BAR TODAY || Official Quarterly Publication of the Bureau of Agricultural Research

Red seaweed cleans up your tanks
by Ma. Lizbeth J. Baroña

January-March 2004
Volume 6 No. 1

Red Seaweed (Gracilaria heteroclada)

Due to the fish food and fish wastes, your living room aquarium ends up getting dirty and murky after sometime. If you also end up getting tired of having to squeeze in cleaning the aquarium in your weekly schedule, you should know how water environment management is like in intensive aquaculture. But there is a phenomenon that can actually reduce, if not totally eliminate, that mundane yet taxing household job.

A study conducted by the Southeast Asian Development Center(SEAFDEC) examined whether a species of algae, Gracilaria heteroclada, can efficiently absorb nitrogen, and in the process improves the water quality in a recirculating system that supports fish life.

In intense cultivation of aquatic plants and marine life, the quality of the water environment usually becomes an issue. Excessive chemical input from feeds to the fishes' wastes, if not properly managed, leads to the deterioration of the water quality. The water becomes virtually toxic, which often leads to mass kills of the cultured fishes.

However, these waste materials also benefit some organisms. This is because the waste matters serve as nourishment for some filter feeding organisms like bivalves. These waste products usually nitrogenous like ammonium, nitrate, and nitrite, although toxic to some marine organisms, are beneficial to seaweeds.

The process of biofiltration uses microorganisms to break down organic compounds (or to transform some inorganic compounds) into carbon dioxide, water, and salts. Since nitrogen limits the growth of algae in marine and freshwater environments, some species of marine algae have been used as biofilter in wastewater treatment in aquaculture.

For example, sea lettuce, scientifically known as Ulva sp., can remove as high as 90% of ammonium from fishponds. Red algae, or Chondrus crispus, remove 53% of nitrogen from wastewater, thereby improving the water quality. These results led to cultivation of seaweeds with fish species.

What is Gracilaria heteroclada?
Gracilaria heteroclada, or red seaweed, is a fast-growing plant in a natural environment. It has high gel strength, thus providing the plant a good environment for culture, that could result to good agar quality. Aside from its water filter capability, the Gracilaria heteroclada, was also observed for its agar quality when reared in a filter tank of finfish broodstock.

In a 500-ton capacity broodstock tank of grouper and milkfish, two 15-day culture trials were conducted. The broodstock tank passes through a sedimentation tank, then through a filter tank before it goes back to the broodstock tank.

The first run had the G. heteroclada stocked at 1.25 kg per square meter, while the second run had the species stocked at 1 kg/sq m. Water samples were collected from the sedimentation tank, which contained unfiltered water from the broodstock tank, and from the flume, which contained filtered water that passed through the filter tank containing seaweeds serving as biofilters. Growth and gel strength was measured in the first trial, while water was analyzed for nitrogen content in the second trial.

After 15 days of cultivating the seaweeds in the broodstock tanks, it was observed that growth rate in the first trial was slightly higher compared with that of the second run, at 12.25 and 9.4%, respectively. Gel strength in the two trials did not differ significantly.

The total ammonia-nitrogen (TAN) content in the sedimentation tank was at 0.06 and 0.33 mg, while the TAN in the filter tank was slightly lower at 0.03 and 0.27 mg. The total TAN removed for 15 days was 2.30 kg.

The total nitrogen absorbed by the seaweed for 15 days was 220.38 g. This was measured by getting the difference of the total nitrogen content of the seaweed before the experiment, which was at 30.72 g, and after, which was at 251.10 g.

The proponents of the study recommended that Gracilaria heteroclada may be used as natural biofilter in an aquaculture environment.

Source:
Growth and agar quality of Gracilaria heteroclada Zhang et Zia grown in filter tank of the finfish broodstock tank, Ma. Rovilla J. Luhan, Aquaculture Department, Southeast Asian Fisheries Development Center, Tigbauan, Iloilo.