Fertilisation de communautés phytoplanctoniques. ii. cas d'un milieu eutrophe: upwelling des côtes du sahara espagnol

Experiments with nutrient enrichment of phytoplankton communities have been carried out on surface waters from the upwelling area off Cap Blanc — Cap Corveiro (Spanish Sahara).None of the control flasks showed phytoplankton development despite the large quantities of nutrient available: this may be due to the presence of an inhibitor in the upwelling waters. On the other hand, most of the enrichments stimulated the autotrophic production. On average, the best growth rates were obtained when a full medium minus Fe-EDTA was added; marked phytoplankton development also took place when a medium without vitamins, or silicates, or phosphates, was added. Lack of metallic compounds (other than iron) and nitrates limit the algal growth.The maximum amounts of chlorophyll a (25–30 mg/m³) are close to the maxima usually observed in this area. The productivity rate (biomass produced/nutrients used) is very high.     

Nutrient limitation of Microcystis aeruginosa in northern California Klamath River reservoirs

Nutrient limitations were investigated in Copco and Iron Gate Reservoirs, on the Klamath River in California, where blooms of the toxin-producing cyanobacterium Microcystis aeruginosa were first reported in 2005. Nutrient enrichment experiments conducted in situ in June and August, 2007 and 2008, determined responses in phytoplankton biomass, Microcystis abundance and microcystin concentration to additions of phosphorus and different forms of nitrogen (NH₄⁺, NO₃, and urea). Microcystis abundance was determined using quantitative PCR targeting the phycocyanin intergenic spacer cpcBA.Total phytoplankton biomass increased with additions of N both before and during Microcystis blooms, with no primary effects from P, suggesting overall N limitation for phytoplankton growth during the summer season. NH₄⁺ generally produced the greatest response in phytoplankton growth, while Microcystis abundance increased in response to all forms of N. Microcystis doubling time in the in situ experiments was 1.24–1.39 days when N was not limiting growth. The results from this study suggest availability of N during the summer is a key growth-limiting factor for the initiation and maintenance of toxic Microcystis blooms in Copco and Iron Gate Reservoirs in the Klamath River.     

Nutrient addition effects on chlorophyll a,phytoplankton biomass,and heterocyte formation in Lake Erie’s central basin during 2014–2017: Insights into diazotrophic blooms in high nitrogen water

1. Phosphorus (P) usually is the primary limiting nutrient of phytoplankton biomass, but attention towards nitrogen (N) and trace nutrients, such as iron (Fe), has surfaced. Additionally, N-fixing cyanobacterial blooms have been documented to occur in N-rich, P-poor waters, which is counterintuitive from the paradigm that low N and high P promotes blooms. For example, Lake Erie's central basin has Dolichospermum blooms when nitrate concentrations are high, which raises questions about which nutrient(s) are selecting for Dolichospermum over other phytoplankton and why an N-fixer is present in high N waters?
2. We conducted a 4-year (2014–2017) study in Lake Erie's central basin to determine which nutrient (P, N, or trace nutrients such as Fe, molybdenum [Mo], and boron [B]) constrained chlorophyll concentration, phytoplankton biovolume, and nitrate assimilation using nutrient enrichment bioassays. The enriched lake water was incubated in 1-L bottles in a growth chamber programmed at light and temperatures of in situ conditions for 4–7 days. We also quantified heterocytes when N-fixing cyanobacteria were present.
3. Compared to the non-enriched control, the P-enriched (+P) treatment had significantly higher chlorophyll and phytoplankton biovolume in c. 75% of experiments. Combination enrichments of P with ammonium-N, nitrate-N, Fe, Mo, and B were compared to the +P treatment to determine secondary limitations. +P and ammonium-N and +P nitrate-N resulted in higher chlorophyll in 50% of experiments but higher phytoplankton biovolume in only 25% of experiments. These results show that P was the primary limiting nutrient, but there were times when N was secondarily limiting.
4. Chlorophyll concentration indicated N secondary limitation in half of the experiments, but biovolume indicated only N secondary limitation in 25% of the experiments. To make robust conclusions from nutrient enrichment bioassays, both chlorophyll and phytoplankton biovolume should be measured.
5. The secondary effects of Fe, Mo, and B on chlorophyll were low (<26% of experiments), and no secondary effects were observed on phytoplankton biovolume and nitrate assimilation. However, +P and Fe resulted in more chlorophyll than +P in experiments conducted during Dolichospermum blooms, and +P and B significantly increased the number of heterocytes in Dolichospermum. These results indicate that low Fe availability might select for Dolichospermum, and low B constrains heterocyte formation in the central basin of Lake Erie. Furthermore, these results could apply to other lakes with high N and low P where diazotrophic cyanobacterial blooms occur.

     

In situ microcosm experiments on the influence of nitrate and light on phytoplankton community composition

Phytoplankton blooms are fundamental features of coastal ecosystems, but the processes that select for blooms of certain species are not well understood. The aim of this work was to investigate experimentally the interaction of light and nutrients (nitrate) in structuring phytoplankton community composition in Pelorus Sound, New Zealand. Microcosm experiments were conducted in situ nine times throughout the year, providing controls and treatments for increased nutrients and decreased light. Nitrate availability was found to be limiting to phytoplankton growth during spring and summer. Small- to medium-sized, chain-forming diatom taxa such as Chaetoceros sp., Skeletonema sp., Pseudonitzschia sp. and Thalassiosira sp. responded most rapidly to nitrate enrichment, increasing their biovolume up to 32-fold during the 5-day experiments. A long-term phytoplankton monitoring database showed that these taxa have historically dominated the phytoplankton assemblage, suggesting that intense competition for nitrate is a key component in structuring the phytoplankton community. Many of the taxa that were able to withstand light reduction in the shaded treatments were rare historically in Pelorus Sound, suggesting that light is secondary to nitrate availability in structuring the phytoplankton community composition in this coastal embayment.     

Alteration of phytoplankton phosphorus status during enrichment experiments: implications for interpreting nutrient enrichment bioassay results

We compared the results of phosphorus-enrichment bioassay experiments with alkaline phosphatase activity (APA) assays as indicators of phosphorus (P) limitation of in situ phytoplankton growth. In 4-d experiments, phytoplankton APA decreased or remained unchanged in P-enriched samples, but increased in unenriched samples, indicating a rapid alteration of the P status of the unenriched algae during the experimental incubations. In direct comparisons of enrichment bioassays and APA assays of reservoir phytoplankton samples, the results of the two methods corresponded in general, although contradictory results were not uncommon. Our data support the conclusion that enrichment experiments can indicate the potential for nutrient limitation of algal growth in the absence of other limiting factors, but do not necessarily demonstrate the occurrence of in situ nutrient limitation of phytoplankton production.     

Microbial Dynamics of an Epilithic Mat Community in a High Alpine Stream

Studies were conducted to examine interrelationships between the heterotrophic and phototrophic populations within an epilithic community in the outlet stream of a high alpine lake. Levels of nitrates, phosphates, and total organic compounds in the lake were consistently near the lower limits of detectability. Microscopic examination of the community by phase-contrast light microscopy and scanning electron microscopy revealed diatoms, filamentous algae, and bacteria embedded within a dense gelatinous matrix. Chlorophyll a and primary productivity measurements had peak values in early August, with subsequent declines. Bacterial heterotrophic activity, as measured by V_max, turnover rate, and relative activity, increased significantly as the phototrophic community declined. This trend in heterotrophic activity was not accompanied by an increase in total bacterial numbers as determined by epi-illuminated fluorescence microscopy. These results suggest that the phototrophic community responded to changes in, or interactions among, various chemical and physical factors throughout the study period. The catabolic activity of the sessile bacteria appeared to be positively influenced by changes in the mat environment resulting from the decline of the phototrophic populations.     

The production ecology of Stephanodiscus Astraea (Ehrenb.) Grun

The production ecology of Stephanodiscus astraea (Ehrenb.) Grun. is discussed with respect to other phytoplankton growths, mixing regimes and nutrient availability. Populations of Stephanodiscus were studied during 1972 and 1973 in an artificially mixed reservoir with a capacity of 3.5 × 10⁷m³. Major nutrients such as soluble reactive phosphates (60–200 μg/1) and nitrates (1–8 mg/1) were not limiting. Large amounts of silicate (approximately 10 mg/1) were utilized to support moderately large populations of S. astraea (1.3 × 10⁷ μm³/ml). Light penetration and mixing regimes strongly affected the periodicity and size of standing crops. It is suggested that heterotrophic production can interact with the mixing environment to influence the production of turbid waters.     

Growth rate of alpine phytoplankton assemblages from contrasting watersheds and N‐deposition regimes exposed to nitrogen and phosphorus enrichments

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Variation in nutrient limitation of lotic and lentic algal communities in a Texas (USA) river

Nutrient limitation of periphyton and phytoplankton was assessed in the Upper Guadalupe River, Texas USA. Nutrient-diffusing substrates with added nitrogen (N) and phosphorus (P) were used to identify the limiting nutrient for lotic algae at three river sites in summer, fall, and winter. Pots enriched with P had significantly higher chlorophyll a concentrations for 7 of 9 trials. Added N alone did not significantly increase algal standing crops, although it was found to be secondarily limiting on one (and possibly two) occasions. Flow-through enrichment experiments were conducted in order to quantify the concentration of P needed to significantly increase algal standing crops. Response to enrichment was rapid when ambient P concentration was low (< 0.010 mg L^–1), but more moderate when ambient P levels were higher (0.015–0.025 mg L^–1). Nutrient limitation of phytoplankton in small surface-release reservoirs varied throughout the study, but N was either primarily or secondarily limiting in 6 of 8 trials; shifts in the limiting nutrient were correlated with fluctuations in flow into the reservoirs. Our enrichment studies show that algal response to nutrient addition was unpredictable as phytoplankton tended to be N-limited while periphyton was mainly P-limited. Further, while discharge apparently dictated the nutrient-biomass relationship for phytoplankton in reservoirs, ambient nutrient level is an important determinant of lotic periphyton response to enrichment.     

Facteurs hydrologiques et phytoplancton en Baie de Morlaix (Manche Occidentale)

Phytoplankton in Roscoff waters (Western English Channel) was studied in spring and summer of 1979 and 1980, parallel to seasonal changes in physical, chemical and biological variables. Strong tides prevent the formation of a seasonal thermocline and the water column remains well mixed throughout the year. Seasonal changes in temperature and salinity show a continuous increase from April to August-September. Dissolved oxygen, dissolved mineral and organic nitrogen and phosphorus vary in relation to phytoplankton production. Silicate shows a seasonal cycle, but is not completely depleted in the sea-water. A decrease of ammonia during the phytoplankton blooms in 1980 suggests that ammonia plays an important role as a nitrogen source. The NO₃/PO₄ ratio indicates that the sea-water is heavily loaded with nitrate, and shows peak values during phytoplankton blooms, due to a fall in the concentration of phosphate, which may become a limiting nutrient. However, this effect may be offset by a rapid recycling of this nutrient. Chlorophyll a and potential primary production show significant variations. The major blooms are observed in June during both years. The mean production/chl. a ratio is around 6 in both years. Phytoplankton development is marked by the succession of three major groups during the spring-summer blooms: Thalassiosira spp, Rhizosolenia spp and Chaetoceros spp. The regularity in phytoplankton species succession over a number of years suggests that the species concerned are autochtonous and well adapted to their environment.     

Grazers and Phytoplankton Growth in the Oceans: an Experimental and Evolutionary Perspective

The taxonomic composition of phytoplankton responsible for primary production on continental shelves has changed episodically through Earth history. Geological correlations suggest that major changes in phytoplankton composition correspond in time to changes in grazing and seawater chemistry. Testing hypotheses that arise from these correlations requires experimentation, and so we carried out a series of experiments in which selected phytoplankton species were grown in treatments that differed with respect to the presence or absence of grazers as well as seawater chemistry. Both protistan (Euplotes sp.) and microarthropod (Acartia tonsa) grazers changed the growth dynamics and biochemical composition of the green alga Tetraselmis suecica, the diatom Thalassiosira weissflogii, and the cyanobacterium Synechococcus sp., increasing the specific growth rate and palatability of the eukaryotic algae, while decreasing or leaving unchanged both parameters in the cyanobacteria. Synechococcus (especially) and Thalassiosira produced toxins effective against the copepod, but ciliate growth was unaffected. Acartia induced a 4-6 fold increase of Si cell quota in the diatom, but Euplotes had no similar effect. The differential growth responses of the eukaryotic algae and cyanobacteria to ciliate grazing may help to explain the apparently coeval radiation of eukaryophagic protists and rise of eukaryotes to ecological prominence as primary producers in Neoproterozoic oceans. The experimental results suggest that phytoplankton responses to the later radiation of microarthropod grazers were clade-specific, and included changes in growth dynamics, toxin synthesis, encystment, and (in diatoms) enhanced Si uptake.     

Determination of growth-limiting nutrients for red species of Oscillatoria and two ‘oligotrophic’ diatoms

Laboratory batch experiments with dilute phytoplankton communities were carried out to determine growth-limiting nutrients and the degree of growth limitation for Asterionella formosa, Hass., Tabellaria fenestrata (Lyngb.) Kütz. and red species of Oscillatoria under specific temperature and light conditions. Water samples from five Norwegian lakes with average epilimnetic total phosphorus concentrations ranging from 5 to 30 µg P per 1 were investigated. Both enrichment and transplant experiments were carried out. In samples from the most oligotrophic lakes (Lake Randsfjorden and Lake Tyrifjorden) phosphorus was found to be the growth-limiting nutrient. In samples from Lake Mjøsa silicate was also growth-limiting for the diatoms, while nitrogen could be limiting for both asterionella and Oscillatoria from Lake Steinsfjorden. Phosphorus was often the only limiting nutrient determined for Oscillatoria in Lake Gjersjøen (the most eutrophic of the lakes). In samples from this lake, however, Asterionella was also growth-limited directly or indirectly as a result of high (pH > 9). This was also found for Asterionella and Tabellaria from lake Mjøsa by means of transplant experiments. Oscillatoria from Lake Gjersjøen could not grow in filtered water from Lake Mjøsa but obtained maximum growth rate after addition of phosphate and chelated iron in combination.     

Nutrient limitation of the primary production of phytoplankton in Lake Baikal

Nutrient limitation of the primary production of phytoplankton at some stations in southern and central Lake Baikal was studied by nutrient enrichment experiments in August 2002. Chlorophyll (Chl.) a concentrations ranged from 0.7 to 5.8μgl⁻¹. Inorganic nutrient concentrations were low: soluble reactive phosphorus ranged from 0.05 to 0.20μmoll⁻¹, ammonia from 0.21 to 0.41μmoll⁻¹, and nitrite plus nitrate from 0.33 to 0.37μmoll⁻¹. In the five enrichment experiments, phosphate spikes and phosphate plus nitrate spikes always stimulated primary production. Nitrate spikes also stimulated primary production in four of the experiments. Significant differences were detected between the controls and phosphate spikes and between the controls and phosphate plus nitrate spikes. Thus, the first limiting nutrient is thought to be phosphorus, but once phosphorus is supplied to the surface water, the limiting nutrient will quickly shift from phosphorus to nitrogen.     

Identification and quantification of phytoplankton groups in lakes using new pigment ratios – a comparison between pigment analysis by HPLC and microscopy

1. Pigment analysis by high‐performance liquid chromatography (HPLC) combined with data analysis using the CHEMTAX program has proven to be a fast and precise method for determining the abundance of phytoplankton groups in marine environments. To determine whether CHEMTAX is applicable also to freshwater phytoplankton, 20 different species of freshwater algae were cultured and their pigment/chlorophyll a (Chl a) ratios determined for exponential growth at three different light intensities and for stationary growth at one light intensity. 2. The different treatments had a relatively insignificant impact on the absolute values of the diagnostic pigment/Chl a ratios, with the exception of cyanobacteria and cryptophytes for which the zeaxanthin/Chl a and alloxanthin/Chl a ratios varied considerably. 3. The pigment ratios were tested on samples collected in six different eutrophic Danish lakes during two summer periods using the CHEMTAX program to calculate the biomass of the phytoplankton groups as Chl a. The CHEMTAX‐derived seasonal changes in Chl a biomass corresponded well with the volume of the microscopically determined phytoplankton groups. More phytoplankton groups were detected by the pigment method than by the microscopic method. 4. Applying the pigment ratios developed in this study, the pigment method can be used to determine the abundance of the individual phytoplankton groups, which are useful as biological water quality indicators when determining the ecological status of freshwater lakes.     

Primary productivity,phytoplankton and limiting nutrient factors in labrador lakes

The primary productivity of some lakes and reservoirs in western Labrador was measured by the ¹⁴C method in order to determine the range of productivities and the effects of impoundment. No primary productivity data previously existed for this part of Canada. Both the primary productivity and standing crops of phytoplankton were found to be low in a newly impounded lake but later rose to levels greater than in surrounding natural lakes. In nutrient enrichment experiments, carbon was never found to be limiting but phosphorus stimulated primary productivity when added alone or in combination with nitrogen.     

Response of Sargasso Sea phytoplankton biomass, growth rates and primary production to seasonally varying physical forcing

The response of phytoplankton biomass, growth rates and primaryproduction to seasonally varying physical forcing was studiedat a station southeast of Bermuda over an 18 month period. Phytoplanktongrowth rates and primary production were measured using thepigment-labeling method, and phytoplankton biomass was calculatedfrom these measurements. Phytoplankton carbon biomass variedsystematically over the year. Highest values were observed duringthe winter and spring. Seasonal variations of chlorophyll (Chi)a in the surface layer could primarily be attributed to variationsin phytoplankton biomass and secondarily to photoacclimation.During the summer period, average values of carbon (C)/Chl ratios(g C g^–1 Chi) ranged from 160 at the surface to 33 atthe 1.6% light level, changes attributed to photoacclimationof the phytoplankton, consistent with the observation that phytoplanktonbiomass did not vary as a function of depth. Phytoplankton growthrates in the surface layer did not vary systematically overthe year, ranging from 0.15 to 0.45 day^–1, in spite ofseasonally varying concentrations of nitrate. Growth rates variedas a function of depth from average values of 0.3 day^–1in the surface layer to <0.1 day¹ at the 1.6% light level.Thus, the primary response of the phytoplankton community tonutrient enrichment during the winter period was an increasein phytoplankton biomass rather than an increase in growth rates.A simple nutrient-phyto-plankton-zooplankton model was usedto explore this phenomenon. The model demonstrated that theobserved response of the phytoplankton to nutrient enrichmentis only possible when phytoplankton growth is not severely limitedby nutrients.     

Nutrient limitation of phytoplankton in a naturally acidic lake (Lake Caviahue, Argentina)

As a result of a low pH, the inorganic carbon of acidic lakes is present as CO₂ at air-equilibrium concentration and is substantially lower than the inorganic carbon concentration in higher-pH waters with bicarbonate. This situation is quite common in artificially acidified lakes and where inorganic carbon is considered the limiting factor in phytoplankton growth. Apart from low inorganic carbon content, Lake Caviahue in Argentina has low nitrogen and high phosphorus content. The aim of this work was to assess the importance of inorganic carbon, phosphorus, and nitrogen, relating data on lake nutrients to phytoplankton species requirements. Lake samples taken in the 2004–2006 period did not show any particular trend in the vertical distribution of the water column of ammonium, inorganic carbon, and phosphorus with reference to either seasonality or depth. A decrease of some 15% in the lake’s phosphorus concentration was observed over the same period. Although the total phytoplankton biomass in Lake Caviahue was similar throughout the period, a seasonal variation was observed. Lab bioassays were carried out with solutions of bicarbonates, ammonium, nitrates, and phosphate. We worked with three species separately, namely, two chlorophytes, Keratococcus rhaphidioides and Watanabea sp.; and one euglenophyte, Euglena mutabilis. Answers to specific nutrient requirements differed for each algal species: both chlorophytes prefer ammonium or nitrates added on their own, whereas the euglenophyte registered a higher growth rate with the joint addition of ammonium and phosphorus. Even when the limiting nutrient(s) for phytoplankton yield and rate varied between species, we observed a tendency for nitrogen limitation in Lake Caviahue.     

Nutrient limitation of Microcystis aeruginosa in northern California Klamath River reservoirs

Nutrient limitations were investigated in Copco and Iron Gate Reservoirs, on the Klamath River in California, where blooms of the toxin-producing cyanobacterium Microcystis aeruginosa were first reported in 2005. Nutrient enrichment experiments conducted in situ in June and August, 2007 and 2008, determined responses in phytoplankton biomass, Microcystis abundance and microcystin concentration to additions of phosphorus and different forms of nitrogen (NH₄⁺, NO₃, and urea). Microcystis abundance was determined using quantitative PCR targeting the phycocyanin intergenic spacer cpcBA.Total phytoplankton biomass increased with additions of N both before and during Microcystis blooms, with no primary effects from P, suggesting overall N limitation for phytoplankton growth during the summer season. NH₄⁺ generally produced the greatest response in phytoplankton growth, while Microcystis abundance increased in response to all forms of N. Microcystis doubling time in the in situ experiments was 1.24–1.39 days when N was not limiting growth. The results from this study suggest availability of N during the summer is a key growth-limiting factor for the initiation and maintenance of toxic Microcystis blooms in Copco and Iron Gate Reservoirs in the Klamath River.     

Studies on phytoplankton pigments in Porto Novo waters (India). I. Mangrove

The annual variations of phytoplankton pigments were studied from January to December, 1971, at two stations of the local mangrove (Pichavaram) environment. At these two stations, chlorophyll a varied from 2.90 to 35.06; chlorophyll b from 0 to 10.02 and chlorophyll c from 0 to 18.12 μg/l. Plant carotenoids varied from 1.56 to 13.83 MSPU/m³ and phaeopigments from 0 to 12.28 μg/l. The main (primary) peak of chlorophyll a was recorded during March at Station 1, and during June at Station 2.Secondary maxima occurred during June and August at Station 1, and during September at Station 2. During the period studied chlorophyll a was the dominant pigment at both the stations, followed by chlorophyll c and chlorophyll b in that order. The increase in the concentration of pigments was mainly due to the presence of phytoplankton species belonging to the genera such as Coscinodiscus, Rhizosolenia, Thalassiothrix, Melosira, Chaetoceros and Biddulphia. During October, phytoplankton was less and the pigment concentration was also low.     

Photoadaptation in marine phytoplankton : changes in spectral absorption and excitation of chlorophyll a fluorescence

The optical properties of marine phytoplankton were examined by measuring the absorption spectra and fluorescence excitation spectra of chlorophyll a for natural marine particles collected on glass fiber filters. Samples were collected at different depths from stations in temperate waters of the Southern California Bight and in polar waters of the Scotia and Ross Seas. At all stations, phytoplankton fluorescence excitation and absorption spectra changed systematically with depth and vertical stability of the water columns. In samples from deeper waters, both absorption and chlorophyll a fluorescence excitation spectra showed enhancement in the blue-to-green portion of the spectrum (470-560 nm) relative to that at 440 nm. Since similar changes in absorption and excitation were induced by incubating sea water samples at different light intensities, the changes in optical properties can be attributed to photoadaptation of the phytoplankton. The data indicate that in the natural populations studied, shade adaptation caused increases in the concentration of photosynthetic accessory pigments relative to chlorophyll a. These changes in cellular pigment composition were detectable within less than 1 day. Comparisons of absorption spectra with fluorescence excitation spectra indicate an apparent increase in the efficiency of sensitization of chlorophyll a fluorescence in the blue and green spectral regions for low light populations.