Eileen E. Hofmann

Climate Change Influences on Marine Infectious Diseases: Implications for Management and Society

Infectious diseases are common in marine environments, but the effects of a changing climate on marine pathogens are not well understood. Here we review current knowledge about how the climate drives host-pathogen interactions and infectious disease outbreaks. Climate-related impacts on marine diseases are being documented in corals, shellfish, finfish, and humans; these impacts are less clearly linked for other organisms. Oceans and people are inextricably linked, and marine diseases can both directly and indirectly affect human health, livelihoods, and well-being. We recommend an adaptive management approach to better increase the resilience of ocean systems vulnerable to marine diseases in a changing climate. Land-based management methods of quarantining, culling, and vaccinating are not successful in the ocean; therefore, forecasting conditions that lead to outbreaks and designing tools/approaches to influence these conditions may be the best way to manage marine disease.

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.

Seasonal variability in the distribution of Antarctic krill, Euphausia superba, west of the Antarctic Peninsula

A unique Antarctic data set from four multidisciplinary cruises (spring, Nov 1991; summer, Jan–Feb 1993; fall, Mar–May 1993; winter, Aug–Sept 1993) was analyzed to provide a description of seasonal variability in the distribution and abundance of Antarctic krill, Euphausia superba, west of the Antarctic Peninsula. Analyses of acoustic observations revealed distinct seasonal variations in estimates of krill abundance, the dimensional parameters used to characterize individual krill aggregations, the geographic area over which krill were acoustically detected, and the depth distribution of krill biomass. Spatially averaged estimates of krill biomass were an order of magnitude higher in spring (32 g m-2) and summer (95 g m-2) compared to fall (12 g m-2) and winter (8 g m-2). In summer, krill were detected throughout the region and most of the krill biomass was associated with aggregations of small cross-sectional area ( 150 g m-3), which were positioned in the upper 50 m of the water column. Winter observations, in contrast, were characterized by the absence of krill throughout most of the region and aggregations occuring deeper than 100 m with large cross-sectional area (>10000 m2) and low mean biomass ( 50 g m-2) at selected locations on the inner shelf and the disproportionate contribution, in terms of total krill biomass (>80%), by a small number of aggregations ( 40 mm) krill positioned offshore of smaller krill (32–38 mm) in all seasons except winter. Synthesis of these data and historical observations suggest that there is a seasonal shift in the primary habitat of krill and that changes in krill behavior are an important factor affecting variations in krill distributions.

Climate change and Southern Ocean ecosystems I: how changes in physical habitats directly affect marine biota

Antarctic and Southern Ocean (ASO) marine ecosystems have been changing for at least the last 30 years, including in response to increasing ocean temperatures and changes in the extent and seasonality of sea ice; the magnitude and direction of these changes differ between regions around Antarctica that could see populations of the same species changing differently in different regions. This article reviews current and expected changes in ASO physical habitats in response to climate change. It then reviews how these changes may impact the autecology of marine biota of this polar region: microbes, zooplankton, salps, Antarctic krill, fish, cephalopods, marine mammals, seabirds, and benthos. The general prognosis for ASO marine habitats is for an overall warming and freshening, strengthening of westerly winds, with a potential pole-ward movement of those winds and the frontal systems, and an increase in ocean eddy activity. Many habitat parameters will have regionally specific changes, particularly relating to sea ice characteristics and seasonal dynamics. Lower trophic levels are expected to move south as the ocean conditions in which they are currently found move pole-ward. For Antarctic krill and finfish, the latitudinal breadth of their range will depend on their tolerance of warming oceans and changes to productivity. Ocean acidification is a concern not only for calcifying organisms but also for crustaceans such as Antarctic krill; it is also likely to be the most important change in benthic habitats over the coming century. For marine mammals and birds, the expected changes primarily relate to their flexibility in moving to alternative locations for food and the energetic cost of longer or more complex foraging trips for those that are bound to breeding colonies. Few species are sufficiently well studied to make comprehensive species-specific vulnerability assessments possible. Priorities for future work are discussed.

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

ENVIRONMENTAL VARIABILITY EFFECTS ON MARINE FISHERIES: FOUR CASE HISTORIES

The changing nature of marine fisheries requires management approaches that recognize and include ecosystem and environmental effects. Therefore, we review some examples of exploited fishery stocks in which environmental control is a major contributor to structuring the abundance and distribution of the stock. Four examples, taken from studies of northern cod (Gadus morhua), cod and haddock (Melanogrammus aeglefinus) larvae, the eastern oyster (Crassostrea virginica), and Antarctic krill (Euphausia superba), are given that clearly illustrate environmental control on the fishery. From these examples, we argue that future management strategies for exploited fisheries must include effects of environmental variability. In particular, management strategies must be flexible enough to include delayed responses to environmental variations that result from the transfer of per- turbations from larger to smaller scales and vice versa. This capability requires an under- standing of where linkages between the physical environment and the species of interest occur. Development of this knowledge requires input from a variety of disciplines, coor- dinated research programs, and considerable cooperation at national and international levels.

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.

Advection, krill, and Antarctic marine ecosystems

Advective processes are recognized as being important in structuring and maintaining marine ecosystems. In the Southern Ocean advective effects are perhaps most clearly observed because the Antarctic Circumpolar Current (ACC) provides a connection between most parts of the system, including open ocean and continental shelf regions. The ACC also provides a mechanism for large-scale transport of plankton, such as Antarctic krill (Euphausia superba Dana), which is an important component of the Southern Ocean food web. This overview provides a summary of recent observational and modelling results that consider the importance of advection to the Southern Ocean ecosystem and, in particular, the role of advection in structuring the large-scale distribution of Antarctic krill. The results of these studies show that advection is a dominant process controlling Antarctic krill distribution and by inference an important process affecting overall structure and production of the Southern Ocean food web. The overview shows that quantifying the roles of advective and retentive physical processes, and population dynamic and behavioural biological processes in determining the regional and local distribution of krill and abundance will be an important research focus. Strategies for future Antarctic multidisciplinary research programmes that are focused on understanding advective processes at a circumpolar scale are suggested.

Models of the early life history of Euphausia superba—Part I. Time and temperature dependence during the descent-ascent cycle

Abstract A time- and temperature-dependent model was developed to simulate the descent-ascent behavior of the embryos and early larval stages of the Antarctic krill, Euphausia superba. This model combines laboratory measurements of temperature effects on developmental times, density and physiology of krill embryos and larvae and the observed water temperature structure in the Bransfield Strait-South Shetland Islands region. Simulations with observed vertical temperature profiles from this region show that embryos that develop at temperatures less than 0°C hatch relatively deep (≈1000 m) or hit the bottom before hatching. The presence of warm (1–2°C) Circumpolar Deep Water (CDW), between 200 and 700 m, results in hatching depths of about 700 m. The sinking rate pattern characteristic of the embryos of Euphausia superba retains the embryos in the CDW, where development is accelerated. Larval ascent rate through the CDW is rapid, so larvae reach the surface before metamorphosing into the first feeding stage, and have sufficient carbon reserves to drift at the surface for several weeks before needing to find food. These results suggest that the sinking rate pattern characteristic of embryos of Antarctic krill may be part of a reproductive strategy that evolved in response to the thermal structure of its environment. The complementary component of this reproductive strategy is the observed correlation between the distribution of krill schools containing reproducing individuals and the presence of CDW. With this reproductive strategy, the spawning regions of Antarctic krill are in areas where oceanic conditions enhance the probability of survival of its embryos and non-feeding larvae.