Habitat, diet and food assimilation of Arctic charr under the winter ice in two subarctic lakes

The Arctic charr Salvelinus alpinus populations of the subarctic lakes Takvatn and Fjellfrøsvatn, north Norway, concentrated in the littoral zones (0–15 m) of the lakes during the entire winter (December to May) despite very low temperatures (0·2 and 0·7° C). High prey availability, low predation and competition and comparatively better light under snow and ice in shallow compared with deep water are probable reasons. At ice break in June, all Arctic charr moved to the profundal zone for a brief period, probably in response to the sudden light increase and a profundal resource peak of chironomid pupae. In the summer, the Arctic charr are found in the pelagic, profundal and littoral zones of the lakes. These populations therefore perform regular habitat shifts between the littoral zone in the winter, the profundal zone at ice break and the whole lake in the summer and autumn. The fish fed continuously during winter despite the cold water and the poor light. Amphipods and chironomid larvae dominated the diet. Catch per unit effort, numbers of stomachs with food and food intake rates varied with the subarctic light cycle but were lowest after the winter solstice. The winter assimilation of energy was about equal to the standard metabolism in Takvatn but was higher in Fjellfrøsvatn. The assimilation increased in both lakes under the spring ice in May. The habitat choice, diet and energy assimilation indicate that the Arctic charr is well adapted to the extreme winter conditions of subarctic lakes.     

Reproductive biology of the threatened golden galaxias Galaxias auratus Johnston and the influence of lake hydrology

Golden galaxias Galaxias auratus (31–235 mm fork length, L _F) were collected monthly from littoral habitats in Lakes Crescent and Sorell, Tasmania, Australia, between July 2000 and December 2002. Spawning habitats were identified and monitored in both lakes, and surveyed in Lake Crescent. Trends in gonado-somatic indices and reproductive stages of development indicated that gonad development in both sexes begins in midsummer and peaks in late autumn to early winter. Males mature at smaller sizes (50% at 52 mm L _F) than females (50% at 76 mm L _F), larger individuals are predominately females (95% of fish ≥138 mm L _F), and overall male to female ratios are female biased ( c . 1:2). Spawning occurs late autumn to early spring (water temperatures = 1·4–9·7° C) with peaks in spawning activity in winter (mean water temperatures <5° C). Demersal adhesive eggs ( c. 1·5 mm diameter) were found on cobble substrata ( c. 20–250 mm diameter) in littoral areas ( c. 0·2–0·6 m deep) and fecundity of fish 71–181 mm L _F ranged from 619 to 14 478 eggs. The rate of change in water level over the 20 days prior to monthly sampling was important in explaining the occurrence of spent fish and this accounted for temporal differences in spawning between the populations. Lake hydrology influences the reproductive cycle of G. auratus by possibly providing a stimulus for spawning and it controls the availability of spawning habitat in Lake Crescent. Seasonal hydrological cycles ( i.e. rises during late autumn to winter) and a minimum water level of 802·20 m Australian Height Datum in Lake Crescent during autumn (above which littoral areas of cobble substratum are inundated) are critical to G. auratus populations.     

Seasonal variation in the lateral distribution of mayfly nymphs in a boreal river

Seasonal variations in the lateral distribution of mayfly nymphs were investigated in the North Swedish river Vindelälven; a river with large seasonal differences in water level, current velocity and ice conditions, A total of 22 mayfly species about equally divided between summer and winter species were found along a single transect. At low water level in late summer and autumn most species were distributed over the whole transect. In winter the populations occupying the littoral moved into deeper water, so avoiding the ice. Only very small mortalities were detected in the ice. Prior to the spring flood most characteristically still water species moved to the shallow uppermost littoral while lotic species and the burrowing Ephemera vulgata L. did not show the same aggregation near the shore. It is suggested that these lateral movements allow the nymphs to avoid adverse environmental conditions and to exploit food resources over the whole transect.     

Shallow males, deep females: sex-biased differences in habitat distribution of the freshwater calanoid copepod Arctodiaptomus alpinus

In a small, circular high-altitude karst lake situated in the northeastern calcareous Alps of Austria the copepod Arctodiaptomus alpinus (Imhof) develops each spring from resting eggs after the lake basin fills in early spring. After mid-summer, late developmental stages and adults concentrate in a thin layer near the sediment throughout the lake basin. During daytime an average 86% and 71% of female and male copepods respectively resided near the bottom. Males exhibited stronger diel vertical migrations than females and part of the population concentrated in near-surface water at night, but the majority of copepods remained at the sediment during darkness. Macro-photographs and core samples revealed that the proportion of epibenthic male A. alpinus decreased constantly from >70% in 3 m depth to <20% below the 8-m isobath. The proportion of ovigerous females increased with depth, whereas non-ovigerous females were comparatively more abundant in shallow water. Concurrently the highest frequencies of copulating copepods were detected in areas of high male density. Above the 3-m isobath male A. alpinus formed dense swarms in scattered patches of the macrophyte Ranunculus eradicatus (>93% males; maximum abundance: 362 600 individuals m⁻²). Adults and larvae of the alpine newt Triturus alpestris are the top predators in this fishless alpine lake, and A. alpinus constitutes a major component of their diet. However, predator densities and different predation rates on male and female copepods were not sufficient to explain the observed horizontal distribution of A. alpinus . We argue that the optimization of successful sexual encounters in copepods is the ultimate driving force behind this segregation of both sexes of A. alpinus ('mate encounter hypothesis').     

Surviving winter hypoxia: behavioral adaptations of fishes in a northern Wisconsin winterkill lake

Synopsis Winterkill lakes often have a characteristic fish community, presumably composed of species able to survive winter hypoxia. Our research on a small winterkill lake in northern Wisconsin indicates that fishes common in winterkill lakes have behavioral adaptations for tolerating or avoiding winter hypoxia. We examined the distribution of fishes within the lake during one winter (December through May), and fish migrations into and out of the lake for two consecutive years. As DO within the lake declined in late fall, adult-sized fishes of four species, brook stickleback, finescale dace, redbelly dace, and fathead minnow, moved to the ice-water interface where DO levels were highest. Stickleback, and to a lesser extent, fathead minnows, also moved toward the more highly oxygenated water near the inlet. During the first year, young-of-the-year fishes of blacknose shiner, Iowa darter, redbelly dace, and fathead minnow, avoided hypoxic conditions by emigrating from the lake via the outlet stream in late fall and early winter while DO within the lake was still relatively high. Blacknose shiner, redbelly dace, and fathead minnow returned to the lake in spring. Almost no fishes were trapped leaving the lake in the second fall-winter season. Central mudminnows neither moved to the ice-water interface nor emigrated from the lake as DO dropped. Mudminnows survive winter hypoxia by breathing oxygen-containing bubbles trapped beneath the ice. These relatively simple behavioral adaptations allow fishes to survive or avoid hypoxic conditions lethal to other species and may help explain the consistency in fish communities of winterkill lakes.     

Impact of fish predation on cladoceran body weight distribution and zooplankton grazing in lakes during winter

1. It is well accepted that fish, if abundant, can have a major impact on the zooplankton community structure during summer, which, particularly in eutrophic lakes, may cascade to phytoplankton and ultimately influence water clarity. Fish predation affects mean size of cladocerans and the zooplankton grazing pressure on phytoplankton. Little is, however, known about the role of fish during winter. 2. We analysed data from 34 lakes studied for 8–9 years divided into three seasons: summer, autumn/spring and winter, and four lake classes: all lakes, shallow lakes without submerged plants, shallow lakes with submerged plants and deep lakes. We recorded how body weight of Daphnia and then cladocerans varied among the three seasons. For all lake types there was a significant positive correlation in the mean body weight of Daphnia and all cladocerans between the different seasons, and only in lakes with macrophytes did the slope differ significantly from one (winter versus summer for Daphnia). 3. These results suggest that the fish predation pressure during autumn/spring and winter is as high as during summer, and maybe even higher during winter in macrophyte‐rich lakes. It could be argued that the winter zooplankton community structure resembles that of the summer community because of low specimen turnover during winter mediated by low fecundity, which, in turn, reflects food shortage, low temperatures and low winter hatching from resting eggs. However, we found frequent major changes in mean body weight of Daphnia and cladocerans in three fish‐biomanipulated lakes during the winter season. 4. The seasonal pattern of zooplankton : phytoplankton biomass ratio showed no correlation between summer and winter for shallow lakes with abundant vegetation or for deep lakes. For the shallow lakes, the ratio was substantially higher during summer than in winter and autumn/spring, suggesting a higher zooplankton grazing potential during summer, while the ratio was often higher in winter in deep lakes. Direct and indirect effects of macrophytes, and internal P loading and mixing, all varying over the season, might weaken the fish signal on this ratio. 5. Overall, our data indicate that release of fish predation may have strong cascading effects on zooplankton grazing on phytoplankton and water clarity in temperate, coastal situated eutrophic lakes, not only during summer but also during winter.     

The reproductive ecology and abundance of Symptevygia bonapavtii endemic to the south-west Atlantic

Logistical ogives gave an estimated L _T50 value (i.e. the total length at which 50% individuals are sexually mature) of 635 mm for female and 650 mm for male Sympterygia bonapartii . Mature individuals of both sexes had significantly larger livers than immature ones and females had a significantly heavier liver than males. Clasper elongation was the first step in male maturation, followed by clasper calcification and the development of alar thorns. In mature females, the right ovary was larger than the left, which was apparently due to differences in stroma tissue. Gonadosomatic index and diameter of ovarian follicles of mature females peaked in late spring and was at a minimum from late summer and through the winter. Juvenile S. bonapartii were more abundant near estuarine areas during winter, and adults appeared in estuaries by late spring and summer. Females carrying egg-cases were found near the shore in late spring and egg-cases were found in benthic samples only in shallow waters suggesting that S. bonapartii deposits egg-cases in shallow waters during late spring-summer and that nursery areas are in outer estuarine zones. In the southern part (38°–42° S) of the study area S. bonapartii showed a strong movement to shallow waters in late spring and summer, spreading over the entire coastal area in winter. These movements are discussed in relation to water temperature and trophic interactions.     

Predation pressure induced by seasonal fishing moratorium changes the dynamics of subtropical Cladocera populations

Seasonal fishing moratorium with selective fish harvesting is a common means of increasing fish production in Asia. Such manipulation might be expected to lead to a seasonal variation in abundance and composition of fish and predation pressure on zooplankton, and consequently a seasonal change in the Cladocera populations. In this study, we examined the seasonal dynamics of cladocerans in a subtropical plateau lake in southwestern China, Lake Erh, in which a fishing moratorium was implemented in spring and early summer. Cladocerans showed a bimodal seasonal distribution linked to variations in predation pressure induced by the seasonal fishing moratorium. When predation was low in winter because of cool water temperatures and decreasing fish density, Daphnia galeata appeared in the water column and increased towards spring. The cladoceran peaked in the middle of the fishing moratorium, coinciding with a clear water phase. The feeding rate increased with temperature in the late moratorium, allowing planktivorous fish to strongly suppress large daphnids. Small cladocerans benefited from the reduced competition with large daphnids by increasing their numbers rapidly, which resulted in a second peak of cladocerans in the fishing season.     

Physical limnological processes under ice

Penetration of solar radiation through ice and snow covering northern lakes produces a gravity current between regions with varying depths. This baroclinic current is a dominant physical process in winter because ice cover insulates lakes from the usual turbulence sources such as breaking surface waves and near-surface shear produced by the wind. The current forms a directed circulation from the littoral zone to the centre of the lake that is an important distribution mechanism for nutrients and other chemical and biological constituents. Heat transported by the current degrades the ice cover and makes surface travel hazardous. The thinning of the ice cover is most severe at the inlet to isolated bays with mean depths that differ significantly from the lake. At the mouth of a bay, the gravity current takes the form of a two-layer flow with inflow to the bay occurring near the surface. The lower layer has the largest temperature gradients and is dominated by a succession of progressive internal bores which decrease in amplitude overnight and with increasing cloud cover. The repetition of the bores occurs very close to the period of the uninodal barotropic seiche which suggests that the internal bores are forced by the surface seiche.     

Interplay between temperature,fish partial migration and trophic dynamics

Whereas many studies have addressed the mechanisms driving partial migration, few have focused on the consequences of partial migration on trophic dynamics, and integrated studies combining the two approaches are virtually nonexistent. Here we show that temperature affects seasonal partial migration of cyprinid fish from lakes to predation refuges in streams during winter and that this migration in combination with temperature affects the characteristics and phenology of lower trophic levels in the lake ecosystem. Specifically, our six‐year study showed that the proportion of fish migrating was positively related to lake temperature during the pre‐migration growth period, i.e. during summer. Migration from the lake occurred later when autumn water temperatures were high, and timing of return migration to the lake occurred earlier at higher spring water temperatures. Moreover, the winter mean size of zooplankton in the lake increased with the proportion of fish being away from the lake, likely as a consequence of decreased predation pressure. Peak biomass of phytoplankton in spring occurred earlier at higher spring water temperatures and with less fish being away from the lake. Accordingly, peak zooplankton biomass occurred earlier at higher spring water temperature, but relatively later if less fish were away from the lake. Hence, the time between phyto‐ and zooplankton peaks depended only on the amount of fish being away from the lake, and not on temperature. The intensity of fish migration thereby had a major effect on plankton spring dynamics. These results significantly contribute to our understanding of the interplay between partial migration and trophic dynamics, and suggest that ongoing climate change may significantly affect such dynamics.     

Seasonal habitat selection by lake trout (<Emphasis Type="Italic">Salvelinus namaycush</Emphasis>) in a small Canadian shield lake: constraints imposed by winter conditions

The need for cold, well-oxygenated waters significantly reduces the habitat available for lake trout (Salvelinus namaycush) during stratification of small temperate lakes. We examined the spatial and pelagic distribution of lake trout over two consecutive summers and winters and tested whether winter increased habitat availability and access to littoral regions in a boreal shield lake in which pelagic prey fish are absent. In winter, lake trout had a narrowly defined pelagic distribution that was skewed to the upper 3 m of the water column and spatially situated in the central region of the lake. Individual core areas of use (50% Kernel utilization distributions) in winter were much reduced (75%) and spatially non-overlapping compared to summer areas, but activity levels were similar between seasons. Winter habitat selection is in contrast to observations from the stratified season, when lake trout were consistently located in much deeper waters (>6 m) and widely distributed throughout the lake. Winter distribution of lake trout appeared to be strongly influenced by ambient light levels; snow depth and day length accounted for up to 69% of the variation in daily median fish depth. More restricted habitat use during winter than summer was in contrast to our original prediction and illustrates that a different suite of factors influence lake trout distribution between these seasons.     

Egg size and emergence timing affect morphology and behavior in juvenile Chinook Salmon,Oncorhynchus tshawytscha

Variation in early life history traits often leads to differentially expressed morphological and behavioral phenotypes. We investigated whether variation in egg size and emergence timing influence subsequent morphology associated with migration timing in juvenile spring Chinook Salmon, Oncorhynchus tshawytscha. Based on evidence for a positive relationship between growth rate and migration timing, we predicted that fish from small eggs and fish that emerged earlier would have similar morphology to fall migrants, while fish from large eggs and individuals that emerged later would be more similar to older spring yearling migrants. We sorted eyed embryos within females into two size categories: small and large. We collected early and late‐emerging juveniles from each egg size category. We used landmark‐based geometric morphometrics and found that egg size appears to drive morphological differences. Egg size shows evidence for an absolute rather than relative effect on body morphology. Fish from small eggs were morphologically more similar to fall migrants, while fish from large eggs were morphologically more similar to older spring yearling migrants. Previous research has shown that the body morphology of fish that prefer the surface or bottom location in a tank soon after emergence also correlates with the morphological variations between wild fall and spring migrants, respectively. We found that late‐emerging fish spent more time near the surface. Our study shows that subtle differences in early life history characteristics may correlate with a diversity of future phenotypes.     

Trading safety for food: evidence from gut contents in roach and bleak captured at different distances offshore from their daytime littoral refuge

1. Regular diel habitat shifts in roach were detected by hydro‐acoustics in five moderately eutrophic, stratifying (maximum depth 24–27 m) and approximately circular lakes (of surface area 15, 75, 125, 300 and 900 ha and diameters 250, 600, 1000, 1700 and 2600 m) in north‐eastern Poland in the years 1998–2000, when the lakes were free of smelt and other typical offshore planktivores, and their offshore areas were completely free of fish during the day. 2. The diel change in roach distribution was shown to assume a similar pattern in each lake: fish migrated from a daytime littoral refuge towards the centre of the lake at dusk, and returned to the littoral refuge at dawn. After sunset, fish gradually dispersed offshore until they covered the entire lake area in each of the three smaller lakes. In each of the two larger lakes, only small numbers of fish were seen in the central area at night, implying that the centre of the lake retained high food availability throughout the summer. 3. Inshore–offshore gradients in zooplankton prey density, body size, and numbers of eggs per clutch were weak or undetectable in the two smallest lakes, but strong and persistent in the three larger lakes, with Daphnia densities 5–30 times as high and body length 1.2–1.5 times as great in the central area as inshore. 4. The likely increase in the potential predation risk with distance from the littoral daytime refuge was found to be compensated by increased food gains in those fish which moved offshore at dusk to feed within a short time window, when light intensity was lower to make the risk reduced, but still high enough to see zooplankton prey. The benefit because of increased prey acquisition was greatest in the centre of the largest lake (at 1300 m from the shore), as revealed from gut inspections of roach and bleak trawl‐sampled at different distances from the edge of the reed belt, and seen as a gradual, order‐of‐magnitude increase in the volume of food in the foregut, The food volume against distance‐from‐shore regression was highly significant on each of the four sampling dates in the largest lake, in spite of the wide variability of food volume in individual fish.     

Night,day, sunrise,sunset: do fish under snow and ice recognize the difference?

1. Although boreal lakes are ice‐covered for several months annually, little is known about the behaviour of fish under ice. To consider the reasons for diel vertical migrations (DVM) it is important to compare periods under ice as opposed to under open water. Echosounding provides a tool for non‐intrusive continuous monitoring of fish, even in winter. 2. Changes in the vertical distribution of fish through six 48‐h periods were monitored using a stationary, mounted echosounder that beamed vertically either from the bottom up or from the surface down from February to April, 2003. The up‐beaming and down‐beaming transducers were run alternately for 24 h each over the 48‐h period. Standard echo analysis software was used to detect fish traces and estimate the vertical and temporal distribution of fish abundance. Fish were sampled with a winter seine. 3. Prominent diel vertical migration in response to changing light level was detected throughout the study period (late winter to spring). Fish were highest in the water column at sunset and sunrise. In daylight, most detected fish were well below 10‐m depth. The number of fish detected was greatest during the night when they occurred throughout almost the whole water column, sometimes with a considerable number very close to the ice. Fish were mostly vendace and whitefish. 4. It became evident from data from the up‐beaming transducer that at night fish may occupy the layer closest to the surface. These fish would not have been detected if we had only used the down‐beaming transducer. The overall pattern of DVM in winter was very similar to that in summer. The results support the suggestion that DVM is a genetically fixed behavioural trait responding to the contemporary level of illumination.     

Metazooplankton dynamics in the Salton Sea, California, 1997–1999

The dynamics of metazooplankton populations were studied over 3 years at the saline (43 g l^–1) Salton Sea, California's largest lake. Total abundance was highest in summer following late winter/early spring phytoplankton blooms. At this time, metazooplankton consisted mostly of the copepod, Apocyclops dengizicus, and the rotifer, Brachionus rotundiformis. In August or September, severe crashes in the metazooplankton populations occurred each year in mid-lake due to strong wind events which increased mixing and caused low oxygen and high sulfide concentrations throughout the water column. Larvae of the polychaete worm, Neanthes succinea and the barnacle, Balanus amphitrite were present mostly in late winter and spring. Their scarcity in summer is due in part to persistent anoxic bottom conditions that decrease adult populations and in part to predation by tilapia, an omnivorous fish that has become abundant in the lake since the 1960s. Two Synchaeta species, rotifers not previously reported from the Sea, were abundant in winter and spring and predation on these may have permitted the copepod to persist at low levels through the winter. There were two major changes in metazooplankton dynamics since 1954–1956 in addition to the appearance of the two synchaetid rotifers in the fauna. First, there are now much lower densities of barnacle and polychaete larvae in the fall, probably due to the invasion of the zooplanktivorous fish, tilapia. Second the precipitous crashes now seen in metazooplankton densities, especially the copepod, in late summer-early fall did not occur in the 1950s possibly because fall overturn events did not result in such high sulfide levels.     

The effect of wintering sites on the survival and reproduction of Gyraulus acronicus (Gastropoda) in a partly frozen river

Summary In a boreal river about 95% of the individuals of Gyraulus acronicus overwinter in the littoral zone which freezes solid each year. These snails were compared with those overwintering in the unfrozen sublittoral area: The littoral snails had a higher survival rate, a higher tissue dry mass/CaCO₃ ratio, and they deposited a higher number of eggs. Littoral snails had a more pronounced tissue degrowth during winter. High winter survival in the frozen littoral zone, a refuge totaly free from predation, indicates that overwintering here is advantageous. However, during frozen periods of short duration (<1 month) the high initial mortality to which the snails were exposed when freezing into the ice was not compensated for by higher survival after the initial phase. Under such conditions when the frozen period is very short the snails would have higher survival in unfrozen parts and are thus expected to avoid the ice.     

Top‐Down Control of Reed Detritus Processing in a Lake Littoral Zone: Experimental Evidence of a Seasonal Compensation between Fish and Invertebrate Predation

We investigated whether predatory fish exert a top‐down control on reed leaf packs processing in a lake littoral zone through a trophic cascade. Exclosure experiments were repeated in summer and winter, under high and low natural fish abundance, respectively. Fish exclusion effects on detritus processing and fungal conditioning were consistent with trophic cascade predictions only in summer. In winter, however, results indicated that a trophic cascade was induced by predatory invertebrates. In both seasons, variations in detritivores abundance generally supported a cascade scenario, whereas several taxon‐specific departures occurred during the experimental periods. We conclude suggesting that predators may continuously regulate leaf detritus processing in lake littoral zones, through a seasonal shift in the relative contribution of fish and invertebrate predation. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Visual conditions and habitat shape the coloration of the Eurasian perch (Perca fluviatilis L.): a trade‐off between camouflage and communication?

In theory, selection for effective camouflage (i.e. dull coloration) in fish should be strongest when the conditions for visual predation are most favourable, such as in structurally simple pelagic habitats. By contrast, in more sheltered (e.g. littoral) habitats, selection may favour effective intra‐specific communication (i.e. bright coloration) (at the expense of crypsis). Poor transparency, as in highly humic waters, should constrain colour adaptations. We investigated phenotypic variation in body coloration of Eurasian perch (Perca fluviatilis L.) in littoral and pelagic habitats of four humic boreal lakes. Perch from the most transparent lake had the lightest and less coloured belly and perch were more colourful in the littoral habitats than in the pelagic areas, with the pattern being clearest in the most transparent lake. In addition, perch in the most transparent lake exhibited sexual dichromatism, with males having a more colourful belly than the females, whereas no indications of sexual dichromatism were found in more humic lakes. Moreover, in the most transparent lake, the condition of fish correlated with bright belly coloration in the littoral, but with dull belly coloration in the pelagic habitat. The results obtained in the present study suggest that selection on perch coloration may differ between lakes as a result of visual properties of the water, and within lakes as a result of divergent selection for camouflage and communication in pelagic and littoral habitats. © 2010 The Linnean Society of London, Biological Journal of the Linnean Society, 2010, 99 , 47–59.     

Effect of water level fluctuation on shore spawning of Mirogrex terraesanctae (Steinitz), (Cyprinidae) in Lake Kinneret, Israel

Mirogrex terraesanctae (Steinitz), is an endemic, open water, zooplanktivorous fish. It spawns in the shallow littoral (0–50 cm) of Lake Kinneret, from November to May with a peak in mid winter (January-February). Spawning begins shortly after nightfall, when schools of the fish move along the shoreline in rocky regions, and release their milt and eggs. The adhesive eggs are attached to the surface of recently inundated, algae-free stones. A negative relationship exists between epilithic growth and egg density and survival. Algal growth may play a key role in the determination of the spawning depth. Breeding success of M. terraesanctae depends on the rate and extent of rise in lake level which, in turn, determines the availability of a suitable spawning substratum.     

Movement of plankton through lake-stream systems

1. River plankton are often assumed to come from upstream lakes, but the factors controlling the movement of plankton between lakes and rivers into outflow streams are unclear. We tested the possibility that the physical structure of the littoral zone near the lake outlet (depth, presence of macrophytes) and diurnal differences in plankton composition at the lake surface influence the movement of plankton from the lake into the stream and determine their persistence downstream. 2. Zooplankton and phytoplankton biomass, community composition and mean body size were compared between two deep lakes without macrophytes at the lake edge and two shallow lakes with macrophytes at the lake edge. Samples were collected day and night on three dates, in the lake centre, in the littoral zone adjacent to the lake outlet, at the outlet and at two sites downstream in Algonquin Park, Ontario, Canada. 3. The morphology of lake edges clearly affects the movement of lake zooplankton into outlet streams. Outlets draining deeper littoral zones had higher zooplankton biomass than shallow littoral outlets (P < 0.0001), but these differences disappeared within 50 m downstream of the lake. There was no difference in mean zooplankton body size among lake outlets or between littoral and outlet samples. However, shallow littoral zones were dominated by cyclopoid copepods and deeper littoral zones were dominated by Bosmina longirostris. In contrast, phytoplankton biomass entering the outlet was similar to that found within the lake and did not vary with lake outlet morphology. These effects were consistent across several sampling weeks and were not affected by surface zooplankton biomass changes associated with diurnal vertical migration in the lake centre. 4. A comparison with published river zooplankton data suggests that zooplankton are rapidly eliminated from shallow outlet streams (≤1 m deep) but persist in most deeper outlet rivers (≥2 m deep). Because the depth of an outlet river determines downstream zooplankton community development, the contribution of lakes to river plankton communities may be influenced by the location of each lake within the drainage basin. These findings suggest that lake and outflow physical structure influences connection strength between spatially successive habitats.