OUTDOOR POND CULTIVATION OF THE SUBTROPICAL MARINE RED ALGA GRACILARIA-TENUISTIPITATA IN BRACKISH-WATER IN SWEDEN - GROWTH, NUTRIENT-UPTAKE, COCULTIVATION WITH RAINBOW-TROUT AND EPIPHYTE CONTROL

Pond cultivation of the subtropical, euryhaline macroscopic red alga Gracilaria tenuisipitata var. liui Zhang et Xia was carried out in brackish seawater (6-7 parts per thousand) in the Gryt archipelago on the east coast of Sweden, using four outdoor tanks of 30-40 m3. Growth rate and nutrient uptake in batch culture were measured with the aim of estimating the water purification capacity of G. tenuisipitata in outdoor conditions. Its ability to withstand epiphytic infections was also studied. An average growth rate of 4% biomass increase per day was recorded during two seasons with a maximum growth rate of 9% d-1. The initial biomass was usually 1 kgFW m-3 (FW, fresh weight). The nutrient uptake capacity was on average ca.1 g N(i)kgFW-1 d-1 and 0.08 g P(i)kg FW-1 d-1 and the uptake rates for NH4+-N were higher than those for NO3--N. Both the growth rate and the nutrient uptake rate were highest at the highest water temperature. Co-cultivation with rainbow trout (Oncorhynchus mykiss) was tested: with trout fodder as the only nutrient input G. tenuistipitata could grow and maintain low levels of N(i) and P(i) with optimum efficiency at a trout:alga ratio of 1:1 (w:w). Epiphytic growth of filamentous green and brown algae was limited, probably as a result of the high pH values caused by inorganic carbon uptake by G. tenuis-tipitata. The growth of Enteromorpha intestinalis, the only significant epiphyte, was completely inhibited and the majority of plants died by a few days treatment with 100 mug l-1 Cu2+, a concentration that did not severely affect G. tenuistipitata. We conclude that G. tenuistipitata can be cultivated in outdoor ponds in southern Sweden during 5-6 months of the year using aerated or unaerated batch cultures and that wastewater from trout cultivation may be used as a nutrient source, resulting in purification with respect to N and P.