Guinardia delicatula, a diatom, is spreading
Guinardia delicatula from net samples off the Helgoland Roads, which are taken weekly to complement the daily water sampling. The right figure shows the cells just beginning to divide. Note the stout, oblique spine on the narrow side of the cell. Photo: Kraberg Alexandra, Alfred Wegener Institute (PLANKTON * NET, planktonnet.awi.de)
The diatom Guinardia delicatula, is a characteristic member of the phytoplankton in temperate latitudes during late spring and summer. Since the 1970s, this alga has become more and more prevalent off Helgoland where it has already been observed in March and as late as October. The reasons for, as well as the environmental consequences of this seasonal change are currently being investigated at the Biological Station Helgoland, of the Alfred Wegener Institute for Polar and Marine Research.
Guinardia delicatula like all diatoms consists of two interlocking silicate frustules. It can occur as a single cell, but mostly forms chains consisting of up to a dozen cylindrical cells.
The long spine on each valve end, and large, greenish-brown chloroplasts make it relatively easy for scientists to identify it. This is not often the case for smaller phytoplankton species.
Guinardia delicatula has long been known from the waters around Helgoland. In 1900 it was first described as Rhizosolenia delicatula by Cleve in plankton of the North Sea. The currently valid name originates from Grethe Hasle.
The species attracted interest, ecologically, due to the results of a long-term study begun in 1962. This still ongoing study, known internationally as the Long-term Helgoland Roads Time Series collects detailed daily data on phytoplankton species composition and salinity, water turbidity, temperature and macro-nutrients (NH4, PO4, Si, NO3). Extensive data sets on zooplankton and bacterioplankton are also available.
The Helgoland Roads long-term data series is one of the most detailed biodiversity data series in Europe, if not worldwide. The scope of this data series goes beyond the creation of an inventory of regional biodiversity, but also facilitates the observation and interpretation of long-term biodiversity change, such as that resulting from immigration and migration of species. Only on the basis of such long-term data is it possible to clearly determine long-term changes in the seasonality of individual diatom species such as Guinardia delicatula.
Cell numbers of the diatom Guinardia delicatula for the month of May, since 1962: The figures plotted with data from the Helgoland long-term series for plankton illustrate the long-term changes in the occurrence of this species. While 40 years ago they were predominantly a summer species and almost never occurred in May, it is now becoming more and more common ever earlier in the year. To what extent this is related to long-term rising water temperatures, is currently under investigation (Wiltshire et al. 2010)
For a few decades, this species has begun to appear as early as March, and often is still quite numerous in October. - Initial laboratory investigations on Guinardia delicatula indicate that its population growth rate increases at higher temperatures. The algae thus appear to be adapted to warmer temperatures, explaining their early appearance at the onset of rapid warming in spring. They can thus prolong their seasonal occurrence, as long as appropriate temperatures prevail. In fact, an increase in annual mean temperature by 1.67 ° C was recorded in the Helgoland time-series from 1962 to 2008. The consequences of the changing seasonality of Guinardia delicatula on the phytoplankton community, have yet to be studied in detail