T.D. Steinke

Leaching losses during decomposition of mangrove leaf litter

Two experiments were conducted in the laboratory to determine leaching losses from submerged mangrove leaves: the first, on freshly-picked and air-dried senescent Avicennia marina (Forrsk.) Vierh. leaves submerged in water of different salinities (16 and 32‰) for up to 14 days; the second, on freshly-picked senescent leaves of A. marina and Bruguiera gymnorrhiza (L.) Lam. submerged in water of 16‰ for up to 7 days. In both experiments, loss of dry mass and changes in nitrogen, phosphorus, potassium and tannins of the leaves were monitored. In the initial experiment there was a mean loss in dry mass of 24.6% after 14 days. The chemical analyses revealed that, although there was no significant decline in nitrogen concentration, there were sharp decreases in the other constituents. The greatest total loss relative to the amount initially present in the litter was in potassium (83 – 95%), while the losses for phosphorus and tannins were 57 – 87% and 74 – 85%, respectively. In general, losses were greater at 16‰ than at 32‰, and air-dried leaves showed greater initial losses than fresh leaves. In the second experiment, trends in leaching losses of both species were similar to those of the first experiment. With the exception of nitrogen, the concentrations of phosphorus and potassium (expressed as percentage oven-dry mass) had decreased significantly after 6 h in both species, while the concentrations of tannins showed a significant decrease only after 24 and 48 h in A. marina and B. gymnorrhiza, respectively. This work has confirmed that significant losses take place in the leaf litter as a result of leaching. It is suggested that those losses could have important consequences for the estuarine ecosystem.

A note on the distribution and approximate areas of mangroves in South Africa

The occurrence of individual species, of which there are five in South Africa, and the approximate total area of mangroves in all known South African estuarine systems are presented for the first time for South Africa as a whole. Natural increase is occurring throughout the region although this is masked in many places by the direct destruction of the mangroves themselves or through adverse effects on their habitats. S. Afr. J. Bot . 1982, 1: 51-53

Degradation of mangrove leaf litter in the St Lucia Estuary as influenced by season and exposure

Rates of degradation of leaves of Avicennia marina (Forssk.) Vierh. and Bruguiera gymnorrhiza (L.) Lam. were investigated in St Lucia Estuary in the warm and cool seasons at three positions on the shore, viz. where leaves were constantly submerged, where they were exposed only at low spring tides and where they were inundated only at high spring tides. Degradation occurred more rapidly in A. marina than in B. gymnorrhiza and leaves constantly submerged decomposed at a faster rate than those exposed for most of the time. Decomposition rates were consistently lower in the cool season. The concentration of nitrogen in all leaves increased over the decomposition period, although there was a decrease in total nitrogen content with rapid decomposition. While under certain conditions degradation proceeds at a rapid rate, because of the lower additions of nitrogen to the system from decomposing leaves, there do not appear to be significant ecological advantages from this rapid decomposition. There were sharp decreases in the phosphorus and potassium contents of the leaves, presumably largely due to leaching. This work has emphasized the importance of mangrove leaf litter as a source of, and substrate for, nutrient release.

A preliminary study of buoyancy behaviour in Avicennia marina propagules

Propagules of A. marina (Forssk.) Vierh. were shown to be either ‘sinkers’, i.e. sink on shedding their pericarp, or ‘floaters’ i.e. remain buoyant after shedding their pericarp. While propagules from different estuaries showed big variations in buoyancy, generally mangroves in the north produced a high proportion of ‘floaters’ while those in the south produced mainly ‘sinkers’. The buoyancy of propagules appeared to be determined by the relative densities of the different parts of the cotyledons. Anatomical sections revealed that there was little difference between the different parts of ‘sinkers’, whereas in ‘floaters’ parts which were buoyant had a high proportion of intercellular air spaces and those which sank had closely- packed cells high in reserves. Some of the ‘sinkers’ were found to become buoyant after a period of submersion, although propagules from all estuaries did not behave in this way. Apparently those propagules which had a density close to that of sea water tended to become buoyant later and it was suggested that utilization of reserves reduced the original density to a level which enabled these propagules to float. The implications of variations in buoyancy of A. marina propagules for the further distribution of this mangrove along the southeast Cape coast are discussed.

Marine and mangrove fungi from the Indian Ocean coast of South Africa

A total of 93 species of marine fungi were identified from three habitats: mangroves, intertidal rocky shores and sandy beaches. The highest number (55 species) were obtained from mangrove wood, while 12 species and 19 species were collected from intertidal wood and sandy beaches, respectively. The high numbers obtained from the mangroves at Beachwood and on wood of Avicennia marina (Forssk.) Vierh. probably reflect the high intensity of sampling. Although some of the marine fungi can be regarded as cosmopolitan, the majority have affinities with tropical/subtropical areas. The high numbers of marine fungi obtained in this survey suggest that these organisms have a significant role to play in our estuarine and marine ecosystems.

In vitro rates of decomposition of leaves of the mangrove Bruguiera gymnorrhiza as affected by temperature and salinity

Decomposition of leaves of Bruguiera gymnorrhiza (L.) Lam. was studied over a period of 96 days under controlled conditions of temperature (16° and 24°C) and salinity (0‰, 15‰ and 30‰). During the experimental period leaves at 24°C and 0‰ showed the greatest loss of dry matter, while the slowest decrease was recorded at 16°C and 30‰. Nitrogen content of decomposing leaves increased during the experiment, while phosphorus and potassium contents showed a steady decrease. The phosphorus content of the water revealed an initial decrease, attributed to microbial activity, followed by a steady increase. Dissolved oxygen content of the water decreased to a constant low level, while pH values fell initially and then increased again. The rate of decomposition in this laboratory study was slower than values obtained in field studies where physical breakdown occurs in addition to microbial action.

Biological nitrogen fixation (acetylene reduction) associated with blue-green algal (cyanobacterial) communities in the Beachwood Mangrove Nature Reserve II. Seasonal variation in acetylene reduction activity

Seasonal variation in nitrogen fixation of blue-green algae associated with Avicennia marina (Forssk.) Vierh. pneumatophores and wet and dry surface sediments were investigated in the Beachwood Mangrove Nature Reserve by means of the acetylene reduction technique. The blue-green algae identified were all non-heterocystous and have been reported to fix nitrogen. The dominant blue-green algae were Lyngbya confervoides C.Ag. ex Gomont, Oscillatoria limosa Ag. ex Gomont and Microcoleus chthonoplastes Thuret ex Gomont in the pneumatophore, wet and dry surface habitats, respectively. A marked seasonal variation in ARA (acetylene reduction activity) was apparent, with summer maxima being recorded. This coincided with maximum nitrogen-fixing blue-green algal numbers, temperature, light intensity and day length. Seasonal variations in organic carbon, inorganic nitrogen, salinity, percentage moisture and bacterial numbers are discussed. It was estimated that nitrogen fixation by blue-green algal communities supplies 24.3% of the annual nitrogen requirements of the mangrove swamp.

Mangroves of the Kosi System, South Africa: Their re-establishment since a mass mortality in 1965/66

The Kosi System, the most northern estuarine system in the Republic of South Africa, supports five species of mangroves: Avicennia marina (Forssk.) Vierh., Bruguiera gymnorrhiza (L.) Lam., Ceriops tagal (Perr.) C.B. Robinson, Lumnitzera racemosa Willd. and Rhizophora mucronata Lam. All populations, particulary those of A. marina, C. tagal and R. mucronata, were severely set back due to large numbers having been killed in 1965/1966. Recolonization by all is taking place, although growth is slow relative to that in many other estuaries in Natal and, further, the recovery has been hindered by biotic disturbance. Nevertheless the situation in April 1985 was one of active growth with, generally, dense stands of relatively short individuals in almost all parts occupied by mangroves prior to 1965, as well as a spread into places not previously recorded as having been occupied by mangroves. This distribution is shown on maps. Data, recorded periodically since 1972, are presented to illustrate structure of stands and changes in sizes of individuals in selected sites.

A preliminary study of the phenology of Scaevola plumieri

Growth of Scaevola plumieri (L.) Vahl, a coastal dune pioneer species, showed seasonal variations in leaf appearance and abscission. Generally more leaves were produced and shed in the warmer months than in the cooler months. However, rate of leaf appearance exceeded rate of leaf abscission in summer, while in winter the reverse applied. Leaves produced in winter showed greater longevity than summer leaves. Flowering commenced in October and all ripe fruits had been shed by March. Maximum seed germination appears to take place during December/January.

Vegetative and floral phenology of the mangrove, Ceriops tagal, with observations on the reproductive behaviour of Lumnitzera racemosa, in the Mgeni Estuary

Phenological observations were conducted on Ceriops tagal Perr. C.B. Robinson (vegetative and floral) and Lumnitzera racemosa Willd. (floral) in the Mgeni Estuary. Leaf emergence in C. tagal was unimodal with high initial values in early summer (December), followed by decreasing numbers of leaves for the remainder of the summer, with the last leaves making their appearance in June/July. Leaf abscision showed high values in August/September but, with the exception of smaller peaks in April, produced no other consistent trends, Leaf/shoot ratios revealed an undulating pattern with high values in summer and decreasing values in the cooler months. Mean longevity of leaves marked in the first two years of the study was approximately 30 and 32 months respectively, It was not possible to follow reproductive behaviour in C, tagal. In L. racemosa a period of 3–4 months was required for development from flowers to the abscission of propagules. The significance of these results for litter production is discussed.