Vertical Migration Patterns Research Articles

Recruitment, growth, and vertical distribution of the endangered mud snail Cerithidea rhizophorarum A. Adams, 1855: implications for its conservation

Some fundamental natural-history traits of the endangered mud snail Cerithidea rhizophorarum were investigated for three years in an open, bare, sandy tidal flat located on the east of Shikoku Island in southwestern Japan. Our study demonstrated a highly aggregated distribution of younger snails and complicated differences in vertical migration patterns between younger and older snails that might have prevented comprehensive understanding of the precise demography of C. rhizophorarum in previous studies. From late summer, juveniles of the first-year cohorts appeared patchily at 12–16 cm lower tidal heights than snails of the oldest cohorts and reached 5.0–7.5 mm in mean shell length by the first mid-autumn. In the second year, juveniles gradually moved upward and appeared at higher tidal heights. The second-year cohorts reached 15.6–19.1 mm in mean shell length by the end of the second autumn and converged into the oldest cohorts during the third year or later. Our results suggest that C. rhizophorarum matures sexually and starts reproduction from the fourth summer at the earliest. On the other hand, older snails showed a similar seasonal migration pattern every year—downward movement in summer and upward movement in autumn—probably associated with their reproductive activity and/or a seasonal change in the maximum tidal height. For conservation of C. rhizophorarum, it is very important to preserve both the higher area as a main habitat for older snails and the lower tidal flat as the nursery ground.

Vertical Migration and Reproductive Patterns of a Long-Term Brooding Freshwater Mussel, Villosa constricta (Bivalvia: Unionidae) in a Small Piedmont Stream

We delineated a permanent 15 m by 9 m reach of a mussel bed in a small piedmont stream in the Cape Fear River Basin of North Carolina, USA. A total of 14 surveys were conducted at the study site from May 2005 to September 2006 at time intervals ranging from 2 weeks to 3 months. The study area was divided into fifteen 1-m-wide transects, and each transect was thoroughly searched twice during each survey event for any mussels on the substrate surface. We recorded species identification, length, gravidity (for known females) and replaced the mussel in the exact spot it was found. A pilot study was conducted to determine detection success with one, two, and three passes per transect and detection success was monitored on all transects throughout the study. We estimate that two passes over these transects yielded approximately 90% of the mussels on the sediment surface. Vertical migration patterns of Villosa constricta, and in particular females, were highly seasonal. Additional within-season variation could not be explained by seasonal patterns alone. Larger individuals were recaptured more frequently. Female mussels became gravid from August through March indicating that spawning and glochidial release took place over an extended period. In 2005, glochidial release was 1-2 months later than in 2006 and lasted through June. In 2006, glochidial release began before 7 February in 2006 and lasted through April. Smaller V. constricta (23-28 mm) were more likely to be gravid, and about half of the individual females were observed to spawn in consecutive years.

Pelagic decapods in the northern Benguela upwelling system: Distribution, ecophysiology and contribution to active carbon flux

Decapods were sampled with a 1m2 MOCNESS (mainly upper 1000m) in the northern Benguela Current during three cruises in December 2009, September/October 2010 and February 2011. Although pelagic decapods are abundant members of the micronekton community, information about their ecophysiology is very limited. Species-specific regional distribution limits were detected for various decapod species (e.g. Plesionika carinata, Sergestes arcticus, Pasiphaea semispinosa). Significant diel vertical migration patterns were determined for three caridean and three penaeiodean species. Biomass was variable and ranged from 23 to 2770mg dry mass m−2 with highest values for P. semispinosa. Fatty acid and stable isotope analyses revealed that the examined decapod species are omnivorous to carnivorous except for the herbivorous to omnivorous species P. carinata. Calanid copepods such as Calanoides carinatus were identified as an important prey item especially for caridean species. Community consumption rates of pelagic decapods derived from respiration rates ranged from 7mgCm−2d−1 (23°S) to >20mgCm−2d−1 (19°S, 17°S). A potential active respiratory carbon flux was calculated for migrating pelagic decapods with 4.4mgCm−2d−1 for the upper 200m and with 2.6mgCm−2d−1 from the base of the euphotic zone to a depth of 600m. Overall, pelagic decapods apparently play a more prominent role in the northern Benguela Current ecosystem than previously assumed and may exert a substantial predation impact on calanid copepods (up to 13%d−1 of standing stock).

Distribution and feeding of Benthosema glaciale in the western Labrador Sea: Fish–zooplankton interaction and the consequence to calanoid copepod populations

This study evaluated the distribution of major calanoid copepods in the western Labrador Sea in relation to that of the myctophid Benthosema glaciale, and investigated patterns of prey composition and feeding periodicity by the latter to assess the potential impact of mesopelagic fish on copepod populations that reside in the deep ocean. Hydroacoustic surveys indicated that B. glaciale and the deep-scattering layer are widely distributed throughout the region with limited evidence of patchiness, with an average abundance of 6 fish m−2 and biomass of 9.3gm−2. There was clear evidence of diurnal variations in feeding activity that was achieved through vertical migration from several hundred meters depths to the surface layer. B. glaciale fed principally on calanoid copepods, with prey size dependent on the length of the fish but the relative variability in prey size was independent of predator length. Average rations were generally less than 1% of body weight per day, and the patterns of diurnal vertical migration by myctophids suggest that individuals fed once every two days rather than daily. The estimated mortality caused by B. glaciale on the calanoid populations, which considers most sources of uncertainty, ranged from 0.002 to 1.8% d−1, with the mid-point of these estimates being ∼0.15% d−1, which is well below the estimated mortality rates of 10–20% d−1 based on vertical life tables. From observations from this and other ecosystems, understanding and contrasting the drivers of population dynamics and productivity of calanoid copepods in different deep basins of the North Atlantic will likely require a more comprehensive characterization of the plankton and pelagic and oceanic fish faunas of the epipelagic and mesopelagic zones and their trophic relationships and interactions.

Analysis of within- and between-day chlorophyll-a dynamics in Mantua Superior Lake, with a continuous spectroradiometric measurement

Eutrophic lakes display unpredictable patterns of phytoplankton growth, distribution, vertical and horizontal migration, likely depending on environmental conditions. Monitoring chlorophyll-a (Chl-a) concentration provides reliable information on the dynamics of primary producers if monitoring is conducted frequently. We present a practical approach that allows continuous monitoring of Chl-a concentration by using a radiometric system that measures optical spectral properties of water. We tested this method in a shallow, nutrient-rich lake in northern Italy, the Mantua Superior Lake, where the radiometric system collected data all throughout the day (i.e. every 5 min) for ~30 days. Here, specifically developed algorithms were used to convert water reflectance to Chl-a concentration. The best performing algorithm (R2 = 0.863) was applied to a larger dataset collected in September 2011. We characterised intra- and inter-daily Chl-a concentration dynamics and observed a high variability; during a single day, Chl-a concentration varied from 20 to 130 mg m–3. Values of Chl-a concentration were correlated with meteo-climatic parameters, showing that solar radiance and wind speed are key factors regulating the daily phytoplankton growth and dynamics. Such patterns are usually determined by vertical migration of different phytoplankton species within the water column, as well as by metabolic adaptations to changes in light conditions.

Analysis on Migration Patterns and the Distribution of Oil-Gas Controlling Factors of Yushulin Area

In order to ascertain the oil and gas migration pattern and its effect on gas/oil distribution of Yushulin area of Sanzhao depression in Northern Songliao Basin, this paper makes a comprehensive analysis of the oil source condition, the source-reservoir-seal assemblage relationship, the oil and gas migration pathway and patterns in Putaohua and Fuyang oil reservoir, summarizes the oil and gas migration patterns of Yushulin area, and analyses different oil and gas migration pattern influence on oil/gas distribution in Yushulin area combined with structural history, hydrocarbon generation and expulsion history, sedimentary microfacies research. The results show that there are mainly four types of oil and gas migration patterns in Yushulin area: indigenous – downward type - vertical migration pattern, indigenous - normal type - vertical migration pattern, proximal - lateral migration pattern and proximal – “U” type - complex migration pattern.

Effects of net cages on the vertical distribution of zooplankton in a semi-arid reservoir, northeastern Brazil

AIM: Carneiro reservoir is an aquatic environment used for net cage aquaculture activities, located in the semi-arid region of northeastern Brazil. Vertical distribution was evaluated in order to determine the effects of organic enrichment induced by aquaculture on zooplankton movements. METHODS: three replicate samples were collected during midday and midnight at each of three depths along the water column (i.e. surface: 0 m; middle: 2 m and bottom: 4 m) at two sites (near and far from the net cages). Samples were collected using a 3 L volume capacity Van Dorn bottle. The collected individuals were preserved in a 4% formaldehyde solution saturated with sugar and, later, identified and counted on a Sedgwick-Rafter chamber under a microscope, considering a minimum of 100 individuals per subsample. RESULTS: higher species richness and density were observed at the N station (nearby the net cages) compared to the F station (distant from the net cages). With few exceptions, Rotifer species did not exhibit typical vertical migration patterns, concentrating at bottom depths during daytime and at intermediate depths during nighttime. Brachionus dolabratus and Hexarthra mira were the only species to show reverse vertical migration at the N station, in opposition to patterns of the cyclopoid copepod Termocyclops crassus. Furthermore, vertical migration patterns were not clear for Cladocera and Copepoda species nearby the net cages, but were rather clear at the F station. CONCLUSIONS: the presence of fish net cages increased zooplankton species richness (mostly rotifers) and density. Vertical movements were more pronounced at sites distant from net cages, suggesting that the increased food availability reduced the need for the typical vertical movements during daytime at sites nearby the net cages.

Vertical migration of Norwegian spring-spawning herring larvae in relation to predator and prey distribution

A diel vertical migration (DVM) pattern of Norwegian spring spawning herring (Clupea harengus) larvae was investigated during 19 and 20 April 2009. Factors influencing DVM included physical and biological properties of the water column. Data on larvae, prey and predators were collected with a depth-stratified multisampling device, inshore of Sklinna bank, close to the Norwegian coast, while light conditions were calculated using a Matlab® algorithm. A type I DVM pattern (i.e. deep during daytime, shallow at night) was observed for herring larvae, mainly occurring above the thermocline. No size-dependent differences were observed for larval vertical positioning. The highest overlap in depth distribution with their main prey was observed during daytime, when larvae were distributed deeper in the water column. From acoustics and macroplankton trawl data, a type I DVM was also observed for krill, although their concentrations were relatively low in the area. Cumulative predator–prey overlap plots suggest that krill most likely forage on copepods and smaller organisms. During the day, larvae concentrate near the thermocline to feed, while they move towards the surface at dusk, possibly to use the remaining light to continuing feeding. During the night, when light levels were too low to feed, larvae spread out in the water column above the thermocline. This migration pattern reduces the overlap between larvae and potential predators such as krill, which also move higher up during nighttime. It is suggested that the pattern of herring larvae DVM is a behavioural response to active pursuit of prey.

Diel vertical migration of European hake Merluccius merluccius and associated temperature histories: insights from a pilot data‐storage tagging (DST) experiment

A pioneering experiment of archival tagging of European hake Merluccius merluccius provided evidence of a diel vertical migration pattern which was analysed using an automatic method, developed and validated through time-frequency analyses. Frequent vertical movements across the thermocline were observed and fish experienced rapid temperature changes >7° C. These tagging-recapture data also suggested a homing behaviour to feeding grounds.

Spatial and temporal patterns of vertical distribution for three planktivorous fishes in Lake Washington

Abstract – We sampled three limnetic fish species: juvenile sockeye salmon (Oncorhynchus nerka), three‐spine stickleback (Gasterosteus aculeatus) and longfin smelt (Spirinchus thaleichthys) in Lake Washington to quantify species‐specific patterns of diel vertical migration (DVM). Catch‐per‐unit‐effort data analysed from 15 years of midwater trawling documented seasonal and diel differences in vertical distributions for each species. These results were consistent with the hypothesis that the patterns of DVM in Lake Washington were affected by life history, size and morphology. Sockeye salmon showed clear DVM in spring but essentially no DVM in fall, remaining in deep water, whereas three‐spine sticklebacks were prevalent at the surface at night in both seasons. In fall, distribution patterns may be explained by differences in thermal performance (e.g., sticklebacks favouring warm water), but the patterns were also consistent with inter‐specific differences in predation risk. Younger sockeye salmon and longfin smelt were present in greater proportions higher in the water column during dusk and night periods than older conspecifics. Compared with sockeye salmon, the greater use by three‐spine sticklebacks of surface waters throughout the diel cycle during weak thermal stratification in spring was consistent with the hypothesis that sticklebacks’ armour reduces predation risk, but use of this warmer, metabolically beneficial stratum may also have promoted growth. This study illustrates variation in the vertical distribution of three sympatric planktivores and offers broader implications for the DVM phenomenon and applied lake ecology.

Variability in positional, energetic and morphometric descriptors of European anchovy Engraulis encrasicolus schools related to patterns of diurnal vertical migration

Acoustic data from summer echo-surveys in the Aegean Sea (Eastern Mediter- ranean) were analysed in an effort to better understand the mechanisms related to diel vertical migration and the schooling behaviour of European anchovy Engraulis encrasicolus in relation to the time of day. Cosine, quadratic and generalized additive models (GAMs) were used to describe the diurnal patterns in the vertical position and the energetic and morphometric characteristics of anchovy schools. The effect of environmental variables on a suite of school descriptors was addi- tionally examined using GAMs. Disruption of schooling at night was followed by school formation just before dawn at shallow layers, usually above the base of thermocline. Schools moved to deeper depths, close to the seabed during daytime and returned close to the surface to disperse after dusk. Surface light intensity and bottom depth explained a significant amount of variability in the vertical position of schools. Higher light intensities urged schools to deeper depths but max- imum downward displacement was constrained by sea bottom in coastal waters (<80 m). Along with positional changes, diurnal patterns in school shape and packing density were also identi- fied. Morphometric and energetic descriptors were mostly influenced by school altitude (distance from bottom), bottom depth and ambient light intensity. They presented high plasticity that could partly be explained in terms of the trade-off between predator avoidance and feeding success. The effect of schooling behaviour and diel vertical migration should be taken into account in pelagic fish bioenergetic models and in acoustic surveys in respect to sampling design and species discrimination.

Vertical patterns of ichthyoplankton at the interface between a temperate estuary and adjacent coastal waters: Seasonal relation to diel and tidal cycles

Vertical distribution and migration pattern of ichthyoplankton assemblage in the Mondego estuary were investigated in relation to diel and tidal cycle. Summer and winter communities were sampled, at surface and bottom, over a diel cycle during spring and neap tides at a fixed station at the mouth of the estuary. Summer presented higher larvae density mainly of Pomatoschistus spp., Gobius niger and Parablennius pilicornis. Main species in winter assemblages were Pomatoschistus spp. and Sardina pilchardus. There were no differences between depth stratums across diel or tide cycle. Nevertheless, main species larval densities showed significant periodic variation associated with tide (M2) and diel (K1) cycles presenting generally, higher density at night and around low tide. Conversely, vertical patterns observed could not be related with diel or tidal cycle. Tough, main species presented some extent of vertical migration. Vertical patterns observed appear to be related to seasonal stratification and river flow, increasing amplitude during periods of less stratification and lower water currents. Present study provides a better understanding of ichthyoplankton vertical movement patterns and of small scale dynamics at the interface of two coastal European systems.

Long-term acoustical observations of the mesopelagic fish Maurolicus muelleri reveal novel and varied vertical migration patterns

Long-term acoustical observations of the mesopelagic fish Maurolicus muelleri reveal novel and varied vertical migration patterns

Size-dependent patterns of diel vertical migration: smaller fish may benefit from faster ascent

Diel vertical migration (DVM) in fish has been studied extensively on the population level, but the individual variability of migration behavior has been widely neglected. However, the individual size of fish modifies the ratio between mortality risk and foraging gain. Therefore, if the rhythmic habitat shifts are considered being adaptive, the average time spent in the nighttime shallow habitat should differ between small and large fish, suggesting that the individual migration trajectories might be size dependent. We studied size-related differences in the vertical migration speed and the onset of DVM in coregonid fishes (Coregonus spp.) in a deep lake. Stationary vertical hydroacoustics was conducted once a month over a 7-month period to track individual fish during the migration periods at dusk and dawn. Quantile regressions revealed that vertical migration speed was higher for smaller than for larger fish. The comparison of detection times of fish migrating upward after sunset revealed an earlier detection of small fish compared with the medium and large fish, thus suggesting that smaller fish ascended earlier and faster at dusk. The reverse trend at dawn was less clearly expressed. Bioenergetics calculations suggest that small fish may profit from a faster and earlier ascent by increasing their daily ration by about 8% relative to fish that would migrate together with the main population. Therefore, plasticity of migration speeds may result in individual competitive advantages and heterogeneous mortality-feeding trade-offs within populations. This variability in individual migration trajectories has to be considered in future studies addressing ultimate causes of DVM. [Behav Ecol]

Seasonally Dynamic Diel Vertical Migrations of Mysis diluviana, Coregonine Fishes, and Siscowet Lake Trout in the Pelagia of Western Lake Superior

AbstractDiel vertical migrations are common among many aquatic species and are often associated with changing light levels. The underlying mechanisms are generally attributed to optimizing foraging efficiency or growth rates and avoiding predation risk (μ). The objectives of this study were to (1) assess seasonal and interannual changes in vertical migration patterns of three trophic levels in the Lake Superior pelagic food web and (2) examine the mechanisms underlying the observed variability by using models of foraging, growth, and μ. Our results suggest that the opossum shrimp Mysis diluviana, kiyi Coregonus kiyi, and siscowet lake trout Salvelinus namaycush migrate concurrently during each season, but spring migrations are less extensive than summer and fall migrations. In comparison with M. diluviana, kiyis, and siscowets, the migrations by ciscoes C. artedi were not as deep in the water column during the day, regardless of season. Foraging potential and μ probably drive the movement patterns of M. diluviana, while our modeling results indicate that movements by kiyis and ciscoes are related to foraging opportunity and growth potential and receive a lesser influence from μ. The siscowet is an abundant apex predator in the pelagia of Lake Superior and probably undertakes vertical migrations in the water column to optimize foraging efficiency and growth. The concurrent vertical movement patterns of most species are likely to facilitate nutrient transport in this exceedingly oligotrophic ecosystem, and they demonstrate strong linkages between predators and prey. Fishery management strategies should use an ecosystem approach and should consider how altering the densities of long‐lived top predators produces cascading effects on the nutrient cycling and energy flow in lower trophic levels.

Diel vertical distribution of early marine phase juvenile pink salmon (Oncorhynchus gorbuscha) and behaviour when exposed to salmon louse (Lepeophtheirus salmonis)

We observed diel vertical migration patterns in juvenile pink salmon ( Oncorhynchus gorbuscha (Walbaum, 1792)) and tested the hypothesis that fish behaviour is altered by exposure to sea lice copepodids. Experiments involved replicated field deployments of a large (9 m) plankton column, which provided a vertical distribution enclosure under natural light and salinity conditions. Diel vertical distributions of juvenile pink salmon were observed during the first 3 weeks of seawater acclimation in both the presence and the absence of the ectoparasitic salmon louse ( Lepeophtheirus salmonis (Krøyer, 1838)). Immediately upon entering seawater, juvenile pink salmon preferred the top 1 m of the water column, but they moved significantly deeper down the vertical water column as seawater acclimation time increased. A significant diel migration pattern was observed, which involved a preference for the surface at night-time, compared with daytime. When fish in the column were exposed to L. salmonis copepodids for 3 h, 43%–62% of fish became infected, fish expanded their vertical distribution range, and significant changes in vertical distribution patterns were observed.

Endogenous versus environmental control of vertical migration by intertidal benthic microalgae

The vertical migratory behaviour of estuarine microphytobenthos, i.e. the biofilm-forming microalgae inhabiting intertidal sediments, is probably a significant factor for their success in this extreme and unstable environment. The present work aimed to assess the relative role of endogenous versus environmental control of benthic microalgal vertical migratory behaviour. This was done by comparing the patterns of vertical migration in undisturbed sediment samples kept under constant conditions of darkness and low light with those in ambient light conditions, by measuring the changes in the surface microalgal biomass during daytime low-tide periods. The results showed that the formation of a biofilm was a two-phase process. It began with a relatively small accumulation of cells at the surface, starting hours before the beginning of the light period and endogenously driven. However, the full formation of the biofilm required exposure to light by the expected beginning of the photoperiod, which further promoted upward migration and accelerated the cell accumulation at the surface. In the absence of light, upward migration was interrupted and the incipient biofilm began to disaggregate. The relative importance of the endogenously controlled behaviour varied during the spring–neap tidal cycle, reaching a maximum on those days when low tide occurred in the middle of the day, suggesting its entrainment by the duration of light exposure on previous days. The regulation of the surface cell concentration during daytime low tides was found to be strongly dependent on exogenous factors, particularly irradiance. The spontaneous disaggregation of the biofilm shortly before the end of the low-tide period (due to tidal flood or sunset), both under constant as well as ambient light conditions, suggested the presence of an endogenously controlled positive geotaxis.

Distribution and behavior of Argentine hake larvae: Evidence of a biophysical mechanism for self-recruitment in northern Patagonian shelf waters

It has been proposed that vertical movements of planktonic organisms coupled to a horizontally or vertically stratified circulation pattern could describe a retention mechanism, with ecological advantages such as favoring recruitment success. The Patagonian stock of the Argentine hake Merluccius hubbsi spawns mainly from January to February in relation to a highly productive tidal frontal system. Although retention of fish eggs and larvae has been previously hypothesized for this system, it has not been fully proven and its mechanisms have not yet been proposed for hake early stages. To better understand the physical and biological processes involved in the survival and distribution of hake larvae, we focused on transport features and associated larval behavior governing the retention of larvae in the spawning area and their subsequent distribution to settlement and nursery grounds. To test this hypothesis at appropriate spatial and temporal scales, we analyzed acoustic records to describe vertical and horizontal distribution patterns of hake larvae, discrete plankton samples to confirm the identity of acoustic targets, and outputs from a numerical circulation model to estimate current patterns in the region during the hake spawning months. Coinciding with the development of a functional swimbladder, hake larvae of 4 mm and larger showed a strong vertical distribution pattern associated with a sound scattering layer migrating from the thermocline during the night to near-bottom depths during the day. This diel vertical migration pattern was associated with the circulation structure, equivalent to a two-layer flow, indicating a recirculation pattern in the vertical plane. The retention of early larval stages in the spawning area was evident from the acoustic data, indicating a persistent location for the bulk of hake larvae at the main spawning ground during summer months. The distribution extends to the rest of the nursery grounds as the breeding season advances in agreement with the general pattern of middle shelf bottom circulation vectors produced by the numerical model. The results are discussed within the framework of Bakun’s fundamental triad, identifying its elements. In addition, a conceptual model for the main biophysical coupling processes during the early life history of M. hubbsi is proposed in order to gain insight about the recruitment mechanism of this species.

Characterizing biological scatter before, during, and after a temporary ice retreat in the Bering Sea.

A three‐frequency echosounder system was integrated into a NOAA oceanographic mooring in the central Bering Sea from 2008–2009. Diel vertical migration patterns were examined before, during, and after the formation and retreat of seasonal ice. Scattering aggregations identified in the year long time series were classified based on the differences in scattering amplitude between the three frequencies. Theoretical scattering curves for four different types of individual scatterers were generated and decibel‐differences at the three acoustic frequencies used in this study were calculated for copepods with lengths of 1 and 8 mm, euphausiids with lengths of 15 and 30 mm, and organisms with a gas‐inclusion such as a swim‐bladdered fish or siphonophore. Aggregations were classified into one of the four categories by determining the shortest geometric distance between the 3 dB differences calculated for the aggregation and that of the theoretical scatterers. If the closest geometric distance exceeded 16 dB, the a...

Vertical migration patterns of phytoflagellates in relation to light and nutrient availability in a shallow microtidal estuary

Physical and chemical vertical gradients in estuaries are often steep, with changes on the scale of cm to m strongly affecting a phytoplankter’s exposure to limiting nutrients and light. Two diel field studies were conducted during June and July 2001 to establish how the composition and productivity of the phytoplankton are influenced by the vertical water column structure within the shallow, eutrophic, microtidal Neuse River Estuary, North Carolina, USA. During both studies, an upper photic mixed layer with low dissolved inorganic nitrogen (DIN) (~ 1 μM) lay above a sub-halocline, aphotic region with elevated DIN (~2 to 5 μM). Phytoflagellates were dominant, and observed diel vertical migration (DVM) patterns are likely an important reason for their success in this N-limited system characterized by strong vertical separation of light and DIN resources. Integrated water column primary productivities during June and July were 15 and 113% greater, respectively, than expected for a vertically homogenous phytoplankton community. The high degree of productivity enhancement in July was due to daytime near-surface aggregation in response to low-light, overcast conditions. Displacements of flagellate populations during DVM were positively correlated with cell size, exposing larger cells to higher light and nutrient levels over the diel period. Since larger phytoplankton are generally less efficient at utilizing scarce resources, such vertical niche partitioning according to cell size should enhance community-level productivity. Interspecific variation in vertical migration patterns is an important trait of phytoplankton that promotes complementarity of resource utilization and contributes to the relationship between diversity and productivity within phytoplankton communities.