Coagulation efficiency and aggregate formation in marine phytoplankton

Flocculation of phytoplankters into large, rapidly sinking aggregates has been implicated as a mechanism of vertical transport of phytoplankton to the sea floor which could have global significance. The formation rate of phytoplankton aggregates depends on the rate at which single cells collide, which is mainly physically controlled, and on the probability of adhesion upon collision (=coagulation efficiency, stickiness), which depends on physico-chemical and biological properties of the cells. We describe here an experimental method to quantify the stickiness of phytoplankton cells and demonstrate that three species of diatoms grown in the laboratory (Phaeodactylum tricornutum, Thalassiosira pseudonana, Skeletonema costatum) are indeed significantly sticky and form aggregates upon collision. The dependency of stickiness on nutrient limitation and growth was studied in the two latter species by investigating variation in stickiness as batch cultures aged. In nutrient repleteT. pseudonana cells stickiness is very low (< 5 × 10^?3), but increases by more than two orders of magnitude as cell growth ceases and the cells become nutrient limited. Stickiness ofS. costatum cells is much less variable, and even nutrient replete cells are significantly sticky. Stickiness is highest (> 10^?1) forS. costatum cells in the transition between the exponential and the stationary growth phase. The implications for phytoplankton aggregate formation and subsequent sedimentation in the sea of these two different types of stickiness patterns are discussed.     

Allelopathic interactions between Skeletonema costatum and Alexandrium minutum

Potential allelopathic interactions between Skeletonema costatum and Alexandrium minutum were investigated using mixed cultures and culture filtrate in nutrient-replete medium. A. minutum growth was inhibited when grown in S. costatum filtrate, with the inhibitory effect directly proportional to the percentage of filtrate added. This demonstrates that the release of allelopathic compounds caused the growth inhibition. In contrast, the filtrate of A. minutum exerted no allelopathic activity on S. costatum. An autoinhibitory compound (15(S)-HEPE) extracted and purified from S. costatum culture was added to cultures of both S. costatum and A. mintum. This substance could depress S. costatum growth, but showed no significant inhibitory activity on A. minutum. This documented a second type of allelochemical interaction, termed auto-allelopathy, caused by a different compound from the one or ones that affected A. minutum in the co-cultures with added crude filtrate. Further studies are needed to explore the relative importance of these two types of allelopathy as factors influencing the competition between S. costatum and A. minutum in the field. Furthermore, given the observed decrease in diatom dominance relative to dinoflagellates with increasing eutrophication, one can predict that toxic species like A. minutum might become more prevalent in the future in the East China Sea if the trend of increasing pollution of coastal waters continues.     

Mechanisms for copper tolerance in Amphora coffeaeformis-internal and external binding

Growth of the ship-fouling diatom Amphora coffeaeformis and accumulation of copper in the cells were evaluated for cultures exposed to copper. Comparisons with literature reports for other species revealed that A. coffeaeformis shows no ability to maintain normal growth rates in the presence of high cellular copper levels. This suggests that internal binding is not the principal copper tolerance mechanism for this species. In addition, the copper complexing capacity of A. coffeaeformis exudates was evaluated. Significant complexing by these exudates was demonstrated by DPASV analysis. When added to the culture medium of another species (Thalassiosira profunda), A. coffeaeformis exudates were also able to reduce copper toxicity and accumulation in the cells of that species. However, the copper tolerance of A. coffeaeformis was greater than that acquired by T. profunda grown with A. coffeaeformis exudates; thus exudate production was deemed not to be a primary tolerance mechanism. Comparison of copper accumulations inside and outside cells of A. coeffeaeformis suggests that binding at the cell surface or to mucilage may be an important factor in the tolerance of this species to copper.     

Total organic sulfur and dimethylsulfoniopropionate in marine phytoplankton: intracellular variations

Cellular levels of particulate organic sulfur (POS) and dimethylsulfoniopropionate (DMSP) were determined in cultures of five species of marine phytoplankton, Amphidinium carterae, Prorocentrum minimum, Emiliania huxleyii, Dunaliella tertiolecta and Skeletonema costatum. The first three are known producers of DMSP while the latter two produce little or non-detectable amounts of DMSP. In those species which produced significant amounts of DMSP, intracellular levels of POS and DMSP varied for the dinoflagellates (A. carterae and P. minimum) while they remained fairly constant for the prymnesiophyte (E. huxleyii) over the growth cycle (until late stationary phase), and DMSP accounted for the majority of the POS (50 to 100%). In species with low levels of DMSP, intracellular POS and DMSP decreased or remained constant over the growth cycle, and DMSP accounted for a much lower percentage of the POS (0 to 40%). Species with high DMSP concentrations had higher POS levels per unit cell volume as well, and DMSP accounted for substantially higher percentages of the particulate organic carbon (POC). Molar N:S ratios suggest a non-protein origin for much of the sulfur in the species producing DMSP. During late stationary phase, an increasing percentage of the DMSP became extracellular in the dinoflagellate and diatom (S. costatum) cultures, suggesting leakage. In a bacterized algal culture, measured quantities were considerably less than in an axenic counterpart, suggesting consumption.     

Growth interactions between marine dinoflagellates in multispecies culture experiments

The growth interactions between the marine dinoflagellates Scrippsiella faeroense (Paulsen) Balech et Soares, Prorocentrum micans Ehrenberg and Gymnodinium splendens Lebour were investigated in batch and continuous-culture combinations. Generation times were 1.3±0.2 days for S. faeroense; 1.6±0.1 days for P. micans and 1.8 ± 0.2 days for G. splendens. In mixed batch cultures, growth depended strongly on the ratio between cell numbers of the two species inoculated at the start of the experiments. In various inocula, the more abundant species prevailed and suppressed the competitor even during exponential growth. Total cell production of the suppressed forms was lowered drastically; initial generation time, however, remained more or less unaffected. In equal inocula, S. faeroense dominated over G. splendens, P. micans over S. faeroense and G. spendens over P. micans. In continuous cultures, the species grew independently of each other during the exponential stage; cell number was regulated only by generation times. Competitive inhibition did not occur until maximum cell densities of the dominating forms had been reached. Filtration experiments indicated inhibiting effects mainly in filtered culture liquids of stagnating cultures: reinoculated cells of all three test populations showed a somewhat reduced total cell production. Short initial lag phases were indicated in filtered culture liquids of P. micans and G. spendens; following this period, generation times of the test population remained unaffected, however. The experiments show that growth in multispecies cultures is regulated during the exponential stage mainly by nutrient competition, while at maximum cell densities an additional effect of inhibiting metabolic products is involved. Whether this effect is caused by toxic algal excretions or by bacterial decomposition products of dead cells could not be determined. Sexual stages occurred in aged cultures of S. faeroense and G. splendens both in monocultures and in multispecies combinations and influenced the equilibrium of the competing species. The results are discussed in the light of the findings of other authors.This paper was presented at a poster presentation of the Second International Conference on Toxic Dinoflagellate Blooms, held in Key Biskayne, Florida, USA, 1978.     

Regulation of photochemical activity in cultured symbiotic dinoflagellates under nitrate limitation and deprivation

The effects of nitrate limitation and nitrate starvation on the photochemical efficiency of photosystem II (Fv/F m) were examined in batch cultures of two species of symbiotic dinoflagellates, Symbiodinium kawagutii and S. pilosum. F v/F m values were determined along growth curves and show that the F v/F m values are negatively correlated with external nitrogen concentrations in cultures of both species. Changes in growth irradiances in the batch cultures due to increments of the cell densities were estimated S. kawagutii cultures showed a negative correlation between F v/F m and growth irradiance. These results indicate that F v/F m is dependent on the light history of the cultures and on the individual sensitivity of each species, and independent of their nutrient status. Nitrate starvation was analyzed by measuring changes in the quantum yield of fluorescence (Fv/F m), electron transport rate (ETR) and non-photochemical quenching (NPQ) at five time points along the growth curves under three conditions: control (C), without nitrogen (N–), and with ammonia (N+) as a nitrogen source sufficient to meet daily nitrogen requirements. Cells collected during the exponential growth phase and exposed to N– and N+ showed significant reductions in their maximum ETR relative to controls (20% in S. pilosum and 40% in S. kawagutii). The loss of electron transport capacity is consistent with a sink limitation rather than the result of nitrogen starvation. Under nitrate-starvation, the induction of NPQ resulted in effective protection against photosystem II damage in S. pilosum. In contrast, S. kawagutii cells failed to induced NPQ resulting in a concomitant increase in the excitation pressure over photosystem II leading to damage. Collectively the data indicate that F v/F m is not a robust indicator of nitrogen limitation in symbiotic dinoflagellates and that protection against photosystem II damage under sink limitations, is largely dependent on the differential capacities of each species to induce NPQ.Communicated by P.W. Sammarco, Chauvin     

Influence of temperature on the growth rate of Skeletonema costatum in response to variations in daily light intensity

Daily light intensities (I _o) can vary 10-fold during the winter-spring and late-summer diatom blooms in New England, USA, coastal waters. Laboratory cultures and natural populations incubated in dialysis sacs were examined to determine the time course of growth rate in Skeletonema costatum (Greville) Cleve in response to variations in daily light intensity during two bloom periods in Narragansett Bay, Rhode Island, USA. Log-phase cultures of S. costatum require 2 d to attain maximum growth rates at 2°C following transfer to saturating intensities. At 20°C, only 1 d is required. As temperature increases, Detonula confervacea (Cleve) Gran, Thalassiosira nordenskiöldii Cleve and Ditylum brightwellii (West) Grunow also exhibit rapid increases in mean daily division rates (K) following transfer to saturating light intensities. Thalassiosira pseudonana Hustedt, however, did not alter the time required to achieve maximum K as temperature varied. Natural populations of S. costatum did not show a well-defined relationship between K and light. Throughout a winterspring bloom, K was limited by low temperatures and exhibited no clear response to variations in I _o. A change in K in response to variation in I _o may occur on a daily basis during the summer, when temperatures are near 20°C; this has yet to be verified for in situ populations.     

Effects of ethyl 2-methyl acetoacetate (EMA) on the growth of Phaeodactylum tricornutum and Skeletonema costatum

We investigated the effects of ethyl 2-methyl acetoacetate (EMA) on growth of the marine diatom algae Phaeodactylum tricornutum (P. tricornutum) and Skeletonema costatum (S. costatum). Growth of P. tricornutum was significantly inhibited by the minimum concentration (3.5 mmol·L ^?1) of EMA at lower initial algal densities (IADs) (3.6×10⁴ and 3.3×10⁵ cells·mL ^?1). However, at the highest IAD, significant growth inhibition was found at above 7 mmol·L ^?1 of EMA exposure. In S. costatum, EMA concentrations of 10.5 mmol·L ^?1 or more significantly inhibited growth at lower IAD (3×10⁴ and 1.8×10⁵ cells·mL ^?1); at the highest IAD, only EMA concentrations above 14 mmol·L ^?1 obviously inhibited the growth of S. costatum. Changes in specific growth rates and pigment were consistent with algal growth, but only at higher EMA concentrations or lower IAD values was the ratio of chlorophyll a (Chla) to carotenoid significantly lower than the control. Medium effective concentration (EC ₅₀) values were in the order 4.07, 8.03 and 12.27 mmol·L ^?1 for P. tricornutum and 7.48, 11.92 and 17.22 mmol·L ^?1 for S. costatum. All these results show that the effect of EMA on the growth of algae was species specific and mainly depended on IAD, which might be an important factor to influence algal growth.     

Interactions between nitrate and ammonium uptake: variation with growth rate,nitrogen source and species

The interaction between nitrate and ammonium uptake was examined as a function of preconditioning growth rate and nitrogen source by adding nitrate, ammonium, or both to nitrogen-sufficient,-deficient, and-starvedSkeletonema costatum (Grev.) Cleve and nitrogen-deficientChaetoceros debilis Cleve. By simultaneously measuring the internal accumulation of intermediates of nitrogen assimilation and the rates of nitrogen assimilation, the metabolic control of nitrogen uptake could be assessed. After the simultaneous addition of nitrate and ammonium to culture, both nitrate and ammonium uptake rates were decreased in comparison with the rates observed when each was added alone, although nitrate uptake was usually decreased more than ammonium uptake. Since both nitrate and ammonium uptake rates vary with time, preconditioning growth conditions, nitrogen sources present, and species, it was necessary to use several different indices to quantify inhibition. In general, ammonium inhibition of nitrate uptake inS. costatum was greatest in cultures preconditioned to ammonium and those at low growth rates, whereas ammonium uptake was inhibited most in cultures preconditioned to nitrate. In nitrogen-deficientC. debilis, nitrate uptake was more inhibited by ammonium, but uptake returned to normal rates more quickly than inS. costatum, whereas inhibition of ammonium uptake was similar. These results explain why the interaction between nitrate and ammonium uptake in the field can be so variable. Inhibition of uptake is not controlled by internal ammonium or total amino acids, nor is it related to the inability to reduce nitrate. Instead, inhibition must be determined in part by the external concentration of nitrogen compounds and in part by some intermediate(s) of nitrogen assimilation present inside the cell.Bigelow Laboratory Contribution No. 82022     

Diurnal and circadian rhythms in the turbidity of growing Skeletonema costatum cultures

Skeletonema costatum (Grev.) Cleve grown in batch culture at low light intensity under a 14 h light: 10 h dark photocycle showed exponential cell proliferation (1.1 doublings d^-1) without significant phasing of the cell division by the light: dark cycle. The growth in carbon concentration was, however, restricted to the light period. The turbidity of the culture closely followed the carbon oattern, and was not affected by the increase in the cell number during the dark period. It was found that a trustule suspension had only approximately 1% of the turbidity of the corresponding intact algae. Culture turbidity was therefore regarded as a biomass parameter similar to the carbon concentration, without direct correlation to the timing of the cell division. The short-time variations in the turbidity of growing algal cultures were further studied in a cage culture turbidostat. The growth rate (based on turbidity) increased rapidly during the first half of the light period, decreased slightly towards the evening and was zero throughout the dark period. When transformed to continuous light, the growth of the culture continued to show damped oscillations for up to 1 wk, but with a period of 26.7 h instead of 24 h. The same circadian rhythm was observed in chlorophyll content, and is thus possibly a reflection of a freely oscillating internal biological clock. The cage culture turbidostat was found to be a suitable device for studies of the photocycle related regulation of biosynthesis in S. costatum.     

Implications of salinity fluctuation for growth and nitrogen metabolism of the marine diatom Ditylum brightwellii in comparison with Skeletonema costatum

In 1987 effects of salinity fluctuations on growth of Ditylum brightwellii (West) Grunow, isolated from the Eastern Scheldt estuary (SW Netherlands) in 1981, were studied. D. brightwellii was grown in a 12 h light: dark cycle at constant salinity in brackish media. Ammonium-limited cultures were subjected to a salinity fluctuation. By decreasing the salinity to 4.8 photosynthesis and cell division were inhibited; cells were deformed. Protein and carbohydrate contents increased slightly, dark respiration was stimulated and cellular levels of glucose decreased at low salinity; this indicated a possible role of sugars in osmoregulation. Ammonium was accumulated in cultures, amino acids may have been stored; the role of the vacuole as a storage compartment was discussed. Both the ammonium uptake capacity and the affinity for ammonium decreased. Nitrogen limitation was relieved in the transient state. [With the activity of the nitrogen assimilation enzymes glutamine synthetase (GS) and glutamate synthase (GOGAT) being uninhibited by lower salinity.] Recovery from hypo-osmotic stress during a salinity increase was initiated by stimulated photosynthesis; chlorophyll a increased, but persistant contractions of cytoplasm (with chloroplasts) may have delayed cell growth. The glutamate dehydrogenase (GDH) activity decreased further whereas the cellular level of alanine increased in the presence of large ammonium pools; this may indicate a temporary activity of ADH (alanine dehydrogenase). Skeletonema costatum (Greville) Cleve, recovered faster from hypoosmotic stress than did D. brightwellii. Due to an osmotic shock from 13.6 to 7.1 S both species excreted amino acids and glucose; S. costatum accumulated more glucose, D. brightwellii accumulated more amino acids. S. costatum may with the competition for nitrogen in waters with an unstable salinity; it will replace D. brightwellii.Contribution no. 427 Delta Institute for Hydrobiological Research, Yerseke, The Netherlands     

Inhibitory effects of copper on marine dinoflagellates

The effect of copper on three species of marine dinoflagellates [Scrippsiella faeroense (Paulsen) Balech et Soares, Prorocentrum micans Ehrenberg, Gymnodinium splendens Lebour] was studied. It inhibited the growth of all species and was lethal to one species in batch cultures. The effect was more pronounced in semicontinuous culture than in batch cultures. Chlorophyll concentrations and rate of uptake of radioactive carbon by cells of S. faeroense were affected in a manner similar to cell numbers. Copper inhibited growth of cells, most probably either by arresting cell division or by penetrating inside the cell and affecting metabolism.     

Sinking rate response to depletion of nitrate,phosphate and silicate in four marine diatoms

The effect of nutrient (N, P, and Si) depletion on sinking rates was studied for two small (Skeletonema costatum (Grev.) Cleve and Chaetoceros gracile Schütt) and two large [Ditylum brightwellii (West) Grun and Coscinodiscus wailesii (Gran et Angst)] centric diatoms obtained from stock cultures. Each diatom was examined under conditions of (1) nutrient repletion (=log growth phase), (2) nutrient depletion (48 h without a given substrate), and (3) recovery (24h after addition of limiting substrate to nutrient-deplete populations). All nutrient-replete cultures displayed low sinking rates despite large differences in cell size. In nutrient-deplete populations, sinking rate was related to the kind of nutrient depleted and varied among species. Silicate depletion elicited by far the greatest increase in sinking rates in all 4 species, indicating that biochemical aspects of silicon metabolism are more important to buoyancy regulation than density-related variations in the amount of silicon per cell. Since N- and P-depletion caused lower sinking rates in 3 of the species, this observation calls for re-evaluation of the axiom that nutrient depletion necessarily causes increased sinking rates. The exception was Coscinodiscus wailesii, which sank faster under all types of nutrient limitation. In most cases, sinking rates typical of log-phase cultures were not regained within 24 h after the addition of limiting nutrient to nutrient-depleted populations. Ultimately, the length of the recovery period may be useful in identifying the metabolic processes responsible for buoyancy regulation in actively growing cells.     

Nutritional value of Phaeocystis pouchetii (Prymnesiophyceae) and other phytoplankton for Acartia spp. (Copepoda): ingestion,egg production,and growth of nauplii

Ingestion and egg production by Acartia hudsonica (5°C) and A. tonsa (10°C) from Narragansett Bay, Rhode Island, USA, was measured in the laboratory over a range of concentrations of the prymnesiophyte Phaeocystis pouchetii and the diatom Skeletonema costatum. Both Acartia species reproduced well when fed unialgal diets of S. costatum. Egg production by females fed P. pouchetii, either as gelatioous colonies (primarily >200 m in diameter) or solitary cells (3 to 5 m), was not significantly different than that of starved females. Acartia spp. fed selectively on S. costatum in multialgal treatments. The presence of P. pouchetii did not reduce ingestion of S. costatum; the availability of S. costatum did not increase ingestion of P. pouchetii. Nauplii of A. hudsonica grew equally well on separate diets of P. pouchetii and Isochrysis galbana, two flagellates of similar size. Adult Acartia spp. reproduced poorly when fed these phytoplankton, suggesting that particle size may be more important than food quality in describing the responses. Grazing by Acartia spp. does not directly impact the dynamics of P. pouchetii, but may indirectly contribute to blooms of this prymnesiophyte by removal of competing phytoplankton.     

Influence of high temperature and residual chlorine on marine phytoplankton

Marine phytoplankton forms are frequently exposed to sudden biological changes such as rapid rise in water temperature and chlorine content of their environment, resulting from the use of sea water for cooling purposes by electric generators. The direct influence of these effluents, i.e. inhibitory effects of high temperature and residual chlorine on growth and photosynthesis of Chlamydomonas sp. and Skeletonema costatum, were investigated experimentally. Chlamydomonas sp. and S. costatum exposed to high temperatures were affected in their growth from 43° and 35°C, respectively, by immersion of the respective cultures in a warm bath for 10 min. Treatment at high temperatures of 40 °C and 30° 35°C for 10 min, influenced their photosynthetic activities, which were completely inhibited immediately after 10 min exposure at 42° and 37 °C, respectively. S. costatum was killed by chlorine at a concentration of 1.5 2.3 ppm when exposed for exactly 5 or 10 min, while Chlamydomonas sp. was not irreversibly damaged even at 20 ppm chlorine or more with the same exposure period. These results lead to the conclusion that the high temperature of, and residual chlorine in, effuents from a power plant discharging into the open sea, should not cause great damage to marine phytoplankton in that area.     

Effect of light and turbulent mixing on the growth of<Emphasis Type="Italic"> Skeletonema costatum</Emphasis> (Bacillariophyceae)

Skeletonema costatum was grown in an outdoor mesocosm to test the hypothesis that fluctuations in irradiance brought about by changes in mixing time and depth can reduce diatom growth and biomass in the turbulent mixed layer. The light environment and mixing regime within the mesocosm were comparable to those in shallow lakes and coastal waters. Experiments showed no significant differences for 24-h mean and 7-day mean chlorophyll a and carbon-specific growth for mixed depths of 1 m and 3 m, and mixing times between 4 min and 65 min. Fluctuations in irradiance brought about by turbulent mixing had no significant effect on specific growth. The relationship between mixed depths and critical depths for S. costatum was therefore independent of fluctuations in irradiance in the turbulent mixed layer. The results indicated that to control growth of S. costatum mixed depths would have to exceed photic depths by a factor of 15, instead of the conventionally accepted factor of 5. Thus, it is likely that artificial mixing of shallow (<10 m) eutrophic waters will be more effective in controlling slow-growing summer biomass than fast-growing spring blooms dominated by diatoms.Communicated by P.W. Sammarco, Chauvin     

Energetics of early development for the sea urchinsStrongylocentrotus purpuratus andLytechinus pictus and the crustaceanArtemia sp.

Embryos and larvae of two species of sea urchin,Strongylocentrotus purpuratus andLytechinus pictus, and larvae of the brine shrimpArtemia sp. (San Francisco brand) were cultured to investigate the contribution of dissolved organic material in seawater to the energetics of early development. When embroys ofS. purpuratus were reared in artificial seawater, a net loss in dry organic mass was observed. In contrast, when sibling embryos were reared to Day 2 under identical conditions in natural seawater, there was either a net increase in dry organic mass or no change. A net decrease in mass was observed in only one of five cultures reared in filtered natural seawater. Energy budgets for each species were determined by giving energy equivalents to the changes in carbohydrate, lipid and protein, and to the rate of oxygen consumption for each day of development. In the case ofS. purpuratus, the use of endogenous reserves accounted for either 0 or 38% of the metabolic demand for two independent cultures reared from Days 0 to 2. For larvae ofL. pictus, reared to 8 d, only 66% of the metabolic demand could be accounted for by the use of endogenous reserves. Sea urchins are capable of transporting dissolved organic material from seawater. Calculations revealed that the energy deficit during the early development of sea urchins (S. purpuratus) could be accounted for by the uptake of dissolved organic matter from seawater. However, for a species that cannot use this resource (Artemia sp.), the metabolic needs during development are supplied through the use of endogenous reserves.     

Lethal effects of Northwest Atlantic Ocean isolates of the dinoflagellate, Scrippsiella trochoidea, on Eastern oyster (Crassostrea virginica) and Northern quahog (Mercenaria mercenaria) larvae

The thecate dinoflagellate Scrippsiella trochoidea is a cosmopolitan, bloom-forming alga that has been generally considered non-toxic. Here, we report that environmentally relevant cell densities (10⁴ cells mL⁻¹) of Scrippsiella trochoidea strains isolated from the Northwest Atlantic Ocean caused 100% mortality in Eastern oyster (Crassostrea virginica) larvae during 3-day exposures while parallel control larvae exhibited 100% survival. S. trochoidea also exhibited lethal effects on Northern quahog (Mercenaria mercenaria) larvae (70% mortality during 3-day exposure) but were non-toxic to juvenile fish (Cyprinodon variegates). The cultures of S. trochoidea were more lethal to Northern quahog larvae than ten other species of harmful algae, including the highly toxic species Cochlodinium polykrikoides. Scrippsiella trochoidea cultures within later stages of growth were more toxic than exponential growth stages to bivalve larvae, and the toxicity was dose dependent. Furthermore, toxicity was maintained in the cultures that were sonicated, boiled, and frozen as well as in resuspended residues of the culture but was significantly lower in cell-free culture media. Collectively, these results suggest that S. trochoidea causes mortality in bivalve larvae through a physicochemical rather than strictly chemical mechanism, such as clogging of larval feeding apparatuses by materials produced by S. trochoidea (e.g., lipids, extracellular polysaccharides, and/or cell debris) which accumulate as cells in culture or blooms age. This is the first report of the lethal effects of Scrippsiella trochoidea on shellfish larvae.     

Adaptive significance of phytoplankton stickiness with emphasis on the diatom Skeletonema costatum

Diatom aggregate formation was analyzed using coagulation theory. Population dynamics models show that coagulation has an important impact on species succession during diatom blooms. When different species collide and form mixed aggregates this process causes interspecific interference competition within the diatom community. The outcome can be predicted by a set of simple differential equations. For a twospecies system the equations reduce to the Lotka-Volterra two-species competition model. The outcome of this interference competition depends on species-specific growth rates, cell sizes, stickiness and on the species composition of the seeding populations of a bloom. Due to mutual flocculation some species may disappear from the environment. Small and fast growing diatoms are favoured by high stickiness coefficients. The impact of stickiness on species succession was found to be most pronounced in eutrophic and hydrographically isolated environments. The sticking properties of the diatom Skeletonema costatum are discussed in an evolutionary context; we suggest that mutual coagulation increases the abundance of S. costatum relative to other diatom species in coastal areas. The model was tested on field data, and the predicted dynamics of a spring bloom was very similar to that observed.     

Competition and facilitation between germlings of<Emphasis Type="Italic"> Ascophyllum nodosum</Emphasis> and<Emphasis Type="Italic"> Fucus vesiculosus</Emphasis>

Competitive interactions between germlings of Ascophyllum nodosum (L) Le Jolis and Fucus vesiculosus L. were studied both in the laboratory and on a shore of the Isle of Man, in the Irish Sea. Both intra- and interspecific competition were investigated by comparing the performance of algal germlings both in monocultures and mixed populations of the two species. The growth of germlings of both species reduced with increasing density. F. vesiculosus always grew faster than Ascophyllum and did best in mixed cultures, whereas Ascophyllum did least well when mixed with Fucus germlings. Clearly the adverse effects of F. vesiculosus on A. nodosum were greater than those of Ascophyllum cohorts. At the same total density, the survival and growth of Ascophyllum declined with an increasing proportion of Fucus germlings, implying that poor recruitment of A. nodosum results from strong competition with F. vesiculosus. However, under desiccation stress on the shore, F. vesiculosus enhanced the survival of A. nodosum at the early germling stage even though competition may occur again at the late stage. Thus, whether interactions between germlings take the form of competition or facilitation depends on the environmental conditions.Communicated by O. Kinne, Oldendorf/Luhe