Helgoland Roads Zooplankton

The Helgoland Roads time series was started in 1975 by Wulf Greve, initially at the Biologische Anstalt Helgoland institute and later continued in cooperation with the German Centre for Marine Biodiversity and the Federal Maritime and Hydrographic Agency. It is now managed by the Biological Station Helgoland again.

Every Monday, Wednesday, and Friday two oblique plankton net samples (150 μm, 500 μm) are collected from the monitoring site (54° 11' 18" N, 7° 54' E). From each sample, almost 400 taxonomic entities of holoplankton and meroplankton (e.g., benthic and fish larvae) are identified and counted for abundance. The purpose of the Helgoland Roads programme is to monitor and document high frequency plankton population dynamics to detect and investigate community dynamics at seasonal and longer time scales and to better understand long-term changes in biodiversity. 

At the Helgoland Roads sampling site, small copepods, mostly Acartia clausi, Temora longicornis, Pseudocalanus sp., represent a significant fraction of the total zooplankton population. Seasonal and interannual variability in the small copepods are large both in timing as well as in magnitude. Maximum density can be found in most years in the mid-summer, and the 40 year time-series shows clear decadal variability. Starting with a negative phase at the beginning of the times series (1975), copepod abundance increased steadily and was consistently higher than the average during much of the 1980s. After a period of transition (1990-1997), copepod density decreased and has remained in a negative phase where abundances are consistently low.

Looking at monthly mean copepod abundance by year, years with strong positive annual anomalies (e.g., 1983-1988) were characterized by an extended period of high abundances in the mid-summer, whereas strong negative annual anomalies years (e.g., 2003-2006) had both a shorter period and lower maximum abundance during the mid-summer. If one then looks at the monthly anomalies, years with a strong positive annual anomaly generally also had positive monthly anomalies for every month in that year, with the opposite also true for years with strong negative annual anomaly (i.e., most months also had negative monthly anomalies). The extent of the peak abundance each year is therefore most greatly influenced by the copepod population density present through the winter months and leading up to the summer peak.