The development and persistence of alternative ecosystem states in a large,shallow lake

1. Palaeolimnological data were used to investigate drivers of the community of primary producers in Lake Mattamuskeet, North Carolina, U.S.A. This is a large, shallow lake with two basins currently dominated by phytoplankton and macrophytes. The two basins were divided in 1940 by the building of a roadway across the lake, which also corresponded with the divergence in their ecosystem state. 2. Photosynthetic pigments, organic matter and nutrients (P, N, C, S) were analysed in sediment cores from each basin to reconstruct the primary producer community over the past c. 100 years. We sought to answer two questions. First, what changes to the ecosystem resulting from the building of the roadway caused the development of different primary producer communities in the two basins? Second, why have the alternative ecosystem states persisted despite a variety of human perturbations since 1940? 3. K‐means cluster analysis and principal component analysis were applied to identify three sediment types based on photosynthetic pigment data: sediments indicating low productivity (low pigment concentrations), sediments associated with macrophytes (chlorophyll a and b) and with phytoplankton (alloxanthin and aphanizophyll). In addition, other palaeolimnological proxies measured, such as loss on ignition, total phosphorus, total organic carbon/total nitrogen and other nutrients, were different in post‐1940 sediments within the two basins. 4. These differences suggest characteristics, such as nutrient cycling, water depth and other physical changes resulting from roadway construction, combined to establish and maintain the differing communities of primary producers in the two basins. Furthermore, Fe/S dynamics and waterfowl herbivory probably contributed to the development of the two ecosystem states.     

Nitrate Uptake by Manganese-Deficient Lemon Plants

The uptake of nitrate and water was followed in Eureka lemon (Citrus limon (L.) Burm. f.) plants grown in solution culture in a greenhouse under short (3–6 months) and long-term (22–24 months) Mn-nutrition stress. Uptake was determined from depletion in the nutrient solution. Under short-term stress, manganese-deficient plants absorbed 14.5% more nitrate and 3.4% more water than the control plants, on a weight basis. Under long-term stress there was a three-fold increase in nitrate and a two-fold increase in water uptake in the Mn-deficient plants. The intensive nitrate uptake under Mn-deficiency stress was more spectacularly demonstrated in plants which were exposed also to low nitrogen supply. The low-nitrogen Mn-deficient plants absorbed more nitrate, had less stunted growth and developed fewer visible symptoms of both N and Mn deficiencies than high-nitrogen Mn-deficient plants.     

The biology of some Argiope species from New Guinea: predatory behaviour and stabilimentum construction (Araneae: Araneidae)

Three species of Argiope: A. picta, A. aemula and A. reinwardti were studied at Wau, New Guinea. The stabilimenta in 548 webs of these species were classified into 18 categories, and the frequency of occurrence of each category was recorded. The data from this study are compared with previous records on the variation in the form of the devices built by spiders of the genera Argiope and Gasteracantha and related to theories about stabilimentum function.
Studies of the predatory behaviour of the three species show that this is, in general, similar to that of Argiope argentata . Data are cited to show that the spiders discriminate lepidopterans from insects of similar size. Particular attention was paid to the question of whether the species interrupt predatory sequences, after the attack phase, with a stage during which they rest at the hub of the web before transporting the prey. Unlike Argiope argentata these species interrupt a high proportion of predatory sequences that are initiated by bite/wrap attacks. This aspect of their behaviour is evaluated and discussed.     

Cold-Inhibited Phloem Translocation in Sugar Beet: II. CHARACTERIZATION AND LOCALIZATION OF THE SLOW-COOLING RESPONSE

Experiments were undertaken on a simplified sugar beet systemto characterize the phloem translocation response to slow coolingtreatments that were applied to the source leaf petiole. Inthese experiments the temperature was decreased by 4°C every16 min, such that the tissue temperature was lowered from 25°Cto 1°C over a period of 80 min. Our results indicated thatan initial slow cooling treatment, on a given test plant, causedno change in the rate of translocation. However, all subsequentslow cooling regimes that were applied to the same petiole positionelicited a characteristic step-type inhibition. This inhibitionaveraged about 10% of the original translocation rate in allcases with no recovery being observed. The data suggest thatthe initial cooling treatment induced an alteration in the petioletissue which facilitated the inhibition phenomenon during subsequentslow coolings. This alteration was shown to be localized withinthe upstream region of the chilled petiole segment, followingan initial slow cooling, or throughout the chilled petiole segmentafter an initial quick cooling from 25°C to 1°C. Resultsalso show that the alteration is a long-lived phenomenon thathas no detectable influence on the quick-cooling induced transientinhibition of translocation. Key words: Phloem, Translocation, Cooling response, Petiole     

TRIANGULAR DIFFERENCE TESTING: REFINEMENTS TO SEQUENTIAL SENSITIVITY ANALYSIS FOR PREDICTIONS FOR INDIVIDUAL TRIADS

Using a model system of 3mM NaCl and purified water, predictions were made for subjects’ability to discriminate the odd sample in a set of nine triadic tasting sequences associated with the triangle test. With such individual triads rather than triangular testing protocols which involve more than one triad, it was found necessary to modify the regular Sequential Sensitivity Analysis. The modification required a consideration of sensory adaptation effects caused by repeated tasting of the same stimulus in some of the sequences. Further confirmation was also obtained for the order of magnitude of signal strengths of water and NaCl stimuli tasted after water or NaCl prerinses.