Freshwater Outflow Research Articles

Arctic freshwater outflow suppressed Nordic Seas overturning and oceanic heat transport during the Last Interglacial

The Last Interglacial period (LIG) was characterized by a long-term Arctic atmospheric warming above the preindustrial level. The LIG thus provides a case study of Arctic feedback mechanisms of the cryosphere-ocean circulation-climate system under warm climatic conditions. Previous studies suggested a delay in the LIG peak warming in the North Atlantic compared to the Southern Ocean and evoked the possibility of southward extension of Arctic sea ice to the southern Norwegian Sea during the early LIG. Here we compile new and published proxy data on past changes in sea ice distribution, sea surface temperature and salinity, deep ocean convection, and meltwater sources based on well-dated records from the Norwegian Sea. Our data suggest that southward outflow of Arctic freshwater supressed Nordic Seas deep-water formation and northward oceanic heat transport during the early LIG. These findings showcase the complex feedback interactions between a warming climate, sea ice, ocean circulation and regional climate.

Long-term observations of the turbid outflow plume from the Russian River, California

Understanding the mechanisms that spread freshwater away from small river systems and form turbid, low-salinity coastal plumes is crucial for assessing water quality in coastal waters. We present an analysis of 15 years (January 2004 to December 2018) of daily MODIS Aqua satellite data and in situ instrument data on the turbid freshwater plume that forms off the Russian River (California, USA), a prototypical Mediterranean-climate, small mountainous river system (SMRS). We present per-pixel statistical metrics and regression analyses to identify and quantify the controls on the extent and configuration of the plume exerted by river discharge, waves, winds, and tides. While freshwater outflow exhibits a persistent signal in nearshore waters, a large-scale plume only extends offshore into coastal waters during high river flow, when plume turbidity can be detected more than 10 km offshore from the river mouth. Our results show times when wave radiation stress exceeds outflow inertia, confining the plume within the surf zone and leading to an absence of detectable plume turbidity in coastal waters. Although tidal currents significantly influence the plume near the inlet, wind forcing is the primary control on plume shape and extent in coastal waters, deflecting the turbid outflow more than 30 km upcoast or downcoast of the river mouth with respective wind directions. Coriolis forcing is also significant and observed most clearly during periods of high river discharge and low wind forcing. In addition to introducing novel remote sensing methodology for SMRS plume analyses, these findings highlight the complex interplay of forcing related to tides, river discharge, winds, and waves in shaping the behavior of SMRS plumes. New insights include the impact of tides on larger discharges, the role of Coriolis forcing in SMRS plumes, and the effect of cross-shore winds on plume compression. Further, by considering the Russian River as a model for SMRS, this study can be used to ground-truth existing numerical models of small river plumes and to contribute to understanding critical for managing coastal water quality and nearshore ecosystems.

Daily variability of estuary-shelf exchange at the Lagoa dos Patos’s mouth

The Lagoa dos Patos is one of the largest coastal lagoons in the world, covering an area of 10,000 km2, with a drainage basin of 200,000 km2, with an average river flow of around 5000 m3/s. The lagoon is of the choked type, and exchanges between the lagoon and the ocean occur through a narrow section with nealy 500 m wide. The hydrodynamics during periods of low river flow are mainly controlled by wind patterns, which can either favor the inflow or outflow of water from the lagoon. A field experiment was conducted to investigate the daily variability of lagoon-ocean exchanges during a period of low flow, fortuitously capturing an extreme condition with a 1 % probability. Flow and salinity data were monitored daily near the lagoon's mouth for one month (from February 23rd to March 23rd, 2020). Additionally, a hydrological balance was calculated using data on river flow from the lagoon's main tributaries (Guaíba river system, Rio Camaquã, and São Gonçalo Channel) and lagoon-atmosphere exchanges through precipitation and evaporation. The average results obtained from direct measurements (∼ 1000 m3/s) agree reasonably well with the data obtained from the hydrological balance (∼850 m3/s), especially considering the limitations of both approaches. It was identified that direct precipitation events over the lagoon can produce rapid responses in the freshwater outflow from the lagoon, often exceeding the values of river flow contribution.

Unraveling the environmental influence on inter-annual fishery yield in a small-scale gillnet fishery under Rio de la Plata influence, South America

The study focuses on understanding the influence of the environment on the relative yields of a gill net fishery operating in San Antonio Cape, Argentina. The research was conducted during five austral warm periods covering spring to summer from 2008 to 2013, aiming to assess the potential impacts of climate change on small-scale fisheries. The hierarchical species composition of 563 fishery landings encompasses 41 registered species. Among these species, Cynoscion guatucupa, Micropogonias furnieri, Mustelus schmitti, and Parona signata were found to be dominant. The species composition showed a similarity ranging from 36.22% to 57.59%, with significant differences observed among most of the warm seasons. The Multivariate Regression Tree analysis showed that the temperature was the variable with the greatest influence on the composition of the landings. During warmer conditions (>20.4 °C), the location of the draft became a significant factor, while variation in turbidity played a role in the northern zone. In colder warm conditions (<20.4 °C), the composition differed based on the volumes of Río de la Plata discharge. Specifically, the freshwater outflow was further differentiated by depth (∼9 m) at lower volumes (<20 649 m3/s) and by turbidity at intermediate values (20 649–22 908 m3/s). The vulnerability of the fishery was explained by a combination of specific ranges of various stressors. These stressors led to significant variations in the composition of fishery landings, ultimately affecting the economic income of the fishermen. By understanding the relationship between environmental factors and fishery yields, this study provides valuable insights into anticipating the potential impacts of climate change on small-scale, vulnerable fisheries.

Identifying minimum freshwater habitat conditions for an endangered fish using life cycle analysis

AbstractIdentifying the most important factors affecting population growth in animal life cycles is an important tool of species conservation. Delta Smelt (Hypomesus transpacificus), an annual fish endemic to the San Francisco Estuary in California (USA), has been provided legal protection since 1993 but 30 years later exists in a conservation‐reliant state on the brink of extinction. Despite considerable controversies about what factors are most responsible for the species' decline, no population growth rate sensitivity comparisons between the most important factors regulating growth have been done. Nor has anyone attempted to quantitatively identify habitat conditions needed to support positive population growth. We developed a set of stage‐structured population models to link habitat indices regulating recruitment of new generations of fish as they metamorphosed into juveniles and the subsequent survival of those fish over several seasons until they reached adulthood. These models are used to quantify drivers of growth rate variation over 30 years. Several complimentary sensitivity analyses indicated freshwater outflow to the estuary during summer had the largest potential to change population growth. Multiple habitat metrics (e.g., food availability, temperature) influencing recruitment and life stage specific survival rates across different seasons interacted in nonlinear ways to determine habitat conditions and water management targets associated with positive population growth. We discuss the implications for freshwater management, Delta Smelt conservation, and the challenges climate change poses for co‐implementation of these two societal priorities.

Whole-genome sequence of Periconia sp. strain TS-2, an ascomycete fungus isolated from a freshwater outflow and capable of Mn(II) oxidation.

Members of the genus Periconia are commonly found as plant-associated filamentous fungi. Here, the first draft genome sequence of a new Periconia strain, TS-2, that was isolated from freshwater outflow sediment and possesses the ability to oxidize dissolved Mn(II), was obtained and has an estimated size of 40.7 Mb.

Influence of Land Surface Temperature and Rainfall on Surface Water Change: An Innovative Machine Learning Approach

The largest reservoir of drinkable water on Earth is surface water. It is crucial for maintaining ecosystems and enabling people to adapt to diverse climate changes. Despite surface freshwater is essential for life, the current research shows a striking lack of understanding in its spatial and temporal dynamics of variations in outflow and storage across a sizable country: India. Numerous restrictions apply to current research, including the use of insufficient machine learning techniques and limited data series. This work uses cutting-edge and SOTA-method to use the available data and machine learning to accurately understand spatial and temporal dynamics of variations in surface freshwater outflow and storage using extended data series. The authors did the examination of thematic maps produced using ArcMap 10.8 from June’2005 to June’2020 using JRC dataset to track changes in the intensity of surface water. Google Earth Engine in Python API has been devised to detect changes in surface water levels and quantifying shifting map trends. Raster image viewing, editing, and calculation are done with ArcMap. For determining the relationship between declines in Surface water levels, changes in rainfall intensity and land surface temperature, variables were averaged over 13 rivers for 15 years. The change in surface water is reliant on independent variables of change in land surface temperature and rainfall intensity. The authors use the correlation between these parameters to achieve an average R-squared adjusted value of 0.402. The study's findings contribute to a better understanding of the matter and can be used across the world.

Satellite telemetry reveals habitat selection decisions by black oystercatchers across seasonal, diel, and tidal cycles.

Habitat use of indicator species is used to prioritize management activities. However, habitat use can vary temporally in response to changes in predation risk and foraging rewards. We deployed satellite tags on 20 black oystercatchers (Haematopus bachmani) in four regions of British Columbia, Canada, to examine habitat use and selection decisions across seasonal, diel and tidal cycles. We characterized the shoreline in each region and used GLMMs to investigate how habitat characteristics influenced shoreline use by tracked birds. For individuals, we estimated home range size and the frequency key features of the shoreline were re-visited. Black oystercatchers generally made greater-than-expected use of rocky islets and shoreline with freshwater outflows, less tree cover and greater intertidal area. However, while black oystercatchers preferred islets and shoreline with less tree cover at most/all time periods, they only exhibited preferences for greater intertidal area during low tides, and preferences for shoreline with freshwater outflows during the nonbreeding season, day, and high tides. Individual home ranges, on average, contained 46 km of shoreline (range: 12-156 km) and individuals used 10.4 km (range: 6.7-13.9 km). Individuals made greater use of larger islets with less tree cover that were closer to outflows, and greater use of outflows associated with larger streams, greater intertidal areas and gravel substrates. Black oystercatchers' habitat preferences likely reduce predation risk (rocky islets and shoreline with less tree cover) and increase foraging rewards (shoreline with freshwater outflows, greater intertidal area, and gravel substrates). However, habitat preferences appear sensitive to constraints on movement in the breeding season and changes in foraging rewards across the diel and tidal cycle, highlighting the importance of examining habitat use at multiple temporal scales. Black oystercatchers are considered indicators of rocky intertidal health; therefore, critical habitat is expected to be important for a suite of wildlife dependent on safe and productive coastline.

Impact of a Tropical Cyclone on Terrestrial Inputs and Bio-Optical Properties in Princess Charlotte Bay (Great Barrier Reef Lagoon)

In January 2013, tropical cyclone Oswald caused widespread flooding in the North-East coast of Australia, and large and highly episodic inputs into Princess Charlotte Bay (PCB, northern Great Barrier Reef). Freshwater outflows from the Normanby and Kennedy rivers, the two main rivers draining the adjacent catchments, resulted in drastic changes in physical, biogeochemical and optical properties within PCB. On 31 January, 2 days after the peak riverine discharge from the Normanby river, nutrients and dissolved organic matter contents peaked under the influence of large outflows from the Kennedy river into the western section of the bay (5.8 μM for dissolved inorganic nitrogen, 6.9 g m−3 for dissolved organic carbon and 6.1 m−1 for the colored dissolved organic matter absorption coefficient at 412 nm). In the eastern section of the bay, the situation appeared more ‘mixed’, with a suspended solids concentration reaching 23.1 g m−3 close to the Normanby river mouth. The main phytoplankton bloom occurred in the transition zone between the Kennedy and Normanby flood plumes, and was dominated by diatoms with a chlorophyll a concentration reaching 14.6 mg m−3. This study highlights the need to better describe the critical spatial and temporal scales of variability of key biogeochemical and optical properties after a major flood event. The data collected is key to improve the accuracy of ocean color remote sensing algorithms and regional biogeochemical budgets following highly episodic inputs.

Freshwater Transport Over the Northeast Greenland Shelf in Fram Strait

AbstractWe assess the contribution of flow over the Northeast Greenland Shelf (NEGS) to the total freshwater transport (FWT) through the Fram Strait. We analyze available observations since 2013 consisting of hydrographic sections, new mooring data, and surface‐geostrophic velocity and ERA5 winds. We provide first seasonal estimates of the FWT over the entire shelf and find a net southward FWT between 47 and 56 mSv (reference salinity 34.9), or ∼40%–45% of the total FWT through the Fram Strait. In summer, the long‐term mean FWT east of 8°W cumulates to 27 mSv southward, there is no trend toward increasing FWT, and new mooring data show a large seasonal cycle. We show that the NEGS contributes significantly to the Arctic freshwater outflow, however, uncertainty is high. To close the Arctic freshwater budget and provide better FWT estimates to the North Atlantic, sustained year‐round observations on the NEGS are essential.

Community Patterns and Environmental Associations for the Early Life Stages of Fishes in a Highly Transformed Estuary

Estuaries are important fish nursery areas, yet little is known of how environmental forcing influences estuarine fishes during their early life stages. We analyzed environmental and larval-juvenile fish community data in the upper San Francisco Estuary (SFE) from spring to early summer 1995–2017, to better understand drivers of spatiotemporal community patterns in this highly modified estuary. We evaluated community patterns based on the relative abundance and diversity of native and introduced fish in the SFE and their predominant distribution (pelagic, demersal). The upper SFE experienced a downward trend of freshwater outflow and upward trends of temperature and salinity intrusion. Fish relative abundance only showed long-term downtrends for native and introduced pelagic fish groups. The most influential habitat components for relative abundance and diversity of fish groups were in decreasing order: temperature, salinity, Secchi depth, bottom depth, and zooplankton biomass. Early life stages of native and introduced fishes were generally segregated spatially and temporally, with native fishes more associated with cooler, saltier, and higher turbidity habitats during early to mid-spring compared to introduced fishes during late spring to early summer. Community ordination showed that environmental (temperature, salinity, outflow, Secchi depth, and zooplankton biomass) and spatiotemporal factors (month and depth), explained nearly 40% of the total variance. Our results suggest that the shorter duration of planktonic and nektonic stages of demersal fish groups results in higher resiliency compared to pelagic fishes. The declining abundance of pelagic fishes overall seems to be linked to drought effects and human-induced synergistic interactions intensified by climate change.

Estuarine Recruitment of Longfin Smelt (Spirinchus thaleichthys) north of the San Francisco Estuary

Longfin Smelt (Spirinchus thaleichthys) was an important forage fish in the San Francisco Estuary (the SFE) but was listed as threatened under the California Endangered Species Act in 2009. This has inspired research within the estuary at the southern edge of their distribution. However, populations also exist in other estuaries along the coast, which are far less described despite their potential importance in a meta-population. We surveyed Longfin Smelt populations along the northern California coast for larval recruitment. We conducted surveys in 2019 and 2020 to (1) identify estuaries north of the SFE where spawning occurs, and (2) evaluate how habitat features (e.g., salinity, temperature, dissolved oxygen, turbidity) influenced Longfin Smelt larvae abundance. We detected larvae in four of 16 estuaries we surveyed, and all were large estuaries north of Cape Mendocino. No larvae were detected in eight coastal estuaries in closer proximity to the SFE. Larvae catch probability increased with turbidity and decreased with salinity with no significant influence of temperature and dissolved oxygen. In the wet winter of 2019, we observed lower densities of larvae in Humboldt Bay and the Eel River, and detected no Longfin Smelt in the Klamath and Mad Rivers; in the dry winter of 2020, we detected larvae in two additional estuaries. Possibly elevated freshwater outflow in 2019 increased transport rates to the sea, resulting in the observed low larval recruitment. Our results suggest that, while populations of Longfin Smelt exist in large estuaries north of Cape Mendocino, coastal estuaries in proximity to the SFE were either under-sampled or are not permanently inhabited by Longfin Smelt. This suggests that the threatened estuary Longfin Smelt population may lack the resiliency afforded by meta-populations and advocates for increased monitoring over a range of hydrologic conditions and improving detection probabilities for future assessments of gene flow between populations.

Reconstructing the Three-Dimensional Structure of Loop Current Rings from Satellite Altimetry and In Situ Data Using the Gravest Empirical Modes Method

The three-dimensional structure of Gulf of Mexico’s warm-core rings, detaching from the Loop Current, is investigated using satellite altimetry and a large set of ARGO float profiles. Reconstruction of the Loop Current rings (LCRs) vertical structure from sea surface height observations is made possible by the use of the gravest empirical modes method (GEM). The GEMs are transfer functions that associate a value of temperature and salinity for each variable pair {dynamic height; pressure}, and are computed by estimating an empirical relationship between dynamic height and the vertical thermohaline structure of the ocean. Between 1993 and 2021, 40 LCRs were detected in the altimetry and their three-dimensional thermohaline structure was reconstructed, as well as a number of dynamically relevant variables (geostrophic and cyclogeostrophic velocity, relative vorticity, potential vorticity, available potential energy and kinetic energy density, etc.). The structure of a typical LCR was computed by fitting an analytical stream function to the LCRs dynamic height signature and reconstructing its vertical structure with the GEM. The total heat and salt contents and energy of each LCR were computed and their cumulative effect on the Gulf of Mexico’s heat, salt and energy balance is discussed. We show that LCRs have a dramatic impact on these balances and estimate that residual surface heat fluxes of −13 W m−2 are necessary to compensate their heat input, while the fresh water outflow of the Mississippi river approximately compensates for their salt excess input. An average energy dissipation of O [10−10–10−9] W kg−1 would be necessary to balance their energy input.

Seasonal variability of carbonate chemistry and its controls in a subtropical estuary

We performed an unprecedented investigation of the seasonal variability in carbonate system parameters (total alkalinity – AT, total dissolved inorganic carbon – CT, pH, and partial pressure of carbon dioxide – pCO2) in the lower zone of the Patos Lagoon Estuary (PLE), the largest choked lagoon in the world. Sampling was conducted monthly from May 2017 to June 2021. AT and pH were measured during the study period, while other carbonate system parameters were estimated using CO2Sys software. The pH distribution reflected the average natural alkaline conditions throughout the year, with an average of 8.0 pH units. The surface waters in the lower zone of the PLE are generally characterized by a supersaturated calcium carbonate environment. However, a susceptibility to undersaturation conditions was observed during winter (calcite and aragonite) and spring (aragonite). Furthermore, the average surface water pCO2 was 394 μatm during the analyzed period, with the highest values recorded in winter and early spring. The predominant estuarine processes governing changes in the carbonate system in the PLE were the dilution and concentration of salts. These processes depend on the complex balance between freshwater outflows and oceanic inflows that change the surface salinity and produce favorable conditions for primary producer development and the input of continental carbon. However, the remineralization of organic matter and CO2 ingassing likely contribute to the deviations in the theoretical mixing line, causing the increased CT in the region. In addition, the nonthermal effects on seasonal changes in the pCO2 prevail over thermal effects, and the region presents an ocean-dominated (riverine-dominated) condition during summer and autumn (winter and spring). The novel results described here reveal the complexity and challenges that still exist to a better comprehension of how carbonate system parameters evolve temporally and spatially in the PLE, especially considering the climate- and anthropogenic-driven stressors. Finally, this study contributes to the understanding of carbonate system variability in coastal ecosystems and highlights the need for more intense and continuous biogeochemical monitoring of Southern Hemisphere estuaries.

Copepod oxygen consumption along a salinity gradient

ABSTRACT Climate change-induced salinity decrease is currently occurring in many estuarine coastal zones, due to increased outflow of freshwater. This freshening can be a problem for brackish-water animals, already living on the edge of their salinity tolerance. We measured oxygen consumption of common copepod Eurytemora affinis along a natural salinity gradient in the western Gulf of Finland. The salinity varied between 3 in the inner bay and 7 in the offshore area along the gradient, pH varied between 7.05 and 7.86. Our results show that respiration increased with decreasing salinity, as expected for a genus more commonly found in estuarine/saline waters, even if it has colonised brackish waters. Our results suggest that future decreasing salinity could enhance respiration rate, and so energy requirements, of large-bodied zooplankton in estuarine areas such as the Baltic Sea and could lead to lower food quality availability for coastal planktivorous fish, such as herring and sprat.

Synoptic Flow Variability in a River-Influenced Inner Shelf off Central Chile

The response of inner shelf circulation and bottom temperature variability to synoptic wind forcing and freshwater outflow is evaluated in an area with a wide continental shelf off central Chile. This forced circulation, with a strong seasonal evolution from upwelling- to downwelling-favorable conditions, is a key process modulating the exchange of water properties in a coastal zone characterized by multiple river outflows and high biological productivity. Ocean currents on the inner shelf (34 m depth) off the Itata River mouth were obtained and analyzed for a nine-month record (December 2008 to September 2009). The synoptic wind and current variability was defined between 2 and 16 days (0.02 to 0.0026 cph). The subinertial coastal circulation within the inner shelf off Itata River was dominated by the along-shelf flow, while cross-shelf flows driven by along-shelf winds were substantially reduced. The alongshore synoptic currents showed two distinct modal structures: (i) A mean two-layer flow field during upwelling-favorable wind stress with northward (southward) flow at the surface (bottom), and (ii) a mean southward flow through the entire water column during downwelling-favorable wind stress periods, which were intensified during peak river discharge events in winter. Calculations of the wind index clarified the relative importance of wind versus buoyancy forcing on the alongshore flow. The wind forcing dominated during summer when the river discharge was minimum, whereas the relative importance of the freshwater outflow from the Itata River became dominant in winter (May-August) when the buoyancy forcing, in conjunction with southward wind events, modulated strong barotropic southward flows. The change in the long-term regime of coastal winds and river discharges, with a dominance of upwelling winds and reduced river outflows in the last decade, is discussed in the context of observations of thinner river plumes under frequent upwelling conditions off central Chile.

Effect of Groundwater Extraction and Artificial Recharge on the Geophysical Footprints of Fresh Submarine Groundwater Discharge in the Western Belgian Coastal Area

Human activities, such as managed aquifer recharge (MAR) and groundwater pumping, are influencing the natural groundwater flow in coastal areas. This might induce saltwater intrusion and impact fresh submarine groundwater discharge (FSGD). The use of resistivity methods (electrical resistivity tomography and continuous resistivity profiling) in coastal studies is very effective to investigate the salt–freshwater distribution, but it can be difficult to interpret quantitatively. In this study, the Western Belgian coast is investigated with resistivity methods, and image appraisal tools are systematically used to quantitatively interpret inversion models. Synthetic resistivity models, which reflect the existing situation at the Western Belgian coast, are first created and assessed quantitatively by means of the model resolution matrix, cumulative sensitivity matrix, and depth of investigation index. They reveal that no quantitative interpretation is possible of the FSGD, although lateral qualitative changes can be deduced from the inversion models. The field data show freshwater outflow from the lower beach to below the low water line, and they indicate that MAR has a positive impact on FSGD, while groundwater extraction reduces the outflow of freshwater to the North Sea. A comparison with existing groundwater models indicates their inability to reproduce the actual FSGD footprints.

Observed Changes in the Arctic Freshwater Outflow in Fram Strait

AbstractWe have updated time series of liquid fresh water transport (FWT) in the East Greenland Current (EGC) in the western Fram Strait with mooring observations since 2015. Novel data have been used to correct earlier estimates when instrument coverage was lower. The updated FWT (reference salinity 34.9) shows that the increased export between 2010 and 2015 has not continued, but FWT has decreased to pre‐2009 levels. Salt transport independent of a reference salinity is shown not to be sensitive to salinity changes. Between 2015 and 2019, the FWT in the Polar Water (PW) decreased to an average of 59.9 (±4.5) mSV, 15% less than the 2003–2019 long‐term mean, however, high FWT events occurred in 2017. The overall decrease is related to a slowdown of the EGC, partly attributed to a decrease of the zonal density gradient, due to stronger salinification of the halocline waters (26.5 &lt; σθ &lt; 27.7 kg/m3) over the shelf. This salinification counterbalances the freshening of the surface layer (σθ &lt; 26.5 kg/m3) and the fresh water content decreases. Our results show changes in the PW between 2003 and 2019: Salinity stratification increased as the salinity difference between 155 and 55 m increased by 0.63 psu, the PW layer became thinner by 40–50 m and the Polar‐Atlantic front moved ∼10 km west in June 2015. All processes point to an “Atlantification” of the western Fram Strait and a reduced Polar outflow. Including the novel data sets reduced the uncertainty of the FWT to an average of 8% after 2015, as opposed to 17% in earlier estimates.

Spatial Heterogeneity in Prey Availability, Feeding Success, and Dietary Selectivity for the Threatened Longfin Smelt

Food availability is a key determinant of the nursery value of a given habitat for larval and juvenile fishes. Growth, survival, and recruitment success are often inter-correlated and influenced by prey availability and associated feeding success. This is likely true for the threatened population of Longfin Smelt (Spirinchus thaleichthys) in the San Francisco Estuary (SFE) which has collapsed in recent decades along with its preferred prey. In years with high precipitation and freshwater outflow, larval Longfin Smelt are found in shallow wetland habitats throughout the SFE, but variation in the availability of food and feeding success in these habitats remains unexplored. To examine spatial variation in the trophic value of different rearing habitats, we quantified variation in prey availability, feeding success, and prey selection for larval and juvenile Longfin Smelt captured in restored tidal marshes, sloughs, and open-water habitats in the northern and southern SFE. Prey abundance varied spatially, with densities approximately tenfold greater in southern sloughs and restored tidal ponds relative to northern and open-water habitats. Feeding success of larval Longfin Smelt was positively correlated with both fish length and prey density. Larval Longfin Smelt fed selectively on the copepod Eurytemora affinis, with larger individuals (> 25 mm total length) exhibiting an ontogenetic diet shift to larger mysid shrimps. Our results suggest that wetland habitats across the SFE vary greatly in their trophic value, with previously unexplored habitats exhibiting the highest densities of prey and the highest foraging success for larval Longfin Smelt.

A Survey of X2 Isohaline Empirical Models for the San Francisco Estuary

This work surveys the performance of several empirical models, all recalibrated to a common data set, that were developed over the past 25 years to relate freshwater flow and salinity in the San Francisco Estuary (estuary). The estuary’s salinity regime—broadly regulated to meet urban, agricultural, and ecosystem beneficial uses—is managed in spring and certain fall months to meet ecosystem objectives by controlling the 2 parts per thousand bottom salinity isohaline position (referred to as X2). We tested five empirical models for accuracy, mean, and transient behavior. We included a sixth model, employing a machine learning framework and variables other than outflow, in this survey to compare fitting skill, but did not subject it to the full suite of tests applied to the other five empirical models. Model performance was observed to vary with hydrology, year, and season, and in some cases exhibited unique limitations as a result of mathematical formulation. However, no single model formulation was found to be consistently superior across a wide range of tests and applications. One test revealed that the models performed equally well when recalibrated to a uniformly perturbed input time-series. Thus, while the models may be used to identify anomalies or seasonal biases (the latter being the subject of a companion paper), their use as inverse models to infer freshwater outflow to the estuary from salinity observations is not expected to improve upon the absolute accuracy of existing outflow estimates. This survey suggests that, for analyses that span a long hydrologic record, an ensemble approach—rather than the use of any individual model on its own—may be preferable to exploit the strengths of individual models.