Maria Vernet

Marine pelagic ecosystems: the West Antarctic Peninsula

The marine ecosystem of the West Antarctic Peninsula (WAP) extends from the Bellingshausen Sea to the northern tip of the peninsula and from the mostly glaciated coast across the continental shelf to the shelf break in the west. The glacially sculpted coastline along the peninsula is highly convoluted and characterized by deep embayments that are often interconnected by channels that facilitate transport of heat and nutrients into the shelf domain. The ecosystem is divided into three subregions, the continental slope, shelf and coastal regions, each with unique ocean dynamics, water mass and biological distributions. The WAP shelf lies within the Antarctic Sea Ice Zone (SIZ) and like other SIZs, the WAP system is very productive, supporting large stocks of marine mammals, birds and the Antarctic krill, Euphausia superba. Ecosystem dynamics is dominated by the seasonal and interannual variation in sea ice extent and retreat. The Antarctic Peninsula is one among the most rapidly warming regions on Earth, having experienced a 2°C increase in the annual mean temperature and a 6°C rise in the mean winter temperature since 1950. Delivery of heat from the Antarctic Circumpolar Current has increased significantly in the past decade, sufficient to drive to a 0.6°C warming of the upper 300 m of shelf water. In the past 50 years and continuing in the twenty-first century, the warm, moist maritime climate of the northern WAP has been migrating south, displacing the once dominant cold, dry continental Antarctic climate and causing multi-level responses in the marine ecosystem. Ecosystem responses to the regional warming include increased heat transport, decreased sea ice extent and duration, local declines in ice-dependent Adélie penguins, increase in ice-tolerant gentoo and chinstrap penguins, alterations in phytoplankton and zooplankton community composition and changes in krill recruitment, abundance and availability to predators. The climate/ecological gradients extending along the WAP and the presence of monitoring systems, field stations and long-term research programmes make the region an invaluable observatory of climate change and marine ecosystem response.

The effects of UV radiation in the marine environment

Preface Stephen de Mora 1. Enhanced UV radiation - a new problem for the marine environment Robert Whitehead, Stephen de Mora and Serge Demers 2. UV physics and optics Susana Diaz, John Morrow and Charles Booth 3. Spectral weighting functions for quantifying effects of ultraviolet radiation in marine ecosystems Patrick Neale 4. Marine photochemistry and its impact on carbon cycling Kenneth Mopper and David Kieber 5. Photochemical production of biological substrates David Kieber 6. Mechanisms of UV damage to aquatic organisms Warwick Vincent and Patrick Neale 7. Strategies for the minimisation of UV-induced damage Suzanne Roy 8. Ultraviolet radiation effects on heterotrophic bacterioplankton and viruses in marine ecosystems Wade Jeffrey, Jason Kase and Steven Wilhelm 9. Effects of ultraviolet radiation on the physiology and ecology of marine phytoplankton Maria Vernet 10. Impact of solar UV radiation on zooplankton and fish Horacio Zagarese and Craig Williamson 11. Implications of UV radiation on the food web structure and consequences on the carbon flow Behzad Mostajir, Serge Demers, Stephen de Mora, Robert Bukata and John Jerome.

Marine Ecosystem Sensitivity to Climate Change

393 M ounting evidence suggests that the earth is experiencing a period of rapid climate change. Never before has it been so important to understand how environmental change influences the earth’s biota and to distinguish anthropogenic change from natural variability. Long-term studies in the western Antarctic Peninsula (WAP) region provide the opportunity to observe how changes in the physical environment are related to changes in the marine ecosystem. Analyses of paleoc limate records (MosleyThompson 1992, Peel 1992, Domack et al. 1993, Thompson et al. 1994, Dai et al. 1995, Domack and McClennen 1996, Leventer et al. 1996) have shown that the WAP region has moved from a relatively cold regime between approximately 2700 BP and 100 BP, to a relatively warm regime during the current century. Air temperature records from the last half-century show a dramatic warming trend, confirming the rapidity of change in the WAP area (Sansom 1989, Stark 1994, Rott et al. 1996, Smith et al. 1996). Significantly, polar ecosystem research over the last few decades (Fraser et al. 1992, Trivelpiece and Fraser 1996) and paleoecological records for the past 500 years (Emslie 1995, Emslie et al. 1998) reveal ecological transitions that have occurred in response to this climate change. In this article, we summarize the available data on climate variability and trends in the WAP region and discuss these data in the context of long-term climate variability during the last 8000 years of the Holocene. We then compare the available data on ecosystem change in the WAP region to the data on climate variability. Both historical and paleoenvironmental records indicate a climate gradient along the WAP that includes a dry, cold continental regime to the south and a wet, warm maritime regime to the north. The position of this climate gradient has shifted over time in response to the dominant climate regime, and it makes the WAP region a highly sensitive location for assessing ecological responses to climate variability. Our findings show that this century’s rapid climate warming has occurred concurrently with a shift in the population size and distribution of penguin species.

Glacial meltwater dynamics in coastal waters west of the Antarctic peninsula

The annual advance and retreat of sea ice has been considered a major physical determinant of spatial and temporal changes in the structure of the Antarctic coastal marine ecosystem. However, the role of glacial meltwater on the hydrography of the Antarctic Peninsula ecosystem has been largely ignored, and the resulting biological effects have only been considered within a few kilometers from shore. Through several lines of evidence collected in conjunction with the Palmer Station Long-Term Ecological Research Project, we show that the freshening and warming of the coastal surface water over the summer months is influenced not solely by sea ice melt, as suggested by the literature, but largely by the influx of glacial meltwater. Moreover, the seasonal variability in the amount and extent of the glacial meltwater plume plays a critical role in the functioning of the biota by influencing the physical dynamics of the water (e.g., water column stratification, nearshore turbidity). From nearly a decade of observations (1991–1999), the presence of surface meltwater is correlated not only to phytoplankton blooms nearshore, but spatially over 100 km offshore. The amount of meltwater will also have important secondary effects on the ecosystem by influencing the timing of sea ice formation. Because air temperatures are statistically increasing along the Antarctic Peninsula region, the presence of glacial meltwater is likely to become more prevalent in these surface waters and continue to play an ever-increasing role in driving this fragile ecosystem.

Dynamics of the vernal bloom in the marginal ice zone of the Barents Sea: Dimethyl sulfide and dimethylsulfoniopropionate budgets

Phytoplankton is known to be a key element in the production and eventual oceanic efflux of dimethyl sulfide (DMS) to the atmosphere. We hypothesized that the alternation of Phaeocystis pouchetii and diatoms, the two major algal components of the spring bloom, would modulate the input of particulate organic sulfur (POS), dimethylsulfoniopropionate (DMSP), and DMS into the mixed layer of the marginal ice zone. A bloom of diatoms is expected to present similar pathways but to have very different rates of POS/DMSP/DMS production and POS/DMSP sinking and no or low DMS flux to the atmosphere as contrasted to the cycling occurring during the P. pouchetii phase of the bloom. Our initial hypothesis cannot be accepted based on our observations in the Barents Sea during the spring of 1993. The contribution of diatoms to the water column budgets of DMSP and DMS was significant and cannot be overlooked. We suggest that the physiological stage of the bloom is perhaps more important to biogeochemical cycling than its phytoplankton species composition in controlling DMSP and DMS fluxes in Arctic waters. Loss of paniculate DMSP in the mixed layer was mainly by release into the dissolved pool and by sedimentation father than by grazing, except in ice-free waters. Cycling of DMS in the mixed layer was predominantly biological in ice-free waters, while in Polar Front waters, ventilation was proportionally more important due to depressed microbiology.

Extreme Anomalous Atmospheric Circulation in the West Antarctic Peninsula Region in Austral Spring and Summer 2001/02, and Its Profound Impact on Sea Ice and Biota*

Exceptional sea ice conditions occurred in the West Antarctic Peninsula (WAP) region from September 2001 to February 2002, resulting from a strongly positive atmospheric pressure anomaly in the South Atlantic coupled with strong negative anomalies in the Bellingshausen-Amundsen and southwest Weddell Seas. This created a strong and persistent north-northwesterly flow of mild and moist air across the WAP. In situ, satellite, and NCEP-NCAR Reanalysis (NNR) data are used to examine the profound and complex impact on regional sea ice, oceanography, and biota. Extensive sea ice melt, leading to an ocean mixed layer freshening and widespread ice surface flooding, snow-ice formation, and phytoplankton growth, coincided with extreme ice deformation and dynamic thickening. Sea ice dynamics were crucial to the development of an unusually early and rapid (short) retreat season (negative ice extent anomaly). Strong winds with a dominant northerly component created an unusually compact marginal ice zone and a major increase in ice thickness by deformation and over-rafting. This led to the atypical persistence of highly compact coastal ice through summer. Ecological effects were both positive and negative, the latter including an impact on the growth rate of larval Antarctic krill and the largest recorded between-season breeding population decrease and lowest reproductive success in a 30-yr Adelie penguin demographic time series. The unusual sea ice and snow cover conditions also contributed to the formation of a major phytoplankton bloom. Unexpectedly, the initial bloom occurred within compact sea ice and could not be detected in Sea-Viewing Wide Field-of-View Sensor (SeaWiFS) ocean color data. This analysis demonstrates that sea ice extent alone is an inadequate descriptor of the regional sea ice state/conditions, from both a climatic and ecological perspective; further information is required on thickness and dynamics/deformation.

Release of ultraviolet-absorbing compounds by the red-tide dinoflagellateLingulodinium polyedra

We tested the hypothesis that ultraviolet-absorbing compounds known as mycosporine-like amino acids (MAAs) are not only synthesized but also excreted by marine phytoplankton. An experiment was performed with cultures of the marine dinoflagellateLingulodinium polyedra (previously known asGonyaulax polyedra) exposed to visible (photosynthetically available, PAR, 400 to 700 nm) and ultraviolet (UV, 290 to 400 nm) radiation. Absorption properties of both particulate and dissolved organic matter pools (POM and DOM, respectively) showed maxima in ultraviolet absorption at 360 nm. Chromatographic analysis confirmed the presence of MAAs in both pools. Release of organic matter byL. polyedra, as measured spectrophotometrically by changes in UV absorption in the surrounding medium, showed a differential increase at 360 nm in cultures exposed to UV-B + PAR radiation. The changes in absorption in the DOM fraction were inversely proportional to intracellular UV absorption. Photodegradation experiments in which the DOM fraction was exposed to visible and UV-B radiation showed a decrease in absorption with dose. First-order photooxidation decay rates varied between − 0.005 and − 0.26 m2 (mol quanta)−1 and were also a function of the initial optical density (OD). These results indicate that UV-absorbing compounds synthesized by phytoplankton, such as certain dinoflagellates, may be a component of the DOM pool in surface waters of the ocean and contribute to the attenuation of UV radiation in the water column. Photooxidation consumes only 3 to 10% of the daily production of the DOM absorbing between 280 and 390 nm (including MAAs). This suggests that MAAs dissolved in seawater may contribute to the decrease of UV transmission through the water column on a time scale representative of phytoplankton growth (days) and bloom development (weeks).

Modeling of light-dependent algal photosynthesis and growth: experiments with the Barents sea diatoms Thalassiosira nordenskioldii and Chaetoceros furcellatus

Abstract The models by Sakshaug et al (1989, Limnology and Oceanography, 34, 198–205) and Webb et al. (1974, Oecologia, 17, 281–291), for prediction of the gross growth rate of phytoplankton and short-term photosynthesis, respectively, have been modified on the basis of experiments with cultures of the centric diatoms Thalassiosira nordenskioeldii and Chaetoceros furcellatus grown at 0.5°C at combinations of two irradiances (25 and 400μmol m−2s−1) and two day-lengths (12 and 24 h). The models have one spectrum, °σ, which represents chlorophyll a (Chla) specific absorption of photosynthetically usable light, and introduces a factor q which represents Chla per PSU, functionally defined. The models describe phytoplankton growth in terms of physiologically relevant coefficients. A properly scaled fluorescence excitation spectrum (°F) represents a more appropriate estimate for °σ than the Chla-specific absorption spectrum °ac judging from calculations of Φmax (=αB/°σ). On the basis of °F, Φmax is 0.04 g-at C(mol photons)−1 for gross growth and about 0.05–0.08 for short-term carbon uptake (unfiltered samples). Calculations based on °ac yield values for Φmax which on average are 44% lower. P vs I (photosynthesis vs irradiance) parameters are relatively independent of day-length and highly dependent on growth irradiance. The product of q [mg Chla (mol PSU)−1] and τ (the minimum turnover time of the photosynthetic unit, h) increases 2–3-fold from high to low irradiance, thus PmB (=Φmax/qτ) and Ik(=1/qτ°σ)decreased. °F decreases from high to low irradiance. Carbon-specific dark respiration rates are Pigment ratios vary inversely with irradiance and day-length. The Chla:C ratio is particularly low under high, strong continuous light; Chlc: Chaa ratios are higher for shalde- than for light-adapted cells, while the converse is true for the ratio of the sum of the photoprotective pigments diadinoxanthin and diatoxanthin to Chla. The fucoxanthin: Chla ratio is virtually independent of the light regime. The two species are similar with respect to variations in growth rate (0.09–0.33 day−1 and Ik (31–36 vs 49–100 μmol m−2 s−1 at low and high irradiance, respectively). PmB and aB for growth as well as °F are systematically higher for C. furcellatus than for T. nordenskioeldii, while the product qτ is lower. C. furcellatus is considerably more plastic than T. nordenskioeldii with respect to pigment composition.

Light-dependence of carbon and sulfur production by polar clones of the genus Phaeocystis

Blooms of the marine prymnesiophyte genus Phaeocystis link the oceanic and atmospheric compartments of the carbon and sulfur cycles. Modeling the fluxes of dimethylsulfide from the ocean to the atmosphere has been limited due to a lack of information on functional responses to environmental variables. In this study, the light-dependence of extracellular carbon production and dimethyl sulfide (DMS) production by non-axenic polar clones of Phaeocystis spp. was examined at different growth stages. Comparative experiments were run with non-axenic arctic clones of the diatoms Thalassiossira nordenskioeldii and Skeletonema costatum. A large portion of carbon incorporated by the colonial stage of Phaeocystis spp. is released extracellularly, in particular in stationary colonies. This extracellular production can be modeled as a function of irradiance, as for carbon incorporation. In Phaeocystis spp., cellular and extracellular carbon incorporation represent different uptake rates, indicating the formation of two distinct carbon pools. The release of extracellular carbon by polar Phaeocystis spp. was not a constant fraction of total production over the irradiance range used. We observed little extracellular carbon production by cells at high irradiance, and maximal rates were observed at intermediate irradiance. Newly incorporated carbon that accumulates in the mucilage of the colonial stage of antarctic Phaeocystis sp. during photosynthesis was not reutilized for cellular growth during the dark period, as observed for temperate clones. In contrast, only a minor fraction of the radiocarbon incorporated by the diatoms was released extracellularly for all growth stages. The production of DMS was an order of magnitude higher for Phaeocystis spp. than for diatoms. The chlorophyll-specific production of DMS and DMSP (dimethylsulphoniopropionate, the precursor to DMS) by Phaeocystis spp. showed a hyperbolic response to irradiance, while arctic diatoms (weak or non-producers of DMS), on the other hand, did not show any light-dependency of DMS production. An inverse relationship between DMS and DMSP production in stationary clones of arctic P. pouchetii was observed, but not for the exponentially growing antarctic clone. Stationary colonies also had higher DMS and dissolved DMSP production rates than exponentially growing ones. These relationships can be extrapolated to the field in areas where Phaeocystis spp. dominates.

Ecological responses of Antarctic krill to environmental variability: can we predict the future?

Abstract Antarctic krill are a key species in the Southern Ocean ecosystem, and their life cycle appears to be correlated with, and by implication dependent upon, seasonal sea ice dynamics. Moving from correlations with environmental parameters to an understanding of the mechanisms that lead to these correlations may allow predictions of the consequences of climate change on the distribution of favourable habitat for Antarctic krill. During winter cruises in 2001 and 2002 in the region west of the Antarctic Peninsula, one of the most rapidly warming regions on the planet, ice camps were established for periods of 3–9 days. Timing of sea ice advance, chlorophyll a concentrations in ice cores, and growth rates and pigment content of larval krill all differed significantly between winters. Growth rates and pigment content of larval krill from the same ice floe were correlated, suggesting that growth rates in winter are a function of the biomass of the sea ice microbial community. Apossible mechanism underlying the correlation between recruitment success and timing of ice advance is proposed. In conjunction with other postulated habitat requirements, this proposed mechanism allows for speculation about future changes in the geographic location of favourable habitat for Antarctic krill.