PhD Abstract

The culture of marine organisms (mariculture) is a growing industry worldwide, and both developing and developed countries are now engaging in it.  While it is a good source of protein as well as income, there is a need to ensure that mariculture is conducted in such a way that it causes as little impact as possible to the marine environment. One important way to do so is to clean effluent water from mariculture to levels that allow either re-circulation or release to the sea according to allowable mariculture effluent standards. A variety of filters such as mechanical devices, plants, animals, and bacteria have been used to strip nutrients from mariculture effluents. The use of seaweeds has developed as one way of biofiltration that can clean mariculture effluents effectively at low costs for profitable and sustainable mariculture operations.

For seaweeds to perform well as biofilters, there should be some water movement that facilitates the diffusion of materials in and out of the seaweeds. This being the case, in most of the reported works, culturing of the seaweed biofilters has involved the use of costly ponds with electrically driven pumps and aeration systems to produce agitation for the seaweed biofilters so as to enhance their performance in nutrient removal. These operations, however, are expensive and not suitable for non-electrified regions in poorer countries. The aim of this study was to understand the processes resulting from air bubbling agitation and to evaluate the performance of seaweeds, particularly Ulva lactuca but also Ulva reticulata, Gracilaria crassa, Gracilaria conferta, Eucheuma denticulatum and Chaetomorpha crassa, as biofilters under culture regimes that do not use electricity to agitate the seaweeds.

Agitation of the seaweed Ulva lactuca by only water exchange and simulated tidal variation in water levels resulted in biofiltration performances that did not differ significantly from aerated systems. The benefit of aeration was apparent only in fishpond effluents with low nutrient concentrations. When cultured under very high and high total ammonia-N (TAN) loads (38 g N m^-2 d^-1, 134 mM concentration and 7 g N m^-2 d^-1, 26 mM, respectively), there were no significant differences in areal biofiltration rates (6-7 g N m^-2 d^-1 and  ~3 g N m^-2 d^-1, respectively) between all treatment regimes. Under low TAN loads (below 3.7 g N m^-2 d^-1, 13 mM), significant differences were found between the treatments. Biofiltration efficiencies of TAN varied from 17% under the very high TAN loading to 66% under low TAN loading. Biomass yields (243-279 g m^-2 d^-1) and protein contents (44% < 20% on a dry weight basis) were not significantly different between the treatments. Likewise, there were no differences in dissolved oxygen concentration, pH, biological oxygen demand, temperature and biomass yields between the treatments. 

When subjected to laboratory conditions where water velocities were determined, growth rates, photosynthetic and biomass yields and protein contents were more affected by water velocity under low than under high nutrient concentrations.  However, the experiments proved too complicated to analyse rigorously the short-term nutrient uptake kinetics for comparison with the outdoor data.

In a water irrigation regime where Ulva lactuca was exposed to air (as opposed to the standard submerged regimes), the biofilter gave removal rates that were not significantly different between the irrigation treatment and the control (5.1 vs. 6.8 g N m^-2 d^-1). Also protein content was not different between the treatment (26.9%) and control (24.7%, all on a dry weight basis). Biomass yield, however, was different between the treatment (171 g m^-2 d^-1) and the control (283 g m^-2 d^-1).

             Under a gravity generated water flow regime in earthen ponds in Zanzibar, Tanzania, seaweeds were cultured in channels at the outflow of fishponds and at a channel where seawater entered the fishponds. The removal of nutrients by the seaweed biofilters was significantly higher in the treatment than in the control (e.g. Ulva reticulata removed 0.43 vs. 0.05 g N m^-2 d^-1). Higher seaweed biomass (72 g m^-2 d^-1) and protein contents (26%) obtained in the treatment than control (13 g m^-2 d^-1 and 20%) show that seaweeds can be cultured and used to remove nutrients from mariculture fishpond effluents by simple facilities, using only the water tidal flow energy.

The removal rates/efficiencies, biomass yields and protein contents obtained here under all regimes, including those that employ low-energy agitation, were comparable to, or higher than, those reported in other works where only high-energy requiring systems were used. It is, therefore, clearly shown that seaweeds can perform efficiently as biofilters of land based integrated mariculture systems when subjected to low energy culture regimes that induce water movement and thenceforth agitation of the seaweed biofilters without aeration - as long as the biofilters are kept wet even if they are not submerged in water. The results show that water movement has a direct effect on the performance of seaweed biofilters in land-based mariculture, but that such water movement can sufficiently be achieved by non-expensive, gravity-driven, systems. From the results of this study, it is also concluded that the effect of biofilter air-agitation is not directly related to photosynthesis and carbon uptake, but to the diffusion and uptake of other nutrients. Therefore, once nutrient concentrations are high enough (above about 7 mM of TAN and other nutrients in accordance with it), aeration per se is not essential for effective biofiltration, at least not with the most effective biofilter Ulva.