Bottom Salinity Research Articles

Salinity regime of the northwestern Bering Sea shelf

The present study has examined the long-term variability of water mass properties and its relation to sea ice production (SIP) in the northwestern Bering Sea shelf using hydrographic and satellite data, and output from atmospheric reanalysis. As per our analysis of hydrographic data in the summer seasons, significant variability in bottom water temperature and salinity was noted. The temperature in 2018 showed particularly remarkable warming of the whole water column throughout the Gulf of Anadyr (GOA) and cold pool (CP) region. These oceanic parameters exhibited correlations with SIP in the Anadyr polynya, which is identified as one of the most active polynyas in the Northern Hemisphere. Specifically, there were more saline and colder water during high-SIP years, suggesting the influence of dense shelf water (DSW) that is formed during freezing processes; however, in low SIP years, bottom salinity has been observed to behave inversely, resulting in more saline water with lower SIP. We suggest that the influence of warm and saline water, originating from the Pacific, transported by ocean currents intensified by wintertime southerly winds in low-SIP years. The properties of bottom water in the semi-closed GOA are influenced by intrusion of the Pacific-originated water and DSW.

Comparison of habitat models in quantifying the spatio-temporal distribution of small yellow croaker (Larimichthys polyactis) in Haizhou Bay, China

Habitat models can be used to analyze the relationships between species distribution and environmental variables. Different models and response variables are usually selected to evaluate the habitat of species according to their ecological traits. In this study, six habitat models were compared to quantify the spatio-temporal distribution of small yellow croaker (Larimichthys polyactis) in Haizhou Bay, including Generalized Additive Model (GAM), K-Nearest Neighbor (KNN), Multivariate Adaptive Regression Splines (MARS), Generalized Boosted regression Model (GBM), Random forest (RF) and Artificial neural network (NN). We also compared the performance of habitat models with four types of response variables, i.e., (1) presence–absence, (2) habitat ranks, (3) probability and (4) abundance. Model performances were evaluated and assessed through cross validations. Principal component analysis (PCA) and coefficient of variation (CV) were applied to compare the performance of these habitat models. Results showed that machine learning algorithms were better than statistical methods. Random forest (RF) performed the best in identifying the spatio-temporal distribution of small yellow croakers based on PCA and CV analysis. The importance of explanatory variables varied with the type of response variables in habitat modeling. In addition, facing different response variables, species showed different response relationships to explanatory variables. This species was found mainly distributed in the central coastal areas of Haizhou Bay at depths between 10 and 30 m according to predicated distribution maps by optimal models. Bottom temperature, bottom salinity and prey had great impacts on spatial distributions of small yellow croaker. This study proposed a framework to evaluate the performance of different habitat models and response variables, which could be extended to other marine organisms and facilitate the application of habitat models. This study enhances our understanding of the underlying ecological mechanisms that drive spatial distribution of marine species.

Investigating Pseudo-nitzschia australis introduction to the Gulf of Maine with observations and models

In 2016, an unprecedented Pseudo-nitzschia australis bloom in the Gulf of Maine led to the first shellfishery closures due to domoic acid in the region's history. In this paper, potential introduction routes of P. australis are explored through observations, a hydrodynamic model, and a Lagrangian particle tracking model. Based on particle tracking experiments, the most likely source of P. australis to the Gulf of Maine was the Scotian Shelf. However, in 2016, connectivity between the Scotian Shelf and the bloom region was not significantly different from the other years between 2012 and 2019, nor were temperature conditions more favorable for P. australis growth. Observations indicated changes on the Scotian Shelf in 2016 preceded the introduction of P. australis: increased bottom salinity and decreased surface salinity. The increased bottom salinity on the shelf may be linked to anomalously saline water observed near the coast of Maine in 2016 via transport through Northeast Channel. The changes in upstream water mass properties may be related to the introduction of P. australis, and could be the result of either increased influence of the Labrador Current or increased outflow from the Gulf of St. Lawrence. The ultimate source of P. australis remains unknown, although the species has previously been observed in the eastern North Atlantic, and connectivity across the ocean is possible via a subpolar route. Continued and increased monitoring is warranted to track interannual Pseudo-nitzschia persistence in the Gulf of Maine, and sampling on the Scotian Shelf should be conducted to map upstream P. australis populations.

Examining Scale Dependent Environmental Effects on American Lobster (Homarus americanus) Spatial Distribution in a Changing Gulf of Maine

The Gulf of Maine (GOM) is a highly complex environment and previous studies have suggested the need to account for spatial nonstationarity in species distribution models (SDMs) for the American lobster (Homarus americanus). To explore impacts of spatial nonstationarity on species distribution, we compared models with the following three assumptions : (1) large-scale and stationary relationships between species distributions and environmental variables; (2) meso-scale models where estimated relationships differ between eastern and western GOM, and (3) finer-scale models where estimated relationships vary across eastern, central, and western regions of the GOM. The spatial scales used in these models were largely determined by the GOM coastal currents. Lobster data were sourced from the Maine-New Hampshire Inshore Bottom Trawl Survey from years 2000–2019. We considered spatial and environmental variables including latitude and longitude, bottom temperature, bottom salinity, distance from shore, and sediment grain size in the study. We forecasted distributions for the period 2028–2055 using each of these models under the Representative Concentration Pathway (RCP) 8.5 “business as usual” climate warming scenario. We found that the model with the third assumption (i.e., finest scale) performed best. This suggests that accounting for spatial nonstationarity in the GOM leads to improved distribution estimates. Large-scale models revealed a tendency to estimate global relationships that better represented a specific location within the study area, rather than estimating relationships appropriate across all spatial areas. Forecasted distributions revealed that the largest scale models tended to comparatively overestimate most season × sex × size group lobster abundances in western GOM, underestimate in the western portion of central GOM, and overestimate in the eastern portion of central GOM, with slightly less consistent and patchy trends amongst groups in eastern GOM. The differences between model estimates were greatest between the largest and finest scale models, suggesting that fine-scale models may be useful for capturing effects of unique dependencies that may operate at localized scales. We demonstrate how estimates of season-, sex-, and size- specific American lobster spatial distribution would vary based on the spatial scale assumption of nonstationarity in the GOM. This information may help develop appropriate local adaptation measures in a region that is susceptible to climate change.

Benthic foraminifera as indicators of water renewal in Bertioga estuarine channel (SP, Brazil).

We use benthic foraminifera and hydrography as environmental indicators to characterize the estuarine channel of Bertioga (SP, Brazil). Salinity and temperature measured in four fixed stations (F1-F4) influence differentiation related to species habitat, fluvial discharge, and environmental quality. Tidal oscillation during neap and spring tides changes the Hansen and Rattray stratification-circulation classification from highly stratified (type 2b) to weakly stratified (type 2a). In the northern entrance close to Itapanhaú River, F1 is highly stratified environment (type 2b), with significant difference between bottom and surface salinities, presenting Pararotalia sp., Pseudononion atlanticum, Hanzawaia boueana, and Quinqueloculina lamarckiana as indicator species. F2 in summer is highly stratified (type 2b) with large difference between surface and bottom salinity, whereas in winter is partially mixed weakly stratified (type 2a) with small difference between salinities and the agglutinated Arenoparella mexicana, Haplophragmoides wilberti, and Trochammina inflata thrives. In summer and winter, F3 is partially mixed weakly stratified (type 2a) with A. mexicana, H. wilberti, Trochammina inflata, Ammobaculites sp., and Ammotium salsum. In the southern entrance, F4 in the winter, is partially mixed, weakly stratified system (type 2a) that changes to highly stratified (type 2b) with large difference between salinities. This indicated low influence of fluvial freshwater and high influence of saline and polluted waters from Santos Channel where Arenoparella mexicana, Gaudryina exilis, and Ammotium cassis inhabits. Presence of Quinqueloculina spp. and Triloculina sp. indicates high saline influence, and presence of Ammonia tepida and Elphidium spp. indicates mixohaline environments which salinity dilution occurs.

Thermohaline conditions and circulation in the Gulf of Thailand during the northeast monsoon

Hydrographic data from the Gulf of Thailand (GoT) reveal two bottom saline water areas (BSWA1 and BSWA2) with salinity (S) ≥ 33.0 psu and where the water columns are highly stratified during December–January. Observational results from a seafloor acoustic doppler current profiler verify that the current is mainly dominated by a barotropic current. The monthly average sea-level-anomaly (SLA) and barotropic current vector maps confirm a cyclonic circulation in the northern GoT during November–January. The BSWA1 is located near the center of the cyclonic circulation. The BSWA1 and cyclonic circulation simultaneously enter their weakening phases, which suggests that the cyclonic circulation convergence plays a key role in maintaining the BSWA1. The BSWA2 occurs as a narrow strip concentrated along the eastern slope of the basin in the southern GoT during December–January. The southern GoT also has higher SLA and forms a congregation area of diluted water with S < 31.0 psu that enters from the north and south of this region during December–January.

Predicting current and future global distribution of black rockfish (Sebastes schlegelii) under changing climate

Changes in the marine environment, particularly climate change, can have large effects on the distribution patterns of various marine species, and alter the biodiversity, structure and functions of the affected ecosystems. Species distribution models (SDM) are tools often used to link species’ ecological niches with their environment. We applied SDM to investigate the effects of five biologically relevant climatic variables from multiple databases, including bottom temperature, bottom salinity, current velocity, depth and primary productivity, on habitat suitability of Sebastes schlegelii in the marine waters of China, Korea and Japan. Nine individual SDM and an ensemble model were used to predict the current and future distribution of S. schlegelii under alternative climate change scenarios (Representative Concentration Pathways, RCP). Results indicated that the ensemble model produced more accurate projections than any individual model. Among the environmental variables investigated, bottom temperature was the most important in determining the range of S. schlegelii. Its current distribution demonstrated that suitable habitat for S. schlegelii was mostly concentrated in the Bohai Sea, coastal areas of the central and northern Yellow Sea, and in the Sea of Japan. Negative effects from climate change on the distribution patterns of S. schlegelii were predicted to lead to varying degrees of habitat reduction, with highest estimate of 45% occurring under RCP8.5 at the end of 2100. Our results illustrate the potential effects of climate change on the future distribution of S. schlegelii populations and can assist with implementing adaptive management measures of this species.

The Irish Atlantic CoCliME case study configuration, validation and application of a downscaled ROMS ocean climate model off SW Ireland

This ocean numerical modelling study is focused on the shelf waters off southwest Ireland. Outputs from the model are used to provide scientists and policy makers with climate change information related to oceanic conditions that influence harmful algal blooms in the region. Four simulations were developed to include a 20-year hindcast simulation (1997–2016), a 31-year historical simulation (1975–2005), and two 30-year future climate model projections (2006–2035) for the representative carbon pathways (RCP) 4.5 and 8.5 scenarios. We evaluated the hindcast model skill by comparing the simulation outputs with measured observational data and calculated statistics such as the bias, and the root mean square error (RMSE). The observations consisted of satellite sea surface temperature and, CTD temperature and salinity profile data collected in Irish waters. The sea surface temperature RMSEs between the 20-year hindcast simulation and the satellite data were ca. 0.50°C. A minimum RMSE of ca. 0.20°C was recorded in December 2015. The basin-averaged bias (difference) and RMSE for near bottom temperature between the RCP 4.5 and the historical simulation was 0.14°C and 0.27°C respectively. The sea surface temperature anomaly pattern for the RCP 8.5 shows warming across the whole model domain. The sea surface salinity and near bottom salinity climate simulation anomaly maps indicate a general freshening on the southwest Irish shelf. A change of ca. 0.2 sea surface salinity and near bottom salinity was observed. The RCP 8.5 simulation shows the highest current velocities when compared to other simulations. The Irish coastal current pattern under the RCP 8.5 scenario appears very intense and well defined with a velocity > 20 [cm/s].

Identification of benthic egg masses and spawning grounds in commercial squid in the English Channel and Celtic Sea: Loligo vulgaris vs L. forbesii

European squid, Loligo vulgaris and veined squid, Loligo forbesii have nearly coinciding distributions in the northeast Atlantic, a similar reproductive seasonality, and largely overlapping depth ranges of spawning grounds. There are no unambiguous criteria to distinguish between egg masses of both species. This pioneering study was focused on the Celtic Sea and western part of the English Channel and combined both research survey data and observations by recreational divers (“citizen science”). Loligo vulgaris was found to reproduce there in late winter – spring; distribution of egg masses coincided with a bottom temperature range of 8.5−10 °C and bottom salinities of 35–35.5 psu. No L.forbesii egg masses were found across the study area though they are known from literature to occur in deeper areas further west. Based on original material and data from the literature, we provide a guideline to distinguish between egg masses of both squids based on egg size and embryonic stage as a tool to map species-specific spawning grounds, towards improved understanding of population structure, migrations and development of fisheries management measures.

Projections of physical conditions in the Gulf of Maine in 2050

The Gulf of Maine (GoM) is currently experiencing its warmest period in the instrumental record. Two high-resolution numerical ocean models were used to downscale global climate projections to produce four estimates of ocean physical properties in the GoM in 2050 for the “business as usual” carbon emission scenario. All simulations project increases in the GoM mean sea surface temperature (of 1.1 °C–2.4 °C) and bottom temperature (of 1.5 °C–2.1 °C). In terms of mean vertical structure, all simulations project temperature increases throughout the water column (surface-to-bottom changes of 0.2 °C–0.5 °C). The GoM volume-averaged changes in temperature range from 1.5 °C to 2.3 °C. Translated to rates, the sea surface temperature projections are all greater than the observed 100-year rate, with two projections below and two above the observed 1982–2013 rate. Sea surface salinity changes are more variable, with three of four simulations projecting decreases. Bottom salinity changes vary spatially and between projections, with three simulations projecting varying increases in deeper waters but decreases in shallower zones and one simulation projecting a salinity increase in all bottom waters. In terms of mean vertical structure, salinity structure varies, with two simulations projecting surface decreases that switch sign with depth and two projecting increases throughout the (subsurface) water column. Three simulations show a difference between coastal and deeper waters whereby the coastal zone is projected to be systematically fresher than deeper waters, by as much as 0.2 g kg–1. Stratification, 50 m to surface, is projected to increase in all simulations, with rates ranging from 0.003 to 0.006 kg m–4 century–1 which are lower than the observed change on the Scotian Shelf. The results from these simulations can be used to assess potential acidification and ecosystem changes in the GoM.

Mean and Seasonal Circulation of the Eastern Chukchi Sea From Moored Timeseries in 2013–2014

AbstractFrom late‐summer 2013 to late‐summer 2014, a total of 20 moorings were maintained on the eastern Chukchi Sea shelf as part of five independent field programs. This provided the opportunity to analyze an extensive set of timeseries to obtain a broad view of the mean and seasonally varying hydrography and circulation over the course of the year. Year‐long mean bottom temperatures reflected the presence of the strong coastal circulation pathway, while mean bottom salinities were influenced by polynya/lead activity along the coast. The timing of the warm water appearance in spring/summer is linked to advection along the various flow pathways. The timing of the cold water appearance in fall/winter was not reflective of advection nor related to the time of freeze‐up. Near the latitude of Barrow Canyon, the cold water was accompanied by freshening. A one‐dimensional mixed‐layer model demonstrates that wind mixing, due to synoptic storms, overturns the water column resulting in the appearance of the cold water. The loitering pack ice in the region, together with warm southerly winds, melted ice and provided an intermittent source of fresh water that was mixed to depth according to the model. Farther north, the ambient stratification prohibits wind‐driven overturning, hence the cold water arrives from the south. The circulation during the warm and cold months of the year is different in both strength and pattern. Our study highlights the multitude of factors involved in setting the seasonal cycle of hydrography and circulation on the Chukchi shelf.

Massive oyster kill in Galveston Bay caused by prolonged low-salinity exposure after Hurricane Harvey

Extreme precipitation events are projected to occur more frequently under a warming climate, posing increasing threats to coastal ecosystems. Hurricane Harvey (2017), the wettest tropical cyclone in the U.S. history causing a 1000-year flood in the Houston metropolitan area, provides an opportunity to study the response of coastal ecosystems to extreme events. As sessile, epibenthic filter-feeding organisms, oysters are inherently sensitive to changes in environmental and water quality conditions, making them a good indicator for ecosystem health. Oyster measurements at 130 sites in Galveston Bay show that the mean oyster mortality drastically increased from 11% before Harvey to 48% after Harvey. Post-Harvey oyster mortality exhibited large spatial variability and was up to 100% at some major reef complexes. For all the oyster sampling sites, brown shells were dominant, while black shells indicating mud burial were rare. Considering the little impact from sediment deposit, we hypothesized the low-salinity exposure as the main cause for the massive oyster kill. The hypothesis is supported by a multidisciplinary (biological-geological-physical) analysis of bay-wide oyster mortality and sediment-core data and results of a previously-validated high-resolution hydrodynamic model. Oyster mortality was found to be significantly and positively correlated with the bottom low-salinity exposure time (duration of bottom salinity continuously less than 5 PSU), while there was no significant relationship with the thickness of storm-induced sediment deposit. The physiological aspects for the impact of low-salinity exposure, the underlying physical mechanisms for the prolonged salinity recovery, and wider implications of oyster kill in Galveston Bay in the context of global oyster reef conditions were discussed. The worldwide reported oyster kills following extreme weather events suggest additional pressure posed by future climate on the native coastal oyster reefs that are already at the brink of functional extinction due to centuries of resource extraction and coastal habitat degradation.

Stygobitic crustaceans in an anchialine cave with an archeological heritage at Vodeni Rat (Island of Sveti Klement, Hvar, Croatia)

A group of four amphoras found in the anchialine cave of Vodeni Rat in the Croatian island of Sveti Klement, on a rocky ledge at 24 m water depth, indicates that a freshwater source was exploited at the bottom of this karstic pit by islanders and/or passing-by sailors from the late Roman Republican Period to the Early Medieval Period. In other words, prior to the 4th–7th century CE, Vodeni Rat was not an anchialine cave but a Pleistocene karstic pit with a freshwater pool at the bottom. Seawater started to infiltrate this cavity via newly opened fissures in the surrounding limestone after the 4th–7th century CE, as the archeometric age of the youngest intact amphora found down in the pit suggests. At present, the 29-m-deep water body is stratified, with a saline water layer at the bottom, a freshwater layer at the surface, and a transitional brackish water layer in the middle. Our study of the physical and biological characteristics of the cave water column revealed diversified and highly partitioned populations of stygobitic crustaceans including ostracods, copepods, amphipods, and thermosbaenaceans. Some species known to be exclusively endemic of highly saline habitats were collected from the bottom saline layer, whereas others known to be of freshwater origin were found in the upper water layer. This suggests that the freshwater dwellers were already present in this cave prior to the early medieval event that caused seepage of seawater into this karstic pit.

Spatial distribution of epifaunal communities in the Hudson Bay system

The seasonal sea ice cover and the massive influx of river runoff into the Hudson Bay System (HBS) of the Canadian Arctic are critical factors influencing biological production and, ultimately, the dynamics and structure of benthic communities in the region. This study provides the most recent survey of epibenthic communities in Hudson Bay and Hudson Strait and explores their relationships with environmental variables, including mean annual primary production and particulate organic carbon in surface water, bottom oceanographic variables, and substrate type. Epibenthic trawl samples were collected at 46 stations, with a total of 380 epibenthic taxa identified, representing 71% of the estimated taxa within the system. Three communities were defined based on biomass and taxonomic composition. Ordination analyses showed them to be associated primarily with substrate type, salinity, and annual primary production. A first community, associated with coarse substrate, was distributed along the coastlines and near the river mouths. This community was characterized by the lowest density and taxonomic richness and the highest biomass of filter and suspension feeders. A second community, composed mostly of deposit feeders and small abundant epibenthic organisms, was associated with soft substrate and distributed in the deepest waters. A third community, associated with mixed substrate and mostly located near polynyas, was characterized by high diversity and biomass, with no clearly dominant taxon. The overall analysis indicated that bottom salinity and surface-water particulate organic carbon content were the main environmental drivers of these epibenthic community patterns. In the face of climate change, projections of increased river inflow and a longer open water season for the HBS could have major impacts on these epibenthic communities, emphasizing a need to continually improve our ability to evaluate and predict shifts in epibenthic richness and distribution.

Using Tracer Variance Decay to Quantify Variability of Salinity Mixing in the Hudson River Estuary

AbstractThe salinity structure in an estuary is controlled by time‐dependent mixing processes. However, the locations and temporal variability of where significant mixing occurs is not well‐understood. Here we utilize a tracer variance approach to demonstrate the spatial and temporal structure of salinity mixing in the Hudson River Estuary. We run a 4‐month hydrodynamic simulation of the tides, currents, and salinity that captures the spring‐neap tidal variability as well as wind‐driven and freshwater flow events. On a spring‐neap time scale, salinity variance dissipation (mixing) occurs predominantly during the transition from neap to spring tides. On a tidal time scale, 60% of the salinity variance dissipation occurs during ebb tides and 40% during flood tides. Spatially, mixing during ebbs occurs primarily where lateral bottom salinity fronts intersect the bed at the transition from the main channel to adjacent shoals. During ebbs, these lateral fronts form seaward of constrictions located at multiple locations along the estuary. During floods, mixing is generated by a shear layer elevated in the water column at the top of the mixed bottom boundary layer, where variations in the along channel density gradients locally enhance the baroclinic pressure gradient leading to stronger vertical shear and more mixing. For both ebb and flood, the mixing occurs at the location of overlap of strong vertical stratification and eddy diffusivity, not at the maximum of either of those quantities. This understanding lends a new insight to the spatial and time dependence of the estuarine salinity structure.

Substrate-dependent fish have shifted less in distribution under climate change

Analyses of the impacts of climate change on fish species have primarily considered dynamic oceanographic variables that are the output of predictive models, yet fish species distributions are determined by much more than just variables such as ocean temperature. Functionally diverse species are differentially influenced by oceanographic as well as physiographic variables such as bottom substrate, thereby influencing their ability to shift distributions. Here, we show that fish species distributions that are more associated with bottom substrate than other dynamic environmental variables have shifted significantly less over the last 30 years than species whose distributions are associated with bottom salinity. Correspondingly, species whose distributions are primarily determined by bottom temperature or ocean salinity have shifted their mean centroid and southern and northern range boundaries significantly more than species whose distributions are determined by substrate or depth. The influence of oceanographic versus static variables differs by species functional group, as benthic species distributions are more associated with substrate and they have shifted significantly less than pelagic species whose distributions are primarily associated with ocean temperatures. In conclusion, benthic fish, that are more influenced by substrate, may prove much less likely to shift distributions under future climate change.

Thin layers of phytoplankton and harmful algae events in a coastal upwelling system

We combined time-series from a monitoring program and specific field observations in order to: 1) describe the characteristics of thin layers of phytoplankton (TLP) in the Galician Rías Baixas (NW Iberian Peninsula), 2) investigate the relationship between spatially-extended TLP (se-TLP) events and environmental variables, and 3) analyze the relationship between TLP occurrence and cell densities of toxin-producing harmful algal bloom (THAB) species. Between 2012 and 2015, a total of 118 TLP were detected in any of the 39 stations weekly sampled in the study area in the frame of the monitoring program, which represents a frequency of occurrence of 2%. Most TLP (84%) were detected between May-August and located slightly below shallow pycnoclines. Eight se-TLP events, i.e., when at least five stations were simultaneously influenced by the occurrence of a TLP, were identified during the period 2012–2015. Six out eight of these events happened during summer-upwelling conditions, and according to a Principal Component Analysis (PCA) they were positively correlated with thermal stratification. The other two events, detected during spring-downwelling, were positively correlated with haline stratification. A logistic regression model including six variables (surface temperature, relative humidity, bottom salinity, a proxy for the intrusion of the Miño River plume into Ría de Vigo, Miño River flow, and bottom temperature) explained 42% of the variance in the occurrence of se-TLP events. The temporal persistence of TLP events was confirmed by observations carried out during specific cruises, which showed how TLP were formed and disappeared over short periods of time in response to changes in vertical mixing. TLP were more frequently observed in Ría de Pontevedra, which was characterized by longer harvesting closures due to detection of Dinophysis toxins (DSP: diarrhetic shellfish poisoning toxins and pectenotoxins) above regulatory levels. About 25% of the TLP detected in the Rías Baixas were associated with increased cell densities of potentially toxic Pseudo-nitzschia species (ASP: amnesic shellfish poisoning) and of D. acuminata. These results suggest that a relationship may be established between the occurrence of TLP and the growth and/or accumulation of the main THAB species in the Galician Rías Baixas.

Seabirds, environmental features and the Argentine anchovy Engraulis anchoita in the southwestern Atlantic Ocean

As top predators in marine environments, seabirds frequently respond to the presence of their main prey, and both predators and prey are usually associated with specific environmental features. We investigated the variability in the presence and density of flying seabirds (mostly Procellariiformes and Charadriiformes) and Magellanic penguinsSpheniscus magellanicusalong the southwestern (SW) Atlantic continental shelf. Five acoustic assessment surveys were conducted to determine the biomass of the Argentine anchovyEngraulis anchoita, and seabird counts and the collection of oceanographic data were conducted simultaneously with the surveys between June and October 2010. Generalized linear mixed models (GLMMs) were fitted to test the effect of anchovy density and environmental variables on seabird density. Sea surface temperature was significant for the presence of flying seabirds. Bottom water temperature and anchovy density were key variables affecting the presence and density of penguins, while bottom water salinity was also important for penguin presence. Based on Akaike’s information criterion, the most important factor explaining the density of flying seabirds was the difference between surface and bottom salinity (ΔS), while for penguin density, the most important factor was anchovy density. These results highlight that the subtropical shelf front in the SW Atlantic Ocean is a key feature influencing the aggregation of flying seabirds and confirm the close association of penguins and anchovies. Bottom water intrusion, originating from the sub-Antarctic shelf water, is an important factor explaining the presence of penguins, which tend to aggregate in areas with high anchovy densities on the SW Atlantic continental shelf.

Estuarine Forecasts at Daily Weather to Subseasonal Time Scales

AbstractMost present forecast systems for estuaries predict conditions for only a few days into the future. However, there are many reasons to expect that skillful estuarine forecasts are possible for longer time periods, including increasingly skillful extended atmospheric forecasts, the potential for lasting impacts of atmospheric forcing on estuarine conditions, and the predictability of tidal cycles. In this study, we test whether skillful estuarine forecasts are possible for up to 35 days into the future by combining an estuarine model of Chesapeake Bay with 35‐day atmospheric forecasts from an operational weather model. When compared with both a hindcast simulation from the same estuarine model and with observations, the estuarine forecasts for surface water temperature are skillful up to about 2 weeks into the future, and the forecasts for bottom temperature, surface and bottom salinity, and density stratification are skillful for all or the majority of the forecast period. Bottom oxygen forecasts are skillful when compared to the model hindcast, but not when compared with observations. We also find that skill for all variables in the estuary can be improved by taking the mean of multiple estuarine forecasts driven by an ensemble of atmospheric forecasts. Finally, we examine the forecasts in detail using two case studies of extreme events, and we discuss opportunities for improving the forecast skill.

Summer Changes in Water Mass Characteristics and Vertical Thermohaline Structure in the Eastern Chukchi Sea, 1974–2017

Hydrographic data from the World Ocean Database 2013 and the Chinese National Arctic Research Expedition were used to investigate the summertime changes in the eastern Chukchi Sea from 1974 to 2017. Owing to the Pacific inflow and timing of the sea ice retreat, water masses and vertical thermohaline structures in the eastern Chukchi Sea have changed but with regional differences. The entire eastern Chukchi Sea warmed up with significant temperature increase in the central shelf; however, the surface and bottom salinity in the southern, central, and northern shelves exhibited different trends. The northward extension of the Pacific Summer Water after 1997 influenced the summer hydrography significantly. Moreover, the data reveal changes in the characteristics of various water masses. Both Bering Summer Water (BSW) and Pacific Winter Water in the deeper layer became saltier, whereas the Alaskan Coastal Water in the upper layer became fresher after 1997. The previous definition of the BSW should be modified to include the warming water mass in the southern Chukchi Sea in the more recent years. Furthermore, the vertical thermohaline structure over the Chukchi shelves experienced considerable changes in its characteristics due to the combined effects of the Pacific inflow and surface forcing.