Heidi M. Dierssen

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.

Bio‐optical properties and remote sensing ocean color algorithms for Antarctic Peninsula waters

Increasing evidence suggests that bio-optical properties of Antarctic waters are significantly different than those at temperate latitudes. Consequently, retrieval of chlorophyll concentrations from remotely sensed reflectance measurements using standard ocean color algorithms are likely to be inaccurate when applied to the Southern Ocean. Here we utilize a large bio-optical data set (>1000 stations) collected in waters west of the Antarctic Peninsula in conjunction with the Palmer Long Term Ecological Research program to assess ocean optical properties and associated ocean color algorithms. We find that the remote sensing reflectance spectrum as a function of chlorophyll concentrations appears significantly different from the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) Bio-optical Algorithm Mini-workshop data set collected from other regions of the worlds oceans. For Antarctic waters, remote sensing reflectance is significantly higher in the blue region and lower in the green region of the spectrum for high chlorophyll concentrations (>1 mg Chl m -3 ). Therefore applying general processing algorithms for both Coastal Zone Color Scanner and SeaWiFS in these Antarctic waters results in an underestimate of chlorophyll by roughly a factor of 2. From modeled estimates of absorption and backscattering we hypothesize that both low chlorophyll-specific absorption and low backscattering contribute to the high reflectance ratios.

Perspectives on empirical approaches for ocean color remote sensing of chlorophyll in a changing climate

Phytoplankton biomass and productivity have been continuously monitored from ocean color satellites for over a decade. Yet, the most widely used empirical approach for estimating chlorophyll a (Chl) from satellites can be in error by a factor of 5 or more. Such variability is due to differences in absorption and backscattering properties of phytoplankton and related concentrations of colored-dissolved organic matter (CDOM) and minerals. The empirical algorithms have built-in assumptions that follow the basic precept of biological oceanography—namely, oligotrophic regions with low phytoplankton biomass are populated with small phytoplankton, whereas more productive regions contain larger bloom-forming phytoplankton. With a changing world ocean, phytoplankton composition may shift in response to altered environmental forcing, and CDOM and mineral concentrations may become uncoupled from phytoplankton stocks, creating further uncertainty and error in the empirical approaches. Hence, caution is warranted when using empirically derived Chl to infer climate-related changes in ocean biology. The Southern Ocean is already experiencing climatic shifts and shows substantial errors in satellite-derived Chl for different phytoplankton assemblages. Accurate global assessments of phytoplankton will require improved technology and modeling, enhanced field observations, and ongoing validation of our “eyes in space.”

Cascading Trophic Impacts of Reduced Biomass in the Ross Sea, Antarctica: Just the Tip of the Iceberg?

BRAD A. SEIBEL* AND HEIDI M. DIERSSEN†Monterey Bay Aquarium Research Institute, Moss Landing, California 95039A significant reduction in phytoplankton biomass in theRoss Sea was reported in the austral summer of 2000–2001,a possible consequence of a disruption in sea-ice retreatdue to the presence of an immense iceberg, B15 (1) (Fig. 1).Our observations in McMurdo Sound suggest temporallyand trophically cascading impacts of that depression inproductivity. Reduced phytoplankton stocks clearly affectedthe pteropod Limacina helicina (Phipps, 1774) (Gastro-poda: Mollusca), an abundant primary consumer in theregion (2, 3), as indicated by depressed metabolic rates in2000–2001. The following season, for the first time onrecord, L. helicina was absent from McMurdo Sound. Manyimportant predators, including whales and fishes, relyheavily on L. helicina for food (3, 4). However, most obvi-ously impacted by its absence was Clione antarctica (Smith,1902), an abundant pteropod mollusc (Gastropoda) thatfeeds exclusively on L. helicina (5). Metabolic rates of C.antarctica were depressed by 50% in 2001–2002. BothL.helicina and C. antarctica are important components ofpolar ecosystems and may be good indicators of overallecosystem “health” in McMurdo Sound and perhaps in theRoss Sea. In this last austral summer, 2002–2003, sea-iceextent was much higher and phytoplankton stocks weredramatically lower than any reported previously, effectspossibly associated with El Nin˜o conditions, and we hypoth-esize that pteropods and their consumers may be furtherimpacted.In the Southern Ocean, phytoplankton production islinked strongly to the seasonal oscillations in the extent ofthe sea ice (6, 7) and survival of higher trophic levels isdependent on reproductive cycles that are synchronous withphytoplankton blooms. This is especially true of the directfood link between L. helicina and C. antarctica. L. helicinalives and feeds in the water column by extending a web ofmucus that traps phytoplankton and, to a lesser extent, smallzooplankton (3). L. helicina is the exclusive food source ofC. antarctica throughout the life cycle, and the two specieshave parallel life histories. They grow in concert, with thepreferred prey size increasing with predator size (3). Suchspecificity within the context of a highly seasonal environ-ment requires precise timing to ensure that predator andprey coexist. The coevolved predator-prey relationship be-tween L. helicina and C. antarctica provides a uniqueopportunity to study the ecological and trophic conse-quences of a depression in primary productivity in the RossSea.A 50% to 75% reduction in phytoplankton biomass, es-timated as chlorophyll a (Chl) concentrations, and highsea-ice cover was observed in December 2000–2001 rela-tive to previous years (Table 1; Fig. 2; 8). A limited bloomdid form by February, but annual primary production wasstill only 60% of the previous year (1). We believe that thereduced phytoplankton stocks in 2000–2001 had pro-nounced impacts on the condition of primary consumers inthe region, causing cascading effects through higher trophiclevels in the following year. This assertion is supported hereby a series of metabolic measurements made on L. helicinaand C. antarctica between 1999 and 2002.Nutritional state is known to be among the primary de-terminants of metabolism in all organisms, including ptero-pods (3), and is especially important in the highly seasonalAntarctic environment (9, 10). Food availability will influ-ence, among other things, the rates of protein synthesis,oxygen consumption, growth, and reproduction (9–11). Wecollected L. helicina and C. antarctica at four samplingstations along Ross Island (Fig. 1) and measured the oxygen

Energetic plasticity underlies a variable response to ocean acidification in the pteropod, Limacina helicina antarctica.

Ocean acidification, caused by elevated seawater carbon dioxide levels, may have a deleterious impact on energetic processes in animals. Here we show that high PCO2 can suppress metabolism, measured as oxygen consumption, in the pteropod, L. helicina forma antarctica, by ∼20%. The rates measured at 180–380 µatm (MO2  = 1.25 M−0.25, p = 0.007) were significantly higher (ANCOVA, p  =  0.004) than those measured at elevated target CO2 levels in 2007 (789–1000 µatm,  =  0.78 M−0.32, p  =  0.0008; Fig. 1). However, we further demonstrate metabolic plasticity in response to regional phytoplankton concentration and that the response to CO2 is dependent on the baseline level of metabolism. We hypothesize that reduced regional Chl a levels in 2008 suppressed metabolism and masked the effect of ocean acidification. This effect of food limitation was not, we postulate, merely a result of gut clearance and specific dynamic action, but rather represents a sustained metabolic response to regional conditions. Thus, pteropod populations may be compromised by climate change, both directly via CO2-induced metabolic suppression, and indirectly via quantitative and qualitative changes to the phytoplankton community. Without the context provided by long-term observations (four seasons) and a multi-faceted laboratory analysis of the parameters affecting energetics, the complex response of polar pteropods to ocean acidification may be masked or misinterpreted.

An Atmospheric Correction Algorithm for Remote Sensing of Bright Coastal Waters Using MODIS Land and Ocean Channels in the Solar Spectral Region

The present operational atmospheric correction algorithm for multichannel remote sensing of ocean color using imaging data acquired with the Moderate Resolution Imaging Spectroradiometer (MODIS) works well over clear ocean but can give incorrect results over brighter coastal waters. This is because: 1) the turbid waters are not dark for the two atmospheric correction channels centered near 0.75 and 0.86 mum; and 2) the ocean color channels (0.488, 0.531, and 0.551 mum) often saturate over bright coastal waters. Here, we describe an atmospheric correction algorithm for multichannel remote sensing of coastal waters. This algorithm is a modification of our previously developed atmospheric correction algorithm for hyperspectral data that uses lookup tables generated with a vector radiative transfer code and multilayered atmospheric models. Aerosol models and optical depths are determined by a spectrum-matching technique utilizing channels located at wavelengths longer than 0.86 mum, where the ocean surface is dark. The aerosol information in the visible spectral region is estimated based on the derived aerosol models and optical depths. Water-leaving radiances in the visible spectral region are obtained by subtracting out the atmospheric path radiances from the satellite-measured total radiances. Applications of the algorithm to two MODIS data sets are presented and compared to field measurements. The water-leaving reflectances retrieved with this algorithm over brighter shallow coastal waters compare closely with those from field measurements. In addition, the retrieved water-leaving reflectances over deeper ocean waters compare well with those derived with the MODIS operational algorithm

Optimizing models for remotely estimating primary production in Antarctic coastal waters

Primary productivity and associated biogeochemical fluxes within the Southern Ocean are globally significant, sensitive to change and poorly known compared to temperate marine ecosystems. We present seasonal time series data of chlorophyll a, primary productivity and in-water irradiance measured in the coastal waters of the Western Antarctica Peninsula and build upon existing models to provide a more optimum parameterization for the estimation of primary productivity in Antarctic coastal waters. These and other data provide strong evidence that bio-optical characteristics and phytoplankton productivity in Antarctic waters are different from temperate waters. For these waters we show that over 60% of the variability in primary production can be explained by the surface chlorophyll a concentration alone, a characteristic, which lends itself to remote sensing models. If chlorophyll a concentrations are accurately determined, then the largest source of error (13–18%) results from estimates of the photoadaptive variable (PBopt). Further, the overall magnitude of PBopt is low (median 1.09 mg C mg chl−1 h−1) for these data compared to other regions and generally fits that expected for a cold water system. However, the variability of PBopt over the course of a season (0.4 to 3 mg C mg chl−1 h−1) is not consistently correlated with other possible environmental parameters, such as chlorophyll, sea surface temperature, incident irradiance, day length, salinity, or taxonomic composition. Nonetheless, by tuning a standard depth-integrated primary productivity model to fit representative PBopt values and the relatively uniform chlorophyll-normalized production profile found in these waters, we can improve the model to account for approximately 72–73% variability in primary production both for our data as well as for independent historic Antarctic data.

LARGE-SCALE DIVERSITY PATTERNS OF CEPHALOPODS IN THE ATLANTIC OPEN OCEAN AND DEEP SEA

Although the oceans cover 70% of the Earths surface and the open ocean is by far the largest ecosystem on the planet, our knowledge regarding diversity patterns of pelagic fauna is very scarce. Here, we examine large-scale latitudinal and depth-related patterns of pelagic cephalopod richness in the Atlantic Ocean in relation to ambient thermal and productive energy availability. Diversity, across 17 biogeochemical regions in the open ocean, does not decline monotonically with latitude, but is positively correlated to the availability of oceanic resources. Mean net primary productivity (NPP), determined from ocean color satellite imagery, explains 37% of the variance in species richness. Outside the poles, the range in NPP explains over 40% of the variability. This suggests that cephalopods are well adapted to the spatial patchiness and seasonality of open-ocean resources. Pelagic richness is also correlated to sea surface temperature, with maximum richness occurring around 15 degrees C and decreasing with both colder and warmer temperatures. Both pelagic and benthos-associated diversities decline sharply from sublittoral and epipelagic regions to the slope and bathypelagic habitats and then steadily to abyssal depths. Thus, higher energy availability at shallow depths seems to promote diversification rates. This strong depth-related trend in diversity also emphasizes the greater influence of the sharp vertical thermal gradient than the smoother and more seasonal horizontal (latitudinal) one on marine diversity.

Measurements and simulations of polarization states of underwater light in clear oceanic waters.

Polarization states of the underwater light field were measured by a hyperspectral and multiangular polarimeter and a video polarimeter under various atmospheric, surface, and water conditions, as well as solar and viewing geometries, in clear oceanic waters near Port Aransas, Texas. Some of the first comprehensive comparisons were made between the measured polarized light, including the degree and angle of linear polarization and linear Stokes parameters (Q and U), and those from Monte Carlo simulations that used concurrently measured water inherent optical properties and particle volume scattering functions as input. For selected wavelengths in the visible spectrum, measured and model-simulated polarization characteristics were found to be consistent in most cases. Measured degree and angle of linear polarization are found to be largely determined by an in-water single-scattering model. Model simulations suggest that the degree of linear polarization (DoLP) at horizontal viewing directions is highly dependent on the viewing azimuth angle for a low solar elevation. This implies that animals can use the DoLP signal for orientation.

Open-ocean fish reveal an omnidirectional solution to camouflage in polarized environments.

Disappearing act Unlike coastal regions and reefs, the open ocean is mostly empty. Many fish species, nonetheless, spend most of their lives there. Such emptiness makes camouflage exceedingly difficult, so how does an organism hide in water filled with bouncing and reflected light? Brady et al. show that some families of fish have evolved skin that reflects and polarizes light, allowing them to blend into their mirrorlike conditions more easily. These results help to explain the silvery coloration found in sea-living fish across the worlds oceans. Science, this issue p. 965 Light-reflecting and -polarizing platelets in their skin permit fish to blend into the mirrorlike open ocean. Despite appearing featureless to our eyes, the open ocean is a highly variable environment for polarization-sensitive viewers. Dynamic visual backgrounds coupled with predator encounters from all possible directions make this habitat one of the most challenging for camouflage. We tested open-ocean crypsis in nature by collecting more than 1500 videopolarimetry measurements from live fish from distinct habitats under a variety of viewing conditions. Open-ocean fish species exhibited camouflage that was superior to that of both nearshore fish and mirrorlike surfaces, with significantly higher crypsis at angles associated with predator detection and pursuit. Histological measurements revealed that specific arrangements of reflective guanine platelets in the fish’s skin produce angle-dependent polarization modifications for polarocrypsis in the open ocean, suggesting a mechanism for natural selection to shape reflectance properties in this complex environment.