Succession Of Blooms Research Articles

Red tide blooms of Cochlodinium polykrikoides in a coastal cove

Successive blooms of the dinoflagellate Cochlodinium polykrikoides occurred in Pettaquamscutt Cove, RI, persisting from September through December 1980 and again from April through October 1981. Cell densities varied from <100 cells L −1 at the onset of the bloom and reached a maximum density exceeding 3.4 × 10 6 cells L −1 during the summer of 1981. The bloom was mainly restricted to the mid to inner region of this shallow cove with greatest concentrations localized in surface waters of the southwestern region during summer/fall periods of both years. Highly motile cells consisting of single, double and multiple cell zooids were found as chains of 4 and 8 cells restricted to the late August/September periods. The highest cell densities occurred during periods when annual temperatures were between 19 and 28 °C and salinities between 25 and 30. A major nutrient source for the cove was Crying Brook, located at the innermost region at the head of the cove. Inorganic nitrogen (NH 3 and NO 2 + NO 3) from the brook was continually detectable throughout the study with maximum values of 57.5 and 82.5 μmol L −1, respectively. Phosphate (PO 4-P) was always present in the source waters and rarely <0.5 μmol L −1; silicate always exceeded 30 μmol L −1 with maximum concentrations reaching 226 μmol L −1. Chlorophyll a and ATP concentrations during the blooms varied directly with cell densities. Maximum Chl a levels were 218 mg m −3 and ATP-carbon was >20 g C m −3. Primary production by the dinoflagellate-dominated community during the bloom varied between 4.3 and 0.07 g C m −3 d −1. Percent carbon turnover calculated from primary production values and ATP-carbon varied from 6 to 129% d −1. The dinoflagellates dominated the entire summer period; other flagellates and diatoms were present in lesser amounts. A combination of low washout rate due to the cove dynamics, active growth, and life cycles involving cysts allowed C. polykrikoides to maintain recurrent bloom populations in this area.

Obligate oil-degrading marine bacteria

Over the past few years, a new and ecophysiologically unusual group of marine hydrocarbon-degrading bacteria - the obligate hydrocarbonoclastic bacteria (OHCB) - has been recognized and shown to play a significant role in the biological removal of petroleum hydrocarbons from polluted marine waters. The introduction of oil or oil constituents into seawater leads to successive blooms of a relatively limited number of indigenous marine bacterial genera--Alcanivorax, Marinobacter, Thallassolituus, Cycloclasticus, Oleispira and a few others (the OHCB)--which are present at low or undetectable levels before the polluting event. The types of OHCB that bloom depend on the latitude/temperature, salinity, redox and other prevailing physical-chemical factors. These blooms result in the rapid degradation of many oil constituents, a process that can be accelerated further by supplementation with limiting nutrients. Genome sequencing and functional genomic analysis of Alcanivorax borkumensis, the paradigm of OHCB, has provided significant insights into the genomic basis of the efficiency and versatility of its hydrocarbon utilization, the metabolic routes underlying its special hydrocarbon diet, and its ecological success. These and other studies have revealed the potential of OHCB for multiple biotechnological applications that include not only oil pollution mitigation, but also biopolymer production and biocatalysis.

Spring bloom succession, grazing impact and herbivore selectivity of ciliate communities in response to winter warming

This study aimed at simulating different degrees of winter warming and at assessing its potential effects on ciliate succession and grazing-related patterns. By using indoor mesocosms filled with unfiltered water from Kiel Bight, natural light and four different temperature regimes, phytoplankton spring blooms were induced and the thermal responses of ciliates were quantified. Two distinct ciliate assemblages, a pre-spring and a spring bloom assemblage, could be detected, while their formation was strongly temperature-dependent. Both assemblages were dominated by Strobilidiids; the pre-spring bloom phase was dominated by the small Strobilidiids Lohmaniella oviformis, and the spring bloom was mainly dominated by large Strobilidiids of the genus Strobilidium. The numerical response of ciliates to increasing food concentrations showed a strong acceleration by temperature. Grazing rates of ciliates and copepods were low during the pre-spring bloom period and high during the bloom ranging from 0.06 (Delta0 degrees C) to 0.23 day(-1) (Delta4 degrees C) for ciliates and 0.09 (Delta0 degrees C) to 1.62 day(-1) (Delta4 degrees C) for copepods. During the spring bloom ciliates and copepods showed a strong dietary overlap characterized by a wide food spectrum consisting mainly of Chrysochromulina sp., diatom chains and large, single-celled diatoms.

The use of fuzzy logic for data analysis and modelling of European harmful algal blooms: results of the HABES project

Fuzzy logic was applied to model blooms of Nodularia spumigena, Dinophysis spp., Alexandrium minutum, Karenia mikimotoi and Phaeocystis globosa at various European sites as part of the Harmful Algal Blooms Expert System (HABES) project. This modelling approach was useful in performing integrated analyses of interacting physical and biological factors involved in HABs. A basic knowledge of HAB formation and sufficient data are a prerequisite for successful bloom prediction.

Comparative population structure, abundance and vertical distribution of six copepod species in the North Atlantic: Evidence for intraguild predation?

The abundance, vertical distribution and stage composition of the copepods Euchaeta norvegica, Metridia lucens and Pleuromamma robusta and the abundance and vertical distribution of Acartia sp., Oithona spp. and Oncaea sp. were analysed for a period of 4.5 years (1971–1974), based on data previously collected at weather station India in the North Atlantic (59°N, 19°W). The results were compared with previously published results on Calanus finmarchicus during the same study. The factor analysis showed two well-differentiated groups. The cohort development and abundance of C. finmarchicus, E. norvegica and Oncaea were related to successive phytoplankton blooms. On the contrary, the cohort development and abundance of M. lucens, P. robusta, Acartia spp. and Oithona spp. were not related to phytoplankton concentration and showed negative relationships with Calanus abundance. We suggest that C. finmarchicus, which largely dominates the biomass at station India, could play a key role in the structuring of the community: as prey for the carnivorous E. norvegica, vectoring the spring bloom to deep waters through faecal pellet consumption by the coprophagous Oncaea spp., and through intraguild predation on the early stages of other omnivorous copepods such as Acartia, Oithona, M. lucens and P. robusta.

Macroalgal blooms on southeast Florida coral reefs: II. Cross-shelf discrimination of nitrogen sources indicates widespread assimilation of sewage nitrogen

Since 1990, coral reefs off southeast Florida have experienced an unprecedented succession of macroalgal blooms and invasions. To determine if anthropogenic land-based nitrogen (N) sources support these HABs, we collected macroalgal tissue for stable nitrogen isotope (δ 15N) analysis at three spatially distinct depths ranging from the shallow subtidal to the shelf break (∼43 m) along seven transects from Jupiter to Deerfield Beach, Florida, USA. This sampling was initiated during a historically significant drought in the spring of 2001 when rainfall, stormwater runoff, and upwelling were at a minimum. The sampling was repeated in the summer wet season following significant stormwater runoff and during a strong upwelling event. Despite significant seasonal changes in N source availability, δ 15N values did not vary between samplings. Collectively, δ 15N values were significantly higher on inshore shallow reefs (+8.1‰) compared to mid (+6.1‰) and deep reefs (+6.7‰). Values were also elevated in the southern portion of the study area (e.g., Boca Raton, +8.5‰) where nearly 1.5 billion l/day of secondarily treated wastewater is discharged into the ocean via coastal outfalls. Codium isthmocladum, a macroalga that assimilates its nutrients entirely from the water column, was the dominant bloom species in the southern study area, where tissue δ 15N values matched source values of nearby sewage outfalls. The northern study area was dominated by species of the genus Caulerpa, particularly the invasive Pacific native C. brachypus var. parvifolia, which are capable of accessing benthic nutrient sources (e.g., submarine groundwater discharge, SGD) by means of root-like rhizoids. The northern area does not have sewage outfalls but features a highly transmissive geologic zone where SGD may be enriched with septic tank leachate and effluent from municipal deep injection wells. Multiple lines of evidence supported the hypothesis that land-based sewage N was more important than upwelling as a N source to these HABs: (1) δ 15N values were highest on shallow reefs and decreased with increasing depth, indicating land-based sources of enrichment; (2) elevated δ 15N values occurred in these HABs during the dry season, prior to the onset of the summer upwelling; (3) elevated NH 4 + concentrations occur on these reefs during both upwelling and non-upwelling periods and are kinetically preferred by macroalgae compared to upwelled NO 3 −. These findings provide a case study of a coupling between increasing anthropogenic activities and the development of macroalgal HABs, including invasive species that threaten economically important reef resources in southeast Florida.

Nannofloral extinction and survivorship across the K/T boundary at Caravaca, southeastern Spain

Detailed qualitative and quantitative studies of the calcareous nannofloral assemblages were carried out on 65 cm across the K/T boundary interval of the Caravaca section. Preservation of nannofloral assemblages is moderate to good. Reworking was identified in the lowermost Danian, but did not essentially influence the rest of the section studied. The nannofloral study shows that most of the Cretaceous taxa disappeared in the “fallout” lamina, a bio-event which is synchronous with the mass extinction of the planktonic foraminifera and with significant geochemical anomalies. Beside these events, a rapid decline of the calcareous nannofossils is recorded in the uppermost 12 cm of the Maastrichtian (about 4 ky prior to the K/T boundary). Such decline becomes more pronounced in the transitional layer, just below the K/T boundary, therefore predating the main extinction event. This pattern is in agreement with other evidences of an unstable ecosystem and inputs of cold-water taxa in this area with typical Tethyan upper Maastrichtian nannofossil assemblages. Two successive blooms, of Thoracosphaera spp., followed by that of Braarudosphaera bigelowii, were identified at the base of the Danian. In the upper part of the studied sequence, nannoflora records are already dominated by survivor as well as by incoming taxa, showing an early pioneer calcareous nannofloral ecosystem about 25 ky after the K/T boundary mass extinction event.

Seasonal variability of plankton blooms in the Ria de Ferrol (NW Spain): I. Nutrient concentrations and nitrogen uptake rates

The frequent plankton blooms in the Ria de Ferrol (Galicia, NW Spain) were analysed by measuring changes in dissolved nutrient concentrations and plankton uptake rates. Four cruises were made during February, May, July and September 2000 to study seasonal differences between blooms. Dissolved inorganic nutrients, oxygen, particulate and dissolved organic carbon and nitrogen, along with chlorophyll- a concentrations were measured in three selected stations representative of inner, middle and outer zones of the ria. In addition, gross uptake of nitrate and ammonium was estimated using 15N-tracers. During February only there were no accumulations of phytoplankton inside the ria, despite the high concentration of nutrients. In contrast, large accumulations of phytoplankton were found during the other three cruises (up to 27.8 mg chlorophyll- a m −3 in September), when nutrient concentrations were much lower; however, inorganic nutrients were never exhausted in surface waters. Values of the inorganic nitrogen-to-phosphate ratio <10 during blooms in the surface during spring and summer suggest that the nitrogen supply control bloom development. In this way, nitrate uptake dominated in February (mean±SE, f- ratio = 0.76 ± 0.01 ) and July (0.64±0.05), while ammonium uptake dominated in May (0.25±0.07) and September (0.42±0.05). Due to the low input of continental water during the study, nutrient dynamics and fluxes were largely dependent on shelf-ria exchanges and in situ regeneration, which helped to sustain a relatively large biomass of plankton in absence of direct effects of the coastal upwelling. The succession of phytoplankton blooms in the Ria de Ferrol produces a progressive transformation of nitrogen from inorganic to organic forms (both particulate and dissolved) from winter to late summer. Such accumulations of organic matter would maintain remineralisation rates matching plankton uptake, with the result of no net accumulation of inorganic nutrients inside the ria.

High frequency monitoring reveals phytoplankton dynamics

Phytoplankton is an important water quality indicator because of its high species differentiation, growth rates and responsiveness to environmental actuators. The new European Water Framework Directive calls for assessment of the duration, intensity and succession of phytoplankton blooms to determine the ecological status of various types of waters. For common phytoplankton growth rates basic signal processing theory yields a minimum monitoring frequency of once per day, which is much more than applied in standard practice. To assess the nature of this discrepancy we followed the behaviour of about 40 groups of organisms/particles found in the Oude Rijn river by a two-week daily cytometric analysis. Particle counts of the 20 most abundant groups are shown. Their variation rate and magnitude confirm that daily sampling is needed to follow such ecosystems in detail. It is shown that limiting the monitoring to the "coarse line" does not allow a correspondingly decreased sampling frequency. Automated systems may fill the gaps between the microscopical examinations by gathering highly frequent information. The information depth of bulk measurements is poor however, and not used as such. The data shown here demonstrate that modern scanning flow cytometry (SFC) offers an information depth close to the taxonomic level. In the past decade, acquisition and operation costs of these systems have come down considerably, whereas operation is hands free, even in situ and submerged, and data analysis has become more efficient. SFC is used most efficiently complementary to microscopical analyses for mutual validation. In these cases it presents a realistic solution to generate the essential high frequency observations required to assess ecosystem variability.

Spring phytoplankton bloom dynamics in Norwegian coastal waters: Microbial community succession and diversity

Most studies of spring bloom succession in Norwegian waters have employed light microscopy and accounted for species composition of phyto‐ and zooplankton. Flow cytometry and molecular tools enable us to extend such investigations to include smaller organisms like bacterio‐ and virioplankton. Here, we describe succession and diversity of algae, bacteria, and viruses in relation to environmental changes from 15 February to 27 April. The spring succession started with an increase in autotrophic picoeukaryotes and Synechococcus sp. The diatoms bloomed around the middle of March and caused nutrient depletion in the upper part of the water column. Upwelling in the beginning of April gave rise to a second bloom, consisting of diatoms and Phaeocystis pouchetii. Numerically, autotrophic picoeukaryotes and Synechococcus sp. dominated the periods between and after these two major blooms. Heterotrophic bacterial abundance increased throughout the experimental period and reached peak values during and after phytoplankton blooms. These bacteria were succeeded by viruses having low DNA fluorescence, whereas viruses with medium DNA fluorescence bloomed during or after blooms of autotrophic picoeukaryotes. High—DNA fluorescence viruses reached maximum concentrations during and after the diatom and Phaeocystis blooms. The diversity of the bacterial community remained relatively stable, whereas viral diversity varied more and increased after major phytoplankton blooms. Our investigation thus demonstrates how virioplankton are important elements of the total microbial diversity and how they are intimately linked to the rest of the microbial community and possibly act as an internal driving force in spring bloom successions.

Spatial and temporal heterogeneity in succession of cyanobacterial blooms in a Spanish reservoir

We describe here the limnological characteristics of the Santillana Reservoir (Spain) and study the phytoplankton distribution during a typical growing season (July to November). The reservoir is a eutrophic stratified waterbody in central part of Spain, in which the average phytoplankton biomass in summer is higher than 20 ug chlorophyll a 1-1. Maximum phytoplankton biomass in 1999 occurred between the end of summer and the beginning of autumn (September-October). Cyanobacteria, that were generally prevalent in phytoplankton community, clearly dominated during the peak of phytoplankton biomass. Microcystis wesenbergii was dominant amongst the cyanobacteria and represented almost 100 % of the biomass at the time of maximum phytoplankton value. The importance of environmental variables triggering cyanobacterial bloom events is discussed. Our results indicate that neither temperature nor N/P ratio are related to the bloom development. However, high nutrient concentra­ tion (ammonium in particular) and stability of the watercolumn seems to coincide with the maximum abundance of Microcystis „ wesenbergii.

Laminations from the Palmer Deep: A diatom‐based interpretation

Highly laminated, Holocene age, diatomaceous sediments are characteristic of the Palmer Deep, western Antarctic Peninsula (Ocean Drilling Program (ODP) Leg 178, Site 1098). From ∼10,000 years B.P. to the present, laminations are comprised of several groups of diatoms. Chaetoceros resting spores, the dominant laminae former, result from intense spring blooms. Rhizosolenia, Proboscia, Thalassiothrix, and Corethron are common also, the consequence of summer production marked by a well‐stratified water column. Each lamination represents a single productivity event, but laminations are not necessarily annual. High concentrations of a subpolar form of Eucampia antarctica, in laminations between ∼9000 and 6700 years B.P., suggest early Holocene warmth, a consequence of southward intrusion of more subpolar waters. The glacial‐interglacial transition is distinguished by pairs of laminae most likely deposited annually. Laminations with an overwhelming dominance of Chaetoceros resting spores alternate with more “terrigenous” laminae, representing alternation of intense spring blooms, with more mixed deposition during the summer. Proximity to retreating glacial ice results in the supply of silt and sand that provides a marker bed between successive blooms.

Livingston, R.J.: Eutrophication Processes in Coastal Systems - Origin and Succession of Plankton Blooms and Effects on Secondary Production in Gulf Coast Estuaries

Livingston, R.J.: Eutrophication Processes in Coastal Systems - Origin and Succession of Plankton Blooms and Effects on Secondary Production in Gulf Coast Estuaries

Late Maastrichtian to early Danian calcareous nannofossils at Elles (Northwest Tunisia). A tale of one million years across the K–T boundary

A detailed study of calcareous nannofossils of the Elles section (Tunisia) was extended over an interval of about 1 Myr including the K–T boundary. The late Maastrichtian assemblages are rich and well preserved. Pulses of surface water cooling, probably related to episodic upwelling are revealed by the increase in abundance of cool water taxa (mainly Kamptnerius magnificus, Nephrolithus frequens, Gartnerago spp.) in certain late Maastrichtian samples, paralleled by a decrease of the warm water species Watznaueria barnesae. A sensible decrease in abundance of Arkhangelskiella cymbiformis and its return to smaller specimens was also recorded at Elles, probably as the result of surface water productivity changes in the latest Maastrichtian. Some Maastrichtian samples at Elles contain Neobiscutum romeinii and Chiasmolithus? sp. These taxa, formerly thought to be Tertiary coccoliths, are here considered as truly Maastrichtian occurrences. As at El Kef, the Danian of Elles is characterised by successive blooms of surviving and newly evolved species. Though the abundance of calcareous nannofossils rapidly reached the same level as in the Maastrichtian, the lower diversity still points to a mesotrophic, poorly stratified marine environment. The reworking versus survivorship of Cretaceous species found in Danian samples remains an unresolved question, but a hypothesis of survivorship is also strengthened by the presence of several coccospheres of W. barnesae and Bidiscus rotatorius well above the boundary. The effect of the marine environmental change across an expanded K–T boundary section as Elles appears complex if seen through calcareous nannofossils. From a geological point of view, the K–T boundary floral turnover at Elles seems dramatically synchronous with the signatures of an extraterrestrial impact, superimposed on long-term, background environmental and climatic changes. However, a precise succession of events based on appearance/disappearance of nannofossil species across the Cretaceous/Paleocene transition is far from being unanambiguous.

Role of transient silicon limitation in the development of cyanobacteria blooms in the Guadiana estuary, south-western Iberia

The Guadiana estuary, located between Portugal and Spain, has the fourth largest drainage basin of Iberian river systems. Up to 75% of the catchment area has been regulated by dams since the early 1970s. During the 1980s and 1990s, an increasing occurrence of summer cyanobacte- ria blooms dominated by the potentially toxic Microcystis spp. was reported. In an effort to under- stand the causes of recurrent noxious blooms in the Guadiana estuary, nutrients (phosphorus, nitro- gen (nitrate, nitrite and ammonium), and silicon (silicic acid), chlorophyll a and phytoplankton abundance, specific composition and biomass were evaluated during field surveys, from April 1997 to March 1998. A pattern of successive blooms of different phytoplankton assemblages was observed throughout this period. Diatoms (nano-sized, chain-forming) dominated an early spring bloom (max. 18 × 10 3 cells ml -1 ) in March and April. Following the decrease in diatom abundance, a chlorophyte bloom (max. 11 × 10 3 cells ml -1 ) and then a cyanobacteria bloom (>6 × 10 4 cells ml -1 ) quickly followed during late spring through to early summer. From July to September, a major cyanobacteria bloom dominated by the potentially toxic Microcystis spp. (> 4 × 10 5 cells ml -1 ) developed in the freshwater zone. The results indicate that high winter loads of nitrogen and phosphorus led to the depletion of silicate (down to as low as 0.2 µM) during the early spring diatom bloom, which conditioned the suc- cessive phytoplankton assemblages during the remaining productive period. Low monthly discharge rates during spring and summer further provided an environment with low Si:N and N:P relative availability which, coupled with high water-column temperature (>21°C), seemed to favour the dominance of cyanobacteria over chlorophytes during the summer.

The Decline and Recovery of a Persistent Texas Brown Tide Algal Bloom in the Laguna Madre (Texas, USA)

The Laguna Madre has experienced a persistent bloom ofAureoumbra lagunensis for over eight years. The persistence of this bloom may be due in part to the often hypersaline conditions in Laguna Madre (40–60 psu) that favor the growth ofA. lagunensis. Above-normal rainfall in the fall of 1997 reduced the salinities in Baffin Bay from >40 to 106 cells ml−1 in July 1997 to c. 200 cells ml−1 in January 1998. During this time of low brown tide density, phytoplankton biomass generally remained high and the Laguna Madre experienced successive blooms of diatoms (Rhizosolenia spp.) and cyanobacteria. Hypersaline conditions returned in 1998 and brown tide densities increased to>0.5 × 106 cells ml−1 by summer. The extraordinary persistence of the brown tide and the unusual sequence of intense blooms may be related in part to the reduction of zooplankton populations. Microzooplankton populations declined following the above-normal rain in the fall of 1997; populations did not recover until fall 1998. Copepod populations also declined sharply and remained low in Laguna Madre, but recovered by summer 1998 in Baffin Bay. Dilution experiments indicated that microzooplankton grazing and phytoplankton growth were usually balanced when measured during our cruises. The rapid recovery of theA. lagunensis bloom suggests that this alga may be a more resilient component of the Laguna Madre flora than previously suspected.

Distribution, dynamics and in situ seeding potential of Scrippsiella hangoei (Dinophyceae) cyst populations from the Baltic Sea

The distribution and seasonal dynamics of cyst populations of the spring bloom dinoflagellate Scrippsiella hangoei were studied in surface sediments on the southwest coast of Finland, Baltic Sea. In situ germination was assessed by monitoring the fraction of empty cysts and chlorophyll a fluorescence in cyst populations at different coastal sites throughout the annual cycle. Scrippsiella hangoei resting cysts were widely distributed in the study area and occurred in exceptionally large numbers (magnitudes of 10 4 -10 6 cysts cm -3 ) at all sampling locations between the innermost parts of the coastal archipelago and the open Gulf of Finland. The decreases in cyst number in winter and the increases occurring in late spring reflected the dynamics of germination and encystment of the species. Chlorophyll fluorescence appeared in mid-winter in -40% of cysts from well-aerated basins and 6-15% of cysts from temporarily anoxic sediments. A generally low increase in the proportion of empty cysts indicated that only a part of the potentially germinable cysts actually germinates. Given the high cyst concentrations in the sediments, the potential for germination is considerable, despite the environmentally and physiologically determined losses. In contrast, the size of the vegetative inoculum is very low, indicating that the survival of germlings is problematic under harsh winter conditions. This is an unusual life cycle strategy; however, the early release of cells into the water column provides a high probability for successful bloom initiation under the unpredictable meteorological conditions in winter and early spring, which often lead to the sudden onset of favourable growth conditions.

Seasonal abundance and toxicity of the dinoflagellate Gambierdiscus spp. (Dinophyceae), the causative agent of ciguatera in Tahiti, French Polynesia

The benthic dinoflagellate Gambierdiscus spp. is the primary causative agent of ciguatera fish poisoning in French Polynesia. However, the initiation of ciguatera outbreaks and the factors that control ciguatoxin production in this dinoflagellate are still poorly understood. In this paper, we present some original data concerning the seasonal abundance and toxicity of natural populations of Gambierdiscus spp. based on a long-term survey in a ciguateric site of the island of Tahiti. From February 1993 to December 1997, Gambierdiscus spp. population densities were monitored weekly in the Atimaono barrier reef of Papara in relation to temperature and salinity. Densities peaked at 4992 cells g−1 in October 1994 and constituted ≥1000 cells g−1 on several occasions. A total of 58 blooms were recorded in the area, of which 65% occurred in 1995 and 1996 alone. Our data confirmed the high endemicity of Gambierdiscus spp. in the Papara area. Refined analysis of our raw data by a principal-component analysis of processes (PCAP) revealed a seasonality in cell densities from February 1993 to May 1995. During this period, Gambierdiscus spp. populations tended to reach maximum abundance at the beginning and end of the hot season. In contrast, salinity did not appear to be a determining factor in the seasonal abundance of this dinoflagellate. The noticeable increase in both peak densities and frequency of blooms further noticed in 1995 and 1996 was preceded by unusually high water temperatures in January to April 1994, concomitant with a severe coral-bleaching episode. During the course of the study, a total of 303 × 106 cells obtained from 48 successive blooms was harvested. Toxicity screening revealed that toxin production was maximum from October 1994 through December 1996. No correlation was found between toxicity of these blooms and their biomass nor the seasonal pattern of temperatures. It is suggested that the toxicity of naturally-occurring blooms of Gambierdiscus spp. and, consequently, the severity of ciguatera incidents in a given area, is mainly dependent on the clonal nature of cells which coexist within local populations of this dinoflagellate.

Seasonal variability in microplanktonic biomasses in the Gironde dilution plume (Bay of Biscay): relative importance of bacteria

Bacterial and phytoplankton biomasses were estimated in a four-year study along a saline gradient (the dilution plume of the Gironde estuary on the Aquitanian continental shelf), by measuring bacterial abundance and mean cell volumes (epifluorescence counts) and chlorophyll- a concentrations (in vitro fluorescence). The spatial and temporal distribution of phytoplankton enabled us to determine the seasonal succession of algal blooms, sprouting in the marine waters of the inner shelf in early spring (picoplanktonic forms), reaching their annual maximum (10 μg L −1) in plume waters in advanced spring, and moving to estuarine waters in the summer (> 20 μm forms). Bacterial biomasses followed mainly a decreasing dilution gradient, especially in winter and early spring, with maximum estuarine values reaching 10 7 cell mL −1 (more than 30 % of small attached cells and more than 60 % of free small cocci), in accordance with estuarine discharges into the inner shelf zone. Moreover, plume waters showed an additionnal peak of bacterial numbers and mean cell volumes in early summer, with increasing proportions of larger rod- and vibrio-shaped bacteria. A clear morphometrical succession was followed in marine waters, from a majority of attached and coccal cells in winter, to maximal proportions of larger vibrio-shaped bacteria, inducing a maximal spring level of total abundance (2 × 10 6 cell mL −1) and high mean cell volumes (0.044 μm 3), coinciding with the marine vernal bloom. Bacterial estimated biomass represented, consequently, a wide range of proportions compared to phytoplankton: only when a good correlation was found between them, bacterial-C/phytoplankton-C ratio was close to 1 4 , a correlation that improved in warmer periods, throughout different water types. Due to the relavitely narrow range of chlorophyll- a values, we could not find a high predictive correlation when pooling all the 4-year data, a relationship that has been outlined in important reviews by pooling together different aquatic systems. We noticed, however, that in the particular environment of our study (high inputs of particulate detritic matter) bacterial-C usually dominated the microbial pool (specially in estuarine and plume waters and in light-limited conditions). This dominance, added to the frequent dominance of small-sized phytoplankton, leads to an enhanced “microbial loop” activity, defining a “maintenance system” (oligotrophic conditions), which alternates with some eutrophic periods.

L'instabilite du phytoplancton et le phenomene des eaux claires dans les etangs de pisciculture

Phytoplankton instability and clear water phenomena in fish ponds. In fish ponds, the specific composition and abundance of phytoplankton present a very stong instability. The different causes are discussed: temperature, light, human interventions (adding of mineral or organic fertilizers), grazing by zooplankton, carbon or other element depletion. The light intensity much more affects species composition than importance of algal biomass. In a pond where the big Cladocerans ( Daphnia) are eliminated by fish predation, the most probable cause of a very fast increase of water transparency (clear water) is an algal collapse. The effects of phytoplankton instability (succession of blooms and collapses) on environmental conditions are also considered: Cyanobacteria blooms following green algae sedimentation, oxygen depletion caused by an important algal collapse, deterioration of chemical quality of water.