In-situ plankton sampling, combined with remotely sensed and ocean Seaglider observations, provided insight into the termination of the winter monsoon bloom and subsequent evolution into a subsurface fluorescence maximum. This subsurface maximum gradually descended, presenting increased fluorescence between 25 and 55 m depth during the spring intermonsoon season. Species diversity decreased by half within the deep fluorescence maximum relative to the bloom. The dinoflagellate Noctiluca scintillans dominated by biomass in all samples collected from the depth of subsurface fluorescence maximum. We show that the subsurface algal bloom persists throughout inter-monsoon seasons, linking algal blooms initiated during the South-West and North-East monsoons. In-situ samples showed a net decrease in Noctiluca cell size illustrating a shift towards a deep chlorophyll maximum adapted community, but did not exhibit any increases in chlorophyll-containing endosymbionts. We put forward that the plankton biomass and estimates of the North-Western Arabian Sea productivity are much greater than estimated previously through remote sensing observations due to the persistence, intensity, and vertical extent of the deep chlorophyll maximum which can be guessed at, but not measured, remotely.
The observational part of the REP14-MED experiment was conducted in June 2014 in the Sardo-Balearic Sea west of Sardinia Island (Western Mediterranean Sea). Two research vessels collected high-resolution oceanographic data by means of hydrographic casts, towed systems, and underway measurements. In addition, a vast amount of data was provided by a fleet of 11 gliders, time series were available from moored instruments, and information on Lagrangian flow patterns were obtained from surface drifters and one profiling float. The spatial resolution of the observations encompasses a spectrum over four orders of magnitude from O(101 m) to O(105 m), and the time series from the moored instruments cover a spectral range of five orders from O(101 s) to O(106 s). The objective of this article is to provide an overview of the huge data set which is utilized by various ongoing studies, focusing on (i) sub-mesoscale and mesoscale pattern analyses, (ii) operational forecasting in terms of the development and assessment of sampling strategies, assimilation methods, and model validation, (iii) modeling the variability of the ocean, and (iv) testing of new payloads for gliders.
Historical data from oceanographic expeditions and remotely sensed data on outgoing longwave radiation, temperature, wind speed and ocean color in the western Arabian Sea (1950–2010) were used to investigate decadal trends in the physical and biochemical properties of the upper 300 m. 72 % of the 29,043 vertical profiles retrieved originated from USA and UK expeditions. Increasing outgoing longwave radiation, surface air temperatures and sea surface temperature were identified on decadal timescales. These were well correlated with decreasing wind speeds associated with a reduced Siberian High atmospheric anomaly. Shoaling of the oxycline and nitracline was observed as well as acidification of the upper 300 m. These physical and chemical changes were accompanied by declining chlorophyll-a concentrations, vertical macrofaunal habitat compression, declining sardine landings and an increase of fish kill incidents along the Omani coast.
In stratified shelf seas, oxygen depletion beneath the thermocline is a result of a greater rate of biological oxygen demand than the rate of supply of oxygenated water. Suitably equipped gliders are uniquely placed to observe both the supply through the thermocline and the consumption of oxygen in the bottom layers. A Seaglider was deployed in the shallow (≈ 100 m) stratified North Sea in a region of known low oxygen during August 2011 to investigate the processes regulating supply and consumption of dissolved oxygen below the pycnocline. The first deployment of such a device in this area, it provided extremely high-resolution observations, 316 profiles (every 16 min, vertical resolution of 1 m) of conductivity, temperature, and depth (CTD), dissolved oxygen concentrations, backscatter, and fluorescence during a 3-day deployment. The high temporal resolution observations revealed occasional small-scale events (< 200 m or 6 h) that supply oxygenated water to the bottom layer at a rate of 2 ± 1 µmol dm−3 day−1. Benthic and pelagic oxygen sinks, quantified through glider observations and past studies, indicate more gradual background consumption rates of 2.5 ± 1 µmol dm−3 day−1. This budget revealed that the balance of oxygen supply and demand is in agreement with previous studies of the North Sea. However, the glider data show a net oxygen consumption rate of 2.8 ± 0.3 µmol dm−3 day−1, indicating a localized or short-lived (< 200 m or 6 h) increase in oxygen consumption rates. This high rate of oxygen consumption is indicative of an unidentified oxygen sink. We propose that this elevated oxygen consumption is linked to localized depocentres and rapid remineralization of resuspended organic matter. The glider proved to be an excellent tool for monitoring shelf sea processes despite challenges to glider flight posed by high tidal velocities, shallow bathymetry, and very strong density gradients. The direct observation of these processes allows more up to date rates to be used in the development of ecosystem models.
The Ross Sea polynya is one of the most productive regions in the Southern Ocean. However, limited access and high spatio-temporal variability of physical and biological processes limit the use of conventional oceanographic methods to measure early season primary productivity. High-resolution observations from two Seagliders provide insights into the timing of a bloom in the southern Ross Sea polynya in December 2010. Changes in chlorophyll and oxygen concentrations are used to assess bloom dynamics. Using a ratio of dissolved oxygen to carbon, net primary production is estimated over the duration of the bloom showing a sensitive balance between net autotrophy and heterotrophy. The two gliders, observing spatially distinct regions during the same period, found net community production rates of -0.9±0.7 and 0.7±0.4 g C m-2 d-1. The difference highlights the spatial variability of biological processes and is probably caused by observing different stages of the bloom. The challenge of obtaining accurate primary productivity estimates highlights the need for increased observational efforts, particularly focusing on subsurface processes not resolved using surface or remote observations. Without an increased observational effort and the involvement of emerging technologies, it will not be possible to determine the seasonal trophic balance of the Ross Sea polynya and quantify the shelf’s importance in carbon export.
High-resolution autonomous glider data (including temperature, salinity, fluorescence, and optical backscatter) collected during the 2010–2011 austral summer identified variations in phytoplankton biomass along two glider sections near 76°40′S. Sea surface temperatures were warmer during the latter, westward section, while mixed layer depths were deeper. Substantial quantities of Modified Circumpolar Deep Water, identified by neutral density criteria, were located within both sections. Chlorophyll (Chl) concentrations computed from fluorescence exhibited daily quenching near the surface, and deep chlorophyll concentrations at 200 m became periodically elevated, suggesting substantial export on small space and time scales. The concentrations of particulate organic carbon (POC) computed from backscatter increased abruptly during the latter, westward section, concurrent with a decrease in chlorophyll. These higher POC:Chl ratios were not strongly correlated with presence of MCDW or with shallower mixed layer depths, but were strongly associated with higher surface temperatures and wind speed. The observed POC:Chl increase suggests a marked spatial and temporal transition between a Phaeocystis antarctica-dominated assemblage characterized by modest POC:Chl ratios to a diatom-dominated assemblage. Finally, a subsampling analysis highlights the capability of high-resolution glider data to resolve these biological/physical parameter correlations that are not discernible from lower frequency data typical of traditional cruise stations.
The Antarctic continental shelves and slopes occupy relatively small areas, but, nevertheless, are important for global climate, biogeochemical cycling and ecosystem functioning. Processes of water mass transformation through sea ice formation/melting and ocean–atmosphere interaction are key to the formation of deep and bottom waters as well as determining the heat flux beneath ice shelves. Climate models, however, struggle to capture these physical processes and are unable to reproduce water mass properties of the region. Dynamics at the continental slope are key for correctly modelling climate, yet their small spatial scale presents challenges both for ocean modelling and for observational studies. Cross-slope exchange processes are also vital for the flux of nutrients such as iron from the continental shelf into the mixed layer of the Southern Ocean. An iron-cycling model embedded in an eddy-permitting ocean model reveals the importance of sedimentary iron in fertilizing parts of the Southern Ocean. Ocean gliders play a key role in improving our ability to observe and understand these small-scale processes at the continental shelf break. The Gliders: Excellent New Tools for Observing the Ocean (GENTOO) project deployed three Seagliders for up to two months in early 2012 to sample the water to the east of the Antarctic Peninsula in unprecedented temporal and spatial detail. The glider data resolve small-scale exchange processes across the shelf-break front (the Antarctic Slope Front) and the front's biogeochemical signature. GENTOO demonstrated the capability of ocean gliders to play a key role in a future multi-disciplinary Southern Ocean observing system.
In 2010–2011, three projects combined to characterize the temporal and spatial distributions of Modified Circumpolar Deep Water (MCDW) in the Ross Sea using icebreaker-based sampling, gliders, instrumented seals, and hindcasts from a numerical circulation model. The fieldwork clearly identified MCDW throughout the Ross Sea, and the data were used to determine its influence on potential heat and nutrient inputs and biotic distributions. Furthermore, the numerical simulations confirm its apparent trajectory and location. Substantial small-scale variability in oceanographic and biological distributions suggests that such variability may play an important role in biogeochemical cycles. Data from the three projects provide a view of hydrographic variability in the Ross Sea that is impossible to obtain using traditional sampling. Multiplatform investigations are promising approaches to future polar experiments where logistical considerations are of paramount importance.
Prompted by recent observations of seasonal low dissolved oxygen from two moorings in the North Sea, a hydrographic survey in August 2010 mapped the spatial extent of summer oxygen depletion. Typical near-bed dissolved oxygen saturations in the stratified regions of the North Sea were 75–80 % while the well-mixed regions of the southern North Sea reached 90 %. Two regions of strong thermal stratification, the area between the Dooley and Central North Sea Currents and the area known as the Oyster Grounds, had oxygen saturations as low as 65 and 70 % (200 and 180 μmol dm−3) respectively. Low dissolved oxygen was apparent in regions characterised by low advection, high stratification, elevated organic matter production from the spring bloom and a deep chlorophyll maximum. Historical data over the last century from the International Council for the Exploration of the Sea oceanographic database highlight an increase in seasonal oxygen depletion and a warming over the past 20 years. The 2010 survey is consistent with, and reinforces, the signal of recent depleted oxygen at key locations seen in the (albeit sparse) historical data.
Over the last couple of decades, autonomous underwater vehicles have become a powerful tool in the investigation of biological, chemical and physical oceanography. Not only do they complement existing technologies, they open up new avenues of investigation through their specific capabilities. For AUVs to benefit from the same success other long term monitoring platforms have had (moorings, ARGO), it is critical to understand their limits in both monitoring and process studies. We present results from several Seaglider deployments by the University of East Anglia where Seagliders were pushed to the limit of their abilities. Comparison of missions in extreme conditions at the limits of their depth range (70 to 1000 m) and battery life shows a need for tailored survey design and flight parameters in order to maximise mission duration, control over the Seaglider and most efficient science sampling. In particular, we look at post-processing of Seaglider data and present aspects of a new MATLAB toolbox which greatly improves on timestamp correction of Seaglider data by accounting for errors introduced by using a single thread processor.
Over the last several decades, numerous approaches have been used to observe the rapid development of the annual phytoplankton bloom in the Ross Sea, including ship-based sampling, moored instrumentation, satellite images, and computer modeling efforts. In the Austral Spring of 2010, our group deployed a pair of iRobot Seagliders equipped with fluorometers, oxygen sensors and CTDs in order to obtain data on this phenomenon over the entire duration of the bloom. Data from these deployments will be used, along with samples from the recovery cruise and satellite data, to model and better understand the dynamics of this phytoplankton bloom.