Low Food Availability Narrows the Tolerance of the Copepod Eurytemora affinis to Salinity, but Not to Temperature

Invasive species perturb food webs, often decreasing resource availability for resident taxa. Low resource availability may interact with abiotic factors to restrict niches, particularly niche axes that influence metabolic demand. The San Francisco Estuary (SFE) provides a case study, as it has low phytoplankton concentrations, likely due to invasive bivalves. We conducted two laboratory experiments to examine how phytoplankton concentrations influence the salinity and temperature tolerance of Eurytemora affinis, an important prey taxon for threatened SFE fish. We found that decreased algal concentration narrowed the tolerance of E. affinis to salinity, but not to temperature. A third experiment revealed that when food concentration was relatively low (chlorophyll-a of 15 mu g L-1) and salinity was elevated (8 vs 4), E. affinis did not compensate for osmotic stress by increasing consumption, halving its growth rate. However, when algal concentration was elevated (chlorophyll-a 55 mu g L-1), E. affinis consumed three times more algae at a salinity of 8 vs 4, allowing copepods to grow equally at both salinities. Our interpretation is that while E. affinis can compensate for increased metabolic demand as temperature increases at low food concentrations, it can only compensate for elevated metabolic demand at hypo- or hyperosmotic salinities when food concentrations are high. We therefore propose the hypothesis that low food concentrations narrow the realized salinity, but not the realized thermal, niche of E. affinis.