The MARE-system (= Marine Artificial Recirculated Ecosystem) represents the first attempt of an indoor low water discharge, multitrophic seawater recirculation system.
The system consisted of several tanks and had a total volume of approx. 5 m³ (Fig. 1).
Two self cleaning conical fish tanks (1), one rectangular tank with baffle plates (2) and one circular tank (3) were connected to form the recirculating system. A pump (4) (type AG8, ITT Hydroair international, Denmark) provided water to two foam fractionators (5) (type Outside Skimmer III; Erwin Sander Elektroapparate GmbH;
Uetze-Eltze, Germany) as well as to the fish tanks. Tanks (2) and (3) received water from the fish tanks (1) by gravitational force. Table 1 presents the technical characteristics of all applied system compartments, such as surface area, water volume and adjusted flow rates.
loop, not included in Fig. 1, containing a holding tank and a pump, was attached to the foam fractionators. The function of this secondary water loop was the automatical rinsing and collection of foam produced by the foam fractionators and thereby collecting the solids removed from the system water.
Fig. 1: Flow-chart of the MARE-system. Solid lines = configuration of the first experimental trial (MARE I), dashed lines = additional components during the second experimental trial (MARE II). (1) = self cleaning tanks used for the cultivation of fish during both experiments, (2) = tank used for the cultivation of N. diversicolor in both trials; (3) = tank used for the cultivation of the seaweed S. chordalis during MARE I and for the cultivation of fish during MARE II; (4) = pump; (5) = foam fractionators;
(6) = photobioreactor system for the cultivation of Nannochloropsis sp. during MARE II: a) pre-treatment unit, b) degassing tower, c) culture units, d) harvesting unit;
(7) = trickling biofilter used during MARE II. Double triangle = tap, arrows = water flow. Dimensions of the components are given in the Tab. 1.
Tab.1: Description of the different compartments of the MARE-system. Water volume and surface area of each compartment are presented for individual compartments such as fish tanks, foam fractionator, microalgae reactor.
system compartment quantity [n]
water volume
[L]
surface area [m²]
adjusted flow rates [L h-1]
fish tank 2 700 0.79 700
worm tank 1 1350 2.08 1400
macroalgae tank 1 2000 2.30 3400
microalgae reactor 3 50 0.03 1.5
foam fractionator 2 55 0.03 1000
biofilter 1 30 35 500 - 1000
Two long-term experiments with different modifications of the system components (details described in 2.1.1 and 2.1.2) were performed: the first trial was done from November, 11th, 2004 until June, 16th, 2005 (217 experimental days) and will be referred to as MARE I. The second trial is indicated as MARE II and was realized from September, 5th, 2005 until February, 15th, 2006 (163 experimental days).
2.1.1 System configuration of MARE I
During MARE I the recirculation system (Fig. 1, solid lines) consisted of two self cleaning conical tanks (1). Each tank was stocked with 85 juvenile Gilthead sea bream (S. aurata, Sparidae) with initial fish weights in the range of 30 to 99 g;
average and standard deviation amounted to 66 ± 13 g. The resulting initial stocking density was 2.5 kg m-³ system volume. Tank 2 was filled with a 0.1 m deep sand layer (grain size ≤ 2 mm; water column 0.7 m) and stocked with the detritivorous worm N. diversicolor (Polychaeta) at abundances of 900 – 950 individuals per m².
Tank 3 was used for the cultivation of macroalgae. This tank was provided with a central aeration device and artificial illumination of 400 àE m-2 s-1 with a day/night cycle of 16:8 hours. The seaweed S. chordalis (Rhodophyta) was cultivated free floating with an initial biomass of 7.3 kg m-² tank surface area.
2.1.2 System configuration of MARE II
Fish biomass was increased compared to MARE I; the conical tanks (1) were each stocked with 35 Gilthead seabream (S. aurata) and 65 additional animals were cultivated in tank 3. Initial fish weights were in the range of 205 to 460 g; average weight and standard deviation amounted to 355 ± 49 g. The resulting initial stocking density was 9.9 kg m-³ system volume. Tank 2 was additionally stocked with a second generation of N. diversicolor, resulting in an estimated initial abundance of approx. 850 individuals per m².
During MARE II two additional components were included (Fig. 1, dashed lines): A photobioreactor system (6a – d) for the continuous cultivation of the microalgae Nannochloropsis sp. (Chrysophyta) was integrated as a bypass between tanks 2 and 3. The photobioreactor system consisted of a disinfection unit, a cultivation and a harvesting unit. In the disinfection unit, water from the recirculation system was pre-
remove residual ozone. Nannochloropsis sp. was cultivated in three acrylic columns (6c). The columns were equipped with light tubes (100 – 400 àE m -2 s-1) and air diffusers. The water flow rate through the photobioreactors was adjusted according to the growth rate of the microalgae. The microalgae were continuously harvested (6d) and treated water flowed back into the main system.
For further details concerning the design and performance of the photobioreactor system, we refer to Kube (2006). Additionally a trickling biofilter (7) was installed in order to maintain low ammonia and nitrite concentrations.