Salinity is a key control on species distribution in estuaries, but interspecific interactions can shift distributions of estuarine species away from physiologically optimal salinities. The distribution of the introduced calanoid copepod Pseudodiaptomus forbesi in the upper San Francisco Estuary (SFE) shifted from brackish to fresh water in 1993 following the introductions of 2 brackish-water copepods, the small but numerically dominant Limnoithona tetraspina (Cyclopoida) and the predatory Acartiella sinensis (Calanoida). The nearly simultaneous timing of these introductions complicated interpretation of the temporal change in distribution of P. forbesi. Although P. forbesi is now uncommon at salinity >~2, which might be interpreted as the result of salinity stress, short-term experiments showed high survival of adults up to salinity ~8 and of nauplii to salinity of at least 14, and reproduction was highest at salinity 5. Feeding experiments showed some overlap in diets of P. forbesi and L. tetraspina, but P. forbesi consumed a broader range of prey than L. tetraspina. Furthermore, feeding rates of the L. tetraspina population appeared insufficient to reduce prey availability to P. forbesi. Previous reports of high consumption of nauplii by A. sinensis and the clam Potamocorbula amurensis suggest that these interspecific interactions are important in constraining the distribution of P. forbesi in the upper SFE. Thus, we interpret the temporal shift in distribution of P. forbesi as due mainly to the introduction of the predatory copepod, whose high abundance may have been facilitated by the availability of a common alternative prey, L. tetraspina.

We examined how freshwater flow and phytoplankton biomass affected abundance and population dynamics of the introduced subtropical copepod Pseudodiaptomus forbesi in brackish and freshwater regions of the San Francisco Estuary, California, USA. This copepod is key prey for the endangered and food-limited delta smelt, Hypomesus transpacificus, in low-salinity water during summerautumn. Long-term monitoring data showed that P. forbesi was most abundant in fresh water, where summer-autumn abundance was invariant with freshwater flow. Abundance was positively related to freshwater flow in low-salinity water. Reproductive rates in both regions during 2010-2012 were low and unresponsive to chlorophyll or freshwater flow. Development indices, calculated as ratios of laboratory-derived to field-derived stage durations, were lowest for nauplii and highest for late copepodites, but averaged below 0.5 for all stages combined. Development indices were weakly related to chlorophyll for late copepodites only, unrelated to freshwater flow, and slightly higher in low-salinity than fresh water. Thus, the principal mechanism by which flow affects the P. forbesi population is apparently transport of copepods from fresh water to low-salinity water, where copepods are available to delta smelt. This work demonstrates how freshwater flow affects estuarine foodwebs through spatial subsidies of food supply.

In 2015, the fourth year of the recent drought, the California Department of Water Resources installed a rock barrier across False River west of Franks Tract to limit salt intrusion into the Delta at minimal cost in freshwater. This Barrier blocked flow in False River, greatly reducing landward salt transport by decreasing tidal dispersion in Franks Tract. We investigated some ecological consequences of the Barrier, examining its effects on water circulation and exchange, on distributions of submerged aquatic vegetation (SAV) and bivalves, and on phytoplankton and zooplankton. The Barrier allowed SAV to spread to areas of Franks Tract that previously had been clear. The distributions of bivalves (Potamocorbula and Corbicula) responded to the changes in salinity at timescales of months for newly settled individuals, to 1 or more years for adults, but the Barrier's effect was confounded with that of the drought. Nutrients, phytoplankton biomass, and a Microcystis abundance index showed little response to the Barrier. Transport of copepods-determined using output from a particle-tracking model-indicated some intermediate-scale reduction with the Barrier in place, but monitoring data did not show a largerscale response in abundance. These studies were conducted separately and synthesized after the fact, and relied on reference conditions that were not always suitable for identifying the Barrier's effects. If barriers are considered in the future, a modest program of investigation should be undertaken that includes adequate replication and ensures that suitable reference conditions are available to allow barrier effects to be distinguished unambiguously from other sources of variability.

We present the longest available dataset (by 15 years) of estuarine zooplankton abundance worldwide. Zooplankton have been monitored throughout the upper San Francisco Estuary from 1972 –present due to its status as a central hub of California water delivery and home to commercially important and endangered fishes. We integrated data from five monitoring programs, including over 300 locations, three size-classes of zooplankton targeted with different gears, over 80,000 samples, and over two billion sampled organisms. Over the duration of this dataset, species invasions have driven community turnover, periodic droughts have occurred, and important fishes have declined, likely due in part to reduced food supply from zooplankton. Data from the individual surveys have been used in prior studies on issues related to species invasions, flows, fish diets and population dynamics, zooplankton population dynamics, and community ecology. Our integrated dataset offers unparalleled spatio-temporal scope to address these and other fundamental ecological questions.