The importance of nutrient co‐limitation in regulating algal community composition,productivity and algal‐derived DOC in an oligotrophic marsh in interior Alaska

1. Compared to lakes and streams, we know relatively little about the factors that regulate algae in freshwater wetlands. This discrepancy is particularly acute in boreal regions, where wetlands are abundant and processes related to climate change (i.e. increased permafrost collapse and soil weathering) are expected to increase nutrient inputs into aquatic systems. To investigate how accelerated nutrient inputs might affect algal structure and function in northern boreal wetlands, we added nitrogen, phosphorus and silica to mesocosms in an oligotrophic marsh in interior Alaska. 2. We conducted two in situ mesocosm enrichment experiments during consecutive summer growing seasons, each lasting 24 days. In 2007, we investigated the effects of +N, +P, +Si and +N+P+Si enrichment on benthic algal biomass (chlorophyll‐a, ash‐free dry mass, biovolume), chemistry (N : P ratio) and community composition. In 2008, we expanded our first experiment to investigate the effects +N+P, +N+Si, +P+Si and +N+P+Si on the same algal parameters as well as productivity (mg C m^?2 h^?1). 3. In both experiments, we measured water‐column dissolved organic carbon (DOC) inside treatment enclosures and related changes in DOC to standing algal biomass. 4. Benthic algal accrual did not increase following 24 days of enrichment with any nutrient alone or with P and Si together (+P+Si), but increased significantly with the addition of N in any combination with P and Si (+N+P, +N+Si, +N+P+Si). 5. Algal productivity (20 mg C m^?2 h^?1) increased between three‐ and seven‐fold (57–127 mg C m^?2 h^?1) with the addition of N in combination with any other nutrient (+N+P, +N+Si, +N+P+Si). Water‐column DOC concentration was significantly higher inside N‐combination treatments compared to the control during each season, and DOC increased linearly with benthic algal biomass in 2007 (r² = 0.89, P < 0.0001) and 2008 (r² = 0.74, P < 0.0001). 6. Taxonomic composition of the wetland algal community responded most strongly to N‐combination treatments in both seasons. In 2007, there was a significant shift from Euglena and Mougeotia in the control treatment to Chroococcus and Gloeocystis with +N+P+Si enrichment, and in 2008, a Mougeotia‐dominated community was replaced by Gloeocystis in the +N+P treatment and by Nitzschia in +N+Si and +N+P+Si treatments. 7. Together, these data provide several lines of evidence for co‐limitation, and the central importance of N as a co‐limiting nutrient for the wetland algal community. Changes in algal dynamics with increased nutrient concentrations could have important implications for wetland food webs and suggest that algae may provide a functional link between increasing nutrient inputs and altered wetland carbon cycling in this region.     

太平洋中西部海域浮游植物营养盐的潜在限制

2009年8月至9月期间在太平洋西部N1站和中部N2站进行现场营养盐加富培养实验。结果显示:N1站,浮游植物生物量对N或者P添加都有较强的响应,其中N+P+Si组和N+P组浮游植物长势迅速,叶绿素a从初始的0.03μg/L分别达到2.12μg/L和1.83μg/L,同时P先于N和Si之前被耗尽;说明N1站为N、P共同限制,P是首要限制因子。而N2站,浮游植物生物量仅对N、P共同添加有明显响应,N先于P和Si被浮游植物消耗殆尽。利用培养过程中营养盐比值变化推断,N1站浮游植物以低于Redfield ratio(16N∶1P)吸收N和P;而N2站浮游植物以高于Redfield ratio(16N∶1P)吸收N和P。这可能解释了太平洋西部的寡营养盐海域为潜在P限制,而在太平洋中部海域则为潜在N限制。     

Tracing Si–N–P ecosystem-pathways: is relative uptake in riparian vegetation influenced by soil waterlogging,mowing management and species diversity?

Despite the growing concern about the importance of silicon (Si) in controlling ecological processes in aquatic ecosystems, little is known about its processing in riparian vegetation, especially compared to nitrogen (N) and phosphorus (P). We present experimental evidence that relative plant uptake of N and P compared to Si in riparian vegetation is dependent on mowing practices, water-logging and species composition. Results are obtained from a controlled and replicated mesocosm experiment, with a full-factorial design of soil water logging and mowing management. In our experiments, the Si excluding species Plantago lanceolata was dominant in the mown and non-waterlogged treatments, while Si accumulating meadow grasses and Phalaris arundinacea dominated the waterlogged treatments. Although species composition, management and soil moisture interacted strongly in their effect on relative Si:N and Si:P uptake ratios, the uptake of N to P remained virtually unchanged over the different treatments. Our study sheds new light on the impact of riparian wetland ecosystems on nutrient transport to rivers. It indicates that it is essential to include Si in future studies of the impact of riparian vegetation on nutrient transport, as these are often implemented as a measure to moderate excessive N and P inputs.     

VARIABILITY IN CELL SIZE,NUTRIENT DEPLETION,AND GROWTH RATES OF THE SOUTHERN OCEAN DIATOM FRAGILARIOPSIS KERGUELENSIS (BACILLARIOPHYCEAE) AFTER PROLONGED IRON LIMITATION1

Diatoms are the main primary producers in the Southern Ocean, governing the major nutrient cycles. Fragilariopsis kerguelensis (O’Meara) Hust. is the most abundant diatom species in the Southern Ocean and its paleo‐oceanographic record is frequently used to reconstruct the past position and nutrient characteristics of the Antarctic polar front. Here we report on the responses of F. kerguelensis on prolonged exposure to a range of iron concentrations, allowing a characterization of morphological and nutrient‐depletion changes in relation to iron status. Under iron limitation, F. kerguelensis grew slower, cells became smaller, chains became shorter, and the nutrient‐depletion ratios changed. Prolonged exposure to iron limitation caused F. kerguelensis to decrease its surface area and volume 2‐fold, and to increase its surface‐to‐volume ratio by 25%. With the decrease in growth rates, silicon (Si) and phosphorus (P) depletion per cell remained fairly constant, but when normalized per surface area (Si) or per cell volume (P), depletion increased. In contrast, nitrogen (N) depletion per cell decreased significantly together with the decrease in growth rates but was constant when normalized per cell volume. The different response in Si, P, and N depletion resulted in changes in the nutrient‐depletion ratios, most notably in the Si:N ratio, which significantly increased, and in the N:P ratio, which significantly decreased with decreasing growth rates. It is concluded that under iron limitation, variation in cell size and/or nutrient depletion ultimately can cause changes in oceanic biogeochemical nutrient cycles. It enables the use of cell size of F. kerguelensis as a paleo‐oceanographic proxy.     

Changes in species composition of phytoplankton due to enrichment by N,P, and Si of water from a North Florida Lake

Water samples were collected from Lake Jackson, near Tallahassee, Florida, and enriched with N, P, N + P, and Si in the laboratory in April, 1969 and January, 1970. After time intervals of 3 and 6 days in April, and 13 days additionally in January, phytoplanktonic algae in the cultures were enumerated as to species composition and cell numbers with an inverted plankton microscope. Controls, to which no nutrients were added, were also maintained to determine changes arising from culture-room conditions. The addition of N and P, alone or in combination, caused increased numbers of algae, along with differences in species composition. This indicated that these two nutrient elements were limiting to production in Lake Jackson besides exerting a chemical control on succession. Si additions increased diatom numbers, but also stimulated Anacystis cyanea to grow in the April sample. Species attributed as indicators of organic pollution were found to be stimulated by inorganic sources of N, P, or Si. Such changes indicated caution must be exercised in interpreting long-term nutrient enrichment experiments using natural phytoplankton communities.     

Copepod growth and diatoms: insensitivity of <Emphasis Type="Italic">Acartia tonsa</Emphasis> to the composition of semi-natural plankton mixtures manipulated by silicon:nitrogen ratios in mesocosms

The feeding selectivity and the growth and reproductive success of the copepod Acartia tonsa have been studied in mesocosms fertilized at different Si:N ratios (0–1.75:1) and, therefore, at different compositions of the phytoplankton communities. Phytoplankton composition showed a strong response to nutrient ratios, with diatoms comprising >90% at Si:N ratios >1:1 of total biomass as opposed to <20% at the lowest ratio. A. tonsa strongly preferred feeding on motile prey (flagellates and ciliates) to feeding on diatoms. Nevertheless, diatoms comprised a substantial part of the diet at the highest Si:N ratios. A. tonsa egg production and the final (after 4 weeks) abundance of adults and copepodites showed no response to Si:N ratios while nauplii production slightly increased with Si:N ratios. It is concluded that the frequently reported deleterious effect of diatoms on copepod reproduction is rather unusual when copepods are confronted with a naturally diverse phytoplankton assemblage instead of clonal cultures in the laboratory.     

The impact of substrate and lake trophy on the biomass and nutrient status of benthic algae

Algal biomass, C:N:P (carbon:nitrogen:phosphorus) ratios and APA (biomass specific alkaline phosphatase activity) were measured in benthic algal communities on living substrates (mussels and macrophytes) and on rocks and stones (epilithon) in three lakes of different trophy. Benthic algal communities on living substrates had lower C:N:P ratios than epilithon, whereas algal biomass was highest on rocks and stones. Benthic algal biomass increased with the trophic level of a lake despite an increase of C:N:P ratios in the benthic community. The differences in C:N:P ratios and algal biomass between lakes of different trophy were higher on inert substrates than on macrophytes and mussels, probably because algae on living substrates could compensate a poor nutrient supply from lake water with substrate nutrients. However, the substrate was not, as expected, the most important nutrient supply in the oligotrophic lake, but in the eutrophic lake. Therefore, differences between inert and living substrates in a single lake were highest in the eutrophic lake. APA values of the oligotrophic lake were very high especially for benthic algae on stones, indicating an ability of the community to take up nutrients from organic sources. In conclusion, living substrates were an important nutrient source for benthic algae and the importance of this nutrient supply did not decrease with increasing lake trophy.     

黄土高原子午岭林区典型树种叶片N、P再吸收特征

为揭示黄土高原子午岭林区不同演替阶段和植被类型主要树种养分再吸收特征,研究选取4种次生植被树种(白桦、山杨、辽东栎和油松)和2种人工植被树种(刺槐和侧柏),测定其成熟叶、凋落叶和林下土壤碳(C)、氮(N)、磷(P)含量,研究了叶片N、P再吸收率及其与养分指标的关系。结果表明:(1)不同树种叶片养分和林下土壤养分含量存在显著差异,土壤C、N含量和C∶N∶P计量比均表现为演替后期林地(辽东栎和油松)>演替前期林地(山杨和白桦)>人工林(侧柏和刺槐);(2)不同树种叶片N、P再吸收率分别为17.18%—43.34%和27.13%—58.12%,均表现为演替后期林地>人工林>演替前期林地,且P的再吸收率总体高于N的再吸收率;(3)不同树种叶片N、P再吸收率与叶片养分指标的关系强于土壤,与养分计量比的相关性大于养分含量的相关性。说明子午岭典型植被会通过叶片N、P再吸收来适应养分限制环境,尤其是演替后期植被再吸收能力更强,研究可为黄土高原植被恢复提供理论依据。     

Silicon application induces changes C:N:P stoichiometry and enhances stoichiometric homeostasis of sorghum and sunflower plants under salt stress

Beneficial effects of silicon (Si) on growth have been observed in some plant species, reportedly due to stoichiometric changes of C, N, and P. However, little is known about the effects on the stoichiometric relationships between C, N, and P when silicon is supplied via different modes in sorghum and sunflower plants under salt stress conditions. Therefore, the current study was performed to investigate the impact of differing modes of Si supply on shoot biomass production and C:N:P stoichiometry in sorghum and sunflower plants under salt stress. Two experiments were performed in a glass greenhouse using the strong Si-accumulator plant sorghum, as well as the intermediate type Si-accumulator sunflower, both of which were grown in pots filled with washed sand. Plant species were cultivated for 30 days in the absence or presence of salt stress (0 or 100 mM) and supplemented with one of four Si treatments: control plants (without Si), 28.6 mmol Si L⁻¹ via foliar application, 2.0 mmol Si L⁻¹ via nutrient solution, and combined application of foliar and nutrient solution, each group with five replications. The results revealed that supplied Si modified the C, N, and P concentrations, thereby enhancing the C:N:P stoichiometry and shoot dry matter of sorghum and sunflower plants under salt stress. Both application of Si via nutrient solution, as well as combined application via foliar and nutrient solution, increased the C:N ratio in both plant species under salt stress, but in sorghum plants decreased the C:P and N:P ratios and increased the shoot biomass production by 39%, while in sunflower plants increased the C:P and N:P ratios and increased the shoot biomass production by 24%. Our findings suggest that salt stress alleviation by Si impacts C:N:P stoichiometric relationships in a variable manner depending on the ability of the species to accumulate Si, as well as the route of Si administration.     

Grazer species effects on epilithon nutrient composition

1. Field and laboratory experiments were conducted to investigate the excretion stoichiometry of nitrogen (N) and phosphorus (P) of two benthic macroinvertebrate grazers, the crayfish Orconectes propinquus and the snail Elimia livescens, that differ in body stoichiometry (mean body molar N : P 18 and 28, respectively). Crayfish excretion had a significantly higher ammonium : soluble reactive phosphorus (SRP) ratio in the laboratory and in three natural streams than did snails, as predicted by ecological stoichiometry theory. 2. In greenhouse recirculating artificial streams, treatments consisting of crayfish, snails, or no grazers were used to examine responses in dissolved nutrient concentrations and epilithon nutrient composition and limitation. SRP concentrations depended upon the grazer species, with the snail treatment having a higher SRP concentration than other treatments (P < 0.05). Dissolved inorganic N was not affected by grazers, but appeared to be rapidly incorporated in epilithon. 3. Epilithon N content was dependent upon the grazer species present, with the crayfish treatment having a significantly higher N content than other treatments (P = 0.001). No grazer species effects on epilithon P content were found. However, both grazer treatments had significantly lower epilithon P content than the no‐grazer treatment. 4. Traditionally, studies have focused on how grazer‐induced structural changes to epilithon can alter epilithon nutrient dynamics, but this structural mechanism could not solely explain differences in epilithon nutrient contents and ratios in the present study. Our results rather suggest that benthic grazers can alter epilithon nutrient composition and limitation via nutrient excretion. Consequently, macroinvertebrate grazers may serve as ‘nutrient pumps’ that partly regulate the availability of nutrients to algae in stream ecosystems.     

Contrasting species responses to continued nitrogen and phosphorus addition in tropical montane forest tree seedlings

Global changes in nutrient deposition rates are likely to have profound effects on plant communities, particularly in the nutrient‐limited systems of the tropics. We studied the effects of increased nutrient availability on the seedlings of six tree species in montane forests of southern Ecuador in situ. After five years of continued N, P, or N+P addition, naturally grown seedlings of each of the two most common species at each elevation (1000, 2000, and 3000 m asl) were harvested for analyses of leaf morphology, nutrient content, herbivory, and tissue biomass allocation. Most species showed increased foliar N and P concentrations after addition of each respective element. Leaf tissue N:P ratios of >20 in the control plants of all species suggest that P is more growth‐limiting in these forests than N. Leaf morphological responses to nutrient addition were species and nutrient specific, with some species (Hedyosmum purparescens, Graffenrieda emarginata) exhibiting increased specific leaf area (SLA), and others (Graffenrieda harlingii) increased leaf area ratios (LAR). Pouteria torta (1000 m) had lower SLA and LAR after P addition. Increased herbivory was only evident in G. emarginata (after N and N+P addition). Only the species from 3000 m asl modified biomass allocation after nutrient addition. In general, N and N+P addition more strongly affected the species studied at the upper elevations, whereas P addition had a similar range of effects on the species at all elevations. We conclude that the responses of the studied tropical montane forest tree seedlings to chronic N and P addition are highly species‐specific and that successful adaptation to increased nutrient availability will depend on species‐specific morphological and physiological plasticity.     

THE Si:C:N RATIO OF MARINE DIATOMS: INTERSPECIFIC VARIABILITY AND THE EFFECT OF SOME ENVIRONMENTAL VARIABLES1

The variability of marine diatom Si:C and Si:N composition ratios was examined to assess their utility as ecological conversion factors. Twenty-seven diatom species grown under an 18:6 h LD cycle and sampled at the end of the light period gave mean ratios, by atoms, of 0.13 ± 0.04 and 1.12 ± 0.33 for Si:C and Si:N ratios, respectively (95% C.I. reported). The mean ratios for 18 species grown under continuous illumination were 0.12 ± 0.03 for Si:C and 0.95 ± 0.23 for Si:N. The mean ratios of the clones grown under constant light were not statistically different from those calculated for the same species grown under an 18:6 h LD photoperiod. The overall mean Si:C and Si:N ratios for the 18:6 h LD and continuous light experiments taken together, weighted by the number of species in each experiment, are 0.13 and 1.05, respectively. The average ratios for the nine nanoplankton species (<20 μm) examined were 0.09 ± 0.03 for Si:C and 0.80 ± 0.35 for Si:N. The eighteen netplankton species (>20 μm) had higher mean ratios, Si:C = 0.15 ± 0.04 and Si:N = 1.20 ± 0.37. Time course sampling throughout a 24 h period revealed twofold variations in both ratios for individual species grown on a 14:10 h LD cycle. Changes in irradiance can also produce factor of two variations, both ratios being higher under low light. Comparisons of these data with those from the literature regarding the effects of temperature and nutrient limitation on diatom elemental composition suggest that use of these ratios to convert field estimates of biogenic silica into nitrogen or carbon units, or to estimate silica production from ¹⁴C data, should yield results accurate to within a factor of three under most circumstances.     

Gastropod grazers and nutrients,but not light,interact in determining periphytic algal diversity

The potential interactions of grazing, nutrients and light in influencing autotroph species diversity have not previously been considered. Earlier studies have shown that grazing and nutrients interact in determining autotroph species diversity, since grazing decreases species diversity when nutrients (i.e. N or P) limit autotroph growth, but increases it when nutrients are replete. We hypothesized that increased light intensities would intensify the interactions between grazing and nutrients on algal species diversity, resulting in even stronger reductions in algal species diversity through grazing under nutrient–poor conditions, and to even stronger increases of algal species diversity through grazing under nutrient-rich conditions. We studied the effects of grazing (absent, present), nutrients (ambient, N + P enriched) and light (low light, high light) on benthic algal diversity and periphyton C:nutrient ratios (which can indicate algal nutrient limitation) in a factorial laboratory experiment, using the gastropod grazer Viviparus viviparus. Grazing decreased algal biomass and algal diversity, but increased C:P and N:P ratios of periphyton. Grazing also affected periphyton species composition, by decreasing the proportion of Spirogyra sp. and increasing the proportion of species in the Chaetophorales. Grazing effects on diversity as well as on periphyton N:P ratios were weakened when nutrients were added (interaction between grazing and nutrients). Chlorophyll a (Chl a) per area increased with nutrient addition and decreased with high light intensities. Light did not increase the strength of the interaction between grazing and nutrients on periphytic algal diversity. This study shows that nutrient addition substantially reduced the negative effects of grazing on periphytic algal diversity, whereas light did not interact with grazing or nutrient enrichment in determining periphytic algal diversity.     

RESPONSE OF A LENTIC PERIPHYTON COMMUNITY TO NUTRIENT ENRICHMENT AT LOW N:P RATIOS1

We examined the effect of nitrogen:phosphorus (N:P) ratios and nutrient concentrations on periphyton when nutrients (N and P) are provided in excess. A gradient of seven N:P ratios ranging from 7.5:1 to 1:7.5 and each at three absolute concentrations, was established using nutrient‐releasing substrata placed in a meso‐oligotrophic lake. Differences in total algal biovolume among nutrient ratios were significant (analysis of covariance [ANCOVA]) when P concentration was entered as the co‐variate. In addition, total algal biovolume was significantly correlated with N concentration but not P. To further evaluate the relationship between nutrient ratios and biovolume, we analyzed (using four 1‐way analysis of variances [ANOVAs]) four subsets of data defined as a series of treatments where one nutrient concentration remained relatively constant as the other changed creating different N:P ratios. Ratios of data subsets ranged from 1:1 to 7.5:1 and 1:1 to 1:7.5 with low and high concentrations of both series. Only diatom biovolume varied with ratio but these differences are most likely related to increased green algal abundance. Species richness and diversity differed among N:P ratios (ANCOVA) when P concentration was used as the co‐variate. Stigeoclonium tenue (Ag.) Gomont, which generally accounted for the increase in green algal abundance, varied with nutrient ratio (ANCOVA) when P was the co‐variate. Based on the ANCOVAs, correlations, and one‐way ANOVAs, periphyton in this system appears to be affected by N concentration but not by N:P treatment ratios under nutrient‐rich conditions. When compared with previous studies, these data also suggest that the response of periphyton to in situ treatments constructed with nutrient‐releasing substrata vary between years.     

Changes in chemical compositions of substrates before and after <Emphasis Type="Italic">Ganoderma lucidum</Emphasis> cultivation

After the commercial cultivation of Ganoderma lucidum, that is becoming increasingly popular because of its valuable medicinal effects, a large amount of Ganoderma spent substrate (GSS) remains as a residual material. It is important to identify differences in the chemical compositions among GSS to indicate their suitability. This study was carried out to characterize and evaluate the substrates and GSS for the some chemical contents such as; pH, EC, organic carbon (C), total nitrogen (N), macro and micro elements and C:N ratios. The substrates were prepared by the mixtures of hornbeam and oak sawdust supplemented with wheat bran in a ratio of 20% (20WB) and tea manufacture waste in different ratios (10, 15, 20 and 25%). Organic C, C:N and pH values decreased in GSS, while N, P, K, Mg, Fe, Mn and Zn contents increased. EC and pH values of GSS varied between 1.13 and 2.33 dS m⁻¹, and 5.14 and 5.69, respectively. GSS was found rich in all investigated mineral nutrients. The highest increases in all nutrient contents were generally determined in hornbeam sawdust with 20TW spent substrate. Initial mineral composition of the wastes influenced the mineral composition of the substrates and spent substrates.     

Physiological integration impacts nutrient use and stoichiometry in three clonal plants under heterogeneous habitats

Physiological integration facilitates clonal plants to deal with heterogeneous resources. However, little is known about how nutrient patchiness affects its use and stoichiometry in clonal plants. We conducted an experiment with Cynodon dactylon, Glechoma longituba, and Potentilla reptans to address the effects of physiological integration on nutrient use efficiency and N:P ratios. For C. dactylon, the effects of nutrient patchiness on N use efficiency (NUE), P use efficiency (PUE), and N:P ratio were stronger in daughter ramets than in parent ramets; for G. longituba, nutrient patchiness affected PUE and N:P ratio of parent and daughter ramets, but not NUE; for P. reptans, nutrient patchiness decreased NUE, PUE, and N:P ratio, regardless of parent or daughter ramets. PUE was associated with N:P ratios in three clonal plants and this association of NUE with N:P ratios varied with species. Our findings suggest that physiological integration alters nutrient use efficiency and N:P ratios of clonal plants under patchy nutrients and that these effects are linked to clonal species identity.     

EFFECT OF NUTRIENT AVAILABILITY ON THE BIOCHEMICAL AND ELEMENTAL STOICHIOMETRY IN THE FRESHWATER DIATOM STEPHANODISCUS MINUTULUS (BACILLARIOPHYCEAE)*

The objective of this study was to examine the differences in the biochemical and elemental stoichiometry of a freshwater centric diatom, Stephanodiscus minutulus (Grun.), under various nutrient regimes. Stephanodiscus minutulus was grown at μ_max or 22% of μ_max under limitation by silicon, nitrogen, or phosphorus. Cell sizes for nutrient‐limited cultures were significantly smaller than the non‐limited cell sizes, with N‐limited cells being significantly smaller than all other treatments. Compared with the nutrient‐replete treatment, both carbohydrates and lipids increased in Si‐ and P‐limited cells, whereas carbohydrates increased but proteins decreased in N‐limited cells. All of the growth‐limited cells showed an increase of carbohydrate and triglyceride, and a decrease of cell size and polar lipids as a percentage of total lipids. The non‐limited cells also had a significantly higher chl a concentration and galactolipids as a percentage of total lipids than any of the limited treatments, and the low‐Si and low‐P cells had significantly higher values than the low‐N cells. The particulate C concentrations showed significant differences between treatments, with the Si‐ and P‐limited treatments being significantly higher than the N‐ and non‐limited treatments. Particulate Si did not show a strong relationship with any of the parameters measured, and it was the only parameter with no differences between treatments. The low‐Si cells had a significantly higher P content (about two times more) than any other treatment, presumably owing to the luxury consumption of P, and a correspondingly high phospholipid concentration. The elemental data showed that S. minutulus had a high P demand with low optimum N:P (4) and Si:P (10) ratios and a C:N:P ratio of 109:16:2.3. The particulate C showed a positive relationship with POM (r = 0.93), dry weight (r = 0.88), lipid (r = 0.87) and protein (r = 0.84, all P < 0.0001). Particulate N showed a positive relationship with galactolipids (r = 0.95), protein (r = 0.90), dry weight (r = 0.78), lipid (r = 0.75), and cell volume (r = 0.64, all P < 0.0001). It is evident that nutrient limitation in the freshwater diatom S. minutulus has pronounced effects on its biochemical and elemental stoichiometry.     

Does the Redfield ratio infer nutrient limitation in the macroalga Spirogyra fluviatilis?

1. The cellular nutrient contents of microalgae, when growing at or approaching maximum rates, approximate the Redfield C : N : P (molar) ratio of 106 : 16 : 1. Deviations from this optimal ratio can be used to infer nutrient limitation of microalgal growth. However, this ratio may not be applicable to macroalgae, which are distinguished from microalgae by forming a thallus that is a discrete structure visible to the naked eye. The utility of the Redfield ratio to infer nutrient limitation of the growth of macroalgae was tested for Spirogyra fluviatilis in a field experiment conducted in tropical Australia. 2. The optimal cellular C : N : P ratio for S. fluvialitis was estimated by means of in situ nutrient addition. This was compared with S. fluvialitis cellular ratios determined from eight sites with a wide range of soluble N concentrations (<1–90 μg L^?1), a smaller range of soluble P concentrations (5–12 μg L^?1), and soluble molar N : P ratios of 0.11– 27. 3. Spirogyra fluviatilis had an optimal molar C : N : P ratio of 1800 : 87 : 1 which differs substantially from the Redfield ratio, and suggests that the latter ratio is not applicable to this macroalga. Concentrations of N and P in the river deviated from the optimal N : P ratio of 87 : 1, inferring nutrient limitation of growth. 4. C : P and C : N ratios of S. fluviatilis varied in accordance with general stoichiometric relationships for autotrophs under nutrient limitation of growth. Ratios of C : P and C : N increased, respectively, with increased severity of P‐ and N‐limitation. Additionally, C : P ratios increased with increased N : P ratios, whilst the C : N ratio increased with decreased N : P ratios. The C : N molar ratio however was an insensitive indicator of nutrient depletion compared with the C : P ratio. Under N‐limitation of growth, luxury amounts of P were stored by S. fluviatilis. 5. In aquatic environments where macroalgae are sufficiently abundant to be sampled, their cellular carbon, nitrogen and phosphorus stoichiometry can be used to infer nutrient limitation of growth when their optimal C : N : P ratio is known.     

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 LIMITATION OF BENTHIC ALGAE IN LAKE MICHIGAN: THE ROLE OF SILICA1

In the Laurentian Great Lakes, phytoplankton growth and biomass are secondarily limited by silica (Si), as a result of phosphorus (P) enrichment. Even modest levels of P enrichment can induce secondary Silimitation, which, in turn, promotes a shift from the native diatom phytoplankton flora to chlorophyte and cyanobacteria species. However, very little is known about the nutritional status of benthic populations and their response to nutrient enrichment. Two experiments were performed in the littoral zone of Lake Michigan where nutrients were delivered to in situ benthic algal (episammic and epilithic) assemblages using nutrient‐diffusing substrata. In order to test the hypothesis that benthic algae in Lake Michigan are Si limited, a 2 × 3 factorial experiment was used to deliver all combinations of Si, N, and P to resident assemblages growing on artificial substrata composed of natural (Si rich) versus calcium carbonate (Si poor) sand. A second experiment utilized a serial enrichment to evaluate the role of Si in mediating changes in taxonomic composition. These findings indicate that benthic algae in Lake Michigan exhibit signs of secondary Si limitation, and that their response to enrichment is similar to the phytoplankton. Moreover, natural sand substrata may provide a source of Si to resident benthic algae.