|
Previous - Next - Table of Contents
4. RESPONSES OF SUBMERSED PLANTS TO ECOSYSTEM CHANGE4.1 COMMUNITY RESPONSES TO THE DURATION OF PERTURBATIONFactors affecting the growth and distribution of submersed plants have been discussed. In this section one example of short term and one of long term fluctuations in macrophyte community structure as related to natural and anthropogenic stresses will be considered. Short term changes described in the literature are often related to a single perturbation with observations before and after the event. The observation period may vary from weeks as for the study by Bilby (1977) of changes in cover and areal distribution of plants following a stream spate to several years as for changes in macrophyte communities in the Currituck Sound of North Carolina (Davis, in preparation). On the other hand, some lakes in the northern U.S.A. have been studied sporadically for over a half century as for the Iowa Great Lakes (Crum and Bachmann, 1973). Compared with responses of submersed plants to short term perturbations which are usually associated with increased suspended sediment loads in the water, long term changes in plant responses generally are harder to relate to specific environmental changes. Although changes in parameters with time, such as water transparencies, have been found during long term studies, short term changes associated with meteorologica1 conditions and human activities such as dredging may be generally more important in effecting changes in submersed plant populations. Short Term PerturbationsThe observations of Steenis (1947) of the decimation of submersed macrophytes in Reelfoot Lake, Tennessee, following heavy rains in June 1945 represent changes which may occur following a single perturbation. The water level rose and extensive siltation resulted from erosion of hills around the lake. Steenis' observations are summarized in Table 2. Ceratophyllum demersum was the dominant submersed plant before the rains, but it appeared to succumb to increased turbidity and wind action that resulted from increased fetch as the water level rose. Lind and Cottam (1969) reported that C. demersum in University Bay of Lake Mendota, Wisconsin, 'anchors poorly' in the sediment and was restricted to areas of low turbulence. Potamogeton pectinatus remained fairly abundant in places through 1945, but was limited in part by feeding activities of carp and competition with filamentous algae. In 1946 growth of P. pectinatus was more luxuriant and widespread than Steenis had ever observed before. Accelerated growth of Najas guadalupensis and P. pusillus also occurred in some regions of the lake in 1946. Potamogeton pusillus was described as the more aggressive of the two. Ceratophyllum demersum came back strongly in the year following the rains while there was a dramatic growth and spread of Zannichellia palustris. Here then, virtual elimination of the dominant species led to changing niches with rapid recovery and increase in macrophyte diversity.
Table 2. Changes in submersed macrophyte populations in Reelfoot Lake, Tennessee (Steenis, 1947)
Other short term changes in species composition and/or biomass have been reported for a shallow pond (Stuckey, 1971), a shallow flood plain (Jackson and Starrett, 1959), a lake (Oglesby et al., 1976), and a reservoir (Peltier and Welch, 1970). In all of these cases an increase in suspended sediment turbidity resulted from changing environmental conditions. Long Term PerturbationsPerhaps the most significant study of long term changes in submersed macrophyte populations was that by Crum and Bachmann (1973) of lakes of the Iowa Great Lakes region. They took advantage of an excellent opportunity to compare the submersed macrophytes in six essentially contiguous lakes which now have varying trophic states. The time element was added to their analysis through reference to three other studies, the first of which was in 1894. Depth distributions of submersed macrophytes given by Crum and Bachmann are included in Figure 7. These data were especially valuable in establishing relationships at the low end of the Secchi depth scale for several individual species (Figure 8). At the time of the study in 1972, somoe of the northern potamogetons remained in Lake West Okoboji (4.2 m Secchi depth) and Big Spirit Lake (2.8 m Secchi depth) while none were in the more turbid lakes (0.7 to 1.6 m Secchi depth range) (Table 3). Generally, the species remaining in the more turbid lakes were those which have been established as turbidity tolerant (Table 1); however, these same species also were found in the clearer lakes. These observations suggest that the northern potamogetons are sensitive to turbidity and associated changes while plants that are turbidity tolerant may thrive under a variety of conditions. Long term changes in macrophyte populations in lakes and bays in northern United States.
The initial invasion of the clearest lake by Potamogeton crispus is surprising since it often grows in disturbed or eutrophic systems. Migration to the other lakes might be expected in the future. The presence of Ruppia maritima in only one lake is also of interest. Since this species is normally found in saline environments (cf. Netcalf, 1931), a comparison of salinities of these lakes would be of interest. Indeed, further analyses of physical and biological parameters that might affect the distribution of submersed macrophytes seems warranted. Based on a study in which macrophytes of Lake East Okoboji were compared with a 1915 survey, Volker and Smith (1965) suggested a number of factors responsible for the observed changes. These include nutrient enrichment from agricultural runoff and from sewage leading to algal blooms, siltation from agriculture, the use of algicides, depletion of dissolved oxygen, and fluctuations in water level. Crum and Bachmann (1973) suggested that stabilization of water levels in some of the lakes may have led to the demise of certain submersed macrophytes. The possible effects of herbicides in areas of intense agriculture might also be considered (Correll et al., 1978). Long term changes in submersed plants for other systems are summarized in Table 3. Environmental degradation associated with shoreline development and intense use of waterways in and around Put-in-Bay Harbor on an island in Lake Erie, Ohio, led to a drastic decrease in macrophyte presence and density over 70 years (Stuckey, 1971). Changes in the submersed macrophyte community over the period described by Stuckey did not begin with a pristine environment which gradually deteriorated due to human activities. Poor agricultural practices on the island apparently contributed to a heavy suspended sediment load in the study area from the beginning of the studies (Thorndale, 1898 as cited in Stuckey, 1971). Stuckey cites Pieters (1901) as reporting that in 1898 it was impossible to see plants in one area deeper than around 0.6-0.9 m due to turbidity. Stuckey suggested that the demise of the plants over time was related to a combination of warming of the water, increased turbidity and decreased dissolved oxygen. Increased turbulence due to extensive bulkheading along the shore could also have been a factor. In the Pamlico River, North Carolina, submersed macrophytes were usually limited to a narrow band in the deeper part of the littoral where bulkheading was extensive (Davis and Brinson, 1976). Lind and Cottam (1969) studied the submersed macrophytes of University Bay of Lake Mendota, Wisconsin in 1966 and compared their results with those of surveys made by Denniston (1921), Rickett (1922), and others. Compared with Rickett's data, they found that the biomass of Myriophyllum exalbescens (or perhaps M. spicatum) had increased drastically while that of Vallisneria americana decreased and Ceratophyllum demersum increased (Table 3) and Cottam (1969) suggested that eutrophication caused the changes observed in University Bay. Bumby (1977) compared the submersed macrophyte biomass and distribution in Green Lake, Wisconsin, in 1971-1974 with that in 1921 (Rickett, 1924). As for University Bay (Lind and Cottam, 1969), Myriophyllum (M. spicatum) had become dominant, but relative biomass (46 percent was not as great as for M. exalbescens in University Bay. Secchi depths were not given, but Bumby suggested that there had been no change measured in light transmission since 1942. However, decreased light due to suspended sediments and seston in the littoral and the abundance of the filamentous alga, Cladophora sp., were suggested as possible factors affecting the observed changes. Lake Wingra is yet another Wisconsin lake in which Myriophyllum has become dominant (Nichols and Mori, 1971). The relative frequency of spicatum was 68 percent and no other submersed species had a relative frequency of over 10 percent. The authors suggested that the lake was once dominated by potamogetons and Vallisneria americana. The species found in 1968-1969 were much the same as described for the other Wisconsin lakes (Table 3). Vallisneria americana and northern potamogetons had disappeared by 1929, as determined by a herbarium survey. Carp were introduced in the 1ate 1800's and practically eliminated submersed macrophytes from the lake from the 1920's through 1955 (Baumann et al., 1974). The demise of V. americana was associated with the carp infestation, apparently due to increased turbidity. All the long-termed changes described thus far are for northern lakes which apparently had no rosulate populations typical of sandy, shallow areas of some northern oligotrophic lakes. Flora of such lakes has been reported for northern Wisconsin (i.e. Steenis, 1932; Wilson, 1935; Potzger and Van Engel, 1942), New Hampshire (Moeller, 1975), Scottish Lochs (Spence, 1967) and English Lakes (i.e., Pearsall, 1920) as well as other northern European countries (Hutchinson, 1975). Hutchinson (1975) suggested that the oligotrophic Weber Lake in northern Wisconsin had the simplest known macrophyte community for northern lakes of moderate altitudes and latitudes. Potzger and Van Engel (1942) found that all species growing in Weber Lake were basically rooted in sandy sediments and suggested that growth of smaller plants (eight of nine species found) in deeper waters was restricted by a cover of organic sediments. Even in the observed depth ranges, Myriophyllum tenellum and Isoetes macrospora Dur. were frequently etiolated up to near the tips due to sediment cover. Other low growing species included Elatine minima, Eriocaulon septangulare, Juncus pelocarpus f. submersus, Gratiola lutea f. pusilla and Lobelia dortmanna. In comparing three northern Wisconsin lakes of varying trophic states, Wilson (1935) found that most of the rosulate forms were not present in the eutrophic lake. In the other lakes they were present in only trace amounts. Some of the northern potamogetons were present at very low relative biomasses where Najas flexilis was dominant with Potamogeton richardsonii a weak subdominant. From 1821 through 1894, several rosulate forms disappeared from Loch Leven, now eutrophic (Jupp et al., 1974). Several of the species described here as northern potamogetons were present, but had disappeared by 1910. By 1966 Ceratophyllum demersum and perhaps Myriophyllum spicatum had disappeared. Potamogeton pectinatus and Zannichellia palustris were adventive by 1959 and P. crispus was first reported in 1966. To summarize the 1ong-term changes in submersed macrophyte communities described above, a survival index has been developed for plants of the northern lakes of Table 3 (Lake West Okoboji and Big Spirit Lake were omitted since there was comparatively little change in submersed species). This is simply a ratio of the number of lakes in which a species was reported in earlier surveys to the number of lakes in which the species was present when last studied. The survival index was calculated for species which were originally found in three or more of the lakes. As for the turbidity tolerance index, other factors in addition to an increase in turbidity surely played a part 1n changes in plant populations observed. Survival indices calculated and turbidity tolerance indices from Table 2 are:
The tabulation of the survival indices merely emphasizes what is apparent in Table 3. The more cosmopolitan species tend to remain with ecosystem change while the northern potamogetons tend to disappear. As would be expected, there is a tendency toward a positive correlation between the survival index and the turbidity tolerance index. 4.2 CATEGORIES OF SPECIES BASED ON RESISTANCE TO ECOSYSTEM ALTERATIONAn arbitrary grouping of submersed macrophytes based on their tendency to decrease in biomass or disappear with increasing alteration of ecosystems has been developed. Ecosystem alteration can be caused by a number of factors, but the net result is usually an increase in water turbidity. Five categories ranked roughly in order of increasing resistance to change are suggested as follows: 1. Rosulate species found mainly in northern lakes. 2. Northern potamogetons. 3. Tolerant species normally with low biomass in disturbed systems. These species may nave relatively high biomass in pristine systems. 4. Tolerant species normally dominant or subdominant in disturbed systems. 5. Adventure species that appear in disturbed systems and may be dominant to rare. Rosulate SpeciesSignificant mixed rosulate populations appear to be restricted to oligotrophic northern lakes. Their absence elsewhere may be associated with factors related to increasing conductivity of the waters (Moyle, 1945) or, as Potzger and Van Engel (1942) suggested, increasing accumulations of finer sediments. Wilson (1935) said that several rosulate species should be considered terrestrial plants which 'go aquatic' with submergence due to rising water level. These are normally small plants found in shallow waters and maximum depth distribution correlates poorly with Secchi depth (Figures 7 and 8). Myriophyllum tenellum, when present, is often included in rosulate populations since it is usually found with the rosulate species. Some of the rosulate species are:
Northern PotamogetonsThere is a preponderance of evidence that the potamogetons which are restricted to or are most widely distributed in northern areas do not survive the long-term changes that lead to eutrophication. Based primarily on the studies of the northern lakes, the potamogetons most sensitive to increasing eutrophication are:
Of these species probably only P. foliosus can be considered truly cosmopolitan in distribution. Potamogeton gramineus is one of the most sensitive of the northern potamogetons to ecosystem change. Stuckey (1971) cited Pieters (1901) as finding this species in 1898 in Put-in-Bay only on a bar where presumably the sediments were sandy and the water was shallow. In comparing three northern Wisconsin lakes, Wilson (1935) concluded that the comparatively low biomass of P. gramineus found in the most eutrophic lake was related to the greater accumulation of organic soils there. Wilson suggested that this species is a colonizer of primitive soils and disappears as the system develops. Bumby (1977) found P. gramineus outside her transects in Green Lake, Wisconsin in 1971, but was unable to find it again through 1973. Of the northern potamogetons, Potamogeton zosteriformis survives best. Just as the disappearance of P. gramineus is an indicator of early ecosystem changes, the continued presence of P. zosteriformis as other northern potamogetons disappear is an indication of further ecosystem change. Tolerant Species Normally with Low Biomass in Disturbed SystemsAny species may be dominant in a part of a system or throughout the system depending on the conditions. However, for the systems reviewed here, there are some species which are resistant to ecosystem changes relating to decreased water transparency but which are commonly minor components of the systems. The species are:
Except for Najas flexilis which tends to have a northern distribution and Potamogeton richardsonii which belongs to a subsection (Perfoliati) which extends southward, the species in this group are widely distributed. Tolerant Species Normally Dominant or Subdominant in Disturbed SystemsA few native species tend to maintain relatively high biomass in disturbed systems. These include:
Ceratophyllum demersum and Potamogeton pectinatus are widespread, Vallisneria americana extends from south to north with rather spotty distribution, P. Perfoliatus var. bupleuroides forms the southern extension of the subsection perfoliati and Najas guadalupensis is the southern counterpart of N. flexilis. Potamogeton pectinatus is probably the most widespread and abundant of all North American submersed species. In the 1930's this species was the most important of the potamogetons as a food source of game ducks. Potamogetons as a group were the most important waterfowl food in six of the eight North American regions described by Martin and Uhler (1939). Only in two areas, the lower Mississippi and Gulf Coast regions, were other species of greater importance. The most used species there were Cyperus esculentus L. and Ruppia maritima, respectively. The resistance of P. pectinatus to suspended sediment loads and short and long-term ecosystem changes for the most part is consistent with the widespread abundance and importance of this species. Adventive SpeciesAs is now apparent, Myriophyllum spicatum has spread in the past quarter century to many areas in the United States and Canada where infestations often cause problems in human uses of lakes, streams, and reservoirs. As reviewed by Davis and Steenis (1973) intertwining mats of M. spicatum may adversely affect swimming, boating and various types of fishing. Quiescent waters in beds of M. spicatum may be conducive to breeding of mosquitoes and decaying plants in windrows along the shore often cause obnoxious odors. There is confusion in distinguishing between M. spicatum and M. exalbescens but M. spicatum is likely the species involved in irruptions. Observations in the Chesapeake Bay region (Steenis, 1970; Steenis et al., 1971) and Currituck Sound-Back Bay (Davis, in preparation) indicate that Myriophyllum spicatum is susceptible at least to short-term perturbations of the ecosystem. Reestablishment of the species as the dominant is highly variable and may not occur for many years. Potamogeton crispus is another widespread adventive, especially in the northern region. This species is well established as an invader of eutrophic or disturbed waters, but in the Chesapeake Bay area it fluctuates widely with changing conditions (Steenis et al., 1971). Potamogeton crispus became a strong dominant in an Indiana lake and measures were taken to control it (McIntosh et al., 1978). Previous - Next - Table of Contents | ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
