John M. Klinck

The linkage between Upper Circumpolar Deep Water (UCDW) and phytoplankton assemblages on the west Antarctic Peninsula continental shelf

Intrusion of Upper Circumpolar Deep Water (UCDW), which was derived from the Antarctic Circumpolar Current (ACC), onto the western Antarctic Peninsula (WAP) shelf region in January 1993 provided a reservoir of nutrient-rich, warmer water below 150 m that subsequently upwelled into the upper water column. Four sites, at which topographically-induced upwelling of UCDW occurred, were identified in a 50 km by 400 km band along the outer WAP continental shelf. One additional site at which wind-driven upwelling occurred was also identified. Diatom-dominated phytoplankton assemblages were always associated with a topographically-induced upwelling site. Such phytoplankton communities were not detected at any other shelf location, although diatoms were present everywhere in the 80,000 km 2 study area and UCDW covered about one-third the area below 150 m. Phytoplankton communities dominated by taxa other than diatoms were restricted to transition waters between the UCDW and shelf waters, the southerly flowing waters out of the Gerlache Strait, and/or the summertime glacial ice melt surface waters very near shore. We suggest that in the absence of episodic intrusion and upwelling of UCDW, the growth requirements for elevated silicate/nitrate ratios and/or other upwelled constituents (e.g. trace metals) are not sufficiently met for diatoms to achieve high abundance or community dominance. One consequence of this is that the ice-free regions of the outer WAP continental shelf will not experience predictable spring diatom blooms. Rather, this region will experience episodic diatom blooms that occur at variable intervals and during different seasonal conditions, if the physical structuring events are occurring. Preferential drawdown of silicate relative to nitrate was observed at each of the offshore upwelling sites and resulted in a reduction in the ambient silicate:nitrate ratio relative to the corresponding value for unmodified UCDW (1.5 versus 3.0 for UCDW). The magnitude of the nutrient drawdown in areas of topographically-induced upwelling suggested that diatom growth had been elevated in response to recent upwelling but that the resulting increased algal biomass was either dispersed by advective processes and/or consumed by the larger krill that were observed to be associated with each offshore upwelling site. Thus, diatom bloom conditions on the outer WAP shelf may not be recognized based on elevated biomass and/or rates of carbon fixation. It was likely that similar physical forcing of significant phytoplankton growth, especially diatoms, may occur but be undetected in regions where the southern boundary of the ACC nears the Antarctic continental shelf edge. Our analyses from the west Antarctic Peninsula demonstrate coupling of the structure of the physical environment with nutrient distributions and phytoplankton assemblages and through to the higher trophic levels, such as Antarctic krill. This environment-trophic coupling may also occur in other regions of the Antarctic, as suggested by correspondences between the distribution of Southern ACC boundary and regions of high concentrations of Antarctic krill. The many mechanisms underlying this coupling remain to be determined, but it was clear that the ecology and biology of the components of the marine food web of the Antarctic continental shelf cannot be studied in isolation from one another or in isolation from the physical environment.

Hydrography and circulation of the West Antarctic Peninsula Continental Shelf

The water mass structure and circulation of the continental shelf waters west of the Antarctic Peninsula are described from hydrographic observations made in March—May 1993. The observations cover an area that extends 900 km alongshore and 200 km o⁄shore and represent the most extensive hydrographic data set currently available for this region. Waters above 100—150 m are composed of Antarctic Surface Water and its end member Winter Water. Below the permanent pycnocline is a modified version of Circumpolar Deep Water, which is a cooled and freshened version of Upper Circumpolar Deep Water. The distinctive signature of cold and salty water from the Bransfield Strait is found at some inshore locations, but there is little indication of significant exchange between Bransfield Strait and the west Antarctic Peninsula shelf. Dynamic topography at 200 m relative to 400 m indicates that the baroclinic circulation on the shelf is composed of a large, weak, cyclonic gyre, with sub-gyres at the northeastern and southwestern ends of the shelf. The total transport of the shelf gyre is 0.15 Sv, with geostrophic currents of order 0.01 m s~1. A simple model that balances across-shelf di⁄usion of heat and salt from o⁄shore Upper Circumpolar Deep Water with vertical di⁄usion of heat and salt across the permanent pycnocline into Winter Water is used to explain the formation of the modified Circumpolar Deep Water that is found on the shelf. Model results show that the observed thermohaline distributions across the shelf can be maintained with a coeƒcient of vertical di⁄usion of 10~4 m2 s~1 and horizontal di⁄usion coeƒcients for heat and salt of 200 and 1200 m2 s~1, respectively. When the e⁄ects of double di⁄usion are included in the model, the required horizontal di⁄usion coeƒcients for heat and salt are 200 and 400 m2 s~1, respectively. ( 1999 Elsevier Science Ltd. All rights reserved.

Circulation near submarine canyons: A modeling study

Circulation near a submarine canyon is analyzed with a numerical model. Previous theoretical work indicated that stratification controlled the interaction of coastal flow with canyons, specifically, the ratio of canyon width to the internal radius of deformation. A wide canyon was thought to merely steer the flow, while a narrow canyon would create substantial cross-shelf exchange. Four cases are analyzed considering two directions of alongshore flow and two choices of initial stratification. The weakly stratified case has an internal radius about equal to the canyon width, while the strongly stratified case has one about 3 times the canyon width. The direction of the alongshore flow is shown in this study to be the more important of the two factors. In particular, right-bounded flow (flow with the coast on the right, looking downstream in the northern hemisphere) leads to shallow downwelling in the canyon and weak exchange across the shelf break, while left-bounded flow creates upwelling at the head of the canyon and strong exchange between the ocean and shelf. In left-bounded flow (upwelling), dense water is pumped onto the shelf, even for strong stratification. However, the stratification limits the vertical extent of the topographic influence so that the alongshore flow above the canyon is only weakly affected in the strongly stratified case. With any level of stratification, the surface temperature (density) is not modified at all by the flow interaction with the submarine canyon. The important dynamics involve pressure gradients and Coriolis acceleration and how they interact with the bathymetric gradients but not advection of momentum. Advection of density is clearly important in the upwelling cases. Finally, continued upwelling onto the shelf acts as a drag mechanism and retards the alongshore coastal flow.

Krill transport in the Scotia Sea and environs

Historical observations of the large-scale flow and frontal structure of the Antarctic Circumpolar Current in the Scotia Sea region were combined with the wind-induced surface Ekman transport to produce a composite flow field. This was usedwith a Lagrangianmodel to investigate transport ofAntarctic krill. Particle displacements from known krill spawning areas that result from surface Ekman drift, a composite large-scale flow, and the combination of the two were calculated. Surface Ekman drift alone only transports particles a few kilometres over the 150-day krill larval development time. The large-scale composite flow moves particles several hundreds of kilometres over the same time, suggesting this is the primary transport mechanism. An important contribution of the surface Ekman drift on particles released along the continental shelf break west of the Antarctic Peninsula is moving them north-northeast into the high-speed core of the southern Antarctic Circumpolar Current Front, which thentransports the particles to South Georgiain about 140-1 60 days. Similar particle displacement calculations using surface flow fields obtained from the Fine Resolution Antarctic Model do not show overall transport from the Antarctic Peninsula to South Georgia due to the inaccurate position of the southern Antarctic Circumpolar Current Front in the simulated circulation fields. The particle transit times obtained with the composite large-scale flow field are consistent with regional abundances of larval krill developmental stages collected in the Scotia Sea. These results strongly suggest that krill populations west of the Antarctic Peninsula provide the source for the krill populations found around South Georgia.

A Simple Model of Fjord and Coastal Circulation Interaction

Abstract The dynamical interaction of a narrow fjord with a wind-driven coastal regime is investigated using a linear, two-layer numerical model. The Coriolis acceleration is important in the coastal regime but assumed to he unimportant in the fjord dynamical because v = 0. For a wide variety of wind conditions, bottom topography and model parameters, the wind-forced coastal circulation, with its geostrophic alongshore currants, has a strong effect on the circulation within the fjord. These geostrophic currents control the free surface and pycnocline displacement at the fjord mouth, thereby strongly affecting fjord circulation. This mechanism is an alternative to the classical idea of hydraulic control at the mouth by sills or constrictions. Model simulators also show that the free surface slope is a baroclinic effect and that alongshore and across-shore winds affect the fjord differently. Alongshore winds produce flooding while up- and down-fjord winds simply sat up the surface. We find that offshore win...

A population dynamics model for the Japanese oyster, Crassostrea gigas

Relationships that describe the growth of the Japanese oyster, Crassostrea gigas, were developed using measurements made from June 1990 to January 1991 in mariculture fields located in Hinase waters of the Okayama Prefecture, Japan. These relationships show that shell length increase for Hinase oyster populations of 50–100 mm in size was similar to that measured for C. gigas populations in the UK; however, the Hinase oysters were lighter for a given length than oysters found in Israel, Canada, Australia and Korea. Increases in live weight were greater in smaller oysters and lower for larger oysters than those observed for C. gigas populations in other areas. This suggests that the linear increase in live weight is a feature of artificially cultured C. gigas populations in Hinase waters. These data were used to calculate regressions between shell length and live weight, wet meat weight and dry meat weight, and dry meat weight and wet meat weight. Additionally, measurements of the gonadal condition of the C. gigas populations indicated that gonadal tissue development occurred when water temperatures were above 23 °C. These relationships were then used as input into a mathematical model that describes the time-dependent evolution of post-settlement oyster populations. A filtration rate relationship was developed for C. gigas, examined for general applicability, and used for the oyster population model. This relationship accounts for the faster growth of C. gigas relative to that of Crassostrea virginica. Similarly, two relationships for respiration rate were examined with the oyster population model. The relationships differed in the amplitude of the respiration rate, namely, one provided a rate 60% lower than the other for a given dry meat weight at 20 °C. The final biological process examined with the model was the reproductive efficiency, which determined the apportionment of net production to somatic and reproductive tissue growth. The simulated post-settlement oyster populations showed growth rates that agreed with those measured for field cultivated populations in Hinase waters when the lower respiration rate was used with a reproductive efficiency that varied from 0.0 to 0.8 between 23 and 27 °C. This modeling effort illustrates the changes that are needed to model the population dynamics of C. gigas in comparison with the similar species, C. virginica. Moreover, the model as now configured can be used to investigate effects of oyster density, local environmental conditions, including flow fields, the distribution of mariculture rafts, and cultivation practices on the growth and development of C. gigas. In addition, this model can provide a framework for predicting potential oyster yield from individual mariculture fields.

Physical Forcing of Phytoplankton Community Structure and Primary Production in Continental Shelf Waters of the Western Antarctic Peninsula

Analyses of a multidisciplinary data set, collected in continental shelf waters of the Western Antarctic Peninsula (WAP) during austral summer of January 1993, identified a previously unrecognized forcing mechanism that sets up a physical and chemical structure that supports and assures site-specific diatom-dominated communities and enhanced biological production (Prézelin et al., 2000). This forcing is active when the southern boundary of the Antarctic Circumpolar Current (ACC) flows along the shelf edge, thereby facilitating onshelf bottom intrusions of nutrient-rich Upper Circumpolar Deep Water (UCDW), which then is upwelled or mixed into the upper water column. At times or locations where UCDW is not introduced to the upper water column, diatoms no longer dominate phytoplankton assemblages over the midto outer WAP continental shelf. This analysis extends the area and seasons studied through similar analyses of multidisciplinary data sets collected on four additional cruises to the WAP that cover all seasons. Results show that onshelf intrusions of UCDW: (1) occur in other regions of the WAP continental shelf; (2) are episodic; (3) are forced by nonseasonal physical processes; and (4) produce areas of diatom-dominated phytoplankton assemblages and enhanced primary production. At times, multiple intrusions are observed on the WAP continental shelf, and each event may be in a different stage. Further, the occurrence of an intrusion event in one area does not necessarily imply that similar events are ongoing in other areas along the WAP shelf. The UCDW bottom intrusions originate along the outer shelf but they can extend into the inner shelf region because the deep troughs that transect the WAP shelf provide connections between the inner and outer shelf. The boundary between the intruded water and the shelf water is variable in location because of the episodic nature of the onshelf intrusions, and is moved farther inshore as an event occurs. These observations show clearly that the phytoplankton community structure on the WAP shelf is determined by physical forcing and that primary production is likely to be considerably greater than previously believed. Moreover, variability in this physical forcing, such as may occur via climate change, can potentially affect the overall biological production of the WAP continental shelf system. 1. Marine Science Institute and Department of Ecology, Evolution and Marine Biology, University of California, Santa Barbara, California, 93106, U.S.A. email: prezelin@lifesci.ucsb.edu 2. Center for Coastal Physical Oceanography, Old Dominion University, Norfolk, Virginia 23529, U.S.A. 3. Biological Sciences Department, California Polytechnic State University, San Luis Obispo, California, 93407, U.S.A. Journal of Marine Research, 62, 419–460, 2004

Flow Near Submarine Canyons Driven by Constant Winds

Circulation over coastal submarine canyons driven by constant upwelling or downwelling wind stress is simulated and analyzed with a primitive equation ocean model. Astoria Canyon, on the west coast of North America, is the focus of this study, and model results are consistent with most major features of mean canyon circulation observed in Astoria Canyon. Near-surface flow crosses over the canyon, while a closed cyclone occurs within the canyon. Upwelling prevails within the canyon and is larger than wind-driven upwelling along the adjacent shelf break. Water rises from depths reaching 300 m to the canyon rim and, subsequently, onto the adjacent shelf. Onshore flow within the canyon is driven by the onshore pressure gradient force, due to the free surface slope created by the upwelling wind, and is enhanced by the limitation to alongshore flow by the canyon topography. Density gradients largely compensate the surface slope with realistic stratification, but continual upwelling persists near the edges of the canyon. Within the upper canyon (50–150 m below the canyon rim) a cyclone is created by flow turning into the canyon mouth, separating from the upstream edge, and advecting toward the downstream rim. Below this layer the cyclone is created by vortex stretching due to the upwelling. Downwelling winds create nearly the opposite flow, in which compression and momentum advection create a strong anticyclone within the canyon. Momentum advection is found to be important both in creating strong circulation within the canyon and in allowing the surface flow to cross the canyon undisturbed. Model results indicate that Astoria-like submarine canyons produce across shore transport of sufficient volume to flush a continental shelf in a few (2–5) years.

Sensitivity of Circumpolar Deep Water Transport and Ice Shelf Basal Melt along the West Antarctic Peninsula to Changes in the Winds

CircumpolarDeepWater (CDW) can be found near the continental shelfbreak around most of Antarctica. Advection of this relatively warm water (up to 28C) across the continental shelf to the base of floating ice shelves is thought to be a critical source of heat for basal melting in some locations. A high-resolution (4 km) regional ocean‐sea ice‐ice shelf model of the west Antarctic Peninsula (WAP) coastal ocean was used to examine the effects of changes in the winds on across-shelf CDW transport and ice shelf basal melt. Increases and decreases in the strength of the wind fields were simulated by scaling the present-day winds by a constant factor. Additional simulations considered effects of increased Antarctic Circumpolar Current (ACC) transport. Increased wind strength and ACC transport increased the amount of CDW transported onto the WAP continental shelf but did not necessarily increase CDW flux underneath the nearby ice shelves. The basal melt underneath some of the deeper ice shelves actually decreased with increased wind strength. Increased mixing over the WAP shelf due to stronger winds removed more heat from the deeper shelf waters than the additional heat gained from increased CDW volume transport. The simulation results suggest that the effect on the WAP ice shelves of the projected strengthening of the polar westerlies is not a simple matter of increased winds causing increased (or decreased) basal melt. A simple budget calculation indicated that iron associated with increased vertical mixing of CDW could significantly affect biological productivity of this region.

On the Role of Coastal Troughs in the Circulation of Warm Circumpolar Deep Water on Antarctic Shelves

AbstractOceanic exchanges across the continental shelves of Antarctica play an important role in biological systems and the mass balance of ice sheets. The focus of this study is on the mechanisms responsible for the circulation of warm Circumpolar Deep Water (CDW) within troughs running perpendicular to the continental shelf. This is examined using process-oriented numerical experiments with an eddy-resolving (1 km) 3D ocean model that includes a static and thermodynamically active ice shelf. Three mechanisms that create a significant onshore flow within the trough are identified: 1) a deep onshore flow driven by the melt of the ice shelf, 2) interaction between the longshore mean flow and the trough, and 3) interaction between a Rossby wave along the shelf break and the trough. In each case the onshore flow is sufficient to maintain the warm temperatures underneath the ice shelf and basal melt rates of O(1 m yr−1). The third mechanism in particular reproduces several features revealed by moorings from M...