Mortality and production of 0+ perch, Perca fluviatilis L., in two Scottish lakes

Mortality of 0+ Eurasian perch was measured directly from the decline in catch in a high-speed plankton sampler and described by a negative exponential model. A mean mortality of 0.063 day ¹ (range=0.020–0.089) was calculated; for 0+ juveniles it was 0.021 day ¹. Mean larval production was 0.85–1.30 mg dry weight m⁻² day⁻¹ in Loch Kinord and 0.08–0.12 in Loch Davan. Annual production of 0+ juveniles was 1.97–7.55 kg wet weight ha ⁻¹ at L. Kinord and 1.01 kg at L. Davan. The contribution of 0+ perch, including the egg stages, to total perch production was 88–96%. Availability of food items on transition to exogenous feeding and through the larval period did not limit survival. Cannibalism by adults accounted for most of larval mortality, and predation by pike and adult perch were responsible for the majority of juvenile losses. The population structure of adult perch, for 12 year classes, was stable at L. Kinord, while year classes at L. Davan exhibited fluctuations in abundance and in two sampling years were eliminated in the first summer. Survival in the eleutheroembryonic phase was variable at L. Davan and observed to be low compared with L. Kinord. It is suggested that this was a result of water turbulence on the exposed and shallow spawning areas at L. Davan.     

Annual changes in the nutritive state of North Sea dab

The nutritive state of dab Limanda limanda was investigated over a 2-year period at a fixed sampling site northwest of Helgoland (German Bight, North Sea), with respect to feeding habits and the accumulation of biochemical storage products. Ophiuroids formed the main weight of food organisms (50%) while polychaetes (10%), molluscs and crustaceans (<5% each) were less frequent. Feeding activity in males varied between summer and winter, while females fed more constantly. The condition factor and the hepatosomatic index showed characteristic seasonal cycles in both sexes. The glycogen content in the liver reached 40–60 mg g⁻¹ FW in summer and fell to about 10–20 mg g⁻¹ FW in late winter. Total lipids of the liver showed a distinct seasonal cycle with 400 mg g⁻¹ FW in summer and a minimum of 50–100 mg g⁻¹ FW in spring. The lipid content of the muscle ranged from 5 to 6 mg g⁻¹ FW and did not vary significantly between seasons.     

On the early growth of 0+ perch, Perca fluviatilis, in Windermere

SUMMARY. The growth in length and weight (wet and dry) of 0+ perch Perca fluviatilis during their first summer of life in Windermere, has been investigated. Two major stanzas, occurring during and after metamorphosis, characterize the growth of 0+ perch in Windermere. The change from one stanza to another takes place between six and eight weeks after hatching. The exponential rate of growth in length was 0.26 and 0.27 mm mm⁻¹ week⁻¹ for the first stanzas of 1975 and 1976, respectively, and decreased to about 0.08 and 0.07 mm mm⁻¹ week⁻¹ for the second stanzas of the same years. Similarly, the growth in both wet and dry weights decreased from the first to the second stanza. Growth of 0+ perch was found to be logistic, and could not be described by the von Bertalanffy growth curve. The relationship between weight and length was allometric and the power values for length were, for wet weight: 4.154 (in 1975) and 4.033 (in 1976) for the first stanza and 2.400 (in 1975) and 2.734 (in 1976) for the second stanza; for dry weight: 3.988 (1975) and 3.971 (1976) for the first stanza and 3.066 (1975) and 2.651 (1976) for the second stanza. Half of the total growth was completed in 47% (1975) and 40% (1976) of the total growth period to the end of the summer.     

Oxygen and nitrate respiration in a reed swamp sediment from a eutrophic lake

Seasonal measurements of the oxygen and nitrate uptake by a reed swamp sediment were carried out in a shallow, eutrophic Danish lake, Arreskov Sø. The oxidation of organic carbon in the sediment by aerobic and nitrate respiration was 290 and 188 g C m⁻² yr⁻¹ respectively. During winter, nitrate respiration amounted to 94% of the total carbon oxidation, whereas it was zero during summer. On an annual basis nitrate respiration constituted 39% of total respiration. Sediment nitrate uptake was correlated to nitrate concentration. In consequence of this the nitrate uptake rates varied during the year from zero in summer to 55 mg N m⁻² d⁻¹ in spring.
Oxygen uptake rates varied from 30 to 250 mg O₂ m⁻² h⁻¹ during the year, with a maximum uptake in August. The oxygen uptake per year was calculated to 860 g O₂ m⁻². The oxygen uptake rate was correlated to lake temperature and Kjeldahl nitrogen content of the sediment. The oxygen uptake rate, however, showed no correlation with loss on ignition of the sediment. A Q₁₀-value of 2.2 was found for lake measurements in the temperature interval of 5–15°C. The corresponding O₁₀-value in the laboratory was 2.6. A high microbial biomass indicated by the maximum content of Kjeldahl nitrogen and the lowest ratio of loss on ignition on Kjeldahl nitrogen appeared in late August, when the maximum oxygen uptake occurred. The oxygen uptake rate increased during the time interval from sampling to the start of the experiments.     

Biomass and oxygen dynamics of the epiphyte community in a Danish lowland stream

SUMMARY. 1. We examined the abundance and oxygen metabolism of epiphytic organisms on the dominant macrophyte, Potamogeton pectinatus , in headwaters of the eutrophic River Suså. Microbenthic algae were abundant in the stream during spring and macrophytes during summer.
2. The low macrophyte biomass in spring supported a dense epiphyte cover whereas the high macrophyte biomass during summer had a thin epiphyte cover of 10–100-fold lower abundance per unit area of macrophyte surface. The epiphyte community was dominated by microalgae in spring and by heterotrophs, probably bacteria, during summer. This seasonal shift was shown by pronounced reductions of the chlorophyll a content (from 2–3% to 0.1–0.7% of organic DW), the gross photosynthetic rate (from 20–85 to 3–15 mg O₂, g-¹ organic DW h⁻¹) and the ratio of gross photosynthesis to dark respiration in the epiphyte community (from 5–18 to 1). The reduced contributions of epiphytic microalgae correlated with reduced light availability during summer.
3. Both the density and the photosynthetic activity of epiphytic algae were low on a stream area basis relative to those of microbenthic algae and macrophytes. Rapid variations in water velocity and extensive light attenuation in water and macrophyte stands probably constrained the development of epiphytic algae. The epiphyte community was more important in overall stream respiration, contributing c. 10% to total summer respiration and c. 20% to summer respiration within the predominantly heterotrophic microbial communities on sediments and macrophyte surfaces.     

The occurrence and ecological importance of dissolved ATP in fresh water

SUMMARY. Dissolved ATP, defined as ATP which passes through 0.2 μm filters, was found in fresh water. During the spring diatom bloom in two eutrophic Danish lakes, concentrations of dissolved ATP varied between 0.1 and 3.8 μgl⁻¹, constituting 14–76% of the total ATP (particulate plus dissolved ATP). The kinetics of the light emission obtained from mixing firefly enzyme with dissolved ATP demonstrated that the major proportion of the dissolved ATP was in fact ATP. Despite some variations, the seasonal changes in dissolved ATP paralleled the changes in the increasing phytoplankton population during the rise of the diatom blooms. The dissolved ATP increased after the diatom peak, indicating that release of ATP from the phytoplankton due to mortality may be a major source of dissolved ATP.
Consumption of dissolved ATP was evaluated in uptake experiments using ³H-ATP. Rates of uptake of ³H-ATP by micro-organisms (diameter 0.2–0.6 μm) proved to be close to the rates for ³H-D-glucose uptake. The variations in ³H-ATP uptake during the diatom blooms showed non-systematic changes and ranged between 1.0 and 15.8% h⁻¹ (mean = 4.9% h⁻¹) of the quantity added. Turnover rates for dissolved ATP varied between 12 and 730 ng l⁻¹ h⁻¹ (mean = 175 ng l⁻¹). These rather high rates of turnover suggest that dissolved ATP is an important compound in the metabolism of freshwater bacteria.     

Phytoplankton production in relation to physico-chemical conditions in a small, oligotrophic subarctic lake in South Greenland

Phytoplankton production and physico-chemical parameters were measured during the summers of 1976 and 1977 in Lake 95 at 60°52'N, 46°01'W (area 8 ha, z 6.9 m). The duration of lake stratification during summer was weather dependent. Concentrations of total-CO₂ ranged from 2.04 mg l⁻¹ (surface) to 4.02 mg l⁻¹ (bottom during stratification). Nutrient concentrations were generally low, and systematic seasonal variation could not be detected with certainty. Vertical distribution of primary production typically showed a distinct maximum followed by a decline with depth. The low production, both per unit area (max. 60 mg C m⁻² d⁻¹) and per unit volume (max, 14.3 mg C m⁻³ d⁻¹), and the annual production of 4.7 g C m⁻² classified the lake as extremely oligotrophic.     

Predicting the growth of age-0 yellow perch populations from measures of whole-lake productivity

SUMMARY. 1. The relationship was examined between four measures of lake productivity [total phosphorus (TP) and chlorophyll a (Chl a) concentrations, zooplankton density and biomass] and growth in length and weight of age-0 yellow perch in ten central Alberta lakes.
2. In these lakes, average summer TP and Chl a levels were in the range 11–51 and 1.4–19.5 μg1⁻¹, respectively. The interaction of TP and Chl a could explain 61% and 57% of the variance in total length and wet weight, respectively, of age-0 yellow perch sampled at the end of August.
3. The ability to predict first year fish growth from lake productivity is strongest at low levels of productivity (TP<35μgl⁻¹). In the lakes studied, fish community structure is more complex at high levels of productivity (TP>35μg1⁻¹), and more data on complex community level interactions seem necessary to predict fish growth in these systems.     

Phytoplankton ecology in an Antarctic lake

SUMMARY. The ecology of the phytoplankton of Heywood Lake, Signy Island, South Orkney Islands, Antarctica was investigated during 1969–72. The lake, which is ice-covered for 8–10 months per year, is moderately eutrophic due to enrichment by seal excreta.
The annual cycle of the phytoplankton is described. During the winter (approximately May-September), very few algal cells could be detected in the water column and ¹⁴C fixation was below measurable limits. In spring (October-November), a rapidly-growing population of algae caused a large increase in the chlorophyll- a concentration (maximum value 170 mg m⁻²) but carbon fixation remained low, with values <500 mg C m⁻² day⁻¹. The algae contributing to this peak were mainly small chlorophytes and chrysophytes. The summer open-water period (December-March) was characterized by a different phytoplankton population dominated by cryptophytes. Chlorophyll levels were lower ( c . 40 mg m⁻²) but ¹⁴C fixation rates >3 g C m⁻² day⁻¹ were measured on bright days. Values for Assimilation Number were very high (maximum value 10.5 mg C h⁻¹ mg⁻¹ (chlorophyll- a ) in January (1971) though temperatures never exceeded 8°C. In autumn, the phytoplankton regressed to winter levels. Both spring and summer algal populations probably overwinter as resting stages.     

The muscle twitch and the maximum swimming speed of giant bluefin tuna, Thunnus thynnus L.

Sustained swimming of bluefin tuna was analysed from video recordings made of a captive patrolling fish school [lengths (L) 1.7–3.3 m, body mass (M) 54–433 kg]. Speeds ranged from 0.6 to 1.2 L s⁻¹ (86–260 km day⁻¹) while stride length during steady speed swimming varied between 0.54 and 0.93 L. Maximum swimming speed was estimated by measuring twitch contraction of the anaerobic swimming muscle in pithed fish 5 min after death. Muscle contraction time increased from the shortest just behind the head (30–50 ms at 20% L) to the longest at the tail peduncle (80–90 ms at 80% L) (all at 28°C). A fish (L = 2.26 m) with a muscle contraction time of 50 ms at 25% L can have a maximum tail beat frequency of 10 Hz and maximum swimming speed of 15m s⁻¹ (54km h⁻¹) with a stride length of 0.65L. With a stride length of 1 L a speed of 22.6 m s⁻¹ (81.4 km h⁻¹) is possible. Power used at maximum speed was estimated for this fish at between 10 and 40 kW, with corresponding values for the drag coefficient at a Reynolds number of 4.43 × 10⁷ of 0.0007 and 0.0027.     

Extracellular release of photosynthetic products by freshwater phytoplankton populations, with special reference to the algal species involved

SUMMARY. Over three successive years, depth profiles of C-fixation and excretion, chlorophyll- a concentrations, phytoplankton species composition and bacterial numbers were determined in Lake Vechten, a slightly eutrophic lake in The Netherlands. Special attention was given to the method used to measure extracellular release.
Excretion of dissolved organic ¹⁴C depended largely upon the photo-synthetic activity of the phytoplankton, ranging from 0–2.5 mg m^-1 h^-1, representing a percentage extracellular release (PER) of 0–25%.
During a period in August, however, a subsurface chlorophyll- a maximum at 5–7 m depth coincided with high excretion rates of up to 10 mg Cm^-3 h^-1 (PER = 55%). Phytoplankton analysis revealed a stratification in numbers of Mallomonas caudata with a maximum at 5–7 m depth.
The results suggest that in these water layers bacterial populations grew at the expense of the dissolved organic carbon compounds excreted by Mallomonas caudata. This means that extracellular release can temporarily function as an important nutrient source for the heterotrophie community in addition to the more or less constant dissolved organic carbon pool.     

Long-term effects of successive Ca(OH)2 and CaCO3 treatments on the water quality of two eutrophic hardwater lakes

1. Whole-lake experiments were conducted in two hardwater lakes (Halfmoon and Figure Eight) in Alberta, Canada, to investigate the effectiveness of repeated lime (slaked lime: Ca(OH)₂ and/or calcite: CaCO₃) treatments (5–78 mg L^–1) for up to 7 years.
2. Randomized intervention analysis of intersystem differences between the experimental and three reference lakes demonstrated a decline in euphotic total phosphorus and chlorophyll a concentrations in the experimental lakes after repeated lime treatments.
3. After the second lime application to Halfmoon Lake, mean winter total phosphorus release rates (TPRR) decreased to < 1 mg m^–2 day^–1 compared with 3.6 mg m^–2 day^–1 during the winter after initial treatment. In the final year of lime application, mean summer TPRR decreased to 4.5 mg m^–2 day^–1 compared with 7.6 mg m^–2 day^–1 in the pre-treatment year.
4. Mean macrophyte biomass declined and species composition was altered at 1 and 2 m depths in Figure Eight Lake during lime application. Over the first 6 years of treatment, macrophyte biomass at 2 m declined by 95% compared with concentrations recorded during the initial treatment year. In the last year of the study, macrophyte biomass at 2 m reached initial treatment concentrations, which coincided with the greatest water transparency. Over the treatment period, macrophyte species shifted from floating to rooted plants.
5. Multiple lime applications can improve water quality in eutrophic hardwater lakes for periods of up to 7 years.     

The nutrient budget of a small eutrophic loch and the effectiveness of straw bales in controlling algal blooms

1. A 1-year intensive study of nutrient flows in Airthrey Loch, central Scotland, a small eutrophic [mean total phosphorus (TP) = 61.1 μg P l^–1] well-flushed freshwater body (area, 6.9 ha; mean depth, 1.85 m; volume, 1.274 × 10⁵ m³; retention time, 0.44 yr), was undertaken.
2. The nutrient budget was dominated by large allochthonous P inputs, equivalent to an areal load to the loch of 8.56 kg P ha^–1 yr^–1, which occurred predominantly during winter. In the summer, when TP inputs were low, water column levels of TP still increased, as a result of aerobic P release from sediments.
3. Sorption experiments indicated potential for sediment P release at water P concentrations of up to 200 μg P l^–1.
4. Aerobic release rate of P from sediments to the water column of Airthrey Loch was estimated to be of the order of 1 mg P m^–2 day^–1, and occurred during periods of elevated water column pH.
5. Straw bales placed in the loch to retard algal blooms were found not to have any demonstrable impact on algal concentrations observed during the study.     

The nutrient budget of a small eutrophic loch and the effectiveness of straw bales in controlling algal blooms

1. A 1-year intensive study of nutrient flows in Airthrey Loch, central Scotland, a small eutrophic [mean total phosphorus (TP) = 61.1 μg P l^–1] well-flushed freshwater body (area, 6.9 ha; mean depth, 1.85 m; volume, 1.274 × 10⁵ m³; retention time, 0.44 yr), was undertaken.
2. The nutrient budget was dominated by large allochthonous P inputs, equivalent to an areal load to the loch of 8.56 kg P ha^–1 yr^–1, which occurred predominantly during winter. In the summer, when TP inputs were low, water column levels of TP still increased, as a result of aerobic P release from sediments.
3. Sorption experiments indicated potential for sediment P release at water P concentrations of up to 200 μg P l^–1.
4. Aerobic release rate of P from sediments to the water column of Airthrey Loch was estimated to be of the order of 1 mg P m^–2 day^–1, and occurred during periods of elevated water column pH.
5. Straw bales placed in the loch to retard algal blooms were found not to have any demonstrable impact on algal concentrations observed during the study.     

The seasonal dynamics and distribution of Chaoborus flavicans larvae in adjacent lake basins of different morphometry and degree of eutrophication

1.  Seasonal dynamics, spatial distribution and population size of the phantom midge Chaoborus flavicans in different parts of the eutrophic Lake Hiidenvesi (30.3 km²) were studied.
2.  Density of larvae was low in the shallow, most eutrophic parts of the lake, while the deep Kiihkelyksenselkä basin was inhabited by a dense population. In the deepest part of Kiihkelyksenselkä (33 m) density was 13 989 ± 3542 m^–2 in May, declined to 1102 ± 274 m^–2 in July and recovered to 7225 ± 1314 m^–2 by October. In spring and autumn the majority of larvae were benthic while, during high summer, few larvae were found in the sediment.
3.  Horizontal distribution fluctuated seasonally. On 3 June < 5% of the population inhabited areas shallower than 10 m. On 6 July the limnetic fraction was still restricted to regions deeper than 10 m, but 43% of benthic larvae were found between 6 and 10 m depths. In October both limnetic and benthic larvae were concentrated in areas deeper than 20 m.
4.  Within the lake, distribution was mainly regulated by stratification characteristics, degree of eutrophy being less important. The seasonal horizontal movements were probably induced by food shortage. Larvae could not meet their energetic demands in stratified areas and dispersed to shallower water, reducing predation risk by use of the benthic habitat.     

Patterns of dispersion, density and dispersal in alpine populations of the land snail Arianta arbustorum (L.) (Helicidae)

In studying populations of the land snail Arianta arbustorum in the eastern Swiss Alps, juvenile and subadult snails were found to be aggregated all the time, while adults were mostly aggregated during summer, the main mating period. The mean size of aggregations was 25 cm × 25 cm but varied, according to the area a grass tuft covered, while the intensity of aggregations was influenced by weather conditions. Densities of 10–15 subadults and 3–6 adults m⁻² were found along ditches and streams, 2–25 subadults and 2–20 adults on uncultivated alpine meadows, while on scree-covered alpine grassland the density of subadults and adults was 0.3–0.5 m⁻². The extent of daily movements varied with microclimatic factors and season and depended on the structure of the habitat. The dispersal rate was greatest in June (2.6 m month⁻¹) and least in August (1.1 m month⁻¹). In summer and autumn, active uphill movements were noted, a process which compensates for the loss of altitude by passive downhill displacements. Roiling down on snowfields and being carried away by avalanches and streams were observed.     

Gas exchange in intact isolated chloroplasts from Chlamydomonas reinhardtii during starch degradation in the dark

During starch degradation in intact isolated chloroplasts from Chlamydomonas reinhardtii gas exchange was studied with a mass spectrometer. Oxygen uptake by intact chloroplasts in the dark never exceeded 1.5% of the starch degradation rate [maximum 15 nmol O₂ (mg Chl)⁻¹ h⁻¹ consumed. 1 000 nmol glucose (mg Chl)⁻¹h⁻¹ degraded]. Evolution of CO₂ under aerobic conditions [9.8–28 nmol (mg Chl)⁻¹ h⁻¹] was stimulated by addition of 0.1–0.5 m M oxaloacetate [393–425 nmol CO₂ (mg Chl)⁻¹ h⁻¹]. Pyridoxal phosphate (5 m M ) inhibited starch degradation by more than 80%, but had no effect on O₂ uptake. Starch degradation rates and CO₂ evolution did not differ under acrobic and anaerobic conditions. Increasing P_i in the reaction medium from 0.5 m M to 5.0 m M stimulated starch degradation by 230 and 260% under aerobic and anaerobic conditions, respectively. A rapid autooxidation of reduced ferredoxin was observed in a reconstituted system consisting of purified Chlamydomonas ferredoxin, purified Chlamydomonas NADP-ferredoxin oxidoreductase (EC 1.6.7.1) and NADPH. Addition of isolated thylakoids from C. reinhardtii did not affect the rate of O₂ uptake. Our results clearly indicate the absence of any oxygen requirement during starch degradation in isolated chloroplasts.     

Recruitment control and production of market-size Oreochromis niloticus with the predator Lates niloticus L. in the Sudan

Experiments were conducted in earth ponds (242–1260m²) to evaluate the Nile perch (Lates niloticus L.) as a predator for recruitment control and the production of marketable Oreochromis niloticus L. in the Sudan. Supply of the predatory fish was maintained by induced spawning of L. niloticus brooders by raising the water level in a 1–0 ha earth pond. Lates niloticus fingerlings (7–5 cm total length) were then stocked with same size O. niloticus at the ratios 1:5, 1:10 and 1:15, Lates:Oreochromis , respectively. Over a period of 7 months Lates niloticus reduced young 50g) Oreochromis population and enhanced the production of preferred-size ( 200 g) Oreochromis. The ratio 1:5 Lates:Oreochromis was established to be the most desirable for Lates -with a total production of 0.2428 kg/m² of O. niloticus with 55.7% (by weight) at the target size averaging 287.5g. The ratios 1:10 and 1:15 produced 0.2106 and 0.2153 kg m² of O. niloticus , with 49.4% and 16.9% averaging 235.0 and 210.0 g, respectively.     

Planktic cyanophytes and their ecology in the shallow Lake Mikri Prespa, Greece

Cyanophytes dominated the phytoplankton of shallow Lake Mikri Prespa during the period May 1990 to September 1992 (76.5 and 52.0% of the total phytoplankton biomass in 1990–1991 and 1991–1992, respectively). Biomass peaks were observed in autumn (from 5.2 to 34.5 g m^-3) when low dissolved inorganic nitrogen, high phosphate phosphorus and low Z_cu to Z_mix ratio prevailed. The dominant species were Microcystis aeruginosa and M. wesenbergii. These represented 64 and 86% of the cyanophyte biomass in 1990–1991 and 1991–1992, respectively and revealed similar patterns of seasonality forming biomass peaks in late summer - autumn. Small chroococcalean cyanophytes (< 2 μm) showed also similar temporal distributions. Of the filamentous cyanophytes, the most important species was Anabaena lemmermannii var. minor which formed peaks in late summer and autumn (2.6 and 1.1 g m^-3 in 1990 and 1992, respectively). Rainfall and the N:P ratio were probably the main factors influencing the seasonality of all of the filamentous cyanophytes in the lake. Cyanophytes, tended to increase at temperatures higher than 16 °C and at inorganic nitrogen concentrations lower than 100 μg 1^--¹. The frequent mixing of the water column did not seem to prohibit the substantial increase of the group.     

Larval settlement of the green sea urchin, Strongylocentrotus droebachiensis, in the southern Gulf of Maine

Abstract. Settlement timing and differential settlement for the larval stage of the green sea urchin, Strongylocentrotus droebachiensis , in the southern Gulf of Maine was studied during the summer of 1996. Settlement densities on astroturf panels were highest in June and early July (13 to 37 m ⁻² d⁻¹), and peaked in mid-June (199 m⁻² d⁻¹). Settlement was low to nonexistent from mid-July through August (0 to 2 m⁻² d⁻¹). During the peak in settlement, no selection for substrate type was observed. In the remainder of the settlement period, differential settlement occurred, with a preference for substrate covered with live coralline algae. Test diameter of newly settled urchins varied among the substrates, with urchins settling on live coralline algae having the largest test diameter (0.43 ± 0.01 mm). There were no differences in test diameter among the different weeks in which sampling was done. Sustained onshore winds occurred only during peak settlement, suggesting that wind drift currents may concentrate larvae and influence patterns of larval settlement.