Aeolian inputs of organic and inorganic nutrients to the ocean are important as they can enhance biological production in surface waters, especially in oligotrophic areas like the Mediterranean. The Mediterranean littoral is particularly exposed to both anthropogenic and Saharan aerosol depositions on a more or less regular basis. During the last few decades experimental studies have been devoted to examining the effect of inorganic nutrient inputs from dust on microbial activity. In this study, we performed experiments at two different locations of the NW Mediterranean, where we evaluated the changes in the quality and quantity of dissolved organic matter due to atmospheric inputs of different origin (Saharan and anthropogenic) and its subsequent transformations mediated by microbial activities. In both experiments the humic-like and protein-like substances, and the fluorescence quantum yield increased after addition. In general, these changes in the quality of dissolved organic matter did not significantly affect the prokaryotes. The recalcitrant character of the fluorescent dissolved organic matter (FDOM) associated with aerosols was confirmed, as we found negligible utilization of chromophoric compounds over the experimental period. We framed these experiments within a two-year time series data set of atmospheric deposition and coastal surface water analyses. These observations showed that both Saharan and anthropogenic inputs induced changes in the quality of organic matter, increasing the proportion of FDOM substances. This increase was larger during Saharan dust events than in the absence of Saharan influence.
Los aportes atmosféricos de nutrientes orgánicos e inorgánicos al océano son importantes ya que pueden aumentar la producción biológica en aguas superficiales, especialmente en las zonas oligotróficas como el Mediterráneo. El litoral del Mediterráneo está particularmente expuesto a aportes de origen antropogénico y a deposiciones de polvo sahariano de forma más o menos regular. Durante las últimas décadas los estudios experimentales se han dedicado, sobre todo, a examinar el efecto de la entrada de nutrientes inorgánicos atmosféricos sobre la actividad microbiana. En este estudio, se realizaron experimentos con comunidades microbianas procedentes de dos zonas del Mediterráneo noroccidental. Se evaluaron los cambios en la calidad y cantidad de la materia orgánica disuelta debido a aportes atmosféricos de distinto origen y sus posteriores transformaciones mediadas por actividades microbianas. En ambos experimentos las sustancias orgánicas fluorescentes y el rendimiento cuántico de fluorescencia aumentaron después de la adición de material atmosférico. En general, estos cambios en la calidad de la materia orgánica no afectaron significativamente a los organismos procariotas. El carácter recalcitrante de la materia orgánica disuelta fluorescente (FDOM) contenida en los aerosoles se confirmó ya que la utilización de compuestos cromóforos durante el período experimental fue insignificante. Los resultados obtenidos se contextualizan en relación con una serie temporal de dos años de datos adquiridos de deposición atmosférica y análisis de agua superficial costera. La variabilidad temporal de estas dos variables mostró que tanto los aportes saharianos como antropogénicos provocaron cambios en la calidad de la materia orgánica disuelta en aguas superficiales, incrementando la fracción fluorescente. Éste aumento resultó ser mayor durante eventos de polvo sahariano que en ausencia de ellos.
The Mediterranean Sea, due to its low nutrient and chlorophyll concentration, is considered one of the most oligotrophic marine systems (
Large Saharan dust transport events over the Mediterranean Sea occur commonly in spring and summer (
The optical properties of CDOM are sensitive to biological and physical processes and thus provide valuable information not only of the biogeochemical processes in aquatic environments, but also of the origin of organic matter (OM).
We collected samples for atmospheric deposition and seawater analyses over a two-year period (September 2012 - July 2014). For atmospheric deposition, one high-density polyethylene (HDPE) container was filled with 500 mL of sterile artificial seawater and left open on the roof of the Institute of Marine Sciences (ICM-CSIC, Barcelona, 41°23′08″N, 2º11′45.5″E) for about one week in summer and two in winter. Upon collection, subsamples for FDOM were analysed after filtering them through Whatman GF/F filters. The fluorescence intensities measured for sterile seawater at time 0 were subtracted from those measured at the end of the exposure period. Seawater samples were taken monthly 0.5 km offshore of Barcelona (NW Mediterranean, 41°22′55″N, 2°11′58″E). Surface water was collected in 2-L acid-cleaned polycarbonate bottles and subsamples for FDOM were analysed freshly.
The aerosols used in the experiments were collected on Munktell quartz filters (quality 360) using an MCV CAV-A/mb high-volume air sampler. The sampler operated for 24 h at 30 m3 h–1. Filter samples for experimental amendments were obtained at different times in January and March 2014 on the roof of the Institute of Marine Sciences in Barcelona (41°23′08″N, 2°11′45.5″E) and on the roof of the Centre for Advanced Studies of Blanes (CEAB, Blanes, 41°40′59.5″N, 2°48′2.6″E). After collection, half of the filters were used for chemical analyses and the other half were employed for the amendment experiments. Collected aerosols tend to be a mix from different sources. The aerosols were classified according to the relative percentage of Saharan dust versus inputs of anthropogenic origin with previous knowledge of the presence of Saharan events based on transport and deposition models and forecasts (
Blanes (BLSp) | Barcelona (BCNSp) | |||
---|---|---|---|---|
A | S | A | S | |
OC | 31.95% | 4.93% | 26.38% | 6.75% |
SiO2 | 4.75% | 40.64% | 13.56% | 27.88% |
Al2O3 | 1.58% | 13.55% | 4.51% | 9.29% |
NO3- | 11.01% | 2.48% | 7.81% | 2.11% |
NH4+ | 2.12% | 0.37% | 1.47% | 0.52% |
P | 0.10% | 0.08% | 0.13% | 0.07% |
Our experiments were conducted with water from two locations that differed in the degree of oligotrophy. The water was collected at the Blanes Bay Microbial Observatory (41°40′0″N, 2°48′0″E) on 8 April 2014 and on the Barcelona coast (41°22′55″N, 2°11′58″E) on 12 May 2014. Blanes Bay Microbial Observatory is characterized as an oligotrophic area with an annual mean of 0.63±0.05 μg L–1 of chlorophyll (
The experiments were termed BLSp and BCNSp for Blanes and Barcelona, respectively. For both, the water was collected from the surface layer (approximately 0.5 m depth) and pre-filtered through a 150-μm nylon mesh to remove the larger zooplankton. The water was then transported to the laboratory in 50-L carboys, which had previously been washed with a dilute solution of sodium hypochlorite and exhaustively rinsed with Milli-Q water and sample water.
In the laboratory, the water was distributed in 15-L cylindrical methacrylate containers, which were subjected to experimental conditions in a light and temperature controlled chamber for 7 days for BLSp and for 5 days for BCNSp. Conditions, in duplicate, were anthropogenic particles enrichment (A), Saharan dust enrichment (S) and control (C) without enrichment. Aerosol concentration added in each container was 0.8 mg L–1. Light conditions were set to 225 μmol photons m–2 s–1 inside the containers and the light:dark cycle (13 h:11 h) and temperature (17.5°C) were adjusted to natural conditions. After placing the containers in the experimental chamber, we left them for acclimation before starting the experiment. Because an in situ Saharan event occurred the day before BLSp water collection, we increased the acclimation period (it lasted 45 hours in BLSp in contrast to 19 hours in BCNSp) to prevent the experimental treatment from being masked by a possible response to the in situ input that occurred in the field. An initial sample was taken and aerosols were subsequently added as single doses.
Samples for DOM analyses (CDOM, FDOM and dissolved organic carbon (DOC)) were filtered by glass fibre Whatman GF/F (combusted at 450°C for 4 hours) prior to analysis. Samples were taken at 0, 4, 49, 97, and 144 h for BLSp and at 0, 4, 49 and 97 h for BCNSp. Samples for chlorophyll
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 nm at a constant room temperature of 20°C. 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)=2.303 A(λ) / L
where A is the absorbance at wavelength λ, L is the optical path length in m, and 2.303 is the factor that transforms decimal logarithms to natural logarithms.
The samples for FDOM were measured immediately after temperature acclimation according to
Finally, the fluorescence quantum yield at 340 nm, defined as the portion of light absorbed at 340 nm that is re-emitted as fluorescence [Φ(340)], was determined using the ratio of the absorption coefficient at 340 nm and the corresponding fluorescence emission between 400 and 600 nm of the water sample and referred to the quinine sulphate standard (QS) ratio (
where aCDOM(340)QS is the absorption coefficient of the QS standard at 340 nm (in m–1); F(400-600) and F(400-600)QS are the average integrated fluorescence spectra between 400 and 600 nm at a fixed excitation wavelength of 340 nm (in QS units) obtained for each sample and the QS standard, respectively (
Samples for DOC were filtered through Whatman GF/F filters using an acid-cleaned glass filtration system. Approximately 10 mL of water was collected in pre-combusted (450°C for 12 h) glass flasks for DOC determination. After acidification with H3PO4 (50 μL) to pH<2 the ampoules were heat-sealed and stored in the dark until analysis. DOC was analysed following the high temperature catalytic oxidation (HTCO) technique (
Heterotrophic prokaryotic cells were quantified by flow cytometry according to the method of
For total Chl
In order to evaluate the potential role of atmospheric deposition on the dynamics of coastal FDOM, we calculated its proportion with respect to the in situ seawater concentration for 23 months (September 2012 to July 2014). High contributions to the DOC pool could be found at all times of the year, although the highest was in summer (July 2013). The results revealed that the deposition of humic-like compounds (peak C and M) and protein-like compounds contributed to an increase in FDOM in surface waters that represented between 0.2% and 3% per m3 per day (
The initial abundance of heterotrophic prokaryotic cells was 8.06×105 cells mL–1 in BLSp and 1.34×106 cells mL–1 in BCNSp (
During BLSp, Chl
Aerosols did not significantly alter the DOC concentration (
We compared the FDOM matrices before addition (C) with the changes in FDOM that occurred in each experiment after the addition (
Our two-year data set on deposition constitutes the first time series that evidences the atmospheric impact on FDOM dynamics in Mediterranean surface waters. Interestingly, several of the highest FDOM deposition values coincided with the Saharan dust events (
Previous studies have shown that prokaryotic abundances increased in response to Saharan dust inputs in oligotrophic systems (
In BLSp, the humic-like fractions of OM increased after the addition of aerosols, and this increase was more conspicuous in treatment A than in treatment S. In fact, peak C/DOC ratios in QSU/µmol C L–1 were 153% higher in A and 33% higher in S than in the control. However, FDOM compounds showed no variation during the incubation time. In BCNSp, FDOM values were also higher after the addition. However the increases in FDOM/DOC ratio after additions did not differ significantly between A and S. In both BLSp and BCNSp, we observed that DOC tended to increase in all treatments during the time of incubation. This increase was larger in BCNSp than in BLSp, which is in accordance with a high activity of phytoplankton (
Regarding the fluorescence quantum yield at 340 nm [Φ(340)], we observed a similar increase in both experiments after the addition was performed, reaching values of about 0.65% (
A two-year time series data shows, for the first time, the influence of atmospheric deposition on the temporal dynamics of FDOM in Mediterranean surface waters. This time series data evidences an increase in the FDOM flux during Saharan events. Our experimental results revealed that aerosol deposition induced an increase in the proportion of FDOM in comparison with DOC. The refractory character of the OM added with aerosols was confirmed from the negligible utilization of this fraction within a short time period (days). Thus, considering our in situ results showing how dust deposition increases the coloured DOC content in surface waters, together with the experimental findings that corroborate the low biological utilization of this coloured fraction, we conclude that the atmospheric deposition could help to explain the exceptionally high CDOM/ chlorophyll values found in the Mediterranean Sea.
We thank Raquel Gutiérrez for her help in the processing of data from atmospheric deposition. We also thank J. Caparros for the analysis of dissolved organic carbon. This study was supported by the projects ADEPT (CTM2011-23458), DOREMI (CTM2012-342949) and ANIMA (CTM2015-65720-R MINECO/FEDER, UE). E.D. Sánchez-Pérez would like to thank the Consejo Nacional de Ciencia y Tecnologia (CONACyT) for their financial support through a PhD fellowship. I. Marín thanks the FPI Spanish scholarship programme for its support (BES 2012-052976).