suggest that horizontal expansion towards deeper water at sheltered sites is limited by unfavourable substrate conditions. Introduction. Wave exposure plays an ...
Freshwater Biology (1987) 18. 537-544
The relation between wave exposure and distribution of emergent vegetation in a eutrophic lake STEFAN E. B. WEISNER Sweden
Limnology, Departmeni of Ecology, University of Lund,
SUMMARY. 1. Maximum water depth penetration and changes in horizontal distribution during 39 years of the emergent vegetation in a eutrophic lake in southern Sweden were investigated. 2. The capacity of the emergent vegetation to penetrate into deeper water areas was found to be higher at wave exposed than al sheltered sites. 3. Differences in biomass and biomass allocation of the dominant speeies. Phnigmiies auxlralis. between an exposed and a sheltered site suggest that horizontal expansion towards deeper water at sheltered sites is limited by unfavourable substrate conditions.
Introduction Wave exposure plays an impiirtani role in structuring the littoral vegetation in lakes. The \cgetaiion is generally considered to be less productive in wave exposed than in sheltered littoral zones, and distributions of emergent and floating-leaved macrophyte species have been found to shift landward with increasing wave exposure (Thunmark. l'-)M: Hutchinson., 1975: Spence. 1982; Keddy, 19K3). These relations have mainly been attributed to mechanical damage lo the vegetation by waves at wave exposed sites, and the coarse and nutrient poor substrates at wave exposed sites resulting in nutrient deficiency (e.g. Hutchinson, 1975; Spence, 1982). Most investigations concerning distribution of littoral vegetation in relation to wave exposure have been done in oligotrophic takes {e.g. Thunmark. 1931; Keddy', 1983). It seems reasonable that decreased nutrient availability in the substrate, with increasing degree of Correspondence: Mr Stef;in Weisner. Limnology. University of Lund. Box 65. S-221 (X) Lund. Sweden.
wave exposure, does not play the same role in euirophie as in oligotrophic lakes. Another pattern of vegetation distribution in relation to wave exposure might therefore appear in eutrophic lakes, compared to oiigotrophic lakes, The purpose of this investigation was to examine the relation of wave exposure to water depth penetration of emergent vegetation in a eutrophic lake.
The Lake
Lake Krankesjbn is 4.2 km^ in area, is shallow (3ni maximum depth), eutrophic (total phosphorus about 50//gr'; chlorophyll a about 20/vgr'; Secchi depth about 0.4m) and situated in a sandy area in southern Sweden. The water level of the lake has been measured weekly in 1933-34 and since 1946. It is generally highest in winter and lowest in the summer. The water level was lowered around 1890 and 1940-44 (Lundh. 1951), The mean water level is, however, only about 0.1 m lower than in 1933-34. Human interference on the lake vegetation is small. Most of the emergent 537.
538
Stefan E. B. Weisner
vegetation in the lake is Phragmites australhi (Cav.) Trin. ex Steud. accompanied by Acorits calamus L., Cladium mariscus (L.) Pahl., Typha angt4StifoUa L., Typha latifolia L.. Scirpus lacitsirb L. and Sparganium erectum L. in limited amounts.
Methods Maps of the emergent vegetation were made from a black-and-white vertical aerial photograph taken on 22 May 1947 (scale 1:20,000; photographing height 46(K)m) and an infra-red vertical aerial photograph taken on 30 June 1986 (scale 1:3O.O(H); photographing height 460(1 m). The photographs were superimposed on a rectified map of scale 1:10,000 in a Bausch & Lomb Stereo Zoom Transfer Scope"^. Correction of scales and image displacements caused by tilt on the photographs were made in the stereoscope. The horizontal expansion towards (or regression from) deeper water of the emergent vegetation from 1947 until 1986 was measured directly under the stereoscope at sixty points around the lake where maximum water depth penetration had been measured in the field (see below). The expansion/regression was measured as the shortest distance from each point at the lakeward edge of the reeds in 1986 to the lakeward edge of the reeds (or if necessary, the shoreline) in 1947. The horizontal expansion from 1975 (black-and-white vertical photograph taken on II June 1975; scale 1:30.000 photographing height 4600 m) to 1986. at the twelve most exposed and the twelve most sheltered points, was measured in the same way. At sheltered sites, standing litter makes interpretation of the aerial photographs independent of time of the year when the photographs were taken. At exposed sites, however, standing litter may be removed by ice scouring during winter-spring. Shoot growth starts at the end of April and at the end of May mean shoot length is about 1.0 m (Graneli, Sytsma & Weisner, 1983). Difficulties in surveying reed vegetation can therefore be expected mainly on the photograph from May 1947. at exposed sites and at water depths exceeding about 1.0m. However, an investigation made around 1945 revealed that the emergent vegetation at
exposed sites was absent or scarce at this time (Lundh, 1951). and it did not reach water depths of 1.0m until around 1975 (see below). Maximum water depth penetration of the emergent vegetation was measured in September 1984, at sixty points regularly distributed along the lakeward edge of the emergent vegetation around the lake, as the water depth at the deepest growing shoot within a distance of 2m from each point. Sites that obviously were affected by boat traffic and river inlets or outlets were avoided. At the twenty-four sites where horizontal expansion from 1975 until 1986 had been measured on aerial photographs, the water depth at the point corresponding to tbe edge of the reeds in 1975 was measured in July 1986. The relative degree of wave exposure at different sites around the lake was calculated by using wind data and fetch measurements. Keddy (1982, 1985) showed that this kind of calculation can be used to rank sites along a lake shore with respect to wave exposure. The relative degree of wave exposure (£) was calculated as H
£ = y^ exceedance45jK) m from the lakeward edge) monospecific stands of P. mi.srralis. Hereafter, the exposed site will be referred to as ES and the sheltered as SS. The above ground biomass was cut at the substrate surface and separated into living shoots and standing Utter. Below ground biomass was
Relaiion of wave exposure to emergent vegetution 539 sampled with a corer (internal diameter 11 cm). At SS two cores, and at ES one core, per square were taken, to a substrate depth of 80 and 60 cm respectively. Half of the cores were divided into 20 cm substrate depth intervals. Below ground biomass was rinsed in tap water and separated into living roots and rhizomes. All biomass samples were dried to constant weight at 85''C. Subsamples were ground and analysed for nitrogen (N) by the Kjcldahl method, phosphorus (P) by the molybdenum blue method after perchloric/ nitric acid digestion and potassium (K). calcium (Ca), iron (Fe), magnesium (Mg), manganese (Mn), zinc (Zn), copper (Cu) by AAS after perchloric/nitric acid digestion. To obtain ash content plant material was combusted at Substrate cores were taken in July at ES and SS with the corer mentioned above. Subsamples for water content and organic matter content were taken at substrate depths 10, 20, 30, 40 cm on three cores from each site. These were dried at 105°C for water content determination. Loss on ignition at 55(rC was used as a measure of organic matter content. Substrate oxidation-reduction (redox) potential was measured with a platinum electrode against a calomel reference electrode on four intact cores at both sites and at the substrate depths mentioned above. The electrodes were allowed to equilibrate for 10 min before recording.
The substrate at the edge of the reeds, at the twelve most exposed and the twelve most sheltered points, was sampled with the same corer as above in July 1986 and inspected visually.
Results Emergent vegetation was absent or scarce along the northern and eastern shores in 1947. These arc the most exposed shores because of predominant south-westerly winds. In 1986 the extension of the reed vegetation along the exposed shores had increased substantially, and there was a continuous reed border around the whole lake. Along the sheltered shores in the southern and western parts of the lake the distribution of the reeds had, however, changed very little (Fig. 1). At fifty-two of the sixty points the deepest growing emergent species was P. auslralis. Maximum water depth penetration of the emergent vegetation and wave exposure were positively correlated (Spearman rank correlation; r=0.569; /*
