Within-population heterogeneity of habitat use by lake trout Salvelinus namaycush

To learn more about the degree of individual variation in resource use by lake trout Salvelinus namaycush , ultrasonic telemetry was used to study their habitat use in a lake without pelagic schooling fish prey. Individuals spent most of their time within the metalimnion in favourable water temperatures. They also made frequent excursions, however, into lake temperatures exceeding their optimum for physiological performance at all temporal scales considered. Their frequent use of nearshore habitats suggested that feeding in littoral areas may be common. Habitat use was highly variable among individuals, but spatial habitat use by individuals showed remarkable consistency between years. In particular, some lake trout exhibited high site fidelity to shallow, nearshore areas, whereas others used deep areas extensively. This level of between-individual variation indicated niche partitioning by depth and the possibility of alternative foraging strategies.     

THE HABITATS AND HABITAT PREFERENCES OF CLARIAS GARIEPINUS (PISCES: CLARIIDAE) IN A CLEAR COASTAL LAKE (LAKE SIBAYA,SOUTH AFRICA)

SUMMARY

The habitat preferences of the sharptooth catfish, Clarias gariepinus, in a clear coastal lake, Lake Sibaya, are described. Larval and small juvenile catfish inhabit flotsam and the rootstocks of plants in very shallow marginal areas. Larger juveniles inhabit densely-vegetated marginal pools, and may venture into open areas at night. Adult catfish (over 350 mm TL) occupy offshore areas, where their density is highest in terrace and sheltered bay habitats, and least in slope and profundal habitats. Adult catfish undergo a diel migration into shallow water at night and into deeper water during the day which is more marked at low lake levels. The methods used to capture catfish, and the main habitats in Lake Sibaya, are also described.     

Wind‐driven thermocline movements affect the colonisation and growth of Achnanthidium minutissimum,a ubiquitous benthic diatom in lakes

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Behavioral and trophic plasticity of juvenileTilapia mossambica in utilization of the unstable littoral habitat

Synopsis JuvenileTilapia mossambica Peters (2.5–10.0 cm standard length) in Lake Sibaya, South Africa, move daily from deep offshore waters to shallow (<0.5 m) littoral areas where they feed for several hours and then return to deep water. The timing of these movements varies in response to changing physical and biological features of the littoral environment. During this study (December 1973 – February 1976), lake level rose more than 1 m. At the start of the study, littoral areas visited by juvenileT. mossambica were free of vegetation and debris, but at higher lake levels trees and brush from the eroding shoreline were common in nearshore littoral waters. When the littoral zone was free of debris, juvenileT. mossambica visited nearshore waters only during daylight hours. After a 0.65 m rise in lake level and considerable accumulation of debris, these fish were abundant in the littoral zone only at night. This reversal can be attributed to efforts by juvenileT. mossambica to avoid their principal predator, the catfishClarias gariepinus. At low lake levelC. gariepinus ventured into the littoral only at night but at higher lake levels this predator was found within flooded vegetation during the day. When further increases in lake level flooded marginal grasslands, juvenileT. mossambica abandoned the littoral zone in favor of this newly created eulittoral habitat where potential fish predators did not occur. With the change in habitat, the diet changed from benthic detrital aggregate to periphyton. These observations suggest the importance of behavioral and throphic plasticity in the ability of tilapia to utilize unstable habitats.     

Comparative predation rates on larval snappers (Lutjanidae) in oceanic, reef, and nearshore waters

The life cycle of most reef fishes involves pelagic larvae entering the nearshore environment to settle to benthic substrates. Settlement is considered to be highly risky as larvae encounter high rates of predation mortality associated with shallow nearshore habitats. This potential bottleneck may be particularly significant for many tropical snapper (Lutjanidae) species which bypass the reef to settle to very nearshore seagrass areas. To test the hypothesis that predation-related mortality increases as naïve late-stage fish larvae leave the offshore oceanic environment and enter the nearshore to settle, relative nocturnal predation rates on tethered late-stage snapper larvae were measured in oceanic, coral reef, and nearshore surface waters of the lower Florida Keys, USA. Both relative predation rate and probability of predation in oceanic areas seaward of the reef was significantly greater than over reef or nearshore seagrass/hardbottom habitats. This surprising result may be due to differences in the density or spatial distribution of potential predators between deep offshore (near flotsam at the surface) and shallow nearshore environments (demersal). These findings suggest that successful late-stage snapper larvae should avoid surface waters in deep oceanic areas and move upward in the water column as they pass over the reef and other shallow nearshore environments prior to settlement.     

Effects of shoreline development on the nearshore environment in large deep oligotrophic lakes

1. In large deep oligotrophic lakes, the shallow nearshore waters may provide the most important habitat for animals to feed and breed, and it is this area of the lake where humans are most likely to have initial impacts as the shoreline is developed. Nutrients in fertilizers, sediments and sewage effluents are likely to be rapidly intercepted by nearshore algae at the lake edge, having heterogeneous effects nearshore before offshore effects are noted. 2. Here we examined the spatially explicit effects of residential development on nearshore periphyton communities in three large deep oligotrophic lakes that have all experienced modest residential development in the Pacific Northwest of the United States. We demonstrate that substantial nearshore changes in the basal food web are detectable even with low levels of shoreline development. These changes can potentially affect whole‐lake food web dynamics. 3. For our primary study site (Lake Crescent, Washington, USA), we found that algal biomass and accumulation of detritus were higher at developed sites. In addition, both macroinvertebrate and periphyton communities exhibited a shift in composition with more detritivores and filamentous green algae at developed sites. These differences were more pronounced during the spring than at other times of year. 4. A complementary investigation of field patterns in Priest Lake and Lake Pend Oreille (Idaho, USA) suggested that, although spatial and temporal patterns were idiosyncratic, indicators of productivity and the presence of filamentous green algae were generally higher at developed sites across lakes. 5. Stable isotope signatures and water column nutrients were not useful in distinguishing developed and undeveloped sites, increasing the potential usefulness of periphyton monitoring during early stages of lake development. 6. A laboratory investigation suggested that common macroinvertebrate grazers assimilated a much greater proportion of diatoms than the filamentous green algae that are associated with fertilization at developed sites. 7. These findings have at least two clear implications: (i) periphyton may be used to detect human impacts before disturbance is evident in offshore monitoring programmes and (ii) nearshore impacts in response to modest residential development have the potential to disrupt lake food web dynamics.     

Biomanipulation-induced reduction of sediment phosphorus release in a tropical shallow lake

Biomanipulation via fish regulation combined with submerged plant introduction is an effective measure to restore eutrophic shallow lakes. Improved water quality and clarity promote growth of benthic algae, which with submerged plants may limit sediment phosphorus (P) release, thereby reinforce lake recovery. Our study sought to evaluate the effect of such a biomanipulation on water quality, benthic algal development and sediment P release in a shallow, tropical lake by (1) comparing porewater and lake water quality, light intensity and benthic algal development in restored and unrestored sections; (2) conducting a ³²P radiotracer experiment to track P release from sediment cores sampled from both sections. The biomanipulation led to lower total P, total dissolved P, and soluble reactive P concentrations in lake water, lower phytoplankton biomass, and increased light intensity at sediment surface, stimulating benthic algal development. Moreover, sediment ³²P release was lower in the restored than unrestored section. Concurrently, dissolved oxygen levels in upper layers of the sediment cores were higher in the restored section. Our study indicates that the biomanipulation improved water quality and enhanced growth of benthic algae, thereby reducing sediment P release, which may be one of the main mechanisms to create successful restoration.     

Partitioning processes controlling water column phosphorus concentrations in a shallow wetland

1. Phosphorus (P) concentrations in the water column of lakes and wetlands are crucial to their trophic status and ecosystem function, but quantifying the processes controlling P concentrations in the field has been a difficult task. A site‐based, in‐lake method is described to partition major field processes controlling P concentration in a shallow lake. 2. It involves (i) in‐lake deployment of a suite of chambers that isolate in‐chamber activities from atmospheric sources, groundwater input and horizontal water movement; (ii) monitoring P concentrations and relevant water properties inside and outside the isolation chambers; and (iii) calculating the contribution of each individual process by simple mathematical deduction, so as to differentiate the contributions from the different sources. 3. The method was applied at nearshore and offshore sites in a seasonal, groundwater‐fed shallow lake on the Swan Coastal Plain, south‐western Australia, during winter refilling. Primary (atmospheric and groundwater) and secondary processes (e.g. circulation and sediment‐water interactions) were partitioned and quantified in terms of their contributions to water column P [as total P (TP; μg m^?2 day^?1)]. 4. Atmospheric and groundwater inputs were the two main processes contributing P loadings (1233 and 1010 μg P m^?2 day^?1), but their influence appeared restricted to the near‐shore site. The estimated influence on TP by mixing‐circulation, atmosphere and groundwater were 2.4–25 times higher near the lake margin as compared with the offshore site. The circulation and sediment‐water interactions decreased water column P at the marginal site, but increased P offshore because of subsequent P release from sediment and a concurrent increase in pH. 5. Results are consistent with data reported elsewhere, and the factors that could affect the accuracy of partitioning are discussed.     

Diatoms as a proxy in reconstructing the Holocene environmental changes in the south-western Baltic Sea: the lower Rega River Valley sedimentary record

This study focuses on diatom assemblages occurring in cores of Late-glacial and Holocene deposits retrieved from the mouth of the lower Rega River valley, of the southern coast of the Baltic Sea. Sediment samples from four cores were the subject of the present study. Diatom-inferred environmental characteristics, e.g., water level; water salinity (conductivity), trophic status and pH, within each core are presented. Diatom assemblage zones (DAZ) were distinguished, based on differences in the distribution of particular ecological groups. Each DAZ appears to be related to environmental changes during the deposition of a given sediment interval. The Late-glacial (Allerød) sediments originated in a shallow lake with increasing concentrations of solutes and nutrients. The Holocene record begins in the early Atlantic Chronozone and the diatoms point to weakly brackish-water sediments deposited in a shallow water environment. During the period of 8,500–5,800 cal year BP sedimentation took place in a shallow embayment and/or lagoon. From ca. 5,800 cal year BP onwards sedimentation took place in a peat bog environment alternating with Aeolian deposition. Changes in diatom community structure imply a close relationship with the climate-controlled eustatic rise of the ocean level and its consequence Littorina transgression. As with other southern Baltic Sea localities, brackish-water diatoms appear in the sediments, signaling the onset of marine transgression somewhat earlier than previously accepted. Differences and similarities in diatom assemblages and the palaeogeographic development of nearby regions within the Baltic Sea basin and lagoons (coastal areas) from different geographic regions, are also discussed.     

Hydrochemistry from Sr and Mg contents of ostracodes in Pleistocene lacustrine deposits,Baza Basin (SE Spain)

A reconstruction of the early Pleistocene paleohydrochemistry based on the Mg, Sr and Ca content of the Cyprideis valves is presented for shallow lacustrine sequences of the Baza basin. A large number of environmental changes in this marginal area has been recorded by the recurrent alternation of two fossil assemblages which differ in their salinity requirements. Measurements of the Sr/Ca and Mg/Ca ratios of individual calcite shells of Cyprideis show that the water in the higher saline stages (with thalassic organisms indicating marine-like conditions) was of non-marine origin. The Sr/Ca values of Cyprideis valves from sands deposited during a saline water phase show lower values than those from an overlying carbonate sequence which was formed under lower salinity conditions. These unexpected values are assumed to be the result of major changes in the chemical composition of the water in shallow, littoral ponded areas of a hydrologically complex lake. In the sequences that originated in these areas, Sr/Ca values may be used only as salinity indicators within each portion of the sequence formed in a single, continuous evolution. In more open areas, the wide fluctuations of Sr/Ca and Mg/Ca recorded in ostracodes from individual layers of rippled ostracode-shell sands probably reflect the mixing of valves from changing short-term environmental conditions.     

Intraannual and interannual changes in the surface area of a closed lake complex in southwestern Siberia using NOAA images

The Lake Chany complex, located in southwestern Siberia, consists of large shallow lakes with an average depth of about 2.2m. The area of the lake fluctuates with the water level, which is closely related to the amount of inflow and evaporation, as the lake complex is endorheic. Using National Oceanic and Atmospheric Administration (NOAA) satellite images of the ice-free periods from 1999 to 2004, we evaluated the seasonal changes in the lake area and surrounding vegetation of the Lake Chany complex. The maximum lake area was observed in early May and decreased until late July. The lake area in August was about 60% of the maximum. Subsequently, the area tended to increase until early October. Compared to ground-truth data, the areas undergoing seasonal fluctuations in the NOAA images corresponded to vast stands of vegetation (Phragmites australis) that were several kilometers wide. These areas seem to be influenced by the inflow of snow-melt water and reed growth. Although interannual differences in the seasonal change in lake area were not great for the whole lake complex, the isolated Yudinskii Pool showed large interannual differences during summer and autumn, suggesting that monitoring this area using NOAA images will be useful for estimating the interannual fluctuations in the southwestern Siberian environment.     

Spatially heterogeneous trends in nearshore and offshore chlorophyll a concentrations in lakes Michigan and Huron (1998–2013)

1. Nutrient abatement programmes have been successfully implemented around the globe to reduce nutrient loading into aquatic ecosystems. Concurrently, the worldwide spread of invasive filter feeders, such as dreissenid mussels, may alter nutrient dynamics in invaded systems by sequestering nutrients away from pelagic zones and reducing primary production in offshore areas. Such is the case in the Laurentian Great Lakes of North America, where decades of nutrient abatement and the establishment of dreissenid mussels have seemingly resulted in more oligotrophic conditions and altered spatial patterns of nutrient availability and primary production.
2. Recent studies have focused on whole lake trends in primary production despite spatial differences in tributary inputs, bathymetry, and other environmental conditions that can affect primary production in nearshore areas. Thus, we hypothesised that trends in nearshore chlorophyll concentrations in different areas may diverge in a manner consistent with spatial differences in nutrient input. To evaluate these differences in surface chlorophyll responses, we assessed temporal trends in four different areas of Lake Michigan and three different areas of Lake Huron.
3. We hypothesised that in lakes Huron and Michigan, nearshore zones have experienced slower declines of chlorophyll concentrations relative to offshore zones. To assess this hypothesis, we estimated temporal trends of surface water chlorophyll concentrations (a proxy for primary production) from satellite imagery from 1998 to 2013. We calculated average surface chlorophyll concentrations for 10-m depth intervals ranging from the shore (0–10 m) to offshore (>90 m) during representative months of May, July, and September. We then analysed these data to determine if long-term trends in surface chlorophyll varied by season, proximity to the shoreline, and water depth.
4. The rates of annual change in chlorophyll concentrations in nearshore areas were markedly different to offshore trends in both lakes. Chlorophyll concentrations declined overtime in offshore areas, but nearshore chlorophyll concentrations were either stable (in May) or increased (in July and September) throughout the time series.
5. Differences in chlorophyll concentrations among areas were prominent in Lake Michigan. While differences between the northern and southern basin have been previously documented, trends in chlorophyll concentrations also differed between the eastern and western sides of Lake Michigan. Despite similar bathymetry and geographic features to Lake Michigan, regional trends were not observed in Lake Huron.
6. The results of this study are generally consistent with the nearshore shunt hypothesis, which predicts that dreissenid filtering, nutrient re-suspension, and continued nutrient loading from tributaries can cause an increase in primary production in nearshore areas during periods of offshore oligotrophication. Thus, the localised effects of nutrient abatement programmes in a given lake will be influenced by complex interactions between lake bathymetry and the presence of non-native filter feeding organisms.

     

Jetski-based plankton towing as a new method of sampling larval fishes in shallow marine habitats

I tested jetski-based plankton towing as a new method of sampling larval fishes in a shallow marine environment in warm temperate South Africa. Larval fishes were collected by towing a stainless-steel ring net behind a three-man jetski. The sampling technique was designed and tested to overcome problematic sampling of shallow sandy beach surf zones and the adjacent, shallow nearshore immediately seaward of the surf zone (behind the breaker line) with the same type of equipment. The composition, density, stage of development, estuary association, and length frequency of larval fish catches obtained by the new sampling method in the surf zone and in the adjacent shallow nearshore were compared. The success of the technique was evaluated by determining the extent of replication of known trends observed in other surf studies in the same climatic region of South Africa using either a seine or pushnet. Preflexion and postflexion larval fishes were collected. Larvae of the family Sparidae dominated the catches. Trends in catch composition, estuary association, and length frequency of the larvae were similar to those found in other studies of warm temperate South African surf zones. Density of larvae varied depending on the type of equipment used, although these results are tentative given the subjectivity of calculation of the water volume sampled and the extent of nursery use by larvae in the habitats sampled in each study. Increased towing times are recommended. Jetski-based plankton towing is a useful alternative technique for sampling larval fishes in shallow habitats and can easily be applied in surf zones, shallow nearshore areas, and estuaries. The technique greatly increases sampling agility in previously problematic sampling areas.     

Saline paleolake of the Cañadón Asfalto Formation (Middle-Upper Jurassic), Cerro Cóndor, Chubut province (Patagonia), Argentina

The carbonate-evaporitic sequence of the Cañadón Asfalto Formation that crops out in Cañadón Carrizal (Chubut province, Argentina) is characterized by a lacustrine facies association. This sequence includes an extensive biohermal belt, flood facies with high lake-level and low lake-level stages, and desiccation–evaporation facies. The biohermal belt shows three growth-stages of microbial communities. This belt controlled sedimentation in nearby areas, favoring the formation of the pan lake of Cañadón Carrizal. High lake-level flood facies represent sedimentation during periods of great water input to the pan lake. Low lake-level flood facies are characterized by low-energy conditions and by the development of a shallow lake with marginal zones and palustrine subenvironments. Desiccation facies represent a dry mudflat that borders the saline lake. Evaporation facies are related to the shrinkage of the water body and to the migration of the shallower facies towards the center of the ephemeral lake.     

Trace metal distribution in the sediments of the whole lake basin for Lochnagar,Scotland: a palaeolimnological assessment

Analyses of trace metals on multiple sediment cores from the whole-lake basin of Lochnagar, Scotland, show that the depth of departure from stable values towards a rapid increase of the Pb/Ti and Hg concentration profiles provides a good dating feature for the 1860s. In relatively shallow areas of the lake, inferred sediment accumulation rates and the trace metal inventories change with water depth, but in the deep water area, sediment accumulation rates are lower than in most other areas of the lake. Mercury, Pb, Cu and Zn inventories in the sediments accumulated since 1860 in the deepest area are 61%, 64%, 73% and 56% of the corresponding average inventories for the whole sediment area of the lake, respectively. This is mainly due to low sediment accumulation in the deep basin. This finding differs from the expected sediment focusing pattern and makes quantitative interpretations of palaeolimnological features using sediments from the deep area of this lake difficult. The influence of sediment focusing from the north-eastern side, the largest portion of the sediment area of the lake basin, on the deepest area of the lake may be limited, so the sediments in the north-east could be difficult to be transported to the deepest area through sediment focusing. Therefore, the sediments in the deepest area of the lake may not represent the whole-lake basin well for the relative abundances of different types of fossils.     

Physical and chemical characteristics of shallow embayments on the southern coast of Finland

Regular water-quality monitoring is yielding a great number of hydrographical and chemical data regarding the brackish Baltic Sea. However, present monitoring programmes often do not include shallow and sheltered near-shore areas, which are adjacent to human population and used for recreation purposes. We collected new near-shore water quality data from the southern coast of Finland that will allow the definition of the present state of these systems and may serve as reference data for possible future monitoring programmes. The data set comprises 45 small and shallow embayments with a mean surface area of 211 ha and a mean depth of 4.3 m. According to redundancy analysis (RDA), catchment land use explained 20.3% of the variance in the measured water chemistry. Agriculture had the strongest influence on the water quality of the study sites, capturing 13.1% of the variance. Nitrogen seemed to be strongly related to agriculture, whereas phosphorus was correlated with urban areas and industry. Higher than average nutrient concentrations regularly occurred at the mouths of large rivers, or close to human settlement, emphasising the significance of local sources to the nutrient concentrations of coastal waters. No increasing trend in nutrient or chlorophyll-a concentrations was observed from west to east in these shallow and sheltered nearshore areas. This contrasts with earlier results from pelagic samplings that report a marked influence of the River Neva and St. Petersburg on the nutrient status of the Gulf of Finland. Because these shallow embayments are strongly influenced by the local nutrient load, continued measures must be undertaken to reduce the nutrient load from land to prevent the continued deterioration and eutrophication of the nearshore, coastal waters.     

Spicule records of <Emphasis Type="Italic">Ephydatia fluviatilis</Emphasis> as a proxy for hydrological and environmental changes in the shallow Lake Trasimeno (Umbria,Italy)

This research shows the results of an analysis of siliceous spicules found in sediment cores collected in Lake Trasimeno (Umbria, Italy), a shallow lake that experienced an important water level lowering during the last century. A morphological analysis of sedimentary sponge records revealed that the spicules accumulated in the lake sediments over the last 150 years are attributable to Ephydatia fluviatilis, the only sponge species found in the lake in recent years. The stratigraphic analysis of the cores showed that the abundance and size of the sponge spicules (megascleres) have remarkably decreased, suggesting that a significant depletion of the sponge fauna occurred, particularly during the first half of the twentieth century. A correlation analysis has identified morpho-hydrological and related variables (the theoretical fraction of lake surface subjected to resuspension and the amount of total suspended solids) as the most significant factors explaining the change in density of sponge spicules. Two ecological explanations of the sponge decline are proposed, based on the sensitivity of the sponge both to the availability of suitable hard substrata for colonization, and to the amount of wind-resuspended solids. One-ended sigmoid response curves were obtained by regression and corresponding transfer functions were derived, which allow the mean water depth of the lake and total suspended solids to be inferred from spicule density records. The results support the use of sponge spicules as a paleohydrological and paleoecological proxy, application of which appears particularly promising for shallow-water systems.     

The catfish fauna of Lake Victoria after the Nile perch upsurge

Stocks of the indigenous catfish species of Lake Victoria have decreased dramatically since the beginning of the 1980s. This decline coincided with the Nile perch boom and concomitant ecological changes in the lake. In deep water, where Nile perch densities were higher, the decline proceeded more quickly than in shallow water. In the former all catfishes eventually vanished. Of the two largest species, Clarias gariepinus and Bagrus docmak, juveniles disappeared faster than adults. This indicates that predation by Nile perch may have played an important role in their decline. Other possible impacts were the deoxygenation of deepwater areas and the decline of haplochromine cichlids which were an important food source for B. docmak, C. gariepinus and Schilbe intermedius. The various catfish species were not all affected to the same extent. The endemic Xenoclarias eupogon, which lived predominantly in deep water, may have become extinct. B. docmak currently seems to be mainly restricted to refugia in rocky habitats. Synodontis victoriae and S. afrofischeri are still present in small numbers in shallow littoral areas. Schilbe intermedius and C. gariepinus seem to be the least affected of the catfishes in littoral and sublittoral areas. This may be caused, among other reasons, by their smaller habitat overlap with Nile perch than the other species. S. intermedius is partly pelagic, and a considerable part of the C. gariepinus stock lives in bodies of water surrounding the lake. The patterns of decline of the catfishes are very similar to those observed for haplochromine cichlids in the lake. The importance of catfishes for the fisheries in the lake is currently negligible.     

Bacterial and archaeal assemblages in sediments of a large shallow freshwater lake, Lake Taihu, as revealed by denaturing gradient gel electrophoresis

Aims:  To explore the association of microbial community structure with the development of eutrophication in a large shallow freshwater lake, Lake Taihu.
Methods and Results:  The bacterial and archaeal assemblages in sediments of different lake areas were analysed using denaturing gradient gel electrophoresis (DGGE) of amplified 16S rDNA fragments. The bacterial DGGE profiles showed that eutrophied sites, grass-bottom areas and relatively clean sites with a eutrophic (albeit dredged) site are three respective clusters. Fifty-one dominant bacterial DGGE bands were detected and 92 corresponding clones were sequenced, most of which were affiliated with bacterial phylotypes commonly found in freshwater ecosystems. Actinobacteria were detected in the centre of the lake and not at eutrophied sites whereas the opposite was found with respect to Verrucomicrobiales . Twenty-five dominant archaeal DGGE bands were detected and 31 corresponding clones were sequenced, most of which were affiliated with freshwater archaeal phylotypes.
Conclusions:  The bacterial community structures in the sediments of different areas with similar water quality and situation tend to be similar in Taihu Lake.
Significance and Impact of the Study:  This study may expand our knowledge on the relationship between the overall microbial assemblages and the development of eutrophication in the shallow freshwater lake.