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To increase the efficiency of computing phytoplankton growth rate ( μ ), chlorophyll-to-carbon ( θ ) and nitrogen-to-carbon ratios ( Q N ) in three-dimensional ocean circulation models, it is preferable to directly calculate θ and Q N from ambient environmental factors instead of treating them as independent tracers. Optimality-based modeling has emerged as a novel and efficient approach to fulfill this task. However, it is still unclear precisely how the response of optimality-based models differs from conventional models. We compare a recent optimality-based phytoplankton model (PAHLOW model), based on which the familiar Droop function can be derived, to a commonly used Monod-type (MONOD) model. The two models generate similar patterns of μ with some important differences. Compared to the MONOD model, the PAHLOW model predicts higher μ under light limitation. The PAHLOW model also predicts that θ decreases with decreasing light under dim light and predicts decreasing Q N with increasing light even at constant nutrient levels. Compared to the MONOD model, these features of the PAHLOW model qualitatively agree better with laboratory data. The PAHLOW model also suffers from a few shortcomings including the underestimation of θ under very low light and two times of computation time compared to the MONOD model. The two models generate striking differences of Q N and θ in a one-dimensional implementation. Validation of such patterns will require more direct in situ measurements of μ , θ and Q N . [ABSTRACT FROM AUTHOR]

Eutrophication and undesired algal blooms in surface water are common and have been linked to increasing nutrient loading. Effects of extreme events such as storms on reservoir nutrient and phytoplankton remain unclear. Here we carried out continuous high-frequency measurements in a long and narrow dam reservoir in southeast China during a storm period in June–July 2015. Our results show a strong nutrient-phytoplankton relationship as well as a very rapid response to storm runoff. We observed an increase in total suspended matter (TSM), ammonium (NH 4 -N), and dissolved reactive phosphate (DRP), with a sharp decline in chlorophyll- a (Chl- a ) in the high flow periods. Afterward, Chl- a , total phytoplankton abundance and Cyanophyta fraction elevated gradually. Nitrate was diluted at first with increasing discharge before concentration increased, likely following a delayed input of groundwater. Physiochemical parameters and Chl- a were evenly distributed in the water column during the flooding period. However, 10% of NH 4 -N and 25% of DRP were removed in surface water (0–1 m) when an algal bloom (Chl- a > 30 μg L − 1 ) occurred 10 days after peak discharge. Conversely, total particulate P (TPP) of surface water was 58% higher than in the deeper water. Dynamic factor analysis (DFA) revealed that TSM, NH 4 -N, DRP, total P and discharge significantly explain Chl- a variations following storms ( C eff = 0.89). These findings highlight that the reservoir ecosystem was vulnerable to pulse input from storm runoff and the Cyanophyta bloom was likely fueled by phosphate and ammonium rather than nitrate. [ABSTRACT FROM AUTHOR]

Seasonal distribution of dissolved nutrients (,,,,) was measured in the Sanggou Bay (SGB), China. In order to understand the impact of rivers and of the Yellow Sea (YS) and the role of integrated multi-trophic aquaculture (IMTA) in nutrient reduction and dynamics. Seasonal surveys were conducted in spring (April), summer (August), autumn (October) and winter (January) in 2011–2012 along 19 stations covering culture areas. The summer was highly impacted by excess water discharge and aquaculture activities than benthic sediments source; however other seasons were mostly influenced by the YS. Results showed that nutrients were vertically mixed and IMTA helped in reducing the elevated nutrient concentration generated by water discharge and the mixing process. The N/P ratio indicated phosphate as a limiting nutrient. Historical values of nutrient data showed that the concentration of,andincreased whiledecreased with slightly higher uptake rate ofin the bay during the last decade. A simple mass-balance model was employed to determine the nutrient budget which showed that nutrients were mostly from the river input. This implies an increase in the anthropogenic activities and assimilation ofin the SGB. Such increasing trends in nutrient concentration could cause eutrophication in the bay. [ABSTRACT FROM AUTHOR]

Coupled physical–biological modelling experiments were made for the period of 1995–2009 to analyse the spatial and interannual variability of nutrients and phytoplankton production in the Gulf of Maine (GOM). The physical model was the Finite-Volume Community Ocean Model (FVCOM) and the biological model was a Nitrogen, Phytoplankton, Zooplankton, and Detritus (NPZD) model. The simulation was carried out with realistic meteorological surface forcing, five major tidal constituents, river discharge, and observation-based open boundary conditions. The results were robust with comparison to SeaWiFS chlorophyll data and historical data of nitrogen. An Empirical Orthogonal Function analysis clearly identified two dominant modes in nutrient and phytoplankton dynamics: (1) sustained nutrient supply and phytoplankton production from spring through autumn, and (2) a dominating phytoplankton bloom in spring, relatively low production in summer, and a noticeable bloom in autumn. Mode 1 was a dominant feature in strong tidal energy dissipation regions such as the southwestern shelf of Nova Scotia, Georges Bank, Nantucket Shoals, the Bay of Fundy, and the coastal regions of GOM, where tidal pumping and mixing were the major drivers for the sustained nutrient supply, and primary production showed certain resilience with less interannual variability. Mode 2 was a characteristic in the deep Gulf, the offshore region of the Scotian Shelf, and in the open sea area, where the timing and amplitude of the spring phytoplankton bloom is essentially controlled by the salinity regime, and its interannual variability was significantly influenced by freshening events controlled by local and remote forcing. [ABSTRACT FROM PUBLISHER]

We consider a phytoplankton-zooplankton interaction model which depends on two complementary nutrients. For a realistic representation in chemostat plankton ecosystem, we have incorporated various natural phenomena such as dissolved limiting nutrients with nutrient uptake functions and yield constants. For the model with two different constant nutrient inputs with same constant washout rate, existence and stability of non-negative equilibria as well as persistence are given. We analyze the behavior of solution of model in order to answer the biological question and seek to determine the limiting behavior of the surviving organisms and the nutrients. It is observed that the constant nutrient inputs of two complementary nutrients play an important role to change steady state behavior of the system. Further it is observed that if the dilution rate of chemostat crosses certain critical value, the system enters into Hopf-bifurcation. Finally, we have derived the explicit algorithm which determines the direction of Hopf-bifurcation. Computer simulations have been carried out to illustrate different analytical results. [ABSTRACT FROM AUTHOR]

Abstract: The Júcar River, characterized by a very irregular freshwater flow, discharges into the Spanish Mediterranean coastal waters. However, the flow at its mouth is usually insignificant due to the overexploitation of upstream waters, except during intense rain periods. The presence or absence of freshwater generates two different scenarios/conditions in this ecosystem. The vertical gradient of salinity due to freshwater flows generates differences in nutrient content and phytoplankton composition and abundance along the water column. Nutrients, in general, increase their concentration in the superficial layers due to the river inputs. However, orthosilicic acid, nitrite and nitrate decrease with depth and when getting closer to the river mouth. The main freshwater phytoplankton groups are Bacillariophyceae, Chlorophyceae and Cyanobacteria. Whether or not freshwater flow reaches the estuary, the water column is almost entirely formed by marine water. Under this condition, nutrient concentrations are lower, and Cryptophyceae, Prymnesiophyceae, Dinophyceae and picocyanobacteria dominate phytoplankton composition. This paper presents the results of two campaigns with different environmental conditions (with and without freshwater flow respectively). Physicochemical and biological variables were studied at three stations placed along the estuary. [Copyright &y& Elsevier]

Abstract: We consider a plankton–nutrient interaction model consisting of phytoplankton, zooplankton and dissolved limiting nutrient with general nutrient uptake functions and instantaneous nutrient recycling. In this model, it is assumed that phytoplankton releases toxic chemical for self defense against their predators. The model system is studied analytically and the threshold values for the existence and stability of various steady states are worked out. It is observed that if the maximal zooplankton conversion rate crosses a certain critical value, the system enters into Hopf bifurcation. Finally it is observed that to control the planktonic bloom and to maintain stability around the coexistence equilibrium we have to control the nutrient input rate specially caused by artificial eutrophication. In case if it is not possible to control the nutrient input rate, one could use toxic phytoplankton to prevent the recurrence bloom. [Copyright &y& Elsevier]

Abstract: Iron [dissolved Fe (D-Fe) and total dissolvable Fe (T-Fe)] and nutrient concentrations in the surface water of the Oyashio region (northwestern North Pacific) were measured before and during spring phytoplankton bloom (March to May in 2007). During the pre-bloom period (middle of March), we observed vertically uniform concentrations of iron (0.3–0.5nM [D-Fe] and 3–5nM [T-Fe]), macronutrients (10–15μM NO3+NO2, 1.0–1.5μM PO4 and 20–30μM Si(OH)4) and chlorophyll a (Chl-a, 0.3–0.4μg l−1) throughout the upper 125 to 150m due to vertical mixing during winter. Water temperature and salinity before the bloom were also vertically uniform with >5°C and S>33.5, the conditions of a modified Kuroshio warm-water (MKW) ring, higher than those of the Coastal Oyashio Water (COW). The cold COW (<2°C, S<33.2) intruded a few times into the surface during the bloom period with high iron (0.4–0.6nM [D-Fe] and 10–25nM [T-Fe]) and Chl-a (10–23μg l−1) concentrations. The high Chl-a observed in the COW is due to the higher original concentrations of iron and macronutrients in the COW than in the MKW. An incubation experiment conducted during a COW intrusion in early April resulted in robust phytoplankton growth and complete exhaustion of nutrients, indicating an adequate supply of bioavailable iron. This result is also consistent with robust in situ phytoplankton growth and Chl-a production due to the presence of high iron levels in the COW. The most important mechanisms transporting iron to the surface water, which would regulate the primary production during spring bloom in the Oyashio region, are the surface intrusions of iron- and nutrient-rich COW derived from vertical and lateral mixing processes and vertical mixing in MKW during winter and spring. [Copyright &y& Elsevier]

Abstract: We investigated the nutrient and diatom dynamics during late winter and spring (9-March to 1-May 2007) in the Oyashio region as part of the OECOS-WEST research cruises. Macronutrients, iron, chlorophyll a (Chl-a) and biogenic silica (BSi) concentrations in the upper mixed layer varied remarkably ranges were 1.88–18.8μmolL−1 for NO3+NO2, 0.64–1.85μmolL−1 for PO4, 3.14–35.7μmolL−1 for Si(OH)4, 0.14–0.54nmolL−1 for D-Fe, 0.64–24.6nmolL−1 for T-Fe, 0.30–17.4μgL−1 for Chl-a, and 0.34–14.1μmolL−1 for BSi. Mixed-layer depth (MLD) varied from 8–190m during the cruises. The growth rate of in situ phytoplankton communities, dominated by centric diatoms, varied in shipboard culture experiments from 0.55d−1 for iron-replete to 0.14d−1 for iron-limited conditions. A relationship between BSi and Chl-a concentrations indicates that the in situ diatom community in the warmer water system (>4°C) was heavily silicified, probably due to iron-limitation. The in situ macronutrient and dissolved iron concentrations below the MLD and estimated macronutrient concentrations during winter were negatively correlated to temperature (1–6°C), that is to the relative proportion of warm modified Kuroshio Water mixed into the colder Oyashio water system. The rate of decrease in Si(OH)4 per °C increase was greater than the rates for NO3+NO2 and PO4 for both in situ and estimated winter values. These results suggest that the spring bloom in the cold-water system with high macronutrients and iron concentrations would progress rapidly and intensely, and then be terminated by nitrogenous nutrient depletion. However, the diatom bloom in warmer waters with lower macronutrients and iron concentrations would be terminated by Si- and/or iron-limitation of heavily-silicified diatoms. In the OECOS study, variation of macronutrients and iron due to the surface intrusions of several water masses and modification from different chemical conditions during winter were the most important factors regulating the progression, magnitude and probably fate of the spring phytoplankton bloom in the Oyashio region. [Copyright &y& Elsevier]

This study was carried out to determine the diurnal distributions of phytoplankton density, bio-volume, and chlorophyll a in relation to nutrients and CTD measurements in surface waters of the coastal area in the Dardanelles during high density of phytoplankton, especially high diatom production (between 3 July and 4 August 2002). Ninety nine surface samples were collected for nutrient, chlorophyll a, and phytoplankton in 3 different time periods of the day (08:00, 13:00 and 18:00 hours) in the sampling period. Based on the sample analyses, average temperature, salinity, pH, DO, TDS, TSS, NO2-+NO3-, PO4-3, SiO4, chlorophyll a, N:P, and Si:P were calculated as 25 °C, 23.3 ppt, 8.42, 9.26 mg L-1, 23.9 g L-1, 36.1 mg L-1, 1.90 μM, 0.24 μM, 3.61 μM, 1.70 μg L-1, 12.5, and 25, respectively. Total phytoplankton density and biovolume varied from 2.86 × 105 to 1.5 × 107 cells L-1 and from 5.98 × 108 to 8.81 × 1010 μm³ L-1, respectively. Contribution rate of Bacillariophyceae (66.5%) to total phytoplankton bio-volume was higher than that of Dinophyceae (31.0%). Phytoplankton community structure was observed to be controlled by 6-7 species. Other species can be considered as insignificant species that do not cause important fluctuations in the phytoplankton density and bio-volume. It has been reported that there were 9-11 population growth slopes for different species at different times and densities. Therefore, life cycles of phytoplankton species were completed between 3 and 4 days. Diurnal variations of biological and physicochemical parameters in the time interval between 0800 and 18:00 were generally much higher than daily variations between 3 July and 4 August 2002 due to the 2 counter flows system of the Dardanelles and domestic inputs of Çanakkale city. [ABSTRACT FROM AUTHOR]

An experiment was conducted with a natural freshwater phytoplankton community from a eutrophic pond to investigate the combined effects of phytoplankton community competitive interactions, nutrient enrichment and zooplankton on Microcystis bloom formation. The pond water initially had a very low concentration of Microcystis, but the total phytoplankton biomass as chlorophyll a reached ∼110 µg L−1 in summer. The pond water was incubated outdoors at natural temperature and light with a 2 × 2 factorial manipulation of nutrient (nutrient additions versus no additions) with and without zooplankton. The interaction of a two-phase nutrient addition (a net increase in concentrations of 250.0 µM N and 16.1 µM P each time) and the presence of zooplankton significantly altered phytoplankton community composition. When the water initially contained zooplankton, nitrogen and phosphorus enrichment promoted a surface Microcystis bloom. However, when the zooplankton was removed from the water at the start of the experiment, no surface Microcystis bloom formed, regardless of nutrient additions. Chlorophyta dominated in the absence of zooplankton, when the same nutrient was provided. Our results demonstrate that Microcystis bloom formation in this eutrophic water body at a mean temperature of about 36°C at 14:00 h was closely related to the initial presence of zooplankton and a sufficient supply of nitrogen and phosphorus. We believe this is one of the first demonstrations of zooplankton controlling Microcystis bloom formation in a water body previously free of surface cyanobacterial blooms. [ABSTRACT FROM PUBLISHER]

Abstract: During an iron-enrichment experiment in the western subarctic Pacific (SEEDS-II), concentrations of dissolved and acid-soluble rare earth elements (REEs) were determined in shallow waters. Detailed vertical profiles of dissolved REEs were compared with those of nutrients in a preliminary survey. The results showed good correlation, except for Ce and Y. Along with phytoplankton growth within the iron-enriched patch, dissolved REEs were decreased in surface waters. The changes in REE concentrations were small but systematic: light REEs were adsorbed preferentially over heavy REEs. This systematic trend is consistent with results of earlier studies of REE adsorption onto biogenic particle surfaces. However, we were unable to detect a decrease of acid-soluble REEs in surface waters. During the phytoplankton bloom decline phase, dissolved REEs were elevated to concentration levels at the preliminary survey, although nutrients were decreased. The liberation of REEs from the suspended particles might be related to disaggregation from large particles to colloid particles, desorption from the particulate phase, and remineralization from organic matter. Outside the Fe-patch, time variations of REEs were also observed during SEEDS-II, indicating that both artificially induced phytoplankton blooms and natural variation of biogenic particles can affect the biogeochemical cycles of REEs in shallow waters within a short time. [Copyright &y& Elsevier]

Dead zones—areas experiencing low levels of dissolved oxygen—are growing in shallow ocean waters around the world. Research has shown that dead zones form as a result of a specific type of pollution, called nutrient enrichment or eutrophication, and are found in almost every coastal zone where humans have large populations. Concepts related to eutrophication are not complicated individually, but they frequently confuse students. The activity presented in this article is a game designed to help middle and high school students understand the processes that encourage formation of dead zones. Postgame discussions encourage students to consider natural and human causes of dead zone formation, as well as ways in which human contributions to it can be reduced. [ABSTRACT FROM AUTHOR]

Abstract: From November 2002 to 2006, five cruises were undertaken in the Yangtze River Estuary and the adjacent East China Sea to compare the nutrient concentrations, ratios and potential nutrient limitation of phytoplankton growth before and after impoundment (June 2003) of the Three Gorges Dam (TGD). Concentrations of dissolved inorganic nitrogen (DIN), soluble reactive phosphorus (SRP) and total nitrogen (TN) exhibited an increasing trend from 2002 to 2006. In contrast, total phosphorus (TP) concentration exhibited a decreasing trend. The mean concentrations of DIN, SRP, and TN in the total study area increased from 21.4 µM, 0.9 µM, and 41.8 µM in 2002 to 37.5 µM, 1.3 µM, and 82.2 µM in 2006, respectively, while TP decreased from 2.1 µM to 1.7 µM. The concentration of dissolved reactive silica (DRSi) had no major fluctuations and the differences were not significant. The mean concentration of DRSi in the total study area ranged from 52.5 to 92.3 µM. The Si:N ratio decreased significantly from 2.7 in 2002 to 1.3 in 2006, while TN:TP ratio increased from 22.1 to 80.3. The area of potential P limitation of phytoplankton growth expanded after 2003 and potential Si limitation appeared in 2005 and 2006. Potential P limitation mainly occurred in an area of salinity less than 30 after 2003, while potential Si limitation occurred where the salinity was greater than 30. By comparison with historical data, the concentrations of nitrate and SRP in this upper estuary during November 1980–2006 increased obviously after impoundment of TGD but DRSi decreased. Meanwhile, the ratios of N:P, Si:N and Si:P decreased obviously. [Copyright &y& Elsevier]

Myall Lakes is a large brackish coastal lake on the east coast of Australia that was considered pristine until the occurrence of blue-green algal blooms in 1999. The temporal and spatial extent of chemical and biological changes to the water column of Myall Lakes was studied intensively after a rain event in 2002. Water quality profiles (T, EC, pH, DO), turbidity (secchi), nutrients (TN, NO x , NH4 +, DON, TP, FRP, DOP, Si), and phytoplankton (chl a and cell counts) were measured at nine sites on eight occasions immediately after the rain event. Freshwater inflows affected a large area of the lake. Greatest changes were seen in areas close to the mouth of the upper Myall River which is the largest freshwater input to the lakes. Here, greatly elevated concentrations of NO x , TP, and FRP (up to two orders of magnitude higher than background) were recorded immediately after the rain event but persisted for only 2 to 8 days. Slightly elevated concentrations of TP and NO x were seen in inflows from the smaller Boolambayte Creek. Stratification was associated with bottom water anoxia and release of ammonia from the sediments. Identification of the sources of nutrient species delivered from different parts of the catchment, combined with studies of nutrient loads can assist managers to develop effective nutrient reduction strategies to reduce the incidence of blue-green algal blooms in Myall Lakes. [ABSTRACT FROM AUTHOR]

Stoichiometric nutrient ratios are the consequence of myriad interacting processes, both biotic and abiotic. Theoretical explanations for autotroph stoichiometry have focused on species' nutrient requirements but have not addressed the role of nutrient availability in determining autotroph stoichiometry. Remineralization of organic N and P supplies a significant fraction of inorganic N and P to autotrophs, making nutrient recycling a potentially important process influencing autotroph stoichiometry. To quantitatively investigate the relationship between available N and P, autotroph N : P, and nutrient recycling, we analyze a stoichiometrically explicit model of autotroph growth, incorporating Michaelis-Menten-Monod nutrient uptake kinetics, Droop growth, and Liebig's law of the minimum. If autotroph growth is limited by a single nutrient, increased recycling of the limiting nutrient pushes autotrophs toward colimitation and alters both autotroph and environmental stoichiometry. We derive a steady state relationship between input stoichiometry, autotroph N : P, and the stoichiometry of organic losses that allows us to estimate the relative recycling of N to P within an ecosystem. We then estimate relative N and P recycling for a marine, an aquatic, and two terrestrial ecosystems. Preferential P recycling, in conjunction with greater relative P retention at the organismal and ecosystem levels, presents a strong case for the importance of P to biomass production across ecosystems. [ABSTRACT FROM AUTHOR]

A comparison between recent data (1997–2002) and older data (1975–1980) related to the Venice lagoon is presented here. These data have been focused on plankton and dissolved nutrient distribution obtained within the framework of differing research projects. Investigations were carried out in the central and northern areas of the Lagoon of Venice. Comparing the two data sets, a general increase in orthosilicates and in dissolved inorganic nitrogen (DIN) was observed. Conversely, phosphates showed a decrease in polyhaline waters (salinity between 18 and 30 PSU) because of the reduced amounts of phosphorous compounds in detergents, following Italian laws enforced in the 1980s. In the recent data set (1997–2002), phytoplankton abundance was higher than in the years 1975–1980 because of more frequent diatom blooms. The increased resuspension processes induced by dredging for molluscs probably favour a mobilisation of benthic epipsamic diatoms from sediments, resulting in a more pelagic life style. Regarding zooplankton, a decrease in the standing stock, more marked in winter, was revealed. These results indicate that the trophic conditions of the Lagoon of Venice are subjected to natural and anthropogenic temporal changes that can be understood only through the analysis of uninterrupted time series. [Copyright &y& Elsevier]