Chronic food limitation of egg production in populations of copepods of the genus Acartia in the San Francisco Estuary

Egg production of planktonic copepods is commonly measured as a proxy for secondary production in population dynamics studies and for quantifying food limitation. Although limitation of copepod egg production by food quantity or quality is common in natural waters, it appears less common or severe in estuaries where food concentrations are often high. San Francisco Estuary, California, has unusually low concentrations of chlorophyll compared to other estuaries. We measured egg production rates of three species of Acartia, which dominate the zooplankton biomass at salinity above 15 psu, on 36 occasions during 1999-2002. Egg production was determined by incubating up to 40 freshly collected individual copepods for 24 h in 140 ml of ambient water. Egg production was less than 10 eggs female 1 d(-1) most of the year, but as high as 52 eggs female(-1) d(-1) during month-long spring phytoplankton blooms. Egg production was a saturating function of total chlorophyll concentration with a mean of 30 eggs female(-1) d(-1) above a chlorophyll concentration of 12 +/- 6 mg chl m(-1). We take chlorophyll to be a proxy for total food of Acartia, known to feed on microzooplankton as well as phytoplankton. These fimdings, together with long-term records of chlorophyll concentration and earlier studies of abundance of nauplius larvae in the estuary, imply chronic food limitation of Acartia species, with sufficient food for maximum egg production < 10% of the time over the last 25 yr. These results may show the most extreme example of food limitation of copepod reproduction in any temperate estuary. They further support the idea that estuaries may provide suitable habitat for Acartia species by virtue of other factors than high food concentration.