Assessment of weekly zooplankton monitoring at coastal station S on the Faroe Shelf

This report is intended to document continuing efforts to optimize the monitoring of zooplankton on the Faroe shelf. The present monitoring system, which enables weekly sampling, is based on sampling using a zooplankton net installation on land coupled to an underwater pumping system. A comparison between samples collected on land and towed samples yielded poor results with the land samples generally showing considerably smaller abundances. However, the zooplankton seasonal succession followed the seasonal succession in chlorophyll a and the land samples of the smallest organisms such as copepod nauplii look realistic. Unfortunately, the towed samples were collected with a 200 μm net whereas a 100 μm net was used for the land samples. This prevented a quantitative comparison for the smallest organisms and may also have affected the results more generally. Another possible reason for the poor correspondence between towed and land samples may be heavy predation in the inlet to the land station. With the inlet located on the bottom at 18 m depth, the land samples are drawn from the benthic boundary layer, whereas the towed samples are from a water column extending from 50 m depth to the surface in deeper water close to the inlet. It has generally been assumed that the strong tidal currents mix the zooplankton both vertically and horizontally to such an extent that the land and towed samples are drawn from the same habitat, but the poor results from the comparison cast this in doubt. If the monitoring at the land station is to be continued, we recommend a further set of comparative sampling. This should include sampling at various depths with a towed multi-net to investigate possible vertical gradients in zooplankton abundance. In addition, comparative sampling on land and at sea should be repeated, but with the same mesh size. The towed sampling should be in triplicate, at the least.