Vertical Distribution Of Zooplankton Research Articles

Sentinel responses to droughts, wildfires, and floods: effects of UV radiation on lakes and their ecosystem services

Environmental drivers such as climate change are responsible for extreme events that are critically altering freshwater resources across the planet. In the continental US, these events range from increases in the frequency and duration of droughts and wildfires in the West, to increasing precipitation and floods that are turning lakes and reservoirs brown in the East. Such events transform and transport organic carbon in ways that affect the exposure of ecosystems to ultraviolet (UV) radiation and visible light, with important implications for ecosystem services. Organic matter dissolved in storm runoff or released as black carbon in smoke selectively reduces UV radiation exposure. In contrast, droughts generally increase water transparency, so that UV radiation and visible light penetrate to greater depths. These shifts in water transparency alter the potential for solar disinfection of waterborne parasites, the production of carcinogenic disinfection byproducts in drinking water, and the vertical distribution of zooplankton that are a critical link in aquatic food webs.

Diel vertical migration of zooplankton at the S1 biogeochemical mooring revealed from acoustic backscattering strength

AbstractWe examined the diel vertical migration of zooplankton by using the backscatter strength obtained from moored acoustic Doppler current profilers at mooring site S1 in the North Pacific subtropical gyre. There was seasonal variability in the vertical distribution and migration of the high‐backscatter layers in that they became deeper than the euphotic zone (<100 m) in winter and were confined above this depth in other seasons. Seasonal changes in daylight hours also affected the timing of the diel migration. We found that lunar cycles affected vertical distributions of zooplankton near the surface by changing the light intensity. Physical events, such as mixed‐layer deepening and restratification and the passage of a mesoscale eddy, also affected zooplankton behavior possibly by changing food environment in the euphotic zone. Since the comparison with net samples indicated that the backscatter likely represents the bulk biomass, the accuracy of biomass estimates based on net samples could be influenced by the high temporal variability of zooplankton distributions.

Measuring the distribution, abundance, and biovolume of zooplankton in an oligotrophic freshwater lake with a 710 kHz scientific echosounder

AbstractAcoustic surveys of the distribution and abundance of freshwater zooplankton were conducted in Lake Giles, an oligotrophic freshwater lake. Volume backscatter data from a 710 kHz scientific echosounder were converted to high‐resolution spatial and temporal numerical density estimates of small zooplankton. Vertical net tows of a 153 μm mesh closing bongo net at multiple depth intervals provided both identification of the types and sizes (0.5–1.5 mm length) of crustacean zooplankton present in the lake as well as an independent measurement of zooplankton numerical density. Net and acoustic estimates of zooplankton abundance, biovolume, and distribution were very similar. The improved resolution of the high‐frequency acoustic sampling provides insight into several aspects of freshwater zooplankton ecology including: separation of migrating and non‐migrating zooplankton, high resolution measurements of in situ zooplankton biovolume, calculation of in situ vertical velocities of migrating zooplankton, and fine‐scale (sub‐meter) horizontal and vertical zooplankton distribution during daytime, nighttime, and vertical migration events. These methods allow for more detailed and accurate estimates of zooplankton distribution than traditional net sampling methods can provide, including determining the total abundance of organisms within a specific habitat. They also provide higher resolution data in both space and time of smaller zooplankton taxa than have been measured previously in freshwater ecosystems.

Control mechanisms of photosynthetic epibionts on zooplankton: an experimental approach

Several top‐down and bottom‐up forces have been put forward to explain variable infestation rates of zooplankton by epibionts. Among top‐down forces, fish predation affects epibiont prevalence on zooplanktonic organisms, either by eliminating more conspicuous, heavily burdened individuals, or by reducing population size of zooplankton hosts, with consequences for substrate availability for epibionts. However, detailed experimental‐based information on the effects of top‐down forces is still lacking. Among bottom‐up forces, light can potentially control populations of photosynthetic epibionts. Therefore, both changes in light penetration in the water column and the vertical position of hosts in the water column could affect the photic conditions in which epibionts live and could thus control their population growth. We tested experimentally the hypothesis that both light limitation and fish predation affect epibiont burden on zooplankton. Moreover, we also tested the hypothesis that zooplanktivorous fish affect the prevalence and burden of the epibiotic alga Colacium sp. (Euglenida) on zooplankton not only by direct predation, but also by affecting the vertical distribution of zooplankton. We analyzed Colacium burden on two zooplankton genera that responded differently to the presence of zooplanktivorous fish by altering their daytime vertical distributions, thus exposing photosynthetic epibionts to different light conditions. Colacium burden on the two zooplankton genera was also compared between enclosures with different degrees of light limitation. Our results suggest that (1) ambient light limitation has the potential to reduce the burden of photosynthetic epibionts on zooplankton in natural conditions, and (2) zooplankton behavior (e.g., daytime refuge use to escape fish predation) can reduce the burden by exposing photosynthetic epibionts to suboptimal light conditions.

Estimating zooplankton vertical distribution from combined LOPC and ZooScan observations on the Brazilian Coast

Although the technology for sampling zooplankton is rapidly evolving, it remains a challenge to determine species distributions with high spatial resolution. This study reports on a technique developed to estimate the mesozooplankton vertical distribution by combining the spatial resolution possible with laser optical particle counter (LOPC) profiles with the taxonomic resolution possible with vertically integrated zooplankton net samples. Data obtained from different coastal and oceanic ecosystems off Brazil were used for our analysis. The aggregate signals from LOPC data sets were removed, and the distributions of the residual Zo-particles, believed to describe zooplankton, were calculated. Significant correlations were then identified between the abundances of the Zo-particles in different size classes and the abundance and size distributions of zooplankton collected in a vertical integrated net sample and classified with a ZooScan system. A principal component analysis was carried out to identify and exclude outliers. After these procedures, multiple linear regression analyses were used to detect relationships among abundances of different zooplankton groups and multiple Zo-particle size classes. The resulting relationships were used to estimate the vertical distributions of the dominant zooplankton taxa—calanoids, poecilostomatoids, shrimp-like, and appendicularians—through the water column from the LOPC data. The results achieved the goal of estimating distributions of dominant zooplankton taxa with an improved spatial resolution. The analytical technique described here enabled the analysis of zooplankton vertical structure in a wide range of marine ecosystems off Brazil and could serve as a basis to revisit historical data collected with similar methods elsewhere.

Stuck between a rock and a hard place: zooplankton vertical distribution and hypoxia in the Gulf of Finland, Baltic Sea

Zooplankton often migrate vertically to deeper dark water during the day to avoid visual predators such as fish, a process which can strengthen benthic–pelagic coupling. In the Gulf of Finland, Baltic Sea, a pronounced hypoxic layer develops when there is an inflow of anoxic bottom water from the Central Baltic Sea, which could be a barrier for vertical migrants. Here, we report an acoustic study of the distributions of crustacean zooplankton (mysid shrimp and the copepod Limnocalanus macrurus), gelatinous zooplankton (Aurelia aurita) and fish. Zooplankton trawl nets were used to ground-truth acoustic data. Vertical profiles of oxygen concentration were taken, and the physiological impact of hypoxia on mysids was investigated using biochemical assays. We hypothesised that the vertical distribution of zooplankton and fish would be significantly affected by vertical heterogeneity of oxygen concentrations because anoxia and hypoxia are known to affect physiology and swimming behaviour. In addition, we hypothesised that mysids present in areas with hypoxia would exhibit a preparatory antioxidant response, protecting them from oxidative damage during migrations. The acoustic data showed peaks of crustacean zooplankton biomass in hypoxic ( 75 m), whereas fish shoals and A. aurita medusae were found in normoxic (5–6 mL L−1) upper water layers (<40 m), with individual fish in deeper water excepting that rule. Mysid shrimp from areas with hypoxia had significantly enhanced antioxidant potential compared with conspecifics from areas with no hypoxia and had no significant indications of oxidative damage. We conclude that mysids can protect themselves from oxidative damage, enabling them to inhabit hypoxic water. Our data suggest that hypoxic and low oxygen zones (up to 4 mL L−1) may provide some zooplankton species with a refuge from visual predators such as fish.

Seasonal and interannual variability of zooplankton vertical distribution and biomass size spectra off Ubatuba, Brazil

The analysis of plankton size distributions in response to physical and biological forcing is relevant to the understanding of marine ecological processes. In this study, we evaluated the influence of seasonal intrusions of the South Atlantic Central Water (SACW) on zooplankton vertical distribution and normalized biomass size spectra (NBSS) on the inner shelf off subtropical Brazil. Monthly sampling was performed at a fixed station for 5 years and zooplankton samples were analyzed with a ZooScan system. SACW intrusions led to increases in nitrate, chlorophyll and zooplankton biomass in different depth strata. NBSS slopes and intercepts varied between SACW intrusion and retraction phases and stratification was a key factor explaining seasonal fluctuations in NBSS parameters. The zooplankton interannual variability was related to the changing strength of SACW intrusions during the time series, as indicated by surfacing 18 and 208C isotherms in certain years and intrusion phases, indicative of transient upwelling. We conclude that the temporal components of zooplankton variability in this subtropical coastal ecosystem were governed by SACW intrusions, affecting the vertical distribution of zooplankton biomass and size structure.

Vertical distribution of zooplankton in Lake Nasser

The composition and distribution of zooplankton communities in three depths (surface, 10–5m and 20–15m depths) along main channel of Lake Nasser were studied in 2013. The density of total zooplankton was increased to maximum during winter and autumn at surface water (39,362 and 63,100Ind.m−3, respectively) and gradually decreased with depth until attaining the lowest average density at 20–15m (12,460 and 8976Ind.m−3). During spring and summer, zooplankton was irregularly distributed through the water profile, where the highest average density was recorded at 10–5m depth (66,007 and 66,734Ind.m−3). Copepoda was the dominant zooplankton group at all depths, it represented about 70–76.2% of the total zooplankton count. Cladocera formed about 13.4%, 14.5% and 11% of total zooplankton density for surface, 10–5m and 20–15m depth. It was decreased with increasing depth during winter and autumn; however it attained its maximum density at 10–5m depth during spring and summer. Rotifera average density decreased with increasing depth. The dominant zooplankton species inhabiting Lake Nasser were strongly temperature-dependent. The study recommends the introduction of some pelagic fish species to consume the high persistence of zooplankton community at the upper 10meters of water column.

Vertical and horizontal distribution of zooplankton and polar cod in southern Baffin Bay (66–71°N) in September 2009

Zooplankton are the link connecting primary producers to higher trophic levels, and knowing their distribution and community is important for predicting the distribution of predator species, like fish, seabirds, and marine mammals. However, data from open Arctic oceans are still scarce. In autumn, tens of millions of the planktivorous little auks (Alle alle) (about 75 % of the world’s population) and millions of thick-billed murres (Uria lomvia) pass through the Baffin Bay. To investigate their potential food sources, we investigated the spatial and vertical distribution of zooplankton and small fishes in the upper 500 m of southern Baffin Bay in September 2009. The zooplankton community was dominated by copepods (55 % of abundance in the upper 500 m), primarily of the genus Calanus. Other important zooplankton taxa included Limacina helicina, Chaetognatha, and Cirripedia nauplii. On the Greenland Shelf, most Calanus were late copepodite stages and most were found at the depths of >200 m, suggesting they were in diapause. On the Canadian Shelf, there were relatively more Calanus in the near-surface layers, which were probably still actively feeding and which were available to visual predators such as seabirds and fish. The acoustic survey showed the highest density of polar cod Boreogadus saida in the upper 50 m on the western part of the Greenland Shelf. A particularly high biomass of both zooplankton and polar cod was found in the central part of the basin in association with a local relatively shallow area.

Zooplankton abundance, species composition and ecology of tropical high-mountain crater lake Wonchi, Ethiopia

The highlands of Ethiopia represent some of the remnants of undisturbed aquatic ecosystems; they are however highly threatened by significant socio–economic developments and associated anthropogenic impacts. Lake Wonchi is one of the few remaining fairly pristine high–mountain crater lakes in the central highlands and has never been investigated in detail. We present a first study on zooplankton taxa composition, abundance and biomass conducted over more than one year including the underlying environmental drivers. The lake is basic (pH 7.9-8.9), dilute (specific conductivity 185-245 µS cm -1 ) and oligotrophic with mean trophic status index of 36. The zooplankton community composition showed low species richness comprising a total of fourteen taxa with six cladocerans, one copepod and seven rotifers. Simpson´s index of diversity with values between 0.6 and 0.8 pointed towards a homogenous taxa occurrence within the single sample units. The overall mean (±SD) standing biomass of zooplankton was 62.02±25.76 mg dry mass m -3 ,which is low compared to other highland and rift valley lakes in Ethiopia. Cyclopoid copepods, in particular Thermocyclops ethiopiensis were the most abundant group and contributed 50% to the total zooplankton abundance followed by cladocerans (38%) and rotifers (12%). Non-metric multi-dimensional scaling resulted in a 3-dimensional model, which revealed similar community composition on successive sampling dates except in December/January and May. Temperature, alkalinity, conductivity and nitrate-N had significant influence on this seasonal pattern. A weak, but significant positive correlation (r=0.482, N=20, P=0.037) between Chlorophyll a and zooplankton biomass mirrors a bottom-up effect of phytoplankton biomass on zooplankton dynamics. The zooplankton of Lake Wonchi displayed some degree of segregation along the epi– and metalimnion during this study, but diel vertical migration was not observed. The results show that fish predation is not the key factor in structuring the vertical distribution of zooplankton in Lake Wonchi.

Fine-scale zooplankton vertical distribution in relation to hydrographic and optical characteristics of the surface waters on the Arctic shelf

One of the main challenges in contemporary biological oceanography is to understand the factors that drive the spatial heterogeneity of pelagic organisms. Our study was performed at 10-m depth intervals within the upper 50 m of the ocean, at two different localities on the West Spitsbergen Shelf in the summers of 2009 and 2010. The fine-scale patterns in vertical zooplankton distribution were studied concurrently with measurements of hydrographic and optical properties of the water. We adopted a novel approach to the acquisition of information on light, phytoplankton and particle distributions by in situ measurements with optical sensors. Based on our data, we distinguished specific zooplankton vertical distribution patterns such as the aggregation of Calanus finmarchicus and small copepods toward the surface and the relatively high contribution of C. glacialis to subsurface layers. A vertical separation of Calanus sp. developmental stages was observed, and zones of rather even zooplankton distributions were also found. The variability in zooplankton distribution and community structure was closely related to the hydrographic and optical characteristics of the water; therefore, we hypothesize that these patterns could be the result of fine-scale microhabitat selection by different zooplankton species and life stages.

Inter- and intra-specific diurnal habitat selection of zooplankton during the spring bloom observed by Video Plankton Recorder

Diel vertical migration (DVM) is a common behavior adopted by zooplankton species. DVM is a prominent adaptation for avoiding visual predation during daylight hours and still being able to feed on surface phytoplankton blooms during night. Here, we report on a DVM study using a Video Plankton Recorder (VPR), a tool that allows mapping of vertical zooplankton distributions with a far greater spatial resolution than conventional zooplankton nets. The study took place over a full day–night cycle in Disko Bay, Greenland, during the peak of the phytoplankton spring bloom. The sampling revealed a large abundance of copepods performing DVM (up during night and down during day). Migration behavior was expressed differently among the abundant groups with either a strong DVM (euphausiids), an absence of DVM (i.e., permanently deep; ostracods) or a marked DVM, driven by strong surface avoidance during the day and more variable depth preferences at night (Calanus spp.). The precise individual depth position provided by the VPR allowed us to conclude that the escape from surface waters during daytime reduces feeding opportunities but also lowers the risk of predation (by reducing the light exposure) and thereby is likely to influence both state (hunger, weight and stage) and survival. The results suggest that the copepods select day and night time habitats with similar light levels (~10−9 μmol photon s−1 m−2). Furthermore, Calanus spp. displayed state-dependent behavior, with DVM most apparent for smaller individuals, and a deeper residence depth for the larger individuals.

Effects of the proximal factors on the diel vertical migration of zooplankton in a plateau meso-eutrophic Lake Erhai, China

To study the proximal factors inducing diel vertical migration (DVM) in large and small zooplankton species in a plateau lake in China, we investigated the DVM of crustacean zooplankton in Lake Erhai bimonthly from November 2009 to September 2010. We hypothesized that the factors affecting DVM behaviour in different-sized zooplankton were different. A linear regression was used to assess the relationships between environmental variables and the vertical distribution of zooplankton. All crustacean zooplankton exhibited normal DVM patterns (down during the day, up at night) across sampling months. The weighted mean depth (WMD) of all zooplankton did not show a significant correlation with the WMD of the dominant phytoplankton and chlorophyll-a. However, a negative relationship was observed between the distribution of zooplankton and water temperature in January, March, and July 2010, but the relationship was relatively weak (R-2 between 0.1 and 0.4). The vertical distribution of zooplankton was primarily affected by water transparency (P0.05), whereas the factors inducing DVM behaviour differed between large and small zooplankton. Predation avoidance and phototactic behaviour may be the dominant factors influencing DVM of large species, whereas only phototaxis contributed to the migratory behaviour of small species.

Variability in the vertical distribution and advective transport of eight mesozooplankton taxa in spring in Rivers Inlet, British Columbia, Canada

Zooplankton vertical distribution data and velocity estimates from a hydrodynamicmodel were employed to determine zooplankton daily exchange during four cruisesconducted from March to June 2010 in Rivers Inlet, a fjord in central BritishColumbia, Canada. Zooplankton transport rates varied temporally, being fastest inMarch when water velocities were highest. The active vertical movement of thezooplankton interacted with the vertically sheared flow field to influence zooplanktonadvection. Surface dwelling plankton such as Acartia longiremis and larvaceans experi-enced the highest advection losses of 20.4 and 20.5 day

Among-lake variation in vertical distribution of invasive, bloom-forming algal speciesGonyostomum semen(Raphidophyceae) in stratified humic lakes of eastern Poland

In recent years, there has been an increasing interest in the ecology of algal species Gonyostomum semen (Ehr.) Diesing, as it is often referred as an invasive species and a climate change indicator. Like many other flagellates, the species has the ability to active swimming, therefore it is often found unevenly distributed in the water column. Although there are several studies focused on the species vertical distribution (VD) and its diurnal changes, there is no general agreement about the causes and mechanisms of the phenomenon. In this paper, we analyzed G. semen VD in three stratified humic lakes in the new area of its spread in Central Europe, aiming to determine the common pattern of this phenomenon as well as to search any relationship with chemical and biotic factors. The results of this investigation showed that VD of the alga during the day varied among three lakes and that Gonyostomum migrated upward in the morning and downward in the afternoon, but it could stay in deep, anoxic and dark layers through most of the diurnal cycle. The analysis of chemical factors revealed that phosphate retrieval from deeper layers is probably not a key driver of the alga uneven VD and its migrations. Diurnal observations showed that three dominating zooplankton species (Asplanchna priodonta, Ceriodaphnia quadrangula, and Eudiaptomus graciloides) changed their vertical position in parallel to Gonyostomum. We have suggested in conclusions that the pattern of G. semen VD in temperate stratified lakes is not universal, but rather depends on particular lake conditions, including phosphate content and light climate. Since the zooplankton VD could play a regulating role in the alga behavior, further research in this field would be of great help in understanding this invasive alga ecology.

Vertical distribution and pigmentation of Antarctic zooplankton determined by a blend of UV radiation, predation and food availability

Selection pressure induced by simultaneously occurring environmental threats is a major evolutionary driver for organisms in terrestrial, as well as in aquatic ecosystems. For example, protection against ultraviolet radiation (UVR) and predation include both morphological and behavioral components. Here we address those selective pressures on zooplankton by performing a latitudinal monitoring, combined with mechanistic experiments in the Antarctic Southern Ocean, where the UVR-threat is extremely high. We assessed vertical distributions of zooplankton along the Antarctic coast showing that animals were most abundant at 20-80 m and tended to avoid the surface at sites with clear water. UVR-threat disappeared at between 9 and 15 m at sites with low and high water transparency, respectively. Light levels were, however, sufficient for visual fish predation down to approximately 19 and 37 m, respectively. The few zooplankton that were present in surface waters had high levels of non-pigmented UVR-protective compounds (mycosporine-like amino acids) compared to deeper dwelling zooplankton. Overall they had low levels of red pigmented UVR-protective compounds (carotenoids), suggesting high predation on pigmented individuals. In a complementary laboratory study we showed that levels of UVR-protective compounds increased considerably when zooplankton were exposed to UVR in the absence of predator cues. The recently developed transparency-regulator hypothesis predicts that UVR avoidance is an important driver to diel vertical migration in transparent waters, such as in Antarctica. We, however, conclude that copepods resided well below the level where UVR had diminished to very low levels and that predator avoidance or food availability are more likely drivers of zooplankton vertical depth distribution in transparent marine systems.

Vertical Distributions of Continental Shelf Zooplankton in Stratified and Isothermal Waters

AbstractFine-scale vertical zooplankton distributions were compared in October 1978 on a cross-shelf transect south of Long Island, New York, at a time-series station south of Long Island, and at a time-series station on Georges Bank. Samples were collected at 5 to 10-m depth intervals with a pumping system and fine mesh (102 μm) nets. The waters south of Long Island were strongly stratified, whereas those of Georges Bank were isothermal. Juvenile or adult copepods accounted for a mean of 91% of the total number of animals collected. Of these, one species (Oithona similis) was overwhelmingly dominant, accounting for a mean of 50% of the total number of zooplankters collected. Comparison of our values for the abundance of O. similis with those obtained with coarser meshes of nets in previous investigations in these waters during the same season suggest that these previous studies may have underestimated the abundance of O. similis by at least a factor of 20.Vertical distributions of some of the same domina...

Vertical distribution of zooplankton in a shallow peatland pond: the limiting role of dissolved oxygen

We investigated the diel vertical distribution patterns of microcrustacean zooplankton (Cladocera, Copepoda) in a shallow pond (max. depth: 70 cm) of the Oreg-turjan peatland (Ocsa, Central Hungary) during three 24-h periods in July (19-20th), August (17-18th) and September (11-12th) 2011. Environmental variables showed remarkable vertical stratification. Oxygen concentration was close to zero in the entire water column from night until sunrise, while the lower strata (from 20 cm below the surface) were close to anoxic during all three diel cycles. It proved to be the main determinant of the vertical distribu- tion of microcrustaceans. Accordingly, the highest proportion of individuals was present in the surface layer. Chlorophyll-a concentration and phytoplankton biomass were inversely distributed compared to zoo- plankton. Microcrustaceans (mainly Daphnia curvirostris) migrated to the middle layer only in August, which could be explained by a trade-off between food resources, dissolved oxygen (DO) and competition with littor- al zooplankters. The diurnal density patterns of microcrustaceans suggested horizontal migration into the aquatic macrophytes during night, which could be a strategy to avoid Chaoborus predation. Our results show that strong vertical gradients of abiotic and biotic factors occur even in such shallow waterbodies. Among them, DO can maintain constant vertical aggregation of zooplankters by limiting their occurrence to the surface layers.

Diurnal vertical distribution of zooplankton in a newly formed reservoir (Tahtalı Reservoir, Kocaeli): the role of abiotic factors and chlorophyll a

Although water level fluctuations and related environmental variations occurring within reservoirs have a considerable effect on the zooplankton community, studies on zooplankton dynamics of reservoirs are very limited. In the present study, we aimed to elucidate the diurnal vertical migration of zooplankton in relation to the concentration of physicochemical factors and chlorophyll a content in a recently formed water reservoir (Tahtalı Reservoir, Kocaeli, northwestern Turkey). During the sampling period 20 zooplankton taxa were identified, which were mostly indicative of eutrophic waters, as confirmed by the physicochemical factors of the water in the reservoir. Rotifera was the most abundant group, followed by Copepoda and Cladocera. Statistical analyses indicated that all the zooplankton species showed a similar distribution pattern throughout the study. They declined in abundance in the water column from the surface to a depth of 6 m, and then increased relatively again towards the deepest part of the water (9 m). Abiotic factors, chlorophyll a, time of the day, and depth significantly impacted the abundance of zooplankton, whereas they jointly had no effect. The results suggested that abiotic factors and chlorophyll a were controlling factors on the vertical distribution pattern of the zooplankton in the Tahtalı Reservoir.

Does the fish presence influence the diurnal vertical distribution of zooplankton in high transparency lakes?

The avoidance of visually feeding fish has long been considered as the primary driver of diel vertical migration of zooplankton. The diurnal vertical distribution of Cyclops gr. abyssorum, Arctodiaptomus alpinus, and Daphnia gr. longispina from 13 alpine lakes with fish (Salvelinus fontinalis) and without, was compared in order to understand whether fish in transparent lakes reduce the presence of large zooplankton from the irradiated zone. We used the light level at each sampling depth and the size of each specimen as proxies of predation risk, and we tested two predictions: (P1) the relative abundance of zooplankton in the well-lit surface waters vs. the darker waters will be greater in fishless lakes; (P2) the size of zooplankton in the well-lit surface waters vs. the deeper, darker waters will be greater in fishless lakes. We did not find any evidence of the validity of P1, but we confirmed P2 for Arctodiaptomus alpinus. These results support with new field data the Transparency Regulator Hypothesis, which argues that in transparent lakes, fish predation is less important for the vertical distribution of zooplankton than ultraviolet radiation, and further suggest that zooplankton size rather than vertical distribution may be more effective against visual predators in transparent lakes.