Presence Of Planktivorous Fish Research Articles

The effects of predation by planktivorous juvenile fish on the microbial food web

The feeding impact of planktivorous fish on microbial organisms is still poorly understood. We followed the seasonal dynamics of the food web in two natural fishponds for two years: one was stocked with planktivorous whitefish while the other had no planktivorous fish. The aim of the study was the simultaneous assessment of the feeding behaviours of planktivorous fish and of bacterivorous meta-/protozooplankters. We hypothesized that in the presence of planktivorous fish there would be fewer metazooplankton, more protozoans and decreased numbers of bacteria. Our results showed that the amount of metazooplankton eaten by the fish was indeed negatively correlated with metazooplankton biomass. The feeding impact of planktivorous fish in shaping the microbial loop was remarkable. The main grazers of bacteria in the fishpond were ciliates, whereas in the pond without fish these were heterotrophic nanoflagellates. In the fishless pond the role of the top predator shifted to the predaceous metazooplankter Leptodora kindtii which controlled the abundance of herbivorous metazooplankters. We found a negative relationship between the number of bacteria and flagellates in the fishless pond, while the number of bacterivorous ciliates was suppressed by predaceous ciliates. Therefore the bacteria-grazing activity was higher in the absence of planktivorous fish.

Zooplankton richness, abundance and biomass of two hypertrophic shallow lakes with different salinity in central Argentina

The zooplankton of lakes is controlled by biological and physico-chemical parameters. Among the former, predation by fish can determine the replacement of large-sized species by small-sized ones and among the latter, salinity exerts negative effects on richness and abundance. Since it has been suggested that saline lakes without fishes have higher zooplankton biomass than low salinity ones, the aim of this study was to determine the richness, abundance and biomass of zooplankton in two lakes with different salinity and test the hypothesis that in the presence of zooplanktivorous fishes and at equal concentrations of nutrients and chlorophyll-a, saline lakes have higher biomass than those with low salinity. The study was conducted in two shallow lakes of the Province of La Pampa (central Argentina): a subsaline lake and a hyposaline lake, which shared high concentrations of chlorophyll-a and total phosphorus, reduced transparency and presence of planktivorous fish. Zooplankton richness was different and higher in the subsaline lake, whereas abundance and total biomass were similar, even when the taxonomic groups were considered separately. It is suggested that the presence of a halotolerant planktivorous fish controlled the size of zooplankton due to the predation on larger species and prevented the development of higher biomass in the saline lake, which is an important difference from previously recorded situations. This study shows that, regardless of the differences in salinity, the top-down effect in the food chain may have been a factor that equalized the zooplankton biomass by allowing only the development of small species and highlights the possible importance of fish predation in determining chlorophyll-a concentrations and water transparency.

Seasonal variation in planktivory risk in a Southern New England coastal habitat

Planktivory is believed to be a major selective force in marine systems, but little is known about how the intensity of planktivory risk varies spatially or temporally. We assessed seasonal daytime planktivory patterns over fifteen months (Apr 2003 to Jun 2004) at a temperate site in eastern Long Island Sound, CT. Planktivory was measured using modified Plankton Tethering Units (PTUs) deployed from floating docks and baited with live brine shrimp. Planktivory risk varied throughout the year. The highest levels of risk occurred in Aug, Sept and Oct (85%, 75% and 42.5% of brine shrimp consumed, respectively), the lowest levels of risk occurred from Dec to Apr. Mean monthly consumption of brine shrimp strongly correlated with the presence of planktivorous fish. Many species of benthic invertebrates exhibited high levels of recruitment during times of the year when planktivory risk was high, although some species recruited when planktivory risk was low. These seasonal planktivory patterns (highest risk occurring during the late summer and early fall) may be typical for many temperate near-shore habitats, especially those associated with man-made structures (e.g., floating docks, pilings).

Community composition of lacustrine small eukaryotes in hyper-eutrophic conditions in relation to top-down and bottom-up factors

Small eukaryotes (0.2-5 microm) in hyper-eutrophic conditions were described using terminal restriction fragment length polymorphism and cloning-sequencing, and were related to environmental variables both by an experimental approach and by a temporal field study. In situ analysis showed marked temporal variations in the dominant terminal restriction fragments (T-RFs), which were related to environmental variables such as nutrient concentrations and metazooplankton composition. To monitor the responses of the small-eukaryote community to top-down (absence or presence of planktivorous fish) and bottom-up (low or high nitrogen and phosphorus addition) effects, a cross-classified design mesocosm experiment was used. Depending on the type of treatment, we recorded changes in the diversity of T-RFs, as well as modifications in phylogenetic composition. Centroheliozoa and Cryptophyta were found in all types of treatment, whereas Chlorophyta were specific to enclosures receiving high nutrient loadings, and were associated either with LKM11 and 'environmental sequences'. Cercozoa and Fungi were not detected in enclosures receiving high nutrient loadings and fishes. Our results showed that resources and top-down factors are both clearly involved in shaping the structure of small eukaryotes, not only autotrophs but also heterotrophs, via complex interactions and trophic cascades within a microbial loop, notably in response to nutrient loading.

The effects of nutrients and zooplankton community structure on phytoplankton growth in a subtropical Australian reservoir: An enclosure study

Patterns of zooplankton–phytoplankton interactions in subtropical lakes of the Southern Hemisphere may deviate from those established for north-temperate lakes. We tested the responses of phytoplankton growth to different community structures of zooplankton and nutrient enrichment in a subtropical Australian reservoir for the prediction of potential outcomes of lake biomanipulation. Two zooplankton communities were created in lake enclosures over 4 weeks: a rotifer-dominated community developed in the presence of planktivorous fish ( Hypseleotris spp.) and a Ceriodaphnia-dominated community developed in the absence of fish. Biomass gradients of both communities were established in 20 L containers and several separate containers received no additions (controls) or were enriched with nitrogen and/or phosphorus. The growth rate of total phytoplankton significantly increased in response to nutrient enrichment, indicating nutrient limitation. Most phytoplankton taxa were not markedly affected by grazing of either zooplankton community. However, both communities had significant stimulatory effects on the growth of inedible chlorophytes. The ability of zooplankton grazing to negatively affect phytoplankton growth during the summer was counteracted regardless of zooplankton community structure, possibly by nutrients regenerated by zooplankton. We hypothesise that in the subtropical system studied, changes in food web nutrient recycling may be more important for the outcome of biomanipulation than grazing impacts.

Changes to zooplankton community structure following colonization of a small lake by Leptodora kindti

The predaceous cladoceran Leptodora kindti (Focke) became established in Third Sister Lake, Michigan, after individuals escaped from experimental enclosures in 1987. By 1988, the Leptodora population exhibited seasonal dynamics characteristic of natural populations. The maximum seasonal abundance of Leptodora increased to 85 individuals m−3 3 yr following the introduction. After the appearance of Leptodora, small‐bodied cladocerans (Ceriodaphnia and Bosmina) virtually disappeared from the lake. There were strong seasonal shifts in the dominance patterns of both cladocerans and copepods, and Daphnia species diversity increased. Results from this unplanned introduction suggest that invertebrate predators can have a rapid and lasting effect on prey populations, even in the presence of planktivorous fish. Small‐scale (<20 km) geographic barriers might be as important as large‐scale barriers to dispersal of planktonic animals.

Photosynthetic carbon allocation: Effects of planktivorous fish and nutrient enrichment

The proportions of lipid, polysaccharide and protein profoundly influence the nutritional value of food for planktonic organisms. Here, we investigated the effects of nutrients (N and P) and planktivorous fish (Phoxinus sp.) on the partitioning of photosynthetically fixed 14C-labelled bicarbonate into macromolecules (lipids, polysaccharides and proteins) and low molecular weight (LMW) metabolites of epilimnetic phytoplankton living in large (8 m diameter) enclosures from July to mid-October 1991. The enclosures were situated in an oligotrophic lake (north of Montreal, Canada) in both shallow (3.5 m) and deep (11 m) locations. The fractions were calculated using the sequential extraction of 14C-labeled phytoplankton samples incubated for short duration (∼2 hours) at maximum photosynthetic rates. Labelling patterns of all end products exhibited great temporal variations. Overall, the LMW, polysaccharide, lipid and protein fractions accounted for 32.1 ± 10.5%, 31.0 ± 8.8%, 19.2 ± 5.3% and 17.8 ± 6.1% of the total 14C fixed respectively. Percent 14C in the LMW fraction was, in general, lower in the nutrient-enriched enclosure and was usually lower in the deep than the shallow enclosures. The effect of fish on the labelling pattern of LMW varied throughout the sampling period but, overall, its impact was non-significant. The proportions of 14C into lipids and polysaccharides did not show the expected increase under nutrient deficiency. In contrast, the polysaccharide fraction increased, for the most part, with nutrient enrichment and was higher in the deep than the shallow enclosures with little changes in the presence of planktivorous fish. Incorporation of 14C into lipids remained relatively uniform throughout the study and averaged less than 20% of total fixed carbon. Nutrient addition increased the lipid fraction in early July but did not have a significant effect thereafter. In general, the presence of planktivorous fish decreased the lipid fraction especially in the deep N + F enclosures. With nutrient enrichment, the proportion of 14C incorporated into protein decreased in the shallow enclosures while it increased in the deep enclosures. During most of the study, there was no significant effect of fish on the distribution pattern of protein. Percent carbon protein averaged 20% of total carbon fixed during the summer and decreased to 10% in October with lower water temperatures. Any fish effects encountered throughout this study were probably mediated indirectly through changes in the species and cell size compositions of phytoplankton. An experiment where samples were incubated with depth (0.5 to 7 m) showed that, for all treatments, the protein fraction increased with depth at the expense of lipid and LMW metabolites syntheses. The polysaccharide fraction demonstrated no apparent relationships with depth. These changes with depth most likely reflect light attenuation rather than a decrease in temperature. Overall, our findings would suggest that the distribution patterns of photosynthetically incorporated carbon into LMW compounds and macromolecules in natural phytoplankton communities are controlled by a combination of both abiotic (e.g., nutrients, light and temperature) and biotic (species and cell size) environmental factors and the interactions between these factors will in essence influence the direction of carbon flow and the food quality of the phytoplankton. These observations are placed in context with a review of the literature on carbon allocation by natural populations of phytoplankton.

Impact of water regime and fish predation on zooplankton resting egg production and emergence

The emergence of zooplankton from ephemeral areas upon re-wetting has been suggested as a significant contributor to billabong productivity. Sixteen experimental billabongs were designed to test two hypotheses (i) that changing the pattern of flooding alters zooplankton abundance within billabongs, and (ii) the presence of small planktivorous fish alters the zooplankton community structure and diversity within billabongs. Results indicated that flooding caused microcrustaceans to increase in abundance but not rotifers. This response was not modified by changing the time of flooding. The presence of the planktivorous fish had only a small influence on the structure of rotifer assemblages, whereas microcrustacean assemblages became dominated by juveniles. Emergence of rotifers and microcrustaceans from sediment taken from the experimental billabongs and incubated under controlled conditions indicated the existence of a large reservoir of resting stages. The number of resting stages in the sediment had not been modified by changing the season of flooding in the experimental billabongs, but it was modified by permanent inundation, which may have removed environmental cues. The presence of planktivorous fish also appeared to influence the number of resting stages that accumulated in the sediment. Emergence under stable laboratory conditions was rapid. This was not the case under more variable natural conditions.

Effects of hydrological variation and planktivorous competition on macroinvertebrate community structure in experimental billabongs

1. To study two factors which are predicted as causing changes to community structure in cut‐off meanders (colloquially known in Australia as billabongs, a term of aboriginal origin), 16 experimental billabongs were constructed. These were designed to test two hypotheses: (a) that the structure of macrophyte and invertebrate communities within billabongs is altered by changing the pattern of flooding; and (b) that the presence of small planktivorous fish alters invertebrate community structure and diversity within billabongs.2. An increase in the duration of flooding seems to favour animals better adapted to a greater availability of macrophyte habitat. Changing the seasonality of flooding resulted in prolonging of the time water was available over the summer months.3. The presence of a planktivorous fish appears to affect macroinvertebrate communities through competition with other planktivores. Variable top‐down pressure may create differing successional patterns and ultimately different communities at lower trophic levels.

Scaling food chains in aquatic mesocosms: do the effects of depth override the effects of planktivory?

To assess the effects of physical dimension and planktivorous fish on phytoplankton standing crop, we repeated an experiment at different scales in plastic enclosures during summer 1995 in Lake Créteil, France. Enclosures were scaled for a constant surface (1.5 × 1.5 m) as depth was increased from 2.5 to 4.5 m. Even-link (zooplankton and phytoplankton) and odd-link (planktivorous fish, zooplankton and phytoplankton) food webs were established in both shallow and deep enclosures. Fish densities in the deep enclosures were scaled to allow comparisons with shallow ones for both in individuals m-2 or individuals m-3. We explicitly designed this experiment to examine the scale-dependent behavior of the top-down mechanism of algal biomass control in lakes, and in particular to test the hypothesis of stronger cascading effects of fish on lower trophic levels at reduced depth. Both fish and enclosure size had highly significant effects on phytoplankton biomass over the duration of the experiment. No depth × fish interaction effects were observed. The presence of planktivorous fish enhanced phytoplankton biomass in both shallow and deep enclosures, although the reduction in depth generally produced a stronger effect. The mean concentration of chlorophyll a in the deep odd-link systems (ca 5 mg m-3) was lower than in the shallow even-link systems (ca 17 mg m-3). Statistical interpretation did not change when data were expressed as phytoplankton biomass per unit of surface area. Light limitation and zooplankton grazing are the most probable mechanisms explaining our results in these nutrient-enriched systems. Moreover, we found that the strength of the cascading effect of fish on plankton was not a function of depth. We believe that further studies on scaling effects should be conducted in order to improve our understanding of ecological patterns and to extrapolate results from micro/mesocosms to natural ecosystems.

Effects of Nutrients and Planktivorous Fish on the Phytoplankton of Shallow and Deep Aquatic Systems

The response of phytoplankton to nutrient (N, P) and planktivorous fish additions (Phoxinus sp.) was compared between two sets of enclosures (8 m in diameter) installed at different depths in a dimictic temperate lake. One set of eight enclosures was located in the deep part of the lake (12 m) where water thermally stratifies during the summer. The other set of eight enclosures was located in a shallow area of the lake (3.5 m) where the water does not thermally stratify. During the summer months, phytoplankton cell number and biomass increased significantly with both nutrient additions (P = 0.004) and the presence of planktivorous fish (P = 0.011) in the deep, thermally stratified enclosures. On average, total algal biomass in the nutrient and fish treatments was 2.6 times higher than in the nutrient—only treatments. In contrast, in the shallow enclosures, total phytoplankton biomass increased with nutrient additions (P = 0.04) but was not significantly affected by the addition of fish (P = 0.34). However, fish additions increased the number of algal cells in both the shallow and deep enclosures, independently of nutrients, because of an increase in small cells: the proportion of nanoplankton cells was significantly greater in the presence of fish. Fertilization increased the large nanoplankton fraction (10—20 μm) in the deep enclosures, but had no significant effect on the size distribution of algal biomass in the shallow enclosures. In general, large (> 64 μ m) cells were a minor component of the phytoplankton biomass in all treatments. Fertilization stimulated the growth of Chlorophyta in the deep (P = 0.018) and in the shallow (P = 0.012) enclosures. Cyanobacteria were conspicuous in the fertilized shallow enclosures, but the effect on total biomass was not statistically significant. The presence of fish stimulated a significant increase in Cryptophyta at both depths and Pyrrophyta were more common. Nutrients and planktivorous fish had different effects on phytoplankton depending on the thermal regime. In terms of total algal biomass there was no significant difference between deep and shallow enclosures within a given treatment, but depth did affect the size distribution of algal biomass among treatments. The effects of fish were more evident with community structure variables (size or taxonomic distribution) than with coarser variables such as algal biomass.

Seasonal and Spatial Overlap between Cladocerans in Humic Lakes

AbstractSeasonal and vertical distribution, migratory patterns and reproductive effort in coexisting cladocerans were investigated in three humic lakes with different, but low phytoplankton abundances and varying fish predation pressure. Seasonal and vertical habitat or niche overlap varied, but were high within most pairs of species in all localities. Migration was conspicuous in presence of planktivorous fish, less so in the fish free lake. Despite algal densities below incipient limiting level (30–200 μg C I−1) and a low ratio (3–10) of algal to cladoceran biomass, zooplankton distribution and reproductive parameters were not clearly related to algal biomass.Bacterial biomass equalled 10–50% of phytoplankton biomass, while detritus by far was the largest of the particulate compartements. It was concluded that with a possible exception of the early summer algal bloom, additional carbon sources (bacteria, detritus) are important to cladoceran nutrition in these humic lakes. A large share of N‐ and P‐poor detritus in the diet would give zooplankton productivity limitation by food quality in terms of elemental composition rather than food quantity. This would permit coexistence even of species with rather high food overlap, but give low production rates for all species in agreement with the observations.

The Effect of Fish Predation on the Zooplankton of Ten Adirondack Lakes, with Particular Reference to the Alewife, Alosa pseudoharengus

Abstract Stomach contents of alewives collected in Black Pond in 1958 consisted primarily of Diaptomus, Epischura, Mesocyclops, and Daphnia. Food of Black Pond alewives in 1966 consisted mainly of Bosmina, Holopedium, cyclopoid copepods (Cyclops, Macrocyclops, Tropocyclops), and chironomid pupae. Alewife stomach contents were compared with limnetic plankton samples to determine whether the alewives were selective in their choice of food organisms. Both species and size selectivity were observed. The evidence collected suggests that alewife predation was responsible for a change in the zooplankton community of Black Pond. The zooplankton communities of 10 Adirondack lakes were compared. The species and size composition of these communities appeared to be influenced by the presence of planktivorous fish.

The distribution and variation of Daphnia lumholtzi (Crustacea: Cladocera) in relation to fish predation in Lake Albert, East Africa

In Lake Albert Daphnia lumholtzi is found in two forms. One has a pointed anterior prolongation, or helmet, on the head. The other has a shorter rounded head, and was originally described as a separate species, D. monacha. The latter form dominates the zooplankton in the middle of the lake where planktivorous fish are rare or absent. The helmeted form becomes commoner near the margins of the lake and reaches its greatest abundance in Ndaiga Lagoon, where planktivorous fish are common. The possession of a helmet is associated with a reduction in the size of the carapace compared to the round headed form. The carapace with its contained eggs is the most conspicuous part of a cladoceran, so that the helmeted forms are at an advantage in the presence of planktivorous fish which locate their prey by sight. The mid‐lake monacha forms are larger than specimens of the same form in Ndaiga Lagoon, where it is shown that Alestes baremose feeds selectively on the larger specimens of the monacha form.The helmeted form produces more, but smaller eggs than the monacha form. The total brood volume (= mean egg volume x mean number eggs per female) is greatest in the midlake monacha forms. The selective advantages of variations in egg size and the possession of a helmet are discussed. It is concluded that the data from Lake Albert support the hypothesis of Brooks (1965) concerning the adaptive significance of helmet development in Daphnia.

The distribution and variation of Daphnia lumholtzi (Crustacea: Cladocera) in relation to fish predation in Lake Albert, East Africa

In Lake Albert Daphnia lumholtzi is found in two forms. One has a pointed anterior prolongation, or helmet, on the head. The other has a shorter rounded head, and was originally described as a separate species, D. monacha. The latter form dominates the zooplankton in the middle of the lake where planktivorous fish are rare or absent. The helmeted form becomes commoner near the margins of the lake and reaches its greatest abundance in Ndaiga Lagoon, where planktivorousfish are common. The possession of a helmet is associated with a reduction in the size of the carapace compared to the round headed form. The carapace with its contained eggs is the most conspicuous part of a cladoceran, so that the helmeted forms are at an advantage in the presence of planktivorous fish which locate their prey by sight. The mid‐lake monacha forms are larger than specimens of the same form in Ndaiga Lagoon, where it is shown that Alestes baremose feeds selectively on the larger specimens of the monacha form.The helmeted form produces more, but smaller eggs than the monacha form. The total brood volume (= mean egg volume x mean number eggs per female) is greatest in the midlake monacha forms. The selective advantages of variations in egg size and the possession of a helmet are discussed. It is concluded that the data from Lake Albert support the hypothesis of Brooks (1965) concerning the adaptive significance of helmet development in Daphnia.