Clonal integration facilitates invasiveness of the alien aquatic plant Myriophyllum aquaticum L. under heterogeneous water availability

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

Zooplankton behavioral responses to solar UV radiation vary within and among lakes

Zooplankton taxa exhibit varying tolerances to ultraviolet radiation(UVR), with rotifers and copepods tending to be more UV tolerantthan cladocerans, regardless of lake UV transparency. On thebasis of these differences, we hypothesized that UV avoidancebehavior would be greatest in zooplankton with low UV tolerance,particularly in high-UV systems. Both downward and upward movementsof zooplankton were examined in cylindrical acrylic columnsthat either transmitted full sunlight or blocked UV-B and shortwavelength UV-A radiation. Columns were suspended verticallyin the surface waters of a high- and low-UV lake at varyingtimes of day and cloud cover. In the high-UV lake, the cladoceranDaphnia catawba displayed a distinct avoidance of the surfacewaters in the UV+ treatment but often showed a preference forthe surface in the UV– treatment. Copepods were less responsiveto UVR with calanoids displaying a somewhat greater UV avoidancethan cyclopoids. In the low-UV lake, no behavioral differenceswere observed among UV treatments. These results suggest thatUVR may influence the vertical distribution and habitat partitioningof certain zooplankton taxa in high-UV lakes while other factors,such as predation and food limitation, may be more importantin low-UV lakes.     

Interactive effects of productivity and predation on zooplankton diversity

Recent studies suggest the necessity of understanding the interactive effects of predation and productivity on species coexistence and prey diversity. Models predict that coexistence of prey species with different competitive abilities can be achieved if inferior resource competitors are less susceptible to predation and if productivity and/or predation pressure are at intermediate levels. Hence, predator effects on prey diversity are predicted to be highly context dependent: enhancing diversity from low to intermediate levels of productivity or predation and reducing diversity of prey at high levels of productivity or predation. While several studies have examined the interactive effects of herbivory and productivity on primary producer diversity, experimental studies of such effects in predator‐prey systems are rare. We tested these predictions using an aquatic field mesocosm experiment in which initial density of the zooplankton predator Notonecta undulata and productivity were manipulated to test their interactive effects on diversity of seven zooplankton, cladoceran species that were common in surrounding ponds. Two productivity levels were imposed via phosphorus enrichment at levels comparable to low and intermediate levels found within neighboring natural ponds. We used open systems to allow for natural dispersal and behaviorally‐mediated numerical responses by the flight‐capable predator. Effects of predators on zooplankton diversity depended on productivity level. At low and high productivity, prey species richness declined while at high productivity it showed a unimodal relationship with increasing the predator density. Effects of treatments were weaker when using Pielou's evenness index or the inverse Simpson index as measures of prey diversity. Our findings are generally consistent with model predictions in which predators can facilitate prey coexistence and diversity at intermediate levels of productivity and predation intensity. Our work also shows that the functional form of the relationship between prey diversity and predation intensity can be complex and highly dependent on environmental context.     

Mathematical modeling of cascading migration in a tri-trophic food-chain system

Diel vertical migration is a behavioral antipredator defense that is shaped by a trade-off between higher predation risk in surface waters and reduced growth in deeper waters. The strength of migration of zooplankton increases with a rise in the abundance of predators and their exudates (kairomone). Recent studies span multiple trophic levels, which lead to the concept of coupled vertical migration. The migrations that occur at one trophic level can affect the vertical migration of the next lower trophic level, and so on, throughout the food chain. This is called cascading migration. In this paper, we introduce cascading migration in a well-known model (Hastings and Powell, Ecology 73:896–903, 1991). We represent the dynamics of the system as proposed by Hastings and Powell as a phytoplankton–zooplankton–fish (prey–middle predator–top predator) model where fish affect the migrations of zooplankton, which in turn affect the migrations of motile phytoplankton. The system under cascading migration enhances system stability and population coexistence. It is also observed that for a higher rate of cascading migration, the system shows chaotic behavior. We conclude that the observations of Hastings and Powell remain true if the cascading migration rate is high enough.     

Relative significance of direct and indirect effects of predation by planktivorous fish on zooplankton

One of the most obvious features of tropical lakes and reservoirs is the small body size of their zooplankton taxa. It is believed that this is the result of high and persistent predation by abundant planktivorous fish, which select large-bodied zooplankton prey thus making them more vulnerable to extinction in tropical as compared to temperate habitats. Do these extinctions result directly from fish predation? Could the high predation-induced mortality alone be responsible for an extermination of the population from a habitat? Or could indirect effects of predation be responsible? Some important indirect effects can be seen at the demographic level; these include reduced reproduction in the population resulting from higher vulnerability of ovigerous females to predation by visually oriented planktivores. Other important indirect effects can be observed at the individual level; these include shifts in behavior (from foraging to predator avoidance) and adjustments in physiology (from high to low feeding rate) in those planktonic animals which detect danger from their predators by sensing either the ‘predator odor’ or an ‘alarm substance’ originating from injured conspecific prey. Although a zooplankton species density may mostly result from the brutal force of direct predator impact on the population (mortality), it is more likely that its distribution in time and space could be attributed to a combination of indirect effects of predation on individual behavior and physiology. An example of periodicity in density and depth distribution patterns of Cahora Bassa zooplankton species and their periodic exterminations seems to confirm the role of indirect effects of predation by planktivorous fish.     

Habitat selection and diel distribution of the crustacean zooplankton from a shallow Mediterranean lake during the turbid and clear water phases

1. The fish fauna of many shallow Mediterranean Lakes is dominated by small‐bodied exotic omnivores, with potential implications for fish–zooplankton interactions still largely unknown. Here we studied diel variation in the vertical and horizontal distribution of the crustacean plankton in Lake Vela, a shallow polymictic and eutrophic lake. Diel sampling was carried out on three consecutive days along a horizontal transect, including an open‐water station and a macrophyte (Nymphaea alba) bed. Since transparency is a key determinant of the predation risk posed by fish, the zooplankton sampling campaigns were conducted in both the turbid (autumn) and clear water (spring) phases. 2. In the turbid phase, most taxa were homogeneously distributed along the vertical and horizontal axes in the three consecutive days. The only exception was for copepod nauplii, which showed vertical heterogeneity, possibly as a response to invertebrate predators. 3. In the clear water phase, most zooplankton taxa displayed habitat selection. Vertically, the general response consisted of a daily vertical migration (DVM), despite the limited depth (1.6 m). Horizontally, zooplankters showed an overall preference for the pelagic zone, independent of the time of the day. Such evidence is contrary to the postulated role of macrophytes as an anti‐predator refuge for the zooplankton. 4. These vertical (DVM) and horizontal (macrophyte‐avoidance) patterns were particularly conspicuous for large Daphnia, suggesting that predation risk from size‐selective predators (fish) was the main factor behind the spatial heterogeneity of zooplankton in the spring. Thus, the difference in the zooplankton spatial distribution pattern and habitat selection among seasons (turbid and clear water phases) seems to be mediated the predation risk from fish, which is directly related to water transparency. 5. The zooplankton in Lake Vela have anti‐predator behaviour that minimises predation from fish. We hypothesise that, due to the distinct fish community of shallow Mediterranean lakes, aquatic macrophytes may not provide adequate refuge to zooplankters, as seen in northern temperate lakes.     

Modelling the influence of copepod behaviour on faecal pellet export at high latitudes

The contribution of faecal pellet (FP) production by zooplankton to the downward flux of particulate organic carbon (POC) can vary from <1 % to more than 90 % of total POC. This results from varying degrees of interception and consumption, and hence recycling, of FPs by zooplankton in the upper mixed layers, and the active transport of FP to depth via diel vertical migration (VM) of zooplankton. During mid-summer at high latitudes, synchronised diel VM ceases, but individual zooplankton may continue to make forays into and out of the surface layers. This study considers the relative importance of different VM behaviours on FP export at high latitudes. We focussed on copepods and parameterised an individual-based model using empirical measures of phytoplankton vertical distribution and the rate of FP production, as a function of food availability. FP production was estimated under three different behaviours common to high-latitude environments (1) no VM, (2) foray-type behaviour and (3) synchronised diel VM. Simulations were also made of how each of these behaviours would be observed by an acoustic Doppler current profiler (ADCP). The model found that the type of copepod behaviour made a substantial difference to the level of FP export to depth. In the absence of VM, all FPs were produced above 50 m, where the probability of eventual export to depth was low. In foray-type scenarios, FP production occurred between 0 and 80 m, although the majority occurred between 30 and 70 m depth. Greatest FP production in the deeper layers (>70 m) occurred when diel VM took place. Simulated ADCP vertical velocity fields from the foray-type scenario resembled field observations, particularly with regard to the occurrence of positive anomalies in deeper waters and negative anomalies in shallower waters. The model illustrates that active vertical flux of zooplankton FP can occur at high latitudes even when no synchronised VM is taking place.     

Distribution and feeding ecology of the Greenland shark (Somniosus microcephalus) in Greenland waters

Greenland sharks are widely distributed and most likely a highly abundant predator in arctic waters. Greenland sharks have previously been considered scavengers, but recent studies suggest that Greenland sharks also predate on live prey. In this study, distribution and feeding ecology in Greenland waters were investigated. Based on data from 25 years of surveys, Greenland sharks were usually caught at 400–700 m but were found at all depths between 100 and 1,200 m. Based on examination of stomachs from 30 Greenland sharks (total length of 258–460 cm), the most important prey items were Atlantic cod (65.6 % IRI), harp seal (9.9 % IRI), skates (5.2 % IRI) and wolffish (4.4 % IRI), but large geographical variations were observed. Prey composition and qualitative observations support the hypothesis of active predation. Consistent with other studies, the results of this work support the notion that the Greenland shark is an apex predator with the potential to influence trophic dynamics in the Arctic.     

Feeding repellence of Antarctic and sub-Antarctic benthic invertebrates against the omnivorous sea star Odontaster validus

Antarctic and sub-Antarctic benthic invertebrates are subjected to intense predation by mobile macroinvertebrates. Accordingly, chemical protection as well as other defensive mechanisms are expected to be common in organisms inhabiting these ecosystems. In order to evaluate anti-predation activities and allocation of chemical defenses within the anatomy of marine benthic Antarctic and sub-Antarctic invertebrates, 55 species were tested for feeding repellence against the sea star Odontaster validus, a common eurybathic sympatric predator. The invertebrates tested were collected from the deep waters of two poorly surveyed areas in terms of chemical ecology studies: the eastern Weddell Sea (Antarctica) and the vicinities of Bouvet Island (sub-Antarctica). Experiments were conducted at the Spanish Antarctic Base in Deception Island. In the feeding deterrence experiments, shrimp pieces were treated with crude lipophilic fractions obtained from each species, and were offered to the sea stars. A total of 29 species (53 %) from 7 different phyla (Porifera, Cnidaria, Chordata, Bryozoa, Echinodermata, Mollusca, and Annelida) showed feeding repellence against O. validus, and are therefore chemically protected against this keystone predator. Furthermore, 25 species were dissected into parts to investigate the possible allocation of defensive compounds. Some of the results obtained from these analyses support the prediction that the most exposed/vulnerable tissues concentrate chemical defenses to avoid predation against the sea stars. In summary, the results obtained in our survey support the hypothesis that deep-water Antarctic and sub-Antarctic benthic invertebrates are well protected chemically against sympatric predators, similarly to what has been reported in previous studies investigating shallow-water Antarctic species.     

Natural variations in estuarine fish,fish odor,and zooplankton photobehavior

ABSTRACT

Fish odor induces predator avoidance behaviors in zooplankton, like vertical migration, by making zooplankton more responsive to light. Odor cues that alter behavior in marine crustacean zooplankton in the laboratory include sulfated glycosaminoglycans (sGAGs) derived from fish body mucus. Few studies quantify these cues in estuarine/marine environments or assess whether laboratory studies reflect natural scenarios. We collected fish and water samples weekly in Broadkill River, Delaware, USA. We used field-collected water in colorimetric assays to determine the concentration of sGAG-equivalent molecules and in behavioral assays with a zooplankton model, brine shrimp (Artemia spp.) nauplii, which only descend in response to downwelling light after fish odor exposure. Fish quantity was positively related to sGAG-equivalents and zooplankton photosensitivity, indicated by descent responses at lower light levels and across a broad intensity range. Our results support that fish odor concentrations used in previous laboratory assays are consistent with levels found in an estuary.     

Factors involved in the distribution pattern of ciliates in the water column of a transparent alpine lake

The recurrent depth preference of three ciliate species (two prostomatids and one haptorid) in a transparent alpine lake indicates the existence of niche partitioning among them involving potential factors such as avoidance of high ultraviolet radiation levels and zooplankton predation, as well as competition for food resources.     

Zooplankton response to climate warming: a mesocosm experiment at contrasting temperatures and nutrient levels

Zooplankton community response to the combined effects of nutrients and fish (hereafter N + F) at contrasting temperatures was studied in a long-term experiment conducted in 24 shallow lake mesocosms with low and high nutrient levels. We found a positive effect of N + F on zooplankton biomass, chlorophyll-a and turbidity. In contrast, zooplankton species and size diversity decreased with added N + F, as did submerged macrophyte plant volume inhabited (PVI). The community composition of zooplankton in high N + F mesocosms was related to chlorophyll-a and turbidity and to macrophyte PVI in the low N + F mesocosms. Macrophytes can protect zooplankton from fish predation. Compared to N + F effects, temperature appeared to have little effect on the zooplankton community. Yet analysis of community heterogeneity among treatments indicated a significant temperature effect at high N + F levels. The results indicate an indirect temperature effect at high N + F levels that can be attributed to temperature-dependent variation in fish density and/or chlorophyll-a concentration.     

Diel vertical migration in zooplankton: rapid individual response to predators

While diel vertical migration in zooplankton has been shownrecently to be a predator avoidance behavior, the mechanismby which predators induce and maintain such behavior has beendebated. We report results of an in situ predator manipulationexperiment during which enclosed populations of the marine planktomccopepod Acaraa hudsonica rapidly changed their vertical distributionand diel migration behavior depending on presence or absenceof the planktivorous fish Casterosteus aculeatus These resultspoint unambiguously to phenotypic behavioral plasticity of individualplanktonic prey, not, as previously hypothesized, population-geneticlevel behavioral changes caused by selective fish predation,as the mechanism underlying changes in diel vertical migrationin this copepod.     

Cyclic temperatures influence growth efficiency and biochemical body composition of vertically migrating fish

1. Diel vertical migrations (DVM) are frequently observed in pelagic fish and zooplankton populations. In addition to predator avoidance and foraging opportunity, bioenergetics optimisation has been hypothesised to cause the selective advantage of migrating fish. However, experimental confirmation of growth advantages of fish held at naturally observed cyclic temperatures, and food densities are surprisingly rare. 2. We compared growth rates, growth efficiencies and energy budgets of vendace (Coregonus albula, Salmoniformes) fed daily rations of 10% body mass and held at low (4.5 °C), high (8 °C) and cyclic (switch between 4.5 and 8 °C) temperatures over a period of 6 weeks. Biochemical body composition was evaluated by bioimpedance analyses and direct determination of water, lipid and protein content in a subset of fish. 3. Growth rate and food conversion efficiency were similar in fish held at high and cyclic temperatures, but higher than those found at low temperatures. Body condition of fish at cyclic temperatures was maximised, but these fish also had the highest water content. The proportion of lipids and proteins was slightly depleted in fish held at cyclic and low temperatures relative to the high temperature treatment. Accordingly, growth and biochemical composition of fish responded specifically to cyclic temperatures and were not simply intermediate between those found at low and high temperatures. 4. We could not confirm a bioenergetics advantage of fish switching between high and low temperatures. However, there were no substantial extra metabolic costs through exposure to cyclic temperatures, and hence, fish performing DVM may benefit from predation avoidance without compromising their metabolic balance and hence growth rates. These results suggest that the evolution of DVM is a multi‐faceted process with no single ultimate explanation.     

Diel vertical migration of the copepod <Emphasis Type="Italic">Thermocyclops inversus</Emphasis> (Kiefer, 1936) in a tropical reservoir: the role of oxygen and the spatial overlap with <Emphasis Type="Italic">Chaoborus</Emphasis>

The effect of water transparency, dissolved oxygen concentration and the invertebrate predator Chaoborus brasiliensis on the day–night vertical distribution of the copepod cyclopoid Thermocyclops inversus was investigated in a shallow tropical reservoir, Nado Reservoir, Belo Horizonte, Brazil. Diel cycles were carried out over a period of 12 consecutive months, between October 1999 and September 2000. The different developmental stages of T. inversus exhibited diel vertical migration (DVM) and displayed a clear ontogenetic trend, with the amplitude of DVM increasing with the age of the organism, and ranging from 0.4 m to 0.8 m for nauplii, 0.4 m to 1.2 m for copepodite, and 1.1 m to 2.1 m for adults. We observed that seasonal changes in dissolved oxygen and C. brasiliensis directly influenced the vertical distribution of the copepod population in this reservoir. Furthermore, it was showed that the diurnal vertical migration is an important predator avoidance behavior since it diminished the spatial overlap between prey and its potential predator. This finding supports the hypothesis that the vertical migration is a defense mechanism against predation. Thus, T. inversus is able to remain in the anoxic layers during day light hours, and at night they move upwards avoiding hypolimnetic waters to escape from predation by Chaoborus.     

Influence of predation pressure on the escape behaviour of Podarcis muralis lizards

Relationships between predator avoidance behaviour and predation pressure were investigated in the wall lizard, Podarcis muralis. The wariness of lizards belonging to high (1185m) and low elevation (308m) populations under two different predation pressure levels was compared. Wall lizards belonging to the lowland population experienced greater predation pressure than those belonging to the highland population. Lizards belonging to the population under higher predation pressure had higher frequency of refuge use, and had longer flight initiation distances (i.e. the distance lizards allowed the observer to approach before fleeing). In contrast, neither the distance fled (i.e. the total distance they fled in one continuous movement from the lizard's initial position until hiding or stopping at a safe distance) nor the distance to the nearest refuge were significantly different between populations. Escape responses were independent of ambient temperature in the lowland population, but animals belonging to the highland population had longer flight initiation distances when the ambient temperatures were higher. These findings suggest that predator avoidance behaviour may vary with predation pressure.     

Mesozooplankton biomass and indices of grazing and metabolic activity in Antarctic waters

Mesozooplankton abundance, body area spectrum, biomass, gut fluorescence and electron transfer system (ETS) activity were studied in the Antarctic Peninsula during the post-bloom scenario in these waters. Values of abundance and biomass were rather low and decreased sharply from the slope waters to the coastal area. In contrast, specific gut fluorescence and ETS activity were high in the coastal area and decreased through the shelf-break. Large copepods were very scarce, similarly to the post-bloom conditions in phytoplankton where large cells are not abundant and small cells such as flagellates dominate the water column. The vertical distribution showed two well defined layers by day, one at the surface which corresponded to krill organisms and a second at depth (>300 m) formed mainly by the large copepod Metridia gerlachei. During the short night, this layer ascended at the time that krill at the surface migrated to deeper waters as observed from acoustics and net sampling. This observation and the absence of large copepods over the shelf suggest that krill consumption on large phytoplankton cells during the bloom is followed by an increase in predation upon mesozooplankton. It also suggests that krill decrease the abundance and biomass of mesozooplankton over the shelf and continues their predation upon mesopelagic copepods during the post-bloom in Antarctic waters. This behaviour could explain the long ago described impoverishment in mesozooplankton south of the Antarctic Circumpolar Current.     

Inhibition of phytoplankton photosynthesis by solar ultraviolet radiation: studies in Lake Titicaca, Bolivia

1. Lake Titicaca is a large, high altitude (3810 m a.s.l.) tropical lake (16°S, 68°W) that lies on the border of Bolivia and Perú, receiving high fluxes of ultraviolet radiation (UVR) throughout the year. Our studies were conducted during September of 1997 with the main objective of studying the impact of solar UVR upon phytoplankton photosynthesis.
2. Water samples were taken daily and incubated in situ (down to 14 m depth) under three radiation treatments to study the relative responses to PAR (Photosynthetic Available Radiation, 400–700 nm), UV-A (320–400 nm), and UV-B (280–320 nm) radiation.
3. Photosynthetic inhibition by UVR in surface waters was about 80%, with UV-A accounting for 60% and UV-B for 20%; the inhibition by high levels of PAR was less than 20%. The inhibition due to UVR decreased with depth so that there were no significant differences between treatments at 8.5 m depth.
4. The amount of inhibition per unit energy received by phytoplankton indicates that even though there was a significant inhibition of photosynthesis due to UVR, species in Lake Titicaca seem to be better adapted than species in high latitude environments.
5. The cellular concentration of UV-absorbing compounds, a possible mechanism of photoadaptation, was low in phytoplanktonic species. However, they were abundant in zooplankton, suggesting a high rate of bioaccumulation through the diet.     

Daphnia magna trade‐off safety from UV radiation for food

Research on diel vertical migration (DVM) is generally conducted at the population level, whereas few studies have focused on how individual animals behaviorally respond to threats when also having access to foraging opportunities. We utilized a 3D tracking platform to record the swimming behavior of Daphnia magna exposed to ultraviolet radiation (UVR) in the presence or absence of a food patch. We analyzed the vertical position of individuals before and during UVR exposure and found that the presence of food reduced the average swimming depth during both sections of the trial. Since UVR is a strong driver of zooplankton behavior, our results highlight that biotic factors, such as food patches, have profound effects on both the amplitude and the frequency of avoidance behavior. In a broader context, the trade‐off between threats and food adds to our understanding of the strength and variance of behavioral responses to threats, including DVM.     

Effects of Ultraviolet Radiation on Diel Vertical Migration of Crustacean Zooplankton: An in situ Mesocosm Experiment

The objective of this study was to expand the spatial scale of previous experiments on the effects of ultraviolet radiation (UVR) on diel vertical migration (DVM) by freshwater zooplankton. We conducted an in situ mesocosm experiment in highly UVR transparent Lake Giles, Pennsylvania, in which we imposed two treatments: ambient UVR and UVR-shielded. Mesocosms (3440 L, 0.74 m diameter, 8 m deep) were large enough to include a spatial refuge from UVR and permit relatively large-scale DVM. Daphnia catawba adopted a significantly deeper distribution during the day in the ambient UVR treatment compared to the UVR-shielded treatment, but effects of UVR were absent at night. In contrast, DVM by Leptodiaptomus minutus was unaffected by the UVR treatment. In both treatments, Leptodiaptomus minutus were most abundant at the bottom of the mesocosms during the day and exhibited a more uniform distribution across depths at night. These results suggest that UVR, along with temperature, algal resources, and predators, may affect zooplankton DVM in aquatic ecosystems.