Multi-scale variability in phytoplankton populations of the North Atlantic basin: from eddies to global change

The Continuous Plankton Recorder (CPR) survey has been deployed since 1931 to describe and analyze plankton variability in the North Atlantic and North Sea. This survey measures the presence and abundance of 437 phytoplankton and zooplankton taxa and provides an assessment of phytoplankton biomass, the Phytoplankton Colour Index (VCI). The diatoms and dinoflagellates are the two main phytoplankton groups identified by the CPR survey. The first part of this work provides insights into the space-time dynamics of phytoplankton communities through an analysis of diatom and dinoflagellate populations in the whole North Atlantic basin. Because the North Atlantic Ocean includes many different biotopes, the second part focuses on the mesoscale variability of phytoplankton species. The long-term fluctuations of the phytoplankton species are studied in the NW and in the NE Atlantic, the two best sampled areas of the CPR survey. In the NE Atlantic, the aim is to determine the contribution of the diatoms and dinoflagellates to the PCI, their fluctuations over 45 years of sampling and their geographical variations. Because local variability in environmental conditions is thought to play a dominant role in temporal fluctuations of phytoplankton biomass, the next part takes advantage to define small areas around the British Isles. This allowed me to study more precisely the processes influencing the long-term variation of phytoplankton assemblages. The North Atlantic Current transports water across the Northern basin of the Atlantic Ocean, along the shelf of Ireland and form the Norwegian current which corresponds to the inflow of oceanic waters into the Norwegian Sea and the North Sea. In this highly hydrodynamic region attention is focused on the fluctuations of plankton species in relation to the currents. The aim of this part is therefore to investigate the fluctuations of phytoplankton biomass, diatoms and dinoflagellates, their geographical distribution and abundance within the area and their relationship with physical processes. The intense hydrodynamic activity observed in the Northwestern Atlantic Shelves Province (NWCS) makes this region especially intriguing from the point of view of physical-biological interactions. The relationship between spatial and temporal structures of eddies (via Sea Surface Heights) and chlorophyll a (from the Sea-viewing Wide Field-of-view Sensor, Sea WiFS) was assessed along the Gulf Stream axis. In particular, the physical structures identified were followed and compared with phytoplankton distribution. In addition, the impact of the LSW changing flow along the Scotian Shelf and the influence of Gulf Stream rings along the George Bank is determined. This work demonstrated that changes are occurring in pelagic ecosystems at different temporal and spatial scales. These changes have been illustrated by the spatial variability induced by eddies and/or currents but also by the regional variability of the hydro-climatic processes, influencing in different ways Sea Surface Temperature, wind-regimes and mixing of local environments. Several different aspects of the North Atlantic Oscillations impact on pelagic ecosystems have been highlighted. In the northeast Atlantic, NA0 fluctuations imply changes in (i) SST in northern Europe, (ii) wind regimes, (iii) Atlantic Water inflow into the North Sea. In contrast, in the northwest Atlantic, the variations of NAO imply changes in (i) SST on the Scotian Shelf, (ii) coastal currents, and (iii) inflow of Labrador Sea Slope Water (LSSW) towards the Scotian Shelf and Georges Bank. These changes in environmental process impact phytoplankton production, abundance, spatial distribution, community structure phenology and ultimately would impact trophodynamics processes. It is, however, still difficult to explain unambiguously all the mechanisms that are involved in the control of the observed patterns

Distribution of picophytoplankton communities from brackish to hypersaline waters in a South Australian coastal lagoon

Background Picophytoplankton (i.e. cyanobacteria and pico-eukaryotes) are abundant and ecologically critical components of the autotrophic communities in the pelagic realm. These micro-organisms colonized a variety of extreme environments including high salinity waters. However, the distribution of these organisms along strong salinity gradient has barely been investigated. The abundance and community structure of cyanobacteria and pico-eukaryotes were investigated along a natural continuous salinity gradient (1.8% to 15.5%) using flow cytometry. Results Highest picophytoplankton abundances were recorded under salinity conditions ranging between 8.0% and 11.0% (1.3 × 106 to 1.4 × 106 cells ml-1). Two populations of picocyanobacteria (likely Synechococcus and Prochlorococcus) and 5 distinct populations of pico-eukaryotes were identified along the salinity gradient. The picophytoplankton cytometric-richness decreased with salinity and the most cytometrically diversified community (4 to 7 populations) was observed in the brackish-marine part of the lagoon (i.e. salinity below 3.5%). One population of pico-eukaryote dominated the community throughout the salinity gradient and was responsible for the bloom observed between 8.0% and 11.0%. Finally only this halotolerant population and Prochlorococcus-like picocyanobacteria were identified in hypersaline waters (i.e. above 14.0%). Salinity was identified as the main factor structuring the distribution of picophytoplankton along the lagoon. However, nutritive conditions, viral lysis and microzooplankton grazing are also suggested as potentially important players in controlling the abundance and diversity of picophytoplankton along the lagoon. Conclusions The complex patterns described here represent the first observation of picophytoplankton dynamics along a continuous gradient where salinity increases from 1.8% to 15.5%. This result provides new insight into the distribution of pico-autotrophic organisms along strong salinity gradients and allows for a better understanding of the overall pelagic functioning in saline systems which is critical for the management of these precious and climatically-stress ecosystems

Prokaryotic aminopeptidase activity along a continuous salinity gradient in a hypersaline coastal lagoon (the Coorong, South Australia)

The distribution and aminopeptidase activity of prokaryotes were investigated along a natural continuous salinity gradient in a hypersaline coastal lagoon, the Coorong, South Australia. The abundance of prokaryotes significantly increased from brackish to hypersaline waters and different sub-populations, defined by flow cytometry, were observed along the salinity gradient. While four sub-populations were found at each station, three additional ones were observed for 8.3% and 13.4%, suggesting a potential modification in the composition of the prokaryotic communities and/or a variation of their activity level along the salinity gradient. The aminopeptidase activity highly increased along the gradient and salinity appeared as the main factor favouring this enzymatic activity. However, while the aminopeptidase activity was dominated by free enzymes for salinities ranging from 2.6% to 13.4%, cell-attached aminopeptidase activity was predominant in more saline waters (i.e. 15.4%). Changes in substrate structure and availability, strongly related to salinity, might (i) modify patterns of both aminopeptidase activities (free and cell-associated enzymes) and (ii) obligate the prokaryotic communities to modulate rapidly their aminopeptidase activity according to the nutritive conditions available along the gradient

Substrate Type Determines Metagenomic Profiles from Diverse Chemical Habitats

Environmental parameters drive phenotypic and genotypic frequency variations in microbial communities and thus control the extent and structure of microbial diversity. We tested the extent to which microbial community composition changes are controlled by shifting physiochemical properties within a hypersaline lagoon. We sequenced four sediment metagenomes from the Coorong, South Australia from samples which varied in salinity by 99 Practical Salinity Units (PSU), an order of magnitude in ammonia concentration and two orders of magnitude in microbial abundance. Despite the marked divergence in environmental parameters observed between samples, hierarchical clustering of taxonomic and metabolic profiles of these metagenomes showed striking similarity between the samples (>89%). Comparison of these profiles to those derived from a wide variety of publically available datasets demonstrated that the Coorong sediment metagenomes were similar to other sediment, soil, biofilm and microbial mat samples regardless of salinity (>85% similarity). Overall, clustering of solid substrate and water metagenomes into discrete similarity groups based on functional potential indicated that the dichotomy between water and solid matrices is a fundamental determinant of community microbial metabolism that is not masked by salinity, nutrient concentration or microbial abundance.</p

2008 Inter-laboratory Comparison Study of a Reference Material for Nutrients in Seawater

Autoclaved natural seawater collected in the North Pacific Ocean was used as a reference material for nutrients in seawater (RMNS) during an inter-laboratory comparison (I/C) study conducted in 2008. This study was a follow-up to previous studies conducted in 2003 and 2006. A set of six samples was distributed to each of 58 laboratories in 15 countries around the globe, and results were returned by 54 of those laboratories (15 countries). The homogeneities of samples used in the 2008 I/C study, based on analyses for three determinants, were improved compared to those of samples used in the 2003 and 2006 I/C studies. Results of these I/C studies indicate that most of the participating laboratories have an analytical technique for nutrients that is sufficient to provide data of high comparability. The differences between reported concentrations from the same laboratories in the 2006 and 2008 I/C studies for the same batch of RMNS indicate that most of the laboratories have been maintaining internal comparability for two years. Thus, with the current high level of performance in the participating laboratories, the use of a common reference material and the adaptation of an internationally accepted nutrient scale system would increase comparability among laboratories worldwide, and the use of a certified reference material would establish traceability. In the 2008 I/C study we observed a problem of non-linearity of the instruments of the participating laboratories similar to that observed among the laboratories in the 2006 I/C study. This problem of non-linearity should be investigated and discussed to improve comparability for the full range of nutrient concentrations. For silicate comparability in particular, we see relatively larger consensus standard deviations than those for nitrate and phosphate

Substrate Type Determines Metagenomic Profiles from Diverse Chemical Habitats

Environmental parameters drive phenotypic and genotypic frequency variations in microbial communities and thus control the extent and structure of microbial diversity. We tested the extent to which microbial community composition changes are controlled by shifting physiochemical properties within a hypersaline lagoon. We sequenced four sediment metagenomes from the Coorong, South Australia from samples which varied in salinity by 99 Practical Salinity Units (PSU), an order of magnitude in ammonia concentration and two orders of magnitude in microbial abundance. Despite the marked divergence in environmental parameters observed between samples, hierarchical clustering of taxonomic and metabolic profiles of these metagenomes showed striking similarity between the samples (>89%). Comparison of these profiles to those derived from a wide variety of publically available datasets demonstrated that the Coorong sediment metagenomes were similar to other sediment, soil, biofilm and microbial mat samples regardless of salinity (>85% similarity). Overall, clustering of solid substrate and water metagenomes into discrete similarity groups based on functional potential indicated that the dichotomy between water and solid matrices is a fundamental determinant of community microbial metabolism that is not masked by salinity, nutrient concentration or microbial abundance

A database of marine phytoplankton abundance, biomass and species composition in Australian waters

There have been many individual phytoplankton datasets collected across Australia since the mid 1900s, but most are unavailable to the research community. We have searched archives, contacted researchers, and scanned the primary and grey literature to collate 3,621,847 records of marine phytoplankton species from Australian waters from 1844 to the present. Many of these are small datasets collected for local questions, but combined they provide over 170 years of data on phytoplankton communities in Australian waters. Units and taxonomy have been standardised, obviously erroneous data removed, and all metadata included. We have lodged this dataset with the Australian Ocean Data Network (http://portal.aodn.org.au/) allowing public access. The Australian Phytoplankton Database will be invaluable for global change studies, as it allows analysis of ecological indicators of climate change and eutrophication (e.g., changes in distribution; diatom:dinoflagellate ratios). In addition, the standardised conversion of abundance records to biomass provides modellers with quantifiable data to initialise and validate ecosystem models of lower marine trophic levels

The impact of turbulence and phytoplankton dynamics on foam formation, seawater viscosity and chlorophyll concentration in the eastern English Channel

The space-time dynamics of chlorophyll a concentration and seawater excess viscosity has been investigated in the hydrographically contrasting inshore and offshore water masses of the eastern English Channel. This was done during the phytoplankton spring bloom dominated by Phaeocystis globosa before and after the very large-scale formation of foam induced by an increase in wind-driven turbulence and the related wave breakings. The results suggest that the dynamics of chlorophyll a concentration and seawater excess viscosity are differentially controlled by the formation of foam through the intensity of the spring bloom and wind-generated turbulence