Limit Phytoplankton Production Research Articles

Hotspots and drivers of compound marine heatwaves and low net primary production extremes

Abstract. Extreme events can severely impact marine organisms and ecosystems. Of particular concern are multivariate compound events, namely when conditions are simultaneously extreme for multiple ocean ecosystem stressors. In 2013–2015 for example, an extensive marine heatwave (MHW), known as the Blob, co-occurred locally with extremely low net primary productivity (NPPX) and negatively impacted marine life in the northeast Pacific. Yet, little is known about the characteristics and drivers of such multivariate compound MHW–NPPX events. Using five different satellite-derived net primary productivity (NPP) estimates and large-ensemble-simulation output of two widely used and comprehensive Earth system models, the Geophysical Fluid Dynamics Laboratory (GFDL) ESM2M-LE and Community Earth System Model version 2 (CESM2-LE), we assess the present-day distribution of compound MHW–NPPX events and investigate their potential drivers on the global scale. The satellite-based estimates and both models reveal hotspots of frequent compound events in the center of the equatorial Pacific and in the subtropical Indian Ocean, where their occurrence is at least 3 times higher (more than 10 d yr−1) than if MHWs (temperature above the seasonally varying 90th-percentile threshold) and NPPX events (NPP below the seasonally varying 10th-percentile threshold) were to occur independently. However, the models show disparities in the northern high latitudes, where compound events are rare in the satellite-based estimates and GFDL ESM2M-LE (less than 3 d yr−1) but relatively frequent in CESM2-LE. In the Southern Ocean south of 60∘ S, low agreement between the observation-based estimates makes it difficult to determine which of the two models better simulates MHW–NPPX events. The frequency patterns can be explained by the drivers of compound events, which vary among the two models and phytoplankton types. In the low latitudes, MHWs are associated with enhanced nutrient limitation on phytoplankton growth, which results in frequent compound MHW–NPPX events in both models. In the high latitudes, NPPX events in GFDL ESM2M-LE are driven by enhanced light limitation, which rarely co-occurs with MHWs, resulting in rare compound events. In contrast, in CESM2-LE, NPPX events in the high latitudes are driven by reduced nutrient supply that often co-occurs with MHWs, moderates phytoplankton growth, and causes biomass to decrease. Compound MHW–NPPX events are associated with a relative shift towards larger phytoplankton in most regions, except in the eastern equatorial Pacific in both models, as well as in the northern high latitudes and between 35 and 50∘ S in CESM2-LE, where the models suggest a shift towards smaller phytoplankton, with potential repercussions on marine ecosystems. Overall, our analysis reveals that the likelihood of compound MHW–NPPX events is contingent on model representation of the factors limiting phytoplankton production. This identifies an important need for improved process understanding in Earth system models used for predicting and projecting compound MHW–NPPX events and their impacts.

Dispersion and Stratification Dynamics in the Upper Sacramento River Deep Water Ship Channel

Hydrodynamics control the movement of water and material within and among habitats, where time-scales of mixing can exert bottom-up regulatory effects on aquatic ecosystems through their influence on primary production. The San Francisco Estuary (estuary) is a low-productivity ecosystem, which is in part responsible for constraining higher trophic levels, including fishes. Many research and habitat-restoration efforts trying to increase primary production have been conducted, including, as described here, a whole-ecosystem nutrient addition experiment where calcium nitrate was applied in the Sacramento River Deep Water Ship Channel (DWSC) to see if phytoplankton production could be increased and exported out of the DWSC. As an integral part of this experiment, we investigated the physical mechanisms that control mixing, and how these mechanisms affect the strength and duration of thermal stratification, which we revealed as critical for controlling phytoplankton dynamics in the relatively turbid upper DWSC. Analysis of a suite of mixing mechanisms and time-scales show that both tidal currents and wind control mixing rates and stratification dynamics in the DWSC. Longitudinal and vertical dispersion increased during periods of high wind, during which wind speed influenced dispersion more than tidal currents. Thermal stratification developed most days, which slowed vertical mixing but was rapidly broken down by wind-induced mixing. Stratification rarely persisted for longer than 24 hours, limiting phytoplankton production in the study area. The interaction between physical mechanisms that control mixing rates, mediate stratification dynamics, and ultimately limit primary production in the DWSC may be useful in informing habitat restoration elsewhere in the Delta and in other turbid aquatic environments.

Phytoplankton community structure of Lake Wushan stocked densely with planktivorous filter-feeding bighead and silver carp, middle and lower reaches of the Yangtze River

武山湖是紧邻长江的通江型富营养化湖泊，是国家级湿地公园.为切实了解该湖在以鲢、鳙养殖为主的情况下浮游植物结构特征，于2017年9月-2018年8月对其浮游植物群落结构特征及水质开展了监测.监测结果表明武山湖水质全年处于轻度富营养到重度富营养水平之间；12次共采集浮游植物7门100种（属），浮游植物优势种共有23种，其中蓝藻门有9种，绿藻门有8种，硅藻门和隐藻门各有3种.夏季和秋季蓝藻门优势种最多且优势度高，冬季和春季绿藻门和硅藻门优势种多且优势度高.武山湖浮游植物每月优势度最大的种类主要有蓝藻门微囊藻和细小平裂藻、绿藻门小球藻以及硅藻门小环藻.浮游植物生物量峰值出现在6月，达34.77 mg/L；丰度峰值出现在7月，达341.46×10⁶ cells/L.冗余分析（RDA）和线性相关分析均表明浮游植物丰度和生物量与总磷、温度和pH均呈显著正相关，且蓝藻门生物量和丰度以及优势属与总磷和温度均呈显著正相关.研究结果表明武山湖浮游植物丰度和生物量在夏季均很高，发生蓝藻水华的风险较大.相对于氮，磷是更重要的限制浮游植物生长的营养元素.;Lake Wushan is a highly eutrophic, shallow lake connected to the Yangtze River, and a national wetland park. In order to study phytoplankton structural characteristics in the lake stocked densely with silver carp (Hypophthalmichthys molitrix) and bighead carp (Aristichthys nobilis), we conducted a field survey on the phytoplankton community and lake trophic status from September 2017 to August 2018. Trophic status indices (TLI) showed the water quality was between mesotrophic and heavy eutrophic state. A total of 100 phytoplankton species (genera) belonging seven phyla were identified; twenty-three dominant species including nine species of Cyanophyta, eight species of Chlorophyta and both three species of Bacillariophyta and of Cryptophyta were observed. The dominant groups were Cyanophyta in summer and autumn, Chlorophyta and Cryptophyta in winter and spring. The phytoplankton size distribution had a trend biased toward miniaturization. Microcystis sp., Merismopedia tenuissima, Chlorella chlorophylla, Chlorella sp., and Cyclotella sp. were the predominant species in our 2017-2018 annual survey. The peak biomass appeared in June, reaching 34.77 mg/L, and the peak abundance appeared in July, reaching 341.46×10⁶ cells/L. Both of redundancy analyses and linear correlation analyses showed that total phosphorus, temperature and pH were positively correlated with phytoplankton abundance and biomass, and total phosphorus and temperature were also positively correlated with the biomass and abundance of Cyanophyta and the dominant genus. Our results showed that the abundance and biomass of phytoplankton were high, indicating high risks of cyanobacterial water blooms in summer months. Nitrogen was at least as important as phosphorus in limiting phytoplankton production in Wuhan lakes.

Sustained Upwelling of Subsurface Iron Supplies Seasonally Persistent Phytoplankton Blooms Around the Southern Kerguelen Plateau, Southern Ocean

AbstractAlthough the supply of iron generally limits phytoplankton productivity in the Southern Ocean, substantial seasonal blooms are observed over and downstream of the Kerguelen plateau in the Indian sector of the Southern Ocean. Surprisingly, of the oceanic blooms, those associated with the deeper southern plateau last much longer (~3 months) than the northern bloom (~1‐month downstream of northern plateau). In this study, iron supply mechanisms around the southern plateau were investigated, obtaining profiles of dissolved iron (<0.2 μm, dFe) to 2,000‐m deep at 25 stations during austral summer 2016. The dFe concentrations in surface waters (≤100‐m depth) ranged from below the detection limit (DL, median of 0.026 nmol/kg) to 0.34 nmol/kg near the Antarctic shelf, with almost half the data points below detection. These low and—with few exceptions—largely spatially invariant concentrations, presumably driven by seasonal drawdown of this essential micronutrient by phytoplankton, could not explain observed patterns in chlorophyll a. In contrast, dFe concentrations (0.05–1.27 nmol/kg) in subsurface waters (100–800 m) showed strong spatial variations that can explain bloom patterns around the southern Kerguelen plateau when considered in the context of frontal locations and associated frontal processes, including upwelling, that may increase the upward supply of dFe in the region. This sustained vertical dFe supply distinguishes the southern blooms from the bloom downstream of the northern Kerguelen plateau and explains their persistence through the season.

Elemental Stoichiometry and Photophysiology Regulation of Synechococcus sp. PCC7002 Under Increasing Severity of Chronic Iron Limitation.

Iron (Fe) is an essential cofactor for many metabolic enzymes of photoautotrophs. Although Fe limits phytoplankton productivity in broad areas of the ocean, phytoplankton have adapted their metabolism and growth to survive in these conditions. Using the euryhaline cyanobacterium Synechococcus sp. PCC7002, we investigated the physiological responses to long-term acclimation to four levels of Fe availability representative of the contemporary ocean (36.7, 3.83, 0.47 and 0.047 pM Fe'). With increasing severity of Fe limitation, Synechococcus sp. cells gradually decreased their volume and growth while increasing their energy allocation into organic carbon and nitrogen cellular pools. Furthermore, the total cellular content of pigments decreased. Additionally, with increasing severity of Fe limitation, intertwined responses of PSII functional cross-section (σPSII), re-oxidation time of the plastoquinone primary acceptor QA (τ) and non-photochemical quenching revealed a shift in the photophysiological response between mild to strong Fe limitation compared with severe limitation. Under mild and strong Fe limitation, there was a decrease in linear electron transport accompanied by progressive loss of state transitions. Under severe Fe limitation, state transitions seemed to be largely supplanted by alternative electron pathways. In addition, mechanisms to dissipate energy excess and minimize oxidative stress associated with high irradiances increased with increasing severity of Fe limitation. Overall, our results establish the sequence of physiological strategies adopted by the cells under increasing severity of chronic Fe limitation, within a range of Fe concentrations relevant to modern ocean biogeochemistry.

Impacts of a buoyant strait outflow on the plankton production characteristics of an adjacent semi-enclosed basin: A case study of the Marmara Sea

Abstract As documented by observational studies, the buoyant outflows emanating from straits and rivers with large amounts of nutrients and biogenic materials cause marked changes in the biochemical characteristics of the adjacent receiving water bodies. Here, using a three-dimensional biophysical model of the Marmara Sea-Bosphorus Strait two-layer exchange flow system and configuring it for the winter phytoplankton blooming period with limited top-down control, we show that complex buoyancy-induced basin-scale circulation is driven by the buoyant jet emanating from a strait, which sustains enhanced production even in the absence of any nutrient flux from its upstream source region and lateral point sources around the sea. In the supercritical flow regime downstream of the strait exit, strong upward motion introduced by the hydraulically controlled outflow dynamics injects subsurface nutrients into the upper layer. Those accumulated within the adjacent anticyclonic bulge to the right of the outflow plume then support relatively high phytoplankton production, whereas strong currents limit phytoplankton production along the main jet axis. Furthermore, topographically controlled anticyclonic circulation within the lower layer around the deep northern basin induces upwelling due to the divergence of cross-isobath, uphill flow and causes the nutrient enrichment of the upper layer. This cumulative response, together with the additional contribution of nutrient recycling and the horizontal distribution of nutrients and biota by the mesoscale-dominated circulation system, maintain a highly productive system within the sea, which is consistent with previous observations.

Effects of CO<sub>2</sub> and iron availability on <i>rbcL</i> gene expression in Bering Sea diatoms

Abstract. Iron (Fe) can limit phytoplankton productivity in approximately 40% of the global ocean, including in high-nutrient, low-chlorophyll (HNLC) waters. However, there is little information available on the impact of CO2-induced seawater acidification on natural phytoplankton assemblages in HNLC regions. We therefore conducted an on-deck experiment manipulating CO2 and Fe using Fe-deficient Bering Sea water during the summer of 2009. The concentrations of CO2 in the incubation bottles were set at 380 and 600 ppm in the non-Fe-added (control) bottles and 180, 380, 600, and 1000 ppm in the Fe-added bottles. The phytoplankton assemblages were primarily composed of diatoms followed by haptophytes in all incubation bottles as estimated by pigment signatures throughout the 5-day (control) or 6-day (Fe-added treatment) incubation period. At the end of incubation, the relative contribution of diatoms to chlorophyll a biomass was significantly higher in the 380 ppm CO2 treatment than in the 600 ppm treatment in the controls, whereas minimal changes were found in the Fe-added treatments. These results indicate that, under Fe-deficient conditions, the growth of diatoms could be negatively affected by the increase in CO2 availability. To further support this finding, we estimated the expression and phylogeny of rbcL (which encodes the large subunit of RuBisCO) mRNA in diatoms by quantitative reverse transcription polymerase chain reaction (PCR) and clone library techniques, respectively. Interestingly, regardless of Fe availability, the transcript abundance of rbcL decreased in the high CO2 treatments (600 and 1000 ppm). The present study suggests that the projected future increase in seawater pCO2 could reduce the RuBisCO transcription of diatoms, resulting in a decrease in primary productivity and a shift in the food web structure of the Bering Sea.

太平洋中西部海域浮游植物营养盐的潜在限制

2009年8月至9月期间在太平洋西部N1站和中部N2站进行现场营养盐加富培养实验。结果显示:N1站,浮游植物生物量对N或者P添加都有较强的响应,其中N+P+Si组和N+P组浮游植物长势迅速,叶绿素a从初始的0.03 μg/L分别达到2.12 μg/L和1.83 μg/L,同时P先于N和Si之前被耗尽;说明N1站为N、P共同限制,P是首要限制因子。而N2站,浮游植物生物量仅对N、P共同添加有明显响应,N先于P和Si被浮游植物消耗殆尽。利用培养过程中营养盐比值变化推断,N1站浮游植物以低于Redfield ratio (16N∶1P)吸收N和P;而N2站浮游植物以高于Redfield ratio (16N∶1P)吸收N和P。这可能解释了太平洋西部的寡营养盐海域为潜在P限制,而在太平洋中部海域则为潜在N限制。;An increase in N₂ fixation by diazotrophic organism due to increased stratification driven by climate changes, may decrease phosphate concentrations and result in P limitation in the oligotrophic upper ocean, which challenges the traditional view that nitrogen is generally the primary nutrient limiting phytoplankton productivity in oceanic waters. Which nutrient, N or P, is the most limiting nutrient for phytoplankton growth in the oligotrophic Pacific Ocean has been on debate over recent years. More studies on nutrient limitation are apparently needed to resolve this debate in the Pacific Ocean. In August and September of 2009, nutrient enrichment bioassays were conducted at two representative stations, N1 (160.58°E, 21.61°N) in the western Pacific Ocean with extremely low nutrient (below detect limit) and Chl a concentrations, and N2 (154.12°W, 10.12°N) in the eastern Pacific Ocean with shallower nutricline due to the influence of equatorial currents, in order to examine the spatial variability in the potential limiting nutrient for phytoplankton growth in the Pacific Ocean. Nutrients were added in 5 combinations in bioassays: control (no addition), NO₃+PO₄ (N+P); NO₃+SiO₄ (N+Si), PO₄+SiO₄ (P+Si), and NO₃+PO₄+SiO₄ (N+P+Si). The limiting nutrient was judged based on the response of algal biomass and nutrient depletion among treatments. Phytoplankton exhibited different response to nutrient additions at two study sites. Phytoplankton biomass increased dramatically in response to both N and P additions at station N1 where the concentrations of Chl a increased from 0.03 μg/L at the beginning to 2.12 μg/L and 1.83 μg/L in N+P+Si and N+P treatments at the end of incubation, respectively. However, the maximum Chl a concentration achieved in N+Si treatment was slight higher than that in and P+Si treatment. Furthermore, P was depleted before N and Si in the N+P and N+P+Si treatments at station N1. In contrast, algal biomass was significantly stimulated only when both N and P were added at station N2, where the concentrations of Chl a increased profoundly from 0.10 μg/L to 0.34 μg/L and 0.40 μg/L at the end in N+P+Si and N+P treatments, respectively, while they was not stimulated in the N+Si and P+Si treatments Furthermore, N always disappeared before P and Si in N+P, N+Si and N+P+Si treatments. These results showed that the limiting nutrient varied spatially during summer in the Pacific Ocean. N and P co-limitation occurred at both stations, with P being the primary limiting nutrient at N1 and N at N2. In addition, changes in the N∶P ratios during the incubation demonstrated distinct patterns, likely due to difference in the phytoplankton composition. N∶P ratios rose from 15.7 at the beginning to 59.2 at the end of the incubation at N1, while N∶P ratios decreased from 15.3 to 0.06 at N2. This implied that the uptake ratio of N∶P was lower than the Redfield ratio (16N∶1P) at N1, but higher than the Redfield ratio at N2. This might explain why P was the primary limiting nutrient in the western Pacific Ocean but N in the central Pacific Ocean. It is speculated that P limitation possibly is associated with N₂ fixation in the oligotrophic western Pacific Ocean.

Simulation of chlorophyll and iron supplies in the Sub Antarctic Zone South of Australia

The Sub Antarctic Zone (SAZ) is part of the high nutrient low chlorophyll region of the Southern Ocean where low iron availability limits phytoplankton productivity. In the Tasman Sea, the SAZ displays a unique and large zonal gradient in the surface chlorophyll concentrations. In situ measurements show higher dissolved iron in the east than in the west which could explain the zonal chlorophyll gradient. To assess and quantify the major sources of iron in this region, we use an eddy-resolving model to simulate the dissolved Fe distribution arising from both the shelf and the dust supply of iron. The simulation showed that dissolved Fe from the shelf is only transported eastward to about 155°E and the shelf source of dissolved iron is unable to supply iron to the entire Tasman Sea SAZ region even when we assume that there no iron losses. The simulation with the atmospheric source of iron showed that through a combination of atmospheric transport and ocean transport, enough dissolved Fe can be supplied to the entire Tasman Sea SAZ region to relieve iron limitation. From the model simulations, we conclude that atmospheric iron supply is essential to generating the observed gradient of the chlorophyll in the SAZ region of the Tasman Sea.

Limnological patterns in northern pantanal lagoons

The aim of this work was to correlate the limnological patterns recorded among ten lagoons in the northern part of the Pantanal of Mato Grosso with some aspects of the lagoons related to their physical and biological characteristics. Some lagoon characteristics, such as connection with the river and the presence of floating macrophytes were important to group similar lagoons into categories. Attempts were also made to determine which kind of the nutrients could be limiting phytoplankton productivity in the lagoons of the northern Pantanal. Data was collected in nine lagoons during the flooding stage in January 2004, and nutrient and chlorophyll a was followed in a tenth lagoon from June 2004 to October 2004. The phytoplankton biomass was significantly associated with the nitrogen concentration during the flooding and drought periods. All the studied lagoons were potentially limited by the nitrogen concentration with a strong negative relationship between the chlorophyll a biomass and ammonium concentration during the drought season. The drought season also had nearly three times more chlorophyll a biomass when compared to the flooding period.

Nutritive Substitution of Zinc by Cadmium and Cobalt in Phytoplankton Isolated from the Lower Great Lakes

Low ambient concentrations of trace metals including Zn may limit phytoplankton productivity in the North American Great Lakes. The microalgae Chlorella sp. UTCC 522 (Chlorophyta) and Cyclotella sp. UTCC 520 (Heterokontophyta), indigenous to the Great Lakes, were assayed for their Zn requirement and the ability for Co and Cd to metabolically substitute for Zn under conditions in which Zn bioavailability was limiting cell growth. Bioassays were conducted in the laboratory using chemically defined media and the metal buffer EDTA to control the free-ion concentrations of Zn 2+ (10 −15.3−10 −10 mol/L), Cd 2+ (10 −15.1−10 −10 mol/L), and Co 2+ (10 −14−10 −10 mol/L). Influence of the metals on the phytoplankton was measured by the effect on specific growth rate. Both microalgae required Zn and could use Co and Cd as Zn substitutes; Cd was used less-effectively, although to a greater extent by the diatom Cyclotella than by the chlorophyte, Chlorella. The observed ability of the phytoplankton to use Cd and Co as Zn substitutes suggests that microalgae play an important role in the geochemical cycling of Zn, Cd and Co in large lakes, and the mobilization of Cd in lake ecosystems impacted by pollution.

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.

Phytoplankton growth and regulation in the Labrador Sea: light and nutrient limitation

The importance of light and nutrients in regulating phytoplankton growth in the Labrador Sea was evaluating using climatological data, 12 years of measurements made as part of the Labrador Sea Monitoring Program (LSMP) and physiological information from the literature. Light limits primary production and phytoplankton growth much of the year, even during summer when surface irradiance is at its seasonal peak. Nutrients, nitrogen (nitrate) and silicate, are reduced to low levels in surface waters in summer/autumn and can limit phytoplankton production and growth at this time of year. Nitrate appears to be the nutrient in shortest supply on the Labrador Shelf while silicate is in shortest supply in the central Labrador Basin. Multiyear trends in regional hydrography (increases in water temperatures, decreases in mixed layer depth) and changes in nitrate and silicate supply over the past decade (i.e. increases in nitrate and decreases in silicate) may be linked to changes in phytoplankton community composition and structure (i.e. decreases in large forms such as diatoms and increases in small picoplankton and nanoplankton forms).

The effect of iron availability on the regulation of inorganic carbon acquisition in the coccolithophore Emiliania huxleyi and the significance of cellular compartmentation for stable carbon isotope fractionation

Iron is limiting phytoplankton productivity in large parts of today’s oceans, the so-called HNLC (high nutrient low chlorophyll) areas. It is a key component in photosynthesis during which inorganic carbon fixation in most phytoplankton species is sustained by so-called carbon concentrating mechanisms (CCMs). Here we investigate CCM regulation in the coccolithophore Emiliania huxleyi in response to varying degrees of iron limitation by means of membrane-inlet mass spectrometry. Compared to iron replete conditions rates of both active CO 2 and HCO 3 - uptake were markedly reduced under iron limitation leading to significantly diminished growth rates. Moreover, there was a concomitant decrease in CCM efficiency, reflected in an increased CO 2 loss from the cell in relation to carbon fixation. Under such conditions higher values for carbon isotope fractionation ( ϵ p) would be expected. However, direct measurements of ϵ p showed that carbon isotope fractionation was insensitive to changes in growth rates and CCM activity. This can be explained by concomitant changes in internal DIC fluxes in and out of the chloroplast as demonstrated with a simple cell model comprising two compartments. Thus, carbon isotope fractionation reflects the ability of phytoplankton to actively control their inorganic carbon acquisition depending on environmental conditions. The insensitivity of carbon isotope fractionation to changes in the availability of iron could be of interest for paleoreconstructions in the HNLC areas of today’s oceans.

The iron CLAW

Environmental context. A ‘climate stabilising’ feedback system known as the CLAW hypothesis, which involves the phytoplankton driven influence on cloud reflectivity through the cycling of sulfur was proposed ~20 years ago, and because of its complexity, it remains unproven today. Since the CLAW proposal, experiments that have added iron to the ocean have proven that iron can significantly limit phytoplankton productivity and can also affect the marine sulfur cycle in a complex manner. Because of a range of possible feedbacks between iron, sulfur and climate, it is likely that future advances in understanding the CLAW hypothesis will require a comprehensive process-based description that can be tested in fully coupled earth-system models.

Iron in ice cores from Law Dome: A record of atmospheric iron deposition for maritime East Antarctica during the Holocene and Last Glacial Maximum

Total dissolvable iron (TDFe) was measured in sections of ice cores recovered from Law Dome on the coast of Wilkes Land, East Antarctica. These samples include ice dating from the Last Glacial Maximum (LGM), the Last Deglaciation, and the early and mid Holocene as well as samples from the Anthropocene that have been dated with seasonal to annual resolution. Combining our TDFe concentration data with estimates of the ice accumulation rate, we estimate the atmospheric iron deposition for Law Dome and the adjacent Southern Ocean during these periods. Our results indicate that the atmospheric iron deposition flux to this region during the LGM (∼8 μmol Fe m−2 yr−1) was an order of magnitude higher than the average Holocene deposition flux (∼0.8 μmol Fe m−2 yr−1). This Holocene flux estimate is significantly higher than recent estimates of atmospheric iron deposition based on the analysis of iron in samples of the Dome C EPICA ice core, implying that there are significant meridional gradients in eolian iron flux to the Antarctic region. Our data also suggest that there have been significant variations in atmospheric iron deposition to Law Dome and adjacent ocean waters over seasonal to decadal timescales during the past century. Analysis of ice samples dating from calendar years 1927 to 1928 indicates an anomalously high flux of TDFe to the Law Dome region, amounting to around half the maximum LGM flux, possibly as a result of severe drought conditions on the Australian continent. Given that chronic iron deficiency is thought to limit phytoplankton production in much of the ocean around Antarctica, such large secular variations in atmospheric iron supply are likely to have had profound impacts on year‐to‐year primary production and ecosystem structure in Antarctic waters.

Development of a decision tree model for the prediction of the limitation potential of phytoplankton in Lake Okeechobee, Florida, USA

Conducting long-term algal bioassays in large, complex systems such as Lake Okeechobee is an expensive and time-intensive undertaking, especially in comparison with physical and chemical monitoring. This paper describes a water quality-based decision tree model for predicting whether the phytoplankton in Lake Okeechobee is limited by light or nutrients. The model was developed and validated using the results of algal bioassays coupled with routinely monitored water quality data. Algal bioassays indicated that the factor most commonly limiting phytoplankton production in Lake Okeechobee for the period of October 1997 to November 2000 was light (59 %) followed by nitrogen (41 %). Limitation status of the phytoplankton was positively correlated with irradiance (in terms of Secchi depth/total depth) and phytoplankton biomass (in terms of chlorophyll-a) and negatively related to dissolved inorganic nitrogen and soluble reactive phosphorus concentrations. A cross-tabulation procedure was used to examine how the frequency of occurrence of light limitation and nutrient limitation varied as a function of these variables. The cross-tabulation procedure was also used to derive the empirical threshold values used to construct the model. This result supports both the accuracy of the derived critical threshold values and the validity of using chemical measurements in predicting whether light is limiting or nutrients are limiting in Lake Okeechobee. The model successfully predicted light limitation versus nutrient limitation in three independent validation data sets 70 % to 85 % of the time. When nutrient limiting conditions prevailed, the model did not successfully predict which nutrient (nitrogen, phosphorus, or a combination of nitrogen and phosphorus) was limiting. Our results suggest that the predictive abilities of the model could be enhanced by using time-specific data rather than averaged monthly data.

Interannual variation of particle fluxes in the eastern Bransfield Strait, Antarctica: A response to the sea ice distribution

Time-series sediment traps were deployed to investigate the temporal evolution of particle fluxes in the eastern Bransfield Strait over a 3-year period, from December 1998 to December 2001. Particle fluxes showed large seasonal and interannual variations. In 1999 and 2000, a seasonal trend of mass fluxes was characterized by highly elevated mass fluxes in the summer season, greater than 300 mg m −2 d −1, two orders of magnitude greater than winter mass fluxes. Such high summer fluxes were not observed in 2001, however, and seasonality was also significantly reduced. The absence of a high summer flux in 2001 might have been due to an abnormal expansion of sea ice to the trap mooring site during the 2001 summer season. The large interannual variation in particle flux is closely related to annual changes in sea ice cover in the eastern Bransfield Strait, which limits phytoplankton productivity in the surface waters. There is a lag of about 1 month between the surface water productivity and the export production in the eastern Bransfield Strait. The delay in the export production might have been due to the favorable environmental conditions for phytoplankton growth in the early summer when most primary production is stored in phytoplankton biomass, and few organic particles sink into the deep ocean. An average biogenic silica:organic carbon (Si:C) ratio of 1.5 was obtained from trap material collected at a depth of 678–1034 m, lower than the ratio obtained from the marginal winter sea ice zone of the Atlantic section and the Antarctic zone of the Pacific section. To produce a Si:C ratio of 1.5 at a depth of 678–1034 m from the surface water ratio of 0.52, at least 65% of the surface organic carbon must be remineralized during the descent to 678–1034 m. The 3-year mean of annual organic carbon fluxes measured in the eastern Bransfield Strait was 4.21±3.16 g C m −2, about two times higher than that of the central Bransfield Strait (2.84±3.33 g C m −2) and an order of magnitude higher than that of the western Bransfield Strait (0.35 g C m −2).

Primary productivity in the Arabian Sea: A synthesis of JGOFS data

Temporal and spatial variations in phytoplankton biomass exist in the Arabian Sea at all scales from the diurnal to the seasonal, and from fine to large scale. Phytoplankton physiological rate parameters and productivity measurements suggest that phytoplankton are not strongly limited by either irradiance or nutrient supply. Grazing activities, in most cases, match phytoplankton net photosynthetic rates and growth. The variations in biomass are therefore explained from variations in mixing at diurnal to seasonal scales, and spatial variations caused by upwelling near the Omani coast and the presence of mesoscale eddies. Vertical mixing regulates the supply of irradiance and nutrients, but vertical mixing is never deep enough to limit phytoplankton productivity, and nitrogen does not appear to be a factor limiting phytoplankton growth. Vertical mixing, however, also affects grazing by diluting micro-grazers along with phytoplankton. It is argued here that mixed layer deepening acts as a natural ‘dilution experiment’ that allows phytoplankton to escape grazing losses and grow, and thereby create the observed variability in phytoplankton biomass.

Iron resources and oceanic nutrients: advancement of global environment simulations

1. IntroductionIn recent years it has been discovered that tracenutrients and in particular iron (Fe) limit phytoplank-ton productivity in 40% of the global oceans. In thoseHigh Nutrient Low Chlorophyll (HNLC) surfacewaters there always remains an excess pool of majornutrients nitrate, phosphate and silicate. These beingin excess, it was hypothesised (Gran, 1931) and muchlater proved (Martin and Fitzwater, 1988; Buma et al.,1991) that iron was the key limiting factor, in additionto light and temperature. It is therefore more realisticto speak of co-limitation; iron, light and majornutrients in concert would limit phytoplankton growthin vast regions of the remaining 60% of surface waters(Blain et al., 2004). In the early 1990s, the Iron Age ofOceanography started (Coale et al., 1999). Allexperimental evidence on Fe (co)-limitation publisheduntil about 2001–2002 has recently been synthesisedin several reviews (De Baar and Boyd, 2000; De Baarand La Roche, 2003).The century-old paradigm of a single limitingfactor for some or all marine ecosystems has thusgiven way to the awareness of co-limitation byseveral nutrients simultaneously (De Baar, 1994),where grazing and auto-mortality losses furthermoreplay a very important role. Iron is a crucial tracenutrient regulating photosynthetic efficiency andelectron transport, hence the fixation rate of CO