Keeping the water clean — Seaweed biofiltration outperforms traditional bacterial biofilms in recirculating aquaculture
Introduction
Reducing the negative environmental impacts associated with aquaculture activities is key to ensuring the long-term sustainability of the industry (Troell et al., 2003). One potential avenue is the increased use of land-based recirculating aquaculture systems. Such systems allow more effective control of culture conditions, and permit many negative impacts on the surrounding environment to be minimised (Piedrahita, 2003). Waste products can be contained within the facility, habitat degradation is decreased or eliminated, and decreased interaction between culture organisms and wild organisms prevents spread of disease or escape of exotic species (Emerson, 1999, Piedrahita, 2003).
However, the accumulation of nitrogenous wastes is a major drawback of recirculating systems; such wastes are typically excreted by the culture species as ammonium (Randall and Wright, 1987, Wood, 1958, Wright, 1995), which is acutely and chronically toxic to aquatic organisms (Randall and Tsui, 2002, Thurston and Russo, 1983). Therefore, some form of filtration is required to keep these wastes below levels that cause biological and economic harm. Traditionally, biofilm filtration has been employed to control ammonium concentrations (Greiner and Timmons, 1998). Such systems exploit the action of Nitrosomonas and Nitrobacter bacteria (van Rijn, 1996, Watten and Sirbrell, 2006), which work in tandem to oxidise highly toxic ammonium to largely inert nitrate in a two part process. However, this system has several disadvantages because i) nitrifying bacteria compete with the culture species for oxygen (Greiner and Timmons, 1998) ii) nitrate can be converted to the highly toxic compound nitrite under anoxic conditions (van Rijn, 1996, Watten and Sirbrell, 2006) and iii) systems using biofilm filtration typically acidify over time due to respiration of both the culture species and the biofilm (van Rijn, 1996). These factors can all contribute to decrease survival, growth and reproduction of the culture organism. An additional issue is that biofilm filtration does not make commercial use of waste nitrogen.
In contrast seaweeds are able to convert nutrient-rich effluents into biomass, which can then be harvested and used to feed the aquaculture species or sold for human consumption, pharmaceuticals, etc. (Chopin et al., 2001, Neori et al., 2004). Thus, the integrated growth of seaweeds can add significant revenue to an aquaculture concern (Chopin et al., 2001, Shpigel and Neori, 1996). Several studies have investigated integrated multi-trophic aquaculture (IMTA), co-culturing Gracilariopsis bialinae Zhang and Zia and Chanos chanos Forsskal (Alcantara et al., 1999), Ulva rigida Agardh and Sparus aurata L. (Ramazanov and Garcia-Reina, 1994), Ulva latuca L. and S. aurata (Ellner et al., 1996), Palmaria mollis Stearn and Haliotis rufescens Swainson (Evans and Langdon, 2000), S. aurata, Haliotis tuberculata L. and U. lactuca (Neori et al., 1996, Neori et al., 1998, Neori et al., 2000, Neori et al., 2003). The success of many of these preliminary studies indicates that the use of seaweeds as biofilters is likely to have significant industrial applications for a range of aquaculture species.
While many studies have proven the feasibility of seaweed biofiltration, none so far have directly compared the efficacy of such systems to biofilm filtration. The aim of the current study, therefore, was to directly compare the efficacy of two seaweed species (U. lactua and Undaria pinnatifida Suringar) in assimilating nitrogenous wastes, produced by blackfoot abalone (Haliotis iris Gmelin), to biofilm filtration.
Section snippets
Experimental systems
All experiments were carried out at Portobello Marine Laboratory, University of Otago, Dunedin, New Zealand. The experimental tanks were situated in a semi-enclosed glass house, which provided protection from the weather while allowing natural light.
The experimental setup consisted of four identical systems, each comprising of an upper filtration tank and a lower culture tank (both 70 l plastic bins), giving a total volume of approximately 80 l. Water was pumped from the lower culture tanks into
Stocking rates and tank effect
There was no significant difference between the biomass of H. iris added to each treatment (DF = 11, F = 0.46, p = 0.716). Similarly, there was no significant difference in the average starting and finishing weight or length distributions of H. iris between the four treatments; in effect, growth (length, weight) was near-zero in all groups (average change in length: 0.05 ± 0.02 mm, average change in weight: − 0.01 ± 0.01 g). The similar stocking rates and size distributions of H. iris added to each
Discussion
The aim of this study was to compare the ability of seaweed and biofilm filtration in controlling ammonium concentrations in a recirculating marine aquaculture system. It was observed that biofiltration is a powerful means to maintain ammonium concentrations within low to undetectable levels, and seaweeds, under the conditions used, are better than biofilm filtration in this capacity. Indeed, seaweeds exhibited an ability to maintain ammonium at a lower concentration than did the biofilm. As
Acknowledgements
A special thanks to Dr. Rodney Roberts from Oceanz Blue NZ Ltd for offering to donate the abalone used in our study, and for the advice offered on various aspects of abalone culture. Staff at Portobello Marine Laboratory (especially Bev Dickson, Rene van Balen and Dave Wilson) are acknowledged for making facilities and human resources available for our experiment. We are also grateful to Mrs Nicky McHugh, Dr. Christoph Mathaei and Mr Ken Miller (all Department of Zoology, University of Otago,
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