The marsh vegetation of Kleinmond Lagoon

The vegetation of Kleinmond Lagoon suggests that this system is in transition from an estuary to a coastal lake. Two major types of vegetation were recognized, one which is subjected to soil and water conditions o f marine origin and the other which is subjected to conditions of terrestrial origin. These vegetation types are discussed and compared to the vegetation o f other estuarine systems. Artificial manipulations o f the mouth seem to have resulted in sediment deposition and a freshening o f the system. These unseasonable manipulations also threaten the continued existence o f a number o f species in the system. UITTREKSEL Die plantegroei van Kleinmondvlei dui daarop dat hieidie vlei in oorgang is van "n getyrivier tot "n strandmeer. Twee hcx)fsoorte plantegroei is erken: die wat onderworpe is aan gronden watertoestande van mariene oorsprong. en die wat onderworpe is aan toestande van land<x>rspn)ng. Hierdie plantegroeitipes word bespreek en met die van ander getyriviere vergelyk. Die plantegroei toon dat die kunsmatige manipulasie van die mond tot afsetting van sediment en vervarsing van die stelsel gelei het. Hierdie buitenstydse nianipulasies bedreig ook die voortbestaan van sommige spesies in die stelsel.


INTRODUCTION
The lerm 'lagoon' is usually applied to a semi-enclosed marine system with a permanent (albeit restricted) con nection to the sea and with little or no fresh water input (Caspers 1967).This definition does not apply to Klein mond.hut the term is retained for historical reasons (Bally 1985).This system receives fresh water from various small streams, more saline water as an overflow from the adjacent Bot River Lagoon, and marine water in the form of overtopping and during short periods w hen the mouth is open to the sea.However, the stabilization of dunes and overflow areas by alien acacias and the manipulation of the Bot River mouth have somewhat reduced the saline input into Kleinmond Lagoon.
The hydrological regime of this lagoon differs substan tially from that of Langebaan Lagoon, the Berg River and Uilkraals River (O'Callaghan 1994a. b. c).The mouth of the Kleinmond Lagoon is closed for most of the year.There is usually a connection to the sea after the first winter rains (June) and again in December.These breachings are usually man-induced, and remain for a period varying from a few days to a few weeks.Unfortunately, until recently, no record was kept regarding the dates or duration of these connections to the sea.nor of the depth of the water at the time of opening (D.Coetzee.Klein mond Municipality, pers.comm.).The reasons given for opening the mouth are related to either a threat to adjacent recreational developments (approximately 2 m above mean sea level (MSL)| or the presence of decaying mats of green filamentous algae (Entenmorpha, Chaetomorpha and/or Cladophora) which detract from the aesthetic ap peal of the area.This ad hoc management strategy.
together with other developments have led to a silting up and freshening of the lagoon, especially the western parts.The more the lagoon silts up. the lower the volumes of sea water which can flow into the lagoon w hen the mouth is open, suggesting an evolution towards a coastal lake.This will be advantageous to the growth of plants which thrive under less saline conditions.The spread of Phrag mites has already been noted (O'Callaghan 1982).This scenario is in opposition to the opinions of Van Heerden (1985) who suggests that the lagoon is becoming deeper.The aim of this study is to gain a clearer understanding of the relative distribution and structure of these vegeta tion types.

METHODS
After studying aerial photographs, orthophotographic maps and following field reconnaissance, six transects were demarcated across the marshes of Kleinmond Lagoon (Figure 1).The siting of these transects was deter mined subjectively according to variability in species composition and the relatively undisturbed nature of the vegetation.Details of these transects are presented in Table 1.Elevation profiles of the transects were surveyed using a theodolite, and at least one point on each transect was surveyed to sea level.
Sampling took place on four occasions during 1987 (March, May, September and November) in order to in clude all bulbous and annual plants.Contiguous 1 x 1 m plots were laid along each transect.The cover-abundance of each species within the plots was estimated according to normal phytosociological methods (Braun-Blanquet 1965).Excessive repetition was avoided by not sampling plots in which it was deemed that the floristic data were simply repetitions of data already recorded from adjacent plots.Taxon names follow Arnold & De Wet (1993) and voucher specimens are housed at the herbarium of the National Botanical Institute at Stellenbosch (STE), the Na tional Herbarium (PRE) and at the Stress Ecology Re search Unit at Kirstenbosch.These voucher specimens are listed by O'Callaghan (1994d).The letters MOC refer to indeterminate voucher specimens.
As classical Braun-Blanquet values cannot be manipu lated mathematically, these values were converted accord ing to Table 2.
To plot the distribution of species, each transect was divided into elevation classes of 10 cm.The converted factors were averaged within each 10 cm class and further averaged over the four sampling periods.As some of the species have annual geophytic or hemicryptophytic life cycles, the number next to the species name on Figures 2 to 7 indicates the number of times this species was lo cated through the year.The order in which the species occur along the transect is primarily determined by its lowest starting point and secondarily by its termination point along the elevation gradient.

RESULTS AN D DISCUSSION
The distribution of species along elevation gradients on Transects K1 to K6 are shown in Figures 2-7.Addi tional species in Transects K4 to K6 are listed in Tables 3-5.
At first glance, these transects seem to fall into two groups: Transects K1 to K3 and Transects K4 to K6.

Transects K 1 to K3
These transects were exposed to stronger marine in fluences, albeit seasonal.The soils were coarse, of marine origin, alkaline and contained very little organic matter.

Phragmites australis
Potamogeton pectinatus Sporobolus virginicus dominated the vegetation, par ticularly near the bottom of the transects.
Relatively few species were found on these shorter transects when compared with K4 to K6 (Figures 2-7).Phragmites australis was only found at K3.This species was restricted to the Lamloch area (the eastern part of the lagoon) until 1983, with a small patch at the mouth of the Isaacs River.A number of factors have led to its spread: a) during the 1980/1981 summer season, the footbridge embankment was extended towards the seaward side.This restricted the outflow channel to an area underlain by a rocky sill.The amount and rate of outflow during sub sequent openings of the mouth was thus reduced.As a result, previously transient sediments accumulated in the western parts of the lagoon as they are not being washed out to sea; b) the regular opening of the mouth prevented the water in the lagoon from rising more than 2 m above MSL.An increased frequency of mouth opening (pers.obs.) resulted in a retention of sediments in the system as outflowing water velocities would be reduced: c) the stabilization of the dunes by alien shrubs drastically reduced the amount of saline water entering the western parts of the lagoon.These shrubs trap wind-blown sand, but do not have the soil binding capabilities of the natural

Transects K 4 to K6
The soils were of terrestrial origin, acidic and with a higher organic content.The soils at K6 were slightly dif ferent in that they were acidic at the lower reaches of the transect, but became more alkaline near the top.
Although Juncus kraussii dominated, Sporobolus virginicus was present and sometimes co-dominated, par ticularly on the upper parts of the transect where the effects of the acidic lagoon water were not as pronounced.
Sporobolus virginicus was rare at K4 when compared to the other transects.This was the only transect on the northern side of the lagoon (between the lagoon and the mountains).Preliminary soil analyses indicated differen ces in soil conductivity at this transect relative to K5 and K6.This might indicate a qualitative difference in the mineral content of the soils between K4 and the others.These developments had the nett effect of altering the equilibrium between fresh water and marine input in the MOC number refers to voucher specimen.-I-----T----T-----»---- Much seepage was also noted along the northern shore, resulting in a permanently high water table .Although Sporobolus virginicus might withstand seasonal flooding, it might not withstand a permanently flooded rhizosphere.

Chara globulans
When comparing the vegetation of this system with that of Langebaan Lagoon, the Berg River and Uilkraals River (O'Callaghan 1994a.b, c), the following are imme diately noticeable: I, an increased preponderance of sedges.In the other systems, sedges are mostly restricted to less saline areas and largely consist of Juncus kraussii.At Kleinmond Lagoon, much of the vegetation is dominated by this species, but many other sedges are also present.This increased dominance by sedges indicates the decreased saline conditions which prevail in this system; 2, an increased proportion of annual and ephemeral herbs.This indicates a less predictable system (limited tidal interaction, seasonality, irregular mouth-opening).
Aquatic vegetation is important in this system.Pota mogeton pectinatus was found at K3 to K6.This species disappeared from the western parts during spring and sum mer when salinities increased and the water levels dropped as a result of mouth-opening.This species was present throughout the whole year at K6.In contrast, Chara globularis is a seasonal plant.It was found at K4 during autumn, at K5 during late summer and autumn, and was only absent from K6 during midwinter.
Although Ruppia maritima is the most important aquatic species in the adjacent Bot River Estuary (Bally et al. 1985), it was only found at K4 and K.5 during late summer.Even though it is regarded as a perennial plant (Obermeyer 1966), it can only grow when favourable con ditions prevail: when local depressions fill with fresh water after the mouth closes.It attained its greatest cover after winter, but it died off in summer as the depression again dried out.Under these conditions, Ruppia maritima might be regarded as a 'facultative annual', i.e. a plant which is seasonally absent due to the seasonal demise of its habitat.On these transects, Triglochin bulbosa and Sar cocomia natalensis might also be regarded as facultative annuals with an opposite seasonal phase.They were found in these depressions when the water was low.
This induced seasonal demise of habitat should be an important management consideration.The flowering season for Ruppia maritima is early summer (Bond & Goldblatt 1984), when conditions at Kleinmond Lagoon are not favourable for growth and it is not found in a flowering state.A compromise should be sought between the needs of winter facultative annuals, summer faculta tive annuals and obligate annuals.The induced seasonal conditions prevailing at this site were unsuitable for both the facultative and obligate annuals.The correct manage ment option would be to allow this area to remain flooded until early midsummer, after which it should remain ex posed until autumn.

FIGURE 2 .
FIGURE 2. Distribution o f species along an elevation gradient onTransect K l. 1-4, number o f times species located through year.

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FIGURE 4.-Distribution of species along an elevation gradient on Transect K3. 1-4.number of times species located through year.
FIGURE 5.-Distribution o f species along an elevation gradient on Transect K4. 1 -4, number o f times species located through year.
FIGURE 6.-Distribution of species along an elevation gradient on Transect K5. 1-4, number o f times species located through year.
FIGURE 7.-Distribution o f species along an elevation gradient on Transect K6. 1-4.number o f times species located through year.