Studies on the Ecology and Production of Seaweeds of Economic and Potential Economic Importance on the Namibian Coast.

ABSTRACT

The seaweeds of the Namibian coast have received little attention from researchers

despite the fact that there is a successful industry based on seaweed exploitation. Beach

cast Graci/aria verrucosa (Hudson) Papenfuss is collected (9000-15000 t (wet) y-1) in

the Luderitz area for the production of agar, Laminaria schinzii Foslie was harvested

for a short period from April 1987 to March 1989 and there is an increasing interest in

utilizing other seaweed species on the coast.

The natural beds of Graci/aria verrucosa in the Luderitz area were mapped and

biomass estimated in summer (August 1991) and winter (January1992) and beach cast

collections were analyzed to ascertain patterns. The distribution of G. verrucosa was

influenced by sediment type with most occurring on firm (constituting at least 60%

106-212 um) sediments. The largest surface area and biomass of the beds occurred in

the northern portion of the Lagoon/Bay system, where the largest biomass of beach

cast occurs. Most of the beds occur between 4-8m depth.

Beach cast follows a distinct pattern at most sites in the northern, more wave exposed

portion of the Bay/ Lagoon system, being highest in autumn/winter when swell is

generally highest. In the southern half of the Bay/Lagoon system beach cast did not

follow an annual pattern. Though the southern porti9n of the system is more wave

'

protected, strong wind driven currents operate which were recorded at 0.44 m s-1

during 40 km h-1 south winds. The northern portion of the system is deeper than· the

southern and thus most of the beds in the northern portion occur deeper than in the

southern portion. It is postulated that the deeper beds are less susceptible to wave and

wind chop action and can attain a large biomass before the plant reaches a size that the

point of attachment can no longer hold (usually during large winter swell).

Gracilaria fragments were grown on ropes to investigate various aspects of growth.

The optimal depth for growth was found to be 0.5-2.5m and the highest (specific

growth rate 11-12 % d-1) growth rate occurred in summer and extended throughout

autumn in years where high temperatures occurred in autumn. Low growth rates

occurred in winter (8 % d-1). Growth rate varied with different seeding weights; large

seeding weights had lower growth rates over 11 days, 12.1 % d-1and5.7% d-1 for

lOg and 70g seeding weights respectively. The reduction was attributed to self shading

and fragmentation of the plants. Seasonal agar yield was highest in autumn/winter and

. lowest in summer/autumn and correlated negatively with,daylength. In plants grown at

various depths the agar yield correlated negatively with growth rate.

Using growth rate (incorporating the factors that effect it) and swell as variables, the

beach cast of Gracilaria verrucosa from 1988 to 1991 was simulated using a matrix

model. The model accounted for over 57% of the variation. By using the model as an

investigative tool it was found that the timing of swell as well as its size were very

important to the total annual beach cast. Largest simulated annual beach cast occurred

during the absence of large swell and the smallest annual beach cast occurred during a

year with large autumn swell. Warm water events increased growth rate and as a result

beach cast was higher. Given the temperatures for the first few months of a year the

beach cast for that year can be predicted for a range of different swell scenarios using

the model.

Seasonal growth rate of Laminaria schinzii varied from 5-13mm d-1, the period of

highest growth was in autumn/summer and lowest in winter and growth in the more

exposed site tended to be lower. Photoperiod was hypothesized to be the most

important controlling factor. Plants in more wave exposed sites had a lower frond

surface area, narrower and thicker fronds, and longer and heavier stipes than those

growing in sheltered sites; these differences were more marked in deep water. The

attachment strength of the plants did not vary with degree of exposure indicating that a

reduction in the surface area offered to wave action was the adaptive route chosen to

cope with wave exposure. In the presence of Ecklonia maxima (Osbeck) Papenfuss the

stipes of L. schinzii tended to be shorter and to have a more solid stipe, approaching

that of L. pallida Greville ex J. Ag.

Percentage cover of Porphyra capensis Kutzing, Aeodes orbitosa (Suhr) Schmitz,

Gigartina radula (~sper) J. Ag. Gigartina stiriata (Turner) J. Ag. and other dominant

intertidal seaweeds showed a marked seasonal growth pattern with a maximum in

summer/autumn and minimum in winter. Peak cover of Aeodes and Gigartina tended

to occur later on the more sheltered shore (Halifax) and throughout the year Aeodes

was more abundant on the more sheltered shore and Gigartina was more abundant on

the more exposed shore (Grossebucht). Sea temperature and light were the most

important factors influencing the seasonal pattern of percentage cover.

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APA

Molloy, F (2021). Studies on the Ecology and Production of Seaweeds of Economic and Potential Economic Importance on the Namibian Coast.. Afribary. Retrieved from https://afribary.com/works/studies-on-the-ecology-and-production-of-seaweeds-of-economic-and-potential-economic-importance-on-the-namibian-coast

MLA 8th

Molloy, Fergus "Studies on the Ecology and Production of Seaweeds of Economic and Potential Economic Importance on the Namibian Coast." Afribary. Afribary, 15 May. 2021, https://afribary.com/works/studies-on-the-ecology-and-production-of-seaweeds-of-economic-and-potential-economic-importance-on-the-namibian-coast. Accessed 21 Jul. 2024.

MLA7

Molloy, Fergus . "Studies on the Ecology and Production of Seaweeds of Economic and Potential Economic Importance on the Namibian Coast.". Afribary, Afribary, 15 May. 2021. Web. 21 Jul. 2024. < https://afribary.com/works/studies-on-the-ecology-and-production-of-seaweeds-of-economic-and-potential-economic-importance-on-the-namibian-coast >.

Chicago

Molloy, Fergus . "Studies on the Ecology and Production of Seaweeds of Economic and Potential Economic Importance on the Namibian Coast." Afribary (2021). Accessed July 21, 2024. https://afribary.com/works/studies-on-the-ecology-and-production-of-seaweeds-of-economic-and-potential-economic-importance-on-the-namibian-coast