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

Abstract

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 water's 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.

Keywords:

• closed estuary mouth
• mangrove habitat
• population structure
• salinity
• sediment characteristics