A Study Of The Distribution Of Algae In The Two Oceans Aquarium, Cape Town

ABSTRACT

Five years hav~ passed since the Two Oceans Aquarium, Cape Town, opened

it doors for the first time in November, 1995. Since then, a seaweed community

has established itself in the Kelp tank. The seaweeds were brought in as either

epiphytes on mature macro algae or as epiphytes on boulders that were placed

on the floor of the tank. Thus, the community in the tank can be assumed to

have established itself naturally. This study looked at the extent of the species

diversity, zonation patterns in the tank (with particular reference to wall

orientation), depth substrate type, plane (i.e. vertical of horizontal surfaces) and

species distribution with depth. This was done by placing quadrats in transects

down each wall and noting the percentage cover of each species. Ouadrat

depth, substrate type, wall orientation, and plane were noted. The percentage

cover values as well as the environmental data were entered into CANOCO, an

ordination programme for windows. Ordination showed that the tank was

essentially a homogeneous environment with no difference in species

distribution with regard to wall orientation, depth, substrate type, or plane. It

was noted that self-seeded Ecklonia maxima were very unhealthy, small,

deformed and had many epiphytes, with the exception of three plants that were

growing under the plunger. From this it was concluded that water motion, and

not light or nutrients, was limiting in the tank. Schimmelmannia elegans was

found growing in the kelp tank. It had never been described as growing in

South Africa previously. One theory was that the increased daylight length in

the tank, caused by the fact that lights over the tank were often left on for

extended periods, triggered a photoperiodic reproductive cue from the crustose

phase (which was assumed to be growing unrecorded on the Cape Peninsular)

for the sporophyte to grow. However, daylight length in the Kelp tank and

Tristan Da Cunha (where it was first recorded as growing) were the same so

that theory was discarded. Another theory was that the algae was introduced

into the tank in some way. This, however requires further investigation.

Environmental conditions in the tank were investigated using aquarium records.

Light intensity and duration, water temperature, pH, salinity, dissolved oxygen

concentration were all found to be optimal for photosynthesis and growth to

occur. Water motion was seen as the only limiting factor for photosynthesis and

growth as it also has an effect on nutrient uptake. Grazer intensity was also

examined. It was found that grazing by fish was stable; however, grazing by

invertebrates and particularly mesa-herbivores could be increased, by adding

more of them into the tank. It was found that with light being left on for functions

there was no real seasonal change in light duration and this may be a problem

for seaweeds that experience seasonal growth as they are missing seasonal

cues. A management option of having a strictly controlled 11 hours of daylight

during a winter month was proposed as a solution. This study is a good

reference point for future work to be done on seaweed succession in the Kelp

tank.