Population and Production Ecology of Zooplankton in Ogac Lake, a Landlocked Fiord on Baffin Island

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Abstract
Collections were analysed from the three basins of the lake in 1957, and from the middle basin of the lake and a fertilized polyethylene column in 1962. The abundant copepod, Pseudocalanus minutus, was basically annual in life cycle, spawning a number of almost synchronous broods (egg sacs) in the short period of abundant food supply in early summer. The number of young per brood was dependent on size of females, and the number of broods was independent of the amount of food during the reproductive season. The species was concentrated in rich food supply at the halocline and at depth. There was no diurnal vertical migration in summer, but ontogenetic descent of older stages appeared adapted to "seeking" colder water. The species is much smaller in the lake than in the nearby sea, and this, as well as seasonal size changes, can be attributed to temperature effects. Females, but not males, roughly doubled weight ("Brooks' Law") between stages in constant conditions. Maximum development rate was about 0.4 stages/day at 6–7 C, similar to laboratory estimates reported for temperate populations. Growth is not isometric; P. minutus grows stouter as it grows longer. Net production (some 30% as eggs) ranged from 410 to 510 mg C/m2 per year in various basins and years, and was less than doubled in the fertilization experiment.The copepod Oithona similis was strictly annual and produced almost synchronous broods (two egg sacs), which were successfully recruited only in late summer, on different food requirements from those of P. minutus. It was nonmigratory in summer, and its vertical distribution, like its size, was little influenced by temperature. Weight increments were only about 50% between copepodite stages, and development between egg and adult took at least 1.2–1.6 months. Production ranged from 202 to 347 mg C/m2 per year and was not increased in the fertilization experiment.The chaetognath Sagitta elegans was annual in the lake, reaching full size but suspending development during summer. Much reproduction occurred before and after the short period of successful recruitment of young, and size of new generation was proportional to the number of adults but not the abundance of food for the young present during this period. Overwinter mortality was less in the shallow outer basin than in the middle and inner basins, where S. elegans lived below their food species. Ontogenetic ascent of young was followed by descent of older individuals, but immatures showed marked migration into the warm, food-rich halocline at night. The species was much smaller in the lake than in the nearby sea, where it is biennial. In another landlocked fiord, temperatures intermediate between those of Ogac Lake and the nearby sea thwarted a successful annual or biennial cycle by promoting maturation at an inappropriate time of year. Net production ranged from 49 to 318 mg C/m2 per year; the wide range is attributable to variable numbers of spawning adults at time of recruitment.The medusa Aglantha digitale had two generations per year in the lake. Successful reproduction may have depended on abundant small particles (phytoplankton) as food for the young, but control of recruitment and subsequent survival remains obscure. Its scarcity in the outer basin may have been related to forced coexistence with S. elegans. Aglantha digitale generally occupied mid-depths in the water column, sinking slightly at night, its vertical distribution being little related to temperature. The species grew about four times as long in the sea outside, but the same relationship prevailed between gonad and body size. Production ranged widely between basins and years from 13 to 263 mg C/m2 per year, and was not increased by the fertilization experiment.Other holoplanktonic species in the lake are treated briefly. Of these, the ctenophores (Beroe sp.) may have been of some importance in production, and the copepod Eurytemora americana was opportunistically abundant and productive in the halocline in 1962. Others, including microzooplankton, added little production.Among the planktonic larvae of benthos, bivalve (mostly Mya truncata?) larvae spent a year in the plankton, as did some polychaetes. These larvae showed marked vertical migrations, especially the older ones. The larvae only achieved an estimated 13% of the production shown by P. minutus and O. similis combined.Allochthonous zooplankters brought in by the tides seemed ill-adapted to the lake. The few surviving allochthonous P. minutus became smaller in the warm waters, and may have insured gene flow between the lake and the sea. The allochthonous zooplankton were important as organic enrichment, equivalent to about one-half the annual autochthonous zooplankton production.Some general conclusions follow. Niche diversification is clear in the lake, and some widespread arctic species may be excluded competitively. The marked seasonality of the lake (and other waters of middle and high latitudes) means that controls of population size and growth occur when food is in excess. The studies of vertical distribution and migration demonstrate the necessity for close vertical spacing of samples, and detailed consideration of different stages and size classes. Support is partial at best for the theory that vertical migration is adapted to control growth and development and maximize rates of increase in thermally stratified waters. The results nevertheless stress the over-riding influence of temperature on body size of zooplankters. Overall zooplankton production was about 1 g C/m2 per year. "Herbivore" production was about 7% of primary production in 1962, and was little enhanced by a seven-fold increase of primary production in the fertilization experiment. Predator production ranged between 16 and 68% of "herbivore" production. Production/biomass ratios averaged about 3:1, but wide variations occurred, such that the ratios cannot be considered to be "physiological" or determinate for a species or a system. It is argued that production cannot be measured or "explained" in highly seasonal environments like Ogac Lake without detailed quantitative studies of life histories.