Copepod and microzooplankton grazing in mesocosms fertilised with different Si:N ratios: no overlap between food spectra and Si:N influence on zooplankton trophic level

We hypothesized that the trophic level of marine copepods should depend on the composition of the protist community. To test this hypothesis, we manipulated the phytoplankton composition in mesocosms and measured grazing rates of copepods and mesozooplankton in those mesocosms. Twelve mesocosms with Northeast Atlantic phytoplankton were fertilised with different Si:N ratios from 0:1 to 1:1. After 1 week, ten of the mesocosms were filled with natural densities of mesozooplankton, mainly calanoid copepods, while two remained as mesozooplankton-free controls. Both before and after the addition of copepods there was a positive correlation of diatom dominance with Si:N ratios. During the second phase of the experiment, copepod and microzooplankton grazing rates on different phytoplankton species were assessed by a modification of the Landry-Hassett dilution technique, where the bottles containing the different dilution treatments were replaced by dialysis bags incubated in situ. The results indicated no overlap in the food spectrum of microzooplankton (mainly ciliates) and copepods. Ciliates fed on nanoplankton, while copepods fed on large or chain-forming diatoms, naked dinoflagellates, and ciliates. The calculated trophic level of copepods showed a significantly negative but weak correlation with Si:N ratios. The strength of this response was strongly dependent on the trophic levels assumed for ciliates and mixotrophic dinoflagellates.     

农田生态系统N、P营养平衡及其肥料效应

９年的农田生态系统Ｎ、Ｐ营养平衡定位试验研究表明,Ｎ、Ｐ肥配合施用,实行秸秆还田,可提高土壤有机质和全Ｎ含量,使土壤速效和迟效Ｐ库容量得到补充．最后几年的Ｐ肥利用率达２１．１—６５．１％,Ｎ肥为４９—７５％．储备性施Ｐ第１年的利用率只有６．５－７．３％,而前３年之和达到２４—２８．５％,９年３个时段平均利用率２７．９—４２．８％,补偿性施Ｐ的１９．５—４２．９％．施Ｐ６５．５ｋｇ·ｈａ－１·ｙｒ－１时,既能满足作物对Ｐ的需求又能补充土壤速效Ｐ库容量．     

Invertebrate communities in dry-season pools of a large subtropical river: patterns and processes

In order to develop an optical model to map the extent of coastal waters, the authors analyzed variations in bio-optical constituents and submarine optical properties along a transect from the nutrient-enriched coastal bay, Himmerfjärden, out into the open Baltic Sea. The model is a simple implementation of the “ecosystem approach,” because the optical constituents are proxies for important components of ecosystem state. Yellow substance or colored dissolved organic matter (CDOM) is often a marker for terrestrial freshwater or decay processes in the littoral zone. Phytoplankton pigments, especially chlorophyll a, are used as a proxy for phytoplankton biomass that may be stimulated by fluvial or coastal inputs of anthropogenic nutrients. Suspended particulate matter (SPM) is placed in suspension by tidal or wind-wave stirring of shallow seabeds, and is therefore an indicator for physical forcing. It is the thesis of this article that such constituents, and the optical properties that they control, can be used to provide an ecological definition of the extent of the coastal zone. The spatial distribution of the observations was analyzed using a steady-state model that assumes diffusional transport of bio-optical variables along an axis perpendicular to the coast. According to the model, the resulting distribution along this axis can be described as a low-order polynomial (of order 1–3) when moving from a “source” associated with land to the open-sea “sink.” Order 1 implies conservative mixing, and the higher orders imply significant biological or chemical processes within the gradient. The analysis of the transect data confirmed that the trend of each optical component could be described well using a low-order polynomial. Multiple regression analysis was then used to weigh the contribution of each optical component to the spectral attenuation coefficient K _d(490) along the transect. The results showed that in this Swedish Baltic case study, the inorganic fraction of the SPM may be used to distinguish between coastal and open-sea waters, as it showed a clear break between coastal and open-sea waters. Alternative models may be needed for coastal waters in which fronts interrupt the continuity of mixing.     

Ecosystem-level effects of re-oligotrophication and N:P imbalances in rivers and estuaries on a global scale

Trends and ecological consequences of phosphorus (P) decline and increasing nitrogen (N) to phosphorus (N:P) ratios in rivers and estuaries are reviewed and discussed. Results suggest that re-oligotrophication is a dominant trend in rivers and estuaries of high-income countries in the last two–three decades, while in low-income countries widespread eutrophication occurs. The decline in P is well documented in hundreds of rivers of United States and the European Union, but the biotic response of rivers and estuaries besides phytoplankton decline such as trends in phytoplankton composition, changes in primary production, ecosystem shifts, cascading effects, changes in ecosystem metabolism, etc., have not been sufficiently monitored and investigated, neither the effects of N:P imbalance. N:P imbalance has significant ecological effects that need to be further investigated. There is a growing number of cases in which phytoplankton biomass have been shown to decrease due to re-oligotrophication, but the potential regime shift from phytoplankton to macrophyte dominance described in shallow lakes has been documented only in a few rivers and estuaries yet. The main reasons why regime shifts are rarely described in rivers and estuaries are, from one hand the scarcity of data on macrophyte cover trends, and from the other hand physical factors such as peak flows or high turbidity that could prevent a general spread of submerged macrophytes as observed in shallow lakes. Moreover, re-oligotrophication effects on rivers may be different compared to lakes (e.g., lower dominance of macrophytes) or estuaries (e.g., limitation of primary production by N instead of P) or may be dependent on river/estuary type. We conclude that river and estuary re-oligotrophication effects are complex, diverse and still little known, and in some cases are equivalent to those described in shallow lakes, but the regime shift is more likely to occur in mid to high-order rivers and shallow estuaries.     

Fluxes of dissolved organic carbon,other nutrients and microbial communities in a water-filled treehole ecosystem

Water-filled treeholes are temporally and spatially variable habitats that consist of communities of a limited number of insect orders, namely dipterans and beetles. Since these systems are largely heterotrophic, treeholes are dependent on the surrounding terrestrial ecosystem for their basal energy input. In this study, we observed a cyclic succession of three system states in a water-filled Fagus grandifolia treehole: ‘connected’ during rainfall; ‘isolated’ during periods without rain; and ‘dry’ when no freestanding water was present. During the isolated phase, a rapid, microbially mediated turn-over of nitrogen and sulphate took place, coincident with an accumulation of orthophosphate. Ammonium was the dominant form of nitrogen in the treehole water, and a net decrease in its concentration was observed when the water volume decreased. Normally, nitrate concentration showed only minor fluctuations (0.3–1.3 mg l⁻¹) and concentrations of nitrite were very low (3–18 μg l⁻¹). Concentration of sulphate showed a net decrease, coincident with an increase in sulphide. During the connected phase, the effect of stemflow and throughfall on the nutrient concentrations in the treehole water proved to be variable. Over time, both dilution of, and increase in, treehole water nutrient concentrations were observed. Dissolved organic carbon (DOC) concentration showed a net decrease during the isolated phase. Spring coarse particulate organic matter input into the treehole was variable (0.20–1.74 mg cm⁻² week⁻¹). The observed variability in precipitation inputs as well as fall-in of organic matter underlines the pulsed character of basal energy entering the treehole food-web. DOC concentrations varied mainly with depth (15–57.4 mg l⁻¹) as concentrations were almost three times greater within the detritus than within the water column. Bacterial abundances ranged from 7.3 × 10⁵ to 9.3 × 10⁶ cells ml⁻¹ and did not appear to vary within the water column or in the detrital sediment. Rates of bacterial production increased 24 h after rain events, suggesting that the combined nutrient fluxes due to increased stemflow stimulate the microbial community. Handling editor: J. Cole     

Nutrient content of the seagrass Thalassia testudinum reveals regional patterns of relative availability of nitrogen and phosphorus in the Florida Keys USA

Between 1992 and 2000, we sampled 504 randomly chosen locations in theFlorida Keys, Florida, USA, for the elemental content of green leaves of theseagrass Thalassia testudinum. Carbon content ranged from29.4–43.3% (dry weight), nitrogen content from 0.88–3.96%, andphosphorus content from 0.048–0.243%. N and P content of the samples werenot correlated, suggesting that the relative availability of N and P variedacross the sampling region. Spatial pattern in C:N indicated a decrease in Navailability from inshore waters to the reef tract 10 km offshore;in contrast, the pattern in C:P indicated an increase in P availability frominshore waters to the reef tract. The spatial pattern in N:P was used to definea P-limited region of seagrass beds in Florida Bay and near shore, and anN-limited region of seagrass beds offshore. The close juxtaposition ofN–and P-limited regions allows the possibility that N loading from thesuburban Florida Keys could influence the offshore, N-limited seagrass bedswithout impacting the more nearshore, P-limited seagrass beds.     

Effects of low nitrogen-phosphorus ratios in the phytoplankton community in Laguna de Bay, a shallow eutrophic lake in the Philippines

The effects of low nitrogen-phosphorus ratios on microalgae from a large eutrophic freshwater lake in the Philippines were investigated. Natural microalgal populations from Laguna de Bay, the largest lake in the Philippines, were cultured using three different nitrogen-phosphorus weight ratios (2N:1P; 6N:1P and 12N:1P) at two phosphorus concentrations (0.25 and 0.5 mg l^–1) in each case. The growth and genera composition of the cultures under the different treatments were followed for a 12-week period. Community level responses were assessed based on species richness (s), Shannon-Wiener Index (H), Simpson Index () and Evenness (J). Among the different microalgal groups, only the chlorophytes showed a significantly higher density in response to the 12N:1P treatment at the higher P concentration, indicating that the nutrient ratio had a significant interaction with the nutrient levels used in the experiments. The genera found in the different treatments were generally similar; however, the degree of dominance of some varied with treatment during the experiment. The succession of dominant genera also differed among the N:P treatments. The diatoms like Fragilaria, Aulacoseira (= Melosira) and Nitzschia dominated the lowest N:P. On the other hand, chlorophytes (Kirchneriella and Scenedesmus) dominated the highest N:P treatment, particularly from the second to the seventh week of the experiments with the diatoms becoming co-dominant only towards the eighth week until the end of the experimental. The 6N:1P treatment showed a mixed dominance between the diatoms and the chlorophyte genera. The various indices of diversity indicate significantly lower diversity only in the 12N:1P at 0.5 mg l^–1 P and not in 12N:1P at 0.25 mg l^–1 P.     

Spatial and temporal variability of nutrient retention in river basins: A global inventory

Nutrient export by rivers may cause coastal eutrophication. Some river basins, however, export more nutrients than others. We model the Basin-Wide Nutrient Export (BWNE) Index, defined as nutrient export by rivers as percentage of external nutrient inputs in the basins. We present results for rivers worldwide for the period 1970–2050. The results indicate that nutrient retentions differ largely among basins. They indicate that BWNE increases with nutrient inputs to the land, indicating that the percentage of, for instance, fertilizers exported to sea increases with fertilization rate. We argue that a better understanding of the BWNE Index might help to identify where measures and technologies to reduce nutrient inputs to coastal waters are most effective.     

Lake Kinneret phytoplankton: Response to N and P enrichments in experiments and in nature

A series of water samples from Lake Kinneret was supplemented with 100 µM N (as NH₄ or as NO₃ and/or 10 µM orthophosphate-P. The yield of phytoplankton both as chlorophyll and in cell numbers of major species was determined after a two-week incubation. During these experiments, some of the algae present initially never multiplied (e.g. Peridinium and Peridiniopsis spp. Cryptomonas spp., Rhodomonas spp. and Crysochromulina parva); others e.g. Anomoeoneis exilis, Synedra sp., Chlamydomonas sp., Elakatothrix gelatinosa), undetected in the original sample, grew out during the incubation. Chlorophyte species (the majority of commonly observed forms in the lake) responded most readily to added nutrients.The results of these enrichment experiments were related to the long-term record of phytoplankton populations observed in the lake and suggest that through summer and fall, when ambient levels of both P and N are minimal, P was generally, but not always, the most limiting nutrient for algal growth. In the spring, after the decline of the Peridinium bloom, P appeared to be limiting the growth of Chlorophyta. Although most algal species grew equally well on NH₄ or NO₃, some species appeared to respond preferentially either to the former (Coelastrum, Chodatella) or to the latter (Chroococcus, Anomoeoneis) source of N.     

Delivery of N2 to bacteroids in simulated soybean nodule cells: consideration of gradients of concentration of dissolved N2 in cell walls,cytoplasm, and symbiosomes

Summary The previously published simulation of physiological functions occurring in infected cells of soybean nodules has been extended to include consideration of the diffusion of N₂ from the outside of a nodule to the nitrogen-fixing bacteroids, in relation to published values for the apparentK _m(N₂) for nitrogen fixation in the soybean nodule system. Nitrogen fixation is driven by bacteroid respiration, so increases in the average relative oxygenation (Y) of cytoplasmic leghaemoglobin lead to increased bacteroid respiration, increased nitrogen fixation, and greater differences in concentration of dissolved N₂ between the cell surface and the innermost bacteroids (d[N₂]). Over the range ofY considered, values for d[N₂] were from 5.2- to 6.2-fold greater than the corresponding values for d[O₂], because of facilitation of O₂ flux by cytoplasmic leghaemoglobin. Gradients of [N₂] within symbiosomes are small relative to cytoplasmic values and at the symbiosome surface [N₂] was greater than 0.4 mol/m³ at the greatest rates of nitrogen fixation calculated. Therefore, it is unlikely that values for [N₂] anywhere in the infected cell are low enough to affect rates of nitrogen fixation significantly, unless low external atmospheric N₂ pressures are used experimentally.Abbreviations Lb leghaemoglobin - LbO₂ oxyleghaemoglobin - [O₂], [N₂ concentrations of free, dissolved oxygen and nitrogen - Y fractional oxygenation of leghaemoglobin     

Benthic macroinvertebrate assemblages of coastal and continental streams and large rivers of southwestern British Columbia, Canada

In southwestern British Columbia (BC, Canada) and within a relatively small geographic area, lotic environments range from streams in coastal rainforests, to streams in arid continental grasslands, to very large rivers. Little is known about the invertebrate communities in large rivers in general, or in the streams of continental BC. The purpose of this study was to determine whether the benthic invertebrate community structure changes spatially between small coastal and small interior streams; between small streams versus large rivers; and whether changes in the benthic community are related to the environmental conditions. Kicknet samples and environmental data were collected from three coastal streams, three continental streams and two large rivers (discharge of 781 and 3620 m³/s, respectively). The large river sites had low invertebrate abundance, species richness and diversity, relative to the small streams. The coastal streams had the highest species richness and the continental streams had the highest invertebrate abundance. A number of taxa were specific to each class of stream. Invertebrate abundance decreased with river size, and increased with elevation, pH, conductivity, alkalinity, NO₂NO₃-N, total Kejldahl nitrogen and percent carbon in suspended solids.     

海南不同生活型植物叶片和根系C、N、P化学计量特征

为了解热带地区植物的营养元素利用策略,对海南3个生活型的9种植物的叶片和根系碳(C)、氮(N)、磷(P)含量及化学计量比进行分析,包括车前(Plantago asiatica)、蒭雷草(Thuarea involuta)、木耳菜(Psidium guajava)、桑(Morus alba)、臭黄荆(Premnaligustroides)、彩叶朱槿(Hibiscusrosa-sinensis)和厚叶榕(Ficusmicrocarpavar.crassifolia)、海岸桐(Guettarda speciosa)和番石榴(Psidium guajava)。结果表明,不同生活型间的元素含量和化学计量比没有显著差异;叶片C、N、P含量高于根系,叶片的C、N含量与根系的呈显著正相关,P含量与根系的呈显著负相关。C∶P与N∶P低于全国和全球尺度,说明该地区植物具有较高的生长速率。小于14的N∶P表明海南热带植物的生长主要受N限制。这揭示了海南热带植物不同生活型的营养元素利用策略相似,虽然受N限制,海南的植物仍具有较高的N、P固持能力和生长速率。     

Trophic-functional patterns of biofilm-dwelling ciliates at different water depths in coastal waters of the Yellow Sea,northern China

Vertical variations in trophic-functional patterns of biofilm-dwelling ciliates were studied in coastal waters of the Yellow Sea, northern China. A total of 50 species were identified and assigned to four trophic-functional groups (TFgrs): algivores (A), bacterivorous (B), non-selective (N) and raptors (R). The trophic-functional structures of the ciliate communities showed significant variability among different water depths: (1) with increasing water depth, relative species numbers and relative abundances of groups A and R decreased sharply whereas those of groups B and N increased gradually; (2) in terms of the frequency of occurrences, group A dominated at depths of 1–3.5 m whereas group B dominated at 5 m, while in terms of the probability density function of the trophic-functional spectrum, group A was the highest contributor at 1 m and group B was highest at the other three depths; (3) distance-based redundancy analyses revealed significant differences in trophic-functional patterns among the four depths, except between 2 and 3.5 m (P > 0.05); and (4) the trophic-functional trait diversity increased from 1 to 3.5 m and decreased sharply at 5 m. Our results suggest that the biofilm-dwelling ciliates maintain a stable trophic-functional pattern and high biodiversity at depths of 1–3.5 m.     

Concentrations and controls of dissolved organic matter in a constricted-channel region of the Ohio River

A 12-month study was conducted to measure the concentrations ofdissolved organic matter (DOC, TDN, TDP) in four sites within a119 km long reach of the Ohio River, near Louisville, KY. In thisstudy we test whether specific geomorphological and biologicalfactors influenced variations in dissolved organic matter.Concentrations of DOC in the river averaged 1200mol/L, and varied by nearly two orders of magnitudeseasonally (mean DOC during base flow 620 mol/L).Peak periods for DOC at all sites were during April–May. Thesite nearest a navigation dam (deeper, lower current velocities)had significantly lower concentrations of TDN and greater C:Nratios than upstream sites. The largest tributary entering thisreach (Kentucky River) had no significant effect on levels of DOMin the main river, despite having significantly greaterconcentrations of TDN and lower levels of DOC during most monthsof the year. Concentrations of DOC, TDN, and TDP were notsignificantly different in littoral and pelagic habitats at allsites studied, suggesting little floodplain influence on DOM inthis constricted-channel section of the Ohio River. C:N ratios ofDOM in the Ohio were significantly different among seasons; C:Nexceeded or equaled Redfield ratios in summer and fall (6 to 10),but were below Redfield (1.8 to 3.0) during winter and spring.Regression models suggest that total phytoplankton densities andflow conditions are the two most important factors regulating DOMin this very large river.     

Canopy N and P dynamics of a southeastern US pine forest under elevated CO2

Forest production is strongly nutrient limited throughout the southeastern US. If nutrient limitations constrain plant acquisition of essential resources under elevated CO₂, reductions in the mass or nutrient content of forest canopies could constrain C assimilation from the atmosphere. We tested this idea by quantifying canopy biomass, foliar concentrations of N and P, and the total quantity of N and P in a loblolly pine (Pinus taeda) canopy subject to 4 years of free-air CO₂ enrichment. We also used N:P ratios to detect N versus P limitation to primary production under elevated CO₂. Canopy biomass was significantly higher under elevated CO₂ during the first 4 years of this experiment. Elevated CO₂ significantly reduced the concentration of N in loblolly pine foliage (5% relative to ambient CO₂) but not P. Despite the slight reduction foliage N concentrations, there were significant increases in canopy N and P contents under elevated CO₂. Foliar N:P ratios were not altered by elevated CO₂ and were within a range suggesting forest production is N limited not P limited. Despite the clear limitation of NPP by N under ambient and elevated CO₂ at this site, there is no evidence that the mass of N or P in the canopy is declining through the first 4 years of CO₂ fumigation. As a consequence, whole-canopy C assimilation is strongly stimulated by elevated CO₂ making this forest a larger net C sink under elevated CO₂ than under ambient CO₂. We discuss the potential for future decreases in canopy nutrient content as a result of limited changes in the size of the plant-available pools of N under elevated CO₂.     

Erosion,phosphorus and phytoplankton response in rivers of South-Eastern Norway

The development of P fractions and phytoplankton was studied in three rivers with varying concentrations of seston.Less than 1% of the yearly TP transport may take place during periods with high algal biomass.The observation of a high growth rate of phytoplankton in the rivers coinciding with high concentrations of RP, low content of seston and high TP:Chl a ratio, indicate that the growth was often not P-limiting. During short periods with high phytoplankton biomass the ratio TP:Chl a may be low, indicating that a high fraction of TP was available.The content of P in soil samples and in samples with high seston content was about 0.1% of dry weight, and the algal availability of P often varied between 25 and 75% of TP for both types of samples.Decreasing biomass or low growth rates were observed at secchi depths less than 0.5 m and seston concentrations less than about 25 mg dry weight 1^–1. High flow rate also depressed the development of the total phytoplankton biomass. The assimilation of available P is incomplete under such conditions, i.e. under conditions of light limitation and high dilution rate.The availability of P for phytoplankton in rivers with different length, light conditions and stream velocity is discussed.     

Global climate change in large European rivers: long‐term effects on macroinvertebrate communities and potential local confounding factors

Aquatic species living in running waters are widely acknowledged to be vulnerable to climate‐induced, thermal and hydrological fluctuations. Climate changes can interact with other environmental changes to determine structural and functional attributes of communities. Although such complex interactions are most likely to occur in a multiple‐stressor context as frequently encountered in large rivers, they have received little attention in such ecosystems. In this study, we aimed at specifically addressing the issue of relative long‐term effects of global and local changes on benthic macroinvertebrate communities in multistressed large rivers. We assessed effects of hydroclimatic vs. water quality factors on invertebrate community structure and composition over 30 years (1979–2008) in the Middle Loire River, France. As observed in other large European rivers, water warming over the three decades (+0.9 °C between 1979–1988 and 1999–2008) and to a lesser extent discharge reduction (?80 m³ s^?1) were significantly involved in the disappearance or decrease in taxa typical from fast running, cold waters (e.g. Chloroperlidae and Potamanthidae). They explained also a major part of the appearance and increase of taxa typical from slow flowing or standing waters and warmer temperatures, including invasive species (e.g. Corbicula sp. and Atyaephyra desmarestii). However, this shift towards a generalist and pollution tolerant assemblage was partially confounded by local improvement in water quality (i.e. phosphate input reduction by about two thirds and eutrophication limitation by almost one half), explaining a significant part of the settlement of new pollution‐sensitive taxa (e.g. the caddisfly Brachycentridae and Philopotamidae families) during the last years of the study period. The regain in such taxa allowed maintaining a certain level of specialization in the invertebrate community despite climate change effects.