Photogrammatic methods spanning a 47-year period were used in conjunction with field studies to document the influence of altered catchment hydrology on the dynamics of Zostera capensis Setchell in the Kromme estuary, South Africa. Reduced freshwater inflow and associated lack of sedimentary disturbances, stable salinities and reduced turbidities are considered to have contributed to the increased Z. capensis population. Over the last nine years there has been a fourfold increase in the total standing biomass of Z. capensis. Biomass density (g m-2) increased by a factor of 2.4 and areal cover by a factor of 1.6. The effect of all floods smaller than the 1-in-30 year flood has been reduced by the construction of a recent dam, the Charlie Malan Dam, four km upstream of the estuary. A moderate flood in November 1989 failed to alter the Z. capensis population, in contrast with the 50% reduction recorded, following a flood of similar dimensions prior to the construction of the Charlie Malan Dam.

This paper describes the vegetation of bushpockets in the active coastal dunefields of the eastern Cape, South Africa, and provides an inventory of the bushpockets of the area. The term 'bushpocket' which was previously applied loosely to all shrubland/thicket patches of coastal dunefields, should be used in a generic sense. Two distinct types of bushpockets with different floristics and geomorphology exist within the dunefields. These have been shown to exist in all four dunefields studied. The different types are termed either shrubland or thicket pockets on the basis of the vegetation. The shrubland pockets have several elements of dune asteraceous fynbos, but are dominated by Myrica cordifolia L. The thicket pockets are dominated by broad-leaf sclerophylls, with elements of the Sundays River Scrub as a minor component. The patches of shrubland are distinctly elongated in the direction of the dune crests and normal to the prevailing winds. Thicket pockets are distinct, being elliptical, with the long axis in the direction of the prevailing wind.

Different vegetation types have been identified in the Alexandria dunefield, in relation to geomorphic history of the substrata. Using ordination and classification, these types have been recognized as Dune Scrub, Dune Thicket, Dune Forest and Sundays River Scrub; each is representative of a different stage of vegetation development. Multivariate analysis separated five sites along a geological gradient, enabling an indication of different degrees of vegetation and soil succession. A decrease in Asteraceae and Poaceae together with an increase in the number of herbaceous and succulent species indicate increasing geomorphic age. While none of the sites could be recognized as a specific vegetation, each was characterized by a unique combination of these four vegetation types. Two successional pathways were observed to operate over different time-scales. The first stage of the succession is from Dune Scrub to Dune Forest, while the second is from Dune Forest to Sundays River Scrub. These successions are seen as simple changes in the relative dominance of the four vegetation types, rather than as the replacement of each towards a single climax community. Our proposal is that these specific combinations of community structure can be used as indicators of geological age in eastern Cape coastal environments.

Etude des modifications dans les populations de plusieurs macrophytes dans les estuaires Nahoon et Kwelera, et de la reponse des populations de phanerogames marines et de macroalgues aux flux des marees

Phytoplankton studies in shallow coastal regions have been restricted largely to a select group of species, collectively known as surf-zone diatoms, that accumulate into dense patches along exposed sandy coastlines. Field-work has concentrated on establishing the distribution of these populations, the dynamics of population movement, and the causal factors. The dynamics of population movement include (a) an endogenously controlled vertical migration between the water surface during the day and the sediment at night and (b) a horizontal movement between the surf zone during medium to high energy conditions and the nearshore during calm periods. Much of the work, however, has been restricted to Anaulus australis, and it remains to be shown whether these behaviour patterns are characteristic of surf-zone diatoms as a whole.

The vertical and horizontal movements of the surf-zone diatom Anaulu australis Drebes et Schulz ( = birostratus) (Bacillariophyceae) in response to environmental factors were investigated during a 10-day storm-calm-storm cycle. Evidence conclusively indicated the role of decreasing wave height in transferring the population from the surf zone to nearshore. Progressively fewer cells sedimented out of the water column during the first 7 days when wave height decreased from 3.5 to 1.3 m. As a result, 51% of the surf-zone population was removed from the surf zone on a daily basis, to accumulate behind the breaker line. Beach morphodynamic state, percent break of incident waves on the outer breaker line and rip-current frequency all varied with the wave regime, implying that the diatom population may not be responding directly to changes in wave energy per se. Cell-division frequency (ƒ) varied with the habitat occupied by the cells but remained fairly constant from day to day, irrespective of prevailing environmental conditions. Since the population showed distinct changes in its distribution across the various habitats throughout the 10 days, the growth rate of the whole population varied from a maximum of 0.33 ·. day−1 when the population was largely in the inner surf zone to 0.01 · day−1 when the cells had moved seaward.

Detrital standing mass measured along the Sundays River Beach, South Africa, was consistently high within the surf-zone, exceeding values recorded in the immediate offshore by a factor of 4. An average detrital load of 3.5 kg C per running meter of beach (· m−1) was estimated for the beach-surf ecosystem, making up 91% of the total POC. This component would appear to represent a major C source to the food chains of the ecosystem, contrary to the current concept where the ecosystem is considered to be fuelled primarily by the conspicuous surf-diatom population. A different situation exists within the inner surf-zone, where nearly 50% of the POC is living material consisting of high concentrations of the surf diatom Anaulus birostratus (Grunow) Grunow. This 1:1 ratio is restricted to daylight hours when dense phytoplankton patches are present within the inner surf-zone. At night when the surf-diatom cells become epipsammic, the ratio shifts in favour of detritus. The trophic significance of these POC components is not commensurate with their respective contribution to the POC pool, since A. birostratus cells are of an enhanced value to consumers because they accumulate into patches, reaching concentrations of 109 cells · 1−1. A sharp decrease in the shore-normal distribution of detritus and inorganic matter in the water column occurs at the junction between the rip-head zone and the nearshore. The sudden changes provide further evidence that the seaward limit of rip-heads is the outer boundary of beach-surf ecosystems.

The spatial patterns of N fixation in the salt marshes of the Swartkops estuary, Republic of South Africa, were established by investigating acetylene reduction activity (ARA) along four environmental gradients. Most sites investigated showed high levels of ARA. Surface sediment and rhizosphere ARA were mainly found in the middle regions of the intertidal which correlate with high organic C and moisture levels. Activity along Spartina maritima tillers showed > 98% restricted to the stem region. The Spartinetum community was the major contributor of fixed N to the salt marsh and appears to be responsible for the marsh acting as an exporter of N to Algoa Bay, Republic of South Africa.

We report on the spatio-temporal distribution of Anaulus birostratus (Grunow) Grunow within a beach/surfzone ecosystem as observed during five case studies. The first three exercises consisted of a time-series of observations where the coastal zone was stratified into sub-environments and sampled concurrently. The results indicate definite instances of major temporal changes in the position and structure of the surf diatom population. On certain occasions these changes were accounted for by onshore/offshore shifts in the population, i.e. decreases in the surfzone population size being accounted for by simultaneous increases in the nearshore population and vice versa. In contrast to the above, the entire population was occasionally restricted to the surfzone. The source of incoming cells responsible for the early morning increases in cell concentration could not be established, nor was it possible to trace the exodus of cells in the late afternoon when cell patches disappeared. It is proposed, therefore, that the dynamics of surf diatom populations are more complex than had been anticipated, involving more than one mode of cell advection. The remaining two case studies provided conclusive evidence on the general impermeability of the breaker line to surfzone A. birostratus cells during the day time. This filter action is proposed as the mechanism for patch formation because it allows for the build-up of a concentration gradient in the inner surfzone. A. birostratus cells were restricted to the surfzone and rip-head region. Within this area cell division was restricted largely to the inner surfzone and the nearshore fraction is therefore expected to represent a dormant population.

Article The Response of Surf Diatom Populations to Environmental Conditions. Changes in the Extent of the Planktonic Fraction and Surface. Patch Activity was published on January 1, 1988 in the journal Botanica Marina (volume 31, issue 2).

We describe a shift in our understanding of cell patch dynamics in the surf diatom Anaulus birostratus (Grunow) Grunow as a result of recent findings concerning their vertical distribution. An extension of previous investigations to include the psammic or sediment habitat of the surfzone has shown this surf diatom to alternate from a planktonic and neustonic life mode during the day to epipsammic at night. The pattern and magnitude of this shift are discussed in the light of existing hypotheses, and form the basis of a new approach to the question of patch formation and decay by surf diatoms. Briefly, cells acquire a positive buoyancy during the day, enabling them to occupy the air/water interface despite their high specific gravity. This positioning allows for the build-up of a concentration gradient in the inner surfzone in spite of the excessive offshore drainage brought about by rip activity. The resultant concentration of cells at the surface is a major causal factor involved in patch formation. Evidence is provided to show that during the late afternoon cells switch adherence from air bubbles to sediment particles in order to enter the sediment. In this way the patch disappears but the cells are retained within the inner surfzone. The cycle is repeated when the epipsammic cells elute out of the sand in the early morning. In the absence of cell patches there is still a resident sediment population, although cell numbers are two orders of magnitude lower. We also observed that under these circumstances the elution process in the early morning was suppressed. Preliminary data obtained for other surf diatoms at the Sundays River Beach, South Africa, suggest the dynamics described above for A. birostratus to be common to this group of marine phytoplankton.

Photogrammetric measurements of dye dispersion in a high-energy surf zone provided semi-quantitative measurements of the pattern and flux of water exchange between the surf zone and nearshore. The intensity of current flow, the rip morphology and its position within the surf zone have important ecological implications for the surf zone and nearshore biota. A variety of water-movement patterns was found, ranging from currents which remained confined to the beach terrace to those which involved substantial exchange across the breaker line. In considering rips as exchange mechanisms, two rip types are recognized. Depending on the exchange of water across the breaker line, a rip may be classified as ‘exchange’ or ‘non-exchange’. The ecological significance of these current types is discussed and a classification scheme for rip currents is proposed. Offshore fluxes across the breaker line by rip currents ranged from negligible to 80 m3s−1 rip−1. The estimated maximum flux per running meter of the Sundays River Beach was calculated as 0·0.32m 3 s−1. The half-residence period of surf-zone water ranged from as little as 22 min t0 5 days, averaging 3·6 h.

A new technique of rip-current recording has been employed to provide quantitative data on rip frequency and intensity during twelve aerial surveys along the shores of Algoa Bay, South Africa. Distinct alongshore gradients in surfzone width and beach morphodynamic state were recorded. These gradients, themselves related to the geography of the bay, wind direction and the angle of deep-sea swells, result in a gradient of rip frequency and intensity in the bay. Rip currents always appear to be present along the Sundays River Beach (central and northern shores), but their presence along the southwest shore was restricted to easterly wind-wave conditions. Both rip frequency and intensity were positively and significantly correlated with surfzone width and beach state over the range of sea conditions encountered. Beach state accounted for 62% of the alongshore variability in rip frequency and 35% of the variability in intensity.

Characterishcs of the surf diatom Anaulus blrostratus have been recorded over two 28 h studies in an effort to understand the factors responsible for the formation and decay of cell patches In the surfzone.A sequence of anatomical and rnorpholog~cal changes characterising the process of asexual reproduction were observed in field samples.Division frequency, cell dimensions and cell cond~hon all exhibited diel periodicities which were synchronized with observed variations in cell concentrahon within the inner surfzone.Cell division was restricted largely to early morning.Initiated just before dawn, it reached a maximum frequency of 53 % at 0830 h and ceased by 1900 h.The first appearance of recently divided cells lagged behind cell division by approximately 3 h and coincided with the exponential increase in cell numbers in the inner surfzone.Changes have been observed in the outer appearance of cell frustules, which were smooth-walled during the period of maximum d~vislon.During the late afternoon when division frequency was less than 5 %, more than 30 % of all cell margins appeared irregular.There was a strong diel periodicity in the dimension of the pervalvar axis which increased by 52 "/o between 0300 and 0800 h ; t h ~s increase was in phase with cell division.The formation of cell patches does not constitute a blooming phenomenon, but is, rather; the result of an advection process which appears to be controlled by or linked to the process of asexual reproduction.