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721.
722.
F. I. M. Thomas 《Marine Biology》1994,119(4):525-534
The morphology of organisms can influence fluid flow near their surfaces which, in turn, can influence the rates of mass exchange as well as the ability of the animals to capture and retain particles. In this paper, the morphologies and orientations of extensions on the tubes of aggregated individuals of the sabellariid polychaete Phragmatopoma californica from different habitats are described. The effects of these structures on fluid exchange and feeding-tentacle deflection are also assessed. The tube shape of aggregated P. californica varies among habitats. In exposed habitats, extensions on tubes (flares) encircle the entire circumference of the worms' apertures. In more protected habitats, extensions (hoods) partially encircle the circumference of apertures and are oriented so that the opening of the hood faces into the predominant direction of water flow over the aggregation. After the surface structures are damaged, hoods are repaired more quickly than flares. In this study, all damaged hodds were repaired within 24 h while flares were fully repaired only after 240 h. The repaired hoods are oriented in the same direction as the original structures Both hoods and flares decrease the rate of fluid exchange between the aggregation surface and the mainstream water flow. These structures also decrease the deflection of the feeding tentacles by the water flow. The orientation of the hoods relative to the direction of water flow affects both the rates of fluid exchange and the deflection of the tentacles.
Present address: Hawaii Institute of Marine Biology, P.O. Box 1346, Kaneohe, Hawaii 96744, USA 相似文献
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Fresh water, a fundamental element of all estuarine ecosystems, is South Africa’s most limited natural resource. Recent projections
indicate that by the year 2020 the country will be utilizing all its exploitable freshwater sources. Steeply increasing demands
by a rapidly growing population on this limited commodity have already resulted in a severe reduction of water supplies to
natural users such as estuaries — this trend is predicted to increase in the future. Concurrent with excessive water abstraction,
poor land husbandry (e.g. soil erosion) in many catchment basins and pollution (e.g. salinization) in return flows have led
to a serious deterioration in water quality. In contrast, a review of estuarine responses to varying flow regimes stresses
the strong dependence of local systems on riverine fresh water inputs of adequate quantity and quality. Freshwater dependence
is i.a. expressed in: flooding events that scour accumulated sediments, riverine nutrient input to drive estuarine phyto-
and zooplankton production, axial salinity gradients that increase habitat and species diversity, and maintenance of open
tidal inlets that prevent salinity and temperature extremes and facilitate larval exchange, fish migrations and tidal flushing
of salt marshes. Thus, estuarine conservation will have to encompass management of rivers and watersheds and play an increasingly
political role in decision processes concerning water allocations among ‘human’ and ‘natural’ users. 相似文献
725.
Benjamin?TooheyEmail author Gary?A.?Kendrick Thomas?Wernberg Julia?C.?Phillips Sairah?Malkin Jane?Prince 《Marine Biology》2004,144(5):1019-1027
Kelp canopies have long been recognised for their influence on the structure of algal assemblages on sublittoral reefs. In Marmion Lagoon, Western Australia, we investigated how a canopy of the small kelp Ecklonia radiata affected the associated foliose algal assemblage and what potential mechanisms were responsible for the effects we observed. Light levels and physical abrasion (thallus scour) by an E. radiata canopy were manipulated in an orthogonal fixed-factor experiment that revealed that changes in light, but not thallus scour, had a significant effect on the composition of the algal assemblage. Reduced light levels were associated with a decrease in the Shannon-Wiener diversity index and an increased dominance of the foliose algae Pterocladia lucida and Rhodymenia sonderi. Photobiological investigations of three foliose species, P. lucida, R. sonderi and Chauviniella coriifolia indicated that they were able to photoacclimate to low light levels by increasing thallus absorption by up to 11%. Photoacclimation was also evident by an increase in the maximum rate of electron transport under low-light conditions. We conclude that the E. radiata canopy in Marmion Lagoon structures the foliose algal assemblage through the modification of the light environment and that this effect may be mediated by differences in the ability of different species of foliose algae to photoacclimate.Communicated by M.S. Johnson, Crawley 相似文献
726.
Few studies have investigated the long-term fitness consequences of nestling size hierarchies in altricial birds. In this
study, we investigated whether or not the size rank order of siblings influences subsequent breeding success in the little
egret, Egretta garzetta. From a marking program allowing individual recognition of wild birds, we obtained data on the breeding success of 56 pairs
comprising individuals for which the size rank order was known. The breeding success in these pairs was positively influenced
by the age of the marked bird but negatively affected by the laying date of the pair and the size rank order of the marked
individual. There was also a significant difference between breeding colonies. We suggest two main hypotheses for a link between
size rank order of individuals and their breeding success and we discuss our results in relation to current hypotheses on
the adaptive value of hatching asynchrony.
Received: 10 August 1998 / Accepted after revision: 13 December 1998 相似文献
727.
Attention is focused on biological systems which are describable in terms of ordinary differential equations subject to human control inputs. The concept of an isochronal system is introduced in order to include systems for which the differential equations are valid only over regularly reoccurring time intervals.It is assumed that the control inputs are to be chosen so that an integral cost function of the state of the system, the control used, current time, and the time interval of the control program is minimized. Problems associated with minimizing this cost function over an infinitely long time interval is then considered. Difficulties inherent with minimizing a cost integral on an infinite time interval are shown to be avoided by minimizing an average of the cost function over an unknown but periodic time interval. Under proper circumstance, the optimal control program for the average cost function is either identical to or a good approximation to the optimal control program for the original cost function over an infinitely long time interval.Necessary conditions are obtained for minimizing an average cost function over an unspecified time interval subject to the system equations. For a given problem the necessary conditions will yield but a single system trajectory in the state space. For management purposes this trajectory may be thought of as a target to which the system should be driven and maintained.A number of examples illustrate the use of the necessary conditions to obtain control targets. Certain problems associated with the stability of the target solutions are illustrated with the examples. 相似文献
728.
Bone charcoal is being developed as a treatment for decontaminating polluted water. In particular, its potential to adsorb metal species from contaminated water supplies is being examined. In this study, bone charcoal was used in batch and column experiments designed to investigate the sorption of two cationic metals Cu and Zn. The data from batch experiments fitted the Langmuir isotherm and so theoretical maximum binding capacities could be obtained. These values were compared with experimental values derived from the column experiments. The mechanisms involved in the immobilisation were also investigated. 相似文献
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730.