Nutrient Enrichment Bioassays Research Articles

Predicting phytoplankton assemblages growth and estimating critical N: P ratios with experimental nutrient additions in a drinking water reservoir of central China

The purpose of this study was to evaluate the effects of limiting nutrients and the N: P ratios on the growth of phytoplankton. In this study, we used the nutrient enrichment bioassays (NEB), Multivariate analysis of variance, and regression analysis. The results showed that the affected change of phytoplankton assemblages growth was the concentrations of nitrogen and phosphorous. During the same period, the response time of phytoplankton to the input of external nutrients was similar. The growth rates of phytoplankton were not persistent. This is related to the differences in environmental conditions, dominant species composition, and phytoplankton growth strategies. Different dominant species also had different response mechanisms to nutrient addition. The growth effect of phytoplankton in the preliminary experiment in the laboratory was more significant than that in the field experiment. Water temperature and light were also important environmental factors affecting the growth of phytoplankton. Multivariate analysis of variance showed that phosphorus addition alone had a more significant stimulation on phytoplankton growth than that of nitrogen, especially for Synedra. But addition of both nitrogen and phosphorus had the most stimulating effect on Cyanophyta and Chlorophyta. The phytoplankton had the maximum growth in TN: TP = 19.87, phytoplankton growth was promoted with 0.9 ∼ 13.59 mg/L TN and 0.02 ∼ 0.684 mg/L TP. When the total phosphorus in summer was greater than 0.04 mg/L, the possibility of algal bloom and will be greater. The linear regression fitted chlorophyll a and phosphorus well, and the regression equation was as follows: Log10(Chl.a) = 2.154 + 0.5103 × Log10(TP). The binary regression equation of chlorophyll a and nitrogen and phosphorus was as follows: Chl.a = 33.84 + 1.769TN + 123.9TP.

Phosphorus limitation in low nitrogen lakes in New Zealand

ABSTRACT Lakes on volcanic soils of the North Island of New Zealand, compared to north temperate lakes, have low total nitrogen:total phosphorus (TN:TP) ratios, low dissolved inorganic nitrogen concentrations during summer stratification, and have positive chlorophyll a responses to N additions more frequently than to P in nutrient enrichment bioassays. This response has resulted, in some cases, to the imposition of N loading caps on some lake catchments, in contrast to nutrient management in north temperate lakes focused more on P management. To explore this possible difference in limiting nutrients, a suite of nutrient status indicators based on measurement of ambient seston stoichiometry and metabolic activity, which have been widely used in north temperate lakes, were applied to 8 lakes on volcanic terrain with previously reported N limitation. These nutrient status measurements were previously calibrated to inform whether in situ phytoplankton are growth-rate limited and, if so, which nutrient is limiting growth rate. In austral summer 2015, all 8 lakes showed indications of P limitation, and P limitation was more extreme, pervasive, and persistent (among samplings) than N limitation. Indications of N limitation were not observed without contemporaneous evidence of P limitation, but P limitation was frequently observed without evidence of N limitation. One lake, Rotorua, was not strongly nutrient limited, and phytoplankton were likely growing at or near optimum growth rates. In this study the commonly used TN:TP ratio was a poor predictor of which nutrient was potentially limiting in situ phytoplankton.

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.

Statistical Optimization of Microcystis Growth and Microcystin Production in Natural Phytoplankton Community using Microcosm Bioassays

Eutrophication of freshwater lakes and reservoirs causing toxic cyanobacterial blooms has become a global health concern. Commonly used approaches of statistical modelling have not fully captured the complex response of cyanobacterial biomass and mi- crocystin concentration in response to stochastic variation of ambient conditions. This study applied statistically-based experimental design to screen out critical environmental factors, and to investigate the interactive effects of total nitrogen (TN), total phosphorus (TP), temperature (T) and light intensity (L) on Microcystis (MA) growth and microcystin (MC) production in the natural phytoplankton com- munity. A batch of central composite designed nutrient enrichment bioassays (CCD-NEBs) were conducted in an ambient-controlled microcosm based on pneumatic annular flume system. Second-order polynomial CCD regression models predicting MA density and MC concentration were established and validated. MC concentration was found positively related to MA dominance. The interactive effects of TN-TP and TN-L on both MA density and MC concentration were determined to be more significant than other factors. Ranges of the optimum TN/TP ratio for MA growth and MC production under various environmental conditions were defined as 9.5 ~ 10.2 and 9.9 ~ 10.4, respectively. The ranges of nutrient thresholds for MA density of 2.00 × 104 cells mL-1 and MC concentration of 1.0 μg L-1 under different temperature and light conditions were evaluated, respectively. This study revealed that nutrient thresholds for MA blooming events and cyanotoxins safety guideline were fluctuant under lake-dependant conditions of temperature and light intensity, and the optimum TN/TP mass ratios should be alerted for the eutrophication management of blooming formation and microcystin safety.

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

Abstract 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? 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. 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. 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. 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.

The influence of nutrients limitation on phytoplankton growth and microcystins production in Spring Lake, USA

Due to excessive loadings of nitrogen (N) and phosphorus (P), frequent blooms of harmful cyanobacteria and their associated cyanotoxins pose serious threats to recreational usage and human health. However, whether cyanobacteria growth and toxin production are limited by N, P, or both N + P is still not clear. Thus, we conducted a nutrient enrichment bioassay in situ in Spring Lake, a eutrophic lake in west Michigan, USA, to examine the influence of nutrient limitation on the proliferation of algal blooms and the production of microcystins (MC). N or P addition alone resulted in a slight increase in the concentration of chlorophyll-a (Chl-a), suggesting a positive effect on phytoplankton growth, but alone, neither were sufficient to induce algal blooms. In contrast, the combination of N and P had a significant and positive influence on phytoplankton growth and MC production. Compared to controls, the N + P treatment resulted in high concentrations of Chl-a and MC, as well as high pH and dissolved oxygen. In addition, significant increases were observed in different MC analogues for each treatment; the highest concentrations of intracellular MC-LR, -RR, -YR, and TMC (total MC) were found in the N + P treatment with values of 9.16, 6.10, 2.57, and 17.82 μg/L, respectively. This study suggests that at least in this temperate coastal lake, cyanobacterial blooms and associated MC are influenced more by combined N and P enrichment than by N or P alone, indicating that managing both nutrients is important for effectively reducing algal blooms and MC production.

Spatiotemporal Variability in Autotrophic and Heterotrophic Microbial Plankton Abundances in a Subtropical Estuary (Galveston Bay, Texas)

Williams, A.K. and Quigg, A., 2019. Spatiotemporal variability in autotrophic and heterotrophic microbial plankton abundances in a subtropical estuary (Galveston Bay, Texas). Journal of Coastal Research, 35(2), 434–444. Coconut Creek (Florida), ISSN 0749-0208.The subtropical estuary Galveston Bay (Texas) is influenced by frequent riverine pulses from two important rivers in its northern sector and by ocean tides from the Gulf of Mexico in its southern sector. This study examined combinations of abiotic and biotic factors that synergistically influence spatiotemporal variability of autotrophic and heterotrophic microbial plankton abundances (0.2–20 µm). Potential nutrient limitation observed in situ was supported by in vitro enrichment bioassays. Shifts in the relative in situ abundance of autotrophs and heterotrophs, stained with SYBR Green I and enumerated on a flow cytometer, were significantly related to temperature, total nitrogen, dissolved inorganic nitrogen-to-phosphorus ratios, and total organic carbon concentrations along estuarine gradients. Observations reveal a pattern of serial colimitation of the microbial plankton community. Among the variables tested, nitrogen became the principal limiting factor for growth if phosphorous was available and temperatures were warm. In vitro nutrient enrichment bioassays performed with inorganic nitrogen (as nitrate) and Pi revealed variations in the responses between autotrophic and heterotrophic microbes under nutrient-limiting conditions, particularly during co-occurring phytoplankton blooms. The measured dynamics in planktonic relationships will have important impacts on understanding estuarine nutrient processing.

Biological Assessment of Water Toxicity and Fertility of Wastewater Omer Bek Drain Discharging into Receiving Water of the River Nile in Egypt

Water toxicity and fertility of a drain wastewater discharging wastewater directly to theDamietta branch of the River Nile were seasonally investigated from winter to autumn,2016. Nutrient enrichment bioassays to define the algal growth limiting nutrients, theirbioavailability and heavy metal toxicity of the sampled water, were carried out with the ISOstandard test alga Pseudokirchneriella subcapitata. The algal growth potential (AGP) testexhibited a wide range of growth potential ranging from 0.4 mg.l-1 to 19.42 mg.l-1 algal dryweight. At drain, mixing and recovery stations the algal growth was mainly limited byheavy metals toxicity with inhibition percent at day 14 of growth (%I14) ranged between27.9 and 58.6 while at the upstream reference station, %I14 ranged between 20 and 37.Thealgal growth was also limited by N and P depending on site and season. In all seasons, butonly at drain station, the expected Chl. a was significantly higher than the observed Chl. a,however, the reverse was held true for other sampling stations during this study. Therelationships between the chemically analyzed nutrients (P and N) and their bioavailableconcentrations were greatly affected by heavy metal toxicity. The results indicated that theobserved chlorophyll a showed significant correlations with TDP(r=0.82), while theexpected chlorophyll a correlated significantly with NH4-N(r=0.73). The algal growthpotential test proved a sensitive and reliable mean to assess water fertility and toxicity ofpolluting drains and the receiving waters.

NUTRIEN PEMBATAS DI EAST RIVER, KOREA

This study presents information on physico-chemical and nutrients that limit the water of East River, Korea. Collected water samples from 16 main streams and 7 branches in MayNovember 2008 were measured for their heavy metal and nutrients concentration. Nutrient enrichment bioassay tests with the use of Nitzschia pungens was also conducted. This threeseason study indicated that the phosphorus was the limiting factor for algal growth.

Unbalanced reduction of nutrient loads has created an offshore gradient from phosphorus to nitrogen limitation in the North Sea

AbstractMeasures to reduce eutrophication have often led to a more effective decline of phosphorus (P) than nitrogen (N) concentrations. The resultant changes in riverine nutrient loads can cause an increase in the N : P ratios of coastal waters. During four research cruises along a 450 km transect, we investigated how reductions in nutrient inputs during the past 25 yr have affected nutrient limitation patterns in the North Sea. This revealed a strong offshore gradient of dissolved inorganic N : P ratios in spring, from 375 : 1 nearshore toward 1 : 1 in the central North Sea. This gradient was reflected in high nearshore N : P and C : P ratios of particulate organic matter (mainly phytoplankton), indicative of severe P deficiency of coastal phytoplankton, which may negatively affect higher trophic levels in the food web. Nutrient enrichment bioassays performed on‐board showed P and Si limitation of phytoplankton growth nearshore, co‐limitation of N and P in a transitional region, and N limitation in the outer‐shore waters, confirming the existence of an offshore gradient from P to N limitation. Different species were limited by different nutrients, indicating that further reductions of P loads without concomitant reductions of N loads will suppress colonial Phaeocystis blooms, but will be less effective in diminishing harmful algal blooms by dino‐ and nanoflagellates. Hence, our results provide evidence that de‐eutrophication efforts in northwestern Europe have led to a large imbalance in the N : P stoichiometry of coastal waters of the North Sea, with major consequences for the growth, species composition, and nutritional quality of marine phytoplankton communities.

Determining critical nutrient thresholds needed to control harmful cyanobacterial blooms in eutrophic Lake Taihu, China.

Nutrient overenrichment has led to dramatic increases in harmful cyanobacterial blooms, creating serious threats to drinking water supplies, ecological and economic sustainability of freshwater ecosystems. Nutrient-cyanobacterial bloom interactions were examined in eutrophic Lake Taihu, China. In situ microcosm nutrient dilution bioassays and mesocosm nutrient addition experiments were conducted to determine nitrogen (N) and phosphorus (P) concentration and load thresholds needed to control cyanobacterial bloom formation. Blooms were dominated by toxic, non N2 fixing Microcystis spp, from May to December. Dilution bioassays showed seasonality in nutrient limitation, with P-availability controlling prebloom spring conditions and N-availability controlling summer-fall blooms. Nutrient dilution and enrichment bioassays indicated that total nitrogen (TN) and total phosphorus (TP) concentration thresholds should be targeted at below 0.80 mg L(–1) and 0.05 mg L(–1), respectively, to limit intrinsic growth rates of Microcystis dominated blooms. Based on estimates of nutrient loading and observed stoichiometry of phytoplankton biomass, 61–71% TN and 20–46% TP reduction are necessary to bring Taihu’s phytoplankton biomass to “acceptable” sub-bloom conditions of less than 20 μg L(–1) chlorophyll a.

Eutrophication and nutrient limitation in the aquatic zones around Huainan coal mine subsidence areas, Anhui, China.

The eutrophication of three small lakes in the aquatic zones at the Huainan coal mine subsidence areas, designated as east site (ES), central site (CS), and west site (WS), were studied. Nutrient content, species, and nitrogen (N) to phosphorus (P) ratios were obtained through water quality analyses. Nutrient limitation was evaluated by nutrient enrichment bioassays (NEBs) in the autumn of 2012 and spring of 2013. Average annual concentrations of total phosphorus (TP) were 0.05, 0.08, and 0.10 mg/L, and total nitrogen (TN) concentrations were 0.77, 1.95, and 2.06 mg/L in the water column at CS, ES, and WS, respectively. All of the three lakes exhibited 'meso-eutrophic' states and the TN:TP ratio ranged from 25:1 to 74:1 with variability between seasons and sites. NEBs verified that primary productivity in the lakes at ES and WS were mainly limited by P, while N limitation or N and P co-limitation was present in the aquatic zones at CS due to unavailable dissolved inorganic nitrogen. In the studied lakes, the blue-green algae, which comprised 70% of all identified species, was the predominant taxa, while the micro-zooplankton taxa was dominant, indicating a typical trophic structure of eutrophic lakes.

Summer phytoplankton nutrient limitation in Maumee Bay of Lake Erie during high-flow and low-flow years

Algal production in Maumee Bay in western Lake Erie is highly affected by inputs of nitrogen (N) and phosphorus (P) from the Maumee River, which drains predominantly agricultural lands, leading to the formation of cyanobacterial blooms. In a 3-year study, precipitation and discharge ranged from relatively low (2012) to relatively high (2011) with corresponding changes in the size of the cyanobacterial bloom. This study aimed to quantify the relation between river discharge and algal nutrient limitation in Maumee Bay. During the summer growing seasons, 20 nutrient enrichment bioassays were performed to determine which nutrient (P or N) might limit phytoplankton growth; and ambient N and P concentrations were monitored. The bioassays suggested that phytoplankton growth shifted from P-limited to N-limited during summer of the low and intermediate discharge years (2012 and 2010, respectively), whereas during the high discharge year (2011) phytoplankton were nutrient-replete before becoming N-limited. Phosphorus-replete growth during the high discharge year likely was due to high P loads from the river and dissolved P concentrations greater than 1μmol/L. Symptoms of N-limited growth occurred during August and September in all three years and during July of 2012 when NO3− plus NH4+ concentration was less than 7.29μmol/L suggesting low or no correspondence between N-limitation and size of the cyanobacterial bloom. Occurrence of a relatively small cyanobacterial bloom in 2012 following the record-breaking bloom in 2011 suggests the possibility of fast-reversal of eutrophication in Maumee Bay if P loading from the watershed could be decreased.

N:P Ratio 조절에 의한 단기 영양염 첨가 바이오에세이(NEBs) 및 1차 생산력(엽록소-a)의 반응성 테스트

N:P Ratio 조절에 의한 단기 영양염 첨가 바이오에세이(NEBs) 및 1차 생산력(엽록소-a)의 반응성 테스트

Nutrient Enrichment Bioassay: A Study with Indigenous Phytoplankton of a Lagoon Receiving Sugar Mills Effluents.

The response of indigenous phytoplankton population to nutrients enrichment bioassay of phosphate-phosphorus and nitrate–nitrogen of different concentrations were studied for three locations created as “before the effluents discharge; point of effluents discharge and after the effluents discharge. For the present study, physico-chemical properties of sugar milling effluents recorded 1.50 mg/L of dissolved oxygen concentration; biological oxygen demand value was 150 mg/L while chemical oxygen demand recorded 59.0 mg/L with the pH of 4.82. Four divisions, Bacillariophyta, Cyanophyta, Chlorophyta and Euglenophyta were predominantly recorded for the combinations while the optimal growth was recorded for all the stations at 100 P /50 N nutrients combination.

Eutrophication and nutrient enrichment bioassays in the waters of the Huainan and Huaibei coal mining subsidence areas,Anhui Province

选取两淮采煤沉陷区内3个不同营养水平的水域为研究站点,即淮北南湖站(HBNH)、淮南潘谢顾桥站(PXGQ)和潘谢谢桥站(PXXQ),首先分析了水体营养盐含量、比例结构和营养状态指数,3个站点TP浓度的年均值分别为0.056、0.064和0.092 mg/L,TN浓度年均值则为1.00、0.94和2.67 mg/L,3个站点水体呈现中营养-轻度富营养和中度富营养2种营养状态,总体上表现出P相对N缺乏的特征.设置对照组、加氮组、加磷组和加氮磷组开展秋季氮、磷限制模拟实验研究.结果表明:HBNH、PXXQ两个站点为P限制,而PXGQ站点则为N限制.尽管水体正磷酸盐浓度较低,但由于藻类具有利用有机磷或储备P库的能力,3个研究站点依然保持了较高的初级生产力,HBNH、PXGQ和PXXQ 3个站点的叶绿素a浓度年均值分别为13.07、26.95和46.25 mg/m³,与各水体的营养水平保持一致.两淮采煤沉陷区水体富营养化控制关键可能在于调控磷元素的水平.;Three waters of different trophic states in the Huaibei and Huainan coal mining subsidence areas,including sites HBNH, PXGQ and PXXQ,were chosen to address their nutrient content,ratio of nitrogen to phosphorus (N: P),and nutrient limitation.Averaged annual concentrations of total phosphorus (TP) in the three sites are 0.056,0.064 and 0.092 mg/L,while the total nitrogen (TN) concentrations are 1.00,0.94 and 2.67 mg/L,respectively.The three studied waters present two trophic levels of meso-trophic to light eutrophication and meso-eutrophciation with deficiency of phosphorus.Nutrient enrichment bioassays were performed by four groups' experiments.Three treatments were cultured with additions of nitrogen (+N),phosphorus (+P),and nitrogen plus phosphorus (+N+P),with raw water without additions as control.HBNH and PXXQ show features of P limitation,and the other one,PXGQ presents N limitation due to its lower available inorganic nitrogen.Although very low phosphate concentrations,the three sites still have rather high primary production,with their chlorophyll-a concentrations of 13.07 (HBNH),26.95(PXGQ) and 46.25 mg/m³ (PXXQ),which are in accordance with their nutrient states.The results of this research give implication of the importance of P control to these waters in the Huainan and Huaibei coal mining subsidence areas.

Nitrogen Constrains the Growth of Late Summer Cyanobacterial Blooms in Lake Erie

Phosphorus (P) is generally considered to be the main limiting nutrient to freshwater phytoplankton productivity. However, recent research is drawing attention to the importance of nitrogen (N) in freshwater eutrophication and N often constrains growth of cyanobacteria in small lakes. In this study we determined phytoplankton nutrient limitation in a large lake, Lake Erie during two growing seasons. During 2010 and 2011, nutrient enrichment bioassays (+P, +N and, +P and N) were conducted monthly from June through September with water collected in Maumee Bay (site MB18) and in the center of the western basin (site WBC). Nutrient concentrations were monitored every other week. At MB18, total P concentration was often >3 mmol/L and nitrate concentration decreased from >250 mmol/L in early summer to mmol/L in late summer. Nitrogen and P levels were about five-fold less at WBC. Bioassays indicated that phytoplankton nutrient limitation varied in summer, spatially, and even among phytoplankton groups. For site MB18, +P increased chlorophyll concentration in one of the eight bioassays, indicating that P did not typically limit production. For site WBC, +P increased chlorophyll concentration in six of the eight bioassays. As a result of very low ambient nitrate concentration (mmol/L) in late summer, +N (without P) increased chlorophyll concentration, suggesting symptoms of N-limitation. The N-fixing cyanobacterium Anabaena became dominant following N-limitation. This study highlights the need to reduce P loading to restore water quality. Furthermore, due to low nitrate concentration, the severity of the cyanobacterial blooms could be worse if not for N-limitation in western Lake Erie.

Phosphorus and nitrogen limitation of phytoplankton growth in eutrophic Lake Inba, Japan

Lake Inba is one of the most eutrophic lakes in Japan. In this study, field sampling and nutrient enrichment bioassays were conducted to determine the seasonal patterns of nutrient limitation for phytoplankton growth in this lake. Phytoplankton biomass increased significantly with the additions of phosphorus (P) on almost all sampling dates, indicating P limitation of phytoplankton growth from spring to autumn. However, nitrogen (N) limitation was also observed during summer (i.e., 19 August). On 10 August, a typhoon struck Lake Inba. After this event, dissolved inorganic nitrogen (DIN) and phosphorus concentrations increased, probably because of increased river discharge. At the same time, phytoplankton growth in the control treatment became relatively high, with the addition of neither P nor N stimulating the growth. However, 10 days after the typhoon, the phytoplankton growth rate in the control treatment decreased, with only the addition of N having a significant positive effect on phytoplankton growth. N limitation during summer is caused by the low concentrations of DIN, as well as changes in the N:P ratio due to allochthonous nutrient loads. These results indicate that a reduction of both P and N input is necessary to control phytoplankton blooms in Lake Inba.

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0.03 μg/L分别达到2.12 μg/L和1.83 μg/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。;An increase in N₂ fixation by diazotrophic organism due to increased stratification driven by climate changes, may decrease phosphate concentrations and result in P limitation in the oligotrophic upper ocean, which challenges the traditional view that nitrogen is generally the primary nutrient limiting phytoplankton productivity in oceanic waters. Which nutrient, N or P, is the most limiting nutrient for phytoplankton growth in the oligotrophic Pacific Ocean has been on debate over recent years. More studies on nutrient limitation are apparently needed to resolve this debate in the Pacific Ocean. In August and September of 2009, nutrient enrichment bioassays were conducted at two representative stations, N1 (160.58°E, 21.61°N) in the western Pacific Ocean with extremely low nutrient (below detect limit) and Chl a concentrations, and N2 (154.12°W, 10.12°N) in the eastern Pacific Ocean with shallower nutricline due to the influence of equatorial currents, in order to examine the spatial variability in the potential limiting nutrient for phytoplankton growth in the Pacific Ocean. Nutrients were added in 5 combinations in bioassays: control (no addition), NO₃+PO₄ (N+P); NO₃+SiO₄ (N+Si), PO₄+SiO₄ (P+Si), and NO₃+PO₄+SiO₄ (N+P+Si). The limiting nutrient was judged based on the response of algal biomass and nutrient depletion among treatments. Phytoplankton exhibited different response to nutrient additions at two study sites. Phytoplankton biomass increased dramatically in response to both N and P additions at station N1 where the concentrations of Chl a increased from 0.03 μg/L at the beginning to 2.12 μg/L and 1.83 μg/L in N+P+Si and N+P treatments at the end of incubation, respectively. However, the maximum Chl a concentration achieved in N+Si treatment was slight higher than that in and P+Si treatment. Furthermore, P was depleted before N and Si in the N+P and N+P+Si treatments at station N1. In contrast, algal biomass was significantly stimulated only when both N and P were added at station N2, where the concentrations of Chl a increased profoundly from 0.10 μg/L to 0.34 μg/L and 0.40 μg/L at the end in N+P+Si and N+P treatments, respectively, while they was not stimulated in the N+Si and P+Si treatments Furthermore, N always disappeared before P and Si in N+P, N+Si and N+P+Si treatments. These results showed that the limiting nutrient varied spatially during summer in the Pacific Ocean. N and P co-limitation occurred at both stations, with P being the primary limiting nutrient at N1 and N at N2. In addition, changes in the N∶P ratios during the incubation demonstrated distinct patterns, likely due to difference in the phytoplankton composition. N∶P ratios rose from 15.7 at the beginning to 59.2 at the end of the incubation at N1, while N∶P ratios decreased from 15.3 to 0.06 at N2. This implied that the uptake ratio of N∶P was lower than the Redfield ratio (16N∶1P) at N1, but higher than the Redfield ratio at N2. This might explain why P was the primary limiting nutrient in the western Pacific Ocean but N in the central Pacific Ocean. It is speculated that P limitation possibly is associated with N₂ fixation in the oligotrophic western Pacific Ocean.

Nutrients enrichment and algal response based on <i>in situ</i> experiments of NEB in summer:a comparative study of Lakes Dianshan, Xiaoxingkai and Erhai

淀山湖、小兴凯湖和洱海分别处在不同的营养阶段,夏季都存在蓝藻水华现象,有效控制蓝藻水华应控氮还是控磷一直存在争议.本研究采用营养物加富生物测试的试验方法,研究和比较三个湖泊限制性营养元素(N和P)夏季对浮游藻类生长的刺激作用,并采用多因素方差分析和两两比较方法(LSD)检验试验结果的显著性.结果表明:淀山湖(TN/TP=10左右)、洱海(TN/TP=29左右)、小兴凯湖(TN/TP=9左右)在夏季分别表现出显著的氮响应、氮磷双重响应、氮响应;较低氮磷比的营养程度较高的湖泊(淀山湖和小兴凯湖)夏季应控制氮含量,若添加氮更容易引发蓝藻(微囊藻)水华,而高氮磷比的初期富营养化湖泊(洱海)夏季应同时控制氮磷含量,同时添加氮磷的交互作用容易引发蓝藻(微囊藻)水华.;Lakes Dianshan, Xiaoxingkai and Erhai represent different trophic status respectively, and each of them is suffering severe cyanobacteria blooms in summer. Nutrient enrichment bioassays were conducted in the fields to compare the effects of nutrients (N and P) on cyanobacteria growth in the three lakes in summer. Significant differences between/among the treatments of the experiments were tested by Univariate and LSD. The results showed that Lakes Dianshan, Erhai and Xiaoxingkai have significant responses to N, N and P, N in summer, and add N to the Lake Dianshan and Lake Xiaoxingkai with low N:P ratio which may cause cyanobacteria(Microcystis) blooms easily, while N and P interaction may induce cyanobacteria bloom in lake(Lake Erhai) with high N:P ratio.