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The diel vertical migration (DVM) of zooplankton has been extensively studied and reviewed. Yet the controlling mechanisms for DVIVI are still uncertain, although several hypotheses, e. g. predator evasion, hunger - satiation, light avoidance, have been proposed. This is particularly so for krill. An important part of understanding krill DVM depends on explaining the factors which drive krill to the surface waters at night. It is frequently speculated that krill migrate to the surface layers to feed. Although there is a vast literature on krill feeding (and the pattern of krill DVM) there has been little attempt to establish the role of feeding in DVIVI. Consequently, the main aims of this thesis were to further explore the mechanisms for krill DVIVI and also to explain the feeding strategy of krill in order to understand the role of feeding in DVM, using Northern krill Meganyctiphanes norvegica as a model system. These aims were achieved by examining the following: whether krill are selective feeders and also whether the morphology of the feeding basket constrains the food types that can be handled by krill; whether krill feed throughout DVM; the relationship between krill metabolism and feeding during DVM. Krill showed significantly greater feeding rates with larger food types compared with smaller food types and this size selection appeared to be at least in part related to the morphology of the feeding basket. Above all it seemed that krill were opportunistic omnivores and the food types handled by krill were affected by the morphology of the feeding basket. Krill also showed significantly greater feeding rates when' offered food types available during the night compared with during the day. Gut contents from field caught individuals supported that krill did not feed extensively during the day as day caught individuals had significantly less stomach pigment content compared with night caught individuals. As krill appeared to not feed extensively on day time available food types it raised the question 'is there a cost to not feeding extensively during the day'. There did appear to be a cost to the lower daytime feeding than compared with the greater feeding shown both with night time available food types and from night captured individuals. It was hypothesized that krill may break down their respiratory pigment, haemocyanin (Hc) possibly for nutrition during these periods of low feeding during the day. In a field experiment, day captured krill had significantly lower Hc concentrations ([Hcl) than individuals captured at night. There was a clear cost to the lower [Hc] of day caught krill as concentrations of lactic acid in the haemolymph (indicating an 02 debt) were significantly greater in these day captured krill than compared with night captured krill. Consequently it seems that krill break down Hc during the day probably for nutrition because, for whatever reason, they do not feed extensively on the food types available to them in the deeper depths they reside within during the day. As they ascend to the surface layers at night, where they feed to significantly greater levels on the available food types, they appeared to rebuild their [Hc] and recover from the 02 debt they incurred during the day. Feeding experiments examining the recovery of [Hc] with food types available during either the day or night showed that after starvation krill recovered their [Hc] significantly quicker (and possibly to higher levels) with night available food types compared with day available food types. As they appear to be opportunistic omnivores it is proposed that this feeding strategy would facilitate the recovery of their daytime incurred debts. Krill appeared to show an asynchronously DVIVI and in particular female krill appeared to ascend to the surface layers of the water column earlier than males. In fact female krill showed a more extreme pattern of metabolism during DVIVI, with significantly greater [Hc] (ca. twice that of males) but also greater lactate debts with the breakdown of their Hc during the day. The earlier ascent to the surface layers and also the much greater [Hc] of females may indicate that they have greater metabolic demands than males. The asynchronous pattern of krill DVIVI supports the hunger - satiation hypothesis for DVM. If satiation is modified to also include the recovery of daytime incurred debts the findings of this thesis do indeed fit this hypothesis. A tentative model is proposed for krill DVM where krill break down their Hc during the day and then recover at night with feeding in the surface layers of the water column.

This quantitative study investigated the contribution of discrete littoral habitats, the whole littoral and the Pelagic habitat to zooplankton abundance, biomass and diversity within shallow lakes. Intensive sampling was carried out in the main study lake, Enagh Lough East where samples were collected biweekly in the Alisma, Elodea, Gravel, Mud, Phalaris and Stone habitats using a quantitative littoral sampling method, the Active Tube and fifteen replicates were collected biweekly in the Pelagic habitat. In 2009 sampling was also carried out in Ballysaggart Lough and Lough Creeve to test the findings from the main study lake. The Active Tube has been designed to sample zooplankton quantitatively in discrete habitats within the littoral zone of lakes. It collects a representative sample of zooplankton within lake littoral habitats compared with the high margins of uncertainty associated with semi quantitative sampling methods. When compared with a commonly used method the Horizontal Timed Net Sweep, the Active Tube collected 50% more cladoceran species over three plant habitats than the Timed Net Sweep method and the Active Tube samples had cladoceran abundances per unit volume that were at least 2000 times greater than those collected using the Timed Net Sweep. The Active Tube method produces a more detailed description of littoral zooplankton assemblages than semi quantitative methods and it allows discrete littoral habitats to be described. In Enagh Lough East the discrete littoral habitats and the whole littoral habitat provided a greater contribution to zooplankton abundance, biomass and diversity than the Pelagic habitat per unit volume and in absolute values. The higher abundance, biomass and diversity values in the littoral habitats were due to the presence of resident littoral cladoceran species and not a result of peIagic cladoceran visitors undergoing diel horizontal migration. Littoral cladocerans made up half of the assemblage within the Pelagic habitat and 20% of the Pelagic habitat biomass. Cladoceran assemblages differed between each of the habitats. Findings from Enagh Lough East were tested in Ballysaggart Lough and Lough Creeve. Results from these test lakes strongly supported the findings in Enagh Lough East and indicated that a similarly important role of littoral zooplankton could be expected in other shallow lakes.

Bibliography: p. 145-153. / v, 153 p. : ill. ; 30 cm. / Title page, contents and abstract only. The complete thesis in print form is available from the University Library. / Thesis (Ph.D.)--University of Adelaide, Dept. of Applied Mathematics, 1996?

Ship-board experiments in the North Atlantic were used to study how food quality influences the egg production of Calanus finmarchicus feeding on natural planktonic diets. Food quality was expressed in terms of carbon (C), nitrogen (N), and the essential fatty acids eicosapentaenoic acid (EPA; 20:5(n-3)) and docosahexaenoic acid (DHA; 22:6(n-3)). Five consecutive 24 hr bottle incubations were conducted in April and July/August 2002 under in situ conditions to determine egg production rates and the ingested quantities of C, N, EPA and DHA. Biomass contributions towards growth were determined and the biochemical composition of the eggs was examined. In order to accurately determine ingestion rates, a method to account for microzooplankton grazing in particle removal experiments was developed. Balanced physiological budgets were compiled for C. finmarchicus in both seasons. The input terms of these budgets consisted of ingestion and the use of biomass, and the outputs were growth, respiration, excretion and egestion. Respiration and excretion were not determined experimentally, and were therefore determined by mass balance and compared to literature-derived values. In April, close agreement between literature- and mass balance-derived rates of respiration and excretion demonstrated that the experimentally determined components of the budget were accurate. Ingestion rates were low, and > 80 % of the C utilised was derived internally from somatic biomass. The absence of storage fatty acids and the low C:N ratio (~ 4 µg µg-1) of the biomass lost from the females indicated that these animals had been catabolising structural protein and were close to exhaustion. This suggests that when food is scarce, C. finmarchicus adopts a semelparous reproductive strategy. In July/August, the observed growth exceeded the estimated ingestion rates. This shortfall was possibly provided by cannibalising eggs. Assuming that EPA and DHA were used with high efficiency (0.9), the stoichiometric analysis predicted that these compounds were non-limiting in April. Using typical maximum growth efficiencies for C (< 0.6) and N (0.4), the former was predicted to be limiting because the biomass utilised was rich in N, EPA and DHA relative to the demand for C.

Zooplankton data from the 1988-2007 Station L4 time-series were used to determine inter-and intra-annual patterns of meroplankton community change at Station L4, Plymouth, UK. Abundances were calculated for five groups: Cirripedia, Decapoda, Polychaeta, Echinodermata and Bivalvia. Analyses showed that, while there is some annual variability, seasonal variation accounts for the major changes in the meroplanktonic community composition throughout the time-series. Cirripedia were the only group to show any significant change in abundance over the time-series. Further sampling at the study site produced data at a finer taxonomic resolution allowing for the analyses of the seasonal cycles of abundance of previously unresolved families and genera from the 1988-2007 time-series. A similar pattern was shown to that seen in the earlier time-series with no evidence of major changes in the meroplanktonic community being found. Comparison with historical data taken from Lebour (1947) showed little evidence of major variation in meroplanktonic species composition. The limitations of traditional methods of taxonomic resolution were highlighted during these analyses and led to the investigation of molecular techniques as a viable aid to identification. Bivalve larval samples were identified to species in many cases using PCR and sequencing reactions focussing on the 18S rRNA gene. Larvae of Phaxas pellucidus were shown to be the most common, and further analysis revealed the presence of two groups of species within the data over the time sampled, comprising the larvae of several hard-substrata species. Development of an RFLP technique focussing on the mtCOI gene, allowed for the successful discrimination of porcellanid larvae to species that were unable to be resolved morphologically. Long-term data-sets play an important role in revealing the long-term patterns of community composition and abundance of meroplanktonic larvae and should be continued to identify those patterns not evident over the time-period studied. Molecular techniques were valuable in aiding the taxonomic resolution of meroplanktonic larvae, allowing previously unknown patterns of species diversity and richness to be ascertained.