Ocean Surface Layer Research Articles

Contribution of atmospheric nitrogen deposition to new production in the nitrogen limited photic zone of the northern Indian Ocean

Primary productivity in the sunlit surface layers of tropical oceans is mostly limited by the supply of reactive nitrogen (Nr) through upwelling, N2 fixation by diazotrophs, riverine flux and atmospheric deposition. The relative importance of these processes varies from region to region. Using recent data on the nitrogen content of aerosols over the ocean and marine new production in parts of the northern Indian Ocean for the period 1994–2006 CE, a quantitative assessment of the contribution of atmospheric deposition to new production in the two biogeochemically different basins of the northern Indian Ocean, viz., the Arabian Sea and the Bay of Bengal, is presented. By suitably converting the measured concentrations of aerosol nitrogen into fluxes and comparing them with 15N tracer‐based direct new and primary production measurements, it is inferred that the contribution of atmospheric deposition to new production in the northern Indian Ocean could at best be ∼3%. Our estimate of ∼1.39 Tg N year−1 of Nr flux into the northern Indian Ocean through aerosols is a step toward significantly reducing the uncertainty in the global nitrogen budget.

The influence of Langmuir turbulence on the scaling for the dissipation rate in the oceanic boundary layer

A model for estimating the turbulent kinetic energy dissipation rate in the oceanic boundary layer, based on insights from rapid‐distortion theory, is presented and tested. This model provides a possible explanation for the very high dissipation levels found by numerous authors near the surface. It is conceived that turbulence, injected into the water by breaking waves, is subsequently amplified due to its distortion by the mean shear of the wind‐induced current and straining by the Stokes drift of surface waves. The partition of the turbulent shear stress into a shear‐induced part and a wave‐induced part is taken into account. In this picture, dissipation enhancement results from the same mechanism responsible for Langmuir circulations. Apart from a dimensionless depth and an eddy turn‐over time, the dimensionless dissipation rate depends on the wave slope and wave age, which may be encapsulated in the turbulent Langmuir numberLat. For large Lat, or any Lat but large depth, the dissipation rate tends to the usual surface layer scaling, whereas when Lat is small, it is strongly enhanced near the surface, growing asymptotically as ε ∝ Lat−2 when Lat → 0. Results from this model are compared with observations from the WAVES and SWADE data sets, assuming that this is the dominant dissipation mechanism acting in the ocean surface layer and statistical measures of the corresponding fit indicate a substantial improvement over previous theoretical models. Comparisons are also carried out against more recent measurements, showing good order‐of‐magnitude agreement, even when shallow‐water effects are important.

Horizontal Density Structure and Restratification of the Arctic Ocean Surface Layer

Abstract Ice-tethered profiler (ITP) measurements from the Arctic Ocean’s Canada Basin indicate an ocean surface layer beneath sea ice with significant horizontal density structure on scales of hundreds of kilometers to the order 1 km submesoscale. The observed horizontal gradients in density are dynamically important in that they are associated with restratification of the surface ocean when dense water flows under light water. Such restratification is prevalent in wintertime and competes with convective mixing upon buoyancy forcing (e.g., ice growth and brine rejection) and shear-driven mixing when the ice moves relative to the ocean. Frontal structure and estimates of the balanced Richardson number point to the likelihood of dynamical restratification by isopycnal tilt and submesoscale baroclinic instability. Based on the evidence here, it is likely that submesoscale processes play an important role in setting surface-layer properties and lateral density variability in the Arctic Ocean.

A satellite ocean color observation operator system for eutrophication assessment in coastal waters

During the INSEA project the potential positive role that remote sensing products can play in coastal eutrophication assessment systems using assimilation into coupled hydrodynamic–biogeochemical models has been shown. However, products derived from satellite ocean color data continue to suffer from high levels of inaccuracy when compared with in situ measurements of the surface layer of the ocean. This has been particularly pronounced for coastal waters and waters optically classified as Case-II. The early success of using empirical relationships between chlorophyll and simple band ratios to derive estimates of surface layer chlorophyll from the first ocean color satellite sensors' data (i.e. CZCS), has led mainstream ocean color remote sensing and standard ocean color products towards following this approach for subsequent sensors (e.g. SeaWiFS and MODIS). Chlorophyll has continued to be the main focus product but is only related to one of the optical properties of sea water, namely the absorption of light by phytoplankton, whereas empirical band ratio approaches use wavelength banded water leaving radiance resultant from all absorption and scattering of light by all the optically active components of the ocean surface layer. We suggest that using approaches that do not fully exploit remote sensing optical data through a parameterization of the optical properties of sea water, is the main reason for the poor performance of many ocean color products when compared with in situ data. This is in concordance with the International Ocean Color Coordinating Group (IOCCG) and following their recent guidelines, novel inherent optical properties approaches (e.g. for MERIS) and the lines of research that are being used in atmospheric remote sensing, we present a demonstration ‘observation operator’ system that is based on biogeochemical model output, optical properties (apparent and inherent), and radiative transfer modeling. In the forward mode we demonstrate the system by producing MODIS and SeaWiFS synthetic images of water leaving radiance for the coastal test sites of INSEA. We show that the observation operator approach has the potential to allow the consistent mapping of model variables into observed quantities which simplifies the transport of measurement errors and reduces the need for approximations inherent in previous approaches. In conclusion we discuss the future development and potential of inversion of the system in order to obtain more accurate ocean color biogeochemical products (including chlorophyll) from satellite radiance data for eutrophication assessment. We also highlight the additional advantages there may be for ecological models from having stronger links to bio-optics.

Relationship between size of oil droplet generated during chemical dispersion of crude oil and energy dissipation rate: Dimensionless, scaling, and experimental analysis

Droplet formation mechanisms during the chemical dispersion of crude oil were investigated using both theoretical and experimental approaches. Dimensionless and force balance analysis identified four distinct regimes of droplet formations. For d>η, d scales either with (ε−2/5) or (ε−1/4) or and for d<η, d scales either with (ε−1/2) or (ε−1/4) depending on whether the main restoring force against droplet breakage is provided by surface tension or oil viscosity. The symbols d, η, and ε represent the droplet diameter, the Kolmogorov length scale, and energy dissipation rate, respectively. For d>η and <η, the external force, which tries to deform and break the droplet is provided by the pressure difference across the droplet diameter and viscous shear, respectively. Identification of the relationship d∼(ε−1/4) for d<η is a new contribution of this present study. The validity of this relationship was also proven by our experimental observations over a range of physical properties (dynamic viscosity 0.015–8.6Pas; oil–water interfacial tension 0.0001–0.015N/m) and mixing energies (0.00075–0.16W/kg), similar to those in real environmental settings (e.g., estuary, surface layer of oceans). All these above findings and observations are vital from the stand point of appropriately scaling droplet formation process, during chemical dispersion of crude oil, and in the development of reliable predictive models.

On the Connection between Dissipation Enhancement in the Ocean Surface Layer and Langmuir Circulations

Abstract A mechanism for the enhancement of the viscous dissipation rate of turbulent kinetic energy (TKE) in the oceanic boundary layer (OBL) is proposed, based on insights gained from rapid-distortion theory (RDT). In this mechanism, which complements mechanisms purely based on wave breaking, preexisting TKE is amplified and subsequently dissipated by the joint action of a mean Eulerian wind-induced shear current and the Stokes drift of surface waves, the same elements thought to be responsible for the generation of Langmuir circulations. Assuming that the TKE dissipation rate ɛ saturates to its equilibrium value over a time of the order one eddy turnover time of the turbulence, a new scaling expression, dependent on the turbulent Langmuir number, is derived for ɛ. For reasonable values of the input parameters, the new expression predicts an increase of the dissipation rate near the surface by orders of magnitude compared with usual surface-layer scaling estimates, consistent with available OBL data. These results establish on firmer grounds a suspected connection between two central OBL phenomena: dissipation enhancement and Langmuir circulations.

Mg2+ as an indicator of nutritional status in marine bacteria

Cells maintain an osmotic pressure essential for growth and division, using organic compatible solutes and inorganic ions. Mg(2+), which is the most abundant divalent cation in living cells, has not been considered an osmotically important solute. Here we show that under carbon limitation or dormancy native marine bacterial communities have a high cellular concentration of Mg(2+) (370-940 mM) and a low cellular concentration of Na(+) (50-170 mM). With input of organic carbon, the average cellular concentration of Mg(2+) decreased 6-12-fold, whereas that of Na(+) increased ca 3-4-fold. The concentration of chlorine, which was in the range of 330-1200 mM, and was the only inorganic counterion of quantitative significance, balanced and followed changes in the concentration of Mg(2+)+Na(+). In an osmotically stable environment, like seawater, any major shift in bacterial osmolyte composition should be related to shifts in growth conditions, and replacing organic compatible solutes with inorganic solutes is presumably a favorable strategy when growing in carbon-limited condition. A high concentration of Mg(2+) in cells may also serve to protect and stabilize macromolecules during periods of non-growth and dormancy. Our results suggest that Mg(2+) has a major role as osmolyte in marine bacteria, and that the [Mg(2+)]/[Na(+)] ratio is related to its physiological condition and nutritional status. Bacterial degradation is a main sink for dissolved organic carbon in the ocean, and understanding the mechanisms limiting bacterial activity is therefore essential for understanding the oceanic C-cycle. The [Mg(2+)]/[Na(+)]-ratio in cells may provide a physiological proxy for the transitions between C-limited and mineral nutrient-limited bacterial growth in the ocean's surface layer.

The influences of interannual stratification variability and wind stress forcing on ENSO before and after the 1976 climate shift

In order to understand the change in oceanic variability associated with the climate shift of the mid-1970s, we analyze the contribution of momentum forcing to the leading baroclinic modes over the tropical Pacific using Simple Ocean Data Assimilation (SODA, version 2.0.2) for the period of 1958–1997. Specifically, we look at the statistical relationship between the wind projection coefficients and climate indices and attempt to provide a physical explanation for the observed changes. It is found that the wind stress projection coefficients according to the oceanic baroclinic modes are different in terms of their magnitude and phase in the tropical Pacific, reflecting a specific forcing associated with each mode before and after the 1976 climate shift. Compared to that before the 1970s, the first baroclinic mode is had a greater effect on the interannual sea surface temperature due to equatorial wave dynamics, and there was an increased delayed response of the second baroclinic mode variability to the interannual atmospheric forcing after the late 1970s. This reflects changes in ENSO feedback processes associated with the climate shift. Our analysis further indicates that, after the late 1970s, there was a decrease in the wind stress forcing projecting onto the Ekman layer, which is associated with increased mixed-layer depth. This result suggests that the changes in the ENSO properties before and after the late 1970s are largely associated with the changes in the way in which the wind stress forcing is dynamically projected onto the surface layer of the tropical Pacific Ocean over interannual timescales.

Role of Westerlies and Thermohaline Characteristics on Sea-Ice Extent in the Indian Ocean Sector of Antarctica

Abstract Satellite-derived sea-ice extent in the Indian Ocean Sector during the period November 1978 to December 2006 was studied in relation to the atmospheric forcing and oceanic thermohaline structure. The study revealed that sea-ice extent increased when the ocean exhibited higher stability. Low sea-ice extent was observed during 1985 to 1993, when the zonal winds and latent flux was relatively weak and when the ocean exhibited strong vertical mixing facilitated by low stability thereby, deepening the mixed layer to ~250 m. This was reflected in the ocean surface layer temperature, which was relatively warm (–0.3°C). Winds increased during 1996 to 2000, but due to higher oceanic stability mixed layer depth shallowed (&amp;lt; 200 m) leading to reduced vertical mixing of deep warmer layers with the surface water, leading to an enhancement in the sea-ice extent.

The influence of crosswind tidal currents on Langmuir circulation in a shallow ocean

[1] Langmuir circulation (LC) is a turbulent process driven by wind and surface waves that plays a key role in transferring momentum, heat, and mass in the oceanic surface layer. On the coastal shelves the largest-scale LC span the whole water column and thus couple the surface and bottom boundary layers and enhance turbulent mixing. Observations and large eddy simulations (LES) of a shallow coastal ocean demonstrate that these relatively large scale Langmuir cells are strongly influenced by crosswind tidal currents. Two mechanisms by which crosswind tidal shear may distort and disrupt Langmuir cells are proposed. The first mechanism involves cell shearing due to differential advection across the whole cell. For the second mechanism, middepth vertical LC currents advect sheared mean crosswind current, leading to the attraction of upwelling and downwelling regions, so that LC cells are unsustainable when both regions overlap. Scaling arguments indicate that LC cells are more susceptible to crosswind shear distortion for smaller LC surface velocity convergence and greater cell aspect ratio (vertical to horizontal LC scale), which is consistent with the results obtained from the observations and LES. These results imply that scaling of LC characteristics in a coastal ocean differs from that in the open ocean, which has important practical implications for parameterizing enhanced mixing due to LC.

Wind Stress Curl and Coastal Upwelling in the Area of Monterey Bay Observed during AOSN-II

Abstract Aircraft measurements obtained during the 2003–04 Autonomous Ocean Sampling Network (AOSN-II) project were used to study the effect of small-scale variations of near-surface wind stress on coastal upwelling in the area of Monterey Bay. Using 5-km-long measurement segments at 35 m above the sea surface, wind stress and its curl were calculated with estimated accuracy of 0.02–0.03 N m−2 and 0.1–0.2 N m−2 per 100 kilometers, respectively. The spatial distribution of wind speed, wind stress, stress curl, and sea surface temperature were analyzed for four general wind conditions: northerly or southerly wind along the coastline, onshore flow, and offshore flow. Wind stress and speed maxima frequently were found to be noncollocated as bulk parameterizations imply owing to significant stability and nonhomogeneity effects at cold SST pools. The analyses revealed that complicated processes with different time scales (wind stress field variation, ocean response and upwelling, sea surface currents, and heating by solar radiation) affect the coastal sea surface temperature. It was found that the stress-curl-induced coastal upwelling only dominates in events during which positive curl extended systematically over a significant area (scales larger than 20 km). These events included cases with a northerly wind, which resulted in an expansion fan downstream from Point Año Nuevo (wind speed peaks greater than about 8–10 m s−1), and cases with an offshore/onshore flow, which are characterized by weak background upwelling due to Ekman transport. However, in general, observations show that cold pools of sea surface temperature in the central area of Monterey Bay were advected by ocean surface currents from strong upwelling regions. Aircraft vertical soundings taken in the bay area showed that dominant effects of the lee wave sheltering of coastal mountains resulted in weak atmospheric turbulence and affected the development of the atmospheric boundary layer. This effect causes low wind stress that limits upwelling, especially at the northern part of Monterey Bay. The sea surface temperature is generally warm in this part of the bay because of the shallow oceanic surface layer and solar heating of the upper ocean.

Effects of Physical Properties and Dispersion Conditions on the Chemical Dispersion of Crude Oil

Abstract Effects of oil type and mixing energy on the performance of chemical dispersants were investigated in a baffled flask mixing system using the commercial dispersant Corexit 9500. Effects of viscosity and interfacial tension were investigated by testing three crude oils (Arabian Light, Mars, and Lloyd) at two dispersant-to-oil ratios (DOR; 1:100 and 1:25) and three energy dissipation rates (0.00075, 0.016, and 0.16 W/kg). Dispersion effectiveness (i.e., fraction entrained as droplets in the water column) of all three oils was maximal at a mixing energy of 0.016 W/kg, which is similar to the energy dissipation rate in the surface layer of the open ocean. For Arabian Light and Mars, dispersion effectiveness was affected by DOR only at the lowest mixing energy, but it was proportional to DOR at all mixing energies for the more viscous oil, Lloyd. Droplet-size distributions were multimodal for all oil–dispersant combinations that were tested, indicating the involvement of multiple droplet-formation mec...

Steady states and sensitivities of commonly used pelagic ecosystem model components

Pelagic, coupled ocean circulation-ecosystem models, are widely used in climate research. These tools aim to quantify fluxes of nutrients and carbon in the ocean and are, increasingly, the base of future projections. For this purpose it is crucial to quantify and identify the sources of uncertainties. In contrast to physical models, the underlying equations for ecosystem models are derived from empirical relationships rather than based on first principles. This resulted in the development of a multitude of different ecosystem models – different in respect to both, underlying principles and complexity. Clearly, the question arises, to what extent the sensitivities of these models are comparable. This study focuses on the intrinsic dynamics of some widely used, simple (containing 2–3 prognostic variables) ecosystem models in a 0-D framework (i.e., comprising only the well-mixed oceanic surface layer). A suite of differing model approaches is tuned such that their behavior is similar. The setup resembles the well-mixed oceanic surface layer in the Baltic proper. It is illustrated that strong differences between the model approaches appear due to exemplary, anticipated changes in the external nutrient and light conditions. Herewith, we demonstrate the well-known, but rarely demonstrated fact that, apparent consistency between modeled prognostic variables with today's data bases is not necessarily a good measure of forecast skill. The causes which lead to the different sensitivities are illustrated by considering the steady state solutions. It is pointed out, that apparently small changes in the model formulations can result in very different dynamical behavior and an enormous spread between the model approaches, despite the feasibility to tune a common behavior in a limited range of light and nutrient supply. In our examples, the sensitivity is mainly a function of the formulation of the loss rate of phytoplankton. It is thus, in particular, the formulation of highly unknown heteorotrophic processes that determines the model sensitivity.

The correctness to the spuriously simulated semi-annual cycle of the sea surface temperature in the equatorial eastern Pacific

One of the challenges faced by the climate model of the Community Climate System Model version 3 (CCSM3) is the spuriously simulated semi-annual cycle of the sea surface temperature (SST) in the equatorial eastern Pacific. This model bias has limited the performance of the climate simulation and prediction. Based on the surface wave-circulation coupled theory, an atmosphere-wave-ocean coupled model was developed, which incorporates the MASNUM (key laboratory of Marine Sciences and Numerical Modeling) wave number spectral model into CCSM3. The new coupled atmosphere-wave-ocean model successfully removes the spurious semi-annual cycle simulated by the original CCSM3 and reasonably produces an SST annual cycle with warm and cold phases in April and August, respectively. The correlation between the simulated and observed SST in the equatorial eastern Pacific is improved from 0.66 to 0.93. The ocean surface layer heat budget analysis indicates that the wave-induced vertical mixing is responsible for improving the simulation of the SST seasonal cycle in the equatorial eastern Pacific.

Temporal and spatial dynamics of CO2air-sea flux in the Gulf of Maine

[1] Ocean surface layer carbon dioxide (CO2) data collected in the Gulf of Maine from 2004 to 2008 are presented. Monthly shipboard observations are combined with additional higher-resolution CO2 observations to characterize CO2 fugacity (fCO2) and CO2 flux over hourly to interannual time scales. Observed fCO2 and CO2 flux dynamics are dominated by a seasonal cycle, with a large spring influx of CO2 and a fall-to-winter efflux back to the atmosphere. The temporal results at inner, middle, and outer shelf locations are highly correlated, and observed spatial variability is generally small relative to the monthly to seasonal temporal changes. The averaged annual flux is in near balance and is a net source of carbon to the atmosphere over 5 years, with a value of +0.38 mol m−2 yr−1. However, moderate interannual variation is also observed, where years 2005 and 2007 represent cases of regional source (+0.71) and sink (−0.11) anomalies. We use moored daily CO2 measurements to quantify aliasing due to temporal undersampling, an important error budget term that is typically unresolved. The uncertainty of our derived annual flux measurement is ±0.26 mol m−2 yr−1 and is dominated by this aliasing term. Comparison of results to the neighboring Middle and South Atlantic Bight coastal shelf systems indicates that the Gulf of Maine exhibits a similar annual cycle and range of oceanic fCO2 magnitude but differs in the seasonal phase. It also differs by enhanced fCO2 controls by factors other than temperature-driven solubility, including biological drawdown, fall-to-winter vertical mixing, and river runoff.

B304 自然エネルギーを利用したグライダー型海中ロボットの制御(OS10 海洋におけるシステムと制御2)

Power supply is one of the most important issues for autonomous underwater vehicles (AUVs) which have no umbilical cable connecting with their mother ships. Underwater robotic glider can go forward without any propulsive machinery and it is extremely efficient in energy consumption. The purpose of this research is to study the feasibility of a solar-powered underwater robotic glider so as to improve its long-term endurance, The SORA glider equipped with two thrustors and amorphous-silicon solar panels was developed as a test-bed vehicle of the solar-powered underwater robotic glider, and the authors have demonstrated that SORA has an ability to go underwater using solar energy. This paper deals with development of the SORA glider and the feasibility of the solar-powered underwater robotic glider use in the surface layer of ocean.

Assimilation of sea surface temperature in the Northwest Pacific Ocean and its marginal seas using the ensemble Kalman filter

Satellite-borne sea surface temperature (SST) data were assimilated with the ensemble Kalman filter (EnKF) in a Northwest Pacific Ocean circulation model to examine the effect of data assimilation. The model domain included the northwestern part of the Pacific Ocean and its marginal seas, such as the Yellow Sea and East/Japan Sea. The performance of the data assimilation was evaluated by comparing the simulated ocean state with that observed. Spatially averaged root-mean-squared errors in the SST and sea surface height (SSH) decreased by 0.44 °C and 4 cm, respectively, by the assimilation. The results of the numerical experiments substantiated the effectiveness of the SST assimilation via the EnKF for all marginal seas, as well as the Kuroshio region. The benefit of the data assimilation depended on the characteristics of each marginal sea. The variation of the SST in the East/Japan Sea and the Kuroshio extension (KE) region were improved 34% and those in the Yellow Sea 12.5%. The variation of the SSH was improved approximately 36% in the KE region. This large improvement was achieved in the deep-water regions because assimilation of SST data corrected the separation point of the western boundary currents, such as the Kuroshio and the East Korea Warm Current, and the associated horizontal surface currents. The SST assimilation via the EnKF also improved the subsurface temperature profiles. The effectiveness of SST assimilation was seasonally dependent, with the improvement being relatively larger in winter than in summer, which was related to the seasonal variation of the vertical mixing and stratification in the ocean surface layer.

An Enhancement of Low-Frequency Variability in the Kuroshio–Oyashio Extension in CCSM3 owing to Ocean Model Biases

Abstract Enhanced decadal variability in sea surface temperature (SST) centered on the Kuroshio Extension (KE) has been found in the Community Climate System Model version 3 (CCSM3) as well as in other coupled climate models. This decadal peak has higher energy than is found in nature, almost twice as large in some cases. While previous analyses have concentrated on the mechanisms for such decadal variability in coupled models, an analysis of the causes of excessive SST response to changes in wind stress has been missing. Here, a detailed comparison of the relationships between interannual changes in SST and sea surface height (SSH) as a proxy for geostrophic surface currents in the region in both CCSM3 and observations, and how these relationships depend on the mean ocean circulation, temperature, and salinity, is made. We use observationally based climatological temperature and salinity fields as well as satellite-based SSH and SST fields for comparison. The primary cause for the excessive SST variability is the coincidence of the mean KE with the region of largest SST gradients in the model. In observations, these two regions are separated by almost 500 km. In addition, the too shallow surface oceanic mixed layer in March north of the KE in the subarctic Pacific contributes to the biases. These biases are not unique to CCSM3 and suggest that mean biases in current, temperature, and salinity structures in separated western boundary current regions can exert a large influence on the size of modeled decadal SST variability.

Arctic Ocean Warming Contributes to Reduced Polar Ice Cap

Abstract Analysis of modern and historical observations demonstrates that the temperature of the intermediate-depth (150–900 m) Atlantic water (AW) of the Arctic Ocean has increased in recent decades. The AW warming has been uneven in time; a local ∼1°C maximum was observed in the mid-1990s, followed by an intervening minimum and an additional warming that culminated in 2007 with temperatures higher than in the 1990s by 0.24°C. Relative to climatology from all data prior to 1999, the most extreme 2007 temperature anomalies of up to 1°C and higher were observed in the Eurasian and Makarov Basins. The AW warming was associated with a substantial (up to 75–90 m) shoaling of the upper AW boundary in the central Arctic Ocean and weakening of the Eurasian Basin upper-ocean stratification. Taken together, these observations suggest that the changes in the Eurasian Basin facilitated greater upward transfer of AW heat to the ocean surface layer. Available limited observations and results from a 1D ocean column model support this surmised upward spread of AW heat through the Eurasian Basin halocline. Experiments with a 3D coupled ice–ocean model in turn suggest a loss of 28–35 cm of ice thickness after ∼50 yr in response to the 0.5 W m−2 increase in AW ocean heat flux suggested by the 1D model. This amount of thinning is comparable to the 29 cm of ice thickness loss due to local atmospheric thermodynamic forcing estimated from observations of fast-ice thickness decline. The implication is that AW warming helped precondition the polar ice cap for the extreme ice loss observed in recent years.

Impacts of climate warming on polar marine and freshwater ecosystems

Polar regions have been identiWed as being most sensitive to climate warming in the planet. The Arctic is experiencing the highest warming on Earth with air temperature increasing at a rate of two to three times that of the global average temperature (International Panel on Climate Change 2007). Atmospheric warming has increased Arctic Ocean temperature and accelerated ice melting, with sea ice decreasing in thickness and extent at a rate of 10% per decade (Comiso et al. 2008). Warming and the associated ice loss appears to have been accelerating in the past few years in the Arctic and in some regions of Antarctica as well (Steig et al. 2009). The Arctic Ocean experienced a massive loss of ice in the summer of 2007, representing the lowest ice cover in the last 40 years, and an unusually warm Arctic Ocean surface layer were recorded (Steele et al. 2008; Zhang et al. 2008). Provided the critical role of ice cover in determining the functioning and traits of polar marine and freshwater ecosystems, rapid ice loss is expected to severely impact these ecosystems at multiple levels. The impact of warming and ice loss will aVect, in turn, the radiative balance of polar waters, aVecting their thermal and light, including UV, environments. Recent overviews of results from observational studies of atmospheric and climate-sensitive variables (e.g., sea ice, snow cover, river discharge, glaciers and permafrost) concluded that, taken together, a reasonably coherent portrait of recent change in the northern high latitudes is apparent. Ice loss will intensify air–water interactions in polar ecosystems and will also severely impact all species, from microbes to megafauna, that depend on ice as a habitat. Warming and ice loss will aVect key biological and biogeochemical processes of aquatic polar ecosystems and may induce ecological regime shifts, associated with possible losses of biodiversity and an increased vulnerability to invasions of species from lower latitudes. The goal of this special issue is to bring together recent research results addressing impacts of warming and ice loss in both Antarctic and Arctic aquatic ecosystems. “Impacts of Climate Warming on Polar Marine and Freshwater Ecosystems” was the title of a topical session of the Aquatic Science Meeting of the American Association of Limnology and Oceanography (ASLO), held in Nice (France) from 23 to 30 January 2009, during which results from both benthic and pelagic, freshwater and marine polar ecosystems were presented. Susana Agusti and Mikael K. Sejr acted as Chairs of this ASLO session, which received Wnancial support by the Prince Albert II of Monaco Foundation. Some papers presented in the session are published in this special issue of Polar Biology. Results from Antarctic aquatic ecosystems presented in this special issue highlight the temporal variability of marine and freshwater systems of the Antarctic Peninsula in the west of the continent, which is the region experiencing the strongest warming (Steig et al. 2009). The signiWcant increase in temperature across most of West Antarctica, exceeding 0.1°C per decade over the past 50 years, is attributed to global warming and to the increased strength of the circumpolar westerlies in response S. Agusti (&) · C. M. Duarte Department of Global Change Research, Instituto Mediterraneo de Estudios Avanzados IMEDEA, CSIC–UIB, Mallorca, Spain e-mail: sagusti@imedea.uib-csic.es