Effects of light on nitrogen and carbon uptake during a Prorocentrum minimum bloom

During a 4-week period in late spring 1998 an extensive Prorocentrum minimum (Pavillard) Schiller bloom developed in several tributaries of the Chesapeake Bay. Experiments were carried out in one of these tributaries using ¹³C and ¹⁵N isotopic techniques to characterize C and N uptake as a function of irradiance during the course of this bloom. Uptake rates of N substrates (NO₃⁻, NH₄⁺, urea, and an amino acid mixture) and C substrates (bicarbonate and urea) were measured. For each N substrate, short-term uptake rates (0.5 h) were not substantially different over the irradiance range measured, suggesting that N uptake of this dinoflagellate was not strongly light-dependent over this time scale. Dark uptake rates of all N substrates ranged between 35 and 113% of light uptake rates. Over the duration of the P. minimum bloom, however, total ambient N uptake rates increased with increasing natural irradiance. Uptake of bicarbonate showed typical light-dependent photosynthetic characteristics and the measured photosynthetic parameters suggested that at least on the short time scale (0.5 h), P. minimum cells were adapted to high light. Rates of C uptake from the substrate urea were minimal, <1% of total C uptake from photosynthesis, but doubled over the course of the bloom, and like N uptake, were not strongly light-dependent on the short time scale (0.5 h). Significant N dark uptake by P. minimum was likely to have been important by providing N sources over the daily scale to sustain the bloom.     

Use of a real-time remote monitoring network (RTRM) and shipborne sampling to characterize a dinoflagellate bloom in the Neuse Estuary, North Carolina, USA

The spatial-temporal distribution of a dinoflagellate bloom dominated or co-dominated by Prorocentrum minimum was examined during autumn through early spring in a warm temperate, eutrophic estuary. The developing bloom was first detected from a web-based alert provided by a network of real-time remote monitoring (RTRM) platforms indicating elevated dissolved oxygen and pH levels in upper reaches of the estuary. RTRM data were used to augment shipboard sampling, allowing for an in-depth characterization of bloom initiation, development, movement, and dissipation. Prolonged drought conditions leading to elevated salinities, and relatively high nutrient concentrations from upstream inputs and other sources, likely pre-disposed the upper estuary for bloom development. Over a 7-month period (October 2001–April 2002), the bloom moved toward the northern shore of the mesohaline estuary, intensified under favorable conditions, and finally dissipated after a major storm. Bloom location and transport were influenced by prevailing wind structure and periods of elevated rainfall. Chlorophyll a within bloom areas averaged 106 ± 13 μg L⁻¹ (mean ± 1 S.E.; maximum, 803 μg L⁻¹), in comparison to 20 ± 1 μg L⁻¹ outside the bloom. There were significant positive relationships between dinoflagellate abundance and TN and TP. Ammonium, NO₃⁻, and SRP concentrations did not decrease within the main bloom, suggesting that upstream inputs and other sources provided nutrient-replete conditions. In addition, PAM fluorometric measurements (09:00–13:00 h) of maximal PSII quantum yield (F_v/F_m) were consistently 0.6–0.8 within the bloom until late March, providing little evidence of photo-physiological stress as would have been expected under nutrient-limiting conditions. Nitrogen uptake kinetics were estimated for P. minimum during the period when that species was dominant (October–December 2001), based on literature values for N uptake by an earlier P. minimum bloom (winter 1999) in the Neuse Estuary. The analysis suggests that NH₄⁺ was the major N species that supported the bloom. Considering the chlorophyll a concentrations during October and December and the estimated N uptake rates, phytoplankton biomass was estimated to have doubled once per day. Bloom displacement (January–February) coincided with higher diversity of heterotrophic dinoflagellate species as P. minimum abundance decreased. This research shows the value of RTRM in bloom detection and tracking, and advances understanding of dinoflagellate bloom dynamics in eutrophic estuaries.     

INFLUENCE OF UV-B RADIATION ON NITROGEN UTILIZATION BY A NATURAL ASSEMBLAGE OF PHYTOPLANKTON

A 7‐day mesocosm experiment was conducted in July 1996 to investigate the effects of ambient UV‐B radiation (UVBR) exclusion and two UVBR enhancements above ambient levels on NO₃^?, NH₄⁺ and urea utilization in a natural plankton community (<240 μm) from the Lower St. Lawrence Estuary. The phytoplankton community was dominated by diatoms during the first 3 days and, afterward, by flagellates and dinoflagellates. The results of 4‐h incubations just below the water surface show that, compared with ambient UVBR conditions, UVBR exclusion generally increased NO₃^?, NH₄⁺, and urea uptakes. During the last 4 days of the experiment, the percent increase in the specific uptake rate of urea under excluded UVBR conditions varied between 17% and 130% and was a linear function of the ambient UVBR dose removed. During the first 3 days, the phytoplankton community dominated by diatoms was able to withstand UVBR enhancements without any perceptible effect on nitrogen uptake. However, during the post‐diatom bloom period, UVBR enhancements resulted in decreases in NO₃^?, NH₄⁺, and urea uptake compared with ambient UVBR conditions. The reduction of urea uptake under UVBR enhancements during the last 3 days varied between 23% and 64% and was linearly related to the enhanced UVBR dose. However, the different UVBR treatments did not affect the internal organic nitrogen composition (internal urea, free amino acids, and proteins) of the phytoplankton community experiencing vertical mixing in the mesocosms. The discrepancy between short‐term uptake measurements at the surface and long‐term effects in the mesocosms emphasizes the importance of vertical mixing on UVBR effects in natural ecosystems. This suggests that an increase in ambient UVBR would have a minimal effect on nitrogen utilization by natural phytoplankton assemblages if these are vertically mixed.     

Nutrient limitation of Microcystis aeruginosa in northern California Klamath River reservoirs

Nutrient limitations were investigated in Copco and Iron Gate Reservoirs, on the Klamath River in California, where blooms of the toxin-producing cyanobacterium Microcystis aeruginosa were first reported in 2005. Nutrient enrichment experiments conducted in situ in June and August, 2007 and 2008, determined responses in phytoplankton biomass, Microcystis abundance and microcystin concentration to additions of phosphorus and different forms of nitrogen (NH₄⁺, NO₃, and urea). Microcystis abundance was determined using quantitative PCR targeting the phycocyanin intergenic spacer cpcBA.Total phytoplankton biomass increased with additions of N both before and during Microcystis blooms, with no primary effects from P, suggesting overall N limitation for phytoplankton growth during the summer season. NH₄⁺ generally produced the greatest response in phytoplankton growth, while Microcystis abundance increased in response to all forms of N. Microcystis doubling time in the in situ experiments was 1.24–1.39 days when N was not limiting growth. The results from this study suggest availability of N during the summer is a key growth-limiting factor for the initiation and maintenance of toxic Microcystis blooms in Copco and Iron Gate Reservoirs in the Klamath River.     

Differential Drawdown of Ammonium,Nitrate, and Urea by Freshwater Chlorophytes and Cyanobacteria1

The chemical form of nitrogen (N) is deemed to be decisive in shaping the composition of the primary producer community. Recently, there has been a shift in the dominant form of N delivered to agricultural landscapes. Urea-based fertilizers are a mainstay in modern agriculture, and their ubiquitous use has increased the likelihood of urea export to nearby freshwaters. The shift to urea fertilizers has coincided with the recent expansion of cyanobacteria harmful algal blooms (cyanoHABs). This study investigated N drawdown patterns between two major freshwater phytoplankton groups—chlorophytes and cyanobacteria. Experiments were designed to understand if different patterns of N drawdown occurred among taxa and the potential synergistic effects of multiple N substrates. Nitrate (NO₃⁻), ammonium (NH₄⁺), and urea were supplied in a series of paired combinations, and N concentrations were monitored to track N drawdowns. We did not find significant differences between phytoplankton classes when supplied with a single N substrate. However, we found that when N substrates were supplied in combination, significant differences in N drawdown patterns were observed. Urea was consumed more rapidly among cyanobacteria, being drawn down at significantly higher rates relative to inorganic N substrates. In contrast, inorganic N substrates were drawn down more rapidly among chlorophytes relative to urea. Our findings support the emerging urea–cyanoHAB link and the potential importance of urea in freshwater eutrophication. As society becomes increasingly dependent on urea for agricultural crops, the need to understand how urea influences phytoplankton community composition may be instrumental in predicting bloom dynamics.     

CHARACTERIZATION OF NITROGEN UPTAKE BY NATURAL POPULATIONS OF AUREOCOCCUS ANOPHAGEFFERENS (CHRYSOPHYCEAE) AS A FUNCTION OF INCUBATION DURATION,SUBSTRATE CONCENTRATION,LIGHT, AND TEMPERATURE1

Nitrogen uptake studies were conducted during an aestival “brown tide” bloom in Shinnecock Bay, Long Island, New York. The same station was sampled in late July and mid-August 1995 when Aureococcus anophagefferens composed >90% and 30–40% of the total cell density, respectively. Experiments were designed to examine the effect of incubation duration on the uptake kinetics, and the effect of light and temperature dependencies of NH₄⁺, urea, and NO₃^? uptake. Maximum specific uptake rates (V'_max) decreased in the order NH₄⁺, urea, NO₃^? and were nonlinear with time for NH₄⁺ and urea, both of which exhibited an exponential decline between 1 and 10 min and then did nut significantly change for 60 min. Nitrogen uptake kinetic experiments exhibited a typical hyperbolic response for urea and NO₃^?. Half-saturation constants. (K_s) were calculated to he 0.03 and 0.12 μmol · L^?1 for urea and NO₃^?; respectively, but could not be calculated for NH₄⁺ under these experimental conditions. Nutrient uptake rate versus, irradiance (NI) experiments showed that maximum uptake rates occurred at ≤% of incident irradiance on both sampling dates and that values of V′_max-cell (NH₄⁺) were on average 30% greater than V′_max-cell (urea). A7°–9°C temperature decrease in incubation temperature between the two NI experiments in August resulted in a 30% decrease in V′_max-cell(NH₄⁺), no change in V′_max-cell(urea), and a 3–4-fold decrease in calculated K_lt values for both NH₄⁺ and urea. The results from these experiments demonstrate that A. anophagefferens has a higher affinity for NH₄⁺ and urea than for NO₃^? and that this particular species is adapted to use these substrates at low irradiances and concentrations. The data presented in this study are also consistent with the hypothesis that A. anophagefferens may be an oceanic clone that was displaced by an anomalous oceanographic event.     

Prorocentrum minimum blooms: potential impacts on dissolved oxygen and Chesapeake Bay oyster settlement and growth

The cosmopolitan dinoflagellate Prorocentrum minimum is a recurrent bloom forming species in the Chesapeake Bay and its tributaries, generally observed at its highest levels in late spring and summer. Laboratory studies were conducted to assess potential bloom impacts on diel oxygen concentrations in shallow littoral zones as well as settlement success and post-set growth of the eastern oyster Crassostrea virginica. Using light–dark and dark cultures and periodic diel sub-sampling, bloom levels of P. minimum produced supersaturated oxygen levels at the end of each day while darkened cultures were typified by rapid decreases in dissolved oxygen (DO) (1.1–1.3 mg L⁻¹ h⁻¹) to hypoxic and anoxic levels within 4 days. These data suggest shallow, poorly flushed systems and the biota in them will experience rapid and large diel variations in oxygen, implying recurrent P. minimum blooms need be considered as short-term oxygen stressors for Bay oyster spat and other living resources. Direct effects of P. minimum impacts on oysters were not as expected or previously reported. In one experiment, pre-bloom isolates of P. minimum were grown and then exposed to polyvinyl chloride (PVC) settlement plates to see whether dinoflagellate preconditioning of the hard substrate might affect oyster sets. No differences were noted between set on the PVC with P. minimum exposure to set recorded with filtered seawater, Instant Ocean^®, or Isochrysis. In the second oyster experiment, spat on PVC plates were exposed to field collected P. minimum blooms and a commercial mixture of several other food types including Isochrysis. Oyster growth was significantly higher in P. minimum exposures than noted in the commercial mix. These results, compared to results with other isolates from the same region, indicate substantial positive impact from some of the P. minimum blooms of the area while others separated in space, time, or nutrient status could severely curtail oyster success through toxin production induced by nutrient limitation.     

Nitrogen utilization by the raphidophyte Heterosigma akashiwo: Growth and uptake kinetics in laboratory cultures

The nitrogen uptake and growth capabilities of the potentially harmful, raphidophycean flagellate Heterosigma akashiwo (Hada) Sournia were examined in unialgal batch cultures (strain CCMP 1912). Growth rates as a function of three nitrogen substrates (ammonium, nitrate and urea) were determined at saturating and sub-saturating photosynthetic photon flux densities (PPFDs). At saturating PPFD (110 μE m⁻² s⁻¹), the growth rate of H. akashiwo was slightly greater for cells grown on NH₄⁺ (0.89 d⁻¹) compared to cells grown on NO₃⁻ or urea, which had identical growth rates (0.82 d⁻¹). At sub-saturating PPFD (40 μE m⁻² s⁻¹), both urea- and NH₄⁺-grown cells grew faster than NO₃⁻-grown cells (0.61, 0.57 and 0.46 d⁻¹, respectively). The N uptake kinetic parameters were investigated using exponentially growing batch cultures of H. akashiwo and the ¹⁵N-tracer technique. Maximum specific uptake rates (V_max) for unialgal cultures grown at 15 °C and saturating PPFD (110 μE m⁻² s⁻¹) were 28.0, 18.0 and 2.89 × 10⁻³ h⁻¹ for NH₄⁺, NO₃⁻ and urea, respectively. The traditional measure of nutrient affinity—the half saturation constants (K_s) were similar for NH₄⁺ and NO₃⁻ (1.44 and 1.47 μg-at N L⁻¹), but substantially lower for urea (0.42 μg-at N L⁻¹). Whereas the α parameter (α = V_max/K_s), which is considered a more robust indicator for substrate affinity when substrate concentrations are low (<K_s), were 19.4, 12.2 and 6.88 × 10⁻³ h⁻¹/(μg-at N L⁻¹) for NH₄⁺, NO₃⁻ and urea, respectively. These laboratory results demonstrate that at both saturating and sub-saturating N concentrations, N uptake preference follows the order: NH₄⁺ > NO₃⁻ > urea, and suggests that natural blooms of H. akashiwo may be initiated or maintained by any of the three nitrogen substrates examined.     

Nitrogen uptake and regeneration (ammonium regeneration,nitrification and photoproduction) in waters of the West Florida Shelf prone to blooms of Karenia brevis

The West Florida Shelf (WFS) encompasses a range of environments from inshore estuarine to offshore oligotrophic waters, which are frequently the site of large and persistent blooms of the toxic dinoflagellate, Karenia brevis. The goals of this study were to characterize the nitrogen (N) nutrition of plankton across the range of environmental conditions on the WFS, to quantify the percentage of the plankton N demand met through in situ N regeneration, and to determine whether planktonic N nutrition changes when high concentrations of Karenia are present. In the fall of 2007, 2008, and 2009 we measured ambient nutrient concentrations and used stable isotope techniques to measure rates of primary production and uptake rates of inorganic N (ammonium, NH₄⁺, and nitrate, NO₃⁻), and organic N and carbon (C; urea and amino acids, AA) in estuarine, coastal, and offshore waters, as well as coastal sites with Karenia blooms present. In parallel, we also measured rates of in situ N regeneration – NH₄⁺ regeneration, nitrification, and photoproduction of NH₄⁺, nitrite and AA. Based on microscope observations, ancillary measurements, and previous monitoring history, Karenia blooms sampled represented three bloom stages – initiation in 2008, maintenance in 2007, and late maintenance/stationary phase in 2009. Nutrient concentrations were highest at estuarine sampling sites and lowest at offshore sites. Uptake of NH₄⁺ and NO₃⁻ provided the largest contribution to N nutrition at all sites. At the non-Karenia sites, in situ rates of NH₄⁺ regeneration and nitrification were generally sufficient to supply these substrates equal to the rates at which they were taken up. At Karenia sites, NO₃⁻ was the most important N substrate during the initiation phase, while NH₄⁺ was the most important N form used during bloom maintenance and stationary phases. Rates of NH₄⁺ regeneration were high but insufficient (85 ± 36% of uptake) to support the measured NH₄⁺ uptake at all the Karenia sites although nitrification rates far exceeded uptake rates of NO₃⁻. Taken together our results support the “no smoking gun” nutrient hypothesis that there is no single nutrient source or strategy that can explain Karenia's frequent dominance in the waters where it occurs. Consistent with other papers in this volume, our results indicate that Karenia can utilize an array of inorganic and organic N forms from a number of N sources.     

Nutrient uptake and primary productivity in an urban estuary: using rate measurements to evaluate phytoplankton response to different hydrological and nutrient conditions

The northern San Francisco Estuary (nSFE) is an urban estuary supplied with anthropogenic nutrient inputs, yet spring blooms are uncommon and phytoplankton biomass is low. The low levels of chlorophyll (<5 µg L^?1) have likely contributed to declines in several native fishes, and there is a need to evaluate the conditions that could allow for increased phytoplankton. Increased ammonium (NH₄) loads have been hypothesized to modulate the magnitude of blooms in nSFE (the “NH₄ hypothesis”) as a result of inhibition of phytoplankton NO₃ uptake that limits access to the greater nitrogen (N) pool of nitrate (NO₃). This hypothesis, tested in enclosures, but not in the field until now, is that lack of access to NO₃ limits primary production and consequently the accumulation of chlorophyll. Here, we test this in the field with the following aims: (1) to observe the uptake response of phytoplankton in different flow and N loading conditions, (2) determine whether the sequence of uptake rates suggested by the “NH₄ hypothesis” occurs and (3) obtain depth-integrated nutrient uptake rates to better constrain published criteria for bloom formation. Weekly measurements of NH₄ and NO₃ uptake, and primary production rates were made during spring 2011–2012, along with nutrient and chlorophyll concentrations during two contrasting hydrological conditions of high vs low freshwater flow. In conditions with high freshwater flow (maximum of 2405 m³ s^?1), there were lower nutrient concentrations than with low/normal flows (e.g., NO₃ of 10 µmol L^?1 compared to 30 µmol L^?1), with low N uptake and primary production rates. With low flow (maximum of 1304 m³ s^?1), there was elevated chlorophyll and blooms occurred, especially in shallow well-lit shoals where chlorophyll reached 60 µg L^?1. The higher levels of chlorophyll and primary productivity resulted from uptake of ambient NO₃ by phytoplankton, and f-ratios >0.5. This was enabled by phytoplankton uptake of NH₄ to below inhibitory levels, as proposed by the “NH₄ hypothesis.” The depth-integrated uptake rate data were used to refine a model that yields flow and nutrient concentration criteria necessary for bloom formation and confirmed that washout flows were the most useful predictor of blooms. Understanding the interaction of phytoplankton biomass with nutrient variability requires evaluating changes in C and N uptake rates and river flow. These dynamic changes are central to understanding why some urban estuaries have lower productivity than expected, and would be difficult to evaluate using biomass data alone. This study points to the importance of treating inorganic N separately as NH₄ and NO₃ rather than lumping together as DIN and to use rate process data as a mechanistic way to understand, predict and minimize cultural eutrophication impacts.     

Using quantitative real-time PCR to study competition and community dynamics among Delaware Inland Bays harmful algae in field and laboratory studies

The Delaware Inland Bays (DIB) have experienced harmful algal blooms of dinoflagellates and raphidophytes in recent years. We used quantitative polymerase chain reaction (QPCR) techniques to investigate the community dynamics of three DIB dinoflagellates (Karlodinium veneficum, Gyrodinium instriatum, and Prorocentrum minimum) and one raphidophyte (Heterosigma akashiwo) at a single site in the DIB (IR-32) in summer 2006 relative to salinity, temperature and nutrient concentrations. We also carried out complementary laboratory culture studies. New primers and probes were developed and validated for the 18S rRNA genes in the three dinoflagellates. K. veneficum, H. akashiwo, and G. instriatum were present in almost all samples throughout the summer of 2006. In contrast, P. minimum was undetectable in late June through September, when temperatures ranged from 20 to 30 °C (average 25.7 °C). Dissolved nutrients ranged from 0.1 to 2.8 μM PO₄³⁻ (median = 0.3 μM), 0.7–30.2 μM NO_x (median = 12.9 μM), and 0–19.4 μM NH₄⁺ (median = 0.7 μM). Dissolved N:P ratios covered a wide range from 2.6 to 177, with a median of 40. There was considerable variability in occurrence of the four species versus nutrients, but in general P. minimum and H. akashiwo were most abundant at higher (>40) N:P ratios and dissolved nitrogen concentrations, while K. veneficum and G. instriatum were most abundant at low dissolved N:P ratios (<20) and dissolved nitrogen concentrations < 10 μM. The semi-continuous laboratory competition experiment used mixed cultures of K. veneficum, P. minimum, and H. akashiwo grown at dissolved N:P ratios of 5, 16, and 25. At an N:P of 16 and 25 P. minimum was the dominant alga at the end of the experiment, even at a temperature that was much higher than that at which this alga was found to bloom in the field (27 °C). P. minimum and H. akashiwo had highest densities in the N:P of 25. K. veneficum grew equally well at all three N:P ratios, and was co-dominant at times at an N:P of 5. H. akashiwo had the lowest densities of the three algae in the laboratory experiment. Laboratory and field results showed both interesting similarities and significant differences in the influences of important environmental factors on competition between these harmful algal species, suggesting the need for more work to fully understand HAB dynamics in the DIB.     

Nutrient limitation of Microcystis aeruginosa in northern California Klamath River reservoirs

Nutrient limitations were investigated in Copco and Iron Gate Reservoirs, on the Klamath River in California, where blooms of the toxin-producing cyanobacterium Microcystis aeruginosa were first reported in 2005. Nutrient enrichment experiments conducted in situ in June and August, 2007 and 2008, determined responses in phytoplankton biomass, Microcystis abundance and microcystin concentration to additions of phosphorus and different forms of nitrogen (NH₄⁺, NO₃, and urea). Microcystis abundance was determined using quantitative PCR targeting the phycocyanin intergenic spacer cpcBA.Total phytoplankton biomass increased with additions of N both before and during Microcystis blooms, with no primary effects from P, suggesting overall N limitation for phytoplankton growth during the summer season. NH₄⁺ generally produced the greatest response in phytoplankton growth, while Microcystis abundance increased in response to all forms of N. Microcystis doubling time in the in situ experiments was 1.24–1.39 days when N was not limiting growth. The results from this study suggest availability of N during the summer is a key growth-limiting factor for the initiation and maintenance of toxic Microcystis blooms in Copco and Iron Gate Reservoirs in the Klamath River.     

Effects of a Prorocentrum minimum bloom on light availability for and potential impacts on submersed aquatic vegetation in upper Chesapeake Bay

Extraordinary spring blooms of the dinoflagellate Prorocentrum minimum have been a recurring feature of upper Chesapeake Bay for many years. Though not thought to be toxic in Chesapeake Bay, these blooms produce extraordinarily high concentrations of chlorophyll, thereby increasing light attenuation. A particularly large event occurred in the spring of 2000. Here, we assess the impact of the spring 2000 P. minimum bloom on habitat quality for submerged aquatic vegetation (SAV) in the mesohaline region of Chesapeake Bay and its tributaries. We determined the light absorption and scattering spectrum of P. minimum on a per cell basis by analyzing inherent optical properties of natural samples from the Rhode River, Maryland, which were overwhelmingly dominated by P. minimum. Using these per cell properties, we constructed a model of light penetration incorporating observed cell counts of P. minimum to predict the impact of the bloom on other tributaries and main stem locations that experienced the bloom. Model estimates of diffuse attenuation coefficients agreed well with the limited measurements that were available. Impacts of the mahogany tide on diffuse attenuation coefficient ranged from negligible (10–30% increase above the seasonal median in the Patapsco and Magothy rivers), to a greater than six-fold increase (Potomac River). Attenuation coefficients in tributaries to the north and south of the bloom region either decreased or were unchanged relative to seasonal medians. Segments with SAV losses in 2000 were mostly the same as those that experienced the P. minimum bloom. Segments north and south of the bloom area mostly had SAV increases in 2000. Though all of the segments that experienced a decline in SAV area after the spring 2000 bloom showed an increase in 2002, the 2000 setback interrupted what otherwise has been a slow recovery in mid-Bay SAV, demonstrating the adverse impact of P. minimum blooms on SAV populations in Chesapeake Bay.     

Nitrogen metabolism by heterotrophic bacterial assemblages in Antarctic coastal waters

Field studies to examine the in situ assimilation and production of ammonium (NH₄ ⁺) by bacterial assemblages were conducted in the northern Gerlache Strait region of the Antarctic Peninsula. Short term incubations of surface waters containing ¹⁵N-NH₄ ⁺ as a tracer showed the bacterial population taking up 0.041–0.128 g-atoms Nl^–1d^–1, which was 8–25% of total NH₄ ⁺ uptake rates. The large bacterial uptake of NH₄ ⁺ occurred even at low bacterial abundance during a rich phytoplankton bloom. Estimates of bacterial production using ³H-leucine and -adenine were l.0gCl^–1 d^–1 before the bloom and 16.2 g Cl^–1 d^–1 at the bloom peak. After converting bacterial carbon production to an estimate of nitrogen demand, NH₄ ⁺ was found to supply 35–60% of bacterial nitrogen requirements. Bacterial nitrogen demand was also supported by dissolved organic nitrogen, generally in the form of amino acids. It was estimated, however, that 20–50% of the total amino acids taken up were mineralized to NH₄ ⁺. Bacterial production of NH₄ ⁺ was occurring simultaneously to its uptake and contributed 27–55% of total regenerated NH₄ ⁺ in surface waters. Using a variety of ¹⁵N-labelled amino acids it was found that the bacteria metabolized each amino acid differently. With their large mineralization of amino acids and their relatively low sinking rates, bacteria appear to be responsible for a large portion of organic matter recycling in the upper surface waters of the coastal Antarctic ecosystem.     

Impacts and potential effects due to Prorocentrum minimum blooms in Chesapeake Bay

The dinoflagellate Prorocentrum minimum (P. minimum) can be found in all seasons and over a broad range of habitat conditions in the Chesapeake Bay and its tributaries. Blooms (>3000 cells ml⁻¹), locally referred to as ‘mahagony tides’, were restricted to salinities of 4.5–12.8 psu, water temperatures of 12–28 °C, and occurred most frequently in April and May. P. minimum blooms have been detected at routine water quality monitoring stations located in the main channel of the Bay and tidal tributaries. Nearshore investigations of bloom events, however, have accounted for the majority of events recorded in excess of 10⁵ cells ml⁻¹. Mahogany tides were associated with widespread harmful impacts including anoxic/hypoxic events, finfish kills, aquaculture shellfish kills and submerged aquatic vegetation losses. We summarize the state of knowledge regarding physical and chemical factors related to P. minimum blooms, their abundance, distribution and frequency, and ecological effects in Chesapeake Bay.     

Stable isotopes as indicators for seasonally dominant nitrogen cycling processes in a subarctic lake

Nitrogen stable isotopes (δ¹⁵N) of dissolved inorganic nitrogen (DIN = NH₄⁺ and NO₃^–), dissolved organic nitrogen (DON), and particulate organic nitrogen (PON) were measured in Smith Lake, Alaska to assess their usefulness as proxies for the biological nitrogen cycling processes, nutrient concentration, and lake productivity. Large seasonal variations in δ¹⁵NH₄⁺, δ¹⁵NO₃^– and δ¹⁵N_PON occurred in response to different processes of nitrogen transformation that dominated a specific time period of the annual production cycle. In spring, ¹⁵N depletion in all three pools was closely related to the occurrences of a N₂‐fixing cyanobacterial bloom (Anabaena flos‐aquae). In summer, δ¹⁵N_PON increased as phytoplankton community shifted to use NH₄⁺ and decreased as a brief N₂‐fixing bloom (Aphanizomenon flos‐aquae) occurred in August. In early and mid‐winter, microbial nitrogen processes were dominated by nitrification that resulted in the largest isotope fractionation between NO₃^– and NH₄⁺ in the annual cycle. This was followed by denitrification that led to the highest ¹⁵N enrichment in NO₃^–. A peak of NH₄⁺ assimilation by phytoplankton along with the elevated δ¹⁵N_PON and Chl a concentration occurred just before the ice break due to increased light penetration. The δ¹⁵N_DON displayed little temporal and spatial variations. This suggests that the DON pool was not altered by biological transformations of nitrogen as the results of its large size and possibly refractory nature. There was a positive correlation between Chl a concentration and δ¹⁵N_PON, and a negative correlation between NH₄⁺ and δ¹⁵N_PON, suggesting that δ¹⁵N_PON is a useful proxy for nitrogen productivity and ammonium concentration. (© 2012 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Feeding by ciliates on two harmful algal bloom species, Prymnesium parvum and Prorocentrum minimum

We have focused on ciliates as potential grazers on toxic phytoplankton because they are major herbivores in aquatic food webs. Ciliates may exert top down control on toxic phytoplankton blooms, potentially suppressing or shortening the duration of harmful algal blooms (HABs). We measured the growth rates of several ciliate species on uni-algal and mixed diets of both HAB and non-HAB algae. The tintinnids Favella ehrenbergii, Eutintinnus pectinis and Metacylis angulata and the non-loricate ciliates Strombidinopsis sp. and Strombidium conicum were isolated from Long Island Sound (LIS), and fed HAB species including the prymnesiophyte Prymnesium parvum (strain 97-20-01) and the dinoflagellate Prorocentrum minimum (strains Exuv and JA 98-01). Ciliates were fed algal prey from cultures at various growth phases and at varying concentrations. We observed no harmful effects of P. minimum (Exuv) on any of the ciliates. However in a comparison of strains, P. minimum (Exuv) supported high growth rates, whereas P. minimum (JA 98-01) supported only nominal growth. P. parvum was acutely toxic to ciliates at high concentrations (2×10⁴–3×10⁴ cells ml⁻¹). At low concentrations (5×10³–1×10⁴ cells ml⁻¹), or in culture filtrate, ciliates survived for at least several hours. In mixed diet experiments, as long as a non-toxic alga was available, ciliates survived and at times grew well at concentrations of P. parvum (5×10²–3×10⁴ cells ml⁻¹) that would otherwise have killed them. The present study suggests that prior to the onset of toxicity and bloom formation ciliates may exert grazing pressure on these HAB species, potentially contributing to the suppression or decline of P. minimum and P. parvum blooms.     

COMPARISONS OF NITRATE UPTAKE, STORAGE, AND REDUCTION IN MARINE DIATOMS AND FLAGELLATES

Diatoms, but not flagellates, have been shown to increase rates of nitrogen release after a shift from a low growth irradiance to a much higher experimental irradiance. We compared NO₃ ^? uptake kinetics, internal inorganic nitrogen storage, and the temperature dependence of the NO₃ ^? reduction enzymes, nitrate (NR) and nitrite reductase (NiR), in nitrogen‐replete cultures of 3 diatoms (Chaetoceros sp., Skeletonema costatum, Thalassiosira weissflogii) and 3 flagellates (Dunaliella tertiolecta, Pavlova lutheri, Prorocentrum minimum) to provide insight into the differences in nitrogen release patterns observed between these species. At NO₃ ^? concentrations <40 μmol‐N·L ^{? 1}, all the diatom species and the dinoflagellate P. minimum exhibited saturating kinetics, whereas the other flagellates, D. tertiolecta and P. lutheri, did not saturate, leading to very high estimated K _s values. Above ～60 μmol‐N·L ^{? 1}, NO₃ ^? uptake rates of all species tested continued to increase in a linear fashion. Rates of NO₃ ^? uptake at 40 μmol‐N·L ^{? 1}, normalized to cellular nitrogen, carbon, cell number, and surface area, were generally greater for diatoms than flagellates. Diatoms stored significant amounts of NO₃ ^? internally, whereas the flagellate species stored significant amounts of NH₄ ⁺ . Half‐saturation concentrations for NR and NiR were similar between all species, but diatoms had significantly lower temperature optima for NR and NiR than did the flagellates tested in most cases. Relative to calculated biosynthetic demands, diatoms were found to have greater NO₃ ^? uptake and NO₃ ^? reduction rates than flagellates. This enhanced capacity for NO₃ ^? uptake and reduction along with the lower optimum temperature for enzyme activity could explain differences in nitrogen release patterns between diatoms and flagellates after an increase in irradiance.     

Nitrogen Utilization and Toxin Production by Two Diatoms of the Pseudo‐nitzschia pseudodelicatissima Complex: P. cuspidata and P. fryxelliana

The toxigenic diatom Pseudo‐nitzschia cuspidata, isolated from the U.S. Pacific Northwest, was examined in unialgal batch cultures to evaluate domoic acid (DA) toxicity and growth as a function of light, N substrate, and growth phase. Experiments conducted at saturating (120 μmol photons · m^?2 · s^?1) and subsaturating (40 μmol photons · m^?2 · s^?1) photosynthetic photon flux density (PPFD), demonstrate that P. cuspidata grows significantly faster at the higher PPFD on all three N substrates tested [nitrate (NO₃^?), ammonium (NH₄⁺), and urea], but neither cellular toxicity nor exponential growth rates were strongly associated with one N source over the other at high PPFD. However, at the lower PPFD, the exponential growth rates were approximately halved, and the cells were significantly more toxic regardless of N substrate. Urea supported significantly faster growth rates, and cellular toxicity varied as a function of N substrate with NO₃^?‐supported cells being significantly more toxic than both NH₄⁺‐ and urea‐supported cells at the low PPFD. Kinetic uptake parameters were determined for another member of the P. pseudodelicatissima complex, P. fryxelliana. After growth of these cells on NO₃^? they exhibited maximum specific uptake rates (V_max) of 22.7, 29.9, 8.98 × 10^?3 · h^?1, half‐saturation constants (K_s) of 1.34, 2.14, 0.28 μg‐at N · L^?1, and affinity values (α) of 17.0, 14.7, 32.5 × 10^?3 · h^?1/(μg‐at N · L^?1) for NO₃^?, NH₄⁺ and urea, respectively. These labo‐ratory results demonstrate the capability of P. cuspidata to grow and produce DA on both oxidized and reduced N substrates during both exponential and stationary growth phases, and the uptake kinetic results for the pseudo‐cryptic species, P. fryxelliana suggest that reduced N sources from coastal runoff could be important for maintenance of these small pennate diatoms in U.S. west coast blooms, especially during times of low ambient N concentrations.