Nutrient Addition Bioassays Research Articles

Patterns of (micro)nutrient limitation across the South Pacific Ocean

Nutrient limitation regulates phytoplankton growth throughout much of the global ocean and its assessment is important for our understanding of future changes in ocean productivity. The South Pacific Ocean represents a vast region where limiting nutrients have so far been investigated with only a handful of experiments. Here we report the results of eleven nutrient addition bioassay experiments conducted across the South Pacific Ocean at ca. 30°S as part of the GEOTRACES GP21 expedition. Nitrogen addition alone stimulated chlorophyll-a accumulation at all sites, whilst increases were larger following supplementary addition of iron in the eastern and central basin, and phosphate in the western basin. Enhanced iron stress following nitrogen addition in the eastern and central basin further pointed towards ambient iron concentrations approaching co-limiting levels. The overall east-to-west shift in serial limiting nutrients from iron to phosphate was attributed to a broadscale east-west increase in iron availability relative to nitrogen.

Managing a cyanobacteria harmful algae bloom “hotspot” in the Sacramento – San Joaquin Delta, California

Cyanobacterial harmful algal blooms (CHABs) have become a persistent seasonal problem in the upper San Francisco Estuary, California also known as the Sacramento-San Joaquin Delta (Delta). The Delta is comprised of a complex network of open water bodies, channels, and sloughs. The terminus of the Stockton Channel is an area identified as a CHAB “hotspot.” As CHABs increase in severity, there is an urgent need to better understand CHAB drivers to identify and implement mitigation measures that can be used in an estuarine complex like the Delta. We investigated water quality conditions and nutrient dynamics in the Stockton Channel by measuring nutrients in the water column, sediments, and pore waters. In situ nutrient addition bioassay experiments were used to assess the effects of nutrient enrichment on total algal/cyanobacterial growth and pigment concentrations. In both June and September, relative to unamended controls, total chlorophyll and cyanobacterial pigment concentrations were unaffected by nutrient additions; hence, the study area showed signs of classical hypereutrophication, with ambient nitrogen and phosphorus present in excess of algal growth requirements. A cyanobacterial bloom, dominated by Microcystis spp. was present throughout the study area but was most severe and persistent at the shallowest site at the channel terminus. At this site, Microcystis spp. created water quality conditions that allowed for a prolonged bloom from June through September. While targeted nutrient reductions are recommended for long term mitigation, on a shorter timescale, our findings suggest that physical/mechanical controls are the more promising alternative approaches to reduce the severity of CHABs in the terminus of the Stockton Channel.

Nutrient limitation of phytoplankton in three tributaries of Chesapeake Bay: Detecting responses following nutrient reductions

Many coastal ecosystems suffer from eutrophication, algal blooms, and dead zones due to excessive anthropogenic inputs of nitrogen (N) and phosphorus (P). This has led to regional restoration efforts that focus on managing watershed loads of N and P. In Chesapeake Bay, the largest estuary in the United States, dual nutrient reductions of N and P have been pursued since the 1980s. However, it remains unclear whether nutrient limitation – an indicator of restriction of algal growth by supplies of N and P – has changed in the tributaries of Chesapeake Bay following decades of reduction efforts. Toward that end, we analyzed historical data from nutrient-addition bioassay experiments and data from the Chesapeake Bay long-term water-quality monitoring program for six stations in three tidal tributaries (i.e., Patuxent, Potomac, and Choptank Rivers). Classification and regression tree (CART) models were developed using concurrent collections of water-quality parameters for each bioassay monitoring location during 1990-2003, which satisfactorily predicted the bioassay-based measures of nutrient limitation (classification accuracy = 96%). Predictions from the CART models using water-quality monitoring data showed enhanced nutrient limitation over the period of 1985-2020 at four of the six stations, including the downstream station in each of these three tributaries. These results indicate detectable, long-term water-quality improvements in the tidal tributaries. Overall, this research provides a new analytical tool for detecting signs of ecosystem recovery following nutrient reductions. More broadly, the approach can be adapted to other waterbodies with long-term bioassays and water-quality data sets to detect ecosystem recovery.

Variabilities in autumn cyanobacterial responses to ecosystem external enrichments based on nutrient addition bioassay in Pengxi River, Three Gorges Reservoir, China

Nutrient availability, is a crucial anthropogenic stressor promoting freshwater eutrophication and rapid expansion of harmful algal blooms (HABs), deteriorating water quality and threatening public health worldwide.The estimation of the HABs community responses to diel changes in the nutrients while characterizing the ecosystem growth limiting factors, is key to prudent watershed management. The present study investigated the short-term variabilities in autumn cyanobacterial responses to the external nutrient inputs into the Pengxi River using the nutrient addition bioassay approach. Results reveal phytoplankton community structure dominated by the cyanobacteria: Anabaena and Aphanizomenon spp. (relative abundance = 46.20% equilibrium abundance), followed by the diatoms, out of which Lindayia bodaniica, are preponderant. Nutrient enrichment triggered strong variabilities in dominance and successions among the cyanobacterial group, with maximum dominance (76.34%) exhibited by the Aphanizomenon sp. upon NH4 addition. Fe enrichment led to the succession of cyanobacteria, Leptolyngbya tenuis, which was below the detectable limit in the control, indicating the role of Fe in its proliferation. Studies on nutrient limitation demonstrated P/NH4 co-limited ecosystem, with P as the primary and NH4, a secondary limiting factor. The nitrate preference index (NO3−RPI = 0.991) shows a high preference for NH4 while NO3 constitutes the bulk of the ecosystem TN. Considering the elevated NO3 concentration, we posit that a shift in the phytoplankton community structure from cyanobacteria to diatoms dominated ecosystem, is expected following Fe depletion and a further stretch on the current ecosystem NH4 limitation. The study provides useful and first-ever insights for nutrient reduction in the middle Three Gorges Reservoir (TGR) before the onset of the heavy HABs during spring in the Pengxi River.

Storm and floods increase the duration and extent of phosphorus limitation on algal blooms in a tributary of the Three Gorges Reservoir, China

Excessive anthropogenic nutrient input has resulted in eutrophication and algal blooms which have severely impacted the function and sustainability of aquatic ecosystems, underscoring the need to implement nutrient management strategies. It was assumed that the increasing rainfall during flood season would affect the stoichiometric ratio of total nitrogen (TN): total phosphorus (TP), driving the nutrient limitation of algal growth. In order to test this concept and explore corresponding nutrient management strategies, nutrient addition bioassays were carried out in Xiangxi Bay, one of the largest tributaries of the Three Gorges Reservoir (TGR), China. Results indicated that nutrient limitation on algal growth fluctuated from nitrogen (N) to phosphorus (P) limitation. N limitation dominated in the early flood season. However, the reduction of dissolved P, accompanied with an increase of TN: TP caused by an increase in extreme rainfall events intensified P limitation throughout the bay. Then P limitation was alleviated due to the reduction of rainfall and the process of impoundment after the flood season. The variation of TN: TP caused by the increasing of rainfall and flooding could be the main driving factor of the nutrient limitation shift in aquatic ecosystems mainly affected by external nutrient inputs. Nutrient dilution and enrichment bioassays showed that TN and TP concentration thresholds should be targeted at below 0.55 mg/L and 0.057 ∼ 0.064 mg/L respectively, to limit the growth of algae and maintain chlorophyll a below 30 μg/L. Dual nutrient (N & P) reductions were required for long-term bloom mitigation in the entire basin. This study provided a scientific basis for a nutrient management strategy to combat eutrophication and reduce algal bloom potentials in the tributaries of the TGR. We recommend that long-term determinations of nutrient limitation and nutrient threshold will be needed to control algal growth, considering future anticipated changes in land use, population density and the impacts of climate change.

Variation in iron and its potential relevance to phytoplankton ecology in Oklahoma reservoirs

The effects of nitrogen (N) and phosphorus (P) on productivity have been well characterized in freshwater ecosystems, including in the abundance of diazotrophic cyanobacteria that cause harmful algal blooms. However, there is much residual variation in these relationships. Because iron (Fe) is required for N-fixation, we tested whether Fe impacts productivity and cyanobacterial abundance using in situ P and Fe addition experiments. While P had the largest effect on total primary productivity, Fe increased cyanobacterial abundance at nearly the same rate as P. Moreover, algal Fe content was strongly correlated with cyanobacterial abundance. These results indicate that cyanobacterial abundance is positively related to total Fe in reservoirs, although there is little known about the variability in Fe among freshwater ecosystems. We surveyed 25 Oklahoma, U.S.A, reservoirs to capture variation in total dissolved Fe, and conducted nutrient addition bioassays to understand the role of Fe in productivity. We found three orders of magnitude variation in total Fe among reservoirs, although Fe alone did not limit primary production. Together, these results indicate that attention to heterogeneity in the supply of elements beyond those typically quantified in freshwaters (i.e. N and P) could be useful in understanding harmful algal blooms.

Variabilities in Autumn Cyanobacterial Responses to Ecosystem Enrichments Based on Nutrient Addition Bioassay in Pengxi River, Three Gorges Reservoir, China

Variabilities in Autumn Cyanobacterial Responses to Ecosystem Enrichments Based on Nutrient Addition Bioassay in Pengxi River, Three Gorges Reservoir, China

Study on nutrient limitation of phytoplankton growth in Xiangxi Bay of the Three Gorges Reservoir, China

After the impoundment of the Three Gorges Reservoir (TGR), algal blooms in the sidearm tributaries have resulted from increasing nutrient loads along the major tributaries. Field sampling and in situ nutrient addition bioassay were implemented to examine the nutrient limitation of phytoplankton growth and bloom initiation during autumn in Xiangxi Bay of the TGR. Result shows that P is the primary limiting nutrient for algal growth and bloom in Xiangxi Bay during autumn. The treatment involving the combination of N, P and Si had a significant (p < .05) additional effect on the growth of phytoplankton. The N, P, Si combined treatment increased growth by 10–50% relative to the N and P treatments from day 1 to day 4, respectively. Trace metal additions involving Fe, Zn, Mn, and Cu and/or in combination with N, P, and Si initially resulted in an extremely low growth rate which later increased significantly (p < .05) towards the end of the study. The present study provides an insight into the responses of different phytoplankton taxa in autumn under nutrient conditions in the tributary bay. The nutrient limitation study is recognized as the first step to mitigating the bloom while proposing an effective nutrient control strategy. The outcome of which can provide the basis for formulating sustainable watershed management. Multiple nutrients reductions with P as primary concern are required for a lasting management solution to the risk of bloom in the TGR.

Nutrient addition bioassay and phytoplankton community structure monitored during autumn in Xiangxi Bay of Three Gorges Reservoir, China

The increasing freshwater ecosystem nutrient budget is a critical anthropogenic factor promoting freshwater eutrophication and episodic bloom of harmful algae which threaten water quality and public health. To understand how the eutrophic freshwater ecosystem responds in term of phytoplankton community structure dynamics to a sudden rise in nutrient concentrations, a microcosm study by nutrient addition bioassay was implemented in Xiangxi Bay (XXB) of Three Gorges Reservoir, China. Our results showed that dissolved trace elements supply adequately altered the phytoplankton community structure creating a regime shift from cyanobacteria-dominated to essentially Chlorophytes-dominated system, relative abundance (>70%). Combined N, P, and Si led to maximum growth stimulation accompanied by the highest chlorophyll yield (82.7 ± 14.01 μgL−1) and growth rate (1.098 ± 0.12 μgL−1d−1). N separate additions resulted in growth responses which did not differ while P -addition differed significantly (p∠0.05) with the control justifying a P limited system. Si enrichment stimulated diatom growth, relative abundance (20.62%) and maximum utility rate (USi = 83.37 ± 0.33%). This study also reveals that increasing nutrient loading from anthropogenic sources adequately decrease the ecological diversity (H < 1) and community overlap (CC ≤ 0.5) intensifying competition and succession which then select the fast-growing taxa to dominate and expand. Result points to the need for multiple nutrient control of N, P and Si loading into XXB through a prudent nutrient management protocol for lasting bloom mitigation in the tributary bay.

Nutrient addition bioassay and phytoplankton community structure monitored during autumn in Xiangxi Bay of Three Gorges Reservoir, China

Nutrient addition bioassay and phytoplankton community structure monitored during autumn in Xiangxi Bay of Three Gorges Reservoir, China

Nutrient regulation of late spring phytoplankton blooms in the midlatitude North Atlantic

AbstractThe duration and magnitude of the North Atlantic spring bloom impacts both higher trophic levels and oceanic carbon sequestration. Nutrient exhaustion offers a general explanation for bloom termination, but detail on which nutrients and their relative influence on phytoplankton productivity, community structure, and physiology is lacking. Here, we address this using nutrient addition bioassay experiments conducted across the midlatitude North Atlantic in June 2017 (late spring). In four out of six experiments, phytoplankton accumulated over 48–72 h following individual additions of either iron (Fe) or nitrogen (N). In the remaining two experiments, Fe and N were serially limiting, that is, their combined addition sequentially enhanced phytoplankton accumulation. Silicic acid (Si) added in combination with N + Fe led to further chlorophyll a (Chl a) enhancement at two sites. Conversely, addition of zinc, manganese, cobalt, vitamin B12, or phosphate in combination with N + Fe did not. At two sites, the simultaneous supply of all six nutrients, in combination with N + Fe, also led to no further Chl a enhancement, but did result in an additional 30–60% particulate carbon accumulation. This particulate carbon accumulation was not matched by a Redfield equivalent of particulate N, characteristic of high C:N organic exudates that enhance cell aggregation and sinking. Our results suggest that growth rates of larger phytoplankton were primarily limited by Fe and/or N, making the availability of these nutrients the main bottom‐up factors contributing to spring bloom termination. In addition, the simultaneous availability of other nutrients could modify bloom characteristics and carbon export efficiency.

The influences of nitrogen form and zooplankton grazing on phytoplankton assemblages in two coastal southeastern systems

AbstractCoastal environments are vulnerable to land development due to human population growth that has increased nitrogen and phosphorus loading into receiving estuaries. As a result, phytoplankton blooms have proliferated as “bottom‐up” (resource) processes outweigh “top‐down” (removals). This study investigated phytoplankton biomass and assemblage regulation by both N‐form (ammonium, nitrate, urea) and zooplankton at two southeastern United States estuaries (the urban Charleston Harbor and less‐developed Ashepoo‐Combahee‐Edisto Basin) with contrasting land cover patterns through in situ nutrient addition bioassays (spring and summer, 2015–2016) and dilution experiments (summer 2016). Phytoplankton responses to N exhibited no clear patterns during spring. By comparison, summer chlorophyll a (Chl a) increases were significant (p &lt; 0.05) in response to all provided N forms, and diatoms contributed most to total biovolume with biovolume increases higher at the more developed site per estuary. Phytoplankton Chl a and biovolume were generally greatest when all nutrients were added in combination, but nitrate added alone frequently elicited the lowest response. Ciliate abundances were high at most sites with lower mesozooplankton abundances. Accordingly, microzooplankton grazing rates were highest for &lt; 5 μm and &lt; 20 μm Chl a fractions, often corresponding with the highest phytoplankton growth rates. Ciliate abundances tracked Chl a during bioassays, highlighting their role as important biomass controls in these estuaries. Results can be used to inform nutrient management decisions, as continued development will increase nutrient loading to developed and developing systems.

Spring phytoplankton community response to an episodic windstorm event in oligotrophic waters offshore from the Ulleungdo and Dokdo islands, Korea

We investigated the phytoplankton distribution and its relationship to environmental factors at 40 stations in oligotrophic waters offshore from the Ulleungdo and Dokdo islands (hereafter Ulleungdo or Dokdo) in Japan/East Sea, prior to and following an episodic windstorm event. Nutrient addition bioassay experiments (control, +N, +P, and +NP, in both the presence and absence of added Fe) were also conducted to investigate the growth response of the phytoplankton assemblage and its nutrient consumption, using surface seawater collected from stations 36 and 40, which are in the vicinity of the Dokdo. Field measurements showed that the surface water temperature ranged from 13.33°C to 16.18°C and the salinity ranged from 34.03 to 34.55. The nitrate+nitrite, phosphate, and silicate concentrations varied from 0.07 to 2.22μM, 0.01μM to 0.19μM and 0.76 to 6.93μM, respectively. The Chl-a concentration varied from 0.36 to 15.97μgL−1 (average 2.66±3.26μM), but was significantly higher in Zone III-a (Dokdo) than in Zone I-b (between Ulljin and Ulleungdo, prior to the windstorm), Zone I-a (between Ulljin and Ulleungdo, following the windstorm), and Zone II-a (Ulleungdo) (F=17.438, p<0.001; ANOVA). Diatoms and Raphidophyta were the dominant phytoplankton types. Following episodic windstorm events the abundance of the raphidophyte Heterosigma akashiwo was maintained at high levels in the offshore oligotrophic area around the Ulleungdo and Dokdo, particularly in Zone III-a (F=16.889, p<0.001; ANOVA). In the algal bioassays conducted with and without added Fe, the in vivo fluorescence values in the +N and +NP treatments were higher than in the control and the +P treatments, which suggests that plankton biomass production was stimulated by N availability. In the +N and +NP treatments, H. akashiwo typically dominated in the initial, logarithmic, and stationary growth phases. The growth rate of the phytoplankton community in the presence of added Fe was not statistically different (p>0.05) from that in the treatments without added Fe. The results suggest that in this area natural phytoplankton communities, including those dominated by H. akashiwo, respond rapidly to pulsed nitrogen loading events. The episodic windstorm event probably resulted in vertical mixing that brought nutrients into the euphotic upper layer. The results suggest that such events are important in triggering spring phytoplankton blooms in potentially oligotrophic waters, such as those in the vicinity of the Dokdo in the Japan/East Sea.

Stimulation of Phytoplankton Production by Anthropogenic Dissolved Organic Nitrogen in a Coastal Plain Estuary.

There is increased focus on nitrogen (N)-containing dissolved organic matter (DOM) as a nutrient source supporting eutrophication in N-sensitive estuarine ecosystems. This is particularly relevant in watersheds undergoing urban and agricultural development, leading to increased dissolved organic N (DON) loading. To understand how this shift in N-loading influences estuarine phytoplankton production, nutrient addition bioassays were conducted in the N-limited Neuse River Estuary, North Carolina from 2014 to 2015. Additions included N-rich DOM sources characteristic of urban and agricultural development, including chicken and turkey litter leachate, wastewater treatment facility effluent, and concentrated river DOM (used as a reference). Each DOM addition was coupled with an inorganic nutrient treatment to account for inorganic nutrient concentrations (NO2/3, NH4, PO4) in each respective DOM addition. Repeated measures analysis of variance (RM-ANOVA) showed that chicken litter leachate stimulated phytoplankton growth greater than its coupled inorganic nutrient treatment. Wastewater treatment facility effluent, turkey litter leachate, and concentrated river DOM did not stimulate phytoplankton growth greater than their respective inorganic nutrient controls. DOM fluorescence (EEM-PARAFAC) indicated the chicken litter contained a biologically reactive fluorescent DOM component, identified as the nonhumic, biologically labile, "N-peak", which may be responsible for stimulating the observed phytoplankton growth in the chicken litter leachate treatments.

Critical nutrient thresholds needed to control eutrophication and synergistic interactions between phosphorus and different nitrogen sources.

Eutrophication is one of the greatest threats to global freshwater ecosystems. The phytoplankton responses to nutrient inputs vary in different water bodies, so it is particularly important to determine the nutrient thresholds and synergistic interactions between nutrients in different freshwater ecosystems. Field sampling and bioassay experiments were conducted to determine the thresholds of soluble reactive phosphorus (SRP), nitrate-nitrogen (NO3-N), and ammonium-nitrogen (NH4-N) in Miyun Reservoir. A separate nutrient addition bioassay was designed to assess the synergistic interactions between these nutrients. Chlorophyll a (Chl a) concentrations were used to estimate phytoplankton biomass. The results showed the following: (1) nutrient threshold bioassay indicated that eutrophication thresholds of SRP, NO3-N, and NH4-N should be targeted at below 0.04mg P L-1, 0.5mg NL-1, and 0.3mg NL-1, respectively, to limit the growth of phytoplankton. (2) The stimulatory effect of "NH4-N plus P" on phytoplankton biomass was greater than "NO3-N plus P" at the same N concentration, and "NH4-N plus NO3-N" did not show such associated stimulatory effect as "NH4-N plus P" or "NO3-N plus P". (3) The average concentrations of total phosphorus (TP), NO3-N, and NH4-N in Miyun Reservior were 0.017mg P L-1, 0.620mg NL-1, and 0.143mg NL-1, respectively. The reservoir-wide average Chl a is below 20μgL-1 on an annual basis. (4) Ammonium was an important factor for the growth of phytoplankton and inputs of both NH4-N and NO3-N should be reduced to control bloom formation. Our findings imply that although P load reduction is important, appropriate reductions of all forms of N in watershed is recommended in the nutrient management strategy for Miyun Reservoir.

Manipulating nutrient limitation using modified local soils: A case study at Lake Taihu (China)

The effect of geo-engineering materials of chitosan modified local soil (MLS) on nutrient limitation was studied in comparable whole ponds in Lake Taihu in October 2013. After 20 kg MLS were sprayed in the whole water pond (400 m2), the chlorophyll-a (Chl-a) concentration was decreased from 42 to 18 μg L−1 within 2 h and remained below 20 μg L−1 in the following 15 months, while the average Chl-a was 36 μg L−1 in the control pond throughout the experiment. In situ nutrient addition bioassay experiments indicated that the nutrient limitation was shifted from nitrogen (N) and phosphorus (P) co-limitation to P limitation after MLS treatment from October 2013 to March 2014 compared to the control pond. In the cyanobacterial bloom season of June 2014, N and P co-limitation remained and N was the primary limiting nutrient and P was a secondary one in the control pond where phytoplankton biomass showed significant increase by N addition and further increase by N + P additions, while both N and P became the limiting nutrient for phytoplankton growth where only combined N and P additions showed significant Chl-a stimulation in the treatment pond. In the next summer (June 2014), a cyanobacteria-dominated state still remained in the control pond but chlorophytes, bacillariophytes and cyanophytes distributed equally and submerged vegetation was largely restored in the treatment pond. Meanwhile, the upper limiting concentration of DIN was enhanced from 0.8 to 1.5 mg L−1 and SRP from 0.1 to 0.3 mg L−1 compared to the control pond. This study indicates that nutrient limitation can be manipulated by using MLS technology.

Algal blooms, circulators, waterfowl, and eutrophic Greenfield Lake, North Carolina

ABSTRACTMallin MA, McIver MR, Wambach EJ, Robuck AR. 2016. Algal blooms, circulators, waterfowl, and eutrophic Greenfield Lake, North Carolina. Lake Reserv Manage. 32:168–181.Lake rehabilitation measures can sometimes have unintended consequences, including increasing certain eutrophication symptoms. An example involves Greenfield Lake, an urban blackwater impoundment within the City of Wilmington, North Carolina. This popular recreational impoundment has experienced noxious phytoplankton and macrophyte blooms, anoxia and hypoxia, and fish kills for many years. Chlorophyll a and biological oxygen demand are strongly correlated within the lake, indicating that decaying algal blooms are an important source of oxygen demand resulting in low dissolved oxygen. Nutrient addition bioassays demonstrated that the phytoplankton strongly responded to nitrogen (N) inputs. Spatially, ammonium, nitrate, and N/P ratios were higher in the upper lake compared with the lower lake, indicating tributary input of inorganic N, whereas phosphorus (P) was higher in the lower lake. A year-long survey indicated that waterfowl, especially cormorants, contributed somewhat to the lake's total N load but a considerable amount of P, particularly in winter. The lake sediments apparently function as P reservoirs, supplying P to the water column during summer, setting the stage for runoff-induced nitrate pulses and subsequent algal blooms. Lake restoration measures initiated in 2005 included installation of solar-powered circulators, introduction of grass carp, and herbicide treatments. These measures resulted in loss of surface macrophyte and algal mats and reduced dissolved oxygen violations but led to a significant increase in phytoplankton chlorophyll a and a tripling of chlorophyll a standard violations in comparison with pre-restoration years. The increased chlorophyll a violations have led to inclusion of the lake on the North Carolina 303(d) list of impaired waters.

Nutrient limitation dynamics examined on a multi-annual scale in Lake Taihu, China: implications for controlling eutrophication and harmful algal blooms

Rapidly increasing urban, agricultural, and industrial growth in the Taihu basin during the past four decades has led to accelerated nitrogen (N) and phosphorus (P) loading to the lake. This has caused the lake to shift from oligo-mesotrophic to hypertrophic conditions, symptomized by toxic cyanobacterial blooms, dominated by the non-N2 fixing genus Microcystis. From 2008 to 2013, a series of in situ microcosm and mesocosm nutrient addition bioassays were conducted that were focused on the heavily polluted northern region (i.e., Meiliang Bay) and other lake locations. Bioassays showed that phytoplankton production, as chlorophyll a and photopigments diagnostic of major phytoplankton groups, was controlled by P inputs from spring to early summer, while N played a more dominant controlling role in summer–fall. In most cases, combined N and P additions promoted maximum growth. This pattern proved true for both the highly eutrophic northern region and the less-eutrophic central and southern regions. Cyanobacteria, chlorophytes, and cryptophytes all showed the strongest positive responses to N and N+P enrichment during the summer bloom period, while diatoms were the least abundant then and just moderately stimulated by nutrient additions. Cyanobacteria failed to selectively respond to P inputs during the summer bloom period, contradicting the paradigm that selective P enrichment will favor them, especially the N2-fixing genera. Rather, Microcystis-dominated blooms remained N-limited during summer months and were not replaced by N2-fixing genera, indicating that internal N and P regeneration of previously loaded nutrients must be sustaining blooms. Successful ‘de-eutrophication’ of Taihu will require reductions of both N and P inputs in all lake regions in order to control blooms and counter the legacy of several decades of nutrient over-enrichment.

Effects of nutrient loading, temperature regime and grazing pressure on nutrient limitation of periphyton in experimental ponds

Summary We studied nutrient limitation of periphytic algae (henceforth periphyton) in 24 mesocosms simulating shallow lakes with two nutrient levels, enriched (with added nitrogen, N, and phosphorus, P) and unenriched (control), and three temperature scenarios, ambient, A2 from the Intergovernmental Panel on Climate Change (IPCC) and A2 + 50%. Periphyton growth (measured as chlorophyll a) was investigated four times in situ using nutrient‐diffusing substrata. The effect of grazing was also manipulated using exclusion cages. We found that periphyton responded differently to nutrient addition bioassays (N and P) depending on the background nutrient concentration and warming scenario. Our results indicate that single‐nutrient limitation prevailed for periphyton in our experimental temperate shallow lakes. The responses were season sensitive. Periphyton in the unenriched mesocosms were P‐limited in early summer in the ambient and A2 scenarios, N‐limited in late summer in these two climate scenarios, not nutrient‐limited in autumn and P‐limited in spring in all climate scenarios. Periphyton in the A2 + 50% scenario showed a positive response to N and P added together in early summer. In contrast, periphyton in the enriched mesocosms showed no clear nutrient limitation, except for short‐term periods of P limitation in the warmer systems. Grazers did not affect the quantitative response of periphyton to nutrient addition, and the concentrations of P and N as well as mean monthly temperature were the main environmental factors driving P or N limitation. We conclude that warming in low‐productivity lakes affects the seasonality of N limitation and changes the single‐nutrient limitation of periphyton into NP co‐limitation. This last observation suggests that warming reduces the sensitivity of temperate shallow lakes to bottom‐up perturbations.

Responses of maximum photosystem II photochemical efficiency of phytoplankton communities to nutrient limitation in the coastal sea of Qingdao, China

Nutrient enrichment experiments with nitrogen (N) and phosphorus (P) were conducted with samples from two stations in the coastal waters of Qingdao, China, during summer to identify limiting nutrients. In late July of 2009, low P concentrations and the maximum photochemical efficiency of photosystem II (F v /F m ) in the initial samples together with F v /F m and chlorophyll a (Chl a) responses to P addition indicated P limitation at the two stations. In early August, low P levels still limited phytoplankton growth at station A. F v /F m and Chl a were the highest in the NP treatments at station B, suggesting an N/P co-limitation. In mid-September, nutrient concentrations and F v /F m were elevated and phytoplankton communities were healthy. Greater F v /F m and Chl a in the treatments with added P than those without the addition suggested potential P limitation at station A. Lack of F v /F m and Chl a responses following nutrient additions indicated N and P repletion at station B. At the end of July 2010, neither N nor P was limited at station B. Additionally, F v /F m coupled with 24-h-long nutrient enrichment experiments can be used to detect P limitation and N/P co-limitation to natural populations. This method can be more accurate for assessing co-limitation than the use of criteria of nutrient concentrations and ratios as indicators, and can provide more rapid results than nutrient addition bioassays using chlorophyll response as an indicator, when a population is potentially limited. Compared with the two conventional methods, the results based on F v /F m can also provide more detailed information about physiological states of the populations.