Seawater samples were collected monthly in surface waters (2 and 5 m depths) of the Bay of Marseilles (northwestern Mediterranean Sea; 5°17'30" E, 43°14'30" N) during one year from November 2007 to ...December 2008 and studied for total organic carbon (TOC) as well as chromophoric dissolved organic matter (CDOM) optical properties (absorbance and fluorescence). The annual mean value of surface CDOM absorption coefficient at 350 nm aCDOM(350) was very low (0.10 ± 0.02 m−1) in comparison to values usually found in coastal waters, and no significant seasonal trend in aCDOM(350) could be determined. By contrast, the spectral slope of CDOM absorption (SCDOM) was significantly higher (0.023 ± 0.003 nm−1) in summer than in fall and winter periods (0.017 ± 0.002 nm−1), reflecting either CDOM photobleaching or production in surface waters during stratified sunny periods. The CDOM fluorescence, assessed through excitation emission matrices (EEMs), was dominated by protein-like component (peak T; 1.30–21.94 QSU) and marine humic-like component (peak M; 0.55–5.82 QSU), while terrestrial humic-like fluorescence (peak C; 0.34–2.99 QSU) remained very low. This reflected a dominance of relatively fresh material from biological origin within the CDOM fluorescent pool. At the end of summer, surface CDOM fluorescence was very low and strongly blue shifted, reinforcing the hypothesis of CDOM photobleaching. Our results suggested that unusual Rhône River plume eastward intrusion events might reach Marseilles Bay within 2–3 days and induce local phytoplankton blooms and subsequent fluorescent CDOM production (peaks M and T) without adding terrestrial fluorescence signatures (peaks C and A). Besides Rhône River plumes, mixing events of the entire water column injected relative aged (peaks C and M) CDOM from the bottom into the surface and thus appeared also as an important source of CDOM in surface waters of the Marseilles Bay. Therefore, the assessment of CDOM optical properties, within the hydrological context, pointed out several biotic (in situ biological production, biological production within Rhône River plumes) and abiotic (photobleaching, mixing) factors controlling CDOM transport, production and removal in this highly urbanized coastal area.
The 2009–2010 period was marked by an episode of intense drought known as the El Niño Modoki event. Sampling of the Términos Lagoon (Mexico) was carried out in November 2009 in order to understand ...the influence of these particular environmental conditions on organic matter fluxes within the lagoon's pelagic ecosystem and, more specifically, on the relationship between phyto- and bacterioplankton communities. The measurements presented here concern biogeochemical parameters (nutrients, dissolved and particulate organic matter POM, and dissolved polycyclic aromatic hydrocarbons PAHs), phytoplankton (biomass and photosynthesis), and bacteria (diversity and abundance, including PAH degradation bacteria and ectoenzymatic activities). During the studied period, the water column of the Términos Lagoon functioned globally as a sink and, more precisely, as a nitrogen assimilator. This was due to the high production of particulate and dissolved organic matter (DOM), even though exportation of autochthonous matter to the Gulf of Mexico was weak. We found that bottom-up control accounted for a large portion of the variability of phytoplankton productivity. Nitrogen and phosphorus stoichiometry mostly accounted for the heterogeneity in phytoplankton and free-living prokaryote distribution in the lagoon. In the eastern part, we found a clear decoupling between areas enriched in dissolved inorganic nitrogen near the Puerto Real coastal inlet and areas enriched in phosphate (PO4) near the Candelaria estuary. Such a decoupling limited the potential for primary production, resulting in an accumulation of dissolved organic carbon and nitrogen (DOC and DON, respectively) near the river mouths. In the western part of the lagoon, maximal phytoplankton development resulted from bacterial activity transforming particulate organic phosphorus (PP) and dissolved organic phosphorus (DOP) to available PO4 and the coupling between Palizada River inputs of nitrate (NO3) and PP. The Chumpan River contributed only marginally to PO4 inputs due to its very low contribution to overall river inputs. The highest dissolved total PAH concentrations were measured in the El Carmen Inlet, suggesting that the anthropogenic pollution of the zone is probably related to the oil-platform exploitation activities in the shallow waters of the southern of the Gulf of Mexico. We also found that a complex array of biogeochemical and phytoplanktonic parameters were the driving force behind the geographical distribution of bacterial community structure and activities. Finally, we showed that nutrients brought by the Palizada River supported an abundant bacterial community of PAH degraders, which are of significance in this important oil-production zone.
UV radiation (UVR) is a significant ecological factor in the marine environment that can have important effects on planktonic organisms and dissolved organic matter (DOM). The penetration of UVR into ...the water column is likely to change in the near future due to interactions between global warming and ozone depletion. In this study we report underwater instruments employed for the measurement of UVR and we review data dealing with the depth of UVR penetration in different oceanic areas including the open ocean, Antarctic waters and coastal waters. We provide the 10% irradiance depth (Z10%) for UV-A and UV-B as well as for DNA damage effective dose (DNA), which we calculated from the values of diffuse attenuation coefficients or vertical profiles reported in the literature. We observe a clear distinction between open ocean (high Z10%, no variation in the ratio UV-B/UV-A), Antarctic waters (increase in the ratio UV-B/UV-A during ozone hole conditions) and coastal waters (low Z10%, no variation in the ratio UV-B/UV-A). These variations in the penetration of UVR could lead to differences in the relative importance of photobiological/photochemical processes. We also compare in this study the penetration of UV-B (unweighted and weighted by the Setlow action spectrum) and DNA damage effective dose.
Diel variability of heterotrophic bacterial production (BP) was investigated in the eastern South Pacific from October to December 2004. Three sites differing by their trophic status were studied: ...Marquesas Islands, the center (GYR) and the eastern South Pacific Gyre. By using a Lagrangian approach and high frequency measurements, an important increase (2- to 4-fold) in BP was observed at the 3 sites during the afternoon–sunset period within surface layers. To evaluate the impact of solar UV radiation on this variability, we determined, fromin situoptical measurements, the mean UV-B (at 305 nm) and UV-A (at 380 nm) doses received within the mixed layer at a daily scale. At GYR, the doses were as high as 0.3 and 11 kJ m–2nm–1for the whole day, respectively due to high surface irradiances and very low light attenuations in the water column. The UV-B/UV-A tri-hourly dose ratios (Q) displayed substantial variations during the daytime, with highest values recorded during the periods 9:00 to 12:00 h or 12:00 to 15:00 h. The negative linear correlation observed betweenQand BP in the surface waters of GYR suggests that changes in the balance between DNA damages and photorepairs (reflected by changes in theQ-ratio) could have a significant influence on the diel variability of BP in open oceans. However, assessing the effects of UV radiation on diel variability of BP through anin situmeasurement approach, independently from other causes like availability of resources, is not so evident, even in these clearest waters of the world ocean.
The semi-enclosed nature of the Mediterranean Sea, together with its smaller inertia due to the relative short residence time of its water masses, make it highly reactive to external forcings, in ...particular variations of water, energy and matter fluxes at the interfaces. This region, which has been identified as a “hotspot” for climate change, is therefore expected to experience environmental impacts that are considerably greater than those in many other places around the world. These natural pressures interact with the increasing demographic and economic developments occurring heterogeneously in the coastal zone, making the Mediterranean even more sensitive. This review paper aims to provide a review of the state of current functioning and responses of Mediterranean marine biogeochemical cycles and ecosystems with respect to key natural and anthropogenic drivers and to consider the ecosystems’ responses to likely changes in physical, chemical and socio-economical forcings induced by global change and by growing anthropogenic pressure at the regional scale. The current knowledge on and expected changes due to single forcing (hydrodynamics, solar radiation, temperature and acidification, chemical contaminants) and combined forcing (nutrient sources and stoichiometry, extreme events) affecting the biogeochemical fluxes and ecosystem functioning are explored. Expected changes in biodiversity resulting from the combined action of the different forcings are proposed. Finally, modeling capabilities and necessity for modeling are presented. A synthesis of our current knowledge of expected changes is proposed, highlighting relevant questions for the future of the Mediterranean ecosystems that are current research priorities for the scientific community. Finally, we discuss how these priorities can be approached by national and international multi-disciplinary research, which should be implemented on several levels, including observational studies and modeling at different temporal and spatial scales.
► Distribution of AHs and PAHs from Marseilles coastal area, NW Mediterranean Sea. ► Enrichment factor of the SML (versus the SSW) reaching up to 1138 for particulate AHs. ► Marseilles coastal area ...was moderately contaminated from rather petrogenic sources. ► Rains increased hydrocarbon concentrations up to 11.5-fold in the dissolved phase.
Aliphatic (AHs) and polycyclic aromatic hydrocarbons (PAHs) were analyzed in dissolved and particulate material from surface microlayer (SML) and subsurface water (SSW) sampled at nearshore observation stations, sewage effluents and harbour sites from Marseilles coastal area (Northwestern Mediterranean) in 2009 and 2010. Dissolved and particulate AH concentrations ranged 0.05–0.41 and 0.04–4.3μgl−1 in the SSW, peaking up to 38 and 1366μgl−1 in the SML, respectively. Dissolved and particulate PAHs ranged 1.9–98 and 1.9–21ngl−1 in the SSW, amounting up 217 and 1597ngl−1 in the SML, respectively. In harbours, hydrocarbons were concentrated in the SML, with enrichment factors reaching 1138 for particulate AHs. Besides episodic dominance of biogenic and pyrogenic inputs, a moderate anthropisation from petrogenic sources dominated suggesting the impact of shipping traffic and surface runoffs on this urbanised area. Rainfalls increased hydrocarbon concentrations by a factor 1.9–11.5 in the dissolved phase.
Coastal sediments are considered to be final receptacles for organic and inorganic contaminants. Characterizing those sediments and assessing their biodegradation potential have become a great ...challenge in recent years. In this study, the chemical composition, including the content in polycyclic aromatic hydrocarbons, the microbial community abundance and diversity (using culture-independent approaches targeting 16S rRNA genes), and the aerobic/anaerobic biodegradation potential of coastal sediments collected in the Sfax coastal area (Gulf of Gabès, Southern Mediterranean Sea) were investigated. The highest concentration of total polycyclic aromatic hydrocarbons (981 µg kg
−1
dw) was recorded in Sidi Mansour harbor sediment, emphasized pyrogenic and petrogenic hydrocarbon sources. Organic matter, including total organic carbon, and the ultimate aerobic biodegradability, with 30% as the highest value in Sidi Salem channel sediment, were in a positive accordance with bacterial communities assigned within
Actinobacteria, Clostridia
and
Flavobacteria
classes. The correlation noticed between
Thermocladium
and
Thermogladius
genera and sulfate content explained that Sidi Mansour and PK4 sediments are located in terrestrial acid–sulfate areas. The highest cumulative methane produced with Marseille inoculum and Tunisian inoculum was recorded in Sidi Salem sediment and strongly correlated with methanogens among
Methanobacteria
,
Methanococci
and
Methanomicobia
classes showing the presence of industrial and municipal sources. The bioavailability of low and moderate polycyclic aromatic hydrocarbons in the current study may explain the occurrence of
Methanobacterium
which positively correlated with the anaerobic biodegradability using Tunisian inoculum with 50% as the highest value in Sidi Mansour sediment.
Model simulations and remote sensing observations show that ocean dynamics at fine scales (1–100 km in space, day–weeks in time) strongly influence the distribution of phytoplankton.
However, only a ...few in situ-based studies at fine scales have been performed, and most of them concern western boundary currents which may not be representative of less energetic regions.
The PROTEVSMED-SWOT cruise took place in the moderately energetic waters of the western Mediterranean Sea (WMS), in the region south of the Balearic Islands.
Taking advantage of near-real-time satellite information, we defined a sampling strategy in order to cross a frontal zone separating different water masses.
Multi-parametric in situ sensors mounted on the research vessel, on a towed vehicle and on an ocean glider were used to sample physical and biogeochemical variables at a high spatial resolution.
Particular attention was given to adapting the sampling route in order to estimate the vertical velocities in the frontal area also.
This strategy was successful in sampling quasi-synoptically an oceanic area characterized by the presence of a narrow front with an associated vertical circulation.
A multiparametric statistical analysis of the collected data identifies two water masses characterized by different abundances of several phytoplankton cytometric functional groups, as well as different concentrations of chlorophyll a and O2.
Here, we focus on moderately energetic fronts induced by fine-scale circulation.
Moreover, we explore physical–biological coupling in an oligotrophic region.
Our results show that the fronts induced by the fine-scale circulation, even if weaker than the fronts occurring in energetic and nutrient-rich boundary current systems, maintain nevertheless a strong structuring effect on the phytoplankton community by segregating different groups at the surface.
Since oligotrophic and moderately energetic regions are representative of a very large part of the world ocean, our results may have global significance when extrapolated.
The surface ocean receives important amounts of organic carbon
from atmospheric deposition. The degree of bioavailability of this source of
organic carbon will determine its impact on the marine ...carbon cycle. In this
study, the potential availability of dissolved organic carbon (DOC) leached
from both desert dust and anthropogenic aerosols to marine heterotrophic
bacteria was investigated. The experimental design was based on 16 d
incubations, in the dark, of a marine bacterial inoculum into artificial
seawater amended with water-soluble Saharan dust (D treatment) and
anthropogenic (A treatment) aerosols, so that the initial DOC concentration
was similar between treatments. Glucose-amended (G) and non-amended
(control) treatments were run in parallel. Over the incubation period, an
increase in bacterial abundance (BA) and bacterial production (BP) was
observed first in the G treatment, followed then by the D and finally A
treatments, with bacterial growth rates significantly higher in the G and D
treatments than the A treatment. Following this growth, maxima of BP reached
were similar in the D (879 ± 64 ng C L−1 h−1; n=3) and G (648 ± 156 ng C L−1 h−1; n=3) treatments and were significantly
higher than in the A treatment (124 ng C L−1 h−1; n=2). The DOC
consumed over the incubation period was similar in the A (9 µM;
n=2) and D (9 ± 2 µM; n=3) treatments and was significantly
lower than in the G treatment (22 ± 3 µM; n=3). Nevertheless,
the bacterial growth efficiency (BGE) in the D treatment (14.2 ± 5.5 %; n=3) compared well with the G treatment (7.6 ± 2 %;
n=3), suggesting that the metabolic use of the labile DOC fraction in both
conditions was energetically equivalent. In contrast, the BGE in the A treatment was lower (1.7 %; n=2), suggesting that most of the used labile
DOC was catabolized. The results obtained in this study highlight the
potential of aerosol organic matter to sustain the metabolism of marine
heterotrophs and stress the need to include this external source of organic
carbon in biogeochemical models for a better constraining of the carbon
budget.
We assessed the influence of the marine diazotrophic cyanobacterium
Trichodesmium on the bio-optical properties of western tropical South
Pacific (WTSP)
waters (18–22∘ S, 160∘ E–160∘ W) during
the ...February–March 2015 OUTPACE cruise. We performed measurements of
backscattering and absorption coefficients, irradiance, and radiance in the
euphotic zone with a Satlantic MicroPro free-fall profiler and took
Underwater Vision Profiler 5 (UPV5) pictures for counting the largest
Trichodesmium spp. colonies. Pigment concentrations were determined
by fluorimetry and high-performance liquid chromatography and picoplankton
abundance by flow cytometry. Trichome concentration was estimated from
pigment algorithms and validated by surface visual counts. The abundance of
large colonies counted by the UVP5 (maximum 7093 colonies m−3) was
well correlated to the trichome concentrations (maximum
2093 trichomes L−1) with an aggregation factor of 600. In the
Melanesian archipelago, a maximum of 4715 trichomes L−1 was enumerated
in pump samples (3.2 m) at 20∘ S, 167 30∘ E. High
Trichodesmium abundance was always associated with
absorption peaks of
mycosporine-like amino acids (330, 360 nm) and high particulate
backscattering, but not with high Chl a fluorescence or blue particulate
absorption (440 nm). Along the west-to-east transect, Trichodesmium
together with Prochlorococcus represented the major part of total
chlorophyll concentration; the contribution of other groups were relatively
small or negligible. The Trichodesmium contribution to total
chlorophyll concentration was the highest in the Melanesian archipelago
around New Caledonia and Vanuatu (60 %), progressively decreased to the
vicinity of the islands of Fiji (30 %), and reached a minimum in the
South Pacific Gyre where Prochlorococcus dominated chlorophyll
concentration. The contribution of Trichodesmium to zeaxanthin was
respectively 50, 40 and 20 % for these regions. During the OUTPACE
cruise, the relationship between normalized water-leaving radiance
(nLw) in the ultraviolet and visible and chlorophyll
concentration was similar to that found during the BIOSOPE cruise in the
eastern tropical Pacific. Principal component analysis (PCA) of OUTPACE data
showed that nLw at 305, 325, 340, 380, 412 and 440 nm was
strongly correlated to chlorophyll and zeaxanthin, while nLw at
490 and 565 nm exhibited lower correlations. These results, as well as
differences in the PCA of BIOSOPE data, indicated that nLw
variability in the greenish blue and yellowish green during OUTPACE was
influenced by other variables associated with Trichodesmium
presence, such as backscattering coefficient, phycoerythrin fluorescence
and/or zeaxanthin absorption, suggesting that Trichodesmium
detection should involve examination of nLw in this spectral
domain.
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