STEADY-STATE LUXURY CONSUMPTION AND THE CONCEPT OF OPTIMUM NUTRIENT RATIOS: A STUDY WITH PHOSPHATE AND NITRATE LIMITED SELENASTRUM MINUTUM (CHLOROPHYTA)1

Selenastrum minutum (Naeg.) Collins was grown over a wide range of growth rates under phosphate or nitrate limitation with non-limiting nutrients added to great excess. This resulted in saturated luxury consumption. The relationships between growth rate and cell quota for the limiting nutrients were well described by the Droop relationship. The observed variability in N cell quota under N limitation as reflected in k_Q·Q_max^?1*, was similar in magnitude to previously reported values but k_Q·Q_max^?1* for P under P limitation was greater than previously reported for other species. These results were evaluated in light of the optimum ratio hypothesis. Our findings support previous work suggesting that the use of a single optimum ratio (k_Qi·K_Qj^?1) is inappropriate for dealing with a species growing under steady-state nutrient limitation. Under these conditions the optimum ratio should be viewed as a growth rate dependent variable. Two approaches for testing the growth rate dependency of optimum ratios are proposed. The capacity for luxury consumption differed between nutrients and was growth rate dependent. At low growth rates, the coefficient of luxury consumption (R_sat) for P was ca. four times that for N. The set of all possible relationships between N and P cell quota under these conditions was reported and these values were then used to establish the cellular N:P niche boundaries for S. minutum. Cell quotas of non-limiting nutrients were not described by the Droop equation. Analysis showed that as the cellular N:P ratio deviates from the optimum ratio, the ability of the Droop equation to describe the relationship between growth rate and non-limiting cell quotas decreases. When non-limiting nutrient cell quotas are saturated, the Droop equation appears to be invalid. Previously reported patterns of non-limiting nutrient utilization are summarized in support of this conclusion. The physiological and ecological consequences of luxury consumption and growth rate dependent optimum ratios are considered.     

HOW CRITICAL IS THE CRITICAL N:P RATIO?1

The value of cellular N:P that corresponds to co‐limitation by N and P, the critical (R_crit) or optimum ratio, has been used to infer the competitive advantage of phytoplankton growing in P‐impoverished systems. Using a revised quota model, with a normalized quota function and capable of simulating surge transport, the interactions between the minimum P‐quota (PCo), the shape of the P‐quota–cell growth relationship (affected by constant KQP), and transport kinetics in affecting the utility of R_crit are considered. For a low PCo to endow an organism with a high R_crit over a wide range of growth rates, the P‐quota curve must be more hyperbolic (KQP low) rather than linear (KQP high). PCo and KQP also affect the half saturation constant for growth, KgP ; this and the capacity to transport nutrients at rates above those required to sustain steady‐state growth endows a competitive advantage. However, the kinetics of transport into the organism have a greater potential for affecting KgP than changing the kinetics of internal P usage. Thus, the value of R_crit is not a critical factor affecting competition except in extreme oligotrophic conditions. For competition between species, nutrient transport, accumulation, and resource utilization are all important. However, the efficiency of internal resource utilization is of lesser importance, and certainly not of greater importance, than resource acquisition. Multinutrient models intended to describe competition need to recognize these interactions; the traditional quota model is poorly equipped to do so.     

Monitoring cellular C:N ratio in phytoplankton by means of FTIR‐spectroscopy

Statistical growth rate modelling can be applied in a variety of ecological and biotechnological applications. Such models are frequently based on Monod or Droop equations and, especially for the latter, require reliable determination of model input parameters such as C:N quotas. Besides growth rate modelling, a C:N quota quantification can be useful for monitoring and interpretation of physiological acclimation to abiotic and biotic disturbances (e.g., nutrient limitations). However, as high throughput C:N quota determination is difficult to perform, alternatives need to be established. Fourier‐transformed infrared (FTIR) spectroscopy is used to analyze a variety of biochemical, chemical, and physiological parameters in phytoplankton. Hence, a quantification of the C:N quota should also be feasible. Therefore, using FTIR spectroscopy, six phytoplankton species from among different phylogenetic groups have been analyzed to determine the effect of nutrient limitation on C:N quota patterns. The typical species‐specific response to increasing nitrogen limitation was an increase in the C:N quota. Irrespective of this species specificity, we were able to develop a reliable multi‐species C:N quota prediction model based on FTIR spectroscopy using the partial least square regression (PLSR) algorithm. Our data demonstrate that the PLSR approach is more robust in C:N quota quantification (R² = 0.93) than linear correlation of C:N quota versus growth rate (R² ranges from 0.74 to 0.86) or biochemical information based on FTIR spectra (R² ranges from 0.82 to 0.89). This accurate prediction of C:N values may support high throughput measurements in a broad range of future approaches.     

Steady-State Growth and Chemical Composition of the Marine Chlorophyte Dunaliella tertiolecta in Nitrogen-Limited Continuous Cultures

The marine chlorophyte Dunaliella tertiolecta was grown in continuous cultures under NH₄⁺-N, NO₂⁻-N, NO₃⁻-N, and urea-N limitations. The effect of the nitrogen cell quota (Q_n) on the steady-state growth rate (μ) was the same regardless of the N source. The relationship between μ and Q_n was well described by the Droop equation, but only up to the true maximum growth rate ^μ (= cell washout rate). The ratio between the minimum cell quota (k_Q) and the maximum cell quota (Q_m) was 0.19. Hence, there is no substitute for determining ^μ experimentally. That there was no difference in growth response to different N sources suggests that no internal pooling of inorganic nitrogen occurred. Both the carbon (Q_c) and phosphorus (Q_p) cell quotas under N limitation increased with increasing μ in a threshold fashion: virtually no change in either cell quota up to ~0.8 ^μ, followed by a rapid and large increase up to ^μ. In addition, in the region of low μ, there was an increase in Q_p with a decreasing medium N/P ratio of between 15 and 5 (by atoms). The results generally indicate the physiological limits in cellular constituency under N limitation. The usefulness of this information, however, in describing the response of natural populations of marine phytoplankton to transient nutrient exposures on the temporal and spatial microscales that most likely exist is of limited value.     

Temperature effects on steady-state growth,phosphorus uptake,and the chemical composition of a marine phytoplankter

The marine chrysophyteMonochrysis lutheri was grown in phosphorus-limited continuous cultures at temperatures of 15°, 18.8° and 23°C. The effect of temperature on the maximum growth rate was well-defined by the Arrhenius equation, but the Q₁₀ for this alga (1.7) was somewhat lower than has been determined previously for many other phytoplankton species (2.0–2.2). The minimum phosphorus cell quota was relatively unaffected by temperature at 18.8°C and 23°C, but doubled in magnitude at 15°C. As a result, the internal nutrient equation of Droop described the relationship between specific growth rate and phosphorus cell quota well at 18.8° and 23°C, but was less successful at 15°C. The major limitation in using the Droop equation is that the ratio between the minimum and maximum cell quotas must be known, thus necessitating the need to establish the true maximum growth rate by the cell washout technique. In addition, the phosphorus uptake rate on a cell basis at a given steady state growth rate (=specific uptake rate) increased dramatically at 15°C, whereas the turnover rate of total available phosphorus was unaffected by temperature. Both the nitrogen and carbon cell quotas were relatively unaffected by growth rate at a given temperature, but the average values increased slightly with decreasing temperature. The overall conclusion is that phytoplankton growth and limiting-nutrient uptake rates are only synchronous at or near the optimum temperature. Because these types of responses are species specific, much additional data on temperature effects will be required before the importance of including such effects in phytoplankton-nutrient models can be determined.     

Nutrient limitation of phytoplankton communities in Subarctic lakes and ponds in Wapusk National Park, Canada

We determined the limiting nutrient of phytoplankton in 21 lakes and ponds in Wapusk National Park, Canada, using nutrient enrichment bioassays to assess the response of natural phytoplankton communities to nitrogen and phosphorus additions. The goal was to determine whether these Subarctic lakes and ponds were nutrient (N or P) limited, and to improve the ability to predict future impacts of increased nutrient loading associated with climate change. We found that 38% of lakes were not limited by nitrogen or phosphorus, 26% were co-limited by N and P, 26% were P-limited and 13% were N-limited. TN/TP, DIN/TP and NO₃ ⁻/TP ratios from each lake were compared to the Redfield ratio to predict the limiting nutrient; however, these predictors only agreed with 29% of the bioassay results, suggesting that nutrient ratios do not provide a true measure of nutrient limitation within this region. The N-limited lakes had significantly different phytoplankton community composition with more chrysophytes and Anabaena sp. compared to all other lakes. N and P limitation of phytoplankton communities within Wapusk National Park lakes and ponds suggests that increased phytoplankton biomass may result in response to increased nutrient loading associated with environmental change.     

珠江口及毗邻海域营养盐对浮游植物生长的影响

基于2006年7月(夏季),10月(秋季)和2007年3月(春季)的现场调查数据,对珠江口及毗邻海域中的营养盐和叶绿素a等环境生态因子的时空分布特性进行了对比分析,研究了氮磷比与叶绿素a含量和种群多样性之间的联系,探讨了该海域营养盐对于浮游植物生长的影响。结果表明:(1)研究海域营养盐表现出较强的季节和空间差异性,总氮(TN)和总磷(TP)浓度均值春季(1.545 mg/L、0.056 mg/L)和夏季(1.570 mg/L、0.058 mg/L)均大于秋季(1.442 mg/L、0.034 mg/L),且春夏季浓度空间差异更明显。(2)调查期间海域营养盐含量超标现象突出,夏季尤为明显。无机氮(DIN)总体均值0.99 mg/L,超四类海水标准限值1倍,活性磷酸盐(PO4-P)总体均值0.021 mg/L,DIN∶PO4-P平均值为130;叶绿素a浓度与营养盐、p H、温度有较显著的相关性。(3)叶绿素a浓度较高的站位,具有较高的DIN∶PO4-P值,但浮游植物多样性指数偏低,优势种明显,主要为中肋骨条藻。氮磷比的改变会影响不同生长特性的浮游植物间的竞争和种群结构的改变;今后海洋污染治理中,在控制氮、磷污染时要注意氮磷比的改变可能造成的浮游生态影响。     

Seasonal pattern in nutrient limitation and grazing control of the phytoplankton community in a non-stratified lake

1. The relative importance of zooplankton grazing and nutrient limitation in regulating the phytoplankton community in the non-stratified Lake Kvie, Denmark, were measured nine times during the growing season.
2. Natural phytoplankton assemblage bioassays showed increasing importance of nutrient limitation during summer. Growth rates at ambient nutrient concentrations were continually below 0.12 per day, while co-enrichment with nitrogen (N) and phosphorus (P) to above concentration-saturated conditions enhanced growth rates from May to the end of July.
3. Stoichiometric ratios of important elements in seston (C : N, C : P, N : P), in lake water (TN : TP), in external loading (TN : TP) and in internal loading (DIN : DIP) were measured to determine whether N or P could be the limiting nutrient. TN : TP molar ratio of both lake water, benthic fluxes and external loading suggested P limitation throughout the growing season. However, seston molar ratios suggested moderate P-deficiency only during mid-summer.
4. Abundance and community structure of the zooplankton varied considerably through the season and proved to be important in determining the responses of algal assemblages to grazing. High abundance of cladocerans and rotifers resulted in significant grazing impact, while cyclopoid copepods had no significant effect on the phytoplankton biomass.
5. Regeneration of ammonium and phosphate by zooplankton were periodically important for phytoplankton growth. A comparison of nutrient regeneration by zooplankton with nutrient inputs from sediment and external sources indicated that zooplankton may contribute significantly in supplying N and P for the growth of phytoplankton.     

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

相似文献   

Performance of the Redfield Ratio and a Family of Nutrient Limitation Indicators as Thresholds for Phytoplankton N vs. P Limitation

We aim to define the best nutrient limitation indicator predicting phytoplankton biomass increase as a result of nutrient enrichment (N, P, or both). We compare the abilities of different indicators, based on chemical measurements of nitrogen (N) and phosphorus (P) fractions in the initial plankton community, to predict the limiting factor for phytoplankton growth as inferred independently from short-term laboratory experiments on the same natural communities in a data set from NE Baltic Sea (Tamminen and Andersen, Mar Ecol Prog Ser 340:121–138, 2007). The best indicators had a true positive rate of about 80% for predicting both N and P limitation, but with a higher false positive rate for N than for P limitation (25 vs. 5%). Estimated threshold ratios for total nutrients (TN:TP) were substantially higher than the Redfield ratio, reflecting the relatively high amounts of biologically less available dissolved organic N in the study area. The best overall performing indicator, DIN:TP, had chlorophyll-response based threshold ratios far below Redfield, with N limitation below 2:1 and P limitation above 5:1 (by atoms). On the contrary, particulate N:P ratio was the overall worst predictor for N or P limitation, with values clustering around the Redfield N:P ratio (16:1, by atoms) independent of the limiting factor. Estimated threshold ratios based on inorganic nutrients (DIN:DIP) and so-called biologically available nutrients (BAN:BAP = (PON + DIN):(POP + DIP)) were also generally clearly above 16:1, indicating that the Redfield ratio rather reflects the transition from N limitation to combined N + P limitation, than to single limitation by P. Coastal systems are complex systems with regard to nutrient dynamics, historically considered to represent the transition from P-limited freshwater to N-limited marine systems. Our analysis shows that rather simple ratios reflect phytoplankton requirement for nutrients. Based on the high prediction performance, analytical considerations, and general data availability, the DIN:TP ratio appears to be the best indicator for inferring in situ N vs. P limitation of phytoplankton from chemical monitoring data.     

EFFECTS OF FLOWING WATER ON NITROGEN- AND PHOSPHORUS-LIMITED PHOTOSYNTHESIS AND OPTIMUM N:P RATIOS BY SPIROGYRA FLUVIATILIS (CHAROPHYCEAE)1,2

The effects of flowing water on net photosynthesis, dark respiration, specific growth rate, and optimum N:P ratios by Spirogyra fluviatilis Hilse were assessed. The alga was cultivated under nitrogen or phosphorus limitation in laboratory streams at three flow velocities: 3, 12, and 30 cm·s^?1. The Droop equation adequately described respiration and photosynthesis (PS_net) as a function of N or P cell quota (Q^N or Q^p). The data show that for N- or P-limited Spirogyra fluviatilis, flowing water is physiologically costly. Generally, flowing water had little effect on respiration rates; however, the proportion of gross photosynthesis devoted to dark respiration did increase with flow velocity. For photosynthesis, the minimum N and P cell quotas increased with velocity, and the theoretical PS_net maxima for N and P both appeared greatest at 12 cm·s^?1. The Droop models showed that for any given Q^N or Q^p, PS_net, was reduced by the 30-cm·s^?1 treatment. Consistent with this finding, independent estimates of specific growth rates for P-limited S. fluviatilis in the laboratory streams were inversely related to flow velocity when ambient PO₄^?3 was undetectable. However, growth was not diminished at the fastest velocity when PO₄^?3 was available for uptake. Thus, the increase in cellular phosphorus demand can be offset by flow-enhanced P uptake when conditions permit; otherwise, growth will be impaired. The optimum N:P ratios for S. fluviatilis at 3, 12, and 30 cm·s^?1 were 50, 58, and 52 by atoms, respectively, when calculated for PS_net= 0. The optimum ratios were inversely related to PS_net and decreased to approximately 20 when PS_net was near maximum. The potential for flowing water to mediate nutrient partitioning among lotic algae by altering growth rates and optimum nutrient ratios is discussed.     

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.     

Modeling continuous cultures of microalgae colimited by nitrogen and phosphorus

It is well documented that the combination of low nitrogen and phosphorus resources can lead to situations where colimitation of phytoplankton growth arises, yet the underlying mechanisms are not fully understood. Here, we propose a Droop-based model built on the idea that colimitation by nitrogen and phosphorus arises from the uptake of nitrogen. Indeed, since N-porters are active systems, they require energy that could be related to the phosphorus status of the cell. Therefore, we assumed that N uptake is enhanced by the P quota. Our model also accounts for the biological observations that uptake of a nutrient can be down-regulated by its own internal quota, and succeeds in describing the strong contrast for the non-limiting quotas under N-limited and P-limited conditions that was observed on continuous cultures with Selenastrum minutum and with Isochrysis affinis galbana. Our analysis suggests that, regarding the colimitation concept, N and P would be better considered as biochemically dependent rather than biochemically independent nutrients.     

INFLUENCE OF IRRADIANCE ON CELL VOLUME AND CARBON QUOTA FOR TEN SPECIES OF MARINE PHYTOPLANKTON1

Ten species of marine phytoplankton were grown under a range of photosynthetic photon flux densities (PFDs) and examined for variation in cell volume and carbon quota. Results suggest that in response to low PFDs phytoplankton generally reduce their cell volume and frequently reduce their carbon quota. A significant linear relationship between the log of PFD (I) and cell volume (in nine of ten species) and log I and carbon quota (four of ten species) was demonstrated. When exposed, to a transient in light intensity, Thalassiosira pseudonana (Hustedt, clone 3H) Hasle and Heimdal underwent a rapid adaptation in cell volume and carbon quota. Cells going from low light to high light reached maximum mean cell volume within 5 h, and cells going from high light to low light reached a minimum mean cell volume within 12 h. The resulting kinetic constant (k; a measure of the rate of adaptation) was considerably larger than previously reported k values. Ditylum brightwellii (West) Grunow increased in length but did not increase in width during a transient to increased irradiance. Nutrient limitation was shown to override PFD in determining cell volume and carbon quota for Heterosigma akashiwo Hada. Cells grown at equivalent irradiances but N-limited, were smaller than light-limited and nutrient-saturated cells. Therefore, cell volume and carbon quota do not have the same relationship with PFD when factors other than PFD control growth rate. The ecological implications of reduced cell volumes and carbon quotas with decreasing PFD include possible impacts on CO₂ budgets, an influence on sinking rates, potential changes in predation rates, and surface area/cell volume benefits.     

Taxonomic and Environmental Variability in the Elemental Composition and Stoichiometry of Individual Dinoflagellate and Diatom Cells from the NW Mediterranean Sea

Here we present, for the first time, the elemental concentration, including C, N and O, of single phytoplankton cells collected from the sea. Plankton elemental concentration and stoichiometry are key variables in phytoplankton ecophysiology and ocean biogeochemistry, and are used to link cells and ecosystems. However, most field studies rely on bulk techniques that overestimate carbon and nitrogen because the samples include organic matter other than plankton organisms. Here we used X-ray microanalysis (XRMA), a technique that, unlike bulk analyses, gives simultaneous quotas of C, N, O, Mg, Si, P, and S, in single-cell organisms that can be collected directly from the sea. We analysed the elemental composition of dinoflagellates and diatoms (largely Chaetoceros spp.) collected from different sites of the Catalan coast (NW Mediterranean Sea). As expected, a lower C content is found in our cells compared to historical values of cultured cells. Our results indicate that, except for Si and O in diatoms, the mass of all elements is not a constant fraction of cell volume but rather decreases with increasing cell volume. Also, diatoms are significantly less dense in all the measured elements, except Si, compared to dinoflagellates. The N:P ratio of both groups is higher than the Redfield ratio, as it is the N:P nutrient ratio in deep NW Mediterranean Sea waters (N:P = 20–23). The results suggest that the P requirement is highest for bacterioplankton, followed by dinoflagellates, and lowest for diatoms, giving them a clear ecological advantage in P-limited environments like the Mediterranean Sea. Finally, the P concentration of cells of the same genera but growing under different nutrient conditions was the same, suggesting that the P quota of these cells is at a critical level. Our results indicate that XRMA is an accurate technique to determine single cell elemental quotas and derived conversion factors used to understand and model ocean biogeochemical cycles.     

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.

     

KINETICS OF NUTRIENT UPTAKE AND GROWTH IN PHYTOPLANKTON1

Microscopic algae can grow rapidly in natural waters that are extremely low in essential macro and micro nutrients. Yet, their nutrient uptake systems exhibit only mediocre nutrient affinities, the saturation constants being often 10–1000 times the (estimated) ambient concentrations. The large difference which exists between the saturation constants for growth (K_u) and short term uptake (K_p) are due to the acclimation capabilities of the organisms. Over the acclimation range, K_u, to K_p the algae can maintain maximum growth rate by modulating both their internal nutrient quotas (Q) and their maximum short term nutrient uptake rates (P_max) in response to variations in external nutrient concentrations. The commonly assumed hyperbolic relationships for steady growth and uptake (viz “chemostat theory”) are coherent with a hyperbolic expression for short term uptake including a variable maximum (P_max). The ratio of the saturation constants for growth and uptake is then directly related to the extreme in quotas and maximum uptake rates: K^μ/K^ρ= Q^min/Q^maxρ^max/ρQ^max. This result is applicable even when the exact hyperbolic laws are not. Published data on Fe, Mn, P and N limitation in algae are generally in accord with the theory and demonstrate a wider acclimation range for trace than for major nutrients.     

KINETICS OF NUTRIENT UPTAKE AND GROWTH IN PHYTOPLANKTON1

Microscopic algae ran grow rapidly in natural waters that are extremely low in essential macro and micro nutrients. Yet, their nutrient uptake systems exhibit only mediocre nutrient affinities, the saturation constants being often 10–1000 times the (estimated) ambient concentrations. The large difference which exists between the saturation constants for growth (K_μ) and short term uptake (K_ρ) are due to the acclimation capabilities of the organisms. Over the acclimation range, K_μ to K_ρ, the algae can maintain maximum growth rate by modulating both their internal nutrient quotas (Q) and their maximum short term nutrient uptake rates (ρ_max) in response to variations in external nutrient concentrations. The commonly assumed hyperbolic relationships for steady growth and uptake (viz “chemostat theory”) are coherent with a hyperbolic expression for short term uptake including a variable maximum (ρ_max). The ratio of the saturation constants for growth and uptake is then directly related to the extreme in quotas and maximum uptake rates: K_μ/K_ρ= Q_min/Q_max·ρ^lo_max/ρ^hi_max. This result is applicable even when the exact hyperbolic laws are not. Published data on Fe, Mn, P and N limitation in algae are generally in accord with the theory and demonstrate a wider acclimation range for trace than for major nutrients.     

Interactive effect of warming,nitrogen and phosphorus limitation on phytoplankton cell size

Cell size is one of the ecologically most important traits of phytoplankton. The cell size variation is frequently related to temperature and nutrient limitation. In order to disentangle the role of both factors, an experiment was conducted to determine the possible interactions of these factors. Baltic Sea water containing the natural plankton community was used. We performed a factorial combined experiment of temperature, type of nutrient limitation (N vs. P), and strength of nutrient limitation. The type of nutrient limitation was manipulated by altering the N:P ratio of the medium (balanced, N and P limitation) and strength by the dilution rate (0% and 50%) of the semicontinuous cultures. The negative effect of temperature on cell size was strongest under N limitation, intermediate under P limitation, and weakest when N and P were supplied at balanced ratios. However, temperature also influenced the intensity of nutrient imitation, because at higher temperature there was a tendency for dissolved nutrient concentrations to be lower, while the C:N or C:P ratio being higher…higher at identical dilution rates and medium composition. Analyzing the response of cell size to C:N ratios (as index of N limitation) and C:P ratios (as index of P limitation) indicated a clear dominance of the nutrient effect over the direct temperature effect, although the temperature effect was also significant.     

Nutrient pulsing as a regulator of phytoplankton abundance and community composition in Galveston Bay, Texas

Galveston Bay, Texas, is a large shallow estuary with a watershed that includes 60% of the major industrial facilities of Texas. However, the system exhibits low to moderate (2-20 μg l⁻¹) microalgal biomass with sporadic phytoplankton blooms. Both nitrogen (N) and phosphate (P) limitation of phytoplankton growth have been proposed for the estuary. However, shifts between N and P limitation of algae growth may occur due to annual fluctuations in nutrient concentrations. The primary goal of this work was to determine the primary limiting nutrient for phytoplankton in Galveston Bay. Nutrient addition bioassays were used to assess short-term (1-2 days) phytoplankton responses (both biomass and community composition) to potentially limiting nutrients. The experimental bioassays were conducted over an annual cycle using natural water collected from the center to lower part of the estuary. Total phytoplankton biomass increased in the nitrate (10 μM) additions in 11 of the 13 bioassays, but no significant increases were detected in the phosphate (3 μM)-only additions. Bioassay results suggest that the phytoplankton community was usually not phosphate limited. All major groups increased in biomass following nitrate additions but diatoms increased in biomass at a faster rate than other groups, shifting the community composition toward higher relative abundance of diatoms. The results of this study suggest that pulsed N input events preferentially favor increases in diatom biomass in this estuary. The broader implications of this study are that N pulsing events, primarily due to river discharge, play an important role in structuring the phytoplankton community in the Galveston Bay estuary.