Effect of eland density and foraging on Combretum apiculatum physiognomy in a semi-arid savannah

We studied the impacts of eland density on the physiognomy of Combretum apiculatum . We divided the study area into two zones: high eland density zone (≤8 km from watering point) and low eland density zone (≥8 km from watering point). Eland density was determined in each zone using walking transects. Combretum apiculatum height and diameter and levels of eland damage were measured in each zone. Eland density was 1.92 and 0.86 km⁻² for high and low-density zones respectively. We recorded 239 C. apiculatum trees, 138 in low-density zone and 101 in high-density zone. Mean C. apiculatum height was 2.88 ± 0.67 m and 5.05 ± 1.04 m for high and low eland density zones respectively. High eland density prevented the recruitment of C. apiculatum from the 2.6–5.5 m height class to the >5.6 m height class. Although extensive tree damage occurred at <2.5 m height stratum, C. apiculatum showed resilience as recruitment into the 2.6–5.5 m height class continued. We concluded that high eland density prevents recruitment of C. apiculatum to higher height classes while at the same time causing extensive tree damage at lower height strata.     

How does the vertical distribution of chlorophyll vary in littoral sediments of small lakes?

1. In this study, sediment chlorophyll profiles at twenty littoral stations in three oligo‐mesotrophic lakes were compared to test whether the vertical distribution of chlorophyll is related to site characteristics (light availability, temperature, physical disturbances) and whether these profiles differ between shallow and deep portions of the littoral zone.
2. The magnitude of chlorophyll peaks at the sediment surface did not vary with light intensity. Chlorophyll peaks in the shallow littoral zone had a weak tendency to decrease with increasing effective fetch. The magnitude of chlorophyll peaks at deeper sites was more closely related to water temperature than to substrate slope.
3. High chlorophyll concentrations were measured down to 1–3 cm in the sediments, both at shallow (< 2.5 m) and deep (4–10 m) stations. The depth to which high chlorophyll was found in sediments did not vary with effective fetch or sediment water content, two indices of wave disturbance in the shallow littoral zone, or with substrate slope, an index of sediment stability in the deep littoral zone. Sediment mixing is apparently not related to common indices of physical disturbances.
4. Between 8 and 100% of sediment surface chlorophyll was 'retained' 4–5 cm into the sediments. The proportion of chlorophyll 'retained' in littoral sediments increased with increasing depth, increasing lake productivity (total phosphorus concentration) and increasing lake pH.
5. Among‐core variability (standard error/mean) in chlorophyll concentration at the sediment surface ranged from less than 1% to 33% at different stations and was highest at shallow, exposed sites. These levels of variability are similar to those found in other periphytic communities.     

Habitat utilization by sympatric arctic charr Salvelinus alpinus L. and brown trout Salmo trutta L. in Lake Atnsjø, south-east Norway

SUMMARY. 1. Habitat utilization, as well as inter- and intraspecific relations of different size groups of arctic charr (Salvelinus alpinus (L.)) and brown trout (Salmo trutta L.) in Lake Atnsjø, south-east Norway, were investigated by analysing food and spatial niches from monthly benthic and pelagic gillnet catches during June-October 1985.
2. Small individuals (150–230 mm) of both arctic charr and brown trout occurred in shallow benthic habitats. However, they were spatially segregated as arctic charr dominated at depths of 5–15 m and brown trout at depths of 0–5 m.
3. Larger (>230 mm) arctic charr and brown trout coexisted in the pelagic zone. Both species occurred mainly in the uppermost 2-3 m of the pelagic, except in August, when arctic charr occurred at high densities throughout the 0–12 m depth interval. On this occasion, arctic charr were segregated in depth according to size, with significantly larger fish in the top 6 m. This was probably due to increased intraspecific competition for food.
4. The two species differed in food choice in both habitats, Arctic charr fed almost exclusively on zooplankton, whereas brown trout had a more variable diet, consisting of surface insects, zooplankton. aquatic insects and fish.
5. The data suggest that the uppermost pelagic was the more favourable habitat for both species. Large individuals having high social position occupied this habitat, whereas small individuals lived in benthic habitat where they were less vulnerable to agonistic behaviour from larger individuals and less exposed to predators. The more aggressive and dominant brown trout occupied the more rewarding part of the benthic habitat.     

Multi-mesh gillnets to estimate species composition and catch per unit effort of fish in a small water body in Zambia

A standard method comprising multi-mesh monofilament nylon gillnets and depth stratification, the Drottningholm method, was employed in test fishing of a small Zambian reservoir (32 ha) on eight occasions over 1 year. The catch per unit effort (cpue) from the 0–3 and 3–6 m zone was timated with benthic and pelagic nets, while in the 6–12 m zone only benthic nets were used. Results include species caught and cpue with coefficients of variation and confidence intervals, for net types and depth zones. A total of 11 species were caught, of which Burbus paludinosus , B. marequensis , Laheo cylindricus , and Oreochromis sp. dominated. Coefficients of variation did not indicate a preferable season for test fishing. During the cold season some species were not caught. The applicability of the method in estimation of species composition and cpue is discussed.     

Constraining rooting depths in tropical rainforests using satellite data and ecosystem modeling for accurate simulation of gross primary production seasonality

Accurate parameterization of rooting depth is difficult but important for capturing the spatio-temporal dynamics of carbon, water and energy cycles in tropical forests. In this study, we adopted a new approach to constrain rooting depth in terrestrial ecosystem models over the Amazon using satellite data [moderate resolution imaging spectroradiometer (MODIS) enhanced vegetation index (EVI)] and Biome-BGC terrestrial ecosystem model. We simulated seasonal variations in gross primary production (GPP) using different rooting depths (1, 3, 5, and 10 m) at point and spatial scales to investigate how rooting depth affects modeled seasonal GPP variations and to determine which rooting depth simulates GPP consistent with satellite-based observations. First, we confirmed that rooting depth strongly controls modeled GPP seasonal variations and that only deep rooting systems can successfully track flux-based GPP seasonality at the Tapajos km67 flux site. Second, spatial analysis showed that the model can reproduce the seasonal variations in satellite-based EVI seasonality, however, with required rooting depths strongly dependent on precipitation and the dry season length. For example, a shallow rooting depth (1–3 m) is sufficient in regions with a short dry season (e.g. 0–2 months), and deeper roots are required in regions with a longer dry season (e.g. 3–5 and 5–10 m for the regions with 3–4 and 5–6 months dry season, respectively). Our analysis suggests that setting of proper rooting depths is important to simulating GPP seasonality in tropical forests, and the use of satellite data can help to constrain the spatial variability of rooting depth.     

Ultrastructure of the chorion and its micropyle apparatus in the mature Fundulus heteroclitus (Walbaum) ovum

The ovum 'membranes' and the micropyle apparatus of mature, extruded ova of Fundulus heteroclitus were examined by scanning electron microscopy. The ovum is covered by a thin jelly coat comprised of a dense mat of 0.3-0.5 urn fibrils, except for a 50–100 um fibril-free zone surrounding the micropyle apparatus. The micropyle apparatus consists of a 50–100 um diameter funnel-shaped vestibule, at the bottom of which is a circular aperture 4–5 μ in diameter leading to the micropyle canal which traverses the entire chorion layer. The inner micropyle aperture, 2–3 um diameter, apposes the inner ovoplasm mass.
The chorion is the major protective coating of the ovum. It consists of a thin (0.4 μm) outer zone, a thicker (9–12 μ), lamellated inner zone with 4–10 lamellae, and sometimes an innermost crystalline zone, varying in thickness from 1–13 μm. The extreme variability in the structure of the lamellated and crystalline zones of the chorion suggests that generalizations concerning ovum membranes can be misleading.     

Seasonal pattern of depth selection in smallmouth bass

The current study used a stationary acoustic telemetry array to monitor the depth selection of adult smallmouth bass Micropterus dolomieu in a large, oligotrophic lake in Ontario, Canada. At an annual scale, smallmouth bass demonstrated regular, seasonal changes in inhabited depths: fish occupied shallow water during the summer (2–5 m depth) and descended to deeper water (12–15 m depth) during winter under ice. Smallmouth bass remained above the thermocline in the summer, seasonal depth patterns did not vary across fish size and movements to and from seasonal depths were closely linked to the development and degradation of the thermocline. At finer time scales, smallmouth bass exhibited diel vertical migrations in summer, with fish moving to <2 m at night, and then descending to 3–5 m during the day. This pattern remained constant during the summer period examined, and varied with size such that larger fish remained deeper than smaller fish. During winter, depth did not vary across the 3-month monitoring period ( c . 14 m), but small (<2 m) changes in depth were observed periodically, suggesting limited movements were occurring. Results are further discussed in the context of climate change and reproductive success for this species.     

The deposition and mortality of the eggs of the Baltic herring Clupea harengus membras L., on different substrates in the south-west archipelago of Finland

The environmental factors affecting the deposition and mortality of the eggs of the spring-spawning Baltic herring were studied in the inner Archipelago Sea of south-western Finland. On four spawning grounds, 27 study squares (area 1 m²) were surveyed by divers. In each square, one quantitative egg sample was taken and the following data were recorded: depth, temperature, bottom quality of the square and under the eggs, egg substrate, cover (%) of plants and/or the bivalve Mytilus edulis , and the heterogeneity of the environment, expressed as the total number of all bottom materials, plants and Mytilus found in the square.
Eggs were found in the depth zone of 1–4 m. Their density ranged from 6000 to 2·3 millions of eggs per square metre. Egg number had a significant negative correlation with both depth (r=-0·545, P <0·01, d.f. = 25) and temperature of the sea water ( r =-0.479,. P <0·01,d.f. = 25), and a positive correlation with the total cover (%) in the squares ( r = 0·375, P < 0·05, d.f. = 25).
The highest numbers of eggs were found on Cladophora glomerata, which the most preferred spawning substrate of those present. Mortality of the eggs varied according to the substrate. In eggs attached to Cladophora, Potamogeton and Mytilus, the proportion of dead eggs was 0.15·5%; in those attached to red algae it was significantly higher ( Furcellaria , 5·0–63·2%; Phyllophora , 0–95·2%).     

Life history and distribution of the small south-western Atlantic octopus, Octopus tehuelchus

Although this species has been reported from shallow waters down to 90 m depth, knowledge is almost entirely based on intertidal samples. In this study both intertidal and subtidal samples were taken during 1982–1987, in northern San Matias Gulf (41° S, 63° 30' W). This is a large-egged (eggs: 9–12 mm long ×–5 mm wide, stack of 4–6 mm long) and small-sized (up to 150 g) octopus. Egg laying occurs between autumn and winter. Embryonic development takes about four months (water temperature:–19°C). Large hatchlings (DML: 5·64 mm, TL: 14·23 mm, TW: 0·139 g) emerge over spring and early summer, and development is direct. Maximum size is reached after 17–18 months; mating takes place in summer. Females reduce their feeding activity when they reach maturity, and cease eating while brooding. Mean life-span is two years, but some individuals (mostly females) may live up to three years. Females approaching the beginning of the brooding period move to the subtidal zone, where males outnumber females until the end of summer and females (mostly brooders) then outnumber males. In the intertidal zone sex ratio was 1:1 from December to late March, but in April males outnumber females.
These life-history traits are compared with those of other large-egged octopuses and are discussed in relation to environmental conditions prevailing in the San Matias Gulf.     

Diving patterns and performance in the Antarctic blue-eyed shag Phalacrocorax atriceps

The pattern and characteristics of diving of two male blue-eyed shags Phalacrocorax atriceps were studied, using continuous-recording time-depth recorders, for a total of 15 consecutive days during which the depth, duration, bottom time, ascent and descent rates and surface intervals of 674 dives were recorded. Deep dives (> 35 m, averages80–90 m, max. 116 m) were twice as common (64% versus 34%) as shallow dives (< 21 m and 90% < 10 m). Deep dives were long (averages 2.7-4.1 min, max. 5.2 min) with half the time spent near maximum depth and fast travel speeds (averages 1.0-2.4 m s⁻¹). Shallow dives were short (average 0.5 min, max. 1.3 min), without bottom time and with slow travel speeds (0.1–0.6 m s⁻¹). The time spent at depth and the diet (mainly benthic fish and octopus) is consistent with benthic foraging; the function of shallow dives is uncertain. Male shags forage mainly in the afternoon in3–5 distinct bouts of diving. Within bouts (and shorter homogeneous sequences of diving) surface intervals are consistently2–3 times the preceding dive duration; in other shags the reverse is the case. Blue-eyed shag diving depth, duration and pattern is extreme amongst shags; and the relationship between dives and surface intervals suggests that they may regularly exceed their aerobic dive limit.     

Depth distribution and biological characteristics of the European eel Anguilla anguilla in Lough Ennell, Ireland

Using a longline survey, a total of 196 European eels Anguilla anguilla were collected at different depths in Lough Ennell (maximum depth 30 m), central Ireland. The catch per unit of effort of A. anguilla that were caught from 1 to 25 m depths was lowest at 0·5–5·0 m and greatest at the deepest depth range (22·5–25·0 m). Sub-samples of A. anguilla from depths of <15 m showed little or no difference in size, sex ratio, age, growth rate, condition factor, length–mass relationship, gonado-somatic index, fin index or eye index with fish from depths of >15 m. All fish examined were female yellow-phase A. anguilla that had ages from 7 to 20 years (mean ± s . d . = 10·3 ± 2·9 years), with growth rates from 24·0–60·8 mm year⁻¹ (mean ± s . d . = 40·7 ± 8·5 mm year⁻¹). Variations in the growth rates were greater in the shallow group than that of the deep group. This study suggested that deeper regions are important feeding habitats for A. anguilla and that fish in this lake were growing moderately fast compared to similar habitats and areas in the species' range.     

Behaviour and diet of sea trout post-smolts in a Norwegian fjord system

In order to study the behaviour of sea trout Salmo trutta L., in the early post-smolt phase, reared smolts were tagged with acoustic transmitters and released in the estuary of the River Aurland (Western Norway) and observed for 31–137·5 h. The study was conducted in May-June 1991 and 1995 on a total of seven tagged smolts. The fish moved back and forth along the littoral zone close to the surface and migrated net distances of 100–8000 m. In 1991 the tagged fish were usually observed in schools averaging 24–55 fish, while the tendency to school was less pronounced in 1995. Post-smolts of wild and reared origin were caught by 25 m long gillnets in the littoral zone. In 1991 the mean distance to shore at capture was 4·3 m and the mean depth 0·6 m, and the corresponding numbers in 1995 were 8·0 and 0·8 m. The distribution of the post-smolts is explained by a general preference for shallow water caused by osmoregulatory problems in high-salinity deep water, together with a preference for the sheltered littoral zone and a strong predation pressure in the open sea from fast swimming pelagic fish predators and gulls Larus sp. Schooling seemed to restrict predation by cod Gadus morhua L., in the littoral zone in 1991, while no cod were observed in this habitat in 1995. In both years the stomach contents of the netted fish consisted mainly of terrestrial insects. Reared fish chose prey items similar to those taken by wild fish, but consumed less food.     

Benthic resting periods of pelagic cyclopoids in an oligotrophic lake

The benthic resting stages of pelagic cyclopoids were studied in the oligotrophic lake Pääjärvi (maximum depth 87 m), southern Finland. Stage 5 copepodids of Thermocyclops oithonoides were found in the bottom from September to April, with highest abundances in the littoral at 1–2 m. Dormant Mesocyclops leuckarti (mainly stage 5 copepodids) were found from mid-August so April, with a strong concentration at the depth of 1.5 m, and resting stage 4 copepodids of Cyclops kolensis from mid-summer to late winter at the depths of 1–13 m. Single stage 4 and 5 copepodids of C. strenuus and C. lacustris were also found in the bottom during autumn and winter. Available information on the periodicity of these species conforms well with the hypothesis of a temperature-adjusted photoperiodic control of diapause.
In the sediment, the resting stages of pelagic cyclopoids were confined to the uppermost 2–3 cm, C. kolensis penetrating slightly deeper than the smaller Thermocyclops and Mesocyclops species.
Losses during the resting period were highest in T. oithonoides , in which the seasonal mortality rates were significantly correlated with temperature. Its overall mortality was slightly lower at 2 m than at 13 or 40 m. C. kolensis had the lowest mortality, with no clear relation to temperature.
The mean winter biomass of the benthic resting stages of pelagic cyclopoids exceeded that of the true benthic copepods, and the winter losses of the former were equal to about one fifth of the total annual production of the true benthic copepods.     

Depth equilibration by two predatory cichlid fish from Lake Malawi

The cichlid fish, Haplochromis polystigma and Haplochromis livingstonii are piscivorous ambush predators endemic to Lake Malawi, Africa. Experimentally it was found that large adult H. livingstonii could equilibrate to 38 m, but that juveniles were restricted to shallower depths. Mean depth attained by 8 H. polystigma was 45 m but two individuals exceeded the maximum simulated depth (59 m). H. livingstonii had a mean equilibration rate of 4–7 m day⁻¹ for the first 10 m declining to 2·4 m day⁻¹ thereafter. The mean rate of descent for H. polystigma was 3·8 m day⁻¹ for the first 10 m and 2·8 m day⁻¹ thereafter. The fish were decompressed at a rate of a 20% decrease every 12 h. Although cichlid fishes occur down to 200 m depth in Lake Malawi, experimental and field data indicate that the depth distribution of individual species is restricted and that the restriction is largely related to swimbladder physiology. All cichlid species so far tested equilibrate slowly to depth (2 to 5 m a day) and are not capable of rapid decompression.     

A new genus and species of deep demersal fish (Teleostei: Stephanoberycidae) from the tropical eastern North Atlantic

A novel stephanoberycid genus and species, Abyssoberyx levisquamosus Merrett & Moore is reported from bottom‐trawl abyssal captures in 4490–4640 m depth. It has cycloid scales, differing from Stephanoberyx , Acanthochaenus and Hispidoberyx with spinoid scales; has 1–2 dorsal fin spines, differing from Acanthochaenus and Malacosarcus with 0, Gibberichthys with 5–6 and Hispidoberyx with 3–5 spines; has 5–7 + 14–15 gill rakers, differing from Stephanoberyx with 12–15 + 25–27 and Acanthochaenus and Malacosarcus with 8 + 17–20 gill rakers. Selected aspects of the osteology of A. levisquamosus are described, to confirm that it shares several features in common with other stephanoberycid genera and allied stephanoberycoids. Aspects of its biology and ecology are discussed.     

Habitat selection by grayling—I. Spawning habitats

Grayling spawning sites were investigated in two French rivers, the Pollon (1 year) and the Suran (2 years) and described by current velocity, water depth, and substrate composition, completed by an assessment of bottom shear stress with FST-hemispheres. A comparison was made between used and available habitats, the latter being characterized by random sampling of 300 m long (Pollon) and 510 m long (Suran) river sections, both including three riffle/pool sequences. Mean velocities observed on spawning sites did not differ significantly between rivers or years (overall mean 48–9 cm s^-1 S.D. = 11.9, range 25.8–91.7 cm s^-1, n =150). Most water depths ranged from 10 to 40 cm in both rivers, but mean depths were significantly different ( P <0.05). Substratum of spawning grounds was dominated by gravel and pebbles (2–64 mm) in both rivers. Most spawning sites (99%) were characterized by a narrow range of hemispheres (nos 9–13), i.e. a range of shear stress of about 5–16 dyn m^-2. In the Pollon, spawners between spawning acts were found in a resting pool located immediately downstream from the spawning area and characterized by slow-flowing water (mainly <20 cm s^-1) and great depth (mainly >60 cm), with cover provided by overhanging branches and tree roots.     

Fish assemblages of Cais do Carvão Bay (Madeira Island) determined by the visual census technique

Fish assemblages in Cais do Carvão Bay, Madeira Island, a proposed marine protected area (MPA), were determined from a diver visual census. A total of 32 transect counts were performed. Habitats sampled included sandy bottom, rocky boulders, vertical walls and rocky outcrops. Species richness, diversity, density, trophic structure, size and spatial organization were documented for the fish assemblages. Forty‐four species from 23 families were encountered; 32% belonged to Sparidae (10) and Labridae (four). The greatest species richness (25) was observed in rocky boulder habitat at 10–15 m depth, while the lowest (five) occurred over a deeper sand habitat. The greatest density (760·5 individuals per 100 m²) was recorded over rocky outcropping (20–25 m deep), and the lowest of 11·6 individuals per 100 m² was over a sand bottom at 10–15 m depth. Thalassoma pavo , Abudefduf luridus and Chromis limbata had higher densities on hard bottoms, while Heteroconger longissimus was the most abundant species in sand bottom habitats. No significant differences were detected for all indices calculated among depth intervals for sand and rocky boulder stations. Sand and rock boulder substratum, however, differed significantly for the 10–15 m depth stratum.     

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.     

Vertical partitioning of the phytoplankton assemblage in ultraoligotrophic Crater Lake, Oregon, U.S.A.

SUMMARY 1. The summertime phytoplankton assemblage in abysmally deep (Z_max: 589 m) Crater Lake, Oregon, consists of over 100 species, which are variously distributed in the upper 200 m of the vertical water column. The depth distribution of the lake's three most prevalent species follows a predictabk pattern: Nitzschia gracilis in the 0–20 m stratum, Tribonema sp. at mid–depth (80–20 m), and Stephanodiscus hantzschii in the lowermost stratum (160–200 m). These major species, which account for approximately 80% or more of the lake's total phytoplankton biomass and primary production, exist under atypical temperature, light, and nutrient conditions.
2. The spatial distribution of phytoplankton in Crater Lake resembles a three-tier structure. Unlike most lakes, where the entire phytoplankton communities exist in less disparate environmental conditions, or are vertically mixed periodically by storm events and seasonal lake turnover. the Crater Lake community is partitioned into stratified environments.
5. The disparate and unusual characteristics of these environments, and the hydrological and limnological stability of the lake basin, are perhaps important factors regulating the diversity, dominance. and partitioning of the lake's phytoplankton populations.     

An application of the plaster dissolution method for quantifying water velocity in the shallow hyporheic zone of an Appalachian stream system

1. A method for quantifying interstitial water velocity based on the dissolution rate of plaster of Paris standards was developed as part of a study of vertical, longitudinal (1–4 order sites) and seasonal variation in the biotic and physical characteristics of the shallow hyporheic zone (0–30 cm) of a headwater stream system in West Virginia, U.S.A.
2. A calibration model was developed using a water velocity simulation tank to relate mass loss of plaster standards to water velocity and temperature. The model was then used to calculate water velocity through artificial substrata embedded in the shallow hyporheic zone of four stream reaches based on in situ mass loss of plaster standards.
3. Water velocity in the hyporheic zone increased with stream order, was highest in early spring and winter during high stream base flows, and decreased with depth into the substratum. There was a strong interaction between depth and season: during periods of high stream discharge, water velocity through the upper level of the shallow hyporheic zone (0–10 cm into the substrate) increased disproportionately more than velocity at greater depths. Mean interstitial velocity in March ranged from 0 cm s^–1 in the lowest level (20–30 cm) to 3.5 cm s^–1 at the upper level (0–10 cm) at the first‐order site, and from 2.5 cm s^–1 (20–30 cm) to 9.5 cm s^–1 (0–10 cm s^–1) at the fourth‐order site. Gradients in stream discharge and sediment permeability accounted for treatment effects.
4. Use of calibrated data improved the ability to resolve among‐season differences in interstitial water movement over the use of uncalibrated mass loss data. For some applications of the plaster standard method, empirical calibration may not be necessary.