Bjørn Rørslett

Principal determinants of aquatic macrophyte richness in northern European lakes

Abstract Determinants of macrophyte species richness were evaluated using data from 641 lakes in Norway, Sweden, Denmark and Finland. The species pool of aquatic macrophytes was carefully delineated prior to establishing the recorded number of species (S) for any lake. The expected richness E[S∣ X ] for a vector of environmental variables, X, related to lake area, altitude, trophic state and several water quality variables. Neither the size of the regional species pool nor the latitude affected S appreciably. Area (A) contributed most to explain variation in species richness. E[S∣A] could be modelled by a Weibull-type curve. The power function employed by the theory of island biogeography is a limiting case of the Weibull model. Residual analysis identified factors which either increased or decreased species richness. A number of cases of low species richness and low floral diversity related to lake regulation, low pH or hypertrophication. Elevated species richness occurred preferably within quite small, often mesotrophic lakes which had undergone lowering of their surface levels. Lakes exhibiting a highly assorted macrophyte flora often experience intermediate disturbance and stress from moderate alterations of their water levels (1–3 m year−1). Overall, meso-eutrophic lakes supported higher species richness than did either oligotrophic or highly eutrophic lakes.

Strategy analysis of submerged lake macrophyte communities: an international example

Abstract The plant strategy approach was used to assess relationships between the submerged macrophyte vegetation and environmental (natural and anthropogenic) pressures acting in four contrasting lakes: Lake Nasser (Egypt), a large man-made water body, and three North European natural or semi-natural lakes, Steinsfjord (Norway), and Loch Dee and Lairds Loch (Scotland). These lakes provide a high-variance data set over large physical, geographical and climatic gradients. Two complementary approaches were used to classify the established-phase strategy types of populations of euhydrophytes found in these water bodies. The relative contribution of each strategy element to the “community strategy” of the whole lake euhydrophyte flora was determined using a simple strategy index technique. The overall set of strategies prevailing in the submerged macrophyte community of each lake was visualised by plotting the percentage occurrence of each strategy type into the appropriate compartment of the triangular established-phase strategy model of Grime (1979). Comparisons were drawn both between lakes and, in the case of Steinsfjord, between the lake euhydrophyte communities present on five sampling occasions over a 58-year period. The strategy approach provided insight into the interaction between environmental pressures (such as trophic state and water level regime) and the macrophyte communities. Thus a high incidence of stress-tolerant traits occurred in the plants of habitats stressed by low nutrient availability in conjunction with low pH. Competition- and disturbance-tolerant traits were more frequent in the plant communities of two productive lakes impacted by regular fluctuation of water level.

Environmental factors and aquatic macrophyte response in regulated lakes - a statistical approach

Abstract In regulated lakes, it is necessary to distinguish between depth and relative elevation. Depth is the height of water overlaying the plants at any time, thus the instantaneous value may be nil. The optimal “depth” and elevation datum is shown to be the median water level. However, depth in its time-averaged sense is not equivalent to relative elevation when the water level fluctuates throughout time. The magnitude of the discrepancy is generally large in shallow waters. Environmental stress factors, e.g. ice-scour, influence aquatics over a much larger part of the “depth” gradient with a variable water level. Factors operating through threshold statistics, e.g. subsurface irradiance, are most adversely influenced by a time-varying water level. In general, regulation impact results in a compressed vertical niche. Simultaneously, the potential niche is shifted into deeper waters. Vegetation data from ninne oligotrophic, regulated Norwegian lakes show significant correlation to the reduced vertical niche defined by ice-scour and threshold irradiance. Water-level schedules determine the actual vegetational response; thus the total probability distribution of water levels should replace the regulation height or mean annual range of water level variation in an analysis of response features.

Lake enrichment by submersed macrophytes: A Norwegian whole-lake experience with Elodea canadensis

The submerged macrophyte Elodea canadensis Michx. heaviliy infested the mesotrophic lake Steinsfjord (SE Norway, area 13.9 km2) in the late 1970s. This study reports an assessment of the impact of Elodea, primarily focussing on the lakes response at the ecosystem level. The species colonized 79% of the lake floor within the 0–6-m depth range. Averaged over the infested area, Elodea attained a peak biomass up to 310 g DW m−2. The combined production of phytoplankton and macrophytes increased significantly, but we could not demonstrate major nutrient enrichment of the lake waters, and in general pelagial primary production declined. However, some short-term dieback incidents, mainly in the summer, clearly enriched the lake waters. The impacted ecosystem rapidly stabilized after the initial perturbation. Sufficient depletion of sediment nutrients, coupled with slow in situ regeneration of nutrients allows for effective negative feedback. Ecosystem complexity (species richness) was largely unaffected, although one aspect of complexity, i.e. evenness, in fact decreased. The observed production enhancement is due to the invaders superior utilization of space and sediment nutrient resources.

An integrated approach to hydropower impact assessment

The submerged aquatic vegetation of 17 Norwegian lakes is described and related to the environmental impacts that result from hydro-electric power (HEP) use of these lakes. Largely based upon physiognomical features, three main community types are discerned. These are denoted as (a) shallow-water, (b) mid-depth, and (c) deep-water community, respectively. The aquatic macrophytes are classified into a plant strategy framework. This classification suggests that these macrophytes frequently exhibit combined traits of the ‘S’ (stress-tolerating), ‘R’ (ruderal), and ‘C’ (competitive) strategies. A plant-strategy index for the lakes is derived from the species classification and related to their HEP use.Broadly, the response features of hydrolake vegetation are: (1) a decline in species richness; (2) the gradual disappearance of the shallow-water and mid-depth communities; (3) a conspicuous absence of vascular submerged macrophytes in storage hydrolakes when lake levels change more than 7 m annually, and; (4) an increased incidence of species possessing plant strategies of the ruderal (R) type. The implications of these results for an environmental impact assessment of hydropower schemes are discussed.

The genus Isoëtes in Scandinavia: An ecological review and perspectives

Abstract Information relating to the Scandinavian quillwort species, Isoetes lacustris L. and I. setacea Lam. (syn. I. echinospora Durieu), is reviewed emphasizing their ecological adaptation to infertile and shaded habitats, where stress results from inadequate levels of light, available carbon and nutrients. In addition, their habitats can feature enhanced disturbance and stress arising from man-made and natural impacts; for example, acidification, eutrophication, water-level alteration, ice-scour and sediment instability. Quillworts have evolved a variety of mechanisms by which these adverse impacts can be mitigated. Such adaptations comprise carbon acquisition through crassulacean acid metabolism (CAM), high root biomass, slow turnover, resource reallocation on the vertical gradient of their habitat and other related features typical for an S-strategist (stress-tolerator). It is emphasized that success of this strategy by no means is guaranteed. In fact, the quillworts are ultimately victims of their own, insufficiently flexible, growth strategy under sustained adverse environmental impacts. Evidently I. setacea fares better than I. lacustris in this respect. This review identified a number of research areas which ought to be further advanced. Specifically we would like to emphasize the need for better insights into the extent and rate of vegetation changes driven by acidification. Contradictory evidence seems to exist on this issue.

Death of submerged macrophytes—Actual field observations and some implications

Using a photographic sampling method, the spatial distribution of dead submerged macrophytes was investigated in lakes Tyrifjord and Steinsfjord, in South Norway. Death incidents were observed in less than 5% of the samples (N = 3257). Distributed sites with unstable sediments had dead plants occuring at all depths, whereas at normal sites, the incidence of dead plants was higher towards deeper waters. In this case, the estimated mortality rose sharply, close to the lower limits of depth extension for the annual Najas flexilis (Willd.) R.&S. and the perennial Isoetes lacustris L. The statistical analysis of death incidence for the predominant macrophyte Isoetes lacustris showed that neither depth nor light intensity accounted adequately for the observed deep-water mortality increase. However, probit-transformed mortality correlated significantly (P < 0.001) to a hazard function, derived from light threshold statistics. Mortality magnitude was sufficient to explain the observed steep deep-water decline of I. lacustris. It is concluded that the commonly-observed double-exponential depth decline of submerged macrophytes could well be invoked by light-induced mortality.

REMEDIAL MEASURES CONNECTED WITH AQUATIC MACROPHYTES IN NORWEGIAN REGULATED RIVERS AND RESERVOIRS

Enhanced growth of aquatic macrophytes have been recorded from some Norwegian rivers regulated for generating hydropower, and reducing the aquatic vegetation to manageable levels is desirable. Weir building is a common practice on Norwegian low flow reaches, but this in turn can result in silting and nuisance growth of macrophytes. Using herbicides to treat aquatic systems is not allowed by the Norwegian environmental authorities and alternative methods of macrophyte control have to be employed. In contrast, many regulated rivers and reservoirs are devoid of macrophytes and the management aim then is to restore native aquatic vegetation. Mechanical methods using harvesters and cutting devices are often difficult to implement in running water. Better results have been achieved utilizing flushing flows to scour sediments and associated macrophytes. Using a sharply peaking flow gives a more efficient control of excessive macrophyte growth because only the initial surge actively scours the river bed. Moss quantities flushed out in a number of controlled flows exhibited a strong log–linear relationship to significant flow, Q, in River Suldalslagen, south-western Norway. Temporarily halting hydroelectric facilities during cold spells to drain the river and thereby expose submerged macrophytes to frost, has been applied to the River Otra in southern Norway. An attendant ice run resulted in heavy loss of macrophytes in this river. The benefit of winter draw-downs combined with flushing flow is partly offset by downstream accumulation of the removed plants. The potential to revegetate shores of reservoirs and impoundments has been studied in Meltingen, a regulated lake in mid-Norway. This field study indicates that amending the shore substrate by light dressing with fertilizer results in enhanced survival of amphibious species. This study also demonstrates the need for sediment seed banks or nearby sanctuaries from which aquatic species can extend into the reservoir during years with a favourable high water level. Reducing the designated fluctuation range of reservoir elevation from 7 to 1.6 m resulted in recolonization by submerged macrophytes in Hartevatn, a reservoir on the upper River Otra.

An integrated approach to hydropower impact assessment. I. Environmental features of some Norwegian hydro-electric lakes

This paper launches concepts instrumental to environmental impact assessment (EIA) studies on hydropower schemes and lake regulations. Norwegian hydro-electric lakes (hydrolakes) and their environmental features are described, and evaluated against non-manipulated waters. A tentative classification of hydrolakes vs. natural waters is proposed. The need for a multiple approach to habitat classification is emphasized. Recommendations for future biological impact assessment approaches are suggested.Hydrolakes differ broadly from natural lakes by combining physical features not ordinarily co-occurring in non-manipulated water bodies. Storage type hydrolakes (reservoirs) feature winter draw-downs and enhancement of yearly level fluctuations; whereas other types of hydro-electric lakes have elevated water levels throughout winter. Hydrochemistry and optics of the studied hydrolakes exhibited no clear differences to non-impacted Norwegian inland waters. All lakes had signs of sublacustrine erosional activity related to internal waves and thermocline movements.

Stereophotography as a tool in aquatic biology

Abstract The details of an underwater stereophotographic system is outlined. This system is based on 35 mm cameras, using high-resolution colour film. A modular construction enables adaptation of the photographic system to a wide range of applications. Interfaced to a computerised data base, the system offers quick and fairly accurate information feedback on the investigated biological communities. Objects down to about 0.5mm natural size are detectable. Spatial relationships and the state of the organisms are easily documented. By time-lapse photography, changes in aquatic communities, e.g. due to pollution impacts, may be well monitored.