The decline in aquatic macrophytic vegetation in a number of water bodies is well-documented. The eutrophication is always indicated as a main cause.
(mostly reed, Phragmites australis)
In many parts of Europe pollution and eutrophication of water bodies started at the end of the 19th century. Slight eutrophication benefits the vegetation growth. Although reed can endure relatively high nutrient loading, already during the 1940s Hurlimann (1951) found declines of the reed beds due to eutrophication in the Zurcher
See and the Vierwaldstattersee. Since the 1960s very severe declines were reported from quite different localities (Table 4).
Table 4. Rates of decrease in the surface area of reed (Phragmites australis) at different localities. The surface area at the beginning of the period is fixed on 100%. The emphasized numbers are the rates of decrease per year (comparable with inflation rates). (1): changes due to natural succession
LOCALITY |
PERIOD |
% DECREASE |
AUTHOR (S) | |
In period |
Per year |
|||
Norfolk Broads |
1880 - 1905 |
100 - 113 |
0.5 |
Boorman & Fuller (1981) |
1905 - 1946 |
113 - 56 |
-1.7 |
||
1946 - 1958 |
56 - 44 |
-2.1 |
||
1958 - 1963 |
44 - 31 |
-6.7 |
||
1963 - 1970 |
31 - 29 |
-1.1 |
||
1970 - 1977 |
29 - 23 |
-3.2 |
||
Lake Constanz |
1926 - 1974 |
100 - 65 |
-0.9 |
Schroeder (1979) |
Pfaffikersee |
1954 - 1966 |
100 - 70 |
-3.0 |
Burgermeister & Lachavanne (1980) |
Havel lakes |
1962 - 1967 |
100 - 69 |
-7.2 |
Sukopp & Markstein (1981) |
1967 - 1972 |
69 - 50 |
-6.3 |
||
1972 - 1977 |
50 - 39 |
-4.8 |
||
1977 - 1982 |
39 - 28 |
-6.3 |
Markstein & Sukopp (1983) | |
NW. Dummer |
1968 - 1974 |
100 - 83 |
-3.0 |
Akkermann (1978) |
Dgal Wielky |
1966 - 1978 |
100 - 33 |
-8.7 |
Krzwosz et al. (1980) |
Vechten |
1973 - 1980 |
100 - 13 |
-25.7 |
Best (1982) |
The rate of decrease ranged from 0.9% to 24.7% per year. The rates were not constant during the whole period. In the Pfaffikersee the main decrease occurred between 1954 and 1966, while in Lake Vechten the reed-covered surface area decreased by 83% in one year (Best, 1982). In Lake Mikolajkie (Ozimek & Kowalczewki, 1984) and Hickling Broad (Boorman & Fuller, 1981), where rapid changes in submersed aquatic macrophytes occurred, the reed beds virtually stayed unaffected.
In the Northbrandenburgian lakes a relatively slight decrease in emergent plants was found compared to the strong decrease in submersed aquatic macrophytes (Jeschke & Müther, 1978)
Most losses of reed progress from the water side of the reed bed to the shore, sometimes followed by expansion of the emergent Typha sp. (Sukopp, 1971; Dinka et al. 1979) or floating-leaved Nuphar lutea (Cragg et al. 1980).
Denmark (Randers Fjord)
Sand-Jensen (1977) mentioned Danish investigations in the Randers Fjord where large areas with richly developed submersed macrophytes, described in the 1950s, had disappeared in 1975 from the innermost 17 km of the Fjord.
Finland
Toivonen (1985) compared data from the late 1940s with records of non-rooted, floating-leaved and submersed vascular plants made between 1976 and 1979 in 54 different lakes. They found that five lakes had become hypertrophic. The submersed vegetation had greatly decreased or was restricted to small lagoons. Lemna minor and Ceratophyllum demersum appeared to be the only species highly resistant to eutrophication.
German Democratic Republic (Northbrandenburgian lakes)
Jeschke & Müther (1978) describe the changes in the vegetation of two lakes (Grienerick-See, 96 ha and Rheinsberger See, 269 ha) between 1964 and 1974. They distinguish 25 types of aquatic vegetation. One vegetation type, characterized by large leaved Potamogeton species P lucens and P. perfoliatus), became extinct while 8 types were in a regression phase. Communities with Characeae nearly became extinct while communities with Stratiotes aloides and Hydrocharis morsus-ranae decreased.
Vegetation types, in which Myriophyllu spicatum and Potamogeton pectinatus together dominated, had expanded since 1966. The community dominated by Myriophyllum spicatum mixed with Nuphar lutea was the most stable, while the monospecific vegetation of M. spicatum considerably decreased.
The eutrophication stronger influenced the submersed than the emergent macrophytes and degeneration of the reed beds was also reported.
The Netherlands
(Loenderveen- and Loosdrecht Lakes)
Between 1949 and 1980/82 23% of the submersed, 40% of the floating-leaved and 48% of emergent plant species disappeared in the slightly eutrophicated Loenderveen Lakes. Between 1961 and 1980 the Characeae became almost extinct. Close to these lakes are the more polluted.
Loosdrecht Lakes, which are strongly affected by recreational activities. On the average the decline in submersed aquatic macrophytes was far greater in these lakes than in the Loenderveen Lakes (Best et al.. 1984).
(Lake Vechten and Lake Maarsseveen) In two small Dutch meso- to eutrophic sand pits the decline of submersed macrophytes has been accurately documented. Within 6 (or 10 in Vechten) years of the study the area covered by submersed vegetation declined by a factor 6 to 570. In lake Vechten (4.7 ha) the total area of submersed macrophytes decreased by 16.7% per year (Elodea canadensis alone by 23.1%) from 1973 to 1983. In lake Maarsseveen-I (70 ha) the total area covered by submersed macrophytes nearly halved (49.1%) each year between 1977 to 1983. (Elodea canadensis decreased by 60.8% per year, Potamogeton lucens by 34.2%, the Characeae by 65.3%) (Best & Meulemans, 1984)
Poland (Mazurian lakes)
The decline in aquatic vegetation of lake Mikolajskie (460 ha) is well-documented (Ozimek & Kowalczewki. 1984). Although these authors did not found a decrease in the number of plant species, they recorded a strong decrease in biomass by 16.6% per year in the period 1963- 1980 (= 94% over the entire period).
The area covered with submersed macrophytes decreased by 1.1% per year from 1963 to 1971 and further to 4.2% per year from 1971 to 1980. The Characeae, Fontinalis antipyretica, Ceratophyllum demersum nearly disappeared. The total area covered by Utricularia vulgaris, Sagittaria sagittifolia and Stratiotes aloides remained constant while that of Potamogeton obtusifolius, P. lucens, P. perfoliatus, P. compressus, P. pectinatus, and Lemna trisulca increased.
In polluted areas the submersed macrophytes obviously established relatively stable communities composed by Potamogeton perfoliatus, P. pectinatus and Ceratophyllum demersum, while more distant from sources of pollution the changes were greater
Switzerland and the German Federal Republic (Perialpine lakes)
Several Swiss lakes are changing from the meso-eutrophic to the hypertrophic phase. The meso-eutrophic phase is characterized by abundant growth of submersed macrophytes down to 5 to 6 m depth. The Characeae are still present and Potamogeton pectinatus and P. erfoliatus as the dominant submersed species. Well-developed reed beds fringe the lake.
In the hypertrophic stage the submersed macrophytes (like Najas marina and Potamogeton crispus) become scarce or are completely absent at depths greater than 2 m (Lachavanne, 1985). The stands of Phragmites decrease while floating-leaved species like Nuphar lutea, are the only common aquatic macrophytes outside the reed beds. The Characeae disappeared from the Pfaffikersee (Burgermeister & Lachavanne. 1980), Lake Morat (Murtensee) and the Burgaschsee (Lachavanne 1979a&b). In Lake Constance the depth of occurrence of the
Characeae changed from a maximum depth of 25 m in the 1950s to 5-7 m in 1978 (Deufel, 1978).
United Kingdom
(Norfolk Broads, England)
Moss (1980) extensively documented the decline of aquatic macrophytes in the Norfolk Broadlands, an area with ca. 50 mostly small (1- 20 ha) shallow lakes, interconnected by channels with a few rivers. From three Broads the pollen and algal remains in sediment cores were analysed. These remains were dated by the Pb-210 and Cs-137 method or by means of estimation of the sedimentation rate. It became apparent
Fig. 4. Decline of submersed aquatic plant populations in Hickling Broad, reflected in the quantities removed each summer to maintain open the navigation channel through the Broad. Weights are approximate and based on the average weight of each truck load removed (Moss & Leah, 1982; with permission of the authors)
from this paleolimnological study that before 1800 AD Strumpshaw Broad (ca. 12 km from the city of Norwich) was clear with submersed macrophytes rooted in the bottom and virtually no phytoplankton. From 1800 to 1900 the epiphytes and Characeae increased. Moss et al. (1985) call this situation Phase I. In this situation the fisheries are productive and the area is rich in wildlife. In the beginning of the 20th century the Characeae are replaced by taller macrophytes like Potamogeton pectinatis, Myriophyllum spicatum and Ceratophyllum demersum. During that period the eutrophication started by sewage effluents from the nearby city and cultivation and fertilization of the surrounding land. This is called Phase II, the phase with abundant growth of taller submersed aquatic angiosperms together with epiphytic and filamentous algae. The fisheries are still productive and the area is considered important for nature conservation.
Phase II gradually ended in Strumpshaw Broad between 1912 and 1950, while Broads without sewage discharges retained submersed vegetation (Characeae and Najas marina) until the early 1970s. Since the 1960s in much of the Broads system the aquatic macrophytes disappeared (Fig. 4) and phytoplankton started to predominate. This is Phase III, in which also the fisheries became less productive. Increased sedimentation (mud flats) and erosion of the banks became problematic. Some isolated patches of Potamogeton pectinatis, Hippurus vulgaris and Myriophyllum spicatum in the least affected lakes were the poor remains of Phase II between 1974 and 1980.
Hickling Broad (the largest lake; 120 ha)is not affected by human sewage effluent input. However this lake is "guanotrophic" because large flocks of gulls, Larus ridibundus but also starlings, Sturnus vulgaris roost in or near this lake during nighttime. Fish-kills occur almost annually since 1969. These kills are caused by substances, secreted by the phytoplanktonic species Prymnesiurn parvum, which are toxic for fish (Moss, 1980).
(Loch Leven, Scotland) A comparison could be made between data from 1910 and field studies during 1972-74 in Loch Leven (shallow eutrophic, 1330 ha). Jupp & Spence (1977a) recovered only 12 species in 1974 of the 23 aquatic macrophytes documented in 1910. Four species were still common, 8 were scarce. Submersed vegetation occurred to a depth of 5 m in 1910, while in 1974 the maximum depth of plant distribution was 1 m, a five fold decrease! There was a continuous decline in the the number of Chara aspera beds, while Nitella sp. slightly increased. Potamogeton filiformis and f. pectinatus became more abundant. The turbidity of the lake strongly increased with the number of blue-green phytoplanktonic (esp. Anabaena) and filamentous algae (Cladophora, Oedogonium and Enteromorpha).
(Llangorse Lake, Wales) Only one submersed plant species (Zannichella palustris) was found in 1977 at Lake Llangorse (150 ha), whereas in 1972 still 10, and in 1964,12 species were documented. (Cragg et al. 1980). Potamogeton crispus, P. pectinatus, Elodea canadensis, Ranunculus circinatus and the Characeae disappeared between 1972 and 1977.
United States of America (Chesapeake Bay)
Davis (1985) reported a paleolimnological study of fossil seed assemblages in the sediment of the Upper Chesapeake Bay, an estuary with a gradient in salinity. The major changes in the aquatic macro- phytic community were related to human impacts, because immediately after the colonist settlements in 1730 the vegetation started to change in species composition. According to observations performed between 1968 and 1975 the area covered by submerged aquatic plants in the Upper Chesapeake Bay decreased by 30% per year Only two (Myriophyllum spicatum and Vallisneria Americana) of the 11 species recorded in 1971 still remained in 1978 (Orth & Moore, 1984).
For the lower (brackish) bay region Orth & Moore (1983) reported an overall decline in eel grass, Zostera marina in the 1930s. After this pandemic decline (also occurring in Europe the vegetation recovered within about five years. The area covered with Z. marina increased on the average by 1.3% per year between 1937 and 1965. A far more drastic decline started at the end of the 1960s and accelerated during the 1970s. The mean rate of decrease was 9% per year but this could go up to 21% per year at sates that became completely bare. In contrast with the 1930s this decline occurred regionally (was not pandemic) and there was no recovery.
Summary:
There is a characteristic pattern in the events preceding the complete disappearance of aquatic macrophytes:
(i) Optimal conditions for growth occur when the light penetration .is still high in mesotrophic to moderately eutrophic conditions. In this situation (phase II) the water is clear (light penetration down to the bottom or Secchi disc readings more than -: 2 metres). There is a high diversity in species composition and variation in habitat structure in stands of aquatic macrophytes. Dependent on management goals, some waters may show "nuisance growth".
(ii) In a translation phase the species diversity decreases, but some submersed aquatic species can stand high nutrient loadings and still may be a "nuisance".
(iii) Very often a rather sudden change to phase III occurs. The water becomes very turbid (Secchi reading 0.20 to 0.40 m) because the phytoplankton starts to dominate while the submersed aquatic macrophytes become virtually absent.
(iv) In many hyper- or eutrophic lakes the reed vegetation is, decreasing by a median rate of 3% year (range 0.9 to 26%).
(v) Phytoplankton, especially blue-green algae and some macroscopic filamentous algae, will predominate.
(vi) Only some types of vegetation with floating-leaved plants, especially Nuphar lutea and/or lemna spp. remain or are not affected in the same degree as the submersed and emergent are.
