Restoring lakes by using artificial plant beds: habitat selection of zooplankton in a clear and a turbid shallow lake

1. Return of large‐bodied zooplankton populations is of key importance for creating a shift from a turbid to a clear‐water state in shallow lakes after a nutrient loading reduction. In temperate lakes, recovery is promoted by submerged macrophytes which function as a daytime refuge for large zooplankton. However, recovery of macrophytes is often delayed and use of artificial plant beds (APB) has been suggested as a tool to enhance zooplankton refuges, thereby reinforcing the shift to a clear‐water state and, eventually, colonisation of natural plants. 2. To further evaluate the potential of APB in lake restoration, we followed the day–night habitat choices of zooplankton throughout summer in a clear and a turbid lake. Observations were made in the pelagic and littoral zones and in APB in the littoral representing three different plant densities (coverage 0%, 40% and 80%). 3. In the clear lake, the zooplankton (primarily Daphnia) were mainly found in the pelagic area in spring, but from mid‐May they were particularly abundant in the APB and almost exclusively so in mid‐June and July, where they appeared in extremely high densities during day (up to 2600 ind. L⁻¹). During night Daphnia densities were overall more equally distributed between the five habitats. Ceriodaphnia was proportionally more abundant in the APB during most of the season. Cyclopoids were more abundant in the high APB during day but were equally distributed between the five habitats during night. 4. In the turbid lake, however, no clear aggregation was observed in the APB for either of the pelagic genera (Daphnia and Bosmina). This may reflect a higher refuge effect in the open water due to the higher turbidity, reduced ability to orient to plant beds and a significantly higher fish density (mainly of roach, Rutilus rutilus, and perch, Perca fluviatilis) in the plant beds than in the clear lake. Chydorus was found in much higher proportions among the plants, while cyclopoids, particularly the pelagic Cyclops vicinus, dominated in the pelagic during day and in the pelagic and high density plants during night. 5. Our results suggest that water clarity is decisive for the habitat choice of large‐bodied zooplankton and that introduction of APB as a restoration measure to enhance zooplankton survival is only a useful tool when water clarity increases following loading reduction. Our results indicate that dense APB will be the most efficient.     

Seasonal variations in the diel vertical distribution of phytoplankton and zooplankton in a shallow pond

The seasonal succession of phytoplankton diversity, and the variations in the diel vertical distribution of phyto‐ and zooplankton were investigated in a small shallow pond (1.7 m water depth) in 2003. It was inferred that the water tended to stratify weakly in the daytime from February to June. In February and April, the green alga Golenkinia radiata Chodat dominated the phytoplankton assemblage. The cell density of G. radiata greatly decreased in April, when rotifers increased near the bottom. The vertical mixing was attenuated in June, large populations of the euglenoids (Lepocinclis salina Fritsch, Phacus acuminatus Stokes, Trachelomonas hispida (Perty) Stein et Deflandre) developed, and the cyanobacterium Aphanizomenon flos‐aquae var. klebahnii Elenk. appeared at low density. Euglenoids and A. flos‐aquae were mostly distributed in the bottom layer. In late September, when the water was mixed throughout the day, euglenoids and A. flos‐aquae were distributed evenly throughout the water column. The zooplankton (cyclopoid copepods and rotifers) densities in September were the lowest throughout the year. The vertical mixing increased in November, and the phytoplankton community was composed of A. flos‐aquae, P. acuminatus, T. hispida and the green alga Ankistrodesmus falcatus (Corda) Ralfs. In November, at the final stage of water bloom of A. flos‐aquae, its population density decreased with depth. The two euglenoids exhibited similar cell distributions at 0.8 m and 1.6 m during 1–3 November. A. falcatus was distributed evenly throughout the water column; however, when the vertical mixing lessened, the cells at the surface started to sink. Copepod nauplii and rotifers appeared at high densities in November. Seasonal variation in the phytoplankton community structure in the pond seemed to be related to the vertical mixing of the water. In addition, zooplankton, especially rotifers, might play an important role in initiating a spring clear‐water phase and in the bloom collapse of A. flos‐aquae.     

Prey selection and diel feeding of the freshwater jellyfish, Craspedacusta sowerbyi

1. The diet of the invasive freshwater jellyfish, Craspedacusta sowerbyi (Lankester), was assessed by analysing its stomach contents. 2. The medusae ingested various zooplankton and benthic prey in the 0.1–3.0 mm size range. The selectivity indices for prey showed that larger zooplankton (0.4–1.4 mm) and active prey such as copepods were preferred; small potential prey, and the loricate rotifer Keratella cochlearis in particular, were almost never consumed. Measurements suggest that spacing between the tentacle branches could determine prey selection. Another possibility is that some prey are too small or slow to activate nematocysts after collision with the predator. Prey > 1.4 mm can probably escape or are otherwise too large to be handled. 3. The medusae of Craspedacusta medusae stayed in deep water during daytime and migrated upwards at night. The greatest number of prey in freshwater jellyfish stomachs was detected at night. 4. Calculation of consumption rates, based on mean stomach contents and digestion times of 4–5 h, showed that, on average, one medusa takes 190 zooplankton prey day^??1 at a mean prey density of about 100 L^??1. At the observed medusae density of 1 m^??3, the cropping rate was about 0.2 zooplankton prey L^??1 day^??1.     

Response of waterbirds to alternating clear and turbid water phases in two shallow Mediterranean lakes

Albufera de Adra (Southern Spain) constitutes an internationally-recognised marsh for waterbirds; important populations of some endangered species such as White-headed Duck and Red-crested Pochard overwinter and breed in its two shallow permanent lakes (Lake Honda and Lake Nueva). In a recently published article, we revealed the factors responsible for the irregular alternation between phytoplankton-dominated turbid phases and macrophyte-dominated clear water phases in Lake Honda and Lake Nueva. In this note, we try to clarify the impact of such an alternation of equilibrium states on the waterbird dynamics. Marked increments in abundance, and brood recruitment of dabbling and diving waterbirds were recorded during the clear water phases in contrast with the turbid water phases, as the increase of macrophytes associated with increased water transparency attract waterbirds for available food. Implications for ecosystem management, restoration and conservation are identified.     

Abrupt shift from clear to turbid state in a shallow eutrophic, biomanipulated lake

Monitoring data were used to assess causes behind a recent shift from a clear-water to a turbid-water state in Lake Major, a 10 ha shallow lake in Hungary. In 1999–2000, fish manipulation was conducted in this hypertrophic lake. Reduced fish stock resulted in clearing water and the development of a dense (>80% coverage) submerged vegetation in 2005. During the recent abrupt shift, which occurred in 2007, submerged vegetation subsequently declined after a two-year period of clear water and abundant vegetation. An intense decay of macrophytes within the lake produced a rapid transition between the clear- and turbid-water states. During the clear-water state in 2005–2006, the most important variables predominantly correlating with macrophyte cover were Secchi transparency, temperature and TN, while TN, temperature, Secchi depth and chlorophyll-a were the most significant variables during the turbid-water state in 2007. Nitrogen may play a significant role in the cover of submerged macrophytes when TP is moderate. We argue that several factors in concert are necessary to initiate a shift. Water temperature likely has contributed to triggering shift through inter-year-dependent changes in cover of macrophytes, with fish recruitment having key roles in the dynamics of shallow lakes. Handling editor: Luigi Naselli-Flores     

Long-term changes in crustacean zooplankton and water quality in a shallow, eutrophic Chinese lake densely stocked with fish

From surveys made in 1962–1963, 1973–1974, 1979–1996 at two Stations in Lake Donghu, a shallow eutrophic water body near Wuhan, P. R. China, the authors, derive long-term changes in species composition, standing crop and body-size of planktonic crustaceans. The species number decreased from the 1960s to the 1990s. The cladocerans dropped from 46 (1960s) to 26 (1980s) to 13 (1990s); the copepods decreased from 14 (1960s) to 10 (1980s) to 7 (1990s). From the mid-1980s on, the dominant crustaceans also changed: Daphnia hyalina and D. carinata ssp. were replaced by Moina micrura and Diaphanosoma brachyurum at Stations 1 and 2, respectively; Cyclops vicinus replaced Mesocyclops leuckarti.Densities and biomass of Cladocera decreased markedly after 1987. Annual average densities and biomass of cladocerans were statistically differences between 1962–1986 and 1987–1996 (P > 0.01). Annual average densities of Daphnia (Station 1 + Station 2) were negatively correlated with fish yield .Since the 1980s, annual average body length of Cladocera and Calanoida decreased, while annual average body length of Cyclopoida increased. In the same years, average body length of copepods was lower during May–October than during January–April and November–December.A 12-yr data analysis showed annual average concentration of chlorophyll^-a (Chl^-a) to be negatively correlated with annual average density of Daphnia, whilst lake transparency was positively correlated with annual average densities of Daphnia. The results imply that, since Daphnia feeds efficiently on phytoplankton, it could decrease concentration of Chl^-a, and enhance water transparency.     

浮游动物昼夜垂直迁移机理的主要假说及其研究进展

对有关浮游动物昼夜垂直迁移(DVM)机理的实验、假说以及理论模型方面的研究进.展进行了综述。昼夜垂直迁移通常指常规迁移(傍晚上升,拂晓下降),其行为过程不仅影响浮游动物的垂直分布,而且也间接地影响其水平分布及生活史特征,对浮游动物在一定水域的种群维持和补充具有重要的意义。与垂直迁移机理有关的假说主要有光驱动假说、逃避捕食者假说、能量和资源利用假说等,其中逃避捕食者假说已得到大量的实验证实。其理论模型研究一般在两个时间尺度上进行,一个是短期的行为机制研究,另一个从长期的生活史策略的角度考虑。随着实验技术的进步和理论及建模工具的发展,这两个时间尺度的研究正逐渐统一到一个框架下进行。     

Vertical distribution and diel migration of flagellated phytoplankton in a small humic lake

The vertical distributions and migrations are described of the most abundant flagellated phytoplankton species from the summer community of a small forest lake in southern Finland. The lake showed a steep and stable thermal stratification with a shallow oxygenated epilimnion. Horizontal variation of phytoplankton distribution within the lake was tested on two scales and found to be statistically significant only in the case of Mallomonas reginae. The vertical distribution of flagellated phytoplankton was assessed by reference to the distribution of a non-motile, neutrally buoyant species Ankyra judayi. Statistically significant, active vertical positioning was demonstrated for all the flagellates examined with the exception of Spiniferomonas bourrellyi. Diel vertical migrations were apparent for all species showing active positioning and the pattern of an evening descent and a morning ascent was ubiquitous. The extent and timing of diel migrations varied between species. The most extensive migrations were by Cryptomonas marssonii which crossed a temperature gradient of 14 °C and penetrated far into the anoxic hypolimnion. Several categories of competitive advantage can be gained by species undertaking such diel vertical migrations.     

Spatial and temporal distribution of fish and zooplankton in a shallow lake

Summary 1. We performed both a large- and a small-scale echo sounding study on the spatial and temporal distribution of fish (mainly roach, Rutilus rutilus and perch, Perca fluviatlis ), as well as a small-scale study of zooplankton distribution in the small, shallow and eutrophic Lake Hanebjerg in Denmark. In the small-scale study, sampling was conducted in open water as well as in the edge zone immediately outside two different types of vegetation.
2. Fish daytime abundances differed between the northern and the southern parts of the lake and, on a small scale, small fish aggregated in the edge zones during day, preferably outside dense emergent vegetation. Copepods avoided emergent vegetation, while cladocerans showed no habitat preference. Both small fish and cladoceran numbers were found to be higher during night than day.
3. The relative abundance (number per sample) of cladocerans in the edge zone immediately outside vegetation was negatively correlated with the relative abundance of fish in that zone. There was no correlation between cladoceran and fish abundance in open water, or between the relative abundance of copepods and fish.
4. The presence of pelagic piscivores in combination with avoidance behaviour of both fish and zooplankton is a likely explanation for the observed distribution of small fish and cladocerans in Lake Hanebjerg. Both small- and large-scale distribution patterns may be dependent on the type and distribution of complex structure in the lake. Even in a small lake, large-scale patterns may affect the interpretation of small-scale data.     

Existence of a macrophyte-dominated clear water state over a very wide range of nutrient concentrations in a small shallow lake

Little Mere, a small shallow lake, has been monitored for four years, since its main source of nutrients (sewage effluent) was diverted. The lake has provided strong evidence for the persistence of a clear water state over a wide range of nutrient concentrations. It had clear water at extremely high nutrient concentrations prior to effluent diversion, associated with high densities of the large body-sized grazer, Daphnia magna, associated with low fish densities and fish predation. Following sewage effluent diversion in 1991, the nutrient concentrations significantly declined, the oxygen concentrations rose, and fish predation increased. The dominance of large body-sized grazers shifted to one of relatively smaller body-sized animals but the clear water state has been maintained. This is probably due to provision of refuges for grazers by large nymphaeid stands (also found prior to diversion). There has been a continued decrease in nutrient concentrations and expansion of the total macrophyte coverage, largely by submerged plants, following effluent diversion. The grazer community of Little Mere has also responded to this latter change with a decline in daphnids and increase in densities of weed-associated grazers. The presence of large densities of such open water grazers was the apparent main buffer mechanisms of the clear water state until 1994. The lake has, so far, maintained its clear water in the absence of such grazers. Thus, new buffer mechanisms appear to operate to stabilize the ecosystem. Little Mere appears to have shifted from previous top-down controlled clear water state to a bottom-up controlled clear water state.     

The role of macroinvertebrates and fish in regulating the provision by macrophytes of refugia for zooplankton in a warm temperate shallow lake

1. The zooplankton often undergoes diel horizontal migration (DHM) from the open water to the littoral of shallow lakes, thus avoiding predators in the former. This behaviour has functional impacts within the lake, as it enhances zooplankton survival, increases their control of phytoplankton and tends to stabilise the clear water state. However, most of the evidence supporting this migration pattern comes from cold north temperate lakes, and more evidence from tropical and subtropical areas, as well as from southern temperate areas, is needed. 2. We conducted a field study of the diel horizontal and vertical migration of zooplankton, and the horizontal distribution of potential predatory macroinvertebrates and fish, over two consecutive days in the summer in a temperate lake in the southern hemisphere. We took zooplankton samples at two depths, at three sampling stations (inside beds of aquatic macrophytes, at their edge and in open water) along three transects running from the centre of a bed of Ceratophyllum demersum to open water. At each sampling station, we also took samples of macroinvertebrates and fish and measured physical and chemical environmental variables. 3. Zooplankton (pelagic cladocerans, calanoid copepods and rotifers) avoided the shore, probably because of the greater risk from predators there. Larger and more vulnerable cladocerans, such as Diaphanosoma brachyurum and Moina micrura, were two to four times more abundant in open water than at the edge of or inside beds of macrophytes, respectively, by both day and night. Less vulnerable zooplankton [i.e. of medium body size (Ceriodaphnia dubia) or with the ability to swim fast (calanoid copepods)] were distributed evenly between open water and the edge of the plant beds. Small zooplankton, Bosmina huaronensis and pelagic rotifers, showed an even distribution among the three sampling stations. Accordingly, no DHM of zooplankton occurred, although larger organisms migrated vertically inside C. demersum stands. 4. Macrophytes contained high densities of predatory macroinvertebrates and fish. The predator assemblage, composed of large‐bodied macroinvertebrates (including odonates and shrimps) and small littoral fish, was permanently associated with submerged macrophytes. None of these groups moved outside the plant beds or changed their population structure (fish) over the diel cycle. 5. Submerged macrophyte beds do not represent a refuge for zooplankton in lakes where predators are numerous among the plants, implying a weaker top‐down control of phytoplankton biomass by zooplankton and, consequently, a more turbid lake. The effectiveness of macrophytes as a refuge for zooplankton depends on the associated assemblage of predatory macroinvertebrates and fish among the plants.     

Small-scale distribution and diel vertical migration of zooplankton in a shallow lake (Lake Naardermeer,the Netherlands)

Groundwater is a major influence on the hydrological, chemical and thermal regime of chalk streams in the southern U.K. However, little is currently known about the nature of the sediment delivery system within these chalk stream systems, even though sediment-related problems have been increasingly cited as a cause of habitat degradation and of declining salmonid stocks. To address this knowledge gap, suspended sediment fluxes were monitored at 4 sites within the Hampshire Avon catchment between February 1999 and August 2000. Maximum suspended sediment concentrations ranged from nearly 45 mg l^–1 to 260 mg l^–1. Over the study period, annual suspended sediment loads ranged from 644 to 6215 t yr^–1 and annual specific sediment yields ranged from 1.4 to 12.5 t km^–2 yr^–1. The results show that, relative to other U.K. rivers, the study chalk streams are characterised by low suspended sediment concentrations and loads and less episodic behaviour.     

Vertical distribution and diel migration of rotifers in a Parana River floodplain lake

The vertical distribution of zooplankton rotifers in the open waters of Laguna El Tigre was investigated. Rotifers showed a relatively uniform distribution throughout the water column. This pattern of distribution was maintained during the year and did not show variations in relation to hydrologic phases of inundation and isolation of the lake. Diel vertical migration of rotifers from the limnetic and the littoral area was investigated too. In littoral area rotifers exhibited a reverse migration, whereas in the limnetic the movements were less conspicuous. Horizontal migration was observed too, and there were interactions between horizontal and vertical distribution. Predation and competition offer a possible explanation.     

夏季南黄海浮游动物的垂直分布与昼夜垂直移动

对 2 0 0 1年 8月南黄海青岛外海至济州岛断面 ,浮游动物的日垂直变化、断面垂直分布及其与海区水文特征的关系进行了研究。共鉴定浮游动物 40种。断面浮游动物的平均丰度为 13 62 ind/ m 3 ;种类丰富度与暖水种数呈正相关 ,分布趋势为测区东部高、西部低 ;优势种为中华哲水蚤 (Calanus sinicus)、小拟哲水蚤 (Paracalanus parvus)、拟长腹剑水蚤 (Oithona similis)、强壮箭虫 (Sagitta crassa)、近缘大眼剑水蚤 (Corycaeus affinis)。在不同水层 ,浮游动物丰度分布不同 ,随水深增加而减少 ,表层丰度最高 (3 2 2 1ind/ m3) ,温跃层和底层分别为 743 ind/ m3、43 8ind/ m3;种类组成也不相同 ,表层数量最多的种类为鸟喙尖头(Penilia avirostris)、温跃层及其以下水体为中华哲水蚤 ,其它主要种类亦表现不同水层取向。在连续观测站 ,根据浮游动物主要种类的昼夜垂直移动与温跃层关系 ,将之分为表层分布 ,近温跃层分布 ,底层分布及全水层分布 4种类型     

Spatial structure and ecological efficiency in the summer zooplankton of a glacial lake

In June–July of 1993 and 1995, pelagic zooplankton dynamics of aglacial lake was investigated. In those years weather conditions diffferedsubstantially. This resulted not only in a great difference in the watersurface temperature (in 1993, <17.8± 0.47 °C; in 1995, 20.4± 0.33 °C), but also in an increase in the depth of metalimnionzone in 1995 in comparison with 1993. The ecological efficiency(K_e, the ratio of secondary to primary consumer productions,was 0.28 ± 0.031 in 1993 and 0.21± 0.034 in 1995. The higherK_e was accompanied by lowering of spatial structurization,primary and secondary consumer production. This revised version was published online in July 2006 with corrections to the Cover Date.     

Factors influencing the spatial distribution of zooplankton and fish in Loch Ness, UK

• 1 The vertical and horizontal distribution of phytoplankton, zooplankton and fish in Loch Ness, Scotland, were monitored during one day‐time and one night‐time survey in July 1992. The vertical samples were collected at a site located at the northern end of the loch and the horizontal samples along a longitudinal transect.
• 2 The vertical distribution surveys demonstrated that the phytoplankton, the zooplankton and the fish were concentrated in the top 30 m of water above the seasonal thermocline. Within this layer, Cyclops stayed much closer to the surface than Eudiaptomus but both species moved towards the surface at night.
• 3 The most important factor influencing the horizontal distribution of the phytoplankton was the north‐ south gradient in productivity. The sub‐catchments surrounding the north basin contain a greater proportion of arable land than those to the south and the concentrations of nitrate‐nitrogen and phytoplankton chlorophyll increased systematically from south to north.
• 4 Zooplankton distribution patterns were influenced by wind‐induced water movements and the dispersion of allochthonous material from the main inflows. The highest concentrations of Cyclops were recorded in the north, where there was more phytoplankton, and the highest concentrations of Eudiaptomus in the south, where there were higher concentrations of non‐algal particulates.
• 5 There was no spatial correlation between total zooplankton and total fish abundance but the highest concentrations of small (1–5 cm) fish were recorded in the south where there was a large patch of Eudiaptomus. The number of Eudiaptomus at specific locations within this patch were, however, negatively correlated with the numbers of small fish. These results suggest that the fish were actively foraging within the patch and were depleting their zooplankton prey in the areas where they were most abundant.

     

Spatial overlap of a predatory copepod,Acanthocyclops robustus,and its prey in a shallow eutrophic lake

Spatial overlap between Acanthocyclops robustus, with special emphasis on the adult females, and other zooplankton in one basin of a shallow (approximate depth of 2 m) eutrophic lake was studied.Horizontal distribution patterns were analysed on two dates. On both dates, most taxa examined showed large-scale patchiness between the three sections of the lake basin (approximate length of 1.2 km). Similarly, most taxa, with the important exception of the adult female Acanthocyclops robustus, were significantly patchily distributed on the small-scale (i.e. within sections). However, the intensity of such patchiness was, in general, relatively low. There was no consistent evidence of aggregation by the adult females or copepodites and adult males (the latter two were considered together) of the predator in such small-scale prey patches.Diurnal vertical distribution patterns were studied on two 24–25 hour periods. The first period was characterized by calm weather. Adult female, and perhaps male, Acanthocyclops robustus, Chydorus sphaericus, Bosmina Coregoni, Keratella cochlearis, Asplanchna species, Polyarthra vulgaris and Pompholyx sulcata seemed to show diurnal migration patterns, while seven other taxa showed consistent preferences for particular depths. Only copepod nauplii and Daphnia species were approximately evenly distributed. Negative correlations were found between the vertical distributions of the adult female predator and seven of the seventeen potential prey recognized.The first half of the second period was characterised by strong winds which abated during the second half. Most zooplankton taxa showed inconsistent heterogeneous vertical distributions or were homogeneously distributed with vertical heterogeneity developing towards the end of the period. Only Bosmina longirostris and Daphnia species seemed to show vertical migration patterns. Thus, no consistent vertical segregation between predator and prey was detected.     

Seasonal dynamics of rotifer and crustacean zooplankton populations in a eutrophic,monomictic lake with a note on rotifer sampling techniques

The abundances, biomass, and seasonal succession of rotifer and crustacean zooplankton were examined in a man-made, eutrophic lake, Lake Oglethorpe, over a 13 month period. There was an inverse correlation between the abundance of rotifers and crustaceans. Rotifers were most abundant and dominated (>69%) the rotifer-crustacean biomass during summer months (June–September) while crustacean zooplankton dominated during the remainder of the year (>89%). Peak biomasses of crustaceans were observed in the fall (151 µg dry wt l^–1 in October) and spring (89.66 µg dry wt l^–1 in May). Mean annual biomass levels were 46.99 µg dry wt l^–1 for crustaceans and 19.26 µg dry wt l^–1 for rotifers. Trichocerca rousseleti, Polyarthra sp., Keratella cochlearis and Kellicottia bostoniensis were the most abundant rotifers in the lake. Diaptomus siciloides and Daphnia parvula were the most abundant crustaceans. Lake Oglethorpe is distinct in having an unusually high abundance of rotifers (range 217–7980 l^–1). These high densities can be attributed not only to the eutrophic conditions of the lake but also to the detailed sampling methods employed in this study.The research was supported by National Science Foundation grants DEB 7725354 and DEB 8005582 to Dr. K. G. Porter. It is lake Oglethorpe Limnological Association Contribution No. 25 and Contribution No. 371 of the Harbor Branch Foundation, Inc.     

Interannual and between-site variability in the occurrence of clear water phases in two shallow Mediterranean lakes

It is widely accepted that clear water phases constitute a regular stage in the seasonal succession of plankton in dimictic lakes and reservoirs (i.e. PEG Model). The occurrence of such a phenomenon in Mediterranean shallow lakes is characterised by a marked interannual variability, which makes it difficult to establish reliable predictions on the dynamics and functioning of plankton in these ecosystems. In the present paper we analyse the factors influencing the occurrence of the clear water phases in the two shallow lakes of the Albufera of Adra, a coastal wetland region of south-eastern Spain: Lake Honda and Lake Nueva. Despite their geographical proximity, both lakes depicted large hydrological and limnological differences. Lake Honda is an epigenic and recharge lake that is strongly influenced by the hydrological conditions in its watershed, while Lake Nueva can be classified as a hypogenic and discharge lake and, as such, is less affected by the hydrological regime. In contrast, the morphometry, exposure and fetch of Lake Nueva make this ecosystem especially sensitive to wind forcing. Clear water phases in these shallow lakes were linked with periods of low thermal stability and the dominance of small-edible algae in the phytoplankton community, both of which allowed a Daphnia magna population to grow up and induce the algae collapse by grazing. In Lake Honda, those conditions were met during the spring of 2002 under the influence of intense rainfall-events, while in Lake Nueva the clear water phase was induced in the spring of 2003 by the occurrence of strong and frequent wind events. In both lakes, a relatively high water column thermal stability and the abundance of cyanobacteria early in the spring prevented the development of the Daphnia magna population and the occurrence of the clear water phase.     

Grazing and food size selection by crustacean zooplankton compared to production of bacteria and phytoplankton in a shallow Norwegian mountain lake

Food size selection by suspension-feeding crustaceans in a shallowmountain lake was investigated with radiolabelled algae andbacteria, and with a series of monodisperse fluorescent latexbeads with diameters ranging from 0.25 to 5.18 µm. JuvenileDaphnia longispina was a more efficient grazer on bacteria-sizedparticles than adults. Bosmina longispina and Cyclops scutiferwere both able to ingest bacteria-sized food particles. Thecrustaceans harvested {small tilde} 100% of the primary productionduring the day investigated, but only 20% of the bacterial production.To keep the bacterial biomass in or near steady state, the microzooplanktonmust be harvesting most of the bacterial production.