Variation in horizontal zooplankton abundance in mountain lakes: shore avoidance or fish predation?

Hypothesizing that fish predation, active shore avoidance and outlet stream avoidance may be separately affecting horizontal zooplankton distribution, the effects of fish presence, sampling location (midlake, outlet and non-outlet shore) and time (day or night) on zooplankton abundance and body size were tested. Statistically significant horizontal zooplankton abundance gradients occurred in both fish-present and fish-absent lakes. Fish may strengthen zooplankton spatial patterns common to both fish-present and fish-absent lakes, as abundance differences among locations were often greater in fish-present systems compared to fish-absent systems. Horizontal zooplankton abundance gradients differed through a diel cycle, but were species specific with some species exhibiting gradients only during the day, while others exhibit gradients only during the night. Avoidance of the outlet over and above active shore avoidance appeared to take place in Daphnia sp. Other taxa provided equivocal support of active outlet avoidance with most showing no significant difference between shore and outlet abundance (seven of nine), one taxa showing a significant decrease and one a significant increase in outlet compared to shore abundance. No gradients in zooplankton body size were found.      

The impact of zooplankton status on the management of Lake Kinneret (Israel)

Monthly averages of standing stock wet biomass of zooplankton in Lake Kinneret (Israel) varied between 11 and 76 g m^–2 during 1969–1981, with the exception of two months. Averaged contributions of different groups were: Cladocera 58%, Copepoda 35% and Rotifera 7%. Total standing crop wet biomass is highest during January–June, averages varied between 35 and 50 g m^–2, and decreases during summer–fall (23–36 g m^–2). The winter biomass of Cladocera fluctuated between 22 and 35 g m^–2 and dropped to a range of 9–23 g m^–2 in summer, whereas copepod biomass varied very little around an average of 18 g (ww) m^–2 with the exception of low values from April to June. The stock biomass of Rotifera is relatively high during winter floods season (December-March) whilst in summer it is very low.Young stages of fish in Lake Kinneret feed mostly on zooplankton and zoobenthic forms. The most abundant fish in the Kinneret ecosystem, Mirogrex terraesanctae terraesanctae, also feed on zooplankton at the adult stage throughout the year, and herbivorous fish consume zooplankton during the summer when lake plankton resources are limited.The summer ecosystem of Lake Kinneret is characterised as a steady state type, in which the impact of the zooplankton-chain is of great importance. Increase of predation pressure on zooplankton by fish can disequilibrate the balanced trophic relations existing between nannoplankton production and zooplankton grazing capacity. Such a situation can lead to organics accumulation as nannoplankton blooms, resulting in water quality deterioration. Management options aimed at preventing collapse of zooplankton populations are discussed.     

<Emphasis Type="Italic">Daphnia</Emphasis> versus copepod impact on summer phytoplankton: functional compensation at both trophic levels

Here we report on a mesocom study performed to compare the top-down impact of microphagous and macrophagous zooplankton on phytoplankton. We exposed a species-rich, summer phytoplankton assemblage from the mesotrophic Lake Schöhsee (Germany) to logarithmically scaled abundance gradients of the microphagous cladoceran Daphnia hyalina×galeata and of a macrophagous copepod assemblage. Total phytoplankton biomass, chlorophyll a and primary production showed only a weak or even insignificant response to zooplankton density in both gradients. In contrast to the weak responses of bulk parameters, both zooplankton groups exerted a strong and contrasting influence on the phytoplankton species composition. The copepods suppressed large phytoplankton, while nanoplanktonic algae increased with increasing copepod density. Daphnia suppressed small algae, while larger species compensated in terms of biomass for the losses. Autotrophic picoplankton declined with zooplankton density in both gradients. Gelatinous, colonial algae were fostered by both zooplankton functional groups, while medium-sized (ca. 3,000 µm³), non-gelatinous algae were suppressed by both. The impact of a functionally mixed zooplankton assemblage became evident when Daphnia began to invade and grow in copepod mesocosms after ca. 10 days. Contrary to the impact of a single functional group, the combined impact of both zooplankton groups led to a substantial decline in total phytoplankton biomass.     

Some ecological aspects of Lake Qarun,Fayoum, Egypt. Part II. Production of plankton and benthic organisms

Distribution and abundance of phyto-, zooplankton and benthic organisms in Lake Qarun were investigated during the period from January 1974 to December 1977.Average number of phytoplankton cells was 152,300 cells/L and its biomass was 0.365 g/C/m³; average number of zooplankton was 31.44 × 10³/m³ and its biomass was 194.19 mg/m³. The average number of benthic fauna was 19889/m² and its biomass was 400.22 g/m² (dry wt.). Therefore, Lake Qarun may be considered as a highly eutrophic body of water.Freshwater planktonic species, that used to inhabit the lake, such as Diaptomus salinus and the cladoceran Moina salinarum, disappeared completely when the salinity of the lake water reached 30–34 However, some Rotatoria were able to withstand the high salinity. The new composition of the zooplankton community shows that the marine zooplankton species include not only Acartia latisetosa and Cirripedia nauplii, but also other species such as Polychaeta, Obelia medusae, etc.The benthos of Lake Qarun is characterised by an intensive growth of few species. The major part (i.e. 93.54% by weight) of bottom fauna in the lake is Mollusca, mainly Cerastoderma glaucum (69·84% by weight).     

Temporal variability and production of Euterpina acutifrons (Copepoda: Harpacticoida) in the Cananéia Lagoon estuarine system,São Paulo,Brazil

Diel and seasonal variations in abundance, population structure, biomass and production rate of the harpacticoid copepod Euterpina acutifrons were studied in the Cananéia Lagoon estuarine system, São Paulo, Brazil. Zooplankton samples were collected at 4-h intervals during multiple 24-h periods, from February 1995 to January 1996. Copepodites and adults of E. acutifrons were present in the plankton throughout the year (temperature, 18.6–29.4 °C; salinity, 4.5–33.0 psu; chlorophyll-a concentration, 1.32–20.42 g l^–1). Abundance of E. acutifrons showed considerable diel variations. On most sampling dates, higher abundances were recorded at times when salinity was higher. Biomass varied from 0.044 ± 0.046 (daily mean ± SD) to 5.264±3.425 mg C m^–3. The estimated production rates (minimum ± SD–maximum ± SD) were 0.034±0.035–4.95±3.25 (Ikeda-Motoda model), 0.035±0.036–5.123±3.347 (Huntley-Lopez model), and 0.016±0.017–2.101±1.372 mg C m^–3 d^–1 (Hirst-Sheader model).     

Autotroph:herbivore biomass ratios; carbon deficits judged from plankton data

A survey on phytoplankton:zooplankton biomass ratios was performed in 342 Norwegian lakes, covering a wide range in lake size and productivity (total phosphorus: 3–246 g l^–1), but with most localities being oligo- to mesotrophic. Mean phytoplankton biomass was 88 g C l^–1, yet with the majority below 50 g C l^–1and a median of 25 g C l^–1. Total zooplankton biomass displayed a mean and median of 37 and 26 g C l^–1, respectively. Cladocerans were by far the dominant group, making up a median of almost 60% of total zooplankton biomass. Total zooplankton biomass as well as that of major aggregated metazoan taxa (cladocerans, calanoid copepods, cyclopoid copepods and rotifers) all showed a positive, but weak correlation with total phytoplankton biomass. These weak correlations suggest that algal biomass per se is a poor predictor of zooplankton biomass. An average phyto-:zooplankton biomass ratio (C:C) of 2.8 (SD±4.7) was found. 30% of the lakes had a phyto-:zooplankton biomass ratio below unity. While there was no correlation between the phyto-:zooplankton biomass ratio with increasing productivity in terms of P concentration, there was a higher biomass ratio in lakes with high fish predation pressure. The low ratio of phyto-:zooplankton biomass suggest major requirements from non-algal sources of C in the zooplankton diet. The need for dietary subsidizing is also supported by the fact that more than 75% of the lakes had algal biomass less than the estimated threshold for net positive growth of zooplankton, although it should be kept in mind that a high share of picoplankton would imply an underestimation of autotroph biomass in these lakes. Since the C-deficiency apparently is most pronounced in oligotrophic systems, it contradicts the view that the detritus pathways plays a predominant role in highly productive systems only, but while the source of detritus probably is mostly of autochthonous origin in eutrophic lakes, allochthonous detritus will be more important in oligotrophic systems.     

Zooplankton of Lake Kandrykul (Republic of Bashkortostan,Russia) under Conditions of Anthropogenic Eutrophication

The species diversity, abundance, and biomass of zooplankton in the pelagic and coastal zones of Lake Kandrykul were studied in 2007–2012. The community was dominated by large Cladocera. The maximum abundance of zooplankton was observed in the anomalously warm 2010. In July, the highest abundance of zooplankton (1300 thousand ind./m³) was recorded near the southern coast in stands of mare′s-tail Hippurus vulgaris; that of biomass (9 g/m³) was found near the northern shore in stands of narrow-leaved cattail Typha angustifolia. The lowest values of the number and biomass of aquatic invertebrates were observed in the pelagial (32 thousand ind./m³ and 0.1 g/m³) and along the M5 motorway stretching aside the northeastern coast (188 thousand ind./m³ and 0.5 g/m³). The Shannon index value (1.3–2.1) corresponded to the meso-eutrophic type of water bodies. In 2007, according to the Mjaemets trophicity index (E), the lake ecosystem was oligotrophic (E 0.11); in 2010–2012 it was mesotrophic (in the pelagial, E value was 0.54; in the open littoral it was 0.76) or weakly eutrophic (E values of protected littoral were 1.52). The estimates of water trophy as assessed by zooplankton are close to those assessed by the number and biomass of phytoplankton (meso-eutrophic type). The rapid eutrophication of the lake ecosystem was revealed. In 6 years the trophic status of the lake changed from oligo-mesotrophic to meso-eutrophic.     

Composition and sources of near reef zooplankton on a Jamaican forereef along with implications for coral feeding

Nocturnal near-reef zooplankton from the forereef of Discovery Bay, Jamaica, were sampled during winter and summer 1994 using a diver-operated plankton pump with an intake head positioned within centimeters of benthic zooplanktivores. The pump collected zooplankton not effectively sampled by conventional net tows or demersal traps. We found consistently greater densities of zooplankton than did earlier studies that used other sampling methods in similar locations. There was no significant difference between winter (3491±578 m^–3) and summer (2853±293 m^–3) zooplankton densities. Both oceanic- and reef-associated forms were found at temporal and spatial scales relevant to benthic suspension feeders. Copepods were always the most abundant group, averaging 89% of the total zooplankton, and most were not of demersal origin. The cyclopoids, Oithona spp., were the numerically dominant organisms, with an average density of 1684±260 m^–3. Other zooplankton (e.g., shrimp larvae, crab larvae, polychaetes, chaetognaths, amphipods, and isopods) were highly variable and much less abundant. Near-reef zooplankton abundances were high throughout the night sampling period, not just after sunset and before sunrise as previously described. Mean biomass was 4.5 mg C m^–3, with values ranging from 1.0 to 15.6 mg C m^–3. This work has important implications for evaluating which zooplankton types are available to benthic suspension feeders, including corals.     

Life history and production of Neomysis mercedis in two British Columbia coastal lakes

The life history and production characteristics of Neomysis mercedis from two British Columbia lakes were examined for their potential influence on zooplankton and limnetic fish communities. During the day, mysids in shallow Muriel Lake (45 m) were on or near the bottom; in Kennedy Lake (> 100 m), mysids remained deeper than 50 m. In both lakes, mysids spent summer nights at 0–15 m depths despite > 20 °C temperatures. Mysid density was not strongly correlated with lake depth. Mysids generally displayed spring to early summer minima and late summer to fall maxima in numbers and biomass. Single peaks in gravid females and juvenile mysid abundance, and the absence of pronounced seasonal size changes of gravid females suggest that N. mercedis produced a single generation each year. Fecundities of study lake mysids are the lowest on record, and although size-dependent, exhibited unusually high variability. Annual productivity of mysids averaged 485 mg m^–2 y^–1 (range 205–690). Calculations indicate mysids consume several times more zooplankton per annum than limnetic fish do. N. mercedis is likely an important competitor of juvenile sockeye salmon (Oncorhynchus nerka) since: (i) sockeye exhibit food limited growth and survival patterns in coastal lakes, (ii) mysids and sockeye consume similar zooplankton prey and (iii) mysids do not contribute greatly to sockeye diet (i.e. < 26% of summer and fall diets by numbers or weight).     

Contemporary state of the zooplankton in Lake Peipsi

L. Peipsi is one of the richest fish lakes in Europe. Planktivorous smelt dominates in the fish fauna. The abundance of zooplankton fluctuates between 43 600–2241 500 ind m^–3, with the average 974 000 ind m^–3, biomass ranges from 0,09–3,69 g m^–3, with the average 1,86 g m^–3. Since the 1960s the abundance of rotifers has risen considerably while the mean zooplankter weight (B/N) has decreased from 0.005 mg to 0.004 mg. Zooplankton production (herbivores 20.6, predators 1.8, whole zooplankton community 22.4 g C m^–2 per period between May and October) can be considered high. Predatory zooplankton eats on an average 50% of the production of herbivorous zooplankton; about 50% of the whole zooplankton production (P_{Filt + Pred}) reaches fishes. The production of herbivorous zooplankton constitutes 10.1% of primary production. This ratio indicates a direct relationship between zoo- and phytoplankton in the food chain; the detrital food chain seems of little importance. About 6% of phytoplankton energy reaches fishes. The transformation of energy in the food web is efficient. On the basis of zooplankton L. Peipsi can be considered a moderately eutrophic or meso-eutrophic lake.     

A Short-Term Study of Vertical and Horizontal Distribution of Zooplankton During Thermal Stratification in Lake Kinneret,Israel

This study documents for the first time both vertical and horizontal distribution patterns of the zooplankton community in Lake Kinneret during the period of thermal stratification. The zooplankton distribution patterns were explored in relation to abiotic (temperature, oxygen) and biotic (picocyanobacteria, ciliates, flagellates, phytoplankton, fish) environmental gradients. Sampling was carried out on 6–7 July 1992 at five stations and six depths from nearshore to offshore. Zooplankton abundance and biomass varied from 5 to 267 ind. l^–1(mean: 95 ind. l^–1), and from 0.1 to 65 d.w. mg m^–3(mean: 24 d.w. mg m^–3). Zooplankton taxonomic groups (Rotifera, Cladocera, Cyclopoida, Calanoida) and size classes (micro-, meso- and macrozooplankton) showed peaks of maximal density and biomass in the epilimnetic and metalimnetic strata (5 and 14 m). Depth, accounting for 31–39% of total spatial variation, reflected the vertical distribution of zooplankton in relation to temperature and oxygen declines, and the higher concentration of food resources (protists and phytoplankton) in the epilimnion and metalimnion. Onshore–offshore distance, accounting for 17–22% of the total spatial variance, reflected different distribution patterns shown among zooplankton groups and size classes. The macrozooplankton (Copepoda, Cladocera) was more abundant offshore, whereas microzooplankton (Rotifera and nauplii) predominated nearshore. These horizontal distribution patterns were related to small increases in temperature and phytoplankton biomass, and higher concentrations of fish in the littoral zone. Although limited to a short temporal scale, our study indicated that zooplankton spatial distribution in Lake Kinneret during the period of thermal stratification was related to physicochemical, food and predation factors, manifested differently along the vertical and nearshore–offshore gradients.     

The zooplankton of Lake Mikri Prespa

A study from June 1990 to October 1992 revealed thatthe pelagic zooplankton community of Lake Mikri Prespaconsists of 45 invertebrate species (28 Rotifera, 11Cladocera, 6 Copepoda and 1 mollusc). It includes A. steindachneri, an endemic calanoid of the WesternBalkans. Half of the species are first records for thelake.The total abundance and biomass ranged from 61 to 905 indl^–1 and 58 to 646 µg l^–1,respectively. Seasonal fluctuations agreed with thePEG-model. Copepods prevailed throughout the year. Thespecies composition and seasonal variability wereaffected by temperature-dependent food conditions,competition and predation. These factors induced aseasonal succession which involved a replacement oflarger species or individuals by smaller ones duringsummer. The cladoceran population, as a whole, wascharacterised by small size classes of the speciespresent, probably due to fish predation. Anexamination of the horizontal distribution showedthat, although seasonal variability was the same atall stations sampled, the central station showedhigher abundance. Inshore-offshore diel migrations ofjuvenile fish is considered to account for this unevendistribution. Shannons diversity index ranged from0.98 to 2.95 and evenness from 0.3 to 0.9. Both wereinfluenced by temperature.     

Impact of submerged macrophytes on fish-zooplanl phytoplankton interactions: large-scale enclosure experiments in a shallow eutrophic lake

1. The impact of changes in submerged macrophyte abundance on fish-zooplankton-phytoplankton interactions was studied in eighteen large-scale (100 m²) enclosures in a shallow eutrophic take. The submerged macrophytes comprised Potamategon pectinatus L., P. pusillus L. and Callitriche hermaphroditica L. while the fish fry stock comprised three-spined sticklebacks, Gasterosteus acuteatus L., and roach, Rutilus rutilus L. 2. In the absence of macrophytes zooplankton biomass was low and dominated by cyclopoid copepods regardless of fish density, while the phytoplankton biovolume was high (up to 38 mm³1) and dominated by small pennate diatoms and chlorococcales. When the lake volume infested by submerged macrophytes (PVI) exceeded 15–20% and the fish density was below a catch per unit effort (CPUE) of 10 (approx. 2 fry m^?2), planktonic cladoceran biomass was high and dominated by relatively large-sized specimens, while the phytoplankton biovolume was low and dominated by small fast-growing flagellates. At higher fish densities, zooplankton biomass and average biomass of cladocerans decreased and a shift to cyclopoids occurred, while phytoplankton biovolume increased markedly and became dominated by cyanophytes and dinoflagellates. 3. Stepwise multiple linear regressions on log-transformed data revealed that the biomass of Daphnia, Bosmina, Ceriodaphmia and Chydorus were all significantly positively related to PVI and negatively to the abundance of fish or PVI x fish. The average individual biomass of cladocerans was negatively related to fish, but unrelated to PVI. Calculated zooplankton grazing pressure on phytoplankton was positively related to PVI and negatively to PVI x fish. Accordingly the phytoplankton biovolume was negatively related to PVI and to PVI x zooplankton biomass. Cyanophytes and chryptophytes (% of biomass) were positively and Chlorococcales and diatoms negatively related to PVI, while cyanophytes and Chlorococcales were negatively related to PVI x zooplankton biomass. In contrast diatoms and cryptophytes were positively related to the zooplankton biomass or PVI x zooplankton. 4. The results suggest that fish predation has less impact on the zooplankton community in the more structured environment of macrophyte beds, particularly when the PVI exceeds 15–20%. They further suggest that the refuge capacity of macrophytes decreases markedly with increasing fish density (in our study above approximately 10 CPUE). Provided that the density of planktivorous fish is not high, even small improvements in submerged macrophyte abundance may have a substantial positive impact on the zooplankton, leading to a lower phytoplankton biovolume and higher water transparency. However, at high fish densities the refuge effect seems low and no major zooplankton mediated effects of enhanced growth of macrophytes are to be expected.     

Diel vertical and horizontal distribution of crustacean zooplankton and young of the year fish in a sub-alpine lake: an approach based on high frequency sampling

Understanding the spatial dynamics of predators and their preyis one of the most important goals in aquatic ecology. We studiedspatial and temporal onshore–offshore distribution patternsin young of the year (YOY) Eurasian perch (Perca fluviatilis)and crustacean zooplankton (Daphnia hyalina, Cyclops prealpinus)along a transect in Lake Annecy (France). Our study representsa first attempt at coupling hydroacoustic fish survey and highfrequency zooplankton recording to assess simultaneously thelarge-scale distribution patterns of YOY fish and their zooplanktonprey over a diel cycle (day, dusk and night sampling). We hypothesizedthat the spatial distribution of zooplankton could be shapedby both anti-predator behaviour (horizontal and vertical migrations)and predation losses. Fish biomass, size structure and dietwere assessed from split-beam echosounding and net trawlingsamples, whereas crustacean abundances were estimated with asmall modified Longhurst–Hardy continuous plankton recorder.We evaluated the diel changes in the spatial distribution patternsof fish and zooplankton and determined the overlap between theirdistributions. Fish biomass was dominated by YOY perch in upperwarmer layers and salmonids (Coregonus lavaretus and Salvelinusalpinus) in the colder and oxygenated deep layers. YOY perchwere aggregated in dense schools in the epilimnion during theday and dispersed at night. Fish biomass was distributed alonga strong increasing onshore–offshore gradient at night,whereas crustacean prey showed a decreasing gradient. This onshore–offshorenegative gradient in crustacean distribution, expressed on ashorter scale during the day, shifted toward the surface watersat night. A distinct kinetic of diel vertical migration (DVM)patterns was exhibited by daphnid and cyclopoid populationsand resulted in distinct vulnerability to perch predation. Spatio-temporaldistribution of crustaceans in Lake Annecy during the diel cyclestudy was probably shaped both by predation loss to YOY perchand by anti-predator behaviour (DVM, DHM) by zooplankton. Theimplications for fine-scale studies of fish-zooplankton interactionsare discussed.     

马鞍列岛多种生境中鱼类群聚的昼夜变化

为了解岛礁水域鱼类群聚的昼夜变化特征,以便更全面地设计采样方法和掌握采样的时间尺度,于2009年9月对马鞍列岛7种生境进行了共计24网次的刺网昼夜采样,结合排序和聚类方法,从种类组成、相对生物量和丰度、种类丰富度、多样性和相似性等方面对研究海域鱼类群聚特征的昼夜变化作了探讨.在采获的55种鱼类中,昼夜出现的分别为41和46种,数量差别不大,但其昼夜组成却随栖息水层的变化而不同,底层鱼类更趋向于夜间在硬相生境集群活动;近底层鱼类的昼夜集群随生境变化而变化,在同一生境中既有偏向白天也有趋向夜间的;中上层鱼类更多地出现在白天的人工生境(AH).AH白天的丰度渔获率显著大于晚上,而天然生境(NH)昼夜差别不大;生物量渔获率无论NH还是AH皆无显著昼夜差异.具体到种类,仅有小黄鱼Larimichthys polyactis和赤鼻棱鳗Thryssa kammalensis等少数种类的数量在AH有显著的昼夜差别,其他多数种类虽然昼夜的出现率大多有别,但渔获率昼夜差异皆不明显.多样性差异更多的表现在不同生境之间,而同一生境的昼夜差异往往不甚显著.各个生境中鱼类的昼夜种类交替现象非常明显,形成了以褐菖(鲐)Sebastiscus marmoratus和鳗鲇Plotosus anguillaris为代表的夜间优势类群为主的硬相生境群聚格局、以丝背细鳞鲀Stephanolepis cirrhifer和细刺鱼Microcanthus strigatus为代表的白天优势类群为主的硬相生境群聚格局以及缺乏底层优势类群、以石首鱼科鱼类为代表的近底层鱼类为绝对优势类群的软相生境群聚格局.因此,采用被动性渔具在近岸典型生境进行鱼类等相关生物调查时,应使采样时间覆盖昼夜两个时段,且至少保证24h.     

Horizontal distribution of pelagic zooplankton in relation to predation gradients

An investigation was conducted examining the horizontal and vertical distribution of zooplankton in Lake Miramar, a southern California reservoir. Daphnia and Mesocyclops populations were most abundant offshore and in deeper water during the day but appeared to move toward shore and upward at night. The results of inshore zooplankton sampling provided no evidence chat the diel horizontal migration pattern was a result of sampler avoidance by zooplankton. Inshore-offshore differences in Daphnia and Mesocyclops abundance and diel migrations were reduced during winter and early spring. Rotifer zooplankters exhibited less seasonal variation in their horizontal distributions than did the large crustacean zooplankters at all times of the year. It is hypothesized that the spatial distribution of zooplankton is related to predation gradients in Lake Miramar. The dominant planktivore in the reservoir, young-of-the-year Micropterus salmoides. was abundant from late May through December and much less so from January to early May. They were largely restricted to the littoral zone and this produced horizontal gradients of planktivory which varied in strength seasonally and from day lo night. It appears that crustacean zooplankton in Lake Miramar avoid areas with abundant planktivores during the day but migrate into these areas at night when the intensity of planktivory is reduced. Rotifers exhibit less horizontal heterogeneity and no significant diel migrations, which is attributed to the reduced risk of predation that rotifers experience relative to crustacean zooplankters. A graphical model is proposed to integrate our understanding of diel vertical and horizontal migrations of zooplankton. In this model, gradients of predation are completely vertical in offshore areas and strongly horizontal in near shore areas. Gradients of food availability are roughly similar to those of predation intensity. Plankiers respond to these gradients by migrating in a path parallel to gradients of predation at dawn and parallel to gradients of food availability after dark.     

Spatial and seasonal variation of the zooplankton in the coastal zone and main khors of Lake Nasser (Egypt)

A prominent feature of Lake Nasser is the presence of khors (dendritic side extensions). We studied the zooplankton of the larger khors and coastal zone of the main lake in 1996 and 1997, and found an assemblage of rotifers, cladocerans and copepods that was partly tropical, partly temperate, at relatively high biomasses. Spatial differences were weak, but the upstream khors (Toushka and Korosko) were consistently richer than the downstream khors (El-Ramla and Kalabsha), with a rather sudden transition around km 150 at El Madiq. Summer standing crops were higher than those in winter by a factor 2–3. The zooplankton of the littoral of the main channel showed the same spatial pattern as that in the khors, being more abundant in spring (average 82300 ind m^–3) than in autumn (average 72700 ind m^–3). Zooplankton dry weight increased from 4 g m^–2 at khor El-Ramla to 7 g m^–2 at khor Toushka. These rather high values had low variation. The number of species, diversity and evenness all showed a high degree of similarity among the khors and in the littoral of the main lake. The lake fish fauna is poor, lacking a pelagic planktivore. The predominance of medium-sized Copepoda (one calanoid, two cyclopoids) in the zooplankton suggests that fish predation is moderate. This is confirmed by the persistence of two Daphnia species at low abundance, although rather strong variations in time suggest that Cladocera suffer from summer predation by invertebrates (copepods) as well as vertebrates (mostly larval fish). Because the zooplankton is underutilised by higher trophic levels, we suggest to assess the benefits of introducing a pelagic zooplanktivorous fish.     

Predator-induced bottom-up effects in oligotrophic systems

Five treatments (replication n=2) were applied to mesocosms in an oligotrophic lake (TP=6–10 µg 1^-1) to assess the effects of fish on planktonic communities. The treatments were: (1) high fish (30 kg ha^–1 Lepomis auritus, Linnaeus), (2) low fish (10 kg ha^–1), (3) high removal of zooplankton, (4) low removal of zooplankton and (5) control. Total phosphorus, chlorophyll a, zooplankton biomass, and species richness decreased from high fish > low fish > control > low removal > high removal treatments. The fish treatments were dominated by crustacean zooplankton, while rotifers outnumbered the other zooplankters in the removal treatments. Calculations of zooplankton grazing rates suggested that clearance rates seldom exceeded 2% of the enclosure volume d^–1 and were unlikely to have had much influence on phytoplankton biomass. Calculations from a phosphorus bioenergetics model revealed that when fish were present, their excretion rates were higher than the rates ascribed to zooplankton. Diet analysis showed that the fish derived most of their energy from the benthos and periphyton, and that fish excretion and egestion made significant contributions to the very oligotrophic pelagic phosphorus pool. In the absence of fish, zooplankton excretion was highest in the control treatments and lowest in the zooplankton removal treatments. Our results suggest that in oligotrophic systems, planktivorous fish can be significant sources of phosphorus and that fish and zooplankton induced nutrient cycling have significant impacts on planktonic community structure.     

Near-bottom depletion of zooplankton over coral reefs: III: vertical gradient of predation pressure

Many diurnal planktivorous fish in coral reefs efficiently consume zooplankton drifting in the overlying water column. Our survey, carried out at two coral reefs in the Red Sea, showed that most of the diurnal planktivorous fish foraged near the bottom, close to the shelters from piscivores. The planktivorous fish were order of magnitude more abundant near (<1.5 m) the bottom than higher in the water column. The predation pressure exerted by these fish was assessed by measuring the consumption of brine shrimps tethered at different heights above the bottom on a vertical line which was pulled over the reef. Below 1.5 m above bottom, the shrimps survival probability sharply decreased toward the bottom. Higher in the water column, survivorship was nearly 100% with little vertical variation. Our results indicate that near-bottom depletion of zooplankton in coral reefs is likely due to intense predation at that boundary layer. Risk of predation by piscivorous fish apparently restricts planktivorous fish to forage near the bottom, with a distribution pattern greatly deviating from ideal-free distribution.     

Strength of coupling between phyto- and zooplankton in Lake Lucerne (Switzerland) during phosphorus abatement subsequent to a weak eutrophication

The strength of coupling between phyto- and zooplankton was measured from 1961 to 1995 by comparing the grazing effect of zooplankton (visible as clear-water phase only in 1968-1994) and also by excluding zooplankton in limnocorral experiments (1980-1984). Although long-term (1961-1995) measurements show little evidence of temporal changes in total biomass of phytoplankton or zooplankton, there is strong evidence of changes in the strength of coupling due to top-down effects. The ratio of change in biomass caused by cladocerans in the intensive grazing period of each year (May/June) and the recovery of netplankton after this period seems to be strongly influenced by the trophic state of the lake. When Lake Lucerne was mesotrophic (1971-1982), the annual mean of monthly changes in phytoplankton biomass was in the range of 1-2, indicating that the biomass more than doubled (or halved) from month to month (no change = 0). Under oligotrophic conditions, the annual average of monthly changes in biomass was below 0.5. Grazing measurements in limnocorrals at 2 m depth with labelled food (Rhodomonas lacustris) showed distinct diel rhythms, with maximum community grazing rate at dusk and dawn. These diel changes were caused by vertical migration of the zooplankton. Grazing rate and zooplankton biomass were strongly coupled, with a maximum rate of 100-200 ml day^-1 mg^-1 (zooplankton biomass) when daphnids were dominant. The decrease in biomass caused by excessive grazing shows parallel trends in nanoplankton and netplankton. However, the increase in biomass after the clear-water phase was largely caused by netplankton.