Acoustic backscatter and copepod secondary production across the Subtropical Front to the east of New Zealand

Abstract

The magnitude and spatial distribution of zooplankton biomass and secondary production across the Subtropical Front (STF) to the east of New Zealand is virtually unknown. We used conductivity‐temperature‐depth data to locate the STF and described the pattern of mixed layer depth, current shear, and Richardson number across the frontal zone. The meridional distributions of surface chlorophyll, acoustic backscatter, optical particle counts, and sizes across the STF were determined from a survey in austral spring 1999. Chlorophyll, acoustic backscatter, and optical biomass all were higher in subtropical water to the north of the STF. Copepod‐sized particles were ≈50 μm larger in the frontal zone (ESD ≈ 400 μm) than to the north and south of the front. Neither acoustic backscatter nor optically estimated copepod biomass was higher in the frontal zone. Particle counts and temperature were used to estimate copepod secondary production. Median potential copepod production was 9.6 mg Carbon m−3d−1. The trend of copepod production across the front was similar to chlorophyll a and acoustic backscatter. Median copepod production to biomass ratio (P/B) was 0.137 d−1. Median turnover time for copepod biomass (B/P) was 7.3 days. This turnover rate is the same order as the range for most marine copepods (≈5–10 days), which leads us to question the idea that the STF is a region of higher than average copepod secondary production in the New Zealand region.