Ivona Cetinić

Eddy-driven subduction exports particulate organic carbon from the spring bloom

Down with atmospheric carbon dioxide How does the ocean move carbon from surface waters to its deep interior? Current understanding is that carbon dioxide is removed from the atmosphere by phytoplankton that are eaten, and in turn their predators die and sink into deep water and seafloor sediments. In addition to this route, Omand et al. show that downwelling caused by ocean eddies 1 to 10 km across can deliver much of the carbon produced in spring to the deep sea. The eddies entrain small particles and dissolved organic carbon to augment the flux of large sinking particles. Science, this issue p. 222 Ocean eddies can transport appreciable quantities of organic carbon from the surface to depth. The export of particulate organic carbon (POC) from the surface ocean to depth is traditionally ascribed to sinking. Here, we show that a dynamic eddying flow field subducts surface water with high concentrations of nonsinking POC. Autonomous observations made by gliders during the North Atlantic spring bloom reveal anomalous features at depths of 100 to 350 meters with elevated POC, chlorophyll, oxygen, and temperature-salinity characteristics of surface water. High-resolution modeling reveals that during the spring transition, intrusions of POC-rich surface water descend as coherent, 1- to 10-kilometer–scale filamentous features, often along the perimeter of eddies. Such a submesoscale eddy-driven flux of POC is unresolved in global carbon cycle models but can contribute as much as half of the total springtime export of POC from the highly productive subpolar oceans.

Phytoplankton seasonality in a highly stratified karstic estuary (Krka, Adriatic Sea)

The abundance of phytoplankton and the composition of nutrients were analysed at three stations in the highly stratified karstic Krka Estuary (east Adriatic coast), in the period March 2000—February 2001. The phytoplankton assemblages were analysed by cluster analysis (average linkage), applied to the Bray—Curtis dissimilarity index computed on density data. Seven groups were identified, which can be combined into two major groups. The spring period was influenced by high riverine water inflow and characterised by the development of cocolithophorids and autotrophic nanoplankton, and small diatoms in winter. In the summer—autumn period domination of one large group occurred, comprising dinoflagellates (micro and nano) and colonial diatoms. Three small groups were also present in that period, each with a specific phytoplankton composition. Variance explained by the Canonical Correspondence Analysis (CCA) of the time-position data was low but corresponded with clustering of samples. CC analysis revealed temperature and salinity as the most important indicators of riverine water impact that influenced species composition and phytoplankton seasonality. The nutrient regime was highly influenced by river/seawater exchange, except in the case of orthophosphates, which showed no correlation with salinity.

Calibration procedure for Slocum glider deployed optical instruments

Recent developments in the field of the autonomous underwater vehicles allow the wide usage of these platforms as part of scientific experiments, monitoring campaigns and more. The vehicles are often equipped with sensors measuring temperature, conductivity, chlorophyll a fluorescence (Chl a), colored dissolved organic matter (CDOM) fluorescence, phycoerithrin (PE) fluorescence and spectral volume scattering function at 117 degrees, providing users with high resolution, real time data. However, calibration of these instruments can be problematic. Most in situ calibrations are performed by deploying complementary instrument packages or water samplers in the proximity of the glider. Laboratory calibrations of the mounted sensors are difficult due to the placement of the instruments within the body of the vehicle. For the laboratory calibrations of the Slocum glider instruments we developed a small calibration chamber where we can perform precise calibrations of the optical instruments aboard our glider, as well as sensors from other deployment platforms. These procedures enable us to obtain pre- and post-deployment calibrations for optical fluorescence instruments, which may differ due to the biofouling and other physical damage that can occur during long-term glider deployments. We found that biofouling caused significant changes in the calibration scaling factors of fluorescent sensors, suggesting the need for consistent and repetitive calibrations for gliders as proposed in this paper.

Algal toxins and reverse osmosis desalination operations: Laboratory bench testing and field monitoring of domoic acid, saxitoxin, brevetoxin and okadaic acid

The occurrence and intensity of harmful algal blooms (HABs) have been increasing globally during the past few decades. The impact of these events on seawater desalination facilities has become an important topic in recent years due to enhanced societal interest and reliance on this technology for augmenting world water supplies. A variety of harmful bloom-forming species of microalgae occur in southern California, as well as many other locations throughout the world, and several of these species are known to produce potent neurotoxins. These algal toxins can cause a myriad of human health issues, including death, when ingested via contaminated seafood. This study was designed to investigate the impact that algal toxin presence may have on both the intake and reverse osmosis (RO) desalination process; most importantly, whether or not the naturally occurring algal toxins can pass through the RO membrane and into the desalination product. Bench-scale RO experiments were conducted to explore the potential of extracellular algal toxins contaminating the RO product. Concentrations exceeding maximal values previously reported during natural blooms were used in the laboratory experiments, with treatments comprised of 50 μg/L of domoic acid (DA), 2 μg/L of saxitoxin (STX) and 20 μg/L of brevetoxin (PbTx). None of the algal toxins used in the bench-scale experiments were detectable in the desalinated product water. Monitoring for intracellular and extracellular concentrations of DA, STX, PbTx and okadaic acid (OA) within the intake and desalinated water from a pilot RO desalination plant in El Segundo, CA, was conducted from 2005 to 2009. During the five-year monitoring period, DA and STX were detected sporadically in the intake waters but never in the desalinated water. PbTx and OA were not detected in either the intake or desalinated water. The results of this study demonstrate the potential for HAB toxins to be inducted into coastal RO intake facilities, and the ability of typical RO operations to effectively remove these toxins.

Autonomous data describe North Atlantic spring bloom

Each spring, increasing sunlight and associated changes in the ocean structure trigger rapid growth of phytoplankton across most of the North Atlantic Ocean north of 30°N. The bloom, one of the largest in the world, is a major sink for atmospheric carbon dioxide and a prototype for similar blooms around the world. Models of the ocean carbon cycle, a necessary component of climate models, need to accurately reproduce the biological, chemical, and physical processes occurring during these blooms. However, a paucity of detailed observations severely limits efforts to evaluate such models.

Net community production and export from Seaglider measurements in the North Atlantic after the spring bloom

Mean rates of net community production (NCP) and particulate organic carbon (POC) export were estimated from sensor measurements of dissolved oxygen (O2), chlorophyll fluorescence (chl F), and particulate backscatter (bbp700) collected from three Seagliders that surveyed a 20 × 20 km area in the North Atlantic subsequent to a large diatom bloom. Since the Seagliders sampled geographically fixed patterns, care was taken in the calculation of all terms applicable to the Eulerian reference frame, including local rate of change, vertical mixing, air-sea exchange, and horizontal advection. Although similar studies of NCP in the open ocean have generally assumed advection to be insignificant, we have found that this term cannot be ignored when dealing with temporal scales of ≤1 month and/or spatial scales ≤20 km. The overlapping sampling pattern of the Seagliders was sufficiently rapid such that 4–5 day time scales observed in the O2 and POC data were adequately resolved and variations were not a consequence of aliasing spatial variability. During the study period, ratios of chlorophyll fluorescence-to-particulate backscatter (chl:bbp700) were lower than values encountered during the spring diatom bloom, suggesting the phytoplankton community was predominantly composed of smaller cells (picoplankton and nanoplankton) and/or coccolithophorids. Coupled budgets of oxygen and POC indicated a net community production of 1.0 mol C m−2 and carbon export of 0.6 mol C m−2, respectively, over a period of 23 days. Thus, the production and export of carbon that occurred over the month-long experiment period was comparable to that encountered during the spring bloom.