A study of a calanoid copepod population in an arctic lake

Comita, Gabriel

The life cycle of an arctic calanoid copepod, Limnocalantus johanseni Marsh, was studied in samples taken from a small shallow lake called Imikpuk, located at Point Barrow, Alaska, during the summer months of 1952 from the time the lake ice melted until the lake froze over again. Other animals found in this lake were Cyclops scutifer Sars, Eurytemora canadensis Marsh, Daphnia pulex var. tenebrosa Sars, two species of Anostraca, one notostracan, Apus, and two other calanoids, Diaptomus glacialis Sars and Diaptomus alaskaensis M. S. Wilson. Rotifers of the genera Kellicottia, Synchaeta, Notholca, and Keratella were found to be abundant in the samples. As far as is known there were no fish in Imikpuk. Imikpuk is a freshwater lake separated from the Arctic Ocean by a shallow marsh. Its temperature cycle in 1952 was as follows: the winter ice melted about July 22 and the lake temperature reached a maximum of 10.5░C by August 10, then rapidly dropped to freezing again by September 27. The lake was free of ice for a period of 65 days in 1952. The plankton of Imikpuk was studied by taking quantitative hauls at time intervals that varied from one to five days. Two consecutive hauls were made on each date. A Clarke-Bumpus plankton sampler was used for making all hauls. The constant, liters per revolution, for this particular instrument was found to be 4.6. When the plankton was counted the population deviation from the mean for the two samples taken on each date was computed. Expressed as a percentage this was found to average 8.54% of the mean for all samples and it ranged from 0.158 to 35.3%. The instar populations were determined by counting and identifying the stages. Each of the stages was recognized by the appearance and state of development of the appendages as well as the body form. Hatching of the eggs occurred during the time the ice was melting. All six nauplius stages were found in the first samples taken. The life cycle was found to consist of six nauplius and six copepodid stages in addition to the egg. The duration of each instar was obtained by estimation of the time interval between peaks of each stage. These time intervals in days were estimated for all stages as follows: for nauplius stages I, Ii, and III, less than four days; for nauplius IV, 2 days; N V, 2.5; N VI, 5; for copepodid I, 3.5; C II, 3; C III, 3; C IV, 5; C V, 16; C VI (adult), approximately 120 days. Data taken on the reproductive activities indicated that 16 days or possibly less were required for sexual maturation in the adult females. Based upon the time of appearance of spermatophores on the females it is certain that males required more than 16 days for sexual maturation. Sexual reproduction occurred over a period known to be at least 47 days after September 10. Some observations were made of environmental conditions existing while egg hatching occurred in the spring and after egg production in the fall. These observations indicate that a slight increase in temperature may have initiated egg hatching. Other possible stimuli for hatching are also discussed. Median lengths for all stages in the samples are presented. The means of these were found to be as follows (mm): N I, 0.229; N II, 0.290; N III, 0.364; N IV, 0.434; N V, 0.434; N VI, 0.759; C I, 0.821; C II, 0.990; C III, 1.21; C IV 1.29; C V (male), 1.42; C V (female), 1.46; C VI (male), 1.74; C VI (female), 1.79. (abstract continued in notes)

A study of a calanoid copepod population in an arctic lake

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