Bachelor's programme in Environmental Sciences: 58 pp. (first reviewer)

In Lake Banter, both the phytoplankton and bacterioplankton distribution was always heterogeneous over the years. A cyanobacterial bloom occurred briefly during the study period. As the investigations began in May, the bloom that occurred in June acted as a disruptive factor in the pure investigations into the distribution of the plankton, but also allowed conclusions to be drawn about the dynamics of the phytoplankton.

The individual large groups of plankton were analysed from various points of view and the results obtained provide the following key findings on phytoplankton dynamics in Lake Banter:
The distribution of phytoplankton is spatially very heterogeneous. There were constantly locations with higher and lower individual densities. A precisely defined distribution pattern was only found in the green algae, but not definitively confirmed. Whether the phytoplankton is distributed more at the edge of the lake or in the open water could not be clarified, as on the one hand the edge stations were only sampled on one day in 2008 and on the other hand the results from previous years did not yet show any regularity.
The parameters analysed obviously have a clearly different effect on individual plankton. This results in areas in Lake Banter that favour development and areas where development is less successful. The previous studies on the phytoplankton in Lake Banter also do not allow any linear derivation of the population densities of individual factors. This seemingly arbitrary dynamic is compounded by the fact that the main species colonising Lake Banter was represented by a different genus in each year.

However, some factors also remained constant over a period of many years. These included the occurrence of diatom species, which showed comparable results in all years (2002 - 2008). Regularities in plankton dynamics were also observed at certain stations. The process of brackish water formation appears to have stabilised as early as 2002. In the same year (2002), however, there were significantly more marine species in the intermediate harbour than in the following years.

Overall, it became clear that Lake Banter is a very complex ecological system. The changing dominance ratios of phytoplankton, which were observed in all years, as well as the unlikely occurrence of a cyanobacterial bloom and the highly uneven distribution of individual genera indicate that further processes in Lake Banter have not yet been completed.


Outlook:
This study has shown that the ecological processes in Lake Banter have not yet stabilised. Further investigations in subsequent years are important in order to understand these processes. The study period of five weeks chosen here merely reflected the state of the water body during this time. Based on the resulting distribution pattern of the phytoplankton, further investigations are required to answer the questions raised in this study. For example, subsequent work could provide information on the influence of turbulence on phytoplankton dynamics in Lake Banter, which has not yet been investigated in detail and may be greater than previously assumed. The low correlability of the phytoplankton numbers with the nutrient salinity data in particular requires an investigation from a different perspective. In earlier years, there was talk of indirect effects that have a major influence on the organisation of species communities. In future, these effects must be linked to phytoplankton dynamics. Further investigations are therefore not concerned with strictly answering individual questions, but rather with establishing correlations and being able to derive a distribution from this. As the phytoplankton dynamics deviate from a strict regularity, Lake Banter remains an interesting object of study.

Diploma programme Landscape Ecology: 99 p. (second reviewer)

An area in the Wadden Sea north of Ostbense on the North Sea coast of Lower Saxony was investigated. Here, areas have been exposed by erosion for some time that have a visually different structure to the neighbouring areas. The clearly visible ripples led to the question of whether this was a furrow structure. Although the general soil parameters showed clear differences between the consolidated study sites and the control sites in fresh silt, almost all of these were directly or indirectly attributable to the different grain sizes. However, this allows the conclusion that the investigated sites A to C were subject to a different pedogenesis than the control sites D and E. The very low CaCO₃ content of sites A and C, which can be attributed to decalcification, also provides an indication of this, as a constant CaC0₃ content is generally present in tidal flat sediments in the same area.

In order to provide evidence of anthropogenic use, the stability of the organic matter (OS), phosphate and phytolith contents as well as pollen were analysed. Furthermore, shell dating was carried out to determine a time window of utilisation. However, the results of the selected investigation methods did not provide evidence of arable utilisation. This may be due to the fact that these methods have proved successful for anthropogenic utilisation of terrestrial soils, but it cannot be ruled out that the influence of the marine environment over centuries has had an impact on some of the parameters investigated. It can also be ruled out that the structures are typical ripples of the Wadden Sea. They correspond neither to oscillation ripples nor to current ripples. Although the shaping processes are probably sedimentation and erosion, they have different effects than in typical mudflat or wet beach areas due to the more aggregated soil material of the former marshland. According to the current state of research and the evaluation of the results, it can be assumed that the visible structures were not created by arable farming.

Diploma programme in Biology at Leibniz Universität Hannover: 100 pp. (second reviewer)

In recent years, utilisation of the area has been limited to the southern half and the area close to the dyke in the form of mowing. Developments typical of fallow land have been observed. In the northern part there is a large population of Halimione portulacoides, which has expanded and spread southwards in recent years. The Andel grassland is heavily overgrown with Atriplex prostrata and Festuca rubra in many areas, while couch grass has spread, especially in the southern part, and in some cases has penetrated far into the lower salt marsh. The reed population has more than doubled in the last 3 years. The beach aster is present in the study area - also close to the dyke - with a high cover. Silt grass has migrated into the Andel grassland and has more than tripled its area, although this is probably also due to the changed water conditions in the soil. The location of a plant community is not exclusively due to anthropogenic utilisation, but also to (a-)biotic factors. The redshank breeds in the Idagroden with a relatively high density, although hatching success is low due to high predation. If the proportion of light in the nest is ≤ 5 % compared to the surroundings, the probability of hatching success is higher. In contrast to the Andel grassland, the couch grass community was positively selected for clutch selection. In general, vegetation units with tall growth are favoured when selecting clutch sites, as the greater height of the surrounding vegetation provides visual protection from predators. Therefore, hatching success was higher in Agropyretum littoralis than in the other vegetation units. The majority of clutches were found in the area close to the dyke, as the vegetation was highest there.

Diploma programme Marine Environmental Sciences: 85 p. (second reviewer)

Humic substances are an important abiotic factor in nature. They can cause stress in organisms. The extent to which this is the case in the coccal green alga Pseudokirchneriella subcapitata was investigated by analysing growth rate and growth progression as well as some fluorescence parameters (yield, electron transport rate, chlorophyll content) and biochemical stress indicators (concentration of total ascorbate, free proline, free total amino acids).

The results showed no changes in growth under the influence of humic substances, regardless of their concentration. Accordingly, the yield remained unchanged. The same applies to the concentrations of total ascorbate and amino acids, but not to the content of proline. The accumulation of this compatible and signalling substance can be interpreted as a consequence of stress. Additional stress caused by NaCl led to an additive effect in the case of proline. In combination with strong radiation, the stress effect of the humic substances was also expressed in photosynthetic electron transport.

From these results it can be concluded that humic substances do exert stress on P. subcapitata, but only to a small extent. Apparently, this alga possesses effective strategies that ensure a high tolerance to the effects of humic substances of the composition used here. This could enable P. subcapitata to survive in waters rich in humic substances in nature and give it an advantage over other organisms.

Diploma Programme Marine Environmental Sciences: 72 p. (second reviewer)

The blue mussel Mytilus edulis L. is a very important species for European aquaculture. However, conflicts of interest and a very limited supply of seed mussels in coastal areas make the expansion of conventional mussel farming considerably more difficult. Offshore areas, on the other hand, offer better opportunities. Here, it is primarily the areas of the planned offshore wind farms whose open space between the individual wind turbines could create an optimal breeding area. However, very harsh environmental conditions prevail here with currents of over 1.5 m/s, wind speeds of over 30 m/s and resulting wave heights of up to approx. 10 metres. It is not only the breeding facility but also the mussel that must be able to cope with these harsh physical conditions. For this reason, this Diplom thesis deals with the search for a suitable collector material that offers the mussel the best possible properties to adhere to it with its byssus threads.

Four substrates were tested, some of which are already used in mussel farming. They were stocked with mussels in the laboratory and then deployed at three test sites in the German Bight. One of these was two protected stations, which were set up near the Jade Bay at the Niedersachsenbrücke (Wilhelmshaven) and on the Helgoländer Reede (Helgoland). The other station is very exposed in a restricted area within sight of the "Roter Sand" lighthouse, about 17 nautical miles off the coast.
The physical characteristics of the fine structures of the collector surfaces, as well as the adhesive discs of the byssus threads on these, were examined visually under a scanning electron microscope. The findings were then compared with adhesive force measurements of the individual mussels on the different substrates.

Although the adhesive strengths were within a wide range, forces of up to 10.42N were found for individual mussels. The maximum values for all substrates in the jade were between 8.48N and 10.42N. Furthermore, it was found that the connection between the adhesive disc of the byssus thread and the substrate surface only caused complete loss of adhesion in a maximum of 50% of cases. Both the fine structure and the coarser structure of the substrates were responsible for this. All results show that the tested structures can guarantee the mussels secure adhesion even under the expected stresses in offshore mussel farming.

Diploma programme in Biology at Leibniz Universität Hannover: 93 p. (second reviewer)

By comparing the vegetation maps, the development of the vegetation of the "Großer Schlopp" within the last two years could be traced. Particularly striking is a change in the complex of the Festuca rubra association, Ononido-Caricetum and Juncetum gerardii. The Festuca rubra association is displacing the Ononodi-Caricetum to the edge of the grassland beds - probably due to eutrophication and a reduction in grazing intensity. Here the community is apparently at an advantage due to the more frequent flooding. Juncetum gerardii also benefited from the more frequent flooding and displaced the Festuca rubrra community in some places. In other areas, however, the latter was at an advantage due to the reduced grazing pressure.

If the intensity of grazing and the frequency of flooding remain unchanged in the coming years, the development of the vegetation will probably continue as described above. However, maximising the frequency of flooding and slightly increasing the grazing pressure could lead to the Ononido-Caricetum and Juncetum gerardii spreading again. This could restore the conditions that prevailed in the area before 2005. With a lower frequency of flooding and a constant or reduced grazing intensity, a further spread of the species-poor Festuca rubra community would be likely. In addition, the Atriplici-Agropyretum pungentis would probably also spread in the area.

If we now consider the influence of the storm surges on the analysed soil parameters, the data are of varying significance with regard to the individual characteristics: The grain size fraction shows a clear change after the storm surge. In large parts of the area there was mainly sedimentation of silt and, to a lesser extent, clay. A few exceptions show an increase in the proportion of sand. This sand was possibly washed from the dunes at the edge of the "Großer Schlopp" to the edge of the study area.

The storm surge also had a very clear influence on the conductivity measured in the soil. This has clearly increased as expected. This influence is particularly high in the area of the upper salt marsh, as the conditions here vary particularly drastically over the course of the year. Here, the soil samples make it clear for the first time how important the flooding factor and the associated conductivity are for the vegetation. All plants that can tolerate such extreme fluctuations have an advantage over other competitors.

This also appears to be the case with Festuca rubra and Ononis spinosa, for example. The latter was displaced to the edge of the grassland beds, where it has an advantage due to the fluctuating salinity. As expected, the proportion of water also increased with the storm surge. However, with this parameter, which is particularly important for the vegetation pattern as already emphasised, the influence of the grain size fractions must also be taken into account. As soils particularly rich in fine sediments have a high water capacity, the increased water content could also be due to a small extent to the sedimentation of fine sediments. Both effects cannot be separated here.

Nitrogen, which is used for the synthesis of osmotically active substances, is a limiting factor in the salt marsh. However, as only sample series C could be analysed for its nitrogen concentration, no statement can be made here about the influence of the storm surge. Only the relatively high ammonium concentration compared to nitrate is noticeable. This arises because a higher water saturation of the soil leads to anaerobic conditions. This in turn inhibits nitrification and promotes denitrification. It is also difficult to make a clear statement about the influence of the storm surge on the nitrogen concentration, as the nitrogen cycle is made up of many reactions and can be influenced by a wide variety of factors. Despite many years of research into salt marshes, no agreement has yet been reached on the extent to which nutrients and organic material are washed out of the system or carried in by tides.

The measurements of phosphate concentration also do not allow any concrete conclusions to be drawn, as the phosphate cycle is also influenced by a wide variety of factors. A correlation between the phosphate concentration and the silt content of the soil could not be established. The phosphate concentration, on the other hand, could be dependent on the proportion of organic matter in the soil.
What is striking about the change in phosphate concentration, however, is that the phosphate concentrations, which were very different before the storm surge, almost levelled out. This at least shows the drastic differences that can occur at a site. It is not possible to determine exactly which factors have the greatest influence on this change.

As the nutrient concentrations in the area are difficult to interpret, it is also not possible to explain their influence on the vegetation distribution in more detail. In contrast, the frequency of flooding - and the associated factors of water content, conductivity and proportion of grain size fractions - appear to have a clear influence. The favoured locations of the individual plant communities resulting from these factors have already been explained. This pattern is additionally varied by the different grazing intensity. Juncetum gerardii, for example, is found particularly on soils with a high proportion of fine sediments, which are waterlogged due to the higher water capacity and are grazed.

In conclusion, it can be stated that frequent flooding and extensive grazing would be necessary for the preservation and spread of the species-rich vegetation of the upper salt marsh. The Ononido-Caricetumand Juncetum gerardii should be emphasised here in particular. If the frequency of flooding continues to increase in the coming years, it is also possible that other halophytes will increasingly appear in the study area. Grazing in the eastern part of the "Großer Schlopp" should also be maintained, as it restricts the spread of species-poor communities such as Atriplici-Agropyretum pungentis.

It should be noted that changes in the salt marsh are due to a complex interplay of changes in geomorphology, flooding frequency, the sediment load of the water and its salinity, soil properties and grazing.

Diploma Programme Biology, Leibniz University Hannover: 113 p. (second reviewer)

Salinity and temperature of seawater can be used as parameters to monitor exchange processes between the Wadden Sea and the North Sea. The current velocity, which is increased during upwelling and downwelling, showed positive correlations with turbidity, seston content and abundances of microphytobenthos in the water column. Due to the topography of the area, a nutrient gradient runs from the coast to the North Sea with increasing concentrations towards the mainland, which is expressed in a pronounced tidal dependence of all investigated nutrient salts (forms of nitrogen, phosphate and silicate) with maximum concentrations during the NW and minimum concentrations at the time of the HW.

The Redfield ratio revealed a marked P limitation of the water column on 24 May, while all other samples were largely N-limited, as the lowest P concentrations of the entire year were measured here due to unclear circumstances. The phytoplankton in the Spiekeroog back mudflats is characterised by diatoms and exhibits seasonal succession. For example, a monodominant Rhizosolenia imbricata bloom was recorded on 21 June, while increased abundances of the dinoflagellate Gymnodium splendens were detected in September. In addition, a tidal dependence of the phytoplankton became apparent, which is expressed in increased cell numbers of diatoms and biomasses during the HW. This means that holoplanktonic species from the neighbouring North Sea were transported to this area, where they were unable to reproduce due to increased feeding pressure and nutrient salt concentrations as well as reduced light conditions. The statement by GÄTJES (1998) that the back mudflats are a sink for holoplankters and an export area for meroplankters could thus be verified.

A benthopelagic exchange takes place: Microphytobenthos makes up 2-50 % of the water column and shows a strong tidal dependence and seasonality - the highest concentrations were detected in September. A succession of microphytobenthos is evident - dominant species only appeared in the water column from July onwards. The tidal dependence is expressed in increased abundances during high current velocities, as this is when the resuspension of benthic material into the water column is at its highest. This confirms the positive correlation between current velocity and resuspension of microphytobenthos postulated by DE JONGE & VAN BEUSEKOM (1995) and others.

Diploma programme in Biology at Leibniz Universität Hannover: 69 p. (second reviewer)

The study of phytoplankton plays an important role in understanding the relationships in a marine ecosystem and a change in species composition can draw attention to possible anthropogenic influences, such as eutrophication or climate change, at an early stage.

Very little is known about the phytoplankton in the Wadden Sea of Lower Saxony. From April to August 2007, water samples were therefore taken weekly from three water depths in the Otzumer Balje between Spiekeroog and Langeoog and analysed for their nutrient content, chlorophyll a and carbon concentration as well as the concentration and species composition of the phytoplankton. Both the differences in the water depths and seasonal changes were to be emphasised.

The investigation of the water depths shows that the water column in the Otzumer Balje is evenly mixed and that both the nutrients and the phytoplankton are evenly distributed everywhere. There is a continuous change in the number of individuals and dominant species over the course of the seasons. The trend already observed in other marine areas is also confirmed for the Wadden Sea of Lower Saxony: A spring bloom at the end of March, during which the highest cell numbers of the year are reached, is followed by a second, much smaller diatom bloom in June. Only from July onwards do dinoflagellates make up a significant proportion of the phytoplankton population, before that diatoms and small flagellates dominate.

The connection between plankton blooms and geochemical and sedimentological processes becomes clear through the parallel investigation of the concentration of dissolved molybdenum. This is presumably removed from the open water by aggregate formation after the summer bloom. Although a similar succession of phytoplankton has been observed, too little is known about the individual species, their ecological requirements and the reason for their dominance. In addition, regular studies over many years are required to recognise changes in the plankton population, which could serve as an indicator of a change in the tidal flat ecosystem.