In situ growth rate and distribution of the ichthyotoxic dinoflagellate Gyrodinium corsicum Paulmier in an estuarine embayment (Alfacs Bay, NW Mediterranean Sea)

Gyrodinium corsicum Paulmier produced massive blooms in Alfacs Bay (Ebro Delta, NW Mediterranean Sea) from December 1994 to April 1995, a period characterized by an initial period of water stability and low outward flux of water during subsequent months. The highest cell densities of G.corsicum were found at the bottom of the bay during the population maintenance period (from January to March). During the day, no differences in the vertical distribution of G.corsicum were observed and no diel in situ vertical migration of the population was recorded. The in situ growth rate of G.corsicum was estimated using the cell cycle method since the cell division cycle was well phased with the daily light cycle. The S phase fraction of G.corsicum reached a maximum near the middle of the dark period. The G2M peak usually occurred in the dark period. The duration of the S phase was 2.1-8.7 h and the duration of the G2M phase was 2.6-7.7 h. The estimated specific in situ growth rate of G.corsicum ranged from 0.94 day^-1 during the initial phases of the bloom to 0.3 day^-1 when the bloom was well developed. Growth rates, measured in different locations at the same time, varied by a factor of 1.5, suggesting that different parts of the same blooming population were at different stages of growth. The persistence of the G.corsicum bloom with high cell densities (10⁵-10⁶ cells l^-1) at the bottom of the bay is discussed in relation to the low cell losses by physical dispersion in this area and the lack of upward vertical migration of G.corsicum.      

Response of an estuarine ecosystem to reduced organic waste discharge

Since the mid-19^th century large amounts of organic wastewere sluiced out into the Ems Estuary on the border between Germany and TheNetherlands. This waste originated from the regional potato flour and cardboardindustries making the inland waterways completely anoxic. In the estuaryseriousoxygen depletion occurred, especially during autumn. Most of the organic wasteentered the estuary in the Dollard, a brackish embayment of the Ems Estuary. Anintensive sanitation scheme was started in the 1970s, leading to a stepwisereduction of the organic waste load on the estuary. In this paper, a review isgiven of the response of the benthos living at intertidal mudflats,viz. microphytobenthos (diatoms), meiofauna (nematodes)andmacrozoobenthos. The benthos response is described mainly on the basis of dataobtained under conditions of high (ca. 1980), intermediate (1987) and largelyreduced (1993) organic waste loading.Reduction of organic loading caused significant changes in abundance, speciescomposition and standing stock of diatoms and nematodes. Macrobenthicpopulations recovered from being severely reduced regularly during the autumnalwaste discharges towards a more stable situation. Altogether, the intertidalmudflat benthos changed from organic waste loading stress towards a normal,estuarine environmental stress. This was especially the case at high and muddyflats in the southeast of the Dollard. At mixed sand/mud flats in the centre ofthe Dollard, a response of macrozoobenthos may have been obscured by theinvasion of the polychaete Marenzelleria cf.wireni.     

Life cycle of the ichthyotoxic dinoflagellate Cochlodinium polykrikoides in Korean coastal waters

Since 1995, blooms of the harmful dinoflagellate, Cochlodinium polykrikoides, have caused considerable mortality of aquatic organisms and economic loss in Korea. However, little is known about the life cycle of the species, except for the planktonic vegetative stage; therefore, the aim of this paper was to elucidate the life cycle of C. polykrikoides. Its life cycle has two morphologically different stages: an armored and an unarmored vegetative stage. Armored vegetative cells were found in seawater samples collected in late-November and developed into four-cell chained, unarmored vegetative cells under laboratory culture. In samples collected in late-May, both the armored and unarmored types (vegetative swimming stage) occurred; the former easily developed into an unarmored vegetative cell type, suggesting that the armoured–unarmored transition occurs as early as May. A presumptive resting cyst, round but folded at one side, was produced from armored type cells in laboratory conditions. It was also collected from natural bottom sediments, which suggests it is the dormant resting cyst of C. polykrikoides.     

Differential susceptibility to grazers in two phases of an intertidal alga: Advantages of heteromorphic generations

The intertidal red alga Gigartina papillata (C. Ag.) J. Ag. alternates between two major forms in its life cycle, an upright blade and a fleshy crust. The evolution and functional significance of such alternation of heteromorphic generations is little understood. This study, done on the outer coast of Washington state (U.S.A.), considers the rôle of physical factors, measured by intertidal position, and a biological factor, interactions with herbivores, as selective pressures potentially responsible for this pattern. Observations along transects and a herbivore exclusion experiment showed that the two phases of this alga simultaneously occupy similar intertidal positions but differ significantly and oppositely in their response to grazing. The number of crust plants declined in the absence of grazers, probably due to being overgrown and shaded by a thick algal film. It is, therefore, suggested that the crust form is grazer-dependent. In contrast, the number of blade phase plants increased in the absence of grazers. “Bet-hedging” is suggested in explanation of the maintenance of two phases in one cycle; the crust is more persistent but susceptible to overgrowth, while the more productive blade is more susceptible to grazing.     

Effects of gastropod grazers on recruitment and succession of an estuarine assemblage: a multivariate and univariate approach

Grazers have been shown to affect assemblages of species in many habitats. Here we studied the effects of the gastropod grazers, Austrocochlea porcata and Bembicium auratum, on intertidal estuarine assemblages in a sheltered bay in New South Wales, Australia. We examined the effects of gastropods on individual species and on the assemblage as a whole. The multivariate response was compared with data on succession in these assemblages to estimate potential effects of grazers on succession. The experiment was repeated several times to determine the generality of grazer effects in the light of possible variation in the timing or intensity of recruitment. There were different responses of individual species to the presence of grazers. Grazers reduced the abundance of ephemeral algal species, bryozoans, copepods, insect larvae and Balanus spp. barnacles. They had a positive effect on oysters and spirorbids and no effect on the barnacles Elminius covertus and Hexaminius spp. These effects were consistent through time. Multivariate analyses confirmed that grazers caused significant changes to whole assemblages and that these effects were far-reaching and not only caused by changes to algal species. The removal of grazers appeared to neither speed up nor slow down succession, but rather caused a completely different assemblage to develop. Apparent important mechanisms affecting the composition of animal species when grazers were removed included accumulation of sediments and detritus and pre-emption of space by algae. Received: 13 May 1996 / Accepted: 1 September 1996     

Pigment profile of the ichthyotoxic dinoflagellate Gymnodinium sp. from a massive bloom in southern Chile

The pigment profile of the ichthyotoxic dinoflagellate Gymnodiniumsp. from a bloom occurred in southern Chile contained fucoxanthinand 19'-acyloxyfucoxanthin derivatives, including novel formsof these latter compounds. This profile resembled those foundin other ichthyotoxic Gymnodinium species.     

Immuno-flow cytometric identification and enumeration of the ichthyotoxic dinoflagellate Gyrodinium aureolum Hulburt in artificially mixed algal populations

Flow cytometric identification and enumeration of Gyrodiniumaureolum Hulburt (Dinophyceae) were performed in artiticiallymixed algal populations using direct immunofluorescence. Calibrationof the flow cytometer, performed with a mixed algal populationspiked with immuno-fluorescently labelled G.aureolum cells,showed that selection of target cells after analysis on greenand orange fluorescence can be done with a recovery of 91.8%[coefficient of variation (CV) = 0.09]. Other selection methodswere less good, with 67.4% (CV = 0.16) and 58.4% (CV = 0.43)recovery based on green and red fluorescence or green fluorescenceand perpendicular light scattering. For mixed algal populationsspiked with unlabelled G.aureolum cells, the quantificationof target cells was quite good (recovery of 76.7%; CV = 0.20).The percentage of total cell loss was high (58.0–92.0%),but this was caused mostly by loss of species smaller in sizethan G.aureolum. Estimates of the relative contribution of G.aureolumin labelled samples were therefore often far too high, but detectionand quantification were not affected. The methodological underestimation(23.3%) was partly caused by gating on green and orange fluorescence(inaccuracy 8.2%).     

Dynamics of Amoebophrya parasites during recurrent blooms of the ichthyotoxic dinoflagellate Cochlodinium polykrikoides in Korean coastal waters

During the bloom events of the harmful dinoflagellate Cochlodinium polykrikoides in August and October, 2012, infections by two different Amoebophrya species were observed in Korean coastal waters. To investigate the dynamics of the two parasites and their relative impact on the host populations, a quantitative real-time PCR (qPCR) method was applied to detect and quantify the parasites in the free-living and parasitic stages. Each specific primer set of the target species, Amoebophrya sp. 1 and sp. 2 was designed on the large subunit (LSU) and the first internal transcribed spacer (ITS1) of ribosomal RNA (rRNA) gene, respectively. Dynamics of the two Amoebophrya species via qPCR assay showed distinct patterns during the C. polykrikoides bloom events. Amoebophrya sp. 1 showed peaks during both bloom events in August and October with relatively low copies (10⁶ to 10⁷ copies L⁻¹), while Amoebophrya sp. 2 appeared only during the bloom event in October with very high copies (10⁹ to 10¹⁰ copies L⁻¹). Overall, the qPCR measurements for the dynamics of two Amoebophrya species in the parasitic stage (> 5 μm fractions) were consistent with parasite prevalence through microscopic observations. Amoebophrya sp. 1 infections were observed during both bloom events in August and October with relatively low parasite prevalence (0.1–1.5%), while Amoebophrya sp. 2 infections were detected only during the bloom event in October with high prevalence (up to 45%). Taken together, Amoebophrya sp. 1 may be a generalist and C. polykrikoides may not be its primary host, while Amoebophrya sp. 2 may be a specialist which can substantially impact host population dynamics.     

Control of ichthyotoxic Cochlodinium polykrikoides using the mixotrophic dinoflagellate Alexandrium pohangense: A potential effective sustainable method

Red tides dominated by Cochlodinium polykrikoides often lead to great economic losses and some methods of controlling these red tides have been developed. However, due to possible adverse effects and the short persistence of their control actions, safer and more effective sustainable methods should be developed. The non-toxic dinoflagellate Alexandrium pohangense is known to grow well mixotrophically feeding on C. polykrikoides, and populations are also maintained by photosynthesis. Thus, compared with other methods, the use of mass-cultured A. pohangense is safer and the effects can be maintained in the long term. To develop an effective method, the concentrations of A. pohangense cells and culture filtrate resulting in the death of C. polykrikoides cells were determined by adding the cells or filtrates to cultured and natural populations of C. polykrikoides. Cultures containing 800 A. pohangense cells ml⁻¹ eliminated almost all cultured C. polykrikoides cells at a concentration of 1000 cells ml⁻¹ within 24 h. Furthermore, the addition of A. pohangense cultures at a concentration of 800 cells ml⁻¹ to C. polykrikoides populations from a red-tide patch resulted in the death of most C. polykrikoides cells (99.8%) within 24 h. This addition of A. pohangense cells also lowered the abundances of total phototrophic dinoflagellates excluding C. polykrikoides, but did not lower the abundance of total diatoms. Filtrate from 800 cells ml⁻¹ A. pohangense cultures reduced the population of cultured C. polykrikoides by 80% within 48 h. This suggests that A. pohangense cells eliminate C. polykrikoides by feeding and releasing extracellular compounds. Over time, A. pohangense concentrations gradually increased when incubated with C. polykrikoides. Thus, an increase in the concentration of A. pohangense by feeding may lead to A. pohangense cells eliminating more C. polykrikoides cells in larger volumes. Based on the results of this study, a 1 m³ stock culture of A. pohangense at 4000 cells ml⁻¹ is calculated to remove all C. polykrikoides cells in ca. 200 m³ within 6 days. Furthermore, maintenance of A. pohangense populations through photosynthesis prepared A. pohangense to eliminate C. polykrikoides cells in future red-tide patches. Moreover, incubation of A. pohangense at 2000 cells ml⁻¹ with juvenile olive flounder Paralichthys olivaceus for 3 days did not result in the death of fish. Therefore, the method developed in this study is a safe and effective way of controlling C. polykrikoides populations and can be easily applied to aqua-tanks on land.     

Temporal and spatial distribution of the ichthyotoxic dinoflagellate Gyrodinium aureolum Hulburt in the St Lawrence, Canada

The occurrence of the potentially ichthyotoxic dinoflagellateGyrodinium aureolum Hulburt is reported for the first time inthe Lower Estuary and Gulf of St Lawrence. Taxonomic identificationusing an iramunochemical tagging method suggests a close taxonomicproximity between the species found in the St Lawrence and G.aureolumpresent in European waters. In 1993, G.aureolum was observedalong the entire coast of Quebec and in offshore waters of theGulf of St Lawrence. At the coastal stations, G.aureolum showedlarge week-to-week variations, but higher densities were observedduring late August and early September. Bloom concentrations(10⁶ cells 1^–1) were only observed in the GaspéCurrent at Mont-Louis, on the north coast of the Gaspd peninsula.In the central part of the Gulf, G.aureolum was concentratedin the upper 5 m of the stations directly influenced by thefreshwater run-off of the St Lawrence Estuary. Results fromprincipal component analysis indicate that G.aureolum favorsenvironments with high nitrogen recycling activity.     

Hindgut fermentation in the wombats: two marsupial grazers

Summary The wombats Vombatus ursinus and Lasiorhinus latifrons have a capacious proximal colon with only a vestigial caecum. The pattern of microbial fermentation in the hindgut of both species was studied in captive animals fed a pelleted straw diet and in wild wombats feeding on their natural winter diets. Digesta pH was low in the stomach but near neutrality along the hindgut, indicating effective absorption and/or buffering of the colonic contents. Initial proportions and production rates of short chain fatty acids in vitro reflected the fermentation of plant cell walls. Proportions of isobutyrate, isovalerate and n-valerate increased towards the distal colon indicating proteolysis and subsequent fermentation of amino acids. The low ammonia content of digesta fluid suggested that ammonia released from these amino acids was absorbed and utilized by the wombats and their gut microbes. Wild wombats had higher concentrations and production rates of short chain fatty acids than captive animals, which was consistent with the higher apparent digestibility of their natural diet. The energy from short chain fatty acids in captive animals was 30–33% of digestible intake. Energy intakes were low and similar to resting metabolic rates estimated for marsupials. Actual resting metabolic rates of the wombats are probably lower than these estimates, and the proportion of energy derived from fermentation substantially higher than the 53–61% estimated in wild wombats. The energy from fermentation clearly enables wombats to utilize diets high in fibre.Abbreviations DEI digestible energy intake - DM dry matter - NDF neutral detergent fibre - SCFA short chain fatty acids - SD standard deviation - SMR standard metabolic rate     

Cooccurrence of elevated urea levels and dinoflagellate blooms in temperate estuarine aquaculture ponds

In hybrid striped bass aquaculture ponds, dinoflagellate blooms were found on 10 of 14 occasions to co-occur with concentrations of urea in excess of 1.5 microM nitrogen. When urea levels were <1.5 microM nitrogen, on seven occasions, no evidence of dinoflagellate blooms was observed in these ponds.     

Plant responses to fertilization and exclusion of grazers on an arctic tundra heath

This study investigated the impacts of fertilization and grazing by Norwegian lemmings (Lemmus lemmus), grey‐sided voles (Clethrionomys rufocanus), and reindeer (Rangifer tarandus) on a diverse tundra plant community dominated by deciduous shrubs. Four out of eight study areas, having a size of 2500 m² each, were fertilized with a N‐P‐K fertilizer and four areas served as unfertilized controls. Two types of exclosures were used within each study area, one to exclude solely reindeer, and one to exclude both rodents and reindeer. Open, grazed plots served as controls. During 5 years following the fertilization event the changes in vegetation inside and outside the exclosures were monitored using a point frequency method. The densities of rodents on the fertilized and unfertilized areas were investigated by live trapping and by counting nests of overwintering individuals. Reindeer do not graze on the study area during the growing season but migrate through this area in autumn and spring. Fertilization increased the abundance of vascular plants while grazing by reindeer and rodents decreased the abundance of vascular plants significantly on both fertilized and unfertilized areas. Rodents preferred clearly the fertilized areas during winter, decreasing the abundance of Vaccinium myrtillus and Vaccinium vitis‐idaea, while very little grazing occurred during summer. Graminoids showed the strongest positive response to fertilization and dominated the plant community on ungrazed plots, while winter grazing by both reindeer and rodents significantly decreased the abundance of graminoids. Deciduous shrubs (Betula nana, Vaccinium myrtillus) increased slightly but significantly due to fertilization and evergreen dwarf shrubs showed no response to fertilization. However, the use of functional growth forms for predicting the responses of nutrient enrichment and grazing must be questioned, as responses to fertilization as well as preferences by herbivores were shown to be species‐specific rather than uniform within functional groups based on plant growth forms.     

The effect of zooplankton grazing on estuarine blooms of the toxic dinoflagellate Gonyaulax tamarensis

A series of short-term ^{in situ} experiments was conducted intwo Cape Cod embayments to estimate mortality rates of the toxicdinoflagellate, Gonyaulax tamarensis, resulting from grazingby zooplankton. Rates of grazing by the whole zooplankton communityand by specific zooplankton populations were measured at variouspoints in the G. tamarensis bloom cycle. The planktonic larvaeof the spionid polychaete Polydora ligni and the tintinnid ciliateFavella sp. were important grazers in the systems studied. Gonyaulax-specificclearance rates effected by Polydora ranged from 0.02 to 0.5ml individual^–1 h^–1; for Favella the range was aboutan order to magnitude lower. Peak population densities wereclose to 900 and 400 individuals 1^–1 for P. ligni andFavella, respectively. Whether measured directly or predictedas the product of individual clearance rates and numerical abundance,rates of grazing were often higher than estimated algal divisionrates in years when blooms failed to develop. A simulation modelcorroborated the results of the field study, demonstrating thatgrazing can be a significant source of mortality during blooms,and can suppress bloom development when grazers are abundant.     

Biotic resistance to invasion along an estuarine gradient

Biotic resistance is the ability of native communities to repel the establishment of invasive species. Predation by native species may confer biotic resistance to communities, but the environmental context under which this form of biotic resistance occurs is not well understood. We evaluated several factors that influence the distribution of invasive Asian mussels (Musculista senhousia) in Mission Bay, a southern California estuary containing an extensive eelgrass (Zostera marina) habitat. Asian mussels exhibit a distinct spatial pattern of invasion, with extremely high densities towards the back of Mission Bay (up to 4,000 m⁻²) in contrast with near-complete absence at sites towards the front of the bay. We established that recruits arrived at sites where adult mussels were absent and found that dense eelgrass does not appear to preclude Asian mussel growth and survival. Mussel survival and growth were high in predator-exclusion plots throughout the bay, but mussel survival was low in the front of the bay when plots were open to predators. Additional experiments revealed that consumption by spiny lobsters (Panulirus interruptus) and a gastropod (Pteropurpura festiva) likely are the primary factors responsible for resistance to Asian mussel invasion. However, biotic resistance was dependent on location within the estuary (for both species) and also on the availability of a hard substratum (for P. festiva). Our findings indicate that biotic resistance in the form of predation may be conferred by higher order predators, but that the strength of resistance may strongly vary across estuarine gradients and depend on the nature of the locally available habitat.     

Nutrient concentrations in two savannah grass species and their implications for grazers

The diversity of grazers on savannahs exerts pressure on the grass to provide nutrition. In this study the concentrations of major and trace animal nutrients in above ground grass tissue as influenced by grazing were examined. Two palatable grass species grazed by both livestock and wild grazers were sampled: Brachiaria nigropedata and Eragrostis lehmanniana. Samples of grass tissue were collected from widely spread sampling sites in both livestock and wildlife grazing sites, at the end of the growing season. Although B. nigropedata had higher nutrient concentrations, there was general covariance in nutrient levels in the two grasses. The covariance indicated that the respective sites generally had location context-dependent low or high nutrient concentrations in all grasses. The adequacy of the nutrient concentrations for grazer nutrition was assessed using literature-sourced minimum and maximum tolerable limit requirements of cattle and the high population gazelle-like species as illustration. The results showed that, in these nitrogen-limited savannahs, the grass generally met grazer requirements in terms of crude protein, potassium, molybdenum, nickel and zinc but that it had deficiencies in phosphorous, sodium, and boron, as well as grazing intensity-related localised toxic levels of potassium, phosphorous, magnesium, iron and manganese. The deficiencies and toxicities were inferred to result in abnormalities in grazer physical condition and reproduction. Given the importance of livestock in semiarid savannahs, these effects on livestock grazers were inferred as threatening human food security. The study shows that high grazing intensity can raise the concentrations of some animal nutrients in above ground grass tissue to levels above grazer tolerable limits. Grazing rotation to reduce grazing intensity is recommended as a proactive grazing management strategy to limit the effects.     

Feeding by the heterotrophic dinoflagellate Oxyrrhis marina on the red-tide raphidophyte Heterosigma akashiwo: a potential biological method to control red tides using mass-cultured grazers

As part of the development of a method to control the outbreak and persistence of red tides using mass-cultured heterotrophic protist grazers, we measured the growth and ingestion rates of cultured Oxyrrhis marina (a heterotrophic dinoflagellate) on cultured Heterosigma akashiwo (a raphidophyte) in bottles in the laboratory and in mesocosms (ca. 60 liter) in nature, and those of the cultured grazer on natural populations of the red-tide organism in mesocosms set up in nature. In the bottle incubation, specific growth rates of O. marina increased rapidly with increasing concentration of cultured prey up to ca. 950 ng C ml(-1) (equivalent to 9,500 cells ml(-1)), but were saturated at higher concentrations. Maximum specific growth rate (mumax), KGR (prey concentration sustaining 0.5 mumax) and threshold prey concentration of O. marina on H. akashiwo were 1.43 d(-1), 104 ng C ml(-1), and 8.0 ng C ml(-1), respectively. Maximum ingestion and clearance rates of O. marina were 1.27 ng C grazer(-1) d(-1) and 0.3 microl grazer(-1) h(-1), respectively. Cultured O. marina grew well effectively reducing cultured and natural populations of H. akashiwo down to a very low concentration within 3 d in the mesocosms. The growth and ingestion rates of cultured O. marina on natural populations of H. akashiwo in the mesocosms were 39% and 40%, respectively, of those calculated based on the results from the bottle incubation in the laboratory, while growth and ingestion rates of cultured O. marina on cultured H. akashiwo in the mesocosms were 55% and 36%, respectively. Calculated grazing impact by O. marina on natural populations of H. akashiwo suggests that O. marina cultured on a large scale could be used for controlling red tides by H. akashiwo near aquaculture farms that are located in small ponds, lagoons, semi-enclosed bays, and large land-aqua tanks to which fresh seawater should be frequently supplied.