DIURNAL CHANGES IN SHALLOW AQUATIC HABITATS

Diurnal changes were studied at different times of the year in a variety of Louisiana ponds, swamps, ditches and streams by the following procedure. Collections were made between 0500 Md 0600 and between 1500 and 1600 on one day, and at the same times scvcral days later. Each sample was analyzed chemically and biologically. The times chosen measure the approximate high and low values of dissolved substances in the daily chemical cycle. The extent of change in chemical factors and plankton indicated the degree and role of oxidative processes in a shallow habitat, and provided a measure for comparing diffcrent habitats. Plankton populations, predominantly protozoa, often changed during the day, and sometimes changed significantly within 2 hr after sunrise. The conditions and organisms found in shallow habitats may thus depend upon the time of sampling. MATERIALS AND METHODS A variety of ponds, ditches, swamps, and streams were studied in southeastern Louisiana during 1959-61 to discover the variations that can occur in shallow aquatic habitats. The maximal and minimal points on the daily dissolved oxygen cycle were chosen to represent the extremes of diurnal variation A series o,f observations showed that the minimal point occurs just before sunrise and the maximal 2 to 3 hr before sunset. In southeastern Louisiana these times are approximately 0630 and 1500 in winter and 0500 and 1600 in summer. Ten shallow habitats were studied at different times of the year by the following procedure. Collections were made between 0500 and 0600 and between 1500 and 1600 on one day, and again at the same times several days later. Cosllections for chemical and plankton analyses were made by lowering quart jars just below the water surface. Water for dissolved oxygen analyses was collected in glass stoppered bottles. Collections of bottom fauna were made by use of Lund’s (1942) apparatus, consisting of a thistle tube passing through a two-hole stopper into a collecting jar. A second tube passed from the jar through the other hole to a small hand pump, so that suction could be applied to draw water into the jar. All parts of the apparatus except the pump were polyethylene. Dissolved olxygen samples were fixed, and temperature and pH readings were taken in the field at time of collection, The latter were determined by a Beckman model 180 pocket pH meter. Chemical analyses were performed within 3 hr of collection. The following analyses (with methods in parcnthescs ) were performed according to American Public Health Association ( 1955) procedures: alkalinity (methyl-orange titration), chloride ( Mohr ) , dissolved oxygen ( azide modification of Winkler method ) , nitrate ( phenoldisulfonic acid), nitrite ( diazotization ) , ammonia ( direct Nesslerization ) , phosphate ( stannous chloride), silicon ( calorimetric silicomolybdate ) , total iron ( phenanthroline), and hardness ( EDTA). Carbon dioxide was found from a graph ( Betz 1955) showing the relationships of carbon dioxide, alkalinity, and pH. Samples for biological analysis were centrifuged and the concentratcs examined in a Sedgewick-Rafter counting cell, This procedure was adapted from Lund ( 1942). In many collections, marked population changes occurred between morning and afternoon. The nature and causes of these changes were studied by the following procedure. On each occasion, 5 collections were made; early in the evening, just before sunrise the next day, about an hour later, another hour later, and early afternoon. Temperature, pH, and dissolved oxygen were measured in the field at each collec-