Andrew J. Lucas

Distinguishing random environmental fluctuations from ecological catastrophes for the North Pacific Ocean.

The prospect of rapid dynamic changes in the environment is a pressing concern that has profound management and public policy implications. Worries over sudden climate change and irreversible changes in ecosystems are rooted in the potential that nonlinear systems have for complex and ‘pathological’ behaviours. Nonlinear behaviours have been shown in model systems and in some natural systems, but their occurrence in large-scale marine environments remains controversial. Here we show that time series observations of key physical variables for the North Pacific Ocean that seem to show these behaviours are not deterministically nonlinear, and are best described as linear stochastic. In contrast, we find that time series for biological variables having similar properties exhibit a low-dimensional nonlinear signature. To our knowledge, this is the first direct test for nonlinearity in large-scale physical and biological data for the marine environment. These results address a continuing debate over the origin of rapid shifts in certain key marine observations as coming from essentially stochastic processes or from dominant nonlinear mechanisms. Our measurements suggest that large-scale marine ecosystems are dynamically nonlinear, and as such have the capacity for dramatic change in response to stochastic fluctuations in basin-scale physical states.

Functional tradeoffs underpin salinity-driven divergence in microbial community composition.

Bacterial community composition and functional potential change subtly across gradients in the surface ocean. In contrast, while there are significant phylogenetic divergences between communities from freshwater and marine habitats, the underlying mechanisms to this phylogenetic structuring yet remain unknown. We hypothesized that the functional potential of natural bacterial communities is linked to this striking divide between microbiomes. To test this hypothesis, metagenomic sequencing of microbial communities along a 1,800 km transect in the Baltic Sea area, encompassing a continuous natural salinity gradient from limnic to fully marine conditions, was explored. Multivariate statistical analyses showed that salinity is the main determinant of dramatic changes in microbial community composition, but also of large scale changes in core metabolic functions of bacteria. Strikingly, genetically and metabolically different pathways for key metabolic processes, such as respiration, biosynthesis of quinones and isoprenoids, glycolysis and osmolyte transport, were differentially abundant at high and low salinities. These shifts in functional capacities were observed at multiple taxonomic levels and within dominant bacterial phyla, while bacteria, such as SAR11, were able to adapt to the entire salinity gradient. We propose that the large differences in central metabolism required at high and low salinities dictate the striking divide between freshwater and marine microbiomes, and that the ability to inhabit different salinity regimes evolved early during bacterial phylogenetic differentiation. These findings significantly advance our understanding of microbial distributions and stress the need to incorporate salinity in future climate change models that predict increased levels of precipitation and a reduction in salinity.

Horizontal internal-tide fluxes support elevated phytoplankton productivity over the inner continental shelf

ORIGINAL ARTICLE Horizontal internal-tide fluxes support elevated phytoplankton productivity over the inner continental shelf Andrew J. Lucas 1 , Peter J. S. Franks 1 , and Christopher L. Dupont 1,2 Abstract The narrow continental shelf of the Southern California Bight (SCB) is characterized by elevated primary productivity relative to the adjacent open ocean. This persistent gradient is maintained by the nitrate fluxes associated with internal waves of tidal frequency (the internal tide). Here we provide the first estimates of the internal-tide –driven horizontal fluxes of nitrate, heat, energy, and salinity, calculated from high-resolution, full water-column data gathered by an autonomous wave-powered profiler and a bottom-mounted current meter. The vertically integrated nitrate, heat, and energy fluxes were onshore over the 3-week period of the field experiment. The inner-shelf area- and time-averaged dissipation rate due to the onshore horizontal energy flux, 2.25 £ 10 27 W kg 21 , was elevated relative to open ocean values. The magnitude of the vertically integrated horizontal nitrate flux (136.4 g N m 21 d 1 ) was similar to phytoplanktonic nitrate uptake rates over the inner-shelf. This nitrate flux was variable in time, capable of supporting 0 – 2800 mg C m 22 d 21 (mean approx. 774 mg C m 22 d 21 ) of “new” primary productivity, depending on the energetics of the internal tide and the cross-shore distribution of nitrate. We postulate that the horizontal, internal-tide –driven nitrate flux is the primary cause of the persistently elevated phytoplankton biomass and productivity over the narrow SCB inner shelf. Furthermore, these results suggest that horizontal fluxes of nutrients driven by internal waves may contribute significantly to primary productivity along the boundaries of aquatic environments. Keywords: internal waves, mixing, new productivity, nitrate flux, nutrient dynamics, Reynolds fluxes Scripps Institution of Oceanography, University of California, San Diego, La Jolla, California 92093, USA Present address: Microbial and Environmental Genomics, J. Craig Venter Institute, San Diego, California 92121, USA Correspondence to Andrew J. Lucas, ajlucas@ucsd.edu Introduction [1] In nutrient-limited oceanic environ- ments, the rate of nutrient supply to the euphotic zone—typically mediated by physical dynamics—controls the rate of phytoplankton productivity and the character of the phyto- plankton community and, at steady-state, sets the proportion of that productivity which can be exported to higher trophic levels and out of the euphotic zone (“new” productivity, sensu Dugdale and Goering 1967). Quantifying the physical supply of nutrients to the sunlit surface ocean is therefore of fundamental importance in understanding oceanic ecosystem function. [2] The elevated primary productivity of the coast ocean relative to adjacent offshore waters is due to the operation of physical dynamics that act to inject nutrients into the Limnology and Oceanography: Fluids and Environments † 1 (2011): 56–74 q 2011 by the American Society of Limnology and Oceanography, Inc. DOI 10.1215/21573698-1258185 Downloaded at UNIV CA- SAN DIEGO on May 26, 2011

Dynamics of oxygen depletion in the nearshore of a coastal embayment of the southern Benguela upwelling system

Acquisition of high resolution time series of water column and bottom dissolved oxygen (DO) concentrations inform the dynamics of oxygen depletion in St Helena Bay in the southern Benguela upwelling system at several scales of variability. The bay is characterized by seasonally recurrent hypoxia (<1.42 ml l−1) associated with a deep pool of oxygen-depleted water and episodic anoxia (<0.02 ml l−1) driven by the nearshore (<20 m isobath) decay of red tide. Coastal wind forcing influences DO concentrations in the nearshore through its influence on bay productivity and the development of red tides; through shoreward advection of the bottom pool of oxygen-depleted water as determined by the upwelling-downwelling cycle; and through its control of water column stratification and mixing. A seasonal decline in bottom DO concentrations of ∼1.2 ml l−1 occurs with a concurrent expansion of the bottom pool of oxygen depleted water in St Helena Bay. Upwelling of this water into the nearshore causes severe drops in DO concentration (<0.2 ml l−1), particularly during end-of-season upwelling, resulting in a significant narrowing of the habitable zone. Episodic anoxia through the entire water column is caused by localized degradation of red tides within the confines of the shallow nearshore environment. Oxygenation of the nearshore is achieved by ventilation of the water column particularly with the onset of winter mixing. No notable changes are evident in comparing recent measures of bottom DO concentrations in St Helena Bay to data collected in the late 1950s and early 1960s.

Mixing to Monsoons: Air-Sea Interactions in the Bay of Bengal

More than 1 billion people depend on rainfall from the South Asian monsoon for their livelihoods. Summertime monsoonal precipitation is highly variable on intraseasonal time scales, with alternating “active” and “break” periods. These intraseasonal oscillations in large-scale atmospheric convection and winds are closely tied to 1°C–2°C variations of sea surface temperature in the Bay of Bengal.

ASIRI : an ocean–atmosphere initiative for Bay of Bengal

AbstractAir–Sea Interactions in the Northern Indian Ocean (ASIRI) is an international research effort (2013–17) aimed at understanding and quantifying coupled atmosphere–ocean dynamics of the Bay of Bengal (BoB) with relevance to Indian Ocean monsoons. Working collaboratively, more than 20 research institutions are acquiring field observations coupled with operational and high-resolution models to address scientific issues that have stymied the monsoon predictability. ASIRI combines new and mature observational technologies to resolve submesoscale to regional-scale currents and hydrophysical fields. These data reveal BoB’s sharp frontal features, submesoscale variability, low-salinity lenses and filaments, and shallow mixed layers, with relatively weak turbulent mixing. Observed physical features include energetic high-frequency internal waves in the southern BoB, energetic mesoscale and submesoscale features including an intrathermocline eddy in the central BoB, and a high-resolution view of the exchange...

Observations of Nonlinear Internal Wave Runup to the Surfzone

AbstractThe cross-shore evolution of nonlinear internal waves (NLIWs) from 8-m depth to shore was observed by a dense thermistor array and ADCP. Isotherm oscillations spanned much of the water column at a variety of periods. At times, NLIWs propagated into the surfzone, decreasing temperature by ≈1°C in 5 min. When stratification was strong, temperature variability was strong and coherent from 18- to 6-m depth at semidiurnal and harmonic periods. When stratification weakened, temperature variability decreased and was incoherent between 18- and 6-m depth at all frequencies. At 8-m depth, onshore coherently propagating NLIW events had associated rapid temperature drops (ΔT) up to 1.7°C, front velocity between 1.4 and 7.4 cm s−1, and incidence angles between −5° and 23°. Front position, ΔT, and two-layer equivalent height zIW of four events were tracked upslope until propagation terminated. Front position was quadratic in time, and normalized ΔT and zIW both decreased, collapsing as a linearly decaying funct...

Submesoscale processes at shallow, salinity fronts in the Bay of Bengal: Observations during the winter monsoon

AbstractLateral submesoscale processes and their influence on vertical stratification at shallow salinity fronts in the central Bay of Bengal during the winter monsoon are explored using high-resol...

A reflecting, steepening, and breaking internal tide in a submarine canyon

Submarine canyons are common features of the coastal ocean. Although they are known to be hotspots of turbulence that enhance diapycnal transport in their stratified waters, the dynamics of canyon mixing processes are poorly understood. Most studies of internal wave dynamics within canyons have focused on a handful of canyons with along-axis slopes less steep than semidiurnal (D2) internal wave characteristics (subcritical). Here, we present the first tidally-resolving observations within a canyon with a steeply sloping axis (supercritical). A process study consisting of two 24-hour shipboard stations and a profiling mooring was conducted in the La Jolla Canyon off the coast of La Jolla, CA. Baroclinic energy flux is oriented up-canyon and decreases from 182 ± 18 W m−1 at the canyon mouth to 46 ± 5 W m−1 near the head. The ratio of horizontal kinetic energy to available potential energy and the observed group speed of each mode are lower than expected for freely propagating D2 internal waves at each station, indicating partial reflection. Harmonic analysis reveals that variance is dominated by the D2 tide. Moving up-canyon, the relative importance of D2 decreases and its higher harmonics are needed to account for a majority of the observed variance, indicating steepening. Steep internal tides cause large isopycnal displacements (∼50 m in 100m water depth) and high strain events. These events coincide with enhanced O(10−7 -10−5 m2 s−3) dissipation of turbulent kinetic energy at mid-depths.