Chapter 6: Zooplankton

6 Zooplankton R. G . Stross, M. C. Miller, and R . J. Daley COMMUNITIES, LIFE CYCLES, AND PRODUCTION Community Composition and Life Cycles Crustaceans dominate the zooplankton of polygon center ponds and also of pools in nearby troughs which form between polygons . Rotifers are present but rare. The zooplankton species found at Barrow (Table 6-1, Figure 6-1) are commonly found throughout northern Alaska, as shown by collections from the Colville River to the east (Reed 1962) and from Cape Thompson to the west (Hilliard and Tash 1966) . Despite the similarity of species over the entire region, there are local differences in the community structure between the zooplankton of these shallow waters and the zooplankton of nearby lakes . In the shallow ponds and trough pools the Crustacea are quite large and often heavily pigmented . For example, the fairyshrimp frequently reach 15 mm and the Heterocope is the largest freshwater copepod. The species of the genus Daphnia, D. middendorffiana and D. pulex, are two of the largest known . In contrast, smaller forms of zooplankton are found in lakes containing planktivorous fish . Ikroavik and Sungoroak Lakes, for example, have no fairyshrimp but do have a small, transparent Daphnia (D. longiremis) . A third large lake, Imikpuk, has no fish ; it contains the same species of large Crustacea as the ponds . From this evidence, it is likely that size-selective predation by fish may prevent the larger species of zooplankton from inhabiting deep lakes. In addition to the differences in zooplankton species between lakes and ponds, there are also differences between the ponds and trough pools at the IBP site (Figure 6-1). Calanoid copepods (Diaptomus and Heterocope) and D. middendorffiana are present in the ponds but not in the trough pools ; fairyshrimps are present in both but are rare in pools . Daphnia pulex, present only in the troughs, is somewhat smaller than D. middendorffiana and has only a small amount of pigmentation (Edmondson 1955) . Cyclopoid copepods (Cyclops) are abundant in both habitats . These tundra ponds are as species-rich as alpine ponds at mid- latitudes (Dodson 1974, Sprules 1972) . * R. Stross 251 252 R. G. Stross et al . TABLE 6-1 Species of Crustacea in the Plankton and Their Life Cycles in a Typical Polygon Pool (IBP Pond CJ on the Tundra near Barrow Overwintering Start of Species stage reproduction Copepoda Cyclops vernalis Fischer copepodid June 15 (pre-adult) Cyclops magnus Marsh copepodid June 15 (pre-adult) Diaptomus bacillifer Kolbel embryo July 20 (early) Diaptomusglacialis Lilljeborg embryo late July (early) Diaptomus alaskaensis M . S . Wilson Heterocope septentrionalis embryo late July-August Juday and Matthowski Branchiopoda Branchinecta paludosa embryo July 20 O .F . Miller (early) Polyartemiella hazeni Murdoch - Daphnia middendorffiana embryo July 15 Fischer (early) All Crustaceans except for the Daphnia are monovoltine (one generation per year); offspring of the overwintering generation enter diapause to survive through the next winter . Usually, the embryonic stages enter diapause, but in the cyclopoid copepods a copepodid (pre-adult) instar overwinters . (The precise overwintering instar was not determined.) In some species, such as Cyclops strenuus, nearly all copepodid instars have the ability to encyst (Elgmork 1959, Szlauer 1963). The advanced stage of development of the overwintering cyclopoids enables them to reach adulthood and reproduce shortly after the ice melts in June . The Daphnia are the only taxa at Barrow to produce a generation that does not overwinter . The sequence of events in the Daphnia life cycle follows: first, the overwintering embryos (formerly called ephippial eggs) hatch shortly after the ice thaws ; second, the hatchlings, which are all female, reach maturity and in mid-July release a brood of young (Stross 1969, Stross and Kangas 1969) ; third, after this brood of young is produced, the females continue to reproduce but now produce diapausing embryos (Edmondson 1955, Stross 1969) . If food is exceptionally abundant, the brood of young (called young-of-the-year) will also produce diapausing embryos . Only females are known for D. middendorffiana but Zooplankton 253 Polygon Pond Trough Pool Cyclops verna/is i Cyclops magnus '~ - Daphnia middendorffiono D. pu/ex Branch/necto pa/udosa Po/yorfemie/% hozeni Heferocope septentriona/is Very rare if present Dioptomus glacial/s Dioptomus baci//ifer FIGURE 6-1 . Crustacea found in the plankton of ponds and trough pools at Barrow, Alaska . in D. pulex a small number (4%) of the young-of-the-year may be infertile males (Brooks 1957) . Stross (1969) has shown that the switch froth production of young to production of diapausing embryos is facultative and under environmental control . In an experiment with constant light and constant temperature (12°C) in the laboratory, the females that hatched from the overwintering eggs produced only young ; at 20 hours or less of light per day only diapausing embryos were produced but this switch occurred only in animals which had already produced a brood of young . The critical photoperiod determined in the laboratory of L22 :D2 may well be acting in the populations of lakes too, as these animals made the switch in late August when the ratio was about L22 :D2. On the other hand, populations in pools and ponds make the switch in mid-July (Stross and Kangas 1969) . This earlier switch may be caused by the temperature regime of the ponds which is warmer and more variable than that of the lakes . Thus, pond temperatures may change as much as 10° in a day and may fall to 5°C on any day of the summer . These low temperatures, below 10°C, may act as 254 R . G. Stross et al . some sort of physiological "dark interval ." Other arthropods have a similar interval timer that determines when they can respond to a photoperiodic induction (Lees 1960) ; in the Daphnia, the production of a brood of young may also be an interval timer. From the foregoing account it is clear that the initial density of zooplankton populations in spring is determined by the number of overwintering animals that emerge . An upper limit to density is thus placed on all monovoltine species and therefore on the annual production . The Daphnia have an additional production capability from the young produced by the overwintering generation . Standing Crops The Crustacea were collected by pouring 8, 10, or 12 liters of water through a No . 25 mesh plankton net . The net could not be towed in the pond because it would have disturbed the sediment ; instead, 1- or 3-liter water samples were dipped from the pond with a plastic bucket mounted on a long pole. This procedure may have resulted in underestimates of the fast moving animals such as the calanoid copepods and of the bottom huggers such as the fairyshrimps . Triplicate samples were taken at one or more stations, depending on the pond being sampled . The distribution of the zooplankton within a pond was not random . Fairyshrimps and Daphnia were frequently clumped, either as a dense swarm (Daphnia) or as a reproductive group (fairyshrimps) . The two species of fairyshrimps were segregated one from the other in the mating swarms and the individuals or attached pairs often hovered in rows . Obviously, there was poor sampling precision for these forms . The 80 i I I I - N (I) Eggs V (2) Adult Females a (3) Adult Males -60 6 v c (4) Copepodids - 72 (5) Nauplii • 4 - - 40 •EE D V • ~~ n N o 2- 5` `; - 20 z • w 3 , \ V o V i 7 ~~ a0 ' I N 0 r i 0 20 10 20 10 20 W 0 Jun Jul Aug FIGURE 6-2 . Densities of various stages of Cyclops vernalis in Pond C, 1971 . Sample points are means of four to eight 10-liter samples. Zooplankton 255 precision was better for the copepods, with only a small variance between samples at the same station . The variance was greater between stations, however, and there was also a tendency for the copepodids to cluster at the margins of the ponds in the spring. In a later study of Daphnia distribution, Haney and Buchanan (personal communication) showed a correlation between the location of most of the D. middendorffiana in a pond and the sun on clear or hazy days. The population seemed to move toward the margin of the pond farthest from the sun and in midsummer actually circled the entire pond margin over 24 hours . The cyclopoid copepods were numerically more abundant than other zooplankton forms, especially early in the summer (Figure 6-2). As the ice melted in 1971, the late juvenile and adult stages of Cyclops vernalis and C. magnus appeared in the water. By mid-June the adults of C . vernalis were carrying egg sacs and the first nauplii were seen on 21 June . By the last week in June, most of the overwintering animals had been transformed into adults which reached a maximum of 2 .5 females and 1 .7 males liter - ' . The maximum number of nauplii, 35 liter -', were found on 13 July . After 25 July no more than 10 liter' were collected . The copepodids began to appear in early July and were most abundant during the first third of July . From late July to mid-August, there was approximately 1 .0 animal liter' . Other species of cyclopoid copepods were present, but the 10-liter samples were not large enough to permit quantitative estimates . Although the copepodids of all the cyclopoid species may enter diapause in the sediment (Elgmork 1959, Szlauer 1963) their entry was not examined in the ponds at Barrow . 40 I I I I r I I (I) D . bacillifer (2) D. glacialis fl). alaskensis (3) Heterocope -0 N 1 20 10 20 l0 0 Jun Jul Aug FIGURI 6-3 .

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