Based on the results of the experimental and long-term analysis (two years monitoring campaign and multivariate statistical analysis) I will now sum up the most important environmental parameters determining the occurrence of Paralia sulcata at Helgoland Roads. Based on the results discussed in the previous sections, I suggest a distinct change in the annual phase of P. sulcata at Helgoland Roads, which is influenced by different environmental parameters and also shows the adaptation of this diatom due to the changing habitat conditions (Fig. 1).
As shown by the growth experiments the best temperature and nutrient conditions were detected in spring and autumn with moderate temperature and higher amounts of
GENERAL DISCUSSION
nutrients in the water column favouring the growth and development of P. sulcata. I suggest that two phases of P. sulcata can be observed at Helgoland Roads related to the benthic-pelagic life style of this diatom. It seems to be the case that spring a change from the pelagic to benthic phase and in autumn a change from the benthic to the pelagic phase takes place. Especially the increasing storm activity in autumn leads to a well mixed water column again and therefore to a resuspension of P. sulcata from the sediment which can explain the higher abundances of P. sulcata in the water column in autumn and winter.
As described in the literature and also shown by the results of our long-term analysis P. sulcata is adapted to live at lower temperatures, especially when nutrient concentrations were high and the light intensities were lower with short day lengths. In winter periods the occurrence of P. sulcata was supported by higher storm activities and mixing of the water column in the surface water. Typical for temperate coastal waters like the North Sea, there are generally lower abundance of phytoplankton in the water column (Wiltshire & Manly 2004). Hence, the biotic interactions e.g., competition with other diatoms is less or reduced. Thus, P. sulcata is able to survive in the water column very well (Table 1, Fig. 1, Chapter I, II).
In spring the increasing light conditions near the surface water have negative effects on the occurrence of P. sulcata (Table 1, Fig. 1) and thus, leading to a shift from the pelagic to the benthic phase. Factors that seemed to exert a positive effect on P. sulcata abundance were the high nutrient concentrations (especially silicate is needed for the growth) as well as to a slight degree the increasing temperature (see Chapter II, growth experiments). The high abundance of P. sulcata provided a continuous food source from the winter to the spring transition as pre-bloom species (see Chapter IV). Further, the biotic interactions in the water column strengthen due to competition for nutrients and a high grazing of the micro- and mesozooplankton on the phytoplankton community as a whole. The ecological tolerance of P. sulcata in this period is wider compared to the tolerance in the other seasons indicating that in this time P. sulcata can cope with a wide varying range of environmental conditions (Chapter II).
GENERAL DISCUSSION
Figure 1: Schematic description of the results from the experimental and long-term data analysis of the ecological role of Paralia sulcata in the water column and sediment (as tychoplankter) and the most important environmental parameters influencing the occurrence at Helgoland Roads separated by the different seasons. The colour of the squares indicated the temperature whereas the orange colour means codler or decreasing temperatures (autumn-winter) and the red colour mean higher or increasing temperatures (bright and dark red) (from winter-spring-summer-autumn). The black dashed line from the sediment to the water surface indicated the resuspension of P. sulcata. The light conditions are shown as sun: low light conditions expressed as small sun symbol, higher light conditions are shown as larger sun symbols, note that the light conditions are spring is higher compared with the autumn. The nutrients (N = nitrogen, P = phosphate, Si = silicate) in bold indicate high concentrations, nutrients in plain font mean low concentrations. Biotic interactions were shown as copepod (grazing) and phytoplankton (competition). The abundance of P. sulcata is reflected in large or small pictures of this diatom in the different seasons.
In summer higher cell numbers were detected in the bottom water samples compared with the surface water samples. The summer period could therefore be characterised by the benthic phase of P. sulcata which is supported by the low light intensities on the bottom of the sediment surface. Temperature displayed no significant influence but also higher temperatures did not inhibit the growth of P. sulcata. Interestingly, all nutrients exhibited a significant negative influence on the abundance of P. sulcata at
GENERAL DISCUSSION
this time. One possibility of this correlation could be the occurrence of humic acids in the sediment, providing benthic diatoms species with additional nutrients due to the formation of complexes with nutrient ions which could affect the transport of the ions to the diatoms (Lund 1990). We suggested that the nutrients can be taken up very fast by benthic diatoms (such as P. sulcata) so they were not detectable within the water column as dissolved inorganic nutrients. Furthermore, on the sediment re-mineralisation processes by bacteria are an important process (Bidle & Azam 1999) and quickly provides nutrients for the benthic diatoms. The nitrogen and phosphate re-mineralisation in the microbial loop is supported by the micro- and mesozooplankton (biologically mediated) and very rapidly (Officer & Ryther 1980). The recycling of silicate is strongly dependent on bacterial activity because the latter promote the silicate recycling near the sea surface bottom especially at higher temperatures (Bidle
& Azam 1999).
The autumn is characterised by increasing storm activity and nutrients due to re-mineralisation processes as well as decreasing temperature and light conditions.
Therefore, good conditions for the growth of P. sulcata were provided and a change from the benthic to the pelagic life style occurred. Due to turbulence the abundance of P. sulcata increased within the water column which is in line with the increasing nutrient amounts supporting the growth and development of this species. Furthermore, the day length and light availability decreased provided favourable conditions within the water column for P. sulcata.