A simple and reliable turbidimetric and kinetic assay for alpha-amylase that is readily applied to culture supernatants and cell extracts

Summary A simple, reliable and sensitive assay for alpha-amylase activity is reported, together with its theoretical derivation, that overcomes many of the problems encountered with other assays, especially when attempting to assay alpha-amylase activity in crude cell extracts or culture supernatants. The method relies on the reduction in turbidity that occurs upon digestion of a starch suspension with alpha-amylase. The initial rate of decrease in turbidity is shown to be proportional to a wide range of enzyme concentrations, permitting a rapid spectrophotometric and kinetic determination of alpha-amylase activity.     

Migration of smolts of Atlantic salmon (Salmo salar L.) and sea trout (Salmo trutta L.) in a chalkstream

Synopsis Migration of smolts in the River Piddle, Dorset, was studied over three years in relation to factors that could influence downstream movement. The river originates mostly from groundwater springs, resulting in stable flows and low turbidity except in very rainy weither. Fish were intercepted at the tidal limit in a fixed trap-net, and measurements of water temperature, discharge, turbidity, barometric pressure, rainfall and solar radiation taken nearby. Slightly increased turbidity and discharge following heavy rain initiated major movements during two nights of the total of 55 days studied. At other times large-scale movements rock place during sunny warm afternoons. Both solar radiation and water temperature were correlated with intensity and timing of movement. The pattern of migration is different from that reported on other rivers, reflecting the relatively stable flow regime of the chalkstream.     

OBSERVATTVNS ON THE UNDERWATER LIGHT FIELD OF TWO HWERTROPHIC,SOUTH AFRICAN IMPOUNDMENTS

SUMMARY

The spectral composition of the underwater light field was examined in two hypertrophic South African Impoundments (Hartbeespoort and Roodeplaat Dams) under a range of inorganic turbidities and chlorophyll α concentrations. The data indicated that inorganic turbidity and gilvin were dominant to chlorophyll in regulating underwater light attenuation during the present study. Under all conditions the wavelengths between 405 and 510 nm were attenuated more rapidly than near UV and the wavelengths above 510 nm and the 623 nm component penetrated deepest. Under low turbidities the 546 nm wavelength was the next most penetrating component, but its attenuation increased with increasing turbidity. This characteristic of the underwater light field may be important to the cyanobacteria which dominate in these hypertrophic lakes.     

Predator–prey encounter rates in freshwater piscivores: effects of prey density and water transparency

One of the most fundamental components of predator–prey models is encounter rate, modelled as the product of prey density and search efficiency. Encounter rates have, however, rarely been measured in empirical studies. In this study, we used a video system approach to estimate how encounter rates between piscivorous fish that use a sit-and-wait foraging strategy and their prey depend on prey density and environmental factors such as turbidity. We first manipulated prey density in a controlled pool and field enclosure experiments where environmental factors were held constant. In a correlative study of 15 freshwater lakes we then estimated encounter rates in natural habitats and related the results to both prey fish density and environmental factors. We found the expected positive dependence of individual encounter rates on prey density in our pool and enclosure experiments, whereas the relation between school encounter rate and prey density was less clear. In the field survey, encounter rates did not correlate with prey density but instead correlated positively with water transparency. Water transparency decreases with increasing prey density along the productivity gradient and will reduce prey detection distance and thus predator search efficiency. Therefore, visual predator–prey encounter rates do not increase, and may even decrease, with increasing productivity despite increasing prey densities.     

Impact of climatic fluctuations on Characeae biomass in a shallow,restored lake in The Netherlands

External phosphorus load to a wetland with two shallow lakes in the Botshol Nature Reserve, The Netherlands, was reduced, resulting in a rapid reduction of phytoplankton biomass and turbidity, and after 4 years, explosive growth of Characeae. The clear water state was unstable, however, and the ecosystem then alternated between clear, high-vegetation and turbid, low-vegetation states. A model of water quality processes was used in conjunction with a 14-year nutrient budget for Botshol to determine if fluctuations in precipitation and nutrient load caused the ecosystem instability. The results indicate that, during wet winters when groundwater level rose above surface water level, phosphorus from runoff was stored in the lake bottom and banks. Stored phosphorus was released the following spring and summer under anaerobic sediment conditions, resulting in increased phytoplankton density and light attenuation in the water column. During years with high net precipitation, flow from land to surface water also transported humic acids, further increasing light attenuation. In years with dry winters, the phosphorus and humic acid loads to surface water were reduced, and growth of submerged macrophytes was enhanced by clear water. Thus, the temporal pattern of precipitation and flow from land to water gave a coherent, quantitative explanation of the observed dynamics in phosphorus, phytoplankton, turbidity, and Characeae. Global warming has caused winters in The Netherlands to become warmer and wetter during the last 50 years, increasing flow from land to water of humic acids and phosphorus and, ultimately, enhancing instability of Characeae populations. In the first half of the 20th century interannual variation in precipitation was not sufficient to cause large changes in internal P flux in Botshol, and submerged macrophyte populations were stable.     

In situ feeding rates and grazing impact of Mesopodopsis africana O. Tattersall in the St Lucia Estuary, South Africa

Mesopodopsis africana is an important mysid in southern African coastal zooplankton and a key species in the St Lucia estuarine lake, which is currently undergoing severe desiccation owing to freshwater deprivation. M. africana populations through much of the system are consequently under severe environmental stress. This study investigates the grazing dynamics of this mysid species, in relation to autotrophic food availability and other environmental constraints in two contrasting areas of the St Lucia Estuary, Charters Creek, heavily affected by the desiccation process and the Mouth, virtually under unchanged conditions. Gut evacuation experiments were conducted once each during the day and the night. Evacuation rates were consistently higher during the night, ranging from 0.27 to 0.33 h^-1 at Charters Creek and from 1.13 to 1.24 h^-1 at the Mouth. Ingestion rates were, therefore, higher at the Mouth resulting in population grazing impacts of 2.5% of the total microalgal biomass, while the grazing impact at Charters Creek was only 0.5%. The spatial variation in ingestion rates could be attributed to seasonal differences in gut evacuation rates, differences in the mean size of mysids used, or the physicochemical conditions present at the two stations. It is suggested that mysid populations at Charters Creek are predominantly driven by bottom-up forces, initiated by the harsh environmental conditions. Despite the lower ingestion rates exhibited at Charters Creek, results indicate that these mysids are capable of meeting all their energetic requirements from a microalgal diet alone, although they may also utilise a heterotrophic diet.     

Determining effects of suspended sediment on condition of a suspension feeding bivalve (Atrina zelandica): results of a survey, a laboratory experiment and a field transplant experiment

The horse mussel Atrina zelandica (Gray) is a large, suspension feeding pinnid bivalve, common in coastal and estuarine areas of northern New Zealand. As a suspension feeder, Atrina is likely to be influenced by suspended sediment loads. We conducted a laboratory experiment to determine the effect of short-term elevations in turbidity levels, such as those commonly recorded during storms, on the physiological condition and clearance rates of Atrina. We also conducted a field survey and a 3-month transplant experiment at multiple sites along a gradient of increasing suspended sediment load in a New Zealand estuary. Laboratory clearance rates of Atrina declined above a threshold suspended sediment concentration, and Atrina physiological condition at the end of this experiment was lower in high cf. low turbidity treatments. Decreases in Atrina condition were detected after exposure to elevated levels for only 3 days. The field survey and transplant experiment provided empirical evidence of a strong, negative effect of increasing suspended sediment flux on the physiological condition of Atrina. We suggest that relationships between the physiological condition of suspension feeders and sediment settling flux could provide a link between sediment inputs, which commonly occur as a result of catchment runoff during rainfall events, and the ecological health of estuarine and shallow coastal areas. Our study also demonstrated that Atrina have a natural distribution limit controlled by suspended sediment load. Thus, there is potential for larger-scale functional and structural effects on benthic communities in estuarine and coastal areas with high rates of sedimentation.     

Thorn fish Terapon jarbua (Forskål) predation on juvenile white shrimp Penaeus indicus H. Milne Edwards and brown shrimp Metapenaeus monoceros (Fabricius): the effect of turbidity, prey density, substrate type and pneumatophore density

A series of laboratory experiments was conducted at Inhaca Island Marine Biological Station, Mozambique, in order to assess the separate effects of turbidity, prey density, substrate type, pneumatophore density, and the combined effects of turbidity with the latter three, on rate of predation by the thorn fish Terapon jarbua (Forskål, 1775) on white shrimp Penaeus indicus and brown shrimp Metapenaeus monoceros.Significant interactions between turbidity and the other three factors on shrimp predation for both prey species were detected. Regardless of prey density, increasing turbidity decreased predation on P. indicus, but not on M. monoceros, for which increasing densities reduced the protective effect of turbidity. Increasing prey density increased predation on P. indicus in clear water, and increased predation on M. monoceros in low and high, but not in intermediate turbidity or clear water. The presence of a substrate suitable for burying decreased predation on M. monoceros in clear water, but not in the turbidity levels used. In clear water, solely sandy-shell substrate afforded protection to P. indicus, while in turbid water, no substrate offered significant protection and muddy substrate even increased prey vulnerability to fish probably as a result of increased preys' locomotor activity. Raising pneumatophores density seems to lower the protective value of turbidity for both species. In clear water, only low and high structure density provided a deterrent effect on predation on P. indicus; in turbid water, intermediate and higher structure density increased predation. Increasing structural complexity reduced predation on M. monoceros linearly in clear water; but in low turbid water it increased. In high turbid waters, the increase was only significant in intermediate pneumatophore density. High structural complexities impair the pursuing capacity of fish and thus decreased predation rates. The results indicate that the effective provision of shelter of different habitats depends not only on the various environmental parameters analysed, but also on the way they interact and on the behaviour of prey and predator as well.     

Spatial variability in the abundance of pelagic invertebrate predators in relation to depth and turbidity

The abundance of pelagic invertebrate predators in relation to turbidity and depth gradients in Lake Hiidenvesi (southern Finland) were studied. In the shallow (<5 m) and the most turbid (up to 75 NTU) part of the lake, the community of invertebrate predators consisted of cyclopoid copepods (max biomass >500 μg dw l⁻¹) and Leptodora kindtiii (Focke) (17 μg dw l⁻¹), while in the less turbid (10–40 NTU) stratifying area Chaoborus flavicans (Meigen) dominated (max 146 μg dw l⁻¹). In the temporarily stratifying and moderately turbid basin Chaoborus and small-bodied invertebrate predators co-existed. Mysis relicta (Lovén) occurred only in the stratifying area (max 15 μg dw l⁻¹). The results suggested that both water depth and turbidity contributed to the community structure of Chaoborus flavicans. Depth great enough for stratification was of special importance and its effect was amplified by elevated turbidity, while high turbidity alone could not maintain chaoborid populations. Mysis relicta also requires a hypolimnetic refuge but is more sensitive to low oxygen concentrations and may therefore be forced to the epilimnion where it is vulnerable to fish predation. Cyclopoids as rapid swimmers can take advantage at elevated turbidity levels and coexist in high biomass with fish even in shallow water. Leptodora kindtii can form high biomass despite planktivorous fish providing that turbidity exceeds 20 NTU. The results demonstrated that depth and water turbidity can strongly regulate the abundance and species composition of invertebrate predators. These factors must thus be taken into account when applying food web management, which aims to reduce phytoplankton biomass by depressing planktivorous fish.     

Cryptic assemblages in coral-rubble interstices along a terrestrial-sediment gradient

The assemblage composition of cryptic mobile animals inhabiting coral rubble was sampled using mesh traps containing clean coral rubble, and used as indicators of land-based pollution at 14 sites in three coral lagoons at Ishigaki Island, southern Japan. Cluster analyses identified three groups of large mobile animal assemblages (molluscs, echinoderms, fishes, decapod and stomatopod crustaceans). Using a distance-based redundancy analysis (db-RDA) there was a significant relationship between the assemblage composition and environmental variables. The 1st axis of the db-RDA ordination was regarded as the land-based pollution gradient because of the strong relationship with silicate sediment, turbidity, and salinity, indicating effects of terrestrial-sediment runoff. Species response curves were derived from a plot of the number of individuals against the 1st axis of the db-RDA sites sample score. The response curves of Galathea mauritiana, an indicator species for the intermediate sites, were unimodal along the land-based pollution gradient. This study demonstrates the use of traps containing clean coral rubble for nondestructive quantitative sampling and environmental monitoring in coral lagoons, and their potential for monitoring changes in the reef environment. Communicated by Environmental Editor Prof. Rob van Woesik.