ALBERT

Description: In order to improve robustness of remote sensing algorithms for lakes, it is vital to understand the variability of inherent optical properties (IOPs) and their mass-specific representations (SIOPs). In this study, absorption coefficients for particulate and dissolved constituents were measured at 38 stations distributed over a biogeochemical gradient in Lake Balaton, Hungary. There was a large range of phytoplankton absorption ( a ph (λ)) over blue and red wavelengths ( a ph (440)=0.11-4.39 m −1 , a ph (675)=0.048-2.52 m −1 ), while there was less variability in chlorophyll-specific phytoplankton absorption ( a* ph (λ)) in the lake ( a* ph (440)=0.022±0.0046 m 2 mg −1 , a* ph (675)=0.010±0.0020 m 2 mg −1 ) and adjoining wetland system, Kis-Balaton ( a* ph (440)=0.017±0.0015 m 2 mg −1 , a* ph (675)=0.0088±0.0017 m 2 mg −1 ). However, in the UV, a* ph (350) significantly increased with increasing distance from the main inflow (River Zala). This was likely due to variable production of photoprotective pigments (e.g. MAAs) in response to the decreasing gradient of colored dissolved organic matter (CDOM). The slope of CDOM absorption ( S CDOM ) also increased from west to east due to larger terrestrial CDOM input in the western basins. Absorption by non-algal particles ( a NAP (λ)) was highly influenced by inorganic particulates, as a result of the largely mineral sediments in Balaton. The relative contributions to the absorption budget varied more widely than oceans with a greater contribution from NAP (up to 30%), and wind speed affected the proportion attributed to NAP, phytoplankton or CDOM. Ultimately, these data provide knowledge of the heterogeneity of (S)IOPs in Lake Balaton, suggesting the full range of variability must be considered for future improvement of analytical algorithms for constituent retrieval in inland waters. This article is protected by copyright. All rights reserved.

Description: Phytoplankton chlorophyll-a (Chl) seasonal cycles of the North Atlantic are described using satellite ocean color observations covering the 1980s and the 2000s. The study region is where warmer SST and higher Chl in the 2000s as compared to the 1980s have been reported. It covers latitudes from 30°N–50°N and longitudes from 60°W–0°W, where two phytoplankton blooms take place: a spring bloom that follows stratification of upper layers, and a fall bloom due to nutrient entrainment through deepening of the mixed layer. In the 1980s, spring and fall blooms were of similar amplitude over the entire study region. In the 2000s, the fall bloom was weaker in the eastern Atlantic (east of 40°W), because of a delayed deepening of the mixed layer at the end of summer (mixed layer depth (MLD) determined from in situ data). Conversely, the spring bloom of the eastern Atlantic was stronger in the 2000s than it was in the 1980s, because of a deeper MLD and stronger winds in winter. In the Northwestern Atlantic (northwest of 38°N–40°W), little differences are observed for spring and fall blooms, and for the wintertime MLD. Our results show that the links between upper layer stratification, SST changes, and biological responses are more complex than the simple paradigm that sequentially relates higher stratification with warmer SST and an enhanced (weakened) growth of the phytoplankton population in the subpolar (subtropical) region.

Description: Determination of the relative inputs of aquatic autochthonous and terrestrial allochthonous organic matter into marine and lacustrine environments is essential to understanding the global carbon budget. A variety of proxies are used for this purpose, including the Branched and Isoprenoid Tetraether (BIT) index. This is calculated from the concentrations of branched glycerol dialkyl glycerol tetraethers (GDGTs), derived from unidentified terrestrial bacteria, and crenarchaeol, a marker for aquatic mesophile Thaumarchaeota (Crenarchaeota group I). As the index is a ratio, its value depends on both the crenarchaeol aquatic in situ production and the soil-derived branched GDGT input. Therefore, the BIT index reflects not only changes in the input of terrestrial or soil organic matter but also relative variations in aquatic Thaumarchaeota abundance in the water column. In fact, we show that in oceanic and lacustrine settings, the BIT index can be dominated by the aquatic end-member of the ratio. Consequently, the BIT index by itself can be an unreliable proxy to compare the input of terrestrial matter between sites and over time, and we propose that the quantification of branched GDGT fluxes or concentrations may instead be a better indicator of soil terrestrial inputs.

Description: High spatial resolution hydrographic data, including Lowered Acoustic Doppler Current Profiler (LADCP) measurements, were acquired along a meridional section at 24.5°W in October 2009. The data are analyzed in detail with the purpose of definitively defining and quantifying the zonal Azores Current System. The Azores Current and Azores Countercurrent are delimited, each extending meridionally for 110 km. The Azores Current is located between 33.5°N and 34.5°N, flanked to the north by the Azores Countercurrent (35.25°–36.25°N). Vertically, both currents reach the γn = 27.975 kg m−3 level (∼2000 m depth), their mass transports ranging across thermocline as well as intermediate layers. The Azores Current transports 13.9 Sv (1 Sv = 106 m3 s−1 ≈ 109 kg s−1) eastward with its maximum associated with the Azores Front (33.75°N). The Azores Countercurrent flows below the surface, transporting 5.5 Sv westward. This contributes to a net eastward flow of 8.4 Sv across the section. At intermediate layers, the Azores Countercurrent transports mixed Mediterranean Water to the west, and the Azores Current transports mixed Sub-Arctic Intermediate Water to the east. Shipboard ADCP and satellite-derived geostrophic velocity are used to confirm the transports revealed by the hydrographic data.

Description: The study of air-sea CO 2 fluxes ( F CO 2 ) in the coastal region is needed to better understand the processes which influence the direction and magnitude of F CO 2 and to constrain the global carbon budget. We implemented a one-year (January through December, 2009) paired study to measure F CO 2 in the intertidal zone (the coastline to 1.6 km offshore) and the near-shore (~3 km offshore) off the north-western coast of Baja California (Mexico); a region influenced by year-round upwelling. F CO 2 was determined in the intertidal zone via eddy covariance; while in the near-shore using mooring buoy sensors then calculated with the bulk method. The near-shore region was a weak annual net source of CO 2 to the atmosphere (0.043 mol CO 2 m -2 y -1 ); where 91% of the outgassed F CO 2 was contributed during the upwelling season. SST and Δ p CO 2 (from upwelling) showed the strongest relationship with F CO 2 in the near-shore suggesting the importance of meso-scale processes (upwelling). F CO 2 in the intertidal zone were up to four orders of magnitude higher than F CO 2 in the near-shore. Wind speed showed the strongest relationship with F CO 2 in the intertidal zone suggesting the relevance of micro-scale processes. Results show that there are substantial spatial and temporal differences in F CO 2 between the near-shore and intertidal zone; likely a result of spatial differences. We suggest that detailed spatial and temporal measurements are needed across the coastal oceans and continental margins to better understand the mechanisms which control F CO 2 , as well as reduce uncertainties and constrain regional and global ocean carbon balances.

Description: A South Atlantic ring is studied through remote sensing altimetry, hydrographic stations and drifters' trajectories. The ring's core was characterized by warmer and saltier Indian Ocean waters. At the time of the cruise the ring's signature extended out 110 km and down to 2000 m, and its core absolute dynamic topography (ADT) exceeded the surrounding Atlantic Ocean waters in 0.4 m. The geostrophic velocities were anticyclonic with maximum velocities about 35 cm s −1 at 100 m and reaching negligible values near 4500 m. The rotational transport inside the ring was 33 Sv in the thermocline and intermediate layers. The drifters' data distinguishes a 30-km core revolving as a solid body with periodicity near 5 days and a transitional band that revolves with constant tangential velocity, resembling a Rankine vortex. The ADT data identifies the ring's track, showing that it was shed by the Agulhas Current retroflection in November 2009 and propagated northwest rapidly during the first two months (mean speed ca. 10 cm s −1 ) but slowed down substantially (3-4 cm s −1 ) between March and July 2010, when it was last detected. The altimetry data also outlines the evolution of the ring's core ADT, radius, vorticity and, through a simple calibration with the cruise data, rotational transport. In particular, the ring surface and mean vorticity decay with time scales of 373 and 230 days, respectively, indicating that most of the property anomalies contained by the ring are diffused out to the subtropical gyre before it reaches the western boundary current system.

Description: Southern Ocean phytoplankton assemblages acclimated to low-light environments that result from deep mixing are often sensitive to ultraviolet and high photosynthetically available radiation. In such assemblages, exposures to inhibitory irradiance near the surface result in loss of photosynthetic capacity that is not rapidly recovered and can depress photosynthesis even after transport below depths penetrated by inhibitory irradiance. We used a coupled biophysical modeling approach to quantify the reduction in primary productivity due to photoinhibition based upon experiments and observations made during the spring bloom in Ross Sea Polynya (RSP). Large eddy simulation (LES) was used to generate depth trajectories representative of observed Langmuir circulation that were passed through an underwater light field to yield time series of spectral irradiance representative of what phytoplankton would have experienced in situ. These were used to drive an assemblage-specific photosynthesis-irradiance model with inhibition determined from a biological weighting function and repair rate estimated from shipboard experiments on the local assemblage. We estimate the daily depth-integrated productivity was 230 mmol C m −2 . This estimate includes a 6-7% reduction in daily depth-integrated productivity over potential productivity (i.e., effects of photoinhibition excluded). When trajectory depths were fixed (no vertical transport), the reduction in productivity was nearly double. Relative to LES estimates, there was slightly less depth-integrated photoinhibition with random walk trajectories and nearly twice as much with circular rotations. This suggests it's important to account for turbulence when simulating the effects of vertical mixing on photoinhibition due to the kinetics of photodamage and repair.

Description: The potential for a dynamical impact of Saharan mineral dust on the North Atlantic Ocean large-scale circulation is investigated. To this end, an ocean general circulation model forced by atmospheric fluxes is perturbed by an idealized, seasonally varying, net shortwave flux anomaly, as it results from remote sensing observations of aerosol optical thickness representing Saharan dust load in the atmosphere. The dust dynamical impact on the circulation is assessed through a comparison between perturbed and an unperturbed run. Results suggest that, following the dust-induced shortwave irradiance anomaly, a buoyancy anomaly is created in the Atlantic offshore the African coast, which over the course of the time propagates westward into the interior Atlantic while progressively subducting. Changes in the large-scale barotropic and overturning circulations are significant after 3 years, which coincides with the elapsed time required by the bulk of the buoyancy perturbation to reach the western boundary of the North Atlantic. Although small in amplitude, the changes in the meridional overturning are of the same order as interannual-to-decadal variability. Variations in the amplitude of the forcing lead to changes in the amplitude of the response, which is almost linear during the first 3 years. In addition, a fast, but dynamically insignificant, response can be observed to propagate poleward along the eastern boundary of the North Atlantic, which contributes to a nonlinear response in the subpolar region north of 40°N.

Description: Subsurface waters from both hemispheres converge in the Western Equatorial Pacific (WEP), some of which form the Equatorial Undercurrent (EUC) that influences equatorial Pacific productivity across the basin. Measurements of nitrogen (N) and oxygen (O) isotope ratios in nitrate (δ 15 N NO3 and δ 18 O NO3 ), the isotope ratios of dissolved inorganic carbon (δ 13 C DIC ), and complementary biogeochemical tracers reveal that northern and southern WEP waters have distinct biogeochemical histories. Organic matter remineralization plays an important role in setting the nutrient characteristics on both sides of the WEP. However, remineralization in the northern WEP contributes a larger concentration of the nutrients, consistent with the older “age” of northern thermocline- and intermediate-depth waters. Remineralization introduces a relatively low δ 15 N NO3 to northern waters, suggesting the production of sinking organic matter by N 2 fixation at the surface – consistent with the notion that N 2 fixation is quantitatively important in the North Pacific. In contrast, remineralization contributes elevated δ 15 N NO3 to the southern WEP thermocline, which we hypothesize to derive from the vertical flux of high-δ 15 N material at the southern edge of the equatorial upwelling. This signal potentially masks any imprint of N 2 fixation from South Pacific waters. The observations further suggest that the intrusion of high δ 15 N NO3 and δ 18 O NO3 waters from the eastern margins is more prominent in the northern than southern WEP. Together, these north-south differences enable the examination of the hemispheric inputs to the EUC, which appear to derive predominantly from southern hemisphere waters.

Description: The tropical Atlantic exerts a major influence in climate variability through strong air-sea interactions. Within this region, the eastern side of the equatorial band is characterized by strong seasonality, whereby the most prominent feature is the annual development of the Atlantic Cold Tongue (ACT). This band of low sea surface temperatures (∼22-23°C) is typically associated with upwelling-driven enhancement of surface nutrient concentrations and primary production. Based on a detailed investigation of the distribution and sea-to-air fluxes of N 2 O in the eastern equatorial Atlantic (EEA), we show that the onset and seasonal development of the ACT can be clearly observed in surface N 2 O concentrations, which increase progressively as the cooling in the equatorial region proceeds during spring-summer. We observed a strong influence of the surface currents of the EEA on the N 2 O distribution, which allowed identifying “high” and “low” concentration regimes that were, in turn, spatially delimited by the extent of the warm eastward-flowing North Equatorial Countercurrent and the cold westward-flowing South Equatorial Current. Estimated sea-to-air fluxes of N 2 O from the ACT (mean 5.18±2.59 µmol m −2 d −1 ) suggests that in May-July 2011 this cold-water band doubled the N 2 O efflux to the atmosphere with respect to the adjacent regions, highlighting its relevance for marine tropical emissions of N 2 O. This article is protected by copyright. All rights reserved.