The results of the niche analysis by Gebühr et al. (2009) illustrated a changing ecological niche of Paralia sulcata over the last 38 years from a more specialised niche in the 1980s to a generalised niche since the middle of the 1990s. The main factors influencing this niche were temperature and light regime in the North Sea.
Temperatures at Helgoland Roads increased by about 1.67°C over the last 40-50 years (Wiltshire et al. 2008, Wiltshire et al. 2010). The canonical correspondence analysis applied by Gebühr et al. (2009) showed that P. sulcata is highly adapted to low temperatures and light conditions. However, higher abundances were detected during the summer months, which is in contrast with the increasing warming trend in the North Sea. Therefore, not the absolute temperature, but rather the temperature range is important for the determination of the ecological niche of P. sulcata. This study showed clearly that the tolerance range of temperature is important for the abundance of P. sulcata in the North Sea. It could be possible that warmer winter and summer temperatures lead to the optimal growth temperatures of P. sulcata at Helgoland Roads.
Furthermore, the ecological niche of P. sulcata is positively influenced by high concentrations of silicate and phosphate (Gebühr et al. 2009). This coincides with the results found in this study, where higher nutrient concentrations enhanced the growth of P. sulcata in the laboratory experiments as well as for the observations of the positive correlations of high abundances and nutrient concentrations in the field, especially in spring. The long-term data set, however, showed a significantly decreasing trend for the phosphate concentrations in the North Sea and it was shown that phosphate concentrations seemed to reach limiting concentrations rapidly (Wiltshire et al. 2008, Gebühr et al. 2009). The effects of the silicate and phosphate concentrations as well as light conditions (Secchi depth and sunshine duration) reflected the same influences on the ecological niche of P. sulcata within the two year investigation in comparison with the long-term analysis (Gebühr et al. 2009).
AUTECOLOGY OF PARALIA SULCATA
Two observed niche ranges for P. sulcata within the water column (bottom and surface waters) were detected at Helgoland Roads in the North Sea. The broad niche of P. sulcata in the bottom water sample in spring could be explained by the strong influence of salinity, Secchi depth and high concentrations of dissolved inorganic nutrients showing an adaptation of P. sulcata to low light conditions and high nutrient supply in the bottom water. The species tolerance of P. sulcata for the present environmental parameters from summer to winter was more or less the same with a tendency to a more generalised niche. It is important to notice that the niches from summer to winter were affected only by the Secchi depth. The fact that only a few environmental parameters affected the ecological niche might be an explanation for the generalised niche of P. sulcata with a good adaptation to its marine habitat. In contrast to the bottom water niche, the ecological niche in the surface water in spring was different, showing a really narrow species tolerance which could not be explained with the measured parameters, as none of them influenced the niche. One explanation for this narrow species tolerance may be the development of the spring bloom and therefore the intensive biotic interactions (competition) along with the grazing of the micro- and mesozooplankton on the phytoplankton as a whole (own data and e.g.
Smetacek 1981, Sommer et al. 1986, Sommer 1996, Löder 2010b). The changing species tolerance and niche position of P. sulcata in summer times could be accompanied by the decrease of the phytoplankton spring bloom species. Another aspect could be the negative influence of the temperature and the positive influence of high nitrogen concentrations on the abundance of P. sulcata, which means that with lower summer temperatures and higher nitrogen concentrations the abundance was increasing. This fact was supported by the growth experiments displaying a good growth at 10°C and 16°C even when silicate availability is limited.
To summarise, the results from the laboratory experiment showed that the autecological study on P. sulcata revealed an optimal growth at higher temperatures (ranging from 10 to 16°C), especially when silicate concentrations were not limiting.
No growth occurred at 4°C. This is in contrast to the temperature range in field observations where highest abundances of P. sulcata were found in autumn and winter, demonstrating that they can tolerate lower temperatures very well. P. sulcata as a benthic-pelagic species is well adapted to survive on the sediment, a trait which is supported by the adaptation to the low light conditions and higher nutrient availability (provided e.g. by the humic acids). This is in agreement with observations from both
AUTECOLOGY OF PARALIA SULCATA
the laboratory and the field. Furthermore, the seasonal observations at Helgoland Roads indicated a wide range of different environmental parameters that can be tolerated and therefore also possible changes in the marine environment.
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CKNOWLEDGMENTSWe thank the crew of the R.V. Aade for the help by taking the water samples over two years. We are very thankful to our technical assistants Kristine Carstens for measuring the nutrients and salinity (surface and bottom water samples) and for her help in the analytical procedure during the growth experiments (nutrients and HPLC), also to Silvia Peters for counting the abundance of Paralia sulcata in the surface water samples due to the long-term data analysis and for her grateful help during the growth experiments. Furthermore we are grateful to Simone Beatrice Moos for her helping during her student internship (practical course) in one part of the growth experiments.
We are grateful to Nicole Aberle-Malzahn, Martin Löder, Katherina Schoo for inspiring discussions and improving the English on the manuscript as well as to Gunnar Gerdts for his help with the niche calculation. This study was part of the PhD thesis in the Food Web project of the Alfred Wegener Institute for Polar and Marine Research in the PACES programme.
AUTECOLOGY OF PARALIA SULCATA
GENETIC DIVERSITY OF PARALIA SULCATA