Sediment Diatom Assemblages Research Articles

Diatom Assemblages in Surface Sediments Along Nutrient and Salinity Gradients of Thi Vai Estuary and Can Gio Mangrove Forest, Southern Vietnam

In order to assess the current ecological status of the tidal Thi Vai Estuary and the Can Gio Mangrove Forest in Southern Vietnam, we evaluated the spatial diatom composition and abundance in surface sediments and their relation to physical and chemical parameters of the water. Major changes in the distribution of diatom assemblages were determined by multivariate statistical analyses. Principal component analysis (PCA) was used to ordinate environmental variables of the water, showing a clear gradient along the estuary, with higher nutrient levels (total phosphorus and total nitrogen) and lower dissolved oxygen (DO) concentrations at upstream sites. Canonical correspondence analysis (CCA) was used to determine the species-environmental relationship. Total phosphorus (TP) and DO (p < 0.001) as well as salinity and pH (p < 0.002) were the most significant variables related to diatom composition. The neritic planktonic species Thalassionema nitzschioides (Grunow) Mereschkowsky was the most widespread and abundant species in the data set, presumably also reflecting significant influence of oceanic water. Thalassiosira cedarkeyensis Prasad, a marine benthic species, was the second most abundant species and seems to be related to higher nutrient levels, particularly at upstream sites. High abundances of Cyclotella meneghiniana Kutzing and Cyclotella atomus Hustedt were also recorded upstream sites. Their association to high TP levels and lower DO concentrations suggests deterioration of water quality. This study shows the potential of diatoms as indicators of environmental change even in dynamic systems such as estuaries.

Seasonal and decadal patterns in Discostella (Bacillariophyceae) species from bi-weekly records of two boreal lakes (Experimental Lakes Area, Ontario, Canada).

A recent rise in the relative abundance of Discostella species (D.stelligera and D.pseudostelligera) has been well documented from sedimentary diatom assemblages across the Northern Hemisphere. This unprecedented change over the last ~150years has been linked to rises in atmospheric temperatures, changes in ice cover, and/or increases in thermal stability, among other factors. The bi-weekly monitoring data from two boreal lakes at the Experimental Lakes Area (ELA) in northwestern Ontario were analyzed across seasons (spring, summer, and fall) and decades (1970s-2000s). We found that Discostella species are primarily spring/early summer bloomers (i.e., late April to June) in these lakes and changes in concentrations of Discostella over time were most pronounced in the spring or early summer months. Increases in Discostella abundance over time may be linked to earlier ice-off and a longer period of spring turnover, resulting from increased winter and spring temperatures. It is also possible that a trophic mismatch between the spring diatom bloom and zooplankton is occurring, thus reducing diatom loss rates, and resulting in greater overall abundance. Moreover, the spring dominance of Discostella in our study lakes occurred at a time of the year when nutrient concentrations were at their highest seasonally, suggesting that these taxa are neither limited directly by nutrients, nor responding to enhanced stratification during the summer months in these lakes.

Surface–sediment diatom assemblages of the urban ponds of St. Petersburg, Russia

The diatom assemblages of the surface sediments have been studied in 53 urban ponds and lakes of St. Petersburg for the first time. In total, 350 species and infraspecific taxa have been registered; the species of Achnanthidium, Cocconeis, Cyclostephanos, Cyclotella, Gomphonema, Lemnicola, Navicula, Nitzschia, and Stephanodiscus genera were the most common. The wide distribution of small centric planktonic taxa, Cyclostephanos dubius, C. invisitatus, Cyclotella pseudostelligera, Stephanodiscus hantzschii, and S. minutulus, is preconditioned by the depletion of dissolved silica, which is a result of the high phosphorus load. The high abundance of macrophytes in the shallowest sites favors the dominance of epiphytic Fragilaria, Staurosira, and Staurosirella taxa. Bottom-living diatoms in the shallow eutrophic and hypereutrophic urban ponds are light-limited; this is a result of either macrophytes or phytoplankton development. Alkaliphilous and alkalibiontic species prevail among the pH-indicative species, while eutraphentic and hypereutraphentic diatoms, as well as those thriving in a wide range of trophic conditions, dominate over other trophic groups. The β- and α-mesosaprobic species are the most common saprobity-indicative taxa.

Distribution and diversity of diatom assemblages in surficial sediments of shallow lakes in Wapusk National Park (Manitoba, Canada) region of the Hudson Bay Lowlands.

The hydrology of shallow lakes (and ponds) located in the western Hudson Bay Lowlands (HBL) is sensitive to climate warming and associated permafrost thaw. However, their biological characteristics are poorly known, which hampers effective aquatic ecosystem monitoring. Located in northern Manitoba along the southwestern coast of Hudson Bay, Wapusk National Park (WNP) encompasses numerous shallow lakes representative of the subarctic zone. We analyzed the distribution and diversity of diatom (microscopic algae; class Bacillariophyceae) assemblages in surficial sediments of 33 lakes located in three different ecozones spanning a vegetation gradient, from NE to SW: the Coastal Fen (CF), the Interior Peat Plateau (IPP), and the Boreal Spruce Forest (BSF). We found significant differences (P < 0.05) in diatom community composition between CF and IPP lakes, and CF and BSF lakes, but not between IPP and BSF lakes. These results are consistent with water chemistry measurements, which indicated distinct limnological conditions for CF lakes. Diatom communities in CF lakes were generally dominated by alkaliphilous taxa typical of waters with medium to high conductivity, such as Nitzschia denticula. In contrast, several IPP and BSF lakes were dominated by acidophilous and circumneutral diatom taxa with preference for low conductivity (e.g., Tabellaria flocculosa, Eunotia mucophila, E. necompacta var. vixcompacta). This exploratory survey provides a first detailed inventory of the diatom assemblages in the WNP region needed for monitoring programs to detect changes in shallow lake ecosystems and ecozonal shifts in response to climate variations.

The fate of diatom valves in the Subantarctic and Polar Frontal Zones of the Southern Ocean: Sediment trap versus surface sediment assemblages

An array of deep ocean sediment traps was initiated in 1997 within the Australian sector of the Southern Ocean, and serviced annually for over a decade (the Subantarctic site is ongoing as part of the Southern Ocean Time Series). Here, we expand on previous findings obtained from the shallow traps (~1000m depth) in the Polar Frontal Zone (PFZ) and Subantarctic Zone (SAZ) by examining the chemical composition and diatom assemblages collected by sediment traps deployed at bathypelagic depths in the PFZ (1500m depth; years 1997–1998, 1999–2000 and 2003–2004) and SAZ (2000 and 3800m; year 1999–2000). Additionally, the diatom assemblages of the surface sediments below the traps were analyzed to document how the seasonal signal is recorded in the sedimentary record. Analysis of the changes of the BSi:POC ratios with depth at both sites confirms previous work suggesting that the decoupling between Si and C cycles in the Southern Ocean is not different from other biogeochemical provinces of the world's ocean. Comparison of the seasonal flux pattern registered by the traps indicates that the diatom assemblages in the sediments of the Antarctic Zone and PFZ mainly represent the summer period. In contrast, the assemblages found in the sediments of the SAZ are a reflection of the spring and summer months, a period characterized by a larger variability in chemical and physical parameters. The strong correlation between the POC fluxes and the relative abundance of a group of diatom species (mainly Pseudo-nitzschia and small Fragilariopsis) in the PFZ traps suggest that there is an intimate relationship between the development of these species and the POC pulses to the deepest layers of the water column in this region. Interestingly, analysis of the diatom assemblages in the surface sediments reveals that the strong dissolution in the sediment-water interface removed the signal of these “carbon sinkers” from the sedimentary record leaving the sediment assemblage enriched in heavily silicified species.

SPATIO-TEMPORAL RESPONSE OF SEDIMENTARY DIATOMS TO WATER LEVEL IN A SHALLOW LAKE

In recent years, human activities have resulted in variations of hydraulic conditions in shallow lakes, thereby affecting interactions between environmental variables and aquatic plants. This feedback of plants on environmental changes is largely recorded in lake sediments. In this study we collected surface sediments in different seasons and a sediment core from a shallow lake Baiyangdian. We investigated the seasonal effects of water level and six physicochemical variables in surface sediments, as well as their impacts on the distribution of diatom assemblages. Therein the response of diatom assemblages to water level and the spatio-temporal effects of water level are highly emphasized. Results indicate that water level significantly changes between seasons, however there is no significant seasonal effects for the com- position of grain size. The lake productivity increases with the vigorous growth of aquatic plants. Water level largely determines the distribution of diatom assemblages in surface sediments and the sediment core. Spatial isolation causes the difference between the habitats and alters the relation between water level and diatom taxa. Human activities also affect the relevance of water level with diatom assemblage. The results can provide the helpful information for the local water resource management.

HISTORIA AMBIENTAL DE UN LAGO ALPINO EN EL CENTRO DE MÉXICO (1230-2010)

High altitude lakes, with a low mineralization, alkalinity and nutrient levels, are sensitive ecosystems to natural or anthropogenic disturbances and currently are in risk due to global warming. There are only two high altitude lakes (> 3,800 m asl) in Mexico, La Luna and El Sol, in the Nevado de Toluca crater. Chemical parameters and surface sediment diatom assemblages show differences between both lakes, La Luna has lower pH and mineralization, and in spite of their geographic proximity, each lake has a characteristic diatom assemblage, with lower diatom concentration and species richness in La Luna. In this lake Encyonema perpusillum and Psammothidium helveticum are dominant while in El Sol are Cavinula pseudoscutiformis, Psammothidium levanderi and Aulacoseira distans. This ecological distribution is the basis for the interpretation of the diatom record from La Luna, from a 57cm long core dated with 210Pb y 14C for which cladocera and magnetic susceptibility data are also available. These proxies allowed identifying the beginning of a trend towards colder and dryer climates around 1350-1510. This period correlates with the Little Ice Age (LIA) in which the coldest and driest conditions were from 1660 to 1760, during the Maunder solar minimum. Cooler and drier conditions are recoded until the begining of the 20th century. Correlation with other paleoenvironmental records show that there was a regional trend to drier climates during the LIA in central Mexico.

Diatoms in Sub-Surface Sediment Cores from Mangrove Forest Floors of Deltaic Islands in Sundarbans, India

Species constituting diatom assemblages in sediments provide important information regarding the past and present environmental conditions related to soil and water physico-chemistry and also biodiversity dynamics data. This information can reveal the developmental history of the region. Diatoms are sensitive to stochastic changes in the environment, making them extremely reliable bio-indicators. The purpose of the present study was to decipher such biodiversity related information provided by the diatom assemblages from the Indian Sundarbans. Diatoms were identified in the top 50 cm layer of two sediment cores: a newly silted up deltaic landmass with mangroves in early succession stage, and an island within the Protected Area Network with mangroves in late succession stage. Fifteen diatom species, with no past records in the sediments of Indian Sundarbans were identified and described. Four species among them are sporadically mentioned in previous phytoplankton records fi-om the Sundarbans, but these are not confirmed taxonomically. This paper provides identification and nomenclatural notes on all 15 species. High abundance and species richness of the centric forms Coscinodiscus spp., Cyclotella spp., and Thalassiosira spp. at both the sites is attributed to their proximity to estuarine rivers and their regular inundation, leading to settling of planktonic forms within sediment cores. Many pennate forms, namely Amphicampa eruca, Amphora holsatica, Diploneis spp. (except Z). smithii), Epithemia turgida, Eunotiapectinalis, Giffenia cocconeiformis , and Rhaphoneis rhombus which are being reported for the first time fi-om this region have never been part of described planktonic communities in estuarine waters, validating their' soil diatom' status. Typical fi-eshwater diatoms, namely Aulacoseira granulata and Epithemia turgida, in the sediments indicate lower salinity in these areas, not much earlier than present times.

Diatom species abundance and morphologically-based dissolution proxies in coastal Southern Ocean assemblages

Taphonomic processes alter diatom assemblages in sediments, thus potentially negatively impacting paleoclimate records at various rates across space, time, and taxa. However, quantitative taphonomic data is rarely included in diatom-based paleoenvironmental reconstructions and no objective standard exists for comparing diatom dissolution in sediments recovered from marine depositional settings, including the Southern Ocean’s opal belt. Furthermore, identifying changes to diatom dissolution through time can provide insight into the efficiency of both upper water column nutrient recycling and the biological pump. This is significant in that reactive metal proxies (e.g. Al, Ti) in the sediments only account for post-depositional dissolution, not the water column where the majority of dissolution occurs.In order to assess the range of variability of responses to dissolution in a typical Southern Ocean diatom community and provide a quantitative guideline for assessing taphonomic variability in diatoms recovered from core material, a sediment trap sample was subjected to controlled, serial dissolution. By evaluating dissolution-induced changes to diatom species’ relative abundance, three preservational categories of diatoms have been identified: gracile, intermediate, and robust. The relative abundances of these categories can be used to establish a preservation grade for diatom assemblages.However, changes to the relative abundances of diatom species in sediment samples may reflect taphonomic or ecological factors. In order to address this complication, relative abundance changes have been tied to dissolution-induced morphological change to the areolae of Fragilariopsis curta, a significant sea-ice indicator in Southern Ocean sediments. This correlation allows differentiation between gracile species loss to dissolution versus ecological factors or sediment winnowing. These results mirror a similar morphological dissolution index from a parallel study utilizing Fragilariopsis kerguelensis, suggesting that results are applicable to a broad spectrum of diatoms typically preserved in the sediments.

Spatial distributions and environmental implications of diatom assemblages in surface sediments of Hulun Lake, China

We examined the spatial distributions of diatom assemblages in surface sediments of Hulun Lake and explored the potential relationships between diatom distributions and environmental variables using multivariate analyses. These results may provide a basis for fossil record interpretation. Surface sediment samples were taken at 30 sites throughout the lake to analyse diatom assemblages, and environmental variables potentially influencing diatom distributions were taken concurrently. Principal components analysis (PCA) and redundancy analysis (RDA) were conducted to explore the spatial characteristics of diatom assemblages in the lake surface sediment and their relationship with the main environmental variables. Analyses showed remarkable spatial differentiation such that epiphytic and benthic diatoms dominated surface sediments in the eastern littoral region, while planktonic diatoms were dominant in the central and western profundal region. Additionally, in the southern part of the central and west profundal region, Stephanodiscus neoastraea was more abundant than Aulacoseira perglabra. RDA showed that the total nitrogen of surface sediments (TNs), which acts as a nitrogen source and stays balanced with lake water nitrogen levels, is the primary environmental variable influencing diatom assemblages in the surface sediments. S. neoastraea and A. perglabra in particular may be boosted by high nitrogen content in the sediment. Additionally, correlations of water depth with the distribution of sample sites and the ratio of planktonic to benthic diatoms indicated an inherent relationship between diatom distribution and water depth, where deep water drove a high planktonic/benthic ratio. This result further supports the use of the planktonic/benthic ratio in the fossil record to reconstruct historical change in lake depth.

Tracking the hydro-climatic signal from lake to sediment: A field study from central Turkey

Palaeo-hydrological interpretations of lake sediment proxies can benefit from a robust understanding of the modern lake environment. In this study, we use Nar Golu, a non-outlet, monomictic maar lake in central Turkey, as a field site for a natural experiment using observations and measurements over a 17-year monitoring period (1997–2014). We compare lake water and sediment trap data to isotopic, chemical and biotic proxies preserved in its varved sediments. Nar Golu underwent a 3 m lake-level fall between 2000 and 2010. δ18Olakewater is correlated with this lake-level fall, responding to the change in water balance. Endogenic carbonate is shown to precipitate in isotopic equilibrium with lake water and there is a strong relationship between δ18Olakewater and δ18Ocarbonate, which suggests the water balance signal is accurately recorded in the sediment isotope record. Over the same period, sedimentary diatom assemblages also responded, and conductivity inferred from diatoms showed a rise. Shifts in carbonate mineralogy and elemental chemistry in the sediment record through this decade were also recorded. Intra-annual changes in δ18Olakewater and lake water chemistry are used to demonstrate the seasonal variability of the system and the influence this may have on the interpretation of δ18Ocarbonate. We use these relationships to help interpret the sedimentary record of changing lake hydrology over the last 1725 years. Nar Golu has provided an opportunity to test critically the chain of connection from present to past, and its sedimentary record offers an archive of decadal- to centennial-scale hydro-climatic change

The diatom distribution in the surface sediments of the Eastern Arctic seas of Russia

The distribution features of diatoms in the surface sediments of the Laptev, East Siberian, and Chukchi seas were studied. The sediment diatom content varied widely, being lowest in the East Siberian Sea and highest in the Chukchi Sea. Cluster analysis revealed several diatom assemblages in the surface sediments of the studied region; their distribution depended on the hydrological conditions. A benthic Amphora libyca assemblage, a freshwater Aulacoseira subarctica one, and a brackish-water Thalassiosira hyperborea assemblage were identified in the Laptev Sea. A cold-water Th. antarctica assemblage was distinguished in the eastern part of the East Siberian Sea and in the northern Chukchi Sea; a highly productive Chaetoceros assemblage was noted in the Herald Canyon area; a sublittoral Odontella aurita assemblage occurred in sandy sediments of the Herald Shoal; a Paralia sulcata assemblage, which is an indicator of low salinity waters, was found in the eastern Chukchi Sea near Alaska; and ”sea-ice” Th. nordenskioeldii and Fragillariopsis oceanica assemblages occurred in the southern Chukchi Sea.

Diatoms indicate that calcium decline, not acidification, explains recent cladoceran assemblage changes in south-central Ontario softwater lakes

In recent decades, softwater lakes across Canada have experienced a wide array of anthropogenic influences, with acidification and climate warming of particular concern. Here, we compare modern and pre-industrial sedimentary diatom assemblages from 36 softwater lakes located on the Canadian Shield in south-central Ontario to determine whether lake acidification or reduced calcium availability was the main stressor responsible for recent declines in Ca-sensitive cladoceran taxa. Regional surveys of south-central Ontario water chemistry have identified the pH recovery of many formerly acidified lakes, and our fossil diatom-inferred pH analyses indicate that modern lakewater pH in the 36 study lakes is similar to (or higher than) pre-industrial levels, with diatom assemblages from both time periods dominated by taxa with similar pH preferences. In addition, modern diatom assemblages compared to pre-industrial assemblages contained higher relative abundances of planktonic diatom taxa (e.g. Asterionella formosa and the Discostella stelligera complex) and lower relative abundances of heavily silicified diatoms (e.g. Aulacoseira taxa) and benthic fragilarioid taxa. These taxonomic shifts are consistent with warming-induced changes in lake properties including a longer ice-free period, decreased wind speed and/or increased thermal stability. We conclude that recent changes observed within the cladoceran assemblages of these lakes are not a response to acidification, but are likely a consequence of Ca declines. In addition, our data suggest that regional climate warming is now responsible for the diatom changes observed in this region.

Exploring the environmental context of recentDidymosphenia geminataproliferation in Gaspésie, Quebec, using paleolimnology

Management authorities often perceive Didymosphenia geminata (didymo) as an invasive, nuisance diatom species that has been introduced to eastern Canada; however, observations from early 20th century diatom surveys challenge this characterization. We apply paleolimnological techniques to place recent didymo blooms from Gaspésie, Quebec, into historical and environmental context. Sedimentary diatom assemblages were examined from Lac Humqui (a headwater lake) and Lac au Saumon (a lake with an inflowing river currently supporting blooms). The Lac Humqui assemblage experienced a broad-scale shift in their dominant life strategy with declines in fragilarioid taxa and increases in planktonic diatoms (i.e., Cyclotella–Discostella species) that began ∼1970 and increased to modern abundances ∼1990. Strong relationships between this diatom shift and increases in regional air temperatures and earlier river ice-out dates are consistent with longer growing seasons and enhanced thermal stability in Lac Humqui. Didymo was observed throughout the Lac au Saumon core, demonstrating that it has been present in the region since at least ∼1970. Our paleolimnological evidence indicates that blooms likely form in response to regional consequences of climate warming, rather than human introduction.

The influence of basin morphometry on the regional coherence of patterns of diatom-inferred salinity in lakes of the northern Great Plains (USA)

Sedimentary diatom profiles from saline lakes are frequently used to reconstruct lakewater salinity as an indicator of drought. However, diatom-inferred salinity (DI-salinity) reconstructions from geographically proximal sites in the Great Plains (USA) have yielded disparate results. This study explores how physical changes in lake habitat resulting from drought may affect climate inferences from salinity reconstructions. Differences in relationships among drought, lake-level change, and diatom community structure over the last century were examined for three saline lakes in the northern Great Plains with dissimilar DI-salinity records. At each site, models were developed relating available planktic:benthic (P:B) habitat area to lake-level change, and models were compared with instrumental drought records and fossil diatoms to understand how drought conditions were recorded in sedimentary diatom assemblages. The degree to which DI-salinity tracked drought variation was affected by site-specific physical characteristics that influenced the relationship between lake-level change and P:B habitat zonation within each lake. Moon Lake showed the strongest correlation between drought and DI-salinity, although this relationship was weaker during wetter conditions, as highstands resulted in a larger influx of benthic diatoms. At Coldwater Lake, a dual-basin system, P:B varied depending on lake level, which apparently reduced the correlation between DI-salinity and drought. At Lake Cochrane, the simplest and freshest of the three basins, the P:B of fossil diatoms was a better proxy for drought than DI-salinity. The integration of additional ecological characteristics into interpretations of paleoclimate records, particularly for biologically-based proxies, may improve reconstructions of regional patterns of climate variation.

Long-term changes in sedimentary diatom assemblages and their environmental implications in the Changjiang (Yangtze) River estuary, China

Long-term data on diatom assemblages in a sediment core (60 cm) obtained from the Changjiang (Yangtze) River estuary were analyzed in order to assess the environmental changes that took place in the approximately 38 years (as determined by 210Pb measurements), i.e., between 1974 and 2012, of sediment accumulation. From the sediment core, 62 diatom taxa and genera were identified. The diatom biomass in the core generally increased beginning in the mid-1990s (core depth: 35 cm), accompanied by a shift in the dominant species from Podosira stelliger and two species of Cyclotella (C. stylorum and C. striata) to Paralia sulcata, three species of Thalassiosira (T. eccentria, T. oestrupii, and T. excentrica), Actinoptychus undulates, and Thalassionema nitzschioides. The changes in both species diversity and abundance suggested that since the 1980s the estuary has undergone extensive eutrophication. This conclusion was supported by the increased proportion of planktonic species, another indicator of high nutrients inputs, in the Changjiang River estuary.

Long-term transformation of diatom assemblages in Lakes Boroe and Glubokoe

A method of graphical analysis of the diatom assemblages’ taxonomic structure allows revealing two main scenarios of their transformations. The first scenario is determined by the external abiotic influence during which the internal ecosystem parameters remain stable. The second scenario is determined by the change in the internal ecosystem parameters and corresponds to the transitional periods in the ecosystem development. The diatom assemblages of the bottom sediments from Lake Boroe (Novgorod oblast) and Lake Glubokoe (Moscow oblast) have been studied. Throughout the entire studied period (the last 500–550 years), only the first scenario was typical of the Lake Boroe ecosystem. As for Lake Glubokoe, we have revealed two periods during which the second scenario occurred and two transitional periods when the second scenario dominated. Thus, the ecosystem of Lake Boroe remained stable during the last 500 years, whereas the ecosystem of Lake Glubokoe underwent serious modifications.

The diversity of diatom assemblages in ships' ballast sediments: colonization and propagule pressure on Canadian ports

This research focused on the specific diversity of diatoms carried in ballast sediments of ships reaching Canadian ports on the Pacific, Atlantic and the Great Lakes during 2007 and 2009. The inventory of 180 taxa included Thalassiosira, Chaetoceros spores, Cyclotella, Actinocyclus, Aulacoseira, Melosira and Pseudo-nitzschia as the most species-rich genera found in 142 samples of the matter settled at the bottom of ship tanks. We also tested and showed evidence that diatom species composition identified in any given tank was the product of: (i) species intrinsic survival strategies, (ii) the most recent ballast water exchange (BWE), and (iii) the cumulative end result of past ballast operations. Multivariate analysis reduced our data set to three assemblages. Propagule pressure of these potentially colonizing assemblages may be enhanced due to repeated introduction attempts by ships belonging to different ballast management categories (transoceanic, intra-coastal with our without BWE) but that actually carried the same assemblage type. A non-linear relationship between colonization and propagule pressure confirms that the early stages of invasion (ballast uptake, survival in the tank) structure the later stages. Coastal floras in tanks that underwent offshore BWE (particularly persisting freshwater species) corroborate concern with the low efficiency of this management option.

Composition and distribution of diatom assemblages in the surface sediments of the Bering Sea

Diatoms collected from the top 1 cm of the surface sediment layer at seven Bering Sea stations during the Fourth Chinese National Arctic Research Expedition (4th CHINARE-Arctic) in 2010 were studied. In total, 101 taxa belonging to 38 genera were found. The species were divided into four groups: eurythermal species, boreal-temperate species, polar species, and sea-ice species. The diatom assemblages at the stations in the southwestern basin area were primarily composed of boreal-temperate species, such as Neodenticula seminae , Thalassiosira trifulta , Rhizosolenia hebetata f. hiemalis , and Actinocyclus curvatulus . The northeastern shelf stations were dominated by polar species, including Fragilariopsis oceanica , Thalassiosira antarctica spora, Thalassiosira nordenskioeldii , and Thalassiosira hyalina . The overall abundance was highest at the basin stations with 3.7 ×106 cells·g-1 of wet sediment, whereas the eastern shelf stations had the lowest abundance of 0.7×106 cells·g-1 of wet sediment (excluding the resting spores of Chaetoceros spp.). The relationship between the distribution of the surface sediment diatom assemblages and the environment is discussed.

Diatom fluxes in the NW Mediterranean: evidence from a 12-year sediment trap record and surficial sediments

We examined the total diatom flux and species composition, total coccolith flux and total mass flux collected with a sediment trap between October 1993 and January 2006 in the northeastern entrance of the Gulf of Lions (North Western Mediterranean). The average daily diatom and coccolith fluxes (3 × 107 valves m2 d–1 and 6.1 × 108 coccoliths m2 day−1, respectively) are comparable in magnitude with previously reported data sets in other high productivity areas of the Western Mediterranean. The temporal particle flux pattern reflected the variations in surface oceanographic conditions and primary productivity, which showed strong annual cycling. Highest diatom, coccolith and total mass fluxes always occurred during the winter–spring transition, while minima were observed during summer. Changes in the diverse diatom communities reflected the water column conditions throughout the record. The intensity of the diatom winter–spring blooms seemed to be enhanced in those years with intense and cold winds during winter, whereas years with low winter wind stress were liable to be less productive for diatoms. Coccolith fluxes exhibited a more stable interannual pattern than diatom fluxes. Significant discrepancies were found between the sediment trap and surficial sediment diatom assemblages.