THE STRUCTURE AND FUNCTION OF ZOOPLANKTON IN THINGVALLAVATN, ICELAND

The zooplankton of Thingvallavatn is dominated by three species of rotifers and another three species of crustaceans. The rotifer Polyarthra dolichoptera (Idelson) constitutes up to 90% of the rotifer biomass in summer, while in autumn Keratella cochlearis (Gosse) and Conochilus unicornis (Rousselet) together contribute 55-80% of the rotifer biomass. Cyclops abyssorum G. 0. Sars s.l. varies from a minimum of 30% in September to 100% of crustacean numbers, during winter. Leptodiaptomus minutus (Lillj.) reaches maximum or 50% by numbers in August and Daphnia longispina (0. F. Mull.) s.str. 60% by numbers in September. L. minutus and C. abyssorum have univoltine life cycles. The populations of L. minutus and D. longispina overwinter as diapause eggs/ephippia, but C. abyssorum as nauplii 1 to 3. L. minutus tends to stay in the intermediate layers of the lake, with an overall downward trend of movement in depth from May to October. The nauplii and copepodite 1 larvae of C. abyssorum have their maxima in the deeper water layers, while older copepodite stages, especially 4 and 5, stay in the uppermost regions. The total population tended to ascend from June to December, coinciding with the decrease in insolation. D. longispina tends to stay in the surface layers during spring peak production, after which the population descended. The diel pattern varies seasonally between species and stages. The migration pattern of the older stages was clearly to avoid predation. The fecundity of crustacean species is low compared to more eutrophic lakes. The recruitment of L. minutus, C. abyssorum and D. longispina was estimated at 1.3-1.4, 1.2-1.8 and 0.45 to 0.47 million ind m-2 yr-1, respectively. The birth rates of D. longispina peaked in the early summer (max. 0.2), after the spring peak in primary production. The birth rate for C. abyssorum peaked in late September (max. 0.7). The daily mortality rate of L. minutus was 0.16% for the nauplius stages, but increased to 0.3% for the copepodite stages. The survivorship curves for C. abyssorumn were similar in successive years. The average mortality rate was approximately 0.5% in nauplius and the first copepodite stages, increasing to 3% for copepodite 4, as a result of increased predation by arctic charr on later copepodite stages. The total annual production of crustaceans was stable during the research period. approximately 9 g DW m-2. The production of rotifers was estimated at 1.5 g DW m-2. The total secondary production in the pelagic zone was therefore ca. 10 g D\\ m-2. The turnover ratio (P/B) for the crustaceans during 1975 to 1977 was 3.97, 4.27, and 4.32, respectively. The crustacean zooplankton contributed 50 to 100% of the observed food of pelagic planktivorous arctic charr Salvelinus alpinus (L.) in the pelagic zone. C. abyssorium and D. longispina were the dominant food species, Cyclops being dominant in summer and Daphnia in autumn. On all occasions the composition of the stomach contents differed significantly from that of the plankton. the fish selecting always the biggest stages. In the autumn. when Daphnia became the dominant food , the size selection mechanisms for Cyclops were weakened.