Temporal mismatch between chlorophyll and organic matter
4.4. Seasonal dynamics of CDOM fractions
4.5. Hypothetical conceptual framework
5. CONCLUSIONS
ACKNOWLEDGEMENTS
REFERENCES
CHAPTER III. SEASONAL VARIABILITY AND CHARACTERIZATION OF THE DISSOLVED ORGANIC POOL (CDOM/FDOM) AT AN OFFSHORE STATION (NW MEDITERRANEAN SEA)
ABSTRACT
1. INTRODUCTION
2. MATERIAL AND METHODS
2.1. Sampling strategy
2.2. PHYSICAL AND CHEMICAL PARAMETERS
2.2.1. Nutrients analyses
2.2.2. Total chlorophyll a
2.2.3. Dissolved organic carbon analyses
3. OPTICAL PROPERTIES OF CDOM
3.1. Absorption measurements
3.2. Fluorescence measurements
4. STATISTIC AND GRAPHIC TOOLS
5. RESULTS
5.1. Hydrological conditions
5.2. Nutrients and phytoplankton
5.3. Variability of the DOM stock and composition in 2013 and 2014 .
5.3.1. Dissolved organic carbon distribution
5.3.2. FDOM and CDOM variability in 2013 and 2014
6. DISCUSSION
6.1. Deep versus weak winter convection. Nutrient and phytoplankton distributions
6.2. Seasonal patterns of DOC dynamics
6.3. Distribution of CDOM and FDOM
6.4. Optical characterization of CDOM
7. CONCLUSIONS
REFERENCES
CHAPTER IV. DUST INPUTS AFFECT THE OPTICAL SIGNATURES OF DISSOLVED ORGANIC MATTER IN NW MEDITERRANEAN COASTAL WATERS
ABSTRACT
1. INTRODUCTION
2. MATERIALS AND METHODS
2.1. TIME SERIES SAMPLING
2.2. AEROSOLS COLLECTION FOR EXPERIMENTS
2.3. WATER SAMPLING AND EXPERIMENTAL DESIGN
2.4. Analytical procedures
2.4.1. CDOM measurements
2.4.2. FDOM measurements
2.4.3. DOC analysis
2.4.4. Prokaryotic abundance and chlorophyll a determination
3. RESULTS
3.1. FDOM TIME SERIES
3.2. MICROCOSMS EXPERIMENTS
3.2.1. PROKARYOTIC ABUNDANCE AND CHLOROPHYLL A
3.2.2. DOC and FDOM dynamics
4. DISCUSSION
4.1. ATMOSPHERIC DEPOSITION INFLUENCE IN SURFACE WATERS
4.2. EFFECTS OF AEROSOLS ADDITIONS ON THE PROKARYOTIC ABUNDANCES .
4.3. DOM OPTICAL PROPERTIES TRANSFORMATIONS AFTER DUST ADDITIONS .
5. SUMMARY
ACKNOWLEDGEMENTS
REFERENCES
CHAPTER V. CONCLUSIONS GÉNÉRALES ET PERSPECTIVES
1. CONCLUSIONS
2. PERSPECTIVES
ANNEXES
RÉSUMÉ
ABSTRACT

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