Thesis (Ph. D.)--Massachusetts Institute of Technology and Woods Hole Oceanographic Institution, 1979. / Vita. Grant no.: OCE 77-22511 and OCE 75-21715. Includes bibliographical references (p. 141-153).

Thesis (M.S.)--Rhodes University, 2002. / Title from PDF t.p. (viewed on July 22, 2006). Includes bibliographical references (p. 68-88).

Thesis (M. Sc. (Zoology and Entomology))--Rhodes University, 2006.

University, Diss., 2004--Kiel.

University, Diss., 2004--Kiel.

In recent years, evidence has been found of Diel Vertical Migration (DVM) in zooplankton during the Polar Night in the Arctic Ocean. However, the drivers of this behaviour during an apparent lack of illumination and food are poorly understood, as is its spatial extent across the Arctic Ocean. A novel dataset comprising 58 deployments of moored Acoustic Doppler Current Profilers is used in this study to observe the vertical migratory behaviour of zooplankton on a pan-Arctic scale. Methods of circadian rhythm analysis are applied to detect synchronous activity. South of 75°N, DVM continues throughout winter (albeit with reduced vertical amplitude when compared to other times of the year). DVM is seen to cease for a short period of time (up to 50 days) at latitudes between 75° and 82°N. The duration for which DVM ceases is controlled primarily by latitude (and therefore the altitude of the sun), but is modified by sea-ice presence and other environmental parameters. A DVM pattern is not seen at 90°N at any time of the year, but ~365 day periodicity is detected in the surface backscatter levels at this latitude. During the Polar Night, the moon controls the vertical positioning of zooplankton across the Arctic. Aggregations at depth coincide with an avoidance of the surface for several (< 6) days over the full moon. The deepest aggregation is seen at 110 m, indicating a depth limit of moonlight perception. A new type of migratory behaviour is described: Lunar Vertical Migration (LVM) which exists as LVM-day (24.8 hour periodicity) and LVM-month (29.5 day periodicity) on a pan-Arctic scale. The strongest LVM-day is seen at 50 m depth. Sea-ice and cloud are found to modify both of these lunar responses. The results presented by this thesis show continuous activity throughout winter, and challenge assumptions of a quiescent Polar Night.

No description available.

The community structure and predation impact of carnivorous macrozooplankton (>2 cm; chaetognaths, medusae, ctenophores and mysids), with particular emphasis on the chaetognaths Eukrohnia hamata and Sagitta gazellae, were investigated during three surveys conducted in late austral summer (April/May) of 2001, 2004 and 2005 in the Polar Frontal Zone in the vicinity of the Prince Edward Islands (46º45’S, 37º50’E), Southern Ocean. The 2001 survey formed part of the Marion Offshore Variability Ecosystem Study (MOVES II), while the 2004 and 2005 surveys formed part of the Dynamics of Eddy Impacts on Marion’s Ecosystem study (DEIMEC III and IV respectively). Macrozooplankton samples were collected using WP-2, RMT-8 and Bongo nets. Results of the hydrographic survey indicated that the region of investigation, the Polar Frontal Zone (PFZ), is an area of high mesoscale variability. During the 2004 survey the Antarctic Polar Front (APF) and the Subantarctic Front (SAF) merged to form an intense frontal feature with subsurface temperature and salinity ranging from 8.5-7.5ºC and 34.15-33.88, respectively. A cyclonic cold core eddy, believed to have been spawned from the APF, was observed during the 2005 survey. Macrozooplankton abundance and biomass ranged from 0 to 43.731 ind. m⁻³, and from 0 to 41.55 mg wwt m⁻³ respectively, during the three surveys. Among the carnivorous macrozooplankton, chaetognaths (Eukrohnia hamata and Sagitta gazellae) were most prominent, contributing up to 85% of the total biomass during all three surveys. Elevated biomass values were found near and within the frontal feature during the 2004 survey, and also along the eddy edge during the 2005 survey. However, hierarchical cluster analysis did not reveal the presence of distinct zooplankton groupings associated with the various water masses encountered during the surveys and this is probably due to the high mesoscale variability in oceanographic conditions that are characteristic of the PFZ. The total average predation impact of the selected carnivorous macrozooplankton during the 2001, 2004 and 2005 surveys accounted for 4.93 ± 6.76%, 0.55 ± 0.51% and 4.88 ± 4.45 of the mesozooplankton standing stock, respectively. S. gazellae had the highest consumption rate in all three surveys, consuming up to 800 g Dwt 1000m⁻³d⁻¹ during the study. Of the two chaetognaths, E. hamata dominated the chaetognath standing stock. The combined abundance and biomass values of E. hamata and S. gazellae ranged from 0 to 43.73 ind. m⁻³ and from 0 to 41.551 mg wwt m⁻³ respectively, during the three surveys. Inter-annual variability in the chaetognath densities was apparent. Highest abundances and biomasses tended to be associated with specific water masses, confirming the existence of a relationship between zooplankton community structure and hydrographic conditions. Generally, about 90% of the chaetognaths contained no food in their guts. S. gazellae consumed a wider variety of prey. Oil droplets occurred in the guts of ≈ 51% of E. hamata. Cannibalism was low in both species, but greater in S. gazellae than E. hamata. During the three surveys, the feeding rate values of E. hamata and S. gazellae went up to 0.48 and 2.099 prey d⁻¹ respectively. S. gazellae also had a greater predation impact on the mesozooplankton standing stock than E. hamata. The mean predation impact of the chaetognaths combined was 0.31 ± 0.291%, 0.52 ± 0.28% and 0.53 ± 0.56% of the mesozooplankton standing stock during the 2001, 2004 and 2005 surveys, respectively. During all three surveys, the majority of individuals (≈ 76%) of the chaetognaths were at stage I maturity, suggesting that during the time of study the chaetognaths were not reproducing. In both species a significant difference (log-linear analysis, p < 0.05) in maturities between the years investigated was observed. In general, there were no differences in lengths and maturities between the different water masses encountered during the surveys. The lengths of E. hamata and S. gazellae ranged from 5 to 24 mm and from 9.4 to 63.6 mm, respectively.

No description available.

Feeding habits of two coexisting species of Chaoborus, C.trivittatus and C.americanus, were investigated in lake and in laboratory experiments, and by an extensive survey of larval crop contents. The field study showed that C.trivittatus has a two year life cycle, migrates dielly during the summer season down to 20 m, and is the more abundant of the two Chaoborus species. C.americanus has a one year life cycle and migrates at most over a distance of only 5 m. Seasonal abundance and distribution of most plankton types found in Eunice Lake were monitored during 1971 and 1972. Zooplankton had a low standing crop and were mostly found above 6 m. Zooplankton size and swimming velocity and size of larval head parts were measured in the laboratory and, together with the above data on vertical distribution, were used to evaluate the availability of zooplankton as prey for Chaoborus. The lake and laboratory experiments showed that rates of larval feeding and digestion increase significantly with temperature. Temperature, however, does not seem to affect feeding rate of the 4th instar C.trivittatus larvae. Feeding rates increase with larval age and vary with prey type and density. From analysis of chaoborid crop contents, diet differences were found among all instars of the two species, and these were related to prey size, abundance, and distribution. The calculated percent of prey standing crop that the Chaoborus in Eunice Lake could potentially remove ranged from a minimum of 3% for nauplii to a maximum of 20% for Diaptomus kenai. Seasonal changes in Choaborus instar composition and in zooplankton species composition result in reduced predation on any single zooplankton group. The observed diet of Chaoborus larvae in Eunice Lake was shown to be closely related to the morphology of Chaoborus and their prey and to the relative distribution of predator and prey. / Science, Faculty of / Zoology, Department of / Graduate