Silicate Limitation Research Articles

The subpolar gyre regulates silicate concentrations in the North Atlantic

The North Atlantic is characterized by diatom-dominated spring blooms that results in significant transfer of carbon to higher trophic levels and the deep ocean. These blooms are terminated by limiting silicate concentrations in summer. Numerous regional studies have demonstrated phytoplankton community shifts to lightly-silicified diatoms and non-silicifying plankton at the onset of silicate limitation. However, to understand basin-scale patterns in ecosystem and climate dynamics, nutrient inventories must be examined over sufficient temporal and spatial scales. Here we show, from a new comprehensive compilation of data from the subpolar Atlantic Ocean, clear evidence of a marked pre-bloom silicate decline of 1.5–2 µM throughout the winter mixed layer during the last 25 years. This silicate decrease is primarily attributed to natural multi-decadal variability through decreased winter convection depths since the mid-1990s, a weakening and retraction of the subpolar gyre and an associated increased influence of nutrient-poor water of subtropical origin. Reduced Arctic silicate import and the projected hemispheric-scale climate change-induced weakening of vertical mixing may have acted to amplify the recent decline. These marked fluctuations in pre-bloom silicate inventories will likely have important consequences for the spatial and temporal extent of diatom blooms, thus impacting ecosystem productivity and ocean-atmosphere climate dynamics.

Nutrient ratios influence variability in Pseudo-nitzschia species diversity and particulate domoic acid production in the Bay of Seine (France)

The population dynamics of different Pseudo-nitzschia species, along with particulate domoic acid (pDA) concentrations, were studied from May 2012 to December 2013 in the Bay of Seine (English Channel, Normandy). While Pseudo-nitzschia spp. blooms occurred during the two years of study, Pseudo-nitzschia species diversity and particulate domoic acid concentrations varied greatly. In 2012, three different species were identified during the spring bloom (P. australis, P. pungens and P. fraudulenta) with high pDA concentrations (∼1400ngl−1) resulting in shellfish harvesting closures. In contrast, the 2013 spring was characterised by a P. delicatissima bloom without any toxic event. Above all, the results show that high pDA concentrations coincided with the presence of P. australis and with potential silicate limitation (Si:N<1), while nitrate concentrations were still replete. The contrasting environmental conditions between 2012 and 2013 highlight different environmental controls that might favour the development of either P. delicatissima or P. australis. This study points to the key role of Pseudo-nitzschia diversity and cellular toxicity in the control of particulate domoic acid variations and highlights the fact that diversity and toxicity are influenced by nutrients, especially nutrient ratios.

Metacaspases and programmed cell death in Skeletonema marinoi in response to silicate limitation

Diatoms comprise up to 40% of annual productivity in the modern ocean, and some stress factors may alter their physiology and survival. Here, we describe the morphological, biochemical and molecular responses that occur during metacaspase-mediated programmed cell death (PCD) in response to silicate limitation in the ubiquitous diatom Skeletonema marinoi. Transmission electron microscopy revealed that vacuolization occurred while the plasma membranes remained intact. Caspase 3-like activity, antioxidant gene expression, phosphatidylserine externalization and activated caspase-like expression were measured using in vivo staining. The results show that each was significantly increased under silicate limitation conditions. Using quantitative reverse transcription-PCR, we identified six distinct putative metacaspase proteins in S. marinoi, and we show that each contains a conserved caspase-like domain and is differentially expressed and that some of their expression profiles were correlated with PCD. Silicate limitation could trigger PCD in S. marinoi as well as an increase of antioxidant genes. PCD could be employed as a strategy to decrease silicate demand of the whole population and probably regulate the fate of S. marinoi.

The Diatom/Dinoflagellate Index as an Indicator of Ecosystem Changes in the Baltic Sea 1. Principle and Handling Instruction

An ecosystem shift in the Baltic Proper at the end of the 1980s could be identified by a decreasing springtime diatom/dinoflagellate ratio. Therefore the suggested diatom/dinoflagellate index (Dia/Dino index) was developed as a descriptive indicator of ecosystem changes for applications pertaining to the Marine Strategy Framework Directive (MSFD). Changes in the dominance of these two phytoplankton classes impact the food web because both their quality as a food source for grazers and their periods of occurrence differ, which may lead to a mismatch in zooplankton growth. The rapid sinking of diatoms results in a high Dia/Dino index, indicative of low-level food availability for zooplankton but high-level food delivery to the zoobenthos. Consequently, the Dia/Dino index can be used to follow the food pathway (Descriptor 4 of MSFD: “food web”). Moreover, a low Dia/Dino index may indicate silicate limitation caused by eutrophication, whereas a high Dia/Dino index supports mitigated eutrophication, as the strong sedimentation of nutrients removes them from the water and deposits them in the sediment (Descriptor 5 of MSFD: “eutrophication”). Diatom dominance, and thus a high Dia/Dino index, is typical in historical data and is therefore assumed to reflect good environmental status (GES). The principles of the Dia/Dino index are explained herein using examples from two very different regimes, the Eastern Gotland Basin and Kiel Bay. In the assessment of the environmental status of these water bodies, GES boundaries of 0.5 and 0.75, respectively, are suggested. The conditions for calculating the Dia/Dino index are described and the limitations and advantages of this indicator are discussed.

Environmental and biological factors controlling the spring phytoplankton bloom at the Patagonian shelf-break front – Degraded fucoxanthin pigments and the importance of microzooplankton grazing

The aim of this study was to investigate the biotic and abiotic factors controlling the spring phytoplankton blooms at the Patagonian shelf-break front (PSBF). Using a CHEMTAX analysis of HPLC pigment data and other methods, the biomass and spatial variability of plankton communities were studied in four sections (39–48°S) across the PSBF during October 2005. Environmental factors and the biomass and composition of plankton communities exhibited a marked spatial heterogeneity. The latitudinal and cross-shelf progression in the timing of the spring bloom initiation and the nutritive properties of the water masses (Subantarctic Shelf Waters and Malvinas Current Waters) seemed to be the key factors. Three plankton regions were distinguished: (a) Outer shelf (OS), (b) Shelf-break front (SBF) and (c) Malvinas Current (MC). At the highly stratified OS region, the post-bloom community showed low-biomasshigh-phytoplankton diversity formed mainly by small cells (haptophytes 30–62%, diatoms 17–49%, chlorophytes 0–34%, and prasinophytes 0–21% of total Chl a). High amounts of degraded fucoxanthin were found associated with the heterotrophic dinoflagellate, Protoperidinium capurroi. Grazing by this microheterotroph on the diatom population seemed to be the most important factor for the spring bloom decay at the OS. A remarkable quasi monospecific bloom (∼90%) of a nanodiatom (Thalassiosira bioculata var. raripora) associated with high Chl a (up to 20mgm−3) occurred along (∼1000km) the SBF and in the most northern extension of the MC. In the southern region, the bloom was developed under absent or incipient density stratification, increasing solar irradiance, high nitrate and phosphate availability, and low numbers of phytoplankton grazers. The average mixedlayer PAR irradiance (<2.0mol quanta PAR m−2d−1) and Si:N ratios (<0.2) were low, suggesting a diatom population limited by light and under progressive silicate limitation. The more stratified northern region of the SBF showed a later stage of the bloom development, but the large population of diatoms under Si limitation was not in senescence and losses from microzooplankton grazing were minor. The observed high proportion of Chl a below a shallow upper mixed layer (up to 85%) could directly reach the bed, favoring the development of epibenthic communities and the formation of seed diatom banks and organic iron-rich sediments. The upwelling along the SBF provides a large source of macronutrients and probably the dissolved iron needed to sustain the intense diatom bloom, but also diatom resting stages that could act as seeds for the next spring bloom. The macronutrient-rich MC region showed low chlorophyll (Chl a<0.8mgm−3) and a highly diverse phytoplankton community, mainly composed of small cells (diatoms 20–70%, haptophytes 20–40%, chlorophytes 2–25%, prasinophytes 2–18%, and cryptophytes 3–12% of total Chl a).

Evidence of intensified biogenic silica recycling in the Black Sea after 1970

The Black Sea has been subject to increased levels of nitrogen and phosphorus loading and a decrease in silicate input after around 1970. Changes in phytoplankton community composition from diatoms to non-diatom groups have been attributed to the decrease in silicate. However, a discrepancy between the decreasing silicate input and the increasing silicate pool in the deep sea reported elsewhere implies that another silicate source exists which challenges the current paradigm of widespread silicate limitation. In this study, we investigate changes in the dissolution state of siliceous protists over the last 140 years and show that siliceous protists became significantly more dissolved after the late 1960s indicating a reduction of the silicate pool preserved in the deep sea sediment. We hypothesize that the decline in the dissolution state is caused by increased recycling of biogenic silica in the water column due to an increased annual production driven by nitrogen enrichment.

Effect of silicate limitation on growth, cell composition, and lipid production of three native diatoms to Southwest Texas desert

To understand the potential of Nitzschia spp. in the biofuels industry, growth characteristics and fatty acid composition of three strains of Nitzschia grown at five silicate concentrations were investigated. The algae were grown in two stages with temperatures and light: dark cycles designed to mimic spring/fall conditions in the southern United States. First, cells were grown on Si concentrations ranging from 0.2 to 10.6 mol m−3. Second, they were grown for four more days with half of the initial medium replaced by a Si-free medium. Growth rates ranged from 0.16 to 0.30 day−1 in Si media and 0.07–0.39 day−1 in Si-free media. The total lipid content in the Si-free media ranged from 26.6 to 255 mg g−1 AFDW. In all cases, the fatty acid distribution was similar between strains, with palmitic acid (16:0) and palmitoleic acid (16:1) dominant. The results show that lipid production of native Nitzschia is strain-specific, and all three strains are potential candidates for the biofuel industry.

Global discovery and characterization of small non-coding RNAs in marine microalgae.

BackgroundMarine phytoplankton are responsible for 50% of the CO2 that is fixed annually worldwide and contribute massively to other biogeochemical cycles in the oceans. Diatoms and coccolithophores play a significant role as the base of the marine food web and they sequester carbon due to their ability to form blooms and to biomineralise. To discover the presence and regulation of short non-coding RNAs (sRNAs) in these two important phytoplankton groups, we sequenced short RNA transcriptomes of two diatom species (Thalassiosira pseudonana, Fragilariopsis cylindrus) and validated them by Northern blots along with the coccolithophore Emiliania huxleyi.ResultsDespite an exhaustive search, we did not find canonical miRNAs in diatoms. The most prominent classes of sRNAs in diatoms were repeat-associated sRNAs and tRNA-derived sRNAs. The latter were also present in E. huxleyi. tRNA-derived sRNAs in diatoms were induced under important environmental stress conditions (iron and silicate limitation, oxidative stress, alkaline pH), and they were very abundant especially in the polar diatom F. cylindrus (20.7% of all sRNAs) even under optimal growth conditions.ConclusionsThis study provides first experimental evidence for the existence of short non-coding RNAs in marine microalgae. Our data suggest that canonical miRNAs are absent from diatoms. However, the group of tRNA-derived sRNAs seems to be very prominent in diatoms and coccolithophores and maybe used for acclimation to environmental conditions.Electronic supplementary materialThe online version of this article (doi:10.1186/1471-2164-15-697) contains supplementary material, which is available to authorized users.

Effects of surface current patterns on spatial variations of phytoplankton community and environmental factors in Sunda shelf

Phytoplankton community and environmental factors were investigated in the surface water from eastern Indian Ocean (EIO) to southern South China Sea (SCS) during April–May monsoon transition period both in 2010 and 2011. Our results indicated that the surface current patterns were different in the two cruises. Sea surface salinity (SSS) was lower in Java Sea (JS), but its salinity front obviously moved to the middle of Sunda Strait when surface waters flowed from JS to EIO in May of 2010. Nutrient concentrations were generally higher in Sunda Strait and JS. Silicate concentrations were significantly lower in EIO suggesting a possibility of silicate limitation for the growth of phytoplankton, which were less than 1µmolL−1 in the offshore stations of EIO in May of 2010. Phytoplankton abundance and biodiversity were significant higher in Sunda strait and its adjacent stations, with larger cell-size and higher diatoms proportion. More offshore species were found in Sunda Strait when surface waters flowed from EIO to JS in April of 2011. Nutrient source can be different in Sunda Strait during inflow or outflow of surface waters. The spatial variation of phytoplankton community was greatly determined by Chaetoceros spp., Navicula sp., Pleurosigma affine, Thalassionema frauenfeldii and nano-dinoflagelage. Salinity, nitrate and silicate were the most important environmental factors regulating the variation of phytoplankton community. This study suggests that spatial distribution of phytoplankton and environmental factors are significantly influenced by the surface current patterns and river discharges, and these influences should be greater in dry season.

Cellular metabolic responses of the marine diatom Pseudo-nitzschia multiseries associated with cell wall formation

In this study a comparative proteomics approach involving a mass spectrometric analysis of synchronized cells was employed to investigate the cellular-level metabolic mechanisms associated with siliceous cell wall formation in the pennate diatom Pseudo-nitzschia multiseries. Cultures of P. multiseries were synchronized using the silicate limitation method. Approximately 75% of cells were arrested at the G2+M phase of the cell cycle after 48h of silicate starvation. The majority of cells progressed to new valve synthesis within 5h of silicon replenishment. We compared the proteome of P. multiseries at 0, 4, 5, and 6h of synchronization progress upon silicon replenishment using two-dimensional gel electrophoresis. Forty-eight differentially expressed protein spots were identified in abundance (greater than two-fold change; P<0.005), some of which are predicted to be involved in intracellular trafficking, cytoskeleton, photosynthesis, lipid metabolism, and protein biosynthesis. Cytoskeleton proteins and clathrin coat components were also hypothesized to play potential roles in cell wall formation. The proteomic profile analysis suggests that P. multiseries most likely employs multiple synergistic biochemical mechanisms for cell wall formation. These results improve our understanding of the molecular mechanisms underlying silicon cell wall formation and enhance our understanding of the important role played by diatoms in silicon biogeochemical cycling.

Silicate consumption: an indicator for long-term trends in spring diatom development in the Baltic Sea

Long-term data sets from 1979 to 2011 were analysed for trends in spring diatom development. As irregular phytoplankton sampling cannot record phytoplankton blooms completely, diatom growth during spring was estimated by silicate consumption. The spring diatom biomass decreased sharply at the end of the 1980s in the Baltic Proper but not in the neighbouring western Baltic area. The diatoms, mainly Thalassiosira spp. and Chaetoceros spp., reappeared after a series of cold winters. A strong negative correlation between the minimum water temperature and the magnitude of the diatom growth in the southern Baltic Proper confirms that diatom growth has decreased after mild winters, when motile phytoplankton, such as dinoflagellates and Mesodinium rubrum, may form blooms instead of diatoms. Silicate limitation did not occur in the Baltic Proper. Decreased convective mixing after mild winters may hamper diatom growth (stratification hypothesis), but this effect could not be proved. Our study supports the hypothesis that increased selective grazing pressure after mild winters may control diatom spring blooms (feeding hypothesis).

High CO2 and Silicate Limitation Synergistically Increase the Toxicity of Pseudo-nitzschia fraudulenta

Anthropogenic CO2 is progressively acidifying the ocean, but the responses of harmful algal bloom species that produce toxins that can bioaccumulate remain virtually unknown. The neurotoxin domoic acid is produced by the globally-distributed diatom genus Pseudo-nitzschia. This toxin is responsible for amnesic shellfish poisoning, which can result in illness or death in humans and regularly causes mass mortalities of marine mammals and birds. Domoic acid production by Pseudo-nitzschia cells is known to be regulated by nutrient availability, but potential interactions with increasing seawater CO2 concentrations are poorly understood. Here we present experiments measuring domoic acid production by acclimatized cultures of Pseudo-nitzschia fraudulenta that demonstrate a strong synergism between projected future CO2 levels (765 ppm) and silicate-limited growth, which greatly increases cellular toxicity relative to growth under modern atmospheric (360 ppm) or pre-industrial (200 ppm) CO2 conditions. Cellular Si∶C ratios decrease with increasing CO2, in a trend opposite to that seen for domoic acid production. The coastal California upwelling system where this species was isolated currently exhibits rapidly increasing levels of anthropogenic acidification, as well as widespread episodic silicate limitation of diatom growth. Our results suggest that the current ecosystem and human health impacts of toxic Pseudo-nitzschia blooms could be greatly exacerbated by future ocean acidification and ‘carbon fertilization’ of the coastal ocean.

Optimizing models of the North Atlantic spring bloom using physical, chemical and bio-optical observations from a Lagrangian float

Abstract. The North Atlantic spring bloom is one of the main events that lead to carbon export to the deep ocean and drive oceanic uptake of CO2 from the atmosphere. Here we use a suite of physical, bio-optical and chemical measurements made during the 2008 spring bloom to optimize and compare three different models of biological carbon export. The observations are from a Lagrangian float that operated south of Iceland from early April to late June, and were calibrated with ship-based measurements. The simplest model is representative of typical NPZD models used for the North Atlantic, while the most complex model explicitly includes diatoms and the formation of fast sinking diatom aggregates and cysts under silicate limitation. We carried out a variational optimization and error analysis for the biological parameters of all three models, and compared their ability to replicate the observations. The observations were sufficient to constrain most phytoplankton-related model parameters to accuracies of better than 15 %. However, the lack of zooplankton observations leads to large uncertainties in model parameters for grazing. The simulated vertical carbon flux at 100 m depth is similar between models and agrees well with available observations, but at 600 m the simulated flux is larger by a factor of 2.5 to 4.5 for the model with diatom aggregation. While none of the models can be formally rejected based on their misfit with the available observations, the model that includes export by diatom aggregation has a statistically significant better fit to the observations and more accurately represents the mechanisms and timing of carbon export based on observations not included in the optimization. Thus models that accurately simulate the upper 100 m do not necessarily accurately simulate export to deeper depths.

2003년 하계 장목만 단일정점에서 종속영양 와편모류와 광합성 와편모류 현존량의 시간적 변화

Abstract We investigated the temporal variations of heterotrophic dinoflagellates (hereafter HDNF) and photosynthetic dinoflagellates (hereafter PDNF) from 14 June to 4 September 2003 at a single station in Jangmok Bay. We took water samples 47 times from 2 depths (surface and bottom layers) at hide tide. A total of 63 species were encountered and in general the most abundant genera were Prorocentrum and Protoperidinium. The abundance of PDNF and HDNF was in the range of 0.04~55.8x10 4 cells/L and in the range of 0.01~4.35x10 4 cells/L, respectively. The mean abundance of PDNF was approximately 7 times higher than that of HDNF, and was higher in the surface layer where has enough irradiance for photosynthesis than in the bottom layer. The total dinoflagellate abundance was higher in the NLP (nitrogen limitation period) than in the SLP (silicate limitation period), and the abundance in the hypoxic conditions was similar to that in the normal conditions. The Shannon-Weaver species diversity index were slightly higher in the bottom layer, the SLP and the hypoxic conditions. The PDNF abundance were correlated with temperature, DO, total inorganic nitrogen and phosphate in the whole water column, and the HDNF abundance was significantly correlated with temperature, salinity and DO. This study shows that the dinoflagellate abundance might be affected by abiotic factors such as irradiance, temperature, salinity, DO and the concentrations of inorganic nutrients, and provides baseline information for further studies on plankton dynamics in Jangmok Bay. Key Words :Jangmok Bay, Heterotrophic dinoflagellates, Photosynthetic dinoflagellates, Inorganic nutrients, Hypoxia

Temporal and spatial distribution of biogenic silica and its significance in the Wuliangsuhai Lake and Daihai Lake

In this work, the concentrations, temporal and spatial distributions and the relationship between biogenic silica (BSi) and primary productivity are discussed on the basis of the geochemistry character of BSi in the water–sediment system of the Wuliangsuhai Lake and Daihai Lake. The results show that the average concentrations of SiO 3 2 - and BSi are 3.0 mg/L and 3.5 mg/g in the overlying water and sediments from the Wuliangsuhai Lake, respectively, while they are 1.0 mg/L and 7.5 mg/g, respectively, in the Daihai Lake. It is the uptake and assimilation of diatom phytoplankton which results in the significant difference of the SiO 3 2 - concentrations between the two lakes, and the inputs of surface runoff is one of the important factors in impacting the spatial distributions of SiO 3 2 - in the overlying water. The spatial distributions of BSi suggest the Si source of the two lakes and indicate the differences of eutrophication types and ancient primary producer between the two lakes. The eutrophication precesses and ancient primary productivity of diatom phytoplankton are reconstructed by applying the geochemistry information of BSi archived in the vertical concentration profiles in the two lake sediments. The geochemistry information of BSi well responds to the paleoenvironment and paleoclimate of the Daihai drainage basin indicating silicate limitation of primary productivity by diatoms phytoplankton in the Daihai Lake.

Variation in diatom biochemical composition during a simulated bloom and its effect on copepod production

The biochemical quality of phytoplankton depends in part upon nutrient availability, which has implications for the population dynamics of grazers. Here, we examined how nutrient availability influenced the growth dynamics and biochemical content of the marine diatom Thalassiosira weissflogii and how these biochemical changes impacted the vital rates of a calanoid copepod (Acartia tonsa). Changes in biochemistry protein, carbohydrate and fatty acids) were compared in diatom cultures that simulated bloom conditions (B-algae) and those maintained in near exponential growth (E-algae) over the course of a 16-day experiment. Egg production rates (EPRs, eggs female -1 day -1 ) and the developmental success of copepodite stages of A. tonsa fed these different diets were quantified. Copepod EPR was significantly lower (reduced by half) when B-algae entered the senescent phase due to silicate limitation. In a crossover (diet switch) experiment, EPR increased when copepods fed B-algae were switched to E-algae and vice versa. Copepodites of A. tonsa developed normally and reached the adult (C6) stage when fed E-algae, but ceased development (approximately at stage C2) when reared on senescent phase B-algae. Given the importance of copepods as prey for higher trophic levels, our results highlight how nutritional changes that naturally occur during a phytoplankton bloom may influence the productivity of copepods and higher trophic levels.

Si cycle in the Cariaco Basin, Venezuela: Seasonal variability in silicate availability and the Si:C:N composition of sinking particles

A 9‐year time series of water column and sediment trap measurements was used to examine silicon cycling within the anoxic Cariaco Basin. The dynamic hydrographic regime within Cariaco Basin results in strong seasonal changes in nutrient availability and the Si:C:N of sinking particles. Upwelling in early winter injects silicic acid (Si(OH)4= ) and nitrate (NO3−) rich waters into the photic zone which stimulates primary production and results in opal fluxes in excess of 4 mmol Si m−2 d−1. However, even during upwelling, surface waters tend to be depleted (&lt;1 μM) in both Si(OH)4= and NO3− as a result of rapid utilization. In most years, the upper water column during winter and spring is marked by Si(OH)4= :NO3− and Si* values of less than 1. This indicates that silicate limitation in Cariaco Basin is most severe during upwelling and may restrict diatom production. Conversely, during the summer and fall when upwelling is reduced, Si(OH)4= :NO3− ratios in the upper 50 m of the water column exceed 10, implying that nitrate rather than silicate is acting to limit production during this time of year. On average, sinking particles collected at 150‐m depth in the Cariaco Basin have Si:C and Si:N values of 0.17 ± 0.01 and 1.14 ± 0.10, respectively. These ratios increase with depth to 400 m and then remain relatively constant, suggesting minimal selective removal of elements with remineralization in the anoxic portion of the water column. Similar depth‐dependent changes in these ratios are seen in surface sediments from the basin. Seasonally, particulate Si:C and Si:N are highest during the early part of the year when upwelling is most intense, while both ratios decrease to their lowest values during summer and fall. The observed seasonal variability in these ratios is due to changes in both nutrient utilization by diatoms and the contribution of diatoms to the total phytoplankton. The high ratios during upwelling suggest enhanced export of Si relative to C and N during this time of year.

Nutrient-Replete Benthic Microalgae as a Source of Dissolved Organic Carbon to Coastal Waters

Dissolved organic carbon (DOC) flux dynamics were examined in the context of other biogeochemical cycles in intertidal sediments inhabited by benthic microalgae. In August 2003, gross oxygenic photosynthetic (GOP) rates, oxygen penetration depths, and benthic flux rates were quantified at seven sites along the Duplin River, GA, USA. Sediments contained abundant benthic microalgal (BMA) biomass with a maximum chlorophyll a concentration of 201 mg chl a m−2. Oxygen microelectrodes were used to determine GOP rates and O2 penetration depth, which were tightly correlated with light intensity. Baseline and 15N-nitrate amended benthic flux core incubations were employed to quantify benthic fluxes and to investigate the impact of BMA on sediment water exchange under nitrogen (N)-limited and N-replete conditions. Unamended sediments exhibited tight coupling between GOP and respiration and served as a sink for water column dissolved inorganic nitrogen (DIN) and a source of silicate and dissolved inorganic carbon (DIC). The BMA response to the N addition indicated sequential nutrient limitation, with N limitation followed by silicate limitation. In diel (light–dark) incubations, biological assimilation accounted for 83% to 150% of the nitrate uptake, while denitrification (DNF) and dissimilatory nitrate reduction to ammonium (DNRA) accounted for 40% of the NO3− uptake. The N addition shifted the metabolic status of the sediments from a balance of autotrophy and heterotrophy to net autotrophy under diel conditions, and the sediments served as a sink for water column DIN, silicate, and DIC but became a source of DOC, suggesting that the increased BMA production was decoupled from sediment bacterial consumption of DOC.

Phytoplankton response to climate warming modified by trophic state

We investigated the combined effect of reduced phosphorus supply and warmer winter and spring conditions on the diatom spring bloom of a shallow lake. Simulations with a simple dynamic model indicated that reduced ice cover and increasing water temperatures resulted in a more intense and earlier bloom independently of phosphorous concentrations. However, whereas the collapse of the bloom was caused by silicate limitation under high phosphorus supply, it was caused by Daphnia grazing under reduced phosphorus supply. This switch from a bottom‐up to a top‐down driven collapse of the diatom spring bloom explains why, despite similarly mild winters, the bloom was observed earlier under high than under reduced phosphorus supply in the lake studied. Thus, an assessment of possible changes in nutrient loading is crucial when anticipating how phytoplankton could evolve under future climate warming.

Real-time forecasting of ecosystem dynamics during the CROZEX experiment and the roles of light, iron, silicate, and circulation

Real-time coupled physical and biological forecasting was conducted prior to and during the CROZet natural bloom and EXport experiment (CROZEX) programme in the Southern Ocean (SO) between November 2004 and January 2005. The programme was aimed at investigating, through both measurements and modelling, the origin and fate of an intense and long-lasting phytoplankton bloom that is unusual for the otherwise high-nutrient low-chlorophyll conditions that characterise the SO. It was hypothesised that dissolved iron supplied to the system from shallow sediments accumulates downstream of the Crozet Islands during winter and creates conditions that favour phytoplankton production. Regular 14-day forecasts and reanalysis of the biogeochemical fields were issued on a weekly basis using a 3D primitive equation model coupled with an 11-component ecosystem model, along with assimilation of remotely sensed and in situ data. Real-time modelling assisted not only in optimisation of the cruise strategy, but also provided an ongoing synthesis of observations and test of the working hypotheses proposed to explain bloom dynamics in the study area.The model results indicated that a complex interplay between light, iron, and silicate limitation on primary production and grazing control determined the spatial extent and dynamics of the phytoplankton bloom. The model suggested that natural fertilisation of the area by dissolved iron was insufficient on its own to initiate a phytoplankton bloom. The persistence of deep mixing in the southern part of the iron-enriched waters was delaying, or in some areas preventing, the development of the bloom.