Large Volumes Of Seawater Research Articles

Rapid removal and replacement of dissolved organic matter during circulation through ultramafic crust

Large volumes of seawater have passed through the rocky subseafloor throughout Earth's history. The scale of circulation is sufficiently large to impact the cycling of marine dissolved organic carbon (DOC), one of the largest pools of reduced carbon on Earth whose sources and sinks remain enigmatic, and to sequester carbon over geologic timescales. While the fate of DOC in numerous mafic systems has been examined, no previous reports are available on the less studied but still abundant ultramafic systems. We analyzed the concentration and composition of DOC from the Lost City hydrothermal field (30°N, Mid-Atlantic Ridge), a long-lived ultramafic system with minimal magmatic input. We show that per liter of seawater, more DOC is removed and a rate >650 times faster rate than in mafic ridge flank systems. Simultaneously, newly synthesized 14C-free organics are exported into the water column, adding a pre-aged component to the deep DOC pool. The sequestration of oceanic organic molecules onto minerals could partially account for the substantial total organic carbon present in ultramafic rocks, which is currently interpreted as evidence of chemoautotrophy or abiotic synthesis.

In Situ Capture and Real-Time Enrichment of Marine Chemical Diversity.

Analyzing the chemical composition of seawater to understand its influence on ecosystem functions is a long-lasting challenge due to the inherent complexity and dynamic nature of marine environments. Describing the intricate chemistry of seawater requires optimal in situ sampling. Here is presented a novel underwater hand-held solid-phase extraction device, I-SMEL (In Situ Marine moleculELogger), which aims to concentrate diluted molecules from large volumes of seawater in a delimited zone targeting keystone benthic species. Marine benthic holobionts, such as sponges, can impact the chemical composition of their surroundings possibly through the production and release of their specialized metabolites, hence termed exometabolites (EMs). I-SMEL was deployed in a sponge-dominated Mediterranean ecosystem at a 15 m depth. Untargeted MS-based metabolomics was performed on enriched EM extracts and showed (1) the chemical diversity of enriched seawater metabolites and (2) reproducible recovery and enrichment of specialized sponge EMs such as aerothionin, demethylfurospongin-4, and longamide B methyl ester. These EMs constitute the chemical identity of each targeted species: Aplysina cavernicola, Spongia officinalis, and Agelas oroides, respectively. I-SMEL concentrated sponge EMs from 10 L of water in a 10 min sampling time. The present proof of concept with I-SMEL opens new research perspectives in marine chemical ecology and sets the stage for further sustainable efforts in natural product chemistry.

Comparison of methods to determine extraction efficiencies of Ra isotopes and 227Ac from large volume seawater samples

Radium isotopes, other than 226Ra, and 227Ac are typically present at low activities in the open ocean. The analysis of these isotopes thus requires the collection of large volumes of seawater and high sensitivity, low background instruments. To obtain the required large volumes (hundreds to thousands of liters), these radionuclides are typically preconcentrated on cartridge-style filters impregnated with MnO2 (Mn-cartridges) deployed on in-situ pumps. This technique, however, requires the determination of the extraction efficiency of the Mn-cartridges for the radionuclides of interest. For Ra isotopes, we used two methods to estimate the extraction efficiency of these Mn-cartridges at two stations on the South-West Indian Ridge in the Southern Ocean (GEOTRACES GS02). Method (1) compares the 226Ra activities recovered on the Mn-cartridges versus the activities determined in Mn-fibers, through which seawater was passed at a flow rate < 1 L min−1 to quantitatively sorb Ra (Mn-fiber method) while method (2) combines the 226Ra activities determined from two Mn-cartridges placed in series on in-situ pumps (A-B method). The second method is also applied to determine the 227Ac extraction efficiency. We find a relatively wide-range of Ra and 227Ac extraction efficiencies across the dataset (from 44.8% to 99.6% for Ra, and from 23.7% to 77.5% for 227Ac). Overall, the yield of 227Ac extraction is lower than that of Ra (mean value of 49.3 ± 19.0% for 227Ac, n = 10, mean value of 79.2 ± 10.3% for Ra, n = 13, using the Mn-fiber method; and a mean value of 63.9 ± 12.5%, n = 11 using the A-B method). Our dataset suggests that the Ra extraction efficiencies using either the A-B method or the Mn-fiber method are in relatively good agreement. Consequently, the 223Raex, 224Raex and 228Ra activities determined from the Mn-cartridges by applying the two Ra extraction yields are similar. We also show that the 227Ac extraction efficiency can be estimated from the Ra extraction efficiency allowing the use of a single Mn-cartridge. Finally, we recommend to determine the Ra and 227Ac extraction efficiencies in each individual Mn-cartridge, rather than applying a single extraction efficiency to all the Mn-cartridges, since a significant variability in the extraction efficiencies was observed between the different Mn-cartridges.

Radionuclides' Recovery from Seawater Using FIC and FIC A Sorbents.

To solve radioecological and oceanological problems (estimate the vertical transport, flows of particulate organic carbon, phosphorus biodynamics, submarine groundwater discharge, etc.), it is necessary to determine the natural values of the radionuclides' activity in seawater and particulate matter. For the first time, the radionuclides' sorption from seawater was studied using sorbents based on activated carbon modified with iron(III) ferrocyanide (FIC) and based on activated carbon modified with iron(III) hydroxide (FIC A-activated FIC) obtained by FIC sorbent treatment with sodium hydroxide solution. The possibility of trace amounts of phosphorus, beryllium, and cesium recovery in laboratory conditions has been investigated. Distribution coefficients, dynamic, and total dynamic exchange capacities were determined. The physicochemical regularities (isotherm and kinetics) of sorption have been studied. The results obtained are characterized via Langmuir, Freindlich, and Dubinin-Radushkevich isotherm equations, as well as pseudo-first and pseudo-second-order kinetic models, intraparticle diffusion, and the Elovich model. Under expeditionary conditions, the sorption efficiency of 137Cs using FIC sorbent, 7Be, 32P, and 33P-using FIC A sorbent with a single-column method by adding a stable tracer, as well as the sorption efficiency of radionuclides 210Pb and 234Th with their natural content by FIC A sorbent in a two-column mode from large volumes of seawater was assessed. High values of efficiency of their recovery by the studied sorbents were achieved.

Multimodal image and spectral feature learning for efficient analysis of water-suspended particles.

We have developed a method to combine morphological and chemical information for the accurate identification of different particle types using optical measurement techniques that require no sample preparation. A combined holographic imaging and Raman spectroscopy setup is used to gather data from six different types of marine particles suspended in a large volume of seawater. Unsupervised feature learning is performed on the images and the spectral data using convolutional and single-layer autoencoders. The learned features are combined, where we demonstrate that non-linear dimensional reduction of the combined multimodal features can achieve a high clustering macro F1 score of 0.88, compared to a maximum of 0.61 when only image or spectral features are used. The method can be applied to long-term monitoring of particles in the ocean without the need for sample collection. In addition, it can be applied to data from different types of sensor measurements without significant modifications.

Phaeogromids of the mesopelagic marine plankton: Temporal variability of concentrations and observations of feeding structures of four species from the mesopelagic in the Mediterranean Sea.

Challengerids, phaeogromids rhizarian protists, are emblematic protists of the deep sea but are also enigmatic as they occur in very low concentrations. In previous studies, we reported on temporal changes in abundance at a near-shore mesopelagic site, but only as part of sampling of the entire microplankton assemblage, not well-suited for examining phaeogromids. Consequently, we turned to using a closing plankton net to provide material from large volumes of seawater, thus allowing for more robust estimates of concentrations and material for observations of living cells, to our knowledge the first made. Here, we report our results on the four most commonly occurring species: Challengeranium diadon, Challengereron willemoesii, Challengeria xiphodon, and Euphysetta lucani. In contrast to our previous report, we found that changes in concentrations were not related to water column stratification, and the four species roughly co-varied with time. Observations of live cells revealed that all four species deploy tentacle-like pseudopods and also very large unstructured webs of fine pseudopods. The similarities in feeding webs suggest similar prey are exploited, and the similar temporal changes in abundances suggest a common factor or factors (unknown at this time) govern their concentrations. Films of live cells are provided in Supplementary Files.

Determination of tritium in large volume of seawater using electrolytic enrichment and LSC and its application for the East China Sea water

Determination of tritium in large volume of seawater using electrolytic enrichment and LSC and its application for the East China Sea water

Sequential scavenging and measurement of seawater radiocesium concentrations and plutonium isotopic ratios offshore Fukushima

The scientific interest in radiocesium and plutonium found in the oceans and seas has increased enormously in the past years as a consequence of the Fukushima Dai-ichi Nuclear Power Plant (FDNPP) accident and is expected to be ongoing due to many unresolved questions. Hence, continuous development of new and verification of old analytical methods should be at the top of the list of the community, working on the topic. In this study, we processed and analyzed several seawater samples, collected in different time frames (2011–2015) from the North Pacific Ocean offshore Fukushima, to determine their radiocesium activities, 134Cs/137Cs activity ratios and 240Pu/239Pu isotopic ratios using the sequential scavenging method, gamma spectrometry and accelerator mass spectrometry (AMS). The observed radiocesium levels in seawater (0.07–0.042 Bq L−1) clearly indicated that the investigated region remained impacted by releases from the damaged power plant even after four years after the accident. Regarding plutonium, its successful separation from large volume seawater samples was confirmed by detection of 240Pu by AMS. However, several problems emerged during the analyzes, which we tried to address with the use of additional methods (e.g., measurements of uranium by ICPMS). The efficiencies of the applied methods and other issues are also discussed.

The KM3NeT potential for the next core-collapse supernova observation with neutrinos

The KM3NeT research infrastructure is under construction in the Mediterranean Sea. It consists of two water Cherenkov neutrino detectors, ARCA and ORCA, aimed at neutrino astrophysics and oscillation research, respectively. Instrumenting a large volume of sea water with sim {6200} optical modules comprising a total of sim {200{,}000} photomultiplier tubes, KM3NeT will achieve sensitivity to sim {10} mathrm{MeV} neutrinos from Galactic and near-Galactic core-collapse supernovae through the observation of coincident hits in photomultipliers above the background. In this paper, the sensitivity of KM3NeT to a supernova explosion is estimated from detailed analyses of background data from the first KM3NeT detection units and simulations of the neutrino signal. The KM3NeT observational horizon (for a 5,sigma discovery) covers essentially the Milky-Way and for the most optimistic model, extends to the Small Magellanic Cloud (sim {60} mathrm{kpc}). Detailed studies of the time profile of the neutrino signal allow assessment of the KM3NeT capability to determine the arrival time of the neutrino burst with a few milliseconds precision for sources up to 5–8 kpc away, and detecting the peculiar signature of the standing accretion shock instability if the core-collapse supernova explosion happens closer than 3–5 kpc, depending on the progenitor mass. KM3NeT’s capability to measure the neutrino flux spectral parameters is also presented.

Determination of low-level 135Cs and 135Cs/137Cs atomic ratios in large volume of seawater by chemical separation coupled with triple-quadrupole inductively coupled plasma mass spectrometry measurement for its oceanographic applications

Radioisotopes of cesium are powerful tracer for oceanographic studies. In this work, a novel method was developed for determination of ultra-low level 135Cs and 137Cs in seawater using triple-quadrupole inductively coupled plasma mass spectrometry (ICP-MS/MS). Cesium was pre-concentrated from up to 45 L seawater samples using ammonium molybdophosphate (AMP) adsorption, following a selective leaching of cesium using Sr(OH)2. The cesium was further purified from interfering elements using AMP-PAN and cation-exchange chromatography. Sr(OH)2 leaching was found to be an effective approach for selective exchange of cesium from the AMP sorbent without dissolution, which avoids the problem of separation of huge amount of NH4+ and MoO42− in the following steps. The decontamination factors for barium and rubidium with the developed method were more than 4 × 107 and 800, respectively. The separated 135Cs and 137Cs were measured using ICP-MS/MS by employing N2O as reaction gas to further elimination of isobaric (i.e. 135Ba and 137Ba) and polyatomic ions interferences. A detection limit of 1.5 × 10−16 g L−1 for 135Cs in seawater was achieved. The concentrations of 135Cs in seawater from Baltic Sea, Danish straits and Roskilde Fjord were determined using the developed method to identify the sources of 135Cs, the water masses exchange in this region was investigated using 135Cs and 137Cs.

Enhance Ocean Carbon Observations: Successful Implementation of a Novel Autonomous Total Alkalinity Analyzer on a Ship of Opportunity

Over recent decades, observations based on merchant vessels (Ships of Opportunity—SOOP) equipped with sensors measuring the CO2 partial pressure (pCO2) in the surface seawater formed the backbone of the global ocean carbon observation system. However, the restriction to pCO2 measurements alone is one severe shortcoming of the current SOOP observatory. Full insight into the marine inorganic carbon system requires the measurement of at least two of the four measurable variables which are pCO2, total alkalinity (TA), dissolved inorganic carbon (DIC), and pH. One workaround is to estimate TA values based on established temperature-salinity parameterizations, but this leads to higher uncertainties and the possibility of regional and/or seasonal biases. Therefore, autonomous SOOP-based TA measurements are of great interest. Our study describes the implementation of a novel autonomous analyzer for seawater TA, the CONTROS HydroFIAⓇ TA system (-4H-JENA engineering GmbH, Germany) for unattended routine TA measurements on a SOOP line operating in the North Atlantic. We present the installation in detail and address major issues encountered with autonomous measurements using this analyzer, e.g., automated cleaning and stabilization routines, and waste handling. Another issue during long-term deployments is the provision of reference seawater in large-volume containers for quality assurance measurements and drift correction. Hence, a stable large-volume seawater storage had to be found. We tested several container types with respect to their suitability to store seawater over a time period of 30 days without significant changes in TA. Only one gas sampling bag made of polyvinylidene fluoride (PVDF) satisfied the high stability requirement. In order to prove the performance of the entire setup, we compared the autonomous TA measurements with TA from discrete samples taken during the first two trans-Atlantic crossings. Although the measurement accuracy in unattended mode (about ± 5 μmol kg^-1) slightly deteriorated compared to our previous system characterization, its overall uncertainty fulfilled requirements for autonomous TA measurements on SOOP lines. A comparison with predicted TA values based on an established and often used parameterization pointed at regional and seasonal limitations of such TA predictions. Consequently, TA observations with better coverage of spatiotemporal variability are needed, which is now possible with the method described here.

Spatiotemporal variation of paralytic shellfish toxins in the sea area adjacent to the Changjiang River estuary

The Changjiang (Yangtze River) River estuary (CRE) and its adjacent coastal waters is a notable region for nutrient pollution, which results in severe problems of coastal eutrophication and harmful algal blooms (HABs). The occurrence of HABs, particularly those of dinoflagellate Alexandrium spp. capable of producing paralytic shellfish toxins (PSTs), has an increasing risk of contaminating seafood and poisoning human-beings. The investigation of PSTs, however, is often hampered by the relatively low abundance of Alexandrium spp. present in seawater. In this study, a monitoring strategy of PSTs using net-concentrated phytoplankton from a large volume of seawater was employed to examine spatiotemporal variations of PSTs in the CRE and its adjacent waters every month from February to September in 2015. Toxins in concentrated phytoplankton samples were analyzed using high-performance liquid chromatography coupled with a fluorescence detector (HPLC-FLD). The results showed that PSTs could be detected in phytoplankton samples during the sampling stage in the CRE and its adjacent waters. Toxin content increased gradually from February to May, reached the peak in June, and then decreased rapidly from July to September. The maximum value of PST content was 215 nmol m−3 in June. Low-potency toxins N-sulfocarbamoyl toxins 1/2 (C1/2) were the most dominant components of PST in phytoplankton samples from February to June in 2015, while high-potency gonyautoxin 4 (GTX4) became the dominant component from July to September. Toxins were mainly detected from three regions, the sea area north to the CRE, the sea area east to the CRE, and sea area near Zhoushan Island south to the CRE. Based on the results of this study, it can be inferred that the three regions around the CRE in May and June is of high risk for PST contamination and seafood poisoning.

Development, validation, comparison, and implementation of a highly efficient and effective method using magnetic solid-phase extraction with hydrophilic-lipophilic-balanced materials for LC-MS/MS analysis of pesticides in seawater

To achieve multi-pesticides residue analysis in seawater, hydrophilic-lipophilic-balanced magnetic particles were designed and fabricated by swelling polymerization of divinyl benzene (DVB) and N-vinyl pyrrolidone (NVP) on the surface of Fe3O4@SiO2 magnetic particles. The ratio of DVB to NVP was adjusted to achieve a proper balance in hydrophilicity and lipophilicity. The obtained magnetic particles were systematically characterized by TEM, SEM, FT-IR and vibrating sample magnetization. Based on the optimized magnetic nanoparticles, a sensitive magnetic solid-phase extraction method was developed for the simultaneous pre-concentration and determination of 96-pesticide residues from large-volume seawater samples prior to being detected by liquid chromatography-tandem mass spectrometry. Recoveries of pesticides in spiked seawater samples (0.001, 0.01, 0.1, 1.0 μg L−1) ranged from 62% to 112% with RSDs less than 21%. The method limits of detection of 96 pesticides ranged from 0.13 to 0.42 ng L−1, the method limits of quantification of 96 pesticides ranged from 1.0 to 10 ng L−1. The method was successfully applied to pesticide residue analysis in water samples from Jiulong River Estuary of China, demonstrating the prospects of this technique as a potential method for the rapid determination of trace levels of multi-pesticide residues in seawater.

Colloidal Organic Matter and Metal(loid)s in Coastal Waters (Gulf of Trieste, Northern Adriatic Sea)

Large volumes of seawater were sampled in the Gulf of Trieste (northern Adriatic Sea) in order to study the interactions between colloidal organic matter (COM) and metal(loid)s (Me) in coastal waters. COM (> 5 kDa) was isolated by ultrafiltration and characterized using 1H NMR spectroscopy and elemental Corg. and Ntot. analyses. COM in the gulf represents about one quarter of the dissolved organic carbon (DOC), and according to 1H NMR analysis, it is composed of polysaccharides (30–45%), lipids (30–55%), proteins and carboxyl-rich alicyclic molecules (CRAM) (15–20%), and humics (< 1%). An accumulation of COM was observed in the late spring–early summer. The polysaccharide and lipid fractions increased up to twofold and the protein fraction decreased, reflected in a higher Corg./Ntot. (28, molar) ratio. Higher concentrations of humics were observed due to local freshwater discharges in spring. COM from the Isonzo/Soca River differed from the marine COM exhibiting higher protein/CRAM and higher humic contents. COM from the Isonzo/Soca mouth at salinities 16–33 was compositionally similar to marine COM. Analysis of Me, performed by ICP-MS and CVAFS (Hg), showed that Hg (nearly 100%), Cu (20%), Cr (10%), and Se (10%) have the highest Me affinity to colloids. Similar to COM, Hg and Cu rapidly increased till summer due to their sequestration in accumulated COM (transfer to particulate phase). The observed Me/Corg. ratios (Co, Cd, Hg < U, Cr, Ni, Mn < As, Zn, Cu, V < Se, Al, Fe) differ somewhat from those of the Irving–Williams series and can be explained by the composition of COM and variable background concentrations of studied Me in the northern Adriatic. Data from the salinity gradient in the metal-contaminated (especially Hg, Pb, Zn) Isonzo/Soca mouth showed flocculation of Al and Ba and desorption of V, Co, As, Se, Cs, U, and Hg, from the riverine particles with increasing ionic strength, while Fe, Mn, Cu, Cr, Ni, Zn, Cd, and Pb did not correlate with salinity.

Offshore concentration of caesium radioisotopes from large volume seawater samples using KNiFC-PAN

A method for caesium concentration from North Sea and Baltic Sea seawater samples was tested and optimised for offshore concentration of radiocaesium and seawater volumes up to 150 L. The composite ion-exchanger PotassiumNickel Hexacyanoferrate in a Polyacrylnitrile binding matrix (KNiFC-PAN) with 80% of powdered KNiFC per gram of dry residue was used for this study. The optimised method achieved recoveries of around 99% with a bed volume (BV) of 50 mL of KNiFC-PAN and average flow rates of seawater of around 182 BV per hour (e.g. 9.1 L per hour).

Effect of seawater components on corrosion behavior of SUS316L in nitric acid solution containing metal ions

ABSTRACT During the accident that occurred at the Fukushima Daiichi nuclear power plant, a large volume of seawater was introduced as coolant into the storage pools for spent nuclear fuel. If this fuel is reprocessed, some components of seawater will be mixed with the nitric acid solution containing metal ions in the reprocessing process where stainless steels are used as structural material. In this study, we investigated the effect of seawater components in high active liquid waste (HAW) containing nitric acid and metal ions as fission products on the corrosion behavior of SUS316L stainless steel. Corrosion tests were conducted in surrogate HAW containing artificial seawater (ASW). Intergranular corrosion was observed in the HAW with ASW, where Ru increased the corrosion potential to the transpassive region. An increase in the amount of ASW led to a decrease in the corrosion rate and suppression of intergranular corrosion. Interactions between Ru ions and seawater components, such as chloride ions, were indicated by the results of extended X-ray absorption fine structure spectroscopy and cyclic voltammetry analyses of the solution containing ASW and HAW.

Determination of 137Cs in large volume seawater using Cu-hexacyanoferrate cartridge filters

A simple method to determine 137Cs in seawater has been developed based on the use of a Cu-hexacyanoferrate scavenger. The Cu-hexacyanoferrate supported on cotton wound cartridge filter was used to absorb 137Cs from seawater by passing large volumes over the cartridge filters with flowrate of 240 L hr-1. Results from the Cu-hexacyanoferrate method were proved acceptable for accuracy with bias below ± 20 % i.e. - 9.16 to + 18.55 % when compared with the traditional ammonium molybdophosphate pre-concentration method. This developed method is cost-effective and less time consuming. In addition it can be easily performed at sampling fields.

Numerical Techniques for the Analysis of HVDC Sea Electrodes

This paper presents an analysis of high-voltage direct current sea electrodes using different numerical techniques. The large volume of sea water that must be included in the analysis is represented using infinite (inclined layer model) and finite models (finite volume and hemispheroidal models). The comparison between the results obtained using these models is based on the potential rise of the electrode, the sea potential rise in the vicinity of the electrode, and the computational burden for utilizing each model. The analysis shows that the results obtained using finite models can predict the performance of the electrode more accurately and realistically.

Variation in Colloidal Organic Matter Composition and Aggregation in Coastal Waters (Gulf of Trieste, Northern Adriatic Sea)

Large volumes of seawater were sampled monthly from December 2011 to October 2012 in the southeastern part of the Gulf of Trieste (northern Adriatic Sea) in order to study the seasonal changes of colloidal organic matter (COM) concentrations, its origin and composition. Isolation of COM was performed by ultrafiltration with molecular weight cut-off membranes of 5 kDa and final desalinization by dialysis. COM was characterized using 1H NMR and FT-IR spectroscopy, elemental C and N and 13C and 15N isotope analyses and high-pressure size exclusion chromatography. COM represents about 1/4 of dissolved organic carbon (DOC) and appears to be mainly of marine origin. COM is mainly composed of polysaccharides, lipids, proteins and humics. FT-IR analysis indicated the presence of OH (carbohydrates and lipids), COOH (proteins and lipids) and NH (proteins) containing functional groups. Using the NMR technique, it was possible to semiquantitatively determine variations of four main biochemical constituents present in our COM samples. In the late spring - summer an accumulation of COM was observed, reaching up to 25 μmol L−1 Corg. and representing about 1/3 of DOC in the Gulf waters. The lipid fraction increased up to 2-fold and the polysaccharide fraction remained nearly constant while protein fraction decreased, reflected in a higher C/N (28) molar ratio. Also, higher concentrations of humics were observed in late spring – early summer probably due to local freshwater discharges in spring. An increase of lipid fraction and nearly constant polysaccharide content in late spring - early summer, in parallel with agglomeration of high molecular weight (>200 kDa) macromolecules, indicates the possible formation of macroaggregates, which has been periodically occurred in the northern Adriatic Sea in the past.

Underestimation of microzooplankton grazing in dilution experiments due to inhibition of phytoplankton growth

AbstractMicrozooplankton dilution grazing experiments conducted in phytoplankton rich waters, particularly in polar and subpolar seas, often result in calculation of nonsignificant or negative grazing coefficients. We hypothesized that preparation of filtered seawater (FSW) from water containing high biomass of phytoplankton results in release of allelochemicals that inhibit phytoplankton growth, lowering the net growth of phytoplankton in the more diluted treatments. We tested this hypothesis during blooms of Skeletonema marinoi and Phaeocystis pouchetii in a nutrient‐enriched mesocosm in the Raunefjord, Norway. During the S. marinoi bloom, inhibition of phytoplankton growth occurred in the diluted treatments. Simultaneously the concentration of total as well as dissolved polyunsaturated aldehydes (PUAs) was elevated. Passage of the FSW through a carbon cellulose cartridge to remove dissolved organic material reduced, or eliminated, the inhibition. In the early phase of the P. pouchetii bloom that followed the diatom bloom in the mesocosm, PUA concentration was relatively low and the untreated FSW had a less drastic, but often significant, inhibitory effect on phytoplankton growth. Laboratory experiments with cultures of S. marinoi and P. pouchetii confirmed that material present in filtrate prepared from diluted cultures was self‐inhibitory. Many phytoplankters, particularly during late stages of a bloom, produce inhibitory metabolites that may be released during filtration of the relatively large volumes of seawater needed for dilution experiments. Under some conditions, dilution grazing experiments may underestimate phytoplankton growth coefficients and microzooplankton grazing coefficients.