Inorganic Nutrient Pools Research Articles

Brownification affects phytoplankton community composition but not primary productivity in eutrophic coastal waters: A mesocosm experiment in the Baltic Sea

Climate change is projected to cause brownification of some coastal seas due to increased runoff of terrestrially derived organic matter. We carried out a mesocosm experiment (15 d) to test the effect of this on the planktonic ecosystem expecting reduced primary production and shifts in the phytoplankton community composition. The experiment was set up in 2.2 m3 mesocosm bags using four treatments, each with three replicates: control (Contr) without any manipulation, organic carbon additive HuminFeed (Hum; 2 mg L−1), inorganic nutrients (Nutr; 5.7 μM NH4 and 0.65 μM PO4), and combined Nutr and Hum (Nutr + Hum) additions. Measured variables included organic and inorganic nutrient pools, chlorophyll a (Chla), primary and bacterial production and particle counts by flow cytometry. The bags with added inorganic nutrients developed a phytoplankton bloom that depleted inorganic N at day 6, followed by a rapid decline in Chla. Brownification did not reduce primary production at the tested concentration. Bacterial production was lowest in the Contr, but similar in the three treatments receiving additions likely due to increased carbon available for heterotrophic bacteria. Picoeukaryotes clearly benefited by brownification after inorganic N depletion, which could be due to more effective nutrient recycling, nutrient affinity, light absorption, or alternatively lower grazing pressure. In conclusion, brownification shifted the phytoplankton community composition towards smaller species with potential effects on carbon fluxes, such as sinking rates and export to the sea floor.

Soil biogeochemistry across Central and South American tropical dry forests

AbstractThe availability of nitrogen (N) and phosphorus (P) controls the flow of carbon (C) among plants, soils, and the atmosphere, thereby shaping terrestrial ecosystem responses to global change. Soil C, N, and P cycles are linked by drivers operating at multiple spatial and temporal scales: landscape‐level variation in macroclimate and soil geochemistry, stand‐scale heterogeneity in forest composition, and microbial community dynamics at the soil pore scale. Yet in many biomes, we do not know at which scales most of the biogeochemical variation emerges, nor which processes drive cross‐scale feedbacks. Here, we examined the drivers and spatial/temporal scales of variation in soil biogeochemistry across four tropical dry forests spanning steep environmental gradients. To do so, we quantified soil C, N, and P pools, extracellular enzyme activities, and microbial community structure across wet and dry seasons in 16 plots located in Colombia, Costa Rica, Mexico, and Puerto Rico. Soil biogeochemistry exhibited marked heterogeneity across the 16 plots, with total organic C, N, and P pools varying fourfold, and inorganic nutrient pools by an order of magnitude. Most soil characteristics changed more across space (i.e., among sites and plots) than over time (between dry and wet season samplings). We observed stoichiometric decoupling among C, N, and P cycles, which may reflect their divergent biogeochemical drivers. Organic C and N pool sizes were positively correlated with the relative abundance of ectomycorrhizal trees and legumes. By contrast, the distribution of soil P pools was driven by soil geochemistry, with larger inorganic P pools in soils with P‐rich parent material. Most earth system models assume that soils within a texture class operate similarly, and ignore subgrid cell variation in soil properties. Here we reveal that soil nutrient pools and fluxes exhibit as much variation among four Neotropical dry forests as is observed across terrestrial ecosystems at the global scale. Soil biogeochemical patterns are driven not only by regional differences in soil parent material and climate, but also by local‐scale variation in plant and microbial communities. Thus, the biogeochemical patterns we observed across the Neotropical dry forest biome challenge representation of soil processes in ecosystem models.

The elemental stoichiometry (C, Si, N, P) of the Hebrides Shelf and its role in carbon export

A detailed analysis of the internal stoichiometry of a temperate latitude shelf sea system is presented which reveals strong vertical and horizontal gradients in dissolved nutrient and particulate concentrations and in the elemental stoichiometry of those pools. Such gradients have implications for carbon and nutrient export from coastal waters to the open ocean. The mixed layer inorganic nutrient stoichiometry shifted from balanced N:P in winter, to elevated N:P in spring and to depleted N:P in summer, relative to the Redfield ratio. This pattern suggests increased likelihood of P limitation of fast growing phytoplankton species in spring and of N limitation of slower growing species in summer. However, as only silicate concentrations were below potentially limiting concentrations during summer and autumn the stoichiometric shifts in inorganic nutrient N:P are considered due to phytoplankton nutrient preference patterns rather than nutrient exhaustion. Elevated particulate stoichiometries corroborate non-Redfield optima underlying organic matter synthesis and nutrient uptake. Seasonal variation in the stoichiometry of the inorganic and organic nutrient pools has the potential to influence the efficiency of nutrient export. In summer, when organic nutrient concentrations were at their highest and inorganic nutrient concentrations were at their lowest, the organic nutrient pool was comparatively C poor whilst the inorganic nutrient pool was comparatively C rich. The cross-shelf export of these pools at this time would be associated with different efficiencies regardless of the total magnitude of exchange. In autumn the elemental stoichiometries increased with depth in all pools revealing widespread carbon enrichment of shelf bottom waters with P more intensely recycled than N, N more intensely recycled than C, and Si weakly remineralized relative to C. Offshelf carbon fluxes were most efficient via the inorganic nutrient pool, intermediate for the organic nutrient pool and least efficient for the particulate pool. N loss from the shelf however was most efficient via the dissolved organic nutrient pool. Mass balance calculations suggest that 28% of PO43−, 34% of NO3− and 73% of Si drawdown from the mixed layer fails to reappear in the benthic water column thereby indicating the proportion of the nutrient pools that must be resupplied from the ocean each year to maintain shelf wide productivity. Loss to the neighbouring ocean, the sediments, transference to the dissolved organic nutrient pool and higher trophic levels are considered the most likely fate for these missing nutrients.

Elevated specific conductance enhances productivity and biomass of periphytic cyanobacteria from Lake Tahoe and Lake Tanganyika

:Productivity of attached algae is limited by macronutrients such as nitrogen and phosphorus when light is saturating. Attached algae can supplement the available dissolved inorganic nutrient pool through the production of intracellular and extracellular enzymes in order to sequester otherwise bio-unavailable forms of nitrogen and phosphorus. Increased cation concentration can potentially stimulate attached algal productivity through increased enzyme activity and increased nutrient uptake efficiency. We conducted a full factorial laboratory experiment to test the effects of increased cation concentration, measured as specific conductance, and phosphorus source (inorganic or organic) on productivity of algae collected from Lake Tahoe and Lake Tanganyika. We manipulated cation concentration using commercially available lake salts, primarily magnesium, calcium, and potassium. Algal dry mass and productivity were significantly higher in the elevated conductivity treatments regardless of whether the phosphorus provided was in organic or inorganic form. No difference in dry mass was observed between phosphorus source and lake of origin but inorganic phosphorus treatments had significantly higher productivity. Our findings suggest that increased specific conductance in nutrient limited conditions can stimulate attached algal biomass and productivity.

C : N : P stoichiometry at the Bermuda Atlantic Time-series Study station in the North Atlantic Ocean

Abstract. Nitrogen (N) and phosphorus (P) availability, in addition to other macro- and micronutrients, determine the strength of the ocean's carbon (C) uptake, and variation in the N : P ratio of inorganic nutrient pools is key to phytoplankton growth. A similarity between C : N : P ratios in the plankton biomass and deep-water nutrients was observed by Alfred C. Redfield around 80 years ago and suggested that biological processes in the surface ocean controlled deep-ocean chemistry. Recent studies have emphasized the role of inorganic N : P ratios in governing biogeochemical processes, particularly the C : N : P ratio in suspended particulate organic matter (POM), with somewhat less attention given to exported POM and dissolved organic matter (DOM). Herein, we extend the discussion on ecosystem C : N : P stoichiometry but also examine temporal variation in stoichiometric relationships. We have analyzed elemental stoichiometry in the suspended POM and total (POM + DOM) organic-matter (TOM) pools in the upper 100 m and in the exported POM and subeuphotic zone (100–500 m) inorganic nutrient pools from the monthly data collected at the Bermuda Atlantic Time-series Study (BATS) site located in the western part of the North Atlantic Ocean. C : N and N : P ratios in TOM were at least twice those in the POM, while C : P ratios were up to 5 times higher in TOM compared to those in the POM. Observed C : N ratios in suspended POM were approximately equal to the canonical Redfield ratio (C : N : P = 106 : 16 : 1), while N : P and C : P ratios in the same pool were more than twice the Redfield ratio. Average N : P ratios in the subsurface inorganic nutrient pool were ~ 26 : 1, squarely between the suspended POM ratio and the Redfield ratio. We have further linked variation in elemental stoichiometry to that of phytoplankton cell abundance observed at the BATS site. Findings from this study suggest that elemental ratios vary with depth in the euphotic zone, mainly due to different growth rates of cyanobacterial cells. We have also examined the role of the Arctic Oscillation on temporal patterns in C : N : P stoichiometry. This study strengthens our understanding of the variability in elemental stoichiometry in different organic-matter pools and should improve biogeochemical models by constraining the range of non-Redfield stoichiometry and the net relative flow of elements between pools.

Soil nutrients and microbial biomass in three contrasting Mediterranean forests

The extent to which the spatial and temporal patterns of soil microbial and available nutrient pools hold across different Mediterranean forest types is unclear impeding the generalization needed to consolidate our understanding on Mediterranean ecosystems functioning. We explored the response of soil microbial, total, organic and inorganic extractable nutrient pools (C, N and P) to common sources of variability, namely habitat (tree cover), soil depth and season (summer drought), in three contrasting Mediterranean forest types: a Quercus ilex open woodland, a mixed Q. suber and Q. canariensis woodland and a Pinus sylvestris forest. Soil microbial and available nutrient pools were larger beneath tree cover than in open areas in both oak woodlands whereas the opposite trend was found in the pine forest. The greatest differences in soil properties between habitat types were found in the open woodland. Season (drought effect) was the main driver of variability in the pine forest and was related to a loss of microbial nutrients (up to 75 % loss of Nmic and Pmic) and an increase in microbial ratios (Cmic/Nmic, Cmic/Pmic) from Spring to Summer in all sites. Nutrient pools consistently decreased with soil depth, with microbial C, N and P in the top soil being up to 208 %, 215 % and 274 % larger than in the deeper soil respectively. Similar patterns of variation emerged in relation to season and soil depth across the three forest types whereas the direction and magnitude of the habitat (tree cover) effect was site-dependent, possibly related to the differences in tree species composition and forest structure, and thus in the quality and distribution of the litter input.

The effect of biological activity, CaCO3 mineral dynamics, and CO2 degassing in the inorganic carbon cycle in sea ice in late winter‐early spring in the Weddell Sea, Antarctica

A large‐scale geographical study of the ice pack in the seasonal ice zone of the Weddell Sea, Antarctica, took place from September to October 2006. Sea ice brines with a salinity greater than 58 and temperature lower than −3.6°C were sampled from 22 ice stations. The brines had large deficits in total alkalinity and in the concentrations of the major dissolved macronutrients (total dissolved inorganic carbon, nitrate, and soluble reactive phosphorus) relative to their concentrations in the surface oceanic water and conservative behavior during seawater freezing. The concentration deficits were related to the dissolved inorganic carbon‐consuming processes of photosynthesis, CaCO3 precipitation, and CO2 degassing. The largest concentration deficits in total dissolved inorganic carbon were found to be associated with CaCO3 precipitation and CO2 degassing, because the magnitude of the photosynthesis‐induced concentration deficit in total dissolved inorganic carbon is controlled by the size of the inorganic nutrient pool, which can be limited in sea ice by its openness to exchange with the surrounding oceanic water.

Recent evolution of river discharges in the Gulf of Trieste and their potential response to climate changes and anthropogenic pressure

Freshwater and nutrient discharges by rivers were analysed in the northeastern Adriatic continental shelf from 1998 to 2008, in order to assess their role in the biogeochemistry of this coastal zone, as well as their potential future changes due to the effects of climate variability and anthropogenic pressure. River water loads (up to 6.05 km3 yr−1) and transport of nutrients (up to 13,200 t N yr−1 for TN, 86 t P yr−1 for TP and 12,400 t Si yr−1 for Si(OH)4) were high from 1998 to 2002, but they decreased by over 70% during the driest years 2003, 2005, 2006 and 2007. The precipitation in the surrounding continental region (average of 1371 mm yr−1) is the major forcing that regulates the runoff in this area, with peaks in early spring and autumn that are associated respectively to negative phases of Mediterranean Oscillation index and positive phases of West Mediterranean Oscillation index. This finding, together with the minor contribution of snowmelt in early spring (up to 3.2·10−5 kg m−2 s−1), indicates that the dynamics of the natural water cycle still overtake in this coastal zone the effects of anthropogenic usage of continental waters.During the last four decades, the northeastern Adriatic continental shelf has been subjected to an increasing pressure due to a high river transport of nitrogen and, currently, only a deep phosphorus deficiency in total (TN/TP = 49–405) and inorganic (DIN/PO4 = 37–418) river nutrient pools prevents its severe eutrophication. By contrast, the decrease of river loads of nutrients from 2003 to 2007 indicates that recurrent water crisis might significantly lower the trophic level in this coastal zone in the future. In this perspective, other allochthonous sources of nutrients, like sewage loads, atmospheric deposition and benthic fluxes might become more important for their balance, with possible implications on the structure of this ecosystem.

River water and nutrient discharges in the Northern Adriatic Sea: Current importance and long term changes

Runoff and nutrient transport by rivers were analysed in the Northern Adriatic continental shelf, in order to evaluate their interannual and multidecal variability, as well as their current contribution to determine freshwater and nutrient budgets in this marine region. During the years 2004–2007, the runoff in the basin (34.1–64.6 km 3 yr −1) was highly imbalanced, being 84% of freshwater discharged along the western coast, because of the contributions of Po, Adige and Brenta rivers. In the northern and eastern sections of the coast, freshwater discharge by rivers was less important (10 and 6%, respectively), but not negligible in determining the oceanographic properties at sub-regional scales. The oscillations of the transport of biogenic elements (124–262×10 3 t N yr −1 for TN, 72–136×10 3 t N yr −1 for DIN, 4.5–11.1×10 3t P yr −1 for TP, 2.2–3.5×10 3 t P yr −1 for PO 4 and 104–196×10 3 t Si yr −1 for SiO 2) were strictly dependant to the differences in the annual runoff. A strong excess of N load in comparison to P load characterised all rivers, both in inorganic nutrient (DIN/PO4=37–418) and total (TN/TP=48–208) pools, particularly in the northern and eastern areas of the basin. The annual runoff showed significant oscillations for Po on multidecadal time scale, whereas a general decrease (−33%) was observed for the other N Adriatic rivers as the recent discharges were compared to those before the 1980s. During the dry years 2005–2007, a strong reduction of river water flows and nutrient loads was experienced by the N Adriatic ecosystem with respect to years characterised by medium-high regimes. An increased frequency of similar drought periods, due to ongoing climate changes or to a larger human usage of continental waters, would be easily able to significantly change the biogeochemistry of this basin.

Seasonal variability in nutrient regeneration by mussel Mytilus edulis rope culture in oligotrophic systems

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 431:137-149 (2011) - DOI: https://doi.org/10.3354/meps09095 Seasonal variability in nutrient regeneration by mussel Mytilus edulis rope culture in oligotrophic systems Henrice M. Jansen1,2,3,*, Øivind Strand1, Tore Strohmeier1, Cathinka Krogness1, Marc Verdegem3, Aad Smaal2,3 1Institute of Marine Research, Nordnesgaten 50, 5817 Bergen, Norway 2Wageningen IMARES–Institute for Marine Resources and Ecosystem Studies, Korringaweg 5, 4401 NT Yerseke, The Netherlands 3Department of Aquaculture and Fisheries, Wageningen University, Marijkeweg 40, 6709 PG Wageningen, The Netherlands *Email: henrice.jansen@imr.no ABSTRACT: Blue mussel Mytilus edulis cultures contribute to nutrient cycling in coastal ecosystems. Mussel populations filter particulate nutrients from the water column and inorganic nutrients are regenerated by excretion of metabolic wastes and decomposition of (pseudo-)faeces. The objective of this study was to determine the intra-annual variability in nutrient regeneration by mussel rope cultures in oligotrophic fjord systems. In situ respiration and nutrient uptake and release rates of 1 m mussel ropes were measured using 250 l pelagic chambers. There was a 20-fold difference between winter and summer respiration and nutrient release rates. Inorganic nitrogen release ranged from 50 to 1000 µmol h–1 per meter rope. These variations were mainly related to mussel growth but were also related to changes in water temperature and biofouling biomass (organisms that colonized the mussel ropes). In total 24 genera of fouling organisms were observed, diversity increased over time (ranging from 2 to 12 genera m–1), and fouling biomass was mainly characterised by ascidians (max. 37 ± 14 g m–1). However, mussels dominated the culture ropes, representing >90% of total faunal biomass. The amount of organic material associated with the ropes was stable (6.9 ± 0.3 g m–1). At the scale of one mussel farm, nutrient regeneration by mussel rope cultures increased inorganic nitrogen concentrations by 20% and inorganic phosphorus concentrations by 5% during summer conditions. During winter, there was no significant effect of mussel cultures on the inorganic nutrient pools. Nutrient regeneration by mussel cultures also affected stoichiometry as nutrients were excreted in dissimilar proportions (nitrogen > phosphate > silicate). The increased nutrient availability may contribute to primary production, especially in nutrient-limited (oligotrophic) fjord ecosystems. However, fjord-scale effects are largely dependent on hydrographic conditions of the fjord system. KEY WORDS: Nutrient dynamics · Respiration · In situ · Suspended mussel culture · Biofouling · Pelagic chamber Full text in pdf format PreviousNextCite this article as: Jansen HM, Strand Ø, Strohmeier T, Krogness C, Verdegem M, Smaal A (2011) Seasonal variability in nutrient regeneration by mussel Mytilus edulis rope culture in oligotrophic systems. Mar Ecol Prog Ser 431:137-149. https://doi.org/10.3354/meps09095 Export citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 431. Online publication date: June 09, 2011 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2011 Inter-Research.

Long-term microbial control of nutrient availability and plant biomass in a subarctic-alpine heath after addition of carbon, fertilizer and fungicide

Abstract A long-term field experiment lasting more than a decade was conducted on a subarctic fellfield to investigate effects of changes in nutrient availability on soil microbial C, N and P, soil nutrients, vascular plant biomass and plant-microbial interactions. Additions of NPK fertilizer, labile C (sugar) and fungicide (benomyl) were done in a fully factorial design, replicated in six blocks. The treatments were run for ten years and soil and vegetation samples were collected four years after initiating the experiment, and again after an additional 12 years, to evaluate the long-term effects. Labile C addition resulted in increased microbial biomass and nutrient immobilization after four years, and a long-term decrease in vascular plant biomass, thus suggesting the microorganisms to strongly control soil nutrient availability in periods of high microbial biomass. Fertilization increased the inorganic and total soil nutrient pools of N and P and the fine root biomass, but not the total aboveground vascular plant biomass. The vascular plant biomass increased due to benomyl addition thus indicating the plants to be strongly affected by the microbial community. Overall, the effects of benomyl resulted in more lasting changes in the soil compared to labile C and fertilizer addition. In relation to environmental changes, the indicated strong microbial control of the available nutrients in the fellfield ecosystem might limit ecosystem changes due to increased soil nutrient availability as otherwise expected in arctic soils.

Functional diversity of microbial decomposers facilitates plant coexistence in a plant-microbe-soil feedback model.

Theory and empirical evidence suggest that plant-soil feedback (PSF) determines the structure of a plant community and nutrient cycling in terrestrial ecosystems. The plant community alters the nutrient pool size in soil by affecting litter decomposition processes, which in turn shapes the plant community, forming a PSF system. However, the role of microbial decomposers in PSF function is often overlooked, and it remains unclear whether decomposers reinforce or weaken litter-mediated plant control over nutrient cycling. Here, we present a theoretical model incorporating the functional diversity of both plants and microbial decomposers. Two fundamental microbial processes are included that control nutrient mineralization from plant litter: (i) assimilation of mineralized nutrient into the microbial biomass (microbial immobilization), and (ii) release of the microbial nutrients into the inorganic nutrient pool (net mineralization). With this model, we show that microbial diversity may act as a buffer that weakens plant control over the soil nutrient pool, reversing the sign of PSF from positive to negative and facilitating plant coexistence. This is explained by the decoupling of litter decomposability and nutrient pool size arising from a flexible change in the microbial community composition and decomposition processes in response to variations in plant litter decomposability. Our results suggest that the microbial community plays a central role in PSF function and the plant community structure. Furthermore, the results strongly imply that the plant-centered view of nutrient cycling should be changed to a plant-microbe-soil feedback system, by incorporating the community ecology of microbial decomposers and their functional diversity.

Nitrogen uptake by Carex aquatilis during the winter–spring transition in a low Arctic wet meadow

Summary 1. Processes of decomposition occur year‐round in tundra ecosystems and respond quickly to seasonal changes. Characterizing the phenology of plant nutrient uptake in relation to these processes is essential to understanding the current and future productivity of Arctic ecosystems. 2. In wet sedge meadows located near Churchill, Manitoba, Canada, soil microbial biomass as well as inorganic and organic nutrient pools fluctuate seasonally, with late‐winter peaks followed by declines of these variables during the early stages of soil thaw; however, it is unknown if the dominant plant in this community takes up nitrogen when levels of this nutrient are high but soil temperatures are 0 °C or below. 3. Stable isotope tracing was utilized by injecting 15NH4Cl into soil cores and incubating for 1 or 8 days during spring thaw to determine the short‐term capacity for uptake and transport of inorganic nitrogen into Carex aquatilis (roots, shoots and rhizomes), moss and soil micro‐organisms during this transitional time of year. 4. During three 8‐day experimental trials in April and May 2007, C. aquatilis roots accumulated a substantial amount of the added nitrogen (33.5% increasing to 63.4%), when inorganic nitrogen was readily available in the soil, but declining. A smaller proportion of injected nitrogen was recovered from soil microbes (30% decreasing to 7%), and only trace amounts of injected 15N were measured in plant shoots, shoot bases, rhizomes and mosses (2% or less). 5. Synthesis. Shifting seasonal patterns in northern ecosystems resulting from climate change are likely to alter the progression of events that lead up to the summer growing season. A substantial pool of inorganic nitrogen resides temporarily in the soil at the end of winter, and we have shown here that plants are able to take up nitrogen at this time. Increases in the frequency and temperature highs of late‐winter warming events are likely to trigger early episodes of soil thaw, potentially reducing the capacity of plants to take up this large ephemeral supply of nitrogen in early spring.

Allochthonous organic matter supplements and sediment transport in a polymictic reservoir determined using elemental and isotopic ratios

Because allochthonous organic matter (OM) loading supplements autochthonous OM in supporting lake and reservoir food webs, C and N elemental and isotopic ratios of sedimenting particulate OM were measured during an annual cycle in a polymictic, eutrophic reservoir. Particulate organic C and N deposition rates were greatest during winter and lowest during spring. C:N ratios decreased through our study indicating that OM largely originated from allochthonous sources in winter and autochthonous sources thereafter. δ13C were influenced by C4 plant signatures and became increasingly light from winter through autumn. δ15N indirectly recorded the OM source shift through nitrate utilization degree with maximum values occurring in May as nitrate concentrations decreased. Unlike relationships from stratified systems, δ13C decreased with increasing algal biomass. This relationship suggests that minimal inorganic C fixation relative to supplies maintained photosynthetic isotopic discrimination during productive periods. Water column mixing likely maintained adequate inorganic C concentrations in the photic zone. Alternatively, OM isotopic composition may have been influenced by changing dissolved inorganic nutrient pools in this rapidly flushed system. δ15N also recorded increased N2 fixation as nitrate concentrations declined through autumn. Secondary sediment transport mechanisms strongly influenced OM delivery. Particulate organic C and N deposition rates were 3× greater near the sediment-water interface. Isotopic ratio mixing models suggested that river plume sedimentation, sediment resuspension, and horizontal advection influenced excess sediment deposition with individual mechanisms being more important seasonally. Our findings suggest that allochthonous OM loading and secondarily-transported OM seasonally supplement phytoplankton production in productive reservoirs.

The interactive effects of elevated carbon dioxide and water table draw-down on carbon cycling in a Welsh ombrotrophic bog

The effects of elevated atmospheric CO 2 (eCO 2) and water table draw-down on soil carbon sequestration in an ombrotrophic bog ecosystem were examined. Peat monoliths (11 cm diameter, 25 cm deep) with intact bog vegetation were exposed to ambient or elevated (ambient + 200 mg l −1) atmospheric CO 2, combined with a natural water table (level with the peat surface) or a water table draw-down (−5 cm). Eight observations per treatment were included in the study, which was conducted over a 12 week period. Concentration of dissolved organic carbon (DOC), phenolic compounds and the fluxes of CO 2 and CH 4 were measured. The eCO 2 treatment caused an increase in the CH 4 and CO 2 fluxes and a small decrease in both the DOC and phenolic concentrations. The water table draw-down invoked decreases in phenolic and DOC concentrations, a decrease in CH 4 flux and a small increase in CO 2 flux. The combined (eCO 2 + water table draw-down) treatment caused a larger than expected CH 4 flux decrease and CO 2 flux increase and an increase in DOC concentration. Our results suggest very different effects on the system dependent on the treatment applied. The draw-down treatment principally increased oxidation of the rhizosphere resulting in increased decomposition and as such a removal of material from the dissolved carbon pool. The data also suggest labile carbon availability may be limiting the rate of decomposition and so slowing inorganic nutrient and carbon pool turn-over. The elevated CO 2 addressed the labile-carbon limitation. Under the environment of the combined treatment, these limitations were effectively removed, culminating in a destabilisation of the carbon-sequestering environment to a weaker sink (or even a source) of atmospheric carbon.

Modelling the relative impact of rivers (Scheldt/Rhine/Seine) and Western Channel waters on the nutrient and diatoms/ Phaeocystis distributions in Belgian waters (Southern North Sea)

The coastal areas of the Southern North Sea (SNS) experience eutrophication problems resulting from freshwater nitrogen (N) and phosphorus (P) inputs from rivers. In particular, massive blooms of Phaeocystis colonies occur in Belgian waters. In this region, water masses result from the mixing of Western Channel (WCH) waters transported through the Straits of Dover with nutrient-rich freshwater from the Scheldt, the Rhine and Meuse, the Seine, the Thames and other smaller rivers. However, the relative contribution of the WCH and each river to the inorganic nutrient pool and the impact on the phytoplankton community structure (diatoms and Phaeocystis) are not known. In order to effectively manage the eutrophication problems, it is necessary to know: (i) the relative contribution of the WCH and of each river impacting the region and (ii) the relative effect of a N and/or P nutrient reduction on the Phaeocystis blooms. To answer these questions, sensitivity tests (1% nutrient reduction) and nutrient reduction scenarios (50% nutrient reduction) have been performed with a three-dimensional (3D) coupled physical–biogeochemical model (MIRO&CO-3D). MIRO&CO-3D results from the coupling of the COHERENS 3D hydrodynamic model with the ecological model MIRO. The model has been set up for the region between 48.5°N, 4°W and 52.5°N, 4.5°E and run to simulate the annual cycle of carbon, inorganic and organic nutrients, phytoplankton (diatoms and Phaeocystis), bacteria and zooplankton (microzooplankton and copepods) in the SNS under realistic forcing (meteorology and river inputs) for the period 1991–2003. The relative contribution of the WCH waters and of the different rivers on the inorganic nutrient pool available for phytoplankton (diatoms and Phaeocystis) growth is assessed by decreasing by 1% the nutrient (dissolved inorganic nitrogen, DIN and inorganic phosphate, PO 4) inputs from the WCH and from, respectively, the Scheldt (and smaller Belgian rivers), the Rhine/Meuse and the Seine (and smaller French rivers) [sensitivity tests]. The relative role of N and P reduction on the diatoms/ Phaeocystis distribution is further explored by simulations with 50% reduction of the total (inorganic and organic) N and total P river inputs [nutrient reduction scenarios]. These scenarios allow assessing the impact of the expected 50% reduction of river nutrient inputs resulting from the implementation of nutrient reduction policy. Results of the sensitivity tests suggest that the impact of a 1% reduction of river nutrient inputs on surface nutrients (DIN and PO 4) over the Belgian Exclusive Economic Zone (EEZ) area is similar for the Seine and the Scheldt, which are in turn greater than for the Rhine. However, a hypothetical 1% reduction of nutrient input from the WCH boundary would have a higher impact than for the Scheldt. The impact of nutrient reduction is higher for DIN than for PO 4 whatever the river (contrary to the WCH). DIN is more sensitive to riverine nutrient reduction because the rivers are over enriched in DIN compared to PO 4. The sensitivity tests suggest also that a PO 4 river input reduction would result in a N:P increase and a DIN river input reduction would result in a N:P decrease but that a combined (PO 4 and DIN) input reduction would reduce the N:P ratio at sea. From 50% nutrient reduction scenarios, model results suggest that a total P reduction would induce a significant decrease of diatoms and a small (coast) to negligible (offshore) decrease of Phaeocystis biomass. On the contrary, a total N reduction would induce a significant decrease of Phaeocystis biomass and a moderate increase of diatoms. When N and P river input reductions are combined, the model predicts a significant decrease of Phaeocystis biomass in Belgian waters and a significant decrease of diatom biomass in the coastal waters and a small increase offshore. A future management plan aiming at Phaeocystis reduction should thus prioritise N reduction.

The effect of nutrient additions on the partitioning of nutrients in an experimental coastal Mediterranean system

The hypothesis that the importance of dissolved organic matter (DOM) as a reservoir of C, N, and P declines, relative to that of the particulate pool, with increasing nutrient inputs was tested using mesocosms exposed to a gradient of nutrient inputs in the Spanish Mediterranean. The nutrient additions included a treatment equivalent to the loading in the coastal ecosystem studied (5 mmol N m−2 d−1), and mesocosms receiving half , 2-, 4-, 8-, and 16-fold this value, as well as a mesocosm to which no nutrients were added. Nutrients were added at ratios of 20 N (as ammonium) : 7 Si : 1 P. The initial concentration of dissolved inorganic nutrients was very low (dissolved inorganic nitrogen 75%). Particulate and dissolved organic matter was depleted in N (C/N ratio > 15) and, particularly, P (C/P ratio > 1000), indicative of a strongly nutrient, particularly phosphorus, deficient ecosystem. Experimental nutrient additions lead to a parabolic change in C/N and C/P ratios in the dissolved organic matter with increasing nutrient inputs, which approached the Redfield stoichiometry at nutrient inputs > 8 fold above the ambient loading. The relative size of the dissolved inorganic nutrient pools (about 20% of the N and P) did not vary, but there was a tendency towards an increase in the relative size of the particulate pool at the expense of a decrease in the relative importance of DOM as a reservoir of N, P and C, with increasing nutrient inputs. The production of nutrient-depleted organic matter at low nutrient inputs likely prevents efficient recycling, leading to the dominance of nutrients in DOM in the system.

Misuse of inorganic N and soluble reactive P concentrations to indicate nutrient status of surface waters

Dissolved inorganic N (DIN) and soluble reactive P (SRP) have been used by some to indicate the trophic status of waters, and concentration ratios (DIN:SRP) to indicate nutrient deficiency. The utility of such measurements should be questioned, particularly based on well-known problems associated with determination of the concentration of SRP, which is commonly assumed to represent PO43−. Another potential problem with using inorganic nutrient pools to represent trophic state and nutrient availability ratios arises because concentration values are in units of mass per unit volume, and cannot be used with certainty to estimate supply (i.e., turnover rate of the nutrient pool, expressed either in mass per unit volume per unit time or simply as per unit time) to organisms without information on uptake and remineralization. Two data sets with lotic water-column nutrient values were explored, a large, continental-scale data set with analyses and collections done by many laboratories, and a more limited data set collected and analyzed by the same laboratory. In concert, the data sets indicated that at high total N (TN) (i.e., >5 mg/L) and total P (TP) (i.e., >2 mg/L) concentrations, >60% of the nutrient is usually made up of dissolved inorganic forms, but at low levels the ratio of dissolved inorganic to total nutrients is highly variable. Last, DIN:SRP is a weak surrogate for TN:TP and thus should be used with caution to indicate nutrient limitation.

Responses in plant, soil inorganic and microbial nutrient pools to experimental fire, ash and biomass addition in a woodland savanna.

In order to investigate the effects of savanna fires on nutrient cycling a field experiment was carried out in an open woodland savanna of southwest Ethiopia. This involved manipulations of fire, fuel load and ash fertilisation in a fully factorial design, and recording of responses in plants, soil inorganic and microbial nutrient pools up to 1year after the disturbances. As plant biomass nitrogen (N) was only 3.5% of that in topsoil the N loss in a single fire event was relatively small. The microbial N pool size in the topsoil was similar to the N pool size in the aboveground part of the plants. Soil microbial biomass carbon increased slightly 12days after the low severity fire, but the effect was transient and was not accompanied by an increase in microbial N. Instead, the soil inorganic N concentration increased strongly 1day after the fire, remained higher up to 3months after the fire and probably caused the 40% higher grass biomass in burned than unburned plots, and the similar sized increase in grass nitrogen, phosphorus and potassium pools in the following rainy season. In contrast, broad-leaved herbs showed less strong increments in biomass and nutrient pool sizes. Fire interacted with fuel load, as burning of plots with double plant biomass led to reduced microbial biomass, plant nutrient pools and herb (but not grass) biomass. Low-severity-fire nutrient losses appear to be moderate and may be replenished from natural sources. However, in areas with frequent fires and high grass biomass (fuel) loads, or with late fires, nutrient losses could be much larger and non-sustainable to the persistence of the woodland savanna ecosystem.

Phenotypic Diversity and Stability of Ecosystem Processes

The resistance of an ecosystem to perturbations and the speed at which it recovers after the perturbations, which is called resilience, are two important components of ecosystem stability. It has been suggested that biodiversity increases the resilience and resistance of aggregated ecosystem processes. We test this hypothesis using a theoretical model of a nutrient-limited ecosystem in a heterogeneous environment. We investigate the stability properties of the model for its simplest possible configuration, i.e., a system consisting of two plant species and their associated detritus and local resource depletion zones. Phenotypic diversity within the plant community is described by differences in the nutrient uptake and mortality rates of the two species. The usual measure of resilience characterizes the system as a whole and thus also applies to aggregated ecosystem processes. As a rule this decreases with increased diversity, though under certain conditions it is maximum for an intermediate value of diversity. Resistance is a property that characterizes each system component and process separately. The resistance of the inorganic nutrient pools, hence of nutrient retention in the ecosystem, decreases with increased diversity. The resistance of both total plant biomass and productivity either monotonically decreases or increases over part of the parameter range with increased diversity. Furthermore, it is very sensitive to parameter values. These results support the view that there is no simple relationship between diversity and stability in equilibrium deterministic systems, whether at the level of populations or aggregated ecosystem processes. We discuss these results in relation to recent experiments.