Differential responses of populations of the copepod Acartia hudsonica to toxic and nutritionally insufficient food algae

Nutritional insufficiency and toxicity are deleterious effects of phytoplankton on grazers. We hypothesize that toxic food is likely to have stronger evolutionary selective effects on grazers than nutritionally insufficient food. We explore this hypothesis in comparative studies of egg production and egg hatching of the copepod Acartia hudsonica challenged with both a toxic and a nutritionally insufficient alga. Experiments lasting 6 days, in which mixtures of different proportions of the suspect and a control alga were offered as food to female copepods, showed that the dinoflagellate Alexandrium fundyense, which bears paralytic shellfish toxins, was toxic to A. hudsonica. In contrast, the diatom Phaeodactylum tricornutum was nutritionally insufficient to A. hudsonica. In another set of experiments, the effects of A. fundyense and P. tricornutum, respectively, as sole foods on egg production and egg hatching success of two geographically separated populations (Maine and Connecticut) of the copepod A. hudsonica were examined in common-environment experiments, after being raised under identical conditions for two generations. The location in Maine regularly experiences toxic blooms of Alexandrium sp. whereas the location in Connecticut does not. During a 6-day period, A. fundyense reduced the egg production rates of the Connecticut copepod population, but not of the Maine population. In contrast, the diatom P. tricornutum reduced the egg production of both populations. These results of this study are consistent with the hypothesis of local adaptation to toxic food, but not to nutritionally insufficient food.     

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.     

Ingestion of the dinoflagellate, Pfiesteria piscicida, by the calanoid copepod, Acartia tonsa

The dinoflagellate, Pfiesteria piscicida, can form harmful algal blooms in estuarine environments. The dominant copepod species usually found in these waters is Acartia tonsa. We tested the ability of A. tonsa to graze the non-toxic zoospore stage of P. piscicida and thus serve as a potential biological control of blooms of this algal species. A. tonsa grazed the non-toxic zoospore stages of both a non-inducible P. piscicida strain (FDEPMDR23) and a potentially toxic strain (Tox-B101156) at approximately equal rates. Ingestion of P. piscicida increased with cell concentration and exhibited a saturated feeding response. Both the maximum number of cells ingested (I_max) and the slope of the ingestion curve (α) of A. tonsa feeding on P. piscicida were comparable to these ingestion parameters for A. tonsa fed similar-sized phytoplankton and protozoan species. When these laboratory ingestion rates were combined with abundance estimates of A. tonsa from the Pocomoke Estuary and Chesapeake Bay, we found that significant grazing control of the non-toxic zoospore stage of P. piscicida by A. tonsa would only occur at high copepod abundances (>10 copepods L⁻¹). We conclude that under most in situ conditions the potential biological control of blooms of P. piscicida is exerted by microzooplankton grazers. However, in the less saline portions of estuaries where maximum concentrations of copepods often occur with low abundances of microzooplankton, copepod grazing coefficients can be similar to the growth rates of P. piscicida.     

Prorocentrum minimum (Dinophyceae) in the Baltic Sea: morphology, occurrence—a review

The potentially toxic dinoflagellate Prorocentrum minimum (Pavillard) Schiller has successfully established in the Baltic Sea in the last two decades. A review of the invasion history is presented as well as new data on the spatial and inter-annual variability of this species and its relation to salinity, temperature, and nutrient concentrations. A short literature review of the morphological characters of the Baltic P. minimum is also included.From 1993 to 2002, P. minimum was a regular component of the summer and autumn plankton flora of the Baltic Sea proper and the Gulf of Finland. Its abundance varied considerably inter-annually and did not show any clear trends during the period. Abundance of P. minimum was significantly higher in the nutrient-enriched Bay of Mecklenburg (German coast) and the southern Baltic proper than in the central and northern Baltic proper and the Gulf of Finland, where its abundance was mostly sparse. In coastal waters P. minimum occasionally reached densities of several million cells per litre and dominated phytoplankton biomass (>90%).Abundance of the Baltic P. minimum was generally not related to salinity or temperature. It could be a dominant species at both high and low salinity (over 15 and 4.8 PSU), and its temperature range was broad (from 2.7 to 26.4 °C). However, dense populations usually occurred from July to October at temperatures above 10 °C.Further, there appears to be a positive correlation between the success of P. minimum in the Baltic Sea and high concentrations of total phosphorus and nitrogen.This tolerant and morphologically variable dinoflagellate seems to be a morphospecies without subtaxa, which can expand its range in the Baltic Sea, especially in nutrient-rich coastal waters.     

Delayed luminescence of Prorocentrum minimum under controlled conditions

Delayed luminescence (DL), also termed delayed fluorescence or delayed light emission, is the phenomenon of long-lived light emission by plants and cyanobacteria after being illuminated with light and put into darkness. Culture growth of three Prorocentrum minimum strains was studied with DL measurements. DL decay kinetics was measured from 1–60 s after a pulse of white light. The strains used were from the Adriatic Sea (PmK), from Chesapeake Bay, USA (D5), and from the Baltic Sea (BAL), cultured at salinity of 32, 16, and 8 (practical salinity scale), respectively. The strains differed in cell size and chlorophyll a content (PmK > D5 > BAL), as well as in DL parameters. The DL results were compared to standard measurements of culture density and carbon content (calculated from biovolumes). DL decay curves had a specific peak, which changed with culture growth and showed more similarities between the strains PmK and D5. The DL intensity increased with cell density and carbon content in a two-stage process, corresponding to the lag and exponential phases of growth. DL intensity was best correlated with carbon content irrespective of strain and is proposed as an estimate of biomass and for differentiating between lag and exponential phases of growth.     

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.     

Characterization of the affinity for nitrogen, uptake kinetics, and environmental relationships for Prorocentrum minimum in natural blooms and laboratory cultures

During the late spring and early summer of 1998, an extensive bloom of the dinoflagellate Prorocentrum minimum (>93% of phytoplankton cell density) developed in several tributaries of the Chesapeake Bay, USA. In January 1999, a bloom of mixed dinoflagellates (Heterocapsa rotundata, H. triquetra and P. minimum, with P. minimum forming 21% of total phytoplankton cells and 39% of the total biovolume) developed in the mesohaline Neuse Estuary, North Carolina, USA. During these blooms, experiments were carried out to characterize the nitrogen uptake kinetics of these assemblages with ¹⁵N isotopic techniques. Four nitrogenous substrates (NO₃⁻, NH₄⁺, urea, and a mixed amino acids substrate) were used to determine uptake rate and substrate preference. Rates of nitrogen uptake were also measured in P. minimum cultures grown on varying growth nitrogen substrates. The calculated kinetic parameters determined for the P. minimum-dominated field assemblages and the cultures indicated a preference for NH₄⁺. NH₄⁺ was also the primary nitrogen source supporting the blooms. In addition, a high affinity for urea was also found, and urea contributed significantly to the Neuse Estuary bloom. Furthermore, results showed that the regulation of uptake for each of the substrates was different: strong positive relationships between affinity and temperature were found for NH₄⁺ and amino acids, while a negative response was found for NO₃⁻, and very little response to temperature was noted for urea. These differences suggest that a diversity of nitrogen uptake mechanisms may aid the development and maintenance of P. minimum blooms.     

Occurrences of Prorocentrum minimum (Pavillard) in México

Prorocentrum minimum is a planktonic dinoflagellate known to produce red tides that can be harmful. To recognize localities and understand occurrences of Prorocentrum minimum blooms in Mexico, published data of plankton from 1942 to present, as well as unpublished data from the authors, were reviewed. Studies and reports covered marine and coastal waters of México during different periods. Presence of P. minimum were reported in the Pacific coast, Gulf of California, Gulf of México, and the Caribbean, but blooms have been only reported since 1990. Thirteen bloom events were recorded. Six occurred in shrimp ponds and seven near aquaculture regions or coastal areas where intensive agriculture is practiced. Most of the blooms can be associated with damage to the surrounding marine biota either in aquaculture ponds or open waters. Direct toxicity has not been fully evaluated, but data suggest that low oxygen may not easily explain all of the damage. Interestingly, for yet unknown reasons, cells belonging to the triangular morphotype have seldom been reported in México.     

Pathology and immune response of the blue mussel (Mytilus edulis L.) after an exposure to the harmful dinoflagellate Prorocentrum minimum

The harmful dinoflagellate Prorocentrum minimum has different effects upon various species of grazing bivalves, and these effects also vary with life-history stage. Possible effects of this dinoflagellate upon mussels have not been reported; therefore, experiments exposing adult blue mussels, Mytilus edulis, to P. minimum were conducted. Mussels were exposed to cultures of toxic P. minimum or benign Rhodomonas sp. in glass aquaria. After a short period of acclimation, samples were collected on day 0 (before the exposure) and after 3, 6, and 9 days of continuous-exposure experiment. Hemolymph was extracted for flow-cytometric analyses of hemocyte, immune-response functions, and soft tissues were excised for histopathology. Mussels responded to P. minimum exposure with diapedesis of hemocytes into the intestine, presumably to isolate P. minimum cells within the gut, thereby minimizing damage to other tissues. This immune response appeared to have been sustained throughout the 9-day exposure period, as circulating hemocytes retained hematological and functional properties. Bacteria proliferated in the intestines of the P. minimum-exposed mussels. Hemocytes within the intestine appeared to be either overwhelmed by the large number of bacteria or fully occupied in the encapsulating response to P. minimum cells; when hemocytes reached the intestine lumina, they underwent apoptosis and bacterial degradation. This experiment demonstrated that M. edulis is affected by ingestion of toxic P. minimum; however, the specific responses observed in the blue mussel differed from those reported for other bivalve species. This finding highlights the need to study effects of HABs on different bivalve species, rather than inferring that results from one species reflect the exposure responses of all bivalves.     

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.     

Latitudinal differentiation in the effects of the toxic dinoflagellate Alexandrium spp. on the feeding and reproduction of populations of the copepod Acartia hudsonica

Blooms of the dinoflagellate Alexandrium spp. increase in their frequency, toxicity and historical presence with increasing latitude from New Jersey (USA) to the Gaspé peninsula (Canada). Biogeographic variation in these blooms results in differential exposure of geographically separate copepod populations to toxic Alexandrium. We hypothesize that the ability of copepods to feed and reproduce on toxic Alexandrium should be higher in copepods from regions that are frequently exposed to toxic Alexandrium blooms. We tested this hypothesis with factorial common environment experiments in which female adults of the copepod Acartia hudsonica from five separate populations ranging from New Jersey to New Brunswick were fed toxic and non-toxic strains of Alexandrium, and the non-toxic flagellate Tetraselmis sp. Consistent with the hypothesis, when fed toxic Alexandrium we observed significantly higher ingestion and egg production rates in the copepods historically exposed to toxic Alexandrium blooms relative to copepods from regions in which Alexandrium is rare or absent. Such differences among copepod populations were not observed when copepods were fed non-toxic Alexandrium or Tetraselmis sp. These results were also supported by assays in which copepods from populations both historically exposed and naïve to toxic Alexandrium blooms were fed mixtures of toxic Alexandrium and Tetraselmis sp. Two-week long experiments demonstrated that when copepods from populations naïve to toxic Alexandrium were fed a toxic strain of Alexandrium they failed to acclimate, such that their ingestion rates remained low throughout the entire two-week period. The differences observed among populations suggest that local adaptation of populations of A. hudsonica from Massachusetts (USA) to New Brunswick (Canada) has occurred, such that some populations are resistant to toxic Alexandrium.     

Prorocentrum minimum bloom and its possible link to a massive fish kill in Bolinao, Pangasinan, Northern Philippines

For the first time, a Prorocentrum minimum bloom at a maximum cell density of 4.7 × 10⁵ cells/L was recorded on January 31 to February 4, 2002 at Bolinao, Pangasinan, Northern Philippines where intensive and extensive aquaculture of Chanos chanos (milkfish) in fish pens and cages has been practiced for years now. The fish kill, which lasted almost simultaneously with the bloom of the organism had its peak when the organisms bloom was declining. Lack of oxygen in the cages and pens was the fundamental cause of the fish kill. Losses due to the fish kill were estimated at six million pesos (equivalent to US$ 120,000), which includes only the worth of dead cultivated fish. Lack of oxygen in the cages and pens was the fundamental cause of the fish kill, and toxicity of the Prorocentrum could not be confirmed. The cells had minute spinules equally all over the surface of valves. Intercalary striae were wide with many ridges perpendicular to valve margin. Outline of cells was rounder than typical P. minimum cells and similar to P. balticum. Recommendations for future research on the organism are incorporated together with monitoring and management interventions in order to mitigate or possibly prevent damages in similar future events.     

Influence of humic, fulvic and hydrophilic acids on the growth, photosynthesis and respiration of the dinoflagellate Prorocentrum minimum (Pavillard) Schiller

Blooms of the dinoflagellate Prorocentrum minimum often occur in coastal regions characterized by variable salinity and elevated concentrations of terrestrially derived dissolved organic carbon (DOC). Humic, fulvic and hydrophilic acid fractions of DOC were isolated from runoff entering lower Narragansett Bay immediately after a rainfall event and the influence of these fractions upon P. minimum growth, cell yield, photosynthesis and respiration was examined. All organic fractions stimulated growth rates and cell yields compared with controls (no organic additions), but the extent of stimulation varied with the fraction and its molecular weight. Greatest stimulations were observed with humic and fulvic acids additions; cell yields were more than 2.5 and 3.5 times higher than with hydrophilic acid additions while growth rates were 21 and 44% higher, respectively. Responses to additions of different molecular weight fractions of each DOC fraction suggest that growth rate effects were attributable to specific molecular weight fractions: the >10,000 fraction of humic acids, both the >10,000 and <500 fractions of fulvic acids and the <10,000 fraction of hydrophilic acids. The form and concentration of nitrogen (as NO₃⁻ or NH₄⁺) present also influenced P. minimum response to DOC; 10–20 μg ml⁻¹ additions of fulvic acid had no effect upon growth rates in the presence of NH₄⁺ but significantly increased growth rates in the presence of NO₃⁻, a relationship probably related to fulvic acid effects upon trace metal bioavailability and subsequent regulation of the biosynthesis of enzymes required for NO₃⁻ assimilation. The influence of DOC additions on P. minimum respiration and production rates also varied with the organic fraction and its concentration. Production rates ranged from 1.1 to 3.4 pg O₂ cell⁻¹ h⁻¹, with highest rates observed upon exposure to fulvic and hydrophilic acid concentrations of >10 μm ml⁻¹. Low concentrations (5–10 μg ml⁻¹) of humic acid had no statistically significant effect upon production, but exposure to concentrations >25 μg ml⁻¹ resulted in a 30% decrease in O₂ evolution, probably due to light attenuation by the highly colored humic acid fraction. Respiration rates ranged from 1.2 to 2.7 pg O₂ cell⁻¹ h⁻¹ and were elevated upon exposure to both fulvic and hydrophilic acids, but not to humic acid. These results demonstrate that terrestrially derived DOC fractions play an active role in stimulation of P. minimum growth via direct effects upon growth, yield and photosynthesis as well as via indirect influences such as interactions with nitrogen and effects upon light attenuation.     

Effects of natural and field-simulated blooms of the dinoflagellate Prorocentrum minimum upon hemocytes of eastern oysters, Crassostrea virginica, from two different populations

Oysters, Crassostrea virginica, from two populations, one from a coastal pond experiencing repeated dinoflagellate blooms (native), and the other from another site where blooms have not been observed (non-native), were analyzed for cellular immune system profiles before and during natural and simulated (by adding cultured algae to natural plankton) blooms of the dinoflagellate Prorocentrum minimum. Significant differences in hemocytes between the two oyster populations, before and after the blooms, were found with ANOVA, principal components analysis (PCA) and ANOVA applied to PCA components. Stress associated with blooms of P. minimum included an increase in hemocyte number, especially granulocytes and small granulocytes, and an increase in phagocytosis associated with a decrease in aggregation and mortality of the hemocytes, as compared with oysters in pre-bloom analyses. Non-native oysters constitutively had a hemocyte profile more similar to that induced by P. minimum than that of native oysters, but this profile did not impart increased resistance. The effect of P. minimum on respiratory burst was different according to the origin of the oysters, with the dinoflagellate causing a 35% increase in the respiratory burst of the native oysters but having no effect on that of the non-native oysters. Increased respiratory burst in hemocytes of native oysters exposed to P. minimum in both simulated and natural blooms may represent an adaptation to annual blooms whereby surviving native oysters protect themselves against tissue damage from ingested P. minimum.     

A comparative approach to study inhibition of grazing and lipid composition of a toxic and non-toxic clone of Chrysochromulina polylepis (Prymnesiophyceae)

Since the massive bloom in 1988 in the North Sea, the prymnesiophyte flagellate Chrysochromulina polylepis Manton et Parke has been known for its ichtyotoxicity. Laboratory experiments using two different clones of C. polylepis were conducted in a comparative approach. Both clones were nearly similar in size and shape, but differed in their toxicity, as demonstrated by the Artemia bioassay. In order to study the effects of toxic C. polylepis on protozooplankton grazers, grazing experiments were performed with the heterotrophic dinoflagellate Oxyrrhis marina Dujardin as grazer. A first experiment was carried out in order to follow batch culture growth and initial grazing of O. marina when fed toxic or non-toxic clones of C. polylepis. Ingestion of the toxic clone was 27% of ingestion when fed with the non-toxic clone. When O. marina was fed with the toxic clone, vacuoles within O. marina contained fewer food particles per cell and the cells attained slower division rate (58% of the division rate estimated for the non-toxic clone). A second experiment was conducted to determine the grazing and growth response of O. marina as a function of algal food concentration. Profound differences in ingestion, clearance, division and gross growth efficiency of O. marina when fed the two clones of C. polylepis again were apparent. However, even at algal concentrations of 400×10³ ml⁻¹, O. marina is not killed by the presence or by the ingestion of toxic C. polylepis, indicating that the toxin deters grazers. In addition to grazing experiments, lipid classes and fatty acids of both algal clones were analysed and compared in order to follow the hypothesis that toxicity of C. polylepis is caused by liposaccharides, lipids, or fatty acids. However, the chemical composition with respect to lipid classes and fatty acids of both clones were quite similar, making an involvement of these substances in the toxicity towards Artemia and O. marina unlikely.     

Perkinsosis in the Manila clam Ruditapes philippinarum affects responses to the harmful-alga, Prorocentrum minimum

The dinoflagellate Prorocentrum minimum is increasingly recognized as a harmful algal bloom (HAB) species that affects filter-feeding shellfish. An experiment was done to investigate possible interactions between parasitic diseases and exposure to P. minimum in Manila clams, Ruditapes philippinarum. Manila clams, with variable levels of infection with Perkinsus olseni, were exposed for three or six days to the benign phytoplankton species Chaetoceros neogracile or a mixed diet of C. neogracile and P. minimum. After three or six days of exposure, clams were assessed individually for condition index, parasite status, and plasma and hemocyte parameters (morphological and functional) using flow-cytometry. Histological evaluation was also performed on individual clams to assess prevalence and intensity of parasitic infection, as well as other pathological conditions.Prorocentrum minimum caused several changes in Manila clams, especially after six days of exposure, such as decreased hemocyte phagocytosis and size and clam condition index. Pathological conditions observed in Manila clams exposed to P. minimum were hemocyte infiltration in the intestine and gonad follicles, myopathy, and necrosis of the intestine epithelial cells. The parasite P. olseni alone had no significant effect on Manila clams, nor did it modulate the hemocyte variables in clams exposed to P. minimum; however, the parasite did affect the pathological status of Manila clams exposed to the P. minimum culture, by causing atrophy and degeneration of residual ova in the gonadal follicles and hyaline degeneration of the muscle fibers, indicating synergistic effects of both stressors on the host over a short period of time. Additionally, an in vitro experiment also demonstrated detrimental effects of P. minimum and exudates upon P. olseni cells, thus suggesting HAB antagonistic suppression of transmission and proliferation of the parasite in the natural environment over a longer period of time. The results of this experiment demonstrate the complexity of interactions between host, parasite, and HAB.     

Kryptoperidinium foliaceum blooms in South Carolina: a multi-analytical approach to identification

Observations following the discovery of Kryptoperidinium foliaceum blooms in South Carolina (SC), USA, suggest that a multi-analytical approach, using a standard, minimal set of criteria, should be adopted for determining dinoflagellate species identity and taxonomic placement. A combination of morphological, molecular, and biochemical analyses were used to determine the identity of this “red tide” dinoflagellate, first documented in SC waters in the spring of 1998. Results from thecal plate tabulations (based on scanning electron and epifluorescence microscopy), gene sequence data, species-specific PCR probe assays, and microalgal pigment profiles were analyzed and compared to reference cultures of K. foliaceum. Comparative data showed marked inconsistencies among the K. foliaceum reference culture isolates. In addition, the SC bloom isolate was shown to be mononucleate, contrary to previous reports for K. foliaceum, suggesting a more transient endosymbiotic association than previously considered.     

Egg production of the copepod Acartia tonsa: The influence of hypoxia and food concentration

Low dissolved oxygen conditions, or hypoxia, occur in estuaries and impact more than just the obvious commercially important species. Copepods are an important link in the food web, and the influence of hypoxia upon them is relatively unstudied. Using the copepod Acartia tonsa, a study of the impact of hypoxia on egg production was conducted. A. tonsa had reduced egg production at low dissolved oxygen concentrations (DO), with the lowest egg production occurring at 0.53-ml/l O₂ concentration. Another experiment was conducted to determine if, by increasing food concentration, the affect of hypoxia could be mitigated. The results indicate that increased food did not offset the impact that hypoxia had on egg production of A. tonsa. These results suggest that as A. tonsa experiences hypoxia in the wild, population numbers will decrease. Thus, if hypoxic conditions increase in scope and duration, declines in copepod abundance may lead to a decline in the abundance of species that depend on them as food. These species may be of commercial importance (e.g., fish, crabs, and oysters).     

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.