IN-SITU GRADOSTAT FOR THE STUDY OF A NATURAL PHYTOPLANKTON COMMUNITY WITH AN EXPERIMENTAL NUTRIENT GRADIENT

A new in situ gradostat was devised to study the dynamics of natural c ommunities having nutrient gradients within aquatic habitats. This sem i-enclosed mesocosm consisted of three components: i.e. three transpar ent, concentric cylindrical culture chambers, with diameters of 70, 60 and 40 cm, and 20 cm deep, supported by four flotation buoys, a mediu m reservoir, and a pump to supply the medium to the mesocosm. Water ex change between compartments was achieved via holes in the cylindrical walls, the number and size of the holes being chosen to regulate the r ate of exchange. Sterile culture medium was pumped into the central (f irst) chamber and diffused outwards into the second and third chambers through the perforated walls. Suitable perforations could achieve a c oncentration gradient of up to 9.5 times between the first and third c hambers within 12 h. The system was designed with the following charac teristics: (1) an experimental semi-enclosed system in the natural env ironment; (2) to maintain equivalence with the external water except f or the experimental factors; and (3) to form a manipulable gradient of experimental factors across the mesocosm. Using the in situ gradostat , the response of a natural phytoplankton community to a nitrate gradi ent was determined. A gradient from oligotrophic to eutrophic was main tained for 10 days in an oligotrophic pond. The mean growth rate of ph ytoplankton was lower in the eutrophic zone than that in the mesotroph ic zone, where the highest mean growth rate was observed. The growth r ate of each dominant phytoplankter showed its own pattern, but all rat es were highest in the mesotrophic environment of the second chamber. (C) 1998 Elsevier Science Ltd. All rights reserved.