Temporal and spatial variation in the sizes of California current macrozooplankton: analysis by optical plankton counter

Macrozooplankton in the southern California sector of the California Current had been reported to decrease from 1951 to 1998. We analyzed some of the same preserved samples of macrozooplankton taken in non-El Niño years 1955, 1956, 1966, 1981, 1984, 1991, 1995, and 1996, and also 1999, with the optical plankton counter (OPC) to determine whether all size categories changed similarly over time. The results from 1955 to 1996 could be interpreted either as a linearly decreasing trend (total biovolume decreased by 45%) or as a regime shift (decrease of 38% from pre- to post-1975 regimes). The largest zooplankters (>2.7 mm equivalent circular diameter, ecd) were relatively more important, in terms of biovolume, at night than by day, and offshore than onshore. Their biovolume decreased by the greatest relative amount, and biovolume of the smallest zooplankters (0.75–0.80 mm ecd) decreased hardly at all. The decrease in relative importance of large zooplankters was most evident in winter and spring, and was qualitatively similar by day and at night and in nearshore and offshore regions. Total biovolume increased in 1999 to the pre-1975 level, consistent with a possible shift to a new regime. Our results are consistent with (i) a change in biovolume of the large zooplankton over the sampled period; and (ii) a regime shift in the mid-1970s and, possibly, the late 1990s. The laboratory OPC is a useful instrument for the rapid and cost-effective analysis of preserved samples of zooplankton.