Nutrient Limitation Of Primary Production Research Articles

Knowing your limits: Patterns and drivers of nutrient limitation and nutrient–chlorophyll relationships in US lakes

AbstractAlthough understanding nutrient limitation of primary productivity in lakes is among the oldest research priorities in limnology, there have been few broad‐scale studies of the characteristics of phosphorus (P)‐, nitrogen (N)‐, and co‐limited lakes and their environmental context. By analyzing 3342 US lakes with concurrent P, N, and chlorophyll a (Chl a) samples, we showed that US lakes are predominantly co‐limited (43%) or P‐limited (41%). Majorities of lakes were P‐limited in the Northeast, Upper Midwest, and Southeast, and co‐limitation was most prevalent in the interior and western United States. N‐limitation (16%) was more prevalent than P‐limitation in the Great Basin and Central Plains. Nutrient limitation was related to lake, watershed, and regional variables, including Chl a concentration, watershed soil, and wet nitrate deposition. N and P concentrations interactively affected nutrient–chlorophyll relationships, which differed by nutrient limitation. Our study demonstrates the value of considering P, N, and environmental context in nutrient limitation and nutrient–chlorophyll relationships.

Nutrient limitation of primary production in rivers along a land use gradient in the Lake Biwa Basin, Shiga Prefecture, Japan

Benthic microalgae (BMA) biomass and community structure in freshwater lotic systems are often limited by inorganic nutrient availability. We examined nutrient limitation of benthic algae biomass and community structure using nutrient diffusing substrates in four rivers along a land use gradient in the Lake Biwa basin, Japan. Ambient in-stream nutrient concentrations were correlated to catchment land use, with the highest nitrogen and phosphorus concentrations being found in rivers draining more forested catchments. Nutrient limitation of primary producer biomass and nutrient-driven changes in community structure were evident in all four rivers, regardless of in-stream nutrient concentrations and surrounding land use. BMA biomass (measured as chlorophyll log-response ratio) exhibited the greatest nutrient limitation in rivers with higher in-stream nutrient concentrations. The relationship to catchment land use was less clear, with the highest nutrient limitation being observed in the catchment with moderate amounts of agricultural and forested land use. Nutrient additions resulted in a shift from dominance by Bacillariophyceae (diatoms) to a mixed Bacillariophyceae-Chlorophyceae (chlorophyte) community in all rivers and this shift was most pronounced in the forested catchments. Changes within the diatom community with nutrient additions were also observed, although the shifts in diatom community within a river in response to nutrient additions were much smaller than the differences in diatom community composition among rivers. Diatom taxa classified as highly motile increased with nutrient additions in all rivers. Our results suggest that primary producer community in rivers may be sensitive to nutrient inputs even in areas with elevated nutrient concentrations and catchments dominated by agricultural land use. There is likely widespread nutrient limitation in rivers of Japan, across both in-stream nutrient and land use gradients and any increases in nutrient loading will likely stimulate benthic algal growth. Our findings highlight the importance of looking at both biomass and species composition to assess ecosystem-level impacts of elevated nutrient levels.

Smoke from regional wildfires alters lake ecology

Wildfire smoke often covers areas larger than the burned area, yet the impacts of smoke on nearby aquatic ecosystems are understudied. In the summer of 2018, wildfire smoke covered Castle Lake (California, USA) for 55 days. We quantified the influence of smoke on the lake by comparing the physics, chemistry, productivity, and animal ecology in the prior four years (2014–2017) to the smoke year (2018). Smoke reduced incident ultraviolet-B (UV-B) radiation by 31% and photosynthetically active radiation (PAR) by 11%. Similarly, underwater UV-B and PAR decreased by 65 and 44%, respectively, and lake heat content decreased by 7%. While the nutrient limitation of primary production did not change, shallow production in the offshore habitat increased by 109%, likely due to a release from photoinhibition. In contrast, deep-water, primary production decreased and the deep-water peak in chlorophyll a did not develop, likely due to reduced PAR. Despite the structural changes in primary production, light, and temperature, we observed little significant change in zooplankton biomass, community composition, or migration pattern. Trout were absent from the littoral-benthic habitat during the smoke period. The duration and intensity of smoke influences light regimes, heat content, and productivity, with differing responses to consumers.

Altered leaf elemental composition with climate change is linked to reductions in photosynthesis, growth and survival in a semi‐arid shrubland

Abstract Climate change will increase heat and drought stress in many dryland areas, which could reduce soil nutrient availability for plants and aggravate nutrient limitation of primary productivity. Any negative impacts of climate change on foliar nutrient contents would be expected to negatively affect the photosynthetic capacity, water use efficiency and overall fitness of dryland vegetation. We conducted a 4‐year manipulative experiment using open top chambers and rainout shelters to assess the impacts of warming (~2°C, W), rainfall reduction (~30%, RR) and their combination (W + RR) on the nutrient status and ecophysiological performance of six native shrub species of contrasting phylogeny in a semi‐arid ecosystem. Leaf nutrient status and gas exchange were assessed yearly, whereas biomass production and survival were measured at the end of the study. Warming (W and W + RR) advanced shoot growth phenology and reduced foliar macro‐ (N, P, K) and micronutrient (Cu, Fe, Zn) concentrations (by 8%–18% and 14%–56% respectively), net photosynthetic rate (32%), above‐ground biomass production (28%–39%) and survival (23%–46%). Decreased photosynthesis and growth in W and W + RR plants were primarily linked to enhanced nutritional constraints on carbon fixation. Poor leaf nutrient status in W and W + RR plants partly decoupled carbon assimilation from water flux and led to drastic reductions in water use efficiency (WUEi; ~41%) across species. The RR treatment moderately decreased foliar macro‐ and micronutrients (6%–17%, except for Zn) and biomass production (22%). The interactive impacts of warming and rainfall reduction (W + RR treatment) on plant performance were generally smaller than expected from additive single‐factor effects. Synthesis. Large decreases in plant nutrient pool size and productivity combined with increased mortality during hotter droughts will reduce vegetation cover and nutrient retention capacity, thereby disrupting biogeochemical processes and accelerating dryland degradation with impending climate change. Increased macro‐ and micronutrient co‐limitation of photosynthesis with forecasted climate change conditions may offset any gains in WUEi and productivity derived from anthropogenic CO2 elevation, thereby increasing dryland vegetation vulnerability to drought stress in a warmer and drier climate. The generalized reduction in leaf nutrient contents with warming compromises plant nutritional quality for herbivores, with potential cascading negative effects across trophic levels.

Nutrient limitation of plant productivity in scrubby flatwoods: does fire shift nitrogen versus phosphorus limitation?

Differences in the biogeochemistry of nitrogen (N) and phosphorus (P) lead to differential losses and inputs during and over time after fire such that fire may affect nutrient limitation of primary productivity. We conducted a nutrient addition experiment in scrubby flatwoods, a Florida scrub community type, to test the hypothesis that nutrient limitation of primary productivity shifts from N limitation in recently burned sites to P limitation in longer unburned sites. We added three levels of N, P, and N and P together to sites 6 weeks, 8 years, and 20 years postfire and assessed the effects of nutrient addition on above- and belowground productivity and nutrient concentrations. At the community level, nutrient addition did not affect aboveground biomass, but root productivity increased with high N + P addition in sites 8 and 20 years after fire. At the species level, N addition increased leaf biomass of saw palmetto (Serenoa repens) in sites 6 weeks and 20 years postfire, while P addition increased foliar %P and apical shoot growth of scrub oak (Quercus inopina) in sites 8 and 20 years postfire, respectively. Contrary to our hypothesis, nutrient limitation does not appear to shift with time after fire; recently burned sites show little evidence of nutrient limitation, while increased belowground productivity indicates that scrubby flatwoods are co-limited by N and P at intermediate and longer times after fire.

The influence of seasonality on benthic primary production in a Red Sea coral reef

Northern Red Sea coral reefs experience pronounced seasonal variations in environmental factors such as water temperature, light intensity, and nutrient availability. This allows studying related effects on primary production by different functional groups. The present study therefore quantified primary production of all dominant benthic primary producers from a Jordanian fringing reef (29° 27′ 31″ N, 34° 58′ 26″ E) by measuring net photosynthesis (P n) and dark respiration (R) using stirred respirometry chamber incubations during all four seasons of 2013. Annual mean P n was highest for the macroalga Caulerpa (901 nmol O2 cm−2 h−1) and lowest for both the soft coral Sarcophyton and sedimentary microphytobenthos (212 and 223 nmol O2 cm−2 h−1, respectively). Sedimentary microphytobenthos exhibited the strongest response to seasonality with 5.7 times higher P n in spring than in winter. R was highest in hard corals among all groups in every season, likely due to nighttime calcification and heterotrophic activity. Gross photosynthesis-to-respiration ratios (P g:R) were highest for turf algae and macroalgae as well as cyanobacterial mats. While R was primarily positively related to PAR and temperature and P g:R to inorganic nutrients, few groups revealed significant relations between P n and environmental parameters. Seasonal budgets found daily gross primary production and respiration to be dominated by hard and soft corals. Total reef gross primary production was comparable between less oligotrophic spring and more oligotrophic summer. This indicates that processes such as heterotrophic feeding and microbial dinitrogen fixation may help the functional groups overcome potential nutrient limitation of primary production in summer.

Assessing nutrient limitation in complex forested ecosystems: alternatives to large-scale fertilization experiments.

Quantifying nutrient limitation of primary productivity is a fundamental task of terrestrial ecosystem ecology, but in a high carbon dioxide environment it is even more critical that we understand potential nutrient constraints on plant growth. Ecologists often manipulate nutrients with fertilizer to assess nutrient limitation, yet for a variety of reasons, nutrient fertilization experiments are either impractical or incapable of resolving ecosystem responses to some global changes. The challenges of conducting large, in situ fertilization experiments are magnified in forests, especially the high-diversity forests common throughout the lowland tropics. A number of methods, including fertilization experiments, could be seen as tools in a toolbox that ecologists may use to attempt to assess nutrient limitation, but there has been no compilation or synthetic discussion of those methods in the literature. Here, we group these methods into one of three categories (indicators of soil nutrient supply, organismal indicators of nutrient limitation, and lab-based experiments and nutrient depletions), and discuss some of the strengths and limitations of each. Next, using a case study, we compare nutrient limitation assessed using these methods to results obtained using large-scale fertilizations across the Hawaiian Archipelago. We then explore the application of these methods in high-diversity tropical forests. In the end, we suggest that, although no single method is likely to predict nutrient limitation in all ecosystems and at all scales, by simultaneously utilizing a number of the methods we describe, investigators may begin to understand nutrient limitation in complex and diverse ecosystems such as tropical forests. In combination, these methods represent our best hope for understanding nutrient constraints on the global carbon cycle, especially in tropical forest ecosystems.

Nutrient Limitation in Northern Gulf of Mexico (NGOM): Phytoplankton Communities and Photosynthesis Respond to Nutrient Pulse

Although the Mississippi-Atchafalaya River system exports large amounts of nutrients to the Northern Gulf of Mexico annually, nutrient limitation of primary productivity still occurs offshore, acting as one of the major factors controlling local phytoplankton biomass and community structure. Bioassays were conducted for 48 hrs at two stations adjacent to the river plumes in April and August 2012. High Performance of Liquid Chromatography (HPLC) combined with ChemTax and a Fluorescence Induction and Relaxation (FIRe) system were combined to observe changes in the phytoplankton community structure and photosynthetic activity. Major fluorescence parameters (Fo, Fv/Fm) performed well to reveal the stimulating effect of the treatments with nitrogen (N-nitrate) and with nitrogen plus phosphate (+NPi). HPLC/ChemTax results showed that phytoplankton community structure shifted with nitrate addition: we observed an increase in the proportion of diatoms and prasinophytes and a decrease in cyanobacteria and prymnesiophytes. These findings are consistent with predictions from trait-based analysis which predict that phytoplankton groups with high maximum growth rates (μmax) and high nutrient uptake rates (Vmax) readily take advantage of the addition of limiting nutrients. Changes in phytoplankton community structure, if persistent, could trigger changes of particular organic matter fluxes and alter the micro-food web cycles and bottom oxygen consumption.

Effects of a dry period on the limnological characteristics of a Mediterranean high mountain lake

This investigation assesses the effects of a drought period on the physical and chemical properties and on the plankton commu­nity of a Mediterranean high mountain lake (Rio Seco, Sierra Nevada, Spain). The study was performed over two consecutive years, 2005 and 2006. Both years were characterised by strong differences associated with rainfall patterns. The first year was extremely dry and had rainfall values far below the mean annual average of 813 mm that has been observed over the previous twenty years. The results of the study confirm that the dissolved inorganic nitrogen:soluble reactive phosphorus ratio (DIN:SRP) showed notable differences throughout both study years, with a greater nitrogen limitation during 2005. The ra­tio of total biovolume of autotrophic to heterotrophic organisms (A:H) also presented interannual differences. The A:H ratio was generally below 1 during 2005, whereas it was above this value in 2006. Therefore, the main plankton component was mesozooplankton in 2005 and phytoplankton in 2006. Ciliates and bacteria showed a mean biovolume significantly higher in 2005, whereas the biovolume of heterotrophic nanoflagellates was similar during both years. These results indicate a prevalent top-down control by zooplankton during the dry year and, on the contrary, a predominant bottom-up control during the wetter year as a consequence of the nutrient limitation of primary production.

Going West: Nutrient Limitation of Primary Production in the Northern Gulf of Mexico and the Importance of the Atchafalaya River

To investigate controls on phytoplankton production along the Louisiana coastal shelf, we mapped salinity, nutrient concentrations (dissolved inorganic nitrogen (DIN) and phosphorus (Pi), silicate (Si)), nutrient ratios (DIN/Pi), alkaline phosphatase activity, chlorophyll and 14C primary productivity on fine spatial scales during cruises in March, May, and July 2004. Additionally, resource limitation assays were undertaken in a range of salinity and nutrient regimes reflecting gradients typical of this region. Of these, seven showed Pi limitation, five revealed nitrogen (N) limitation, three exhibited light (L) limitation, and one bioassay had no growth. We found the phytoplankton community to shift from being predominately N limited in the early spring (March) to P limited in late spring and summer (May and July). Light limitation of phytoplankton production was recorded in several bioassays in July in water samples collected after peak annual flows from both the Mississippi and Atchafalaya Rivers. We also found that organic phosphorus, as glucose-6-phosphate, alleviated P limitation while phosphono-acetic acid had no effect. Whereas DIN/Pi and DIN/Si ratios in the initial water samples were good predictors of the outcome of phytoplankton production in response to inorganic nutrients, alkaline phosphatase activity was the best predictor when examining organic forms of phosphorus. We measured the rates of integrated primary production (0.33–7.01 g C m−2 d−1), finding the highest rates within the Mississippi River delta and across Atchafalaya Bay at intermediate salinities. The lowest rates were measured along the outer shelf at the highest salinities and lowest nutrient concentrations (<0.1 μM DIN and Pi). The results of this study indicate that Pi limitation of phytoplankton delays the assimilation of riverine DIN in the summer as the plume spreads across the shelf, pushing primary production over a larger region. Findings from water samples, taken adjacent the Atchafalaya River discharge, highlighted the importance of this riverine system to the overall production along the Louisiana coast.

Nitrogen versus phosphorus limitation in a subtropical coastal embayment (Moreton Bay; Australia): Implications for management

An approach combining nutrient budgets, dynamic modelling, and field observations of phytoplankton and nitrogen (N 2)-fixing Lyngbya majuscula following changes in wastewater N loads, was used to demonstrate that Moreton Bay is potentially phosphorus (P) limited. Modelling and nutrient budgeting shows that benthic N-fixation loads are high, allowing the system to overcome any potential N-limitation. Phytoplankton biomass has shown little change from 1991 to 2006 in the sections of Moreton Bay most impacted by wastewater effluents, despite a large reduction in wastewater N loads from 2000 to 2002. This is consistent with modelling that also showed no reduction in primary productivity associated with reduced N loads. Most importantly, there have been rapid increases in the occurrence of N-fixing L. majuscula in Moreton Bay as wastewater P loads have increased relative to wastewater N loads. This is also consistent with modelling. This work supports the premise that there may be fundamental differences in nutrient limitation of primary production between subtropical and temperate coastal systems due to differences in the importance of internal nitrogen sources and sinks (N-fixation and denitrification). These differences need to be recognised for optimum management of coastal systems.

Controlling Eutrophication along the Freshwater–Marine Continuum: Dual Nutrient (N and P) Reductions are Essential

Expanding human activities along the freshwater to marine continuum of coastal watersheds increasingly impact nutrient inputs, nutrient limitation of primary production, and efforts to reduce nutrient over-enrichment and eutrophication. Historically, phosphorus (P) has been the priority nutrient controlling upstream freshwater productivity, whereas nitrogen (N) limitation has characterized coastal waters. However, changing anthropogenic activities have caused imbalances in N and P loading, making it difficult to control eutrophication by reducing only one nutrient. Furthermore, upstream nutrient reduction controls can impact downstream nutrient limitation characteristics. Recently, it was suggested that only reducing P will effectively control eutrophication in both freshwater and coastal ecosystems. However, controls on production and nutrient cycling in estuarine and coastal systems are physically and chemically distinct from those in freshwater counterparts, and upstream nutrient management actions (exclusive P controls) have exacerbated N-limited downstream eutrophication. Controls on both nutrients are needed for long-term management of eutrophication along the continuum.

Nutrient limitation of primary productivity in the Southeast Pacific (BIOSOPE cruise)

Abstract. Iron is an essential nutrient involved in a variety of biological processes in the ocean, including photosynthesis, respiration and dinitrogen fixation. Atmospheric deposition of aerosols is recognized as the main source of iron for the surface ocean. In high nutrient, low chlorophyll areas, it is now clearly established that iron limits phytoplankton productivity but its biogeochemical role in low nutrient, low chlorophyll environments has been poorly studied. We investigated this question in the unexplored southeast Pacific, arguably the most oligotrophic area of the global ocean. Situated far from any continental aerosol source, the atmospheric iron flux to this province is amongst the lowest of the world ocean. Here we report that, despite low dissolved iron concentrations (~0.1 nmol l−1) across the whole gyre (3 stations located in the center and at the western and the eastern edges), primary productivity are only limited by iron availability at the border of the gyre, but not in the center. The seasonal stability of the gyre has apparently allowed for the development of populations acclimated to these extreme oligotrophic conditions. Moreover, despite clear evidence of nitrogen limitation in the central gyre, we were unable to measure dinitrogen fixation in our experiments, even after iron and/or phosphate additions, and cyanobacterial nif H gene abundances were extremely low compared to the North Pacific Gyre. The South Pacific gyre is therefore unique with respect to the physiological status of its phytoplankton populations.

E21 Habitat Connectivity and Ecosystem Productivity: Implications from a Simple Model

The import of resources (food, nutrients) sustains biological production and food webs in resource‐limited habitats. Resource export from donor habitats subsidizes production in recipient habitats, but the ecosystem‐scale consequences of resource translocation are generally unknown. Here, I use a nutrient‐phytoplankton‐zooplankton model to show how dispersive connectivity between a shallow autotrophic habitat and a deep heterotrophic pelagic habitat can amplify overall system production in metazoan food webs. This result derives from the finite capacity of suspension feeders to capture and assimilate food particles: excess primary production in closed autotrophic habitats cannot be assimilated by consumers; however, if excess phytoplankton production is exported to food‐limited heterotrophic habitats, it can be assimilated by zooplankton to support additional secondary production. Transport of regenerated nutrients from heterotrophic to autotrophic habitats sustains higher system primary production. These simulation results imply that the ecosystem‐scale efficiency of nutrient transformation into metazoan biomass can be constrained by the rate of resource exchange across habitats and that it is optimized when the transport rate matches the growth rate of primary producers. Slower transport (i.e., reduced connectivity) leads to nutrient limitation of primary production in autotrophic habitats and food limitation of secondary production in heterotrophic habitats. Habitat fragmentation can therefore impose energetic constraints on the carrying capacity of aquatic ecosystems. The outcomes of ecosystem restoration through habitat creation will be determined by both functions provided by newly created aquatic habitats and the rates of hydraulic connectivity between them.

Habitat Connectivity and Ecosystem Productivity: Implications from a Simple Model

The import of resources (food, nutrients) sustains biological production and food webs in resource-limited habitats. Resource export from donor habitats subsidizes production in recipient habitats, but the ecosystem-scale consequences of resource translocation are generally unknown. Here, I use a nutrient-phytoplankton-zooplankton model to show how dispersive connectivity between a shallow autotrophic habitat and a deep heterotrophic pelagic habitat can amplify overall system production in metazoan food webs. This result derives from the finite capacity of suspension feeders to capture and assimilate food particles: excess primary production in closed autotrophic habitats cannot be assimilated by consumers; however, if excess phytoplankton production is exported to food-limited heterotrophic habitats, it can be assimilated by zooplankton to support additional secondary production. Transport of regenerated nutrients from heterotrophic to autotrophic habitats sustains higher system primary production. These simulation results imply that the ecosystem-scale efficiency of nutrient transformation into metazoan biomass can be constrained by the rate of resource exchange across habitats and that it is optimized when the transport rate matches the growth rate of primary producers. Slower transport (i.e., reduced connectivity) leads to nutrient limitation of primary production in autotrophic habitats and food limitation of secondary production in heterotrophic habitats. Habitat fragmentation can therefore impose energetic constraints on the carrying capacity of aquatic ecosystems. The outcomes of ecosystem restoration through habitat creation will be determined by both functions provided by newly created aquatic habitats and the rates of hydraulic connectivity between them.

The ocean as a net heterotrophic system: Implications From the carbon biogeochemical cycle

The global ocean apparently consumes more organic carbon than it produces. The excess heterotrophy probably occurs in the nearshore zone. This nearshore heterotrophy has significant implications with respect to processes such as organic matter transport from the nearshore zone to the adjacent open ocean, nutrient limitation of primary production, and the role of the coastal zone as a short‐term sink for anthropogenic CO2.

Utilization of nitrogen and phosphorus by primary producers in warm-core ring 82-B following deep convective mixing

Rates of utilization for NO 3 −, NH 4 +, urea, and PO 4 − by primary producers and the abundances of particulate C, N, and P were determined for the euphotic zone of warm-core ring 82-B in April and early May 1982. The ring had formed in late February, and our study took place between a period of deep convection and the formation of a stable seasonal thermocline. Deep convection enriched the surface waters with NO 3 − and PO 4 −, which remained in sufficient quantities to preclude nutrient limitation of primary production during the course of the study. Concentrations of HN 4 + and urea were always less than or equal to the limit of detection (0.03 μmol kg −1). The fraction of total nitrogen taken up as NO 3 − (ƒ ratio) was high (0.62–0.66) in the core region of the ring. During the course of this study, the concentration of NO 3 − decreased near the surface, allowing a NO 3 − gradient to develop, while the concentration of particulate N (PN) and the rate of N uptake increased. However, these processes were not in balance. Only about 16% of the NO 3 − based production remained in the euphotic zone. The PN:Chl a ratio was higher at the bottom of the euphotic zone than at the top. This indicates that detrital PN was being formed faster than it was exported by either sinking or mixing processes. The overall reduction in euphotic zone NO 3 − concentrations was about one third the measured consumption rate. Midway through the study a storm was apparently responsible for redistributing NO 3 − in the euphotic zone, and perhaps mixing NO 3 − up from greater depths. After that event, the mean NO 3 − uptake rate was about twice the rate of NO 3 − disappearance. Upward flux of NO 3 − associated with the relaxation of depressed density surfaces as the ring lost kinetic energy could have supplied NO 3 − at a rate equivalent to 40–110% the mean NO 3 − uptake rate for the period. The highest rates of NO 3 − uptake were observed at the end of the study, and were in excess of the calculated rates for upward flux. This may explain the increasing magnitude of NO 3 − gradients observed durinng the final few days of this study.

In situ nutrient assays of periphyton growth in a lowland Costa Rican stream

Nutrient limitation of primary production was experimentally assessed using an in situ bioassay technique in the Quebrada Salto, a third-order tropical stream draining the northern foothills of the Cordillera Central in Costa Rica. Bioassays employed artificial substrata enriched with nutrients that slowly diffuse through an agar-sand matrix (Pringle & Bowers, 1984). Multiple comparisons of regression coefficients, describing chlorophyll-a accrual through time for different nutrient treatments, revealed positive micronutrient effect(s). Micronutrient treatment combinations (Fe, B, Mn, Zn, Co, Mo, EDTA), supplemented with and without nitrate and phosphate, exhibited significantly greater chlorophyll-a accrual over all other treatments (P < 0.05), supporting over three times that of the control after 14-d of substratum colonization. Neither of the major nutrients (N or P) produced a significant stimulation, although the N treatment displayed ≃50% more chlorophyll-a than the control after 14-d. Similarly, Si, EDTA, and Si + N + P treatments did not exhibit chlorophyll-a response curves that were significantly different from the control. During the experiment, mean NH4-N and (NO2 + NO3)-N concentrations in the Salto were 2.0 µM (28.6 µg · l−1) and 7.2 µM (100.2 µg · l −1), respectively. High concentrations of PO4-P ( $$\bar X$$ = 2.0 µM; 60.9 µg · l−1) and TP ( $$\bar X$$ = 3.0 µM; 94.0 µg · l−1) were also found, and consequently low molar N:P ratios $$\bar X$$ = 4.7). Despite the potential for N limitation in the system, both N and P appear to be at growth saturating levels. This may be due to micronutrient limitation and/or light limitation of periphyton growth in densely shaded upstream portions of the stream.

Nutrient Limitation of Primary Production in a Sub-Arctic Salt Marsh

Les effets des apports d'azote et de phosphore sur la production primaire des parties aeriennes d'un marais sale domine par Puccinellia phryganodes et Carex sulspathacea a La Perouse Bay (Manitoba, Canada)