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 most aquatic ecosystems, copepod nauplii outnumber all other mesozooplankton. Although thousands of studies have examined feeding by later life history stages, the feeding habits of nauplii are poorly known. We offered conspecific adult and naupliar stages of the current-feeding calanoid copepod Pseudodiaptomus marinus and the ambush-feeding cyclopoid copepod Oithona davisae 14 species of phytoplankton from various functional and taxonomic groups that spanned a wide size range. Using a novel epifluorescence microscopy method, we calculated an index of gut pigment for copepods fed each phytoplankton species. We also measured adult and naupliar feeding rates on three species of phytoplankton: the cryptomonad Rhodomonas salina, the prasinophyte Tetraselmis suecica, and the diatom Thalassiosira pseudonana, by an improved gut fluorescence method using a microplate reader. Despite their smaller size, weaker swimming and sensory capabilities, and rudimentary feeding apparatus, nauplii fed on a large range of cell sizes and were capable of consuming many of the same phytoplankton as adults.

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