Circatidal activity rhythms in ovigerous blue crabs, <Emphasis Type="Italic">Callinectes</Emphasis><Emphasis Type="Italic">sapidus</Emphasis>: implications for ebb-tide transport during the spawning migration

Female blue crabs (Callinectes sapidus Rathbun) with mature embryos have a spawning migration in which they: (1) undergo ebb-tide transport for movement seaward from estuaries, (2) release their larvae, and (3) reverse direction by undergoing flood-tide transport for up-estuary movement. The study determined whether ebb-tide transport during the spawning migration is based upon an endogenous rhythm in vertical migration. Under constant conditions in a rectangular container, which limited horizontal and vertical movements, females with young and mature embryos had circatidal rhythms (periods=12.11-12.95 h) in migratory restlessness (swimming activity) and egg maintenance behavior (abdominal pumping). However, the rhythms were out of phase, as migratory restlessness occurred during the expected time of ebb tide in the field, and egg maintenance behavior, during the time of flood tide. Under constant conditions in vertical columns (1.32 m high), crabs with mature embryos had a circatidal rhythm (periods=12.2-13.7 h) in which they had frequent bouts of swimming to the surface of the column during the expected time of ebb tide in the field and remained on the bottom during the time of flood tide. This rhythm was not present in crabs with young embryos and disappeared after larval release. Thus, an endogenous rhythm in vertical migration does underlie the ebb-tide transport behavior of ovigerous blue crabs with mature embryos during their spawning migration.     

Seasonal and daily dynamics in pelagic larvae of marine shelf bottom invertebrates in nearshore waters of Kandalaksha Bay (White Sea)

Seasonal and daily population dynamics have been studied in pelagic larvae of littoral and upper-sublittoral bottom invertebrates in the plankton of the shallow, narrow Velikaya Salma Sound, which connects the inner and outer areas of the Kandalaksha Bay in the western part of the White Sea. Hydrologically, this Sound is characterised by a clearly defined cycle of great seasonal variations in water temperature coupled with more or less stable salinities and regular, pronounced semi-diurnal tides corresponding to daily and lunar monthly tidal cycles. The seasonal dynamics of larvae in the Sound reflect differences in occurrence of spawning periods in local waters of various species and systematic groups of bottom invertebrates. These differences are caused by the correlation of spawning periods of local species of different zoogeographical origin with the different water temperatures. They reflect, also, lunar periodicities of spawning and larval hatchings. The daily dynamics of larval abundancies are related to the daily spawning rhythms of many species with pelagic development affected by the daily tidal cycles of the Velikaya Salma Sound. A daily invasion of the Sound by pelagic larvae of bottom invertebrates from the inner and the outer parts of the Kandalaksha Bay occurs at ebb tide, and also at flood tide; the rhythms of the invasions coincide with the daily spawning rhythms of the Sound's invertebrates. From literature data summarized by Mileikovsky (1958a, b, 1960a, b, c, 1961, 1965, 1968, 1970), it is concluded that seasonal, lunar and daily (tidal) reproductive periodicities for the marine shallowshelf bottom invertebrates concerned, follow world-wide ecological patterns. It is evident that the effects of these rhythms upon the population dynamics of pelagic invertebrate larvae, as demonstrated by the present data on the Velikaya Salma Sound (White Sea), must also follow world-wide regularities.     

Vertical distribution of bivalve larvae along a cross-shelf transect during summer upwelling and downwelling

Previous time-series studies of meroplankton abundances in the LEO-15 research area off Tuckerton, New Jersey, USA (39°28′N, 74°15′W) indicated short-lived (6–12 h) pulses in larval surfclam (Spisula solidissima Dillwyn) concentration often associated with the initiation of downwelling. To examine possible larval surfclam (and other bivalve) concentrating mechanisms during upwelling and downwelling, six sets of adaptive mobile zooplankton pump samples were taken in July 1998 at different depths at five to six stations along a 25-km transect perpendicular to the coastline and crossing Beach Haven Ridge at LEO-15. Sampling was guided by near real-time, satellite imagery of sea surface temperature overlain by sea surface currents from a shore-based ocean surface current radar (OSCR) unit. A Seabird CTD on the mobile pump frame near the intake provided information on thermocline depth, and sampling depths were adjusted according to the temperature profiles. Near shore, the thermocline was tilted down during downwelling, and up during upwelling. The highest concentrations of surfclam larvae occurred near the bottom at a station near Beach Haven Ridge during downwelling, and just above the thermocline 3 km further offshore during well-developed upwelling. For other bivalve taxa, the larvae were concentrated near the thermocline (Anomia simplex Orbigny and Pholadidae spp.) or concentrated upslope near the bottom (Mytilidae spp.) during upwelling, and the larvae were concentrated near the bottom or were moved downslope during downwelling. Donax fossor Say larvae were found near the surface or above the thermocline during upwelling and downwelling. The general patterns of larval bivalve distribution appear to be influenced by water mass movement during upwelling and downwelling. The larval concentration patterns of individual species are likely a consequence of advection due to upwelling and downwelling circulation, vertical shear in the front region, species-specific larval behaviors, and larval sources.     

Oceanographic habitat,growth and mortality of larval anchovy (<Emphasis Type="Italic">Engraulis encrasicolus</Emphasis>) in the northern Aegean Sea (eastern Mediterranean)

Data from two ichthyoplankton surveys carried out during June 1995 and June 1996 were used to study the broad scale distribution patterns of anchovy eggs and larvae over the northern Aegean Sea continental shelf and the regional/inter-annual variability in growth and mortality rates of larvae. Two major spawning grounds were identified. One in the east, located in the area influenced by the Samothraki gyre (SG), in which a large amount of enriched, modified Black Sea water (BSW) is entrapped and one in the west, associated with zooplankton-rich waters in the semi-enclosed Thermaikos gulf close to several river mouths. In the NE Aegean, anticyclonic gyres generated over the continental shelf and fed by the circulating stream of BSW (like the SG) may act as retention areas for larval anchovy. In the west, the high enclosure of the Thermaikos Gulf contributes to reducing offshore dispersal. Major changes were observed in egg and larval abundance as well as larval mortality between June 1995 and June 1996 in both the western and eastern part of the continental shelf. Mean abundance of eggs and early larvae was >5 times higher in 1996 than in 1995, when waters were significantly cooler, fresher and richer in mesozooplankton. Larval survival decreased from 79 to 69% day⁻¹ in the east and from 89 to 74% day⁻¹ in the west between 1995 and 1996. Hence increased egg production was coupled with higher larval mortality during June 1996. Furthermore, a highly significant positive relationship between larval mortality (Z) and mean egg abundance (A) emerged (Z = −0154 + 0.205 log[A], r ² = 0.96, n = 7) when data from this study and a similar study in the NW Mediterranean were regressed. Mean growth rate of anchovy larvae in the study area (∼0.5 mm day⁻¹) did not differ significantly between areas/years. A marked ontogenetic change was observed in the otolith size/recent otolith growth-on-fish size relationships, which exhibited significant inflection points at ∼6 mm formalin preserved length. This change seems to coincide with performance (e.g., catchability) and behavioral changes (e.g., onset of vertical migrations) in European anchovy associated with the development of the caudal fin (the flexion stage).     

Circatidal swimming behavior of brachyuran crab zoea larvae: implications for ebb-tide transport

Larvae of the blue crab Callinectes sapidus and fiddler crab Uca pugilator are exported from estuaries and develop on the continental shelf. Previous studies have shown that the zoea-1 larvae of some crab species use selective tidal-stream transport (STST) to migrate from estuaries to coastal areas. The STST behavior of newly hatched larvae is characterized by upward vertical migration during ebb tide followed by a descent toward the bottom during flood. The objectives of the study were (1) to determine if newly hatched zoeae of U. pugilator and C. sapidus possess endogenous tidal rhythms in vertical migration that could underlie STST, (2) to determine if the rhythms persist in the absence of estuarine chemical cues, and (3) to characterize the photoresponses of zoeae to assess the impact of light on swimming behavior and vertical distribution. Ovigerous crabs with late-stage embryos were collected from June to August 2002 and maintained under constant laboratory conditions. Following hatching, swimming activity of zoeae was monitored in darkness for 72 h. U. pugilator zoeae displayed a circatidal rhythm in swimming with peaks in activity occurring near the expected times of ebb currents in the field. Conversely, C. sapidus zoeae exhibited no clear rhythmic migration patterns. When placed in a light field that simulated the underwater angular light distribution, C. sapidus larvae displayed a weak positive phototaxis at the highest light levels tested, while U. pugilator zoeae were unresponsive. Swimming behaviors and photoresponses of both species were not significantly influenced by the presence of chemical cues associated with offshore or estuarine water. These results are consistent with predictions based on species-specific differences in spawning and the proximity of hatching areas to the mouths of estuaries. U. pugilator larvae are released within estuaries near the adult habitat. Thus, ebb-phased STST behavior by zoeae is adaptive since it enhances export. Selective pressures for a tidal migration in C. sapidus larvae are likely weaker than for U. pugilator since ovigerous females migrate seaward prior to spawning and hatching occurs near inlets and in coastal waters.     

Relation of spawning of the giant scallop,Placopecten magellanicus,to temperature fluctuations during downwelling events

We examined the relation of spawning to biological and physical factors for the scallop Placopecten magellanicus (Gmelin, 1791) over 8 yr (1984 to 1991) in the Baie des Chaleurs, southwestern Gulf of St. Lawrence, Canada. Spawning was always abrupt and occurred between July and mid-September. It did not appear to be related to the abundance of phytoplankton or particulate organic carbon and nitrogen in the water. It further showed no relationship to lunar or tidal phascs or to current velocity. Spawning consistently occurred during the summer temperature maximum, but did not coincide with any critical temperature or cumulative temperature threshold. All but one of the 33 spawning events, for which temperature data were recorded, were associated with temperature changes; 25 of these were sharp temperature increases and 7 were during strong temperature fluctuations when the mean temperature was 9 to 14°C. Both types of temperature changes were caused by downwelling of warm surface water. The delay by about 1 d in time of spawning between sites coincided with the rate at which downwelling events propagated into the bay. Virtually all of the spawning events resulted in gametes being ejected into warm water masses where conditions are likely to favour larval development.     

Diving characteristics of southern rockhopper penguins (Eudyptes c. chrysocome) in the southwest Atlantic

The diving behaviour of southern rockhopper penguins (Eudyptes c. chrysocome) was studied at two breeding sites in the Southwest Atlantic: the Falkland Islands and Staten Island, Argentina. Incubating and brooding birds were equipped with time-depth recorders to monitor their foraging activities. Rockhopper penguins from Staten Island started their breeding season about 3 weeks earlier than their conspecifics from the Falkland Islands. The foraging area used by incubating males from the Falkland Islands comprised about 150,000 km² to the northeast of the breeding site and was characterised by shelf and slope waters, whereas the foraging area of incubating males from Staten Island comprised 350,000 km² of oceanic waters to the southeast of the breeding site. A number of dive parameters were measured and compared between the four study groups: Incubating males and brooding females from the Falkland Islands, and incubating males and females from Staten Island. In all study groups, dive depth correlated positively to light intensity, dive duration and vertical velocity. However, significant differences between various diving parameters of the study groups were noted, not only in terms of diving performance, but also as regards diving efficiency (DE). A principal component analysis (PCA) on 16 variables revealed that 75% of the variance could be explained by only two principal components: diving pattern (PC1) and diving effort (PC2). PC1 indicated that the birds from Staten Island, both males and females, dived deeper, covered a greater vertical distance per hour and had higher ascent rates, but spent less time underwater and at the bottom of a dive, and had a lower DE than conspecifics from the Falkland Islands. PC2, which included the percentage of foraging dives, the number of dives per hour, dive duration, bottom time and descent rate, differed significantly between incubating males from the Falkland Islands and the other three groups, which were all very similar. Overall, the diving behaviour was notably similar to that of conspecifics from the Indian and Pacific Oceans. The implications of the results in terms of intra-specific adaptations as well as potential threats from human activities are discussed.     

Spatial Variations of Heavy Metals in the East China Sea Continental Shelf Surface Sediments

Surface sediments collected from continental shelf of the East China Sea were analyzed for heavy metals (iron, manganese, copper, zinc, lead, cadmium), carbonate, organic carbon contents, and grain sizes. the range of concentrations observed were iron: 0.3-1.3 wt%, manganese: 2.3-14 μmole/g, copper: 7.1-184 μmole/g, zinc: 0.16-0.77μmiole/g, lead: 15-98 μmole/g, cadmium: 0.17-3.9 μmole/g, carbonate: 3.6-87 wt%, sand: 10-100%, silt: 0-70% and clay: 0-50%.

A zonal distribution pattern of the heavy metals was found in the East China Sea Continental shelf sediments. High concentrations of most heavy metals, organic carbon and fine-grained sediments were observed in the inner shelf zone, especially those near the discharge of the Yangtze River. Concentrations of these heavy metals decreased from the inner shelf to the shelf break region. High concentrations of metals were also found in sediments near Taiwan. Iron concentrations decreased north-east of the central shelf region. High concentrations of cadmium were found in the shelf break region where biogenic carbonate is predominant. This study showed that biogenic carbonate in the East China Sea shelf break region and the terrigenous sediments from the Yangtze River and island of Taiwan were the major sources of heavy metals. Heavy metal concentrations were strongly influenced by the content of the coarse-grained quartz sand present in the sediments.     

Spatial Variations of Heavy Metals in the East China Sea Continental Shelf Surface Sediments

Surface sediments collected from continental shelf of the East China Sea were analyzed for heavy metals (iron, manganese, copper, zinc, lead, cadmium), carbonate, organic carbon contents, and grain sizes. the range of concentrations observed were iron: 0.3-1.3 wt%, manganese: 2.3-14 μmole/g, copper: 7.1-184 μmole/g, zinc: 0.16-0.77μmiole/g, lead: 15-98 μmole/g, cadmium: 0.17-3.9 μmole/g, carbonate: 3.6-87 wt%, sand: 10-100%, silt: 0-70% and clay: 0-50%.

A zonal distribution pattern of the heavy metals was found in the East China Sea Continental shelf sediments. High concentrations of most heavy metals, organic carbon and fine-grained sediments were observed in the inner shelf zone, especially those near the discharge of the Yangtze River. Concentrations of these heavy metals decreased from the inner shelf to the shelf break region. High concentrations of metals were also found in sediments near Taiwan. Iron concentrations decreased north-east of the central shelf region. High concentrations of cadmium were found in the shelf break region where biogenic carbonate is predominant. This study showed that biogenic carbonate in the East China Sea shelf break region and the terrigenous sediments from the Yangtze River and island of Taiwan were the major sources of heavy metals. Heavy metal concentrations were strongly influenced by the content of the coarse-grained quartz sand present in the sediments.     

Self-contamination of aquaculture cages in shallow water

Fish farms, which initially colonized quiet and protected natural coastal areas, are now frequently installed in open flow zones, due to the lack of space along coasts and to the emergence of new environmental constraints. For the past two decades, a salmon fish farm has been located inside the roadstead of Cherbourg (France) to benefit from both sea protection and tide currents which regularly refresh the water. In spite of these favourable environmental conditions, periods of non-negligible fish mortalities have been observed to occur without clear evidence of their origin. This motivated the turbidity measurements and the numerical simulations presented in this paper. Firstly, it is shown that high turbidities in the farm site under study are mainly due to the flow acceleration under the cages, which causes the re-suspension of sediments and bio-deposits. Secondly, particles which enter the fishnet can have different origins (external source, bottom, or the net itself). Numerical simulations, based on the Reynolds equations and on the discrete random walk model for particle dispersion, suggest that the rear area of the net can be reached by particles emerging from below the net. It is observed that turbulent dispersion is a key ingredient for such a behaviour, as it can lead particles towards a large recirculation cell behind the net. Dispersion by realistic unsteady vortices has also been analysed by means of a Lattice-Boltzmann model. Though these computations involve smaller Reynolds numbers, they confirm qualitatively the observations of the random walk model. In addition, they suggest that vortex shedding and unsteady recirculation cells near the bottom can force particles from the sand bed to be lifted up and reach the rear of the net.     

Reproductive cycle,larval development,juvenile growth and population dynamics of Patiriella pseudoexigua (Echinodermata: Asteroidea) in Taiwan

A field study was conducted at Wanlitung, southern Taiwan, in 1986–1089, to determine the reproductive cycle, development mode, growth rate and population dynamics of the small seastar Patiriella pseudoexigua (Dartnall), which occurs in highly stressful and disturbed intertidal pools in this area. An inverse relationship between gonad index and pyloric-caccum index was only recorded immediately prior to spawning. A short, well-synchronized seasonal spawning occurs in October. When reared at 25 °C, lecithotrophic larvae develop directly, lack a bipinnaria stage, and metamorphose completely on the seventh day after fertilization. The growth curves of field juveniles are linear, those of laboratory-reared juveniles are sigmoid. Juveniles appear in tide pools in spring-early summer of each year. Adults spawn mainly in late fall, enabling spawning to occur in time for the larvae to benefit from the environmentally favorable winter season. Populations in high-tidal pools decrease in later summer, but remain more stable in lowtidal pools and lagoons.     

Enrichments of Trace Metals in Particulate Matter From the Southern East China Sea,North of Taiwan

Abstract

This study investigates the distributions and enrichments of trace metals in suspended and sinking particulate matter from southern East China Sea (ECS) north of Taiwan during the period April 1992 to April 1993. According to these results, concentration of suspended particulate matter in the inner shelf of southern China Sea, the upwellinginfluenced shelf break, and in Kuroshio water are 1.30 (surface)–4.2 (bottom) mg1^?1, ca. 0.4 mg1^?1 and 0.1–0.2 mg1^?1, respectively, reflecting various influences of terrestrial inputs. A benthic nepheloid layer (BNL), apparently owing to resuspension of local and/or remote bottom sediments, formed over the shelf region. Temporal variations in trace metal contents and enrichments in suspended matter from the shelf region reflect the variation of metal inputs from Chinese rivers, particularly from the Changjiang runoff. the enriched metals are more likely to be derived from anthropogenic input, rather than from biological accumulation. in addition, a decrease in metal contents and an increase in salinity confirm the transport of suspended particulate metals from the East China Sea shelf to the open ocean. the feature of metal plume in the intermediate layer (550–800m) of Kuroshio water also verifies this occurrence. Moreover, the sinking particles collected from a sediment trap on the upper slope are relatively enriched in lithogenic matter and trace metals, suggesting the deposit of anthropogenic metals in the slope area.     

Enrichments of Trace Metals in Particulate Matter From the Southern East China Sea, North of Taiwan

This study investigates the distributions and enrichments of trace metals in suspended and sinking particulate matter from southern East China Sea (ECS) north of Taiwan during the period April 1992 to April 1993. According to these results, concentration of suspended particulate matter in the inner shelf of southern China Sea, the upwellinginfluenced shelf break, and in Kuroshio water are 1.30 (surface)-4.2 (bottom) mg1^-1, ca. 0.4 mg1^-1 and 0.1-0.2 mg1^-1, respectively, reflecting various influences of terrestrial inputs. A benthic nepheloid layer (BNL), apparently owing to resuspension of local and/or remote bottom sediments, formed over the shelf region. Temporal variations in trace metal contents and enrichments in suspended matter from the shelf region reflect the variation of metal inputs from Chinese rivers, particularly from the Changjiang runoff. the enriched metals are more likely to be derived from anthropogenic input, rather than from biological accumulation. in addition, a decrease in metal contents and an increase in salinity confirm the transport of suspended particulate metals from the East China Sea shelf to the open ocean. the feature of metal plume in the intermediate layer (550-800m) of Kuroshio water also verifies this occurrence. Moreover, the sinking particles collected from a sediment trap on the upper slope are relatively enriched in lithogenic matter and trace metals, suggesting the deposit of anthropogenic metals in the slope area.     

Habitat use pattern of tidal mud and sand flats by shorebirds (charadriiformes) wintering in southern India

Habitat use pattern and status of shorebirds in southern India were investigated in six tidal flats comprising three mudflats and three sand flats on the east coast of southern India. Totally, 7757 shorebirds belonging to 21 species were recorded during September 2000–March 2002. The birds preferred mudflats over sand flats. Density, diversity and richness of shorebirds were relatively higher in tidal flats, especially in mudflats. The shorebirds preferred mudflats over sand flats as stopover sites and sites for refueling the required energy during migration. Rhythmic changes in tidal pattern also influenced the allochthonous nutrients and prey species, which also attracted the migratory shorebirds. However, the abundance of shorebirds with reference to tidal flats *season*tide and habitats*seasons * tide had strong variations (P < 0.001). During migratory season, bird density, diversity and species richness were observed higher than in other seasons.     

Mechanisms to initiate a Heterosigma akashiwo red tide in Osaka bay

The ecological role of diel vertical migration of Heterosigma akashiwo Hada to initiate red tide was investigated in Tanigawa Fishing Port and Sano Harbor, Osaka Bay, Japan during red tide seasons in 1979 and 1980. This species migrated toward the surface early in the morning at a velocity of 1.0 to 1.3 m h^-1. Downward migration was found in the afternoon, and more than 2.0×10³ cells ml^-1 aggregated in the bottom layer at night. The upward migration started before sunrise and downward shifting occurred prior to sunset. The movement is presumably associated with circadian rhythm, which is known as one of the biological periodicities. H. akashiwo crossed steep temperature and salinity gradients (6.5°C and 5.7 S) during the diel vertical migration. High values of particulate organic carbon and nitrogen concentrations were obtained in dialysis bags suspended in situ at identical layers with high cell concentration, while the values for surface and bottom bags were comparatively low. The results reveal that H. akashiwo migrates toward the sea surface to carry out photosynthesis effectively, and to the bottom to utilize nutrients efficiently.     

The generation of internal waves by tidal flow over continental shelf/slope topography

Experiments were performed to investigate the generation of internal waves induced by a barotropic tidal flow over continental shelf/slope topography in a two-layer stratified fluid. The interaction of the barotropic tide with the continental slope resulted in the formation of both linear and nonlinear waveforms, ranging from a linear wave of depression to a highly nonlinear internal bolus. The type of wave response was strongly dependent on the tidal forcing intensity and the position of the density interface relative to the shelf depth. Based on the values of the internal Froude number and the layer depth ratio, we delineate four distinct generation regimes, each with a distinct wave response.     

Seasonal migrations of plaice (Pleuronectes platessa) through the Dover Strait

Analysis of paired-haul mid-water trawling experiments conducted in the five months November to March during the years 1985 and 1988 revealed the seasonal pattern of migration of plaice (Pleuronectes platessa L.) moving through the Dover Strait between the Southern Bight of the North Sea and the English Channel. Mid-water catches of maturing fish were highest on south-going tidesprior to peak spawning (late January), while catches of spent fish, were highest on north-going tidesafter peak spawning. Detailed analysis of the catch data provided evidence that males complete their pre-spawning migration earlier, and their post-spawning migration later, than female fish and consequently spend more time on the spawning ground. Catches of immature female fish indicated a migration south following that of the mature fish. This dummy run may be a method by which immature plaice use the presence of high densities of mature and spawning fish to learn the location of the spawning ground. The analysis also indicated that differences in catch rates between day and night are unlikely to be the result of gear avoidance by day. The results confirm that selective tidal-stream transport is a key migratory mechanism for plaice in areas of fast tidal streams on the European continental shelf.     

Mitochondrial DNA and electronic tracking reveal population structure of Atlantic bluefin tuna (Thunnus thynnus)

Population subdivision was examined in Atlantic bluefin tuna (Thunnus thynnus) through sequencing of the control region of the mitochondrial genome. A total of 178 samples from the spawning grounds in the Gulf of Mexico, Bahamas and Mediterranean Sea were analyzed. Among the samples from these locations were 36 electronically tagged bluefin tuna that were tagged in the North Atlantic and subsequently traveled to one of these known spawning grounds during the spawning season. Bluefin tuna populations from the Gulf of Mexico and the Mediterranean Sea were found to be genetically distinct based on Φ_st, and sequence nearest neighbor analyses, showing that these two major spawning areas support independent stocks. Sequence nearest neighbor analysis indicated significant population subdivision among the Gulf of Mexico, western Mediterranean and eastern Mediterranean Sea. However, it was not possible to find significant pairwise differences between any sampling areas when using all samples. If only samples that had a high likelihood of assignment to a specific spawning site were used (young of the year, spawning adults), the differentiation increased among all sampling areas and the Western Mediterranean Sea was distinct from the Eastern Mediterranean Sea and the Gulf of Mexico. It was not possible to distinguish samples from the Bahamas from those collected at any of the other sampling sites. These data support tagging results that suggested distinctness of the Gulf of Mexico, Eastern and Western Mediterranean Sea spawning areas. This level of stock differentiation is only possible if Atlantic bluefin tuna show strong natal homing to individual spawning grounds.     

Dissipation of Tidal Energy and Associated Mixing in a Wide Fjord

This paper sets out to test the hypothesis that vertical mixing due to the dissipation of the internal tide accounts for a significant proportion of the total vertical mixing in a fjordic basin during a period of deep water isolation. During July and August 1999 two locations in the Clyde Sea were instrumented with moored RD Instruments Acoustic Doppler Current Profilers (ADCPs) and conductivity-temperature-pressure chains: Station C2, near the shallow entrance sill (55 m water depth), and station C1 in the deep basin (155 m water depth). A bottom pressure recorder was also deployed at station C3 by the seaward entrance to the Clyde Sea in the North Channel of the Irish Sea. A Free-falling Light Yo-yo shear microstructure profiler (FLY) was used to measure the dissipation rate of turbulent kinetic energy (TKE) throughout the water column over 25 h at both C1 and C2. These were interspersed with two-hourly conductivity-temperature-depth casts at both sites. The observations show agreement between the dissipation rate of TKE estimated by using a microstructure profiler and that estimated from the decay of the internal tide as measured by the two ADCPs. However, to account for all the implied mixing it is necessary to invoke an additional source of buoyancy flux, the most probable candidate mechanism is enhanced internal wave breaking near the sill and at the sloping boundaries of the deep basin. In addition, the vertical eddy diffusivity estimated from the micro-structure profiler (O(0.5 cm² s^–1) indicates that internal tide induced mixing away from any boundaries contributed significantly to the overall level of mixing which is required to account for the observed evolution of the deep basin water properties.     

The diving behaviour of brooding king penguins (<Emphasis Type="Italic">Aptenodytes patagonicus</Emphasis>) from the Falkland Islands: variation in dive profiles and synchronous underwater swimming provide new insights into their foraging strategies

The diving behaviour of king penguins (Aptenodytes patagonicus) was studied on the Falkland Islands, where a small population (ca. 300 fledglings year^–1) is located at the geographical limit of their breeding range. King penguins rearing newly hatched chicks were equipped with time-depth recorders before leaving for sea. In total, 20,175 dives >3 m were recorded from 12 birds during 15 foraging trips with a mean duration of 5.7±2.3 days. The majority of the trips was directed up to 500 km to the northeast of the breeding colony in slope waters of, and oceanic waters beyond, the Patagonian shelf. Mean time spent underwater accounted for 42±9% of the foraging trip. Mean dive depth achieved was 55±16 m; maximum dive depth recorded was 343 m. Mean dive duration was 159±25 s; maximum dive duration was 480 s. The mean vertical distance covered was 140±65 km trip^–1; and on average birds covered 25 km day^–1. Synchronous diving behaviour was observed in two birds for a period of about 24 h after leaving the colony. Dive depth correlated positively with: (1) light intensity, (2) dive duration and (3) vertical velocities, thus confirming previous findings obtained from conspecifics at other breeding sites and indicating comparable diving behaviour. However, separation of dives according to their profile—V-, U-, or W-shaped—revealed significant differences between certain dive parameters. For a given depth range, bottom time was longer and vertical velocities higher in W-dives than in U-dives. This, together with a higher number of W-dives at dawn and dusk, suggests that foraging is more effective during W-dives than U-dives, and during twilight. These findings imply that king penguins have to make more complex decisions, individually and socially, on the performance of the subsequent dive than previously thought.Communicated by O. Kinne, Oldendorf/Luhe