Control of phytoplankton in a shelf sea: Determination of the main drivers based on the Helgoland Roads Time Series

Studies on what controls phytoplankton in shallow coastal seas are increasingly important in the context of current climate change. We analysed the daily long term data for Helgoland Roads, since 1962, to determine the main drivers of phytoplankton growth. Helgoland Roads is one of the few marine time series with an unbroken, highly resolved (work-daily) record of abiotic, biotic and hydrological parameters. All data for 50years (1962–2011) of phytoplankton, diatoms, flagellates, zooplankton, nutrients, temperature and salinity, as well as light were analysed. The dominating factors, both with regard to timing and diatom numbers were light availability (Secchi depth), temperature and zooplankton. Annual patterns of temperature, Secchi, radiation and sunshine hours were found to be related. Salinity reduction in spring was attributed to riverine input in late winter. Secchi depth is driven mainly by meteorological and hydrographic conditions. We found that with a shift from low to high Secchi between weeks 12–18, the diatom spring bloom usually starts. The timing of phytoplankton occurrence was significantly correlated to Secchi values. Cumulative abundance growth patterns of microalgae are low/high with low/high temperatures and Secchi. In the early weeks of the year high temperatures lead to lower phytoplankton abundance. Zooplankton growth was found to follow diatom growth. Cumulative algal abundance growth patterns were low/high with high/low zooplankton. In the early and late weeks of the year, low zooplankton leads to longer algal growing periods. High zooplankton in spring results in a later start of algal growth. Nutrients were taken up from the water column in the following order: Silicate, phosphate, and nitrate, and this uptake follows the algal growth curves. Different algal species show different growth requirements for underwater light and temperature. The overall phytoplankton growth has shifted in 2001–2011 to longer growing periods with a less steep incline/decline in spring/autumn and significantly higher abundances.