Southern California Bight Research Articles

Observations of Elevated Mixing and Periodic Structures Within Diurnal Warm Layers

AbstractSurface drifters (SWIFTs) equipped with down‐looking high‐resolution acoustic doppler current profilers (ADCPs) were used to estimate the turbulent kinetic energy (TKE) dissipation rate within highly stratified diurnal warm layers (DWLs) in the Southern California Bight. Over a 10‐day period, five instances of DWLs were observed with strong surface temperature anomalies up to 3°C and velocity anomalies up to 0.3 m . Profiles of in the upper 5 m suggest turbulence is strongly modulated by the DWL stratification. Burst‐averaged (8.5 min) is stronger than predicted by law‐of‐the‐wall boundary layer scaling within the DWLs and suppressed below. Predictions for within the DWLs are improved by a shear‐production scaling using observed shear and linearly decaying turbulent stress. However, is still under‐predicted. Examination of the un‐averaged acoustic backscatter data suggests elevated is related to the presence of turbulent structures in the DWLs which span the layer height and strongly modulate TKE. Evolution in the bulk Richardson number each day suggests the DWLs become unstable to layer‐scale overturning and entrainment each afternoon, thus the turbulent structures may result from shear‐driven instability. This interpretation is supported by a conditional average of the data during a burst characterized by strongly periodic structures. The structures resemble high‐frequency internal waves with strong asymmetry in the along‐flow direction (steepening) which suggests they are unstable. Coincident asymmetric patterns in upwelling/downwelling and corresponding regions of strong vertical convergence/divergence suggest that both vertical transport and local TKE generation are plausible sources of elevated in the DWLs.

Effects of urban eutrophication on pelagic habitat capacity in the Southern California Bight

Land-based nutrient inputs to the ocean have been linked to increased coastal productivity, subsurface acidification and O2 loss, even in upwelling systems like the Southern California Bight. However, whether eutrophication alters the [environment’s] capacity to support key taxa has yet to be evaluated for this region. Here, we assess the impact of land-based nutrient inputs on the availability of aerobic and calcifying habitat for key pelagic taxa using ocean model simulations. We find that acute, lethal conditions are not commonly induced in epipelagic surface waters, but that sublethal, ecologically relevant changes are pervasive. Land-based nutrient inputs reduce the potential aerobic and calcifier habitat during late summer, when viable habitat is at its seasonal minimum. A region of annually recurring habitat compression is predicted 30 – 90 km from the mainland, southeast of Santa Catalina Island. Here, both aerobic and calcifier habitat is vertically compressed by, on average, 25%, but can be as much as 60%. This effect can be traced to enhanced remineralization of organic matter that originates from the coast. These findings suggest that effects of land-based nutrients are not restricted to chemistry but extend to habitat capacity for multiple taxa of ecological and economic importance. Considerable uncertainty exists, however, in how this habitat compression translates to population-level effects.

Altimeter-derived poleward Lagrangian pathways in the California Current System: Part 1

We use altimeter-derived geostrophic velocities, with and without the addition of surface Ekman transports, to create trajectories for virtual parcels in the California Current System (CCS). The goal is to investigate the poleward transport of passive water parcels in the surface 50–100 m of the nominally equatorward system. Motivation for the study is provided by observations of anomalous biomass of copepods with warm water affinities along the Newport Hydrographic Line off central Oregon (44.7°N) during El Niño years, as well as during and following the 2014–2016 marine heat wave. By backward tracking virtual parcels from 44.7°N, we find that the most distant source of passive water parcels in the upper ocean during a one-year period of travel is from within the Southern California Bight (SCB), north of 30°N. To make that journey, parcels use the Inshore Countercurrent off southern and central California during summer–winter and the Davidson Current off northern California and Oregon during autumn–winter. The inclusion of small-scale eddy diffusion usually increases the number of parcels that reach more northern latitudes, while the inclusion of Ekman velocities more often reduces those numbers. Even so, parcels can travel from the SCB to central Oregon in either the Ekman layer or beneath it in the geostrophic flow. Using backward tracking, we find that parcels arrive at 44.7°N most often in winter–spring, least often in autumn. They arrive from within the large-cape region off northern California (41°–42°N) during all years and all months, from just south of the large-cape region (38°–39°N) during most years but seldom in autumn, from south of Monterey Bay along central California (36°N) and within the SCB (34.5°N) during a third (or less) of the years and only in winter-spring. The shortest average transit times are found in winter: for parcels reaching 44.7°N in February, the average transit time is 2 months for parcels coming from 41°–42°N, 4 months for parcels coming from 38°–39°N, and 5–6 months or more for parcels coming from south of 36°N. Transit times increase as the arrival time progresses from winter to autumn. The longest average transit times are for parcels reaching central Oregon in autumn (9–12 months in October for parcels coming from south of 39°N). This makes the journey a multi-generational task for the copepods. Interannual variability in the observed southern copepod species biomass off central Oregon correlates highly with years when more virtual parcels from the south reach central and northern Oregon, providing increased confidence in the results found with the altimeter-derived parcel trajectories.

Temporal and oceanographic factors differentially affect two size classes of white shark at a Southern California aggregation site

Ontogenetic habitat shifts are a common feature of many marine species, including sharks, which face conservation threats when their distributions overlap with human resource extraction and habitat modification. White sharks Carcharodon carcharias, for example, exhibit a distinctly coastal phase as juveniles, with a limited distribution compared to the basin-scale range of adults. Using an unoccupied aerial vehicle (UAV), we studied a coastal aggregation site within a Southern California Bight nursery area to determine how fine-scale temporal and oceanographic factors affect white sharks at different developmental stages. White shark density, as measured via UAV, was highly variable across time of day and day of year, with modest variation across years. Typically, more sharks were observed in the late afternoon hours. Sharks, especially those <3 m total length, were observed more often during periods of colder seafloor temperatures, potentially reflecting avoidance of these colder, deeper waters by more cold-intolerant smaller white sharks. Alternate models incorporating sea surface temperature showed a very small but significant association between surface temperatures and <3 m total length white sharks for the months we surveyed, but no such association for larger sharks. There were no or only modest effects of visibility, swell height, chl a levels, sea state, and tidal height on UAV-observed shark density. Understanding how temporal patterns and oceanographic predictors of density change over time as well as how shark ontogeny interacts with these factors can help us to better understand how this species uses coastal habitats and predict when they may be more likely to share marine space with humans.

Biogeographic patterns of Pacific white‐sided dolphins based on long‐term passive acoustic records

AbstractAimThis study investigates the biogeographic patterns of Pacific white‐sided dolphins (Lagenorhynchus obliquidens) in the Eastern North Pacific based on long‐term passive acoustic records. We aim to elucidate the ecological and behavioural significance of distinct echolocation click types and their implications for population delineation, geographic distribution, environmental adaptation and management.LocationEastern North Pacific Ocean.Time Period2005–2021.Major Taxa StudiedPacific white‐sided dolphin.MethodsOver 50 cumulative years of passive acoustic monitoring (PAM) data from 14 locations were analyzed using a deep neural network to classify two distinct Pacific white‐sided dolphin echolocation click types. The study assessed spatial, diel, seasonal and interannual patterns of the two click types, correlating them with major environmental drivers such as the El Niño Southern Oscillation and the North Pacific Gyre Oscillation, and modeling long‐term spatial‐seasonal patterns.ResultsDistinct spatial, diel and seasonal patterns were observed for each click type. Significant biogeographical shifts in presence were observed following the 2014–2016 marine heatwave event.Main ConclusionsDistinct spatial distributions of the two click types support the hypothesis that Pacific white‐sided dolphins produce population‐specific echolocation clicks. Seasonal and diel patterns suggest spatiotemporal niche partitioning between the populations in Southern California. Interannual changes, notably initiated during the 2014–2016 marine heatwave, indicate climate‐driven range expansions and contractions related to gradual tropicalization of the Southern California Bight.

Ziphius cavirostris presence relative to the vertical and temporal variability of oceanographic conditions in the Southern California Bight.

The oceanographic conditions of the Southern California Bight (SCB) dictate the distribution and abundance of prey resources and therefore the presence of mobile predators, such as goose-beaked whales (Ziphius cavirostris). Goose-beaked whales are deep-diving odontocetes that spend a majority of their time foraging at depth. Due to their cryptic behavior, little is known about how they respond to seasonal and interannual changes in their environment. This study utilizes passive acoustic data recorded from two sites within the SCB to explore the oceanographic conditions that goose-beaked whales appear to favor. Utilizing optimum multiparameter analysis, modeled temperature and salinity data are used to identify and quantify these source waters: Pacific Subarctic Upper Water (PSUW), Pacific Equatorial Water (PEW), and Eastern North Pacific Central Water (ENPCW). The interannual and seasonal variability in goose-beaked whale presence was related to the variability in El Niño Southern Oscillation events and the fraction and vertical distribution of the three source waters. Goose-beaked whale acoustic presence was highest during the winter and spring and decreased during the late summer and early fall. These seasonal increases occurred at times of increased fractions of PEW in the California Undercurrent and decreased fractions of ENPCW in surface waters. Interannual increases in goose-beaked whale presence occurred during El Niño events. These results establish a baseline understanding of the oceanographic characteristics that correlate with goose-beaked whale presence in the SCB. Furthering our knowledge of this elusive species is key to understanding how anthropogenic activities impact goose-beaked whales.

Observation and modeling of an unusual spatiotemporal pattern in bioacoustic chorusing

This paper describes an unusual underwater biological chorus recorded in the Southern California Bight and presents a numerical modeling approach that replicates aspects of the chorus. During one experiment, the evolution of the directionality of the chorusing region over time is suggestive of “The Wave”, the human waves performed by fans periodically standing and sitting in sports stadia around the world; here, the region of chorusing periodically propagated upcoast over 20 km of coastline at nearly 1.5 km/s. The chorus occurs predominantly at night in spring and summer, mostly in very shallow waters near the coast. It increases the underwater sound field levels within chorusing regions by 20–30 dB in the 50 Hz to 1 kHz frequency band. The chorus is composed of three parts; 1) a “sunset chorus” which is a 20-to-30 min continuous roar around sunset with received spectral levels up to 100 dB re 1 μPa2/Hz, 2) a “sunrise chorus” of lower level than the sunset chorus, and 3) an all-night-long cycling chorus. The cycling portion is made up of 15-to-20 s periods of higher received spectral levels (up to 90 dB re 1 μPa2/Hz within the chorusing region) each followed by a 10-to-20 s lull in which the spectral levels drop by 4–10 dB. This alternating pattern repeats every 30–40 s throughout the night. The numerical modeling approach is based on the physics of excitable media. A cellular automaton is used to model the two-dimensional spatial grid occupied by the calling animals (units), with each unit being either in the “resting”, “excitable”, or “active (calling)" state at each time step. Transition from resting to excitable and from active back to resting occurs automatically after a fixed period of time in the present state, whereas the probability of transitioning from excitable to active is determined not only by the elapsed time since entering the excitable state, but also by the received sound level at the unit location, creating a non-linear acoustics-based coupling between units. With appropriate inputs, many determined from measurements of the chorus properties and the individual animal calls themselves, simulations with the model can replicate the cycling levels in the night-long chorus, the continuous din of the sunset chorus, and (once properly initialized) the periodic upcoast evolution of the chorusing region (“The Wave”). When noise from a transiting ship is included in the simulations, the spatiotemporal characteristics of the chorus change appreciably, in ways similar to changes observed during the experiment.

Evaluating the time to detect biological effects of ocean acidification and warming: an example using simulations of purple sea urchin settlement

Ocean acidification (OA) and ocean warming driven by climate change are important stressors for marine species and systems, but documenting and detecting their long-term impacts on biological responses outside of laboratory settings are challenging due to natural variability caused by complex processes and interactions. We used settlement of purple sea urchins Strongylocentrotus purpuratus in the Southern California Bight (USA) over 6 yr as an example data set to parameterize a simulation model for exploring the time needed to detect environmental effects on a biological response. A generalized linear model was used to describe an index of urchin settlement as functions of pH, sea surface temperature (SST), sea surface salinity (SSS), and spatio-temporal factors, demonstrating that settlement was negatively associated with pH (i.e. lower settlement at higher pH) and positively associated with SST and SSS. Monte Carlo simulations were developed from this base model under a variety of alternative scenarios to estimate the time to detect: (1) annual trends in pH and SST time series, (2) pH and SST effects on urchin settlement, and (3) annual trends in urchin settlement. Time to detect pH and SST effects was predominantly influenced by the underlying strength of the relationships and the model uncertainty. Time to detect annual trends in settlement was more sensitive to the severity of long-term OA and warming trends, which had cumulative (at times opposing) effects. This study highlights the variable time scales (2-60+ yr) that may be necessary to detect biological responses to OA and ocean warming and the sensitivity to different assumptions of the study system.

Statistical downscaling of coastal directional wave spectra using deep learning

The modelling of coastal Directional Wave Spectra (DWSs) often requires downscaling techniques integrating DWSs from open ocean boundaries. Dynamic downscaling methods reliant on numerical wave models are often computationally expensive. In coastal areas, wave dynamics are strongly influenced by the bathymetry and coastal morphology, implying that once the DWSs at the open ocean boundary are known, the DWSs at various locations along the coast are almost determined. This property can be utilized for statistical downscaling of coastal DWSs. This study presents a deep learning approach to compute coastal DWSs from open ocean DWSs. The performance of the proposed downscaling model was evaluated using both numerical wave model data and buoy data in the Southern California Bight. The results show that the deep learning approach can effectively and efficiently downscale coastal DWSs without relying on any predefined spectral shapes, thereby showing potential for coastal spectral wave climate studies.

Identification of DDT+ in Deep Ocean Sediment and Biota in the Southern California Bight

Identification of DDT+ in Deep Ocean Sediment and Biota in the Southern California Bight

Kelps may compensate for low nitrate availability by using regenerated forms of nitrogen, including urea and ammonium.

Nitrate, the form of nitrogen often associated with kelp growth, is typically low in summer during periods of high macroalgal growth. More ephemeral, regenerated forms of nitrogen, such as ammonium and urea, are much less studied as sources of nitrogen for kelps, despite the relatively high concentrations of regenerated nitrogen found in the Southern California Bight, where kelps are common. To assess how nitrogen uptake by kelps varies by species and nitrogen form in southern California, USA, we measured uptake rates of nitrate, ammonium, and urea by Macrocystis pyrifera and Eisenia arborea individuals from four regions characterized by differences in nitrogen availability-Orange County, San Pedro, eastern Santa Catalina Island, and western Santa Catalina Island-during the summers of 2021 and 2022. Seawater samples collected at each location showed that overall nitrogen availability was low, but ammonium and urea were often more abundant than nitrate. We also quantified the internal %nitrogen of each kelp blade collected, which was positively associated with ambient environmental nitrogen concentrations at the time of collection. We observed that both kelp species readily took up nitrate, ammonium, and urea, with M. pyrifera taking up nitrate and ammonium more efficiently than E. arborea. Urea uptake efficiency for both species increased as internal percent nitrogen decreased. Our results indicate that lesser-studied, more ephemeral forms of nitrogen can readily be taken up by these kelps, with possible upregulation of urea uptake as nitrogen availability declines.

A simple method for the quantification of amidic bioavailable dissolved organic nitrogen in seawater

AbstractA targeted method for the quantification of bioavailable amide N found in marine DON (bDON) is presented. The method utilizes mild acid hydrolysis to convert amide N found in proteins and N‐acetyl amino polysaccharides to primary amine containing products that are measured using a highly sensitive (nanomolar range and precision) fluorometric technique with addition of O‐phthaldialdehyde. We find amidic bDON concentrations ranging from 0.08 to 1.82 μM N within waters from the upper 300 m in the southern California Current, Southern California Bight, and subtropical North Pacific representing 15–33% of bulk DON concentrations. Bioassay experiments from the North Pacific revealed consumption of ~20% of the in situ bDON within 5 days. The method represents a simple and rapid tool for the quantification of bioavailable DON concentrations in seawater with improved analytical precision over traditional estimates of bulk DON concentrations.

Estimating whisker growth rates and foraging profiles for northern fur seal pups and adult females using stable isotope analysis

Changing ratios of stable carbon ( δ13C) and nitrogen ( δ15N) isotopes in consumer tissues reflect foraging patterns over time and can be used to inform studies on historically vulnerable populations. Northern fur seals (NFS; Callorhinus ursinus) at San Miguel Island (SMI), California, have been severely impacted by climate events such as El Niño and remain susceptible to future environmental disturbances, necessitating the need to further understand their foraging ecology. We analyzed sequential δ13C and δ15N values along single NFS whiskers sampled from five pups and five unrelated post-parturient females at SMI and reconstructed temporally aligned stable isotope profiles for each whisker. Pup profiles consistently showed increases in postnatal δ13C and δ15N values that likely stem from pup consumption of maternal milk and changes in maternal foraging locations after parturition. Adult female profiles indicated predominantly heterogeneous foraging strategies and minimal foraging outside the Southern California Bight. These findings provide initial characterizations of longitudinal, stable isotope-derived foraging data for NFS from SMI and offer insight into the inference of otariid foraging ecology with sequential δ13C and δ15N values.

Exempted and Research Deep-Set Fishing Trials for Swordfish, Xiphias gladius, in the Southern California Bight, 2017–21

Exempted and Research Deep-Set Fishing Trials for Swordfish, Xiphias gladius, in the Southern California Bight, 2017–21

Cross-shore transport and eddies promote large scale response to urban eutrophication

A key control on the magnitude of coastal eutrophication is the degree to which currents quickly transport nitrogen derived from human sources away from the coast to the open ocean before eutrophication develops. In the Southern California Bight (SCB), an upwelling-dominated eastern boundary current ecosystem, anthropogenic nitrogen inputs increase algal productivity and cause subsurface acidification and oxygen (O2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{2}$$\\end{document}) loss along the coast. However, the extent of anthropogenic influence on eutrophication beyond the coastal band, and the physical transport mechanisms and biogeochemical processes responsible for these effects are still poorly understood. Here, we use a submesoscale-resolving numerical model to document the detailed biogeochemical mass balance of nitrogen, carbon and oxygen, their physical transport, and effects on offshore habitats. Despite management of terrestrial nutrients that has occurred in the region over the last 20 years, coastal eutrophication continues to persist. The input of anthropogenic nutrients promote an increase in productivity, remineralization and respiration offshore, with recurrent O2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{2}$$\\end{document} loss and pH decline in a region located 30–90 km from the mainland. During 2013 to 2017, the spatially averaged 5-year loss rate across the Bight was 1.3 mmol m-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-3}$$\\end{document} O2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{2}$$\\end{document}, with some locations losing on average up to 14.2 mmol m-3\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{-3}$$\\end{document} O2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$_{2}$$\\end{document}. The magnitude of loss is greater than model uncertainty assessed from data-model comparisons and from quantification of intrinsic variability. This phenomenon persists for 4 to 6 months of the year over an area of 278,40 km2\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$^{2}$$\\end{document} (∼\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\sim$$\\end{document}30% of SCB area). These recurrent features of acidification and oxygen loss are associated with cross-shore transport of nutrients by eddies and plankton biomass and their accumulation and retention within persistent eddies offshore within the SCB.

Disentangling the History of Deep Ocean Disposal for DDT and Other Industrial Waste Off Southern California.

Ocean disposal of industrial waste from technical DDT [mainly 1,1'-(2,2,2-trichloroethane-1,1-diyl)bis(4-chlorobenzene), or 4,4'-DDT] manufacture occurred historically in the Southern California Bight. However, the paucity of historical records highlights uncertainties as to the mode, location, and timing of disposal or ongoing ecological effects of these wastes. This study combines sampling, chemical analysis, and numerical modeling of deep San Pedro Basin sediments revealing substantial DDT contamination that extends at least 25 km from the mainland. These findings narrate bulk DDT waste disposal to the offshore that peaked in the 1950s, prior to the onset of formal regulations; was agnostic to later-designated disposal sites; and has experienced sluggish transformation. Our findings further indicate an attenuating secondary source for the DDT daughter product, 1-chloro-4-[2,2-dichloro-1-(4-chlorophenyl)ethenyl]benzene (4,4'-DDE), which still deposits into deep San Pedro Basin sediments. While demonstrating the severity of DDT contamination to the region, these findings further define the burial potential of DDT wastes and inform the past, present, and future contamination potential that is needed to understand and predict ecological consequences. This work also points firmly to bulk, not containerized, disposal of DDT waste and to potential alternative contents of collocated waste.

Coastal Data Information Program: advances in measuring and modeling wave activity, climate, and extremes

ABSTRACT The Coastal Data Information Program (CDIP) provides wave data to the public in near real-time, maintains an operational wave model for the California coast, and is engaged in ocean wave research on a global scale. CDIP’s array of moored wave buoy stations are instrumented with Datawell Waveriders. CDIP’s recent work to improve monitoring capabilities for directional surface wave spectra, surface current, and temperature is described. This includes data quality assurance and control, real-time alerts, telemetry and dissemination information technology infrastructure and methodology, data visualization tools, and mooring and instrumentation innovation. CDIP’s goal is to maximize data reliability, availability, accuracy, and precision. Validation of commonly used wave models – operational, hindcast, and forecast – with CDIP data is automated to provide assessments of relative skills at a variety of coastal locations and wave conditions. Wave heights measured by the buoys typically exceed modeled heights during the most energetic events, such as hurricanes, nor’easters, and bomb cyclones. In a practical validation example, the global wave model used to drive CDIP’s California coastal wave forecasts was recently changed, based on comparisons against buoy data. Forecast performance has improved for swell events, which are a challenge to model accurately in the Southern California Bight.

Effect of ocean outfall discharge volume and dissolved inorganic nitrogen load on urban eutrophication outcomes in the Southern California Bight

Climate change is increasing drought severity worldwide. Ocean discharges of municipal wastewater are a target for potable water recycling. Potable water recycling would reduce wastewater volume; however, the effect on mass nitrogen loading is dependent on treatment. In cases where nitrogen mass loading is not altered or altered minimally, this practice has the potential to influence spatial patterns in coastal eutrophication. We apply a physical-biogeochemical numerical ocean model to understand the influence of nitrogen management and potable wastewater recycling on net primary productivity (NPP), pH, and oxygen. We model several theoretical management scenarios by combining dissolved inorganic nitrogen (DIN) reductions from 50 to 85% and recycling from 0 to 90%, applied to 19 generalized wastewater outfalls in the Southern California Bight. Under no recycling, NPP, acidification, and oxygen loss decline with DIN reductions, which simulated habitat volume expansion for pelagic calcifiers and aerobic taxa. Recycling scenarios under intermediate DIN reduction show patchier areas of pH and oxygen loss with steeper vertical declines relative to a “no recycling” scenario. These patches are diminished under 85% DIN reduction across all recycling levels, suggesting nitrogen management lowers eutrophication risk even with concentrated discharges. These findings represent a novel application of ocean numerical models to investigate the regional effects of idealized outfall management on eutrophication. Additional work is needed to investigate more realistic outfall-specific water recycling and nutrient management scenarios and to contextualize the benefit of these management actions, given accelerating acidification and hypoxia from climate change.

Dynamics of pH at Santa Catalina Island

The local expression of ocean acidification may depend on local oceanographic features in addition to global forcings. Our objective is to provide a baseline of pH behavior at Santa Catalina Island, situated within the unique oceanographic characteristics of the Southern California Bight, and to gain insight into ocean acidification at the island. Measurements of the upper water column (to 30-m depth) of pH, temperature, conductivity, chlorophyll and dissolved oxygen at Santa Catalina were made from a fixed mooring and by profiling the water column from a boat and on Self-Contained Underwater Breathing Apparatus (SCUBA). The average pH (8.095 at 18-m depth) was found to be higher than that reported off the nearby mainland and the Northern Channel Islands. The higher value is thought to result from both downwelling produced by internal waves as well as less upwelling at the island compared to other locations. Large modulations in pH at depth corresponded to advection of gradients by internal waves. Within the accuracy of the sensors there was no seasonal dependence detected at near-surface, nor a pH signal associated with the sub-surface chlorophyll and oxygen maxima. We conclude that marine life living at depths affected by internal waves experience significant variation in pH.

Fifty-Year Review of Effluent Discharge Trends to the Southern California Bight from Large Municipal Wastewater Treatment Facilities from 1971 to 2020

Historically, Southern California’s four largest publicly owned treatment works (POTWs) were major sources of contaminants in the Southern California Bight (SCB), home to over 22 million residents and diverse marine life. This study conducted a fifty-year review (1971–2020) to assess these facilities’ performance in protecting human and aquatic life. We analyzed effluent discharge flow, total annual mass emissions into SCB, and the rate of change of mass emissions in five ten-year timeframes. Results show a significant decrease in contaminant concentrations (43% to 99%) from 1971 to 2020, with most reaching historically low levels. Despite a growing population, effluent volume decreased by 23%, reflecting successful local water conservation and recycling efforts. Some constituents experienced temporary increases that later decreased. Overall, the decline in mass emissions for all constituents over fifty years highlights the POTWs’ efforts to treat effluent and reduce their impact on the SCB’s ecosystem.