Hugo Coops

Growth and morphological responses of four helophyte species in an experimental water-depth gradient

Abstract The distribution on shorelines of four helophyte species (two gramineous species, viz. Phalaris arundinacea L. and Phragmites australis (Cav.) Trin. ex Steudel and two cyperacean species, viz. Scirpus maritimus L. and S. lacustris L.) was studied in relation to growth responses in the water-depth gradient. Stands of S. lacustris were found at lower depths relative to the mean water level (average fringe depth 69 ± 19 cm) than stands of Phragmites australis (av. 45 ± 20 cm), S. maritimus (av. 36 ± 8 cm) and Phalaris arundinacea (av. 25 ± 8 cm). The growth responses to a gradient of water depth were studied by planting the four species at five distinct water depths in outdoor basins, and determining morphological parameters and biomass distributions of the species grown for two consecutive years. The biomass of Phalaris arundinacea was reduced below 30 cm water depth, while Phragmites australis and S. maritimus showed reduced biomass at 80 cm water depth. S. lacustris showed no biomass reduction even at 80 cm water depth. An increased above-ground: below-ground biomass ratio in deeper water was demonstrated for each of the species under study; however, the modification of biomass distribution in the gramineous species occurred abruptly in very shallow water contrary to the cyperacean species. Mean basal stem diameter increased with water depth in all four species. Mean stem length increased with water depth in three of the four species. Stem elongation with increasing water depth was strongest in the cyperacean species. The gramineous species showed enhanced formation of adventitious roots at submerged nodes. The similarity of responses to water depth was greatest within each of the groups of gramineous and cyperacean species. The responses reflect the zonation of the species along the water-depth gradient: S. lacustris in relatively deep water, Phragmites australis and S. maritimus in shallower water, and Phalaris arundinacea in very shallow water.

Biomanipulation in shallow lakes: state of the art

The current state of biomanipulation was the subject of muchdiscussion at Shallow Lakes ‘95. This led to a workshop focusing onthe factors influencing the establishment of macrophytes and themechanisms responsible for their stability followingbiomanipulation. The purpose of the current paper is to distilcurrent knowledge on biomanipulation in shallow lakes gleaned fromdiscussions at the conference and recent literature.Biomanipulation should be used in the theoretical context of twoaltemative stable equilibria, as the extreme perturbation requiredto move from a phytoplankton dominated state to one dominated bymacrophytes. Understanding the nature of the factors and mechanismsresponsible for turbid water, is critical if biomanipulation is tobe appropriate. We suggest that with sufficient information,particular components of the fish community may be targeted andprecise figures for removal, designed to exceed critical thresholdvalues, may be set. Without this knowledge, a ‘play-safe’ strategyshould be adopted and at least 75% of the fish removed. Stockingwith piscivores may be a useful additional measure to fish removal.The principal objective of biomanipulation in shallow lakes is togenerate a period of clear water of sufficient length to allowmacrophytes to establish. To this aim, as well as for technicalreasons, biomanipulation is best undertaken in winter and earlyspring to generate clear water as early as possible in the season.In the cases where grazing is important, this coincides with thespring peak of Daphnia spp. Biomanipulation may have to berepeated if macrophytes do not colonise effectively within thefirst season. The factors responsible for the lag in response ofmacrophytes in some cases and the potential mechanisms contributingto the maintenance of clear water in macrophyte beds are discussed.From empirical data sets from many lakes, both a relative increasein the piscivorous fish stock and a reduction in nutrient levelsare thought to be important in stabilising the system in thelong-term. Whether biomanipulation may lead to alternativeequilibria (i.e. high diversity macrophyte communities withpiscivorous fish at high P) is unknown. Further study ofexceptional cases, theoretical modelling and development andanalysis of more long-term (>10 years) case histories isrecommended.

Clear Water Associated with a Dense Chara Vegetation in the Shallow and Turbid Lake Veluwemeer, The Netherlands

The presence of submerged aquatic macrophytes in lakes is affected by the underwater light climate. Lakes with clear water can show abundant macrophyte vegetation, whereas lakes with turbid water usually have a poor submerged vegetation (Moss, 1990; Scheffer et al., 1993). Moreover, macrophytes improve their own light climate by enhancing the water transparency.

Macroinvertebrate communities in relation to submerged vegetation in two Chara-dominated lakes

Relationships between macroinvertebrates and the presence ofsubmerged vegetation were studied in two shallow eutrophiclakes inThe Netherlands, Lake Veluwemeer and Lake Wolderwijd. A shiftfromturbid water with sparse macrophyte cover (Potamogetonperfoliatus, Potamogeton pectinatus) towards clearwaterwith a dense cover of submerged vegetation (Chara spp.)hasbeen observed in the lakes over the past 10 years. RelativelylargeChara meadows (300–500 ha) have recently developed inbothlakes. The composition of macroinvertebrate fauna wasdetermined atsites varying in cover and dominant vegetation type bysamplingsediment and water during 1992 and 1994. Macrophyte biomass,sampling year and vegetation type were the major determinantsofmacroinvertebrate community composition. Valvatapiscinalis,Bithynia tentaculata, Gammarus tigrinus and Chironomussp.characterized the sites with high charophyte biomass, whereasPotamopyrgus antipodarum, Cladotanytarsus sp., Stictochironomus sp. dominated the samples with lowcharophytebiomass. Chara vegetation was different from Potamogeton sp. by showing lower densities of the midgelarvaeEinfeldia dissidens and Cricotopus gr. sylvestris.Seasonal variations in densities of the dominant molluscspecies(V. piscinalis, P. antipodarum) were closelyrelated tothe development of Chara biomass as well as toperiphytoncover on charophytes. Thus, changes of the light climate inbothlakes, which have led to an increase in colonization bysubmergedvegetation (particular Chara meadows), indirectly had alargeimpact on macroinvertebrate communities.

Vegetated areas with clear water in turbid shallow lakes

Abstract During the summer of 1993 large areas of clear water have persisted for months in the turbid shallow lakes Wolderwijd and Veluwemeer (Netherlands). The areas coincide with submerged plant stands dominated by Chara contraria A. Braun ex Kutzing. These observations show that the two contrasting states that have been described as alternative equilibria for shallow lakes, a turbid state and a clear vegetation dominated one, can actually coexist within a lake.

The role of characean algae in the management of eutrophic shallow lakes.

Characeae (charophytes or stoneworts) are anatomically highly developed green algae. The family is divided into two tribes, the Chareae and Nitelleae, which together contain six genera and about 200 species worldwide (Moore 1986). They grow mainly in alkaline freshwater lakes and ponds. The algae are fixed to the sediment by rhizoids, and several species tend to cover the sediment in dense mats, which often are referred to as meadows. Eutrophication has led to a decline in the charophyte vegetation in many lakes. Over the past decade, numerous restoration projects have been carried out to reduce the negative effects of eutrophication. The main purpose of such projects in shallow lakes has been to change the state of turbid water with dominance by phytoplankton into an alternative stable state with clear water and dominance by macrophytes, including charophytes (Scheffer et al. 1993, Moss et al. 1996). Biomanipulation, mostly comprising a temporary strong reduction of the fish stock, may help to induce the switch to clear water, but a fast return of macrophytes seems crucial for the stabilization of the clear water state (Meijer et al. 1994). Charophytes often play an important role in such projects because they are notoriously rapid colonizers (Crawford 1977, Simons et al. 1994, Beltman and Allegrini 1997). Furthermore, dominance by charophytes can be of special value in recreational waters because the plants normally do not reach the water surface. Such meadows cause less nuisance to swimmers or boats than stands of canopy-forming angiosperms. This minireview presents some ecological aspects of charophytes and their implications for management of shallow lakes. The first section presents the

INTERACTIONS BETWEEN WAVES, BANK EROSION AND EMERGENT VEGETATION : AN EXPERIMENTAL STUDY IN A WAVE TANK

Abstract Emergent vegetation development, wave extinction and soil erosion are strongly interrelated processes in exposed riparian zones. The above-ground parts of the vegetation reduce wave energy, while the below-ground parts strengthen the soil. On the other hand, vegetation development may be restricted as a result of wave stress. Interactions between waves, soil erosion, and emergent vegetation were studied during three consecutive years. Two helophyte species, Phragmites australis (Cav.) Trin. ex Steudel and Scirpus lacustris L., were planted in separate bank sections on two types of sediment, sand and silty sand, in a wave tank. Regular waves were transmitted through 4 m wide bank sections with and without helophytes growing on a horizontal part. Bank profiles, wave transmission patterns and vegetation parameters were measured after exposure to waves with a height of 10 cm (Year 1) and 23 cm (Years 2 and 3). Both 10 cm and 23 cm waves affected bank profiles. Erosion of the banks occurred due to downslope transport of sediment. Soil erosion patterns closely reflected the patterns of standing waves over the horizontal part of the bank. Emergent vegetation influenced the erosive impact of waves by both sediment reinforcement and wave attenuation. A smaller amount of net erosion was measured in the wave-exposed sections covered by vegetation than in the unplanted sections. The stands of Scirpus lacustris were damaged due to uprooting of rhizome parts by 23 cm waves, followed by increased erosion of the soil. No damage occurred to the Phragmites australis stands. The greatest wave attenuation (measured as relative wave height reduction) was measured in the fully developed vegetation in August of each year in both types of vegetation.

Dominance of charophytes in eutrophic shallow lakes—when should we expect it to be an alternative stable state?

Abstract Submerged plant dominance and a turbid state with few submerged plants have been hypothesized to represent alternative stable states in eutrophic shallow lakes. Here, we analyze the conditions for occurrence of alternative stable states in shallow lakes further, using Charisma, a simulation model describing the growth of Chara aspera. The model includes seasonality and spatial structure, aspects which were absent in earlier models predicting alternative equilibria. The parameterization of the model is largely based on experimental results and field observations. Over a range of conditions, the model does indeed predict alternative stable states. The range of conditions over which alternative stable states exist, appeared most sensitive to the assumed reduction of local turbidity by plants and the maximum growth rate of the plants. Aboveground biomass disappears during winter in most lakes in temperate regions. Our analysis indicates that from an evolutionary perspective there is an optimum biomass allocation strategy with respect to investment in overwintering structures. Too little investment reduces chances to regain dominance in the subsequent year, whereas too much investment in dormant overwintering structures such as seeds and tubers reduces photosynthesis. Interestingly, the optimal investment is lower for obtaining maximal summer biomass than for realizing the maximum stability of the vegetated state. The model also suggests that a short clear-water phase enhances the probability of vegetation survival. The optimal timing for a clear-water phase is at the end of May or in June, as is indeed the case in many lakes. In line with earlier theory, shallow lakes with a ‘flat’ depth profile are predicted to have the strongest hysteresis. In lakes with a depth gradient, the response to changes in turbidity is predicted to depend strongly on horizontal mixing of the water between vegetation stands and the open water. Hysteresis disappears predominantly due to a strong horizontal mixing of water. In case of little mixing, on the other hand, local alternative stable states are predicted to occur.

Dynamics and stability of Chara sp. and Potamogeton pectinatus in a shallow lake changing in eutrophication level

The submerged vegetation in Veluwemeer (The Netherlands) has shown large changes over the past 30 years. Potamogeton pectinatus L. remained present in the lake even during the hypertrophie period with total phospohorus (P) levels of about 0.6 mg P 1−1. Furthermore, the data suggest that there may be one critical level of Secchi depth for the presence of charophytes (about 0.4 m), whilst there may be two critical P levels (0.3 mg P 1−1 for loss and 0.1 mg P 1−1 for their return). Logistic regression of detailed vegetation maps over the past 10 years (1988–1997) showed that occurrence of Chara spp. was positively related with spring averages of Secchi depth, while the occurrence of P. pectinatus was slightly negatively related with Secchi depth. Both species had a negative sigmoid response to water depth. Sites without vegetation can be easily predicted by water depth and Secchi depth (97% for Chara and 99% for P. pectinatus correctly predicted cases). The sites where the species will appear are difficult to predict (50% for Chara and 3% for P. pectinatus correctly predicted cases, respectively). If the vegetation of a year prior to the census was included as variable, the predictability of the occurrence of P. pectinatus and Chara can be improved to 71% and 53% correctly predicted cases, respectively. Grid cells which had been covered 1 year earlier with dense Chara vegetation showed a different relationship with Secchi depth than the grid cells without earlier cover. Previous dense Chara (>50% cover) stands returned always in the subsequent year independent on water depth or Secchi depth. Measurements in 1995 showed that dense charophyte beds decrease the turbidity to below 1 m−1 (estimated light attenuation, K d ) in high summer, while outside the vegetation the water remained turbid (estimated K d ~3 to 5 m−1). The positive effect on transparency, together with formation of dense propagule banks, may stabilize the Chara vegetation. Dominance of P. pectinatus may be restricted to turbid water, since Chara appears to be a stronger competitor than P. pectinatus in clear water.

Competition between Chara aspera and Potamogeton pectinatus as a function of temperature and light

Abstract Two experiments were carried out to study the interaction between Chara aspera Deth. ex Willd. and Potamogeton pectinatus L. The purpose of the first experiment was to assess the effect of temperature on the rate of emergence and the second was designed to study the effect of light on the competition during the established phase. P. pectinatus tubers sprouted after about 4 days at 16°C and 9 days at 10°C, which was sooner than either oospores or bulbils of C. aspera (about 15 days at 16°C and 27 days at 10°C). When the irradiance was 36 μ mol m −2 s −1 , the ash-free dry weight biomass of both species was about 80% lower than at 416 μ mol m −2 s −1 and the biomass was not affected by neighbouring plants. However, in the higher light treatment the presence of P. pectinatus suppressed the biomass of C. aspera by maximally 63%. The individual biomass of P. pectinatus at high initial plant densities was reduced by maximally 70% by other plants of the same species, but was not affected by C. aspera . It is concluded that P. pectinatus rather than C. aspera has an advantage in the phase of emergence during spring. Furthermore, under higher light conditions and high inorganic carbon concentrations P. pectinatus may be a better competitor for light than C. aspera , because of its canopy placement near the water surface.