Now showing items 21-40 of 83

• #### Numerical modeling study of the circulation of the Greenland Sea

This study is a simulation of the circulation of the Greenland Sea aimed at modeling some of the issues related to the Great Salinity Anomaly (GSA) and deep water formation using a primitive equation ocean general circulation model (Semtner, 1974). The features of the model include: (1) a high resolution, (2) real topography, (3) open boundaries at the south and north, and (4) temporally variable wind and thermohaline forcing. The model is used to study: (1) the spreading of a fresh water anomaly, (2) the mechanisms of cross frontal mixing that lead to deep water formation, (3) the general circulation of the deep and upper layers of the ocean and their dependence on wind and thermohaline forcing, and (4) the possible implications of meso-scale and large-scale variability on climate change. One of the major results of this work is the simulation of continental shelf waves propagating along the shelf slope of Greenland between 77$\sp\circ$N and 72$\sp\circ$N. Waves with a subinertial period of 17.2 hrs, a wavelength of 363 km, a phase speed of 586 cm/s and a group velocity of 409 cm/s, are found. Possible mechanism for generation of shelf waves is presented. It is suggested that some energy related with wave activity may support cross-frontal mixing in the East Greenland Current (EGC), where formation of the two main sources of North Atlantic Deep Water (e.g. Norwegian Sea Deep Water and Denmark Strait Overflow Water) have been reported. The results from the GSA simulation suggest that during the early stage of the GSA (e.g. during its propagation with the EGC to the south, in the late 1960s) when no observations are available, the fresh water signal is not being mixed into the interior circulation of the Greenland Sea gyre. The second experiment, representing recirculation of the GSA from the North Atlantic back into the Greenland Sea, in the late 1970s, shows freshening in the Greenland Sea gyre of comparable magnitudes ($-$0.05 to $-$0.1 psu) to the observed ones. These results agree with the earlier indirect measurements (Rhein, 1991; Schlosser et al., 1991) indicating dramatic reduction of deep water renewal in the Greenland Sea in the late 1970s and early 1980s. From the general circulation experiments it has been found that the ocean response to seasonal forcing is mainly barotropic. This implies a strong topographic control in the distribution of currents and hydrographic variables. Most of the areas of topographic steering which are simulated in the region have been reported in the literature. The so-called Molloy Deep eddy shows its direct dependence on the large scale dynamics affecting the northward flow of the West Spitsbergen Current (WSC), controlling this way a net mass transport into the Arctic Ocean. Simulations with different wind forcing suggest dependence of the Greenland Sea gyre circulation on the variations with time of the local wind forcing. Results indicate that monthly mean wind stress forcing probably underestimate wind forcing in the model. Analysis of surface, intermediate and deep ocean velocity fields compare reasonably well with observations.
• #### Physiological and ecological implications of hemorheological variations in marine and terrestrial mammals

The possible significance of variations in interspecific hemorheological properties related to diving behavior was studied in eight species of marine mammals with humans and pigs as terrestrial controls. Diving duration was positively correlated with elevated blood hemoglobin, oxygen capacity and viscosity among animals of the same class. No acclimatization response to activity was evident from studies of blood drawn from newly-captured northern elephant seals and sea otters and those in captivity for extended periods which justified the use of captive animals for rheological studies. Adaptations of marine mammals to diving were evident from comparisons of phocid seal and pig hemorheology. Seals had increased oxygen storage (six times) with less viscosity-dependent reductions in oxygen transport ($-$22%) when compared to pigs at equal packed cell volume. Phocid seal blood samples were compared with those of pigs and humans for erythrocyte aggregation and blood viscoelasticity to study the mechanics of viscometric variations. Viscous and elastic components of seal blood viscosity were 20 to 73% lower than those of pigs due to decreased aggregation extent and rate (P $<$ 0.05). Lower plasma fibrinogen and increased erythrocyte electrophoretic mobility are believed to contribute to lowered seal blood aggregation. Comparisons of the in vivo effects of blood viscosity on whole body and myocardial oxygen consumption by manipulation of whole body hematocrit in seals and pigs revealed that optimal hematocrit ranges for seals were shifted to the right of those from pigs (SEALS: 25%-55%; PIGS: 25%-45%; P $<$ 0.05). Seals showed significantly less viscosity-dependence in total body oxygen transport and oxygen consumption than did pigs. Myocardial oxygen consumption data were variable and showed no statistically significant differences among seals and pigs. The seals' lower erythrocyte aggregation, decreased low-shear viscosity and a greater ability to compensate for viscosity changes may represent adaptations to reduce the stress necessary to reinitiate flow in stagnant venous sinuses thereby reducing blood-flow resistance during dive-recovery. These adaptations may help maintain circulatory perfusion to vital organs, while flow is restricted to less oxygen-dependent tissues during underwater submergence without sacrificing the advantage of increased blood oxygen storage.
• #### Influences of abiotic factors on the return, ocean abundance, and maturity of sockeye salmon (Oncorhynchus nerka) in the northern North Pacific Ocean

The fluctuations in return, ocean abundance, and maturity of sockeye salmon (O. nerka) were examined and related to wind stress curl, sea suface temperature (SST), sea level pressure, and cloudiness, in the area between 40$\sp\circ$N-60$\sp\circ$N and 160$\sp\circ$E-140$\sp\circ$W. Historical records, during two periods, 1971-76 and 1955-86, were the primary source of data. Spectral analysis of a 360-month period of mean wind stress curl during 1955-85 showed 3.1- and 5.3-year cycles. The 5.3-year cycle was correlated (r =.32 to.44, P $<$.10) with the return of Bristol Bay sockeye salmon mostly at 0- (the year of spawning migration) and 1-year lag (the first year of lake residence). The relative ocean abundance of sockeye salmon in the northwestern Northern Pacific during 1971-76 was lowest during the three periods: 1961-70, 1971-76, and 1977-85. Mature Kamchatka sockeye salmon were 24% more abundant than mature Bristol Bay sockeye salmon during 1971-76. A significant relationship was found between the May-June mean SST and abundance of sockeye salmon (r =.56 to.66, P $<$.01) during 1961-85. In the northern North Pacific, the SST was positively (r =.73 to.86, P $<$.001) related with the gonad weight of sockeye salmon. The results indicated a close relation between the return, ocean abundance, and maturity of sockeye salmon and most of the abiotic factors.
• #### The paradox of pelagic food webs on the Bering-Chukchi continental shelf

Prolific primary production and spectacular populations of marine birds and mammals in the northern Bering Sea were for many years considered to be a paradox of an environment that should have had low production, as is typical of shallow continental shelves elsewhere. However, a "river" of oceanic water, Anadyr Water, originating along the continental slope of the Bering Sea carries a perpetual supply of nutrients and biota onto this northern shelf that transforms part of the region into one that is extremely productive at all trophic levels. Diatoms grow profusely throughout the ice-free season and, together with oceanic zooplankton advected in the Anadyr stream, provide the energy base for rich pelagic and benthic food webs. Contrasting with the highly productive pelagic regime is one associated with Bering Shelf Water and Alaskan Coastal Water. Both of these water masses originate over the shallow shelf of the northern and eastern Bering Sea, and are typically nutrient-poor following the spring phytoplankton bloom. Terriginous nutrients introduced by the Yukon and other rivers are not sufficient to elevate primary production above a low level typical of inner shelf regions. The oceanic zooplankton are excluded from this environment, and populations at higher trophic levels are small. The consequence of these contrasting physical regimes is that discrete oceanic and inner shelf food webs coexist in a small geographic region where only a coastal ecosystem is expected.
• #### On the dynamics of the Alaska coastal current

The Alaska Coastal Current (ACC) in the northern Gulf of Alaska is a wind- and buoyancy-driven near-surface jet primarily maintained by the horizontal salinity gradient due to fresh water entering at the coast. It serves as the major source of fresh water to the North Pacific Ocean. The buoyancy driving force is the major focus of this investigation. The study area is situated just "downstream" of Prince William Sound (PWS), a large estuary whose surface outflow is seen to occupy a narrow inshore band after joining the ACC. The effect of this band appears to be the formation of an occasional double maximum in the ACC. The period focused on in this study was selected on the basis of weak windstress but large fresh water input in order to emphasize the buoyancy forcing. The TS characteristics and a water mass tracing technique are used to separate the thermal and haline signals in the buoyancy forcing and to track the origin and fate of the source waters of the study area. The buoyancy driving force is shown to be primarily haline, with temperature playing a secondary, moderating role. Because of the large topographic variability and sloping density interfaces, and in order to exploit the available data, a diagnostic model retaining the baroclinicity and bottom topography terms was chosen to study the dynamics. Model premises are verified by results from hydrographic surveys, moored current meters, and a profiling current meter. The model predicts a midshelf region of negligible sealevel gradient, with a nearshore ($\approx$70 km wide) band over which the sealevel changes by about 25 cm. The sloping surface drives a strong ($\approx$100 cm/s) surface flow, which decreases to zero and reverses below about 100 m due to the opposing baroclinic pressure gradient. The flow splits around a shoal region. The onshore portion joins the outflow from PWS and accelerates downstream forming a double maximum. The offshore segment forms a large meander before rejoining the rest of the ACC, advecting midshelf water shoreward. The momentum balance is dominated by the JEBAT terms, which primarily determine the flow along and across contours of f/H.
• #### Nitrogen flux in the northern Bering Sea

Much of the primary production occurring over the Bering Sea continental shelf is thought to be associated with both ice edge and spring blooms. The nature of summer production over the shelf is now being addressed. A general model is presented for summer phytoplankton production along the Bering Sea shelf break front and subsequent transport of phytodetritus into the northern Bering Sea. Production associated with the shelf break front is estimated to be 200 g $\cdot$ C $\cdot$ m$\sp{-2}$ over a 120-day growing season and is supported by nutrients from the Bering Slope Current. A portion of the biomass accumulating over the front is advected into the Chirikov Basin, supplying 26% of the daily carbon demand of the benthos. The Bering Slope Current bifurcates at Cape Navarin and one branch, referred to as the Anadyr Current, flows north through Anadyr and Bering Straits. Nutrients in the Anadyr Current support an intense surface bloom over the western Chirikov Basin where total nitrogen uptake rates are $>$6.0 mg-at N $\cdot$ m$\sp{-2} \cdot$ h$\sp{-1}$ and nitrate contributes up to 50% of the total nitrogen uptake. Modified Bering Shelf water contains phytoplankton at two depths: both a surface accumulation and a deep layer. Nitrate contributes $<$35% to total nitrogen uptake rates of 1.80 mg-at N $\cdot$ m$\sp{-2} \cdot$ h$\sp{-1}$ in this water. Nitrogen productivity is lowest in Alaskan Coastal water (1.0 mg-at N $\cdot$ m$\sp{-2} \cdot$ h$\sp{-1}$) where nitrate uptake averages only 15% of the total. A simple nitrogen budget suggests that 29% and 62% of the annual nitrogen productivity in modified Bering Shelf and Anadyr waters, respectively, is exported through Bering Strait into the southern Chukchi Sea for deposition. Improved estimates of the rates of urea production and uptake by phytoplankton in the northern Bering Sea were made after determining the change in $\sp{15}$N-atom % enrichment of urea during incubations. Estimates of uptake rates increased by up to 83% using a $\sp{15}$N accumulation model and by $>$210% using a $\sp{15}$N disappearance model. However, a discrepancy exists between the $\sp{15}$N-urea removed from the aqueous phase and the $\sp{15}$N accumulated in the particulate phase. The ability to find in the particulate fraction the $\sp{15}$N removed from solution as $\sp{15}$N-urea was improved by 72% following removal of the $>$20-$\mu$m particulate fraction.
• #### The significance of marine-derived biogenic nitrogen in anadromous Pacific salmon freshwater food webs

The natural abundance of the stable isotope ratios $\sp{15}$N/$\sp{14}$N and $\sp{13}$C/$\sp{12}$C expressed as $\delta\sp{15}$N and $\delta\sp{13}$C was used to trace biogenic nutrients delivered by returning adult anadromous Pacific salmon into freshwater systems. These systems were Sashin Creek, a rapidly flushing stream located on Baranof Island, southeastern Alaska and Iliamna Lake, the major sockeye salmon, Oncorhynchus nerka, nursery lake in the Kvichak River watershed, Bristol Bay, southwestern Alaska. Marine-derived nitrogen (MDN) was quantifiable by use of an isotope mixing model based on comparison of biota $\delta\sp{15}$N in areas used for spawning by anadromous salmon with salmon-free controls within the same watershed. Control periphyton (benthic primary producers) $\delta\sp{15}$N values $\sim$0 suggested that the control N pool was derived from N$\sb2$ fixation without significant recycling. In contrast, periphyton abundant in areas of intense spawning activity or carcass aggregation had $\delta\sp{15}$N $\sim$ +7. These two values were the basis for comparison of $\delta\sp{15}$N values of higher trophic level biota. A mixing model relating $\delta\sp{15}$N to MDN with trophic level was used to estimate consumer MDN through incorporation of a priori isotopic trophic enrichment factors established in the literature. Distinctive $\delta\sp{13}$C signatures along the Sashin Creek stream gradient and between Iliamna Lake littoral and limnetic production were used in concert with $\delta\sp{15}$N. Sashin Creek fishes reflected isotopic signatures of periphyton and thus production within the same stream section. Isotopic data suggested an overall importance of limnetic production in Iliamna Lake resident fish and juvenile sockeye salmon diets. Salmon eggs and emergent fry retaining the parental marine isotopic signature were distinguishable from autochthonous production derived from marine N, and appear to be a minor dietary component in both Sashin Creek or Iliamna Lake fishes. The proportion of MDN in resident fish N, including juvenile salmon after turnover of the natal N pool, was proportional to the escapement of spawners. Thus there is now direct evidence for a significant natural fertilization process: the flow of remineralized marine-derived biogenic nutrients from returning anadromous Pacific salmon into freshwater food webs.
• #### Numerical modeling study of the circulation in the Gulf of Alaska

A series of numerical experiments are performed to simulate the Gulf of Alaska circulation and to examine the dynamical ocean response to the annual mean and seasonal forcing using a primitive equation model (Semtner 1974). The model domain encompasses the North Pacific north of 45$\sp\circ$ N and east of 180$\sp\circ$ and is surrounded by artificial walls in the south and west. Biharmonic diffusion is used in the interior to excite mesoscale eddies. A sponge layer with high Laplacian diffusion is incorporated near the western boundary. Horizontal resolution of 30$\sp\prime$ x 20$\sp\prime$ and 20 vertical levels are used to resolve the mesoscale topography and eddies. Wind stress computed from sea level atmospheric pressure and temperature and salinity data of Levitus (1982) are used. A diagnostic model produces a circulation in the Gulf of Alaska which agrees with observed patterns. In a three-layer flat-bottom baroclinic model, baroclinic Rossby waves propagate at 0.8 cm/sec and it takes a decade for spin-up to be completed. Baroclinic models forced by the annual mean wind and thermohaline forcings show the generation of eddies by baroclinic instability. The eddies in the flat-bottom model have a period of 75 days and are interpreted as barotropic Rossby waves. In the model with topography, the period of dominant eddies is 3-4 years and they are interpreted as baroclinic Rossby waves. Anticyclonic eddies near Sitka show similar characteristics as the Sitka eddy. They propagate westward and cause meanders in the Alaska Stream near Kodiak Island. The abnormal shift of the Alaska gyre in 1981 is probably due to the presence of one of these anticyclonic eddies. A flat-bottom model with seasonal forcing shows a large seasonal variability. When bottom topography is present, however, seasonal response is greatly reduced due to the dissipation of barotropic response by bottom topography. The seasonal baroclinic model shows a similar seasonal variability to the seasonal barotropic model indicating that the seasonal response is mainly barotropic. Eddies are also excited in the seasonal case and are almost identical to those of the annual mean case.
• #### Competition between two aquatic microorganisms for oscillating concentrations of phosphorus

The availability of limiting nutrients is a critical factor regulating growth of aquatic microorganisms. In at least some aquatic systems the frequency of addition rather than the absolute concentration of nutrients controls community structure. Gnotobiotic continuous cultures were used to examine the growth characteristics of a green alga (Selenastrum capricornutum) and a heterotrophic yeast (Rhodotorula rubra) when phosphorus-limited steady-state populations were subjected to varying concentrations of pulsed phosphorus. The responses of these organisms to phosphorus additions were measured both in single and dual species continuous cultures. Both organisms exceeded the maximum transport rates for phosphorus predicted from batch and steady-state continuous cultures. Carbon limitation did not cause a decline in phosphorus accumulation in R. rubra. Carbon-limited yeast cultures perturbed with phosphorus attained the highest phosphorus per cell values seen in these studies. The phosphorus pool was not significantly diminished in these cultures only because the total yeast biomass was limited by carbon. These results suggest that carbon-limitation of heterotrophic populations may be essential to the existence of phytoplankton in low-nutrient aquatic environments.
• #### Assessing The Health Of Harbor Seals In Alaska

Declining populations of pinnipeds in the Gulf of Alaska, possibly resulting from changes in prey quality, prompted research to determine the population health status of harbor seals (Phoca vitulina) using blood chemistry and digestive constraints. Blood chemistry and morphology reference range values between two harbor seal pup populations in Alaska, one population in continued decline, Prince William Sound, and another in recent increase, Tugidak Island, offered clues that blood values can vary on the population scale and that health assessment must utilize an appropriate set of reference values for valid comparisons. Subsequently, a captive study involving harbor seals yielded changes in ten blood chemistry or hematology values as a function of season and diet. These data provided evidence that populations may have distinct "identities" based on blood chemistry values. The "metabolic identity" of a population provides evidence of the relationship between environmental stressors and the genetic capacity of the animal to respond to metabolic demands. This made it possible to better understand population level differentiation in plasma chemistry values and thus assess the health of animals occupying the outlier regions of populations, since these regions are often suggestive of poor health. A captive study involving harbor seals, which are known to consume the low quality prey (pollock) implicated in the declines of many species of birds and mammals in the Gulf of Alaska, yielded consistent dry matter digestibility resulting in greater gut fill from pollock than from herring. Digestible energy intakes from pollock were greater than from either herring or the mixed diet. Lipid digestibility of herring declined from 90% to 50% when lipid intake exceeded 60 g kg -0.75 d-1. Results of this study imply that a flexible digestive system for harbor seals can compensate for ingesting a prey of low energy density by increasing gut fill and enhancing protein and lipid assimilation, to sustain digestible energy intake. In other words, harbor seals can offset differences in prey quality if prey availability and abundance does not limit the physiological plasticity of their digestive system to maintain their supply of energy and nutrients.
• #### Tracing Amino Acid Metabolism Of Harbor Seals (Phoca Vitulina) Using Stable Isotope Techniques

Compound specific isotope techniques were used to trace amino acid metabolism in captive harbor seals (Phoca vitulina) through a two-year controlled feeding trial with either Pacific herring (Clupea pallasi ) or walleye pollock (Theragra chalcogramma). Techniques were developed for measuring carbon and nitrogen isotope ratios of individual amino acids. Carbon and nitrogen trophic enrichments in serum of captive harbor seals varied with the two fish diets, which might have resulted from the changes in metabolic pathway due to the differing dietary protein intake between herring and pollock. Data on serum free amino acid compositions also showed, from a different perspective, that changes in seal metabolism occurred in response to these different feeding regimes. Carbon and nitrogen isotopic composition of individual amino acids varied much more within an organism than across trophic levels, reflecting the distinct amino acid biosynthetic pathways. The similar patterns in relative amino acid carbon isotopic composition at different trophic levels indicated a conservative transfer of delta13C from primary producers to top predators. Nitrogen trophic enrichments in different amino acids were not uniform, depending upon the extent to which a given amino acid was transaminated or deaminated, with several essential amino acids showing lesser variations than most non-essential and branched-chain amino acids. The differences in amino acid isotope ratios among phocids from the North Pacific or Atlantic and their counterparts from the Antarctic reflected the geographic variations in isotopic composition of phytoplankton. The striking similarities in relative amino acid isotopic composition among phocids from the three distinct geographic locations indicated that phytoplankton worldwide had similar biosynthetic pathways during initial amino acid biosynthesis. This has important implications for using individual amino acid isotope ratios in studies of modern and prehistoric marine organisms. Amino acid metabolic pathways governed the varying patterns of 15N enrichments following 15N-labeled amino acid tracer infusions. Tracer experiments further confirmed that phenylalanine, threonine, lysine and probably histidine may be useful as relatively conservative natural biomarkers. This study provided new insight into mechanisms of isotopic trophic dynamics in food web studies and improved our understanding of seal protein metabolism.

• #### Carbon And Nitrogen Flows In Zero -Water Exchange Shrimp Culture: Inferences Using Stable Isotope Tracers

Nutrient and energy flow in cultures of Pacific White Shrimp, Litopenaeus vannamei, were examined in zero-water exchange, 1200--1300 L mesocosms at the Oceanic Institute (OI), Waimanalo, Hawaii. A technique was developed for monitoring shrimp use of formulated feeds through the addition of stable isotopically labeled nutrients to the feed ingredients. Crystalline amino acid compounds were ineffective as labels due to their rapid dissolution in the tank water with feed pellet break-up. Labels which were 'packaged' as algal cells prior to addition to the feed pellets were more effectively incorporated into shrimp tissues than crystalline label (approximately 27% versus 8% for crystalline label). The 'packaged' label technique was also used to test soluble proteins from pollock processing wastes (stickwater) as a feeding stimulant for Litopenaeus vannamei. Indoor controlled condition experiments and outdoor experiments with natural pond biota compared stickwater amended feed to squid liver powder amended feed for growth and assimilation by the shrimp. Initial results indicated that pollock processing by-products might function as a feeding stimulant in shrimp aquaculture. The addition of 15N-ammonium to outdoor shrimp tanks showed that natural tank production contributed significantly to shrimp growth requirements providing between 17 and 77% of the growth nitrogen. When labeled ammonium was added to black covered tanks, shrimp had slower growth rates (0.5 g/wk as compared to 0.7 g/wk for uncovered ammonium addition tanks) but significant uptake of this label, with a tank biota contributing 23%. This finding supported a bacterial role in shrimp nutrition that will require further study. Isotopic analysis of individual amino acids in shrimp muscle from outdoor tanks with and without added 15N-ammonium further established the role of tank natural populations to shrimp nutrition. Rapid increases in delta 15N for threonine one day after label addition suggested an increased requirement for this essential amino acid. Further identification of the contribution of tank biota to shrimp amino acid profiles will require profiles of the delta 15N of specific amino acids for suspended particulate organic matter.

• #### Carbon And Nitrogen Isotope Ratios In Bowhead Whales (Balaena Mysticetus) And Their Zooplankton Prey As Indicators Of Feeding Strategy And Environmental Change.

This study details regional, seasonal and inter-annual differences in $\delta\sp{13}$C and $\delta\sp{15}$N of zooplankton in the Bering and Chukchi Seas. These isotopic variations were correlated with inter-annual and long-term variations in $\delta\sp{13}$C and $\delta\sp{15}$N of bowhead whale baleen.<p> Statistical analyses indicate significant $\sp{13}$C-enrichment in Bering/Chukchi seas zooplankton relative to Beaufort Sea zooplankton, supporting the observed trends of $\sp{13}$C-depletion at higher latitudes and enrichment at lower latitudes. Bering/Chukchi seas zooplankton show a weak trend in $\sp{13}$C-depletion between 1987-1990, possibly due to inter-annual environmental changes affecting phytoplankton $\delta\sp{13}$C. The $\delta\sp{13}$C of baleen produced in fall/winter months is inversely correlated to sea surface temperature trends in the Bering Sea. Long-term changes in baleen $\delta\sp{13}$C may be helpful in assessing past climatic change.<p> The observed $\delta\sp{15}$N oscillations in baleen may be due to changes in trophic level of prey between seasonal feeding grounds or from physiological cycles in the bowhead whale. <p>
• #### Seasonal Migration And Distribution Of Female Red King Crabs In A Southeast Alaska Estuary.

Seasonal movements and distribution of primiparous and multiparous red king crabs (Paralithodes camtschaticus) were monitored approximately weekly for one year in Auke Bay, Alaska, using ultrasonic biotelemetry. Patterns of seasonal movements were generally similar for all crabs, although movements of multiparous crabs were more conservative and coordinated between individuals. Groups of crabs remained in relatively discrete areas for several weeks before moving, usually as a group, to a different area. The annual range of primiparous crabs (x = 11.9 km$\sp2$) exceeded (P $<$ 0.025) that of multiparous crabs (x = 3.6 km$\sp2$). All crabs displayed distinct seasonal shifts in depth distribution and habitat use. Depth distribution was significantly correlated with photoperiod and the abrupt, synchronous movement of crabs between habitats was coincident with thermohaline mixing. Females displayed a highly aggregated distribution, especially during winter in shallowwater areas. Podding behavior of adult crabs was documented for the first time. Possible causes and functions of this highly specialized behavior are discussed. <p>