Interaction effects of zooplankton and CO2 on phytoplankton communities and the deep chlorophyll maximum

相似文献   

Grazing effect of the invasive reef-forming polychaete Ficopomatus enigmaticus (Fauvel) on phytoplankton biomass in a SW Atlantic coastal lagoon

In this work we evaluate the potential grazing impact of the invasive reef-forming polychaete Ficopomatus enigmaticus in a SW Atlantic coastal lagoon (Mar Chiquita coastal lagoon; 37° 40' S, 57° 23' W; Argentina). This gregarious species feeds on suspended detritus and phytoplankton. Given the large area dominated by reefs, suspension feeding by this species might reduce the lagoon phytoplankton concentration and even hinder local eutrophication. To evaluate this hypothesis in situ replicated mesocosm experiments were performed in spring 2005 and, summer and winter 2006. Mesocosms enclosing reefs and without reefs were installed and grazing intensity was measured as the difference in chlorophyll a concentration and turbidity between the reef-treatment and the treatment without reefs. Reefs of F. enigmaticus decreased the mean chlorophyll a concentration, more during summer (56% decrease) than spring (25% decrease) and winter (19% decrease). Reefs also decreased water turbidity during summer (54% decrease) and spring (23% decrease). While previous studies indicate that the physical structure of these reefs alters water flow increasing water turbidity, our evidences show that their suspension-feeding activity can counteract this effect. Water turbidity was positively correlated with chlorophyll a concentration, which suggests that phytoplankton grazing by F. enigmaticus decreased light attenuation through the water column, with a potential for enhancement of benthic primary productivity. Therefore, our results suggest that grazing by this polychaete affects overall estuarine primary production as well as the relative importance of planktonic and benthic carbon sources to higher trophic levels.     

Ultraviolet-B effects on photosystem II efficiency of natural phytoplankton communities from Antarctica

The impact of UVB on the Antarctic phytoplankton photosystem II repair cycle, involving the rapidly cycled D1 protein, was studied during summer 2002. On sunny and overcast days, phytoplankton (from 1-m depth) were exposed to natural light (+UVB) and Mylar-screened (–UVB) conditions. Half of the samples from each treatment were inoculated with lincomycin, an inhibitor of synthesis of chloroplast-encoded proteins including the D1 protein. Blocking D1 repair caused significant F_v/F_m depressions on sunny days but had not effect on the overcast day. Most of the F_v/F_m depression was caused by PAR and UVA with a non-significant contribution from UVB. In the presence of D1 repair, suppressing UVB had no effect on F_v/F_m when the samples originated from a weakly stratified water column with no defined upper mixed layer (UML) while it alleviated F_v/F_m depression when the phytoplankton samples originated from within an UML deeper than the depth of UVB penetration. These results suggest that UVB had more effect on the D1 repair process than on the damage process itself but that phytoplankton sensitivity to surface UVB exposure was influenced by their previous light history, partly determined by the vertical structure of the water column.     

Étude expérimentale de la nutrition d'un copépode commun (Temora stylifera Dana). Effets de la température et de la concentration de nourriture

The ingestion rate of phytoplankton by adult Temora stylifera Dana changes with concentration of algae and with temperature. From the experimental results, two types of curves were fitted to the data. The effects of these two variables are generally considered independently in grazing models. Thus, the maximal ingestion parameter of the relationship between ingestion and phytoplankton concentration is expressed as a function of temperature. It is therefore possible to formulate simultaneously the effect of these two factors. This formulation of grazing, after normalization of units, can be easily generalized to other filter-feeding copepods.     

Nutrients and phytoplankton in Lake Peipsi during two periods that differed in water level and temperature

Data for the vegetation periods (May–November) of 1985–2003 were used to collate the nutrient content and biomass of the most important phytoplankton groups in Lake Peipsi (Estonia). Two periods differing in external nutrient load and water level were compared by analysis of variance. The years 1985–1988 were characterized by the highest loads of nitrogen and phosphorus, high water level and cool summers. The years 2000–2003 were distinguished by low or medium water levels and warm summers. The first period showed statistically significantly higher values of total nitrogen (N_tot) and a higher N_tot:P_tot mass ratio. The second period showed a higher content of total phosphorus (P_tot), a higher ratio of dissolved inorganic compounds N to P and higher phytoplankton and cyanobacterial biomasses. Comparison between parts of the lake demonstrated that the differences between the two periods were more evident in the shallower and strongly eutrophic parts, Lake Pihkva and Lake Lämmijärv, than in the largest and deepest part, the moderately eutrophic Lake Peipsi s.s. Temperature and water level acted synergistically and evidently influenced phytoplankton via nutrients, promoting internal loading when the water level was low and the temperature high. The effect of water level was stronger in the shallowest part, Lake Pihkva. The difference in P_tot content between the southern and northern parts was twofold; the N_tot:P_tot mass ratio was significantly lower in the southern parts, and phytoplankton biomass (particularly the biomass of cyanobacteria) was significantly higher for Lake Pihkva and Lake Lämmijärv than for Lake Peipsi s.s.     

Zooplankton-phytoplankton interactions in a eutrophic lake

Enclosure experiments were made in a cyanobacteria dominatedlake (Lake Rotongaio) to assess the impact of zooplankton (>150µm) grazing on algal growth rates and determine the effectof diel and vertical changes in zooplankton grazing intensityand nutrient (NH₄-N) regeneration upon abundance of phytoplankton.The filamentous cyanobacterium Anabaena minutissima var. attenuataand diatom Cyclotella meneghiniana showed a negative linearchange in abundance with a gradient in zooplankton grazing intensity.Phytoflagellates were not grazed and showed a positive linearchange in abundance with increasing zooplankton biomass. Theseeffects, as well as shortening of filament length of Anabaena,were caused by raptorial feeding by the alanoid copepod Boeckellapropinqua which dominated the zooplankton. Phytoplankton growthwas not stimulated by addition of nutrients, suggesting nutrientregeneration was not important. Diel and vertical changes infeeding and NH₄-N regeneration rates were measured in Marchand June 1988. Diel differences were more pronounced in Marchwhen the water column was stratified. Specific feeding rateswere more important than vertical changes in zooplankton biomassin determining community grazing rates in March, but in Junewhen the water column was mixed, vertical distribution of zooplanktonbiomass was important. Zooplankton grazing was an importantloss process for phytoplankton in the lower part of the epilimnionin Lake Rotongaio.     

Some ecological effects of artificial circulation on a small eutrophic lake with particular emphasis on phytoplankton

A small eutrophic New Hampshire lake was artificially circulated from July 16 to September 12, 1968. Artificial circulation destratified Kezar Lake completely; the stability of stratification was reduced to zero when the lake became isothermous. Mixing caused an increase in the heat budget. Water transparency also increased after mixing.Inverse clinograde distributions of Fe, Mn, ammonia-N, CO₂, alkalinity and conductivity were ameliorated after mixing by reoxygenation of stagnant bottom water. The chemical nutrients Ca, Mg, K, Cl, and SiO₂ were little influenced, but a marked increase in total-P occurred when artificial circulation transferred suspended organic detritus into the water column from agitated profundal muds. The effects of mixing on Na, Cu, Zn, NO₂-N, NO₃- N, organic-N and orthophospate are also discussed. Most chemical nutrients were distributed isometrically in the water column after mixing. The supply of chemical nutrients was sufficient to support large populations of phytoplankton.During stagnation a dense bloom ofAphanizomenon flos-aquae occurred. Mixing caused a uniform vertical distribution of this alga and its large population eventually dissipated. The phytoplankton then became dominated by chlorophycean taxa. The variations in chlorophyll-a followed closely changes in phytoplankton abundance. Chlorophyll-a levels are shown to be typical of other eutrophic lakes. Primary production in surface waters decreased markedly subsequent to destratification, but it increased at lower depths in agreement with vertical expansion of the euphotic zone.     

Macrozooplankton and the persistence of the deep chlorophyll maximum in a stratified lake

相似文献   

Phytoplankton seasonal and vertical variations in a tropical shallow reservoir with abundant macrophytes (Ninféias Pond,Brazil)

This study aimed at analyzing the environmental factors which determine the structure and dynamic of phytoplankton in a shallow reservoir with abundant macrophyte flora, Ninféias Pond (Brazil). It is hypothesized that, although its shallowness, periodic stratifications play an important role on its phytoplankton community. Water samples were collected monthly, from January to December 1997, in four depths (sub-surface, 1 m, 2 m, and bottom) of pelagic zone (Z _max = 3.6 m). Community seasonal and vertical variations followed a hot-rainy season with water column stratification (phase 1; Q index: medium), alternating with a cool-dry season with water column mixing (phase 2: Q index: excellent). Nanoplanktonic flagellates dominated, mainly mixotrophic species. During phase 1, Chlamydomonas sp. (G) was the main species, dominating at the anoxic and nutrient-rich hypolimnion. At the same time, richness and diversity were relatively lower. During phase 2, lower water temperatures and higher dissolved oxygen concentrations favoured the prymnesiophyte Chrysochromulina cf. breviturrita (X2). Sequence of functional groups over phases 1 and 2 was: phase 1 = G → transition = Y/P/E/D/F/W2/X3 → phase 2 = X2/Lo/X1; most of these groups have been associated to oligo-mesotrophic systems. Seasonal stratifications played a decisive role in determining the structure and dynamic of phytoplankton in the Ninféias Pond. However, in such a complex and heterogeneous system, other compartments of the food web (macrophytes, zooplankton, fishes) may also act as relevant driving forces, in synergy with the physical and chemical environment.     

Long‐term phytoplankton community changes in a deep subalpine lake: responses to nutrient availability and climatic fluctuations

1. In natural lakes, modifications in the species composition and abundance of phytoplankton communities may ultimately be responses to changes in nutrient availability and climatic fluctuations. Phytoplankton and associated environmental factors were collected at monthly intervals from the beginning of the 1990s to 2007 in the large subalpine Lake Garda (z_max = 350 m, V = 49 × 10⁹ m³). In this study period, the lake showed a slight and continuous increase of total phosphorus (TP) in the water column, up to concentrations of 18–20 μg P L^?1. This increase represented the last stage of a long‐term process of enrichment documented since the 1970s, when concentrations of TP were below or around 10 μg P L^?1. 2. At the community level, annual phytoplankton cycles underwent a unidirectional and slow shift mainly due to changes in the species more affected by the nutrient enrichment of the lake. After a first and long period of dominance by conjugatophytes (Mougeotia) and diatoms (Fragilaria), phytoplankton biomass in recent years was sustained by cyanobacteria (Planktothrix). Other important modifications in the development of phytoplankton were superimposed on this pattern due to the effects of annual climate fluctuations principally mediated by the deep mixing events at spring overturn and, secondarily, by temperature and thermal stability of the water column during the growing season. 3. Interannual variations in the stability and temperature of the water column appeared to influence the development of a few subdominant flagellates (dinophytes and cryptophytes). Nevertheless, the major impact of climate on phytoplankton was indirect, and mediated through the effects of winter climatic conditions on deep mixing dynamics. Winter climatic fluctuations proved to be a key element in a linked chain of causal factors including cooling of hypolimnetic waters, deep vertical mixing and epilimnetic nutrient replenishment. The process of fertilisation was measurable both for TP and dissolved inorganic nitrogen, although only the first had a large effect, reinforcing the seasonal growth of a few dominant groups. The degree of nutrient replenishment further increased the spring development of large diatoms and the increase of Planktothrix in summer and autumn. 4. Currently, changes in nutrient concentrations have the greatest effect on the phytoplankton community, while direct effects due to the interannual variations in the thermal regime are of secondary importance compared with the indirect effects mediated through deep water mixing and spring fertilisation. Overall, the results demonstrate that the consequences of climatic fluctuations and climate warming on phytoplankton communities need to be studied at different levels of complexity and integration, from the direct effects of temperature and thermal regime, to the indirect effects mediated by the physiographic characteristics of water bodies.     

Measurement of in situ growth rates of phytoplankton under conditions of simulated turbulence

In situ measurement of the growth rates of planktonic populationscan be improved by using dialysis chambers (‘cage cultures’)to avoid shifts in the chemical environment during incubation.Vertical mixing and small-scale turbulence affect the growthof planktonic populations, there fore natural mixing conditionsshould be simulated as closely as possible during the incubation.A new device is described here which combines the advantagesof a dialysis chamber with a programmable vertical mixing regime.Realistic phytoplankton growth rates can thus be measured insitu under con ditions of vertical mixing and small-scale turbulence.The chamber made of transparent, UV-transmitting acrylic glasswas fitted at both ends with permeable polycarbonate membranes.It was moved vertically through the water column by a pocket-sizedlift and rotated simultaneously on its central axis. The methodwas applied to two typos of experiments on growth and lossesof phytoplankton in the River Severn, UK. The first one comparedchanges in biovolume of phytoplankton in a water parcel flowingdownstream (6% h^–1 decline) with those in a simultaneouslyincubated dialysis chamber moved between water surface and riverbottom (7% h^–1 increase). The difference equates to algallosses prevented in the chamber but suffered along the river(mainly sedimentation and grazing of benthic filter feeders).Loss rate of diatoms was three times higher than those of chlorophytes.In another experiment growth of phytoplankton from the mainstream and lateral dead zone was compared under different mixingconditions. Algae from the main stream grew faster than fromthe dead zone. Only cryptophytes preferred calm conditions,all the other algal groups grew faster in chambers moved throughthe water column than in stationary ones. Further possible applicationsin both standing and flowing waters are discussed.     

Some ecological effects of artificial circulation on a small eutrophic lake with particular emphasis on phytoplankton II. Kezar lake experiment, 1969

A small eutrophic New Hampshire lake was artificially circulated (mixed) from 28 May to 15 September, 1969, inclusive, to impede the annual bloom of a noxious blue-green alga; yet Aphanizomenon flos-aquae bloomed immediately after mixing was commenced. The bloom collapsed in early July; it was succeeded by heavy growth of predominantly chlorophycean taxa. In an in vitro experiment Aphanizomenon flos-aquae did reattain bloom proportions when the influence of artificial circulation was removed. Other phytoplankton exhibited population pulses only when the dominance of cyanophycean and chlorophycean taxa were in transition. Mixing maintained uniform vertical populations of all phytoplankton. Changes in water transparency attended fluctuations in phytoplankton abundance. An isothermic condition was maintained over the test period, which increased the lake's heat budget, and most chemical nutrients were distributed isometrically in the water column. Increased concentrations were exhibited by Ca, Cl, Cu, K, Mg, SiO₂ and Zn. Sodium was not affected by mixing. Levels of Fe, Mn, Zn, phosphate and ammonia, nitrate, and organic nitrogen were influenced by phytoplankton. Mixing could not maintain orthograde profiles of dissolved O₂ and CO₂ when dense populations of phytoplankton prevailed. Variations in chlorophyll-a followed closely changes in phytoplankton abundance. Its degradation to pheo-pigments appeared to be less for a bloom of Aphanizomenon flos-aquae than during dense growth of chlorophycean taxa. Rates of photosynthesis were considerably greater when the latter algae were predominant. Extracellular release of organic carbon usually increased with depth; it amounted to 19.4 percent of the total carbon fixed in the euphotic zone.     

ANTARCTIC AQUATIC ECOSYSTEMS AS HABITATS FOR PHYTOPLANKTON

1. The Southern Ocean is a large-scale, relatively homogeneous upwelling ecosystem whose phytoplankton apparently grows suboptimally over much of its area. By contrast there is a wide variety of freshwater habitats in the Antarctic and in some of these phytoplankton growth efficiency is very high. The two habitats share similar temperature and irradiance regimes, but differ markedly in availability of inorganic nutrients, in grazing pressure and in the time- and space-scales on which various physical processes act. 2. Concentrations of inorganic nutrients in the marine ecosystem have been represented as being in excess of phytoplankton requirements, but the ionic composition of some nutrient pools may not conform to phytoplankton preferences. 3. Nutrient-limitation determines phytoplankton production in Antarctic lakes and gives rise to gross differences between lakes. 4. Irradiance in the water column varies greatly over the year in both marine and freshwater ecosystems. Most algae are shade-adapted, with the ability to utilize low irradiance but with sub-optimal response to high irradiance. However, local phytoplankton maxima may attain very high carbon fixation and growth rates. 5. Consistently low temperatures characterize both systems. Their effects on photo-synthetic carbon uptake mirror shade-adaptation. Division rates of marine phytoplankton may however be very much higher than predicted for ambient temperatures. 6. Vertical mixing is important in both ecosystems and influences the environment experienced by phytoplankton cells. This appears to have little effect on the average performance of phytoplankton in the strongly mixed surface water column of the Southern Ocean, where the mixed depth may exceed 100 m. This can be related partly to the shade-adapted photosynthetic response. Euphotic depths range from 20 to 100 m. 7. Strong vertical mixing under ice-free conditions in lakes may maximize photosynthetic efficiency, whilst distinct vertical stratification in permanently ice-covered lakes gives rise to segregation of nutrient uptake and regeneration. 8. Physical removal of phytoplankton biomass by grazing is locally important in the Southern Ocean, in contrast to the estimated mean mesoscale impact of grazing. Vertical sedimentation losses appear important in the context of mixing depth and generation time, and may be modified by vertical circulation of water. 9. Loss of phytoplankton biomass from lakes during the ice-free period is dominated by physical removal via the lake outflow. Grazing is generally unimportant, except where larvae of otherwise nektobenthic zooplankton hatch in synchrony with a phytoplankton maximum. Sedimentation is important under ice-cover.     

Winter condition of marine plankton populations in Saanich inlet,B.C., canada. I. Phytoplankton and its surrounding environment

Weekly sampling was carried out in Saanich Inlet, British Columbia throughout the winter of 1975–1976. The surface water column was characterized by exposure to low solar radiation energy (<150 g cal·cm^?2 · day^?1), slight stratification with occasional vertical mixing, and abundant algal nutrients. Phytoplankton were mostly distributed above 5 m in the water column, with a fairly low biomass averaging <1 μgchla·1^?1. Dominant phytoplankton organisms were nanoflagellates occasionally accompanied by dinoflagellates as the second dominant. Centric diatoms, which were dominant in the blooms, were always present but less than a few percentage of the total phytoplankton biomass. Daily photosynthetic productivity was exclusively limited by available radiant energy. Low solar radiation and occasional mixing of the surface zone prohibited the centric diatoms from becoming dominant.     

Effects of water-column mixing on bacteria,phytoplankton, and rotifers under different levels of herbivory in a shallow eutrophic lake

Water-column mixing is known to have a decisive impact on plankton communities. The underlying mechanisms depend on the size and depth of the water body, nutrient status and the plankton community structure, and they are well understood for shallow polymictic and deep stratified lakes. Two consecutive mixing events of similar intensity under different levels of herbivory were performed in enclosures in a shallow, but periodically stratified, eutrophic lake, in order to investigate the effects of water-column mixing on bacteria abundance, phytoplankton abundance and diversity, and rotifer abundance and fecundity. When herbivory by filter-feeding zooplankton was low, water-column mixing that provoked a substantial nutrient input into the euphotic zone led to a strong net increase of bacteria and phytoplankton biomass. Phytoplankton diversity was lower in the mixed enclosures than in the undisturbed ones because of the greater contribution of a few fast-growing species. After the second mixing event, at a high biomass of filter-feeding crustaceans, the increase of phytoplankton biomass was lower than after the first mixing, and diversity remained unchanged because enhanced growth of small fast-growing phytoplankton was prevented by zooplankton grazing. Bacterial abundance did not increase after the second mixing, when cladoceran biomass was high. Changes in rotifer fecundity indicated a transmission of the phytoplankton response to the next trophic level. Our results suggest that water-column mixing in shallow eutrophic lakes with periodic stratification has a strong effect on the plankton community via enhanced nutrient availability rather than resuspension or reduced light availability. This fuels the basis of the classic and microbial food chain via enhanced phytoplankton and bacterial growth, but the effects on biomass may be damped by high levels of herbivory. Received: 3 May 1999 / Accepted: 13 April 2000     

Short term spatial and temporal variation of phytoplankton in a shallow tropical oligotrophic reservoir,southeast Brazil

Locally called IAG pond, system is a small, oligotrophic, shallow, urban reservoir located in the Parque Estadual das Fontes do Ipiranga Biological Reserve, south of São Paulo municipality, southeastern Brazil. Study was carried out in two phases to cover the two climatic periods (dry and rainy) that characterize the area. Daily samples were collected on seven consecutive days (dry = 20–26 August 1996; rainy = 22–28 January 1997), three times a day (07:00 h, 13:00 h, and 19:00 h), following the water column vertical profile at reservoir’s deepest site (Z _max = 4.7 m). The phytoplankton’s community structure’s diurnal variation was compared with environmental variables (thermal structure of the water column, pH, CO₂, dissolved oxygen, total phosphorus, and NH₄). Analysis of variance (ANOVA) showed significant variation among sampling hours and days but not among depths for most species, thus revealing a tendency towards a homogeneous vertical distribution of phytoplankton in the reservoir. Canonical Correspondence Analysis (CCA) showed a significant relationship between distribution of phytoplankton species and environmental variables used for ordination. Water temperature was the environmental variable that best explained distribution of sample units. Atelomixis was the only possible explanation for how a non-motile alga as, for example, the chlorococcal Oocystis lacustris Chodat and the diatom Cyclotella stelligera Cleve &; Grunow, can be so widespread and abundant in the IAG pond during the stratified period.     

Phytoplankton seasonal variation in a shallow stratified eutrophic reservoir (Garças Pond, Brazil)

This study aimed at describing the phytoplankton dynamics and structure in a shallow eutrophic reservoir, the Garças Pond, located in the Parque Estadual das Fontes do Ipiranga (23°38′40.6′′ S, 46°37′28.0′′ W), in the Municipality of São Paulo, southeast Brazil. Samples were collected monthly from January to December 1997 in five depths (subsurface, 1 m, 2 m, 3 m, and 20 cm above the bottom) in the pelagic zone (Z _max = 4.7 m). Abiotic variables studied were: water temperature, turbidity, transparency, conductivity, pH, dissolved oxygen, alkalinity, inorganic carbon, and N and P dissolved and total forms. Altogether 236 phytoplankton taxa distributed among 10 classes were identified. Phytoplankton seasonal and vertical variation was related to shifts in the water chemical features as a consequence of a warm-wet season with stratified water column (phase 1, January–March and September–December) alternating with a cool-dry season with mixed water column (phase 2, April–August). There were shifts in cyanobacterial dominance over the entire year. During phase 1, Raphidiopsis/Cylindrospermopsis was one of the most important taxon. During phase 2, Raphidiopsis/Cylindrospermopsis biomass decreased, whereas richness and diversity increased and diatoms were relatively abundant. In September, when the water column was markedly stratified, a cyanobacterial bloom (Sphaerocavum brasiliense) occurred. Changes in water chemical variables caused by the bloom allowed recognition of a phase 3, in which pH and chlorophyll a, TP and CO₃ ^2? concentration reached their highest values. According to Reynolds and collaborators’ functional groups approach, phase 1 was marked by groups S/W1/W2/H1/Y, phase 2 by groups K/L _M /L _O /D/P/X1/F, and phase 3 by group M. This sequence was corroborated by canonical correspondence analysis (CCA) results.     

Different photochemical responses of phytoplankters from the large shallow Taihu Lake of subtropical China in relation to light and mixing

The maximum quantum yield of photosystem II was estimated from variable chlorophyll a fluorescence in samples of phytoplankton collected from the Taihu Lake in China to determine the responses of different phytoplankters to irradiance and vertical mixing. Meteorological and environmental variables were also monitored synchronously. The maximum quantum yield of three phytoplankton groups: cyanobacteria, chlorophytes, and diatoms/dinoflagellates, showed a similar diurnal change pattern. F _v/F _m decreased with a significant depth-dependent variation as irradiance increased during the morning and increased as irradiance declined in the afternoon. Furthermore, the rates of F _v/F _m depression were dependent upon the photon flux density, whereas the rates of recovery of F _v/F _m were dependent upon the historical photon density. Moreover, photoinhibition affected the instantaneous growth rates of phytoplankton. Although at noon cyanobacteria had a higher photoinhibition value (up to 41%) than chlorophytes (32%) and diatoms/dinoflagellates (34%) at the surface, no significant difference in diurnal growth rates among the three phytoplankton groups were observed indicating that cyanobacteria could photoacclimate better than chlorophytes and diatoms/dinoflagellates. In addition, cyanobacteria had a higher nonphotochemical quenching value than chlorophytes and diatoms/dinoflagellates at the surface at noon, which indicated that cyanobacteria were better at dissipating excess energy. The ratios of enclosed bottle samples F _v/F _m to free lake samples F _v/F _m showed different responses for the three phytoplankton groups to irradiance and vertical mixing when wind speed was approximately constant at about 3.0 m s⁻¹. When wind speed was lower than 3.0 m s⁻¹, cyanobacteria accumulated mainly at the surface and 0.3 m, because of their positive buoyancy, where diurnal growth rates of phytoplankton were relatively higher than those at 0.6 m and 0.9 m. Chlorophytes were homogenized completely by vertical mixing, while diatoms/dinoflagellates avoided active high irradiance by moving downward at noon, and then upward again when irradiance decreased. These results explain the dominance of cyanobacteria in Taihu Lake. Handling editor: L. Naselli-Flores     

THE KINETICS OF OSMOTIC SWELLING IN LIVING CELLS

The rate of swelling of unfertilized sea urchin eggs in hypotonic sea water was investigated. Analysis of curves leads to the following conclusions. 1. The rate of swelling follows the equation, See PDF for Equation where V _eq., V ₀, and V_t stand for volume at equilibrium, at first instant, and at time t, respectively, the other symbols having their usual significance. This equation is found to hold over a wide range of temperatures and osmotic pressures. This relation is the one expected in a diffusion process. 2. The rate of swelling is found to have a high temperature coefficient (Q ₁₀ = 2 to 3, or µ = 13,000 to 19,000). This deviation from the usual effect of temperature on diffusion processes is thought to be associated with changes in cell permeability to water. The possible influence of changes in viscosity is discussed. 3. The lower the osmotic pressure of the solution, the longer it takes for swelling of the cell. Thus at 15° in 80 per cent sea water, the velocity constant has a value of 0.072, in 20 per cent sea water, of 0.006.