Nitrogen cycling and anthropogenic impact in the tropical interamerican seas

We discuss the mechanisms leading to nutrient limitation in tropical marine systems, with particular emphasis on nitrogen cycling in Caribbean ecosystems. We then explore how accelerated nutrient cycling from human activities is affecting these systems.Both nitrogen and phosphorus exert substantial influence on biological productivity and structure of tropical marine ecosystems. Offshore planktonic communities are largely nitrogen limited while nearshore ecosystems are largely phosphorus limited. For phosphorus, the ability of sediment to adsorb and store phosphorus is probably greater for tropical carbonate sediments than for most nearshore sediments in temperate coastal systems. However, the ability of tropical carbonate sediments to take up phosphorus can become saturated as phosphorus loading from human sources increases. The nature of the sediment, the mixing rate between nutrient-laden runoff waters and nutrient-poor oceanic waters and the degree of interaction of these water masses with the sediment will probably control the dynamics of this transition.Nearshore tropical marine ecosystems function differently from their temperate counterparts where coupled nitrification/denitrification serves as an important mechanism for nitrogen depuration. In contrast, nearshore tropical ecosystems are more susceptible to nitrogen loading as depurative capacity of the microbial communities is limited by the fragility of the nitrification link. At the same time, accumulation of organic matter in nearshore carbonate sediments appears to impair their capacity for phosphorus immobilization. In the absence of depurative mechanisms for either phosphorus or nitrogen, limitation for both these nutrients is alleviated and continued nutrient loading fuels the proliferation of nuisance algae.     

The importance of nitrogen in Pyramid Lake (Nevada,USA), a saline,desert lake

The increase in human development in the downstream portion of the Pyramid Lake drainage basin has resulted in increased nutrient loading to the lake. Since this is a deep, terminal lake, concern over nutrient build up and change in trophic status exists. On the basis of lake chemistry which shows consistently high concentrations of total reactive-P (mean = 55 µg P l^–1) relative to dissolved inorganic-N (DIN) (mean = 15 µg N 1^–1), it has been hypothesized that Pyramid is N-limited. However, no systematic study of nutrient limitation had been undertaken. Nutrient enrichment bioassays conducted throughout an entire year clearly showed that additions of DIN resulted in a 350–600% stimulation of chlorophyll production. Phosphate, when added singly or in combination with DIN, had no effect. This positive response to N-addition was significant at all times of the year except, (1) immediately after complete lake mixing in February when a large pool of hypolimnetic nitrate was injected into the euphotic zone, and (2) during a fall bloom of the nitrogen fixing species Nodularia spumigena. The positive response to N-addition in the bioassay experiments was strong between March and November. However, the seston exhibited only a gradual depletion of nitrogen relative to carbon over this same period. PN:PC ratios suggested no N-deficiency in phytoplankton biomass in February, March and April, moderate N-deficiency in May, June and July and, severe N-deficiency from August until winter turnover. The appearance of nitrogen fixing blue-green algae in September supports the hypothesis of N-limitation in the summer-autumn. In evaluating the nutrient status of a lake, the concepts of nutrient stimulation versus nutrient deficiency versus nutrient limitation must clearly be defined.This paper is dedicated to G. Evelyn Hutchinson who first visited Pyramid Lake in 1933.     

Nutrient limitation of phytoplankton growth in Waquoit Bay, Massachusetts, USA: a nutrient enrichment study

We conducted nutrient enrichment experiments and field sampling to address three questions: (1) is there nutrient limitation of phytoplankton accumulation within an estuary whose waters are exposed to relatively high nitrogen loading rates, (2) where in the salinity gradient from fresh to seawater (0 to 32‰) is there a shift from phosphorus to nitrogen limitation of phytoplankton accumulation, and (3) is there a seasonal shift in limiting function of phosphorus and nitrogen anywhere in the estuarine gradient. Nitrogen and phosphorus enrichment experiments in the Childs River, an estuary of Waquoit Bay, Massachusetts, USA, showed that the accumulation of phytoplankton biomass in brackish and saline water was limited by supply of nitrate during warm months. The effects of enrichment were less evident in fresh water, with short-lived responses to phosphate enrichment. There was no specific point along the salinity gradient where there was a shift from phosphorus- to nitrogen-limited phytoplankton accumulation; rather, the relative importance of nitrogen and phosphorus changed along the salinity gradient in the estuary and with season of the year. There was no response to nutrient additions during the colder months, suggesting that some seasonally-varying factor, such as light, temperature or a physiological mechanism, restricted phytoplankton accumulation during months other than May-Aug. There was only slight evidence of a seasonal shift between nitrogen- and phosphorus-limitation of chlorophyll accumulation. Phytoplankton populations in nutrient-rich estuaries with short flushing times grow fast, but at the same time the cells may be advected out of the estuaries while still rapidly dividing, thereby providing an important subsidy to production in nearby deeper waters. This revised version was published online in August 2006 with corrections to the Cover Date.     

Lake‐type‐specific seasonal patterns of nutrient limitation in German lakes,with target nitrogen and phosphorus concentrations for good ecological status

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Phosphatase activity and nitrogen fixation reflect species differences,not nutrient trading or nutrient balance,across tropical rainforest trees

A fundamental biogeochemical paradox is that nitrogen‐rich tropical forests contain abundant nitrogen‐fixing trees, which support a globally significant tropical carbon sink. One explanation for this pattern holds that nitrogen‐fixing trees can overcome phosphorus limitation in tropical forests by synthesizing phosphatase enzymes to acquire soil organic phosphorus, but empirical evidence remains scarce. We evaluated whether nitrogen fixation and phosphatase activity are linked across 97 trees from seven species, and tested two hypotheses for explaining investment in nutrient strategies: trading nitrogen‐for‐phosphorus or balancing nutrient demand. Both strategies varied across species but were not explained by nitrogen‐for‐phosphorus trading or nutrient balance. This indicates that (1) studies of these nutrient strategies require broad sampling within and across species, (2) factors other than nutrient trading must be invoked to resolve the paradox of tropical nitrogen fixation, and (3) nitrogen‐fixing trees cannot provide a positive nitrogen‐phosphorus‐carbon feedback to alleviate nutrient limitation of the tropical carbon sink.     

Characterisation of the type and extent of nutrient limitation in grassland vegetation using a bioassay with intact sods

An important methodological problem in plant ecology concerns the way in which the type and extent of nutrient limitation in terrestrial communities should be assessed. Conclusions on nutrient limitation have been founded mainly on soil extractions, fertiliser trials and tissue nutrient concentrations. In order to avoid some of the problems associated with these methods, we employed a special technique using intact sods which rooted both in the intact soil and in a nutrient solution, from which N, P and K were omitted stepwise. The method was applied to hay-field communities which differed in their history of fertiliser application. Four fields were compared which were not fertilised for 2, 6, 19 or 45 years, while hay making continued. This was done to restore former species-rich grassland communities. We tested the hypothesis that the increase in species diversity in these grasslands was attended by an increase in the number of limiting nutrients.We observed clear shifts in the type and extent of nutrient limitation. Fields which were recently fertilised were characterised by nitrogen and potassium limitation, while phosphorus limitation increased in importance towards the later stages of succession. In the last field (45 years unfertilised) N, P and K equally limited production at the community level. These conclusions differed from those drawn from a fertiliser trial in these same four fields, which failed to detect phosphorus limitation.It is concluded that the use of this method provides a valuable extra source of information while studying relationships between nutrient limitation and species diversity in grassland communities.     

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.     

Nutrient stoichiometry of linked catchment‐lake systems along a gradient of land use

1. Catchments export nutrients to aquatic ecosystems at rates and ratios that are strongly influenced by land use practices, and within aquatic ecosystems nutrients can be processed, retained, lost to the atmosphere, or exported downstream. The stoichiometry of carbon and nutrients can influence ecosystem services such as water quality, nutrient limitation, biodiversity, eutrophication and the sequestration of nutrients and carbon in sediments. However, we know little about how nutrient stoichiometry varies along the pathway from terrestrial landscapes through aquatic systems. 2. We studied the stoichiometry of nitrogen and phosphorus exported by three catchments of contrasting land use (forest versus agriculture) and in the water column and sediments of downstream reservoirs. We also related stoichiometry to phytoplankton nutrient limitation and the abundance of heterocystous cyanobacteria. 3. The total N : P of stream exports varied greatly among catchments and was 18, 54 and 140 (molar) in the forested, mixed‐use and agricultural catchment, respectively. Total N : P in the mixed layers of the lakes was less variable but ordered similarly: 35, 52 132 in the forested, mixed‐use and agricultural lake, respectively. In contrast, there was little variation among systems in the C : N and C : P ratios of catchment exports or in reservoir seston. 4. Phytoplankton in the forested lake were consistently N limited, those in the agricultural lake were consistently P limited, and those in the mixed‐use lake shifted seasonally from P‐ to N limitation, reflecting N : P supply ratios. Total phytoplankton and cyanobacteria biomass were highest in the agricultural lake, but heterocystous (potentially N fixing) cyanobacteria were most abundant in the forested lake, corresponding to low N : P ratios. 5. Despite large differences in catchment export and water column N : P ratios, the N : P of sediment burial (integrated over several decades) was very low and remarkably similar (4.3–7.3) across reservoirs. N and P budgets constructed for the agricultural reservoir suggested that denitrification could be a major loss of N, and may help explain the relatively low N : P of buried sediment. 6. Our results show congruence between the catchment export N : P, reservoir N : P, phytoplankton N versus P limitation and the dominance of heterocystous cyanobacteria. However, the N : P stoichiometry of sediments retained in the lakes was relatively insensitive to catchment stoichiometry, suggesting that a common set of biogeochemical processes constrains sediment N : P across lakes of contrasting catchment land use.     

The influence of mesozooplankton on phytoplankton nutrient limitation: a mesocosm study with northeast Atlantic plankton

We used marine phytoplankton from mesocosms seeded with different zooplankton densities to study the impact of mesozooplankton on phytoplankton nutrient limitation. After 7 d of grazing (copepod mesocosms) or 9 d (appendicularian mesocosms) phytoplankton nutrient limitation was studied by enrichment bioassays. After removal of mesozooplankton, bioassay bottles received either no nutrients, phosphorus or nitrogen alone, or a combination of nitrogen and phosphorus and were incubated for 2 d. Phytoplankton reproductive rates in the bottles without nutrient addition were calculated after correction for grazing by ciliates and indicated increasing nitrogen limitation with increasing copepod abundance. No nutrient limitation was found in the appendicularian mesocosms. The increase of nutrient limitation with increasing copepod density seems to be mainly the result of a trophic cascade effect: Copepods released nanoplankton from ciliate grazing pressure, and thereby enhanced nitrogen exhaustion by nanophytoplankton and reduced nitrogen excretion by ciliates. Nitrogen sequestration in copepod biomass, the mechanism predicted by the ecological stoichiometry theory, seems to have been a weaker effect because there was only little copepod growth during the experiment.     

Micronutrient and phosphorus limitation of phytoplankton abundance in Gem Lake,Sierra Nevada,California

Nutrient addition experiments conducted during the ice-free seasons of 1983 and 1984 in Gem Lake, an alpine lake in the Sierra Nevada mountains of California, indicate that algal biomass is limited by phosphorus, in combination with iron or copper. Phosphorus additions were always required to stimulate growth, but did not do so when phosphorus was the only nutrient added. Simultaneous additions of phosphorus and iron resulted in increased levels of chlorophyll, particulate carbon, particulate nitrogen and particulate phosphorus. Simultaneous additions of phosphorus and copper resulted in increases in chlorophyll, particulate nitrogen and particulate phosphorus, not in particulate carbon. Neither iron nor copper by itself stimulated growth.Particulate N : P ratios from all seasons in Gem Lake suggest that simultaneous micronutrient and phosphorus limitation exists throughout the summer, when nutrient and biomass levels remain low; limitation by phosphorus alone may appear in the fall and spring, when biomass and major ion concentrations increase dramatically.     

大沙河水库主要入库河流氮磷营养输入分析

为了解大沙河水库流域内营养盐输入对水库水质的影响,以 2011年3月~2012年2月大沙河水库5条主要入库河流(大沙河、白沙河、双石河、富食河和沃江河)的水文水质监测数据为依据,分析了这些入库河流的流量和氮磷营养盐浓度,并估算了外源负荷总量,旨在为水库进行高效和合理的流域规划以及水质保护方案的制定提供科学依据。结果表明:位于西南方向的白沙河年平均流量最大(1.01 m³·s^-1),西北部富食河流量最小(0.23 m³·s^-1)。各入库河流总氮平均浓度变化范围为1.62~4.37 mg·L^-1,总磷平均浓度范围为0.08~0.36 mg·L^-1,其中富食河氮和磷营养盐的浓度最高,大沙河总氮浓度最低,白沙河总磷浓度最低,总体上西北部河流的氮磷浓度高于西南部河流。全年大沙河水库总氮输入量为176.7 t,总磷输入量为13.7 t。在所有入库河流中,位于水库北部的沃江河对水库营养盐输入量贡献最大,氮、磷负荷分别占总输入量的33%和32%,位于西部的双石河氮负荷最小(12%),西南方向大沙河磷负荷最小(9%)。     

降水量变化对藏北高寒草地养分限制的影响

生态系统净初级生产养分限制的模式是现代生态学关注的重要问题。养分的可利用性是草原生态系统生产力动态变化的关键决定因素, 但土壤养分可利用性与整个生态系统中养分限制之间的关系尚不清楚。该研究通过在藏北降水梯度上4种类型高寒草地(从东到西依次是高寒草甸、高寒草甸草原、高寒草原和高寒荒漠草原)设置氮磷养分添加试验, 系统研究氮磷养分添加对不同类型高寒草地的影响, 并探讨降水梯度上高寒草地的氮磷限制模式。结果表明: (1)氮磷添加对不同高寒草地的影响存在差异: 氮添加显著提高了高寒草甸和高寒草甸草原地上生产力, 而对高寒草原和高寒荒漠草原无影响; 单独磷添加对4种高寒草地均无显著影响, 而氮磷添加对4种高寒草地地上生产力均有促进作用。(2)通过计算氮磷共同限制指数发现: 随着降水量减少, 高寒草地氮限制指数从1.18逐渐降低到0.52-0.64, 养分限制模式从氮限制过渡到氮磷共同限制; 磷限制指数在高寒草甸草原和高寒草原为负值, 说明单独磷添加对高寒草甸的生产力有负向作用, 高寒草甸主要受氮限制; 高寒草甸草原介于氮限制与氮磷限制之间, 受到氮磷共同限制, 单独磷添加有负向作用; 高寒荒漠草原受到氮磷共同限制。研究表明, 高寒草地氮磷限制模式存在环境梯度上的递变规律, 随着降水量减少, 高寒草地养分限制模式从氮限制逐渐过渡到氮磷共同限制。由此推断, 未来气候变化条件下氮沉降增加对不同类型高寒草地的影响可能存在差异。同时, 利用养分添加恢复不同类型退化高寒草地时也应将氮磷限制模式的差异考虑进去。     

Contrasting mechanisms of proteomic nitrogen thrift in Prochlorococcus

Organisms limited by carbon, nitrogen or sulphur can reduce protein production costs by transitions to less costly amino acids, or by reducing protein expression. These alternative mechanisms of nutrient thrift might respond differently to selection, but this possibility remains untested. We hypothesized that relatively invariant sequence composition responds to long-term variation in nutrient concentrations, whereas dynamic expression profiles vary with nutrient predictability. Prolonged nutrient scarcity favours proteome-wide nutrient reduction. Under stable, nonfluctuating nutrient availability, reduction of nutrient content typically occurs in proteins upregulated when nutrient availability is low, e.g. assimilation and catabolism. We suggest that fluctuating nutrient availability favours mechanisms involving short-term downregulation of nutrient-rich proteins. We analysed protein nitrogen content in six high-light, low-nutrient adapted (HL) vs. six low-light, high-nutrient adapted (LL) Prochlorococcus (marine cyanobacteria) strains, alongside expression data under experimental nitrogen and phosphorus limitation in two strains, MED4 (HL) vs. MIT9313 (LL). HL strains contained less nitrogen, but DNA GC content confounded this relationship. While anabolic and catabolic proteins had normal nitrogen content, most strains showed reduced nitrogen in typical nitrogen stress response proteins. In the experimental data set, though, proteins upregulated under nitrogen limitation were nitrogen-poor only in MIT9313, not MED4. MIT9313 responded similarly to nitrogen and phosphorus limitation, with slow, sustained downregulation of nitrogen-rich ribosomal proteins. In contrast, under nitrogen but not phosphorus limitation, MED4 rapidly downregulated ribosomal proteins. MED4's specific, rapid nitrogen response suggests adaptation to fluctuating conditions, supporting previous work. Thus, we identify contrasting proteomic nitrogen thrift mechanisms within Prochlorococcus consistent with different nutrient regimes.     

Variability of nutrient limitation in the Archipelago Sea,SW Finland

Over a two year study period, zooplankton was sampledin Gazi Bay, Kenya, using a 335 μm mesh size Bongonet. Two Way Indicator Species Analysis (TWINSPAN)classification technique demonstrated that rainfalland tidal regime had substantial influence on thezooplankton community structure. Samples collectedduring the rainy season months clustered together whentreated with TWINSPAN. Furthermore, theclustering was more pronounced for neap tidesamples than for spring tide ones. Samples obtainedduring spring tide did not give a clear cut pattern. Canonical Correspondence Analysis (C.C.A.) confirmedthese findings, a clustering together of rainy/neaptide samples; and little separation (based onenvironmental variables) between samplingstations. This revised version was published online in August 2006 with corrections to the Cover Date.     

Plant–herbivore interactions and ecological stoichiometry: when do herbivores determine plant nutrient limitation?

Recent studies on plant–herbivore indirect interactions via nutrient recycling have led to the hypothesis that herbivores with a low nitrogen: phosphorus ratio, feeding on plants with a higher nitrogen: phosphorus ratio, recycle relatively more nitrogen, driving plants into phosphorus limitation. We demonstrate in this paper that such a hypothesis is valid only under restricted conditions, i.e. the nitrogen: phosphorus ratio of inorganic nutrients supplied to the system must be neither too high nor too low compared with the nitrogen: phosphorus ratio of the whole plant + herbivore biomass. If plants have a greater affinity for phosphorus than for nitrogen, low herbivore nitrogen: phosphorus ratio can even promote nitrogen limitation. These results are qualitatively robust, whether grazing functions are donor-controlled or recipient-controlled. We present a graphical analysis of these conditions based on the Zero Net Growth Isocline method.     

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

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山美水库氮营养盐滞留特征及其影响因素的多时间尺度分析

水库对氮营养盐的滞留效应对水体富营养化防控有重要影响,为揭示不同时间尺度下水库氮营养盐的滞留特征及影响因素的差异,通过构建山美水库流域生态动力学模型,在统计水库氮滞留量和滞留率的基础上,从年、月、日不同时间尺度对氮营养盐的滞留效应进行分析;并通过多元线性相关分析,对入库流量、出入库流量比、水力滞留时间、流速、温度5种因素的影响进行研究.结果表明: 不同时间尺度下山美水库的氮滞留过程均呈现波动特征.在年尺度上,水库主要呈现正滞留效应,是流域重要的氮汇;在月尺度上,氮滞留呈现丰枯期变化特征,水库在源和汇的角色上发生转变;在日尺度上,枯水年氮滞留过程波动剧烈,滞留率在-300%～100%之间变化.不同时间尺度下,出入库流量比、流速因子均是影响水库氮滞留的重要因素;随着时间尺度的降低,入库流量和温度因子对氮滞留的影响程度有所增强;水力滞留时间对氮滞留的影响程度则随时间尺度变化而有正负差异.     

Alternative criteria for assessing nutrient limitation of a wetland macrophyte (Peltandra virginica (L.) Kunth)

Various nutrient incorporation and plant production parameters were measured to assess their relative usefulness in determining possible nutrient limitation of the wetland plant Peltandra virginica (L.) Kunth. From four stations located along a transect in a tidal freshwater marsh, we documented spatial differences in peak standing biomass of plants. Plant biomass was positively correlated with porewater concentrations of both ammonium and phosphate, but not with sediment concentrations of total nitrogen and phosphorus. Tissue nitrogen and phosphorus concentrations decreased significantly over the growing season, but there were no differences among plants from the four stations, and correlations between plant biomass and ratios of carbon to nitrogen and carbon to phosphorus were weak. Because in situ fertilization of plants had no effect on either peak biomass or tissue concentrations of nitrogen and phosphorus, growth of Peltandra was probably not nutrient limited. Other criteria did predict nitrogen or nitrogen and phosphorus limitation, however, demonstrating that application of parameters used by ecologists to support contentions of nutrient limitation can yield conflicting results. Assessment of nutrient limitation of primary producers may be an ambiguous and unnecessary task in some environments where these criteria are utilized.     

Contrasting effects of N addition on the N and P status of understory vegetation in plantations of sapling and mature Larixprincipis-rupprechtii

Aims The productivity of forest plantations in temperate areas is often limited by nitrogen (N), but may shift towards phosphorus (P) limitation with increasing atmospheric N deposition. Nutrient resorption is a nutrient conservation strategy in plants. Although data on nutrient resorption are available for overstory trees, there are few data for understory vegetation.     

SEASONAL CYCLING OF ALGAL NUTRIENT LIMITATION IN CHAUTAUQUA LAKE,NEW YORK1

Algal nutrient enrichment bioassays were conducted between May 1975 and August 1978 using water samples collected from Chautauqua Lake, New York. Photosynthetic fixation rates of natural phytoplankton assemblages were enhanced by additions of phosphorus and nitrogen, although enrichment with other nutrients had no significant stimulatory effect on algal photosynthesis. Whereas phosphorus stimulated in spring and early summer, both nitrogen and phosphorus enhanced photosynthesis in midsummer and fall. Relative to the effect of phosphorus enrichment, enhancement of photosynthesis by nitrogen during the summer and fall was highest in the northern part of the lake. During the period of ice cover, photosynthesis did not appear to be limited by nutrients in that nutrient additions (P, N, Si, C, Fe, trace metals) did not enhance fixation rates. Observed temporal fluctuations in the response of the algae to P and N correlated with changes in the lake water N:P ratio as well as with temporal changes in dissolved orthophosphate and nitrate-nitrite nitrogen. The N:P ratio decreased drastically in the summer and remained at ca. 10 or less through mid-fall, suggesting that N concentrations were inadequate for the non-N-fixing phytoplankton. Studies over 3 yr indicate that states of P and N limitation undergo time-space fluctuations that occur in a cyclic pattern in the surface waters of Chautauqua Lake.