Vertical Distribution of Zooplankton in a Strongly Stratified Hypertrophic Lake

The vertical and temporal distribution of metazooplankton in the small hypertrophic, strongly stratified, temperate Lake Verevi (Estonia) was studied during 1998–2001. The zooplankton of Lake Verevi is characteristic of hypertrophic lakes, with a small number of dominant species, rotifers being the main ones, and juveniles prevailing among copepods. In 1999–2001, the average abundance of metazooplankton in the lake was 1570 × 10³ ind m⁻³; in the epilimnion 2320 × 10³ ind m⁻³, in the metalimnion 2178 × 10³ ind m⁻³, and in the hypolimnion 237 × 10³ ind m⁻³. The average biomass of metazooplankton was 1.75 g m⁻³; in the epi-, meta- and hypolimnion, accordingly, 2.16, 2.85 and 0.26 g m⁻³. The highest abundances – 19,136 × 10³ ind m⁻³ and 12,008 × 10³ ind m⁻³ – were registered in the lower half of the metalimnion in 24 May and 5 June 2001, respectively. Rotifer Keratella cochlearis f. typica (Gosse, 1851) was the dominating species in abundance. In biomass, Asplanchna priodonta Gosse, 1850, among the rotifers, and Eudiaptomus graciloides (Lilljeborg, 1888), among the copepods, dominated. According to the data from 2000–2001, the abundance and biomass of both copepods and rotifers were highest in spring. Zooplankton was scarce in the hypolimnion, and no peaks were observed there. During the summers of 1998 and 1999, when thermal stratification was particularly strong, zooplankton was the most abundant in the upper half of the metalimnion, and a distinct peak of biomass occurred in the second fourth of the metalimnion. Probably, the main factors affecting the vertical distribution of zooplankton in L. Verevi are fish, Chaoborus larvae, and chemocline, while food, like phytoplankton, composition and abundance may affect more the seasonal development of zooplankton.     

The Seasonal Dynamics and Trophic Relations of the Plankton Components in Lake Peipsi (Peipus)

The seasonal dynamics of the biomass and production of phyto-, zoo- and bacterioplankton was investigated during the vegetation periods (from May to November) in 1985 and 1986 in the pelagial of the large eutrophic lake Peipsi (Estonia). The average values of productions per vegetation period for the investigation years were as follows: phytoplanktion − 203.5 gC · m⁻²; bacterioplankton − 37.9 gC · m⁻²; filter-feeding zooplankton − 20.6 gC · m⁻² and predatory zooplankton − 1.5 gC · m⁻². The herbivorous zooplankton production constituted 10.1% of primary production. This ratio indicates a direct relationship between zoo- and phytoplankton in the food chain — filtrators are feeding mostly on living algae and the detrital food chain seems of little importance. The dominance of large forms (Melosira sp., Aphanothece saxicola), in the phytoplankton during the major part of the vegetation period is assumed to be a result of high grazing pressure on small algae. Zooplankton grazing was investigated in situ in a specially constructed twin bathometer. Experimental measurements revealed, that zooplanktion presence in the experimental vessel actually stimulated the phytoplankton growth in many cases — the negative grazing values have been registered. That could be caused by the stimulation effect of nutrients (N, P), excreted by the concentrated zooplankton in the grazing chamber, which led to an increase of the nongrazed phytoplankton production. Bacteria have satisfied the zooplankton food requirements on average by 11%. Grazing on bacteria increased, when grazing on phytoplankton was somehow disturbed.     

The role of cladocerans reflecting the trophic status of two large and shallow Estonian lakes

The role of pelagic cladoceran communities is discussed on the basis of a comparative study conducted in two Estonian lakes, the moderately eutrophic Lake Peipsi (N_tot 700, P_tot 40 μg l^?1 as average of ice-free period of 1997–2003) and in a strongly eutrophic Lake Võrtsjärv (N_tot 1600, P_tot 54 μg l^?1). The cladoceran community was found to reflect the differences in the trophic state of these lakes. In L. Peipsi, characteristic species of oligo-mesotrophic and eutrophic waters co-dominated (making up 20% or more of total zooplankton abundance or biomass), whereas in L. Võrtsjärv only species of eutrophic waters occurred. In L. Peipsi, the dominant cladocerans were Bosmina berolinensis and Daphnia galeata, while Chydorus sphaericus was the most abundant cladoceran in L. Võrtsjärv. The cladocerans of L. Peipsi (mean individual wet weight 25 μg) were significantly (threefold) larger than those of L. Võrtsjärv (8 μg). The mean wet biomass of cladocerans was higher and total cladoceran abundance was lower in L. Peipsi compared to L. Võrtsjärv (biomass varied from 0.133 to 1.570 g m^?3; mean value 0.800 g m^?3 in L. Peipsi and from 0.201 to 0.706 g m^?3, mean 0.400 g m^?3 in L. Võrtsjärv; the corresponding data for abundances were: 8,000–43,000 ind. m^?3, mean 30,000 ind. m^?3 for L. Peipsi, 50,000–100,000, mean 52,000 ind. m^?3 for L. Võrtsjärv). Based upon differences in body size, cladocerans were more effective transporters of energy in L. Peipsi than in L. Võrtsjärv. Cladocerans proved to be informative indicators of the trophic status and of the efficiency of the food web in studied lakes.     

Plankton and primary productivity of Lake Manzala,Egypt

Primary production and distribution and abundance of phyto- and zooplankton of lake Manzala were investigated from June 1985 to June 1986.Primary production varied from 4.1 to 28.7 g O₂ m^–2 d^–1 with the highest values 24.2 and 28.7 g O₂ m^–2 d^–1 recorded in the eastern and southern sectors and the lowest value 4.1 g O₂ m^–2 d^–1 recorded in the northern sector. The seasonal mean production for the lake was estimated at 13.3 g O₂ m^–2 d^–1.Mean phytoplankton abundance ranged from 32.7 10⁷ to 76.1 10⁷ cells m^–3 with a mean value of 48.10⁷ cells m^–3. Diatoms were the dominant phytoplankton group comprising 52 to 90 % by number. The greatest relative abundance (87 to 90%) was recorded in the southern sector.Mean zooplankton abundance ranged from 30.1 10³ to 44.4 10³ organisms m^–3 in the eastern sector to 5.5.10³ in the northern sector. In response of eutrophication, the species composition changed significantly over the last 20 years. Cladocerans represented less than 1% of zooplankton during 1959/60, but 75% in 1985/86. Rotifers constituted 40% in 1959/60, and only 1% in 1985/86. Cirriped larvae declined from 21% to 1%.     

Phytoplankton and zooplankton (Cladocera,Copepoda) relationship in the eutrophicated River Danube (Danubialia Hungarica,CXI)

The seasonal variation in primary production, individual numbers, and biomass of phyto- and zooplankton was studied in the River Danube in 1981. The secondary production of two dominant zooplankton species (Bosmina longirostris and Acanthocyclops robustus) was also estimated. In the growing season (April–Sept.) individual numbers dry weights and chlorophyll a contents of phytoplankton ranged between 30–90 × 10⁶ individuals, l^–1, 3–12 mg l^–1, and 50–170 µg l^–1, respectively. Species of Thalassiosiraceae (Bacillariophyta) dominated in the phytoplankton with a subdominance of Chlorococcales in summer. Individual numbers and dry weights of crustacean zooplankton ranged between 1400–6500 individuals m^–3, and 1.2–12 mg m^–3, respectively. The daily mean gross primary production was 970 mg C m^–3 d^–1, and the net production was 660 mg C m^–3 d^–1. Acanthocyclops robustus populations produced 0.2 mg C m^–3 d^–1 as an average, and Bosmina longirostris populations 0.07 mg C m^–3 d^–1. The ecological efficiency between phytoplankton and crustacean zooplankton was 0.03%.     

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.     

Can suspension feeding by bivalves regulate phytoplankton biomass in Lake Waccamaw, North Carolina?

Suspension feeding by bivalves has been hypothesized to control phytoplankton biomass in shallow aquatic ecosystems. Lake Waccamaw, North Carolina, USA is a shallow lake with a diverse bivalve assemblage and low to moderate phytoplankton biomass levels. Filtration and ingestion rates of two relatively abundant species in the lake, the endemic unionid, Elliptio waccamawensis, and an introduced species, Corbicula fluminea, were measured in experiments using natural phytoplankton for durations of 1 to 6 days. Measured filtration and ingestion rates averaged 1.78 and 1.121 ind.^–1 d^–1, much too low to control phytoplankton at the observed phytoplankton biomass levels and growth rates. Measured ingestion rates averaged 4.80 and 1.50 µg chlorophyll a ind.^–1 d^–1, too low to support individuals of either species. The abundance of benthic microalgae in Lake Waccamaw reaches 200 mg chlorophyll a m^–2 in the littoral zone and averages almost an order of magnitude higher than depth-integrated phytoplankton chlorophyll a. Total microalgal biomass in the lake is therefore not controlled by suspension feeding by bivalves.     

Trophic structure and productivity of the lagoonal communities of Tikehau atoll (Tuamotu Archipelago,French Polynesia)

Carbon standing stocks and fluxes were studied in the lagoon of Tikehau atoll (Tuamotu archipelago, French Polynesia), from 1983 to 1988.The average POC concentration (0.7–2000 µm) was 203 mg C m^–3. The suspended living carbon (31.6 mg C m^–3) was made up of bacteria (53%), phytoplankton < 5 µm (14.2%), phytoplankton > 5 µm (14.2%), nanozooplankton 5–35 µm (5.7%), microzooplankton 35–200 µm (4.7%) and mesozooplankton 200–2000 µm (7.9%). The microphytobenthos biomass was 480 mg C m^–2.Suspended detritus (84.4% of the total POC) did not originate from the reef flat but from lagoonal primary productions. Their sedimentation exceeded phytobenthos production.It was estimated that 50% of bacterial biomass was adsorbed on particles. the bacterial biomass dominance was explained by the utilisation of 1) DOC excreted by phytoplankton (44–175 mg C m^–2 day ^–1) and zooplankton (50 mg Cm^–2 day^–1)2) organic compounds produced by solar-induced photochemical reactions 3) coral mucus.50% of the phytoplankton biomass belongs to the < 5 µm fraction. This production (440 mg C m^–2 day^–1) exceeded phytobenthos production (250 mg C m^–2 day^–1) when the whole lagoon was considered.The zooplankton > 35 µm ingested 315 mg C m^–2 day^–1, made up of phytoplankton, nanozooplankton and detritus. Its production was 132 mg C m^–2 day^–1.     

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.     

The diversity,biomass and production of zooplankton in Lake Inarijärvi

About 650 zooplankton samples were collected from Lake Inarijärvi in 1977–1979 from the littoral and pelagial zones of the lake. One hundred and twenty-three zooplankton taxa were found and most of them can be considered euplanktonic.The most important species were Holopedium gibberum, Daphnia cristata, Cyclops spp. and Eudiaptomus spp. Mean pelagial zooplankton biomass was 0.29 g m^–3 in the 0–5 m depth zone, 0.17 g m^–3 in 5–10 m and 0.11 g m^–3 in 10–20 m.The zooplankton biomass at a sandy shore was about 0.09 g m^–3, at a stony shore 0.05 g m^–3 and at a vegetated shore 0.76 g m^–3. About 70% of the whole zooplankton production consisted of crustaceans.The sum of herbivore and carnivore zooplankton production in the pelagial area during the summer was 210–330 kg ha^–1 × 3 months.     

Primary production of phytoplankton and standing crops of zooplankton and zoobenthos in hypertrophic Lake Teganuma

Primary production of phytoplankton and standing crops of zooplankton and zoobenthos were intensively surveyed in Lake Teganuma during May 1983–April 1984. The annual mean chlorophyll a concentrations were as high as 304 µg · l^–1–383 µg · l^–1. The daily gross primary production of phytoplankton was high throughout the year. The peak production rate was recorded in August and September, when blue-green algae bloomed. The annual gross primary production was estimated as 1450 g C · m^–2 · y^–1, extremely high as compared with other temperate eutrophic lakes. Zooplankton was predominantly composed of rotifers. The annual mean standing crop of zooplankton was 0.182 g C · m^–2 around the middle between the inlets and the outlet and was lower than in most other temperate eutrophic lakes. Zoobenthos was mostly composed of Oligochaeta and chironomids. The annual mean standing crop of zoobenthos ranged from 0.052 g C · m^–2 to 0.265 g C · m^–2, the lowest values among temperate eutrophic lakes, which is in contrast to the high primary production.     

Impact of long-term alternations of discharge and spate on the chironomid community in the lowland Widawka River (Central Poland)

Spatial and temporal distribution, abundance and production of macroinvertebrate communities were estimated over two years in a fifth-order section of the Widawka River. Discharge of this river has been increased artificially by coal mine water inputs. Additionally, during the second year, one of the highest discharges of the current 20-year period was recorded. Chironomidae were co-dominant in macrobenthos, both in a straight reach (WIA) and in a meandering site (WIB). More mosaic habitats resulted in higher densities of midges, reaching 6215 ind.m^–2 in year 1 and 1141 ind.m^–2 in year 2 at WIA, while at WIB 896 densities were ind.m^–2 and 257 ind.m^–2, respectively. Flooding affected the distribution and abundance of the chironomid assemblages. Recolonization by psammophilous Polypedilum began after the various microhabitats were buried with sand. Chironomid production was estimated on a species-specific basis for the dominant taxa. In year 1 (mean annual water temperature 10.0° C) chironomid production was 12.4 g dry wt m^–2 yr^–1 1 at WIA and 1.9 g dry wt m^–2 yr^–1 at WIB. These values sharply decreased in year 2 (mean annual water temperature 9.8° C) reaching 1.9 g dry wt m^–2 yr^–1 at WIA and 0.4 g dry wt m^–2 yr^–1 at WIB, as effects of the high spate.     

Characteristics of plankton communities in Chinese integrated fish ponds: effects of excessive grazing by planktivorous carps on plankton communities

Biomass and production of plankton communities were investigated in two Chinese integrated fish culture ponds in August, Dianshanhu Pond (with high density of planktivorous carp) and Pingwang Pond (with low density of planktivorous carp). The plankton communities were composed of rotifers, protozoans, phytoplankton (<40 µm) and bacteria. The large phytoplankton (>40 µm), cladocerans and copepods were rare because of grazing pressure by the carp. The density or biomass of bacteria (1.93 × 10⁷ and 2.20 × 10⁷ cells ml^–1 on average in Dianshanhu and Pingwang Ponds, respectively), picophytoplankton (24.6 and 18.5 mg m^–3 Chla on average) and rotifers (5372 and 20733 ind. 1^–1 on average) exceeded the maximum values reported for natural waters.The average [³H]thymidine uptake rates were 694 and 904 pmoles 1^–1 h^–1 (13.4 and 20.6 µgC 1^–1) and the bacterial production by the >2 µm fraction amounted 21–28% of total [³H] thymidine uptake rate in both ponds. The mean chlorophylla concentrations were 59.1 and 183 mg m^–3 in Dianshanhu and Pingwang Ponds, respectively. 82.4% and 65.3% of the total Chla was contributed by the <10 µm nano- and picophytoplankton in each pond, respectively. In particular, the picophytoplankton contribution amounted 41.2% of thtal Chla in Dianshanhu Pond. Primary production was 2.5 and 3.4 gC m^–2 d^–1 in each pond, respectively, and >50% of production was contributed by picophytoplankton. The mean biomasses of protozoa were 168 µg 1^–1 and 445 µg 1^–1 and those of rotifers were 763 µg 1^–1 and 1186 µg 1^–1 in Dianshanhu and Pingwang Ponds, respectively. The ecological efficiencies expressed in terms of the ratios of primary production to zooplankton production were 0.22 and 0.31, for the two ponds.     

The dynamics and role of limnetic zooplankton in Loosdrecht lakes (The Netherlands)

The paper summarizes the results of a ten-year (1981–1991) zooplankton research on the Lake Loosdrecht, a highly eutrophic lake. The main cause of the lake's eutrophication and deteriorating water quality was supply up to mid 1984 of water from the River Vecht. This supply was replaced by dephosphorized water from the Amsterdam-Rhine Canal in 1984. The effects of this and other restoration measures on the lake's ecosystem were studied. Despite a reduction in the external P-load from ca. 1.0 g P m^–2 y^–1 to ca. 0.35 g m^–2 y^–1 now, the filamentous prokaryotes, including cyanobacteria and Prochlorothrix, continue to dominate the phytoplankton.Among the crustacean plankton Bosmina spp, Chydorus sp. and three species of cyclopoid copepods and their nauplii are quite common. Though there was no major change in the composition of abundant species, Daphnia cucullata, which is the only daphnid in these lakes, became virtually extinct since 1989. Among about 20 genera and 40 species of rotifers the important ones are: Anuraeopsis fissa, Keratella cochlearis, Filinia longiseta and Polyarthra. The rotifers usually peak in mid-summer following the crustacean peak in spring. The mean annual densities of crustaceans decreased during 1988–1991. Whereas seston (< 150 µm) mean mass in the lake increased since 1983 by 20–60%, zooplankton (> 150 µm) mass decreased by 15–35%.The grazing by crustacean community, which was attributable mainly to Bosmina, had mean rates between 10 and 25% d^–1. Between 42 and 47% of the food ingested was assimilated. In spring and early summer when both rotifers and crustaceans have their maximal densities the clearance rates of the rotifers were much higher. Based on C/P ratios, the zooplankton (> 150 µm) mass contained 2.5 times more phosphorus than seston (< 150 µm) mass so that the zooplankton comprised 12.5 % of the total-P in total particulate matter in the open water, compared with only 4.5% of the total particulate C. The mean excretion rates of P by zooplankton varied narrowly between 1.5 and 1.8 µg P 1^– d^–1, which equalled between 14 and 28% d^–1 of the P needed for phytoplankton production.The lack of response to restoration measures cannot be ascribed to one single factor. Apparently, the external P-loading is still not low enough and internal P-loading, though low, may be still high enough to sustain high seston levels. Intensive predation by bream is perhaps more important than food quality (high concentrations of filamentous cyanobacteria) in depressing the development of large-bodied zooplankton grazers, e.g. Daphnia. This may also contribute to resistance of the lake's ecosystem to respond to rehabilitation measures.     

Seasonal and annual variations in biological regime of lakes in an ultracontinental climate (Trans-Baikal Region of U.S.S.R)

Summary The lakes are covered 7 months with ice, but under the transparent ice (up to 180 cm thickness) a rich vegetation of phytoplankton, phytomicrobenthos and macrophyta develop and activate the bacterial and animal population. Winter production of the phytoplankton reaches 36 g/m² C and that of the phytomicrobenthos 70 g/m² C.The water levels of the lakes show fluctuations with an amplitude of 2–4 m, affecting the whole trophic system inclusive species composition, proportion and abundance of individual aquatic organisms as well as related abiotic conditions.Co-authors: E. I. Bondereva, T. N. Morozova, (primary production), A. A. Topolov, K. A. Shishkina (microbiology), V. P. Gorlachov (zooplankton), I. M. Shapovalova (zoobenthos), N. M. Pronin (fish and their parasites), V. N. Kuzmich (nutrition of fish).Co-authors: E. I. Bondereva, T. N. Morozova, (primary production), A. A. Topolov, K. A. Shishkina (microbiology), V. P. Gorlachov (zooplankton), I. M. Shapovalova (zoobenthos), N. M. Pronin (fish and their parasites), V. N. Kuzmich (nutrition of fish).     

Predator-prey relationships within the pelagic community of Neusiedler See

Neusiedler See, a shallow alkaline lake, has become increasingly eutrophic; this enrichment improved the nutritive situation of the herbivorous zooplankton leading to a higher standing stock. A multiple regression analysis of the long-term development of the crustacean plankton indicates that abiotic factors (i.e. wind, temperature) have the most important impact on the community in spring and autumn, biotic factors (i.e. food, predation) during the summer months. Currently an invertebrate (Leptodora kindti) and two planktivorous fish (Pelecus cultratus and Alburnus alburnus) control the population development of Diaphanosoma mongolianum during summer. L. kindti predation acts on immature stages, whereas the fishes consume adult stages. L. kindti densities of 100 to 200 ind. × m^-3 affect the prey population to a negligible extent; densities between 300 and 500 ind. × m^-3 result in elimination rates of 5% to >40% of the juvenile Diaphanosoma stock. The impacts by the invertebrate predator are pronounced but short-term events. Juvenile and underyearling fish eliminate 4–13% of the zooplankton in the open lake and 5–33% inshore; their predation pressure acts on all younger stages of the crustaceans. Planktivorous fish older than 0 + concentrate on the large food items (adult stages of the crustaceans). A. alburnus is able to consume 6–16% of the zooplankton standing stock during summer. P. cultratus eliminates about 1–49% of D. mongolianum, 1–4% of A. spinosus and 1–31% of L. kindti. From May until August the position of the dominant predator changes, beginning with juvenile fish which are then followed by P. cultratus and then by A. alburnus. Predation by L. kindti is of importance in July and August; at that time it causes suppression of the juveniles of D. mongolianum, but, on the other hand, the older stages of L. kindti are endangered themselves by the predatory impact of planktivorous fish.     

The productivity and carbon budget of a natural population of Daphnia lumholtzi Sars

D. lumholtzi in Lake Samsonvale, Queensland, Australia, is a small species (max. size approx. 7 µgC) that occurs in low abundance (max. abundance 6400 m^–3), with an average daily biomass of 3.32 mgC m^–3. Its annual rates of carbon assimilation, production and respiration, are 166, 110, and 56 mgC m^–3 y^–1 respectively. Annual biomass turnover (annual production/average daily biomass) is 33 and production efficiency is 50–66%. The population may consume 1.65–2.20 mgC m^–3 daily, equivalent to about 1% of the average daily standing crop of phytoplankton. Clutch size is small, 2 eggs, but represents 30–80% of a female's weight. A female may only produce 8–10 offspring in a full lifespan, nevertheless egg production may account for 56% of total production. The population shows autumn and spring peaks in abundance, and is believed to oversummer (4 months) as ephippia.     

Production of a fish taxocene in a Texas pond

Synopsis Production rates of seven species of fishes were determined for one year in a 0.95 ha pond. Total annual production of all fishes in the pond was 209 g m^–2y^–1 (after the subtraction of 16.2 g m^–2y^–1 of negative production). Over 90% of the production occurred in the months of June through September, with young-of-the-year fishes contributing 84% of the total. When Y of Y fishes shifted to benthic diets in the spring, the small biomass of benthos available could not sustain the large biomas of fish. Severe competition for the available food then resulted in decreased production rates for the entire fish taxocene.     

Secondary production of the brackish copepod communities and their contribution to the carbon fluxes in the Westerschelde estuary (The Netherlands)

The zooplankton community of the brackish part of the Westerschelde estuary (November 1989–October 1990) was dominated by two calanoid copepods, Eurytemora affinis and Acartia tonsa. Eurytemora was present during a longer period of the year and was much more important in terms of total abundances and biomasses than Acartia.The secondary production of these species was estimated by means of the growth rate method, using weight-specific growth rates obtained from laboratory cultures (Eurytemora) or from the literature (Acartia).Due to the substantially higher growth rates of Acartia compared to Eurytemora, total yearly productions of both communities were comparable, notwithstanding the large discrepancies in biomass. They amounted to about 5 and 6 g C m^–2 y^–1 by Acartia and Eurytemora respectively.The food needed to realise this production was estimated to be about 14 and 17 g C m^–2 y^–1 by Acartia and Eurytemora respectively. Provided that the copepods are able to selectively ingest the phytoplankton, in situ net production provides sufficient carbon for zooplankton demands for a short period of the year only. As phytoplankton standing stock is very low and net phytoplankton productivity is negative from late fall to early spring, nutritional demands of the copepods have to be fulfilled by other than algal food at least during this period of the year.Although the copepods in the brackish part can have an important impact on some food items, their contribution to total carbon fluxes in the brackish zone is negligible: each year some 6% of all consumed carbon in the brackish part of the estuary passes through the copepod food web.     

Life-history strategies and production of Tinodes waeneri (L.) (Trichoptera) in Lake Esrom,Denmark

The life cycle and production of Tinodes waeneri (L.) was examined at five shallow littoral localities in Lake Esrom, Denmark, during 1979/80. Five larval instars were demonstrated. The differences in temporal composition of instars indicated three different life cycles. A bivoltine strategy was shown for the most exposed locality which also had the highest average abundance equal to 11 500 ind. m^–2. A univoltine population was found in the more sheltered NW part of the lake with an average density of 1 500 ind. m^–2. At the three remaining stations the life cycles were partly bivoltine with wintering populations of mixed cohorts and densities between 10 500 and 3 000 ind. m^–3. Two distinct flight periods with maxima in June and August were demonstrated for bi- and partly bivoltine populations. Differences in rates of growth and elimination added to the variability in life cycle. Growth rates peaked in August (5.0–7.4% d^–1) at an average temperature of 20°C. Estimates of production ranged from 1.9 to 17.5 g AFDW m^–2 a^–1 with an average of 8.0 g AFDW m^–2. The P/B ratios, which were within the expected range for uni- and bivoltine populations of trichopterans, were 5.73 for the bivoltine population and 3.31 for the univoltine population.Estimates of consumption revealed that the populations could be sustained by autochthonous epilithic production in the habitats. Variability in the quality and seasonal availability of food were judged to influence the type of life cycle.Lake Esrom Littoral Research Publ. No. 8. Publication No. 385 from the Freshwater Biological Laboratory, University of Copenhagen, 51, Helsingørsgade, DK-3400 Hillerød, Denmark.Lake Esrom Littoral Research Publ. No. 8. Publication No. 385 from the Freshwater Biological Laboratory, University of Copenhagen, 51, Helsingørsgade, DK-3400 Hillerød, Denmark.