Water quality and zooplankton monitoring at the seawater circuit of a mollusk hatchery.

HOFFMEYER, M. S.; PASCUAL, M. S.

WORLD AQUACULTURE

World Aquaculture Society

Lugar: Baton Rouge, Louisiana, USA; Año: 2006 vol. 37 p. 31 - 36

1041-5602

Statistical results obtained from the analysis of variance (ANOVA) performed on data from all the studied variables suggest a marked seasonal variability between dates of these, with the exception of phaeopigments. Also, a marked spatial variability between sites exists in all variables, except chlorophyll a, phaeopigments and POC, which vary less. Seasonal differences in intake water follow natural environmental fluctuations (Krepper and Bianchi 1982, Scasso and Piola 1986, Rivas and Beier 1990, Ramírez, 1996). This behavior is found at the remaining sites with some distortions, as a result of the particular conditions and water management procedures used in each portion of the supply circuit. Ups and downs of water temperature in reservoir water are usually a response to seasonal variations in air temperature. Salinity at intake probably increases in summer due to evaporation resulting from high air temperature and heating of the incoming water running through the wide rocky intertidal.  Chlorophyll a, phaeopigments and POC varied among sites, “hatchery” sites reaching much higher values than those recorded at intake. These increments suggest that from the reservoir on, mainly in the elevated tank, several biological events are taking place: microalgal growing, degradation of chlorophyll into phaeopigments, resulting from consumption or algal alteration, consumption of herbivorous zooplankton and accumulation of alochtonous material, faecal pellets and remains of benthic algae, zooplankton and benthic animals, all contributing with chlorophyll a, other pigments,  particulate organic carbon and probably new nutrients (N - P) formed by regeneration, though this issue was not addressed, to the system. Zooplankton species richness is higher at intake than in the other sites. Conversely, zooplankton abundance shows a marked increase inside the hatchery, representing  fewer species (O. nana, T. varians, P. parvus and larvae of cirripeds, bivalves or copepods). The behavior of these two variables confirms the existence and active reproduction of populations of these species in the external tanks (reservoir) as well as in the elevated tank, more than in the remaining portions of the water supply system.   Zooplankton at the intake comprises species belonging to coastal-estuarine assemblages from temperate-cold latitudes commonly found in argentine locations (Hoffmeyer 1983, Boltovskoy 1981, 1999). Seasonal succession agrees in general terms with that is known from the only background information for the San Matías gulf (Ramírez 1996), partly with that of Bahía Blanca estuary (Hoffmeyer 1994) and with the one observed in Nueva Bay (Nuevo gulf) in Patagonia, reported in Esteves et al. (1996). Samples included an important fraction of surfing elements and benthic fauna, harpacticoid copepods and amphipods, resulting from water mix in this particular environment. Zooplankton species richness and total zooplankton abundance found in the intake water was lesser than that mentioned by Ramírez (1996), 39 taxons, 15 belonging to copepods, in around 36 stations allocated in different areas within San Matías gulf (surveys onboard B/I Cruz del Sur, 1974-1975). In addition, while the copepods O. helgolandica (=O. similis), P. parvus, A. tonsa and C. vanus had been previously reported as the most abundant species in this gulf (Ramírez 1996), these species showed low abundance during the present study. O. nana mentioned as rare in the gulf by this author was dominant in the seawater intake of the hatchery. The lack of information corresponding to intermediate months plus the variable conditions found in the water supply into the hatchery at different dates, caused by operation/managment differences, makes the interpretation of some results rather difficult.  Nevertheless, and even when the results obtained show a high seasonal and spatial variability, they provided important information regarding the efficiency of the filtration systems and the management protocols in use at the hatchery.  Water treatment methods and management protocols were reformed based on these results and others suggested in the literature to improve water quality and biological production in this hatchery (Utting et al. 1983, 1985, Chen 1984, Jaeckle and Manahan 1989, Paillard et al. 2001, Renoult and Arzul 2001).