Physico-chemical factors influencing the spatial and temporal distribution of the seagrass. Zostera capensis. in Langebaan Lagoon

Abstract

Seagrass meadows have important ecological roles in coastal ecosystems and provide high-value

ecosystems services compared to other marine and terrestrial habitats. Despite this, there is

growing evidence that this key ecosystems is declining on a global scale. Much of these declines

have been attributed to the effects of human alterations of marine habitats. Continued destructive

activities, in both South Africa and elsewhere, if not managed properly will result in the demise of

seagrass beds from coastal areas. With this in mind, the aim of the project is to determine the main

physico-chemical factors influencing Zostera capensis biomass and distribution in Langebaan

Lagoon. Five sites within the Langebaan Lagoon were selected and within these sites a minimum of

three and maximum of ten Z. capensisbeds were sampled, depending on the availability of the beds.

Five 10 cm core samples were collected from each bed, from which seagrass biomass, density, leaf

length, leaf width and epiphytic algal biomass were estimated. Physico-chemical data were

collected with a CTD to test for the differences in: temperature, turbidity, pH, salinity, dissolved

oxygen and dissolved chlorophyll levels. The results indicated that temperature and salinity are the

main factors driving biomass and distribution along Langebaan Lagoon. Increasing temperature

appears to have a negative effect on both leaf length and leaf width, while increasing salinity has a

positive effect on seagrass biomass. Although, the ancestral seagrass populations thrived in

conditions of higher temperatures, today the rates of change along the coast is so rapid that

seagrass populations cannot keep up with this change. In addition, sites near the open ocean

experienced a higher seagrass biomass than the sites further away, indicating that salinity is an

important factor driving growth and distribution. However, temperature and salinity are not the

sole factor responsible for the distribution of Z. capensis at Langebaan lagoon. Turbidity and

dissolved chlorophyll also appear to impact certain sites more than others. It is thus clear that at

least a few other factors must be considered: the interaction between nutrients, epiphytes and

macrograzers and the plants tolerance to the sediment quality. These different aspects need to be

studied in concert in order to perceive the full range of impacts on the Z. capensis beds and to better

manage these ever-declining key ecosystems.