Study of the daily fluctuations of dissolved oxygen in fish ponds by the moving-average method Relation with temperature and solar radiations
Dissolved oxygen is a very important factor in fish ponds, particularly under intensive conditions. Thus, the daily fluctuations of dissolved oxygen have been studied in different countries and under different conditions (lakes, ponds, rivers), but with data series generally limited to a few measurements per day. The automatic data recorder of the IRRA'S experimental station provides very long series of data concerning of dissolved oxygen, temperature and meteorological factors through hourly measurements which allow the use of mathematical analysis like the moving average method. This automatic data recorder bas been in use since September 1987 and the data obtained have been analysed in research on summer oxygen deficit prevention. After 3 years' measurements it was interesting to investigate the annual change in the daily fluctuations of the dissolved oxygen. For this, daily hourly measurements of dissolved oxygen, temperature and solar radiations were taken and examined during 22 weeks from January 1988 to December 1989 (one week under ice cover). Oxygen data were analysed using seasonal decomposition by the moving average method, and the results were compared with the temperature and solar radiation measurements. The measurements were taken from 6 of the 16 IRRA'S experimental ponds (in the Dombes region, to the North-East of Lyon, France) and in a fish pond. All these ponds were semi-intensive fish-ponds stocked with Carp (Cyprinus carpio) Tench, (Tinca tinca), Grass Carp (Ctenopharingodon idella) and Silver Carp (Hypophthalmichthys molitrix). The seasonal decomposition through the moving average method devides the time series data into a trend (long term fluctuations), seasonal indices (or daily component) and residuals, but only seasonal indices, representing the daily variations, are studied here. The daily component was next spin into oxygen production and consumption if respiration is taken to be constant over one day and can be evaluated through the night oxygen decrease (EDELINE et al., 1968; ROMAIRE and BOYD, 1978; BARTHELEMY and COUTURE, 1990). Through this method, it appeared that the daily fluctuations of dissolved oxygen had the same classical aspect all the year round, except under ice cover, where it was impossible to observe a significant oxygen production. Such fluctuations, which are the result of the effect of oxygen production and consumption, have also been observed under different conditions (BOUD and LICHTKOPPLER, 1979; SIN and CHIU,1982; KEPENYES and VARADI, 1984; MARZOLF and SAUNDERS, 1984; GINOT, 1988; COUTURE, 1988). The aspect of the daily cycles was relatively constant but it was possible to distinguish 3 periods according to the amplitude of these cycles : - a winter period with less than 2 mg/l daily variations, - a summer period when these variations were greater titan 4 or 7 mg/l, - an intermediate period in spring and autumn. As these 3 periods did not occur at the same time of the year for 1988 & 1989, it appeared that the meteorological conditions (temperature and solar radiations) were important factors for determining these periods in the year. The study of oxygen production and consumption gave the same conclusions. These results have been compared with meteorological data : mean water temperature over 7-day periods (168 measurements), plus mean cumulative daily solar radiations over 7 days. There appeared significant correlations between the oxygen production and the consumption, with these meteorological factors, but these relations were different for the « summer » period (June to October) and the rest of the year. There was a delay between the period when temperature and solar radiations are best for primary production (June and July) and the period when the pond’s metabolism is maximum. There have been no measurements on primary production or algal population dynamics to explain these results clearly, but considering the observations of GRYGIEREK and WASILEWSKA (1979) and ELORANTA (1985) it is possible to give an explanation. In May-June the ponds are characterized by clear water (a higher than 60 cm transparency was observed) with low density algal populations, thus the pond ecosystem is not ready to use thermic and light energy. This delay between meteorological conditions and oxygen dynamics have been observed by ELORANTA (1985) and BOYD (1985). These results are useful in pond management, particularly when it is necessary to prevent oxygen depletion. The risk of oxygen deficits is greater in August-September, when the ecological conditions are very unstable owing to the important daily oxygen fluctuations. In this case, meteorological changes like temperature decreases or rain-fall can perturb the ecosystem and cause algal die-off with a great risk of oxygen depletion. The aim of this study was to show the possibilities of seasonal decomposition by the moving average method in the analysis of oxygen measurements. When it is possible to get long serie data, with constant skip, this method is very valuable for studying long term and daily fluctuations. The decomposition of the daily component through the method of EDELINE et al. (1968) also gives indications on primary production and oxygen consumption.