Biological productivity of Lake Towada,a north temperate,oligotrophic, kokanee fishery lake

During the Lake Towada survey from April through October 1998, we measured primary production at shore and offshore stations, and calculated crustacean zooplankton production from samples collected at the offshore station. We then analyzed these data and compared them with commercial fishery data in order to discuss the energy flow in this lake where kokanee (Oncorhynchus nerka) fisheries are one of the main businesses. At all stations, primary production was relatively high: 150–300mgCm^–2day^–1 in April–mid-June and lower at ca. 100mgCm^–2day^–1 thereafter. The variation in primary productivity could largely be explained by multiple regression models that included phytoplankton biomass and ambient nutrient conditions as independent variables. Among zooplankton, rotifers had their peak abundance in May, before the crustacean zooplankton (Daphnia longispina, Bosmina longirostris, and Acanthodiaptomus pacificus) population was well established. D. longispina dominated the crustacean zooplankton community in terms of biomass and production; their production during the study period made up 80% of crustacean community production (19.6gdry-wtm^–2), which was 40% of primary production during the survey. In July, when the abundance of D. longispina was particularly high, their daily production slightly exceeded daily primary production, which resulted in ca. 30% and 75% reduction in the amount of particulate organic carbon and chlorophyll a, respectively, during this period. The community ingestion rate of crustacean zooplankton, calculated from their daily production (D. longispina accounted for 90%) and the assumption that their gross production efficiency (K₁) was 60%, could roughly explain this reduction of particulate matter, corroborating previous studies that the grazing of D. longispina can significantly improve the water transparency of this lake. The catch of kokanee and pond smelt (Hypomesus transpacificus nipponensis, another important fish in the lake) during the survey corresponded to 1.1% of crustacean community production, and corresponded to 0.45% of the primary production, which is one of the highest recorded values. Bearing in mind that D. longispina was the major food item of planktivorous fish such as kokanee and pond smelt, the present study suggests that the energy transfer from phytoplankton to zooplankton to fish is outstandingly efficient, compared with other aquatic ecosystems, when D. longispina dominates in the lake.     

Experimental analysis of the predation impact of the larvae of pond smelt (<Emphasis Type="Italic">Hypomesus transpacificus nipponensis</Emphasis>) on zooplankton populations established in mesocosms

The predation impact of the larvae of pond smelt Hypomesus transpacificus nipponensis on a zooplankton community was studied using mesocosms. The fish significantly depressed the abundances of copepod nauplii and rotifers, especially Hexarthra mira. The vulnerabilities of these prey might be determined by their swimming behavior and population density, suggesting that larval fish selectively prey on zooplankton that have a high encounter rate with the predator. The larvae did not have a negative effect on the densities of cladocerans, but fish predation altered the cladoceran community structure from the dominance of B. longirostris to that of B. fatalis. This result suggests that larval fish predation is an important factor that shifts the species composition of Bosmina in some lakes, the shift occurring in the season when fish larvae are abundant. Our results have shown that predation by the larval fish would control not only the abundance, but also the community structure of the small-sized zooplankton prey.     

Variations of antioxidant enzymes in Daphnia species and populations as related to ambient UV exposure

To assess their role in photoprotection in the crustacean zooplankton Daphnia spp., activities of the antioxidants catalase (CAT), superoxide dismutase (SOD), glutathione transferase (GST) and content of carotenoids (CAR) were studied in 4 Daphniaspecies from a total of 50 populations. Included in the survey were alpine and lowland populations of both pigmented and non-pigmented D. longispina, rock-pool and laboratory clones of D. magna and Arctic populations of the D. pulex/D. middendorffianacomplex. Most of the surveyed populations inhabit shallow ponds, and are thus highly exposed to UV-radiation during the summer. The results are primarily discussed in relation to antioxidants as possible protection against UV-radiation. D. pulex, D. middendorffianaand D. longispinaoccurred both as hyaline and pigmented (carapace melanization) morphs. The most notable inter-species differences were the high activity of CAT in the D. magnalab-clone, the low activity of GST in melanic animals from the D. pulexgroup and the high activity of SOD in D. longispinafrom a lowland humic pond. Contrary to expectations, we found no differences in antioxidant capacity between melanic and non-pigmented alpine D. longispina. Intraspecies variation in antioxidant activities was studied in relation to pond characteristics. Among the alpine populations of D. longispina there was a significant positive correlation between absorbance (300 nm) of the pond water and CAT activity, which could be related to ambient levels of photoinduced hydrogen peroxide production in these small water bodies. The data from this comparative study provide insight in photo-protective mechanisms in cladocera, and constitute a valuable basis for future research on pro-oxidant and antioxidant processes in alpine and arctic freshwater zooplankton.     

A description of pelagic zooplankton in the large lakes of the Todzha Depression (Bolshoi Yenisey River basin, Tuva)

The zooplankton structure of large lakes (Noyon-Khol’, Shuram-Khol’, Borzu-Khol’, Many-Khol’, Kadysh, and Todzha) of the Todzha Depression has been studied. A comparison of the species composition, abundance, and biomass of zooplankton in the lakes was made. Rotifers have the largest species diversity: from 10 to 16 species, with Conochilus sp., Collotheca sp., Kellicottia longispina (Kellicott), and Asplanchna priodonta Gosse being the most abundant. Copepods are represented mostly by Arctodiaptomus paulseni (Sars), Eudiaptomus graciloides (Lilljeborg), Cyclops scutifer Sars, and Mesocyclops leuckarti Claus. Daphnia galeata Sars and Bosmina longispina Leydig are the dominant cladoceran species. It is shown that the zooplankton communities of the lakes in the Todzha Depression are characterized by a high degree of species composition similarity. Factors forming the taxonomic structure of pelagic zooplankton in the system of these lakes are displayed.     

Impact of cyprinids on zooplankton and algae in ten drainable ponds

To study the impact of cyprinids on algae, zooplankton and physical and chemical water quality, ten drainable ponds of 0.1 ha (depth 1.3 m) were each divided into two equal parts. One half of each pond was stocked with 0 + cyprinids (bream, carp and roach of 10–15 mm), the other was free of fish. The average biomass of the 0 + fish at draining of the ponds was 466 kg ha^–1, to which carp contributed about 80%.The fish and non-fish compartments showed significant differences. In the non-fish compartments the density of Daphnia hyalina was 10–30 ind. l^–1 and that of Daphnia magna 2–4 ind. l-^–1, whereas in the fish compartments densities were c. 1 ind. l^–1. Cyclopoid copepods and Bosmina longirostris, however, showed higher densities in the fish compartments. The composition of algae in the two compartments differed only slightly, but the densities were lower in the non-fish compartments. The significant difference in turbidity was probably caused by resuspension of sediment by carp. No significant difference in nutrient concentration between the compartments was found.     

Annual variation in biomass and the community structure of crustacean zooplankton over 5 years in Lake Toya, Japan

The biomass and population dynamics of crustacean zooplankton were determined in oligotrophic Lake Toya in Japan over 5 years from May 1992 to May 1997. In 1992 and 1993, zooplankton biomass was up to 4.3 g dry weight m^?2, whereas it decreased to <1 g dry weight m^?2 after 1994. This extreme change in biomass was associated with the succession of dominant species from larger ones, such as Daphnia longispina and Cyclops strenuus (s. lat.), to smaller ones, such as Eubosmina tanakai and Bosmina longirostris. Consequently, this biomass change seemed to cause an increase in the chlorophyll a concentration in the euphotic zone and a decline in lake transparency. Because the birth rates of the dominant species were somewhat higher after 1994, the decline in the populations of larger crustaceans seemed to depend more on their rate of death rather than rate of birth, and this higher death rate is not considered to be attributed to food shortage. Although these results strongly suggest a top-down cascading effect of fish predation upon crustaceans, annual catches of two commercially important planktivorous fish species have also decreased in the lake, coincidentally with decreases in zooplankton biomass. This may be attributable to fishing regulations that prohibit catching smaller fish, implying that such smaller fish affect zooplankton and phytoplankton, as well as lake transparency.     

The effects of selective fish predation on the horizontal distribution of pelagic Cladocera in a southern French reservoir

A four-year study showed a clear seasonal succession of species within the cladoceran community of the large, oligotrophic Sainte-Croix reservoir (S.E. France). Diaphanosoma brachyurum and Ceriodaphnia pulchella were strictly limited to the warm stratified period (July to October), whereas Bosmina longirostris and Bosmina coregoni were dominant during spring and autumn. Daphnia longispina was the only species to occur throughout the year with higher densities in spring.In spring and late autumn, the discharge of the inflowing river Verdon was high and the abundance of all species showed a gradient over the whole lake with lower densities close to the inflow. During the stratified period, water inflow was very low and species showed different patterns. Densities of the small form Ceriodaphnia pulchella were similar all along the long axis of the lake, whereas Daphnia densities were significantly higher near the dam. The distribution pattern of Diaphanosoma, an intermediate-sized species, showed similar trends to that of Daphnia. The only planktivorous fish in the pelagic zone, the bleak (Alburnus alburnus), fed mostly on large-bodied species (> 1.0 mm) and was more abundant close to the inflow current. A comparison between the length frequency distributions of cladocera upstream and downstream provided a clear demonstration of the effects of size-selective predation on prey populations. Finally, the interactions between spatial heterogeneity and long-term development of the zooplankton community and the indirect effects of predation are discussed.     

Trophic interactions of the freshwater jellyfish Craspedacusta sowerbii

The predatory impact and the trophic role of the freshwaterjellyfish, Craspedacusta sowerbii, was studied using microcosmand enclosure experiments as well as a 3-year pond survey. Theresults showed a significant decrease of small herbivorous crustaceans,i.e. Bosmina longirostris and juvenile cyclopoid copepods, inthe medusa treatments of the microcosms and the enclosure experiments.Chlorophyll concentrations in the enclosure experiment weresignificantly increased in the medusa treatment, suggestingthat C. sowerbii may cause cascading effects in the food chain.A comparison of daily zooplankton losses during the pond surveycaused by medusae and fish (roach, Rutilus rutilus), and theirfood selectivities suggest food separation of these two predatorsand reveal a strong negative impact of medusae on the copepodpond community. In the case of a jellyfish bloom, our resultsshow that both food chains can co-occur in lakes because ofa weak interaction between these top predators, fish and jellyfish,with simultaneous impacts on the zooplankton structure.     

The spatial distribution of zooplankton populations in the presence and absence of fish

The spatial distribution patterns of the main zooplankton groups in two small experimental ponds, one which contained fish and one which did not, were measured over a twelve month period from March 1983. Populations of all zooplankton groups were generally distributed non-randomly and with varying degrees of aggregation. The aggregation ofDaphnia longispina populations was inversely related to ambient wind speed, indicating a breakdown of behavioural clumping by wind-induced water currents. Evidence that populations ofDaphnia aggregate in response to fish predation was inconclusive. In contrast cyclopoid nauplii showed no evidence of behavioural clumping and the degree of aggregation was not related to wind speed. Nauplii aggregations were however greater in the absence than in the presence of fish.     

The diet of Abramis brama (L.) larvae reared in illuminated cages

The paper presents the results of investigations on the feeding of bream larvae reared in illuminated cages in a mesotrophic lake. The same species of zooplankton were found in both the lake and the fish cages. Copepoda predominated in abundance and biomass. During the mixed feeding the bream larvae diet consisted of rotifers (45.8%), small cladocerans (41.6%) and phytoplankton (13.6%). According to Ivlev's selectivity indices (E), the most important food species were: Keratella cochlearis (E = +9.94), Kellicotia longispina (E = +0.93), K. quadrata (E = +0.92), Bosmina longirostris (E = +0.92), Trichocerca sp. (E = +0.89), and Chydorus sphaericus (E = +0.88). The total length (TL) of bream larvae consuming their first nauplii and copepodites of Copepoda was 8.9 mm. As bream larvae grew, cladocerans were more often selected than rotifers. Copepods usually made up an insignificant component of the diet. Bream larvae in illuminated cages generally fed at night; their alimentary tracts were filled to the maximum in the period from sunset to sunrise. Indices thereof reached 755.6 at night and 278.0 ? in the daytime.     

Did chironomid emergence release Daphnia from fish predation and lead to a Daphnia-driven clear-water phase in Lake Towada,Japan?

The emergence of the chironomid pupae into the water column may potentially allow Daphnia to be released from positively size-selective fish predation and indirectly cause a Daphnia-driven clear-water phase in lakes. We studied these processes in Lake Towada, Japan, from April to October 1998. The biomass of the Daphnia longispina population was stable from April through June and increased by 3-fold in mid-July. This coincided well with the onset of the clear-water phase in the lake. The sharp decline of chironomid larvae in the sediment in June suggested that the major emergence occurred. Stomach contents of kokanee (Oncorhynchus nerka) and pond smelt (Hypomesus transpacificus nipponensis) also shifted from D. longispina to the chironomid pupae at that time. However, mean individual body size, mean ovigerous female size and clutch size of D. longispina declined rapidly in mid-June and stayed smaller thereafter compared to those in April–May, suggesting that fish predation was stronger after mid-June. It is probable that lower water temperatures in April–May suppressed fish feeding activity as well as reproduction in D. longispina, which resulted in the stable biomass of D. longispina. The increased water temperatures resulted in the stable biomass of D. longispina. This would lead to a mid-July peak of less predation-sensitive young adults. Therefore, the chironomid emergence seemed neither to release D. longispinafrom fish predation nor to be associated with the clear-water phase in the present study.     

Zooplankton community composition of lakes in the Yukon and Northwest Territories (Canada): relationship to physical and chemical limnology

We analysed associations between zooplankton species composition and local abiotic factors in 30 lakes located along a 900 km south-north transect from Whitehorse (Yukon Territory) to Inuvik (Northwest Territories). The lakes were situated in three broadly defined vegetation zones: (i) Boreal forest (between Whitehorse and Dawson City), (ii) alpine tundra (Ogilvie mountains north of Dawson City) and (iii) subarctic forest-tundra (near Inuvik). Lakes in the alpine tundra were characterised by lower conductivity, temperature, chlorophyll a and nutrients than those in the other two zones. Those in the forest-tundra were generally small and shallow, and had higher chlorophyll a concentrations than lakes further south. Lakes in forested catchments spanned a larger latitudinal range and exhibited a greater variety of physical and chemical characteristics. However, they were generally deeper, with higher conductivity, temperature and ionic concentrations. Forty-one zooplankton taxa were identified from the 30 lakes, of which the most frequently occurring were the rotifers Conochilus unicornis, Kellicottia longispina, Keratella cochlearis and Polyarthra vulgaris, the cladocerans Daphnia middendorffiana and Bosmina longirostris, and the copepods Leptodiaptomus pribilofensis, Heterocope septentrionalis and Cyclops spp. The lakes contained between two and fifteen species (mean = 6.9). Alpine tundra lakes contained slightly less species (mean = 5.8) than those at lower elevations; in particular the cladoceran fauna was depauperate or absent. Relationships among the lakes, species and environmental factors were examined using canonical correspondence analysis, with forward selection and associated Monte Carlo permutation tests. Chloride, silica and temperature showed statistically significant relationships with species distribution, and together these abiotic factors explained 25% of the variation in zooplankton communities within Yukon and Northwest Territories lakes.     

Bacterivory of metazooplankton, ciliates and flagellates in a newly flooded reservoir

Bacterial consumption by metazoan zooplankton and phagotrophic protists was measured in situ during the period of thermal stratification in the epilimnion (1 m) and metalimnion (7 m) of a newly flooded reservoir (Sep reservoir, France). The mean bacterial consumption was 2.53 x 10⁶ bacterial l^-1 h^-1 at 1m and 0.97 x 10⁶ bacteria l^-1 h^-1 at 7m. The main consumers over the whole study period were the cladocerans Daphnia longispina and Ceriodaphnia quadrangula, accounting on average for 72% of the potential total predation of bacteria at 1 m and 56% at m, especially during the months of May-June and August. Heterotrophic nanoflagellates (HNF), which accounted for 12% estimated total predation of bacteria at 1 m and 13% at 7m, only exerted a limited predation, mainly by a Monas-type cell. Ciliates, dominated in terms of abundance by Pelagohalteria viridis, accounted for 4% of total predation in the epilimnion (0.00-0.42 x 10⁶ bacteria l^-1 h^-1). In a newly flooded reservoir, metazoan zooplankton appear to be the main consumers of bacteria. Predation of ciliates and HNF by zooplanktonic crustaceans could account for the low contribution of components of the microbial loop to bacterial consumption.      

Factors influencing the spatial distribution of zooplankton and fish in Loch Ness, UK

• 1 The vertical and horizontal distribution of phytoplankton, zooplankton and fish in Loch Ness, Scotland, were monitored during one day‐time and one night‐time survey in July 1992. The vertical samples were collected at a site located at the northern end of the loch and the horizontal samples along a longitudinal transect.
• 2 The vertical distribution surveys demonstrated that the phytoplankton, the zooplankton and the fish were concentrated in the top 30 m of water above the seasonal thermocline. Within this layer, Cyclops stayed much closer to the surface than Eudiaptomus but both species moved towards the surface at night.
• 3 The most important factor influencing the horizontal distribution of the phytoplankton was the north‐ south gradient in productivity. The sub‐catchments surrounding the north basin contain a greater proportion of arable land than those to the south and the concentrations of nitrate‐nitrogen and phytoplankton chlorophyll increased systematically from south to north.
• 4 Zooplankton distribution patterns were influenced by wind‐induced water movements and the dispersion of allochthonous material from the main inflows. The highest concentrations of Cyclops were recorded in the north, where there was more phytoplankton, and the highest concentrations of Eudiaptomus in the south, where there were higher concentrations of non‐algal particulates.
• 5 There was no spatial correlation between total zooplankton and total fish abundance but the highest concentrations of small (1–5 cm) fish were recorded in the south where there was a large patch of Eudiaptomus. The number of Eudiaptomus at specific locations within this patch were, however, negatively correlated with the numbers of small fish. These results suggest that the fish were actively foraging within the patch and were depleting their zooplankton prey in the areas where they were most abundant.

     

Contrasting Zooplankton Assemblages in Two Oxbow Lakes with Low Transparencies and Narrow Emergent Macrophyte Belts (Krapina River,Croatia)

The aim of this study was to examine the combined effect of water transparency and narrow macrophyte belts on zooplankton assemblages in two oxbow lakes (Krapina River, Croatia). Samples were collected in open water and among helophytes in the littoral zone from April until September 2008. Rotifers were the most abundant group of zooplankton in both lakes, and dominated in the Krapina oxbow lake 1 (KO1). Lake KO1 had significantly lower transparency, lower percentage macrophyte cover and higher chlorophyll a concentration than Krapina oxbow lake 2 (KO2). In lake KO1, variation in the horizontal distribution of cladocerans and rotifers in terms of their abundance seemed to be determined by competition between Bosmina longirostris and Keratella cochlearis, initiated by oscillation in transparency and detritus availability. In lake KO2, with higher transparency and higher percentage macrophyte cover, the abundance of small‐ and large‐bodied cladocerans increased in the littoral zone simultaneously with higher transparency, suggesting fish predation. Results of this study indicated that small differences in transparencies between the two lakes caused significant differences in horizontal distribution of the zooplankton assemblage. Even narrow helophyte belts offered a refuge to zooplankton, although lower transparencies reduced the effectiveness of macrophytes as a refuge from predators. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Phosphorus Flux Through a Littoral Ecosystem: The Importance of Cladoceran Zooplankton and Young Fish

Foodchain relationships in water within a helophytic vegetation stand were experimentally investigated using the radioactive isotope ³³P as a tracer. The results proved the importance of cladoceran zooplankton (mainly Bosmina longispina LEYDIG) in the control of phytoplankton and the importance of young fish in the control of Bosmina abundance in the littoral water.     

Effects of fish density on planktonic communities and water quality in a manipulated forest pond

We examined the effects of fish on lower trophic levels in a small pond in eastern Finland. The pond was divided into four sections with plastic curtains and stocked with crucian carp (Carassius carassius); two sections had low (4.4–5.5 g m^–3) and two high (10.4–13.7 g m^–3) densities of fish. In the summer of 1987, the pond was sampled weekly for phyto- and zooplankton until the fish were removed by rotenone in September after a three month experiment. Fish density as well as the extent of macrophyte cover had a considerable impact on planktonic communities and water quality. Mean zooplankton biomass was significantly lower and phytoplankton biomass higher, at high fish density. Water transparency was correlated negatively with chlorophyll-a at low fish density but turbidity appeared to reduce transparency at high fish density. The composition and dynamics of the plankton also differed at different fish densities. The mechanisms behind these effects, and the influences of habitat and fish behaviour on the results, are discussed.     

Vertical distribution of pelagial zooplankton in a middle-sized dimictic valley reservoir

Our observations indicate the vertical distribution of zooplankton and its seasonal changes in Dubník II reservoir (Slovakia) are determined mainly by the thermal regime of the reservoir, by transparency, and by fish and invertebrate predation. During periods of circulation, zooplankton vertical distribution in the whole water column was more homogeneous, whilst during summer temperature stratification zooplankton concentrated in the epilimnion — rotifers in higher layers than crustaceans. During summer stagnation a steep thermal gradient occurred at the boundary of the epi-and hypolimnion and low temperature and low dissolved oxygen in hypolimnion offered a refuge for Chaoborus flavicans larvae against fish, enabling coexistence of vertebrate and invertebrate predation. This evidence supports our previous findings concerning dominance of rotifers in zooplankton and representation of crustaceans by small-bodied species in the study reservoir. Steep thermal gradient and the presence of Chaoborus larvae caused very low zooplankton abundance in the lower part of the water column and a reduction of cladocerans refuges against fish to layers of thermocline or closely under thermocline where Daphnia cucullata and Daphnia parvula were found. Our previous assumptions about the high density of zooplanktivorous fish in Dubník II reservoir are supported by the fact that these small cladocerans are represented by smaller individuals in the upper layers and bigger individuals in deeper layers.     

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.     

Some studies on the ecology of limnetic crustacean zooplankton in Lake Begnas and Rupa,Pokhara Valley,Nepal

Monthly zooplankton samples taken during the period February, 1977 to February, 1978 in the deepest portion in two shallow sub-tropical Lakes, Begnas and Rupa in the Pokhara Valley, Nepal were studied. Four peaks of zooplankton abundance were noted. Adult and copepodid Copepoda were numerically dominant in both lakes with 56% Copepoda, 24% Cladocera and 20% nauplii in Lake Begnas and 48% Copepoda, 36% Cladocera and 16% nauplii in Lake Rupa. Other forms like Chaoborus larvae occurred sporadically in both lakes. An occurrence of the rare Limnocnida nepalensis (Coelenterata: Limnomedusae) in Lake Rupa was also noted during April and May, 1977. Although both of these lakes had already been ranked as eutrophic, the absence of calanoids, relative abundance of Bosmina longirostris and higher gross primary production in Lake Rupa is an indication of a higher trophic condition than that of Lake Begnas.