Sediment processing by gizzard shad, Dorosoma cepedianum (Lesueur), in Acton Lake, Ohio, U.S.A.

Gizzard shad are primarily detritivorous in Acton Lake, a 253-ha impoundment in southwestern Ohio, U.S.A. To determine the magnitude of sediment utilization by the gizzard shad population in Acton Lake. I used data on population density and age structure, daily ration, and feeding selectivity in estimating the mass of sediments processed by shad daily from April through November. At densities of 4595–10 645 fish ha⁻¹(wet weight biomass = 90–121 kgha ¹), gizzard shad could process 3.8–23.0 kg of dry sediments ha⁻¹ day ¹. On average throughout the growing season, gizzard shad could process a dry mass of sediments each day equivalent to 13% of shad wet weight biomass. Because of the high rate of sedimentation (> 700 kg dry sediment ha⁻¹ day⁻¹) in Acton Lake, gizzard shad can process < 4% of the freshly deposited sediments each day, and therefore are likely to have little effect on benthic community dynamics in the system.     

Climate-induced changes in high elevation stream nitrate dynamics

Mountain terrestrial and aquatic ecosystems are responsive to external drivers of change, especially climate change and atmospheric deposition of nitrogen (N). We explored the consequences of a temperature-warming trend on stream nitrate in an alpine and subalpine watershed in the Colorado Front Range that has long been the recipient of elevated atmospheric N deposition. Mean annual stream nitrate concentrations since 2000 are higher by 50% than an earlier monitoring period of 1991–1999. Mean annual N export increased by 28% from 2.03 kg N ha⁻¹ yr⁻¹ before 2000 to 2.84 kg N ha⁻¹ yr⁻¹ in Loch Vale watershed since 2000. The substantial increase in N export comes as a surprise, since mean wet atmospheric N deposition from 1991 to 2006 (3.06 kg N ha⁻¹ yr⁻¹) did not increase. There has been a period of below average precipitation from 2000 to 2006 and a steady increase in summer and fall temperatures of 0.12 °C yr⁻¹ in both seasons since 1991. Nitrate concentrations, as well as the weathering products calcium and sulfate, were higher for the period 2000–2006 in rock glacier meltwater at the top of the watershed above the influence of alpine and subalpine vegetation and soils. We conclude the observed recent N increases in Loch Vale are the result of warmer summer and fall mean temperatures that are melting ice in glaciers and rock glaciers. This, in turn, has exposed sediments from which N produced by nitrification can be flushed. We suggest a water quality threshold may have been crossed around 2000. The phenomenon observed in Loch Vale may be indicative of N release from ice features such as rock glaciers worldwide as mountain glaciers retreat.     

Piscivory and trophic position of Anguilla anguilla in two lakes: importance of macrozoobenthos density

The feeding habits of the European eel Anguilla anguilla (>300 mm total length, L _T) were compared in two lakes of different environmental state: Lake Großer Vätersee (LGV), Germany (clear water, mesotrophic and submerged macrophytes), and Lake Vallum (LV), Denmark (turbid, eutrophic and no submerged macrophytes). The density of macrozoobenthos was higher in LV (3500 individuals m⁻²) than in LGV (1500 individuals m⁻²). The abundance of small prey fishes (40–99 mm L _T) was highest in LV. In LV, A. anguilla fed on macrozoobenthos, in particular, chironomid larvae. In LGV, A. anguilla used fishes as the main food component. Stable isotope analyses confirmed the stomach contents dietary results. The estimated mean ± s.d . trophic positions of A. anguilla in LGV (3·7 ± 0·2) was one level higher than those of fish in LV (2·7 ± 0·2). Based on these results, it is concluded that piscivory among A. anguilla was generally controlled by the density of macrozoobenthos. Stable isotope analysis further indicated that A. anguilla may act as integrators between benthic and pelagic food webs when density of insect larvae is low.     

Abrupt change in primary productivity in a littoral zone of Lake Biwa with the development of a filamentous green-algal community

1. Some characteristics of the photosynthesis and primary production of benthic and planktonic algal communities were investigated in a littoral zone covered with gravel in the north basin of Lake Biwa, paying special attention to the recent development of filamentous green algae (FGA) in the benthic algal community.
2. P_max (maximum gross photosynthesis rate) values of the benthic algal community (0.1–1.2 mg C mg chl. a ⁻¹ h⁻¹) obtained from photosynthesis–irradiance (P–I) curves were lower than those of the planktonic algal community (2.4–11.5 mg C mg chl. a ⁻¹ h⁻¹). This is apparently a result of the high degree of self shading in the benthic algal community and its low turnover as compared with that of the planktonic algal community.
3. Relatively high I_k values (150–200 μmol photon m⁻² s⁻¹) were observed in the benthic algal community only in June–July when a FGA, Spirogyra sp., was abundant. This reflected a photosynthetic characteristic of the Spirogyra itself, in which photosynthesis was saturated at high light intensity.
4. The FGA community established in the layer between planktonic and sessile (benthic algae except for FGA) algal communities. It brought about extraordinarily high organic matter production in the littoral zone at the expense of production in the sessile algal community.     

Activity of methanotrophic bacteria in Green Bay sediments

Abstract Sediment pore water samples obtained from a 19 m station in Green Bay in Lake Michigan were examined for levels of ambient dissolved methane and copper, and for the potential for in situ methane oxidation by methanotrophs found within surface sediments. The in situ methane concentration in the upper oxic sediment layer ranged from 20–150 μmol · 1⁻¹ at this station. The activity of methanotrophs and the kinetics of methane oxidation in these sediments were demonstrated by the uptake of radiolabeled methane. K_s values varied between 4.1–9.6 nmol · cm³ of sediment slurry. High V_max values (12.7–35.2 nmol · cm⁻³ · h⁻¹) suggest a large population of methanotrophs in the sediments. An average methane flux to the oxic sediments of 0.24 mol · m⁻² · year⁻¹ was calculated from the pore water methane gradients. Pore water concentrations of copper in the upper sediment layer ranged from 10–120 nmol · 1⁻¹. Based upon the copper concentration, other measured parameters, and equilibrium conditions defined by WATEQF4, an estimate for dissolved free Cu²⁺ concentration of 5–38 nmol · 1⁻¹ pore water was obtained. Several factors control the rate of methane oxidation, including oxygen, methane, and the bioavailability of free Cu²⁺.     

Factors related to gizzard shad and the threadfin shad occurrence and abundance in Florida lakes

Gizzard shad Dorosoma cepedianum were collected in 23 and threadfin shad D. petenense were collected in 22 of the 60 Florida lakes sampled. Logistic regression equations were 94% effective for predicting gizzard shad occurrence from chlorophyll and lake surface area, and 84% effective for predicting threadfin shad occurrence from lake surface area and lake volume inhabited (PVI). Occurrence of both shad species was related positively to lake size. In lakes where gizzard shad or threadfin shad were collected, shad density and biomass of both shad species were related positively to chlorophyll. Gizzard shad populations were generally vulnerable to predation in lakes, with the per cent of gizzard shad ≤200mm L _T values exceeding 60% with few exceptions. Effects of gizzard shad and threadfin shad on fish community dynamics may be confined to relatively large (>100 ha) and fertile (chlorophyll >20–30μg l⁻¹) Florida lakes.     

Tree population dynamics in Kibale National Park, Uganda 1975–1998

Changes in species composition, stem abundance, and basal area of trees taller than or equal to 10 m in a medium altitude tropical rain forest at the Ngogo study area, Kibale National Park, Uganda are described for the period between 1975 and 1998 ( n  = 23 years). Two enumeration episodes were conducted in 263 plots of 5 m by 50 m during 1975–80 and 1997–98. During this period, species richness decreased by 3% (from 92 to 89). Species diversity (H') also declined slightly from 2.97 to 2.86. The number of trees recorded in the sample plots decreased by 8% (from 2545 to 2329), while basal area decreased from 49.48 m² ha⁻¹ to 48.68 m² ha^−1. However, stem abundance and basal area increased for some species.     

Terrestrial export of highly bioavailable carbon from small boreal catchments in spring floods

1. We assessed the terrestrial export of organic carbon, which effectively supported aquatic bacterial production (BP), from small boreal catchments during spring flood. We analysed stream runoff from nine small catchments with different proportions of peat mires and coniferous forests by monitoring the dissolved organic carbon (DOC) flux in combination with conducting bacterial bioassays.
2. Multiple linear regression analysis showed that BP during 7-day-dark bioassays (BP₇; μg C L⁻¹day⁻¹) was explained by both the quantity and quality (low-molecular weight fractions) of the DOC. BP₇ can be used as a measure of export of terrestrial organic carbon that is highly bioavailable.
3. Total export of DOC during spring flood from the different catchments ranged from 20 to 27 kg ha⁻¹ and was negatively correlated to forest cover (%). However, the export of BP₇ carbon was positively correlated to forest cover and varied from about 0.1 kg ha⁻¹ in mire-dominated streams to about 0.2 kg ha⁻¹ in forest-dominated streams.
4. The high bioavailability of forest carbon suggests that forests are the main contributors of BP-supporting carbon in boreal streams although mires have higher area-specific export of DOC.     

Decline of the African lungfish (Protopterus aethiopicus) in Lake Victoria (East Africa)

Catch and effort data for the period 1973–1990 demonstrate a dramatic decline of lungfish in the Tanzanian waters of Lake Victoria. Bottom trawl catches in the Mwanza Gulf showed a decline in catch rates from 67.5 kg h⁻¹ in 1973 to 5.5 kg h⁻¹ in 1986. Trawling of commercial vessels in the Speke Gulf revealed a decline in lungfish catches from 1.3 kg h⁻¹ in 1986 to 0.07 kg h⁻¹ in 1990. The development of anoxia in the deeper waters of Lake Victoria, the algal blooms, and the decline of water transparency, all associated with eutrophication, are not likely to have contributed to the decreased catch rate. However, the lungfish decline may reflect the interaction of overexploitation by the fishery and a low level of Nile perch predation that restricts lungfish to wetland refugia. We suggest that this may have been reinforced over the past few decades by large-scale conversion of wetlands to agricultural land and harvesting of nest-guarding male lungfish leading to decreased recruitment of young.     

A preliminary assessment of the biomass of the pelagic sardine Limnothrissa miodon in Lake Kariba

The sardine Limnothrissa miodon was very patchily distributed in Lake Kariba in 1981–1983 and its mean biomass was 59 23 kg ha⁻¹, declining from 90–91 kg ha⁻¹ in 1981 to 38–66 kg ha⁻¹ in 1983. In addition, the mean size of the fish fell from 55–2 mm in 1981 to 49–9 mm in 1983. These changes are attributed to the effects of drought and reduced river flows as well as to the high level of commercial fishing for this species. There was a correlation between commercial catches and biomass estimates which allowed estimation of biomass in previous years from fishing statistics. Fishing effort appeared to be a major influence on sardine abundance: total biomass in 1985 was about 10% of what it was in 1974 when the fishery began. When expressed in terms of unit area, as is usual for African lakes, the pelagic biomass appeared to be greater in Lake Tanganyika, where Limnothrissa is endemic, than in Lake Kariba, although there is little difference in their primary productivity, but consideration in terms of volume rather than area shows that this was not the     

The population biology of pike, Esox lucius L., in two gravel pit lakes, with special reference to early life history

The population biology of pike in two gravel pit lakes of contrasting habitat type were studied. The density of pike (1 + and over) in November 1987 in the Main Lake was 7.7 fish ha⁻¹ (19 kg ha⁻¹) and in St Peter's was 71 fish ha'(53 kg ha⁻¹). The age distribution showed that recruitment was consistent in St Peter's but variable in the Main Lake. The main factor causing this difference in recruitment is abundance of aquatic vegetation. Survival up to November 1987 of 0-group pike in the Main Lake was 0.00175% from potential egg deposition, producing 6.6 fish ha⁻¹ (0.83 kg ha⁻¹). In St Peter's survival was 0.00315%, producing 23 fish ha⁻¹ (3.04 kg ha ¹).     

Seasonal nitrogen dynamics of Miscanthus×giganteus and Panicum virgatum

There is a tradeoff to consider when harvesting perennial biomass crops: harvest too late and yield declines, harvest too early and risk higher mineral contents, particularly nitrogen (N). Allowing the crop to completely senesce and recycle nutrients has several advantages, including improved feedstock quality and reduced fertilizer requirements, but it comes at a risk, particularly in temperate climates where snow and ice can reduce or destroy harvestable biomass. The effect of harvest time on the N concentration ([N]) and biomass of Panicum virgatum and Miscanthus × giganteus was evaluated at three sites in Illinois over two years. In both species [N] of standing biomass significantly declined with time ( P <0.0001). Interestingly, there was no significant interaction effect of species and sample date on [N] ( P =0.2888), but there was a highly significant interaction effect on the total N in standing biomass ( P <0.0001). The amount of standing N was directly related to biomass yield. Seasonal changes in standing N differed among locations, suggesting that harvest time recommendations for N management depend on location. P. virgatum would have potentially removed as much as 187 kg N ha⁻¹ if harvested green, and as little as 5 kg N ha⁻¹ if harvested in late winter. Because of higher biomass yields, M . × giganteus standing N ranged from 379 kg N ha⁻¹ in June to <17 kg N ha⁻¹ in February. Importantly, there was little overall change in [N] between an early winter (December) harvest and a late winter (February/March) harvest, indicating the benefits of N cycling in the system can be realized by end of the growing season and thus, at least from an N economy perspective, there is no reason to risk yield losses by delaying harvest over the winter.     

Wild coffee management and plant diversity in the montane rainforest of southwestern Ethiopia

Coffea arabica occurs naturally in the montane rainforests of Ethiopia, but large areas of these unique forests have been converted to other land-uses. In the remaining forest, wild coffee is managed and harvested with increasing intensity because of rising coffee prices in the world market. This study evaluated the impact of coffee management on wild coffee populations and the forest vegetation as a basis for conservation planning in southwestern Ethiopia. Vegetation surveys and yield assessments were carried out in unmanaged natural forest and in managed semi-forest coffee (SFC) systems. Analyses show that wild coffee density and coffee yields were low in natural forest (max. 15 kg ha⁻¹ year⁻¹). In SFC systems, 30% of the canopy trees and most undergrowth vegetation were removed. This stimulated wild coffee growth and strongly enhanced yields (max. 54 kg ha⁻¹ year⁻¹), but severely disturbed forest structure. Species richness increased by 26% because of an increase in species of ruderal and secondary vegetation; however, species richness and abundance of typical forest species declined. Conservation of the natural forest therefore requires the control of wild coffee management. Wild coffee certification is discussed as one tool to reconcile conservation measures and the interests of local farmers.     

Environmental factors affecting the occurrence of mycobacteria in brook sediments

The occurrence of mycobacteria was studied in aerobic brook sediments from 39 drainage areas in Finland. The culturable counts of mycobacteria were related to climatic conditions, characteristics of the drainage area, chemical characteristics of the sediment and water, culturable counts of other heterotrophic bacteria, and microbial respiration rate in the sediment. The counts of mycobacteria varied from 1·1 × 10² to 1·5 × 10⁴ cfu g⁻¹ dry weight of sediment. They correlated positively with the proportion of the drainage area consisting of peatland, total content of C, content of Pb, microbial respiration rate in the sediment, and chemical oxygen demand of the water. The correlations of the mycobacterial counts with pH of sediment and alkalinity of water were negative. The results of the present sediment study and of the forest soil study published earlier strongly suggest that an increase in acidity increases the counts of mycobacteria and decreases the counts and activity of other heterotrophic bacteria. Mycobacterial counts were more than 100 times higher (per dry weight) in forest soils with pH 3·5–4·3 than in sediments with pH 4·5–6·3.     

Prey consumption rates and growth of piscivorous brown trout in a subarctic watercourse

Prey consumption rates of piscivorous brown trout Salmo trutta were studied in the Pasvik watercourse, which forms the border between Norway and Russia. Estimates of food consumption in the field were similar to or slightly less than maximum values from a bioenergetic model. The piscivore diet consisted mainly of vendace Coregonus albula with a smaller number of whitefish Coregonus lavaretus . Individual brown trout had an estimated mean daily intake of c . 1·5 vendace and 0·4 whitefish, and a rapid annual growth increment of 7–8 cm year⁻¹. The total population of brown trout >25 cm total length was estimated as 8445 individuals (0·6 individuals ha⁻¹), giving a mean ±  s . e . annual consumption of 1553880 ± 405360 vendace and 439140 ± 287130 whitefish for the whole watercourse. The rapid growth in summer of brown trout >25 cm indicated a high prey consumption rate.     

Lacustrine growth of juvenile pink salmon and a comparison with sympatric sockeye salmon

The growth rates of naturally sympatric juvenile pink Oncorhynchus gorbuscha and sockeye Oncorhynchus nerka salmon were compared in a common lacustrine environment in south‐west Alsaka, an unusual opportunity given the normal disparity in freshwater residence time of these two species. Fork length ( L _F) frequency distributions of juvenile pink salmon caught in the lake during the summer in 1991 and 1999–2003 indicated a growth rate of 0·54 mm day⁻¹, 54% greater than the estimated growth rate of juvenile sockeye salmon sampled from 1958 to 2003 (0·35 mm day⁻¹). Examination of daily growth rings on otoliths indicated that pink salmon in Lake Aleknagik grew an average of 1·34 mm day⁻¹ in 2003 but sockeye salmon grew only 0·63 mm day⁻¹(average specific growth rates were 3·0 and 1·8% day⁻¹, respectively). Pink salmon increased from c . 32 mm L _F and 0·2 g at emergence to 78 mm L _F and 3·0 g within 3–4 weeks. After experiencing these rapid growth rates, the pink salmon appeared to leave the lake by late July in most years. The diets of pink and sockeye salmon in the littoral zone of the lake were very similar; >80% of the stomach contents consisted of adult and pupal insects and the remainder was zooplankton. This high degree of diet overlap suggested that the observed differences in growth rate were not attributable to variation in prey composition.     

Holocene carbon burial by lakes in SW Greenland

The role of the Arctic in future global change processes is predicted to be important because of the large carbon (C) stocks contained in frozen soils and peatlands. Lakes are an important component of arctic landscapes although their role in storing C is not well prescribed. The area around Kangerlussuaq, SW Greenland (66–68°N, 49–54°W) has extremely high lake density, with ∼20 000 lakes that cover about 14% of the land area. C accumulation rates and standing stock (kg C m⁻²), representing late- to mid-Holocene C burial, were calculated from AMS ¹⁴C-dated sediment cores from 11 lakes. Lake ages range from ∼10 000 cal yr  bp to ∼5400 cal yr  bp , and reflect the withdrawal of the ice sheet from west to east. Total standing stock of C accumulated in the studied lakes for the last ∼8000 years ranged from 28 to 71 kg C m⁻², (mean: ∼42 kg C m⁻²). These standing stock determinations yield organic C accumulation rates of 3.5–11.5 g C m⁻² yr⁻¹ (mean: ∼6 g C m⁻² yr⁻¹) for the last 4500 years. Mean C accumulation rates are not different for the periods 8–4.5 and 4.5–0 ka, despite cooling trends associated with the neoglacial period after 4.5 ka. We used the mean C standing stock to estimate the total C pool in small lakes (<100 ha) of the Kangerlussuaq region to be ∼4.9 × 10¹³ g C. This C stock is about half of that estimated for the soil pool in this region (but in 5% of the land area) and indicates the importance of incorporating lakes into models of regional C balance at high latitudes.     

Habitat use and trophic position determine mercury concentration in the straight fin barb Barbus paludinosus, a small fish species in Lake Awassa, Ethiopia

The diet, habitat use and mercury concentration of the small fish species, the straight fin barb Barbus paludinosus , were studied in Lake Awassa, Ethiopia, for a period of 1 year from February 2003 to January 2004. Stable isotope signatures of nitrogen and carbon in different total length ( L _T) classes were used to determine trophic positions and organic carbon sources, respectively. Barbus paludinosus mainly occupied the protected benthic habitats (littoral and profundal) of the lake. The δ¹³C values were in the range from −24 to −19‰, indicating that the carbon source for B. paludinosus was benthic, as well. Small individuals (≤ 60 mm L _T) mainly preyed upon ostracods, intermediate sizes (60–100 mm) on aquatic insects and gastropods, while a tiny cyprinodont fish Aplocheilichthys antinorii dominated the diet of large individuals (100–160 mm). The progressively increase in δ¹⁵N with increasing L _T also indicated a diet shift towards piscivory in larger individuals. The mercury concentration ranging from 0·02 to 0·74 mg kg⁻¹ wet mass (wm), was unexpectedly high in this small species, and was significantly positively related to L _T, as well as to δ¹⁵N. Some large individuals had mercury concentrations < 0·1 mg kg⁻¹ wm, and low δ¹⁵N, indicating substantial variations in diet between individuals of same size. The study suggests that other piscivorous species which include B. paludinosus in their diet may have a high mercury intake risk.     

Migratory patterns and contribution of stocking to the population of European eel in Lithuanian waters as indicated by otolith Sr:Ca ratios

Otolith Sr:Ca ratios were examined to evaluate the contribution of the stocked eel Anguilla anguilla elvers, which have been stocked in Lithuanian waters and mixed with naturally recruited eels for several decades, to the native eel population. Stocked eels were identified by the freshwater signature (Sr:Ca ratios <2·24 × 10⁻³) on the otolith after the glass eel stage. Naturally recruited eels, that had migrated through the North and Baltic Seas, were characterized by an extended seawater and brackish-water signature (Sr:Ca ratios >3·23 × 10⁻³) after the glass eel stage. Of 108 eels analysed, 21 eels had otolith Sr:Ca ratio profiles consistent with stocking while 87 showed patterns of natural recruitment. The ages of naturally recruited eels arriving in Lithuanian fresh waters varied from 1 to 10 years, with a mean ±  s.d . age of 5·2 ± 2·1 years. Eels from the inland Lake Baluošai were all freshwater residents of stocked origin. Stocked eels, however, accounted for only 20% of the eels from the Curonian Lagoon and 2% of eels sampled in Baltic coastal waters. This finding does not support the hypothesis that the eel fishery in the Curonian Lagoon depends mostly on stocking.     

The relative abundance of brown trout in acidic softwater lakes in relation to water quality in tributary streams

The influence of the water quality of tributary streams on the relative abundance in benthic gillnet catches (catch per unit effort, cpue) of allopatric brown Salmo trutta was assessed in associated acidic, softwater lakes. The study was carried out over 6 years (1989–1994) in 15 lakes located at altitudes between 230–715 m a.s.l. in two Norwegian catchments. The water quality of the main inlets and outlets varied little, as indicated by their of pH range (4·93–5·51) and calcium concentrations (0·19–0·44 mg 1⁻¹), but varied more with respect to concentrations of inorganic, monomeric aluminium (7·0–41·0 μg l⁻¹). Most of the lakes were also fed by secondary streams with better water quality: a maximum pH of 6·56, calcium levels of up to 0·74mg 1⁻¹, and inorganic aluminium levels as low as l·0 μg 1⁻¹. The cpue was inversely correlated with lake altitude ( r ²=0·50), and thus was adjusted to a mean altitude. The calcium concentration in the richest secondary stream to each lake, its richness judged on the basis of its acid-neutralizing capacity, had the highest predictive power of the variability in cpue ( r ² = 0·49).The calcium content in the other secondary streams or in the main inlets and outlets did not correlate with cpue. Alkalinity in the main outlets correlated to some extent with cpue ( r ² = 0·27). It is suggested that secondary streams with good water quality provide important refuges for the recruitment of brown trout in acidic softwater lakes.