Charophytes in man-made lakes, Germany

Charophytes in man-made lakes in Bremen, Germany

Charophytes in man-made lakes in Bremen, Germany

Sebastian Trapp (trapp@zfn.uni-bremen.de) #

In the lakes of Bremen and the neighbouring Lower-Saxon a great number of macrophyte species can be found. Among those are rare and threatened species of charophytes.

Bremen is situated in the northern part of Germany, about 70 kilometres south of the Northsea. The glaciers of the last ice-ages did not reach this far, so the land was exposed to the harsh climate with freezing and thawing of the soil which levelled the landscape. Due to this natural lakes are very rare.

All 11 lakes investigated in this study are of anthropogenic origin. They are gravel-pits which were digged for road construction. Most lakes are close to the motorways A 1 and A 27. The age of the lakes ranges from six to forty years. Their greatest length ranges from 300 m to 700 m, depth of most of the lakes is about 15 m. The majority of the lakes is used for recreation, but two are natural reserves. All recreational lakes are used by angling associations too.

The majority of the lakes is more or less eutrophic, with a lack of oxygen in the hypolimnion at the end of the summer. The amount of total phosphorous during circulation ranges from 0.01 to 0.08 mg l^-1, showing considerable differences from year to year. Due to a salt-dome the electric conductivity in the lakes close to the A 27 is pretty high. It ranges from about 800 µS cm^-1 in the northernmost lake to up to 2800 µS cm^-1 in the natural reserve Kuhgrabensee.

Fig 1 Chara connivens, Gebogen kransblad

Most of the lakes host a rich submerged vegetation. It was investigated using SCUBA equipment. This was necessary since the plants can grow in a depth of up to 11 m. The lakes were mapped using the method of Melzer (1976, 1985). The use of SCUBA allows for a detailed search for small plants which are otherwise easily overlooked. For charophytologists this can be of crucial importance.

The mapping took place in successive years from 1993 to 1997. One lake was investigated from 1991 to 1998. The data collected represent the vegetational dynamics on a year to year basis. Repeated mapping of two lakes during a single year gives a clear impression of the dynamics on a bimonthly scale.

The following species of charophytes have been found during these investigations: 1 Chara canescens 2 Chara delicatula* 3 Chara globularis 4 Chara vulgaris 5 Nitella capillaris 6 Nitella flexilis 7 Nitella mucronata 8 Nitella translucens 9 Nitellopsis obtusa 10 Tolypella nidifica The names follow Krause (1997). * In The Netherlands this taxon is regarded as Chara globularis var. virgata. The vegetation in the lakes is pretty dynamic, and the charophytes are no exception. In fact, since "Charophytes can be viewed as a ruderal group relative to other macrophytes" ( van den Berg et al. 1998) this can be expected, even more so, it seems, if the age of the lakes is considered. Since the lakes are "young", the vegetation could be considered as still being in a primary stage. The investigations clearly showed that this is not the case. The results indicate that after construction of the lakes this primary phase lasts for five years at most. This is underlined by the fact that invasions of charophytes took place more often in lakes which are 20 to 30 years of age than in younger ones. More important, the charophytes in the younger lakes didn’t last, while in older lakes they still spread further at the end of the investigation. For instance, Nitella capillaris in the youngest lake grew only for the first two years of the lakes existence, while it lasted in a 25 years old one for the whole period of investigation.

Chara delicatula, Chara globularis, Chara vulgaris and Nitella flexilis in Bremen and Lower-Saxon cannot be considered especially rare (Vahle 1990, Becker 1997). Their distribution in the area investigated will not be elaborated here. It has been described by Trapp (1995) and remains more or less unchanged. Here I will concentrate on three of the lakes, which lie in close vicinity to each other. The distance between the natural reserve Kuhgrabensee and the recreational lake Stadtwaldsee is barely 300 m. The Waller Feldmarksee (WFS) is 3 km away. This lake, too, is a heavily used recreation area.

Chara canescens grew in the Kuhgrabensee for about a decade. It was first described by Winter, Kuhbier and Kirst (1987). Since 1997 it has vanished. Its numbers have been decreasing for years.

A surprising invasion occurred in the WFS, when in 1996 vast numbers of Nitella mucronata appeared. It grew in great densities together with other plants with a frequency of 15%. The species colonized the deeper parts of the lake. It had not been recorded at all a year before. In 1997 it reached a frequency of 27% and in some parts of the lake it covered the sediment completely. The following year the species started to decline; the frequency dropped to 7% and the coverage of N. mucronata decreased accordingly.

Tolypella nidifica is a very rare species in northern Germany. From here, it had been recorded for the last time at the end of the 19th century (by Migula 1897; see Vahle 1990). In 1991, it was found in two lakes in Bremen (Trapp 1995). In the Stadtwaldsee at this time it was already abundant. T. nidifica is an early species which grows partly in places which at the end of May are not yet overgrown by other plants.

For this reason it is able to grow in the bathing area of the lake. This part of the lake is more or less sheltered from the wind, but the sunshine readily comes in. T. nidifica can grow here in a depth of 0.1 m to 2.0 m. This is the range that cannot be inhabited by higher plants due to the mechanical disturbance by swimmers in summer. The situation is somewhat different in the WFS, where in 1993 there was only a single mass of T. nidifica to be found. It grew in a very dense stand of Chara vulgaris. In the following years the species became more frequent, but numbers and growth places differed considerably. It appears that T. nidifica has to conquer anew its habitats every year. The most clearly marked spread of any Charophyte in Bremen was shown by Nitellopsis obtusa. A detailed discussion is given by Trapp and Kirst (1999). The species is known from the mesotrophic natural reserve Kuhgrabensee since 1980 (Winter et al. 1987). It is still growing in the lake, but only in small numbers in a very restricted area. The spot where it grows has changed, probably two times in the last twenty years. Other than that, the dynamics of the species in this lake is remarkably small. This is decisively different in the other two lakes mentioned. While in the Stadtwaldsee N. obtusa is the most abundant macrophyte at least since the beginning of the nineties, in the WFS it was recorded for the first time in 1995 with about 20 specimens. Since then, it has spread in the whole lake, growing often in dense stands and reaching a frequency of 21%.

There is something puzzling about this. Nitellopsis obtusa in Bremen grows much more vividly in lakes that are heavily used for recreation. Their trophic state is higher than that of the natural reserve. This is in accordance with the findings of Krause (1995). That there is more to this than the changed ecological behavior of one species is clearly shown by the findings of Tolypella nidifica. Even though in 1996 a single mass of the species could be found in the natural reserve Kuhgrabensee, it could not take hold here like it did in the other lakes mentioned.

Clearly, in the recreational lakes there is a factor that favours the growth of charophytes. In the WFS occurs the highest number of charophyte species of all the lakes investigated. Here, eight species can be found. In the Stadtwaldsee it is four, and in the Kuhgrabensee after the disappearance of C. canescens only three. The reason for this is other than in the trophic situation, since the recreational lakes are eutrophic. Rather, the usage of the lakes seems to hold the key. It can be shown that the lakes with the most charophyte species and with the highest coverage belong to those lakes with a very dynamic vegetation. The great dynamics appear to be caused by frequent disturbance, especially by cyprinid fishes. Digging for food, these fishes uproot the macrophytes, as could be frequently seen during the investigations. The fishes are introduced by anglers. Therefore, the natural reserves lack this kind of ichtyocoenosis. These are the lakes with the least dynamic vegetation.

By digging, the fishes create open spots. These open spots appear during the vegetational period. Here, charophytes seem to be at an advantage over higher plants. Carpenter and McCreary (1985) have shown how spots cleared from vegetation by fishes can sustain species which otherwise could not grow in a given lake. Especially Nitellopsis obtusa can benefit from this mechanism, since in the lakes investigated it grows for a longer period of the year than higher plants.

Of course, one has to bear in mind that high fish densities have detrimental effects on water quality by reducing water transparency and resuspending nutrients from the sediment (Richardson et al. 1995). Still, the investigation indicates that under certain circumstances the introduction of cyprinid fishes by anglers can have advantageous effects for ruderal plants like charophytes. How these circumstances are to be defined has yet to be investigated.

Bibliography

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Carpenter, S.R., McCreary, N.J. (1985): Effects of Fish Nests on Pattern and Zonation of Submersed Macrophytes in a Softwater Lake. Aquatic Botany 22, 21-32.

Krause, W. (1985): ‹ber die Standortanspr¸che und das Ausbreitungsverhalten der Stern-Armleuchteralge Nitellopsis obtusa (Desvaux) J. Groves. carolinea 42, 31-42.

Krause, W. (1997): Charales (Charophyceae). In: Ettl, H., G‰rtner, G., Heynig, Mollenhauer, D. (Eds.): S¸sswasserflora von Mitteleuropa, Bd. 18. Jena, Stuttgart, L¸beck, Ulm.

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Richardson, M.J., Whoriskey, F.G., Roy, L.H. (1995): Turbidity generation and biological impacts of an exotic fish, Carassius auratus, introduced into shallow seasonally anoxic ponds. Journal of Fish Biology 47, 576-585.

Trapp, S. (1995): Die Characeen in Bremer Seen. Floristische Rundbriefe 29 (2), 207-211.

Trapp, S., Kirst, G.O. (1999): Nitellopsis obtusa in Bremen. Mitteilungen des Naturwissenschaftflichen Vereines zu Bremen, 42 (2) Festschrift Kuhbier. In print.

van den Berg, M.S., Scheffer, M., Coops, H., Simons, J. (1998): The role of Characean algae in the management of eutrophied shallow lakes. Journal of Phycology 34

VAHLE, H.-C. (1990): Armleuchteralgen (Characeae) in Niedersachsen und Bremen. Verbreitung, Gef‰hrdung und Schutz. In formationsdienst Naturschutz Niedersachsen 5/90, 85-130. Hannover.

Winter, U., Kuhbier, H., Kirst, G.-O. (1987): Characeen-Gesellschaften im oligohalinen Kuhgrabensee und benachbarten Gew‰ssern. Abhandlungen des naturwissenschaftlichen Vereines zu Bremen 40/4, 381-394. Bremen.

Sebastian Trapp

#)trapp@zfn.uni-bremen.de
University of Bremen
FB 2, Marine Botany 
Prof.G.O. Kirst
Postbox 330440
D-28334 Bremen