Limitation Of Algal Growth Research Articles

Storm and floods increase the duration and extent of phosphorus limitation on algal blooms in a tributary of the Three Gorges Reservoir, China

Excessive anthropogenic nutrient input has resulted in eutrophication and algal blooms which have severely impacted the function and sustainability of aquatic ecosystems, underscoring the need to implement nutrient management strategies. It was assumed that the increasing rainfall during flood season would affect the stoichiometric ratio of total nitrogen (TN): total phosphorus (TP), driving the nutrient limitation of algal growth. In order to test this concept and explore corresponding nutrient management strategies, nutrient addition bioassays were carried out in Xiangxi Bay, one of the largest tributaries of the Three Gorges Reservoir (TGR), China. Results indicated that nutrient limitation on algal growth fluctuated from nitrogen (N) to phosphorus (P) limitation. N limitation dominated in the early flood season. However, the reduction of dissolved P, accompanied with an increase of TN: TP caused by an increase in extreme rainfall events intensified P limitation throughout the bay. Then P limitation was alleviated due to the reduction of rainfall and the process of impoundment after the flood season. The variation of TN: TP caused by the increasing of rainfall and flooding could be the main driving factor of the nutrient limitation shift in aquatic ecosystems mainly affected by external nutrient inputs. Nutrient dilution and enrichment bioassays showed that TN and TP concentration thresholds should be targeted at below 0.55 mg/L and 0.057 ∼ 0.064 mg/L respectively, to limit the growth of algae and maintain chlorophyll a below 30 μg/L. Dual nutrient (N & P) reductions were required for long-term bloom mitigation in the entire basin. This study provided a scientific basis for a nutrient management strategy to combat eutrophication and reduce algal bloom potentials in the tributaries of the TGR. We recommend that long-term determinations of nutrient limitation and nutrient threshold will be needed to control algal growth, considering future anticipated changes in land use, population density and the impacts of climate change.

Nitrogen modulation under chemostat cultivation mode induces biomass and lipid production by Chlorella vulgaris and reduces antenna pigment accumulation

Algal growth limitation in large-scale cultivation mostly results from high level synthesis of photosynthetic pigments, owing to self-shading effects and attenuation of light distribution. To overcome this problem, here we investigated the influence of nitrogen modulation on changes in antenna pigments as well as biomass and lipid production by Chlorella vulgaris under a chemostat continuous cultivation mode. The production of algal antenna pigments, including chlorophylls and carotenoids, was promoted in a total nitrogen (TN) concentration-dependent manner. Maximum algal biomass and lipid production were obtained from 70 mg/L of TN concentration along with a significant increase in light transmittance and reduction in antenna pigments. Furthermore, the composition of polyunsaturated fatty acids remarkably augmented at low TN concentrations. These results suggest that the reduction in algal antenna pigment synthesis via modulation of nitrogen concentration may serve as an effective strategy to enhance algal biomass and lipid production.

On improvement of water quality of a reservoir by optimizing water exchange

Reservoirs provide important water resources for economic and social sustainable development. However, many shallow reservoirs are confronted with the problem of severe eutrophication. In this study, a method of optimizing the water exchange of reservoir was proposed to improve the water quality. The improvement was evaluated by the spatial and temporal eutrophic state of the reservoirs predicted by the water quality parameters, algal growth limitation factors and eutrophic state level. Three optimized schemes were designed, as an example, to improve the water exchange of Tianjin Erwangzhuang Reservoir. The results showed that the water quality of the reservoir is improved by the increase of water exchange. The best optimized design scheme for improving water quality was to open multiple gates and lift the amount of water in and out of the reservoir. The result indicated that the Trophic State Index was reduced from the range of 37.8 and 43.1 of the original scheme to that of 35.20 and 26.50 after about 160 day operations of the best optimized design scheme. © 2017 American Institute of Chemical Engineers Environ Prog, 37: 399–409, 2018

Contrasting responses of phytoplankton and benthic algae to recent nutrient enrichment in Arctic tundra ponds

Summary Tundra ponds are a dominant feature on the Arctic Coastal Plain, but their response to warming, especially in the long‐term, is largely unknown. Our study assessed changes in algal nutrient limitation over a 40‐year period in historically studied ponds that may have been affected by either human development in the region or increased temperatures leading to permafrost thaw and nutrient release. We also compared nutrient limitation of algae in the pelagic and benthic zones at a landscape level. Nutrient diffusing substrata (NDS) and bottled nutrient incubations, which expose natural algal communities to known quantities of nitrogen (N) and phosphorus (P), were utilised to determine benthic and phytoplankton algal nutrient limitation, respectively, in a series of ponds on the north slope of Alaska, USA. This included ponds where phytoplankton nutrient limitation was examined in 1971–1973 as part of the International Biological Program (IBP), as well as ponds in a remote protected area and others near the village of Barrow. Nutrient limitation status of phytoplankton has changed since the original IBP study 40 years ago from P limitation to NP co‐limitation. One‐third of regional phytoplankton enrichment experiments indicated no nutrient limitation of algal growth, and none exhibited single‐nutrient P limitation. This shift in nutrient limitation was coincident with increased water column nutrients due to degrading permafrost, and to the expansion into the ponds of macrophytes, which may compete with algae for available nitrogen. A comparison of pelagic and benthic experiments across the landscape revealed differences among these zones with a predominance of NP limitation of phytoplankton and absence of nutrient limitation in the benthic zone, reflecting contrasting nutrient limitation status within the same ponds. Permafrost thaw is probably reintroducing previously frozen stores of P or N to the sediment surface, which are quickly taken up by periphyton but become limited in the water column. Grazing by invertebrates, which were not excluded from the benthic assays, may also have influenced the results. While ponds within the village of Barrow had higher nutrient and algae levels, there was no obvious effect of urban development on nutrient limitation status, nor did development appear to have influenced the historic IBP pond sites.

Nutrient ratios, differential retention, and the effect on nutrient limitation in a deep oligotrophic lake

Nutrient ratios have been related to nutrient limitation of algal growth in lakes. Retention of nutrients in lakes, by sedimentation and by denitrification, reduces the nutrient concentrations in the water column, thereby enhancing nutrient limitation. Differential retention of nitrogen and phosphorus alters their ratios in lakes and thereby contributes to determine whether nitrogen or phosphorus limits algal growth. We examined the relationships between differential nutrient retention, nutrient ratios, and nutrient limitation in Lake Brunner, a deep oligotrophic lake. The observed retention of nitrogen (20%) and phosphorus (47%) agreed with predictions by empirical equations from literature. As a result of differential retention with a much larger proportion of phosphorus retained than that of nitrogen, the nitrogen:phosphorus ratio was higher in the lake (69) than in the inflows (46). While the mean ratio in the inflows suggested no or only moderate phosphorus limitation, the lake appeared to be severely phosphorus limited. Combining empirical equations from literature that predict nitrogen and phosphorus retention suggests that the nitrogen:phosphorus ratio is enhanced by greater retention of phosphorus compared to nitrogen only in deep lakes with relatively short residence times, such as Lake Brunner. In contrast, in most lakes differential retention is expected to result in lower nitrogen:phosphorus ratios.

Strong interactions between stoichiometric constraints and algal defenses: evidence from population dynamics of Daphnia and algae in phosphorus-limited microcosms

The dynamic interactions among nutrients, algae and grazers were tested in a 2 × 3 factorial microcosm experiment that manipulated grazers (Daphnia present or absent) and algal composition (single species cultures and mixtures of an undefended and a digestion-resistant green alga). The experiment was run for 25 days in 10-L carboys under mesotrophic conditions that quickly led to strong phosphorus limitation of algal growth (TP ≅ 0.5 μM, N:P 40:1). Four-day Daphnia juvenile growth assays tested for Daphnia P-limitation and nutrient-dependent or grazer-induced algal defenses. The maximal algal growth rate of undefended Ankistrodesmus (mean ± SE for three replicate microcosms; 0.92 ± 0.02 day−1) was higher than for defended Oocystis (0.62 ± 0.03 day−1), but by day 6, algal growth was strongly P-limited in all six treatments (molar C:P ratio >900). The P-deficient algae were poor quality resources in all three algal treatments. However, Daphnia population growth, reproduction, and survival were much lower in the digestion-resistant treatment even though growth assays provided evidence for Daphnia P-limitation in only the undefended and mixed treatments. Growth assays provided little or no support for simple threshold element ratio (TER) models that fail to consider algae defenses that result in viable gut passage. Our results show that strong P-limitation of algal growth enhances the defenses of a digestion-resistant alga, favoring high abundance of well-defended algae and energy limitation of zooplankton growth.

A simple Lagrangian model to simulate temporal variability of algae in the Elbe River

We present a five-year (1997–2001) numerical simulation of daily mean chlorophyll a concentrations at station Geesthacht Weir on the lower Elbe River (Germany) using an extremely simple Lagrangian model driven by (a) water discharge, global radiation, water temperature, and (b) silica observations at station Schmilka in the upper reach of the Elbe River. Notwithstanding the lack of many mechanistic details, the model is able to reproduce observed chlorophyll a variability surprisingly well, including a number of sharp valleys and ascents/descents in the observed time series. The model's success is based on the assumption of three key effects: prevailing light conditions, sporadic limitation of algal growth due to lack of silica and algae loss rates that increase above an empirically specified temperature threshold of 20 °C. Trimmed-down model versions are studied to analyse the model's success in terms of these mechanisms.In each of the five years the model consistently fails, however, to properly simulate characteristic steep increases of chlorophyll a concentrations after pronounced spring minima. Curing this model deficiency by global model re-calibration was found to be impossible. However, suspension of silica consumption by algae for up to 10 days in spring is shown to serve as a successful placeholder for processes that are disregarded in the model but apparently play an important role in the distinctly marked period of model failure. For the remainder of the year the very simple model was found to be adequate.

Nitrogen isotope constraints on subantarctic biogeochemistry

We report nitrate (NO3−) nitrogen isotope ratios for seawater samples collected in the Subantarctic Zone of the Southern Ocean during both winter and summer as part of the Australian Antarctic CRC Subantarctic Zone (SAZ) Project. The concentration and 15N/14N of the wintertime surface nitrate are very close to those of the subantarctic thermocline. The 15N/14N of nitrate in the surface increases sharply into the summer even though there is little seasonal change in nitrate concentration. There are two possible end‐member explanations for this observation. First, there may be significant equatorward nitrate transport during the summer, including a supply from the Antarctic surface. Second, the isotope effect of algal nitrate assimilation may be higher than has been estimated elsewhere, for example, for the seasonal sea ice zone of the Antarctic. We use a simple geochemical box model of the SAZ surface mixed layer as it evolves over the course of the summer to simulate salinity, nitrate concentration, and the 15N/14N of nitrate and sinking N. Our results suggest that a significant portion (∼30%) of the summertime SAZ nitrate is supplied from south of the Subantarctic Front and that N export is ≥3.5 mmol N m−2 d−1. Our approach also identifies the necessity of an isotope effect for nitrate assimilation in the SAZ of ≥7‰ and probably 8–9‰. Comparison to laboratory results suggests that this relatively high isotope effect may result from light limitation of algal growth in the SAZ.

Response of Periphytic Algae to Gradients in Nitrogen and Phosphorus in Streamside Mesocosms

In this study we manipulated both nitrogen and phosphorus concentrations in stream mesocosms to develop quantitative relationships between periphytic algal growth rates and peak biomass with inorganic N and P concentrations. Stream water from Harts Run, a 2nd order stream in a pristine catchment, was constantly added to 36 stream-side stream mesocosms in low volumes and then recirculated to reduce nutrient concentrations. Clay tiles were colonized with periphyton in the mesocosms. Nutrients were added to create P and N concentrations ranging from less than Harts Run concentrations to 128 μg SRP l−1 and 1024 μg NO3-N l−1. Algae and water were sampled every 3 days during colonization until periphyton communities reached peak biomass and then sloughed. Nutrient depletion was substantial in the mesocosms. Algae accumulated in all streams, even streams in which no nutrients were added. Nutrient limitation of algal growth and peak biomass accrual was observed in both low P and low N conditions. The Monod model best explained relationships between P and N concentrations and algal growth and peak biomass. Algal growth was 90% of maximum rates or higher in nutrient concentrations 16 μg SRP l−1 and 86 μg DIN l−1. These saturating concentrations for growth rates were 3–5 times lower than concentrations needed to produce maximum biomass. Modified Monod models using both DIN and SRP were developed to explain algal growth rates and peak biomass, which respectively explained 44 and 70% of the variance in algal response.

Modeling ice algal growth and decline in a seasonally ice‐covered region of the Arctic (Resolute Passage, Canadian Archipelago)

We have developed a coupled snow‐ice–ice algae model to investigate the importance of different ice algal growth limitation terms, as well as different loss terms, in regulating the ice algal biomass accumulation at the bottom of landfast ice in the Canadian Archipelago. The model results are compared with data collected from May to July 2002 at a station near Resolute in Barrow Strait. Our results show that ice algae are light limited at the beginning of the bloom, then fluctuate between light and nutrient limitation, finally remaining nutrient limited toward the end of the bloom. The fortnightly tide modulates the ice algal biomass through the supply of nutrient to the ice algal layer but mainly through modulation of the bottom ice melt rate. We also demonstrate that the bottom ice melt rate regulates the maximum biomass attained in the region and that a rapid increase in ice temperature can lead to a significant decline in ice algal biomass. The eventual termination of the bloom is triggered by melting of the snow cover and results from (1) increased ice algal losses due to high bottom ice melt rate and (2) decreased ice algal growth due to nutrient limitation caused by the formation of a meltwater lens below the ice. Finally, our results show that the snow cover controls the length of the bloom, such that earlier snowmelt that is expected to accompany climate warming may lead to a reduction in ice algal production.

Utilization of flue gas for cultivation of microalgae Chlorella sp.) in an outdoor open thin-layer photobioreactor

Flue gas generated by combustion of natural gas in a boiler was used for outdoor cultivation of Chlorella sp. in a 55 m2 culture area photobioreactor. A 6 mm thick layer of algal suspension continuously running down the inclined lanes of the bioreactor at 50 cm s−1 was exposed to sunlight. Flue gas containing 6–8% by volume of CO2 substituted for more costly pure CO2 as a source of carbon for autotrophic growth of algae. The degree of CO2 mitigation (flue gas decarbonization) in the algal suspension was 10–50% and decreased with increasing flue gas injection rate into the culture. A dissolved CO2 partial pressure (pCO2) higher than 0.1 kPa was maintained in the suspension at the end of the 50 m long culture area in order to prevent limitation of algal growth by CO2. NO X and CO gases (up to 45 mg m−3 NO X and 3 mg m−3 CO in flue gas) had no negative influence on the growth of the alga. On summer days the following daily net productivities of algae [g (dry weight) m−2] were attained in comparative parallel cultures: flue gas = 19.4–22.8; pure CO2 = 19.1–22.6. Net utilization (η) of the photosynthetically active radiant (PAR) energy was: flue gas = 5.58–6.94%; pure CO2 = 5.49–6.88%. The mass balance of CO2 obtained for the flue gas stream and for the algal suspension was included in a mathematical model, which permitted the calculation of optimum flue gas injection rate into the photobioreactor, dependent on the time course of irradiance and culture temperature. It was estimated that about 50% of flue gas decarbonization can be attained in the photobioreactor and 4.4 kg of CO2 is needed for production of 1 kg (dry weight) algal biomass. A scheme of a combined process of farm unit size is proposed; this includes anaerobic digestion of organic agricultural wastes, production and combustion of biogas, and utilization of flue gas for production of microalgal biomass, which could be used in animal feeds. A preliminary quantitative assessment of the microalgae production is presented.

Phytoplankton and bacterial alkaline phosphatase activities in relation to phosphate and DOP availability within the Gironde plume waters (Bay of Biscay)

Previous studies conducted on the continental shelf in the Southeast Bay of Biscay influenced by Gironde waters (one of the two largest rivers on the French Atlantic coast) showed the occurrence of late winter phytoplankton blooms and phosphorus limitation of algal growth thereafter. In this context, the importance of dissolved organic phosphorus (DOP) for both algae and bacteria was investigated in 1998 and 1999 in terms of stocks and fluxes. Within the mixed layer, although phosphate decreased until exhaustion from winter to spring, DOP remained high and phosphate monoesters made up between 11 to 65% of this pool. Total alkaline phosphatase activity (APA, V max) rose gradually from winter (2–8 nM h −1) to late spring (100–400 nM h −1), which was mainly due to an increase in specific phytoplankton (from 0.02 to 3.0 nmol μgC −1 h −1) and bacterial APA (from 0.04 to 4.0 nmol μgC −1 h −1), a strategy to compensate for the lack of phosphate. At each season, both communities had equal competitive abilities to exploit DOP but, taking into account biomass, the phytoplankton community activity always dominated (57–63% of total APA) that of bacterial community (9–11%). The dissolved APA represented a significant contribution. In situ regulation of phytoplanktonic APA by phosphate (induction or inversely repression of enzyme synthesis) was confirmed by simultaneously conducted phosphate-enrichment bioassays. Such changes recorded at a time scale of a few days could partly explain the seasonal response of phytoplankton communities to phosphate depletion.

Long-term land-based and internal forcing of the nutrient state of the Gulf of Riga (Baltic Sea)

The long-term nutrient trends (N, P, Si) in the Gulf of Riga and their driving factors were studied based on monitoring data from 1974 to 2000. Each nutrient showed individual temporal behaviour. The nitrate-N pool peaked in 1989–1990, but after that, the increasing trend switched to a decrease. The P pool, in turn, constantly increased till the mid-1990s; afterwards, a tendency was unclear. The obvious depletion of the silicate-Si pool reversed after 1995. Algal growth limitation responded to the changes in the nutrient pool. P deficiency during the vernal phytoplankton bloom in 1979–1990 switched to further alternate P or N limitation or co-limitation. Since 1991, Si shortage set in for diatom growth in spring. Both land-based loading and internal processes have contributed to the long-term change of the nutrient pools. The dynamics of nitrate-N was mainly determined by riverine N loading, i.e., N leaching losses from watersheds. On the contrary, the P variation did not correlate with riverine inputs because the external P loading relative to its pool in the water layer and to its sediment–water exchange was less significant. Weakening in agricultural soil leaching that diminished riverine N export to the Gulf occurred 6–7 years after reduction of fertiliser application at the end of the 1980s. Internal processes—P outflux from the sediments and N losses via denitrification—varied in the long term and affected the magnitude of the P and N pools significantly. P bottom supply sustained the upward P trend during the 1990s. The silicate-Si dynamics reflected inverse changes in P concentrations. Current changes in the nutrient state of the Gulf and still declining anthropogenic loading present an optimistic perspective on further water quality improvement.

Pelagic Nitrogen Fixation in Lake Victoria (East Africa)

Between 1994 and 1998 biological nitrogen fixation was measured using the acetylene reduction method at inshore and offshore sites on northern Lake Victoria. Rates of biological N-fixation were high and often exceeded 0.5 μg N/L/h. Average rates of volumetric N-fixation at optimal irradiance were 8 times higher at inshore locations than in the offshore. Rates of annual areal N-fixation, modeled from the N-fixation light-response and light attenuation in the lake were moderate to high (1.8 to 23.1 g N/m 2/y) depending on location within the lake, and were, on average, only twice as high inshore as offshore. Variations in the light extinction coefficient explained a small but significant proportion of the variation in the optimal N-fixation in Lake Victoria. Algal biomass and N-fixation were lower in the more deeply mixing (> 20 m) offshore waters, because of the persistently low mean light intensities (108 ± 41 μE/m 2/s) in the water column most of the year. N-fixation increased with increased light availability, and maximal rates occurred when the lake was shallowly mixing and thermally stratified. At both inshore and offshore stations, minima of algal biomass and N-fixation were consistent with low light availability in July when the lake was most deeply mixing. The ratio of the mean water column irradiance (I 24) to the irradiance at which N-fixation approaches saturation (I k) was often < 1, and provides evidence that N-fixation was light-limited, chronically so in the offshore region. Light limitation of algal growth lessens the algal demand for N, and it constrains algal biomass development and N-fixation more in the offshore compared to the inshore surface waters of Lake Victoria. Biological N fixation is the largest input of fixed N to Lake Victoria, greatly exceeding estimates of atmospheric deposition and river inputs of N.

Iron in the Sargasso Sea: Implications for the processes controlling dissolved Fe distribution in the ocean

In the surface water of the winter Sargasso Sea, “dissolved” Fe concentrations (defined as iron that passes through a filter with an 0.4 micrometer diameter pore size) decrease with increasing latitude from 0.8 nM at 26°N to 0.2 nM at 31°N. It does not appear that this pattern can be explained solely by spatial variations of eolian Fe deposition. Vertical mixing, phytoplankton growth and particle scavenging all appear to play important roles in controlling dissolved Fe distributions in the surface water. Concentrations of dissolved Fe in the subsurface waters increase with increasing depth from ∼0.2 nM at ∼200 m to ∼0.6 nM at depths below 1000 m, reflecting an imbalance between Fe removal by particle scavenging and Fe regeneration from microbial decomposition of organic matter sinking from the euphotic zone. Simple model calculations suggest that scavenging loss of Fe and ventilation of low preformed Fe high P water from source region may create deep ocean Fe:P concentration ratios (5 × 10−4 for the Atlantic and 1.2 × 10−4 for the Pacific) that are much lower than the ratios of Fe to P regenerated from the organic matter sinking from the euphotic zone (24–74 × 10−4 for the Atlantic and 11–32 × 10−4 for the Pacific). We hypothesize that this low Fe:P ratio may be the driving force for Fe limitation of algal growth observed in high nutrient low chlorophyll oceans and that regional changes of Fe:C ratios in organic matter sinking from the euphotic zone may play an important role in regulating dissolved Fe concentrations in the deep waters of the world ocean.

Mixing Patterns and Plankton Biomass of the St. Lawrence Great Lakes under Climate Change Scenarios

This study is part of an assessment of potential effects of climate change on the St. Lawrence Great Lakes. Its purpose is to investigate potential future lake mixing patterns and primary production. Nested physical and biological models were applied to seasonal mixed layer depth, heat content, primary productivity, and to algal biomass measured as particulate chlorophyll. Two independent second generation General Circulation Models provided scenarios for future conditions of cloud cover, air temperature, humidity, and winds. The climate variables were used to force heat balance and surface mixed layer models for Lakes Superior, Michigan, Huron, Erie, and Ontario. Physical models of heat balance and mixed layer dynamics were coupled with a model of primary biological production and growth of phytoplankton. Simulated climate conditions were for time periods centered at 1975, 2030, 2050, and 2090. Climate projections from both GCMs lead to elevated mixed layer and bottom temperatures in all five lakes by as much as 5°C during this century. Both GCMs point to longer duration of thermal stratification in the five lakes, stronger stability of stratification, and deeper daily mixing depths during peak thermal stratification. For Lake Erie, no striking differences in algal biomass are likely according to climate projections of either model, but for the other lakes, either the duration of nutrient limitation of algal growth is projected to increase, or light limitation caused by deeper mixing is projected to limit the development of algal biomass.

Formation of the vertical heterogeneity in the Lake Shira ecosystem: the biological mechanisms and the mathematical model

Data on the seasonal changes in vertical heterogeneity of the physical-chemical and biological parameters of the thermally stratified Shira Lake ecosystem (Khakasia, Siberia) in 1996–2000 have been analyzed. The interaction mechanisms involving: (1) The plankton populations in aerobic and anaerobic zones, involving the cycling of carbon and sulphur, (2) the primary production limitation (by light and phosphorus) and inhibition (by light), and (3) the kinetic characteristics of plankton populations have been elucidated. A mathematical model of the vertical structure of the lake's plankton populations, based on the ecosystem description and on vertical turbulent diffusion of the matter, has been constructed. The green alga Dictyosphaerium tetrachotomum (Chlorophyta) and the cyanobacterium Lyngbya contorta (Cyanophyta), which dominated the phytoplankton biomass, were taken as oxygen producers. Arctodiaptomus salinus (a calanoid copepod) has been assumed as the main grazer in Shira Lake as it dominated the zooplankton biomass. Four groups of microorganisms involved in the sulphur cycle formation have been distinguished: sulphur, sulphur purple, sulphur green and SRB. H2S is oxidized to sulphate (only the green sulphur bacteria oxidize it to sulphur), and sulphate is reduced to H2S, forming neither sulphur nor its water-soluble compounds. The role of grazing, light and nutrient limitation, in forming the vertical inhomogeneities, particularly in lowering the depth of the maximal cyanobacterial biomass, has been demonstrated. When the model takes into account both light limitation and nutrient limitation of algal growth by P and consumption of algae by crustaceans: (a) in the scenario where the P is formed only by the cycling and decomposition of autochthonous organic matter, both the green algae and cyanobacteria are eliminated; (b) in the scenario involving an additional P flux in the deep water layers the peak of the cyanobacteria is at a depth of 10 m, and its amplitude is close to the one observed in the lake. The position of the peak remains stable owing to the `double' limitation mechanism: light `from above' and P `from below'. Another mechanism responsible for the deep position of the peak of cyanobacteria was analyzed mathematically based on the model involving the experimentally proven assumption of the growth inhibition by light in the epilimnion and the light limitation in the hypolimnion. The main result is: the peak is positioned stable at its depth and does not change with time. The analytical and numerical calculations made for this positioning mechanism yielded the formulae relating the depth of the maximum of algal biomass, the `width' of the peak base and the peak amplitude and a number of parameters (algae elimination, turbulent diffusion coefficient, sedimentation rate, light extinction coefficient and light intensity). The theoretical curves for the stratification of chemical and biological parameters have been brought in conformity with field observations, e.g. for the different patterns for the peaks, and the biomass maxima of cyanobacteria, purple and green sulphur bacteria, oxygen, and hydrogen sulphide. The calculations revealed that for an adequate assessment of the parameters for the hydrogen sulphide zone it is necessary to introduce flows of allochthonous organic matter. For the first time, theoretically, based on the form of the sulphur distribution curve, the allochthonous input of organic matter [KEYWORDS: phytoplankton; sulphur cycle; vertical model; stratification control; sulphate-reducing bacteria; hydrogen sulphide]

Phytoplankton processes. Part 2: Rates of primary production and factors controlling algal growth during the Southern Ocean Iron RElease Experiment (SOIREE)

Abstract The Southern Ocean Iron RElease Experiment (SOIREE) fertilised 200 km2 of polar waters (mixed-layer depth, 65 m) south of Australia in February 1999. During the 13-d SOIREE, iron enrichment stimulated primary production, algal growth and biogenic silica production rates. A floristic shift from small to large cells resulted in a greater contribution to community production by diatoms, which contributed >60% to community production towards the end of SOIREE. Column-integrated rates of primary production increased by more than 10-fold, and community production rates (normalised to chlorophyll) had doubled by day 4, but then declined after day 6 to initial rates. Iron enrichment increased the growth rates of the algal community from 0.02 to 0.15 d−1 (based on changes in in-situ chlorophyll concentrations), from 0.02 to 0.2 d−1 (based on algal carbon), and more than doubled initial rates (based on 14C) to 0.2 d−1 during SOIREE. However, these iron-elevated rates were less than the maximum rates (0.69 d−1) predicted for waters at 2.5°C. Community biogenic silica production rates increased by 6-fold, i.e. to a lesser extent than primary production rates, resulting in a 2-fold reduction in the silicic-acid : carbon uptake ratio. Shipboard perturbation experiments indicated that iron, rather than both iron and light, limited algal growth in these waters. However, a significant increase in light attenuation in situ due to algal self-shading, suggested that light limitation of algal growth might become significant after day 13 at this locale. Limitation of algal growth by both iron (and silicic acid and/or zinc) within iron-fertilised waters after day 8 of SOIREE was not evident from the results of a shipboard experiment. A simple mathematical model of particulate organic carbon (POC) potential export during SOIREE predicted large increases in export, due to both elevated production rates and a floristic shift to large cells. However, no significant increase in POC export was observed during SOIREE.

Total nitrogen, total phosphorus, and nutrient limitation in lakes and oceans: Is there a common relationship?

Total nitrogen (TN) and total phosphorus (TP) measurements and contemporaneous measurements of chlorophyll a (Chl a) and phytoplankton nutrient deficiency have been made across a broad range of lakes and ocean sites using common methods. The ocean environment was nutrient rich in terms of TN and TP when compared with most lakes in the study, although Lake Victoria had the highest values of TN and TP. TN concentrations in lakes rose rapidly with TP concentrations, from low values to TN concentrations that are similar to those associated with the ocean sites. In contrast, the TN concentrations in the oceans were relatively homogeneous and independent of TP concentrations. The hyperbolic shape of the TN:TP relationship created a broad range of TN:TP values for both lakes and oceans. The TN:TP ratios of the surface ocean sites were usually well in excess of the Redfield ratio that is noted in the deep ocean. Phytoplankton biomass, as indicated by Chl a, was strongly dependent upon TP in the lakes, and there was a weaker relationship with TN. Oceanic Chl a values showed a positive relationship with TP, but at much higher TP values than were observed in the lakes; there was no relation with TN.P‐deficient phytoplankton growth was inferred using independent indicators when TP was &gt;0.5 µmol L−1 at both freshwater and marine sites. N‐deficiency indicators were highly variable and did not show any clear dependence on TN concentration. The TN:TP ratio was indicative of which nutrient would become limiting for growth in both lakes and oceans. When all sites are compared, N‐deficient growth was apparent at TN:TP µ 20 (molar), whereas P‐deficient growth consistently occurred when TN:TP µ 50 (molar). At intermediate TN:TP ratios, either N or P can become deficient. We conclude that N or P limitation of algal growth is a product of the TN and TP concentration and the TN:TP ratio rather than a product of whether the system of study is marine or freshwater.

Limitation of algal growth by iron deficiency in the Australian Subantarctic Region

In March 1998 we measured iron in the upper water column and conducted iron‐ and nutrient‐enrichment bottle‐incubation experiments in the open‐ocean Subantarctic region southwest of Tasmania, Australia. In the Subtropical Convergence Zone (∼42°S, 142°E), silicic acid concentrations were low (&lt; 1.5 µM) in the upper water column, whereas pronounced vertical gradients in dissolved iron concentration (0.12‐0.84 nM) were observed, presumably reflecting the interleaving of Subtropical and Subantarctic waters, and mineral aerosol input. Results of a bottle‐incubation experiment performed at this location indicate that phytoplankton growth rates were limited by iron deficiency within the iron‐poor layer of the euphotic zone. In the Subantarctic water mass (∼46.8°S, 142°E), low concentrations of dissolved iron (0.05‐0.11 nM) and silicic acid (&lt; 1 µM) were measured throughout the upper water column, and our experimental results indicate that algal growth was limited by iron deficiency. These observations suggest that availability of dissolved iron is a primary factor limiting phytoplankton growth over much of the Subantarctic Southern Ocean in the late summer and autumn.