Natural ultraviolet radiation and photosynthetically active radiation induce formation of mycosporine-like amino acids in the marine macroalga Chondrus crispus (Rhodophyta)

The UV-absorbing mycosporine-like amino acids (MAAs) are hypothesized to protect organisms against harmful UV radiation (UVR). Since the physiology and metabolism of these compounds are unknown, the induction and kinetics of MAA biosynthesis by various natural radiation conditions were investigated in the marine red alga Chondrus crispus collected from Helgoland, Germany. Three photosynthetically active radiation (PAR, 400–700 nm) treatments without UVR and three UV-A/B (290–400 nm) treatments without PAR were given. Chondrus crispus collected from 4–6 m depth contained only traces of the MAA palythine. After 24 h exposure to 100% ambient PAR, traces of three additional MAAs, shinorine, palythinol and palythene, were detected, and their concentrations increased strongly during a one-week exposure to all PAR treatments. The concentration of all MAAs varied directly with PAR dose, with palythine and shinorine being four- to sevenfold higher than palythinol and palythene. Likewise, naturally high doses of both UV-A and UV-B resulted in a strong accumulation of all MAAs, in particular shinorine. While shinorine accumulation was much more stimulated by UVR, the content of all other MAAs was more affected by high PAR, indicating an MAA-specific induction triggered by UVR or PAR. Received: 24 September 1997 / Accepted: 17 December 1997     

Mycosporine-like amino acids in planktonic organisms living under different UV exposure conditions in Patagonian lakes

Mycosporine-like amino acids (MAAs) were studied in zooplanktonfrom 13 Argentinian lakes covering a broad range in altitude,maximum depth and physico-chemical properties of the water.Four to nine different MAAs (predominantly porphyra-334 andshinorine) were found in the copepods Boeckella gibbosa, B.gracilipes, B. meteoris and Parabroteas sarsi, and in the ciliateStentor amethystinus, while MAAs were undetectable in the cladoceranDaphnia middendorffiana. Among the different copepods, maximumMAA concentrations accounted for 0.25–1.31% of the dryweight, and contents were generally about three to seven times(up to 43 times) higher in the animals living in the clearestlakes compared to those occurring in low-UV systems. This variabilityin the content of MAAs was related to the lake altitude (r²= 0.71), and the fraction of the water column to which 1% ofthe surface UV radiation at 320 nm penetrated (r² = 0.57). Ourdata therefore underscore the role of MAAs as sunscreens todecrease the potential negative effects of solar radiation,but they also indicate that other environmental factors besidesUV transparency play a role in determining MAA concentrations.One lake was selected to obtain additional information on thequalitative composition of MAAs in seston of <100 µmbetween two sampling sites and over a 2 month study period (australsummer). Six different MAAs were detected in the samples, withporphyra-334 and palythine being predominant. In the copepodscollected simultaneously, there was low variation in MAA concentrationsbetween the two sites and over time. Thus, our results suggestthat under similar UV exposure conditions MAA contents of planktonicorganisms show low temporal variation.     

Relationships between fecal pellet production and feeding in the calanoid copepod Boeckella

The number, size (mm³ pellet ^–1) and total volume of fecalpellets produced (mm³ h^–1) by Boeckella titicacae increasedwith concentration of Chlorella and natural seston in threedifferent lakewaters from Bahia de Puno and Lago Grande of LakeTiticaca and nearby Laguna Arapa. The three lakewaters differedin the size and number of fecal pellets produced at seston concentrations>0.5 mg dry wt l^–1. However, the total volume of fecalpellets egested varied little. Large-sized food (>10 µg)resulted in larger pellets but smaller total fecal production.Food concentration had parallel effects on the production offeces and on feeding rates. Both fecal production and feedingincreased linearly with food concentration with saturation between5 and 7.5 x 10⁵ Chlorella ml^–1 (3.7–5.6 mg dry wtl^–1). Varied responses in fecal production when fed differentsize fractions of lake seston suggest Boeckella has a complexand flexible feeding behavior. Feeding preferences of B.titicacaeand B.occidentalis (Ivlev index) for two seston size fractionsare generally reflected by fecal production.     

Positive effects of UV radiation on a calanoid copepod in a transparent lake: do competition, predation or food availability play a role?

Zooplankton tolerant to ultraviolet radiation (UVR) could beindirectly affected by UVR through interactions with UV-sensitivespecies in the same ecosystem. In Lake Giles, Pennsylvania,USA, the calanoid copepod Leptodiaptomus minutus is more UVRtolerant than the cohabiting species Daphnia catawba and Cyclopsscutifer. We asked whether L. minutus is affected by UV-inducedmortality of a food competitor (D. catawba) or a predator ofits nauplii (C. scutifer). We conducted two in situ enclosureexperiments with six treatments: L. minutus alone, L . minutus+ Daphnia and L. minutus + Cyclops in the presence and absenceof UVR. There were few differences in survival among treatmentsin Experiment 1, which had enhanced food and a cumulative UVR(320 nm) dose of 9.3 kJ m^–2. In Experiment 2, which hadambient food and a UVR (320 nm) dose of 20.0 kJ m^–2, L.minutus survival and reproduction were higher in the +UVR comparedto –UVR, regardless of competitors or predators. Chlorophylla (Chl a) in Experiment 2 was higher in the +UVR than –UVR.While interactions between zooplankton species of differingUVR tolerances are potentially important, these results insteaddemonstrate that the beneficial UVR effect on L. minutus isindependent of concurrent detrimental UVR effects on competitorsand predators. Further research on the phytoplankton communityis necessary to determine whether UVR alleviates bacterial competition,increases nutrient availability or affects phytoplankton byother mechanisms.     

Isolate-specific effects of ultraviolet radiation on photosynthesis,growth and mycosporine-like amino acids in the microbial mat-forming cyanobacterium <Emphasis Type="Italic">Microcoleus </Emphasis><Emphasis Type="Italic">chthonoplastes</Emphasis>

Microcoleus chthonoplastes constitutes one of the dominant microorganisms in intertidal microbial mat communities. In the laboratory, the effects of repeated daily exposure to ultraviolet radiation (16:8 light:dark cycle) was investigated in unicyanobacterial cultures isolated from three different localities (Baltic Sea = WW6; North Sea = STO and Brittany = BRE). Photosynthesis and growth were measured in time series (12–15 days) while UV-absorbing mycosporine-like amino acids (MAAs) and cellular integrity were determined after 12 and 3 days exposure to three radiation treatments [PAR (22 μmol photon m⁻² s⁻¹) = P; PAR + UV-A (8 W m⁻²) = PA; PAR + UV-A + UV-B (0.4 W m⁻²) = PAB]. Isolate-specific responses to UVR were observed. The proximate response to radiation stress after 1-day treatment showed that isolate WW6 was the most sensitive to UVR. However, repeated exposure to radiation stress indicated that photosynthetic efficiency (F _v/F _m) of WW6 acclimated to UVR. Conversely, although photosynthesis in STO exhibited lower reduction in F _v/F _m during the first day, the values declined over time. The BRE isolate was the most tolerant to radiation stress with the lowest reduction in F _v/F _m sustained over time. While photosynthetic efficiencies of different isolates were able to acclimate to UVR, growth did not. The discrepancy seems to be due to the higher cell density used for photosynthesis compared to the growth measurement. Apparently, the cell density used for photosynthesis was not high enough to offer self-shading protection because cellular damage was also observed in those filaments under UVR. Most likely, the UVR acclimation of photosynthesis reflects predominantly the performance of the surviving cells within the filaments. Different strategies were observed in MAAs synthesis. Total MAAs content in WW6 was not significantly different between all the radiation treatments. In contrast, the additional fluence of UV-A and UV-B significantly increased MAAs synthesis and accumulation in STO while only UV-B fluence significantly increased MAAs content in BRE. Regardless of the dynamic photosynthetic recovery process and potential UV-protective functions of MAAs, cellular investigation showed that UV-B significantly contributed to an increased cell mortality in single filaments. In their natural mat habitat, M. chthonoplastes benefits from closely associated cyanobacteria which are highly UVR-tolerant due to the production of the extracellular UV-sunscreen scytonemin.     

Inhibition of phytoplankton photosynthesis by solar ultraviolet radiation: studies in Lake Titicaca, Bolivia

1. Lake Titicaca is a large, high altitude (3810 m a.s.l.) tropical lake (16°S, 68°W) that lies on the border of Bolivia and Perú, receiving high fluxes of ultraviolet radiation (UVR) throughout the year. Our studies were conducted during September of 1997 with the main objective of studying the impact of solar UVR upon phytoplankton photosynthesis.
2. Water samples were taken daily and incubated in situ (down to 14 m depth) under three radiation treatments to study the relative responses to PAR (Photosynthetic Available Radiation, 400–700 nm), UV-A (320–400 nm), and UV-B (280–320 nm) radiation.
3. Photosynthetic inhibition by UVR in surface waters was about 80%, with UV-A accounting for 60% and UV-B for 20%; the inhibition by high levels of PAR was less than 20%. The inhibition due to UVR decreased with depth so that there were no significant differences between treatments at 8.5 m depth.
4. The amount of inhibition per unit energy received by phytoplankton indicates that even though there was a significant inhibition of photosynthesis due to UVR, species in Lake Titicaca seem to be better adapted than species in high latitude environments.
5. The cellular concentration of UV-absorbing compounds, a possible mechanism of photoadaptation, was low in phytoplanktonic species. However, they were abundant in zooplankton, suggesting a high rate of bioaccumulation through the diet.     

A comparative study of ultraviolet radiation tolerance in different populations of Diaptomus minutus

A series of experiments was conducted to test the hypothesis that populations of Diaptomus minutus routinely experiencing high levels of ultraviolet radiation (UVR) are more tolerant of UVR than are those that routinely experience low levels of UVR. The relative degree of UVR tolerance was determined by monitoring mortality induced by either lamp or solar UVR. Diaptomus minutus from the low-dissolved organic carbon (DOC), high-UVR Lake Giles were consistently more tolerant of lamp and solar UVR than were those from the moderate-DOC, low-UVR Lake Lacawac. This difference in UVR tolerance was apparent throughout the year in freshly collected animals, but it did not persist in cultured animals. The lamp UVR tolerances of cultured D.minutus were similar, except for those initiated from September collections. The September culture of Lake Giles animals was significantly more tolerant than the Lake Giles cultures initiated in December, May and June. The September culture of Lake Lacawac animals was significantly more tolerant than the June culture. The lamp UVR tolerance of freshly collected Lake Giles animals was greatest from late June through October, but the Lake Lacawac population was least tolerant in July and August. No differences in lamp UVR tolerances were detected between animals collected from the surface and those collected from deeper water of either lake. It is concluded that the Lake Giles population was more tolerant than the Lake Lacawac population and that the period of greatest tolerance occurred several months after the time period with the highest ambient levels of solar UVR. Collectively, the experiments with both field and laboratory-cultured animals suggest that acclimation time plays a large role in UVR tolerance.      

Bacterioplankton growth, grazing mortality and quantitative relationship to primary production in a humic and a clearwater lake

Bacterial growth and grazing mortality were estimated from Mayto October in two south Swedish oligotrophic lakes, one beinga clearwater lake (water colour 5–10 mg Pt l^–1 DOC2.9–3.4 mg l^–1, Secchi disk depth 5.0–9.4m) and the other a humic, brownwater lake (water colour 105–165mg Pt l^–1, DOC 13.7–22.7mg l^–1, Secchi diskdepth 1.3–2.1 m). Specific rates of growth and grazingmortality were generally similar for both lakes. However, theabundance of bacteria was consistently 2–3 times higherin the water of the humic lake, suggesting that the total productionand consumption of bacterial cells were also higher than inthe dearwater lake. The ratio of bacterial secondary productionto primary production was higher in the humic lake than in theclearwater lake, indicating that the bacterioplankton of thehumic lake utilize allochthonous substrates, in addition tosubstrates originating from autochthonous primary production.Most of the bacterial loss in both lakes could be attributedto small protozoan grazers. This implies that allochthonousand autochthonous organic carbon fixed by bacterioplankton isless important in terms of carbon flow to higher trophic levelsthan would be expected if macrozooplankton were the dominantbacterivores, providing a more direct and efficient transferof carbon to larger organisms.     

Physiological characteristics of picophytoplankton, isolated from Lake Kinneret: responses to light and temperature

Two different phylogenetic groups of picophytoplankton, namelypicocyanobacteria and picoeukaryotes, are represented in LakeKinneret. Three species were isolated from the lake and identifiedas the picoeukaryote Mychonastes homosphaera and two picocyanobacteria,Synechococcus sp. A and B. Picocyanobacterial and M. homosphaeracultures grew well at light intensities up to 330 and 700 µmolphotons m^-2 s^-1, respectively, but poorly below 10 µmolphotons m^-2 s^-1. Picocyanobacterial and M. homosphaera culturesphotoacclimated to low light by increasing their chlorophyllper cell through increase in photosynthetic unit (PSU) sizeand PSU numbers, respectively. Growth rates of SynechococcusA and B were higher at temperatures characteristic of summer–autumnin the epilimnion, when maximum abundances of picocyanobacteriaoccur. Growth rates of M. homosphaera were higher at 14°C,corresponding to lake water temperatures during their occurrencein winter–spring. Temperature is a dominant factor influencingthe seasonal dynamics of both picocyanobacteria and picoeukaryotesin Lake Kinneret, while the vertical distribution is controlledby acclimation to different light conditions. Differences intemperature tolerance and photoacclimation suggest that SynechococcusA belongs to picocyanobacteria found in summer below surfacewaters, while Synechococcus B represents picocyanobacteria foundthroughout the year at all depths. Photoacclimation to highlight as shown in M. homosphaera cultures, may account for therelatively high abundance of picoeukaryotes in surface watersin Lake Kinneret.     

Response of ciliates and Cryptomonas to the spring cohort of a cyclopoid copepod in a shallow hypereutrophic lake

The impact of a cyclopoid copepod population on the protozoacommunity (two ciliate categories and Cryptomonas) was assessedweekly during the spring cohort of Cyclops vicinus (one monthduration) in hypereutrophic Lake Søbygård by insitu gradient experiments with manipulation of ambient zooplanktonabundance. As C.vicinus always made up >92% of the zooplanktonbiomass, the response of protozoa is assumed to be a resultof predation by the copepod. Significant effects of copepodbiomass on protozoa net population growth rates were obtainedin the four experiments. Copepod clearance rates were significantlyhigher on oligotrichs than on prostomatids and Cryptomonas butdeclined for all three protozoa categories during the firstthree weeks of the copepod cohort, probably because of the changein developmental instar composition of the copepod population.Grazing impact on protozoa at ambient copepod abundance wasconsiderable (range, 0.05–0.87 day^–1) and could,together with the estimated reproductive potential of protozoans(range, –0.20–0.87 day^–1), account for thedecline in abundance and biomass of protozoa during the cohortdevelopment. Carbon flow from the protozoa to C.vicinus (range,2.8–23.5 µg C l^–1 day^–1) documents thepresence of a trophic link between protozoa and the spring cohortof C.vicinus in Lake Søbygård.     

Bacterial growth and losses due to bacterivory in a mesotrophic lake

Bacterial secondary production and rates of bacterivory weredetermined from samples collected from mesotrophic Lake Arlington.Bacterial production and losses were determined by comparingthe growth of natural bacterial assemblages in the presenceof predators (unfiltered samples) to growth in the absence ofpredators (water filtered through 1.0 (im porosity filters).Growth rates of heterotrophic nanoflagellates (HNF) were estimatedfrom growth in the absence of predators (water filtered through5.0 µm porojity filters). Bacterial growth rates rangedbetween 0.002 and 0.069 h^–1 and averaged 0.026 h^–1.HNF grew at rates ranging between 0.003 and 0.107 h^–1and averaged 0.028 h^– Grazing rates ranged between 0.002and 0.043 h^–1, and averaged 0.018 h. The annual averagerate of bacterial biomass synthesis was 3.2 –g Cl^–h^–1 and {small tilde}69% of this production was grazed.Temporal changes in growth and grazing rates suggest a tightlycoupled predator-prey linkage in this lake. ¹Present address: Hydrobiological Institute, Czech Academy ofSciences, Na sddkach 7, 370 05 teski Budjovice, Czech Republic     

Uptake of dissolved nitrogen by phytoplankton in a eutrophic subtropical lake

Uptake rates for ananonium, nitrate, urea and dinitrogen byphytoplankton in Lake Okeechobee ranged from 0.58 to 1.52 µmol1^–1 h^–1 among four representative stations duringa short-term study period. Ammonium accounted for 53% of theuptake rates, followed by nitrate (19%), urea (16%) and dinitrogen(12%). Half-saturation constants for nitrogen (N) uptake rangedfrom 8.70 µmol 1^–1 for ammonium, 2.07 iimol 1^–1for urea and 2.21 µmol 1^–1 for nitrate at Southstation. This study reveals spatially varying N uptake rates,particularly N fixation, within a large eutrophic lake.     

Experimental and Field Studies of Mesocyclops notius (Copepoda: Cyclopoida) from a Subtropical Chinese Lake, Lake Donghu

This paper reports experimental and field studies on the cyclopoidMesocyclops notius from subtropical Lake Donghu close to theYangtze river. Mesocyclops notius, a dominant crustacean zooplankterthroughout tropical Australia, was previously considered tobe endemic to Australia, but recently, Mesocyclops leuckartiin Lake Donghu was re-identified as M. notius. Laboratory cultureexperiments were conducted to reveal the effect of temperature(15, 20, 25 and 30°C) on the development, growth and reproductionof M. notius. Temperature was inversely related to developmenttimes of eggs, nauplii and copepodites, body length and physiologicallongevity of adults, and brood size. Body length and physiologicallongevity of females were greater than those of males at thesame temperature. No reproduction took place at 15°C. Productionand seasonal cycles of M. notius during 1980–1982 werestudied at two sampling stations of Lake Donghu. At the mid-lakestation, the annual production and production/biomass (P/B)ratio of M. notius varied between 6.0 and18.0 g dry wt m^–2year^–1 and 74.6 and 95.5, respectively. Mesocyclops notiusreached their highest density peaks in the warm months (July–October),with a maximum density of 1256 individuals l^–1 at a littoralsite. No reproduction and recruitment by M. notius took placeduring the cold months (December–March) when the temperatureof the lake water was <15°C. Mesocyclops notius weremore abundant at a littoral station than at a pelagic station,possibly due to different food availability. The higher male:femalesex ratio of M. notius at the littoral station was most likelycaused by size-selective fish predation on larger females.     

Are Freshwater Mixotrophic Ciliates Less Sensitive to Solar Ultraviolet Radiation than Heterotrophic Ones?1

We tested whether mixotrophic ciliates are more resistant to solar ultraviolet radiation (UVR) than heterotrophic ones because symbiotic algae can provide self-shading by cell matter absorption and eventually by direct UV screening from mycosporine-like amino acids (MAAs). Sensitivity of a natural assemblage to solar radiation was tested in experiments in the original lake and in a more UV transparent alpine lake after transplantation of the ciliates. In both lakes, the assemblage was exposed either to full sunlight, to photosynthetically active radiation only, or kept in the dark. In each lake, exposure was for 5 h at the surface and at the depth corresponding to the 10% attenuation depth at 320 nm. Overall, when the assemblage was exposed to surface UVR, only one out of four dominant mixotrophic ciliates, Vorticella chlorellata, was more resistant than heterotrophic species. The higher UV resistance in V. chlorellata was related to the presence of MAAs and the high percentage of ciliate volume occupied by algal symbionts. Our results indicate that effects of UVR were species-specific and depended on efficient screening of these wavelengths, but also on the depth preference of the ciliates and thus, on their previous exposure history to UVR.     

Effects of Salinity and Temperature on Survival and Reproduction of Boeckella hamata (Copepoda: Calanoida) from a Periodically Brackish Lake

The freshwater calanoid copepod, Boeckella hamata Brehm, occursperiodically in Lake Waihola, New Zealand, a shallow, tidally-influencedlake that experiences daily and seasonal fluctuations in salinity,and seasonal changes in temperature. To determine whether theseenvironmental changes might contribute to the copepod's disappearancefrom the lake in summer, we tested the survival and egg productionof B. hamata at a range of salinities (20-3000 mg^–1 Cl)and two temperatures (10°C, 20°C). Adults survived for20 days or more only at salinities     

The effects of light and temperature on photosynthate partitioning in Antarctic freshwater phytoplankton

The effects of temperature and radiation flux on the partitioningof photosynthetically fixed carbon into four intracellulai metabolicpools was investigated for natural phytoplankton assemblagesfrom an Antarctic freshwater lake. At ambient temperature, proteinsynthesis was saturated at low photon flux densities (30–40µmol m^–2 s^–1) and above this flux fixed carbonwas increasingly stored as lipid and polysaccharide. Increasingtemperature raised both the saturated rate of protein synthesisand the photon flux at which saturation occurred. There wasa corresponding decline in the accumulation of reserve products,particularly at low radiation fluxes. The consequences of thispattern of uptake for the phytoplankton is discussed.     

Assessment of the potential impact of the mysid shrimp, Neomysis mercedis, on Daphnia

Neomysis mercedis, an important invertebrate predator in somelakes and estuaries in North America, occurred at high densities(>2 mysids m^-3) in Lake Washington in the early 1960s, aperiod when Daphnia were scarce in the lake. Because Neomysisfeed selectively on Daphnia, it was hypothesized that mysidpredation contributed significantly to the scarcity of Daphnia.To evaluate this hypothesis, mysid abundance was monitored inthe lake (July 1989–February 1992), and whole lake predationimpacts on Daphnia were estimated. Mysid abundance varied from319 million, 0.1 mysids m^-3 (February 1992) to 4276 million,1.7 mysids m^-3 ( June 1991), and mysid biomass ranged from 1153(October 1991) to 4700 kg dry weight (November 1990). A peakin Daphnia consumption was noted during autumn, when total mysidbiomass was high. In late autumn/winter mysid consumption month^-1accounted for 28–95% of Daphnia biomass and 13–38%of production. Mysid densities in this study were 18x and 5xlower than in 1962 and 1975, respectively. By extrapolation,at a density of 3.5 mysids m^-3 observed in the early 1960s,total Neomysis consumption demand month^-1 exceeded 100% of Daphniabiomass (late autumn–early spring), and 100% of Daphniaproduction (late autumn–winter). Estimates of the totalmysid consumption demand in this study are thus in accord withthe hypothesis that Neomysis could control Daphnia abundancein Lake Washington.     

The impact of short-term exposure to UV-B radiation on zooplankton communities in north temperate lakes

We exposed natural zooplankton communities to in situ levelsof sunlight for 3 days at different depths in two north temperatelakes: one oligotrophic and one eutrophic. Natural fluxes ofUV-B radiation (280–320 nm) were manipulated with cutofffilters (Mylar®). There was substantial mortality in someof the zooplankton exposed to UV-B in the oligotrophic lake,but not in the eutrophic lake. Reproduction of Diaptomus wassuppressed by UV-B down to 6 m in the oligotrophic lake. Thesedata suggest that natural levels of UV-B radiation in the oligotrophiclake may prevent some species of zooplankton from continuouslyexploiting the warm surface waters during summer stratification.In the more eutrophic lake, UV-B is a less important constraintin the vertical distribution of zooplankton. These differencesin the responses of zooplankton to natural UV-B radiation inlakes may alter their ecological interactions with food resources,predators and other environmental variables in the water column. ¹Present address: Universidad Nacional del Comahue, Centro RegionalUniversitario Bariloche, CC 1336, (8400) Bariloche, Argentina ²Present address: National Academy of Engineering, 2101 ConstitutionAvenue, NW, Washington, DC 20418, USA     

Vertical and temporal heterogeneity of planktonic ciliated protozoa in a humic lake

Seasonal population dynamics and the vertical distribution ofciliates were studied in relation to the particular food resourcesoccurring in a humic and moderately acidic lake (Lake Vassivière).The abundance (1.4 x 10³–20.4 x 10³ cells l^–1 mean= 4.8 x 10³ cells l^–1) and biomass (0.5–34.6 µgC l^–1, mean = 6.0 µg C l^–1) of ciliated protozoawere low and close to values reported for oligotrophic environments.The species composition of the population varied greatly withdepth. Whereas large-sized species of oligotrichs, some of whichwere mixotrophic, dominated at the surface, haptorids were bestrepresented in deep waters. The spatial distribution of thevarious groups of ciliates was largely determined by light andthe vertical distribution of microbial food resources (detritus,bacteria, algae) within the water column of this brown-coloredlake.     

Microbial dynamics during the summer ice-loss phase in maritime Antarctic lakes

The dynamics of bacterioplankton and protozooplankton in twomaritime Antarctic lakes (Heywood Lake and Sombre Lake, SignyIsland, South Orkneys) were studied during the phase of icebreak-out (December and early January 1994/95). The lakes aresuffering animal-induced (fur seal) eutrophication, though HeywoodLake is most severely affected. Both lakes had morphologicallydiverse bacterial communities which increased during the studyperiod, reaching maxima of 80 x 10⁸ l^–1 in Heywood Lakeand 31.8 x 10⁸ l^–1 in Sombre Lake. Heterotrophic nanoflagellates(HNAN) reached a peak in late December with maxima of 40.6 x10⁸ l^–1 in Sombre Lake and 174 x 10⁵ l^–1 in HeywoodLake. Phototrophic nanoflagellates (PNAN) peaked in late Decemberafter ice loss in Heywood Lake (63 x 10⁵ l^–1), which coincidedwith a peak in a bloom of Chroomonas acuta which reached abundancesof 1.0 x 10⁸ l^–1. In Sombre Lake, ice persisted for alonger period and here PNAN reached their highest density atthe end of the study period (around 70.0 x 10⁵ l^–1). Ciliateabundance reached high levels in Heywood Lake (>60001^–1),while in Sombre Lake maximum abundance was 568l^–1. Protozooplanktondiversity was greater in the less-enriched Sombre Lake. Grazingrates of ciliates averaged 70.6 bacteria indiv.^–1 h^–1in Heywood Lake and 119.3 bacteria indiv.^–1 h^–1in Sombre Lake. The difference was a reflection of the differenttaxonomic make-up of the community in the lakes. HNAN grazingrates varied between 0.51 and 0.83 bacteria indiv.^–1 h^–1in Sombre and Heywood Lakes, respectively. Specific growth rates(r) h^–1 in Sombre Lake were 0.028 for ciliates and 0.013for HNAN, and in Heywood Lake 0.010 for ciliates and HNAN 0.012.These growth rates result in doubling times ranging between38 and 69 h for ciliates and around 55 h for HNAN.HNAN grazingon bacteria was curtailed in Heywood Lake in early January asa result of predation by microcrustacean larvae feeding on theplankton. Thus, for a short phase top-down control was apparentin the dynamics of Heywood Lake, a feature uncommon in Antarcticlake ecosystems. The impact of natural eutrophication on thesesystems is discussed in relation to other unaffected Antarcticlakes.