Drifter Studies Research Articles

The successional formation and release of domoic acid in a Pseudo-nitzschia bloom in the Juan de Fuca Eddy: A drifter study

Blooms of Pseudo-nitzschia species are frequent, but presently unpredictable, in the Juan de Fuca Eddy region off the coasts of Washington (US) and British Columbia (Canada). This upwelling eddy region is proposed to be the bloom commencement site, before cells are entrained into the coastal surface currents. During a shipboard study, we characterized the different stages of the Pseudo-nitzschia bloom development from its initiation and intensification, to its eventual sinking and dissipation. Specifically, we followed a water mass using lagrangian ARGOS-tracked drifters released at the eddy water mass and quantified production of dissolved and particulate domoic acid, and the physiological status of the Pseudo-nitzschia cells with regards to photosynthesis, nutrient needs and sinking rates, along with its relationship with competing species – in this case, the marine euglenoid, Eutreptiella spp. The drifter study allows for an interpretation of the presence or absence of Pseudo-nitzschia and domoic acid against active environmental factors – particularly copper and iron.

Tracking the Hercules 265 marine gas well blowout in the Gulf of Mexico

AbstractOn 23 July 2013, a marine gas rig (Hercules 265) ignited in the northern Gulf of Mexico. The rig burned out of control for 2 days before being extinguished. We conducted a rapid‐response sampling campaign near Hercules 265 after the fire to ascertain if sediments and fishes were polluted above earlier baseline levels. A surface drifter study confirmed that surface ocean water flowed to the southeast of the Hercules site, while the atmospheric plume generated by the blowout was in eastward direction. Sediment cores were collected to the SE of the rig at a distance of ∼0.2, 8, and 18 km using a multicorer, and demersal fishes were collected from ∼0.2 to 8 km SE of the rig using a longline (508 hooks). Recently deposited sediments document that only high molecular weight (HMW) polycyclic aromatic hydrocarbon (PAH) concentrations decreased with increasing distance from the rig suggesting higher pyrogenic inputs associated with the blowout. A similar trend was observed in the foraminifera Haynesina germanica, an indicator species of pollution. In red snapper bile, only HMW PAH metabolites increased in 2013 nearly double those from 2012. Both surface sediments and fish bile analyses suggest that, in the aftermath of the blowout, increased concentration of pyrogenically derived hydrocarbons was transported and deposited in the environment. This study further emphasizes the need for an ocean observing system and coordinated rapid‐response efforts from an array of scientific disciplines to effectively assess environmental impacts resulting from accidental releases of oil contaminants.

English

doi: http://dx.doi.org/10.15447/sfews.2014v12iss4art1 In branching channel networks, such as in the Sacramento–San Joaquin River Delta, junction flow dynamics contribute to dispersion of ecologically important entities such as fish, pollutants, nutrients, salt, sediment, and phytoplankton. Flow transport through a junction largely arises from velocity phasing in the form of divergent flow between junction channels for a portion of the tidal cycle. Field observations in the Georgiana Slough junction, which is composed of the North and South Mokelumne rivers, Georgiana Slough, and the Mokelumne River, show that flow phasing differences between these rivers arise from operational, riverine, and tidal forcing. A combination of Acoustic Doppler Current Profile (ADCP) boat transecting and moored ADCPs over a spring–neap tidal cycle (May to June 2012) monitored the variability of spatial and temporal velocity, respectively. Two complementary drifter studies enabled assessment of local transport through the junction to identify small-scale intrajunction dynamics. We supplemented field results with numerical simulations using the SUNTANS model to demonstrate the importance of phasing offsets for junction transport and dispersion. Different phasing of inflows to the junction resulted in scalar patchiness that is characteristic of MacVean and Stacey’s (2011) advective tidal trapping. Furthermore, we observed small-scale junction flow features including a recirculation zone and shear layer, which play an important role in intra-junction mixing over time scales shorter than the tidal cycle (i.e., super-tidal time scales). The study period spanned open- and closed-gate operations at the Delta Cross Channel. Synthesis of field observations and modeling efforts suggest that management operations related to the Delta Cross Channel can strongly affect transport in the Delta by modifying the relative contributions of tidal and riverine flows, thereby changing the junction flow phasing.

Autonomous River Navigation Using the Hamilton–Jacobi Framework for Underactuated Vehicles

The feasibility of drifter studies in complex and tidally forced water networks has been greatly expanded by the introduction of motorized floating sensors. This paper presents a method for such motorized sensors to accomplish obstacle avoidance and path selection using the solutions to Hamilton-Jacobi-Bellman-Isaacs (HJBI) equations. The method is then validated experimentally.

Quasi-Lagrangian drifter studies of iron speciation and cycling off Point Conception, California

The distribution and speciation of dissolved Fe (dFe) were measured during four quasi-Lagrangian drogued drifter studies (~ 4 d duration each) that were conducted in the southern California Current System in May 2006 and April 2007. Three of the four drifter studies were within the coastal upwelling regime and one drifter study was in a warm-core anticyclonic eddy. Incubation bottle experiments were also conducted to determine the degree of phytoplankton Fe limitation and to assess changes in the concentration of Fe-binding ligands. In the coastal upwelling drifter studies, in situ dFe (1.4–1.8 nM) and macronutrients were initially high and declined over time. Fe addition incubation experiments indicated that the phytoplankton community was not Fe limited at the beginning of the coastal upwelling drifter experiments (when μM nitrate:nM dFe ratios were ~ 7–8). By the end of two of the three drifter studies (when μM nitrate:nM dFe ratios were ~ 12–19), Fe addition resulted in larger nitrate and silicic acid drawdown, and larger accumulations in chlorophyll a, particulate organic carbon and nitrogen, and diatom and dinoflagellate-specific carotenoid pigments. Fe speciation was measured in situ in three of the four drifter studies with stronger L 1-type ligands found to be present in excess of dFe in all samples. In Fe speciation incubation experiments, L 1-type ligand production was observed in conjunction with phytoplankton growth under Fe-limiting conditions. The results presented here support and add a quasi-Lagrangian perspective to previous observations of dFe and macronutrient cycling over space and time within the California coastal upwelling regime, including Fe limitation within the phytoplankton community in this region and the biological production of Fe-binding ligands concomitant with Fe limitation.

Temporal and spatial scales of variation in bacterioplankton assemblages of oligotrophic surface waters

Marine bacterioplankton dominate microbial carbon biomass in surface waters of the oligotrophic ocean, yet there have been few studies examining rates of change in bacterioplankton assemblage composition in situ over time and across water masses. Temporal changes in bacterio- plankton assemblage composition were investigated during 7 drifter studies of 24 to 360 h duration in the oligotrophic Gulf of Mexico, the North Pacific and the West Tropical Atlantic in 2001 to 2003, using an assemblage fingerprinting technique, automated rRNA intergenic-spacer analysis (ARISA). The similarity indices between assemblages collected over time in the same drifter changed on aver- age by a Sorensen index of 0.12 d -1 (comparing the presence/absence of operational taxonomic units, OTU) and a Whittaker index (comparing proportions in various OTU) of 0.17 d -1 per fingerprint, across all surface drifter studies. Fingerprints generated from 7 replicate bacterioplankton DNA sam- ples collected at each of 2 stations were remarkably similar to each other, sharing a Whittaker index >0.85. Despite this consistency over small spatial scales (<2 km), no clear relationship was observed between the separation distance of sampling locations and similarity between assemblage finger- prints in the North Pacific and Atlantic gyres over mesoscales (10 to 3000 km), oscillating around a mean of 0.38 to 0.47 for each gyre. Our results suggest that factors ultimately shaping assemblage composition are localized at spatial scales between a few kilometers and about 50 km, i.e. this is a typical horizontal 'patch size', within which communities are relatively homogenous, perhaps because physical mixing (e.g. by eddies) may dominate over biological interactions. Our results also suggest that, while similar environmental factors may cause bacterioplankton assemblages to share a small portion of OTU between different sampling locations, biological factors selecting for parti- cular bacterial types (e.g. viral lysis, grazing, antagonism, nutrition) may cause assemblage composi- tion variability over short geographic distances.

Coupling between primary production and pelagic consumption in temperate ocean margin pelagic ecosystems

Abstract Three fates potentially consume primary production occurring on ocean margins: portions can be oxidized within the water column, portions can sediment to shelf/slope depots, and portions can be exported to the interior ocean. Zooplankton mediate all three of these processes and thus can alter the pathway and residence time of particulate organic carbon. As part of both US DOE- and NSF-sponsored studies on the Cape Hatteras and South Atlantic Bight (SAB) shelves, the role of microzooplankton in these processes was determined by measuring phytoplankton production and its consumption by microzooplankton. Grazing and growth rates were measured during 46 dilution incubation experiments using chlorophyll a (chl a ) as a proxy for phytoplankton (prey) biomass. Chl a production and grazing were determined for the 14 CO 3 − incubations during two Lagrangian drifter studies lasting several days in March and July 1996. From similar measurements during cross-shelf transects over larger spatial scales, primary production was also calculated for the Hatteras study area using a wavelength-resolved bio-optical model. Primary production during the Lagrangian studies was generally 0.5–1.0 gC/m 2 /d in March and 0.5–2.0 gC/m 2 /d in July. Modeled estimates of primary production for the larger Hatteras study region in March and July averaged 1.8 gC/m 2 /d. Typically, a and its grazing by microzooplankton. In the dilution experiments conducted throughout the SAB and Hatteras shelves, microzooplankton grazed 65% of a production, and 81% of a production. These relationships were temperature-dependent: losses of chl a production in both size fractions to microzooplankton herbivory increased with increasing temperature. Higher grazing rates were found in the 14 CO 3 − -derived estimates of primary production in Cape Hatteras shelf waters. Integrated water column grazing removed 40% and 58% of

Near-surface circulation of the northeast Pacific Ocean derived from WOCE-SVP satellite-tracked drifters

Statistics of the near-surface circulation in the northeast Pacific Ocean were derived from the trajectories of nearly 100 surface drifters tracked between August 1990 and December 1995 as part of the World Ocean Circulation Experiment's (WOCE) Surface Velocity Program (SVP). Drifters were drogued within the mixed layer (15 m drogue depth) or near the top of the permanent halocline (120 m). All branches of the Alaskan Gyre were well-sampled at both depths, revealing a weak Subarctic Current, a bifurcation of the Subarctic Current near 48°N, 130°W at 15 m depth, and strong, variable flow in the Alaska Current and Alaskan Stream. At 120 m depth, northward flow in the Alaska Current occurred much farther offshore than within the mixed layer. The drifter trajectories revealed interannual variability, with evidence of an intensified Alaskan Gyre during the winters of 1991–92 and 1992–93 and more southerly transport during winter 1994–95. A minimum in eddy kinetic energy was found at both depths within the northern branch of the Subtropical Gyre. Eddy kinetic energies were nearly twice as high in the mixed layer compared to below, and were 2–3 times larger in winter than in summer throughout most of the near-surface Alaskan Gyre. High eddy energies observed near the eastern perimeter of the Alaskan Gyre may be due to the offshore intrusion of eddies formed by coastal current instabilities. Taylor's theory of single-particle dispersion was applied to the drifter ensembles to estimate Lagrangian decorrelation scales and eddy diffusivities. Both the initial dispersion and random walk regimes were identified in the dispersion time series computed for several regions of both ensembles. The integral time scales and eddy diffusivities computed from the dispersion scale linearly with r.m.s. velocity, which is consistent with drifter studies from the Atlantic. An exception is the meridional integral time scales, which were nearly constant throughout the study area and at both drogue depths. The magnitudes of the derived eddy statistics are comparable to those derived from surface drifters in other parts of the world ocean. These are the first Lagrangian estimates of particle dispersion over a broad region of the near-surface North Pacific, and the consistency of the results with previous studies from the Atlantic lends credence to the idea that the simplifying assumptions of Taylor (1921) (Proceedings of the London Mathematical Society Series A 20, 196–221) are reasonably valid throughout the upper ocean. This bodes well for the effective parameterization of near-surface diffusivities in general circulation models. Finally, the drifter-derived velocity statistics were used to speculate on the source regions of waters of possible coastal origin observed at offshore stations during the field studies of the Canadian Joint Global Ocean Flux Study.

Evidence for a clade‐specific temporal and spatial separation in ribulose bisphosphate carboxylase gene expression in phytoplankton populations off Cape Hatteras and Bermuda

The factors affecting the regulation of photosynthetic carbon fixation in diverse phytoplankton populations are not yet understood. To this end, we have measured the expression of the gene (rbcL) for the major carbon fixation enzyme, ribulose‐1,5‐bisphosphate carboxylase/oxygenase, in coastal phytoplankton populations off Cape Hatteras and in oligotrophic oceanic picoplankton near Bermuda. Using gene probes specific for the cyanobacterial/chloro‐phytic clade and the chromophytic clade (diatoms, chrysophytes, prymnesiophytes, and others) of Form I rbcL genes (“cyano” and “chromo” probes, respectively), we have measured rbcL messenger ribonucleic acid (mRNA) levels in size‐fractionated coastal waters, in a decktop diel incubator and a Lagrangian drifter study, and in vertical profiles in stratified, oligotrophic ocean water. In coastal waters influenced by estuarine plumes, an equal distribution of carbon fixation between the picoplankton and the micro/nannoplankton occurred, with cyano rbcL mRNA coinciding with Synechococcus counts in the &lt;1‐μm fraction, with the majority of the chromo rbcL mRNA expression occurring in the larger sized phytoplankton fraction. In profiles of oligotrophic oceanic waters, the cyano rbcL mRNA was found in the upper water column (∼50‐m depth) and coincided with peaks in Synechococcus counts. The chromo rbcL mRNA was concentrated at the subsurface chlorophyll a (Chl a) maximum (∼85 m) and corresponded to red‐fluorescing cell counts, thought to be picoeucaryotes and diatoms. Photosynthetic carbon fixation and RUBISCO enzyme activity encompassed both cyano and chromo rbcL mRNA peaks, suggesting a near equal contribution to carbon fixation in the water column by these two phytoplankton clades. Both decktop diel incubator studies and a Lagrangian drifter study in coastal waters indicated cyano rbcL transcription in the morning and chromophytic rbcL transcription in the late afternoon/early evening. Thus, the two major clades of RUBISCO‐containing phytoplankton occupy separate niches in time, space, and cell size in the waters off Cape Hatteras. The factors determining such clade‐specific niches may include efficiency of nutrient utilization, differences in relative carboxylase/oxygenase activity (τ values) of cyano (Form IB) and chromophytic (Form ID) RUBISCOs, and differences in pigment composition/adaptation to light regimes. Additionally, we propose that chromo rbcL mRNA may be indicative of new production, whereas cyano rbcL mRNA correlates with recycled production in stratified, oligotrophic oceanic environments.

Spread of oil from theTenyo Maru, off the Southwest coast of Vancouver Island

We provide background oceanography and observations and hindcast simulations of spreading oil leaked from the wreck of theTenyo Maru, a fishing vessel that sank off the western entrance to Juan de Fuca Strait in July 1991. The vessel sank in the region of the Tully Eddy on the continental shelf. Although this eddy is normally present in summer, its location shifts and its ‘trapping time’ varies considerably. The spread of theTenyo Maru oil is represented reasonably well by assuming the eddy to be a littler further to the east and north of its position as observed during drifter studies in 1986. Observations in 1985 showed that the preferred exit route of surface water from the eddy was toward the southeast, the direction followed by theTenyo Maru oil in 1991; however, other drift paths are possible, and under different weather conditions the oil could have drifted toward the Vancouver Island coast.

The tidal and longer-period circulation of Capricornia, southern Great Barrier Reef : Griffin, D.A., J.H. Middleton and Lance Bode, 1987. Aust. J. mar. Freshwat. Res., 38(4):461–474. School of Math., Univ. of New South Wales, P.O. Box 1, Kensington, NSW 2033, Australia

Between June and December 1983, nine current meters and three water level recorders were deployed on the continental shelf and slope of the Capricornia Section of the Great Barrier Reef between Fraser Island (25s.) and the mouth of the Capricorn Channel (23s.) on the east coast of Australia. Tidal analyses of the hourly data set reveal an amplification of the semi-diurnal tides as they propagate northwestward into the Capricorn Channel. The results of a numerical model of tidal flow show excellent agreement with observations. The daily averaged (non-tidal) currents are highly variable and produce complex circulation patterns, but with a mean flow generally alongshore to the north-west. Comparisons with previous drifter studies and satellite-tracked buoy data suggest that the south-eastward flowing East Australian Current drives a large clockwise eddy, in the lee of the Swain Reefs, located east of the study region. It is postulated that this eddy, in addition to the generally north-westward wind stress, contributes to the north-westward flow within the study region. Temperatures recorded by the deployed instruments and temperature profiles from conductivity-temperature-depth casts confirm that tidal and longer period variablity contribute to upwelling onto the continental shelf.

Sediment transport in the Bristol Channel: A review

Investigations into the physical oceanographic characteristics of the Bristol Channel and Severn Estuary are reviewed; these include the general pattern of water movement and the influence of high freshwater discharges on the salinity distribution. Various approaches to the problem of defining sediment transport paths in the macrotidal estuarine system, for fine grained and coarse grained material, are summarised. Transport patterns are examined, for example, through: the distribution of surficial sediments and their associated bedforms; seabed drifter studies; the analysis of tidal current and wave data, separately and in combination, because of their influence on bedload transport paths; satellite imagery, within the visible part of the spectrum; and on the basis of output from depth-averaged numerical models. Comparison between the interpretations shows that there are discrepancies and contradictions in the derived sediment transport paths. In particular, it is difficult to explain the up-channel movement of fine grained material against a background of inferred offshore movement, shown by the orientation of sandwaves and the direction of movement of the surface waters. An early model to overcome this contradiction incorporated ‘two-way differential transport’ throughout the water column. Localised contradictions in the transport paths for sand and ‘fine sediment’ occur in the inner part of the Bristol Channel. The most recent conjectural models for sediment movement incorporate an ebb- dominated zone along the central axis of the Channel, together with flood-dominated coastal zones. This lateral variation in transport paths, across the Channel, constrasts with the earlier proposition for vertical differentiation; it is suggested by seabed drifter recovery patterns and confirmed by the output from one of the numerical models. Transport towards the flood-dominated zones at the boundaries is enhanced under the superimposed influence of waves, which is demonstrated through the use of sediment transport formulae and sand tracer experiments. Although the annual contribution of the rivers to the overall sediment budget is shown to be small, the time of retention of waters and sediment within the system could result in fluviatile sediment inputs being retained for tens of years. In an attempt to extrapolate observations of transport processes into the ancient record, a preliminary facies model for the macrotidal Bristol Channel is presented. The model shows the availability of sand increasing in an onshore direction; progressing in this direction, the facies pass from scoured bedrock and lag deposits, through cross bedded megaripple foresets and the topset planar bedding of intertidal flats, to Holocene fill and supratidal deposits.

The tidal and longer-period circulation of Capricornia, Southern Great Barrier Reef

Between June and December 1983, nine current meters and three water level recorders were deployed on the continental shelf and slope of the Capricornia Section of the Great Barrier Reef between Fraser Island (25�s.) and the mouth of the Capricorn Channel (23�s.) on the east coast of Australia. Tidal analyses of the hourly data set reveal an amplification of the semi-diurnal tides as they propagate north- westward into the Capricorn Channel. The results of a numerical model of tidal flow show excellent agreement with observations. The daily averaged (non-tidal) currents are highly variable and produce complex circulation patterns, but with a mean flow generally alongshore to the north-west. Comparisons with previous drifter studies and satellite-tracked buoy data suggest that the south-eastward flowing East Australian Current drives a large clockwise eddy, in the lee of the Swain Reefs, located east of the study region. It is postulated that this eddy, in addition to the generally north-westward wind stress, contributes to the north-westward flow within the study region. Temperatures recorded by the deployed instruments and temperature profiles from conductivity-temperature-depth casts confirm that tidal and longer period variablity contribute to upwelling onto the continental shelf.

High frequency radar measurements of tidal currents flowing through San Pablo Strait, San Francisco Bay1

High frequency (HF) radar measurements of the surface current averaged over the upper 0.5 m in San Pablo Strait were compared with current meter measurements of the subsurface current made at 9.4 m below mean lower low water (MLLW) over two 12.4‐h tidal cycles. After averaging the radar and current meter data over two tidal cycles, a southerly (ebbing direction) surface current of 32 cm·s−1 was deduced from the radar measurements and a northerly (flooding direction) subsurface current of 7 cm·s−1 from the current meter measurements. This nontidal flow is maintained by freshwater discharge from the Sacramento–San Joaquin Rivers into Suisun and San Pablo Bays. The radar measurement technique provides quantitative estimates of the surface currents that previously were determined only from surface drifter studies.

A seabed drifter study of near-bottom circulation in North Carolina shelf waters

Circulation within North Carolina near-bottom shelf waters was investigated between June 1972 and April 1973 using seabed drifters (SBDs). Seabed drifter trajectories revealed a tendency for onshore components of near-bottom drift in Raleigh Bay; in Onslow Bay, near-bottom currents were complex, generally featuring offshore components in the northern end with a tendency for onshore components in the southern end. A divergence zone separating onshore from offshore near-bottom flow was found in Onslow Bay. The location of this zone (parallel to a line bearing 75 °TN across Onslow Bay) remained constant during the duration of the present study.

Helicopter launching of surface and sea‐bed drifters

The semi‐enclosed nature of Long Island Sound makes it ideal for surface and sea‐bed drifter studies. A U.S. Coast Guard H‐3 helicopter provided a “synoptic” launch within a half tidal cycle. Drifter releases were conducted on 18 June 1973 at a total of 50 stations in Long Island and Block Island Sounds. Drops were made at various altitudes and speeds in order to determine optimum release conditions.

Residual Drift near the Sea Bed in Liverpool Bay: an Observational Study

Summary The results of an extensive sea-bed drifter study are presented and the physical processes producing residual drift of the near-bed water are discussed. The drifter returns show that there is a strong landward movement near the bed throughout the whole of the Bay; also that there are distinct divisions between the estuarial-type systems associated with the Dee, Mersey and Ribble estuaries. The general residual movement is caused primarily by the circulation produced by the combined effect of the tidal currents (with their associated mixing processes) and the longitudinal density-gradients (caused by the input of freshwater from the rivers). Seasonal changes in the near-bed residual drift are considered and the effect of storm conditions is discussed.