Vertical Distribution Of Phytoplankton Research Articles

Asynchronous vertical migration and bimodal distribution of motile phytoplankton

Some motile phytoplankton have the capability to exploit deep sources of nutrients in a vertical migration cycle: photosynthesis in the near-surface layer, transit to depth, uptake of the limiting nutrient and transit back to the surface layer. If all four steps can be completed within 24 h, then migrations can be synchronized to the day/night cycle to maximize photosynthetic efficiency. Alternatively, if physiological, behavioral or environmental factors make it impossible for the cycle to be completed in 24 h, then migration may be asynchronous. Many observations of phytoplankton reveal bimodal vertical distributions of organisms, with maxima near the surface and the nutricline. We demonstrate how bimodal vertical distributions of phytoplankton may be symptomatic of asynchronous vertical migration using a Lagrangian Ensemble numerical model. We simulate vertical migration of the dinoflagellate Alexandrium fundyense in conditions similar to those in the Gulf of Maine, where bimodal distributions of A. fundyense have been observed. Migration is regulated by internal nutritional state—organisms swim down toward the nitracline when depleted of nitrogen, and return to the surface after nutrient uptake. We test the sensitivity of the results to growth rate, nitrogen uptake rate and swimming speed, and find that organism distributions can be bimodal or unimodal depending on conditions. Finally, we develop an analytical estimate for population distribution based on organism characteristics and nutricline depth.

Effects of a blue mussel Mytilus edulis bed on vertical distribution and composition of the pelagic food web

The impact of a mussel bed on the neighbouring pelagic food web is investigated by a comparative study of the vertical distribution of plankton above a mussel bed with that above a bare sandy bottom. The use of conventional water bottles did not reflect the near-bed dynamics. However, use of a high-resolution water sampler in the bottom 1 m of the water column above the sea floor revealed statistically significant differences. The results document for the first time in situ grazing on all major components of the pelagic food web. The vertical distributions of phytoplankton, bacteria, protozoa, meroplankton and copepods were statistically significantly different above the mussel and sand beds. The mussel bed recycled nutrients back to the water column as waste products. The pre- sent investigation stresses the need for a more diverse view of the trophic role of suspension-feeding bivalves in shallow coastal ecosystems.

Contrasting the vertical differences in the phytoplankton biology of a dipole pair of eddies in the south-eastern Indian Ocean

Abstract A dipole pair consisting of one warm-core (WC) and one cold-core (CC) eddy formed off Western Australia in the winter of 2003. Within these eddies the vertical distributions of phytoplankton, both in terms of community and physiology, were characterized in an effort to understand the impact of eddies on the phytoplankton ecology of the region. The WC eddy had a 275-m deep mixed layer in the centre and a vertically dispersed photoautotrophic community dominated by large diatoms, Pelagophyceae and coccolithophorids. The CC eddy had a shallower and more consistent mixed-layer depth with a deep chlorophyll a maximum (chlamax) well developed throughout the eddy. The euphotic zones and mixed layers of both eddies were very low in nitrate. Phytoplankton characteristics that are known to acclimate to variation in irradiance but require different amounts of time to reach steady state were measured at the surface and deep chlamax in both eddies. Differences between the surface and chlamax were found for characteristics that can diverge over a range of time scales from very short (seconds) to much longer (months) for physiological and ecological parameters such as: Ik and fluorescence quantum yield from PAM fluorometry, pigments (photoprotective and photosynthetic), pigment ratios, carbon to chlorophyll a ratios, PB and PBmax (measured over 24 and 4 h, respectively) community composition based on cell counts and pigments, and deep chlamax formation. Absolute time scales for change of these characteristics were obtained from the literature and used to quantify vertical mixing rates in the two eddies. Overall, vertical mixing is shown to be more pronounced, up to 20 times faster and deeper, in the WC eddy than in the CC eddy, especially near the WC eddy centre. This high rate of vertical mixing is hypothesized to allow the development and persistence of a vertically homogeneous community of large diatoms that spreads from the surface down to 275 m, well below the euphotic depth. As nitrate concentrations in the surface mixed layers of both eddies were very low there was no capacity for a significant increase in phytoplankton biomass within the eddies without additional nitrogen inputs. The longer-term biological trajectories of the eddies are discussed.

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

SUMMARYThe 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.

Phytoplankton vertical distributions and composition in Baltic Sea cyanobacterial blooms

We studied the vertical structure of the phytoplankton community in two toxic cyanobacterial blooms in the offshore Baltic Sea. In 1994, vertically separated potentially toxic, diazotrophic and mixotrophic species (belonging to Cyanophyceae, Dinophyceae and Prymnesiophyceae) dominated. In 1997, picocyanobacteria, mainly in colonies, made up 40–50% of the total phytoplankton carbon biomass in the top 20 m both day and night. Colony-forming species of picocyanobacteria seem to be occasionally important and hitherto underestimated in the Baltic Sea. We found species-specific depth distribution patterns. Nodularia spumigena and Anabaena spp. were observed mainly above 10 m depth, while Aphanizomenon sp. was mostly found deeper, especially at night. Dinophysis norvegica was only abundant near the seasonal pycnocline and showed very limited diurnal migration. Other flagellates, including small Cryptophyceae and 10 identified Chrysochromulina species, occurred down to 40 m depth. Their vertical migration may help to retrieve nutrients from below the summer pycnocline. We conclude that considerable differences in dominating functional groups may occur between years/bloom stages, and that the vertical distribution pattern of many species is recurring at similar environmental conditions, suggesting species-specific niche-separation.

Lack of seasonality in phytoplankton standing stock (chlorophyll a) and production in the western Bay of Bengal

The investigations in the western Bay of Bengal (BoB) during summer, winter and spring intermonsoon periods evidenced lack of pronounced seasonal variation in phytoplankton standing stock (chlorophyll a ) and primary production. The supply of riverine materials to the coastal bay is witnessed by the presence of major nutrients (nitrate-NO 3, phosphate-PO 4 and silicate-SiO 4) in the upper layers (up to 20 m) during monsoonal periods. However, it was not found to enhance the phytoplankton production rates to a large extent. The strong stratification due to enormous quantity of fresh water discharge, acts as a barrier to vertical transport of nutrients from the subsurface layers. The average primary production in the inshore and the offshore waters of the BoB were 350 ± 225 and 251 ± 177 mgC m - 2 d - 1 in summer, 252 ± 210 and 231 ± 150 mgC m - 2 d - 1 in winter and 308 ± 120 and 303 ± 95 mgC m - 2 d - 1 in spring intermonsoon periods, respectively. Subsurface chlorophyll maxima (SCM) were observed at 20–50 m in the spring intermonsoon, whereas in summer and winter, the chlorophyll maxima occurred in the upper layers of the euphotic zone ( < 20 m ). The vertical distribution of phytoplankton ( > 5 μ m ) was primarily restricted by the pycnocline, and the bulk of phytoplankton assemblage existed within the surface. The seasonal change in cell numbers, however, did not coincide closely with the changes in the chlorophyll a concentration. Diatoms formed the major group ( > 85 % ) in the total phytoplankton abundance irrespective of seasons. During spring intermonsoon, large patches of Trichodesmium blooms were observed in the coastal as well as open waters of the western BoB.

Vertical distribution of phytoplankton in a clear water lake of Colombian Amazon (Lake Boa, Middle Caquetá)

Vertical distribution of phytoplankton is highly influenced by physical and chemical factors, but the knowledge about these aspects remain unknown in Colombian Amazon lakes. In this work, the relations between the physico-chemical variables and the vertical distribution of phytoplankton community of a clear water lake (Lake Boa) are analyzed. Samples were taken at every 30 cm from surface to bottom in the center of the lake at different times of the hydrological cycle. Phytoplankton transformed from a diluted community dominated by cyanobacteria in high waters to one with elevated abundance of coccal chlorophytes in low waters. In the low water phase, the vertical distribution of Cryptomonas spp., Peridinium cf. umbonatum and Chlamydomonas spp. had significant statistical associations with the redox potential (RP), reactive phosphorus (PO4), and hydrogen sulfide (H2S) (p ≤ 0.05). During the flood stage the conditions were more varied and significant associations were detected between the vertical arrangement of Botryococcus braunii, Oscillatoria splendida, Dinobryon serturlaria, Euglena acus, and Trachelomonas volvocina with conductivity, dissolved oxygen (DO), pH, temperature, dissolved organic carbon (DOC), PO4, RP, and ammonia (NH4). The vertical structure of the Lake Boa’s phytoplankton responds to annual changes produced by the flood pulse, and to the vertical environmental variations, which are stronger during the high water phase.

Effects of Wind and Wind-Induced Waves on Vertical Phytoplankton Distribution and Surface Blooms of Microcystis aeruginosa in Lake Taihu

ABSTRACT Effects of wind and wind-induced waves on vertical phytoplankton distribution and Microcystis aeruginosa bloom formation were studied in Meiliang Bay of Lake Taihu from September 5 to 9, 2004. Concentrations of chlorophyll a, b, and phycocyanin in the water column were used to represent the abundance of total phytoplankton, chlorophytes, and cyanobacteria, respectively. Water samples were taken under different wind and wind-induced wave conditions. Ca. 90% of Microcystis aeruginosa had densities less than that of lake water and were thereby positively buoyant. Vertical distributions of total phytoplankton, chlorophytes, and cyanobacteria were markedly affected by wind and waves, with coefficients of variation of pigment concentrations throughout the water column showing a negative correlation with increasing wind and waves. Surface proportions of M. aeruginosa versus total amounts in the water column also closely correlated with wind and waves, which demonstrated that wind and waves directly influenced surface M. aeruginosa blooms in Lake Taihu. A less significant correlation was found for total phytoplankton, but there was no correlation for chlorophytes in the water column.

Light limitation of primary production in high latitude reservoirs

Abstract. To explore the effects of vertical mixing on the primary production in a northern reservoir, a Lagrangian particle dispersion model was coupled to a 1-D reservoir model where the vertical mixing was calculated using a k-ε model together with an empirically-based deep-water eddy viscosity. The primary production of each phytoplankton cell is assumed to be a function of the ambient light and not to be nutrient limited. The photoadaption follows first-order kinetics where the photoadaptive variables, a, b, and Pm, describe the coefficients of the photosynthesis-irradiance curve. The model is applied to the northern reservoir Akkajaure, which is strongly regulated with a mean and maximum depth of 30 m and 100 m respectively. Based on the release of 1000 particles (plankton), the model calculated the mean primary production of each plankton, during four different growing seasons. Vertical mixing has a substantial effect on the vertical distribution of phytoplankton and, thus, on the primary production in a reservoir. It was found that primary production was greater in a cold summer with weak stratification than in a warm summer when the reservoir was more stratified.

Zooplankton biomass distribution patterns along the western Antarctic Peninsula (December 2002)

The phytoplankton [chlorophyll a (Chl a)], microzooplankton, mesozooplankton and macrozooplankton biomass and distribution were studied as part of a multidisciplinary project (Tempano) along the Antarctic Peninsula during December 2002. Even though the summer phytoplankton bloom was not yet developed in the area, autotrophs dominated the plankton biomass. Phytoplankton vertical distribution was, in general, homogeneous in the upper 40–50 m of the water column, further decreasing with depth. Protozoans showed low biomass; their contribution to the total plankton being one order of magnitude lower than that of autotrophs. The vertical distribution of protozoans was variable among stations with marked peaks at depths ranging from 30 to 80 m. Mesozooplankton-integrated biomass was generally low, although there was a notable increase southward near the ice marginal zone. Macrozooplankton distribution was more variable without any clear zonal distribution pattern. The vertical distribution of meso- and macrozooplankton (>4 mm) biomass showed clear peaks of abundance comprising different species depending on the geographical area. Our biomass distribution data suggest a food-web scenario in which macrozooplankton are preying on mesozooplankton populations only in the northerner stations, and mesozooplankton are, in their turn, shaping the abundance of the emerging populations of microzooplankton. Phytoplankton, on the other hand, seem to be hardly controlled by grazing activity.

Catastrophic shifts in vertical distributions of phytoplankton The existence of a bifurcation set

A model of phytoplankton dynamics within a water column was analyzed with special consideration on the existence of a bifurcation set in the parameter space. We considered two resources, light and a limiting nutrient, for phytoplankton growth and assumed that the water column is separated into two layers by thermal and/or density stratification. It was shown that there exists a bifurcation set in the parameter space when the growth function meets several conditions that are general for growth functions of two essential resources. Specifically, these conditions include that a less abundant of the two resources limits the growth while the effect of the other is sufficiently small. Folded structure with two stable states separated by one unstable state appears in the catastrophe manifold when parameters move to a certain direction with a certain curvature from a point in the bifurcation set. These results suggest that occurrence of discontinuous transition between two alternative vertical patterns is possible nature of phytoplankton dynamics within a stratified water column.

Interannual variability in the distribution of the phytoplankton standing stock across the seasonal sea-ice zone west of the Antarctic Peninsula

The spatial distribution of phytoplankton cell abundance, carbon (C) biomass and chlorophyll a (Chl a) concentration was analysed during three summers (1996, 1997 and 1999) in a seasonal sea-ice area, west of the Antarctic Peninsula. The objective of the study was to assess interannual variability in phytoplankton spatial distribution and the mechanisms that regulate phytoplankton accumulation in the water column. Phytoplankton C biomass and Chl a distributions were consistent from year to year, exhibiting a negative on/offshore gradient. The variations in C concentration had a close and non-linear relationship with the upper mixed layer depth, suggesting that the vertical mixing of the water column is the main factor regulating phytoplankton stock. The magnitude of C gradients was 5-fold higher during 1996 than during 1997 and 1999. This was ascribed to interannual variations in the concentration of diatom blooms in the region influenced by sea-ice melting. Vertical distribution of the phytoplankton, as estimated from Chl a profiles, also varied along an on/offshore gradient: Chl a was distributed homogeneously in the upper mixed layer in coastal and mid-shelf stations and concentrated in the deep layer (40–100 m) occupied by the winter waters (WW, remnants of the Antarctic surface waters during summer) in more offshore stations. The region with a deep Chl a maximum layer (DCM layer) was dominated by a phytoplankton assemblage characterized by a relatively high concentration of diatoms. The extent of this region varied from year to year: it was restricted to pelagic waters during 1996, extended to the shelf slope during 1997 and occupied a major portion of the area during 1999. It is hypothesized that iron depletion in near surface waters due to phytoplankton consumption, and a higher concentration in WW, regulated this vertical phytoplankton distribution pattern. Furthermore, we postulate that year-to-year variations in the spatial distribution of the DCM layer were related to interannual variations in the timing of the sea-ice retreat. The similarity between our results and those reported in literature for other areas of the Southern Ocean allows us to suggest that the mechanisms proposed here as regulating phytoplankton stock in our area may be applicable elsewhere.

Nutrient and phytoplankton distribution in the Loire River plume (Bay of Biscay, France) resolved by a new Fine Scale Sampler

A new Fine Scale Sampler (FSS) was designed and used in the Loire River plume (Bay of Biscay, France) in order to study phytoplankton distribution in highly stratified water columns. The FSS consists of a linear array of 15 sampling bottles, set horizontally at 20 cm intervals. Data acquired from the CTD and fluorescence probe of the FSS enable it to be precisely positioned at the depth of the highest density gradient or at the chlorophyll a maximum. The FSS made it possible to investigate fine scale vertical distribution of phytoplankton and relationships with water density, nutrient concentrations or light availability, for spring and summer conditions. Under spring conditions, the entire upper layer exhibited a dominant diatom population ( Chaetoceros sociale, Thalassiosira rotula) above 15 m with evident uptake in dissolved inorganic nutrients and the maximum concentration of dinoflagellates (mainly small forms of Gymnodinium spp. and Gyrodinium spp.) was located 16 m deep, where light energy began to be a limiting factor for phytoplankton growth. During summer conditions, the maximum phytoplankton biomass (10 μg l −1 of chlorophyll a) was only concentrated in the 2 m thick pycnocline layer, located at a depth of 12 m. The diatom C. sociale was dominant in the phytoplankton peak. Dinoflagellates ( Dinophysis acuminata and Gymnodinium spp.) were also concentrated around the density gradient. Low inorganic nitrogen concentration (<0.2 μmol l −1) was responsible for the limitation of phytoplankton production in the surface layer above the pycnocline, and the phytoplankton growth was irradiance-limited underneath the pycnocline.

Distribution patterns, biomass estimates and diversity of phytoplankton in İzmit Bay (Turkey)

This study was carried out on the qualitative and quantitative characteristics of phytoplankton community structure and the environmental factors that affect its distribution and the changes in İzmit Bay, which is an eutrophic bay of Marmara Sea. The water samples were collected bimonthly between February 1999 and September 2000 from the 10 sampling sites (from surface, −5 m, −10 m, and −20 m). Phytoplankton densities were calculated as cells l −1, biomass was estimated by biovolumes and chlorophyll a content. During the study period, a total of 77 taxa from five algal classes, Bacillariophyceae, Dinophyceae, Chrysophyceae, Haptophyceae and Dicthyochophyceae were determined. Species composition of phytoplankton was typical of eutrophic conditions and was frequently characterized by the presence of Dinophyceae. In general, the most important groups were Dinophyceae and Bacillariophyceae in species numbers and density. The bay was characterized by intensive dinoflagellate (mainly Prorocentrum spp.) dominated bloom in all sampling periods, while the diatoms (shifts were observed in dominant diatoms) contributed most to the settling phytoplankton biomass. Differences among sampling sites and depths were recorded in terms of phytoplankton density, biovolume and chlorophyll a values. Correlation between chlorophyll a, cell count and biomass was significant. In general, the bay sustained a poor phytoplankton community with higher diversity relatively towards the outer than the interior zone in terms of Shannon-Weaver diversity index ( H′). Some environmental parameters seem to play an important role in determining the phytoplankton community succession and then diversity, favoring or limiting the growth of the different groups of the phytoplankton. The Secchi disc depth, an indicator of primary production in the marine environment was comparatively low and limited vertical distribution of phytoplankton. The results indicate that primary production of the bay may be limited by nitrogen and silicate in some periods.

Analysis and numerical simulation of phytoplankton–nutrient systems with nutrient loss

The dynamics of a mathematical model of a layer of single phytoplankton species growing over a pool of nutrients, proposed by [A.H. Taylor, J.R.W. Harris, J. Aiken, The interaction of physical and biological process in a model of the vertical distribution of phytoplankton under stratification, Mar. Int. Ecohyrd., J.C. Nihoul (Ed.) 42 (1986) 313–330] is analyzed. Both cases of presence and absence of a phytoplankton in the water below the layer of interest are considered. Positive and elementary stable nonstandard (PESN) methods, having the same qualitative features as the corresponding continuous models, are formulated and analyzed. Biological implications and a set of numerical simulations supporting the mathematical and numerical analysis are also presented.

Effects of onset of thermal stratification on vertical distribution of phytoplankton and zooplankton species

Click to increase image sizeClick to decrease image sizeKey words: Phytoplanktonzooplanktonstratificationheterogeneity

UV transmission in Norwegian marine waters: controlling factors and possible effects on primary production and vertical distribution of phytoplankton

We investigated the ultraviolet radiation (UVR) transmission properties of Norwegian oceanic, coastal and fjord waters, and how they influence the primary production and vertical distribution of phytoplankton. Values of the 1% UVR attenuation depth and diffuse attenuation coef- ficients (Kd) in the Greenland and Norwegian Seas (GNS), in the coastal waters of south-western Nor- way (SWN) and in the Samnanger fjord (SAF) are presented. Maximum penetration of UVR in the GNS was confirmed by Kd(320) = 0.25 m -1 , and mimimum penetration in the SAF, by Kd(320) = 9 m -1 . In the GNS, Kd and chlorophyll a (chl a) were closely correlated, while coloured dissolved organic matter (CDOM) was the main contributor to ultraviolet (UV) attenuation in the SAF. Also, in SWN waters, CDOM was more important than chl a for UV attenuation, but less important than in SAF waters. In GNS and SAF waters the average vertical distribution of chl a had its maximum in the upper 10 and 7.5 m of the water column, respectively, while in SWN waters it had its maximum at 20 m. The depths with the highest photosynthetic rates per unit volume decreased successively from the oceanic waters of the GNS via the coastal waters of the SWN to the fjord waters of the SAF. Under similar PAR intensities, however, the water column photosynthetic efficiency (integrated carbon assimilation/chl a ratio) was highest in SWN waters. Maximum and mean percentage potential for inhibition of the estimated (from PAR and UV) primary production due to UVR at a depth of 5 m were 11 and 4.3% in the GNS, 3.2 and 0.9% in the SWN and 0.5 and 0.1% in the SAF. The UVR potential for inhibition was significant down to a depth of 10 m in the GNS, down to a depth of 5 m in the waters of the SWN, while it was seldom found deeper than 3 m in the SAF. These variations could be ascribed to differences in CDOM concentrations and mixed-layer depths. The optical properties of the investigated water masses were found to be highly influenced by the circulation patterns.

Acoustic observations of finescale zooplankton distributions in the Oregon upwelling region

Abstract We collected data on two GLOBEC cruises to investigate the ubiquity and character of small and finescale phytoplankton and zooplankton vertical distributions in continental shelf waters. Coincident data from a four-frequency echosounding system (38, 120, 200, and 420 kHz), CTD, 150 kHz ADCP, and a suite of high-resolution bio-optical instruments were collected at three continental shelf stations off the coast of Oregon in June and August of 2000. Temporally and spatially coherent finescale features were identified in the acoustic record at all sites throughout the upper water column and the dominant characteristics were observed. There was evidence of an association between the vertical structure of horizontal velocity and the finescale structure seen in the acoustic data. These observations support the hypothesis that discrete, finescale vertical distributions of phytoplankton and zooplankton may be common in productive upwelling areas and indicate that additional concurrent physical and biological measurements, at appropriate temporal and spatial scales, must be made to further characterize the ecological significance of these finescale distributions.

Deriving phytoplankton biomass in the Marginal Ice Zone from satellite observable parameters

Water mass properties and sea ice history have a significant impact on the vertical distribution of phytoplankton in the Marginal Ice Zone of the northern Barents Sea. A method is suggested to identify the phase and magnitude of the phytoplankton bloom based on satellite observable values of chlorophyll-a, temperature, salinity and sea ice history. For each bloom phase, formulae are provided for calculation of the chlorophyll-a column from satellite-equivalent measurements of chlorophyll-a.

Seasonal vertical distribution of phytoplankton and copepod species in a high-mountain lake

Many zooplankton species migrate to deeper water layers during the day to reduce light-dependent mortality risk at the surface layers. Theories explaining diel vertical migration (DVM) of zooplankters share the assumption that the surface waters are more food-rich compared to the deep-water layers and that zooplankters return at night to the surface to profit from the warmer, food-rich environment. However, evidence suggests that the resources in deep-water layers can be as profitable as in the upper water layers for zooplankton. Those deep-water food maxima have often been overlooked in freshwater. In this study we investigated the vertical distribution of phytoplankton together with the vertical day and night distribution of copepod species in a high-mountain lake with a deep-water phytoplankton maximum dominated by cryptophytes and blue-greens. All copepod species, including the small-sized individuals avoided the upper strata during daytime. Adult Cyclops abyssorum migrated almost throughout the entire year. In contrast, the Eudiaptomus gracilis and copepodites of C. abyssorum and nauplia of both species did not migrate but remained in the same water layers day and night. Secchi depth transparency significantly correlated with daytime depth of adult copepods. Daytime depths are usually associated with high concentration of algal biovolume and moderate temperature. Although food conditions are better in deeper strata, for some species the warmer temperatures in the surface waters may be the reason why they migrate to shallower depths during the night.