Anusha Rajkaran

Mangrove expansion and salt marsh decline at mangrove poleward limits

Mangroves are species of halophytic intertidal trees and shrubs derived from tropical genera and are likely delimited in latitudinal range by varying sensitivity to cold. There is now sufficient evidence that mangrove species have proliferated at or near their poleward limits on at least five continents over the past half century, at the expense of salt marsh. Avicennia is the most cold-tolerant genus worldwide, and is the subject of most of the observed changes. Avicennia germinans has extended in range along the USA Atlantic coast and expanded into salt marsh as a consequence of lower frost frequency and intensity in the southern USA. The genus has also expanded into salt marsh at its southern limit in Peru, and on the Pacific coast of Mexico. Mangroves of several species have expanded in extent and replaced salt marsh where protected within mangrove reserves in Guangdong Province, China. In south-eastern Australia, the expansion of Avicennia marina into salt marshes is now well documented, and Rhizophora stylosa has extended its range southward, while showing strong population growth within estuaries along its southern limits in northern New South Wales. Avicennia marina has extended its range southwards in South Africa. The changes are consistent with the poleward extension of temperature thresholds coincident with sea-level rise, although the specific mechanism of range extension might be complicated by limitations on dispersal or other factors. The shift from salt marsh to mangrove dominance on subtropical and temperate shorelines has important implications for ecological structure, function, and global change adaptation.

Mangrove litter production and organic carbon pools in the Mngazana Estuary, South Africa

Wood harvesting is reducing the density of adult mangrove trees in the Mngazana Estuary. This is expected to decrease the amount of litter produced as well as the availability of organic carbon to the estuary and the nearshore environment. Pools of organic carbon were identified and measured in non-harvested areas of the forest so that the effect of future (mangrove) harvesting on organic carbon production could be determined. Litter production was higher in summer (2.4 ±0.2gm−2 d−1) than in winter (0.3 ± 0.1gm−2 d−1). Leaf litter on the forest floor was minimal, as a result of tidal flushing and consumption by crabs. Using previous studies it was hypothesised that the fate of litter produced was as follows: 43% is consumed by crabs, 10% decomposes on the forest floor and is taken into the sediment by bacteria, forming a sink of dissolved organic carbon, and the remaining 47% is exported out of the creeks and the estuary to the sea. Particulate organic carbon (POC) that is available for export was derived from crab faeces and the mechanical breakdown of leaves by crabs and tidal action. Significantly higher concentrations of POC were found during summer than winter and significantly more during a spring tide (5.3 ± 0.9mg l−1) than a neap tide (3 ± 0.4mg l−1). Dissolved organic carbon (DOC) concentrations were mostly below detectable limits (<0.2mg l−1). The estuary is probably a sink for DOC, as very little to no DOC was exported out of the estuary. Organic matter comprised 12% (0.3g C) and 9% (0.46g C) of the sediment in the mangrove forests in Creeks 1 and 2, respectively, these concentrations being higher at Mngazana than at other estuaries in South Africa. If mangrove harvesting continues at the current rate of 1ha per year, and if there is no re-growth, no mangrove vegetation will be left after 118 years, and as a result no POC would be exported. But if harvesting increases to 3ha per year, the mangrove forest would disappear after just 40 years. Mngazana Estuary is an important source of mangrove litter and POC for the adjacent marine environment, possibly sustaining nearshore food webs.

A method for monitoring mangrove harvesting at the Mngazana estuary, South Africa

The Mngazana estuary supports the third largest area of mangroves, and probably the largest stand of Rhizophora mucronata Lamk., in South Africa. The objective of this study was to determine the extent of harvesting in the Mngazana mangrove forest, using available aerial photographs and ground surveys. At 19 sites where harvesting was evident the number of juveniles, adults and stumps were counted in three replicate 25m2 quadrats. GIS was used to generate maps, which indicated that approximately 80% of the mangrove forest showed signs of medium to high harvesting intensity, and that harvesting was taking place in easily accessible areas, especially where Rhizophora mucronata was the dominant species. The ESRI ArcMap density function was used to illustrate the number of harvested stumps and adults within a specific area. GIS analyses classified the mangrove area according to three harvesting intensity classes: low intensity with a ratio of adults to stumps of 2:1, medium intensity 1:1 and high intensity 1:2. According to the classes, 21% of the mangrove area was classified as harvested at low intensity, 35.5% was medium intensity and 43.5% was high intensity. The results indicate that areas dominated by Rhizophora mucronata showed a high intensity of harvesting, while areas where Avicennia marina was dominant showed a low intensity of harvesting. The information from this study can be used to make recommendations for the conservation and management of mangroves in the estuary. In a management plan, access to areas where harvesting is concentrated should be controlled so as to minimise impacts and to allow regeneration to take place.

Historic and recent (2006) state of mangroves in small estuaries from Mlalazi to Mtamvuna in KwaZulu-Natal, South Africa

The new forest type classification considers mangroves to be the rarest and most threatened of forest types in South Africa. The aim of this study was to determine the change in distribution and the current population and community structure of mangrove forests along the KwaZulu-Natal coastline. Potential threats to mangroves in South Africa include wood harvesting, altered water-flow patterns coupled with salinity changes, and the potential for permanently open-mouth conditions to become prolonged closed-mouth conditions. A literature review and field sampling were undertaken to gather information on historic and current mangrove forest status. Population structure was assessed using height and diameter at breast height in three 25 m2 quadrats per site in four estuaries with mangroves. Mangroves were completely lost from 11 estuaries between 1982 and 1999. These losses can be related to changes in open-mouth conditions and intertidal habitat, which are important for mangrove survival. The mouth condition of these estuaries was observed over a nine-year period from 1996 to 2005. Mangroves only occurred in those estuaries where the mouth was open for more than 56% of the time. Rhizophora mucronata has been lost from the Mkomazi Estuary and almost completely from the Mlalazi Estuary; the distribution of this mangrove species is very limited and density is low at 0.5 ± 0.4 m-2. Most mangrove forests were regenerating as they had inverse J-shaped curves as well as high adult:seedling ratios (1:2 for Bruguiera gymnorrhiza, 1:19 to 1:7 for Avicennia marina, and 1.8 to 3:1 for R. mucronata). None of the forests showed signs of harvesting for poles, and the greatest threat seems to be altered water-flow patterns due to freshwater use in the catchments and the change of land use from wetland to sugar-cane plantations. Effective management of these mangroves begins with determining the freshwater requirements of the estuaries to maintain the mouth dynamics and biotic communities. Further management is needed to ensure that mangroves are cleared of pollutants (plastic and industrial), and any further developments near them should be minimised. Management plans for each mangrove forest should be drafted to ensure long-term conservation of the mangroves in KwaZulu-Natal.

Response of mangroves to drought and non-tidal conditions in St Lucia Estuary, South Africa

The effect of a prolonged closed-mouth state on the condition of mangrove habitats was studied in May 2010 at St Lucia Estuary, KwaZulu-Natal. The mouth had been closed to the sea since 2002 as a result of artificial mouth closure and drought, which had resulted in non-tidal, dry and hypersaline aquatic conditions. Sediment characteristics, mangrove population structure, and snail and crab abundance, were measured at four sites of differing salinity and water level to determine if there was a relationship between environmental characteristics and biotic response. A site fringing the main channel had the lowest elevation and the highest density of mangrove seedlings and saplings at the waters edge, whereas a dry site at the highest elevation had the lowest density of mangroves with mostly tall adult trees and few saplings. At these two sites the salinity (>34.5) was significantly higher and the surface sediment water content (<30%) was significantly lower than at other sites. Highest tree density was found at a freshwater seepage site, where all size classes were represented, while the permanently flooded site between the Mfolozi River and St Lucia Estuary had the tallest trees but no seedlings or saplings. At the freshwater seepage and permanently flooded sites, porewater and sediment salinity (<10) were significantly lower, while sediment moisture (>55.5%) was significantly higher, than at the fringing and dry sites. Low sediment moisture was unfavourable for mangrove growth and recruitment. Long-term monitoring is needed to document the response of the mangroves to closed-mouth, non-tidal conditions.

Mangrove expansion and population structure at a planted site, East London, South Africa

Avicennia marina (Forrsk.) Vierh. was planted in 1969 at Nahoon Estuary, East London, followed a few years later by the planting of Bruguiera gymnorrhiza (L.) Lam. and Rhizophora mucronata (L.) among the larger A. marina trees. This study tested the hypothesis that mangroves have expanded and replaced salt marsh over a 33-year period (1978–2011). It provides important information on mangroves growing at higher latitudes, where they were thought to not occur naturally due to lower annual average temperatures. It further provides insights on future scenarios of possible shifts in vegetation types due to climate change at one of the most southerly distribution sites worldwide. The expansion of mangroves was measured using past aerial photographs and Esri ArcGIS Desktop 10 software. In addition, field surveys were completed in 2011 to determine the population structure of the present mangrove forest and relate this to environmental conditions. The study showed that mangrove area cover increased linearly at a rate of 0.06 ha y−1, while the salt marsh area cover also increased (0.09 ha y−1) but was found to be variable over time. The mangrove area is still relatively small (<2 ha) and expanded mostly over a bare sandflat area. Avicennia marina was the dominant species and had high recruitment (seedling density was 33 822 ± 16 364 ha−1). Only a few Bruguiera gymnorrhiza and Rhizophora mucronata individuals were found (<10 adult trees), although observations indicate that some young plants are becoming established away from the parent plants. The site provides opportunities for studies on mangrove/salt marsh interactions in response to a changing climate. Mangroves should not be planted in non-native areas as they may become invasive outside their natural range. However, future increases in temperature will certainly lead to a southerly expansion of mangoves in South African estuaries.