In this investigation, we have tested the potential usefulness of variable fluorescence, i.e., the potential photochemical efficiency of photosystem II (F(v)/F(m)), determined with a fast repetition rate fluorometer (FRRF), as an indicator of the effect of metal pollution on natural phytoplankton assemblages. Phytoplankton populations were collected from an eutrophic embayment and exposed to different copper concentrations within the ppb range for 4 d. Enhanced photosynthesis was found for Cu concentrations of 10 and 20 microg/L, at very short exposure times (1 and 5 h). However, after 72 h even at 10 microg/L of Cu, F(v)/F(m)-values were significantly lower than those measured in the control. The highest Cu concentration tested (80 microg/L) caused a statistically significant decrease of F(v)/F(m) after 5 h. This response was contrasted with parallel determinations of chlorophyll a concentration, measurements of photosynthetic O2 production rates, and changes in the taxonomic composition and size structure of the microplankton communities. A reduction in overall particle size was found at Cu concentrations of 80 and 40 microg/L. Enhanced abundance of flagellates and the diatom Pseudonitzschia pungens was observed at 20 microg/L of copper. This study demonstrates the utility of this methodological approach as a rapid, nondestructive way for detecting trace-metal toxicity on natural phytoplankton.